Demonstration He-Ne Lasers, Weatherproofing

Putting Together a Demonstration He-Ne 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 He-Ne 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 He-Ne laser laid bare where its sealed (internal mirror) He-Ne tube, ballast resistors, wiring, and power supply (with exposed circuit board), are mounted inside a clear Plexiglas case with all parts labelled. This would allow the discharge in the He-Ne tube to be clearly visible. 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 He-Ne tube when a reflex action results in smashing the entire laser to smithereens!

A He-Ne 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, monochromaticy, shape, and stability, will likely be much higher for the He-Ne laser should you also want to use it for actual optics experiments like interferometry. (However, the first one of these – coherence length – can actually be quite good for even the some of the cheap diode lasers in laser pointers.)
  • For a given power level, a 632.8nm He-Ne laser will appear about 5 times brighter than a 670 nm laser pointer. 635 nm laser pointers are available but still more expensive. However, inexpensive laser pointers with wavelengths between 650 and 660 nm are becoming increasingly common and have greater relative brightness.

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 He-Ne 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!).

He-Ne Tube and Power Supply
He-Ne Tube and Power Supply

For example, this is from a hand-held barcode scanner. A similar unit was separated into its component parts:

Melles Griot He-Ne Tube
Melles Griot He-Ne Tube
He-Ne Laser Power Supply IC-I1
He-Ne Laser Power Supply IC-I1

The power supply 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 12v DC, 1 A wall adapter. Used barcode scanner lasers can often be found for $20 or less.

An alternative is to purchase a 0.5 to 1 mW He-Ne tube and power supply kit. This will be more expensive (figure $5 to $15 for the He-Ne tube, $25 to $50 for the power supply) but will guarantee a circuit board with all parts visible.

The He-Ne 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 He-Ne tube and power supply.

When attaching the He-Ne tube, avoid anything that might stress the mirror mounts. While these are quite sturdy and it is unlikely that any reasonable arrangement could result in permanent damage, even a relatively modest force may result in enough mirror misalignment to noticeably reduce output power. And, don’t forget that the mirror mounts are also the high voltage connections and need to be well insulated from each other and any human contact! The best option is probably to fasten the tube in place using Nylon cable ties, cable clamps, or something similar around the glass portion without touching the mirror mounts at all (except for the power connections).

Provide clearly marked red and black wires (or binding posts) for the low voltage DC or a line cord for AC (as appropriate for the power supply used), power switch, fuse, and power-on indicator. Label the major components and don’t forget the essential CDRH safety sticker (Class II for less than 1 mW or Class IIIa for less than 5 mW).

See:

Sam's Demo He-Ne Laser
Sam’s Demo He-Ne Laser

Above, as an example (minus the Plexiglas safety cover), contructed from the guts of a surplus Gammex laser (probably part of a patient positioning system for a CT or MRI scanner). The discrete line operated power supply is simple with the HV transformer, rectifier/doubler, filter, multiplier, and ballast resistors easily identified. This would make an ideal teaching aid.

See the suppliers listed in the chapter: Laser and Parts Sources.

The Ultimate Demonstration He-Ne Laser

Rather than having a see-through laser that just outputs a laser beam (how boring!), consider something that would allow access to the internal cavity, swapping of optics, and modulation of beam power. OK, perhaps the truly ultimate demo laser would use a two-Brewster tube allowing for interchangeable optics at both ends, be tunable to all the He-Ne spectral lines, and play DVD movies. πŸ™‚ We’ll have to settle for something slightly less ambitious (at least until pigs fly). Such a unit could consist of the following components:

  • One-Brewster He-Ne laser tube or head. This can be something similar to the Melles Griot 05-LHB-570 tube or the Climet 9048 head which contains this tube. These have a Brewster window at one end and an internal HR mirror with a 60 cm Radius of Curvature (RoC) at the other. Their total length is about 10.5 inches (260 mm).
  • Adjustable mirror mount with limited range to permit easy mirror tweaking but with minimal chance of getting alignment really messed up. A basic design using a pair of plates with X and Y adjustment screws and a common pivot with lock washers for the compliance springs would be adequate.
  • Interchangeable mirrors of RoC = 60 cm and reflectance of 98% to 99.5% (OC) and 99.999% (HR in place of OC to maximize internal photon flux). These may be salvaged from a dead 3 to 5 mW He-Ne laser tube. Those from a tube like the Spectra-Physics 084-1 would be suitable. It would be best to install the mirrors in protective cells for ease of handling.
  • Baseplate to mount the laser and optics with the internal HR of the one-Brewster tube/head about 60 cm from the external mirror to create a confocal cavity – about one half of which is external and accessible. An option would be to put the external mirror mount on a movable slide to allow its position to be changed easily.
  • Power supply with adjustable current and modulation capability. This would provide the ability to measure output power versus current and to use the laser as an optical transmitter with a solar cell based receiver.
  • Plexiglas box to house and protect the laser and power supply (as well as inquisitive fingers from high voltage) with part of one side open to allow access to the internal photons.

Everything needed for such a setup is readily available or easily constructed at low cost but you’ll have to read more to find out where or how as each of the components are dealt with in detail elsewhere in Sam’s Laser FAQ (but I won’t tell you exactly where – these are all the hints you get for this one!).

A system like this could conceivably be turned into an interactive exhibit for your local science museum – assuming they care about anything beyond insects and the Internet these days. πŸ™‚ There are some more details in the next section.

Guidelines for a Demonstraton One-Brewster He-Ne Laser

The following suggestions would be for developing a semi-interactive setup whereby visitors can safely (both for the visitor and the laser) adjust mirror alignment and possibly some other parameters of laser operation. The type of one-Brewster (1-B) He-Ne laser tube like the Melles Griot 05-LHB-570. Note that the 05-LHB-570 is a wide bore tube that runs massively multi (transverse) mode with most mirrors configurations unless an intracavity aperture is added. This is actually an advantage for several reasons:

  1. The multi-transverse mode structure is interesting in itself and provides additional options for showing how it can be controlled.
  2. Mirror alignment is easier and the tube will lase over a much wider range of mirror orientation.
  3. Output power is higher for its size and power requirements.

Here are some guidelines for designing an interactive exhibit:

  • Mount the 1-B tube in a clear plastic (Plexiglas) enclosure with some ventilation holes to allow for cooling but make sure any parts with high voltage (anode, ballast resistors if not insulated) are safely protected from the curious. Provide a small hole lined up with the Brewster window for the intracavity beam. However, even if the B-window is at the cathode-end of the tube, don’t allow it to be accessible as the first fingerprint will prevent lasing entirely.
  • Put the power supply in a safe place inside another clear plastic box if desired. I’d recommend controlling it with a time switch that will turn it on for perhaps 10 minutes with a push of a button. This is a tradeoff between wear from running the laser all the time and wear from repeated starts. Don’t forget the fuse!!!
  • Orient the tube so the B-windows is either on the side or facing down. This will minimize dust collection and permit the rig to operate for many hours or days without the need for even dusting.
  • Use an output mirror with an RoC from 50 cm to planar and reflectivity of 98 to 99.5 percent at 632.8 nm. The specific parameters and distance will affect the beam size, mode structure, and output power. A shorter RoC will limit the distance over which lasing will take place but will be somewhat easier to align.
  • Use a decent quality mirror mount like a Newport MM-1 for the output mirror. Once it’s secured, arrange for the adjustment screws to be accessible to visitors but limit the range of rotation to less than one turn and mark the location of each screw where lasing is peaked. That way, no amount of fiddling will lose lasing entirely.
  • The distance between the mirror and tube can be fixed or adjustable:
    • For a fixed location, a distance of a few inches between the laser enclosure and mirror mount is recommended. This is enough space to install an aperture or Brewster plate. Or a hand to show that the beam is only present with the resonator is complete, not just a red light inside! But, it’s short enough that alignment is still easy.
    • For added excitement, put the mirror mount on a precision rail to permit the distance to be varied from 0 to at least 45 cm from the B-window. Then, it will be possible to see how the mode structure changes with distance. This will depend on the RoC of the mirror as well.
  • Another option is to provide various things like an iris diaphragm, thin wires and/or a cross-hair, adjustable knife edge, Brewster plate that can be oriented, etc. However, some care will be needed in making these useful without a lot of hand holding.

Weatherproofing a He-Ne Laser

If you want to use a He-Ne 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 He-Ne 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 He-Ne tube output mirror) is best. However, a Plexiglas 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 He-Ne 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!

Internal Mirror He-Ne Tubes up to 35 mW – Red and Other Colours

Typical He-Ne Tube Specifications

Prior to the introduction of the CD player, the red He-Ne laser was by far the most common source of inexpensive coherent light on the planet. The following are some typical physical specifications for a variety of red (632.8 nm) He-Ne tubes (all are single transverse mode – TEM00):

Where:

  • Power Output is the minimum beam power after a specified warm up period over the spec’d life of the tube.
  • Tube Operating Voltage is the voltage across the bare tube at the nominal operating current.
  • Tube Start Voltage is the minimum voltage across the bare tube required to guarantee starting.
  • Tube Size is generally the maximum diameter of the tube envelope and the total length from the outer surfaces of the mirrors.

Tubes like this are generally available in both random and linearly polarized versions which are otherwise similar with respect to the above characteristics (for red tubes at least, more below).

At least one other basic specification may be critical to your application: Which end of the tube the beam exits! There is no real preference from a manufacturing point of view for red He-Ne lasers. (For low gain “other-colour” He-Ne laser tubes, it turns out that anode output results is slightly higher gain and thus slightly higher output for the typical hemispherical cavity because it better utilizes the mode volume.) However, this little detail may matter a great deal if you are attempting to retrofit an existing barcode scanner or other piece of equipment where the tube clips into a holder or where wiring is short, tight, or must be in a fixed location. For example, virtually all cylindrical laser heads require that the beam exits from the cathode-end of the tube. It is possible that you will be able to find two versions of many models of He-Ne tubes if you go directly to the manufacturer and dig deep enough. However, this sort of information may not be stated where you are buying surplus or from a private individual, so you may need to ask.

The examples above (as well as all of the other specifications in this and the following sections) are catalog ratings, NOT what might appear on the CDRH safety sticker (which is typically much higher). See the section: About Laser Power Ratings for info on listed, measured, and CDRH power ratings.

Note how some of the power levels vary widely with respect to tube dimensions, voltage, and current. Generally, higher power implies a longer tube, higher operating/start voltages, and higher operating current – but there are some exceptions. In addition, you will find that physically similar tubes may actually have quite varied power output. This is particularly evident in the manufacturers’ listings. (See the chapter: A HREF=”laserhcl.htm#hcltoc”>Commercial Unstabilized HeNe Lasers.)

These specifications are generally for minimum power over the guaranteed life of the tube. New tubes and individual sample tubes after thousands of hours may be much higher – 1.5X is common and a “hot” sample may hit 2X or more. My guess is that for tubes with identical specifications in terms of physical size, voltage, and current, the differences in power output are due to sample-to-sample variations. Thus, like computer chips, they are selected after manufacture based on actual performance and the higher power tubes are priced accordingly! This isn’t surprising when considering the low efficiency at which these operate – extremely slight variations in mirror reflectivity and trace contaminants in the gas fill can have a dramatic impact on power output.

I have a batch of apparently identical 2 mW Aerotech tubes that vary in power output by a factor of over 1.5 to 1 (2.6 to 1.7 mW printed by hand on the tubes indicating measured power levels at the time of manufacture).

And, power output also changes with use (and mostly in the days of soft-sealed tubes, just with age sitting on the shelf):

(From: Steve Roberts.)

“I have a neat curve from an old Aerotech catalogue of He-Ne laser power versus life. The tubes are overfilled at first, so power is low. They then peak at a power much higher than rated power, followed by a long period of constant power, and then they SLOWLY die. It’s not uncommon for a new He-Ne tube to be in excess of 15% greater than rated power.”

And the answer to your burning question is: No, you cannot get a 3 mW tube to output 30 mW – even instantaneously – by driving it 10 times as hard!

I have measured the operating voltage and determined the optimum current (by maximizing beam intensity) for the following specific samples – all red (632.8 nm) tubes from various manufacturers. (The starting voltages were estimated.):

Melles Griot, Uniphase, Siemens, PMS, Aerotech, and other HeNe tubes all show similar values.

The wide variation in physical dimensions also means that when looking at descriptions of He-Ne lasers from surplus outfits or the like, the dimensions can only be used to determine an upper (and possibly lower) bound for the possible output power but not to determine the exact output power (even assuming the tube is in like-new condition). Advertisements often include the rating on the CDRH safety sticker (or say ‘max’ in fine print). This is an upper bound for the laser class (e.g., Class IIIa), not what the particular laser produces or is even capable of producing. It may be much lower. For example, that Class IIIa laser showing 5 mW on the sticker, may actually only be good for 1 mW under any conditions! The power output of a He-Ne laser tube is essentially constant and cannot be changed significantly by using a different power supply or by any other means. See the section: Buyer Beware for Laser Purchases.

Also see the section: Locating Laser Specifications.

In addition to power output, power requirements, and physical dimensions, key performance specifications for He-Ne lasers also include:

  • Beam Diameter at the laser’s output aperture and beam profile (Gaussian TEM00 for most small He-Ne laser tubes).
  • Beam Divergence (probably far field ignoring beam waist). Note that this may not always be the same as the expected value from the diffraction limit based on beam/bore diameter as it also depends on the combination of the HR and OC mirror (inside) curvature and the shape of the exterior surface of the OC.
  • Mode Spacing (frequency) between the multiple longitudinal modes that are active simultaneously in all but single mode frequency stabilized lasers.

With manufacturers like Aerotech, Melles Griot, and Siemens, a certain amount of information can be determined from the model number. For example, here is how to decipher most of those from Melles Griot (e.g., 05-LHP-121-278):

  • All Melles Griot He-Ne laser tubes and power supplies start with 05. Matched systems may start with 25 (e.g., laser head and lab-style power supply).
  • The first letter will be an L for all He-Ne laser tubes and heads except for perpendicular window terminated tubes (in which case it will be W – this is inconsistent with the rest of their numbering but who am I to complain!), and some of their self contained lasers where it will be S.
  • The second letter will be one of: H = red (632.8 nm), G = green (543.5 nm), Y = yellow (594.1 nm), O = orange (611.9 nm), or I = infra-red (1,523 or 3,391 nm). A couple of self contained red lasers use R for red but for most, I guess they got stuck using H (presumably denoting He-Ne) before ‘other colour’ He-Ne lasers were part of their product line. And, their stabilized He-Ne lasers use a T here. Confused yet? πŸ™‚
  • The third letter will be one of: R = Randomly polarized, L = linearly polarized, or B = Brewster window at one or both ends.
  • The following three digit number determines the physical characteristics of the laser tube to some extent. Unfortunately, there may be no direct mathematical relationship of this number to anything useful. As will be seen below, for some models, it (or some of its digits) sort of correlates with output power or length but for others, they might as well be totally random! However, it does appear as though an identical set of numbers among different colour tubes (see below) will denote similar physical size tubes at least.
  • If there are additional numbers, they relate to a special variation on the basic design done for a particular customer. For example, this might be a different curvature on the outer surface of the output mirror to provide a non-standard divergence to eliminate the need for an additional lens in a barcode scanner. Or, an external window for protection from the elements or to deliberately reduce output power. Go figure. πŸ™‚ It may also just denote a specific configuration like -249 (meaning 115 VAC operation, kind of arbitrary, huh?) or -55 (meaning 5.5 mA). In these cases, the user may be able to modify the settings (flip a switch or twiddle a pot) but the warranty may then be void.

The vast majority of Melles Griot lasers you are likely to come across will follow this numbering scheme though there are some exceptions, especially for custom assemblies. (Some surplus places drop the leading ’05-‘ when reselling Melles Griot laser tubes or heads so an 05-LHP-120 would become simply an LHP-120.)

For other manufacturers like Spectra-Physics, the model numbers are totally arbitrary!

He-Ne Tubes of a Different Colour

Although a red beam is what everyone thinks of when a He-Ne laser is discussed, He-Ne tubes producing green, yellow, and orange beams, as well as several infra-red (IR) wavelengths, are also manufactured. However, they are not found as often on the surplus market because they are not nearly as common as the red variety. In terms of the number of He-Ne lasers manufactured, red is far and away the most popular, with all the others combined accounting for only 1 to 2 percent of the total production. In order of decreasing popularity, it’s probably: red, green, yellow, infra-red (all IR wavelengths), orange. Non-red tubes are also more expensive when new since for a given power level, they must be larger (and thus have higher voltage and current ratings) due to their lower efficiency (the spectral lines being amplified are much weaker than the one at 632.8 nm). Operating current for non-red He-Ne tubes is also more critical than for the common red variety so setting these up with an adjustable power supply or adjusting the ballast resistance for maximum output is recommended.

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, a lower power tube may be more than adequate for many applications. Yellow (594.1 nm) and orange (611.9 nm) He-Ne lasers appear more visible by factors of about 3 and 2 respectively compared to red beams of similar power.

Infrared-emitting He-Ne lasers exist as well. In addition to scientific uses, these were used for testing in the Telecom industry before sufficiently high quality diode lasers became available.Yes, you can have a He-Ne 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 He-Ne tube. However, by far the most likely explanation for no visible output beam is that the mirrors are misaligned or the tube is defective in some other way. Unfortunately, silicon photodiodes or the silicon sensors in CCD or CMOS cameras do not respond to any of the He-Ne IR wavelengths, so the only means of determining if there is an IR beam are to use a GaAs photodiode, IR detector card, or thermal laser power meter. IR He-Ne tubes are unusual enough that it is very unlikely you will ever run into one. However, they may turn up on the surplus market especially if the seller doesn’t test the tubes and thus realize that these behave differently – they are physically similar to red (or other colour) He-Ne tubes except for the reflectivity of the mirrors as a function of wavelength. (There may be some other differences needed to optimize each color like the He:Ne ratio, isotope purity, and gas fill pressure, but the design of the mirrors will be the most significant factor and the one that will be most obvious with a bare eyeball, though the color of the discharge may be more pink for green He-Ne tubes and more orange and brighter for IR He-Ne tubes compared to red ones, more below.) Even if the model number does not identify the tube as green, yellow, orange, red, or infra-red, this difference should be detectable by comparing the appearance of its mirrors (when viewed down the bore of an UNPOWERED tube) with those of a normal (known to be red) He-Ne tube. (Of course, your tube could also fail to lase due to misaligned or damaged mirrors or some other reason.

As noted above, the desired wavelength is selected and the unwanted wavelengths are suppressed mostly by controlling the reflectivity functions of the mirrors. For example, the gains of the green and yellow lines (yellow may be stronger) are both much much lower than red and separated from each other by about 50 nm (543.5 nm versus 594.1 nm). To kill the yellow line in a green laser, the mirrors are designed to reflect green but pass yellow. I have tested the mirrors salvaged from a Melles Griot 05-LGP-170 green He-Ne tube (not mine, from “Dr. Destroyer of Lasers”). The HR (High Reflector) mirror has very nearly 100% reflectivity for green but less than 25% for yellow. The OC (Output Coupler) also has a low enough reflectivity for yellow (about 98%) such that it alone would prevent yellow from lasing. The reflectivities for orange, red, and IR, are even lower so they are also suppressed despite their much higher gain, especially for the normal red (632.8 nm) and even stronger mid-IR (3,391 nm) line.

However, to manufacture a tube with optimum and stable output power, it isn’t sufficient to just kill lasing for unwanted lines. The resonator must be designed to minimize their contribution to stimulated emission – thus the very low reflectivity of the HR for anything but the desired green wavelength. Otherwise, even though sustained oscillation wouldn’t be possible, unwanted colour photons would still be bouncing back and forth multiple times stealing power from the desired colour. The output would also be erratic as the length of the tube changed during warm up (due to thermal expansion) and this affected the longitudinal mode structure of the competing lines relative to each other. Some larger He-Ne lasers have magnets along the length of the tube to further suppress (mostly) the particularly strong mid-IR line at 3,391 nm. (See the section: Magnets in High Power or Precision HeNe Laser Heads.)

In addition, you can’t just take a tube designed for a red laser, replace the mirrors, and expect to get something that will work well – if at all – for other wavelengths. For one thing, the bore size and mirror curvature for maximum power while maintaining TEM00 operation are affected by wavelength.

Furthermore, for these other colour He-Ne lasers which depend on energy level transitions which have much lower gain than red – especially the yellow and green ones – the gas fill pressure, He:Ne ratio, and isotopic composition and purity of the helium and neon, will be carefully optimized and will be different than for normal red tubes.

Needless to say, the recipes for each type and size laser will be closely guarded trade secrets and only a very few companies have mastered the art of other colour He-Ne lasers, especially for high power (in a relative sort of way) in yellow and green. I am only aware of four companies that currently manufacture their own tubes: Melles Griot, Research Electro-Optics, Uniphase, and LASOS, with the last two having very few models to choose from. Others (like Coherent) simply resell lasers under their own name.

And, the answer to that other burning question should now be obvious: No, you can’t convert an ordinary red internal mirror He-Ne tube to generate some other colour light as it’s (almost) all done with mirrors and they are an integral part of the tube. πŸ™‚ Therefore, your options are severely limited. As in: There are none. (However, going the other way, at least as a fun experiment, may be possible. For a laser with external mirrors, a mirror swap may be possible (though the cavity length may be insufficient to resonate with the reduced gain of other-colour spectral lines once all loses taken into consideration). But realistically, this option doesn’t even exist where the mirrors are sealed into the tube.

There are also a few He-Ne lasers that can output more than one of the possible colors simultaneously (e.g., red+orange, orange+yellow) or selectively by turning knob (which adjusts the angle of a Littrow or other similar dispersion prism) inside the laser cavity using a Brewster window He-Ne tube). But such lasers are not common and are definitely very expensive. So, you won’t likely see one for sale at your local hamfest – if ever! One manufacturer of such lasers is Research Electro-Optics (REO). See the section: Research Electro-Optics’s Tunable HeNe Lasers.

However, occasionally a He-Ne tube turns up that is ‘defective’ due to incorrect mirror reflectivities or excessive gain or magic πŸ™‚ and actually outputs an adjacent colour in addition to what it was designed to produce. I have such a tube that generates about 3 mW of yellow (594.1 nm) and a fraction of a mW of orange (611.9 nm) but isn’t very stable – power fluctuates greatly as it warms up. Another one even produces the other orange line at 611.9 nm, and it’s fairly stable. But, finding magic ‘defective’ tubes such as these by accident is extremely unlikely though I’ve heard of the 640.1 nm (deep red) line showing up on some supposedly good normal red (632.8 nm) He-Ne tubes.

As a side note: It is strange to see the more or less normal red-orange glow in a green He-Ne laser tube but have a green beam emerging. A diffraction grating or prism really shows all the lines that are in the glow discharge. Red through orange, yellow and green, even several blue lines (though they are from the helium and can’t lase under any circumstances)!! The IR lines are present as well – you just cannot see them.

Actually, the colour of the discharge may be subtly different for non-red He-Ne tubes due to modified gas fill and pressure. For example, the discharge of green He-Ne tubes may appear more pink compared to red tubes) which are more orange), mostly due to lower fill pressure. The fill mix and pressure on green He-Ne tubes is a tricky compromise among several objectives that conflict to some extent including lifetime, stability (3.39Β΅m competition), and optical noise. This balancing act and the lower fill pressure are why green He-Ne tubes don’t last as long as reds. Have I totally confused you, colour-wise? πŸ™‚

The expected life of ‘other colour’ He-Ne tubes is generally much shorter than for normal red tubes. This is something that isn’t widely advertised for obvious reasons. Whereas red He-Ne tubes are overfilled initially (which reduces power output) and they actually improve with use to some extent as gas pressure goes down, this luxury isn’t available with the low gain wavelengths – especially green – everything needs to be optimal for decent performance.

The discharge in IR He-Ne tubes may be more orange and brighter due to a higher fill pressure. Again, this is due to the need to optimize parameters for the specific wavelength.

Determining He-Ne Laser Colour from the Appearance of the Mirrors

Although most He-Ne lasers are the common red (632.8 nm) variety (whose beam actually appears orange-red), you may come across unmarked He-Ne tubes and just have to know what colour output the produce without being near a He-Ne laser power supply.

Since the mirrors used in all He-Ne lasers are dielectric – functioning as a result of interference – they have high reflectivity only around the laser wavelength and actually transmit light quite well as the wavelength moves away from this peak. By transmitted light, the appearance will tend to be a colour which is the complement of the laser’s output – e.g., cyan or blue-green for a red tube, pink or magenta for a green tube, blue or violet for a yellow tube. Of course, except for the IR variety, if the tube is functional, the difference will be immediately visible when it is powered up!

The actual appearance may also depend on the particular manufacturer and model as well as the length/power output of the laser (which affects the required reflectivity of the OC), as well as the revision number of your eyeballs. πŸ™‚ So, there could be considerable variation in actual perceived colour. Except for the blue-green/magenta combination which pretty much guarantees a green output He-Ne tube, more subtle differences in colour may not indicate anything beyond manufacturing tolerances.

Appearance of He-Ne Laser Mirrors
Appearance of He-Ne Laser Mirrors

The chart above in conjunction withΒ will help to identify your unmarked He-Ne tube. (For accurate rendition of the graphic, your display should be set up for 24 bit colour and your monitor should be adjusted for proper colour balance.)

The entry labelled ‘Broadband’ relates to the HR mirror in some unusual multiple colour (combinations of red and/or orange and/or yellow) internal mirror tubes as well as those with an internal HR and Brewster window for external OC optics. And, the yellow and orange tubes may actually use broad band HRs. The OCs would then be selected for the desired wavelength(s) and may also have a broad band coating.

For low gain tubes, they play games with the coatings. I guess it isn’t possible to just make a highly selective coating for one wavelength that’s narrow enough to have low reflectivity at the nearby lines so they won’t lase. So, one mirror will be designed to fall off rapidly on one side of the design wavelength, the other mirror on the other side. That’s one reason front and back mirrors on yellow and green tubes in particular have very different appearances.

As noted, depending on laser tube length/output power, manufacturer, and model, the appearance of the mirrors can actually vary quite a bit but this should be a starting point at least. For example, I have a Melles Griot 05-LHR-170 He-Ne laser tube that should be 594.1 nm (yellow) but actually outputs some 604.6 nm (orange) as well. It’s mirror colours for the HR and OC are almost exactly opposite of those I have shown for the yellow and orange tubes! I don’t know whether this was intentional or part of the problem And, while from this limited sample, it looks like the OCs for orange, yellow, and green He-Ne lasers appear similar, I doubt that they really are in the area that counts – reflectivity/transmission at the relevant wavelengths.

More on Other Colour He-Ne Lasers

Here are some comments on the difficulty of obtaining useful visible output from He-Ne lasers at wavelengths other than our friendly red (632.8 nm):

(From: Steve Roberts.)

You do need a isotope change in the gases for green, and a He:Ne ratio change for the other orange and yellow lines. In addition, the mirrors to go to another line will have a much lower output transmission. The only possible lines you’ll get on a large frame He-Ne laser will be the 611.9 nm orange and 594.1 nm yellow. The green requires external mirror tubes in excess of a meter and a half long and a Littrow prism to overcome the Brewster losses and suppress the IR.

The original work on green was done by Rigden and Wright. The short tubes have lower losses because they have no Brewsters and thus can concentrate on tuning the coatings to 99.9999% reflectivity and maximum IR transmission. There is one tunable low power unit on the market that does 6 lines or so, but only 1 line at a time, and the $6,000 cost is kind of prohibitive for a few milliwatts of red and fractional milliwatt powers on the other lines. But, it will do green and has the coatings on the back side of the prism to kill the losses.

Also look for papers by Erkins and Lee. They are the fellows who did the green and yellow for Melles Griot and they published one with the energy states as part of a poster session at some conference. Melles Griot used to hand it out, that’s how I had a copy, recently thrown away.

Even large He-Ne lasers such as the SP-125 (rated at 50mW of red) will only do about 20mW of yellow, with a 35mW SP-127 you’re probably only looking at 3 to 5mW of yellow. And, for much less then the cost of the custom optics to do a conversion, you can get two or three 4 to 5 mW yellow heads from Melles Griot. I know for a fact that a SP-127 only does about 3mW of 611.9 with a external prism and a remote cavity mirror, when it does 32mW of 632.8nm.

So in the end, unless you have a research use for a special line, it’s cheaper to dig up a head already made for the line you seek, unless you have your own optics coating lab that can fabricate state-of-the-art mirrors.

I have some experience in this, as I spent months looking for a source of the optics below $3,000.

(From: Sam.)

I do have a short (265 mm) one-Brewster He-Ne tube (Melles Griot 05-LGB-580) with its internal HR optimized for green that operates happily with a matching external green HR mirror (resulting in a nice amount of circulating power) but probably not with anything having much lower reflectivity to get a useful output beam. In fact, I could not get reliable operation even with the HR from a dead green He-Ne laser tube as the Brewster window would not remain clean enough for the time required to align the mirror.

I would expect an SP-127 to do more than 3 to 5 mW of yellow, my guess would be 10 to 15 mW with optimized mirrors but no tuning prism. If I can dig up appropriate mirrors, I intend to try modifying an SP-127 to make it tunable and/or do yellow or green. πŸ™‚

(From: Lynn Strickland (stricks760@earthlink.net).)

You can find 640.1 nm in a lot of red He-Ne lasers. I have a paper on it somewhere, and cavity design can influence it to a large extent. If you have a decent quality grating, it’s pretty easy to pick up. 629 nm is the one you don’t see too much.

I’m no physicist, but the lower gain lines can lase simultaneously with the higher gain lines, no problem, as long as there is sufficient gain available in the plasma. It’s really pretty easy to get a He-Ne laser to output on all lines at the same time (if you have the right mirrors). The trick is optimizing the bore-to-mode ratio, gas pressure, and isotope mixture to get good TEM00 power. Usually the all-lines He-Ne lasers are multi (transverse) mode. I don’t know of anyone who makes them commercially though – at least not intentionally.

Steve’s Comments on Superradiance and the 3.39Β΅m He-Ne Laser

Generally, when a gas laser is superradiant, there is a limit to its maximum power output (with exceptions for nitrogen and copper vapour laser, although nitrogen’s upper limit is defined by the maximum cavity length into which you can generate a 300ns or less excitation pulse.

The 3.39Β΅m He-Ne laser’s gain is still, like all other He-Ne lines limited by a wall collision to return the excited atoms to the ground state. 3.39 Β΅m He-Ne lasers have larger bores then normal He-Ne lasers, and the bores are acid etched to fog them and create more surface area, but still the most power I’ve ever seen published was 40 mW – nothing to write home about. The massive SP-125, the largest commercial He-Ne laser, could be ordered with a special tube and special optics for 3.39Β΅m, and it still only did about 1/3rd the visible power. Superradiance and ultimate power are not tied together.

The reason 3.39Β΅m got all the writeups it did was that it started on the same upper state as all the other He-Ne lines, was easily noticed when it sapped power from the visible line, and was, at the time, a exotic wavelength for which there were few other sources.

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 nm), yellow (594.1 nm), and orange (604.6 and 611.9 nm) HeNe lasers are also available but are not nearly as ‘efficient’ as the common red type since the spectral lines that need to be amplified are much weaker at these wavelengths. Thus, ‘other color’ HeNe lasers must be much larger for the same output power and use higher quality mirrors. Manufacturing yield is also lower and far fewer of these are produced. Taken together, the bottom line is that they are much more expensive either new or surplus.Note: Since the gain of these wavelengths is so low, they also have a shorter life and the chance of finding working surplus green or yellow HeNe lasers is much lower than for red. I would not recommend bidding on an eBay auction for one of these unless guaranteed to be working. The likelihood of the problem for an “unknown condition” green or yellow HeNe laser being just mirror alignment is small to none!
  • IR (infra-red) HeNe laser tubes are manufactured as well (1,523.1 nm is most common probably because this wavelength is useful for testing of fiber optic data transmission systems). The other two common IR wavelengths are 1,152.3 and 3,391.3 nm. However, an invisible beam just doesn’t seem as exciting and these make truly lousy laser pointers!

Typical maximum output available from (relatively) small HeNe tubes (400 to 500 mm length) for various colors: red – 10 mW, orange – 3 mW, yellow – 2 mW, green – 1.5 mW, IR – 1 mW. Higher power red HeNe tubes (up to 35 mW or more and over 1 meter long) and ‘other-color’ HeNe tubes (much lower – under 10 mW) are also available. However, these will be very large and very expensive.

Tunable HeNe Lasers

If it were possible to select any available wavelength desired, then some people would be content beyond description. πŸ™‚

A few tunable HeNe lasers have been produced commercially. These provide wavelength (color) selection with the turn of a knob. However, due to the low gain of most HeNe lasing lines, producing a useful tunable HeNe laser is not an easy task. Everything must be just about perfect to get the “other color” lines to lase at all, and even more so when a laser is to be designed to work at more than one wavelength with a TEM00 beam. The most widely known such laser (as these things go) is manufactured by Research Electro-Optics, Inc. (REO). It produces at least 5 of the visible wavelengths: normal red, two oranges, yellow, and green. A Littrow (or Brewster) prism with micrometer screw adjusters takes the place of the HR mirror in a normal HeNe laser. See the section: Research Electro-Optics Tunable HeNe Lasers.

There used to be a model ML-500 tunable HeNe laser from Spindler and Hoyer that did *14* lines between 611 nm and 1,523 nm. So no 604 nm orange, 594.1 nm yellow, 543.5 nm green, or 3.39 Β΅m IR. The mirror set had to be changed to go between the visible and IR wavelengths. It used a Birefringent Filter (BRF) for wavelength selection instead of the Littrow prism in the REO tunable laser. A BRF has the advantage that there is no loss from a slightly incorrect Brewster angle for all but one wavelength, unavoidable with a Littrow prism. This is because the BRF is always set at exactly the Brewster angle. The birefringent crystal in the BRF filter produces a different optical delay for polarization components oriented in the direction of its slow and fast axes. Only when this difference is a multiple of a full cycle for any given wavelength, will the polarization be unchanged and thus result in minimal loss through the BRF. By rotating the BRF around its optical axis (still maintaining it at the Brewster angle to the laser’s optical axis), the wavelength where minimum loss occurs can be selected. In 1987, it was only $5,800 for laser with either wavelength range, an additional $750 for the other mirror set

I don’t know why Spindler and Hoyer would have admitted defeat in not including those other wavelengths as they were certainly known at the time. Perhaps, the losses through the two Brewster windows of their laser tube and the Brewster angled plate of the BRF compared to those of the Brewster window and Brewster prism of the PMS/REO tunable laser were just too high. Perhaps, their mirror coating technology was not as good as what PMS/REO had available.

Unfortunately, Spindler and Hoyer no longer makes this laser, only boring normal HeNe lasers and other optical equipment. However, a scan of the original ML-500 product brochure can be found at Vintage Lasers and Accessories Brochures and Manuals. With modern technology, a 17 line tunable HeNe laser should be possible. πŸ™‚ A tube with internal mirrors and a BRF *inside* would reduce the number of Brewster angle reflective surfaces to only 2, compared to the 3 of the PMS/REO design. A magnetic coupling can be used to move the BRF from outside the tube. In addition, the mirrors can be recessed away from the ends of the tube so they don’t experience any high temperatures during the sealing process. The tube itself would be hard-sealed with frit or regular glass. Then optical contacting or leaky Epoxy seals can be avoided. Use a Brewster angle window to pass the laser beam out of the tube. One of the mirror mounts would be attached via a metal bellows to allow for alignment.

Exact Frequency/Wavelength of HeNe Lasers

There is, of course, no single precise HeNe wavelength since any given cavity will only oscillate at the permitted longitudinal modes and the gain curve is something like 1.5 GHz wide. Thus, for a common HeNe laser, there is no single wavelength and those that are present drift over time (mostly due to thermal expansion of the cavity).

For reference, here are the approximate red HeNe parameters close enough for Government work: πŸ˜‰

  • A vacuum wavelength of 633 nm corresponds to an optical frequency of 474 THz.
  • A wavelength change of 1 nm at 633 nm corresponds to an optical frequency change of 749 GHz.
  • An optical frequency change of 1 GHz at 633 nm corresponds to a wavelength change of 1.34 pm (picometers).
  • A one part-per-billion (ppb) change in wavelength at 633 nm corresponds to an optical frequency change of 474 kHz.

Where more decimal points matter, a single mode frequency stabilized HeNe laser will have very nearly a constant single wavelength precise to 9 or more significant figures but it too will be affected by various physical parameters including the exact length of the laser’s cavity, gas pressure and He:Ne fill ratio, and temperature – there is no single correct answer!

For example, one typical stabilized HeNe laser from Hewlett-Packard, has a precise vacuum wavelength of 632.991372 nm. Another one from Melles Griot (as noted below) is 632.991058 nm in vacuum or 632.81644 nm in air (divide by the index of refraction of air, n=1.00027593).

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

The Melles Griot catalog claims a nominal frequency of 473.61254 THz for their 05-STP series of frequency stabilized lasers. (Elsewhere in the same catalog they are more precise and lists 473.612535 THz for the 632.8 nm line.) Anyhow, with c = 2.997925E8 m/s this gives 632.991058 nm in vacuum or 632.81644 nm in air for n = 1.00027593 (formula from J Phys.E, vol. 18, 1985, pp. 845ff). To find reliable values for all the other HeNe lines is quite difficult. One has to compare a number of books to be sure whether the values are for air or vacuum.

(From: D. A. Van Baak (dvanbaak@calvin.edu).)

Well, here it is depending on the level of precision desired:

  • 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 stabilize 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 632.8 nm HeNe laser stabilized to the a-13 component of the R(127) line of the 11-5 transition of the 127-Iodine dimer molecule is:

  • Frequency = 473,612,214,705 kHz.
  • Wavelength = 632,991,398.22 fm (femtometer = 10-15 m).

under certain specified conditions, with uncertainty 2.5×10-11. 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 1.5 mm – the inside bore diameter of the capillary discharge tube.
  • The beam from most HeNe lasers 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.The minimum divergence obtainable is affected mostly by beam (exit or waist) diameter (wider is better). Other factors like the ratio of length to bore diameter (narrower is better) may also affect this slightly. The equation for a plane wave source is:

    So, for an ideal HeNe laser with a .5 mm bore at 632.8 nm, the divergence angle will be about 1.6 mR. Note that although a wider bore should result in less divergence, this also permits more not quite parallel rays to participate in the lasing process. This assumes planar mirrors – which few HeNe lasers use. Where one or both mirrors are curved, the divergence changes. For example, it is common with HeNe tubes for the Output Coupler (OC) mirror to be ground slightly concave and for the High Reflector (HR) mirror to be planar. If the outer surface of the OC glass is not also curved to compensate for the negative lens that results, the beam will diverge at a much higher rate than would be expected for the bore diameter.HeNe laser tubes destined for barcode scanners often have a much higher divergence by design – up to 8 mR or more (where the optimal divergence may be as little as 1.7 mR or less). These tubes either have a negative curvature for the outer surface of the OC mirror glass (concave inward) or even an external negative lens attached with optical cement. See Uniphase HeNe Laser Tube with External Lens. The outer surface of OC in a normal HeNe tube will either be planar or slightly convex depending on whether the OC mirror is planar or slightly concave respectively. In the latter case, the convex surface precisely compensates for the extra divergence produced by the OC mirror curvature and results in a nearly optimally collimated beam. If the outer surface of your HeNe tube’s OC is concave, then it will have the high divergence characteristic. Note that the beam is still of very high quality but an additional positive lens approximately one focal length away from the OC will be required to produce a collimated beam.

    Also see the section: Improving the Collimation of a HeNe Laser with a Beam Expander.

  • Common HeNe lasers are of two types: random polarized and linearly polarized, which refers to the polarization of the output beam. A random polarized laser generally doesn’t produce anything like rapidly fluctuation polarization. It simply means that nothing has been done to control the polarization. And for the red (632.8 nm) wavelength, most HeNe will actually produce two sets of linearly polarized modes that are orthogonal to each other and fixed to the physical structure of the tube. These will change in amplitude as the tube heats up and the cavity expands.For a short tube (e.g., 5 or 6 inches), this is easily observed by placing a polarizer in the beam. At certain orientations, 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.
  • HeNe tubes which generate a linearly polarized beam are also available. Rotating a polarizer in a linearly polarized beam will result in high transmission at one orientation and close to zero transmission 90 degrees to it. These tubes usually include a glass plate oriented at the Brewster angle in the beam path (inside the resonator). This results in the optical resonator favoring one polarization orientation and the beam then becomes almost 100 percent linearly polarized. Melles Griot puts this plate inside next to the HR mirror of a HeNe tube that is otherwise similar to a random polarized model. Other manufacturers like Hughes have used a tube with a Brewster window at the OC-end and fasten the OC mirror to it externally. And, some really old cylindrical Hughes laser heads use tubes with Brewster windows at both ends with the mirrors mounted in the metal end-caps of the case. 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. Some HeNe lasers have an internal High Reflector (HR) mirror at one end of the tube but a Brewster window and external Output Coupler (OC) mirror at the other end. These are also linearly polarized and only half as finicky. πŸ™‚
  • In the trivial triviality department, the largest commercial two-Brewster laser I know of is the Spectra-Physics model 125, rated at 50 mW (red, 632.8 nm) but often producing much more output power when new. The plasma tube in this beast is over 5 feet long. Jodon also manufactures a 50 mW HeNe laser. The smallest two-Brewster plasma tube I’ve ever seen was from a photo in a book on lasers from the 1960s. It was only about 4 inches in length.
  • Inexpensive internal mirror HeNe tubes nearly always operate with multiple longitudinal modes and most have a TEM00 beam profile (though some, designed for maximum power output in a given size package, may have a wider bore and operate with multiple transverse modes – TEMxy where ‘x’ and ‘y’ are integers greater than 0). 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. Not only may one or both mirrors be mounted external to the tube in many of these, even if both mirrors are internal, there may be interesting and strange electrical, optical, electro-optical, or magnetic devices added to implement external modulation, mode locking, stabilization, and additional high performance (and high cost) features. Consider such a HeNe laser to be quite a find! See the sections: Spectra-Physics 120, 124, and 125 HeNe Laser Specifications and Interesting and Strange HeNe Lasers and 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. Maybe even another still weaker one after that.

Most internal mirror HeNe tubes should not have any higher order transverse (non-TEM00) modes. And, for multimode tubes, such modes should show up as part of, or adjacent to the main beam anyhow.

One possible cause for this artifact is that the output-end mirror (Output Coupler or OC) has some ‘wedge’ (the two surfaces are not quite parallel) built in to move any reflections – unavoidable even from Anti-Reflection (AR) coated optics – off to the side and out of harm’s way. Where wedge is present, the small portion of the light that returns from the outer AR coated surface of the OC will bounce back to the mirror itself and out again at a slight angle away from the main beam. In a dark room there may even be additional spots visible but each one will be progressively much much dimmer than its neighbor. Note that if the laser had a proper output aperture (hole), it would probably block the ghost beams and thus you wouldn’t even know of their existence!

Without wedge, these ghost beams would be co-linear with the main beam (exit in the same direction) and thus could not easily be removed or blocked. This could result in unpredictable interference effects since the ghost beams have an undetermined (and possibly varying) phase relationship with respect to the main beam. Sort of an unwanted built-in interferometer! The wedge also prevents unwanted reflections from that same AR coated front surface back into the resonator – perfectly aligned with the tube axis – which could result in lasing instability including cyclic variations in output power.

Thus, the ghost beam off to one side is likely a feature, not a problem! The effects of wedge on both the output beam and a beam reflected from a mirror with wedge is illustrated in Effects of Wedge on Ghost Beams and Normal Reflections. Note that his diagrams shows the effect of a beam coming in from the right and reflecting off the mirror. Where the beam is from the tube itself, the main beam corresponds to the one marked “1st Back Surface”.

If it isn’t obvious from close examination of the output mirror itself that the surfaces are not parallel, shine a reasonably well collimated laser beam (e.g., another HeNe laser or laser pointer) off of it at a slight angle onto a white screen. There will be a pair of reflected beams – a bright one from the inner mirror and a dim one from the outer surface. As above, if the separation of the resulting spots increases as the screen is moved away, wedge is confirmed (there may be higher order reflections as well but they will be VERY weak – see below). Where the mirror is curved, the patterns will be different but the wedge will still result in a line of spots at an angle dependent on the orientation of the tube.

Wedge is often present on the other mirror (High Reflector or HR) as well (in fact, this appears to be more likely than the OC). Wedge at the HR-end won’t affect the output beam at all but performing the reflectance test using a collimated laser (as above) at a near-normal angle of incidence may result in the following:

  • An intense spot in the center due to the reflection of the beam from the actual mirror.
  • A weaker spot on the thinner side of the optic due to the reflection of the beam from its front surface.
  • Several progressively weaker spots on the thicker side of the optic due to multiple internal reflections between its front surface and the mirror.

With the exaggerated amount (angle) of wedge in Effects of Wedge on Ghost Beams and Normal Reflections, another effect becomes evident: The weaker spots are spaced further apart. It is left as an exercise for the student to determine what happens when a laser beam is reflected at an angle from such a mirror! Note that his diagrams shows the effect of a beam coming in from the right and reflecting off the mirror. Where the beam is from the tube itself, the main beam corresponds to the one marked “1st Back Surface”.

The appearance resembles that of a diffraction grating on such a beam (but for entirely different reasons). The behavior will be similar for an OC with wedge but because the HR mirror isn’t AR coated, the higher order spots (from the HR) are much more intense.

It is conceivable that slight misalignment of the mirrors may result in similar ghost beams but this is a less likely cause than the built-in wedge ‘feature’. However, if you won’t sleep at night until you are sure, try applying the very slightest force (a few ounces) to the mirror mounts (the metal, not the mirrors as they are very fragile) in each while the tube is powered (WARNING: High Voltage – Use a well insulated stick!!!!).

  • If the ghost beam or beams are caused by wedge, all the spots will get weaker but their relative intensity and separation won’t change significantly. The peak absolute intensity should be in the relaxed position.
  • If the cause is poor mirror alignment, the shape, position, relative intensity, and even the number of visible ghost beams may change dramatically. The intensity of the main beam may increase when the mirror is deflected certain ways further confirming that a realignment is needed.

Depending on the type of laser you have, see the sections: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes, Quick Course in Large Frame HeNe Laser Mirror Alignment, and External Mirror Laser Cleaning and Alignment Techniques, for more information.

Another much simpler cause of an ugly beam from a HeNe (or other) laser is dirt on the outside of the output mirror since this will decrease the effectiveness of the AR coating. The dirt may also be on other external optics. Some HeNe laser heads have either a debris blocking glass plate glued at an angle to the end-cap or a neutral density filter to adjust output power. Even if AR coated, either of these may also introduce one or more ghost beams and if not perfectly clean, other scatter as well. I’m gotten supposedly bad HeNe lasers where the only problem was dirt on either the output mirror or external plate or filter.

(From: Steve Roberts.)

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.

Other Spectral Lines in HeNe Laser Output

While there is no such thing as a truly monochromatic source – laser or otherwise, the actual output beam of even an inexpensive HeNe laser is really quite good in this regard with a spectral line width of less than 1/500th of a nm. For a frequency stabilized HeNe laser, it can be 1,000 times narrower!

But if you look at the output of a HeNe laser with a spectrometer, there will be dozens of wavelengths present other than one around 632.8 nm (or whatever is appropriate for your laser if not a red one). Close to the output aperture, there will be a very obvious diffuse glow (blue-ish for the red laser) visible surrounding the actual beam. So why isn’t the HeNe laser monochromatic as expected?

With one exception, this is just due to the bore light – the spill from the discharge which makes it through the Output Coupler (OC) mirror. As your detector is moved farther from the output aperture, the glow spreads much faster than the actual laser beam and its intensity contribution relative to the actual beam goes down quickly. It is not coherent light but what would be present in any low pressure gas discharge tube filled with helium and neon. However, the presence of these lines can be confusing when they show up on a spectral printout.

The exception is that with a ‘hot’ (unusually high gain) tube or one with an OC that is not sufficiently narrow-band, one (though probably not more though not impossible) of the neighboring HeNe laser lines (e.g., for other color HeNe lasers) may be lasing though probably much more weakly than the primary line. For example, a red (632.8 nm) laser might also produce a small amount of output at 629.4 or 640.1 nm though this isn’t that common. For many applications, a bit of a “rogue” wavelength output is of little consequence and specifications for general purpose HeNe lasers usually don’t explicitly include any mention of them. However, rogue output will cause reduced accuracy in metrology applications and since they may not be TEM00, even where the beam is simply used for alignment.

I have a couple of 05-LHP-171 lasers that produce up to 10 percent of their output at 640.1 nm. The first is of unknown pedigree obtained in a lot laser junk from a well known laser surplus dealer. It may have been rejected for other reasons since the output at 632.8 nm is only about 4 mW when it should be well over 7 mW. The 632.8 nm is the normal TEM00 but the 640.1 nm beam may be TEM01 or TEM10 (2 modes) or even TEM11 (4 modes) depending on mirror alignment. With optimal mirror alignment for 632.8 nm, there may be no 640.1 nm at all. The other is a 25-LHP-171-249 system sold to a university lab. It has a manufacturing date of 2000, so this isn’t only a problem with old lasers as some people have claimed.

I have one ‘defective’ yellow (594.1 nm) HeNe tube that also produces a fair amount of orange (604.6 nm), and another that produces in addition some of the other orange line (611.9 nm).

While the probability of a commercial HeNe laser outputting at a rogue wavelength is low, where such a laser is used for measurements assuming pure 632.8 nm, errors could result. For more on this topic, see the paper:

  • “Advice from the CCL on the use of unstabilized lasers as standards of wavelength: the helium-neon laser at 633 nm”, J. A. Stone, J. E. Decker, P. Gill, P Juncar, A Lewis, G. D. Rovera and M. Viliesid, 2009 Metrologia 46, 11-18.

In the course of research for this paper, the first author, Jack Stone, borrowed one of my interesting Melles Griot 633 nm lasers that produced 5 to 10 percent of its output at 640.1 nm! πŸ™‚

And sometimes HeNe lasers are designed to produce more than one wavelength. PMS/REO used to produce such lasers, usually a visible and IR line, or a pair of IR lines. But one that I’ve acquired is a combination yellow (594.1 nm) and green (543.5 nm). Unlike many other PMS/REO lasers that produce multiple lines by accident :), this one was either designed that way or their Marketing department decided to convert a bug into a feature, since the laser head has a model number of LHGYR-0300M. Since these wavelengths are so far apart and are low gain, such an occurrence would normally not be likely, but I’ve also seen one where this was the case – a yellow laser that had a tiny bit of green.

I also have one that does orange (612 nm) and green (543.5 nm) but only under special conditions. It is a high mileage PMS/REO LHOR-0150M so it’s presumably supposed to be 612 nm only. But when run at lower than normal current, the green line pops up weakly. Just above the dropout current at 4.5 mA, it produces about 0.5 mW of orange and 0.05 nm of green. At 6.5 mA, it produces around 1 mW of pure 612 nm orange. I rather suspect this is a peculiarity of the tube running near end-of-life with a lower gas pressure. When new, it may not have been so interesting. πŸ™‚

For more on multiline HeNes, see the sections starting with: The Dual Color Yellow/Orange HeNe Laser Tube. And to make your own (sort of), see the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.

(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

For gas lasers the plasma lines are typically 80 dB or more below the output (measured, of course, within the very small laser mode divergence). This is unlike most semiconductor lasers, which typically have broad ‘shoulders’ close in to the line, as well as ‘lines’ due to other modes and instabilities because the initial divergence of the diode is high, and spontaneous emission from the junction high, the broad background tends to be large.

For gas lasers it is usually in the form of narrow lines at remote wavelengths, very easily removed with an interference filter and/or spatial filtering in the *rare* cases where it matters. There is presumably a weak broad background from processes involving free electrons (bound/free and free/free), but I’ve never seen it even mentioned, let alone observed it. More likely to be significant in the high current density argon laser than the very low current density HeNe.

The only cases I have seen where the plasma lines caused problems were Raman measurements on scattering samples with photon counting detection, and weak fluorescence measurements which are similar.

In most cases scattered light in the monochromator is much more of an issue (hence double monochromators for Raman) and will obscure plasma lines in many cases.

Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes

As a practical matter, the only wavelength that is useful from an internal mirror HeNe laser is the one for which it was designed. (Or the pair in the case of a couple of Research Electro-Optics (REO) lasers.) However, it is often possible to at least obtain unstable lasing at other wavelengths by extending the cavity using an external mirror. The output power of the other lines can be anywhere from almost non-existent to greater than the power at the original wavelength. This probably works best obtaining a some red from a long “hot” yellow (594.1 nm) or orange (611 nm) tube since at least one mirror is likely coated broadband to include yellow through red. Due to the low gain of the non-red lines, going the other way – getting yellow from a red tube, for example – is not likely to succeed unless the tube is very long. But obtaining lasing at other red wavelengths – and even orange – may be possible with a moderate size red HeNe laser tube. Even a 1 mW tube may give you 1 or 2 other red lines. I doubt it will work at all with a green HeNe tube having mirrors that appear orange in transmission since both mirrors are probably too transparent at even the yellow wavelength (except possibly if two external mirrors are used). However, if a mirror is more red in transmission, there might be a chance. See the section: Instant HeNe Laser Theory for a table of HeNe lasing wavelengths and relative gains.

I’ve gotten most of the well known HeNe lasing lines in this manner including up to 4 mW of red from a 2 mW yellow HeNe laser, both orange lines, various other red lines, and one of the wavelengths that isn’t even mentioned in most texts dealing with HeNe lasers. More below. I’ve only heard of one instance of any yellow being produced from non-yellow tubes, that being a REO 612 nm laser. And I haven’t even attempted to obtain green from non-green tubes.

Here’s how to get other wavelengths from your HeNe laser. Either a bare tube or complete laser head can be used for these experiments.

  • Position an adjustable mount with a red OC or HR mirror a few inches beyond the OC or HR of the tube. Depending on the actual reflectance curves of the tube’s mirrors, one end will be better than the other. An HR may have a better chance of obtaining the low gain wavelengths if it is broadband, but many found on commercial HeNe lasers have been designed with high reflectivity only at the desired lasing wavelength in order to suppress the others. For the best chance of obtaining the most wavelengths from a red HeNe laser tube, I’d suggest an external red HR mirror beyond the tube’s internal OC mirror. For an “other color” tube, which end is best may be a random function (see below). However, in general, the OC is preferred over the HR since the outer surface almost always has better optical quality and will be AR coated. The outer surface of an HR may have some wedge and its shape and coating are pretty much irrelevant to normal laser operation (some are not even polished), so could be quite poor inside a cavity.The Radius of Curvature (RoC) of the external mirror may need to be consistent with a stable resonator configuration for the overall cavity. I’m not entirely sure this matters that much (and the implication in the next section is that it may not), but I’d still go with a stable configuration given a choice. If you don’t want to perform the calculations, a mirror that should work would be one from a dead red HeNe laser at least as long as the tube you are using. Of those I tried that worked at all, minimizing the distance between the ecternal mirror and tube resulted in the best results but this may not always be true. A dielectric mirror is definitely preferred but a good quality aluminized front surface (planar) mirror should work, though it may not be as good.
  • Place diffraction gratings in the beams from both ends of the laser so the spectra can be projected onto white cards. If you don’t happen to own a “real” diffraction grating, a junk CD or CD-R disc works quite well as a reflection grating. In conjunction with a weak positive lens inserted in the beam before the grating (approximately 1 focal length from the end of the tube), the lines can be narrowed to permit sub-nm resolution. There will be a bit of spread perpendicular to the spectrum due to the bit patterns encoded on the disc but this just makes the lines look more like lines. :)The quality of the beam from the end of the tube opposite where your external mirror is located will probably be better, especially if the mirror is beyond the HR of the tube (which may have some wedge and is not AR coated). However, the beam from the external mirror end is instructive at least in helping to adjust the alignment.
  • Power up the laser and carefully adjust the external mirror so that the beam from the tube (leakage from HR or output beam from OC) reflects back on itself. As the adjustments pass through this exact point, there should be evidence of wavelength competition as a color change and/or intensity change of the beams at one or both ends of the laser. The size of the beam exiting the external mirror will also be a minimum at this point and there may be visible interference effects. As the tube heats and expands, the wavelengths will come and go as the modes compete for attention. Just touching the mirror mount will also result in similar effects. Nothing will likely be stable for more than a few seconds. When lasing at the non-design wavelengths is present, the intensity of the original color(s) will probably decrease, possibly substantially.

Using my Melles Griot 05-LYR-170 yellow HeNe tube which for my “broken” sample, actually lases a combination of yellow (594.1 nm) and orange (604.6 nm) from both ends (see the section: The Dual Color Yellow/Orange HeNe Laser Tube), it was quite easy to achieve red output, and all three colors were occasionally present at the same time – an impressive achievement for a HeNe laser. My setup is shown in 05-LYR-170 HeNe Laser Tube Mounted in Test Fixture for Multiline Experiments. The output from the tube’s OC was directed at an AOL CD used as a reflective diffraction grating with the first-order beam projected on a white card several feet away. An MSN CD would work just as well πŸ™‚ but a CD-R or CD-RW may not. The lens from a pair of eyeglasses (mildly positive, about 4 diopters or 1/4 meter focal length) narrowed the spots to improve spectral resolution. This rig could easily resolve lines separated by less than 1 nm. The first external “red” mirrors I tried were from an SP-084 HeNe laser tube but due probably to their relatively short RoC, the 05-LYR-170 had to be pushed quite close to the mount to get any red output. Mirrors designed for a longer laser worked better but there wasn’t much difference between the behavior using an HR or OC (99 percent).

Then to add to the excitement, with a bit of twiddling, I was able to obtain the other orange line (611.9 nm) as well, and at times, all 4 lines were lasing simultaneously! As expected, this additional line was only present when using an external HR. Depending on the original makeup of the yellow and orange beam (for this tube, their absolute and relative intensities varied with time and were also a very sensitive function of mirror alignment), it was possible to get mostly red or to vary the intensities of the other colors, most easily suppressing yellow in favor of orange and red. The intensity of the red output was never more than 1 mW or so. Its transverse mode structure varied from TEM00 to a star pattern with nothing in the center. Strange. Due to both surfaces of the HeNe tube’s HR mirror reflecting some of the intracavity beam resulting in a multiple cavity interference effect, there was a distinct lack of stability. To help compensate for this, a micrometer screw to precisely adjust cavity length without affecting mirror alignment would have been nice.

I also tried this with the external mirror mounted beyond the tube’s OC mirror but although there was a definite effect on yellow and orange lasing, it wasn’t possible to obtain any red output. (For the 05-LYR-170, the OC already reflects red quite well and the HR doesn’t.) Finally, I replaced the red external mirror with a green HR (from a tube of about the same length) mounted beyond the 05-LYR-170’s OC (since its HR by appearance looked like it might be a good mirror for green). But, not surprisingly, while this could affect the lasing of the yellow and orange lines, I could detect no coherent green photons. However, I would expect that with a appropriately coated mirrors (or possibly two such mirrors, one beyond each end of the tube), obtaining lasing at the relatively high gain 640.1 nm red line would be easy – the usual “red” mirrors may deliberately kill this line to prevent it from lasing. Although I couldn’t detect any evidence of lasing at the other red lines of 629.4 nm and 635.2 nm, these should also be possible with appropriate mirrors as they have higher gain than the yellow and oranges. Another interesting one would be the “Border Infra-Red” line at 730.5 nm. Lasing at the IR lines might also be possible but they are so boring. πŸ™‚

Next, determined to do something with a more normal HeNe laser tube, I tried a Siemens tube but that refused to do anything interesting. Then, I tried a Melles Griot 05-LHR-150 which typically outputs a 5+ mW red (632.8 nm) beam. Since the OC for this laser is probably around 99% reflective at most, peaking at 632.8 nm, I figured that it would be best to place the external mirror beyond the OC rather than the HR. And, with the same external HR as used above, it was possible to obtain 6 lasing lines, count’m 6: 629.4 nm, 632.8 nm, 635.2 nm, 640.1 nm, a line popping up around 650 nm (all variations on red), ****AND**** 611.9 nm orange! However, since the output is being taken from the HR, none of the colors was more than a fraction of a mW.

Lasing of the 650 nm line was hard to obtain – it only showed up for a few seconds off-and-on every few minutes and increasingly rarely after the tube warmed up. The exact wavelength is very close to 650 nm (649.98 nm) as determined later with an Agilant 86140B Optical Spectrum Analyzer (OSA) which is a lot more expensive than my AOL CD. πŸ™‚ (The wavelength was referenced to the 632.8 line from the same laser resulting in a measurement error bound of +/- 0.02 nm assuming the 632.8 nm line is actually 632.8 nm. But since this could also be slightly shifted, the error may be higher.) Getting anything at 650 nm is really puzzling as there are no HeNe lasing lines between 640.1 nm and 730.5 nm. But I have no doubt it is a true lasing line since it was fluctuating independantly of the others (later confirmed, see below). And all those other lines were quite accurately located corresponding to their handbook wavelengths in the diffracted pattern (and later confirmed with the OSA). So there is little reason to suspect that the funny one isn’t as well. When present, it appeared as strong (or weak) as all the expected ones, (except of course, the original 632.8 nm line which was usually, but not always, the strongest). If 650 nm is not a HeNe lasing line – it’s certainly not in the sequence of energy level transitions that produce all the other visible HeNe lines – one possible explanation is that there is some trace element present inside the tube and that is what’s lasing, not neon. I figured this to be a distinct possibility since the particular tube I am using originally had gas contamination and I revived it by heating the getter. (See the section: Repairing the Northern Lights Tube.) Therefore, the 650 nm wavelength may not be present with another more normal tube. But as it turned out, contamination has nothing to do with it.

I don’t think the 730.1 nm line was present but given its low relative perceived brightness, it may not have been visible at all using my AOL Special CD diffraction grating but I couldn’t find it with the OSA either. It took awhile to detect the evidence of the 635.2 nm line which only appeared sporatically (but it is the lowest gain of all the known ones above).

A few days later, I tried the same experiment with a couple of my old Spectra-Physics 084-1 HeNe laser tubes which are of soft-seal design so have almost certainly leaked over time (but still work fine). With my “hottest” SP084-1 (about 2.9 mW), I could almost duplicate the results of the 05-LHR-150 including the funny line around 650 nm but minus anything at 635.2 nm. Using a more normal 2.4 mW SP084-1, it was possible to obtain (non 632.8 nm) lines at 629.4 nm and 640.1 nm. For these, an SP084-1 HR worked almost as well for the external mirror as the longer RoC HR I had been using with the 05-LHR-150. I then installed a SP098-1, a common hard-seal barcode scanner tube (this sample puts out about 1.4 mW). With that, the only additional line was at 640.1 nm. Which particular lines appear in each case seem consistent with the length of the tubes (and thus the single pass gain) and the relative gain of the lasing lines.

Some quick calculations predict that the real effect of the external HR mirrors is the obvious one – to increase the circulating power. A 1 percent OC (typical) followed by even a 90 percent external mirror would result in greater than a 99.9 percent effective mirror for a range of wavelengths/modes. An external 99.9 percent HR would result in an even better effective mirror. It looks like the reflectance peak is relatively broad with respect to wavelength (the transmission peak is rather narrow). Specific modes for each of the wavelengths will be enhanced or suppressed. This would also appear to be consistent with the apparent lack of need for the external mirror to result in a stable resonator. All it has to do is form a Fabry-Perot cavity.

These have to be classified right up there in the really fascinating experiments department. Seeing any HeNe laser operating with multiple spectral lines is really neat.

For more examples of these stunts using an already interesting “defective” HeNe laser, see the sections starting with: Melles Griot Yellow Laser Head With Variable Output and in particular, the section: External Mirror Therapy for Variable Power 05-LYR-171 Yellow Laser Head.

As always, depending on mirror reflectivity and other factors, your mileage may vary. But feel free to try variations on these themes. The results from using an HeNe HR beyond the OC of almost any red HeNe laser tube should be easily replicated (except perhaps for the funny 650 nm line). Almost any mirror will do something since even an aluminized mirror will be returning over 90 percent of the otherwise wasted photons to the cavity – enough to boost the gain of all but the weakest lines enough for lasing if everything lines up just right. Aside from getting zapped by the high voltage or dropping the tube on the floor, they are low risk, high reward experiments.

And, can you believe that people get stuff like this published in scholarly journals? I was recently sent an article entitled: “Yellow HeNe going red: A one-minute optics demonstration” by Christopher Hopper and Andrzej Sieradzan, American Journal of Physics, vol. 76, pp. 596-598, June 2008. Geez, they could have saved a lot of time and effort and come here instead. Or, perhaps they did. πŸ™‚

(From: Bob.)

For neutral neon at low pressure, the lines 640.3 nm, 659.9 nm are listed. For neutral helium, there is one at 667.8 nm. None of the other noble gases have wavelengths listed this short. As far as ionized species go, singly ionized argon has a line at 648.30 nm. Singly ionized krypton has a hand full of lines from 647 nm to 657 nm. Finally, xenon has one at 652 nm.

For atmospheric gases, there is a singly ionized nitrogen line at 648.3 nm. There are no neutral lines of interest for atmospheric gases. The footnotes for the above line were listed as CW lasing in 0.02 torr of krypton. Whats the standard operating pressure of a HeNe laser? Not THAT far out of the ball park I would guess.

(From: Sam.)

The last one sounds promising and would make sense given the history of the particular 05-LHR-150 and the soft-seal design of the SP084-1. Though HeNe lasers operate in the 2 to 3 TORR range – about 100 times higher pressure, the partial pressure of any N2 contamination could very well be down around 0.02 Torr.

However, I now know exactly where the 650 nm line is coming from and it has nothing whatsoever to do with contamination. The exciting writeup from someone who beat me to this by about 15 years follows in the next section preceeded by a condensed version, below.

I’ve also found a commercial laser that appears to produce a very stable 650 nm line. See the section: The PMS/REO External Resonator Particle Counter HeNe Laser.

(From: Stephen Swartz (sds@world.std.com).)

Lasing of certain HeNe tubes at 650 nm is a known phenomenon and not just a hallucination. The 650 nm line which is never discussed in most standard texts is not due to a “normal” transition of neon. It comes instead from a Raman transition. The 650 nm line is not often observed but when it is it will always be seen simultaneously with operation on a multitude of other lines. A large number of other “unusual” colors have been seen over the years. Higher power tubes with mirrors that are excessively broadband are your best bet for observing them. Often these lines flicker on and off over a few seconds to minutes time scale. A diffraction grating is a good way to look for them.

(From: Someone at a major laser company.)

The 650.0 nm Raman line is a known problem in that it competes for power with the 632.8 nm line intermittently, particularly in long tubes with high circulating power. Polarized tubes are much less susceptible to this effect and using a lower reflectance for the OC mirror helps since it reduces circulating power without affecting output very much (over a reasonable range).

Bruce’s Notes on Getting Other Lines from Red (633 nm) HeNe Laser Tubes

This, to make a gross understatement, would appear to be the definitive word on coaxing other colors from surplus HeNe laser tubes. And I thought six lines (including the mysterious 650 nm line) was an achievement. πŸ™‚

(From: Bruce Tiemann (BruceT@ctilidar.com).)

I have gotten many lines from many different HeNe lasers. In my experience almost every tube is capable of giving at least one other line than 633 nm. (Most wavelengths have been rounded to save bits. So, 632.8 nm becomes 633 nm.) I have never tried doing this with lasers that give other lines than 633 nm, but since that line has the highest gain, it should be no mean feat to at least get that line from lasers that are supposed to not give it. It is also not my experience that calculations to ensure resonator stability, etc., are necessary. Just try it! My best results, in terms of output power, were with a flat grating as the external feedback mirror, and my best results in terms of new lines was obtained with a flat dielectric mirror, formerly used as a facet in a polygonal scanning assembly. Flat mirrors are not stable at any separation for a diverging beam, and HeNe lasers are very rare that give converging beams for their output.

The home stuff had the mirrors on blocks, with the steering accomplished by adjusting the HeNe tube by lifting one or the other end of the tube with sheets of paper, and the azimuth by moving the laser tube back and forth. The lab experiments were done with “real” mirror mounts, supplemented by a single PZT that tilted the feedback mirror a few microns.

(I like PZTs a great deal, and would like to observe that you can get PZT elements from little piezo alarms, from which the useful element can be extracted with some hand-tools and the mind-set of a 9-year-old kid dissecting a bug. πŸ™‚ These are only about $1 each, as opposed to tens to hundreds of bucks for “real” PZTs that you buy from Thor, etc. One of them and a 0 to 50 VDC power supply can precision-wiggle a mirror on the micron scale, which is all that is needed for these experiments.)

(From: Sam.)

I have indeed done something similar using the piezo beeper from a dead digital watch to move a mirror in a HeNe laser based Michelson interferometer. With 0 to 25 V, it went through 4+ fringes which means over 2 full wavelengths at 633 nm. The configuration in these is called a “drum head” piezo element because the movement resembles that of a musical (depending on your point of view!) drum head with the most shift in the center. The piezo material itself doesn’t change by very much in thickness but is constructed so it distorts to produce the shape change. With care, the piezo material can be cut to size or drilled to pass light through its center. Much more voltage could have been safely applied if needed.

(From: Bruce.)

Something I also did is cast the spots from a smaller (approximately 3/4 m) spectrometer directly onto the CCD element of a small camera with no lens. I also fabricated a beam block by taping little wires to the side of a block, that would protrude up just in the locations of the very bright lines, like 633, 650, and 612 nm, to block them, but letting light of other colors pass in the ample space between the wires. You could still see when the bright lines were on from light leaking around the wires, but it wouldn’t wash out the image when they were.

In this case, when the feedback mirror was tilted, speckle, which was cast everywhere, would kind of shift around all over the place, but the new lines looked like ghostly bullseyes, which would breathe in and out as the mirror was tilted, but remain in the same location unlike the speckle. This was an easy way to see the weakest lines like 624 nm, and it was also how I discovered 668 nm, the CCD being more sensitive than the eye in the deep red. (I searched for but did not find the normal laser line 730 nm even with this very sensitive method.)

  • “Normal” laser lines: The multiplet that gives 633 nm includes a total of nine lines, ranging from 543 nm at the green end to 730 nm at the far-red end. In between are yellow (594 nm), orange (605 and 612 nm), and several reds (629, 633, 635, and 640 nm). I have never obtained the yellow, green, or far-red line from any 633 laser but I have gotten all the others.
  • Grating feedback: My best results are with a 5 mW Melles Griot 05-LHR-551 (or similar) laser. For maximum power output on these lines, a flat aluminum-coated grating, unblazed and with low (less than 10%) diffraction efficiency, gives near 1 mW power on 612 and 640 nm (as well as 650 nm, see below), when it is used to exactly retro-reflect the 633 nm beam back into the bore. The grating handily disperses the different colors off to the side. Sub-mW output power is also available on 629 and 635 nm. The beams sometimes wink on and off, but contrary to one’s impression they are on more often than they are off, and represent fairly stable and reliable outputs. I have also used a blazed 600 l/mm Edmund grating in Littrow, meaning, slanted so the first order is returned to the output coupler, and have thereby obtained operation at 612 and 640 nm, one at a time, at reduced output power, though of course with 633 nm on all the time.
  • 640.1 nm: This line experiences anomalous dispersion from a nearby line, and therefore experiences gas lensing in the bore. Hence, it will oscillate in some marginally unstable (2-mirror) resonators even when 633 nm won’t. In a 3-mirror system, 2 from the laser tube and a third added by the experimenter, 640 nm is often the line that will oscillate with the least feedback. One of my 5 mW lasers will lase this line with an uncoated glass microscope slide, or even a plastic ruler or plastic box-lid as the third (feedback) mirror, up to a distance of several inches from the output coupler. With an uncoated Edmund 1/10 wave optical flat, 640 nm would oscillate with the surface located up to some 1.2 m away from the output coupler of the laser. That the resulting resonator is unstable can be clearly seen by the fact that the retro-reflected beam from the flat grossly overfills the size of the beam exiting the output coupler, nevertheless, 640 nm operation can be verified by looking at the diffracted beam from a grating, located behind the optical flat.That 640 nm line would lase even with a plastic ruler or similar non-mirror mirror and could be established by hand-holding the piece of plastic in the beam, braced against the laser tube.
  • Dielectric feedback mirrors: Most small, 1 mW metal-ceramic tubes won’t give any other lines with a metal grating as the feedback element, so it is natural to try to coax them out with higher R. With use of a max-R dielectric mirror, almost every laser I have tried has given at least one other line. 640 nm is probably the most common next line to get, though there are some tubes that give many others but not that one, go figure. One small metal-ceramic tube would only give 629 and 635 nm, no others, even though these are weak lines. Probably has to do with the narrow-band coating on the back mirror. Interestingly, this laser would only give them periodically, with the 633 nm mode, normally TEM00, splitting up and becoming complicated, at which point the other lines would come on. When 633’s mode started to simplify, the others would vanish. One is drawn to think that the max-R mirror plus the laser output coupler forms a Fabry-Perot cavity, that when it becomes resonant at 633 nm, loses reflectivity there and thereby gives gain to the other lines, as well as inclines 633 to find a higher-order mode where the cavity is still reflective.
  • 604.6 nm: My “best” tube gives 605 nm, only in one exact position of feedback mirror only a few mm from the output coupler,. and then, curiously, only when the feedback mirror is misaligned such that the appearance of the output, seen in transmission through the dielectric mirror, is a contiguous line of spots, instead of just the dot in the middle. This line competed strongly with 612, trading intensity and almost never being on at the same time. When 605 was on, it was just as bright as 612, which is curious given how reluctant it was to lase.
  • 650.0 nm: Many 5 mw lasers give a line at 650.0 nm as well. This line isn’t a neon (Ne) transition, and isn’t due to an impurity either. Nor is it widely known. I demonstrated this line, and some of the usual other ones, to an astonished audience at Japan’s NIST, the National Research Laboratory for Metrology in Tsukuba, using one of their own “single line” HeNe lasers and one of their dielectric mirrors. It’s an electronic Raman line, pumped by 633 nm. The 2s states of Ne, which are what the 2p lower laser levels dump into, include two metastable states, from which it is forbidden to drop into the ground state. Hence, they build up population in the plasma. If a Ne metastable interacts with a 633 photon, it can take some of the energy to promote the atom to a higher state, and scatter the photon with the correspondingly lesser amount of energy, in this case at 650 nm. The energy difference between the 1S5 and 1S4 states, about 417 cm-1, is the same as the energy difference of the photons, 633 nm compared to 650 nm. This line was only discovered around 1985!The gain-bandwidth of the Raman transition is only 60 MHz wide, so the cavity modes for 633 nm must line up with the cavity modes at 650 nm, to within this uncertainty, in order for 650 nm to oscillate. Considering that the Doppler bandwidth is more like 1,500 MHz (1.5 GHz), and the FSR of the laser is ~hundreds of MHz, that is only rarely the case. Hence, 650 nm comes and goes, most of the time being gone. And when it’s gone, it’s gone. When the laser warms up, however, the cavity expands, and the 633 and 650 nm modes sort of vernier past each other, sometimes bringing them into alignment in difference-frequency space. When they align, 650 nm oscillates. The observed behavior is that 650 nm more rapidly blinks on when the laser is warming up, but only for short periods, and then as the tube comes closer to a steady-state temperature, the periods become less frequent, but 650 nm lasts for a longer duration each time. Eventually, at the steady-state condition, 650 nm will be gone, or more rarely, may persist. However, temperature control of the laser can cause 650 nm to become steady, in the low-tech way of putting a blanket made of paper sheets or something over the laser tube, to stabilize the laser tube temperature to the next-higher value that supports 650 nm oscillation, or in the higher-tech case with a heater tape and thermistor and temperature control unit. When 650 nm goes, it is strong, and one 5 mW tube gives nearly 1.5 mW of output power at 650 nm, when the feedback element was a metal grating, and the output was taken from the first order. It is also perceptibly a deeper red color than 633 or even 640 nm, to me.I (Sam) have tested two external PMS/REO particle counter assemblies lasing on up to 6 normal HeNe lines (605, 612, 629, 633, 635, 640) and the 650 nm Raman line where the 650 nm line was present 100 percent of the time with little variation in intensity. No stabilization of any kind is involved and the behavior is little changed from power-on to thermal equilibrium. This seems to directly contradict the need to simultaneous resonance mentioned above and in the papers. On one sample, I believe the 650 nm line was actually the strongest one, stronger even than the 633 line. On that laser, there was even the occasional hint of another strange line at around 653 nm. Also see the section: The PMS/REO External Resonator Particle Counter HeNe Laser.
  • 4-wave mixing lines: Even less well known than 650 nm is that some HeNe lasers can give 4-wave mixing lines when 650 nm also oscillates. I have gotten 10 such lines from my “good” laser, many at one time. The most easily seen one is at 597 nm, and results from the addition of 417 cm-1 to a 612 nm photon. It can be thought of as a sideband of 612 nm, modulated by the 11.7 THz modulation set up by the difference frequency between 633 nm and 650 nm. Thus, to see 597 nm, one needs both 650 nm and also 612 nm to be oscillating. It also needs the feedback element to be a max-R dielectric mirror – the metal grating doesn’t work, even though it easily gives 612 nm and 650 nm at the same time. The 4-wave outputs, taken in transmission through the max-R feedback mirror, or through the high-R mirror on the back of the laser tube, are VERY weak, measured in 10s of nW at most, and dwindling down into the pW for the weakest ones. (This power level may be compared with the 40 nW that results from a 1 mW HeNe beam reflecting off a 4% reflector, an uncoated glass surface.) The stronger ones, such as 597 and 613 nm, can be easily as dots on white paper in a slightly darkened room, but the weakest ones, such as 589 and 624 nm, are best seen by looking directly into the grating, with dark-adapted eyes in a darkened room, in which case they look like star-disks that come and go as the feedback mirror is wiggled, differently from the 633 speckle. 597 nm is also perceptibly different color than even 612 nm. It looks “yellow” in comparison to the other orange and red spots from the HeNe, though the true color is more orange than that.These lines were best observed with the high-R feedback mirror located within about 6 inches of the output face of the laser, closer tending to be a bit better. Except for 589 nm, which required 605 nm to be oscillating, and this only occurred for one exact spacing of feedback mirror about 1.5 cm away from the output coupler, or about 1 mm away from the output flange of the tube, which I didn’t remove. (I did, however, find out that you can take a laser tube to the university infirmary, and ask to have it X-rayed, to determine the extent of the internal glass envelope within the aluminum outer casing, and they would only charge you $10 for the cost of the X-ray film and processing, which is not bad for a doctor visit including X-rays.)To my knowledge, these lines are my discovery.

    A brief table shows the relationship between “pump” lines and 4-wave mixing lines, observed on one tube. Upper Sideband is toward shorter wavelengths from the pump; Lower Sideband is toward longer wavelengths of the pump (all values in nm):

    (632.8 and 650.0 nm are parenthesized since they are associated with the genesis of the 4-wave mixing lines.)

All in all this laser produced 17 different lines, many at one time, from a “single line” 633 swap-meet laser. πŸ™‚

References:

The 650 nm discovery paper is:

  • Assendrup, J.; Grover, B.; Hall, L.; Jabr, S., “CW Helium-Neon Raman Laser”, Applied Physics Letters; 1/13/86, vol. 48, is. 2, p86, January 1986.Abstract: Continuous lasing has been observed at 650 nm with a helium-neon electrical discharge placed in an ultrahigh finesse optical cavity. This new lasing line is attributed to a Stokes-Raman process between the 1s5 and 1s4 electronic states of neon atoms pumped by the 632.8-nm neon lasing line. A gain calculation based on a near-resonant stimulated electronic Raman process predicts a lasing threshold for the 650-nm line near that measured. Lasing output power was measured as a function of discharge current and helium-neon gas pressure for the pump line and for the Stokes line.
  • Huang, Zhiwen; Zhao, Suitang; Jin, Haoran, “650 nm CW He-Ne Raman Laser”, (Chinese Journal of Lasers, vol. 15, Nov. 1988, p. 648-651), Chinese Physics – Lasers (ISSN 0887-3518), vol. 15, Nov. 1988, p. 803-806. Translation.Abstract: The 650 nm laser line of a simultaneously operating six-wavelength He-Ne laser was studied experimentally. It is shown that the precise lasing wavelength at 650 nm should be 650.00 + or – 0.05 nm and that this laser line is the result of the stimulated Raman scattering of the 632.8 nm transition between the 1S5 and 1S4 states. The characteristics of the Raman emission are studied and the gain is obtained.
  • Peter Franke, Alfred Feitisch, Fritz Riehle, Kegung Zhao and Jurgen Helmcke, “Simultaneous cw laser emission including a Raman line of a He-Ne laser at six wavelengths in the visible range”, Applied Optics, vol. 28, no. 17, 1 September 1989, pp. 3702-3707.Abstract: Simultaneous CW laser emission has been observed in a He-Ne discharge at 611.8-, 629.3-, 632.8-, 635.1-, 640.1-, and 650.0-nm wavelengths. The output power and the mode spectra have been investigated for various operational conditions. Spontaneous mode locking of the different lines has been observed. The Raman transition (650.0 nm) pumped by the strong intracavity radiation at 632.8 nm has been investigated in detail and its relevance for a secondary multiwavelength standard is discussed.

Miscellaneous Comments on Getting Other Lines from HeNe Laser Tubes

(From: Flavio Spedalieri.)

I have a small Yellow Tube – 05-AYR-006 (as a combo with power supply 05-LPM-496-037). This tube is physically the same size at the 1mW reds, but has a larger bore resulting in multimode output.

I have managed to get the red (632.8nm) line to lase and perhaps orange lines by placing a HR from a 632.8nm HeNe at the HR end of the yellow tube.

Further, I obtained a broadband mirror from an Argon Laser tube, the OC worked best at the OC end of the Yellow tube, have got the laser to output green…

The mirrors hand-held – next to build a small external resonator assembly.

About the Waste Beam from a HeNe Laser

The so-called High Reflector (HR) or totally reflecting mirror in a HeNe laser isn’t really perfect, though the actual reflectivity is generally 99.95 percent or better. For a 1 mW laser tube with a 99 percent Output Coupler (OC) mirror, there is about 100 mW of intracavity power. Of this, about 50 uW will exit the rear through a 99.95 percent HR mirror. Unless the back of the HR mirror is painted or covered, there is always some small beam exiting the rear of the laser.

Normally, what comes out in that direction is, well, waste, and is of no consequence. But, there are times where it’s convenient to use this low power beam as a reference, expecting its power to track that of the main output beam. Unfortunately, it is sometimes not well behaved in this regard.

In constructing some amplitude stabilized HeNe lasers which depend on the waste beam feeding a photodiode for their feedback loop, an annoying characteristic of the waste beam has become evident with some otherwise perfectly normal and healthy HeNe laser tubes. Namely, that the relative power in the waste beam and the main beam does not remain constant as the tube warms up. In fact, one tube I was using had a variation of almost 2:1 in relative waste beam and output beam power depending on the tube’s temperature. This is probably due to one or both of the following:

  1. Variation in mirror reflectivity. Designing and manufacturing high reflectivity mirror coatings is somewhat of an art and they don’t always come out right. There may be ripples, a slope, or other variations in the reflectivity-versus-wavelength function. For an HR mirror on a 1 mW tube of, say, 99.97 percent resulting in 30 uW, a change of only 0.01 percent would add 10 uW to the waste beam.
  2. Lack of wedge or insufficient wedge between the inner and outer surfaces of the HR mirror. This will result in an etalon effect, effectively modulating the reflectance as a periodic function of temperature by perhaps 10 or 20 percent, which would appear as a similar change in the waste beam power. From room temperature to the operating temperature of a typical enclosed HeNe laser head, the power variation would go through several cycles.

The coating problem is more likely to result in a strictly increasing, or at least slow change in waste beam power with higher temperature while the etalon would be periodic with temperature going through several cycles, it might be possible to determine which of the two effects is present.

Normally, the waste beam is not used for anything and no one cares. Though there will also be a change in the power of the output beam (inversely relative to the waste beam) from these issues, it will be too small to be detectable without careful measurements, being swamped by the normal mode sweep power variations. But when the waste beam is used as the amplitude reference in a stabilized laser, the supposedly stabilized output will vary based on the relative waste beam power. That 10 uW change would result in the output power changing by 33 percent.

For some plots the mode sweep of normal and naughty tubes, see the section: Plots of HeNe Laser Power Output and Polarized Modes During Warmup. In particular, compare the plots of the Spectra-Physics 088 with those of the tubes that immediately follow it.