10W Copper Vapour Laser Operation: VIDEO

[danielbriggs]

Morning all,
I like the idea+format of Eric's videos as I think it's great way to convey loads of info in a fairly short space of time; so that inspired me to create a basic video of running up a 10W Copper Vapour laser from cold. I've tried to keep it jargon free but still getting the main points of basic operation across.

It's only the 2nd time I've run this particular unit up, the first run a few days ago was a little problematic as the laser had been sat near atmospheric pressure for a while and not been run for well over a year. So it took many hours to get it to lase, but was very very happy when it did. I think this unit needs one more run of a good 3-4 hours and then it will be back to near perfect health again.

I picked up two of these from Ryan some time ago; the 2nd unit needs some repair and bit of TLC, but I'm aiming to get them both running at some point.

These CVL's are vey nice units and the photons it chucks out are superb. It's a little more fiddly to get running than my old friend the CuBr (it's cooler brother ) but when the tube is in good condition, it's not too bad at all. The fact you can refill and re-pump the laser down means the head will last a near lifetime (only really care of the thyratron and servicing the vac pump in the PSU would be required to make it last many thousands of hours).

Anyway, I hope people find it vaguely interesting!

https://www.youtube.com/watch?v=6KDWdloHRvo


Cheers,
Dan

[mixedgas]

I raise my glass to you SIR!


Steve

(PS, The Three Sets of Big Cambridge Galvos are stored in the barn, Call BritFloyd!)

[planters]

Dan,
This was a little like Christmas! I enjoy doing the videos I have done and although it obviously takes work, it's nice to see they are liked when people open them up. But, coming down to the comp and seeing something under the "tree" was sweet. Nicely done. You give a good mix of the practical layout, the technical graphs and some stills where the video fails to reveal the discharge in the tube.

Cu (or any metal vapor) laser is really interesting. The diodes are sure convenient with a low voltage DC current and well...nothing I guess, That's it, they're convenient. But, the marriage of the extremely short pulse electronics, the gas handling system and the discharge tube that, inside its neat finned housing, runs so hot that even after the power it turned off, it glows like the sun, that's beautiful.

If you are not going to need to use that bright two color Cu output immediately then how difficult is the exchange of one metal for another? Beside Pb and Mn, would Hg be an option? Would the cavity optics have to be changed for each metal? Just wondering. Also, is the cavity plano/plano? How is the divergence? In developing my latest pulsed dye (no video yet) I discovered a cavity design for bulk gain media like metal vapor, thick YAG rods and dye cells that reduces divergence without resorting to unstable optics. If we ever do get together, I would like to review this with you.

I'm going to watch this video a few more times. Thanks!

[helium]

Thanks Dan, I found the video both interesting and informative. It's not often you hear the word 'thyratron' here on PL. I used to work on various types of CO2 many years ago and always found the thryratrons (air cooled versions) to be very temprementle, especially Monday mornings! From what I can remember we needed a licence to import them into Ireland from the US, that was back in the 80's. I guess Pink Floyd are the only ones that could afford to shovel gold into the cavity of these lasers. What is the actual output beam diameter?

[danielbriggs]

Thanks Steve + Eric,

If you wanted to put another metal in place, I think it might be easier to put in a new, fresh + clean tube. As I'm convinced it would be nearly impossible to fully remove all traces of copper with ease. And as copper is a high gain medium, any other "exotic" metal would be struggling to compete with the remaining copper.
It's highly likely you'd need to change the bore size as well; I amassed a dozen or so papers on the technicalities of doing such a while back (mainly for copper) but also touched on other metal vapours.
I am far from an expert on this, but it seems the cavity optics aren't really that important at all. These are "thermal lasers" (if that's a thing?) Get the pressure, bore diameter, temperature, voltage and plasma current right then slap some glass on the ends.

Copper or any other metal vapour lasers use a single "big pulse" to excite the Cu atoms (or filling of choice). Metal halide lasers (e.g. CuBr) lower the boiling point significantly (to around 430deg) making the physical problem of heating the tube a little easier. However vaporised Copper Bromide gas does not lase. To get around this, the PSU employs a clever "two pulse trick". The first pulse ionises the Cu and Br, and for a very short time period (few hundred ns?) the ions are separate. Then a second "conversional" pulse excites the Cu while it's ionised before it sticks back to the Br- ions. So CVL = Bang. CuBr = Bang Bang!

I sold my CuBr a while back, and compared to this CVL, it was a like a luxury automatic saloon, with the CVL being a powerful traction engine (in terms of starting the thing).
I took the CuBr cavity mirrors out to clean while it was lasing and with both removed, it STILL lased (poorly and very low power) but copper just wants to lase. The O/C's are around 4-5% I recall; any more and you risk over heating the tube.

Back to the CVL... the cavity is Plano-plano. And as a result gives a fairly high divergence (25mm dia, 4mRad). If you change it to an unstable resonator, you'll drop the 25mm beam down to 0.8mRad, ish, at the expense of roughly halving the average power.

A while back when I was digging for info on these I ended up speaking to a service engineer who more or less knew the location of every Oxford Cu in existence. He was the same guy who toured as one of the two British techs which followed a pair of the larger Cu45's for Pink Floyd, keeping the lasers in tip top condition for the show + operator. He said they had dye modules in the front, such the Cu's were just pumps for 10W-15W or so of nice red.
When I asked about conversion to "Gold" the Robert said "don't bother" a 10W Cu tube will be modified to 2W of 628nm. He said if you wanted red, keep the Cu as is, and pump a dye, you'll get more red, and of better beam quality, and cheaper.

I think these little tidbits you discover "down the rabbit hole" which absolutely make the whole process worth every second.

The Oxford heads look more or less bullet proof, as long as the thyratron is treated well + the rest of the PSU + vac pump well serviced, you can "rebuild/recondition" any Cu head to peak condition in around 1-2 day, 3 days if it's really contaminated, or poor service technique etc.
This is promising for the 2nd CVL; as soon as I get the PSU rebuilt + working and the tube lit, I'm certain the rest is just time + minor fiddling.

Further reading: Copper Vapour kinetic enhanced gas.
Simply by changing the buffer gas, to include a smidge of hydrogen + chlorine can yield around 2x the output power from the same tube + conditions.

All the best,
Dan

[macpod]

This was quite interesting to watch. I had not read about the general operation of one of these before. Thanks for taking the time to make it!

[tribble]

PL needs a "like" button for when you want to show appreciation but don't have anything substantive to say beyond "wow, cool, thanks!" :-)

Wow, cool, thanks!

[laserbiker]

Thank you Dan for sharing this video. The graphs of wavelength VS tube temperature are very interesting.
It makes you wonder about the details of the physics going on in there. Lasers in all their form are such beautiful
and interesting things..

[daction]

I too don't have anything constructive to add. Very cool.

[mixedgas]

Manganese has two green lines.

Strontium is at 430.9 nm with some mid IR Lines.

Lead is at 729 nm with some Mid IR lines.

Mercury would be at so high a vapor pressure in that hot tube that it would not lase.

Barium has some very low power visible lines.

Of all the other metals, Manganese is next choice after Gold.

Planters, the books come from authors named C.E. Little, ‎N.V. Sabotinov and G.G Petrash. Petrash, when translated from the Russian, is very interesting. C.E Little invented CuBr and is worth reading. The Russians and Bulgarians spent MegaRoubles on MVLs, the results were interesting.
So did the US government for pumping certain dye lasers used for isotope separation.

Steve