This partial laser head was found on the side of the road with no markings and no return address! :) (Don't I wish one could find lasers on the curb. Regrettably, all one finds around here are very dead lawn mowers. But I can't reveal the actual source, so some trivial details of this story have been modified in a way that has no effect on its technical accuracy.)
The reason I deduced it must be for a stabilized HeNe laser was the pair of extra wires sticking out of the front of the head with a low resistance between them - the heater. (I knew it wasn't a bomb because it didn't explode when the resistance was checked.) And the rear end-cap has the usual Alden cable as well as another one, exact purpose unknown. It's only a partial head because the front section which presumably would have the beam sampler and possibly other optical or electronic components was missing, apparently removed against the head's wishes with a pipe cutter. :( :)
From the appearance of the front of the laser tube inside the head cylinder, the visible part of a 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 other than its use in a stabilized application.
What ultimately 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.
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 red (633 nm), clean, and well collimated. The output power is around 3 mW and peaks 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. Based on these factors, easy start and run, and low dropout current, it's probably new or near new.
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:
Well no sense is 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 even more interesting and there was no turning back. 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 any 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 steel strip could be used to cut through it all around. And in under 15 minutes, the tube was free 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, glued to the metal tube was a Kapton thin-film heater with what must be a temperature sensor - a 3 pin TO92 package glued near the center, unused with its legs clipped. The magnet is now clearly visible - a ferrite ring pressed onto the tube next to the where the metal part begins towards HR-end of the tube. 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:
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. 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.
Much of the strangeness with the mode sweep is probably due to the rogue spatial modes, increasing in amplitude as the tube warms up and 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 hoping 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 laser tube used in the REO stabilized HeNe laser. (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.) But there is some reason to suggest that it is the same design. One piece of circumstantial evidence is the presence of the temperature sensor, not used in the head I found but perhaps a key part of REO's system. And why change it?
And 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 twonky one was abandoned. Everything visible from the outside is identical and the magnet is present. I have been forbidden from discombobulating this laser head so I can't easily check if the funky HR configuration is present for fear of invoking the wrath of the laser gods. 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. 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 one polarized output mode can be plotted. With the SFPI, there is just the slightest hint of rogue modes at an amplitude between 0.1 to 0.2 percent of that of the normal mode. These may in fact simply be leak-through of the orthogonal mode that is supposed to be blocked, due to an imperfect polarizer or 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 that was supposed to have been smashed, poor thing. :-) Or REO kept the best and sold the rest. :( :)
