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Thread: When Carbon Dioxide Attacks

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    Default When Carbon Dioxide Attacks

    NO, I'm not the one who did this.

    Moral of the story. DON'T SCREW AROUND WHEN WORKING WITH CO2 lasers. Don't align at full power, either. Run System Checks daily and hourly.

    This is what happens when a mirror mount's orientation changes and you get a nearly resonant back reflection off the Laser's OC. That was about 180 watts. One Eighty prior to the nearly resonant bounce condition. Think three or four bounces, maybe more.

    S.
    Attached Thumbnails Attached Thumbnails MELTED1.jpg  

    FriedMirrorMELT.jpg  

    Last edited by mixedgas; 04-09-2020 at 17:13.
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    ... this was a single bounce!

    Viktor
    Attached Thumbnails Attached Thumbnails Defekter Faserlaser2.jpg  

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    Quote Originally Posted by VDX View Post
    ... this was a single bounce!

    Viktor
    Ouch, that means it's its worth it to buy a Faraday Isolator.

    Steve
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    ... common CW-fiberlasers (like the one in the image) didn't have one -- seems to have some disadvantages too?

    Could be, the isolators can handle the pulse peaks of the pulsed fiberlasers, but not the CW-power?

    Did some tests with first-surface-mirrors in a DIY-galvoscanner with a 50Watt CW-fiberlaser -- "exploded" several mirrors with 3W to 20W after some seconds use with milliseconds long pulses ... but had no problems with a 20Watts "pulsed" type, where the 200ns pulses have peak powers of up to 10 Kilowatts

    Viktor
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    OK Steve, I'm going to need help with this one. I still can't figure out what the hell happened.?.

    "Nearly resonant back-reflection to the OC"... If I have this properly pictured, you've now got something similar to a q-switched laser with the switch open (albeit on the wrong end of the laser). So you've dramatically increased the effective length of the optical cavity (and also doubled the optical intensity at the OC face, but whatever). Are you saying that this built up enough energy to MELT both the mirror *and* the mount? Because in the top picture the stage that the mirror would normally be mounted to looks like it caved in - either because it was hit with a hammer or because it got hot enough for the metal to soften and deform. That's crazy...

    And is that a burn mark on the second picture (the one that appears to show the support post for the mount)? If so, how fucking hot did this mirror mount get?

    Also, what happened to the grad student who was adjusting the mount at the time? Is he missing any fingers now? (I'm pretty sure he at least had to change his underwear afterwards!)

    So many questions...

    Adam

    PS: Viktor, I've seen your picture before. Something tells me that was a bad day... ("Well SHIT! This thing is well-and-truly fucked now!")

    EDIT:
    EDIT # 2: Crap - attachment doesn't seem to be working. Link to IMGUR instead - ( https://imgur.com/MhKt69o )
    Attached Thumbnails Attached Thumbnails HomerDynamite.gif  

    Last edited by buffo; 04-11-2020 at 03:37.

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    ... this was not my problem with the exploded fiberlaser

    Received 8 or 9 of them with different "wrecking"-grades and sourced my first fibercoupled IR-laserdiodes with output-powers of 5, 9 25 and 30Watt from them (have some others with 200Watt or 500Watt powers from ebay too).

    Got even two fiber-lasers with all the diodes and fiber-resonator OK, but broken electronics (one with cut output fiber, the other not damaged) ... replaced the electronics with my own drivers ... and have now a working QCW fiberlaser with 130 Watt output power

    Viktor
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    Buffo, Hu[man} did this. Hu was having a bad day with various social frustrations. Hu, tired of the hourly alignment touchups of the motion stage, somehow aimed M2 back into the beam path towards the laser head. Normally M2 aligned the beam along path "A" into the cutting head to hit fold mirror M3, which deflected the beam into the lens, which could also move in three axis. Hu had to contend with 15 optical adjustment screws, not counting the CNC motion, laser power and assist air velocity.

    Remember when I said, almost resonant? Hu's action resulted in the spot "stacking" itself into a row of beams on Laser two's OC mirror and at M2. The beam waist from the laser head focused right before M2, so it had a really small spot size.

    Hu set the power to 100% on both tubes and started a long cutting run.

    Hu was at his part time job, Hu is not a grad student.

    You know when your aligning a cavity with an alignment HENE, and your close to having the mirrors aligned, but one axis is still a little off and you see a close together pattern of Dots in a Row? That is what Hu ended up doing, with the ~ 3 mm diameter beams almost but not quite totally overlapping.

    Eventually the waveplate W1 could not take the flux and turned into a weak lens. Laser tube One's OC is also damaged. So at first it melted the rounded "burn" spot, and then the beam from one tube diverged from the optics damage and heated the whole mirror and mirror mount.

    So 180 Watts minus the per pass adsorption bounced "N" times and built up to a huge amount of power, by reflecting from M2 to the OCs and back. There is a Quantum Mechanical Quirk where in some cases the OC is not all that reflective when it is intracavity and exactly aligned to form an Etalon. So he had a long, long, cavity. But OC two was just perfectly aligned enough to not have damage.

    This was CW Co2, no Q-Switching. Really freaky situation, and I doubt I could ever reproduce it in the lab. I've had a few people with decades of experience look at that picture. They are all scratching their heads wondering just how that happened.


    The three mirror linear cavity is well known to having energy enhancing effects for things like intracavity frequency doubling, when well designed, and is in fact patented long ago. It is just really difficult to "accidently" make one.

    Back of the envelope calculation is 1121'F / 650'C if its aluminum, and 785'F / 385'C if its Zinc Alloy for die casting. I'm not sure it 100% melted, it may have gotten just hot enough for the strain from the springs to warp it. But the "burn spot" is a melt.

    Jury is still out if Hu had a really freaky day and forgot to tighten the M2 mounting post, or if Hu wanted to see what would happen if the beam was folded back on itself. The laser head and M1, M2, M3, move on a gantry with frightening acceleration levels for such an assembly. I'd like to give Hu the benefit of the doubt, but I'm not going to bet either way.

    The system is back up and running on tube two only in an emergency manufacturing situation. That's why I got the call. Tube two was undamaged and running nearly TEM00.
    Tube one still lases at a surprising amount of power, but the beam is divergent crap and is not aligned with Tube two. The laser head is buried deep in the machine, and the judgment call is to run on the remaining tube. Only way to get a new tube is a long wait for one to be flown in from overseas.

    Sorry for using the "woke" New Age sexless Pronoun "Hu", but Hu is reasonably young, and we all make mistakes.

    See Attached.


    Steve
    Attached Thumbnails Attached Thumbnails MIRRAHmeltd.png  

    Last edited by mixedgas; 04-12-2020 at 16:26.
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    Quote Originally Posted by VDX View Post
    ... this was a single bounce!

    Viktor

    - There is no such word as "can't" -
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    ... my "garage-DIY-brew" is more focussed on micro-/nanotech - really hard to find related cook-books

    And my own lasers are in much better condition, than the "exploded", which I'm salvaging for laserdiodes ... but already had two occasions, where two of the CW-fiberlasers (one 50W-type, the other 120W) shut down for "back-reflection >10%" -- what's still in the safe range, so still working

    Viktor
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    Thumbs up This is turning into a deep-dive!

    First, thanks Steve for sharing this story. (Especially with the diagram so it was easy to follow!) It forced me to dust off some things that I haven't thought about it a long time, and for that I'm grateful. However, it's also uncovered some glaring ignorance on my part as I'm trying to understand the physics behind what happened. So please, if you don't mind putting on your teacher hat for a moment, I've got more questions.

    Quote Originally Posted by mixedgas View Post
    You know when your aligning a cavity with an alignment HENE, and your close to having the mirrors aligned, but one axis is still a little off and you see a close together pattern of Dots in a Row? That is what Hu ended up doing, with the ~ 3 mm diameter beams almost but not quite totally overlapping.
    Got it. Thus the "not quite resonant" part.

    Eventually the waveplate W1 could not take the flux and turned into a weak lens. Laser tube One's OC is also damaged. So at first it melted the rounded "burn" spot, and then the beam from one tube diverged from the optics damage and heated the whole mirror and mirror mount.
    I understand how a failure of the waveplate might save the OC on tube 2. So far so good.

    So 180 Watts minus the per pass adsorption bounced "N" times and built up to a huge amount of power, by reflecting from M2 to the OCs and back. There is a Quantum Mechanical Quirk where in some cases the OC is not all that reflective when it is intracavity and exactly aligned to form an Etalon.
    Here's the part that I have trouble with. So you've got (effectively) a larger cavity because the mirror at M2 is acting as the new OC (at least for laser 1), and the former OC for laser 1 is now a shitty etalon (because reasons... it's not at the right angle, it's not thermally stabilized, etc). But how are you building up any *extra* power? There's no pump energy being added to that longer cavity. All you have is the pump energy from the original cavity that is between the (former) OC and the HR. And all the mirrors in the optical path had to be rated for the full 180 watt combined output of the two tubes. So what went wrong?

    Or am I looking at this wrong? Is it because the mirror at M2 was not, in fact a true OC, but more like an HR, which means that instead of merely being exposed to the nominal output power of the laser (which would have leaked through the original OC), instead it now had to contend with the much higher intensity of the intra-cavity flux (which was now also present outside of the original cavity as M2 is now the OC), and the mirror at M2 simply wasn't up to the task? If so, then what if the mirror at M2 had been an amaze-balls HR mirror? Would the damage still have occurred? Or would it have survived?

    Put another way: what if you could remove the OC on the tube and replace it with a second HR? (I know you said sealed optics; work with me here, this is hypothetical) This would effectively make a laser with no real output; all the energy from stimulated emission would be trapped in the cavity. I'm assuming that in this case the tube would overheat, and unless the cooling system could remove the excess heat it would eventually shut down on thermal overload. But is it also possible that the tube would "run away" - that is the internal flux would keep rising until one of the HRs failed?

    My instinct is to assume that such a "run away" condition is not possible, because there's always some leakage on any mirror, so at some point the losses through the two HRs plus heat loss to the surrounding environment would balance out. But I'm not positive about this. Maybe it's just that long before you reach the balance point, the mirror face fails due to the intensity of the intra-cavity flux? And if so, then whether it runs away (or just sits there running a little hotter than normal with no output) is dependent on how much pump energy there is vs how good the mirrors are?

    The three mirror linear cavity is well known to having energy enhancing effects for things like intracavity frequency doubling, when well designed, and is in fact patented long ago. It is just really difficult to "accidently" make one.
    As Bob Ross would say, "Just a happy little accident?" You mention frequency doubling with 3 mirror cavities; this is starting to sound like Greg Makhov's famously creepy explanation of frequency-summation laser theory... I know I'm going to regret asking this, but can you point me towards some light reading on the 3-mirror cavity concept? (No pun intended...)

    Adam

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