I still consider the HeNe laser to be the quintessential laser: An electrically excited gas between a pair of mirrors. It is also the ideal first laser for the experimenter and hobbyist. OK, well, maybe after you get over the excitement of your first laser pointer! :) HeNe's are simple in principle though complex to manufacture, the beam quality is excellent - better than anything else available at a similar price. When properly powered and reasonable precautions are taken, they are relatively safe if the power output is under 5 mW. And such a laser can be easily used for many applications. With a bare HeNe laser tube, you can even look inside while it is in operation and see what is going on. Well, OK, with just a wee bit of imagination! :) This really isn't possible with diode or solid state lasers.
I remember doing the glasswork for a 3 foot long HeNe laser (probably based on the design from: "The Amateur Scientist - Helium-Neon Laser", Scientific American, September 1964, and reprinted in the collection: "Light and Its Uses" [5]). This included joining side tubes for the electrodes and exhaust port, fusing the electrodes themselves to the glass, preparing the main bore (capillary), and cutting the angled Brewster windows (so that external mirrors could be used) on a diamond saw. I do not know if the person building the laser ever got it to work but suspect that he gave up or went on to other projects (which probably were also never finished). And, HeNe lasers are one of the simplest type of lasers to fabricate which produce a visible continuous beam.
Some die-hards still construct their own HeNe lasers from scratch. Once all the glasswork is complete, the tube must be evacuated, baked to drive off surface impurities, backfilled with a specific mixture of helium to neon (typically around 7:1 to 10:1) at a pressure of between 2 and 5 Torr (normal atmospheric pressure is about 760 Torr - 760 mm of mercury), and sealed. The mirrors must then be painstakingly positioned and aligned. Finally, the great moment arrives and the power is applied. You also constructed your high voltage power supply from scratch, correct? With luck, the laser produces a beam and only final adjustments to the mirrors are then required to optimize beam power and stability. Or, more, likely, you are doing all of this while your vacuum pumps are chugging along and you can still play with the gas fill pressure and composition. What can go wrong? All sorts of things can go wrong! With external mirrors, the losses may be too great resulting in insufficient optical gain in the resonant cavity. The gas mixture may be incorrect or become contaminated. Seals might leak. Your power supply may not start the tube, or it may catch fire or blow up. It just may not be your day! And, the lifetime of the laser is likely to end up being only a few hours in any case unless you have access to an ultra-high vacuum pumping and bakeout facility. While getting such a contraption to work would be an extremely rewarding experience, its utility for any sort of real applications would likely be quite limited and require constant fiddling with the adjustments. Nonetheless, if you really want to be able to say you built a laser from the ground up, this is one approach to take! (However, the CO2 and N2 lasers are likely to be much easier for the first-time laser builder.) See the chapters starting with: Amateur Laser Construction for more of the juicy details.
However, for most of us, 'building' a HeNe laser is like 'building' a PC: An inexpensive HeNe tube and power supply are obtained, mounted, and wired together. Optics are added as needed. Power supplies may be home-built as an interesting project but few have the desire, facilities, patience, and determination to construct the actual HeNe tube itself.
The most common internal mirror HeNe laser tubes are between 4.5" and 14" (125 mm to 350 mm) in overall length and 3/4" to 1-1/2" (19 mm to 37.5 mm) in diameter generating optical power from 0.5 mW to 5 mW. They require no maintenance and no adjustments of any kind during their long lifetime (20,000 hours typical). Both new and surplus tubes of this type - either bare or as part of complete laser heads - are readily available. Slightly smaller tubes (less than 0.5 mW) and much larger tubes (up to approximately 35 mW) are structurally similar (except for size) to these but are not as common.
Much larger HeNe tubes with internal or external mirrors or one of each (more than a *meter* in length!) and capable of generating up to 250 mW of optical power have been available and may turn up on the surplus market as well (but most of these are quite dead by now). Even more powerful ones have been built as research projects. The largest HeNe lasers in current production are rated between 35 to 50 mW.
Highly specialized configurations, such as a triple XYZ axis triangular cavity HeNe laser in a solid glass block for an optical ring laser gyro, also exist but are much much less common. Common HeNe lasers operate CW (Continuous Wave) producing a steady beam at a fixed output power unless switched on and off or modulated. (At least they are supposed to when in good operating condition!) However, there are some mode-locked HeNe lasers that output a series of short pulses at a high repetition rate. And, in principle, it is possible to force a HeNe laser with at least one external mirror to "cavity dump" a high power pulse (perhaps 100 times the CW power) a couple of nanoseconds long by diverting the internal beam path with an ultra high speed acousto-optic deflector. But, for the most part, such systems aren't generally useful for very much outside some esoteric research areas and in any case, you probably won't find any of these at a local flea market or swap meet! :)
Nearly all HeNe lasers output a single wavelength and it is most often red at 632.8 nm. (This color beam actually appears somewhat orange-red especially compared to many laser pointers using diode lasers at wavelengths between 650 and 670 nm). However, green (543.5 nm), yellow (594.1 nm), orange (611.9 nm), and even IR (1,1152 and 3,921 nm) HeNe lasers are available. There are a few high performance HeNe lasers that are tunable and very expensive. And, occasionally one comes across laser tubes that output two or more wavelengths simultaneously but this may actually be a 'defect' resulting from a combination of high gain and insufficiently narrow band optics - these tubes tend to be unstable.
Manufacturers include Melles-Griot, Spectra-Physics, Uniphase, and several others. (You may also find Aerotech and Siemens HeNe lasers though these companies have gotten out of the HeNe laser business.) HeNe tubes, laser heads, and complete lasers from any of these manufacturers are generally of very high quality and reliability. A more complete list can be found at Photonics Buyers' Guide - Lasers, HeNe and in the chapter: Laser and Parts Sources.
HeNe lasers have been found in all kinds of equipment including:
Nowadays, many of these applications are likely to use the much more compact lower (drive) power solid state diode laser. (You can tell if you local ACME supermarket uses a HeNe laser in its checkout scanners by the color of the light - the 632.8 nm wavelength beam from a HeNe laser is noticeably more orange than the 660 or 670 nm deep red from a typical diode laser type.)
Melles Griot catalogs used to include several pages describing HeNe laser applications. I know this was present in the 1998 catalog but has since disappeared and I don't think it is on their Web site.
Also see the section: Some Applications of a 1 mW Helium-Neon Laser for the sorts of things you can do with even a small HeNe laser.
Since a 5 mW laser pointer complete with batteries can conveniently fit on a keychain and generate the same beam power as an AC line operated HeNe laser half a meter long, why bother with a HeNe laser at all? There are several reasons:
However, the market for new HeNe lasers is still in the 100,000 or more units per year. What can you say... If you need a stable, round, astigmatism-free, long lived, visible 5 to 10 mW beam for under $500 (new, remember!), the HeNe laser is still the only choice.
Below are just a few possibilities.
(Portions from: Chris Chagaris (pyro@grolen.com).)
For many more ideas, see the chapters: Laser Experiments and Projects and Laser Instruments and Applications and the many references and links in the chapter: Laser Information Resources.
However, unlike those for laser diodes, HeNe power supplies utilize high voltage (several kV) and some designs may be potentially lethal. This is particularly true of AC line powered units since the power transformer may be capable of much more current than is actually required by the HeNe laser tube - especially if it is home built using the transformer from some other piece of equipment (like an old tube type console TV or that utility pole transformer you found along the curb) which may have a much higher current rating.
The high quality capacitors in a typical power supply will hold enough charge to wake you up - for quite a while even after the supply has been switched off and unplugged. Depending on design, there may be up to 10 to 15 kV or more (but on very small capacitors) if the power supply was operated without a HeNe tube attached or it did not start for some reason. There will likely be a lower voltage - perhaps 1 to 3 kV - on somewhat larger capacitors. Unless significantly oversized, the amount of stored energy isn't likely to be enough to be lethal but it can still be quite a jolt. The HeNe tube itself also acts as a small HV capacitor so even touching it should it become disconnected from the power supply may give you a tingle. This probably won't really hurt you physically but your ego may be bruised if you then drop the tube and it then shatters on the floor!
However, should you be dealing with a much larger HeNe laser, its power supply is going to be correspondingly more dangerous as well. For example, a 35 mW HeNe tube typically requires about 8 mA at 5 to 6 kV. That current may not sound like much but the power supply is likely capable of providing much more if you are the destination instead of the laser head (especially if it is a homemade unit using grossly oversized parts)! It doesn't take much more under the wrong conditions to kill.
After powering off, use a well insulated 1M resistor made from a string of ten 100K, 2 W metal film resistors in a glass or plastic tube to drain the charge - and confirm with a voltmeter before touching anything. (Don't use carbon resistors as I have seen them behave funny around high voltages. And, don't use the old screwdriver trick - shorting the output of the power supply directly to ground - as this may damage it internally.)
See the document: Safety Guidelines for High Voltage and/or Line Powered Equipment for detailed information before contemplating the inside or HV terminals of a HeNe power supply!
Now, for some first-hand experience:
(From: Doug (dulmage@skypoint.com).)
Well, here's where I embarrass myself, but hopefully save a life...
I've worked on medium and large frame lasers since about 1980 (Spectra-Physics 168's, 171's, Innova 90's, 100's and 200's - high voltage, high current, no line isolation, multi-kV igniters, etc.). Never in all that time did I ever get hurt other than getting a few retinal burns (that's bad enough, but at least I never fell across a tube or igniter at startup). Anyway, the one laser that almost did kill me was also the smallest that I ever worked on.
I was doing some testing of AO devices along with some small cylindrical HeNe tubes from Siemens. These little coax tubes had clips for attaching the anode and cathode connections. Well, I was going through a few boxes of these things a day doing various tests. Just slap them on the bench, fire them up, discharge the supplies and then disconnect and try another one. They ran off a 9 VDC power supply.
At the end of one long day, I called it quits early and just shut the laser supply off and left the tube in place as I was just going to put on a new tube in the morning. That next morning, I came and incorrectly assumed that the power supply would have discharged on it own overnight. So, with each hand I stupidly grab one clip each on the laser to disconnect it. YeeHaaaaaaaaa!!!!. I felt like I had been hid across my temples with a two by four. It felt like I swallowed my tongue and then I kind of blacked out. One of the guys came and helped me up, but I was weak in the knees, and very disoriented.
I stumbled around for about 15 minutes and then out of nowhere it was just like I got another shock! This cycle of stuff went on for about 3 hours, then stopped once I got to the hospital. I can't even remember what they did to me there. Anyway, how embarrassing to almost get killed by a HeNe laser after all that other high power stuff that I did. I think that's called 'irony'.
A 10 mw HeNe laser certainly presents an eye hazard.
According to American National Standard, ANSI Z136.1-1993, table 4 Simplified Method for Selecting Laser Eye Protection for Intrabeam Viewing, protective eyewear with an attenuation factor of 10 (Optical Density 1) is required for a HeNe with a 10 milliwatt output. This assumes an exposure duration of 0.25 to 10 seconds, the time in which they eye would blink or change viewing direction due the the uncomfortable illumination level of the laser. Eyeware with an attenuation factor of 10 is roughly comparable to a good pair of sunglasses (this is NOT intended as a rigorous safety analysis, and I take no responsibility for anyone foolish enough to stare at a laser beam under any circumstances). This calculation also assumes the entire 10 milliwatts are contained in a beam small enough to enter a 7 millimeter aperture (the pupil of the eye). Beyond a few meters the beam has spread out enough so that only a small fraction of the total optical power could possible enter the eye.
The term laser stands for "Light Amplification by Stimulated Emission of Radiation". However, lasers as most of us know them, are actually sources of light - oscillators rather than amplifiers. (Although laser amplifiers do exist in applications as diverse as fiber optic communications repeaters and multi-gigawatt laser arrays for inertial fusion research.) Of course, all oscillators - electronic, mechanical, or optical - are constructed by adding the proper kind of positive feedback to an amplifier.
All materials exhibit what is known as a bright line spectra when excited in some way. In the case of gases, this can be an electric current or (RF) radio frequency field. In the case of solids like ruby, a bright pulse of light from a xenon flash lamp can be used. The spectral lines are the result of spontaneous transitions of electrons in the material's atoms from higher to lower energy levels. A similar set of dark lines result in broad band light that is passed through the material due to the absorption of energy at specific wavelengths. Only a discrete set of energy levels and thus a discrete set of transitions are permitted based on quantum mechanical principles (well beyond the scope of this document, thankfully!). The entire science of spectroscopy is based on fact that every material has a unique spectral signature.
The HeNe laser depends on energy level transitions in the neon gas. In the case of neon, there are dozens if not hundreds of possible wavelength lines of light in this spectrum. Some of the stronger ones are near the 632.8 nm line of the common red HeNe laser - but this is not the strongest:
The strongest red line is 640.2 nm. There is one almost as strong at 633.4 nm. That's right, 633.4 nm and not 632.8 nm. The 632.8 nm one is quite weak in an ordinary neon spectrum, due to the high energy levels in the neon atom used to produce this line. See: Bright Line Spectra of Helium and Neon. (The relative brightnesses of these don't appear to be accurate though at present.) More detailed spectra can be found at the: Laser Stars - Spectra of Gas Discharges Page. And there is a photo of an actual HeNe laser discharge spectra with very detailed annotation of most of the visible lines in: Skywise's Lasers and Optics Reference Section. The comment about the output wavelength not being one of the stronger lines is valid for most lasers as if it were, that energy level would be depleted by spontaneous emission, which isn't what is wanted!
There are also many infra-red lines and some in the orange, yellow, and green regions of the spectrum as well.
The helium does not participate in the lasing (light emitting) process but is used to couple energy from the discharge to the neon through collisions with the neon atoms. This pumps up the neon to a higher energy state resulting in a population inversion meaning that more atoms in the higher energy state than the ground or equilibrium state.
It turns out that the upper level of the transition that produces the 632.8 nm line has an energy level that almost exactly matches the energy level of helium's lowest excited state. The vibrational coupling between these two states is highly efficient.
You need the gas mixture to be mostly helium, so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state that radiates 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states responsible for non-laser lines.
A neon laser with no helium can be constructed but it is much more difficult without this means of energy coupling. Therefore, a HeNe laser that has lost enough of its helium (e.g., due to diffusion through the seals or glass) will most likely not lase at all since the pumping efficiency will be too low.
However, pure neon will lase superradiantly in a narrow tube (e.g., 40 cm long x 1 mm ID) in the orange (611.9 nm) and yellow (594.1 nm) with orange being the strongest. Superradiant means that no mirrors are used although the addition of a Fabry-Perot cavity does improve the lateral coherence and output power. This from a paper entitled: "Super-Radiant Yellow and Orange Laser Transitions in Pure Neon" by H. G. Heard and J. Peterson, Proceedings of the IEEE, Oct. 1964, vol. #52, page #1258. The authors used a pulsed high voltage power supply for excitation (they didn't attempt to operate the system in CW mode but speculate that it should be possible).
(From: Steve Roberts (osteven@akrobiz.com).)
"Various IR lines will lase in pure neon, and even the 632.8 nm line will lase, but it takes a different pressure and a much longer tube. 632.8 nm also shows up with neon-argon, neon-oxygen, and other mixtures. Just about everything on the periodic table will lase, given the right excitation. See "The CRC Handbook of Lasers" or one of the many compendiums of lasing lines available in larger libraries. These are usually 4 volume sets of books the size of a big phone book just full of every published journal article on lasing action observed. It's a shame that out of these many thousands and thousands of lasing lines, only 7 different types of lasers are under mainstream use.
There are many possible transitions in neon from the excited state to a lower energy state that can result in laser action. The most important (from our perspective) are listed below:
(1) (2) (3) (4) (5) (6)
Output HeNe Perceived Lasing Typical Maximum
Wavelength Laser Name Beam Color Transition Gain (%/m) Power (mW)
------------------------------------------------------------------------------
543.5 nm Green Green 3s2->2p10 0.52 0.59 2 (5)
594.1 nm Yellow Orange-Yellow 3s2->2p8 0.5 0.67 7 (10)
604.6 nm Orange 3s2->2p7 0.6 1.0 3
611.9 nm Orange Red-Orange 3s2->2p6 1.7 2.0 7
629.4 nm Orange-Red 3s2->2p5 1.9 2.0
632.8 nm Red " " 3s2->2p4 10.0 10.0 75 (200)
635.2 nm " " 3s2->2p3 1.0 1.25
640.1 nm Red 3s2->2p2 4.3 2.0 2
730.5 nm Border Infra-Red 3s2->2p1 1.2 1.25 0.3
886.5 nm " " 2s2->2p10 1.2 1.25 0.3
1,029.8 nm Near-IR Invisible 2s2->2p8 ???
1,062.3 nm " " " " 2s2->2p7 ???
1,079.8 nm " " " " 2s3->2p7 ???
1,084.4 nm " " " " 2s2->2p6 ???
1,140.9 nm " " " " 2s2->2p5 ???
1,152.3 nm " " " " 2s2->2p4 ??? 1.5
1,161.4 nm " " " " 2s3->2p5 ???
1,176.7 nm " " " " 2s2->2p2 ???
1,198.5 nm " " " " 2s3->2p2 ???
1,395.0 nm " " " " 2s2->2p? ??? 0.5
1,523.1 nm " " " " 2s2->2p1 ??? 1.0
3,391.3 nm Mid-IR " " 3s2->3p4 ??? 440.0 24
Notes:
Gain at 1,523 nm may be similar to that of 543.5 nm - about 0.5%/m. Gain at 3,391 nm is by far the highest of any - possibly more than 100%/m. I know of one particular HeNe laser operating at this wavelength that used an OC with a reflectivity of only 60% with a bore less than 0.4 m long.
See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge for an easy way to see many of the visible ones.
The most common and least expensive HeNe laser by far is the one called 'red' at 632.8 nm. However, all the others with named 'colors' are readily available with green probably being second in popularity due to its increased visibility near the peak of the of the human eye's response curve (555 nm). And, with some HeNe lasers with insufficiently narrow-band mirrors, you may see 640 nm red as a weak output along with the normal 632.8 nm red because of its relatively high gain. There are even tunable HeNe lasers capable of outputting any one of up to 5 or more wavelengths by turning a knob. While we normally don't think of a HeNe laser as producing an infra-red (and invisible) beam, the IR spectral lines are quite strong - in some cases more so than the visible lines - and HeNe lasers at all of these wavelengths (and others) are commercially available.
The first gas laser developed in the early 1960s was an HeNe laser operated at 1,152.3 nm. In fact, the IR line at 3,391.3 is so strong that a HeNe laser operating in 'superradiant' mode - without mirrors - can be built for this wavelength and commercial 3,391.3 nm HeNe lasers may use an output mirror with a reflectivity of less than 50 percent. Contrast this to the most common 632.8 nm (red) HeNe laser which requires very high reflectivity mirrors (often over 99 percent) and extreme care to mimize losses or it won't function at all.
When the HeNe gas mixture is excited, all possible transitions occur at a steady rate due to spontaneous emission. However, most of the photons are emitted with a random direction and phase, and only light at one of these wavelengths is usually desired in the laser beam. At this point, we have basically the glow of a neon sign with some helium mixed in!
To turn spontaneous emission into the stimulated emission of a laser, a way of selectively amplifying one of these wavelengths is needed and providing feedback so that a sustained oscillation can be maintained. This may be accomplished by locating the discharge between a pair of mirrors forming what is known as a Fabry-Perot resonator or cavity. One mirror is totally reflective and the other is partially reflective to allow the beam to escape.
The mirrors may be perfectly flat (planar) or one or both may be spherical with a typical radius (r = 2 * focal length) equal to the length of the cavity (L). The latter is a configuration called 'confocal'. Curved mirrors result in an easier to align more stable configuration but are more expensive than planar mirrors to manufacture and are not as efficient since less of the lasing medium volume is used (think of the shape of the beam inside the bore). The confocal arrangement represents a good compromise between a true spherical cavity (r = 1/2 * L) which is easiest to align but least efficient and one with plane parallel mirrors (f = infinity) which is most difficult to align but uses the maximum volume of the lasing medium. Based on my experience with commercial HeNe tubes, short ones (less than 8 inches in total length) seem to use planar mirrors while longer ones will tend to have at least one curved mirror. This makes sense since with a short bore, every fraction of a percent of gain is needed (implying the desire to use the maximum volume of the lasing medium) and aligning short resonators is going to be easier anyhow. See the section: Common Laser Resonator Configurations.
These mirrors are normally made to have peak reflectivity at the desired laser wavelength. When a spontaneously emitted photon resulting from the transition corresponding to this peak happens to be emitted in a direction nearly parallel to the long axis of the tube, it stimulates additional transitions in excited atoms. These atoms then emit photons at the same wavelength and with the same direction and phase. The photons bounce back and forth in the resonant cavity stimulating additional photon emission. Each pass through the discharge results in amplification - gain - of the light. If the gain due to stimulated emission exceeds the losses due to imperfect mirrors and other factors, the intensity builds up and a coherent beam of laser light emerges via the partially reflecting mirror at one end. With the proper discharge power, the excitation and emission exactly balance and a maximum strength continuous stable output beam is produced.
Spontaneously emitted photons that are not parallel to the axis of the tube will miss the mirrors entirely or will result in stimulated photons that are reflected only a couple of times before they are lost out the sides of the tube. Those that occur at the wrong wavelength will be reflected poorly if at all by the mirrors and any light at these wavelengths will die out as well.
For most common IR wavelengths, level 4 is the 2s state and level 3 are various 2p states. However, the very strong 3.93 um line originates from the 3s state just like the visible wavelengths - and is the reason it competes with them in long HeNe tubes and must be suppressed to optimize visible output.
The 's' states of neon have about 10 times the lifetime of the 'p' states and thus support the population inversion since a neon atom can hang around in the 2s state long enough for stimulated emission to take place. However, the limiting effect is the decay back to level 1, the ground state, since the 1s state also has a long lifetime. Thus, one wants a narrow bore to facilitate collisions with its walls. But this results in increased losses. Modern HeNe lasers operate at a compromise among several contradictory requirements which is one reason that their maximum output power is relatively low.
While it is commonly believed that the 632.8 nm (for example) transition is a sharp peak, it is actually a Gaussian - bell shaped - curve. (Strictly speaking, it is something called a "Voigt distribution" which is a conbination of Gaussian and Lorentzian - but that's for the advanced course. Gaussian is close enough for this discussion since the discrepency only shows up way out in the tails of the curve.) In order for the cavity to resonate strongly, a standing wave pattern must exist. This will only occur when an integral number of half wavelengths fit between the two mirrors. This restricts possible axial or longitudinal modes of oscillation to:
L * 2 c * n
W = --------- or F = ---------
n L * 2
Where:
The laser will not operate with just any wavelength - it must satisfy this equation. Therefore, the output will not usually be a single peak at 632.8 nm but a series of peaks around 632.8 nm spaced c/(L * 2) Hz apart. Longer cavities result in closer mode spacing and a larger number of modes since the gain won't fall off as rapidly as the modes move away from the peak. For example, a cavity length of 150 mm results in a longitudinal mode spacing of about 1 GHz; L = 300 mm results in about 500 MHz. The strongest spectral lines in the output will be nearest the combined peak of the lasing medium and mirror reflectivity but many others will still be present. This is called multimode operation.
Think of the vibrating string of a violin or piano. Being fixed at both ends, it can only sustain oscillations where an integer number of cycles fits on the string. In the case of a string, n can equal 1 (fundamental) and 2, 3, 4, 5 (harmonics or overtones). Due to the tension and stiffness of the string, only small integer values for n are present with a significant amplitude. For a HeNe laser, the distribution of the selected neon spectral line and shape of the reflectivity function of the mirrors with respect to wavelength determine which values of n are present and the effective gain of each one.
For a typical HeNe laser tube, possible values of n will form a series of very large numbers like 948,123, 948,124, 948,125, 948,126,.... rather than 1, 2, 3, 4. :-) A typical gain function showing the emission curve of the excited neon multiplied by the mode structure of the Fabrey-Perot resonator and the reflectivity curve of the mirrors may look something like the following:
| 632.8 nm
I| .
| | | |
| | | | | |
| | | | | | | |
_______|______.__|__|__|__|__|__|__|__|__|__._______
n=948,125 -5 -4 -3 -2 -1 +0 +1 +2 +3 +4 +5
Since the mode locations are determined by the physical spacing of the mirrors, as the tube warms up and expands, these spectral line frequencies are going to drift downward (toward longer wavelengths). However, since the reflectivity of the mirrors as a function of wavelength is quite broad (for all practical purposes, a constant), new lines will fill in from above and the overall shape of the function doesn't change.
However, for very short HeNe tubes, the gain curve may be narrower than the spacing between modes. The effect is even more likely with short low pressure carbon dioxide (CO2) lasers because for a given resonator length, the ratio of wavelengths (10,600 nm for CO2 compared to 632.8 nm for HeNe means that the longitudinal mode spacing is 16.7 times larger). In these cases, the laser output will actually turn on and off as it heats up and the distance between the mirrors increases due to thermal expansion.
Now for some actual numbers: The Doppler broadened gain curve for the neon in a HeNe laser has a half-width (the gain is at least half the peak value) on the order of 1,500 MHz. So, for a 500 mm long (high gain) tube with its mode spacing of about 300 MHz (similar to what is depicted above), 5 or 6 lines may be active simultaneously and oscillation will always be sustained (though there would be some variation in output power as various modes sweep by and compete for attention). However, for a little 10 cm tube, the mode spacing is about 1,500 MHz. If this laser were to be really unlucky (i.e., the distance between mirrors was exactly wrong) the cavity resonance might not fall in a portion of the gain curve with enough gain to even lase at all! Or, as the tube heats up and expands, the laser would go on and off. There are very few commercial HeNe laser tubes that short. It is possible to widen the gain curve somewhat by using a mixture of neon isotopes (Ne20 and Ne22) rather than a single one since the location of their peak gain differ slightly. This would allow a smaller cavity to lase reliably and/or reduce amplitude variations from mode sweeping in all size HeNe lasers.
A high speed photodiode and oscilloscope or spectrum analyzer can be used to view the frequencies associated with the longitudinal modes of a HeNe laser. The clearest demonstration would be using a short tube where exactly two longitudinal modes are active. This will result in a single difference frequency. A polarized tube is best as it forces both modes to have the same polarization (a photodiode will not detect the difference frequencies for orthogonally polarized modes). But, adding a polarizer can partially compensate for this with a slight loss in signal strength. Without a polarizer, the beat frequencies of a random polarized laser will tend to be at twice the mode spacing.
Passive stabilization (using a structure made of a combination of materials with a very low or net zero coefficient of thermal expansion or a temperature regulator) or active stabilization (using optical feedback and piezo or magnetic actuators to move the mirrors, or a heating element to control the length of the entire structure) can compensate for these effects. An internal etalon will also likely be part of such a system to select a single mode (frequency). However, the added expense is only justified for high performance lab quality lasers or industrial applications like interferometric based precision measurement systems - you won't find these enhancements on the common cheap HeNe tubes found in barcode scanners (which are long enough to not be affected in any case unless possibly if they are old and barely alive)! See the section: Stabilized Single Frequency HeNe Lasers.
Thus, a typical HeNe laser is not monochromatic though the effective spectral line width is very narrow compared to common light sources. Additional effort is needed to produce a truly monochromatic source operating in a single longitudinal mode. One way to do this is to introduce another adjustable resonator called an etalon into the beam path inside the cavity. A typical etalon consists of a clear optical plate with parallel surfaces. Partial reflections from its two surfaces make it act as a weak Fabry-Perot resonator with a set of modes of its own. Then, only modes which are the same in both resonators will produce enough gain to sustain laser output.
The longitudinal mode structure of an optional intra-cavity etalon might look like the following (not to scale):
| 632.8 nm
I| . . .
| | | |
| | | |
| | | |
_______|______|______________|______________|_______
m=13,542 -1 +0 +1
Notice that since the distance between the two surfaces of the etalon is much less than the distance between the main mirrors, the peaks are much further apart (even more so than shown). (The etalon's index of refraction also gets involved here but that is just a detail.) By adjusting the angle of the etalon, its peaks will shift left or right (since the effective distance between its two surfaces changes) so that one spectral line can be selected to be coincident with a peak in the main gain function. This will result in single mode operation. The side peaks of the etalon (-1, +1 and beyond) will only coincide with weak peaks in the main gain function shown above so that their combined amplitude (product) is insufficient to contribute to laser output.
(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)
The standard, small HeNe laser normally lases on only one transition, the well known red line at about 632.8 nm.
The HeNe gain curve is inhomogeneously Doppler broadened with a gain bandwidth of around 1.5 GHz (at 632.8 nm). (The width of the Doppler broadened gain curve depends on the lasing wavelength. At 3,391 nm, it is only about 310 MHz.) For a typical laser, say 30 cm long, the axial modes are separated by about 500 MHz. Typically, two or three axial modes are above threshold, in fact as the laser length drifts you typically get two modes (placed symmetrically about line centre) or three modes (one near centre, one either side) cyclically, and a slow periodic power drift results. Shorter lasers, less modes, more power variation unless stabilized. But it needs a huge HeNe laser to get ten modes, and since they are closer of course they still only spread over the 1.5 GHz line width.
Most HeNe lasers which do not contain a Brewster window or internal Brewster
plate are randomly polarized; adjacent modes tend to be of alternating
orthogonal polarizations. (Note that this is not always the case and can be
overridden with a transverse magnetic field, see below. See the section:
Some frequency stabilized HeNe lasers are NOT single mode, but have two, and
the stabilization acts to keep them symmetrical about line centre - i.e., both
are half a mode spacing off line centre. A polariser will then split off one
of them or a polarizing beamsplitter will separate the two.
(From: Sam.)
The party line is that adjacent modes in a HeNe laser will be of
orthogonal polarization. However, I've seen samples of small (e.g., 5 or
6 inch) random polarized tubes only supporting 2 active modes where this
is not the case - they output a polarized beam that remains stable with
warmup and in any case, applying a strong transverse magnetic field will
override the natural polarization. So, it's not a strong effect. Only if
everything inside the tube is reasonably symmetric, will the modes alternate.
Modes may also remain one polarization as they move through part of the gain
curve and then abruptly - and repeatably - flip polarization. But the
majority of tubes are well behaved in this regard.
For a tutorial on both longitudinal (axial) and spatial (transverse) modes,
see An
Investigation of the Cavity Modes of the HeNe Laser.
(Portions from: Steve Roberts (osteven@akrobiz.com).)
Flames expected, as I'm ignoring some of the physics and am trying to explain
some of this based on what I observe, aligning and adjusting cavities on HeNe
and argon ion lasers as part of repairing them. Anyone who only goes by the
textbooks has missed out on the fun, obviously having never had to work on an
external mirror resonator. It can be quite a education!
Due to the complex number of possible paths down the typical gain medium, you
will see lasing as long as the mirrors are reasonably aligned. The cavity
spacing is not always that critical and will change anyway as the mirror mounts
are adjusted (there will always be some unavoidable translation even if only
the angle is supposed to be changed). No, lasers don't really flash on and off
in interferometric nulls as you translate the mirrors - they instead change
lasing modes. They will find another workable path. You will in some cases
see this as a change in intensity but it is more properly observed on a optical
spectrum analyzer as a change in mode beating. Eventually you can translate
them far apart enough that lasing ceases, but this is a function of your optics
not the resonator expansion.
I have seen what you fear in some cases by adding a third mirror to a two
mirror cavity with a low gain medium such as HeNe where the third mirror can
be positioned in such a way to kill many possible modes. This usually occurs
when I use a HeNe laser to align an argon laser's mirrors and the HeNe laser
will flicker from back reflections. See the section:
External Mirror Laser Cleaning and Alignment
Techniques. But unless you have a extremely unstable resonator design,
translation will just cause mode hopping, this becomes important on a frequency
stabilized or mode locked laser if you have a precision lab application.
Otherwise, most commercial lasers are not length stabilized in the least. There
are equations and techniques for determining if you have a stable optical
design - stable in this case meaning it will support lasing over a broad range
of transverse and longitudinal modes. For examples see any text by A. E.
Siegmund or Koechner. If your library doesn't have any similar texts, find a
book on microwave waveguides. It might aid you in visualizing what is going
on.
Either an intracavity etalon or active stabilization systems are usually
used on single frequency systems anyways, by either translating the mirror
on piezos or by pulling on mirror supports with small electromagnets, or in
the case of smaller units, heaters to change the cavity length on internal
mirror tubes. An etalon is basically a precision flat glass plate in the
lasing path between the mirrors, its length is changed by a oven and it
acts as a mode filter.
Length stabilization to the 50 or 100 nm you might have expected to be needed
would be gross overkill anyhow, and would be impossible to achieve in practice
by stablizing the resonator alone. Depending on the end use of the product,
most lasers are simply built with a low expansion resonator of graphite
composite or Invar, although in many products a simple aluminum block or L
shape is used, a few rare cases use rods made of two different materials
designed to compensate by one short high expansion rod moving the mirror mount
in opposition to the main expansion. A small fraction of a millimeter is a
more reasonable specification.
(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)
The basic idea, that the laser can only work at the frequencies where an
integral number of half waves fit in the cavity, is perfectly correct. The
separation between adjacent modes is just 1/(2*L) where L is the cavity length
in cm. From this we get the separation in 'wavenumbers'. One wavenumber is
30 GHz, so in more usual units it is just 30 GHz/(2*L). Or, to make it easy, in
a 50 cm long laser the modes are 300 MHz apart. That is not very far optically.
The laser operates by some molecule, gas, ion in a crystal, etc. making a
transition between two levels. But those levels are not perfectly 'sharp'; we
say they are 'broadened'. The reason can be many things:
In any case no transition is *perfectly* sharp, the fact that it has a finite
lifetime gives it a certain width, but this is not often the real limit,
something else is usually more important.
These broadening mechanisms 'blur out' the line - we see optical gain over
that *range* of frequencies, the gain bandwidth.
An example is carbon dioxide. The 'natural width' is very small, of order Hz.
The Doppler width at 300 °K is about 70 MHz. The collision broadened
width increases about 7 MHz/Torr; so well below 10 Torr the width is Doppler
limited, ~70 MHz; above 10 Torr pressure broadened (e.g. ~700 MHz at 100 Torr).
If I take a typical HeNe laser it might 'blur' out over a GHz or so - **more**
than that 300 MHz mode spacing - so there are *always* two or thee modes within
the 'gain bandwidth' and it will always lase. For a glass laser there might be
*thousands* of modes, because the glass gain is very wide indeed.
But there *are* cases that go the other way. For carbon dioxide, at low
pressure, the line is Doppler broadened and about 70 MHz wide, much **LESS**
than that 300 MHz mode spacing. So short carbon dioxide lasers really do turn
on and off as the cavity length changes, and you have to 'tune' the cavity
length to get a mode inside the gain width. This mainly happens with short,
gas lasers in the infrared.
For a *high pressure* CO2 laser at 760 Torr (1 atm), the line width is
several GHz, much more than the mode spacing, so the effect disappears.
There are many ways to actually "see" the modes of a laser including the
use of an instrument called a Scanning Fabry-Perot Interferometer (see the
section: Scanning Fabry-Perot
Interferometers). However, for a short tube with only 1 or 2 modes,
it's quite straightforward to interpret what's going on from the output
power and polarization alone. All that's need is a
photodiode and multimeter (or continuous reading laser power meter),
and polarizing filter. (A lens from a pair of polarized Sun glasses
or a photographic polarizing filter will do.) The power monitor can
be set up in the output beam and the polarizing filter in the waste beam
from the HR mirror. Alternatively, a non-polarizing beamsplitter can be used
to provide the two beams. Adding a polarizing beamsplitter oriented
so that it separates the two polarization orientations in one of the
beams can simplify the interpretation of the polarization changes.
Changing the orientation of the polarizer will affect the amplitude of
the intensity variations. For most HeNe lasers, the
longitudinal modes will generally remain at two fixed orthogonal
orientations, with adjacent modes usually being orthogonal to each other.
As the tube heats and the cavity length increases, the modes march along
under the gain curve with those at one end disappearing and new ones appearing
at the other end as described above. But for well behaved tubes, they
don't flip polarization. When the polarizer is oriented at 45 degrees
to the polarization axes of the tube, the reading will remain constant.
When aligned with the polarization axes of the tube, the reading will
fluctuate the most.
As a specific example, consider an HeNe laser tube with a mirror spacing
of 120 mm (about 4.75 inches, one of the shortest commercially available
laser tubes). This corresponds to a mode spacing of
about 1.25 GHz - rather close to the FWHM of 1.5 to 1.6 GHz for the neon
gain bandwidth. With this tube, at most 2 modes will be oscillating
at any given time. When the output power and polarization is monitored
while the tube is warming up, a very distinctive behavior will be observed.
One might think that it should be a periodic variation in output power with
a simple sinusoidal or similar characteristic. However, there will actually
be two peaks for each cycle: A large one corresponding to when there is a
single lasing mode at the center of the gain curve, and a smaller one when
there are two modes symmetric around the center of the gain curve. For
most tubes, the polarization of adjacent modes is orthogonal and will remain
fixed with the mode. So, as the modes cycle under the gain curve successive
large peaks will have opposite polarization. The small peaks will have
equal components of both polarizations. Even though two modes are
oscillating, the gain for each one is so much closer to the lasing
threshold that their combined power is still lower than for the single
mode at the peak of the gain curve. There may also be rather sudden
changes in output power as modes on the tails of the gain curve come and
go. However, for some tubes which are affectionately called "flippers",
the polarization of the modes will tend to suddenly change orientation
as they move through the gain curve. This should also be apparent when
viewing the beam through a polarizing filter.
For more on these types of experiments along with typical plots, see the
section: HeNe Laser Output Power Fluctuation
During Warmup.
When the laser beam hits a high speed photodetector like a photodiode, which
is a non-linear (square law) device, in addition to the DC power term, there
are the primary difference frequencies which are close to multiples of
c/2L (but not exactly due to mode pulling), but also the differences of the
difference frequencies - the second order intermodulation products - which
will be at (relatively) low frequencies compared to c/2L. As the cavity
length changes and the lasing modes drift across the gain curve, the mode
pulling effect on each one varies slightly. But, small differences between
large numbers can result in dramatic changes in these second order terms,
rapidly rising and falling in frequency, and coming and going as modes
drop off one end of the gain curve and appear at the other. The amplitude
of the second order beat will be much lower than that of the primary beat
but is still detectable with a spectrum analyzer, or in some cases with an
audio amplifier.
For a HeNe laser, the range of second order frequencies is typically in the
1 to 100 kHz range while for a solid state laser it will be in the MHz to
10s or 100s of MHz range. Note that there will generally not be any beat
in the range from 0 Hz and some minimum frequency (e.g., 1 kHz or so in the
case of the HeNe laser) as would be expected where the modes are almost
symmetric on either side of the gain curve so there would be very low
second order frequencies. Apparently, a self mode-locking effect occurs
to force these to be exactly zero frequency over a small range of mode
positions.
For the effect to be present, the laser has to be able to oscillate on at
least 3 longitudinal modes simultaneously. (With only 2 modes, there will
be only a single difference frequency.) The doppler broadened gain curve
of neon for the HeNe laser is about 1.5 GHz Full Width Half Maximum (FWHM)
at 632.8 nm. To get 3 modes requires the modes to be less than about
500 MHz apart implying a c/2L tube length of about 30 cm or more -
typical of a 5 mW or more (rated) HeNe laser. It should be polarized
to force all modes to be of the same polarization - orthogonal
polarizations do not mix in a photodetector. For a randomly polarized
laser which typically produces alternating polarizations for adjacent
modes, a longer tube length would be required to guarantee enough
same-polarized modes and/or a polarizer at 45 degrees to the beam
polarizations could be added (but this would cut the power to the
photodiode by 50 percent or more).
This effect can be demonstrated using a medium length HeNe laser, high speed
photodiode, and audio amplifier. Initially when the laser is turned on and
is heating up and expanding the fastest, they may sound like clicks or pops
or just non-random noise. As the expansion slows down, more distinct chirps
and other interesting sounds will appear. The complexity of the symphony will
also depend on the tube length and thus how many modes are oscillating.
(From: Roithner Lasertechnik (office@roithner-laser.com).)
You can "listen" to a single mode HeNe tube: Take an X-rated photodiode
and an AC power amplifier - guide a small part of the HeNe laser beam to the
photodiode (don't let it saturate!) - and listen to the "chirping
oscillations" during warming up with a speaker. Hint: There are no birds
inside the tube. ;-) But it sounds similar! Looks like sin(x)/x.
Here is a rough idea of what transverse modes might look like for a
rectangular cavity:
I have only shown the rectangular case because that's the only one I could
draw in ASCII!
Other (non-cartesian) patterns of modes will be produced depending on
bore configuration, dimensions, and operating conditions. These may have
TEMxy coordinates in cylindrical space (radial/angular), or a mixture of
rectangular and cylindrical modes, or something else!
To achieve high power from a HeNe laser, the tube may be designed with a wider
but shorter bore which results in transverse multimode output. Since these
tubes can be smaller for a given output power, they may also be somewhat less
expensive than a similar power TEM00 type. As a source of bright light - for
laser shows, for example - such a laser may be acceptable. However, the lower
beam quality makes them unsuitable for holography or most serious optical
experimentation or research. An example of a high power multimode HeNe
laser head is the Melles Griot 05-LHR-831 which has a rated output power
of 25 mW. Compared to their 05-LHR-827 which is a 25 mW TEM00 laser head,
the multimode laser is about 2/3rds of the length and runs on about 3/5ths
of the operating voltage at lower current.
(Note that it is easy in principle to convert the output of a TEM00 laser into
multimode by using a length of fiber-optic cable with lenses at each end to
focus the beam into it and collimate the beam coming out. If the core diameter
of the fiber is greater than that needed for the fiber itself to be single
mode, then the result will be that multiple modes will propagate inside
and the output will be multimode. To assure single mode propagation at
632.8 nm with the index of refraction of a typical glass fiber, a 4 um or
smaller core is needed. The actual core diameter of the fiber
will determine how many modes are actually generated. A core diameter of
10 um will result in a few modes while one of 125 um will produce
dozens of modes. Why this would be desired is another matter.)
Sometimes, laser companies don't quite get it right either and a laser
tube that is supposed to be TEM00 may actually be multi-transverse mode
all the time or whenever it feels like it (e.g., after warmup). I have a
13.5 mW Aerotech tube that is supposed to be TEM00 but produces a beam that
has an outer torus (doughnut shape) with a bright spot in the middle. I've
also seen an apparently factory-new Uniphase green HeNe laser that produces
a similar doughnut beam. Both of these are probably the result of one or
both mirrors having a radius of curvature that is
too short for the bore diameter. They may have been manufacturing goofups.
Everyone can have a bad day, even if it results in a bunch of dud lasers. :)
Note that the mode structure implies nothing about the polarization of
the beam. Single mode (TEM00) and multimode lasers can be either linearly
polarized or randomly polarized depending on the design and for the multimode
case, each sub-mode can have its own polarization characteristics. HeNe
(and other) lasers will be linearly polarized if there is a Brewster window
or Brewster plate inside the cavity. The majority of HeNe laser tubes produce
a TEM00 beam which has random polarization. For internal mirror tubes, linear
polarization may be an extra cost option. External mirror HeNe lasers also
generally produce a TEM00 beam but are linearly polarized since the ends of
the tube are terminated with Brewster windows.
A photodiode and oscilloscope or spectrum analyzer can be used to view the
frequencies associated with transverse modes. The transverse difference
frequencies are very low compared to the longitudinal mode spacing so a
really high speed photodiode isn't needed. A response of a few MHz should
be sufficient. Typically less than 2 mm square silicon photodiode will have
an adequate frequency response. But the modes do have to overlap on the
detector so it may be necessary to spread the beam of a multimode HeNe laser
using a lens. A polarized tube is best as it forces the modes to have the
same polarization (a photodiode will not detect the difference frequencies
for orthogonally polarized modes). But, adding a polarizer can partially
compensate for this, though the polarization may drift with a randomly
polarized laser.
For a tutorial on both longitudinal (axial) and spatial (transverse) modes,
see An
Investigation of the Cavity Modes of the HeNe Laser.
All of these are really somewhat equivalent and simply mean that more than
one mode fits inside the available active mode volume.
Where there is access to the inside of the cavity (as with a one-Brewster
tube), a laser that operates multimode can be forced to operate TEM00 with
a stop (aperture) between the external mirror and tube-end. However, there
will be a (possibly substantial) reduction is output power. Where both
mirrors are external, it may be possible to substitute longer RoC mirrors
to force TEM00 mode (again at the expense of some output power).
Note that a speck of dirt or dust on the inside of a mirror or window (if
present), or damage to an optical surface, can result in a multi-transverse
mode beam even if the bore and mirror parameters are correct for TEM00
operation. Unfortunately, convincing a bit of dust to move out of the
way isn't always easy on the inside of an internal mirror HeNe laser
tube! Yes, though not common, it can happen. This is one reason not to
store tubes vertically. I've heard of people successfully using a Tesla
(Oudin) coil to charge up the errant dust particle, causing it
to just out of the way via electrostatic repulsion. Your mileage may
vary. :)
The following actually applies to all lasers using Fabry-Perot cavities
operating with multiple longitudinal modes. It was in response to the
question: "Why does the coherence length of a HeNe laser tend to be about the
same as the tube length?"
(From: Mattias Pierrou).
In a HeNe laser you typically have only a few (but more than one) longitudinal
modes. These cavity modes must fulfill the standing-wave criterion which
states that must be an integer number of half wavelengths between the mirrors.
In the frequency domain this means that the 'distance' between two modes is
delta nu = c/(2L), where L is the length of the laser.
The beat frequency between the modes gives rise to a periodic variation in the
temporal coherence with period 2L/c, i.e. full coherence is obtained between
two beams with a path-difference of an n*2L (n integer).
If you have only one frequency, the coherence length is infinite (that is, if
you neglect the spectral width of this mode which otherwise limit the
coherence length). If you have two modes, the coherence varies harmonically
(like a sinus curve).
The more modes you have in the laser, the shorter is the regions (path-length
differences) of good coherence, but the period is still the same.
You can try this by setting up a Michelson interferometer and start with equal
arm-lengths which of course gives good coherence. Then increase the length of
one arm until the visibility of the fringes disappear. This should occur for a
path-difference slightly less than 2L (remember that the path-difference is
twice the arm-length difference!). If there are only two modes is the laser
the zero visibility of fringes should occur at exactly 2L. Now continue to
increase the path-difference until you reach 4L (arm-length difference of
2L). You should again see the fringes clearly due to the restored coherence
between the beams.
Mode locking is implemented by mounting one of the mirrors of the laser cavity
on a piezo-electric or magnetic driver controlled by a feedback loop which
phase locks it with respect to the optically sensed output beam.
Without mode locking, all the modes oscillate independently of one another
with random phases. However, with the mode locked laser, all the cavity modes
are forced to be in phase at one point within the cavity. The constructive
interference at this point produces a short duration, high power pulse.
Destructive interference produces a power of almost zero at all other points
within the cavity. The mode locked pulse then bounces between the two laser
mirrors, and a portion passes through the output coupler on each pass.
As a practical matter, you probably won't run into a mode locked HeNe laser
at a garage sale!
Note that while the frequency of the power variations in output power of a HeNe
laser goes to beyond the GHz range, the following deals with what can be
seen by human eyeballs with the aid of only a photodiode and multimeter
or chart recorder (or a PC with a data aquisition module).
Thanks to Ryan Haanappel, here is a plot of the measured output power
of a typical HeNe laser tube from power-on to 20 minutes:
Typical HeNe Laser Output Power Versus
Time During Warmup. More plots and photos can be found on
Ryan's
HeNe Lasers Experience Page, and later in this section.
Examining the actual plot of output power versus time such as shown in
HeNe Laser Output Power Fluctuation During Warmup
(or careful observation of laser power meter readings) of a HeNe laser reveals
that the curve is not simple but may include several types of behavior:
There is also usually an increase of power due to the heating
of the laser tube (independent of thermal expansion effects) as
well but this may be only a fraction of the effects of alignment.
I do not know exactly what the underlying cause is, but it has to
do with the lasing process itself.
In addition, especially with soft-seal tubes, there may be a
power increase as the cathode, acting as a weak getter, removes
contaminants from circulation that may have accumulated from
a period of non-use. (Or depending on how far gone they are,
the power may go down!)
Depending on the particular laser, the initial output power can
be very low even where the final output power exceeds rated power.
Goofups in design and manufacturering can result in various combinations of
these and other effects, though for the most part, HeNe laser companies
generally know what they are doing! :) But see the plots below for both
normal and abnormal behavior, and a link near the end of the section for
a case study of one dramatic example of and "oops". :)
For most of the plots, my "instrumentation" consisted of a pair of $2
photodiode feeding two of the analog inputs of a
DATAQ Chart
Recorder Starter Kit attached to my ancient 486DX-75 Kiwi laptop running
Win95. The photodiodes are reverse biased by 30 VDC from a +/-15 VDC power
supply with a variable load resistor to set the calibration. The
output is taken between the junction of the resistor and the photodiode, and
power supply common (0 VDC). One channel is shown below:
The values shown were selected for lasers with a maximum power output of
around 1 mW. For higher power lasers, R2+R3 can be decreased or an attenuation
filter can be placed in the beam. The later is preferred to avoid shifting
the 0 mW reference level, and is what I did for most of the plots.
The capacitor across the input is intended to minimize noise
pickup. The resulting filter rolls off at around 0.1 Hz.
For reasonably well behaved HeNe lasers, even during the initial
warmup period, this bandwidth is more than adequate.
The sampling rate for all the plots is at least 10 Hz to allow for averaging
since the A/D seems to have an uncertainty of about 2 LSBs.
For monitoring power from the waste beam (which is much lower), a dedicated
beam sampler assembly was constructed which along with a photodiode preamp,
enabled power levels as low as a few uW to fully utilize the 20 V p-p range
of the A/D.
Although some of these plots aren't as nicely annotated as the
one above, zero power is near the bottom of the plot so relative power
variations can still be easily seen (who cares about absolute power
anyhow!) and the time/division is indicated. The plots are arranged by
increasing laser tube length.
For the following, "Total" means all the power in the beam; "Polarized"
means a polarizing filter has been inserted in the beam and aligned to
produce the largest difference between minimum and maximum output as the
modes cycle. (Only done for random polarized lasers.) The scale factor
for the "polarized" plot has been adjusted so that the peak amplitude
is approximately the same as for the "total" plot ease of viewing. However,
it should be understood, that the sum of the power in the two orthogonal
polarizations must add up to the total power. All are red (632.8 nm)
HeNe lasers unless otherwise noted.
Plot of Melles Griot 05-LHR-007 HeNe Laser Tube
During Warmup (Polarized). This is the same tube but with a
non-polarizing beamsplitter followed by orthogonal polarizing filters
inserted in the beam. The orientation of the polarizing filters is
adjusted for minimum transmission when its mode is not present since as can
be seen, the power actually goes to 0 mW for about half the period of each
polarized mode. Alternate similar height peaks on the total power plot
correspond to the same mode polarization. A careful examination will
confirm that they actually alternate very slightly in amplitude due to minor
variations in gain as a function of polarization. (I have adjusted
the scale factors to make the plot looks similar.) The reason why the peak
spacing on the two plots differs is that the tube was likely not
quite at the same temperature when each run was started.
Plot of Melles Griot 05-LHR-640 HeNe Laser Tube
During Warmup (Polarized). This is the same tube with the polarized
modes separately plotted. Similar comments apply for this tube as for
the 05-LHR-007, above.
However, the dramatic variation in mode amplitude
over the course of warmup is an artifact of the way that data is being
collected for this run and a peculiarity of the tube that doesn't
noticeably affect its useful output. Rather than using the output beam,
the P and S Modes are taken from the waste beam leaking through
the HR mirror at the back of the laser. The Total Power (Waste) is
then simply the sum (in an op-amp) of the modes. Compare this to the
Total Power (Output) curve, which was measured from the main beam.
The cause of the rear beam power variation is interference from multiple
internal reflections in the HR mirror glass - between the HR coated inner
surface and the uncoated outer surface. The result is a weak Fabry-Perot
etalon which varies the effective reflectance of the HR mirror. It doesn't
take much: A change from 99.975% to 99.95% would double the waste beam power
- from about 15 uW to 30 uW. The 15 uW lost from the main beam power
of about 1 mW is almost undetectable on the plot. The HR mirror glass
is apparently not wedged on these tubes so the surfaces are very parallel.
And indeed there was no ghost beam next to the waste beam as would be the
case if wedge was present. The cause was confirmed by putting a dab of 5
minute Epoxy on the outer surface of the mirror. The Epoxy is smooth and
clear enough to pass sufficient power for the photodiodes (though it is
reduced). But the Epoxy surface is lumpy enough to greatly reduce the power
variation. Why? The glass and Epoxy are fairly closely index matched
so that the dominant reflection is no longer from the planar glass surface
but from the lumpy surface of the Epoxy. There is minimal reflection
directly back along the optical axis and thus minimal etalon effect resulting
in a reduction of power variation from nearly 100 percent to under 10 percent.
Using Norland 65 UV cure optical cement to glue an angled plate to the
HR mirror reduced the ripples even more as shown in
Plot of Siemens LGR-7641 HeNe Laser Tube
With Variable Waste Beam Power During Warmup (Corrected).
More on this phenomenon can be found in the section:
Power Variations Due to Lack of Mirror
Substrate Wedge which explains the cause in more detail and additional
tests that were performed on this specific tube.
Plot of Melles Griot 05-LHR-151 HeNe Laser Head
During Warmup (Polarized). This is the same laser head but with the
two orthogonal polarizitions separated (as described for the shorter tubes,
above) and oriented for maximum variation ("ripple"). They are plotted
separately to reduce clutter. Since there are always modes of both
polarization present regardless of polarizer orientation, the output
power in doesn't go to zero as with the shorter laser but their ripple is
almost perfectly complementary. As expected, the size of the fluctuations
in each polarization - 5 to 10 percent - is more in line with the total power
behavior of a laser with only 2 or 3 modes. Even this amplitude seems
remarkable given the almost perfectly smooth behavior of the total (randomly
polarized) power. If the plots are examined very carefully, it will be noted
that their envelopes are not identical - there is a very subtle slow
variation over the course of the warmup period. This may be attributed to
a small rotation of the polarization axes as the tube expands. With
some samples of these lasers, it can be much more dramatic including
polarization flips whenever it feels like it. But such behavior
is still considered normal since for a random polarized laser, only the total
power really matters, not any peculiar gyrations the modes may go
through.
Plot of Melles Griot 05-LYR-173 HeNe Laser
Head During Warmup (Polarized). The same laser with a polarizing
filter in the beam. The fluctuations are larger as expected both
because of the fewer modes in the polarized beam, and the lower gain
of the 594.1 nm lasing line.
Since the laser head has optics to separate
the modes with orthogonal polarization, the raw beam already varies
by more than 2:1 in output power without any additional polarizer. Yes,
that is the actual spread - the vertical scale hasn't been stretched!
The actual HeNe laser tube inside is a specially selected Melles
Griot 05-LHR-120, which by itself would have a normal mode sweep with a
small ripple. From a cold start to lock takes about 20 minutes.
Plot of Coherent Model 200 Stabilized HeNe Laser
Head Near End of Warmup. This plot zooms in on the last
two cycles. Notice that there is a slight distortion on the rising part
of the second cycle in the plot. That is probably when the active feedback
is switched on. Before then, the heater is simply running at a constant
current to bring the tube up to operating temperature. It only takes
less than one full additional cycle to achieve lock. The amplitude is
then quite stable (uncertainty of less than 0.5 percent on the plot),
but the frequency stability which is d(power)*slope(frequency/power),
will be under 0.125 percent of the mode spacing of around 750 MHz, so
less than 1 MHz.
However, near the very end of the warmup period (measured in terms of
mode cycles, not time) something very interesting
occurs: The tube seems to have reverted to being well behaved! This only
happens when the tube is approaching thermal equilibrium where each complete
mode cycle is taking over 90 seconds. There are perhaps 3 or 4 beyond
what is on the plot but the tube temperature is so close to its final
value that any disturbance like moving near the laser head will disrupt
the sequence. This behavior is consistent from run to run. The cause is
unknown, nor is it known whether the tube would continue to behave if
stabilization was attempted. But it might since the operating temperature
will be somewhat above the natural point of thermal equilibrium.
Plot of "Flipper" Aerotech OEM1R HeNe Laser Head
During First Part of Warmup is a closeup of the mode variations when
flipping. The shapes are nearly identical from the start of warmup until
the transition to normal behavior.
Also note that the frequency of the mode cycles for
a flipper is double that of a normal tube - each mode would normally be
what resulted from tracing the continuous curve and not taking the
discontinuities as is evident in
Plot of "Flipper" Aerotech OEM1R HeNe Laser Head
During First Part of Warmup (Combined). So following red-blue-red, etc.,
ignoring the green lines. And
Plot of "Flipper" Aerotech OEM1R HeNe Laser Head
at Transition to Normal Behavior (Combined) is a closeup of the point
where flipping ceases. Note that the "envelope" of the mode plot is virtually
unchanged at this point but the green transitions have disappeared.
At the transition point, the period of a full
mode sweep cycle is about 80 seconds. There are then an additional 10
full cycles (only 4 or 5 are shown) requiring about an hour until
thermal equilibrium. There is more on flippers in the sectoin:
HeNe Mode Flipper Observations.
Common internal mirror HeNe laser tubes include a specification called
"Mode Cycling Percent" or something similar. This relates to the amount of
intensity variation resulting from changes in longitudinal modes due to
thermal expansion. Typical values range from 20 percent for a small (e.g.,
6 inch, 1 mW) tube to 2 percent or less for a long (e.g., 15 inch, 10 mW)
tube. These take place over the course of a few seconds or minutes and are
very obvious using any sort of laser power meter or optical sensor. Even the
unaided eyeball may detect a 20 percent change. The more modes that can be
active simulataneously, the closer those that are active can be to the same
output power on the gain curve. Very short tubes or those with low gain
(other wavelengths than 632.8 nm or due to age/use or poor design) may vary
widely in output intensity or even cycle on and off due to mode cycling. (Note
that since the polarization for each mode may be different, reflecting the
beam of one of these HeNe lasers from a non-metallic reflective surface (which
acts somewhat as a polarizaer) can result in a large variation in brightness as
the dominant polarization changes orientation over time.) Trading off between
tube size and mode cycling intensity variations is one reason that HeNe tubes
with otherwise similar power output and beam characteristics come in various
lengths.
There are also stabilized HeNe lasers which use optical feedback to maintain
the output intensity with a less than 1 percent variation. (They usually
also have a frequency stabilized mode but can't do both at the same time.)
An alternative to doing it in the laser is to have an external AO modulator
or other type of variable attenuator in a feedback loop monitoring optical
output power. See the next section for more info.
Short term changes in intensity may result from power supply ripple and would
thus be at the frequency related to the power line or inverter. These can
be minimized with careful power supply design.
Intensity variations at 100s of MHz or GHz rates result from beats between the
various longitudinal modes that may be simultaneously active in the cavity.
For most common applications, these can be ignored since they will be removed
by typical sensor systems unless designed specifically to respond to these
high beat frequencies.
Also see the section: Amplitude Noise.
If you have, say, $5,000 to spend on a HeNe laser, you can buy something that
actually produces a single frequency with specifications guaranteed stable for
days and that don't change over a wide temperature range. While the operation
of such a HeNe laser is basically the same as the one in a barcode scanner
(and in fact may use the identical model HeNe laser tube!),
several additional enhancements are needed to eliminate mode sweep and
select a single output frequency. Simply constructing the laser cavity of
low thermal expansion materials isn't enough when dealing with distances
on the order of a fraction of a wavelength of light! Active feedback is
needed. The most common implementation of these lasers starts
with a short tube that can only oscillate on at most 3 longitudinal modes.
It then adds optical feedback to keep them in a fixed location on the HeNe
gain curve by precisely adjusting the distance between the mirrors over
a range of about 1/2 the lasing wavelength. This is most often
done with a heating coil (inside or outside the tube), but a PieZo
Transducer (PZT, an expensive version of the beeper element in a digital watch)
may also be used. The PZT reduces the time for the system to stabilize
to a few seconds, compared to 10 or 20 minutes for the heater. But, for
a laser that will be left on continuously, this probably doesn't matter.
Some lasers use a means of cooling in addition to the heater like a
piezo fan, probably to allow the laser to run stably over a wider temperature
range. And a few including the Melles Griot 05-STP-909/910/911/912 (originally
based on teh Aerotech Syncrolase 100) use a miniature RF induction heater
surrounding the HR mirror mount to control only its length, not that of the
entire tube. With direct heating of such a small mass, the response is quite
fast. This also makes for a more compact package than a full tube heater.
Many schemes work well and it's amazing how dirt simple these really are,
considering their hefty price tags! It's easy to build perfectly usable
systems from a common surplus HeNe laser tube and a few common junk parts.
Note that an etalon inside the laser cavity could also
be used to select out a single longitudinal mode. For high power
lasers which would require long tubes supporting many modes, this
would be needed with both the overall mirror spacing and etalon being
feedback controlled. But for low power lasers (e.g. 1 to 3 mW),
the use of a short tube to limit the number of modes in conjuntion with
basic feedback control is a much less complex lower cost approach.
Commercial stabilized HeNe lasers usually fall into one of two classes:
Some inexpensive (this is relative!) stabilized HeNe lasers only use a
single mode for frequency locking. When on the slope, this will be
reasonably stable after warmup once the output power has reached
equilibrium.
When the best intensity stability of the total output (without regard to
polarization) is desired, a non-polarizing beam sampler is used or the
signals from the two photodiode channels are summed and compared to the
reference.
Most commercial stabilized HeNe lasers for general laboratory applications
are of type (1) and operate with 2 orthogonal modes for frequency
stabilization, though some use 1 mode for intensity stabilization.
These include the Coherent 200, Spectra-Physics 117
and 117A (and the identical Melles Griot 05-STP-901), various models from
Teletrac, Zygo, and others. The interferometry lasers used in metrology
manufactured by Agilent (formerly Hewlett Packard) and others are of type (2).
For example, in the Melles Griot 05-STP series of
frequency and intensity stabilized HeNe lasers, the laser cavity
permits a pair of orthogonal polarized longitudinal modes to be active and can
provide very precise control by straddling these on the steep slopes of the
gain curve (frequency stabilized mode) or positioning one on the flatter
portion of the gain curve (intensity stabilized mode). Those from other
companies are generally similar.
For some photos of the (quite simple) Zeeman split stabilized HeNe tube used
in the Hewlett-Packard 5517 laser head, see the
Laser Equipment Gallery (Version 1.86
or higher) under "Assorted Helium-Neon Lasers". And for more information
on these lasers, see the sections starting with:
Hewlett-Packard HeNe Lasers.
It isn't really possible to convert an inexpensive HeNe tube that operates on
many longitudinal modes into a single frequency laser. Adding temperature
control could reduce the tendency for mode hopping or polarization
changes, and the addition of powerful magnets can force a polarized beam and
probably stabilize the discharge. But, selecting out a single longitudinal
mode would be difficult without access to the inside of the tube. However,
if the HeNe tube is short enough that the mode spacing exceeds about 1/2 the
doppler broadened gain bandwidth for neon (about 1.5 GHz), it will oscillate
on at most 2 longitudinal modes at any given time and these will each be
linearly polarized and usually orthogonal to each-other. Then, stabilization
is possible using very simple hardware. In fact, even if the mode spacing is
a bit smaller - down to 500 or 600 MHz - then only 2 modes will be present
most of the time but 3 may pop up if one is close to the center of the gain
curve. This, too, is an acceptable situation since the tube can be stabilized
with the modes straddling the gain curve and then only 2 modes will oscillate.
For intensity stabilization, 4 modes may even be permitted.
Note that while the modes of a random polarized and
linearly polarized tube are similar (except for polarization), a random
polarized tube is desirable to be able to use a tube that supports 2 modes
to with the benefits they provide, but be able to eliminate the second mode
in the output. Also see the section:
Inexpensive Home-Built
Frequency or Intensity Stabilized HeNe Laser for details.
It may be possible with a combination of what can be done externally, as well
as control of discharge current, to force a situation where gain is adequate
for only 1 or 2 modes even for a longer tube. Whether this could ever be a
reliable long term approach for a HeNe tube that normally oscillates in
many longitudinal modes is questionable.
What I don't think will have much success are optical approaches such as
feeding light back in through the output mirror. Doing this would likely have
the exact opposite of the desired effect but may work in special cases (it's
called injection locking and is used with great success for other
applications).
Coherent, Melles Griot, Spectra-Physics, and others will sell you a small
stand-alone stabilized HeNe laser for $5,000 or so and Agilant (HP) and
others have interferometers and other similar equipment which includes this
type of laser (and are even more expensive!). Other manufacturers includ Zygo,
Teletrac, Nikon, Micro-g Solutions, SIOS, NEOARK, and Nikon. The lab lasers
that I've seen all use short HeNe tubes with feedback thermal control of the
resonator length and all operate at the red HeNe wavelength (632.8xxxxxx nm
to 8 or more significant figures). One typical system is described in the
section: Coherent Model 200 Single Frequency
HeNe Laser. The Spectra-Physics model 117A/118A laser actually uses
a lowly SP088-2 tube similar to those in older grocery store barcode
checkout scanners as its heart. A tube like this is visible in the
Spectra-Physics Model 117 OEM Stabilized HeNe
Laser Assembly. However, some do employ a custom tube with
the heater inside to greatly speed up response and reduce heat dissipation
to the outside. A stabilized HeNe laser for green or other color
visible HeNe wavelength or one of the IR wavelengths is also
possible using the same techniques.
As noted above, the actual stabilization mechanism for the general purpose
stabilized lasers may be the ratio of amplitudes of two longitudinal modes
(which is better for frequency stabilization) or the amplitude of one
mode (which is better for intensity stabilization). These are usually
stable to within a few parts in 109. However, the frequency
drift when intensity stabilized is still not much - probably less than
1 part in 108. Output power variation may be 0.2 percent if
intensity stabilized and 1 percent if frequency stabilized. Some allow
either method to be selected via a switch, as well as providing for an
external tuning input to vary the frequency over several hundred MHz.
(However, due to the thermal control most often used, the response time is
not exactly fast.)
The Zeeman split interferometer lasers may lock the difference frequency to a
crystal clock, though most seem to use the basic polarized modes for
stabilization, with the Zeeman beat used only as the reference for the
interferometer. See the sections starting with:
Hewlett-Packard HeNe Lasers.
A few do lock the Zeeman frequency to a PLL. One of
these was the Laboratory for Science Model 220. (Laboratory for Science
is now out of business.) See the section
Laboratory for Science Stabilized HeNe
Lasers. Another example is the
NEORK Model 262
Transverse Zeeman Laser.
More sophisticated schemes with even better precision
and lower long term drift may lock to the "Lamb Dip" at the center of
the neon gain curve or to one of the hyperfine absorption lines of an iodine
vapor other type of gas cell, achieving stabilities on the order of 1 part in
1010 or even better. See, for example: NPL
Iodine-Stabilized HeNe Reference Lasers and
Winters Electro-Optics, Inc..
Due to the performance, simplicity, reliability, and relatively low cost
of stabilized HeNe lasers, they are still often the preferred frequency
reference for many applications. As noted, a typical system might go for
$5,000. While this may seem high, it is small compared to many other
technologies. The cost is not the result of expensive components or
complex manufacturing, but more to the relatively limited number of units
produced. If stabilized HeNe lasers were as popular as laser pointers,
they would probably cost under $100.
In particular:
See the section: On-Line Introduction to
Lasers for the current status and on-line links to these courses, and
additional CORD LEOT modules and other courses relevant to the
theory, construction, and power supplies for these and other types of lasers.
Several modules would be of particular interest for HeNe lasers.
Unfortunately, the on-line manuals (in PDF format) have disappeared
from the MEOS Web site. But I have found and archived most of them:
If MEOS should complain, these will have to be removed. So, get them while
you can! But I doubt they'll complain. And most are also archived at the
Wayback Machine Web Site.
Early HeNe lasers were also quite large and unwieldy in comparison to modern
devices. A laser such as the one depicted above was over 1 meter in length
but could only produce about 1 mW of optical beam power! The associated RF
exciter was as large as a microwave oven. With adjustable mirrors and a
tendency to lose helium via diffusion under the electrodes, they were a
finicky piece of laboratory apparatus with a lifetime measured in hundreds
of operating hours.
In comparison, a modern 1 mW internal mirror HeNe laser tube can be less than
150 mm (6 inches) in total length, may be powered by a solid state inverter
the size of half a stick of butter, and will last more than 20,000 hours
without any maintenance or a noticeable change in its performance
characteristics.
Older brochures from several manufacturers of HeNe lasers can be found
at Vintage Lasers and
Accessories Brochures
This fabulous ASCII rendition of a typical small HeNe laser tube should make
everything perfectly clear. :-)
The main beam may emerge from either end of the tube depending on its design,
not necessarily the cathode-end as shown. (For most applications it doesn't
matter. However, when mounted in a laser head, it makes sense to put the
anode and high voltage at the opposite end from the output aperture both for
safety and to minimize the wiring length.) A much lower power beam will
likely emerge from the opposite end if it isn't covered - the 'totally
reflecting' mirror or 'High Reflector' (HR) doesn't quite have 100 percent
reflectivity (though it is close - usually better than 99.9%). Where both
mirrors are uncovered, you can tell which end the beam will come from without
powering the tube by observing the surfaces of the mirrors - the output-end
or 'Output Coupler' (OC) mirror will be Anti-Reflection (AR) coated like a
camera or binocular lens. The central portion (at least) of its surface will
have a dark coloration (probably blue or violet) and may even appear to vanish
unless viewed at an oblique angle.
For a diagram with a little more artistic merit, see:
Typical HeNe Laser Tube Structure and Connections.
And, for a diagram of a complete laser head: Typical
HeNe Laser Head (Courtesy of Melles
Griot). For some photos, see: Typical Small to
Medium Size Melles Griot HeNe Laser Tubes. The ratings are guaranteed
output power. These tubes may produce much more when new. Another
type of construction that is relatively common is shown in the
Hughes Style HeNe Laser Tube and a photo
in Hughes 3227-HPC HeNe Laser Tube. These are
probably disappearing though as Melles Griot bought the Hughes HeNe laser
operation and is converting most to their own design but many still show
up on the surplus market, including newer ones with the Melles Griot label.
Another design that is similar is the
NEC Style HeNe Laser Tube.
Some specifications for various NEC HeNe lasers can be found at
SOC
under "Gas Lasers". Most common
higher quality HeNe tubes will be basically similar to one of these two
designs though details may vary considerably. Most have an outer glass
envelope but a few, notably some of those from PMS/REO, may be nearly all
metal (probably Kovar but with an aluminum liner which is the actual cathode)
with glasswork similar to that of Huches or NEC at the anode-end.
Tubes up to at least 35 mW are similar in design but proportionally larger,
require higher voltage and possibly slightly higher current. and of course,
will be more expensive.
The discharge at this end produces little heat or damage due to sputtering.
This is a 'cold' cathode - there is no need to heat it (like the ones
in the electron guns of a CRT) for proper operation and no warmup period
is required before the tube can be started.
The discharge is distributed over the entire area of the can thereby
spreading the heat and minimizing damage due to sputtering which results
from positive ion bombardment. For this reason, although the laser may
appear to work (in fact, starting tends to be easier) a HeNe tube should
not be run with reverse polarity for any length of time (e.g., more than
a minute or so, preferably a lot less) since damage to the anode (now acting
as a cathode) and its mirror would likely result. See the section:
Damage to Mirror Coatings of Internal Mirror
Laser Tubes.
The can-shaped structure is also called a 'hollow cathode' for obvious
physical reasons - it is a tube electrode that is large in diameter and
hollow like a piece of pipe - and because the plasma discharge flows inside
of it. It operates in the abnormal glow current density gas discharge
region (should you care). The surface of the cathode can is also not
pure aluminum as it appears, but is processed with a very thin layer
of oxide which eventually gets depleted, and this is the main determination
of tube life. Hollow cathodes are usually used where a tube needs lots of
slow moving electrons to excite the gas. They are currently used mainly
in HeNe lasers but have been applied to other types of gas lasers having
modest current requirements.
Very old HeNe lasers (and some others, old and new, like argon ion) use a
heated filament which also acts as the cathode instead of the cold cathode
design. This structure can be much smaller than the cold cathode but the
added complexities of manufacture, the additional power supply, and the need
for a warmup period have delicated it only to those applications where there
is no other choice. See the section: Strange
High Power HeNe Laser for an example of this technology.
A very few, very tiny HeNe laser tubes, use a small ring-shaped cathode
made of either zirconium (expensive) or aluminum. These were likely
designed for special applications, presumably requiring very small size
or fast turn-on response (due to the reduced capacitance). The examples
of these HeNe tubes I've seen are about 5" long by 1/2" in diameter. Life
expectancy using the aluminum version (at least) is probably quite limited
due to sputtering (since the electrode is very close to the bore, which
promotes this due to the increased field gradient).
On some (mostly larger) HeNe tubes, the bore may be ground (but not polished)
on the outside, inside, or both:
Note that since the frosting process is done chemically (hydrofluoric
acid etch?), the bore will become marginally wider and care must be taken
that this doesn't result in multimode (non-TEM00) operation if it goes too
far!
HeNe tubes used in barcode scanners tend to use a simpler (possibly cheaper)
design. Some typical examples are the Uniphase 098-1
HeNe Laser Tube and Siemens LGR-7641S HeNe Laser
Tube. A typical small barcode scanner tube is shown in
Uniphase HeNe Laser Tube with External Lens. That
negative lens is used in the barcode application to expand the beam at a
faster rate than with the bare tube. A second positive lens about 4 inches
away is then used to recollimate the beam. (In many cases, the required
curvature is built into the output mirror but not here. The lens was removed
by soaking the end of the tube in acetone overnight.)
CAUTION: While most modern HeNe tubes use the mirror mounts for the high
voltage connections, there are exceptions and older tubes may have unusual
arrangements where the anode is just a wire fused into the glass and/or the
cathode has a terminal separate from the mirror mount at that end of the tube.
Miswiring can result in tube damage even if the laser appears to work
normally. See the section:
Identifying Connections to Unmarked HeNe Tube
or Laser Head if in doubt.
The getter material is then available to chemically combine with residual
oxygen and other unwanted gas molecules that may result from imperfect
vacuum pumps and contamination on the tube's glass and metal structures
(e.g., from the surface as well as in fine cracks and other nooks and
crannies). It will also mop up any intruder molecules that may diffuse
or leak through the walls of the tube during its life. Helium and neon
are noble gases - they ignore the getter and the getter ignores them. :-)
Should the getter spot (if visible) turn to a milky white or red powdery
appearance, it is exhausted and the tube is probably no longer functional.
If you had grown up during the vacuum tube age, the getter would be familiar
to you since nearly all radio and TV tubes had very visible silvery getters
(and CRTs still do).
The getter electrode can be seen in photos of a Typical
Small to Medium Size Melles Griot HeNe Laser Tubes. However, no getter
spots are visible. I have found many tubes where there is a getter electrode
present but the getter spot is undetectable. Some modern getters use a
zirconium based material which is colorless as opposed to old style getters
which were barium based with a very visible spot. (Really long life HeNe
tubes like those from Hewlett-Packard actually use a zirconium cathode. They
are rated for a 100,000 hour life!) It's also possible that the getter
was included as insurance and never activated. I suppose that modern
vacuum systems and processing methods are so good and hard-seal tubes
don't really leak, so there is not as much need for a getter as there
used to be.
Note that a high mileage HeNe (or other gas discharge) tube may exhibit
metallic deposits (usually) near electrodes which look similar to the getter
spot. However, these are due to sputtering and won't change appearance if
there is a leak! The tube is usually near death at this point in any case.
The mirrors used in lasers are a bit more sophisticated than your bathroom
variety:
However, note that for a sufficiently long HeNe tube (one with high enough
gain), it would be possible to use a pair of freshly coated or protected
aluminum mirrors though performance would be pretty terrible. And, getting
a useful beam out of such a laser would be difficult because aluminized
mirrors tend to not be even partially transparent! I've gotten
a 10" long HeNe tube with an internal HR and Brewster window at the other
end to lase using the aluminized mirror from a barcode scanner - just
barely. But the first HeNe laser would not have been possible without
dielectric mirrors despite its length since the wide bore resulted in
very low gain.
I have also come across HeNe laser output mirrors with a slight *negative*
RoC - they are convex rather than concave with respect to inside the cavity.
At first I thought these were a mistake, coating the wrong sides of the
mirror glass or something like that. But the slightly convex curvature
does indeed result in a stable resonator configuration and actually has
a slightly lower divergence than a similar concave mirror when tested in
my one-Brewster external mirror HeNe laser (though I can't
tell if this might also have been more due to the curvature of the outer
surface). I have since found a sample of a HeNe laser tube (probably from
a barcode scanner) that had such a mirror, though it's certainly not a
common configuration.
You may be able to tell which type you have by looking at a reflection off
of the inner surfaces of the mirrors at each end (assuming the one at the
non-output end is not painted or covered). Assuming the outer surfaces
are flat, a concave mirror will reduce the size of the reflection very
slightly compared to a planar mirror. If wedge is present, the reflections
from the front and back (interior) surface of the mirror will shift apart as
you move further away (though this may be tough to see on the Anti
Reflection (AR) coated output mirror since the reflection from the AR
coated surface will be very weak). See the section:
Ghost Beams From HeNe Laser Tubes.
To further complicate matters, the front (outer) surface of the mirror at
the output-end of the tube may be ground to a (slight) convex or concave
shape as well resulting in either a positive lens which aids in beam
collimation or a negative lens with increases the divergence.
Since the reflection peaks at a single wavelength, this type of mirror
actually appears quite transparent to other wavelengths of light. For
example, for common HeNe laser tubes, the mirrors transmit blue light quite
readily and appear blue when looking down the bore of an UNPOWERED (!!) tube.
Blue light from the electrical discharge will also pass out of the
mirrors as a diffuse glow when running. No, you don't have a blue HeNe
laser!
Also see the section: Mirror Reflectances
for Some Typical HeNe Lasers.
There should be no reason for the alignment to have changed unless you
whacked the tube - it was set at the factory. But due to the way some
tubes are constructed, it can creep with multiple thermal cycles over the
years. If you suspect an alignment problem, it is easy to check.
Then, you can decide if attempting an adjustment is worth the risks. See
the section: Checking and Correcting Mirror
alignment of Internal Mirror Laser Tubes.
However, long high power tubes (i.e., 20 mW and up) may require fixtures to
maintain mirror alignment even when the mirrors are internal. For example,
they may need to be securely mounted in their mating laser head cylinders.
Such tubes will not be stable by themselves because thermal expansion will
result in enough change in alignment to significantly alter beam power - even
to the extent of extinguishing the beam entirely at times! There may even be
a 'This Side Up' indication (not related to the orientation for linearly
polarized tubes) on the HeNe tube or laser head as gravity affects this as
well (the alignment and thus power, not the gas, electrons, ions, or light!)
and can significantly affect operation. I do not know if this latter sort of
behavior is common or only likely with tubes that are marginal in some way.
But, there will always be at least a small change in power with orientation
for longer tubes.
There is a slight benefit to having the output coupler mirror at
the anode-end of the tube due to the typical long-radius hemispherical
cavity configuration. With the bore running almost to the mirror mount,
more of the mode volume is inside the bore and thus the gain will be
slightly higher. But the difference is only really significant for "other
color" HeNe laser tubes which have very low gain and these are more likely
to use anode-end output configuration.
The HRs in all cases showed greater than 99.9 percent reflectivity (T less
than 0.001 - virtually undetectable on my fabulous meter).
Due to the behavior of the photodiode at low light levels, the absolute
precision of the readings is somewhat questionable. However, the relative
reflectivities of these mirrors is probably reasonably accurate. Note, in
particular, the high R of 99.4% for the very long external mirror laser
compared to the low R of 97.7% (T of 2.3%) for a shorter internal mirror tube.
I expect that in addition to the length of the bore, part of this difference is
due to the absence of Brewster window losses in the internal mirror tube
resulting in a higher gain so that more energy can be extracted via the OC on
each pass.
Mirrors for non-red HeNe lasers must be of even higher quality due to the
lower gain on the other spectral lines. The OC will also have higher
reflectivity for this reason. For green HeNe tubes (which have the lowest
gain of all the visible HeNe wavelengths), the transmission is about 1/10th
that of a similar length red tube. For example, the reflectivity of a typical
green HeNe tube OC is 99.92 to 99.95 percent (.08 to .05 percent transmission)
at 543.5 nm.
Notes on making these measurements:
Ion Beam Sputtered (IBS) coatings have a much higher packing density, so they
withstand the (i.e., 450 °C) frit sealing temperatures and don't even
shift 1 nm. Nowadays, everything is hard sealed, with the exception of the
high-end (long precision) Brewster tubes. Hard-sealing a BK-7 window puts a
lot of stress on it, and that just isn't acceptable on the high-Q tubes. So,
those get fused silica windows optically contacted (lapped and polished
surfaces that are vacuum tight.) (In fact, with this type of seal, if there is
no adhesive present, the windows can be easily removed from your dead, leaky,
or up-to-air tubes by heating the Brewster stem and window with a heat gun.
The window can then be popped off with your thum bnail!)
The main physical effect resulting in a particular polarization direction
being favored in a random polarized HeNe tube is a slight preferred axis in
the dielectric mirror coatings. Where this is very small or the mirrors at
opposite ends of the tube happen to be oriented so their effects cancel out,
the resulting polarization axes may indeed not be restricted to a fixed
orientation. But most often, they are fixed for the life of the tube.
Most linearly polarized HeNe laser tubes are similar to their randomly
polarized cousins but include a Brewster plate or window inside the cavity
which results in slightly higher gain for the desired polarization orientation
Such tubes produce a highly polarized beam with a typical ratio of 500:1 or
more between the selected and orthogonal polarization. External mirror
HeNe lasers almost always use Brewster windows and so are inherently
linearly polarized. A strong transverse magnetic field can also be
used to force linear polarization and indeed, long before I observed
this phenomenon, some commercial HeNe lasers offered a "polarization
option" which was a set of magnets to be placed next to the bore. See
the section: Unrandomizing the Polarization of a Randomly
Polarized HeNe Tube.
Another way to force linear polarization in a HeNe laser (or any other low
gain laser) is to add a mirror at 45 degrees reflecting to the actual HR
mirror, which is then at 90 degrees to the optic axis (facing sideways).
The 45 degree mirror will have a slight polarization preference
so it's reflectance will be extremely high at the desired
polarization and slightly lossy at the unwanted one. Like the Brewster
plate, this is enough to force linear polarization in low gain lasers.
The undesirable losses from the extra mirror bounce may be less
than the losses through a less than perfect Brewster plate or one with
a slight orientation error, which is particularly important for "other color"
HeNe lasers, especially green, which has the lowest gain. However, this
approach is much less common than using a Brewster plate (even for green).
I've only seen it in PMS green HeNe laser heads. Based on a test of
the mirrors from a broken tube, the reflectance of the 45 degree mirror
was about 99.997% for the preferred polarization orientation and
99.9% at the unwanted one. The 90 degree mirror had a reflectance
of about 99.997% regardless of polarization. This
difference in loss is far less than for a Brewster window but is still
more than adequate for the green laser, though probably not for a
higher gain red one. And the one PMS polarized yellow HeNe laser head
I've had used a Brewster plate.
For more info, see: U.S. Patent #6,567,456: Method and Apparatus for
Achieving Polarization in a Laser using a Dual-Mirror Mirror Mount.
Linearly polarized HeNe lasers tended to be used in older laser printers
(since the external modulator often required a polarized beam) and older
LaserDisc players (because the servo and data recovery optics required a
polarized beam). Randomly polarized lasers were used in older barcode
scanners since polarization doesn't matter there. Note the use of "older".
Nowadays, this equipment all use diode lasers which are inherently polarized.
I've heard of people retrofitting such equipment to use diode lasers without
much difficulty, but your mileage may vary. :)
(Portions from: Lynn Strickland (stricks760@earthlink.net).)
Our testing suggested that adjacent modes always have orthogonal
polarization - (lets go with S and P designations). BUT, in some two-mode
tubes, a given mode doesn't always REMAIN S or P as it changes in frequency
(it flips polarization). In "flippers", certain frequencies only support one
polarization. If this frequency range is around the center of the gain
curve, most power will be of one polarization regardless of temperature (so
it appears to be linearly polarized). (However, the extinction ratio varies
over time, and is generally poor).
Here's a test setup that shows what's going on if you have access to some
nice instrumentation: Send the beam from a two mode, randomly polarized HeNe
tube (Example: 05-LHR-006) into a scanning Fabry-Perot interferometer (this is
mucho more expensive than your basic exorbitantly priced optical spectrum
analyzer). (However, you can build a scanning Fabry-Perot interferometer if
so inclined. See the sections starting with:
Scanning Fabry-Perot Interferometers.
--- Sam.) Put a polarizer in the beam path, aligned to maximize P
polarization (or S polarization, doesn't matter). Normally, the P mode
will remain P polarization at all frequencies under the gain curve. So
as the frequency changes (due to cavity length changes with temperature),
the P mode will trace out a nice pretty sort of bell-shaped curve with a
width of about 1.6 GHz FWHM. Bottom line, you can get P-polarized light
at every frequency under the gain curve.
In a 'flipper', your curve has missing sections. In other words, there are
some frequencies where you cannot get P polarization. When the observed, P
mode reaches one of these frequency ranges, it will flip and become
S-polarized. When the flip occurs, the other, formerly S mode, turns into a
P. If you're just looking at one polarization (as the experiment describes),
the observed P mode disappears and pops up again at a frequency delta equal
to the longitudinal mode spacing (where the S mode used to be). Some call it
mode hop, but it really isn't, because both modes are still there. Both
modes still have, and always had, orthogonal polarization - they just
swapped. Some tubes flip at one point under the gain curve, some flip many
times under the gain curve.
This has to do with gain asymmetry. What brought it to our
attention, is that when the polarizations flip, you get high frequency 'noise'
if you have polarization sensitive components in your beam path. Solutions
are to specify a laser that doesn't flip, go to a three mode (longer) laser,
go to non-polarization sensitive optics all the way through the beam
delivery/detection train, or put a bandwidth filter on your detector.
A magnetic field will sometimes make a flipper stop, and sometimes make a
non-flipper start - but not always. Sans magnetic field, over time (several
thousand operating hours) our test population suggested that flippers always
flip, non-flippers always behave.
There is more on flippers below.
A "flipper" tube is one where the polarization orientation of adjacent
longitudinal modes flip places at a fixed location on the gain curve as
the modes sweep through it. This peculiar behavior may not be detected
where the laser is simply used as a source of photons (for the same reason
that polarization effects of normal mode sweep tend to be minimal - the
total power doesn't vary that much). But if are any polarization optical
elements (intentional or not), significant sudden power fluctuations will
be evident in the polarized beam(s).
While I haven't seen any discussion of flipper theory, here are some
thoughts.
In the absence of external influences like magnetic fields, the mode
orientation in a laser will be determined by two factors:
Since a transverse magnetic field can also introduce a polarization preference,
it is possible to cause a well behaved HeNe laser tube to exhibit flipper
behavior by the careful placement of a strong magnet near the tube. I've
demonstrated this with a normal Uniphase 098 laser. With no magnet, the
mode sweep is perfectly ordinary with no tendency to flipping. By placing
a single rare earth magnet next to the tube near the middle, it can be made
to turn into a flipper with a mode plot very similar to that of a natural
flipper. With too weak a magnetic field, there is no effect or a sort of
shortened aborted flipping. With too strong a magnetic field, the
polarization becomes locked to the magnetic field and the output ends
up being linearly polarized.
For that peculiar tube above which reverts to normal behavior at the
very end of the warmup period, a very weak magnetic field will cause
it to flip after the point of transition.
Plot of "Flipper" Aerotech OEM1R HeNe Laser
Head with Various Magnetic Fields Applied (Combined) shows the
effect of a rare earth magnet at 4 orientations about 4 inches from
the center of the laser head compared with no magnetic field. The
magnetic field axis was horizontally aligned with one of the
polarization axes of the laser. The magnet was rotated 90 degrees
approximately every 30 seconds. The first and last orientation shows
a mode sweep pattern that is relatively normal. They probably differ
slightly because the magnet wasn't in exactly the same position. The
tube was allowed to completely warm up with the magnets in the last
orientation with no significant change in the plot, even after the
transition point where the tube reverts from flipper to normal
behavior with no magnetic field A closeup is shown in
Plot of "Flipper" Aerotech OEM1R HeNe Laser Head
with Magnetic Field Induced Somewhat Normal Behavior (Combined).
While very different than the mode plot of the tube after warmup with
no magnetic field, the flips are gone (no vertical jumps) and it's
relatively well behaved.
Conversely, it should be theoratically possible to suppress flipper behavior
with a suitably placed magnet. Getting this to work is more problematic
since the magnetic field has to exactly counteract the natural polarization
birefringence. But I was able to somewhat do this with my flipper head
so that the mode sweep became well behaved. This was more finicky than
going the other way. Almost any magnetic field did disrupt the normal
flipper behavior. But getting it to be really well behaved was more
difficult.
Of course, a magnetic field will also introduce other effects due to
Zeeman splitting which may be detrimental depending on the application.
Note that mirror alignment which may affect the resonator orientation
preference had no effect on flipper behavior. Pressing on the mirror
mount of my flipper tube in any direction would reduce the output power
significantly due to changing mirror alignment. But the mode flips still
occurred, and appeared to be at approximately the same location on the
gain curve.
But what is the underlying cause?
(From: A. E. Siegman (siegman@stanford.edu).)
The reason that HeNe lasers can run - more accurately,
like to run - in multiple axial modes is associated with inhomogeneous
line broadening (See section 3.7, pp. 157-175 of my book) and "hole burning"
effects (Section 12.2, pp. 462-465 and in
more detail in Chapter 30) in the doppler-broadened laser transitions
commonly found in gas lasers (though not so strongly in CO2) and not in
solid-state lasers.
The tendency for alternate modes to run in crossed polarizations is a
bit more complex and has to do with the fact that most simple gas laser
transitions actually have multiple upper and lower levels which are
slightly split by small Zeeman splitting effects. Each transition is
thus a superposition of several slightly shifted transitions between
upper and lower Zeeman levels, with these individual transitions having
different polarization selection rules (Section 3.3, pp. 135-142,
including a very simple example in Fig. 3.7). All the modes basically
share or compete for gain from all the transitions.
The analytical description of laser action then becomes a bit complex
- each axial mode is trying to extract the most gain from all the
subtransitions, while doing its best to suppress all the other modes -
but the bottom line is that each mode usually comes out best, or
suffers the least competition with adjacent modes, if adjacent modes
are orthogonally polarized.
There were a lot of complex papers on these
phenomena in the early days of gas lasers; the laser systems studied
were commonly referred to as "Zeeman lasers". I have a note that says
a paper by D. Lenstra in Phys. Reports, 1980, pp.
289-373 provides a lengthy and detailed report on Zeeman lasers. I
didn't attempt to cover this in my book because it gets too complex and
lengthy and a bit too esoteric for available space and reader interest.
The early (and good) book by Sargent, Scully and Lamb has a chapter on
the subject. You're probably aware that Hewlett Packard developed an
in-house HeNe laser short enough that it oscillated in just two such
orthogonally polarized modes, and used (probably still uses) the two
frequencies as the base frequencies for their precision metrology
interferometer system for machine tools, aligning airliner and ship
frames, and stuff like that.
(From: Sam.)
Indeed, HP has several models of two-frequency HeNe lasers but the ones
I'm familiar with actually use an external magnet to create Zeeman
splitting. Rather than two longitudinal modes, a PZT or heater is used
to adjust cavity length so that only a single mode is oscillating, which
is split by the Zeeman effect. Then, the difference frequency (in the low
MHz range) is used in the measurement system as a reference and possibly
for stabilizing the (optical) frequency. See the section:
Hewlett-Packard HeNe Lasers.
The Spectra-Physics model 117A frequency stabilized HeNe laser is designed
more like what you are describing - two modes, no magnets. A heater is used
to adjust cavity length in a feedback loop using a pair of photodiodes
to monitor the two orthogonal polarized modes. However, I would assume
that based on its description, the desired operating conditions would
be for it to run with a single mode (which it can with carefully
controlled cavity length). See the section:
Description of the SP-117A Laser.
The Coherent and Melles Griot stabilized HeNe lasers are similar.
Like most low current discharge tubes, the HeNe laser is a negative
resistance device. As the current *increases* through the tube, the
voltage across the tube *decreases*. The incremental magnitude of the
negative resistance also increases with descreasing current.
In the case of a HeNe tube, the initial breakdown voltage is much greater
than the sustaining voltage. The starting voltage may be provided by a
separate circuit or be part of the main supply.
Often, you may find a wire or conductive strip running from the anode
or ballast resistor down to a loop around the tube in the vicinity of the
cathode. (Or there may be a recommendation for this in a tube spec sheet.)
This external wire loop is supposed to aid in starting (probably where a
pulse type starter is involved). There may even be some statistical
evidence suggesting a reduction in starting times. I wouldn't expect there
to be much, if any, benefit when using a modern power supply but it might
help in marginal cases. But, running the high voltage along the body of the
tube requires additional insulation and provides more opportunity for bad
things to happen (like short circuits) and may represent an additional
electric shock hazard. And, since the strip has some capacitance, operating
stability may be impaired. I would probably just leave well enough alone if
a starting strip is present and the laser operates without problems but
wouldn't install one when constructing a laser head from components.
With every laser I've seen using one of these strips, it has either had
virtually or totally no effect on starting OR has caused problems with
leakage to the grounded cylinder after awhile. Cutting away the strip in
the vicinity of the anode has cured erratic starting problems in the latter
case and never resulted in a detectable increase in starting time.
In order for the discharge to be stable, the total of the effective power
supply resistance, ballast resistance, and tube (negative) resistance must be
greater than 0 ohms at the operating point. If this is not the case, the result
will be a relaxation oscillator - a flashing or cycling laser!
Note: HeNe tube starting voltage is lower and operating voltage is higher
when powered with reverse polarity. With some power supply designs, the
tube may appear to work equally well or even better (since starting the
discharge is easier) when hooked up incorrectly. However, this is damaging
to the anode electrode of the tube (and may result in more stress on the
power supply as well due to the higher operating voltage) and must be
avoided (except possibly for a very short duration during testing).
See the chapter: HeNe Laser Power Supplies
for more information and complete circuit diagrams.
A few HeNe lasers - usually larger or research types - have used a radio
frequency (RF) generator - essentially a radio transmitter to excite the
discharge. This was the case with the original HeNe laser but is quite
rare today given the design of internal mirror HeNe tubes and the relative
simplicity of the required DC power supply.
Between dropout and nominal, output power will increase, but not in
proportion to current and not linearly. The usable output power variation
(e.g., for modulation purposes) will be in the 15 to 25 percent range.
Between nominal and the onset of single frequency noise, output will decrease
somewhat, but again not in proportion (or inverse proportion) to current.
Attempting to modulate current symmetrically around the nominal current will
result in a sort of rectification or absolute value effect on the variation
in output power.
Note that the visual effect of increasing current from dropout to cessation
of output will just be a smooth increase and then decrease in coherent optical
output power. To detect the single frequency or broadband noise will require
a sensor and oscilloscope with a bandwidth of at least a few MHz.
Also of note is that the HeNe laser power supply itself will contribute to
optical ripple and noise. A DC input switchmode (inverter) power supply
will have ripple at the switching frequency. This is typically in the range
of 1 to 5 percent of the operating current and will result in an optical
power variation of a few tenths of a percent. An AC input linear power
supply will have some ripple at 1X or 2X of the line frequency (with some
harmonics) even with a regulator. An AC input switcher (most bricks) will
have both types of ripple. Special low noise power supplies are available
for critical applications. However, for most common uses, the additional
cost is not justified. There are some more comments on this topic in the
section: Intensity Stabilized HeNe Laser.
You have probably wondered why the beam from a typical HeNe laser (without
additional optics) is so narrow. Is it that making a tube with larger mirrors
would be more costly?
No, it's not cost. Even high quality and very expensive lab lasers still have
narrow bores. The very first HeNe lasers did use something like a 1 cm bore
but their efficiency was even more mediocre than modern ones. A wide bore
tube would actually be cheaper to manufacture than one requiring a super
straight narrow capillary. However, it wouldn't work too well.
A combination of the current density needed in the bore, optimal gas pressure,
gain/unit length in the bore, the bore wall itself aiding in the depopulation
of lower energy states, and the desire for a TEM00 (single transverse mode)
beam (there are multimode tubes that have slightly wider bores), all interact
in the selection of bore diameter.
In fact, there is a mathematical relationship between bore size,
gas pressure, and tube current resulting in maximum power output and long life.
The optimal pressure at which stimulated emission occurs in a HeNe laser is
inversely proportional to bore diameter. According the one source (Scientific
American, in their Amateur Scientist article on the home-built HeNe laser -
see the chapter: Home-Built Helium-Neon (HeNe)
Laser), the pressure in Torr is equal to 3.6 divided by the ID of the
bore. I don't know whether this exact number applies to modern internal
mirror tubes but it will likely be similar. Power output decreases on either
side of the optimal pressure but a laser with a low loss resonator may still
produce some output above twice and below half this value.
Thus, as the bore diameter is increased, the optimal pressure drops. Aside
from having fewer atoms to contribute to lasing resulting in a decrease in
gain, below a pressure of about .5 to 1 Torr, the electrons can acquire
sufficient energy (large mean-free-path?) to cause excessive sputtering at the
electrodes. This will bury gas atoms under the sputtered metal (which may
also coat the mirrors) leading to a runaway condition of further decreasing
pressure, more sputtering, etc. Even with the large gas reservoir of your
typical HeNe tube (which IS the main purpose of all that extra volume), there
may still be some loss over time. A drop in gas pressure after many hours of
operation is one mechanism that results in a reduction in output power and
eventual failure of HeNe tubes.
As a result, the maximum bore diameter you will see in a commercial HeNe laser
will likely be about 2 mm ID (for those multimode tubes mentioned above where
the objective is higher power in a short tube). Most are in the 0.5 to
1.2 mm range. This results in high enough pressure to minimize sputtering,
maximize life, provide maximum power output, and optimal efficiency (to the
extent that this can be discussed with respect to HeNe lasers! Well, ion
lasers are even worse in the efficiency department so one shouldn't complain
too much. Since total resonator gain is proportional to bore length and
approximately inversely proportional to bore diameter (since the optimal
pressure increases resulting in a higher density of lasing atoms), this favors
tubes with long narrow bores. But these are difficult to construct and
maintain in alignment. Wide bore tubes have lower gain but a higher total
number of atoms participating with potentially higher power output at the
optimal pressure and current density. Everything is a tradeoff!
However, all this does provide a way of estimating the power output and drive
requirements of a HeNe tube or at least comparing tubes based on dimensions.
Assuming a tube with a particular bore length (L) is filled to the optimum
pressure for its bore diameter (D), power output will be roughly proportional
to D * L, discharge voltage will be roughly proportional to L (probably minus
a constant to account for the cathode work function), and discharge current
will be roughly proportional to D. (Note that D instead of the
cross-sectional area is involved because the optimal pressure and thus density
of available lasing atoms is inversely proportional to D.)
So, do the numbers work? Well, sort of. Here are specifications for some
selected Melles Griot red HeNe tubes rearranged for this comparison:
(Bore length was estimated since the cathode-end of the capillary is not
visible without X-raying the tube or by optically determining its position
through the mirror!)
The general relationships seem to hold though large tubes seem to produce
higher output power than predicted possibly constant losses represent a
smaller overhead. As noted elsewhere there is also a wide variation even for
tubes with similar physical dimensions. Oh well...
There are more examples in the section:Typical
HeNe Tube Specifications. You can do the calculations. And, some large
IR HeNe lasers may use a somewhat wider bore. See the section:
Spectra-Physics 120, 124, and 125 HeNe
Laser Specifications for a comparison of visible and IR HeNe tubes for the
same model laser.
Note that there are some multi-mode (non-TEM00) HeNe tubes with wider bores
and a different mirror curvature that produce up to perhaps twice the power
output for a given tube length. However, with multiple transverse modes,
these are not suitable for many applications like interferometry and
holography. They are also not very common compared to single-mode TEM00
HeNe tubes.
The most powerful HeNe laser I have ever seen was 160 mW of real power and was
the only time I've ever seen a HeNe laser burn anything before with raw
beamage. It would slowly burn electrical tape placed in the beam and felt
warm on your skin. It was made of two almost 6 foot long Spectra-Physics
model 125 tubes hooked electrically to separate power supplies and optically
in series in a custom made double-wide sized 125 head. Sadly, it doesn't
work anymore and is currently resting piecefully in the NTC laser
department's laser graveyard. :-(
(From: Steve Roberts (osteven@akrobiz.com).)
I've seen a normal SP-125 break 160 mW on its own. Two tubes at only
160 mW sounds like it was misaligned, not that I'd like to try to align that
one! :)
The current record is for a Chinese researcher using 2 tubes with a
flattened elliptical profile in a V fold resonator to get 330+ mW
into a fiber. The beam shape and divergence from this are not what you
would expect from a typical HeNe laser, even one that runs multi (transverse)
mode. Remember that a HeNe laser's power is limited by collisions with the
tube wall returning Ne atoms to the ground state, so using a flattened tube
means more wall area, hence more power. Optimal gas pressure is a function
of bore diameter as well. So you're limited to about a 1 meter tube in most
cases by other optics reasons and sputtering. With collisions with the wall
increased by a larger wall surface area, what the folks in China did is try
tubes with different cross sections. To get enough length they folded the
resonator using a 3 optic V-fold. You don't want to see the beam profile.
It's nasty! It looks kind of like this: <{[=]}>. And the divergence is
high as the optics need to fill that whole lasing volume.
Please note, however, that going to a large rectangular or star shaped tube
is not possible due to some quirks in the plasma at the pressure required
for HeNe laser operation. Details are in a 1996 issue of Review of Scientific
Instruments. A few years ago, Cornell University attempted to sell the rights
to the unit in the United States, on behalf of the Chinese Inventor.
U.S. patent and marketing were assigned to a group that sadly dropped
the ball. At the time, the picture of the unit looked like one of those
old foldaway sewing machines like my mom used to have, an ornamental blue
box about the size of a PC Tower turned on its side with 4 wooden legs.
Most laser heads include the ballast resistor since it needs to be close to
the HeNe tube anode anyhow (though you may still need additional resistance
to match the tube to your power supply). The ballast resistor may be potted
into the end cap with the HV cable, a wart attached to the HeNe tube, or a
separate assembly. There may be an additional ballast resistor (e.g., 10K)
in the cathode circuit as well.
The majority of laser heads use a HeNe laser tube with the output beam
emerging from the cathode-end of the tube so there is little or no voltage
present on the exposed terminals if the output end-cap is removed. However,
some laser heads will place the anode and ballast resistors at the
output-end. This is particularly true of some "other color" HeNe lasers
(e.g., yellow and green) since there are some subtle advantages to this
arrangement in terms of output power for a given tube size. But, in
some cases, it's just to be able to install a stock tube.
The high voltage cable will likely use an 'Alden' connector which is
designed to hold off the high voltages with a pair of keyed recessed
heavily insulated pins. This is a universal standard for small to medium
size HeNe laser power supplies (the longer fatter pin is negative).
Typical cable length is from 6 inches to 6 feet.
Internal wiring may be via fat insulated cables or just bare metal (easily
broken) strips. Take care if you need to disassemble one of these laser
heads (the round ones in particular) as the space inside may be quite
cramped.
CAUTION: The case, if metal, of the laser head may be wired to the cathode
of the HeNe tube and thus the negative of the Alden connector and power
supply. This is not always the situation but check with an ohmmeter and
keep this in mind when designing a power supply or modulation scheme. The
case should always be earth grounded for safety if at all possible (or
else properly insulated). DO NOT assume that a commercial power supply
is designed this way - check it out and take appropriate precautions.
Note: Depending on design, the laser tube itself may be mounted inside the
laser head in a variety of ways including RTV Silicone (permanent and
alomst impossible to remove), hot-melt glue (permanent but removable), or 3
or 4 set screws at two locations (front and rear) around the outside of
the housing. The latter approach permits precise centering of the beam
but don't overtighten the screws or you WILL be sorry! (Since RTV silicone
has some compliance, very SLIGHT adjustment of alignment may still be
possible even if mounted this way - don't force it, however.)
For example, I found that some recent samples of the popular Melles Griot
05-LHR-911 HeNe laser head, rated at 1 mW minimum power output, were all
made with neutral density filters to assure that the maximum power
output was less than 1.5 mW. With the filters removed, it jumped to between
1.8 and 2.1 mW! Apparently, the filters were individually selected to get
the lasers as close as possible to 1.5 mW without exceeding it since their
attenuations were not all the same and the weakest laser in the batch (with
the filter) actually ended up having the hottest tube.
If you have a laser head that is missing the Alden connector, replacements
should be available from the major laser surplus suppliers or salvage one
from another (dead) head. I also have many available. Where the end-cap
on a cylindrical laser head is also missing, there are no readily available
commercial sources - fabricate one from a block of wood and paint it black
or find some other creative solution. A suitable ballast resistance must
also be installed between the positive power supply output and the HeNe
tube anode.
The cylindrical head serves another purpose besides structural support and
protection. This is the distribution of heat and equalization of thermal
gradients. Thus, removing a long HeNe tube in particular from its laser head
may result in somewhat random or periodic cycling of power output due to
convection and other non-uniform cooling effects.
Often, particularly inside equipment like barcode scanners, you will see
something in between: A HeNe tube wrapped in several layers of thick aluminum
foil probably to help distribute and equalize the heating of the tube for the
reason cited above. However, I haven't really noticed any obvious difference
in stability when this wrap was removed. Spectra-Physics is very fond of
this but others may have copied it to sell compatible tubes.
The operating lifetime of a typical HeNe laser tube is greater than 15,000
hours when used within its specified ratings (operating current, proper
polarity, and not continuously restarting). Under these conditions,
end-of-life occurs when the oxide "pickling" layer of the cathode can
gets depleted. Larger diameter (1.5 or 2 inch) tubes last the longest - up
to 50,000 hours or more. Small diameter (0.75 or 1 inch) tubes have the
shortest lifetime - 10,000 hours or so. Since even 10,000 hours is still
very long - over 1 year of continuous operation - HeNe laser lifetime is
not a major consideration for most hobbyist applications. Chances are that
even a surplus laser will still have thousands of hours of life remaining.
However, the shelf life of the tube depends on types of sealing method used
in the attachment of the optics. There are two types of internal mirror HeNe
tubes:
The frit is basically powdered low melting point glass mixed with a liquid
to permit it to be spread like soft puddy or painted on. The frit can be
fired at a low enough temperature that the mirror mount or glass mirror
itself is not damaged, there is virtually no distortion introduced by the
process, and manufacturing is greatly simplifed compared to using normal
(high temperature) glass or ceramic joints. Some tubes use frit seals at
other locations in addition to the mirrors (like the end-caps) rather than
glass-to-metal seals. The same process is used for other permanently sealed
tubes like those in internal mirror argon ion lasers as well as some xenon
flashlamps and similar devices.
Note that the electrical connections on those tubes that don't use the
mirror mounts will generally be glass-metal seals which do not leak.
Mirrors can't use glass-metal seals since they require high temperatures
to make which would distort or totally destroy the mirrors. You can tell
if a seal is frit or Epoxy by how easily it scratches: Frit is like glass
and requires something hard to make a mark while Epoxy can be scratched
with a good solid fingernail. Another way to tell is the color: Frit is
generally gray or tan while Epoxy is clear or white.
Should you care, the metal parts of the tube are likely made from Kovar, an
alloy commonly used with frit seals since there is a very good CTE
(Coefficient of Thermal Expansion) match of the Kovar to the frit glass.
CAUTION: The frit seal is thin and relatively fragile, even more so than
the fragile optical glass, so avoid placing any stress on the mirrors!
Shelf life of soft-sealed tubes is limited by diffusion of the Helium
atoms out and air leakage in, water vapor from Epoxy seals, etc. Helium
atoms are slippery little devils and diffuse through almost anything.
In the case of HeNe tubes, diffusion
takes place mostly through the Epoxy adhesive used to mount the mirrors in
non-hard sealed tubes (not common anymore) and through the glass itself but
at a much much slower rate. Most of the contamination of air leakage will
be cleaned up by the getter (if present) until it is exhausted. However,
hydrogen may appear, probably from dissociated wate vapor (the getter will
clean up the O2) and hydrogen (1) kills lasing at very low concentrations
and (2) appears virtually impossible to remove. The discharge spectrum
will reveal much about the gasious health of a HeNe laser tube.
See the sections starting with: HeNe
Tube Problems and Testing.
.
CAUTION: Take care in attempting to clean the
Brewster windows or mirror mounts of soft-sealed HeNe or ion laser tubes with
alcohol or other solvents as the result may be immediate air leakage and a
dead tube. The failure mechanism for this isn't clear - after all, it can
take weeks to loosen up these optics by soaking when trying to salvage them
for some other use. However, there is anecdotal evidence to suggest that
instant tube death may result from such cleaning attempts. So, to be safe,
avoid getting the area of the sealing adhesive wet with solvent.
A very few tubes apparently have frit at one end and a soft-seal
at the other so check both ends. This probably applies only to some low
gain "other color" HeNe lasers with a mirror that would be affected by
even the relatively low temperature at which the frit melts.
Note that other parts of most tubes (except for Brewster windows, if present)
use glass-to-metal seals but since these must be manufactured at high
temperature, they are not an option for delicate optics. The very best tubes
with one or two Brewster windows do not use frit because even at the low
temperature at which it is fired, there may still be some unavoidable stresses
introduced - these tubes continued to be soft-sealed even after frit was
common but now use optical contacted seals. With optical contacted seals, the
two pieces are ground and polished optically flat and brought together under
clean room conditions. The resulting seal is gas-tight. Just a bit of Epoxy
is used for mechanical stability but it doesn't do the sealing.
The HeNe gas doesn't 'wear out'. A HeNe tube, when properly connected has a
substantial portion of its power dissipated by the bombardment of positive
ions at the cathode (the big can electrode) which is made large to spread
the effect and keep the temperature down and is "pickled" (coated) to reduce
its work function. Hook a tube up backwards and you may damage it in short
order and excessive current (operating current as well as initial starting
current from some high compliance power supplies) can degrade performance
after a while. Electrode material may sputter onto the adjacent mirrors
(reducing optical output or preventing lasing entirely) or excessive heat
dissipation may damage the electrodes or mirrors directly.
As the tube is used (many thousands of hours or from abuse), operating and
starting voltages may be affected as well - generally increasing with the
ultimate result being that a stable discharge cannot be initiated or maintained
with the original power supply. See the section:
How Can I Tell if My Tube is Good?.
(From: Lynn Strickland (stricks760@earthlink.net).)
Typical failure mechanism in a HeNe is cathode sputtering -- seldom gas
leakage in the newer (like since 1983) tubes. Shelf life is stated to be about
10 years, but it's not uncommon at all to see HeNe lasers built in the early
1980's that still meet full spec.
Interesting lifetime note - it used to be that you left a HeNe 'on' at all
times to prolong life. Since hard-sealing, you should turn it off while not
in use. If it's a 20,000 hour tube, and you only turn it on for a few hundred
hours a year, it will last a heck of a long time. Not uncommon at all for the
HeNe to outlive several power supplies. The larger diameter tubes tend to
last longer, but it also depends on fill pressure and operating current
(higher fill-pressure tubes last longer). The typical 5 mW red HeNe will
commonly live to 40k to 50k operating hours.
As for cathode sputtering, the tube has an aluminum cathode that is 'pickled'
during the production process to add a layer of oxidation about 200 microns
thick. The oxidation layer prevents aluminum from being bombarded away from
the cathode during plasma discharge. As the tube ages, the oxide layer is
depleted until aluminum is exposed. Sputtered aluminum can stick to the
mirror, causing power decline, or to the inside of the glass envelope, causing
the discharge to arc internally. This arcing, if allowed to continue for a
period of time, will also cook the power supply. A tube with no oxidation
layer on the cathode will die in about 200 hours of use. OR, once the
oxidation layer is depleted, the tube will die in about 200 hours. This is
why a HeNe life curve is usually pretty flat, then quickly degrading to
nothing over about a 200 hour period.
As is typical of Spectra-Physics internal mirror HeNe tubes, these have thick
glass walls (at least compared to tubes from most other manufacturers). For
the barcode scanner application (at least) there was an outer wrap (removable)
of several layers of thick aluminum foil, apparently for thermal stabilization
but it would also reduce electrical noise emissions and light spill from the
discharge. (The foil wrap also seems to be common with more modern
Spectra-Physics HeNe barcode scanner tubes when not installed in cylindrical
laser heads.) A 100K ohm ballast resistor stack in heat shrink tubing was
attached with a clip and RTV Silicone to the anode end-plate stud, and both
ends were capped with rubber covers for protection (of the tube and user).
The SP-084-1 is about 9-1/2" (241 mm) by 1" (25.4 mm) in diameter with a bore
length of 5.5" (140 mm). Its output is a TEM00 beam about 0.8 mm in diameter
exiting through a hole in the cover on the cathode-end of the tube. Power
supply connections are made to a stud on the anode end-plate and the
exhaust tube on the cathode end-plate. Their optimal operating point is
around a tube current of 5 mA resulting in a total operating voltage (across
tube + Rb) of about 1.9 to 2.0 kV using the 100K ballast.
Note from the diagram that unlike modern tubes where the mirrors are on
mounts that can be adjusted (by bending) after manufacturer, alignment of
the SP-084-1 would appear to be totally fixed. Some possible ways of setting
alignment might be:
From appearances, I would guess (2). Since the mirrors are slightly curved
(non-planar), their position could be used to adjust alignment slightly - and
some were attached very visibly off-center to compensate for end-plates fused
to the glass tube at a slight angle.
More recent HeNe laser-based laser pointers became more compact and some
ran off a bunch of AA or 9 V batteries. But they never achieved
keychain status, unless they were keys for elephants. :)
It is still possible to buy a HeNe laser in a compact package. The
Metrologic model 811 (red, $399) or 815 (green, $719) is not much
over 1" x 2" x 7" and houses a 5 or 6 inch HeNe laser tube with HV
power supply built-in. However, this is still tethered to
a DC wall adapter, though a bettery box option might be available.
There's not much demand for these as pointers anymore but they are
still cute. :)
Note that the intensity of the light between the mirrors of an HeNe laser
may be on the order of 100 times (or more) that of the output beam. Some
instruments for making scattering measurements or related applications
actually take advantage of this by using this only the 'internal' beam.
Such a device could be constructed using an HeNe tube with at least one
external mirror with optical sensors to observe only the scattered light
from the side. In addition, the amount of attenuation due to the dust will
affect the output beam intensity amplified by the gain of the resonator and
this behavior can also be used in conjunction with various types of studies.
By using these techniques, many of the benefits of a 1 W laser (for example)
are available with only a 10 mW tube and at much lower cost. Such a laser
is also much safer to use since that 1 W beam is in a sense, virtual - if
anything of substantial size intercepts it (like an unprotected eyeball),
lasing simply ceases without causing any harm.
Melles Griot and others offer Brewster window HeNe tubes rated up to 30 mW
or more of output power and 30 Watts of intra-cavity power! As a
rough estimate, a HeNe tube capable of n mW of normal output will be able
to do 1000*n mW of circulating power with high quality HRs at both ends.
Modern one-Brewster HeNe tubes for partical scattering or particulate
monitoring applications may provide as much as 100 Watts of
intra-cavity power using super-polished mirror substrates for the two HRs
with ion beam sputter coatings and an optically contacted fused silica
Brewster window. (The mirrors are about 15 times the cost of those used in
common HeNe lasers. Don't ask about the total tube price!)
Specifications for a variety of one and two-Brewster HeNe tubes can be
found in the section: Melles Griot Brewster
and Zero Degree Window HeNe Tubes.
As noted, the best of these tubes will have optically contacted Brewster
windows (rather than frit seals, more on this below). As frit cools, some
stresses may build up which can distort the window ever so slightly reducing
the tube's performance where hundreds or thousands of paases through the
window are involved. Optical contacting uses lapped and polished surfaces
to form a glass-to-glass vacuum-tight seal. Adhesive is only really needed
for mechanical protection - it doesn't hold the vacuum. Soft-seal windows
don't have the distortion problem but do leak over time.
(From: Lynn Strickland (stricks760@earthlink.net).)
The tube is a Melles Griot model 05-LHB-570. It has an internal HR mirror
and Brewster window at the other end of the tube. The HR is similar to those
on other Melles Griot tubes (including the use of a locking collar) though the
somewhat more silvery appearance of its surface may indicate that it is coated
for broadband reflectivity and/or perhaps for higher reflectivity than
ordinary HRs. (The mirror reflectivity of the HR on at least some versions of
the 05-LHB-570 is greater than 99.9% from 590 to 680 nm but I don't think
this one, which is quite old, has these characteristics.) The total length
is about 265 mm (10.5 inches) from the HR mirror to the Brewster window.
There is also a power sensor inside the head for (I assume) monitoring what
gets through the HR mirror (untested).
CLIMET 9048 One-Brewster HeNe Laser Head shows the
aluminum cylinder with its mounting flange at the Brewster window end, ballast
resistor, and Alden connector. The other black wire attaches to the solar
cell power sensor.
These one-Brewster HeNe tubes are generally used in applications like particle
counting which requires high photon flux to detect specks of dust or whatever.
Access to the inside of the resonator is ideal since with appropriate highly
reflective mirrors at both ends, several WATTs of "virtual" circulating
power can be produced inside the cavity of this HeNe laser. Thus, for these
applications, they have the benefits of a high power laser without the cost or
safety issues. There are even HeNe tubes similar to this that will do up to
45 W using super high quality mirrors and Brewster window. And, of course,
they are also super expensive. Of course, you can't siphon off all that power
- only be extremely envious and frustrated that it is trapped in there - but
also safe from any sneak attacks on an unsuspecting eyeball. :)
A rig similar to the one from which the Climet 9048 was removed is a model
8654, whatever that means. It is shown in Climet
Particle Counter Assembly - Front and Climet
Particle Counter Assembly - Rear. There really isn't much inside -
just some passages for the particle-containing gas which is directed to
through the intracavity beam at one focus of a large aspheric lens which
directs any scattered light onto a PhotoMultiplier Tube (PMT). The PMT is
inside the black box at the lower left with its high voltage power supply
above in the front view. The three-screw (sort of) adjustable mount for the
external HR mirror is visible in the rear view. What's interesting is that
there is really nothing physical to protect either the B-window or mirror
from contamination by the flowing gas, except presumably by the flow pattern
and pressure. There are separate compartments for the B-window and mirror,
but they aren't sealed. However, it appears that during operation,
those compartments are provided with a flow of higher pressure gas,
filtered by the large canister visible in the photos. But, how they are
expected to remain clean when the thing is shut down is a mystery. It is
a particle counter after all. Aren't particles basically dust? :)
OK, well, part of the secret is that apparently these things are intended
to be looking at really clean air without many particles. A typical
use would be in a semiconductor Fab Class 10 cleanroom - 10 or fewer
particles (2 microns or larger) per cubic foot. This isn't your normal
room air, which would be Class 10000 to Class 100000! :) Even so, the
recommended service interval printed on the label is only 6 months.
With its wide bore, this tube has an optimal operating point (maximum power)
of about 7.5 to 8 mA at about 1 kV (though the recommended current is
actually 6.5 mA). This may just be a peculiarity of the sample I tested.
I have constructed a simple mirror mount so that various mirrors could be
easily installed and there is easy access to the inside of the cavity.
See HeNe Laser Tube with Internal HR and Brewster
Window with External OC for a diagram showing this laser assembly.
Using various mirrors, both from deceased HeNe lasers as well as from laser
printers and barcode scanners, output power reached more than 3 mW and the
circulating power inside the resonator peaked at over 1 W (but not with the
same mirrors). With optimum high quality mirrors, it should be capable of
more power in both areas. Photos of this laser are shown in
Sam's External Mirror Laser Using One-Brewster HeNe
Laser Head.
See the section: Sam's Instant External Mirror
Laser Using a One-Brewster HeNe Tube for details on these experiments and
the design of the mirror mount.
I have attempted to get wavelengths other than boring 632.8 nm red out of
this and similar 1-B tubes. However, all attempts have failed but one -
installing a somewhat larger 05-LHB-670 in place of the dead tube of a PMS/REO
tunable HeNe laser. (This 1-B tube did 7.5 mW with the same OC mirror as used
above. The 1-B tube in the Climet head probably woudn't have enough gain.)
The HR mirror on the tuning prism is broadband coated for 543.5 to
632.8 nm. In this case, I was able to convince just a few 611.9 nm orange
photons to cooperate and lase. However, the only way to collect them was
from the reflections off the Brewster surfaces of the tube or prism, or from
the HR mirror of the 1-B tube. The total orange power was around 225
microwatts - 50 uW from the HR mirror, 65 uW reflected from the Brewster prism,
and 110 uW reflected from both surfaces of the tube's Brewster window. When
633 nm was selected, the output from the HR mirrors was about 350 uW (I didn't
measure the red power from the Brewster reflections).
H. Weichel and L.S. Pedrotti put out a good summary paper which includes the
equations used in the design process of a gas laser. In particular, section V
tells you how to calculate mode radius at any point, given mirror curvature,
spacing and wavelength. If you know that, the aperture size (the capillary
bore usually) and the magic number for the ratio between the two, you can
design a TEM00 gas laser. Using a HeNe tube with a Brewster window, you could
do some fun stuff with predicting aperture sizes and locations to force TEM00
operation.
The paper was published by the Department of Physics, Air Force Institute of
Technology, Wright-Patterson Airforce Base, OH. The title is "A Summary of
Useful Laser Equations -- an LIA Report". Don't know where you'd find it, but
the Laser Institute of America (LIA) might
be a good start.
Note: Since the gain of these wavelengths is so low, they also have a
shorter life and the chance of finding working surplus green or yellow
HeNe lasers is much lower than for red. I would not recommend bidding on
an eBay auction for one of these unless guaranteed to be working. The
likelihood of the problem for an "unknown condition" green or yellow HeNe
laser being just mirror alignment is small to none!
Typical maximum output available from (relatively) small HeNe tubes
(400 to 500 mm length) for various colors: red - 10 mW, orange - 3 mW,
yellow - 2 mW, green - 1.5 mW, IR - 1 mW. Higher power red HeNe tubes (up
to 35 mW or more and over 1 meter long) and 'other-color' HeNe tubes (much
lower - under 10 mW) are also available. However, these will be very large
and very expensive.
A few tunable HeNe lasers have been produced commercially. These provide
wavelength (color) selection with the turn of a knob. However, due to the
low gain of most HeNe lasing lines, producing a useful tunable HeNe laser
is not an easy task. Everything must be just about perfect to get the
"other color" lines to lase at all, and even more so when a laser is to be
designed to work at more than one wavelength with a TEM00 beam. The most
widely known such laser (as these things go) is manufactured by
Research Electro-Optics, Inc. (REO).
It produces at least 5 of the visible wavelengths: normal red, two oranges,
yellow, and green. A Littrow (or Brewster) prism with micrometer screw
adjusters takes the place of the HR mirror in a normal HeNe laser. See
the section: Research Electro-Optics
Tunable HeNe Lasers.
There used to be a model ML-500 tunable HeNe laser from
Spindler and Hoyer that did
*14* lines between 611 nm and 1,523 nm. So no 604 nm orange, 594.1 nm yellow,
543.5 nm green, or 3.39 um IR.
The mirror set has to be changed to go between the visible
and IR wavelengths. It used a Birefringent Filter (BRF) for
wavelength selection
instead of the Littrow prism in the REO tunable laser. A BRF has the
advantage that there is no loss from a slightly incorrect Brewster angle
for all but one wavelength, unavoidable with a Littrow prism. This is
because the BRF is always set at exactly the Brewster angle. The
birefringent crystal in the BRF filter produces a different optical
delay for polarization components oriented in the direction of its
slow and fast axes. Only when this difference is a multiple of a
full cycle for any given wavelength, will the polarization be unchanged
and thus result in minimal loss through the BRF. By rotating the BRF
around its optical axis (still maintaining it at the Brewster
angle to the laser's optical axis), the wavelength where minimum loss
occurs can be selected. In 1987, it was only $5800 for laser with either
wavelength range, an additional $750 for the other mirror set
I don't know why Spindler and Hoyer would have admitted defeat in not
including those other wavelengths as they were certainly known at the
time. Perhaps, the losses through the two Brewster windows
of their laser tube and the Brewster angled plate of the BRF
compared to those of the Brewster window and Brewster prism of the
PMS/REO tunable laser were just too high. Perhaps, their mirror
coating technology was not as good as what PMS/REO had available.
Unfortunately, Spindler and Hoyer no longer makes this laser, only
boring normal HeNe lasers and other optical equipment. However, a scan
of the original ML-500 product brochure can be found at
Vintage Lasers and
Accessories. With modern
technology, a 17 line tunable HeNe laser should be possible. :) A tube
with internal mirrors and a BRF *inside* would reduce the number of
Brewster angle reflective surfaces to only 2, compared to the 3 of the
PMS/REO design. A magnetic coupling can be used to move the BRF from
outside the tube. In addition, the mirrors can be recessed away from
the ends of the tube so they don't experience any high temperatures
during the sealing process. The tube itself would be hard-sealed with
frit or regular glass. Then optical contacting or leaky Epoxy seals
can be avoided. Use a Brewster angle window to pass
the laser beam out of the tube. One of the mirror mounts would be
attached via a metal bellows to allow for alignment.
For example, one typical stabilized HeNe laser from Hewlett-Packard, has a
precise vacuum wavelength of 632.991372 nm. Another one from Melles Griot
(as noted below) is 632.991058 nm in vacuum or 632.81644 nm in air (divide by
the index of refraction of air, n=1.00027593).
(Portions from: Jens Decker (Jens.Decker@chemie.uni-regensburg.de).)
The Melles Griot catalog claims a nominal frequency of 473.61254 THz for their
05-STP series of frequency stabilized lasers. (Elsewhere in the same catalog
they are more precise and lists 473.612535 THz for the 632.8 nm line.) Anyhow,
with c = 2.997925E8 m/s this gives 632.991058 nm in vacuum or 632.81644 nm in
air for n = 1.00027593 (formula from J Phys.E, vol. 18, 1985, pp. 845ff). To
find reliable values for all the other HeNe lines is quite difficult. One has
to compare a number of books to be sure whether the values are for air or
vacuum.
(From: D. A. Van Baak (dvanbaak@calvin.edu).)
Well, here it is exact:
The metrologists' answer for a 632.8 nm HeNe laser stabilized to the a-13
component of the R(127) line of the 11-5 transition of the 127-Iodine dimer
molecule is:
under certain specified conditions, with uncertainty 2.5x10-11.
See: "Metrologia", vol. 30., pp. 523-541, 1993-1994.
The minimum divergence obtainable is affected mostly by beam (exit or waist)
diameter (wider is better). Other factors like the ratio of length to bore
diameter (narrower is better) may also affect this slightly. The equation
for a plane wave source is:
HeNe laser tubes destined for barcode scanners often have a much higher
divergence by design - up to 8 mR or more (where the optimal divergence may
be as little as 1.7 mR or less). These tubes either have a negative
curvature for the outer surface of the OC mirror glass (concave inward) or
even an external negative lens attached with optical cement. See
Uniphase HeNe Laser Tube with External Lens. The
outer surface of OC in a normal HeNe tube will either be planar or slightly
convex depending on whether the OC mirror is planar or slightly concave
respectively. In the latter case, the convex surface precisely compensates
for the extra divergence produced by the OC mirror curvature and results in
a nearly optimally collimated beam. If the outer surface of your HeNe
tube's OC is concave, then it will have the high divergence characteristic.
Note that the beam is still of very high quality but an additional positive
lens approximately one focal length away from the OC will be required to
produce a collimated beam.
Also see the section: Improving the
Collimation of a HeNe Laser with a Beam Expander.
Lasers with external mirrors and Brewster windows (plates at the Brewster
angle attached to the ends of the tube) will be linearly polarized and
really expensive. They will also be more finicky as there may be some
maintenance - the optics will need to be kept immaculate and the mirror
alignment may need to be touched up occasionally. However, the fine
adjustments will permit optimum performance to be maintained and changes in
beam characteristics due to thermal effects should be reduced since the
resonator optics are isolated from the plasma tube. Some HeNe lasers have
an internal High Reflector (HR) mirror at one end of the tube but a Brewster
window and external Output Coupler (OC) mirror at the other end. These are
also linearly polarized and only half as finicky. :)
In the trivial triviality department, the largest commercial two-Brewster
laser I know of is the Spectra-Physics model 125, rated at 50 mW (red, 632.8
nm) but often producing much more output power when new. The plasma tube in
this beast is over 5 feet long. Jodon also manufactures a 50 mW
HeNe laser. The smallest two-Brewster plasma tube I've ever seen was from
a photo in a book on lasers from the 1960s. It was only about 4 inches in
length.
Most internal mirror HeNe tubes should not have any higher order transverse
(non-TEM00) modes. And, for multimode tubes, such modes should show up as part
of, or adjacent to the main beam anyhow.
One possible cause for this artifact is that the output-end mirror (Output
Coupler or OC) has some 'wedge' (the two surfaces are not quite parallel) built
in to move any reflections - unavoidable even from Anti-Reflection (AR) coated
optics - off to the side and out of harm's way. Where wedge is present, the
small portion of the light that returns from the outer AR coated surface of the
OC will bounce back to the mirror itself and out again at a slight angle away
from the main beam. In a dark room there may even be additional spots visible
but each one will be progressively much much dimmer than its neighbor. Note
that if the laser had a proper output aperture (hole), it would probably block
the ghost beams and thus you wouldn't even know of their existence!
Without wedge, these ghost beams would be co-linear with the main beam (exit in
the same direction) and thus could not easily be removed or blocked. This
could result in unpredictable interference effects since the ghost beams
have an undetermined (and possibly varying) phase relationship with respect to
the main beam. Sort of an unwanted built-in interferometer! The wedge also
prevents unwanted reflections from that same AR coated front surface back into
the resonator - perfectly aligned with the tube axis - which could result in
lasing instability including cyclic variations in output power.
Thus, the ghost beam off to one side is likely a feature, not a problem!
The effects of wedge on both the output beam and a beam reflected from a
mirror with wedge is illustrated in Effects of Wedge on
Ghost Beams and Normal Reflections. Note that his diagrams shows the
effect of a beam coming in from the right and reflecting off the mirror.
Where the beam is from the tube itself, the main beam corresponds to the
one marked "1st Back Surface".
If it isn't obvious from close examination of the output mirror itself that
the surfaces are not parallel, shine a reasonably well collimated laser beam
(e.g., another HeNe laser or laser pointer) off of it at a slight angle
onto a white screen. There will be a pair of reflected beams - a bright one
from the inner mirror and a dim one from the outer surface. As above, if the
separation of the resulting spots increases as the screen is moved away, wedge
is confirmed (there may be higher order reflections as well but they will be
VERY weak - see below). Where the mirror is curved, the patterns will be
different but the wedge will still result in a line of spots at an angle
dependent on the orientation of the tube.
Wedge is often present on the other mirror (High Reflector or HR) as well (in
fact, this appears to be more likely than the OC). Wedge at the HR-end won't
affect the output beam at all but performing the reflectance test using a
collimated laser (as above) at a near-normal angle of incidence may result in
the following:
With the exaggerated amount (angle) of wedge in Effects of
Wedge on Ghost Beams and Normal Reflections, another effect becomes
evident: The weaker spots are spaced further apart. It is left as an exercise
for the student to determine what happens when a laser beam is reflected at an
angle from such a mirror! Note that his diagrams shows the effect of a
beam coming in from the right and reflecting off the mirror. Where the
beam is from the tube itself, the main beam corresponds to the one marked
"1st Back Surface".
The appearance resembles that of a diffraction grating on such a beam (but for
entirely different reasons). The behavior will be similar for an OC with
wedge but because the HR mirror isn't AR coated, the higher order spots (from
the HR) are much more intense.
It is conceivable that slight misalignment of the mirrors may result in similar
ghost beams but this is a less likely cause than the built-in wedge 'feature'.
However, if you won't sleep at night until you are sure, try applying the very
slightest force (a few ounces) to the mirror mounts (the metal, not the mirrors
as they are very fragile) in each while the tube is powered (WARNING: High
Voltage - Use a well insulated stick!!!!).
Depending on the type of laser you have, see the sections:
Checking and Correcting Mirror Alignment of
Internal Mirror Laser Tubes,
Quick Course in Large Frame HeNe Laser Mirror
Alignment, and
External Mirror Laser Cleaning and Alignment
Techniques, for more information.
Another much simpler cause of an ugly beam from a HeNe (or other) laser is
dirt on the outside of the output mirror since this will decrease the
effectiveness of the AR coating. The dirt may also be on other external
optics. Some HeNe laser heads have either a debris blocking glass plate
glued at an angle to the end-cap or a neutral density filter to adjust
output power. Even if AR coated, either of these may also introduce one
or more ghost beams and if not perfectly clean, other scatter as well.
I'm gotten supposedly bad HeNe lasers where the only problem was dirt on
either the output mirror or external plate or filter.
(From: Steve Roberts (osteven@akrobiz.com).)
The mirror is wedged to cut down on the number of ghost beams, however even
with a wedged mirror there is almost always one ghost. Nothing is wrong with
your coatings on the mirror, it is simply a alignment matter. The mirrors need
to be "walked" into the right position relative to the bore. There are many
many paths down the bore that will lase, but only a few have the TEM00 beam
and the most brightness, this generally corresponds to the one with minimum
ghosts.
See the section: Quick Course in Large Frame
HeNe Laser Mirror Alignment for more information.
I've gotten most of the well known HeNe lasing lines in this manner
including up to 4 mW of red from a 2 mW yellow HeNe laser, both orange
lines, various other red lines, and one of the wavelengths that isn't
even mentioned in most texts dealing with HeNe lasers. More below.
What I don't believe I've seen so far is any yellow from non-yellow
tubes and I haven't even attempted to obtain green from non-green tubes.
Here's how to get other wavelengths from your HeNe laser. Either a bare
tube or complete laser head can be used for these experiments.
The Radius of Curvature (RoC) of the external mirror may need to be consistent
with a stable resonator configuration for the overall cavity. I'm not
entirely sure this matters that much (and the implication in the next section
is that it may not), but I'd still go with a stable configuration given a
choice. If you don't want to perform the calculations, a mirror that should
work would be one from a dead red HeNe laser at least as long as the tube you
are using. Of those I tried that worked at all, minimizing the distance
between the ecternal mirror and tube resulted in the best results but this
may not always be true. A dielectric mirror is definitely preferred but a
good quality aluminized front surface (planar) mirror should work, though
it may not be as good.
The quality of the beam from the end of the tube opposite where your
external mirror is located will probably be better, especially if the
mirror is beyond the HR of the tube (which may have some wedge and is
not AR coated). However, the beam from the external mirror end is
instructive at least in helping to adjust the alignment.
Using my Melles Griot 05-LYR-170 yellow HeNe tube which for my "broken"
sample, actually lases a combination of yellow (594.1 nm) and orange (604.6 nm)
from both ends (see the section: The Dual Color
Yellow/Orange HeNe Laser Tube), it was quite easy to achieve red output,
and all three colors were occasionally present at the same time - an
impressive achievement for a HeNe laser. My setup is shown in
05-LYR-170 HeNe Laser Tube Mounted in Test Fixture
for Multiline Experiments. The output from the tube's OC was directed
at an AOL CD used as a reflective diffraction grating with the first-order
beam projected on a white card several feet away. An MSN CD would work
just as well :) but a CD-R or CD-RW may not. The lens from a pair of
eyeglasses (mildly positive, about 4 diopters or 1/4 meter focal length)
narrowed the spots to improve spectral resolution. This rig could easily
resolve lines separated by less than 1 nm. The first external "red" mirrors I
tried were from an SP-084 HeNe laser tube but due probably to their relatively
short RoC, the 05-LYR-170 had to be pushed quite close to the mount to get
any red output. Mirrors designed for a longer laser worked better but there
wasn't much difference between the behavior using an HR or OC (99 percent).
Then to add to the excitement, with a bit of twiddling, I was able to obtain
the other orange line (611.9 nm) as well, and at times, all 4 lines were
lasing simultaneously! As expected, this additional line was only present
when using an external HR. Depending on the original makeup of the yellow and
orange beam (for this tube, their absolute and relative intensities varied
with time and were also a very sensitive function of mirror alignment), it
was possible to get mostly red or to vary the intensities of the other colors,
most easily suppressing yellow in favor of orange and red. The intensity of
the red output was never more than 1 mW or so. Its transverse mode structure
varied from TEM00 to a star pattern with nothing in the center. Strange.
Due to both surfaces of the HeNe tube's HR mirror reflecting some of the
intracavity beam resulting in a multiple cavity interference effect, there
was a distinct lack of stability. To help compensate for this, a micrometer
screw to precisely adjust cavity length without affecting mirror alignment
would have been nice.
I also tried this with the external mirror mounted beyond the tube's OC mirror
but although there was a definite effect on yellow and orange lasing, it
wasn't possible to obtain any red output. (For the 05-LYR-170, the OC already
reflects red quite well and the HR doesn't.) Finally, I replaced the red
external mirror with a green HR (from a tube of about the same length) mounted
beyond the 05-LYR-170's OC (since its HR by appearance looked like it might
be a good mirror for green). But, not surprisingly, while this could affect
the lasing of the yellow and orange lines, I could detect no coherent green
photons. However, I would expect that with a appropriately coated mirrors
(or possibly two such mirrors, one beyond each end of the tube), obtaining
lasing at the relatively high gain 640.1 nm red line would be easy - the
usual "red" mirrors may deliberately kill this line to prevent it from lasing.
Although I couldn't detect any evidence of lasing at the other red lines of
629.4 nm and 635.2 nm, these should also be possible with appropriate mirrors
as they have higher gain than the yellow and oranges. Another interesting one
would be the "Border Infra-Red" line at 730.5 nm. Lasing at the IR lines might
also be possible but they are so boring. :)
Next, determined to do something with a more normal HeNe laser tube, I tried
a Siemens tube but that refused to do anything interesting. Then, I tried
a Melles Griot 05-LHR-150 which typically outputs a 5+ mW red (632.8 nm) beam.
Since the OC for this laser is probably around 99% reflective at most, peaking
at 632.8 nm, I figured that it would be best to place the external mirror
beyond the OC rather than the HR. And, with the same external HR as used
above, it was possible to obtain 6 lasing lines, count'm 6: 629.4 nm,
632.8 nm, 635.2 nm, 640.1 nm, a line popping up around 650 nm
(all variations on red), ****AND**** 611.9 nm orange! However, since the
output is being taken from the HR, none of the colors was more than a
fraction of a mW.
Lasing of the 650 nm line was hard to obtain - it only showed up for a few
seconds off-and-on every few minutes and increasingly rarely after the tube
warmed up. The exact wavelength is very close to 650 nm (649.98 nm) as
determined later with an Agilant 86140B Optical Spectrum Analyzer (OSA) which
is a lot more expensive than my AOL CD. :) (The wavelength was referenced to
the 632.8 line from the same laser resulting in a measurement error bound of
+/- 0.02 nm assuming the 632.8 nm line is actually 632.8 nm. But since this
could also be slightly shifted, the error may be higher.) Getting anything at
650 nm is really puzzling as there are no HeNe lasing lines between
640.1 nm and 730.5 nm. But I have no doubt it is a true lasing line since it
was fluctuating independantly of the others (later confirmed, see below).
And all those other lines were
quite accurately located corresponding to their handbook wavelengths in the
diffracted pattern (and later confirmed with the OSA).
So there is little reason to suspect that the funny one isn't as well. When
present, it appeared as strong (or weak) as all the expected ones, (except of
course, the original 632.8 nm line which was usually, but not always, the
strongest). If 650 nm is not a HeNe lasing line - it's certainly not in the
sequence of energy level transitions that produce all the other visible HeNe
lines - one possible explanation is that there is some trace element present
inside the tube and that is what's lasing, not neon. I figured this
to be a distinct possibility since the particular tube I am using originally
had gas contamination and I revived it by heating the getter. (See the
section: Repairing the Northern Lights
Tube.) Therefore, the 650 nm wavelength may not be present with
another more normal tube. But as it turned out, contamination has nothing
to do with it.
I don't think the 730.1 nm line was present but given its low relative
perceived brightness, it may not have been visible at all using my AOL
Special CD diffraction grating but I couldn't find it with the OSA
either. It took awhile to detect the evidence of the 635.2 nm line
which only appeared sporatically (but it is the lowest gain of all
the known ones above).
A few days later, I tried the same experiment with a couple of my old
Spectra-Physics 084-1 HeNe laser tubes which are of soft-seal design so
have almost certainly leaked over time (but still work fine). With my
"hottest" SP084-1 (about 2.9 mW), I could almost duplicate the results
of the 05-LHR-150 including the funny line around 650 nm but minus
anything at 635.2 nm. Using a more normal 2.4 mW SP084-1, it was possible
to obtain (non 632.8 nm) lines at 629.4 nm and 640.1 nm. For these,
an SP084-1 HR worked almost as well for the external mirror as the longer
RoC HR I had been using with the 05-LHR-150. I then installed a SP098-1,
a common hard-seal barcode scanner tube (this sample puts out about 1.4 mW).
With that, the only additional line was at 640.1 nm. Which particular lines
appear in each case seem consistent with the length of the tubes (and thus
the single pass gain) and the relative gain of the lasing lines.
Some quick calculations predict that the real effect of the external HR
mirrors is the obvious one - to increase the circulating power. A 1 percent
OC (typical) followed by even a 90 percent external mirror would result in
greater than a 99.9 percent effective mirror for a range of wavelengths/modes.
An external 99.9 percent HR would result in an even better effective mirror.
It looks like the reflectance peak is relatively broad with respect to
wavelength (the transmission peak is rather narrow). Specific modes for
each of the wavelengths will be enhanced or suppressed. This would
also appear to be consistent with the apparent lack of need for the external
mirror to result in a stable resonator. All it has to do is form a
Fabry-Perot cavity.
These have to be classified right up there in the really fascinating
experiments department. Seeing any HeNe laser operating with
multiple spectral lines is really neat.
For more examples of these stunts using an already interesting "defective"
HeNe laser, see the sections starting with:
Melles Griot Yellow Laser Head With Variable
Output and in particular, the section:
External Mirror Therapy for Variable Power
05-LYR-171 Yellow Laser Head.
As always, depending on mirror reflectivity and other factors, your mileage
may vary. But feel free to try variations on these themes. The results from
using an HeNe HR beyond the OC of almost any red HeNe laser tube should be
easily replicated (except perhaps for the funny 650 nm line). Almost any
mirror will do something since even an aluminized mirror will be returning
over 90 percent of the otherwise wasted photons to the cavity - enough to
boost the gain of all but the weakest lines enough for lasing if everything
lines up just right. Aside from getting zapped by the high voltage or
dropping the tube on the floor, they are low risk, high reward experiments.
(From: Bob.)
For neutral neon at low pressure, the lines 640.3 nm, 659.9 nm are listed.
For neutral helium, there is one at 667.8 nm. None of the other noble gases
have wavelengths listed this short. As far as ionized species go, singly
ionized argon has a line at 648.30 nm. Singly ionized krypton has a
hand full of lines from 647 nm to 657 nm. Finally, xenon has one at 652 nm.
For atmospheric gases, there is a singly ionized nitrogen line at 648.3 nm.
There are no neutral lines of interest for atmospheric gases. The footnotes
for the above line were listed as CW lasing in 0.02 torr of krypton.
Whats the standard operating pressure of a HeNe laser? Not THAT far
out of the ball park I would guess.
(From: Sam.)
The last one sounds promising and would make sense given the history of the
particular 05-LHR-150 and the soft-seal design of the SP084-1. Though HeNe
lasers operate in the 2 to 3 TORR range - about 100 times higher pressure,
the partial pressure of any N2 contamination could very well be down
around 0.02 Torr.
However, I now know exactly where the 650 nm line is coming from and it has
nothing whatsoever to do with contamination. The exciting writeup from
someone who beat me to this by about 15 years follows in the next section
preceeded by a condensed version, below.
I've also found a commercial laser that appears to produce a very stable
650 nm line. See the section: The PMS/REO
External Resonator Particle Counter HeNe Laser.
(From: Stephen Swartz (sds@world.std.com).)
Lasing of certain HeNe tubes at 650 nm is a known phenomenon and not just a
hallucination. The 650 nm line which is never discussed in most standard
texts is not due to a "normal" transition of neon. It comes instead from a
Raman transition. The 650 nm line is not often observed but when it is it
will always be seen simultaneously with operation on a multitude of other
lines. A large number of other "unusual" colors have been seen over the
years. Higher power tubes with mirrors that are excessively broadband are
your best bet for observing them. Often these lines flicker on and off
over a few seconds to minutes time scale. A diffraction grating is a good
way to look for them.
(From: Someone at a major laser company.)
The 650.0 nm Raman line is a known problem in that it competes for power
with the 632.8 nm line intermittently, particularly in long tubes with
high circulating power. Polarized tubes are much less susceptible to this
effect and using a lower reflectance for the OC mirror helps since it reduces
circulating power without affecting output very much (over a reasonable range).
(From: Bruce Tiemann (BruceT@ctilidar.com).)
I have gotten many lines from many different HeNe lasers. In my experience
almost every tube is capable of giving at least one other line than 633 nm.
(Most wavelengths have been rounded to save bits. So, 632.8 nm becomes 633
nm.) I have never tried doing this with lasers that give other lines than 633
nm, but since that line has the highest gain, it should be no mean feat to at
least get that line from lasers that are supposed to not give it. It is also
not my experience that calculations to ensure resonator stability, etc., are
necessary. Just try it! My best results, in terms of output power, were
with a flat grating as the external feedback mirror, and my best results in
terms of new lines was obtained with a flat dielectric mirror, formerly used
as a facet in a polygonal scanning assembly. Flat mirrors are not stable at
any separation for a diverging beam, and HeNe lasers are very rare that give
converging beams for their output.
The home stuff had the mirrors on blocks, with the steering accomplished by
adjusting the HeNe tube by lifting one or the other end of the tube with
sheets of paper, and the azimuth by moving the laser tube back and forth.
The lab experiments were done with "real" mirror mounts, supplemented by a
single PZT that tilted the feedback mirror a few microns.
(I like PZTs a great deal, and would like to observe that you can get
PZT elements from little piezo alarms, from which the useful element can
be extracted with some hand-tools and the mind-set of a 9-year-old kid
dissecting a bug. :) These are only about $1 each, as opposed to
tens to hundreds of bucks for "real" PZTs that you buy from Thor, etc. One
of them and a 0 to 50 VDC power supply can precision-wiggle a mirror on the
micron scale, which is all that is needed for these experiments.)
(From: Sam.)
I have indeed done something similar using the piezo beeper from a dead
digital watch to move a mirror in a HeNe laser based Michelson
interferometer. With 0 to 25 V, it went through 4+ fringes which
means over 2 full wavelengths at 633 nm. The configuration in these is
called a "drum head" piezo element because the movement resembles that of
a musical (depending on your point of view!) drum head with the most shift
in the center. The piezo material itself doesn't change by very much in
thickness but is constructed so it distorts to produce the shape change.
With care, the piezo material can be cut to size or drilled to pass light
through its center. Much more voltage could have been safely applied if
needed.
(From: Bruce.)
Something I also did is cast the spots from a smaller (approximately 3/4 m)
spectrometer directly onto the CCD element of a small camera with no
lens. I also fabricated a beam block by taping little wires to the side
of a block, that would protrude up just in the locations of the very bright
lines, like 633, 650, and 612 nm, to block them, but letting light of other
colors pass in the ample space between the wires. You could still see when
the bright lines were on from light leaking around the wires, but it wouldn't
wash out the image when they were.
In this case, when the feedback mirror was tilted, speckle, which was
cast everywhere, would kind of shift around all over the place, but
the new lines looked like ghostly bullseyes, which would breathe in
and out as the mirror was tilted, but remain in the same location
unlike the speckle. This was an easy way to see the weakest lines like
624 nm, and it was also how I discovered 668 nm, the CCD being more
sensitive than the eye in the deep red. (I searched for but did not
find the normal laser line 730 nm even with this very sensitive
method.)
That 640 nm line would lase even with a plastic ruler or similar non-mirror
mirror and could be established by hand-holding the piece of plastic in the
beam, braced against the laser tube.
The gain-bandwidth of the Raman transition is only 60 MHz wide, so the
cavity modes for 633 nm must line up with the cavity modes at 650 nm, to
within this uncertainty, in order for 650 nm to oscillate. Considering that
the Doppler linewidth is more like 1500 MHz (1.5 GHz), and the FSR of the
laser is ~ hundreds of MHz, that is only rarely the case. Hence, 650 nm comes
and goes, most of the time being gone. And when it's gone, it's gone. When
the laser warms up, however, the cavity expands, and the 633 and 650 nm modes
sort of vernier past each other, sometimes bringing them into alignment in
difference-frequency space. When they align, 650 nm oscillates. The observed
behavior is that 650 nm more rapidly blinks on when the laser is warming up,
but only for short periods, and then as the tube comes closer to a
steady-state temperature, the periods become less frequent, but 650 nm lasts
for a longer duration each time. Eventually, at the steady-state condition,
650 nm will be gone, or more rarely, may persist. However, temperature
control of the laser can cause 650 nm to become steady, in the low-tech way
of putting a blanket made of paper sheets or something over the laser tube, to
stabilize the laser tube temperature to the next-higher value that supports
650 nm oscillation, or in the higher-tech case with a heater tape and
thermistor and temperature control unit. When 650 nm goes, it is strong, and
one 5 mW tube gives nearly 1.5 mW of output power at 650 nm, when the feedback
element was a metal grating, and the output was taken from the first order.
It is also perceptibly a deeper red color than 633 or even 640 nm, to me.
These lines were best observed with the high-R feedback mirror located
within about 6 inches of the output face of the laser, closer tending to be
a bit better. Except for 589 nm, which required 605 nm to be oscillating,
and this only occurred for one exact spacing of feedback mirror about 1.5 cm
away from the output coupler, or about 1 mm away from the output flange of the
tube, which I didn't remove. (I did, however, find out that you can take a
laser tube to the university infirmary, and ask to have it X-rayed, to
determine the extent of the internal glass envelope within the aluminum
outer casing, and they would only charge you $10 for the cost of the X-ray
film and processing, which is not bad for a doctor visit including X-rays.)
To my knowledge, these lines are my discovery.
A brief table shows the relationship between "pump" lines and 4-wave mixing
lines, observed on one tube. Upper Sideband is toward shorter wavelengths
from the pump; Lower Sideband is toward longer wavelengths of the pump (all
values in nm):
(632.8 and 650.0 nm are parenthesized since they are associated with the
genesis of the 4-wave mixing lines.)
All in all this laser produced 17 different lines, many at one time, from a
"single line" 633 swap-meet laser. :)
References:
The 650 nm discovery paper is:
Later, in 1989, a Chinese group that doesn't read Applied Physics Letters
published:
The first one, at least, should be available from a university library.
But if you look at the output of a HeNe laser with a spectrometer, there will
be dozens of wavelengths present other than one around 632.8 nm (or whatever
is appropriate for your laser if not a red one). Close to the output aperture,
there will be a very obvious diffuse glow (blue-ish for the red laser) visible
surrounding the actual beam. So why isn't the HeNe laser monochromatic as
expected?
With one exception, this is just due to the bore light - the spill from the
discharge which makes it through the Output Coupler (OC) mirror. As your
detector is moved farther from the output aperture, the glow spreads much
faster than the actual laser beam and its intensity contribution relative to
the actual beam goes down quickly. It is not coherent light but what would
be present in any low pressure gas discharge tube filled with helium and neon.
However, the presence of these lines can be confusing when they show up on
a spectral printout.
The exception is that with a 'hot' (unusually high gain) tube or one with
an OC that is not sufficiently narrow-band, one (though probably not more
though not impossible) of the neighboring HeNe laser lines (e.g., for other
color HeNe lasers) may be lasing though probably much more weakly than
the primary line. For example, a red (632.8 nm) laser might also produce a
small amount of output at 629.4 or 640.1 nm though this isn't that common.
I have one 'defective' yellow (594.1 nm) HeNe tube that also produces a fair
amount of orange (604.6 nm), and another that produces in addition some of
the other orange line (611.9 nm).
(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)
For gas lasers the plasma lines are typically 80 dB or more below the output
(measured, of course, within the very small laser mode divergence). This is
unlike most semiconductor lasers, which typically have broad 'shoulders' close
in to the line, as well as 'lines' due to other modes and instabilities
because the initial divergence of the diode is high, and spontaneous emission
from the junction high, the broad background tends to be large.
For gas lasers it is usually in the form of narrow lines at remote
wavelengths, very easily removed with an interference filter and/or spatial
filtering in the *rare* cases where it matters. There is presumably a weak
broad background from processes involving free electrons (bound/free and
free/free), but I've never seen it even mentioned, let alone observed it. More
likely to be significant in the high current density argon laser than the very
low current density HeNe.
The only cases I have seen where the plasma lines caused problems were Raman
measurements on scattering samples with photon counting detection, and weak
fluorescence measurements which are similar.
In most cases scattered light in the monochromator is much more of an issue
(hence double monochromators for Raman) and will obscure plasma lines in many
cases.
Normally, what comes out in that direction is, well, waste, and is of
no consequence. But, there are times where it's convenient to use this
low power beam as a reference, expecting its power to track that of the
main output beam. Unfortunately, it is sometimes not well behaved in
this regard.
In constructing some amplitude stabilized HeNe lasers which depend on the
waste beam feeding a photodiode for their feedback loop, an annoying
characteristic of the waste beam has become evident with some otherwise
perfectly normal and healthy HeNe laser tubes. Namely, that the
relative power in the waste beam and the main beam does not remain
constant as the tube warms up. In fact, one tube I was using had a
variation of almost 2:1 in relative waste beam and output beam power
depending on the tube's temperature. This is probably due to one or
both of the following:
The coating problem is more likely to result in a strictly increasing, or
at least slow change in waste beam power with higher temperature while
the etalon would be periodic with temperature going through several cycles, it
might be possible to determine which of the two effects is present.
Normally, the waste beam is, well, waste and so no one cares. Though there
will also be a change in the power of the output beam (inversely relative
to the waste beam), it will be too small to be detectable without careful
measurements, being swamped by the normal mode sweep power variations. But
when the waste beam is used as the amplitude reference in a stabilized laser,
the supposedly stabilized output will vary based on the relative waste beam
power. That 10 uW change would result in the output power changing by 33
percent.
Magnets may be incorporated in HeNe lasers for several reasons including the
suppression of IR spectral lines to improve efficiency (such as it is!) and
to boost power at visible wavelengths, for the stabilization of the beam, and
to control its polarization. There are no doubt other uses as well.
The basic mechanism for the interaction of emitted light and magnetic fields
is something called the 'Zeeman Effect' or 'Zeeman Splitting'. The following
brief description is from the "CRC Handbook of Chemistry and Physics":
Magnet fields may affect the behavior of HeNe tubes in several ways:
As a result of the Zeeman Effect, if a gas radiates in a magnetic field,
most of its spectral lines are split into 2 or sometimes more components.
The magnitude of the separation depends on the strength of the magnetic
field and as a result, if the field is also non-uniform, the spectral lines
are broadened as well because light emitted at different locations will see
an unequal magnetic field. These 'fuzzed out' lines cannot participate in
stimulated emission as efficiently as nice narrow lines and therefore will
not drain the upper energy states for use by the desired lines. The
magnitude of the Zeeman splitting effect is also wavelength dependent and
therefore can be used to control the gain of selected spectral lines (long
ones are apparently affected more than short ones on a percentage basis).
Without the use of magnets, the very strong neon IR line at 3.39 um would
compete with (and possibly dominate over) the desired visible line (at 632.8
nm) stealing power from the discharge that would otherwise contribute to
simulated emission at 632.8 nm. However, the IR isn't wanted (and therefore
will not be amplified since the mirrors are not particularly reflective at
IR wavelengths anyhow). Since the 3.39 nm wavelength is more than 5 times
longer than the 632.8 nm red line, it is affected to a much greater extent
by the magnetic field and overall gain and power output at 632.8 nm may be
increased dramatically (25 percent or more). The magnets may be required to
obtain any (visible) output beam at all with some HeNe tubes.
The typical higher power Spectra-Physics HeNe laser will have relatively
low strength magnets (e.g., like those used to stick notes to your fridge)
placed at every available location along the exposed bore along the sides
of the L-shaped resonator frame. They will alternate N and S poles pointing
toward the bore. Interestingly, on some high mileage tubes, brown crud
(which might be material sputtered off the anode) may collect inside the
bore - but only at locations of one field polarity (N or S, whichever would
tend to deflect a positive ion stream into the wall). The crud itself
doesn't really affect anything but is an indication of long use. And
on average, tubes with a lot of brown crud may be harder to start and
require a higher voltage to run.
I do not know how to determine if and when such magnets are needed for long
high power HeNe tubes where they are not part of an existing laser head.
My guess is that the original or intended positions, orientations, and
strengths, of the magnets were determined experimentally by trial and error
or from a recipe passed down from generation to generation, and not through
the use of some unusually complex convoluted obscure theory. :)
The only thing I can suggest other than contacting the manufacturer is to
very carefully experiment with placing magnets of various sizes and
strengths at strategic locations (or a half dozen such locations) to
determine if beam power at the desired wavelength is affected. Just take
care to avoid smashing your flesh or the HeNe tube when playing with
powerful magnets. (Though the magnets used in large-frame HeNe lasers with
exposed bores aren't particularly powerful, to produce the same effective
field strength at the central bore of an internal mirror HeNe tube may
require stronger ones.) Enclosing the HeNe tube in a protective rigid
sleeve (e.g., PVC or aluminum) would reduce the risk of the latter disaster,
at least. :-)
(From: Lynn Strickland (stricks760@earthlink.net).)
However, you'll probably see a benefit from magnets to suppress the 3.39 um
line on the older HeNe tubes."
Where the capillary of the plasma tube is exposed as with many older lasers,
and the magnets can be placed in close proximity to the bore, their strength
can be much lower. Some commercial lasers (like the Spectra-Physics model
132) offered a polarization option which adds a magnet assembly alongside the
tube. However, I doubt that this is done commercially with any modern HeNe
tubes with coaxial gas reservoirs.
In this case, what is required is a uniform or mostly uniform field of the
appropriate orientation rather than one that varies as for IR spectral line
suppression though both of these could be probably be combined.
Also see the section: Unrandomizing the
Polarization of a Randomly Polarized HeNe Tube.
In principle, varying fields from electromagnets could be used for intensity,
polarization, and frequency modulation. I do not know whether any are
implemented in this manner.
(In all of these diagrams, the orientation of the Brewster windows shown is
totally arbitrary - for sealed HeNe tubes with internal mirrors, they would
not be present at all!)
Polarity may alternate with North and South poles facing each other across
the tube forming a 'wiggler' so named since such a they will tend to deflect
the ionized discharge back and forth though there may be no visible effects
in the confines of the capillary:
Alternatively, North and South poles may face each other:
For the magnet configuration used in a commercial laser, see the section:
Description of the SP-124 Laser Head.
Where:
At least one other basic specification may be critical to your application:
Which end of the tube the beam exits! There is no real preference from a
manufacturing point of view for red HeNe lasers. (For low gain "other-color"
HeNe laser tubes, it turns out that anode output results is slightly higher
gain and thus slightly higher houtput for the typical hemispherical cavity
because it better utilizes the mode volume.) However, this little detail
may matter a great deal if you are attempting to retrofit an existing barcode
scanner or other piece of equipment where the tube clips into a holder or
where wiring is short, tight, or must be in a fixed location. For example,
virtually all cylindrical laser heads require that the beam exits from the
cathode-end of the tube. It is possible that you will be able to find two
versions of many models of HeNe tubes if you go directly to the manufacturer
and dig deep enough. However, this sort of information may not be stated
where you are buying surplus or from a private individual, so you may need
to ask.
The examples above (as well as all of the other specifications in this and
the following sections) are catalog ratings, NOT what might appear on the CDRH
safety sticker (which is typically much higher). See the section:
About Laser Power Ratings for info on
listed, measured, and CDRH power ratings.
Note how some of the power levels vary widely with respect to tube dimensions,
voltage, and current. Generally, higher power implies a longer tube, higher
operating/start voltages, and higher operating current - but there are some
exceptions. In addition, you will find that physically similar tubes may
actually have quite varied power output. This is particularly evident in the
Melles Griot listings, below.
These specifications are generally for minimum power over the guaranteed life
of the tube. New tubes and individual sample tubes after thousands of hours
may be much higher - 1.5X is common and a "hot" sample may hit 2X. My guess
is that for tubes with identical specifications in terms of physical size,
voltage, and current, the differences in power output are due to
sample-to-sample variations. Thus, like computer chips, they are selected
after manufacture based on actual performance and the higher power tubes are
priced accordingly! This isn't surprising when considering the low efficiency
at which these operate - extremely slight variations in mirror reflectivity
and trace contaminants in the gas fill can have a dramatic impact on power
output.
I have a batch of apparently identical 2 mW Aerotech tubes that vary in power
output by a factor of over 1.5 to 1 (2.6 to 1.7 mW printed by hand on the
tubes indicating measured power levels at the time of manufacture).
And, power output also changes with use (and mostly in the days of soft-sealed
tubes, just with age sitting on the shelf):
(From: Steve Roberts (osteven@akrobiz.com).)
And the answer to your burning question is: No, you cannot get a 3 mW tube to
output 30 mW - even instantaneously - by driving it 10 times as hard!
I have measured the operating voltage and determined the optimum current (by
maximizing beam intensity) for the following specific samples - all red
(632.8 nm) tubes from various manufacturers. (The starting voltages were
estimated):
Melles Griot, Uniphase, Siemens, PMS, Aerotech, and other HeNe tubes all show
similar values.
The wide variation in physical dimensions also means that when looking at
descriptions of HeNe lasers from surplus outfits or the like, the dimensions
can only be used to determine an upper (and possibly lower) bound for the
possible output power but not to determine the exact output power (even
assuming the tube is in like-new condition). Advertisements often include the
rating on the CDRH safety sticker (or say 'max' in fine print). This is an
upper bound for the laser class (e.g., Class IIIa), not what the particular
laser produces or is even capable of producing. It may be much lower. For
example, that Class IIIa laser showing 5 mW on the sticker, may actually only
be good for 1 mW under any conditions! The power output of a HeNe laser tube
is essentially constant and cannot be changed significantly by using a
different power supply or by any other means. See the section:
Buyer Beware for Laser Purchases.
Also see the section: Locating Laser
Specifications.
In addition to power output, power requirements, and physical dimensions,
key performance specifications for HeNe lasers also include:
With manufacturers like Aerotech, Melles Griot, and Siemens, a certain amount
of information can be determined from the model number. For example, here is
how to decipher most of those from Melles Griot (e.g., 05-LHP-121-278):
The vast majority of Melles Griot lasers you are likely to come across will
follow this numbering scheme though there are some exceptions, especially for
custom assemblies. (Some surplus places drop the leading '05-' when reselling
Melles Griot laser tubes or heads so an 05-LHP-120 would become simply an
LHP-120.)
For other manufacturers like Spectra-Physics, the model numbers are totally
arbitrary! (See the section: Spectra-Physics
HeNe Lasers.)
Maximum available power output is also lower - rarely over 2 mW (and even
those tubes are quite large (see the tables below). However, since the eye is
more sensitive to the green wavelength (543.5 nm) compared to the red (632.8
nm) by more than a factor of 4 (see the section:
Relative Visibility of Light at Various
Wavelengths), a lower power tube may be more than adequate for many
applications. Yellow (594.1 nm) and orange (611.9 nm) HeNe lasers appear more
visible by factors of about 3 and 2 respectively compared to red beams of
similar power. To get an idea of the actual perceived color at each
wavelength, see the section: Color Versus
Wavelength.
Infrared-emitting HeNe lasers exist as well. In addition to scientific
uses, these were sued for testing in the Telecom industry before sufficiently
high quality diode lasers became available.Yes, you can have a HeNe tube and
it will light up inside (typical neon glow), but if there is no output beam (at
least you cannot see one), you could have been sold an infrared HeNe tube.
However, by far the most likely explanation for no visible output beam is
that the mirrors are misaligned or the tube is defective in some other way.
Unfortunately, silicon photodiodes or the silicon sensors in CCD or CMOS
cameras do not respond to any of the HeNe IR wavelengths, so the only means
of determining if there is an IR beam are to use a GaAs photodiode,
IR detector card, or thermal laser power meter. IR HeNe tubes are unusual
enough that it is very unlikely you will ever run into one. However, they
may turn up on the surplus market especially if the seller doesn't test the
tubes and thus realize that these behave differently - they are physically
similar to red (or other color) HeNe tubes except for the reflectivity of
the mirrors as a function of wavelength. (There may be some other differences
needed to optimize each color like the He:Ne ratio, isotope purity, and gas
fill pressure, but the design of the mirrors will be the most significant
factor and the one that will be most obvious with a bare eyeball, though
the color of the discharge may be more pink for green HeNe tubes and more
orange and brighter for IR HeNe tubes compared to red ones, more below.) Even
if the model number does not identify the tube as green, yellow, orange, red,
or infra-red, this difference should be detectable by comparing the appearance
of its mirrors (when viewed down the bore of an UNPOWERED tube) with those of
a normal (known to be red) HeNe tube. See the section:
Determining HeNe Laser Color from the
Appearance of the Mirrors. (Of course, your tube could also fail to lase
due to misaligned or damaged mirrors or some other reason. See the section:
How Can I Tell if My Tube is Good?.)
As noted above, the desired wavelength is selected and the unwanted
wavelengths are suppressed mostly by controlling the reflectivity functions of
the mirrors. For example, the gains of the green and yellow lines (yellow
may be stronger) are both much much lower than red and separated from each
other by about 50 nm (543.5 nm versus 594.1 nm). To kill the yellow line in a
green laser, the mirrors are designed to reflect green but pass yellow. I
have tested the mirrors salvaged from a Melles Griot 05-LGP-170 green HeNe
tube (not mine, from "Dr. Destroyer of Lasers"). The HR (High Reflector)
mirror has very nearly 100% reflectivity for green but less than 25% for
yellow. The OC (Output Coupler) also has a low enough reflectivity for
yellow (about 98%) such that it alone would prevent yellow from lasing.
The reflectivities for orange, red, and IR, are even lower so they are
also suppressed despite their much higher gain, especially for the normal
red (632.8 nm) and even stronger mid-IR (3,391 nm) line.
However, to manufacture a tube with optimum and stable output power, it isn't
sufficient to just kill lasing for unwanted lines. The resonator must be
designed to minimize their contribution to stimulated emission - thus the very
low reflectivity of the HR for anything but the desired green wavelength.
Otherwise, even though sustained oscillation wouldn't be possible, unwanted
color photons would still be bouncing back and forth multiple times stealing
power from the desired color. The output would also be erratic as the length
of the tube changed during warmup (due to thermal expansion) and this affected
the longitudinal mode structure of the competing lines relative to each other.
Some larger HeNe lasers have magnets along the length of the tube to further
suppress (mostly) the particularly strong mid-IR line at 3,391 nm. (See
the section: Magnets in High Power or Precision
HeNe Laser Heads.)
In addition, you can't just take a tube designed for a red laser, replace
the mirrors, and expect to get something that will work well - if at all -
for other wavelenghts. For one thing, the bore size and mirror curvature
for maximum power while maintaining TEM00 operation are affected by wavelength.
Furthermore, for these other color HeNe lasers which depend on energy level
transitions which have much lower gain than red - especially the yellow and
green ones - the gas fill pressure, He:Ne ratio, and isotopic composition
and purity of the helium and neon, will be carefully optimized and will be
different than for normal red tubes.
Needless to say, the recipes for each type and size laser will be closely
guarded trade secrets and only a very few companies have mastered the art
of other color HeNe lasers, especially for high power (in a relative sort
of way) in yellow and green. I am only aware of four companies that currently
manufacture their own tubes: Melles Griot, Research Electro-Optics,
Uniphase, and LASOS, with the last two having very few models to choose
from. Others (like Coherent) simply resell lasers under their own name.
And, the answer to that other burning question should now be obvious: No, you
can't convert an ordinary red internal mirror HeNe tube to generate some other
color light as it's (almost) all done with mirrors and they are an integral
part of the tube. :) Therefore, your options are severely limited. As in:
There are none. (However, going the other way, at least as a fun experiment,
may be possible. See the section: Getting
Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.)
For a laser with external mirrors, a mirror swap may be possible
(though the cavity length may be insufficient to resonate with the reduced
gain of other-color spectral lines once all loses taken into consideration).
But realistically, this option doesn't even exist where the mirrors are sealed
into the tube.
There are also a few HeNe lasers that can output more than one of the possible
colors simultaneously (e.g., red+orange, orange+yellow) or selectively by
turning knob (which adjusts the angle of a Littrow or other similar dispersion
prism) inside the laser cavity using a Brewster window HeNe tube). But such
lasers are not common and are definitely very expensive. So, you won't likely
see one for sale at your local hamfest - if ever! One manufacturer of such
lasers is Research Electro-Optics (REO).
See the section: Research Electro-Optics's
Tunable HeNe Lasers.
However, occasionally a HeNe tube turns up that is 'defective' due to incorrect
mirror reflectivities or excessive gain or magic :) and actually outputs an
adjacent color in addition to what it was designed to produce. I have such a
tube that generates about 3 mW of yellow (594.1 nm) and a fraction of a mW of
orange (611.9 nm) but isn't very stable - power fluctuates greatly as it
warms up. Another one even produces the other orange line at 611.9 nm, and
it's fairly stable. But, finding magic 'defective' tubes such as these by
accident is extremely unlikely though I've heard of the 640.1 nm (deep red)
line showing up on some supposedly good normal red (632.8 nm) HeNe tubes.
As a side note: It is strange to see the more or less normal red-orange glow
in a green HeNe laser tube but have a green beam emerging. A diffraction
grating or prism really shows all the lines that are in the glow discharge.
Red through orange, yellow and green, even several blue lines (though they are
from the helium and can't lase under any circumstances)!! The IR lines are
present as well - you just cannot see them.
See the section: Instant Spectroscope for
Viewing Lines in HeNe Discharge for an easy way to see many of the visible
ones.
Actually, the color of the discharge may be subtly different for non-red HeNe
tubes due to modified gas fill and pressure. For example, the discharge of
green HeNe tubes may appear more pink compared to red tubes) which are more
orange), mostly due to lower fill pressure. The fill mix and pressure on
green HeNes is a tricky compromise among several objectives that conflict to
some extent including lifetime, stability (3.39 um competition), and optical
noise. This balancing act and the lower fill pressure are why green HeNes
don't last as long as reds. Have I totally confused you, color-wise? :)
The expected life of 'other color' HeNe tubes is generally much shorter than
for normal red tubes. This is something that isn't widely advertised for
obvious reasons. Whereas red HeNe tubes are overfilled initially (which
reduces power output) and they actually improve with use to some extent as
gas pressure goes down, this luxury isn't available with the low gain
wavelengths - especially green - everything needs to be optimal for decent
performance.
The discharge in IR HeNe tubes may be more orange and brighter due to a higher
fill pressure. Again, this is due to the need to optimize parameters for
the specific wavelength.
Since the mirrors used in all HeNe lasers are dielectric - functioning as a
result of interference - they have high reflectivity only around the laser
wavelength and actually transmit light quite well as the wavelength moves away
from this peak. By transmitted light, the appearance will tend to be a
color which is the complement of the laser's output - e.g., cyan or blue-green
for a red tube, pink or magenta for a green tube, blue or violet for a yellow
tube. Of course, except for the IR variety, if the tube is functional, the
difference will be immediately visible when it is powered up!
The actual appearance may also depend on the particular manufacturer and
model as well as the length/power output of the laser (which affects the
required reflectivity of the OC), as well as the revision number of your
eyeballs. :) So, there could be considerable variation in actual perceived
color. Except for the blue-green/magenta combination which pretty much
guarantees a green output HeNe tube, more subtle differences in color may
not indicate anything beyond manufacturing tolerances.
The chart below in conjunction with Appearance of HeNe
Laser Mirrors will help to ideentify your unmarked HeNe tube. (For
accurate rendition of the graphic, your display should be set up for 24
bit color and your monitor should be adjusted for proper color balance.)
The entry labeled 'Broadband' relates to the HR mirror in some unusual
multiple color (combinations of red and/or orange and/or yellow) internal
mirror tubes as well as those with an internal HR and Brewster window for
external OC optics. And, the yellow and orange tubes may actually use broad
band HRs. The OCs would then be selected for the desired wavelength(s) and
may also have a broad band coating.
For low gain tubes, they play games with the coatings. I guess it isn't
possible to just make a highly selective coating for one wavelength that's
narrow enough to have low reflectivity at the nearby lines so they won't
lase. So, one mirror will be designed to fall off rapidly on one side
of the design wavelength, the other mirror on the other side. That's one
reason front and back mirrors on yellow and green tubes in particular have
very different appearances.
As noted, depending on laser tube length/output power, manufacturer, and
model, the appearance of the mirrors can actually vary quite a bit but this
should be a starting point at least. For example, I have a Melles Griot
05-LHR-170 HeNe laser tube that should be 594.1 nm (yellow) but actually
outputs some 604.6 nm (orange) as well. It's mirror colors for the HR and OC
are almost exactly opposite of those I have shown for the yellow and
orange tubes! I don't know whether this was intentional or part of the
problem And, while from this limited sample, it looks like the OCs for
orange, yellow, and green HeNe lasers appear similar, I doubt that they
really are in the area that counts - reflectivity/transmission at the relevant
wavelengths.
I do not have any data for the 1,152 nm (IR) HeNe laser wavelength. If you
have access to a 1,152 nm or any other non-red HeNe tube and would like to
contribute or comment on their mirror colors (or anything else), please send
me mail via the
Sci.Electronics.Repair FAQ
Email Links Page.
(From: Steve Roberts (osteven@akrobiz.com).)
You do need a isotope change in the gases for green, and a He:Ne ratio change
for the other orange and yellow lines. In addition, the mirrors to go to
another line will have a much lower output transmission. The only possible
lines you'll get on a large frame HeNe laser will be the 611.9 nm orange and
594.1 nm yellow. The green requires external mirror tubes in excess of a
meter and a half long and a Littrow prism to overcome the Brewster losses and
suppress the IR.
The original work on green was done by Rigden and Wright. The short tubes have
lower losses because they have no Brewsters and thus can concentrate on tuning
the coatings to 99.9999% reflectivity and maximum IR transmission. There is
one tunable low power unit on the market that does 6 lines or so, but only
1 line at a time, and the $6,000 cost is kind of prohibitive for a few
milliwatts of red and fractional milliwatt powers on the other lines. But,
it will do green and has the coatings on the back side of the prism to kill
the losses.
Also look for papers by Erkins and Lee. They are the fellows who did the green
and yellow for Melles Griot and they published one with the energy states as
part of a poster session at some conference. Melles Griot used to hand it out,
that's how I had a copy, recently thrown away.
Even large HeNe lasers such as the SP-125 (rated at 50 mW of red) will only do
about 20 mW of yellow, with a 35 mW SP-127 you're probably only looking at 3
to 5 mW of yellow. And, for much less then the cost of the custom optics to
do a conversion, you can get two or three 4 to 5 mW yellow heads from Melles
Griot. I know for a fact that a SP-127 only does about 3 mW of 611.9 with a
external prism and a remoted cavity mirror, when it does 32 mW of 632.8 nm.
So in the end, unless you have a research use for a special line, it's cheaper
to dig up a head already made for the line you seek, unless you have your own
optics coating lab that can fabricate state-of-the-art mirrors.
I have some experience in this, as I spent months looking for a source of the
optics below $3,000.
(From: Sam.)
I do have a short (265 mm) one-Brewster HeNe tube (Melles Griot 05-LGB-580)
with its internal HR optimized for green that operates happily with a matching
external green HR mirror (resulting in a nice amount of circulating power) but
probably not with anything having much lower reflectivity to get a useful
output beam. In fact, I could not get reliable operation even with the HR from
a dead green HeNe laser tube as the Brewster window would not remain clean enough
for the time required to align the mirror. See the section:
A Green One-Brewster HeNe Laser for more info.
I would expect an SP-127 to do more than 3 to 5 mW of yellow, my guess would
be 10 to 15 mW with optimized mirrors but no tuning prism. If I can dig up
appropriate mirrors, I intend to try modifying an SP-127 to make it tunable
and/or do yellow or green. :)
(From: Lynn Strickland (stricks760@earthlink.net).)
You can find 640.1 nm in a lot of red HeNe lasers. I have a paper on it
somewhere, and cavity design can influence it to a large extent. If you have
a decent quality grating, it's pretty easy to pick up. 629 nm is the one you
don't see too much.
I'm no physicist, but the lower gain lines can lase simultaneously with the
higher gain lines, no problem, as long as there is sufficient gain available
in the plasma. It's really pretty easy to get a HeNe laser to output on all
lines at the same time (if you have the right mirrors). The trick is
optimizing the bore-to-mode ratio, gas pressure, and isotope mixture to get
good TEM00 power. Usually the all-lines HeNe lasers are multi (transverse)
mode. I don't know of anyone who makes them commercially though - at least
not intentionally.
The 3.39 um HeNe laser's gain is still, like all other HeNe lines limited
by a wall collision to return the excited atoms to the ground state. 3.39 um
HeNe lasers have larger bores then normal HeNe lasers, and the bores are acid
etched to fog them and create more surface area, but still the most power I've
ever seen published was 40 mW - nothing to write home about. The massive
SP-125, the largest commercial HeNe laser, could be ordered with a special
tube and special optics for 3.39 um, and it still only did about 1/3rd the
visible power. Superradiance and ultimate power are not tied together.
The reason 3.39 um got all the writeups it did was that it started on the
same upper state as all the other HeNe lines, was easily noticed when it
sapped power from the visible line, and was, at the time, a exotic wavelength
for which there were few other sources.
(Spectra for varioue elements and compounds can be easily found by searching
the Web. The NIST Atomic
Spectra Database has an applet which will generate a table or plot of
more spectral lines than you could ever want.)
The shear number of individual spectral lines present in the discharge is
quite amazing. You will see the major red, orange, yellow, and green lines as
well as some far into the blue and violet portions of the spectrum and toward
the IR as well. All of those shown in Bright Line
Spectra of Helium and Neon will be present as well as many others not
produced by the individual gas discharges. There are numerous IR lines as
well but, of course, these will not be visible.
Place a white card in the exit beam and note where the single red output line
of the HeNe tube falls relative to the position and intensity of the numerous
red lines present in the gas discharge.
As an aside, you may also note a weak blue/green haze surrounding the intense
main red beam (not even with the spectroscope). This is due to the blue/green
(incoherent) spectral lines in the discharge being able to pass through the
output mirror which has been optimized to reflect well (>99 percent) at 632.8
nm and is relatively transparent at wavelengths some distance away from these
(shorter and longer but you would need an IR sensor to see the longer ones).
Since it is not part of the lasing process, this light diverges rapidly and is
therefore only visible close to the tube's output mirror.
(From: George Werner (glwerner@sprynet.com).)
Here is an effect I found many years ago and I don't know if anyone has
pursued it further.
We had a recording spectrometer in our lab which we used to examine the
incoherent light coming from the laser discharge. This spectrum when
lasing was slightly different from the spectrum when not lasing, which one
can expect since energy levels are redistributed. As with most detectors,
ours used a chopper in the spectrometer light beam and a lock-in amplifier.
Instead of putting the chopper in the path of light going to the
spectrometer, I put it in the path of the internal laser beam, so that
instead of an open/closed signal going to the amplifier it was a
lasing/not-lasing signal. What was recorded then was three kinds of
spectrum lines: some deflected positive in the normal way, others deflected
negative, and the third group were those that were unaffected by chopping,
in which case when we passed over the line we only saw an increase in the
noise level. Setting up such a test is easy. The hard part is interpreting
the data in a meaningful way.
For HeNe lasers, the primary line (usually 632.8 nm) is extremely narrow and
effectively a singularity given any instrumentation you are likely to have at
your disposal. Any other lines you detect in the output are almost certainly
from two possible sources but neither is actual laser emission:
Close to the output mirror, you may see some of this light seeping through
especially at wavelengths in the green, blue, and violet, for which the
dielectric mirrors are nearly perfectly transparent. However, such light will
be quite divergent and diffuse and won't be visible at all more than a couple
of inches from the mirror.
The result will be a weak green beam that can sometimes be observed with a
spectroscope in a very dark room room. It isn't really quite as coherent or
monochromatic as the beam from a true green HeNe laser and probably has much
wider divergence but nonetheless may be present. It may be easier to see
this by using your spectroscope to view the bright spot from the laser on
a white card rather than by deflecting the beam and trying to locate the
green dot off to one side.
Note: I have not been able to detect this effect on the short HeNe tubes I
have checked.
Since the brightness of the discharge and superradiance output should be about
the same from either mirror, using the non-output end (high reflector) should
prove easier (assuming it isn't painted over or otherwise covered) since the
red beam exiting from this mirror will be much less intense and won't obscure
the weak green beam.
Note that argon and krypton ion lasers are often designed for multiline output
where all colors are coherent and within an order of magnitude of being equal
to each other in intensity or with a knob to select an individual wavelength.
Anything like this is only rarely done with HeNe lasers because it is very
difficult (and expensive) due to the low gain of the non-red lines. For more
information, see the sections: HeNe Tubes of a
Different Color and Research
Electro-Optics Tunable HeNe Lasers.
The next best thing is a small HeNe laser laid bare where its sealed (internal
mirror) HeNe tube, ballast resistors, wiring, and power supply (with exposed
circuit board), are mounted inside a clear Plexiglas case with all parts
labeled. This would allow the discharge in the HeNe tube to be clearly visible
(and permit the use of the Instant Spectroscope
for Viewing Lines in HeNe Discharge). The clear insulating case prevents
the curious from coming in contact with the high voltage (and line voltage, if
the power supply connects directly to the AC line), which could otherwise
result in damage to both the person and fragile glass HeNe tube when a reflex
action results in smashing the entire laser to smithereens!
A HeNe laser is far superior to a cheap laser pointer for several reasons:
Important: If this see-through laser is intended for use in a classroom, check
with your regulatory authority to confirm that a setup which is not explicitly
CDRH approved (but with proper laser class safety stickers) will be acceptable
for insurance purposes.
For safety with respect to eyeballs and vision, a low power laser - 1 mW or
less - is desirable - and quite adequate for demonstration purposes.
The HeNe laser assembly from a barcode scanner is ideal for this purpose.
It is compact, low power, usually runs on low voltage DC (12 V typical), and is
easily disassembled to remount in a demonstration case. The only problem is
that many of these have fully potted "brick" type power supplies which are
pretty boring to look at. However, some have the power supply board coated
with a rubbery material which can be removed with a bit of effort (well, OK,
a lot of effort!). For example, this HeNe Tube and
Power Supply is from a hand-held barcode scanner. A similar unit was
separated into its Melles Griot HeNe Tube and
HeNe Laser Power Supply IC-I1 (which includes the
ballast resistors). These could easily be mounted in a very compact case (as
little as 3" x 6" x 1", though spreading things out may improve visibility and
reduce make cooling easier) and run from a 12 VDC, 1 A wall adapter. Used
barcode scanner lasers can often be found for $20 or less.
An alternative is to purchase a 0.5 to 1 mW HeNe tube and power supply kit.
This will be more expensive (figure $5 to $15 for the HeNe tube, $25 to $50
for the power supply) but will guarantee a circuit board with all parts
visible.
The HeNe tube, power supply, ballast resistors (if separate from the power
supply), and any additional components can be mounted with standoffs and/or
cable ties to the plastic base. The tube can be separated from the power
supply if desired to allow room for labels and such. However, keep the ballast
resistors as near to the tube as practical (say, within a couple of inches,
moving them if originally part of the power supply board). The resistors may
get quite warm during operation so mount them on standoffs away from the
plastic. Use wire with insulation rated for a minimum of 10 kV. Holes or
slots should be incorporated in the side panels for ventilation - the entire
affair will dissipate 5 to 10 Watts or more depending on the size of the HeNe
tube and power supply. (However, if you want to take this thing outdoors, see
the section: Weatherproofing a HeNe Laser.
When attaching the HeNe tube, avoid anything that might stress the mirror
mounts. While these are quite sturdy and it is unlikely that any reasonable
arrangement could result in permanent damage, even a relatively modest force
may result in enough mirror misalignment to noticeably reduce output power.
And, don't forget that the mirror mounts are also the high voltage connections
and need to be well insulated from each other and any human contact! The best
option is probably to fasten the tube in place using Nylon cable ties, cable
clamps, or something similar around the glass portion without touching the
mirror mounts at all (except for the power connections).
Provide clearly marked red and black wires (or binding posts) for the low
voltage DC or a line cord for AC (as appropriate for the power supply used),
power switch, fuse, and power-on indicator. Label the major components and
don't forget the essential CDRH safety sticker (Class II for less than 1 mW or
Class IIIa for less than 5 mW).
See: Sam's Demo HeNe Laser as an example (minus
the Plexiglas safety cover), contructed from the guts of a surplus
Gammex laser (probably part of a patient positioning system for a CT or MRI
scanner). The discrete line operated power supply is simple with the HV
transformer, rectifier/doubler, filter, multiplier, and ballast resistors
easily identified. This would make an ideal teaching aid.
See the suppliers listed in the chapter: Laser
and Parts Sources.
Everything needed for such a setup is readily available or easily constructed
at low cost but you'll have to read more to find out where or how as each of
the components are dealt with in detail elsewhere in Sam's Laser FAQ (but I
won't tell you exactly where - these are all the hints you get for this one!).
A system like this could conceivably be turned into an interactive exhibit
for your local science museum - assuming they care about anything beyond
insects and the Internet these days. :) There are some more details in
the next section.
Here are some guidelines for designing an interactive exhibit:
Or, for a more aesthetic rendition, see: Helium-Neon
Laser Tube with Segmented Bore.
The third has Brewster angle windows at both ends with an external (fixed) HR
mirror and an external screw-adjustable OC mirror. The cathode is also in
a side-tube rather than the more typical coaxial can type but is otherwise
similar.
Only one of the 3 HeNe tubes of this type that I have works at all and it has
a messed up gas fill probably due to age despite its being hard sealed. Its
output is perhaps 1 or 2 mW (where it should be around 20 mW). However, to
the extent that it works, there doesn't appear to be anything particularly
interesting or different about its behavior. Of the other two tubes, one has
a broken off mirror (don't ask) but before the mishap, did generate some decent
power (perhaps 5 to 10 mW but still nowhere near its 20 mW rating) but
erratically. I suspect this was due to a contaminated gas fill resulting in
low gain rather than the segmented design since a couple of other similar
length tubes of conventional construction behaved in a similar manner. The
funky tube with the external mirrors was not hard-sealed at the Brewster
windows and leaked over time.
The only obvious effect this sort of structure should have on operation would
be to provide gas reservoirs at multiple locations rather than only at the
cathode-end of the bore as is the case with most 'normal' HeNe tube designs.
I do not know whether this matters at all for a low current HeNe discharge.
Therefore, the reason for the unusual design remains a total mystery. It may
have been to stabilize the discharge, to suppress unwanted spectral lines,
easier to maintain in alignment than a single long capillary, or something else
entirely. Then again, perhaps, the person who made the tubes just had a spurt
of excessive creativity. :)
I have also acquired a complete laser head with a similar tube, rated
25 mW max with a sticker that says it did 22 mW at one time. It is
unremarkable in most respects but does have a large number of IR suppression
magnets arranged on 3 sides over most of the length of the tube. Currently,
it does not lase because the gas is slightly contaminated but it is also
misaligned. The discharge color is along the lines of "Minor - Low Outupt"
in Color of HeNe Laser Tube Discharge and Gas Fill
so there may be some hope.
Here is the original description (slightly reformatted):
(From: Chris Chagaris (pyro@grolen.com).)
I have recently acquired what I have been told is a 35 mW Helium Neon laser
head. However, it is unlike anything I have ever seen before. (See the
diagram, below.)
Here is one reply Chris received by email from someone else named Marco. As
you will see, this turns out to be a dead end.
(From: Marco.)
This seems to be a really old one, or from other location than west Europe,
Japan, and the USA. The 'SM' could be an abbreviation for Siemens, they had
manufactured lasers from 1966 to 1993; until last year Zeiss/Jena has taken
over the production; and since 1997 Lasos has overtaken the production by a
kind of management buy-out. You can send them the number, it will be possible
that they know it. Contact Dr. Ledig. I will also look around if I can help
you further.
HeNe lasers with a heated filament are no longer built. To see if it still
runs you can attach a 3.3 V supply to the filament and see if it glows red,
not more, to much heat will destroy it. You could use transformers from
tube amplifiers for the filament and an old HeNe laser power supply for the
anode.
This laser will need around 5,000 V and 10 mA I think. If you could only get a
smaller power supply, you may not see any laser beam, but you can see if it
will trigger."
(From: Sam.)
Here are my 'guesses' about this device. (I have also had email discussions
with Chris.)
I agree with much of what Marco had said.
(From Chris (a few months later).)
Well, tonight while looking through the "Holography Handbook" I spied what
looked suspiciously like that elusive laser I have. It said it was made by
Jodon Engineering Associates of Ann Arbor, MI. I immediately called them and
was fortunate to have the engineer (Bruce) who has built their tubes for the
last 18 years answer the phone. I told him of my plight and read off the
numbers that were on the plasma tube. Sure enough, it was one of their early
lasers. They have been manufacturing HeNe's since 1963. He provided me with
many of the details that I had been searching for.
I finally located a small supply of HeNe gas, just yesterday. While visiting
North Country Scientific to purchase a pair of neon sign electrodes (in
Pyrex), I mentioned my need for a small amount of laser gas for my laser
refurbishing project. (This was formally Henry Prescott's small company that
supplied all the hard to find components for the Scientific American laser
projects.) Lo and behold, there on a shelf, covered with dust, were a few of
the original (1964?) 1.5 liter glass flasks filled with the 7:1 He/Ne gas mix.
He let them go at a very decent price!
(Hopefully, those tiny weeny slippery He atoms have not leaked out! --- sam)
Now, about the magnets:
The magnets are of rectangular shape, one inch long, 3/4 inch in width and 3/8
inch thick. There are a total of 26 magnets placed flat against the top (14)
and flat against the bottom (12) of the plasma tube as viewed from the side.
All but the ones on the very ends of the plasma tube are attached exactly
opposite from one another, top and bottom. (See Jodon
Laser Head for placement and field orientation).
They are placed with the long side (1") parallel to the plasma tube with the
north and south poles along this axis.
They appear to be of ceramic construction and not very powerful. Sorry, I
don't have any means of measuring the actual field strength.
The current status of this project is that the laser needs to be regassed.
Chris is equipped to do this and has acquired the needed HeNe gas mixture.
To be continued....
Photos of a similar but much larger Jodon HeNe laser (3.39 um IR in this case)
can be found in the Laser Equipment Gallery
(Version 1.41 or higher) under "Jodon Helium-Neon Lasers".
The Brewster windows appear to be glued in place. The OC is a normal
7 or 8 mm diameter curved mirror glued to the inside of the output aperture
plate - basically a metal washer. The HR is a square, almost certainly
planar mirror, glued to the outside of a 4 screw adjustable mount of sorts.
Why is the HR square? Probably because it was cut from a large
coated plate, rather than being coated individually. Why 4 screws instead
of 3, making mirror adjustment much more of a pain? Another unsolved
mystery of the Universe. :) Though it's not obvious
from the photo, the Brewster windows aren't quite oriented the same - the
angle differs by perhaps 5 degrees - so the
gain is already slightly reduced from what's possible. However, I have
been assured that this laser did meet specifications when new. The output
is still polarized - probably half way in between - but
the polarization extinction ratio is certainly lower than it could be. If
the laser is still under warranty, it might be worth complaining. ;)
As can be seen, this
sample still lases after refiring the getter and then letting it run for
several hours to allow the cathode to adsorb remaining impurities. The
refiring was actually done using a can crusher demonstration apparatus
and the remains of the getter coating can be seen as the ugly brown ring
encircling the tube just to the left of
the anode connection. I don't know whether the getter coating
was any the worse for wear after that exciting event as I was not present.
What's a "can crusher"? :) Basically an electromagnetic pulse (EMP)
generator: Discharge a really large high voltage capacitor bank into
a couple of turns of wire wrapped around the tube (in this case). Since
the getter electrode in this tube is conveniently oriented as a ring around the
bore and thus acts as the secondary of a transformer, the high current
discharge induced enough current to heat the ring to heat it instantly.
I wish I could have witnessed that!
The output is only about 2 mW though, when the spec is 4 mW. Spectral line
measurements of the discharge in the bore suggest that it's low on helium
and low pressure in general. A helium soak may be in its future.
I have a most likely even earlier Aerotech tube which is constructed along the
same lines as the LS4P except that:
It doesn't lase and has a very pink discharge - running it now to see
if that helps but not much hope by the time it gets that far.
The tube originally put out 22 mW according to a hand-written sticker.
I had picked it up on eBay in a big blue case and substituted another
only slightly newer hard-sealed Aerotech tube which at least
lased - 6 mW, wow. :) Its problem appears to be a bad recipe for the
gas fill, mirrors, or both.
The cover on one unit bears a sticker from El Don Engineering, 2876 Butternut,
Ann Arbor, Michigan 48104, Phone: 1-313-973-0330. The laser was serviced and
repaired on 9/28/80 and its output was 2.3 mW, TEM00. Another one had "Tube
No. 1170, 2.1 mW TEM00, Jan. 13, 1970". I wonder if they still exist. :)
The AO-3100 appears to be made by Gaertner (whoever they are/were, their model
number is not known). Two samples are shown in the
Laser Equipment
Gallery (Version 2.08 or higher) under "Assorted Helium-Neon Lasers".
The bore is about 2.5 mm in diameter which is extremely wide for a red HeNe
laser. I would have expected it to be multi-mode (not TEM00). However, both
samples say TEM00 and they must know. The Brewster
windows are Epoxy-sealed so needless to say, most of these lasers no longer
work (aside from the slight problem that when I received the first tube from
one, it was in pieces. :( Not that I expected it to work, but intact would
have been nicer.
Not surprisingly, most of these lasers no longer lase or even light up
since the tubes are soft-seal and long past their expiration dates.
But if you happen to own a working time machine, it seems that Metrologic was
supplying replacement tubes and power supplies for the AO-3100 as late as
1980. And, a bargain at only $225 and $100, respectively. You'll have
to pay with old bills though. :)
However, I now have obtained an AO-3100 that does still lase. More below.
Lasing specifications:
HeNe laser tube:
Resonator:
Power Supply:
I have acquired a sample of the AO-3100 that was quite battle weary but
the tube did survive cross-county shipping. The case, on the other hand,
looks like it lost a fight with one of those Sherman Tanks. :) It was
bent and dented in multiple places. How the tube didn't turn to a million
bits of glass is amazing.
The better thing about this laser is that the discharge color of the old
soft-seal tube looks pretty good and there is still a very distinct getter
spot. A measurement of the ratio of the He 587.56 nm and Ne 585.25 nm
spectral lines in the discharge show that they are about equal in intensity.
This means that the He:Ne fill pressure is still decent, though compared
to a barcode scanner HeNe laser tube I tested, about 1/2 the helium intensity.
A helium soak might be in its future.
After realigning the mirrors and cleaning the Brewster windows, I now have
0.35 mW of red photons squirting out the front of the laser. Probably only
the front mirror was misaligned originally, but since I had to remove them
both to get the rubber Brewster covers off, realignment of both were required.
Fortunately, getting an alignment laser beam through the wide bore was
straightforward. The HR mirror mount was then installed and adjusted
to return the alignment beam cleanly through the bore. The OC mirror mount
was then installed and that's when it became clear that its alignment was
way off. Now I wonder who did that. :) Once the alignment screws were
tweaked to center its reflected spot, a bit of fiddling resulted in a weak
beam. Some mirror walking and Brewster cleaning helped, but it's not
finished.
The discharge color appears to be improving as it is run as well but output
power has been decreasing as it is run. I hadn't realized that the spec'd
lifetime is only around 100 hours - and I've put on 5 or 10 percent of that
just testing it! It might be a power supply problem though since it produces
a nice bright beam for an instant when started, but then settles down to
perhaps 100 uW on a good day. I do turn it on for a few seconds almost
everyday just to keep it happy.
The photos for "Gaertner/American Optical 3100 Helium-Neon Laser 2"
in the Laser Equipment Gallery are of this
laser in action. The color rendition of my digital camera isn't very good.
The color in the main bore and larger sections of tubing actual should look
close to that in normal HeNe lasers. But the cathode glow (the bright blob)
is actually more yellow, (though not quite the yellow in these photos. :)
The double coiled glowing hot filament is clearly visible in Views 03 to 05.
A careful examination of Views 03 and 06 reveals the scatter from
the Brewster windows at each end of the tube. Note the large difference
in scatter size due to the hemispherical resonator. View 07 shows
that there is indeed a beam from this laser (if that wasn't obvious from
the Brewster windows), though due to its relatively low power, bore light
is competing for attention.
Through screwups in manufacturing (incorrect mirror formula, extra "hot"
emission, etc.), an occasional HeNe laser may produce weak
lasing at one or more ("rogue") wavelengths other than those for which it
was designed. For red tubes, the most likely spurious wavelength is a deeper
red at 640 nm since it is also a fairly high gain line. For a low gain
yellow laser, orange is most likely since it is a relatively close wavelength
and any goofup with the mirror reflectivities may allow it to lase.
I have a tube made by Melles Griot, model number 05-LYR-170, which is about
420 mm long and 37 mm in diameter and can be seen as the middle tube in
Three HeNe Tubes of
a Different Color Side-by-Side. Its only unusual physical characteristics
are that the bore has a frosted exterior appearance (what you see in the photo
is not the reflection of a fluorescent lamp but the actual bore). Apparently,
larger Melles Griot HeNe tubes are now made with this type of bore - it is
centerless ground for precise fit in the bore support. I don't know if the
inside is also frosted; that is supposed to reduce ring artifacts. And, of
course, the mirrors have a different coating for the non-red wavelengths.
According to the the Melles Griot catalog, this is a HeNe laser tube operating
at 594.1 nm with a rated output of 2 mW. However, my sample definitely
operates at both the yellow (594.1 nm) and orange (604.6 nm) wavelengths
(confirmed with a diffraction grating) - to some extent when it feels like
it. The output at the OC-end of the tube is weighted more towards yellow
and has a power output of up to 4 mW or more (you'll see why I say 'up to' in a
minute). The output at the HR-end of the tube has mostly orange and does a
maximum of about 1 mW. Gently pressing on the mirrors affects the power
output as expected but also varies the relative intensities of yellow and
orange in non-obvious ways. They also vary on their own. The mirror
alignment is very critical and the point of optimum alignment isn't
constant. In short, very little about this tube is well behaved. :)
Why there should be this much leakage through the HR is puzzling. The mirror
is definitely not designed for outputting a secondary beam or something like
that as there is no AR coating on its outer surface. Thus, that 1 mW is
totally wasted. Perhaps, this was an unsuccessful attempt to kill any
orange output from the OC. The OC's appearance is similar to that of a
broadband coated HeNe HR - light gold in reflection, blue/green in
transmission. The HR appears similar to one for a green HeNe laser -
light metallic green in reflection, deep magenta in transmission. (However,
it's hard to see the transmission color in the intact tube. The OC may be
more toward deep blue and the HR may be more toward purple.)
As would be expected where two lines are competing for attention in a low gain
laser like this, the output is not very stable. As the tube warms up and
expands - or just for no apparent reason - the power output and ratio of
yellow to orange will gradually change by a factor of up to 10:1. Very gently
pressing on either mirror (using an insulated stick for the anode one!) will
generally restore maximum power but the amount and direction of required
pressure is for all intents and purposes, a random quantity. If the mirror
adjuster/locking collar is tweaked for maximum output at any given time, 5
minutes later, the output may be at a minimum or anywhere in between.
I surmise - as yet unconfirmed - that at any given moment, the yellow and
orange output beams will tend to have orthogonal polarizations. But, as
the distance between the mirrors changes, mode cycling will result in the
somewhat random and unpredictable shifting of relative and total output power
as the next higher mode for one color competes with the opposite polarized
mode of the other. Is that hand waving or what? :)
A few strong magnets placed along-side the tube reduce this variation somewhat.
I'm hoping that adding some thermal control (e.g., installing the tube in an
aluminum cylinder or enclosed case) may help as well. I was even contemplating
the construction of a servo system that would dither the cathode-end mirror
mount to determine the offset direction that increases output and adjusts
the average offset to maximize the output. This might have to be tuned
for yellow or orange - an exclusive OR, I don't know if maximizing total
optical power will also maximize each color individually.
Using an external red HR or OC (99 percent) mirror placed behind the tube's
HR mirror, I was able to obtain red at 632.8 nm as well as a weak output
at the other orange line (611.9 nm), and at times, all four colors were lasing
simultaneously. :) See the section: Getting
Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.
(From: Steve Roberts (osteven@akrobiz.com).)
Ah, the Melles Griot defects... These show up from time to time and are
highly prized in the light show community for digitizing stations and personal
home lumia displays.
The yellow/orange combo is not a goof. I've seen a 7 mW version of that
that was absolutely beautiful, but rejected because it was too hot. It's
probably slight differences in the length of the tube or bore size. They
cut them for a given mode spacing, but fill them all at once with the same
gas mixture. A few companies do make dual line tubes, but you can imagine
the initial cost is murder.
I used to have a short tube that switched from red (632.8 nm) to orange
(611.9 nm) that appeared brighter then the red when it felt like it.
I sometimes wonder if there are a few more HeNe transitions we don't know
about. I know they exist in ion lasers. I have seen a 575 nm yellow line in
krypton that's not on the manufacture's data and a red in Kr that is between
633 and 647 nm. I had that red in my own laser. 575 nm is preferred for show
lasers because it doesn't share transitions with 647 nm like 568 nm does.
When I was interviewing at AVI in Florida they used 4 color 4 scan pair
projectors for digitizing - 6 mW of yellow, 5 mW of green, and 8 mW of red,
all from HeNe lasers. The blue came up from an ILT ion laser in the basement
to each of the four stations via optical fiber. The guy who owned AVI said if
you call Melles Griot and ask nicely they will grade some tubes for you for a
slight extra cost. Methinks they make all the special colors up and tune them
in power somehow, so they can make a price differential, those lines should be
consistent by now.
Every two years of so it seems Melles Griot cleans out their scrap pile, and
somebody always seems to get there hands on them, grades them and sells em.
(From: Daniel Ames (Dlames2@aol.com).)
The yellow and orange HeNe energy transitions are very similar and possibly
competing with each other, especially if the optics are questionable. I have
learned that Melles Griot and other HeNe laser manufacturers sometimes suffer
from costly mistake on a batch of tubes due to the optics being incorrectly
matched to the tube and/or the optics themselves not being correct for the
desired output wavelength. One such batch was supposed to be the common red
(632.8 nm) but the optics actually caused the gain of the orange to be high
enough that the output contained both red and orange (611.9 nm). Then I
believe they are rejected and tossed out, only to be saved by professional
dumpster divers to show up on eBay or elsewhere. Actually, these misfits such
as the yellow/orange tube can be quite fascinating. It would be interesting
to shine a 632.8 nm red HeNe laser right through the bore of that tube while
powered and see what color the output is. I have been told that if you shine a
red HeNe through a green HeNe that it will cause the green wavelength to
cease. I have not had this opportunity to try this, so I do not know for sure
what really happens, maybe the red just overpowered the green beam. This could
be verified with 60 degree prism or diffraction grating on the beam exiting the
opposite end of the green tube. Happy beaming. :)
(From: Sam.)
I have tried the experiment of shining a red HeNe laser straight down the bore
of a green HeNe laser (my green One-Brewster tube setup). I could detect no
significant effect using a low power (1 or 2 mW) laser. This isn't surprising
given that the intracavity power of the green laser was probably in the
hundreds of mW range so the loss from the red beam would be small in a
relative sense. However, wavelength competition effects are quite real as
evidenced from experiments with the two color 05-LYR-170 tube.
Its actual total power output after warmup is over 2.50 mW. The 594.1 nm
(most intense, TEM01* doughnut) and 604.6 nm (TEM10* or TEM10 depending on
its mood) are relatively stable but the 611.9 nm (least intense, TEM01)
visibly fluctuates. Nonetheless, overall power stability and mode cycling
behavior are similar to that of a typical medium power red (632.8 nm) HeNe
laser, which contrasts dramatically with the very unstable yellow/orange
Melles Griot laser described above. REO does have a couple of dual
wavelength HeNe laser heads listed on their
Laser Products Page
but nothing like this. They are 1,152/3,391 nm and 1,523/632.8 nm.
There is also an additional mystery 2 pin connector on this laser head.
The resistance between pins is about 20 ohms and I assume it to be a
heater on the OC mirror, though driving it with about 10 V had no
detectable effect whatsoever.
However, I wonder if there is also some screwup in the REO model
descriptions as the size of this laser head actually matches that of the
REO LHYR-0200M,
being almost 17" in length rather than the 13" listed for the
LHYR-0100M. I kind of doubt that shorter length can be accounted for
by dramatic improvements in HeNe laser technology since my sample was
manufactured (1988), though I suppose that's a possibility. But the
electrical specifications of the two lasers are supposed to be identical,
which doesn't make sense and I don't believe in coincidences. :) And the
output power of my sample peaks at 6.5 mA which isn't consistent with
the specs for either the LHYR-0100M or LHYR-0200M which are both 5.25 mA.
PMS has a patent for this setup - U.S. Patent #4,594,715: Laser With
Stabilized External Passive Cavity. By linearly oscillating the external
mirror at a modest frequency (enough to produce a few cm/sec of movement),
the resulting Doppler broadening of the wavelength spectrum will be
sufficient to effectively decouple the external cavity from the active
cavity. This gets around the stability issues present with open cavity
(e.g., Brewster window) particle counter designs. There is a great deal
of information in the patent on this and other principles of operation.
Any hapless particles that may pass through the beam in the cavity between
the OC of the HeNe laser tube and the external mirror will result in
scatter detectable from the side. A large reflector and aspheric lens
collects the side-scatter and focuses it on another photodiode (under yellow
CAUTION sticker). There is a preamplifier in the box.
It gets better. Viewing the waste beam out the unused HR-end of the tube
(far right) with a diffraction grating reveals that the tube is lasing on
the normal red line, but also on both of the HeNe orange lines (604.6 nm
and 611.9 nm), three other red lines (629.4 nm, 635.2 nm, and 640 nm),
*and* on the very rare Raman shifted red line at 650 nm. And there may
be others but it's difficult to resolve them since the beam is multimode
and the spectra cannot be focused to small spots. This is similar but even
better than what I've observed in my experiments
using external mirrors with normal internal mirror HeNe laser tubes
although this one seems particularly stable with little obvious variation
in the intensities of the lines, at least over a period of a few minutes.
Obtaining the 650 mm line is particularly unusual, especially since it
is so stable. See the section: Getting
Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes. These
non-632.8 nm lines are probably not an objective of the design but are just an
interesting artifact.
I have estimated the reflectivities for the three mirrors which are in this
laser. These values are based on measurements of the output power of the
HeNe laser tube without the external mirror (about 8 mW after warmup) and
the assumption that the internal OC is about 99 percent:
The "Power" refers to the optical power passed by the specified mirror
depending on whether the external HR mirror is present and aligned.
In the case of the HeNe laser tube OC with the external HR, this is the
circulating power in the external cavity which is what's available for
the particle scatter. Note that the circulating power inside the
HeNe laser tube is around 10 WATTS but isn't accessible.
And here are some comments on particle counter technology:
(From: Phil Hobbs (pcdh@us.ibm.com).)
There exist particle counters using external resonant cavities, and also
intracavity Nd:YAG setups. Intracavity measurements *look* as though they
give you amazing sensitivity, but they usually don't. Not only is the
circulating power amazingly sensitive to temperature gradients and tiny
amounts of schlieren from air currents, but the signal you get is wildly
nonlinear and highly position-dependent. Intracavity measurements are a
great way to lose sleep and hair. Passive cavities are usually much better,
and nonresonant multipass cells are better still.
But the remarkable thing about these laser heads revolves around what
is inside: A two-Brewster HeNe laser tube! Except for some very early
units, the tips of the 2-B tube extend to very nearly touch the mirror plates.
On some early ones, the tube is about an inch shorter. (I don't know if
this is just a physical difference or whether the newer tubes are actually
slightly higher power.) So, these are really
external mirror lasers in a nice compact stable package. See
View Inside Hughes Model 3184H HeNe Laser Head
and Hughes Model 3184H HeNe Laser Head Construction.
The end-plates press against aluminum gaskets which allow for mirror
adjustment as well as providing a mostly decent environmental seal.
The mirror glass is held in place in the end-plate with an aluminum ring
press-fit against a rubber cushion. Note the threaded inserts to provide
steel-on-steel contact for the adjustment screws.
The Brewster window and potting material can be seen within the massive
aluminum cylinder - the wall thickness of the sections near each end is
at least 5/16ths inch! It's actually made up of 3 pieces (in addition to the
end-plates) press-fit together along with a rubber O-ring and an additional
rubber ring (maybe just squirted in before completing the press-fit) for
sealing. The center section has thinner walls and I found out that clamping
it in a vice will crunch the tube. :( But at least the broken heads still
make decent hammers. :) The actual tube is the typical Hughes-style
but with B-windows at both ends. Although the potting material is soft
rubber and not RTV, it appears to mostly fill the inner space, just
allowing the Brewster stem at the anode/wiring-end of the tube to poke out
and nearly covering the cathode-end, so removing
the tube intact would be a challenge. More below.
Several other models may also contain 2-B tubes like this including the 3176H
and 3194H.
Unfortunately, dating from the 1970s, most samples are deader than the
standard door nail. They might light up but don't lase. I acquired two
of these awhile ago. One, from 1976, appeared to have approximately the
correct discharge color (as best as I can determine viewing it from the end)
and the tube voltage seemed reasonable. But, no red photons no matter what
I've tried. Another, from 1979, did start a couple years ago, though the
discharge color and tube voltage characteristics were obviously wrong.
But now it only flashes, indicating that it's nearly up to air. However,
several of the oldest lasers, dating from the early 1970s, have
survived and lase and even produce an output power not much different
than what was measured in 1973, the last time they were tested! The
beam is TEM00 with low divergence and less scatter than many modern
HeNe lasers. I suspect that for those fortunate individuals, the
Brewster windows were optically contacted instead of being sealed with Epoxy.
One of the working heads I tested outputs about 3.5 mW at 6.5 mA with an
operating voltage to the head of about 1,610 V. The test power in 1973
was 3.4 mW. Based on the 4 in the model number and a CDRH sticker rating
of 6.5 mW, I suspect that the rated output power is actually 4 mW.
Power continues to increase slightly above 6.5 mA. This may mean that
either the optimal current is higher, or more likely, that the tube is
low on helium or has some other slight gas fill problem, or it's just
high mileage. (Although the power supply that apparently went with
these heads is not very well regulated, its behavior suggests that 6.5 mA
is correct.) Due to the way the tube is potted inside the metal
cylinder, there is no way to easily assess the discharge spectrum
to evaluate the gas fill without test instruments.
The mirrors appear to be hard-coated with the HR being flat and the
OC having an RoC of about 30 cm. This results in a nearly hemispherical
resonator with a mirror spacing just under 30 cm, confirmed by the
very small spot visible on the HR mirror when the laser is operating. The OC
is AR coated on its outer surface (though it is not as robust as modern AR
coatings), and on most of the laser heads, the HR is fine-ground on its outer
surface.
Interestingly, the bore of the 3184H appears to be tapered and is
wider at the OC-end than at the HR-end. This makes sense to more
closely match the mode volume of the hemispherical resonator and thus
increase the gain slightly. A tapered bore was apparently an optimization
that was popular in the early days of HeNe lasers but went out of fashion
due to its higher cost compared to using a uniform size capillary tube
for the bore. I've only come across a tapered bore (or at least noticed it)
in one modern-style HeNe laser tube, a Melles Griot 05-LHP-170, manufacturing
date unknown but it has a serial number of 675P - sounds kind of old!
With this asymmetry, the HR and OC cannot simply be swapped
without likely seeing a severe penalty in output power. It also would likely
not be advantageous to use a confocal or any other symmetric
configuration. However, going to a long-radius hemispherical resonator
might work even better than the existing arrangement.
With 4 screws holding the end-plates in place against the aluminum gasket,
mirror adjustment is somewhat awkward but with persistence, optimal alignment
including mirror walking can be performed relatively quickly. However, the
aluminum gasket isn't ideal, so for testing, I've replaced it with a rubber
O-ring to provide some real compliance. That is, until I decide what to do
with the 2-B tube inside! :)
For a description of several more of these lasers, and a test jig and tests
using external mirrors, see the section:
Some Semi-Antique Hughes Laser Heads.
Where one is really determined to get the tube out, here is more info on
what's involved. But why bother? Aside from aesthetics, it's perfectly
happy in there and very well protected. The risk of destroying the tube
may not be worth the rewards. The press-fit end-sections
must be pulled straight out (not twisted) with something along the lines
of a gear puller as they are a very tight metal-to-metal press fit with ridges
all around. Or, they can be carefully cut off with a metal cutting lathe or
band saw. But serious vibrations will likely destroy the tube. Then,
the rubber potting material would have to be chipped/gouged/cut/sliced
away to actually extract the tube. Then all the remnents of the rubber
stuff must be removed from the tube.
Having said that, I was able to get the end-sections off of a dead laser
head without serious tools. (I'm not about to risk a good one!) Since
the center section has a slightly larger outside diameter than the
end-sections, an aluminum HeNe laser head clamp tightened just snug
around the end-section provided a way of pressing on the center section
to pull the end-sections free. Four clearance holes were drilled in a
1/2" thick piece of aluminum plate and 4-40 screws were then passed
through these holes and threaded into the laser head. By carefully
tightening these screws in a cyclic manner (e.g., 1,2,3,4,1,2...),
the end-section could be pulled out about 1/8". Once this was done,
the HeNe head clamp was removed and shorter screws were used to attach
the 1/2" plate directly to the head. With the plate clamped in a vice,
the entire head could be worked back and forth until it came free.
(Alternatively spacer plates and/or shorter screws could be added/substituted
to continue the original process until the end-section comes free.) This
was not fun, a set of screws survived for only about one end-section, and
as noted, this is really only the beginning of the tube extraction process.
I have not yet attempted to go any further. But someone else has succeeded
in removing the tube. Apparently it wasn't much fun.
Raman spectroscopy is used to identify gases by passing a
laser beam through the unknown sample. Raman scattering results in a
shift toward longer wavelengths depending on the atomic/molecular
composition of the gas. By measuring the intensity of the Raman
scatter at several longer wavelengths, the gas composition can be
determined. For these units, the relevant gases were
apparently N2, O2, and N2O based on "linearization constants"
printed on a label on the lasers.
To get any sort of sensitivity, the beam
must be high power since a very small percentage of photons actually
undergo the Raman shift. For the Ohmeda unit, this is achieved by
utilizing the intracavity power between 2 super polished HR mirrors
and super-polished Brewster window. While I don't know for sure
what the intracavity power should be, based on tests of the mirror
reflectivities and output power with an OC mirror with known reflectivity,
it is at least several watts and could be over 100 W!
The relevant patents include:
The first one describes the principles of Raman spectroscopy, but the
actual drawings do not correspond to the Ohmeda laser assembly. But the
other two have diagrams which closely match the specimen I have though
I'm not sure which it is.
The laser tube is made by PMS and is physically similar to the REO/PMS
tunable 1-B tubes, but its internal HR mirror appears to be coated so that
in conjunction with the HR mirror at the other end of the cavity, the
reflectance for 632.8 nm is maximized.
Using a 60 cm RoC OC mirror with a reflectance of approximately 98 percent
at 632.8 nm, the laser produces about 5.4 mW, multimode. I assume that
with an optimal OC mirror, the power would be somewhat higher. (This
test was done without the Brewster prism assembly. There would be some
loss with the prism present in the cavity.)
At 5 mW - implying 250 mW of intracavity power with the 98 percent OC -
the waste beam is about 5 uW and the reflectivity of the internal HR
mirror is thus about 99.998 percent. There is very little scatter visible
on the B-window under these conditions. (I did have to clean it, but there
is a handy access port that can be used for this purpose.) If there were
no other losses, putting a similar HR at the other end would result in
125 W of intracavity power! Of course, this is impossible as there ARE
other losses, but it is likely to be several watts and perhaps much more.
With an SP-084 HR, the output from this mirror was about 0.5 mW and the
output from the internal HR was 32 uW corresponding to about 1.5 W of
intracavity power. Not too shabby. But with the PMS HR (and Brewster
prism), the waste beam power for 633 nm was a whopping 171 uW implying
about 8.5 WATTs inside. Not too shabby at all. :) I have cleaned the
Brewster prism with no significant change in performance. However, a
careful cleaning of all three surfaces would almost certainly improve
things some more, especially for this case. Interestingly, with the PMS
mirrors, the beams exiting the laser appear to be nice TEM00.
When used in the normal way, there is a 632.8 nm narrow band filter between
the external mirror and a silicon photodiode. So, that is almost certainly
used to monitor the power transmitted by that mirror, and by inference,
intracavity power.
The 632.8 nm intracavity power would no doubt be greater without the prism but
that's where it gets interesting. With the prism in place, the wavelength
is tunable with both orange wavelengths being easily selectable. Here are
the stats for two simmilar laser assemblies with different dates of
manufacture:
Laser 1 (2004):
Laser 2 (2003):
I did not test Laser 2 with non-PMS mirrors, thus the exact reflectivity
and intracavity power is not known.
I do not know what the reflectivity of the internal HR is at 604 nm
and 611.9 nm for either laser so the intracavity power is not known
for these wavelengths either.
The purpose of the Brewster prism is no doubt to select only one of the
possible wavelengths, which based on the specifications of the filter
between the external mirror and photodiode, is no doubt 632.8 nm.
The very nice behavior on the orange lines is thus simply an
artifact of the mirrors being so highly reflective at 632.8 nm.
But note how the power balance between the two mirrors seems to
be more or less reversed for Lasers 1 and 2. So, although the internal
mirror for both lasers is not AR coated and the external mirror is, the
coating formulas appear to have been interchanged.
It would be quite risky to try to run the laser with only the external
PMS HR but no prism as the mirror glass is glued in place. While the
plate it's glued to could be mounted directly on the adjustable mount,
the mirror is very exposed and susceptible to damage. So, I'm probably
not going to attempt that.
Here are how the 8 filters intercepting Raman light from the side of the
lasers were labeled:
I'm deducing the center wavelength based on the part number and observations
of visible light transmittance for those in the 600 to 700 nm range. I don't
think the exact location of the side mirrors matters except to the extent that
it matches up with the appropriate sensor channel.
While these center wavelengths would suggest a rather large wavelength
shift, this apparently is the case for gases. But wouldn't there also
have to be a 632.8 nm rejection filter in front of the detectors or else
that would overwhelm the small Raman signal?
While I had expected the photosensors to be PhotoMultiplier Tubes (PMTs)
as in the similar Raman system using an argon ion laser, these are most
likely Avalanche PhotoDiodes (APDs). They are in TO18 cans clamped to
a ThermoElectric Cooler (TEC, Peltier device) on a large heatsink. Inside
the can, there is a little gold colored block perhaps 1.5 mm square,
with a 0.5 mm blue dot in the middle, which I presume is the active area.
The APD is probably a S9251-05 (or very similar), one of the Hamamatsu
S9251 Series Avalanche Photodiodes. There's a fair amount electronics
to go with them, though nothing obviously recognizable.
More on Resonator Length and Mode Hopping
Here are some additional comments that address the common fear of the novice
laser enthusiast that the resonator length has to be stabilized to the nm or
else the laser will blink off.
Observing Longitudinal Modes of a HeNe Laser
Monitoring the output power of any HeNe laser
while it's warming up will show a variation in output power due to
longitudinal mode cycling. There is even a specification called the
"Mode Sweep Percentage" which indicates how large the variation is
in relation to the output power. For short tubes, the power fluctuations
can approach 20 percent; for long tubes, they may be less than 2 percent.
Longitudinal Mode Pulling
It turns out that most lasers don't actually oscillate on exact multiples of
the cavity resonance frequency, c/2L, as stated in introductory textbooks.
(The exceptions would be where the gain curve is essentially flat but that's
another story.) Longitudinal modes that aren't exactly centered on the gain
curve will be at frequencies very slightly offset from these, pulled toward
the center of the gain curve with those that are farthest away seeing the
most shift. This is a well known effect called "mode
pulling" with highly developed theory to back it up. (Mode pulling
isn't unique to lasers. For example, a quartz crystal oscillator can be
tuned over a small range using an external capacitor even though its resonance
frequency differs from the output frequency.)
Transverse Modes of Operation
Lasers can also operate in various transverse modes. Laser specifications
will usually refer to the TEM00 mode. This means "Transverse Electromagnetic
Mode 0,0" and results in a single beam. The long narrow bore of a typical HeNe
laser forces this mode of oscillation. With a wide bore multiple sub-beams
can emerge from the same cavity in two dimensions. The TEM mode numbers
(TEMxy) denote the number (minus one) or configuration of the sub-beams.
O OO OOO Each 'O' represents
O OO O OO OOO a single sub-beam.
TEM00 TEM10 TEM01 TEM11 TEM21
Multi-Transverse Mode HeNe Lasers
As noted, most HeNe lasers are designed to operate with a single transverse
(spatial) mode or TEM00. However, to obtain the highest power for a given
tube size or by a goof-up in design, a higher order mode structure may be
produced. A non-TEM00 mode may be present if:
Coherence Length of HeNe Lasers
Common HeNe lasers have a coherence length of around 10 to 30 cm. By adding
an etalon inside the cavity to suppress all but one longitudinal mode,
coherences lengths of 100s of meters are possible. Naturally, such HeNe
lasers are much more expensive and are more likely to be found in optics
research labs - not mass produced applications.
What is Mode Locking?
The normal output of a HeNe or other CW laser is a more or less constant
intensity beam. Although there may be long term variations in output power
as well as short term optical noise and ripple from the power supply, these
are small compated to the average intentsity. Mode locking is a technique
which converts this CW beam to a periodic series of very short pulses with a
length anywhere from picoseconds to a fraction of a nanosecond. The
separation of the pulses is equal to the time required for light to make one
round trip around the laser cavity and the pulse repetition rate (PRF) will
then be: c/(2*l). For example, a laser resonator with a distance of 30 cm
(1 foot) between mirrors, would have a mode locked PRF of about 500 MHz.
HeNe Laser Output Power Fluctuation During
Warmup
While not generally visible by eye alone except possibly for very
short or tired (low gain) HeNe lasers, there is a quasi-periodic
variation of output power with time. For the typical HeNe laser
tube shortly after turn-on, the frequency is quite rapid (a cycle
every few seconds) and gradually slows down as the tube temperature
reaches a steady state value (after a half hour or more).
Plots of HeNe Laser Power Output and Polarized Modes During Warmup
Here are some plots I made of power output versus time for several typical
HeNe laser tubes and heads from nearly the shortest available to
mid-size. (Beyond this, the appearance will be very similar, but possibly
with even a smaller fluctuation in power due to mode cycling.)
Most are from Melles Griot but the behavior of lasers from other
manufacturers will be very similar. The majority are healthy samples
but a few show some rather dramatic peculiarities.
There are also plots of a Coherent model 200 and Hewlett Packard model 5517A
frequency stabilized HeNe lasers from power-on to locking.
R1 PD1
+15 VDC o----/\/\----|<|----+
100 |
/
\<----------+----+---o A/D Input (+/-10 V range)
/ R2 | |
\ 25K | /
R3 | C1 _|_ \ 200K ohms (Zin of A/D module)
-15 VDC o-----/\/\----------+ 1uF --- /
68K | \
| |
0 VDC o-------------------------------+----+----o A/D Ground
Intensity Stability of HeNe Lasers
There are at least three kinds of intensity variations present with HeNe
(or other gas) lasers: long term as various longitudinal modes compete for
attention, short term due power supply ripple or discharge instability, and
beat frequencies between modes that are active.
Stabilized Single Frequency HeNe Lasers
The common HeNe laser, while highly monochromatic, may not produce just a
single frequency (or equivalently, wavelength) of light. As noted in the
section: Longitudinal Modes of Operation,
several closely spaced frequencies will generally be active at the same
time and their precise values and intensities will change over time.
For many applications, this doesn't matter. However, for others, it
makes such a laser useless.
On-line Introductions to HeNe Lasers
There are a number of Web sites with laser information and tutorials.
HeNe Laser Tubes, Heads, Structure, Power Requirements, Lifetime
Early Versus Modern HeNe Lasers
In the first HeNe lasers (see the diagram below), exciting the gas atoms to
the higher energy level was accomplished by coupling a radio frequency (RF)
source (i.e., a radio transmitter) to the tube via external electrodes.
Modern HeNe lasers almost always operate on a DC discharge via internal
electrodes.
Bellows Bellows
/\/\/\ Discharge tube with external electrodes /\/\/\
|| \________________________________________________/ ||
|| | | | | | | ||===> Laser
|| ___ __|_|________________|_|______________|_|__ ||===> Beam
|| / || | | | \ ||
\/\/\/ || | o | \/\/\/
Adjustable || +-----------o RF exciter o----------+ Adjustable
totally || partially
reflecting ||<-- to vacuum system reflecting
mirror mirror
Structure of Internal Mirror HeNe Lasers
The following applies to most of the inexpensive internal mirror low to medium
power (0.5 to 5 mW) HeNe tubes available on the surplus market. Depending on
the original application, the actual laser tube may be enclosed inside a laser
head or arrive naked. :-)
____________________________________________
/ _________________ \
Anode |\ Helium+neon, 2-5 Torr Cathode can ^ \ |
.-.---' \.--------------------------------------. '-'---.-. Main
<---| |:::: :======================================: :::::| |===> beam
'-'-+-. /'--------------------------------------' .-.-+-'-'
Totally | |/ Glass capillary ^ _________________/ | | Partially
reflecting | \____________________________________________/ | reflecting
mirror | | mirror
| Rb + - |
+---------/\/\---------o 1.2 to 3 kVDC o-----------+
Beam quality - There is a statistically significant reduction in
diffraction rings (stray light) around the main beam with a frosted bore
ID, though some designs are more susceptible to this than others. However,
sometimes requirements for a particular spot size or output power limit
options and the frosting will help.
Gas Fill and Getter
In order for an HeNe laser to operate efficiently (as such things go) or at
all, there must be a very precise and pure mixture of helium and neon gas in
the tube. The total amount of gas in a typical 1 mW HeNe tube is much less
than 1 cubic cm if it were measured at normal atmospheric pressure. It fills
the tube only because the pressure is very low. However, with this small
amount of gas, it doesn't take much contamination or leakage to ruin the tube.
Mirrors in Sealed HeNe Tubes
(See: Typical Small to Medium Size Melles Griot
HeNe Laser Tubes for views of the types of mirrors and mirror mounts
discussed below.)
Mirror Reflectances for Some Typical HeNe
Lasers
Here are some (approximate) typical OC reflectances for red (632.8 nm) HeNe
lasers determined by measuring the actual transmission (R = 100 - T) of a red
HeNe laser beam through the optic with a simple photodiode based laser power
meter:
More About HeNe Dielectric Mirrors
In the mid 1980s, before Ion Beam Sputtered (IBS) coatings really made their
commercial debut, some mirrors were still Epoxied (soft-sealed), particularly
those with a lot of coating layers (like 20 or 30), mostly green, yellow, and
IR HeNe lasers. These tubes need sharp cutoffs (to kill lasing on unwanted
wavelengths) and/or ultra high reflectivity (due to their very low gain) in
the coatings - which means a lot of layers. The packing density on
Electron-Beam (E-Beam) coatings is not great, so water molecules get into all
the layers. When you hard-seal the mirror by heating the frit, the water
comes out and cracks the coating (called a 'crazed' mirror). Another problem
with mega-stack E-Beam coatings is that the transmittance curve can shift as
much as 10 nm (to longer wavelengths - the layers get thicker) during the oven
cycle (again a water-thing). If you have to, say, highly reflect at 594.1 nm
(for a yellow output tube) and highly transmit beyond 604.6 nm (to kill the
orange and red), and your coating shifts 10 nm in the oven cycle, another
batch of tubes ends up in the dumpster. :( No! Send the my way. :)
Random and Linear Polarized HeNe Tubes
Most common HeNe laser tubes are randomly polarized since for many applications
the polarization of the beam doesn't matter. As noted elsewhere, the term
"random" here really doesn't mean that the polarization is necessarily jumping
around to totally arbitrary orientations. In fact, such behavior would be
rather unusual. It just means that nothing special is done to control the
polarization. The typical HeNe laser will lase on several longitudinal
modes (how many will depend on tube length of the resonator) with adjacent
modes having polarizations orthogonal to each-other. Each of the modes will
change their relative intensities periodically over time or may even switch
polarizations suddenly as the tube heats and expands.
For the special case of a short tube where only two
modes fit under the gain curve (typically 5 or 6 inches in length) at the
instants when they are equal, the output will appear to be non-polarized
(constant intensity as an external polarizer is rotated in the beam) but as
the modes shift under the gain curve, one or the other polarization will
dominate. For longer tubes, there will be much less of an effect because
there will be multiple modes with both polarizations at all times.
More on Mode Cycling in Short HeNe Lasers
As noted, a randomly polarized HeNe laser doesn't really produce arbitrary
polarization but the individual longitudinal modes may switch polarizations
as the tube warms up and expands. Where the distance between the mirrors
is small - 5 or 6 inches as is the case with small HeNe laser tubes, only
two adjacent modes will fit under the inhomogeneously Doppler broadened gain
curve of neon. With only two active modes, effects of mode changes may be
obvious even without anything more than Mark-I eyeballs and a polarizing
filter but fancy equipment may be needed to fully characterize what's going on.
HeNe Mode Flipper Observations
The longitudinal modes of a HeNe laser tube sweep through the gain curve
as the resonator heats and expands. On a random polarized tube, adjacent
modes then to be orthogonally polarized due to non-linear mode competition
(or something). With well behaved tubes, once a mode starts lasing with
a given polarization as it exceeds threshold on one side of the gain curve,
that polarization is fixed until the mode ceases lasing on the other side
of the gain curve.
Polarization of Longitudinal Modes in HeNe
Lasers
It is well known that adjacent longitudinal modes in HeNe lasers (at least)
tend to be orthogonally polarized as discussed above.
This is a weak coupling as a magnetic field, Brewster plate, or even
some asymmetry in the cavity can affect it or kill it entirely. And
some lasers will cause the polarization to suddenly flip as modes
cycle through the gain curve.
Power Requirements for HeNe Lasers
Power for a HeNe laser is provided by a special high voltage power supply
(see the chapter: HeNe Laser Power Supplies
and consists of two parts (these maximum values depend on tube size - a
typical 1 to 10 mW tube is assumed):
Operating Regions of a HeNe Laser Tube
There are several distinct operating regions for a HeNe plasma discharge
as a function of tube current each of which has its own properties. The
following summary is partially extracted from the
HeNe Laser Manual by Elden
Peterson and is mostly just for curiosity sake as there is little reason
to run a HeNe laser tube at anything other than close to the nominal current
(which results in maximum power output and rated life) listed in the tube
specifications except possibly to implement low level modulation for laser
communications.
HeNe Tube Dimensions, Drive, and Power Output
A large number of factors interact to determine the design of a modern HeNe
laser. Beam/bore diameter, bore length, gas fill pressure, voltage, current,
and mirror design, are all critical in determining how much output power will
be produced - or whether a given tube will lase at all. Hundreds (at least)
of technical papers and entire phone book size volumes filled with equations
have no doubt been written on these topics and we can't hope to do anything
serious in a few paragraphs, but at least, may be able to give you a feel for
some of the relationships among power output, bore dimensions, gas pressure,
and drive requirements in particular.
Total Bore Bore --- Ratio of --- Discharge Discharge Output
Lgth Lgth (L) Dia. (D) L D (D * L) Voltage Current Power
------------------------------------------------------------------------------
135 mm 80 mm .46 mm 1 1 1 900 V 3.3 mA .5 mW
177 mm 115 mm .53 mm 1.4 1.15 1.6 1,130 V 4.5 mA 1.0 mW
255 mm 190 mm .72 mm 2.4 1.57 3.7 1,360 V 6.5 mA 2.0 mW
370 mm 300 mm .80 mm 3.8 1.7 6.4 1,800 V 6.5 mA 5.0 mW
440 mm 365 mm .65 mm 4.6 1.4 6.4 2,150 V 6.5 mA 10 mW
930 mm 855 mm 1.23 mm 11.1 2.7 29.9 4,500 V 8.0 mA 25-35 mW
Largest HeNe Laser?
(From: Chris Leubner (cdleubner@ameritech.net).)
Boosting the Power Output of a HeNe Laser?
Unfortunately, given the existing laws of physics, there usually isn't much
you can do to increase the output power of a HeNe laser above its specified
ratings. Unlike an ion laser where higher tube current usually increases
power output (at the expense of tube life), boosting current to a HeNe tube
beyond the optimal amount actually *decreases* power output. Options like
Q-switching don't exist for HeNe lasers.
Bare HeNe Tubes and Laser Heads
What you have may be a 'bare' tube or it may be encased in a cylindrical or
rectangular laser head - or something in between:
HeNe Tube Seals and Lifetime
Neon signs last a long times - years - how about HeNe laser tubes?
An Older HeNe Laser Tube
The Spectra-Physics Model 084-1 HeNe Laser Tube was
popular for applications like barcode scanners. It is rated at 2 to 3 mW
when new. While the main glass tube and end-plates use glass-to-metal (hard)
seals, the mirrors appear to be Epoxied in place (soft sealed). Thus, one
would expect these tubes to leak over time. However, out of 31 that I have
tested, 20 appear to be nearly as good as new showing only slight leakage
which their getters have taken care of nicely and no detectable reduction in
power output. (Of the others, 7 had weak or no output but most could be at
least partially revived - see the section:
Attempting to Revive Some Soft-Seal HeNe
Tubes. The remainder were totally dead.)
HeNe Laser Pointers
While modern laser pointers fit comfortably on a keychain and can be had for
$1 or less if you know where to look, the first laser pointers were, well,
HUGE and at least several hundred dollars. :) One of the earliest laser
pointers using a HeNe laser tube I've seen (dating from the late 1970s) was
about 12 inches long by 1-3/4" in diameter (just like a common HeNe laser
head). The name on it is Bergen Expo Systems, Inc. and it is a model
LP6-227 should want to order one. :) The date of manufacture was 1978.
This pointer was tethered via a six foot cord to a separate high voltage
power unit. The beam on/off button on the side not surprisingly didn't
control the power supply but rather moved a sliding shutter. The actual
manufacturer was probably Spectra-Physics as the tube inside was a SP-084
(a common barcode scanner type) and it has the funny 3 pin power supply
connector mainly used by Spectra-Physics. I don't have the power supply
so can't say what it looked like.
HeNe Lasers using External Mirrors
While most of what you will likely come across are the common internal mirror
HeNe tube, having the optics external to the tube is essential for some
applications.
"Brewster window terminated HeNe tubes are mostly sold into particle counter
applications, where the user pulls an air stream through the cavity. With
ultra low-loss ($$$) High Reflecting mirrors on both ends, massively
multimode, you can develop 10 to 20 Watts of internal cavity power, we've
seen as high as 30 Watts. Selling prices for new tubes is upwards of a
thousand bucks in volume quantity (tubes only). The high-end models have an
optically contacted Brewster window. There are not too many double-Brewster
HeNe laser tubes made anymore, mostly on a special order basis. They're not
that hard to align, if you know some tricks."
A One-Brewster HeNe Laser Tube
I was given a CLIMET 9048 HeNe laser head which contains a Melles Griot HeNe
tube with a normal HR mirror at one end but with a frit-sealed Brewster window
instead of an OC mirror at the other end. In this case, it is the cathode-end
which is nice since there is no high voltage to deal with near the Brewster
window. But identical tubes also come with the Brewster window at the
anode-end but why anyone would want this excapes me. :) (And, several
other models of one-Brewster tubes are common - see the section:
Melles Griot Brewster and Zero Degree
Window HeNe Tubes.)
Designing a Helium-Neon Laser Tube
(From: Lynn Strickland (stricks760@earthlink.net).)
Wavelengths, Beam Characteristics
HeNe Laser Wavelengths
While what comes to mind when there is mention of a HeNe laser is a red beam,
those with other wavelengths are manufactured.
Tunable HeNe Lasers
If it were possible to select any available wavelength desired, then some
people would be content beyond description. :)
Exact Frequency/Wavelength of HeNe Lasers
There is, of course, no single precise HeNe wavelength since any given
cavity will only oscillate at the permitted longitudinal modes and the gain
curve is something like 1.5 GHz wide. Thus, for a common HeNe laser, there
is no single wavelength and those that are present drift over time (mostly due
to thermal expansion of the cavity). A single mode frequency stabilized HeNe
laser will have very nearly a constant single wavelength precise to 9
or more significant figures but it too will depend on the physical size of the
laser's cavity - there is no one correct answer!
HeNe Laser Beam Characteristics
Compared to a diode laser, the beam from even an inexpensive mass produced
HeNe tube is of very high optical quality:
Wavelength * 4
Divergence angle (half of total) in radians = --------------------
pi * Beam Diameter
So, for an ideal HeNe laser with a .5 mm bore at 632.8 nm, the divergence
angle will be about 1.6 mR. Note that although a wider bore should result
in less divergence, this also permits more not quite parallel rays to
participate in the lasing process. This assumes planar mirrors - which few
HeNe lasers use. Where one or both mirrors are curved, the divergence
changes. For example, it is common with HeNe tubes for the Output Coupler
(OC) mirror to be ground slightly concave and for the High Reflector (HR)
mirror to be planar. If the outer surface of the OC glass is not also
curved to compensate for the negative lens that results, the beam will
diverge at a much higher rate than would be expected for the bore diameter.
Ghost Beams From HeNe Laser Tubes
If you project the output from some HeNe laser tubes (as well as other lasers)
onto a white screen a meter or so away, you may see a main beam and a weak beam
off to the side a few cm away from it. Maybe even another still weaker one
after that.
Getting Other Lasing Wavelengths from Internal Mirror
HeNe Laser Tubes
As a practical matter, the only wavelength that is useful from an internal
mirror HeNe laser is the one for which it was designed. (Or the pair in the
case of a couple of Research Electro-Optics
(REO) lasers.) However, it is often
possible to at least obtain unstable lasing at other wavelengths
by extending the cavity using an external mirror. The output power
of the other lines can be anywhere from almost non-existent to greater
than the power at the original wavelength. This probably
works best obtaining a some red from a long "hot" yellow (594.1 nm) or
orange (611 nm) tube since at least one mirror is likely coated broadband
to include yellow through red. Due to the low gain of the non-red
lines, going the other way - getting yellow from a red tube, for example -
is not likely to succeed unless the tube is very long. But obtaining lasing
at other red wavelengths - and even orange - may be possible with a moderate
size red HeNe laser tube. Even a 1 mW tube may give you 1 or 2 other red
lines. I doubt it will work at all with a green HeNe tube
having mirrors that appear orange in transmission since both mirrors are
probably too transparent at even the yellow wavelength (except possibly if
two external mirrors are used). However, if a mirror is more red in
transmission, there might be a chance. See the section:
Instant HeNe Laser Theory
for a table of HeNe lasing wavelengths and relative gains.
Bruce's Notes on Getting Other Lines from Red (633 nm)
HeNe Laser Tubes
This, to make a gross understatement, would appear to be the definitive
word on coaxing other colors from surplus HeNe laser tubes. And I thought
six lines (including the mysterious 650 nm line) was an achievement. :)
Upper Pump Lower
Sideband Wavelength Sideband
--------------------------------
589.7 604.6 -----
596.6 611.9 627.8
613.3 629.4 646.4
616.5 632.8 (650.0)
618.8 635.2 652.5
623.4 640.1 -----
(633.8) 650.0 668.1
Zhiwen, Suitang, and Haoran, "650 nm CW He-Ne Raman Laser",
Chinese Phys-Lasers, v. 15, no. 11, Nov. 1988, p. 803. (From Chin. J.
Lasers 15, pp. 648-651.)
Other Spectral Lines in HeNe Laser Output
While there is no such thing as a truly monochromatic source - laser or
otherwise, the actual output beam of even an inexpensive HeNe laser is really
quite good in this regard with a spectral line width of less than 1/500th of
a nm. For a frequency stabilized HeNe laser, it can be 1,000 times narrower!
About the Waste Beam from a HeNe Laser
The so-called High Reflector (HR) or totally reflecting mirror in a HeNe
laser isn't really perfect, though the actual reflectivity is generally
99.95 percent or better. For a 1 mW laser tube with a 99 percent Output
Coupler (OC) mirror, there is about 100 mW of intracavity power. Of this,
about 50 uW will exit the rear through a 99.95 percent HR mirror. Unless
the back of the HR mirror is painted or covered, there is always some small
beam exiting the rear of the laser.
Magnets in High Power or Precision HeNe Laser Heads
Effects of Magnetic Fields on HeNe Laser
Operation
If you open the case on a higher power (and longer) HeNe laser head or one
that is designed with an emphasis on precision and stability, you may find a
series of magnets or electromagnetic coils in various locations in close
proximity to the HeNe tube. They may be distributed along its length or
bunched at one end; with alternating or opposing N and S poles, or a coaxial
arrangement; and of various sizes, styles, and strengths.
"The splitting of a spectrum line into several symmetrically disposed
components, which occurs when the source of light is placed in a strong
magnetic field. The components are polarized, the directions of polarization
and the appearance of the effect depending on the direction from which the
source is viewed relative to the lines of force."
"They've pretty much nailed the 3.39 micron problem on red HeNes these days so
magnets really aren't needed on them. Even the new green tubes don't have
much of a problem - especially since the optic suppliers have perfected the
mirror coatings. All of the good green mirrors are now done with Ion Beam
Sputtering (IBS), as opposed to run-of-the-mill E-Beam stuff.
Typical Magnet Configurations
Here are examples of some of the common arrangements of magnets that you may
come across. In addition to those shown, magnets may be present along only one
side of the tube (probably underneath and partially hidden) or in some other
peculiar locations. I suspect that for many commercial HeNe lasers, the exact
shape, strength, number, position, orientation, and distribution of the magnets
was largely determined experimentally. In other words, some poor engineer was
given a bare HeNe tube, a pile of assorted magnets, a roll of duct tape, and a
lump of modeling clay, and asked to optimize some aspect(s) of the laser's
output. :-)
N S N S N S N S
|| //======================================================\\ ||
|| //======. .========================================. .======\\ ||
S ||| N S N S N S ||| N
'|' '|'
N S N S N S N
|| //===============================================\\ ||
|| //======. .=================================. .======\\ ||
N ||| S N S N S ||| N
'|' '|'
N N N N N N N
|| //===============================================\\ ||
|| //======. .=================================. .======\\ ||
S ||| S S S S S ||| S
'|' '|'
+--+ +--+ +--+ +--+
N | | S N | | S N | | S N | | S
+--+ +--+ +--+ +--+
|| //==================================================\\ ||
|| //====. .========================================. .====\\ ||
||| +--+ +--+ +--+ +--+ |||
'|' N | | S N | | S N | | S N | | S '|'
+--+ +--+ +--+ +--+
Other axial configurations with opposing poles or radially oriented poles
may also be used or there may be a single long solenoid type of coil
or permanent magnet as for a two-frequency laser interferometer.
Internal Mirror HeNe Tubes up to 35 mW - Red and Other Colors
Typical HeNe Tube Specifications
Prior to the introduction of the CD player, the red HeNe laser was by far the
most common source of inexpensive coherent light on the planet. The following
are some typical physical specifications for a variety of red (632.8 nm) HeNe
tubes (all are single transverse mode - TEM00):
Output Tube Voltage Tube Supply Voltage Tube Size
Power Operate/Start Current (75K ballast) Diam/Length
---------- --------------- ------------ ---------------- -------------
.3-.5 mW .8-1.0/6 kV 3.0-4.0 mA 1.0-1.2 kV 19/135 mm
.5-1 mW .9-1.0/7 kV 3.2-4.5 mA 1.1-1.3 kV 25/150 mm
1-2 mW 1.0-1.4/8 kV 4.0-5.0 mA 1.2-1.8 kV 30/200 mm
2-3 mW 1.1-1.6/8 kV 4.0-6.5 mA 1.4-2.0 kV 30/260 mm
3-5 mW 1.7-1.9/10 kV 4.5-6.5 mA 2.1-2.4 kV 37/350 mm
"I have a neat curve from an old Aerotech catalog of HeNe laser power versus
life. The tubes are overfilled at first, so power is low. They then peak at
a power much higher than rated power, followed by a long period of constant
power, and then they SLOWLY die. It's not uncommon for a new HeNe tube to be
in excess of 15% greater than rated power."
Output Tube Voltage Tube Supply Voltage Tube Size
Power Operate/Start Current (75K ballast) Diam/Length
---------- --------------- ------------ ---------------- -------------
.8 mW .9/5 kV 3.2 mA 1.1 kV 19/135 mm
1.0 mW 1.1/7 kV 3.5 mA 1.4 kV 25/150 mm
1.0 mW 1.1/7 kV 3.2 mA 1.4 kV 25/240 mm
2.0 mW 1.2/8 kV 4.0 mA 1.5 kV 30/185 mm
3.0 mW 1.6/8 kV 4.5 mA 1.9 kV 30/235 mm
5.0 mW 1.7/10 kV 6.0 mA 2.2 kV 37/350 mm
12.0 mW 2.5/10 kV 6.0 mA 2.9 kV 37/475 mm
HeNe Tubes of a Different Color
Although a red beam is what everyone thinks of when a HeNe laser is discussed,
HeNe tubes producing green, yellow, and orange beams, as well as several
infra-red (IR) wavelengths, are also manufactured.
However, they are not found as often on the surplus market because
they are not nearly as common as the red variety. In terms of the number
of HeNe lasers manufactured, red is far and away the most popular, with all
the others combined accounting for only 1 to 2 percent of the total production.
In order of decreasing popularity, it's probably: red, green, yellow, infra-red
(all IR wavelengths), orange. Non-red tubes are also more
expensive when new since for a given power level, they must be larger (and
thus have higher voltage and current ratings) due to their lower efficiency
(the spectral lines being amplified are much weaker than the one at 632.8 nm).
Operating current for non-red HeNe tubes is also more critical than for the
common red variety so setting these up with an adjustable power supply or
adjusting the ballast resistance for maximum output is recommended.
Determining HeNe Laser Color from the Appearance of the
Mirrors
Although most HeNe lasers are the common red (632.8 nm) variety (whose beam
actually appears orange-red), you may come across unmarked HeNe tubes and
just have to know what color output the produce without being near a
HeNe laser power supply.
HeNe Laser High Reflector (HR) Output Coupler (OC)
Color Wavelength Reflection Transmission Reflection Transmission
------------------------------------------------------------------------------
Red 632.8 nm Gold/Copper Blue Gold/Yellow Blue/Green
Orange 611.9 nm Whitish-Gold Blue Metallic Green Magenta
Yellow 594.1 nm Whitish-Gold Blue Metallic Green Magenta
Green 543.5 nm Metallic Blue Red/Orange Metallic Green Magenta
Broadband (ROY) Whitish-Gold Blue
IR 1,523 nm Light Green Light Magenta Light Green Light Magenta
IR 3,391 nm Gold (Metal) Coated Neutral Clear
More on Other Color HeNe Lasers
Here are some comments on the difficulty of obtaining useful visible output
from HeNe lasers at wavelengths other than our friendly red (632.8 nm):
Steve's Comments on Superradiance and the 3.39 um HeNe
Laser
Generally, when a gas laser is superradiant, there is a limit to its maximum
power output (with exceptions for nitrogen and copper vapor laser, although
nitrogen's upper limit is defined by the maximum cavity length into which you
can generate a 300 ns or less excitation pulse.
Viewing Spectral Lines in Discharge, Other Colors in Output
For accurate measurements, you'll need an optical instrument such as a
monochromator or spectrophotometer or optical spectrum analyzer.
See the section: Monochromators.
But to simply see the complexity of the discharge spectrum inside the
bore of a HeNe laser tube, it's much easier and cheaper.
Instant Spectroscope for Viewing Lines in HeNe
Discharge
It is easy to look at the major visible lines. All it takes is a diffraction
grating or prism. I made my instant spectroscope from the diffraction grating
out of some sort of special effects glasses - found in a box of cereal, no
less! - and a monocular (actually 1/2 of a pair of binoculars).
Dynamic Measurement of Discharge Spectra
The following is trivial to do if you have a recording spectrometer and
external mirror HeNe laser. For an internal mirror HeNe laser tube,
it should be possible to rock one of the mirrors far enough to kill lasing
without permanently changing alignment. If you don't have proper measuring
instruments, don't worry, this is probably in the "Gee wiz, that's neat but
of marginal practical use" department. :)
Other Color Lines in Red HeNe Laser Output
When viewing spectral lines in the actual beam of a red HeNe laser, you may
notice some very faint ones far removed from the dominant 632.8 nm line we all
know and love. (This, of course, also applies to other color HeNe lasers.)
Demonstration HeNe Lasers, Weatherproofing
Putting Together a Demonstration HeNe Laser
For a classroom introduction to lasers, it would be nice to have a safe setup
that makes as much as possible visible to the students. Or, you may just want
to have a working HeNe laser on display in your living room! Ideally, this is
an external mirror laser where all parts of the resonator as well as the power
supply can be readily seen. However, realistically, finding one of these is
not always that easy or inexpensive, and maintenance and adjustment of such a
laser can be a pain (though that in itself IS instructive).
The Ultimate Demonstration HeNe Laser
Rather than having a see-through laser that just outputs a laser beam (how
boring!), consider something that would allow access to the internal cavity,
swapping of optics, and modulation of beam power. OK, perhaps the truly
ultimate demo laser would use a two-Brewster tube allowing for interchangeable
optics at both ends, be tunable to all the HeNe spectral lines, and play DVD
movies. :) We'll have to settle for something slightly less ambitious (at
least until pigs fly). Such a unit could consist of the following components:
Guidelines for a Demonstraton One-Brewster HeNe Laser
The following suggestions would be for developing a semi-interactive
setup whereby visitors can safely (both for the visitor and the laser)
adjust mirror alignment and possibly some other parameters of laser operation.
The type of one-Brewster (1-B) HeNe laser tube like the Melles Griot
05-LHB-570. See the sections starting with:
A One-Brewster HeNe Laser Tube
Note that the 05-LHB-570 is a wide bore tube that runs massively multi
(transverse) mode with most mirrors configurations unless an intracavity
aperture is added. This is actually an advantage for several reasons:
Weatherproofing a HeNe Laser
If you want to use a HeNe laser outside or where it is damp or very humid, it
will likely be necessary to mount the tube and power supply inside a sealed
box. Otherwise, stability problems may arise from electrical leakage or the
tube may not start at all. There will then be several additional issues to
consider:
Interesting, Strange, and Unidentified HeNe Lasers
When Your Laser Doesn't Fit the Mold
The vast majority of HeNe tubes and laser heads you will likely come across
will be basically similar to those described in the section:
Structure of Internal Mirror HeNe Lasers.
However, when rummaging through old storerooms or offerings at hamfests or
high-tech flea markets, you may come across some that are, to put it bluntly,
somewhat strange or weird. I would expect that in most cases, these will be
either really old, developed for a specific application, or higher performance
lab quality models which are just not familiar to someone used to surplus
specials. Consider these to be real finds if only for the novelty value!
Refurbishing of the lab-grade lasers may be worth the effort and/or expense
resulting in a truly exceptional (and possibly valuable) instrument. And,
simply from an investment point of view, it is amazing what some old (and even
totally useless dead) but strange lasers have fetched on places like
Ebay Auction recently. See the section:
Auctions.
Here are some descriptions of what I and others have come across:
Segmented HeNe Tubes
I have several medium power HeNe tubes that do not have a single long bore
(capillary) but rather it is split into about a half dozen sections with a
1 or 2 mm gap between them. Each of the short capillaries is fused into
a glass separator without any holes. Two of these tubes look like the more
common internal mirror HeNe tubes except for the multiple segments as shown
below:
____________________________________________
/ | | | _______ \
Anode |\ | | | \ | Cathode
.-.---' \.-----'-----..-----'-----..-----'------. '-'---.-.
<---| |:::: :===========::===========::============: :::::| |===>
'-'---. /'-----.-----''-----.-----''-----.------' .-.---'-'
|/ | | | _______/ |
\_______|____________|____________|__________/
Strange High Power HeNe Laser
This is a on-going project on finding information and restoring a strange
HeNe laser acquired by: Chris Chagaris (pyro@grolen.com). Research to
determine the specifications and requirements involved postings to sci.optics,
email correspondence, and a bit of luck - seeing a photograph of the
mysterious laser in a book on holography.
Capillary tube/external starting electrodes
Starting pulse o-------+----------------------+
_|_ _|_
|| //==================================================\\ ||
|| //=====. .==================. .=================. .=====\\ ||
||| | | |||
Mirror '|' 25K | | 25K '|' Mirror
Anode 1 +---/\/\---o +HV | | +HV o---/\/\---+ Anode 2
.---------------' '--------------.
---|-+ +-|---
| ) Main Spare ( |
---|-+ +-|---
'--------------------------------'
Gas reservoir with heated cathodes and getters
Jodon Laser Head shows the construction in more
detail.
"Hi Chris,
Unfortunately, Chris has determined that regassing will be required and he is
equipped to do this but there will be some delay in the results.....
I explained that I planned on trying to re-gas this antique and he offered to
help with what ever information I needed. It is truly refreshing to find
someone in the industry that is willing to help the amateur without an eye on
just making a profit.
The Aerotech LS4P HeNe Laser Tube
This is a 1970s HeNe laser tube contributed by Phil Bergeron who also refired
the getter (see below) before sending it to me. It was probably manufactured
just before companies realized that putting the mirrors inside the gas
envelope would work just fine and is best and cheapest.
The construction of the LS4P is generally similar to that
of modern tubes with a hollow cold cathode and narrow bore. However, it is
basically a two-Brewster laser with mirrors sealed to short glass extensions
that are the same diameter as the main tube. See
Aerotech LS4P HeNe Laser Tube.
A Really Old HeNe Laser
This one isn't really that strange but it must be quite old. The American
Optical Corporation model 3100 was a red (632.8 nm, the usual wavelength) HeNe
laser that used an external mirror tube with a heated filament for the cathode.
The Dual Color Yellow/Orange HeNe Laser Tube
Multiline operation is common in ion lasers where up to a dozen or more
wavelengths may be produced simultaneously depending on the optics and
tube current. However, most HeNe lasers operate at a single wavelength.
The only commercial HeNe lasers I know of that are designed to produce
more than one wavelength simultaneously are manufactured by
Research Electro-Optics (REO).
They have 1,152/3,390 nm and 1,523/632.8 nm models.
The Weird Three-Color PMS HeNe Laser Head
I recently picked up a surplus PMS (now
Research Electro-Optics) LHYR-0100M
HeNe laser head (with power supply) on eBay for a whopping $30 including
shipping. This model supposedly produces a pure yellow (594.1 nm) multimode
beam with a minimum power output of 1 mW. See
REO LHYR-0100M.
But mine is happily outputting the yellow (594.1 nm) and two orange (604.6
and 611.9 nm) lines (determined by splitting the beam with a diffraction
grating, something I routinely do with all newly acquired HeNe lasers!).
The PMS/REO External Resonator Particle Counter HeNe
Laser
This is a particle counter assembly labeled: ULPC-3001-CPC, 18861-1-16 with
the actual HeNe laser tube labeled: LB/5T/1M/E(HS), PMS-4877P-3594. The unit
is shown in PMS/REO ULPC-3001 Particle Counter HeNe
Laser Assembly. When I found it on eBay, the listing was for a
One-Brewster tube. However, this one is really strange. For one thing, it is
not a Brewster tube but rather a somewhat normal internal mirror
HeNe laser tube. Well, at least normal by PMS/REO standards -
mostly metal with Hughes-style glasswork at the anode-end.
Except it is a very multimode tube having an output that is rather
high (greater than 7.5 mW) for its length (11 inches between mirrors) and
power requirements 1,900 V/5.25 mA. That would be only modestly interesting.
But there is an additional mirror beyond the OC (inside in the area between
the two red dots next to the red sticker at the left) which forms an external
resonant cavity with the (internal) OC mirror. The
external HR mirror is actually coated on the end of a transparent crystal about
1 cm in length, mounted by a pair of electrodes attached to opposite sides
which most likely is piezoelectrically active and probably changes length
when a voltage is applied to it. A photodiode is mounted beyond the crystal
(far left in photo). The signal from the photodiode shows resonance effects
at several relatively low frequencies (two dominant ones are around 175
and 350 kHz). The waste beam from the HeNe laser HR mirror can actually
be seen to flicker and become much lower in power at the resonance points.
The crystal and photodiode may be used to dither the output
so that the effects of the inherent laser noise are eliminated. I doubt its
supposed to be a very high frequency because the wires to the electrodes
are not shielded. It might also be used in a feedback loop at low
frequencies.
Power with external HR?
Mirror Description Reflectivity No Yes
----------------------------------------------------------------------
HeNe laser tube HR 99.99% 0.9 mW 1.00 mW
HeNe laser tube OC 99% (assumed) 8.00 mW 80.00 mW
External HR 99.9% -- 0.09 mW
The Ancient Hughes HeNe Laser Head
These old laser heads have been showing up in various places including
eBay with one particular model number being: 3184H.
See Hughes Model 3184H HeNe Laser Head.
They date from the 1970s, some possibly quite early in the decade.
Their external appearance is unremarkable - a heavy gold-colored
cylinder about 12.25 inches long and 1.75 inches in diameter, with end-plates
each attached with 4 cap screws. Power connections to most are via
a pair of rather thin red and green wires (with red being the positive input),
though Later ones may use an Alden cable. There is
a 30K ohm, 5 W metal film internal ballast resistor which by itself is
insufficient for stable operation with most power supplies -
an external ballast of 50K to 75K is required. The power supply
that appears to be intended to drive this laser head has a 60K ballast
on board. (See the section: Hughes HeNe
Laser Power Supply for the Model 3184H Laser.) So far, ho hum. :)
The Ohmeda Raman Gas Analyzer PMS One-Brewster Laser
This unit is somewhat similar to a particle counter in that there is a very
high-Q 1-B HeNe laser tube with a second HR mirror some distance away.
In between is a space for an absolute filtered unknown gas to pass through
with 8 "viewing ports" - 4 on each side. Sensitive photon counting detectors
would normally go behind individual narrow band filters on each port,
each with a different center wavelength.
Power from <------- External Mirror -------> Intracavity
Wavelength Internal HR Type Reflectivity Power Power
-----------------------------------------------------------------------------
632.8 nm 5 uW 60 cm OC 98.0% 5,400 uW 0.25 W
" " 32 uW SP-084 HR 99.966% 50 uW 1.5 W
" " 172 uW PMS HR 99.9984% 120 uW 8.5 W !!
611.9 nm 147 uW " " --- 1,120 uW ---
604.0 nm 26 uW " " --- 0.410 uW ---
Power from <------- External Mirror -------> Intracavity
Wavelength Internal HR Type Reflectivity Power Power
-----------------------------------------------------------------------------
632.8 nm 381 uW PMS HR ??? 141 uW ???
611.9 nm 1,120 uW " " --- 93 uW ---
604.0 nm 710 uW " " --- 32 uW ---
Location Part Number Wavelength
-----------------------------------------------------------
1A BARR #4 4 375-003 7819 1 1993 781.9 nm
1B BARR #1 2 373-030 7777 2 3100 777.7 nm
1C BARR #9 2 373-024 6938 3991 693.8 nm
1D BARR #8 373-027 7421 2 2093 742.1 nm
2A BARR #8 373-026 7364 1 2293 736.4 nm
2B BARR 373-022 6753 4391 675.3 nm
2C BARR #1 373-021 6629 1 2193 662.9 nm
2D BAR #473 CAVITY 7017 1 2293 701.7 nm
Ext HR BARR #1 2 374-002 6238 4102 632.8 nm
I may be contacted via the
Sci.Electronics.Repair FAQ
Email Links Page.
![Spectra-Physics Model 084-1 HeNe Laser Tube](sp084-1.gif"){kind=link}
