k3pgp.org

Sam's Laser FAQ - Helium-Neon Lasers


  • Back to Sam's Laser FAQ Table of Contents.

    Sub-Table of Contents


  • Back to Sam's Laser FAQ Table of Contents.
  • Back to Helium-Neon Lasers Sub-Table of Contents.

    HeNe Laser Characteristics, Applications, Safety

    Introduction to Helium-Neon Lasers

    A helium-neon (henceforth abbreviated HeNe) laser is basically a fancy neon sign with mirrors at both ends. Well, not quite, but really not much more than this. The gas fill is a mixture of helium and neon gas at low pressure. A pair of mirrors (one totally reflective, the other partially reflective at the wavelength of the laser's output) complete the resonant cavity. This is called a Fabry-Perot cavity (if you want to impress your friends). The mirrors may be internal (common on small and inexpensive tubes) or external (on precision high priced lab quality lasers). Electrodes sealed into the tube allow for the passage of high voltage DC current to excite the discharge.

    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 the easiest type of laser to fabricate which produces 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. 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 may end up being only a few hours. 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 the approach to take! 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 sealed HeNe laser tubes are between 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 (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. 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 - you probably won't find one of these at a local flea market or swap meet!

    Manufacturers include Aerotech, Melles-Griot, Siemens, Spectra-Physics, Uniphase, and others. (Note: Aerotech's HeNe laser division is now part of Melles Griot). Tubes, laser heads, and complete lasers from these companies are generally of very high quality and reliability.

    HeNe lasers used to be found in all kinds of equipment including early laser printers, laserdisc players, small laser shows, optical surveying and tunnel boring systems, medical positioning systems, and supermarket checkout UPC and other barcode scanners.

    Nowadays, most of these applications are likely to use the much more compact lower (drive) power solid state diode laser. (You can tell if you local ACME supermarket uses a HeNe laser in its checkout scanners by the color of the light - the 632.8 nm wavelength beam from a HeNe laser is noticeably more orange than the 670 nm deep red from a typical diode laser type.)

    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:

    • For many applications including holography and interferometry, the high quality stable beam of a HeNe laser is unmatched (at least at reasonable cost, perhaps at all) by laser diodes. In particular, the coherence length and monochromicity of even a cheap HeNe laser are excellent and the beam profile is circular (laser diodes usually have some amount of astigmatism) so that simple spherical optics can be used for beam manipulation.

    • As noted in the chapter: Diode Lasers, it is all too easy to ruin them in the blink of an eye (actually, the time it takes light to travel a few feet). It would not take very long to get frustrated burning out $50 diodes. So, the HeNe laser tube may be a better way to get started. They are harder to damage through carelessness or design errors. Just don't get the polarity reversed or exceed the tube's rated current for too long - or drop them on the floor! And, take care around the high voltage.

    • Laser diode modules at a wavelength of 635 nm (close to the 632.8 nm wavelength of red HeNe lasers) may still be somewhat more expensive than surplus HeNe tubes with power supplies. However, with the introduction of DVD players and DVDROM drives, this situation probably will not last long.

    HeNe Laser Safety

    As with *any* laser, proper precautions must be taken to avoid any possibility of damage to vision. The types of HeNe lasers dealt with in this document are classified as type II, IIIa, or the low end of IIIb (see the section: Laser Safety Classification. For most of these, common sense (don't stare into the beam) and fairly basic precautions suffice since the reflected or scattered light will not cause instantaneous injury and is not a fire hazard.

    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! Use a well insulated 1M resistor made from a string of four 250K, 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 Siemans. 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 after all that other high power stuff that I did. I think that's called 'irony'.

    Comments on HeNe Laser Safety Issues

    (Portions from: Robert Savas (jondrew@mail.ao.net)).

    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.


  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Theory of Operation, Modes, Coherence Length, On-Line Course and Tutorials

    Instant HeNe Laser Theory

    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.

    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 laser (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.

    There are many possible transitions from the excited state to a lower energy state that can result in laser action. The most important (from our perspective) are listed below: 

             Wavelength            Color
    -------------------------------------------
           543.5 nm         Green
           593,9 nm         Yellow
           611.8 nm         Orange
           632.8 nm         Red
         1,152.3 nm         Near Infra-Red
         1,523.1 nm         Near Infra-Red
         3,391.3 nm         Mid Infra-Red
    See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge for an easy way to see many of the visible ones.

    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 more so than the visible lines. In fact, the first HeNe laser operated at 1,152.3 nm. HeNe lasers at all of these wavelengths are commercially available. However, those operating at 632.8 nm are by far the most common and least expensive.

    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 of curvature equal to the length of the cavity. The latter is a configuration called 'confocal'. Spherical mirrors result in an easier to align more stable configuration but are more expensive than planar mirrors to manufacture.

    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.

    Longitudinal Modes of Operation

    The physical dimensions of the Fabry-Perot resonator impose some additional constraints on the resulting beam characteristics.

    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 * 2
    where:
    • L is the distance between the mirrors (m).
    • W denotes the possible wavelengths of oscillation (m).
    • n is a large integer (order of 948,000 for W around 632.8 nm, L = .3 m).
    • F denotes the possible frequencies of oscillation (Hz).
    • c is the speed of light (approximately 300 million m/s).
    The laser will not operate with just any wavelength - it must satisfy this equation. Therefore, the output will not usually be a single peak at 632.8 nm but a series of peaks around 632.8 nm spaced c/(L * 2) Hz apart. Longer cavities result in closer mode spacing and a larger number of modes since the gain won't fall off as rapidly as the modes move away from the peak. For example, a cavity length of 150 mm results in a longitudinal modes spacing of about 1 GHz; L = 300 mm results in about 500 MHz. The strongest spectral lines in the output will be nearest the combined peak of the lasing medium and mirror reflectivity but many others will still be present. This is called multimode operation.

    Think of the vibrating string of a violin or piano. Being fixed at both ends, it can only sustain oscillations where an integer number of cycles fits on the string. In the case of a string, n can equal 1 (fundamental) and 2, 3, 4, 5 (harmonics or overtones). Due to the tension and stiffness of the string, only small integer values for n are present with a significant amplitude. For a HeNe laser, the distribution of the selected neon spectral line and shape of the reflectivity function of the mirrors with respect to wavelength determine which values of n are present and the effective gain of each one.

    For a typical HeNe laser tube, possible values of n will form a series of very large numbers like 948,123, 948,124, 948,125, 948,126,.... rather than 1, 2, 3, 4. :-) A typical gain function showing the emission curve of the excited neon multiplied by the mode structure of the Fabrey-Perot resonator and the reflectivity curve of the mirrors may look something like the following: 

                    |                  632.8 nm
               I|                     .
                |                  |  |  |
                |               |  |  |  |  | 
                |            |  |  |  |  |  |  |  
         _______|______.__|__|__|__|__|__|__|__|__|__._______
           n=948,125  -5 -4 -3 -2 -1 +0 +1 +2 +3 +4 +5
    Since the mode locations are determined by the physical spacing of the mirrors, as the tube warms up and expands, these spectral line frequencies are going to drift downward (toward longer wavelengths). However, since the reflectivity of the mirrors as a function of wavelength is quite broad (for all practical purposes, a constant), new lines will fill in from above and the overall shape of the function doesn't change.

    However, for very short HeNe tubes, the gain curve may be narrower than the spacing between modes. The effect is even more likely with short low pressure carbon dioxide (CO2) lasers because for a given resonator length, the ratio of wavelengths (10,600 nm for CO2 compared to 632.8 nm for HeNe means that the longitudinal mode spacing is 16.7 times larger). In these cases, the laser output will actually turn on and off as it heats up and the distance between the mirrors increases due to thermal expansion. 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. However, the added expense is only justified for high precision lab quality lasers - you won't find these enhancements on the common cheap HeNe tubes found in bar-code scanners (which are long enough to not be affected in any case)!

    Thus, a typical HeNe laser is not monochromatic though the effective spectral line width is very narrow compared to common light sources. Additional effort is needed to produce a truly monochromatic source operating in a single longitudinal mode. One way to do this is to introduce another adjustable resonator called an etalon into the beam path inside the cavity. A typical etalon consists of a clear optical plate with parallel surfaces. Partial reflections from its two surfaces make it act as a weak Fabry-Perot resonator with a set of modes of its own. Then, only modes which are the same in both resonators will produce enough gain to sustain laser output.

    The longitudinal mode structure of an optional intra-cavity etalon might look like the following (not to scale): 

                    |                  632.8 nm
               I|      .              .              .
                |      |              |              |
                |      |              |              |
                |      |              |              |
         _______|______|______________|______________|_______
           m=13,542   -1             +0             +1
    Notice that since the distance between the two surfaces of the etalon is much less than the distance between the main mirrors, the peaks are much further apart (even more so than shown). (The etalon's index of refraction also gets involved here but that is just a detail.) By adjusting the angle of the etalon, its peaks will shift left or right (since the effective distance between its two surfaces changes) so that one spectral line can be selected to be coincident with a peak in the main gain function. This will result in single mode operation. The side peaks of the etalon (-1, +1 and beyond) will only coincide with weak peaks in the main gain function shown above so that their combined amplitude (product) is insufficient to contribute to laser output.

    Transverse Modes of Operation

    Lasers can also operate in various transverse modes. Laser specifications will usually refer to the TEM00 mode. This means Transverse Electromagnetic Mode 0,0 and results in a single beam. The long narrow bore of a typical HeNe laser forces this mode of oscillation. With a wide bore multiple sub-beams can emerge from the same cavity in two dimensions. The TEM mode numbers (TEMxy) denote the number (minus one) of such sub-beams: 
                            O        OO        OOO      Each 'O' represents
     O        OO        O        OO        OOO       a single sub-beam.
   TEM00     TEM10    TEM01    TEM11      TEM21
    Other (non-cartesian) patterns of modes may also be possible depending on tube dimensions and operating conditions.

    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.

    Coherence Length of HeNe Lasers

    Common HeNe lasers have a coherence length of around 10 to 30 cm. By adding an etalon inside the cavity to suppress all but one longitudinal mode, coherences lengths of 100s of meters are possible. Naturally, such HeNe lasers are much more expensive and are more likely to be found in optics research labs - not mass produced applications.

    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.

    On-line Introductions to HeNe Lasers

    There are a number of Web sites with laser information and tutorials.
    • One of the best so far is Course 1: Intro to Lasers which is part of the Laser/Electro-Optics Technology Series being developed by Cord Communications, 324 Kelly Drive, P.O. Box 21206, Waco, Texas 76702-1206. (Intro to Lasers is the only complete course at this time.)

      In particular, Unit 1-10 Helium-Neon Gas Laser--A Case Study, goes into considerable detail on the theory as well as some more practical information related to HeNe lasers.

      This entire course contains great educational content for those who wish to gain a better understanding of the general principles of laser operation. But, it is designed at a level that probably won't put you to sleep with too much heavy math. :-)

    • The HeNe Laser Manaul by Elden Peterson of Voltex, Inc. has a variety of practical information on HeNe lasers including characteristics and power supply considerations.

    • Also see the section: General Laser Information and Tutorial Sites for other sites that may be worth visiting.


  • Back to Helium-Neon Lasers Sub-Table of Contents.

    HeNe Laser Tubes, Heads, Structure, Power Requirements, Lifetime

    Early Versus Modern HeNe Lasers

    In the first HeNe lasers (see the diagram below), exciting the gas atoms to the higher energy level was accomplished by coupling a radio frequency (RF) source (i.e., a radio transmitter) to the tube via external electrodes. Modern HeNe lasers almost always operate on a DC discharge via internal electrodes. 
          Bellows                                                Bellows
      /\/\/\      Discharge tube with external electrodes     /\/\/\
     ||     \________________________________________________/     ||
     ||             | |                | |              | |        ||===> Laser
     ||      ___  __|_|________________|_|______________|_|__      ||===> Beam
     ||     /   ||   |                  |                |   \     ||
      \/\/\/    ||   |                  o                |    \/\/\/
 Adjustable     ||   +-----------o RF exciter o----------+     Adjustable
  totally       ||                                              partially
 reflecting     ||<-- to vacuum system                         reflecting
   mirror                                                        mirror
    Early HeNe lasers were also quite large and unwieldy in comparison to modern devices. A laser such as the one depicted above was over 1 meter in length but could only produce about 1 mW of optical beam power! The associated RF exciter was as large as a microwave oven. With adjustable mirrors and a tendency to lose helium via diffusion under the electrodes, they were a finicky piece of laboratory apparatus with a lifetime measured in hundreds of operating hours.

    In comparison, a modern 1 mW sealed 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.

    HeNe Laser Tubes and Laser Heads

    The following applies to most of the inexpensive sealed low to medium power (.5 to 5 mW) HeNe tubes available on the surplus market. Depending on the original application, the actual laser tube may be enclosed inside a laser head or arrive naked. :-)

    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-----------+
    (Note: the main beam may emerge from either end of the tube depending on its design, not necessarily the cathode end as shown. A much lower power beam will likely emerge from the opposite end if it isn't covered - the 'totally reflecting' mirror isn't perfect.)

    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).

    Tubes up to at least 35 mW are similar in design but proportionally larger, require higher voltage and current, and, of course, will be more expensive.

    • The anode (+) end is simply a small cylindrical metal electrode with a mirror fused or glued to its end.

      The discharge at this end produces little heat or damage due to sputtering.

    • The cathode (-) end is also a cylindrical metal electrode with a mirror fused or glued to its end but in addition, there is a large 'cylindrical can attached to the cathode and extending about half the length of the tube.

      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 since damage to the anode would likely result.

    • The major discharge is forced to take place inside a thick glass capillary tube with an inner bore of .5 to 1.5 mm depending on the power of the tube. This concentrates the discharge forcing operation in the most common and desirable TEM00 mode.

    • An outer glass envelope of much larger diameter than the capillary provides a substantial gas reservoir. While the helium-neon gas mixture doesn't get used up, some unavoiable adsorption (sticking of the gas molecules to the glass and metal parts) and leakage does occur. Having a larger gas supply minimizes any effects on performance.

    Gas Fill and Getter

    In order for an HeNe laser to operate efficiently (as such things go) or at all, there must be a very precise and pure mixture of helium and neon gas in the tube. The total amount of gas in a typical 1 mW HeNe tube is much less than 1 cubic cm if it were measured at normal atmospheric pressure. It fills the tube only because the pressure is very low. However, with this small amount of gas, it doesn't take much contamination or leakage to ruin the tube.
    • The gas fill consists of a mixture of helium and neon in the proportions of about 7:1 (He:Ne) at a pressure of 1.5 to 5 Torr (millimeters of mercury - 1 Torr is approximately 1/760th of standard atmospheric pressure). Note the large amount of helium even though it is the neon that actually emits the coherent light. See the section: Instant HeNe Laser Theory for more information on the principles of operation.

    • Many HeNe tubes will have a ring or rectangular shaped metal structure (probably attached to the cathode) holding a spongy substance in its U-shaped cross-section. This is called the 'getter electrode'. After the tube has been pumped down and sealed, it is heated by RF induction causing the spongy stuff to decompose and release a highly reactive metal like barium - the actual getter - which may be visible as a metallic or dark colored spot on the glass near the getter electrode.

      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. 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 and ignore the getter. :-)

      Should the getter spot turn milky white or red, 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).

    Mirrors in Sealed HeNe Tubes

    These mirrors are a bit more sophisticated than your bathroom variety:
    • The mirrors are not silvered or aluminized (metal coated) but are a type called 'dichroic'. They are made by depositing many alternating layers of hard but transparent materials having different indexes of refraction. The thickness of each is precisely 1/2 the wavelength of the laser light (632.8 nm being the most common for a HeNe laser). This results in reflection by interference with very high (>99.9 %) efficiency - much greater than for even the best metal coated mirrors.

    • The mirrors may be perfectly flat (planar) or one or both may be spherical (concave - positive - with respect to the cavity) with a typical radius of curvature equal to the length of the cavity. The latter is a configuration called 'confocal'. Spherical mirrors result in an easier to align more stable configuration but are more expensive than planar mirrors to grind and polish. I do not know how significant this might be for the common small narrow bore HeNe tubes - I have found tubes with planar mirrors on both ends as well as those with spherical mirrors on the output end.

      You can 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). A concave mirror will reduce the size of the reflection very slightly compared to a planar mirror.

      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.

    • One of the mirrors will be nearly totally reflecting and the other will only be partially reflecting at the laser wavelength. These are called the High Reflector (HR) and Output Coupler (OC) respectively. Note that the HR isn't perfect - there will be a low intensity beam exiting from that end of the tube as well as from the OC end assuming it is not covered with paint or tape.

      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.

    • The mirrors will likely not have any 'user' adjustments. However, the cylindrical end pieces are mounted by thinner sections of metal tubing so that some slight changes to alignment may be possible with appropriate fixtures. I do not recommend this because: (1) grabbing the high voltage electrodes is not likely to be pleasant and (2) it is too easy to break the seal if you get carried away. There should be no reason for the alignment to have changed unless you whacked the tube - it was set at the factory. However, if you suspect an alignment problem, it is easy to check. Then, you can decide if attempting an adjustment is worth the risks. See the section: Checking and Correcting Mirror alignment of Internal Mirror Laser Tubes.

      However, long high power tubes (i.e., 20 mW and up) may require fixtures to maintain mirror alignment even when the mirrors are internal. 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 main beam will emerge from the partially reflecting mirror but this may be at either end of the tube depending on model. For example, where the tube is enclosed in a metal barrel, the HV connections will be to the anode end and the beam will exit from the cathode end. With this arrangement, the positive output of the power supply and ballast resistor can be very close to the tube anode. The entire barrel (cathode) can be connected to earth ground for safety.

    • Unlike common metal coated mirrors, these dichroic types are not perfectly reflective. Thus, there will be a weaker beam visible from the non-output end of the tube if that mirror is not covered (blocked or painted over). One use of this is to permit monitoring of laser power for purposes of optical power regulation or other closed loop applications.

    Power Requirements for HeNe Lasers

    Power for a HeNe laser is provided by a special high voltage power supply (see the chapter: HeNe Laser Power Supplies and consists of two parts (these maximum values depend on tube size (typical 1 to 10 mW tube is assumed):
    • Operating voltage of 1,000 to 3,000 DC at 3 to 8 mA. Like any discharge tube, the HeNe laser is a negative resistance device. As the current *increases* through the tube, the voltage across the tube *decreases*. The incremental magnitude of the negative resistance also increases with descreasing current.

    • Starting voltage of 5 to 12 KV at almost no current.

      In the case of a HeNe tube, the initial breakdown voltage is much greater than the sustaining voltage. The starting voltage may be provided by a separate circuit or be part of the main supply.

    • With a constant voltage power supply, a series ballast resistor is essential to limit tube current to the proper value. A ballast resistor will still be required with a constant current or current limited supply to stabilize operation. The ballast resistor may be included as part of a laser head but will be external for a bare tube.

      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!

    • Power supply polarity is important for HeNe tubes. Electrical behavior may be quite different if powered with incorrect polarity and tube damage (and very short life) will likely be the result from prolonged operation.
      • The positive output of the power supply is connected to a series ballast resistor and then to the anode (small) electrode of the HeNe tube. This electrode may actually be part of the mirror assembly at that end of the tube or totally separate from it.

      • The negative output of the power supply is connected to the cathode (large can) electrode of the HeNe tube (usually directly though there may be exceptions). This electrode may be electrically connected to the mirror mount at that end of the tube but is a separate aluminum cylinder that extends for several inches down the tube.

      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.

    • Every HeNe tube will have a nominal current rating. In addition to excessive heating and damage to the electrodes, current beyond this value does not increase laser beam intensity. In fact, optical output actually decreases (probably because too high a percentage of the helium/neon atoms/ions are in the excited state). You can easily and safely demonstrate this behavior if your power supply has a current adjustment or you run an unregulated supply using a Variac. While the brightness of the discharge inside the tube will increase with increasing current, the actual intensity of the laser beam will max out and then eventually decrease with increasing current. (This is also an easy way of determining optimal tube current if you have not data on the tube - adjust the ballast resistor or power supply for maximum optical output and set it so that the current is at the lower end of the range over which the beam intensity is approximately constant.)

    • The efficiency of the typical HeNe laser is pretty pathetic. For example, a 2 mW HeNe tube powered by 1,400 V at 6 mA has an efficiency of less than .025 %. More than 99.975 percent of the power is wasted in the form of heat and incoherent light (from the discharge)! This doesn't even include the losses of the power supply and ballast resistor.

    Bare HeNe Tubes and Laser Heads

    What you have may be a 'bare' tube or it may be encased in a cylindrical or rectangular laser head - or something in between:
    • Bare tubes require clip-on connections to the power supply or high voltage connector and an external ballast resistor.
      • Advantages: Less expensive, discharge is fully visible resulting in an interesting display.

      • Disadvantages: Fragile, exposed high voltage terminals, need to provide your own mounting, wiring, and ballast resistor.

    • Laser heads should plug right into a suitable power supply with no fuss, mess, or unexpected Zaps. :-)
      • Advantages: High voltage safely insulated, wiring is already done for you, generally very high quality, relatively robust, easily mounted, may include beam shutter and mounting holes or bezels to permit the accurate attachment and alignment of additional optical components..

      • Disadvantages: More expensive, discharge not readily visible, repairs to wiring (unlikely to be needed) difficult, tube replacement may not be possible (at least not easily and/or non-destructively) if mounted using RTV silicone or something similar.

      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.)

    HeNe Tube Lifetime

    Neon signs last a long times - years - how about HeNe laser tubes?

    The operating lifetime of a typical HeNe laser tube is greater than 15,000 hours when used within its specified ratings (operating current, proper polarity, and not continuously restarting). Therefore, this is not a major consideration for most hobbyist applications. However, the shelf life of the tube depends on its construction. There are two types of (sealed) HeNe tubes:

    • Most better HeNe tubes (possibly all tubes manufactured in the last 15 years) are 'hard sealed' - the mirrors are fused to their respective electrodes by a low temperature glass 'frit' - sort of like solder for glass! These do not leak - at least not on any time scale that matters.

      Shelf life of hard sealed tubes is essentially infinite.

    • Older tubes have their mirrors just glued - Epoxied to the end electrodes. This adhesive leaks and such tubes have a shelf life of a few years - they fail by just sitting around doing nothing. This means that a bargain tube may not be such a bargain if it is beyond its expiration date (yes, just like dates on milk containers) as it may have a very limited life, be hard to start, weak or erratic, or may not work at all. You probably won't see any of these - at least not in a working condition.

      Shelf life of non-hard sealed tubes is limited by diffusion of the Helium atoms. 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.

    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?.


  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Wavelengths, Beam Characteristics

    HeNe Laser Wavelengths

    While what comes to mind when there is mention of a HeNe laser is a red beam, those with other wavelengths are manufactured.
    • The most common HeNe lasers by far produce light at a wavelength of 632.8 nm in the red part of the visible spectrum. This is well into the region of the human eye's high sensitivity (but not anywhre as good as green). Thus, a 1 mW red HeNe laser will appear brighter than a 4 mW diode laser operating at 670 nm. Although these are called red HeNe lasers, compared to the color of the 670 nm diode, their beam actually appears somewhat orange-red.

    • Green (543.5), yellow (593.9), and orange (611.8) HeNe lasers are also available but are not nearly as 'efficient' as the common red type. Thus, they are also much more costly for the same power since the spectral lines that need to be amplified are weaker at these wavelengths and therefore, the tubes must be larger.

      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). However, an invisible beam just doesn't seem as exciting!

      Typical maximum output available from (relatively) small sealed 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.

    Exact Frequency/Wavelength of HeNe Lasers

    (From: Jens Decker (Jens.Decker@chemie.uni-regensburg.de)).

    The Melles Griot catalog states 473.612535 THz for the 632.8 line. For their stabilized system they claim 473.61254 THz. 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, p. 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:

    • If you want 4 significant digits, you have to know if it's the wavelength in air, or in vacuum.

    • If you want 6 significant digits, you need a single mode HeNe laser.

    • If you want 7 significant digits, you have to stablize the output to the center of the gain profile and you probably need to know the helium pressure and the neon isotope ratio.

    • If you want 8 significant digits, you have to know the diameter of the beam since diffraction effects will change the wavelength.

    • If you want 9 significant digits, you might need frequency, not wavelength, metrology.

    The metrologists' answer for a 633 nm HeNe laser stabilized to the a-13 component of the R(127) line of the 11-5 transition of the 127-Iodine dimer molecule is:
    • Frequency = 473 612 214 705 kHz
    • Wavelength = 632 991 398.22 fm (femtometer = 10E-15 m)
    under certain specified conditions, with uncertainty 2.5 x 10E-11. See: "Metrologia", vol. 30., pp. 523-541, 1993-1994.

    HeNe Laser Beam Characteristics

    Compared to a diode laser, the beam from even an inexpensive mass produced HeNe tube is of very high optical quality:
    • The width of the beam as it emerges from the tube is typically between .5 mm and 1 mm - the inside bore diameter of the capillary discharge tube.

    • The beam from a HeNe laser is already very well collimated even without external optics (unlike a laser diode which has a raw divergence measured in 10s of degrees). The divergence measured in milliradians (1 mR is less than 1/17th of a degree) is usually one of the tube specifications. A small HeNe tube may have a divergence of 1 to 2 mR. Divergence 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 Diameter
      So, for an ideal HeNe laser with a .5 mm bore at 632.8 nm, the divergence angle will be about 1.6 mR. Note that although a wider bore should result in less divergence, this also permits more not quite parallel rays to participate in the lasing process. Also see the section: Improving the Collimation of a HeNe Laser with a Beam Expander.

    • The cheap surplus HeNe tubes do not generally produce a fixed polarized beam. The polarization will either be random or slowly changing as the tube heats. The latter is easily observed by placing a polarizer in the beam. The beam brightness will then appear to go through cycles - light, dark, light, etc. However, polarization can be affected by external means. See the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.

    • Sealed tubes which generate a linearly polarized beam are also available but are generally more costly (and less common). They include a glass plate oriented at the Brewster angle in the beam path (inside the resonator) near one of the mirrors. This results in the optical resonator favoring one polarization orientation. See the section: What is a Brewster Window? for more information.

      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.

    • These tubes nearly always operate with multiple longitudinal modes and a TEM00 beam profile. See the section: Instant HeNe Laser Theory for more information on laser mode structure.

    • High precision or lab quality HeNe lasers may be of quite unconventional construction incorporating plasma tubes that differ substantially compared to these mass produced HeNe tubes - both electronically and optically. In addition, interesting and strange electrical, optical, electro-optical, or magnetic devices may be added to implement external modulation, mode locking, stabilization, and additional high performance (and high cost) features. Consider one of these to be quite a find! See the section: Interesting and Strange HeNe Lasers for some examples.

    Ghost Beams From HeNe Laser Tubes

    If you project the output from some HeNe laser tubes (as well as other lasers) onto a white screen a meter or so away, you may see a main beam and a weak beam off to the side a few cm away from it.

    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!

    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 45 degree 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.

    It is conceivable that slight misalignment of the mirrors may result in similar symptoms 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, see the section: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes but ONLY apply the very slightest force in each direction to confirm that the beam is cleanest and most intense with the present alignment.

    (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.


  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Magnets in High Power or Precision HeNe Laser Heads

    Effects of Magnetic Fields on HeNe Laser Operation

    If you open the case on a higher power (and longer) HeNe laser head or one that is designed with an emphasis on precision and stability, you may find a series of magnets or electromagnetic coils in various locations in close proximity to the HeNe tube. They may be distributed along its length or bunched at one end; with alternating or opposing N and S poles, or a coaxial arrangement; and of various sizes, styles, and strengths.

    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:

    • HeNe tubes with long discharge paths will tend to amplify the (generally unwanted) IR wavelengths (probably the one at 3.39 um which is one of the strongest, if not the strongest of all lines) at the expense of the visible ones. The purpose of these magnets is to suppress spectral lines that do not contribute to the desired lasing wavelength (usually the visible red 632.8 nm for these long tubes).

      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 output beam 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, 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. Enclosing the HeNe tube in a protective rigid sleeve (e.g., PVC or aluminum) would reduce the risk of the latter disaster, at least. :-)

    • While most inexpensive HeNe tubes that produce linearly polarized light do so because of an internal Brewster plate and lasers with external mirrors have Brewster windows on the ends of the plasma tube, it is also possible to affect the polarization of the beam with strong magnets again using the Zeeman Effect.

      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.

    • Magnets may be present in lab quality lasers (even much smaller ones) possibly to stabilize the discharge and beam power and quality. It is quite unlikely that you will ever come across a fancy bare HeNe tube of this type that requires magnets to operate. The ones I have heard of have been part of expensive high quality lasers and of unconvention construction compared to the common sealed HeNe tubes.

    In principle, varying fields from electromagnets could be used for intensity and polarization modulation. I do not know whether any are implemented in this manner.

    Typical Magnet Configurations

    Here are examples of some of the common arrangements of magnets that you may come across. I suspect that for many commercial HeNe lasers, the exact shape, strength, number, position, orientation, and distribution of the magnets was largely determined experimentally. In other words, some poor engineer was given a bare HeNe tube, a pile of assorted magnets, a roll of duct tape, and a lump of modeling clay, and asked to optimize some aspect(s) of the laser's output. :-)

    (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!)

    • Transverse (varying field) - These will most likely be permanent magnets in pairs, probably several sets.

      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
                     '|'                                 '|'
    • Transverse (uniform field). Here, the objective is to achieve a constant field throughout the entire discharge: 
                         N      N      N      N      N      N      N
        ||     //===============================================\\     ||
        ||   //======. .=================================. .======\\   ||
                   S |||  S      S      S      S      S  ||| S
                     '|'                                 '|'
    • Axial - These will either be permanent magnet toroids (similar to magnetron magnets) or an electromagnetic coil (possibly with adjustable or selectable field strength) surrounding the tube. Thus, the North and South poles will be directed along the tube axis: 
                              +--+      +--+      +--+      +--+
                      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.

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Sealed HeNe Tubes up to 35 mW - Red and Other Colors, SP-124/125

    Typical HeNe Tube Specifications

    Prior to the introduction of the CD player and CDROM drive, the common red HeNe laser was by far the most common source of inexpensive coherent light on the planet. The following are typical specifications for a variety of red (632.8 nm) HeNe tubes (all single mode - TEM00): 
       Output     Tube Voltage       Tube         Supply Voltage     Tube Size
   Power      Operate/Start     Current        (75K ballast)    Diam/Length
 ----------  ---------------  ------------   ----------------  -------------
  .3-.5 mW     .8-1.0/6  KV    3.0-4.0 mA       1.0-1.2 KV       19/135 mm
  .5-1 mW      .9-1.0/7  KV    3.2-4.5 mA       1.1-1.3 KV       25/150 mm
   1-2 mW     1.0-1.4/8  KV    4.0-5.0 mA       1.2-1.8 KV       30/200 mm
   2-3 mW     1.1-1.6/8  KV    4.0-6.5 mA       1.4-2.0 KV       30/260 mm
   3-5 mW     1.7-1.9/10 KV    4.5-6.5 mA       2.1-2.4 KV       37/350 mm
    Melles Griot, Uniphase, Siemens, PMS, and Aerotech all show similar values.

    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. These specifications are generally for minimum power (when new). Individual samples may be much higher. This is particularly evident in the Melles Griot listings, below.

    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 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         37/450 mm
    The following are from Melles Griot. These are all red (632.8 nm) tubes operating single mode - TEM00. Many are actually sold as part of complete laser heads (all those where the model number is odd). You don't even want to think about asking about the prices of these laser heads. Most are still listed in the current catalog.

    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  .46 mm  1.77 mR  1200 MHz  1.10/5  KV   3.2 mA   19/135 mm  002-246*
   .5 mW  .49 mm  1.70 mR  1040 MHz  1.25/5  KV   4.5 mA   25/152 mm    700
   .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   25/177 mm    900
  1.0 mW  .59 mm  1.35 mR   687 MHz  1.79/8  KV   6.5 mA   37/230 mm    111
  2.0 mW  .59 mm  1.35 mR   687 MHz  1.79/10 KV   6.5 mA   37/230 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   25/200 mm    691
  4.0 mW  .80 mm  1.00 mR   435 MHz  2.35/10 KV   6.5 mA   37/351 mm    140
  5.0 mW  .80 mm  1.00 mR   438 MHz  2.29/10 KV   6.5 mA   37/370 mm    151
   7 mW  1.02 mm   .79 mR   373 MHz  2.65/10 KV   6.5 mA   37/420 mm    171
  10 mW   .65 mm  1.24 mR   341 MHz  2.64/10 KV   6.5 mA   37/440 mm    991
  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    927
      (* Model 05-LHR-002-246 is a cute little tube that was found in a hand-held bar code scanner. Specifications for output power, beam diameter, and divergence have been estimated.)
    The operating voltage across the tube itself can be found by subtracting the voltage drop across the ballast resistor (I*Rb), from the value listed in the table. Actual starting voltages are typically 3 to 5 times the tube operating voltage (though the specifications may be higher).

    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 about 10 to 15 percent more expensive.

    HeNe Tubes of a Different Color

    Although a red beam is what everyone thinks of when a HeNe laser is discussed, HeNe tubes producing green, yellow, and orange beams are also manufactured. However, they are probably not found very often on the surplus market because they are not nearly as common as the red variety. Such tubes are also more expensive when new since for a given power level, they must be larger (and thus have higher voltage and current ratings) due to their lower efficiency (the spectral lines being amplified are much weaker than the one at 632.8 nm).

    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.

    There are infrared HeNe tubes 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 do exist and 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 center wavelength of the mirror reflectivity. (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?.)

    Even if the model number does not identify the tube as green, yellow, orange, 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 - red - HeNe tube. Since the mirrors 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 mirrors will tend to appear with 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. The mirrors of an IR tube may appear neutral or even transparent. Of course, except for the IR variety, if the tube is functional, the difference will be immediately visible when it is powered!

    And, the answer to that other burning question should now be obvious: No, you cannot convert a red sealed HeNe tube to generate some other color light. As noted above, the mirror reflectance peak is different (and the cavity length may be insufficient to resonate with the reduced gain other-color spectral lines). For a laser with external mirrors, a mirror swap may be possible (though the Brewster angle windows may then not quite be at the proper angle). However, for sealed HeNe tubes, your options are severely limited - as in there are none :-(.

    The following 'other-color' tubes are listed in the Melles Griot catalog. These are all single mode - TEM00, random polarized. Linearly polarized versions are also available but for some reason, unlike the red tubes (where the output power specifications are identical), have much lower output power. Also note that the output power of these tubes is much much less than that of the red (632.8 nm) models with similar (in some cases identical) dimensions.

    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   30/215 mm    025
   .5 mW  .80 mm  1.01 mR   438 MHz  2.39/10 KV   6.5 mA   37/360 mm    151
   .8 mW  .89 mm   .92 mR   373 MHz  2.62/10 KV   6.5 mA   37/420 mm    173
  1.5 mW  .86 mm   .81 mR   328 MHz  2.75/10 KV   6.5 mA   37/475 mm    193
    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   30/215 mm    025
  .75 mW  .80 mm  1.01 mR   438 MHz  2.43/10 KV   6.5 mA   37/360 mm    151
  2.0 mW  .75 mm   .92 mR   373 MHz  2.59/10 KV   6.5 mA   37/420 mm    173
    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   30/215 mm    025
  2.0 mW  .80 mm  1.01 mR   438 MHz  2.49/10 KV   6.5 mA   37/360 mm    151
    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  .88 mm  2.20 mR   373 MHz  2.97/10 KV   6.0 mA   37/420 mm    171
    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!! 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.

    Steve's Comments on Other Color HeNe Lasers

    (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 612 nm orange and 594 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.

    Even Large HeNe lasers such as the SP 125 (100 to 200 mW of red) will only do about 20 mW of yellow, with a 30 mW 107 your 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 107 only does about 3 mW of 612 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.

    Spectra-Physics 124 and 125 HeNe Laser Specifications

    Here are the nearly complete specifications for a pair of older high quality scientific/research type HeNe lasers which have resonators with external mirrors. The SP-125, in particular, is definitely a BIG laser - size-wise at laest - with a resonator length of over 1-3/4 METERS! These are the type of laser that might turn up in the long forgotten and dusty storage room of a university physics/optics or industrial research lab. Both of these models permit the mirror assemblies to be replaced (or at least as an option at the time of purchase) to select the output 'color' - normal HeNe red (632.8 nm), near IR (1152.3 nm), and medium IR (3391.3 nm). (It is possible that magnets may also need to be added/removed depending on the desired wavelength though this is not mentioned in the spec sheets. For an explanation, see the section: Magnets in High Power or Precision HeNe Laser Heads.)

    I don't know whether it is also possible to obtain orange (611.8 nm), yellow (593.9 nm), or green (543.5 nm) output with similar modifications - the resonator gain may be too low. The 1152.3 nm and 3391.3 nm IR lines are very strong and high gain.

    Note: The specifications below were copied from an almost illegible scan of a fax of a copy of the original product brochure. Corrections are welcome! 

      Spectra-Physics Laser:         Model SP-124B             Model SP-125A
 -----------------------------------------------------------------------------
  OUTPUT
    Wavelength (nm):         632.8  1152.3  3391.3     632.8  1152.3  3391.3
    Minimum Power (mW):       15     2.5     5.0        50     10      10
  BEAM CHARACTERISTICS
    Beam Diameter (mm):       1.1     1.4    2.5        1.8    2.4     4.1
    Beam divergence (mR):      .75    1.0    1.5         .6     .8     1.4
  RESONATOR CHARACTERISTICS
    Transverse Mode:                              TEM00
    Degree of Polarization:                       1E-3
    Angle of Polarization:               Vertical (+/- 5 Degrees)
    Resonator Configuration:                   Long Radius
    Resonator Length:               70.1 cm                  177.0 cm
    Axial Mode Spacing:             214 MHz                   85 MHz
    Plasma Excitation:           5 KV at 15 mA         6 KV at 25 to 35 mA
                                                     (RF Opt: 15 W at 46 MHz)
  AMPLITUDE STABILITY
    Beam Amplitude Noise:           <.3% RMS            <2% RMS (RF: <.5%)
    Beam Amplitude Ripple:          <.2% RMS            <.5% RMS (RF: <.6%)
    Long Term Power Drift:         <5% over 8 hours and 10 Degrees C
    Warmup Time:                   30 Minutes                 1 Hour
  ENVIRONMENTAL CAPABILITY
    Operating Temperature:               10 to 40 Degrees 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%
    Input Power:                    125 W                     156 W
  PHYSICAL CHARACTERISTICS
    Laser Head Weight:          11.4 KG (25 lb)          45.4 KG (100 lb)
    Power Supply Weight:         3.5 KG (7.5 lb)         13.6 KG (30 lb)

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Viewing Spectral Lines in Discharge, Other Colors in Output

    Instant Spectroscope for Viewing Lines in HeNe Discharge

    It is easy to look at the major visible lines. All it takes is a diffraction grating or prism. I made my instant spectroscope from the diffraction grating out of some sort of special effects glasses - found in a box of cereal, no less! - and a monocular (actually 1/2 of a pair of binoculars).
    • If you missed the Kellogg's option, diffraction gratings can be purchased from places like Edmund Scientific. You don't need anything fancy - any of the inexpensive 'transmission replica gratings' on a flat rigid substrate or mounted between a pair of plane glass plates will be fine. In a pinch, a CD disc or other optical media will work but only as a reflection grating so mounting may be a problem. See the section: Use of a CD, CDROM, CD-R, or DVD Disc as a Diffraction Grating. A spectroscope can also be made with a prism of course but a diffraction grating is likely to be less expensive and better for this application since it is much lighter and easier to mount.

    • The plasma tube of a bare HeNe laser is an ideal light source since it provides its own slit as the glow discharge is confined to the long narrow capillary bore. However, this approach can also be used with other lasers as long as the beam can be focused to a spot on a wall or screen. This will produce a 'bright spot spectra' instead of politically correct lines but you can't have everything. :-)

    • The diffraction grating can be used by itself but the additional optics will provide magnification and other benefits for people with less than perfect eyeballs.

    • Glue the diffraction grating to a cardboard sleeve that can be slipped over the (or one) objective of a monocular, binocular, or small telescope - or the telephoto lens of your camera. Orient it so that the dispersion will be vertical (since your slit will be horizontal).

    • Operate the HeNe tube on a piece of black velvet or paper. This will result in optimum contrast. This is best done in a darkened room where the only source of light is the laser tube itself. Just don't trip and zap yourself on the high voltage!

    • A diffraction grating produces several images. The zeroth order will be the original image seen straight ahead. The important ones are the first order spectra. Tip the instrument up or down to see these. The dispersion direction - order of the colors - will depend on which way it is tipped.

    • Any distance beyond the closest focus of your instrument will work but being further away will reduce the effective width of the 'slit' resulting in the ability to distinguish more closely spaced lines.

    The shear number of individual spectral lines present in the discharge is quite amazing. You will see the major red, orange, yellow, and green lines as well as some far into the blue and violet portions of the spectrum and toward the IR as well. All of those shown in Bright Line Spectra of Helium and Neon will be present as well as many others not produced by the individual gas discharges. There are numerous IR lines as well but, of course, these will not be visible.

    Place a white card in the exit beam and note where the single red output line of the HeNe tube falls relative to the position and intensity of the numerous red lines present in the gas discharge.

    As an aside, you may also note a weak blue/green haze surrounding the intense main red beam (not even with the spectroscope). This is due to the blue/green (incoherent) spectral lines in the discharge being able to pass through the output mirror which has been optimized to reflect well (>99 percent) at 632.8 nm and is relatively transparent at wavelengths some distance away from these (shorter and longer but you would need an IR sensor to see the longer ones). Since it is not part of the lasing process, this light diverges rapidly and is therefore only visible close to the tube's output mirror.

    Other Color Lines in Red HeNe Laser Output

    When viewing spectral lines in the actual beam of a red HeNe laser, you may notice some very faint ones far removed from the dominant 632.8 nm line we all know and love. (This, of course, also applies to other color HeNe lasers.)

    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:

    • Plasma discharge - As noted in the section: Instant Spectroscope for Viewing Lines in HeNe Discharge, there are many strong emission lines in the actual discharge - and none of them are actually at the 632.8 nm lasing wavelength! These extend from the mid-IR through the violet.

      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.

    • Superradiance - As we know, HeNe lasers can be made to operate at a variety of wavelengths other than the common 632.8 nm red. The physics for these is still applicable in a red HeNe tube but the mirrors do not have the needed reflectivity at these other wavelengths and therefore the resonator gain is too low to support true laser action. However, stimulated emission can still take place in superradiance mode - one pass down the tube and out, exiting easily for the green wavelength in particular since the dichroic mirrors are quite transparent in that region of the spectrum.

      The result will be a weak green beam that can sometimes be observed with a spectroscope in a very dark room room. It isn't really quite as coherent or monochromatic as the beam from a true green HeNe laser and probably has 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.

    Since the brightness of the discharge and superradiance output should be about the same from either mirror, using the non-output end (high reflector) should prove easier (assuming it isn't painted over or otherwise covered) since the red beam exiting from this mirror will be much less intense and won't obscure the weak green beam.

    Note that argon and krypton ion lasers are often designed for multiline output where all colors are coherent and within an order of magnitude of being equal to each other in intensity. This is not to my knowledge ever done with HeNe lasers, at least not the sealed tube variety.


  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Demonstration HeNe Laser, Weatherproofing

    Putting Together a Demonstration HeNe Laser

    For a classroom introduction to lasers, it would be nice to have a safe setup that makes as much as possible visible to the students. Or, you may just want to have a working HeNe laser on display in your living room! Ideally, this is an external mirror laser where all parts of the resonator as well as the power supply can be readily seen. However, realistically, finding one of these is not always that easy or inexpensive, and maintenance and adjustment of such a laser can be a pain (though that in itself IS instructive).

    The next best thing is a small HeNe laser laid bare with the sealed (internal mirror) HeNe tube, ballast resistors, wiring, and power supply (with exposed circuit board), are mounted inside a clear Plexiglass 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:

    • The discharge and mirrors are clearly visible permitting the lasing process to be described in detail. Compared to this, a diode laser pointer is about as exciting as a flashlight even if you are able to extract the guts.

    • The beam quality in terms of coherence length, monochromicity, shape, and stability, will likely be much higher for the HeNe laser should you also want to use it for actual optics experiments like interferometry.

    • For a given power level, a 632.8 nm HeNe laser will appear about 5 times brighter than a 670 nm laser pointer. (635 nm laser pointers are available but still more expensive.)

    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.

    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.

    Weatherproofing a HeNe Laser

    If you want to use a HeNe laser outside or where it is damp or very humid, it will likely be necessary to mount the tube and power supply inside a sealed box. Otherwise, stability problems may arise from electrical leakage or the tube may not start at all. There will then be several additional issues to consider:
    • Heat dissipation - For a small HeNe tube (say 1 mW), figure this is like a 10 to 15 W bulb inside a plastic box. If you make the box large enough (e.g., 3" x 5" x 10"), there should be enough exterior surface area to adequately remove the waste heat.

    • Getting the beam out - A glass window (e.g., quality microscope slide) mounted at a slight angle (to avoid multiple reflections back to the HeNe tube output mirror) is best. However, a Plexiglass window may be acceptable (i.e., just pointing the laser at a slight angle through the plastic case). A Brewster angle window should be used only if the HeNe tube is a linearly polarized type (not likely for something from a barcode scanner) and then the orientation and angle must be set up for maximum light transmission.

    • Condensation on the optics and elsewhere - This may be a problem on exposed surfaces if they are colder than the ambient conditions. Let the entire laser assembly warm up before attempting to power it up!


  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Interesting, Strange, and Unidentified HeNe Lasers

    When Your Laser Doesn't Fit the Mold

    The vast majority of HeNe tubes and laser heads you will likely come across will be basically similar to those described in the section: HeNe Laser Tubes and Laser Heads. However, when rummaging through old storerooms or offerings at hamfests or high-tech flea markets, you may come across some that are, to put it bluntly, somewhat strange or weird. I would expect that in most cases, these will be either really old, developed for a specific application, or higher performance lab quality models which are just not familiar to someone used to surplus specials. Consider these to be real finds if only for the novelty value! Refurbishing of the lab-grade lasers may be worth the effort and/or expense resulting in a truly exceptional (and possibly valuable) instrument.
    • Really old HeNe tubes are very likely to be non-functional as inadequate seals were probably used (Epoxied, Brewster windows or mirrors) and would need to be regassed, at the very least.

    • Special purpose HeNe lasers could come in a variety of shapes and sizes. I wouldn't even know what to tell you to look for in this area!

    • High performance lab quality lasers will have external mirrors with fine adjustments, more sophisticated internal or external optics, power supplies with tweaks and monitoring meters or test points.

    Here are some descriptions of what I and others have come across:

    Segmented HeNe Tubes

    I have several medium power HeNe tubes that do not have a single long bore (capillary) but rather it is split into about a half dozen sections with a 1 or 2 mm gap between them. Each of the short capillaries is fused into a glass separator without any holes. Two of these tubes look like the more common sealed HeNe tubes except for the multiple segments: 
                    ____________________________________________
               /       |            |            | _______  \
        Anode |\       |            |            |        \  | Cathode
        .-.---' \.-----'-----..-----'-----..-----'------.  '-'---.-.
    <---| |::::  :===========::===========::============:   :::::| |===> 
        '-'---. /'-----.-----''-----.-----''-----.------'  .-.---'-'
              |/       |            |            | _______/  |
               \_______|____________|____________|__________/
    The third has Brewster angle windows and external screw-adjustable mirrors. It also has its cathode in a side-tube rather than the more typical coaxial can type. It 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, one has a broken mirror and the funky tube with the Brewster windows was non-hard sealed and leaked.) The reason(s) for the unusual design are therefore not known at this time. It may have been to stabilize the discharge, suppress unwanted spectral lines, or something else.

    Strange High Power HeNe Laser

    This is a on-going project on finding information and restoring a strange HeNe laser acquired by: Chris Chagaris (pyro@grolen.com). For the complete saga (as of now), see the document: "Interesting Repair Related Stories and Anecdotes", specifically: "Chris's HeNe Laser Adventure". Below is just a description of the laser and estimated specifications and requirements.

    The HeNe laser head was manufactured by Jodan Engineering Associates of Ann Arbor, MI. Output power is 15 to 25 mW, 2 mm diameter beam, probably multimode (not TEM00).

    It requires approximately 2,000 V at 18 to 22 mA (for two discharge feeds, see the diagram below). Starting uses a large trigger transformer to produce an ionizing pulse via external electrodes (rather than a voltage in series with the anode). This is similar the way a xenon flash lamp is triggered.

    The construction is certainly non-standard (at least compared to modern sealed HeNe lasers) and is shown 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 getters
    This tube uses a heated cathode (like an old vacuum tube) requiring a filament supply of 6.3 V at 2.05 A. Only one of the two shown are needed, the other is a spare. Modern HeNe lasers use cold cathodes (the 'can' electrode). However, some other gas lasers (e.g., argon ion) still use heated cathodes.

    A series of magnets (not shown) along axis of discharge: Suppress the 3.39 um line which competes with the 632.8 nm line and can rob up to 25% of its power

    Jodon Laser Head shows the construction in more detail including the magnet locations and orientations.

    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. Stay tuned.

    Hewlett-Packard HeNe Laser

    This 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:

    • The overall size of the aluminum case is 11.5 x 3.5 inches. Part of it has a cylindrical shape.

    • One end of the aluminum case is sealed. It carries the optics. The other end is where the three wires are connected to. This one is open.

    • The laser (glass) tube is about 1/2 inch in diameter and should be about 8 inches long. The exact length is however impossible to measure without making an X-ray (!) since the other end (with the optics) is sealed.

    • The inside diameter of the laser tube itself is difficult to see because everything is sealed inside the aluminum case.

    • On the open end, it can be seen that the laser tube is soldered coaxially to another larger glass tube, about 1.5 inches in diameter. This fits inside the aluminum housing, whose cylindrical bore is about 2.5 inches in diameter.

    • The gap between the two seems actually to be filled with a stack of magnets (like a TWT??!!). The whole assembly is held together by some rubbery material. This also reminds me of how some TWTs are built.

    • The heavily insulated wire goes to an electrode on the smaller glass tube. the two small violet and red wires go to two small electrodes out of the back of the larger tube.

    • The two small wires could NOT be used to monitor the laser's "back-beam" because they aren't connected to anything in the path of that beam. in fact, the actual laser tube sticks out about 1.5 inches out of the back of the larger glass structure, and that's where the back beam should come out from.

    • Another intriguing feature is a small vent drilled in the aluminum case in the front part and sealed with some rubbery material. It might be the same material that can be seen in the back of the assembly or the vent might have been used to pump the air out of the optics compartment(?).

    • The HV connector is made of black plastic with a screw-on ring and it matches the HV connectors I have on several Laser Drive, Inc. potted 1500 V supplies. There is really only one wire inside.

    • The magnets may be toroids. Holding a piece of steel over the outside of the large cylinder shows that there must be magnets all around and along the laser tube - like a Traveling Wave tube.

    • The rear mirror seems to be inside the small tube (not sealed to it as with more common HeNe tubes).

    I have other He-Ne's 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.

  • Back to Sam's Laser FAQ Table of Contents.
  • Back to Helium-Neon Lasers Sub-Table of Contents.
  • Forward to HeNe Laser Testing, Adjustment, Repair .