Power Requirements for He-Ne Lasers

Power for a He-Ne laser is provided by a special high voltage power supply and consists of two parts (these maximum values depend on tube size – a typical 1 to 10 mW tube is assumed):

  • Operating voltage of 1,000 to 3,000v DC at 3 to 8mA.Like most low current discharge tubes, the He-Ne 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 decreasing current.
  • Starting voltage of 5 to 12 kV at almost no current.In the case of a He-Ne tube, the initial breakdown voltage is much greater than the sustaining voltage. The starting voltage may be provided by a separate circuit or be part of the main supply.Often, you may find a wire or conductive strip running from the anode or ballast resistor down to a loop around the tube in the vicinity of the cathode. (Or there may be a recommendation for this in a tube spec sheet.) This external wire loop is supposed to aid in starting (probably where a pulse type starter is involved). There may even be some statistical evidence suggesting a reduction in starting times. I wouldn’t expect there to be much, if any, benefit when using a modern power supply but it might help in marginal cases. But, running the high voltage along the body of the tube requires additional insulation and provides more opportunity for bad things to happen (like short circuits) and may represent an additional electric shock hazard. And, since the strip has some capacitance, operating stability may be impaired. I would probably just leave well enough alone if a starting strip is present and the laser operates without problems but wouldn’t install one when constructing a laser head from components.

    With every laser I’ve seen using one of these strips, it has either had virtually or totally no effect on starting OR has caused problems with leakage to the grounded cylinder after awhile. Cutting away the strip in the vicinity of the anode has cured erratic starting problems in the latter case and never resulted in a detectable increase in starting time.

  • 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 most bare tubes. (The exceptions are larger Spectra-Physics He-Ne lasers where the ballast resistors are also inside a glass tube extension, electrically connected but sealed off from the main 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 ohms 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 He-Ne tubes. Electrical behaviour 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 He-Ne tube. This electrode may actually be part of the mirror assembly at that end of the tube or totally separate from it. The distance from the resistors to the electrode should be minimized – no more than 2 or 3 inches.
    • The negative output of the power supply is connected to the cathode (large can) electrode of the He-Ne tube. This electrode may be electrically connected to the mirror mount at that end of the tube but is a separate aluminium cylinder that extends for several inches down the tube. CAUTION: Some He-Ne tubes use a separate terminal for the cathode and sometimes the anode as well, not the mirror mount(s). Powering one of these via the mirror mounts may result in lasing but will also result in tube damage.

    Note: He-Ne tube starting voltage is lower and operating voltage is higher when powered with reverse polarity. With some power supply designs, the tube may appear to work equally well or even better (since starting the discharge is easier) when hooked up incorrectly. However, this is damaging to the anode electrode of the tube (and may result in more stress on the power supply as well due to the higher operating voltage) and must be avoided (except possibly for a very short duration during testing).

  • Every He-Ne 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 are in the excited state). You can easily and safely demonstrate this behaviour 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.) Optical noise in the output will also increase with excessive current.
  • The efficiency of the typical He-Ne laser is pretty pathetic. For example, a 2 mW HeNe tube powered by 1,400 V at 6mA has an efficiency of less than 0.025%. More than 99.975% 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.

A few He-Ne lasers – usually larger or research types – have used a radio frequency (RF) generator – essentially a radio transmitter to excite the discharge. This was the case with the original He-Ne laser but is quite rare today given the design of internal mirror He-Ne tubes and the relative simplicity of the required DC power supply.

He-Ne Laser Safety

As with *any* laser, proper precautions must be taken to avoid any possibility of damage to vision. The types of He-Ne lasers mostly dealt with in this document are rated Class II, IIIa, or the low end of IIIb (see the section: Laser Safety Classifications. 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, He-Ne 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 He-Ne 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 He-Ne 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 He-Ne tube itself also acts as a small HV capacitor so even touching it should it become disconnected from the power supply may give you a tingle. This probably won’t really hurt you physically but your ego may be bruised if you then drop the tube and it then shatters on the floor!

However, should you be dealing with a much larger He-Ne laser, its power supply is going to be correspondingly more dangerous as well. For example, a 35 mW He-Ne tube typically requires about 8 mA at 5 to 6 kV. That current may not sound like much but the power supply is likely capable of providing much more if you are the destination instead of the laser head (especially if it is a home-made unit using grossly oversized parts)! It doesn’t take much more under the wrong conditions to kill.

After powering off, use a well insulated 1M resistor made from a string of ten 100K, 2 W metal film resistors in a glass or plastic tube to drain the charge – and confirm with a voltmeter before touching anything. (Don’t use carbon resistors as I have seen them behave funny around high voltages. And, don’t use the old screwdriver trick – shorting the output of the power supply directly to ground – as this may damage it internally.)

And only change electrical connections or plug/unplug connectors with power OFF, being aware of the potential for stored charge. In particular, the aluminium cylinder of some HeNe laser heads is the negative return for the tube current via a spring contact inside the rear end-cap. So, pulling off the rear end-cap while the laser is powered will likely make YOU the negative return instead! You will probably then bounce off the ceiling while the laser bounces off the floor, which can easily ruin your entire day in more ways than one. 🙁 🙂 This connection scheme is known to be true for most JDS Uniphase and many Melles Griot laser heads, but may apply to others as well.

Now, for some first-hand experience:

(From: Doug (dulmage@skypoint.com).)

Well, here’s where I embarrass myself, but hopefully save a life…

I’ve worked on medium and large frame lasers since about 1980 (Spectra-Physics 168’s, 171’s, Innova 90’s, 100’s and 200’s – high voltage, high current, no line isolation, multi-kV igniters, etc.). Never in all that time did I ever get hurt other than getting a few retinal burns (that’s bad enough, but at least I never fell across a tube or igniter at startup). Anyway, the one laser that almost did kill me was also the smallest that I ever worked on.

I was doing some testing of AO devices along with some small cylindrical HeNe tubes from Siemens. These little coax tubes had clips for attaching the anode and cathode connections. Well, I was going through a few boxes of these things a day doing various tests. Just slap them on the bench, fire them up, discharge the supplies and then disconnect and try another one. They ran off a 9 VDC power supply.

At the end of one long day, I called it quits early and just shut the laser supply off and left the tube in place as I was just going to put on a new tube in the morning. That next morning, I came and incorrectly assumed that the power supply would have discharged on it own overnight. So, with each hand I stupidly grab one clip each on the laser to disconnect it. YeeHaaaaaaaaa!!!!. I felt like I had been hid across my temples with a two by four. It felt like I swallowed my tongue and then I kind of blacked out. One of the guys came and helped me up, but I was weak in the knees, and very disoriented.

I stumbled around for about 15 minutes and then out of nowhere it was just like I got another shock! This cycle of stuff went on for about 3 hours, then stopped once I got to the hospital. I can’t even remember what they did to me there. Anyway, how embarrassing to almost get killed by a HeNe laser after all that other high power stuff that I did. I think that’s called ‘irony’.

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 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.

He-Ne Lasers – Introduction

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 at first glance (though the design and manufacturing issues which must be dealt with to achieve the desired beam characteristics, power output, stability, and life span, are non-trivial). The gas fill is a mixture of helium and neon gas at low pressure. A pair of mirrors – one totally reflective (called the High Reflector or HR), the other partially reflective (called the Output Coupler or OC) at the wavelength of the laser’s output – complete the resonator assembly. 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.

Note that a true laser jock will further abbreviate “HeNe laser” to simply “HeNe”, pronounced: Hee-nee. Their laser jock colleagues and friends then know this really refers to a laser! 🙂 While other types of lasers are sometimes abbreviated in an analogous manner, it is never to the same extent as the HeNe.

I still consider the HeNe laser to be the quintessential laser: An electrically excited gas between a pair of mirrors. It is also the ideal first laser for the experimenter and hobbyist. OK, well, maybe after you get over the excitement of your first laser pointer! 🙂 HeNe’s are simple in principle though complex to manufacture, the beam quality is excellent – better than anything else available at a similar price. When properly powered and reasonable precautions are taken, they are relatively safe if the power output is under 5 mW. And such a laser can be easily used for many applications. With a bare HeNe laser tube, you can even look inside while it is in operation and see what is going on. Well, OK, with just a wee bit of imagination! 🙂 This really isn’t possible with diode or solid state lasers.

I remember doing the glasswork for a 3 foot long HeNe laser (probably based on the design from: “The Amateur Scientist – Helium-Neon Laser”, Scientific American, September 1964, and reprinted in the collection: “Light and Its Uses” [5]). This included joining side tubes for the electrodes and exhaust port, fusing the electrodes themselves to the glass, preparing the main bore (capillary), and cutting the angled Brewster windows (so that external mirrors could be used) on a diamond saw. I do not know if the person building the laser ever got it to work but suspect that he gave up or went on to other projects (which probably were also never finished). And, HeNe lasers are one of the simplest type of lasers to fabricate which produce a visible continuous beam.

Some die-hards still construct their own HeNe lasers from scratch. Once all the glasswork is complete, the tube must be evacuated, baked to drive off surface impurities, backfilled with a specific mixture of helium to neon (typically around 7:1 to 10:1) at a pressure of between 2 and 5 Torr (normal atmospheric pressure is about 760 Torr – 760 mm of mercury), and sealed. The mirrors must then be painstakingly positioned and aligned. Finally, the great moment arrives and the power is applied. You also constructed your high voltage power supply from scratch, correct? With luck, the laser produces a beam and only final adjustments to the mirrors are then required to optimize beam power and stability. Or, more, likely, you are doing all of this while your vacuum pumps are chugging along and you can still play with the gas fill pressure and composition. What can go wrong? All sorts of things can go wrong! With external mirrors, the losses may be too great resulting in insufficient optical gain in the resonant cavity. The gas mixture may be incorrect or become contaminated. Seals might leak. Your power supply may not start the tube, or it may catch fire or blow up. It just may not be your day! And, the lifetime of the laser is likely to end up being only a few hours in any case unless you have access to an ultra-high vacuum pumping and bakeout facility. While getting such a contraption to work would be an extremely rewarding experience, its utility for any sort of real applications would likely be quite limited and require constant fiddling with the adjustments. Nonetheless, if you really want to be able to say you built a laser from the ground up, this is one approach to take! (However, the CO2 and N2 lasers are likely to be much easier for the first-time laser builder.)

However, for most of us, ‘building’ a HeNe laser is like ‘building’ a PC: An inexpensive HeNe tube and power supply are obtained, mounted, and wired together. Optics are added as needed. Power supplies may be home-built as an interesting project but few have the desire, facilities, patience, and determination to construct the actual HeNe tube itself.

The most common internal mirror HeNe laser tubes are between 4.5″ and 14″ (125 mm to 350 mm) in overall length and 3/4″ to 1-1/2″ (19 mm to 37.5 mm) in diameter generating optical power from 0.5 mW to 5 mW. They require no maintenance and no adjustments of any kind during their long lifetime (20,000 hours typical). Both new and surplus tubes of this type – either bare or as part of complete laser heads – are readily available. Slightly smaller tubes (less than 0.5 mW) and much larger tubes (up to approximately 35 mW) are structurally similar (except for size) to these but are not as common.

Much larger HeNe tubes with internal or external mirrors or one of each (more than a *meter* in length!) and capable of generating up to 250 mW of optical power have been available and may turn up on the surplus market as well (but most of these are quite dead by now). The most famous of these (as lasers go) is probably the Spectra-Physics model 125A whose laser head is over 6 feet in length. It was only rated 50 mW (633 nm), but new samples under optimal conditions may have produced more than 200 mW. Even more powerful ones have been built as research projects. I’ve seen photos of a Hughes HeNe laser with a head around 8 feet in length that required a 6 foot rack-mount enclosure for the exciter.

Monster Vintage Hughes HeNe Laser System
Monster Vintage Hughes HeNe Laser System

Its output power is unknown, but probably less than that of the SP-125A. The largest single transverse mode (SM, with a TEM00 beam profile) HeNe lasers in current production by a well known manufacturer like Melles Griot are rated at about 35 mW minimum over an expected lifetime of 20,000 hours or more, though new samples may exceed 50 mW. However, HeNe lasers rated up to at least 70 mW SM and 100 mW MM are available. Manufacturers include: CDHC-Optics (China), Spectral Laser (Italy), and PLASMA, JSC (Russia). However, the lifetime over which these specifications apply is not known and may be much shorter.

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. Most HeNe lasers operate CW (Continuous Wave) producing a steady beam at a fixed output power unless the power is switched on and off or modulated (or someone sticks their finger in the beam and blocks it!). (At least they are supposed to when in good operating condition!) However, there are some mode-locked HeNe lasers that output a series of short pulses at a high repetition rate. And, in principle, it is possible to force a HeNe laser with at least one external mirror to “cavity dump” a high power pulse (perhaps 100 times the CW power) a couple of nanoseconds long by diverting the internal beam path with an ultra high speed acousto-optic deflector. But, for the most part, such systems aren’t generally useful for very much outside some esoteric research areas and in any case, you probably won’t find any of these at a local flea market or swap meet, though eBay can’t be ruled out! 🙂

Nearly all HeNe lasers output a single wavelength and it is most often red at 632.8 nm. (This color beam actually appears somewhat orange-red especially compared to many laser pointers using diode lasers at wavelengths between 650 and 670 nm). However, green (543.5 nm), yellow (594.1 nm), orange (604.6 and 611.9 nm), and even IR (1,152, 1.523, and 3,921 nm) HeNe lasers are available. There are a few high performance HeNe lasers that are tunable and very expensive. And, occasionally one comes across laser tubes that output two or more wavelengths simultaneously. Although some tubes are designed this way, it is more likely to be a ‘defect’ resulting from a combination of high gain and insufficiently narrow band optics. Such tubes tend to be unstable with the relative power varying among the multiple wavelengths more or less at random.

Note that the single wavelength described above usually consists of more than one longitudinal mode or lasing line (more on this later). However, some HeNe lasers are designed to produce a highly stable single optical frequency or two closely spaced optical frequencies. These are used in scientific research and metrology (measurement) applications, described in more detail below.

Current major HeNe laser manufacturers include Melles-Griot, JDS Uniphase, and LASOS. This is far fewer than there were only a few years ago. So, you may also find lasers from companies like Aerotech, Hughes, Siemens, and Spectra-Physics that have since gotten out of the HeNe laser business or have been bought out, merged, or changed names. For example, the HeNe laser divisions of Aerotech and Hughes were acquired by Melles Griot; Sieman’s HeNe laser product line is now part of LASOS; and Spectra-Physics which was probably the largest producer of HeNe lasers from the very beginning gradually eliminated all HeNe lasers from its product line over the last few years. HeNe tubes, laser heads, and complete lasers from any of these manufacturers are generally of very high quality and reliability.

HeNe lasers have been found in all kinds of equipment including:

  • Consumer: Supermarket checkout UPC and other barcode scanners. early laser printers, early LaserDisc players.
  • Advertising/entertainment: Holography, small laser shows.
  • Measurement: Optical surveying, interferometric metrology and velocimetry, other non-contact measurement and monitoring, ring laser gyro.
  • Construction: Laser level, tunnel boring, alignment of saw mill wood cutting, general surveying.
  • Industrial: Automotive and other alignment; parts detection, counting, and positioning; particle counting.
  • Biotechnology: Blood cell analysis (cytometry), laser induced fluorescence of everything from whole cells to single DNA bases, laser tweezers, confocal microscopy, Raman spectroscopy, anesthesia and other gas analysis.
  • Medical/surgical: Patient positioning systems for diagnostic and treatment machines, alignment of high power CO2 and YAG treatment lasers and pointing beams.

Nowadays, many of these applications are likely to use the much more compact lower (drive) power solid state diode laser. (You can tell if you local ACME supermarket uses a HeNe laser in its checkout scanners by the color of the light – the 632.8 nm wavelength beam from a HeNe laser is noticeably more orange than the 660 or 670 nm deep red from a typical diode laser type.)

Melles Griot (now part of IDEX Optics and Photonics Marketplace. Catalogs used to include several pages describing HeNe laser applications. I know this was present in the 1998 catalog but has since disappeared and I don’t think it is on their Web site.

Also see the section: Some Applications of a 1 mW Helium-Neon Laser for the sorts of things you can do with even a small HeNe laser.

Since a 5 mW laser pointer complete with batteries can conveniently fit on a keychain and generate the same beam power as an AC line operated HeNe laser almost 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 (though this is apparently changing at least for some diode lasers. See the section: Holography Using Cheap Diode Lasers. In particular, the coherence length and monochromicity of even a cheap HeNe laser are excellent and the beam profile is circular and nearly ideal Gaussian TEM00 so that simple spherical optics can be used for beam manipulation. Bare edge emitting laser diodes (the only visible type currently available) on the other hand always produce a wedge shaped beam and have some amount of astigmatism. Correcting this to the equivalent quality of a HeNe laser is difficult and expensive.
  • 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 increasing popularity of DVD players and DVDROM drives, this situation probably won’t last long.

However, the market for new HeNe lasers is still in the 100,000 or more units per year. What can you say? If you need a stable, round, astigmatism-free, long lived, visible 1 to 10 mW beam for under $500 (new, remember!), the HeNe laser is still the only choice.