Tuning a large CO2 laser resonator

[Twizzy]

I am currently trying to tune up a large fast-axial-flow laser that had been sitting around for years and I am looking for advice, I have had a hard time finding guides on this that have much detail. I have re-aligned the mirrors well enough to bring it back to just over half power (~500W so far, rated for 1000W CW, 2500W pulsed) but the beam is super lumpy with multiple hot spots, not just one distorted spot. I have heard that it is difficult to get a perfectly homogenous beam from large bore CO2 lasers but this seems really bad.

I've attached some images of the inner workings but here's the pattern of hot-spots on a concrete target. The melted region on the left best shows what the beam distribution is currently. It's basically a row of 5 dots with the lower dots being too weak to melt the concrete. The line of dots is parallel to the fold mirrors, I don't know if that is relevant though, and they are not adjustable.



I have currently been adjusting the mirrors to optimise output power (sensed by an integrated sensor that receives the small amount of beam that comes through the "back mirror"). I have been able to bring all the spots together with adjustments but the output power seems much lower when I do this. Is there some technique that involves keeping all the spots together and then making adjustments to raise the power? Also is it possible that the sensor could be inaccurate or not calibrated for the mirror?

I have heard that it is difficult to tune a CO2 laser to a good quality over its entire power range and I have noticed that the beam shape changes so I have been tuning it at the highest power that is stable since I believe that is what we would be using. Is this a good idea? It's a bit dangerous and I've already burnt myself since you have to reach over the beam to make adjustments, and it would be a bit of effort to enclose the beampath since the cabinet door would have to pass through the area I would have to build the enclosure in. It would be nice to know if it could be tuned at a lower power since it would be easier to make a target for visualising the beam pattern and I wouldn't have to use welding goggles over the top of the laser goggles.

There is possibly electrical problems with it too, when running at more than 85% current, one discharge in one or both of the 2 tubes will flip around and flow through the opposing discharge path (the layout is two parallel tubes each containing two back-to-back discharge paths, pretty standard for this size). I don't think the gas mixing valves have been touched but that could be adjusted. I have replaced the vacuum pump oil since that seemed a little dark, but I might replace the oil in the roots blower since it has been sitting around for so long and could be introducing some gas contamination. I have not removed any of the resonator mirrors to check their internal surfaces but they were apparently replaced within the year the machine went into storage.

Sorry for the rough and rambly first post but if anyone knows about this stuff, or could guide me to good resources for fixing my gross lumpy beam then I would be very thankful, even just suggestions for things to try would be great.


For background; the laser is a self-contained unit that feeds into a CNC cutting machine, it's a chinese NT-1000 model but the company is gone and there is no info on google. the previous manager (we now operate without a manager lol good riddance you career fraudster) bought it cheap 5 years ago since the previous owner cut wood and never cleared out the catchment tray so this caught fire and burnt out all the wiring around the head assembly, might have been an insurance job. I have replaced all the head carriage wiring and sensors at great effort since the documentation was rather poor. They also left the grease port open to debris on the 50mm ballscrew that drives the main axis so that was just full of shards of ball bearings and I had to re-ball it like holy fuck how did they not realise the horrific grinding sound it was making was a bad thing. I don't see how they could have screwed up the laser though, since it was self contained and professionally serviced, I feel like it should still be in fine condition aside from some bumps in transit.

[mixedgas]

I'll check with my friend the adjuster/rebuilder of Big CO2, on the order of 5KW for you. No promises, as some one here offended him one too many times and he will never post here again. Where are you located?

Generally if the multiple spots will not converge, you either need to regroup and start over, find where the beam is clipping the bore, or find the BAD or REVERSED optic making the ghosts.

Can you machine a set of beam centering disks with a small hole in it ? Then align with a HENE laser with CO2 power off? Bounce the HENE through the system and maintain near center thus way. Adjust the mounts as needed, Then insert the optics one at a time, align as needed, repeat, etc.

A few years ago I did an emergency checkout on something smaller but similar and found a reversed optic in the path. Gain enough to lase, but it cooked the uncoated face, and the designed-in wedge in the optic wrecked havoc on the system alignment path.. Partially melted ZnSe on the transparent face added diffraction to one edge of the beam as well.
It did not help that another optic was missing a spacer between it and the retaining ring. That results in beam power fluctuations as the system moved the table.

One fine point is un-clean, misadjusted, CO2 optics heat up and move the beam position once "warmed up".

Avoid the desire to use a thermal camera to view the beam, more then a few watts of reflected CO2 light results in burned pixels in the camera, even at 20 feet away from the scatter source. I damaged one of my video rate TCs learning that lesson on a 200 watt system. The power levels you have will damage beam viewing cards

At very low powers, B&W photographic print film makes a great one shot beam viewing card. The silver nitrate loaded gelatin cooks very nicely. Works well, better then Zap-it in many cases. Normal orange building brick forms a black obsidian coating if you hit it with enough Co2 power, much better then zapping cheap tile. Firebrick bricks for Kilns often heat to incandesce well before they crack, so that may form a better target. So does long thick polished blocks of clear polycarbonate for "mode burns" in depth, but DO NOT BREATH THE FUMES,

Any dirt, film, or crud at these power levels is fatal to the optic. At these power levels, you buy high quality optics, a replacement cheap optic will haunt you in many ways, including thermal distortion, etc.

Back reflections into high power Co2 also cause strange effects after the hot spot formed on some optic in the cavity, and often NOT the output coupler mirror, is partially melted. So some place along the way, machining the wrong material at exactly the wrong lens focal length or focal spot , can break things internally. Seen that one, too.


Steve

PS, Generally when I find abused abused large CO2, I find this scene useful:
Her is the laser...

https://youtu.be/qz5JmgLQEzs?t=188

There are times I realize that the factory aligns these with a transit, or a source autocollimator, not a visible laser. When you don't have the factory tooling jig what else can you do???

For background; the laser is a self-contained unit that feeds into a CNC cutting machine, it's a chinese NT-1000 model but the company is gone and there is no info on google. the previous manager (we now operate without a manager lol good riddance you career fraudster) bought it cheap 5 years ago since the previous owner cut wood and never cleared out the catchment tray so this caught fire and burnt out all the wiring around the head assembly, might have been an insurance job. I have replaced all the head carriage wiring and sensors at great effort since the documentation was rather poor. They also left the grease port open to debris on the 50mm ballscrew that drives the main axis so that was just full of shards of ball bearings and I had to re-ball it like holy fuck how did they not realise the horrific grinding sound it was making was a bad thing. I don't see how they could have screwed up the laser though, since it was self contained and professionally serviced, I feel like it should still be in fine condition aside from some bumps in transit.[/QUOTE]

[Twizzy]

Cheers for that. That must have been some serious beef. I'm in New Zealand here, so probably a bit far for dropping by.

I forgot to mention that we don't have any delivery mirrors fitted and this melted tile "beam image" was straight from the resonator, I feel a lot more confident about aligning the delivery mirrors since the effects you have to deal with seem a lot less arcane. Also there is a diode laser on the mechanical shutter which is hopefully good enough for the aligning those despite having at least 0.1deg of divergence and awful pattern.
Mirrors are on the way though, we ordered molybdenum mirrors to start with since they are fairly cheap and from what I heard are the most durable to abuse, sounds like they are a little lossy and get hot but I figured I'll just make sure they have good contact with the waterblocks.

For visualising the beam I had found that very fine stainless mesh gives a great thermal image at very low power, but doesn't last long before it oxidises too much and melts (nichrome could be quite good for this). I've just been using the glow from a concrete block so far, viewed through welding goggles, but fire bricks sound good since this is melting a lot.

So would a better strategy be to try maintaining a single spot and then working on raising the power while keeping it together? I might walk it around at a lower power in case I'm running the beam into the tubes.
I feel like I might be in a sort of false maximum where to adjust to the optimal alignment involves moving through an area of much worse performance.

If this doesn't help then could it be a good idea to remove the mirrors to check/clean them? I have been a little hesitant about touching them but that does sound like it could be a cause if the alignment isn't bad.
They were apparently replaced within a year of it going into storage so I wouldn't expect anything bad, but maybe they were running industrial grade gasses or something.

[mixedgas]

My friend's comments... If possible only ever remove one mirror at a time. He suspects that one of your mirrors potentially has the wrong radius of curvature, which is known to happen often when mirrors are replaced. Make sure the gas mix is close to correct, and figure out what commercial laser that unit is a cloned from.

He also says to check the actual power spec, on "import" lasers the warning stickers carry great exaggerations.

So is this AC, DC, or RF excited?

If you have the correct ultra pure cleaning chemicals, cleaning may be needed. Learn about "drop and drag" cleaning techniques and find really good lens tissue. I suspect you will be placing an order to Thorlabs too. You can never have enough lens tissue when working on CO2. Dont use crap from amazon or the local photo shop.

And whatever you do, no moisture or dirt near those optics. 90% of primary cleaning we do with a laser grade air bulb, NO CANNED AIR or Nitrogen, the freon like propellant kills the coatings on the optics. 99.99995% acetone or methanol are your friends, and we store ours under Argon gas to keep moisture away.

I would not want to clean my optics on the shop floor.... Optical Cotton gloves are a must, or dust free finger cots...

Not great technique but is a start:

Drag and wipe cleaning method how-to video - YouTube

This guy is good, except we go out of our way to fold the tissue without finger contact, and with practice it can be done with a second hemostat,
He is using a detergent on a visible light glass optic, which is a no-no for Co2 optics..

https://youtu.be/yANMsCmkqJ0


You want CTA-10 and MC-5oE from Thorlabs. Known good optics grade materials, do not go skimp on these...

Do not use common 100 pack cotton swabs, use individually wrapped wooden stick throat swabs without glue.

Proper type of puffer:

Amazon.com : GearFend Dust Cleaner Kit Strong Cleaning Air Blower + Soft Brush Blower for Camera & Camcorder, Lens, LCD Screens, etc + Microfiber Cloth : Electronics

Puffers are used, because otherwise you need 20 micron or better filtered dry air.

Microsoft Word - LMT Lens Optics Cleaning Procedure COHR.docx (coherent.com)

Practice your drop and drag on a scrap piece of glass or a microscope slide.

Avoid scrubbing large optics with small swabs, DON'T even think about the ultrasonic cleaner or parts washer.

For coated CO2 Transmissive Optics only and NOT for Visible light optics. While very professional, some of these methods run a great deal of risk:

https://youtu.be/X7NduedSVKI


Steve

[Twizzy]

My friend's comments... If possible only ever remove one mirror at a time. He suspects that one of your mirrors potentially has the wrong radius of curvature, which is known to happen often when mirrors are replaced. Make sure the gas mix is close to correct, and figure out what commercial laser that unit is a cloned from.

He also says to check the actual power spec, on "import" lasers the warning stickers carry great exaggerations.

So is this AC, DC, or RF excited?

Cheers for that.

Excitation doesn't look like it's designed for more than a MHz or two but I can't fully identify, HV cables are coaxial though. I think it's DC since it has a switching converter rather than a large traditional transformer but I don't know, could be an inverter without rectification. I could probably set up a probe to measure it if that's important.

Power spec should be close to actual, I find that industrial machines don't tend to be more than 20% off.

I did a quick calculation from the pressures and flow valve settings and it seems the gas mixture is around 53%He 40%N2 7%CO2. It's a bit hard to tell exactly since the CO2 valve is turned down so much and if helium or CO2 has a different viscosity than air then this will be wrong. The pressure on the helium is higher than the others, it might have been turned up on the regulator. Is this a reasonable composition? Anything that could be changed to improve stability and prevent the discharge from flipping around?

I don't have any leads for the design origins. The same model appears to still be produced in India, but it is possible that only the power supply and optical layout has a lineage to a name brand since the other components don't really require very advanced research. I found this wanktext that claims they make the best lasers in China: https://en.m.ofweek.com/showroom/Nan...t-0-0-0-0.html Entirely possible but the company did kinda stop existing so idk lol.

As for the cleaning whoo boy I was really hoping to have some practice with the delivery mirrors before touching the resonator optics. I'll have one more try twiddling the knobs but then I guess that is the next option. I need to get those supplies anyway. Reminds me of the mirror cleaning that was done on the JWST mirrors. They had an assistant just bringing the trays of cloths that get wiped very slowly down the mirror once then discarded. I'll probably try this in a couple of weeks when I have some other work finished.

[Twizzy]

Got the mirrors aligned! With it being close to alignment and without an external alignment beam I found the best way to bring it back was to set the power just high enough to cause a visible thermal glow on concrete and then search around. The refraction/reflection pattern I had from the beam running into the tubes had an obvious centre so once you get a feel for the direction to move things it is actually fairly easy. Top row on the concrete is the roughly aligned beam (before I jacked up the laser unit to not clip into the mirror mount on the lower right corner).

I still have a problem with the discharge flipping around at high power after running for 30s or so (second picture). This happens at above 90% power output, I don't trust the power or current sensors since they seem poorly calibrated but this is supposed to be 900W. Anybody have an idea what could cause this? A rough calculation from the regulator pressures and the restrictor settings should put the gas composition around 53%He 40%N2 7%CO2 which seems a little high on the nitrogen, could it be the gas? I am really hoping it is not electrical.