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 .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 .5 mW) and somewhat larger tubes (up to approximately 35 mW) are structurally similar (except for size) to these but are not quite 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 are also available and may turn up on the surplus market as well. A few smaller HeNe tubes may exist, perhaps 4" long and 0.5" in diameter but these were probably not for continous use.
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.
HeNe lasers have been found in all kinds of equipment including:
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:
Below are just a few possibilities.
(Portions from: Chris Chagaris (pyro@grolen.com).)
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.
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 line will lase, but it takes a different pressure and a much longer tube. 632.8 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) Output HeNe Perceived Lasing Typical Wavelength Laser Name Beam Color Transition Gain (%/m) --------------------------------------------------------------------- 543.5 nm Green Green 3s2->2p10 0.52 594.1 nm Yellow Orange-Yellow 3s2->2p8 0.5 604.0 nm Orange 3s2->2p7 0.6 611.9 nm Orange Red-Orange 3s2->2p6 1.7 629.4 nm Orange-Red 3s2->2p5 1.9 632.8 nm Red " " 3s2->2p4 10.0 635.2 nm " " 3s2->2p3 1.0 640.1 nm Red 3s2->2p2 4.3 730.5 nm Border Infra-Red 3s2->2p1 1.2 1,152.3 nm Near-IR Invisible 2s2->2p4 ??? 1,523.1 nm Near-IR " " 2s2->2p1 ??? 3,391.3 nm Mid-IR " " 3s2->3p4 ???
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.
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 (for example) transition is a sharp peak, it is actually a gaussian - bell shaped - 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 * 2where:
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 +5Since 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, 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 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!
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) 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: Frequency Stabilized Single Mode 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 +1Notice 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 line width of around 1.5 GHz. 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 and are random 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. --- Sam).
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 beam splitter 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 precisely symmetric, the modes will alternate.
(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:
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.
Here is a rough idea of what transverse modes might look like for a rectangular cavity:
O OO OOO Each 'O' represents O OO O OO OOO a single sub-beam. TEM00 TEM10 TEM01 TEM11 TEM21I 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! Some examples can be found on the ECE482/582 - Optical Electronic Systems Course Page (Winter 1997) of Oregon State University:
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 shorter 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.
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.
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!
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, $10,000 to spend on a 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, several additional enhancements are needed to eliminate mode hopping and select a single output frequency. Some of these include:
Optical feedback may then be used to maintain constant frequency or constant intensity by using the sensed output beam to drive the temperature regulator or piezo transducer. For example, the Melles Griot 05-STP series 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 flat portion of the gain curve (intensity stabilized mode). For some photos of the (quite simple) 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".
It isn't really possible to convert the typical inexpensive HeNe tube into a reliable frequency stabilized 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 orthogonal to each-other. Then, stabilization is possible. 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 a single line. Whether this could ever be a reliable long term approach for a HeNe tube that normally oscillates in many longitudinal modes is questionable but the experiments could be quite interesting. However, this may work for very short tubes which may only have 1 or 2 active modes to begin with - or with old and weak ones which now just barely lase in a single mode!
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.
(Note: As of Summer, 2001, the first of these courses (Intro to Lasers) has been removed from the CORD Web site supposedly due to the expiration of their funding. Others may follow. While the courses are available for purchase in print form, It's a pity that this has happened. Print is not the same as on-line, even if it were free. I am looking into hosting them on one of my Web sites but suspect that in the end, such a request will be denied due to commercial interests winning out over availability of information.)
In particular:
See the section: On-Line Introduction to Lasers for additional CORD LEOT modules and other courses relevant to the theory, construction, and power supplies for these and other types of lasers.
The following modules would be of particular interest for HeNe lasers (all in PDF format):
EXP01 Emission and Absorption EXP03 Fabry Perot Resonator EXP06 HeNe Laser EXP20 Laser Safety EXP27 Bar Code Reader
Bellows Bellows /\/\/\ Discharge tube with external electrodes /\/\/\ || \________________________________________________/ || || | | | | | | ||===> Laser || ___ __|_|________________|_|______________|_|__ ||===> Beam || / || | | | \ || \/\/\/ || | o | \/\/\/ Adjustable || +-----------o RF exciter o----------+ Adjustable totally || partially reflecting ||<-- to vacuum system reflecting mirror mirrorEarly 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.
This fabulous ASCII rendition of a typical small HeNe laser tube should make everything perfectly clear. :-)
____________________________________________ / _________________ \ 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-----------+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 Mid-Size Internal Mirror HeNe Tube which is rated 7 mW nominal but as can be seen, was actually measured producing 7.9 mW at the time of manufacture. This is probably similar to the 7 mW Melles Griot HeNe tube listed in the section: Typical HeNe Tube Specifications. Another type of construction that is relatively common (though probably disappearing) is shown in the Hughes Style HeNe Laser Tube and similar NEC Style HeNe Laser Tube. Most common higher quality HeNe tubes will be basically similar to one of these two designs though details may vary considerably.
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.
The discharge at this end 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 it may 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. These 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!
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. 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 gasses - they ignore the getter and the getter ignores them. :-)
Should the getter spot 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 have getters (and CRTs still do).
The getter can be seen in photos of a Typical Mid-Size Internal Mirror HeNe Tube. However, no getter spot is visible. I have found many tubes where there is a getter 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 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. 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. The first HeNe laser would not have been possible without dichroic mirrors despite its length since the wide bore resulted in very low gain.
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 shape as well resulting in a positive lens which aids in beam collimation.
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.
Also see the section: Mirror Reflectances for Some Typical HeNe Lasers.
However, long high power tubes (i.e., 20 mW and up) may require fixtures to maintain mirror alignment even when the mirrors are internal. 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 sort of behavior is common or only likely with tubes that are marginal in some way.
The HRs in all cases showed greater than 99.9 percent reflectivity (T less than .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 thumbnail!)
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 or slight misalignment of the mirrors. Where this is very small or the mirrors at opposite ends of the tube happen to be oriented so they effects cancel out, the resulting polarization may indeed not be restricted to a fixed pair of orthogonal orientations as the tube heats and parts expand. The polarization may be slowly rotating or flip between arbitrary orientations.
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 use Brewster windows and so are inherently linearly polarized. A magnetic field can also be used to force linear polarization. See the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.
Linearly polarized HeNe lasers tended to be used in older laser printers (since the external modulator often required a polarized beam) and 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. Nowadays, this equipment all use diode lasers which are inherently polarized.
(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 set up that shows what's going on if you have access to some nice instrumentation: Send the beam from a two mode, randomly polarized HeNe (Example: 05-LHR-006) into a scanning Fabry-Perot interferometer (this is mucho more expensive than your basic exorbitantly priced optical spectrum analyzer). 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 Gaussian curve, the curve width being 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.
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.
Occasionally, 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.
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 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 short duration during testing).
See the chapter: HeNe Laser Power Supplies for more information and complete circuit diagrams.
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.
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 .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:
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(Bore length was estimated since the cathode-end of the capillary is not visible without X-raying the tube!)
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.
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 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-medium size HeNe laser power supplies (the longer fatter pin is negative).
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), 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.)
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. 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.
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). Therefore, this is not a major consideration for most hobbyist applications. 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 so avoid placing any stress on the mirrors!
Shelf life of non-hard sealed tubes is limited by diffusion of the Helium atoms out and air leakage in. Helium atoms are slippery little devils. They 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.
The gas doesn't 'wear out'. A HeNe tube, when properly connected has much of its heat 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. 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 themselves.
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 .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:
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 and on Optlectra's Open Cavity HeNe Laser Tube Page.
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).)
"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."
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 know if this one, which is quite old, has these characteristics.) The total length of 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. :)
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.
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.
IR (infra-red) HeNe laser tubes are manufactured as well (1,523.1 nm is most common probably because this wavelength is useful for testing of fiber optic data transmission systems). The other two common IR wavelengths are 1,152.3 and 3,391.3 nm. However, an invisible beam just doesn't seem as exciting!
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 'other-color' HeNe tubes (much lower - under 10 mW) are also available. However, these will be very large and very expensive.
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 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:
Wavelength * 4 Divergence angle (half of total) in radians = -------------------- pi * Beam DiameterSo, 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.
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. (However, the chance of this is probably minimal since anything making its way back inside from reflection from an AR coated surface AND through the 99%+ reflective OC would be extremely weak).
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.
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:
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!!!!).
(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.
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) of the neighboring HeNe laser lines (e.g., for other color HeNe lasers) may be lasing though it will probably be much weaker 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 a 'defective' yellow (594.1 nm) HeNe tube that also produces a fair amount of orange (611.9 nm).
(From: Stephen Swartz (sds@world.std.com).)
Lasing of certain HeNe tubes at 640.1 nm is a known phenomenon and not just a hallucination. The 640.1 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 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: 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.
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":
"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."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.
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).)
"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.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 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:
N S N S N S N S || //======================================================\\ || || //======. .========================================. .======\\ || S ||| N S N S N S ||| N '|' '|'Alternatively, North and South poles may face each other:
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.
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 mmWhere:
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 (when new). Individual sample tubes may be much higher. 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).)
"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."
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):
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 mmMelles 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.
Optlectra's Product Search may be able to locate specifications for helium-neon and many low and medium power argon ion laser heads and tubes from multiple manufacturers. These include internal mirror as well as Brewster window and flat window types. (Unfortunately, for some reason the search results don't include the manufacturer. One way to at least increase the odds that a model number applies to your tube is to compare the physical dimensions.) These are based on manufacturer's specifications so they should be accurate. However, even with over 500 models of HeNe lasers in their database, the coverage of the search is still somewhat limited.
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:
For other manufacturers like Spectra-Physics, the model numbers are totally arbitrary! (See the section: Some 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. 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. The IR may be visible with a video camera (assuming it doesn't have an IR blocking filter) or by using one of the IR detector circuits or an IR detector card as discussed with respect to IR laser diodes. 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 only one you can detect with a bare eyeball.) 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. Note that 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.
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 all done with mirrors and they are an integral part of the tube. Therefore, your options are severely limited - as in: There are none. 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 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. But, finding one of those magic 'defective' tubes 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 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.
Since the mirrors used in all HeNe lasers are dichroic - 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.)
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 Broad Band (ROY) Whitish-Gold Blue IR 1,523 nm Light Green Light Magenta Light Green Light Magenta IR 3,391 nm Gold (Metal) Coated Neutral ClearThe entry labeled 'Broad Band' 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.
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 611.9 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 gasses 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 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 a 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. See the section: A Green One-Brewster HeNe Laser for more info.
(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.
Red (632.8 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LHR- ----------------------------------------------------------------------------- .5 mW .47 mm 1.70 mR 1200 MHz 1.29/5 kV 3.3 mA 19/135 mm 002(1) .5 mW .49 mm 1.70 mR 1040 MHz 1.25/5 kV 4.5 mA 29/152 mm 700 .5 mW .46 mm 1.77 mR 1063 MHz 1.32/5 kV 4.0 mA 25/150 mm 213 .8 mW .46 mm 1.77 mR 1063 MHz 1.32/5 kV 4.0 mA 25/150 mm 211 1.0 mW .53 mm 1.50 mR 883 MHz 1.47/8 kV 4.5 mA 29/178 mm 900 1.0 mW .59 mm 1.35 mR 687 MHz 1.79/8 kV 6.5 mA 37/226 mm 111 2.0 mW .59 mm 1.35 mR 687 MHz 1.79/10 kV 6.5 mA 37/228 mm 121 2.0 mW .72 mm 1.10 mR 612 MHz 1.85/10 kV 6.5 mA 29/255 mm 080 2.0 mW .76 mm 1.06 mR 636 MHz 1.71/10 kV 5.0 mA 30/250 mm 073 2.5 mW .52 mm 1.53 mR 822 MHz 1.77/10 kV 4.5 mA 25/198 mm 691 4.0 mW .80 mm 1.00 mR 435 MHz 2.35/10 kV 6.5 mA 37/353 mm 140 5.0 mW .80 mm 1.00 mR 438 MHz 2.29/10 kV 6.5 mA 37/353 mm 151 7 mW 1.02 mm .79 mR 373 MHz 2.65/10 kV 6.5 mA 37/410 mm 171 10 mW .65 mm 1.24 mR 341 MHz 2.64/10 kV 6.5 mA 37/440 mm 991 12 mW 1.20 mm 3.40 mR NA-MM 2.09/10 kV 6.5 mA 37/350 mm 185(2) 16 mW 1.47 mm 1.40 mR NA-MM 2.48/10 kV 7.0 mA 37/464 mm 981(2) 17 mW .96 mm .83 mR 267 MHz 3.70/12 kV 7.0 mA 37/600 mm 925 25 mW 1.23 mm .66 mR 165 MHz 5.10/15 kV 8.0 mA 42/930 mm 827 35 mW 1.23 mm .66 mR 165 MHz 5.10/15 kV 8.0 mA 42/930 mm 927Notes:
Both random and linearly polarized models are available. The only other difference in specifications between these is the price - those that are linearly polarized are considerably more expensive. The price for a complete linearly polarized laser head was 10 to 15 percent higher in a catalog I have so you can imagine how much more the tube itself costs since that price differential is virtually all in the tube (at least in terms of manufacturing cost)!
Green (543.5 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LGR- ----------------------------------------------------------------------------- .2 mW .63 mm 1.26 mR 732 MHz 1.56/8 kV 4.5 mA 29/215 mm 025 .5 mW .80 mm 1.01 mR 438 MHz 2.39/10 kV 6.5 mA 37/351 mm 151 .8 mW .89 mm .92 mR 373 MHz 2.62/10 kV 6.5 mA 37/410 mm 173 1.0 mW 1.3 mm 1.00 mR NA-MM 1.87/10 kV 6.5 mA 37/351 mm 161 1.5 mW .86 mm .81 mR 328 MHz 2.75/10 kV 6.5 mA 37/475 mm 193 2.0 mW .86 mm .81 mR 328 MHz 2.75/10 kV 6.5 mA 37/475 mm 393
Yellow (594.1 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LYR- ----------------------------------------------------------------------------- .35 mW .63 mm 1.26 mR 732 MHz 1.62/8 kV 4.5 mA 29/215 mm 025 .75 mW .80 mm 1.01 mR 438 MHz 2.43/10 kV 6.5 mA 37/351 mm 151 2.0 mW .75 mm .92 mR 373 MHz 2.59/10 kV 6.5 mA 37/410 mm 173 2.0 mW 1.17 mm 1.00 mR NA-MM 2.09/10 kV 6.5 mA 37/351 mm 161
Orange (611.9 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LOR- ----------------------------------------------------------------------------- .5 mW .63 mm 1.26 mR 732 MHz 1.66/8 kV 4.5 mA 29/215 mm 025 2.0 mW .80 mm 1.01 mR 438 MHz 2.49/10 kV 6.5 mA 37/351 mm 151 4.0 mW 1.17 mm 1.00 mR NA-MM 2.07/10 kV 6.5 mA 37/351 mm 161
Infra-Red (1,523 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LIR- ----------------------------------------------------------------------------- 1.0 mW 1.26 mm 1.59 mR 438 MHz 2.49/10 kV 6.5 mA 37/351 mm 151 1.0 mW 1.33 mm 1.48 mR 373 MHz 2.97/10 kV 6.0 mA 37/410 mm 171
Infra-Red (3,391 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LFR- ----------------------------------------------------------------------------- 1.0 mW .83 mm 1.60 mR 438 MHz 2.50/10 kV 6.0 mA 37/351 mm 151
Note: Some of the listed values for divergence in particular appear to be questionable. For example, for the same beam diameter, diffraction limited divergence should be proportional to wavelength. The discrepency for the 3,391 nm IR tube is particularly striking. Either the divergence or beam diameter are almost certainly incorrect.
Brewster angle window HeNe tubes:
Minimum Supply Supply Nominal Number Output Voltage Voltage Tube Tube Size Model of Power Tube Only Rb=68K Current Diam/Lgth 05-LHB- Windows ------------------------------------------------------------------- 1.0 mW 1,430 V 1,870 V 6.5 mA 37/222 mm 270 1 1.0 mW 1,460 V 1,900 V 6.5 mA 37/253 mm 290 2 3.5 mW 1,080 V 1,520 V 6.5 mA 37/265 mm 370 1 4.0 mW 1,030 V 1,470 V 6.5 mA 37/265 mm 570 1 ??? mW 1,??? V 1,??? V 6.5 mA 37/265 mm 580 1 6.0 mW 1,430 V 1,870 V 6.5 mA 37/351 mm 670 1The most common application for one-Brewster HeNe tubes was probably for particle counting since by using an external high quality HR mirror, the intracavity flux can be several watts which makes a speck of anything stand out! (Some large one-Brewster HeNe tubes can do as much as 100 W intracavity, not these!). Passing the air/gas/whatever flow through the cavity of a one-Brewster HeNe laser is similar to passing it through the output beam of a high power laser - at a fraction of the cost (and it's much safer as well since if anything macroscopic in size (like an eyeball or piece of paper) were to block the intracavity beam, lasing simply stops with no damage to vision and no risk of fire!
The LHB models all have HR mirrors that are probably optimal for 632.8 nm (red) though newer versions, at least, may be quite broadband and better than 99.9 percent from 590 to 680 nm so operation at some of the non-632.8 nm wavelengths may be possible. However, older versions may not have such nice HRs.
Other variations on these tubes are also produced (though they may be special order). I was given an 05-LGB-580 which has an HR optimized for 543.5 nm (green). With an external green HR, the behavior is very similar to the red version but with loads of circulating green photons instead of red ones. :)
The 05-LHB-370, 05-LHB-570, 05-LHB-670, and 05-LHB-580 have wide bores and generally operate with multiple transverse modes to achieve maximum intracavity power in particle counting applications. The 05-LHB-270 and 05-LHB-290 have narrow bores like most conventional HeNe tubes. (The 05-LHB-270 appears physically similar to an 05-LHR-120 except for the Brewster window at one end.) The model 05-LHB-570 is the one-Brewster HeNe tube used in the CLIMET 9048 one-Brewster laser head described in the section: A One-Brewster HeNe Laser Tube. You can't tell from the model numbers but both Melles Griot and Hughes style designs may be used. For example, the 05-LHB-570 looks like a normal Melles Griot tube but with a Brewster angle window frit sealed to the metal end-cap instead of an OC mirror. The 05-LHB-580 looks like a Hughes style tube, but with an optically contacted Brewster window instead of an OC mirror (though some Hughes style polarized HeNe tubes are just one-Brewster tubes with an OC mirror attached to a glass tube that slips over the Brewster stem and is itself glued in place). Thus, the 05-LHB-580 is actually a much higher quality (and more expensive) tube than the 05-LHB-570 but you can't tell this from the catalog listing! Here are diagrams of each type:
One possible explanation of why the Hughes style design is used for the high quality tubes with optically contacted Brewster windows is that since Hughes already produced HeNe tubes with a glass Brewster stem (as noted above), when Melles Griot took over the Hughes HeNe laser product line, making the modifications for the graded seal to accommodate the fused silica Brewster stem (needed to match the expansion coefficient of the fused silica window) was probably easier than starting with a metal end-cap.
Zero degree AR coated window HeNe tubes:
Minimum Supply Supply Nominal Number Output Voltage Voltage Tube Tube Size Model of Power Tube Only Rb=68K Current Diam/Lgth 05-WHR- Windows ------------------------------------------------------------------- 4.0 mW 1,030 V 1,470 V 6.5 mA 37/269 mm 570 1 6.0 mW 1,670 V 2,110 V 6.5 mA 37/351 mm 252 2 8.0 mW 1,670 V 2,110 V 6.5 mA 37/351 mm 183 1Rather than mirrors, one or both ends of these HeNe tubes have optical flats with very high quality AR coatings to permit the use of external mirrors. One advantage of this arrangement is that external optics can be used to control polarization (the output beam of Brewster tubes is always linearly polarized and can't be changed).
The 05-WHR-252 and 05-WHR-183 appear to be identical except for the number of windows - and the loss of 2 mW with the two window version!
Minimum Laser Wave- Mirrors Output Exciter Original Model length Int/Ext Power Model Price Description/Comments ------------------------------------------------------------------------------ 107 632.8 nm E 30 mW? 207 $ ?,??? Similar to 127 (1) 115 632.8 nm E 5 mW? 200 $ ?,??? RF excited, 24" resntr. 120 632.8 nm E 6 mW 256 $ 1,980 Small lab laser (2) -01 1,152 nm E 1 mW " $ 2,800 " " -02 3,391 nm E 1.25 mW " $ 2,800 " " 122 632.8 nm E 5 mW? 253A $ ?,??? Short version of 124 (3) 123 632.8 nm E 10 mW? I $ ?,??? Between 120 and 124 124B 632.8 nm E 15 mW 255 $ 4,900 Popular lab laser (3) -01 1,152 nm E 2 mW " $ 5,500 " " -02 3,391 nm E 5 mW " $ 5,500 " " 125A 632.8 nm E 50 mW 261A $ 16,000 Huge-head >125 lbs. (4) -01 1,152 nm E 10 mW " $ 17,500 more than 6 feet long. -02 3,391 nm E 10 mW " $ 17,500 " " 127 632.8 nm E 35 mW I $ ??,??? 39 inch resonator (1) 130B 632.8 nm E 2 mW? I $ ??? Self contained (5) 132 632.8 nm I 1.00 mW I $ ??? Self contained (6) 133 632.8 nm I 0.95 mW? 233 $ ??? Separate rect. head (5) 134 632.8 nm I 3 mW? I $ ??? Self contained 135 632.8 nm I ??? mW ??? $ ??? Separate rect. head (5) 142 632.8 nm I 4 mW ??? $ ??? " " 147 632.8 nm I 8 mW 247? $ ??? Separate cyl. head 155 632.8 nm I 0.5 mW I $ 310 Educational laser (6) 156 632.8 nm I ??? mW I $ ??? " " 157 632.8 nm I 3 mW I $ 525 Self contained 159 632.8 nm I 5 mW I $ 630 " " 102R 632.8 nm I 2 mW 212 $ 610 Cyl. head, rand. pol. 102P 632.8 nm I 1.5 mW " $ ??? Cyl. head, lin. pol. 105R 632.8 nm I 5 mW 215 $ ??? Cyl. head, rand. pol. 105P 632.8 nm I 5 mW " $ ??? Cyl. head, lin. pol. 117A 632.8 nm I 1 mW 117A $ 3,500 Stabilized (7)Notes:
It should be possible to possible to obtain orange (611.9 nm), yellow (593.9 nm), and green (543.5 nm) output with similar modifications (at least for the longer lasers), though the gain of these lines is only a fraction of that for the red or IR lines (1152.3 nm and 3391.3 nm) so output power will be lower.
Some photos of these lasers can be found in Laser Equipment Gallery under "Spectra-Physics Helium-Neon Lasers".
Note: The specifications for the SP-124A and SP-125, below, were copied from an almost illegible scan of a fax of a copy of the original product brochure. Corrections are welcome! The specifications for the SP-120 were copied from an original user manual which, however, didn't list the tube operating voltage or current, so these values were sort of, well, guessed. :)
Spectra-Physics Laser: SP-120 SP-124B SP-125A ------------------------------------------------------------------------------- OUTPUT Wavelength (nm): 632.8 632.8 1152.3 3391.3 632.8 1152.3 3391.3 Minimum Power (mW): 5.0 15 2.5 5.0 50 10 10 BEAM CHARACTERISTICS Beam Diameter (mm): .65 1.1 1.4 2.5 1.8 2.4 4.1 Beam divergence (mR): 1.7 .75 1.0 1.5 .6 .8 1.4 RESONATOR CHARACTERISTICS Transverse Mode: TEM00 Degree of Polarization: 1000:1 Angle of Polarization: Vertical (+/-5 Degrees except SP-120, +/-20 Deg.) Resonator Configuration: Long Radius Resonator Length (cm): 39 70.1 177.0 Longitudinal Mode Spacing: 385 MHz 214 MHz 85 MHz PLASMA TUBE Plasma Excitation: 3.7 kV, 7 mA 5 kV, 15 mA 6 kV at 25 to 35 mA (RF Opt: 15 W at 46 MHz) Starting Method: ~8 kV ~12 kV Trigger pulse on isolated (Direct from Exciter) bar adjacent to tube. AMPLITUDE STABILITY Beam Amplitude Noise: <.3% RMS <.3% RMS <2% RMS (RF: <.5%) Beam Amplitude Ripple: <.5% RMS <.2% RMS <.5% RMS (RF: <.6%) Long Term Power Drift: <5% over 8 hours and 10 °C Warmup Time: 30 Minutes 30 Minutes 1 Hour ENVIRONMENTAL CAPABILITY Operating Temperature: 10 to 40 °C Operating Altitude: Sea Level to 3,000 m (10,000 ft.) Operating Humidity: Below Dew Point POWER REQUIREMENTS Power Supply: 115/230 VAC, 50/60 Hz, +/-10% Exciter Model (DC): SP-256 (1) SP-255 (2) SP-261A Input Power: 50 W 125 W 456 W PHYSICAL CHARACTERISTICS Laser Head Size: 3.26" (W) x 3.26" (W) x ??? (W) x 3.66" (H) x 3.66" (H) x ??? (H) x 18.48" (L) 32.00" (L) ??? (L) Laser Head Weight: 7.5 lb 25 lb 100 lb Power Supply Size: 7.25" (W) x 7.25" (W) x 13" (W) x 3.72" (H) x 3.72" (H) x 6" (H) x 9.88" (D) 9.88" (D) 18" (D) Power Supply Weight: 7.5 lb 7.5 lb 30 lbNotes:
Actual power from these lasers may be much more than their ratings would indicate, especially when new: greater than 35 mW for the SP-124B and up to 200 mW (!!) for the SP-125A. (However, I don't know how likely such 'hot' samples, especially of the SP-125A, really were.)
There is also a model 127 (OEM versions: SP-107 and SP-907) with the following partial specifications (632.8 nm). Beam diameter: 1.25 mm, divergence 0.66 mrad, length 38.75", height and width: about 4", power requirements: 5 kV, 11.5 mA, starting voltage: 12 kV + 6 kV pulse.
Mirror sets for green (543.5 nm), yellow (594.1 nm), and orange (611.9 nm) were available for the longer lasers. (The SP-120 may be too short for the low gain green line.) There were also tunable versions of the SP-125 (and possibly others) which used a Littrow prism in place of the HR mirror.
See the following sections for more information on these lasers.
Depending on specific model, the SP-107/127 has a minimum output power of 25 or 35 mW but may do much more when new. The following is from a Spectra-Physics datasheet (only the specs for red are shown but other wavelengths should be possible by substituting optics):
Spectra-Physics Laser: SP-107B ----------------------------------------------------------------- OUTPUT Wavelength (nm): 632.8 Minimum Power (mW): 25 or 35 BEAM CHARACTERISTICS Beam Diameter (mm): 1.25 Beam divergence (mR): 0.66 RESONATOR CHARACTERISTICS Transverse Mode: TEM00 Degree of Polarization: 500:1 Angle of Polarization: Horizontal (+/-5 Degrees) Resonator Configuration: Long Radius Beam Waist Location: Outer surface of output mirror Resonator Length (cm): 95 Longitudinal Mode Spacing: 161 MHz PLASMA TUBE Type: Hard-seal, cathode in side-arm Operating Voltage: 5 (+/- 0.4) kV, 11.5 (+/- 0.5) mA Starting Voltage: ~15 kV Lifetime: Greater than 20,000 hours AMPLITUDE STABILITY Beam Amplitude Noise: <1% RMS Beam Amplitude Ripple: <1% RMS Warmup Time: 20 Minutes (95% power) ENVIRONMENTAL CAPABILITY Operating Temperature: 10 to 50 °C Operating Humidity: 5-90% non-condensing POWER REQUIREMENTS Power Supply: SP-207A (110/220 VAC +/- 10%) SP-207A-1 (100/200 VAC +/- 10%) SP-207B (90-130 VAC or 180-260 VAC) PHYSICAL CHARACTERISTICS Laser Head Size: 3.7" (W) x 3.7" (H) x 38.75" (L) Laser Head Weight: 23 lb Power Supply Size: 2.4" (W) x 1.4" (H) x 10" (L) Power Supply Weight: 3 lb
Photos of a SP-120 laser head and the SP-120 resonator and tube can be found in the Laser Equipment Gallery (version 1.85 or higher) under "Spectra-Physics Helium-Neon Lasers". The complete user manual for the SP-120 laser with SP-256 exciter can be found at Lasers.757.org, Manuals.
The resonator uses three-screw adjustable mirror mounts for coarse alignment (tweaking these is a true pain!) and pan/tilt adjustments on the mirror mount/tube holders for fine adjustment and tube centering. It is possible to replace the tube in about 5 minutes without requiring major mirror re-alignment.
The resonator is constructed from 4 pieces of thick very nicely machined aluminum plate bolted together to form an L-channel with end-plates. This is a very rigid structure. It is supported at only three points and essentially floats inside the outer case (the "Stabilite" name as discussed for the SP-124 laser, below) which isolates the resonator from external stress (or so it is claimed).
The SP-124 laser head is a box about 76 mm (H) x 76 mm (W) x 813 mm (L) (3" x 3" x 32"), nicely massive for its size. There are threaded beam apertures at both ends though the HR is backed by a solid aluminum plate so I don't think much light would ever get through that even if there was leakage through the mirror!
This is one of SP's "Stabilite" series lasers. This approach to frequency stabilization is based on a mounting system that employs optimally located pivots in an attempt to minimize the coupling of gravitational and vibrational torques and other distorting forces to the resonator cavity itself. In the SP-124, most of the mass of the laser head is in such an optimally mounted heavy solid frame with roughly an L cross section that runs nearly the full distance between the mirror mounts and attached to each of them at three points.
Adjustments accessible externally at each end of the laser allow the beam alignment (X and Y) to be tweaked very accurately by moving the entire optics chassis relative to the head mounting studs (which accept 6-32 screws or rubber feet). The adjustment scheme is sort of interesting (to me, at least): A V-shaped block (bolted to the rosonator and case) sits between a pair of wedges (part of the mounting stud assembly) that can be moved up and down via a pair of screws (call them A and B) and retained in position by a stiff spring. Rotating both A and B equally in the same direction moves the beam in Y; rotating A and B equally in opposite directions moves it in X. The setting may then be locked.
The external mirror HeNe tube is clamped in rubber mounts at its ends and also stabilized at the 1/3 and 2/3 (approximately) positions. Metal bellows join the tube mount brackets to the mirror mounts and, in conjunction with the rubber seals, prevent dust and dirt from getting on the inside surfaces of the mirrors and on the Brewster windows. The mirror mounts have hex head bolts for adjustments with set screws to prevent their settings from changing over time. An additional metal bellows joins the OC to the treaded output aperture.
The HeNe tube itself is a bare capillary about 7 mm OD with a 1.1 mm ID (no, I didn't measure it - just trust the specs!). The cathode, getter assembly, and HeNe gas reservoir is in a side-arm at the output-end of the laser bent to run parallel to the bore. It is about 32 mm x 178 mm (1-1/4" x 7") with the 'can' electrode nearly filling the glass envelope. The anode is (naturally) at the other end of the bore along with the three 9.8K ohm (5 W at least) ballast resistors also in a parallel side-arm inside the gas envelope as apparently is the case with other Spectra-Physics lasers of this era. Interesting, they are just ordinary Ohmite power resistors. I guess this approach does reduce problems with high voltage insulation breakdown but it would be a shame if the laser went bad because a $.50 resistor failed and could not be easily replaced! The total value of about 30K ohms would seem to be rather low but might have been selected to match the needs of the SP-253A exciter (see below) or additional external ballast resistors may be required. The SP-124B version of this laser may use a more normal 81K ballast resistance.
A series of relatively weak (e.g., refrigerator note holder strength) ceramic magnets 14 mm (W) x 22 mm (L) x 6 mm (H) (9/16" x 7/8" x 5/16") are mounted in close proximity under (15 magnets) and on one side (24 magnets) all along the length of the bore wherever they fit. (See the section: Magnets in High Power or Precision HeNe Laser Heads for an explanation of their purpose.) The approximate arrangement is shown below. I may have the poles backwards (which is of course irrelevant). A cheap pocket compass came in handy to determine the pole configuration!: The magnets were positioned with their broad faces about 2 mm from the bore.
Magnets N_S_N_S_N_S_N_S_N_S S_N_S_N_S_N_S_N N_S_N_S_N_S_N_S_N on side |_|_|_|_|_|_|_|_|_| |_|_|_|_|_|_|_| |_|_|_|_|_|_|_|_| (24) ------------------------------------------------------------- HR end ============================================================= OC end of bore ------------------------------------------------------------- of bore Magnets N_S_N S_N_S N_S N_S S_N S_N S_N S_N S_N S_N_S N_S_N below |_|_| |_|_| |_| |_| |_| |_| |_| |_| |_| |_|_| |_|_| (15) N_S_N +-----+-----+ Where: |_|_| = 2 adjacent ceramic magnets: |N S|S N| +-----+-----+I assume that the only reason there aren't 24 magnets below the tube is that the holes in the Stabilite frame got in the way.
Apparently, there must have been a couple of power supply options for the SP-124. Most of these lasers appear to use the Spectra-Physics Model 255 Exciter (SP-255). This is a traditional HeNe power supply providing operating and start voltage through a high voltage BNC connector. However, the laser I have apparently is supposed to use an SP-253A Exciter, a model for which no one (including Spectra-Physics) seems to have any information or even acknowledge exists though I have since found out that the SP-122 laser, a model slightly shorter than the SP-120 but built more along the lines of an SP-124, may have also used the SP-253A. For more information on what I have found out so far about the exciter, see the section: Spectra-Physics Model 253A Exciter (SP-253A).
Unfortunately, on the system I obtained, the boost/start module (which is what I assume was supposed to be inside the head to attach to the exciter) had been ripped out with the cable just chopped off and thus I can't even determine what was there originally. So, I removed the multiconductor cable and replaced it with a HV coax (terminated with a standard Alden connector) and wired it directly to the tube anode terminal and chassis ground (recall that the ballast resistors are inside the tube. Yes, I know, the 30K ballast resistance may be too low for use with the SP-255!)
Using my SP-255 to power the head, I get a nice pink glow in the bore (more red than orange indicating a rise in pressure from slow leakage over the years) but as expected, no coherent light. The low ballast resistance is fine as far as maintaining a stable discharge (I don't know if this would still be the case if the gas pressure in the tube were correct). Maybe someday in the far distant future after that hot place freezes over AND those pigs start flying, I will get around to regassing the tube! :)
The SP-125A tube has a common cathode in the middle of the tube with two anodes, one at each end. The dual discharges are driven from its SP-261A Exciter which provides 6 kV at up to 35 mA. (An SP-250 Exciter may also have been compatible with this laser.)
With a bit of rewiring of the laser head, one could feed the anodes separately reducing the individual current requirements so that a pair of power supplies similar to the SP-255 could be used. With this sort of scheme, it should also be possible to selectively power only one of the discharge paths for reduced beam output if desired. Yes, I know, why would you ever want *less* power? :)
Two sets of ballast resistors in the laser head totaling 87K ohms (75K+12K) provide the operating voltage to each of the anodes of the dual discharge tube. They are located between the anodes and chassis ground (The SP-261A's output is negative with respect to ground. Thus, ground is the positive supply voltage). The HeNe tube's single cathode is attached directly to the negative output of the SP-261A.
The starter operates in a manner similar to that of the method of triggering the xenon flashlamp in a typical electronic flash unit or solid state laser power supply - by pulsing an external electrode in close proximity to the HeNe tube bore. The whole tube is supported by metal rods which are insulated from the cavity structure by nylon disks. One of the rods is the trigger electrode. The starter runs off a voltage from the 75K/12K ohm taps of both ballast resistors ORed together so that it repeatedly generates a trigger pulse until BOTH discharges have been successfully initiated.
The SP-261A also has a low power RF output (this isn't the same as the RF power supply option mentioned below) which drives a pair of plates in proximity to the HeNe tube. The RF is supposed to stabilize the laser power (presumably by some sort of discharge dithering process). However, the RF apparently also results in interference with local radio stations. :(
An RF power supply option is/was also available. (Possibly some version of the SP-200 though the specs don't quite match for the one I have. See the section: Spectra-Physics Model 200 Exciter (SP-200).) This would replace theSP-261A and starter entirely by driving the tube directly with radio frequency energy - 15 W at 46 MHz. Note the greatly reduced power to the tube compared to the 150 to 210 W for the DC discharge! The drive is applied via coax from a BNC connector on the back of the laser to a resonant circuit about midway in the laser head. The two phases of the output of the resonant circuit connect to a pair of 0.1 inch diameter bars running the length of the tube about 0.6 inches from the centerline suspended from insulators.
Unfortunately, many SP-125s that appear as surplus are not good for more than long boat anchors (or as a parts unit for salvage of the optics and frame). Unless the tube has been replaced relatively recently, being soft-seal, it has likely leaked to the point at which the getter can no longer clean up the contamination. Refilling is the only option and that cost would make what you paid for the laser look like pocket change. And, refilling a HeNe tube is generally not a realistic basement activity. So, if you come across an SP-125 at a low price, unless it is guaranteed to lase, buyer beware. An SP-125 sold "as-is" almost certainly means the seller couldn't get it to work (not that everything possible wasn't tried) since they likely know it is worth 10 times as much in operating condition!
Also see the section: Spectra-Physics 120, 124, and 125 HeNe Laser Specifications and Spectra-Physics Model 261A Exciter (SP-261A).
(From: Marco Lauschmann (mla@sbk-ks.de).)
The SP125A is absolutely beautiful with much chrome and a metallic blue cover! It is nearly 2 meters long and looks like an older large-frame argon ion laser. A Spectra-Physica scientist noticed that this device will deliver twice the rated power with no problems. Others have claimed as much as 200 mW for the red (632.8 nm) model!
The laser tube is about 20 inches long with separate bore and gas chambers side-by-side. The bore uses rather thin glass tubing and is a very large diameter for a HeNe laser - about 3 to 4 mm ID - consistent with early HeNe laser technology. The laser head is nicely mounted with lots of fine machined hardware. It has no IR suppression magnets. There are two RF connectors on the side for the Spectra-Physics model 200 RF-type power supply. One of the connectors is for the actual RF signal; the other is for starting. There is an impedance matching network located under the "tube deck". This consists of a series LC circuit (C is adjustable for peaking the tuning) between the RF input and case with the output taken from the junction of the L and C. The RF drives a dozen or so electrodes with alternating polarities in close proximity to the tube bore. The start connection goes to the input of a potted transformer which produces a several kV pulse when the "Start button" on the exciter is pressed. The starting pulse goes to a separate small electrode clamped near the center of the tube bore.
The laser has external adjustable mirrors mounted on the very solid precision milled black anodized aluminum box support structure. Both mirrors have screw adjustments for coarse alignment not accessible from outside the case without removing the end-plates. The front mirror also has external fine adjustments in X and Y via two precision Lufkin micrometers and the rear mirror is mounted on a precision slide with an external micrometer adjustment for mirror separation (try to find that on any modern laser!). I don't know if the intent of this axial adjustment (over 1/2" of travel) was to fine tune the longitudinal or transverse modes or both. Since the resonator frame would experience little if any heating (and expansion), the micrometer could be used to center a longitudinal mode and maximize output for this low gain laser. In addition, the larger movement could possibly be used to select a particular transverse mode pattern, though actually achieving TEM00 operation in such a wide bore laser might not be possible.
The power supply for the SP-115 is a high quality 15 to 25 watt 40.68 MHz RF source consisting of a crystal controlled oscillator and a power amplifier using a 4x150 tube. All active elements are tubes, of course, but out of character for the era, the oscillator and driver are built on a printed circuit board. Overall, the system looks like something straight out of the ARRL Handbook (which is probably where the design came from!).
Not surprisingly, on the sample I have, the tube has leaked and only produces a weak purple glow when the RF is turned on. The getter has the "white cloud of death" syndrome and without an aluminum can cathode, there is no possibility of getter action anywhere else. (Not that a tube this far gone would have any chance of revival in any case. The tube would make an ideal candidate for refilling since the vacuum could be breeched by cutting the exhaust nipples at either end of the gas ballast without contaminating the Brewster windows.) The SP-200 does do a nice job of lighting 20 W fluorescent lamps and most likely screwing up radio reception in the neighborhood. :)
The tube inside the lasers in the photos is the typical small Spectra-Physics side-arm type (like those in the SP-155 and other similar lasers also shown on the Web page above) but with Brewster windows instead of mirrors. However, earlier versions may look a bit different with a side-arm for the anode as well and really early versions (SP-130, no B) actually used a heated filament for the cathode (though for some reason, the schematic of the SP130 with the heated filament is dated slightly later than the schematic of the SP130B with the cold cathode design).
Based on the length of the tube, I would have expected its output power to be in the 2 to 5 mW range. However, from the specifications in the manual, it turns out to be only be 0.75 mW when used with the hemispherical mirror configuration (planar and 30 cm radius of curvature), but capable of a TEM00 beam despite its wide bore (2.5 mm). With a confocal configuration using a pair of 30 cm mirrors, the beam is multimode (non-TEM00) and output power may be as much as 1.5 mW.
When I obtained the first of these lasers (the one in the top two photos), the tube actually still lit up but there was no output beam. At first I thought it might even have a chance of working since the discharge color looked sort of reasonable, though somewhat less intense than I would have expected. Fiddling with the optics didn't yield any positive results. And then, when I wasn't looking, the discharge went out! As best I can tell, a crack must have opened somewhere in the tube and it is now at much higher pressure or up to air - bummer! I can find no visible damage or any evidence of this except that it won't start even on a much larger HeNe power supply and shows no signs of a glow from an RF source. So far, the getter hasn't changed color.
I don't think this laser was ever really alive - the tube was probably gassy or helium deficient or something but I still can't explain what happened. The only place it could have leaked that I can't see is under the anode connection which is kind of potted but there shouldn't have been any heat there to cause such a problem.
And to compound my disappointment, I dinged the OC removing the tube. Enough of it may be left to still work but the optics appear to be soft-coated as the AR coating came off totally by just barely touching it. However, that still hurts. Sometimes, you just have one of those days. :(
The laser in the third photo was DOA with an up-to-air tube, seriously damaged mirrors (coatings mostly gone), and evidence of prior dissection attempts (cut wires, etc.). The tube in that one is probably one of the earliest non-heated filament types with a small cathode and separate side-arm for the anode.
However, I have since obtained a third SP-130B which originally had a red/blue discharge. But while running for a few hours, the color gradually changed to a mostly correct white-ish red-orange. And, with an optics cleaning and alignment, this SP-130B actually lases. The output power is not up to spec - about 0.25 mW at maximum current (it's rated at 0.75 mW) - but that's still a bit amazing considering its age. See the section: Reviving a Spectra-Physics Model 130B Antique Laser for details.
The internal power supply accounts for much of the weight and most of the height of the box and consists of:
For more info and schematics, see the section: Spectra-Physics Model 130 HeNe Laser Power Supply (SP-130).
Now, the question becomes: Do I leave the dead ones intact as examples of antique lasers or replace their tube and optics with modern 2 mW barcode scanner tubes (that's about the largest that would fit height-wise, a 1 inch diameter tube) to have working lasers? I guess there's nothing special about 2 mW HeNe lasers so leaving them intact would be the best option. (I can hear the antique connoiseurs breathing a collective sigh of relief!) Who knows, maybe someone will drop replacement tubes and an OC in my lap someday! :)
Photos of a typical SP-155 can be found in the Laser Equipment Gallery under "Spectra-Physics Helium-Neon Lasers".
The HeNe laser tube is the classic Spectra-Physics side-arm design but with the anode electrode mounted about half way along the length of the bore (the same tube with the anode mounted at the end would produce around 4 to 5 mW).
The power supply is a basic transformer/doubler/multiplier design with a single transistor current regulator.
Note that other manufacturers sell (or have sold) lasers identical in appearance to the SP-155. For example, there is a Uniphase model 115ASL-1 and a Liconix L-388 (even though it is made by Uniphase). However, these use a hard-seal Uniphase barcode scanner HeNe tube (similar to a model 098 with a tiny collimating lens glued to its OC to reduce divergence) rather than the fancy Spectra-Physics side-arm tube we know and love. But their power supplies are similar or identical to that used in the SP-155 and what follows should still apply. (There is also a Spectra-Physics model 155ASL which is physically identical to the Uniphase and Liconix lasers except for the name on the front. I assume it has the same construction though I haven't seen the insides of one up close and personal.)
Also see the section: Spectra-Physics Model 155 HeNe Laser Power Supply (SP-155).
Here are some specifications for a couple of REO tunable lasers. There were two models listed in this 1992 catalog though only the LPTP-1010 shows up in a recent listing. Both linearly polarized (500:1):
(From: Lynn Strickland (stricks760@earthlink.net).)
The five lines are 543.5, 594.1, 604, 611.9, and the common (red) 632.8 nm. You might see a flash at 629.4 nm and at 640.1 nm, but nothing to write home about. The 629 and 640 nm lines are so weak, and so close to 633 that they're sometimes hard to distinguish. There should be nothing at the IR lines (1,153, 1,523 or 3,391 nm).
As originally designed, these lasers used a Brewster window tube with a Littrow prism as the wavelength selection mechanism. The tube's internal mirror was a broad band output coupler. Don't know if it's changed, but I doubt it.
The fundamental design issue is that the optimum bore-to-mode ratio for green is much higher than for red. (BMR is the ratio of limiting aperture size to mode radius. To get TEM00 operation for green, the optimal number is about 4.2, for red it's about 3.5.) If you know the wavelength, mirror curvature, and spacing, you can calculate the mode radius at any point in the cavity. The capillary bore serves as the limiting aperture, so adjusting bore length and bore diameter sets the BMR, which in turn determines transverse mode purity.
Thus, if you optimize the BMR for green power (which you have to do), the red is under-apertured, and has something like 50% off-axis modes. It's getting close to a doughnut-mode.
REO builds some of the highest 'Q' Brewster tubes in the world (probably THE highest), exclusively for the company, Particle Measuring Systems (PMS). REO and PMS used to be one in the same , but the owner sold off the particle counter biz a few years back, for something like $75 million. They now have some sort of supply agreement. The REO tubes aren't the most robust or mechanically stable, but if you get them packaged right, probably some of the highest power you can get from a given tube length. This is mostly due to coatings (all Ion Beam Sputtered), and a super-polishing process they have for substrates. As they say, it's all done with mirrors. ;)
A green Brewster tube IS a bitch! The original REO (PMS) tube was a 5 mW size - about 15" long. They did a soft-seal on the B-window; because it's fused silica. Don't know if they've gone to optical contacting/graded seal now - I'd hope so.
I think REO added a 7 mW, maybe even a 10 mW size for power. I recall seeing some longer ones at a trade show. As for cavity power, I've seen an REO B-tube with 2 HRs do almost 45 Watts of intra-cavity circulating power. They're probably higher than that now. These puppies are like $1,700 each in volume and only sold to PMS - pretty hard to come by.
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 dichroic 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 eNHe laser and probably has wide 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.
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 section: HeNe Tubes of a Different Color.
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 .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 the suppliers listed in the chapter: Laser and Parts Sources.
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. :)
____________________________________________ / | | | _______ \ Anode |\ | | | \ | Cathode .-.---' \.-----'-----..-----'-----..-----'------. '-'---.-. <---| |:::: :===========::===========::============: :::::| |===> '-'---. /'-----.-----''-----.-----''-----.------' .-.---'-' |/ | | | _______/ | \_______|____________|____________|__________/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. :)
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.)
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 gettersJodon Laser Head shows the construction in more detail.
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.)
"Hi Chris,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 first is most of the HP 5501B laser head from the HP 5501A Laser Interferometry Measurement System. Position/distance resolution down to better than 10 nm (that's nanometer as in .000000001 meter!) were possible with this equipment. Of course, only the laser remains) but the specifications say something about the frequency stability of the laser head.
One other thing that is most interesting is that the original list price from the HP catalog for the laser head alone is about $9,000!
(From: Angel Vilaseca (100604.1242@compuserve.com).)
Here is a quick description of the unit:
I have other HeNe lasers but this one really seems to be a class (or several!) above all others...
The label on the unit says:
HENE GAS LASER, Hewlett-Packard, P.N. 05517-60501 Date of mfg. 4-12-93, Date of instl. 4-19-93, Ser. no. 591-3 Made in USA, Licensed by Patiex Corporation, under patent no. 4,704,583.
(From: Sam.)
There are photos of some version of the HP 5501A laser head in the Laser Equipment Gallery (Version 1.87 or higher) under "Assorted Helium-Neon Lasers". The tube used there matches my strange tube but not the description above which appears to be like the tube in the HP 5517A.
(From: Wong Sy Ming (siming@singnet.com.sg).)
I picked up a HP 5517A laser head for S$50 (that's about US$30) and I have to say it's an extremely fine piece of equipment, about the same as the 5501B. The HP 5517A Laser Head Datasheet claims a "vacuum wavelength stability" of 0.002 ppm(!!!) over 1 hour and 0.02 ppm over it's entire 50,000 hour lifetime. Quite incredible, isn't it? It also says it has a wavelength of 632.991372 nm and a wavelength accuracy of 0.1 ppm. (that's for the "consumer grade" model, the "military calibrated" one is 0.02 ppm).
I got a rather more complete version than the one above. It came within its original casing (I believe that HP has the specs for this on it's Website) with an inverter and a whole lot of electronics (don't know what they were for so I just took them out).
The tube is really non-standard, it has only one thick white HV wire coming out of the back and two smaller wires (red and purple, just like the 5501B) and the tube connects to the HV power supply through only the ONE HV cable (for the anode). I discovered later (by poking around with a separate little inverter power supply) that the not-so-obvious cathode connection is via the read wire.)
The two smaller wires are connected to a "connector board" (that's what it says on the PCB) which has a big multiway connector on it, but I just ignored it and connected a 12 VDC power supply to a 470 uF or so capacitor on the board, and the tube lights up! It states a maximum power of 1 mW but the beam looks much brighter than that (probably due to the magnets along the tube which were drawing all my tools to them).
The power supply is a Laser Drive, Inc. model 111-ADJ-1, which appears to be adjustable (due to the model number and the presence of a third wire which goes to a small preset on the PCB) but I didn't fiddle with that. It only takes 0.5 A at 12 VDC which is quite incredible. CAUTION: Do NOT just connect a 12 VDC power supply to the two red and black wires from the power supply or you will get quite a nasty shock. I don't know why.
I wasn't able to trace where the two smaller wires from the tube went. The tube also has additional optics to expand the beam size to 6mm.
(From: Sam.)
I suspect that those two unmarked wires and that stuff you removed were needed to actually obtain the incredible stability that HP claims via some sort of piezo feedback control of the HR mirror.
The only reference to HeNe lasers on HP's Web site are for a several high precision lasers used for interferometry and machine tool (and other) calibration. See: HP Laser Heads.
They use what are called "Continuous Wave Two Frequency Lasers" or more specifically: "Helium-Neon Lasers with Automatically Tuned Zeeman-Split Two-Frequency Output". They have an extremely precise wavelength of: 632.991384 nm and .002 ppm short term wavelength stability.
As best I can determine, a powerful magnet does the Zeeman splitting resulting in orthogonal (polarized) outputs at two very slightly different frequencies, F1 and F2. The distance between the mirrors is feedback controlled by a heating coil wrapped around the bore to force the HeNe laser tube to maintain the position of two adjacent longitudinal modes on the Doppler broadened gain curve resulting to two lines separated by about 1 GHz (for a laser of this length) and with orthogonal polarization. (It may be that the Zeeman splitting aids in stabilizing the laser but may not be fundamental - adjacent longitudinal modes generally have orthogonal polarization without any help.) F1 is reflected from the thing being measured or tested (e.g., disk drive servo writer) and F2 is reflected from a fixed reference. The difference frequencies (F1-F2) and (F1-F2)+dF1 are then analyzed to determine precise position, velocity, or whatever. According to HP, this approach has lower noise, greater stability, and is therefore more accurate than the common single frequency interferometer.
There are some photos of the HP 5517 laser head as well as another strange Hewlett-Packard HeNe laser (with description) in the Laser Equipment Gallery under: "Assorted Helium-Neon Lasers".
If anyone has additional information on this model or even what it means to be a two frequency laser, please send me mail via the Sci.Electronics.Repair FAQ Email Links Page.
The 3100 appears to be similar (or identical) to the Gaertner (model unknown) HeNe laser shown in the Laser Equipment Gallery (Version 1.72 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 and thus it was probably multi-mode (not TEM00). The Brewster windows are Epoxy sealed so needless to say, it no longer worked (aside from the slight problem that when I received the tube, it was in pieces. :( (All I have are the HeNe tube bits, its mounting clamps, mirrors, and mirror mounts, so some of the description below is inferred from limited information.)
HeNe Tube:
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, some larger Melles Griot HeNe tubes are 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 (611.9 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 almost 3 mW (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.
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. Thus, that 1 mW is totally wasted. Perhaps, it was an unsuccessful attempt to kill any orange output from the OC. The HR's appearance is similar to that of a broad band coated HeNe HR - light gold in reflection, blue/green in transmission. The OC appears similar to one for a green HeNe laser - light metallic green in reflection, deep magenta in transmission.
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.
(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 a 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. :)