(16 February, 2010, ff)
Quite a few metal vapor lasers are known; they operate at many wavelengths, from some microns in the IR to fairly shortwave UV. Of these, the best-known (and possibly the most efficient) is copper, which emits self-terminated pulses at 510.6 (green) and 578.2 (yellow) nm, as well as other wavelengths mentioned below.
The first copper-vapor lasers used copper metal, which was added to the buffer gas by heating it up until it reached a temperature at which it has adequate vapor pressure. This is somewhat painful, as it involves temperatures of more than 1000° Celsius. The Cu(I) halides have much lower melting and boiling points, and they soon proved to be suitable, though at low repetition rates they require double-pulsing. (The first pulse dissociates the halide, releasing copper atoms into the buffer gas; the second pulse, a few microseconds later, lases the copper.) A copper halide laser typically operates at 400-470° C. These lasers can be heated externally, but if the pulse repetition rate is high enough, the discharge heats itself.
I think I once saw an article about copper acetate, which was operated at much lower temperatures; but I would guess that it rapidly decomposes, and is suboptimal for that reason.
(Hmmm. Yes, I did see such an article:
Laser action in copper with copper acetate as a lasant
G. Chakrapani, T. A. Prasada Rao, A. A. N. Murty, and D. Ramachandra Rao
Appl. Phys. Lett. V31 (1977), p 832
Looks like it works well at 230° C.)
230° C is not hard to reach, but I am not aware of any DIY copper-vapor laser that uses copper acetate. In any case, there is another interesting approach: sputtering in a hollow-cathode discharge.
Hollow-cathode discharges are well suited to various lasers. They typically sputter the active material into the buffer gas, so they do not require high temnperatures. (Helium-iodine is an exception; the vapor pressure of iodine is high enough at room temperature that it doesn’t need to be sputtered.)
As far as I am aware, no DIYer has ever built a
hollow-cathode metal-vapor laser, and it seems to me
that it is past time for that to happen. This pageset
describes my initial attempt to design and construct a
hollow-cathode copper vapor laser, and possibly
HC helium-zinc and helium-iodine lasers as well.
(16 February, 2010)
Although this may be less true of copper vapor than the other lasers I have mentioned above, in general positive-column (longitudinal) discharge lasers have an unfortunate characteristic: as you increase the current above threshold the power increases at first, but then saturates and begins to decrease again as the current continues to increase. Hollow-cathode designs generally do not do this at reasonable current levels. They show increasing output up to the point at which it becomes difficult to sustain a stable discharge. This makes them attractive from the standpoint of efficiency as well as output power/energy.
There are actually several types of copper vapor laser, at least three of which have been operated in hollow-cathode structures. The first is the CW laser at 780.8 nm, which is an ion laser; the second is the usual green and yellow pulsed laser mentioned above. (As far as I am aware, no DIYer has ever built a 780.8 nm copper vapor laser, so one of the objectives of this project is to see whether the structure I have envisioned will work at that wavelength, assuming it works at all.) There are also more ion laser transitions, in the shortwave UV at 248.6 nm, 260.0 nm, 270.3 nm, and probably one or two other wavelengths; but as far as I’m aware they operate only with neon as the buffer gas. There is, if I recall correctly, at least one other red line, and there may be yet more viable transitions that I am not [yet] aware of. I am primarily concerned with the green and yellow pulsed lines here, at least initially.
(It turns out that at least one group has used copper bromide in a hollow-cathode tube, btw; but I believe they ran the tube at the usual elevated temperatures.)
The first step in a project of this sort is to glean some information from published work. As I write this, I have read several abstracts, but I have not yet managed to get to the University library to read the actual papers. We have just gone through two large snowstorms in a row, and the University was actually closed for a brief period. It is now open again, and I hope to get down there within a few days, after which I will have a much better sense of how (and how well) these things work.
In the meanwhile, though, I have some notions about buildable designs, from the information already on hand. My first idea for a single-section testbed device is about as follows:
anode anode
|| ||
||iii iii||
==================||iii\ /iii||==================
|| ----------- ||
(Pyrex end tube) cathode
|| ----------- ||
==================||iii/ \iii||==================
||iii iii||
|| ||
(Please forgive the ASCII-picture; I will substitute something better when I have a chance.)
The cathode, in this case, is a piece of copper tubing with flared ends, and the anodes are stainless steel washers. "iii" is an insulator. (As it happens, I have acquired two “Roman glass” bead rings for this purpose; this will probably be the only laser in the world with truly antique glass insulators — the shop owner said that these rings could be as much as 1000 years old.) Because I will probably be dissipating a fair amount of power in this device, I will seal it with high-temperature automotive gasket forming silicone rubber. That should permit it to get quite toasty without damage if I fail to cool it adequately. (Permatex® “Ultra Copper®” RTV is rated for 371° C intermittent service. That’s almost enough to let the laser operate in the usual furnace mode!)
[Note, added 24 February, 2010: I have now read several articles, some of which are listed in the References, and there are some photos below.]
The power supply is operated at negative polarity, so that the ends of the active section (the anodes) are at ground potential. This means there won’t be any voltage gradient along the Pyrex™ tubes that lead to the gas inlet and outlet. This helps prevent (or at least slow down) the accumulation of “junk” on the end windows. (Also, it is counterproductive to allow the power supply discharge to the vacuum pump instead of the laser!)
I think I now have nearly everything I need to build this test structure, and I will start constructing and photographing as time permits.
(18 February, 2010, early am)
Here are the anodes, the insulating spacers, and the cathode:
Here is the beginning of assembly:
(As you can see from the copper pipe that I am using to hold the cathode section in a more or less vertical orientation, I have also begun to acquire parts for a larger version with perhaps a dozen sections. That, however, will wait, as I do not yet know how long an individual cathode section can be and still sustain a discharge through its entire length. My guess is that I will be able to use segments with length of several times their internal diameter, but for now that’s just a guess.)
I intend to use helium as the buffer gas for this laser. Neon is reported to be better, but it is also at least 5 times as expensive, and I do not already have it on hand. I may add a small amount of argon, which (according to one of the papers I’ve read) enhances the sputtering a bit.
(18 February, 2010, afternoon)
Although I have found out variously that it doesn’t pay to rush, I’m still impatient. I have put the first spacer on the tube section, and now I need to wait for the RTV to cure before I add a second one. Once all three spacers are in place I can add the outer Pyrex tubes. I am afraid that final assembly will be somewhat tweaky and difficult because the gasket-maker material does not adhere well to glass; we’ll have to see how things go when I tighten the compression fittings.
As of now, I expect to use a large ceramic tile as the baseplate for this tube. If I orient it along a diagonal, I should have room for mirror mounts.
(24 February, 2010, afternoon)
Here is the assembled section. I used a piece of stainless-steel tubing as a mandrel when I added the two pieces of Pyrex tube at the ends, and I left it in when I took the photos; it has since been removed.
I now need to set up a plinth, mount this thing, cobble together a power supply (and work out a way to make connections), and test it.