The Strange Mischievous REO Tube from a Stabilized HeNe Laser Head

This is the mostly true saga of a misbehaving orphaned laser.

I've dreamed of finding lasers on the curb. Regrettably, all one tends to find around here is an occasional very dead rusty lawn mower. In the interests of artistic license I've embellished the initial discovery just a wee bit. ;-) And some other trivial details of this story have been modified in a way that should have no effect on its technical accuracy.


While on my regular afternoon walk one day, I spotted a cylindrical object partially buried under Autumn leaves on the side of the road. What's this? At first I thought it might be a bomb, though there don't tend to be that many abandoned bombs around here. :) But no, it was the mostly intact remains of a HeNe laser head! :-) This can't be! And in remarkably good condition except that the front bezel had been removed against the head's wishes with a pipe cutter. Otherwise, it was without even a scratch or ding (or ant colony in residence). However, there were no labels or other markings and no return address. The back had the normal Alden high voltage cable as well as a second one. The presence of that second cable meant the head was almost certainly intended for a stabilized HeNe laser - the only common type to have additional connections inside the head.

From the appearance of the front of the laser tube almost poking out from inside the head cylinder - the OC mirror frit-sealed to a metal mount attached to the metal tube cylinder - it is clearly from PMS or REO. Being fairly new, that leaves REO. At this point I assumed it would be a bog-standard 2 to 3 mW random polarized laser tube, which isn't all that exciting. But if in good condition and well behaved, such a tube could be useful to build another stabilized HeNe laser or to repair one.

What ultimately really attracted my interest to this laser head (no pun intended) aside from its existence was that a battery literally decided to roll toward the cylinder and then stuck to it. Huh? A magnet inside a cylindrical laser head? Now that's weird. I had never ever seen a cylindrical laser head - stabilized or not - with an internal magnet. Magnets may be present in Zeeman-split two-frequency HeNe metrology lasers or larger (and mostly older) HeNes with exposed bores, but never in modern cylindrical heads.

Instant stabilized HeNe laser tutorial:

The typical stabilized HeNe laser operates at a wavelength of 632.8 nm (red-orange) and uses a random polarized tube with a cavity length of 20 to 25 cm such that it lases on at most two adjacent orthogonally polarized longitudinal modes when they straddle the neon gain curve. Perhaps surprising to the uninitiated, the tube must be "random polarized" which simply means nothing has been done to restrict the output to linear polarization thus allowing for the required pair of orthogonal polarized modes to oscillate. The tube doesn't jump around uncontrollably. :) HeNe Laser Mode Sweep: 200 mm (~9 inch) Cavity Length is a PowerPoint show that demosntrates this for a laser tube similar to the type that would be used in a stabilized HeNe laser. A feedback loop controls the precise cavity length down to a scale of nanometers by regulating current to a heater wrapped around the tube. It maintains the optical power in the two modes to be equal (frequency stabilization as shown in Dual-Mode Single-Frequency Stabilized HeNe Laser) or a single mode to be at a specific optical power (intensity stabilization as shown in Single-Mode Single-Frequency Stabilized HeNe Laser). You can simulate laser stabilization in the PP slide show using the left and right arrow keys to maintain one or both of the modes appropriately positioned on the gain curve. But the electronics does a much better job. ;-)

Commercial models in the USA are manufactured by companies including Melles Griot, REO, and Thorlabs at the present time and by Aerotech, Coherent, Lasangle, Newport, Spectra-Physics, Tropel, and others in the past. There are also several from foreign manufacturers. More information can be found in the section: Stabilized Single Frequency HeNe Lasers and the chapter: Commercial Stabilized HeNe Lasers.

Basic lasing test:

The head ran fine on a HeNe laser power supply intended for 2 to 3 mW lasers, a Melles Griot 05-LPL-379 with an optimal current for maximum power of around 5 mA. The beam is boring pure red (632.8 nm, checked with a diffraction grating), clean, and reasonably well collimated with a divergence of around 1.7 mR. The output power is around 3 mW after warmup which is excellent for a head of this size. The mirror alignment is near optimal as determined by pressing on the exposed OC mirror mount after the tube had reached thermal equilibrium. Based on these factors, easy start and run, and low dropout current, it's probably new or near new. And no, it didn't surprise me at all to discover that the tube was intact and seemed to work fine. ;-) Whoever abandoned the head intended for it to be found.

Lasing modes:

This is where its lasing behavior begins to become unusual. On the Scanning Fabry-Perot Interferometer (SFPI), it's clear that something strange is going on. Using my dual-polarization detector, the display is shown in Dual Polarization SPFI Display of HeNe Laser with Higher Order Spatial Modes. As shown, in addition to the normal expected orthogonally polarized modes (the mostly tall peaks), there are a pair of "rogue" modes at locations that are not a multiple of the longitudinal mode spacing. These are almost certainly higher order spatial modes meaning that the beam is not pure TEM00. However, since the amplitude of the rogue modes is relatively small, any deviation from pure TEM00 is not visible by eye and might not even show up using a fancy beam profiler. No wonder this was abandoned on the side of the road! A stabilized HeNe laser must be pure TEM00 to produce the desired single frequency when one polarized mode is selected at the output.

Removing the tube:

At first, I assumed the tube inside the head would be something similar to what's shown in REO 0.8 mW HeNe Laser Tube, but with a longer metal cylinder (because the power is higher). This is the classic PMS/REO HeNe laser tube design with an HR mirror frit-sealed to a metal mount on the left, a short glass section, a metal cylinder running much of the length of the tube, and an OC mirror frit-sealed to a metal mount on the right that is directly attached to the metal cylinder.

Well no sense in postponing the inevitable: Time to remove the tube from the cylinder. As soon as the rear end-cap was pulled off, it became clear that the tube would be most interesting. Now there was no turning back since the anode connection had popped off and apparently could only be reattached from the inside once the rear end-cap was replaced. :) The actual removal process turned out to not be as terrible as I had feared. Although anchored using RTV Silicone (which doesn't yield to most solvents in finite time that don't also liquify human internal organs), it was soft enough, in small enough beads, and relatively near each end of the cylinder, that a thin metal strip could be used to cut through it all around. And in under 15 minutes, the tube could be slid out and removing the unsightly RTV residue was straightforward. But what a strange tube this is! See REO Tube from Stabilized HeNe Laser Head.

As expected, a Kapton thin-film heater is glued to the metal cylinder over most of its length. What must be a temperature sensor - a 3 pin TO92 package - is stuck on near the center, with wires missing. The magnet is now clearly visible - a ferrite ring press-fit onto the tube next to the where the metal part begins towards HR-end. But rather than the usual HR mirror, this tube has a funny dual HR configuration - one mirror at 45 degrees with another at 90 degrees to the tube axis. What???? Why???? Closeups of all of these "features" are shown in REO Stabilized HeNe Laser Tube Dual HR, Magnet, and Temperature Sensor.

At this point there are at least 3 mysteries:

  1. What's up with the higher order spatial modes? The usual cause in a tube intended to be TEM00 is that the bore is too wide for the mirror geometry. The HRs here are almost certainly planar, so this would mean the Radius of Curvature (RoC) of the OC is too small or the bore is too wide.

  2. What is the purpose of the dual HR? This configuration has sometimes been seen in low gain green REO tubes to introduce a polarization asymmetry in place of a Brewster plate. (The benefit is the potentially reduced losses compared to a Brewster plate that may be imperfectly cleaned and aligned, and have scatter from its surfaces.) That polarization preference is much less than the 7 percent or so introduced by a Brewster plate but adequate to completely linearly polarize a low gain laser. A quick test shortly after the laser was powered up revealed that the waste beam exiting the 45 degree mirror is purely linearly polarized (vertically - parallel to the normal of the 45 degree mirror as expected) with a power varying between 3.1 µW and 4.8 µW during mode sweep. The waste beam exiting the 90 degree mirror contains both polarizations and had a nearly constant power of around 4.4 µW. (They would be somewhat greater after full warmup.) The pure linear polarization might be useful for sensing of its polarized mode but that's like turning a tree trunk on a lathe to manufacture a single toothpick. :) There are much cheaper ways of accomplishing the same thing.

  3. Why is there a magnet stuck to the tube and why doesn't it turn this thing into a Zeeman-split two-frequency laser, which would definitely NOT be desirable in a stabilized HeNe for laboratory use?

Mode sweep:

The next step was to document the mode sweep of the orthogonal polarized modes (assuming that's what they were) from a cold start with and without the magnet in place. The mode sweep is a sort of fingerprint for lasers. :) See Behavior of Tube Used in REO Stabilized HeNe Laser. Red is the horizontal mode and blue is the vertical mode with the 90 degree HR mirror pointing up. The time scale on all four plots is 1.0 second per box (30 boxes total in each plot).

The only conclusions can be that the funky HR configuration is there to break the polarization symmetry by a small amount, but not so much that the tube becomes linearly polarized (as would be the case with an internal Brewster plate). This might both lock the polarization axes to the mirrors as well as prevent a Zeeman laser being created by the magnet, which is intended to kill the flipper behavior but isn't successful in taming the anomalous mode appearance. In fact, using the same magnet on a common barcode scanner tube that is a flipper (1) does not eliminate the flipping behavior and (2) results in a ~100 kHz beat being produced using a fast photodiode - classic Zeeman behavior. No such beat could be detected with the magnet installed on the REO tube.

But is there theory to back this up, or was this excessively complex technique for presumably assuring a mode-flip-free tube with predetermined polarization axes discovered by accident? And for that matter, what's wrong with the method used by every other HeNe laser company for building tubes for stabilized HeNe lasers that need to be well behaved - using a conventional tube design with optics that minimize back-reflections? After all, many if not most of the mass produced bog standard barcode scanner tubes are mode-flip-free and can be used perfectly well in stabilized HeNe lasers. Now true, if the dual-HR-with-magnet technique actually works well, it would be more deterministic than having to test each tube for their polarization axes, as is normally required. But once that's done, they don't usually change with age or use. And the cost here must be much higher. And given the precision with which the polarization needs to be matched to the polarizing optics in the head, that test needs to be done anyhow since eye-balling the orientation is not good enough.

Much of the strangeness with the mode sweep is probably due to the rogue spatial modes, increasing in amplitude as the tube warms up as the output power increases. If there were no rogue modes, the magnet might indeed prevent flipping with the mode sweep remaining normal even after warmup.

This was for a stabilized HeNe laser?:

So how could a laser tube with these faults have ever been intended for use in a stabilized HeNe laser? And if it was, how did it get past basic testing and Quality Assurance? Not only would the peculiar mode shapes make locking difficult, especially if used for intensity stabilization. But the rogue modes would mean that it may never be pure single frequency. So, was this a reject? If so, why weren't the anomalies caught before being installed in the head cylinder? Or was it sold as a stabilized HeNe laser head with full knowledge that it had problems and the hope that no one would notice? A stabilized laser using this tube cannot be as good as if the mode sweep is well behaved even if it does stay locked. But if it locks at a place where there are no rogue modes, it may be acceptable unless nutcases like me look at the mode sweep or academic types obsess over obscure measurements like phase noise in the optical frequency.

Does the stabilized HeNe laser REO is now selling have these same issues? It is not really known whether this IS the same laser tube used in ALL REO stabilized HeNe lasers. These are now sold by REO, as well as Edmund Optics, Newport, and several other companies. (Anyone care to donate one to the cause? Or, simply check if there is a magnet inside the head and report back here. A paper clip would stick to the cylinder 3 or 4 inches from the rear end.) And there is every reason to suggest that it is the same design. Why change it?

I have since tested a complete stabilized HeNe laser which appears to use the same tube design since it was found concealed behind a bush near where the bad one was abandoned. Everything visible from the outside is identical and the magnet is present based on the same battery being attracted to it. :) I have been forbidden from discombobulating this laser head for fear of invoking the wrath of the laser gods. So I can't check if the funky HR configuration is present without an X-ray. But peering through one of the holes in the rear end-cap after its fastening screw has been removed does appear to reveal the same glasswork and by its location, that the 90 degree HR is aligned vertically. The divergence and beam profile of both lasers appear to be the same by eye at least. But this tube has almost no evidence of rogue modes and the mode sweep itself is much more normal and changes character only slightly as the tube warms up. See Behavior of Stabilized HeNe Laser Using a REO Tube. In this plot, the time scale near the start is 0.67 s/div. while at 30 minutes it is 10 s/div. The only anomalies are tiny blips near the bottom excursion which are non-existent near the start but get quite pronounced after warmup. And for some unexplained reason, are slightly larger when the tube is cooling and the cavity is contracting as it is at 30 minutes (for reasons known only to the feedback controller). But the blips are far from the lock point for both frequency and intensity stabilization. Thus they should not affect operation. Since I can't disassemble the head, only the vertical polarized output mode can be plotted, but shape of the horizontal one is probably close to that of its mirror image. With the SFPI, there is just the slightest hint of a single rogue mode at an amplitude between 0.1 to 0.2 percent of that of the normal mode. By its relative location, it may in fact simply be leak-through of the orthogonal mode that is supposed to be blocked, due to a very slight misalignment of the polarizer axis. Clearly, this tube is much better behaved and the stabilized laser it's part of would be perfectly acceptable for most applications. So perhaps the abandoned head was a reject intended to have been destroyed, poor thing. :-)