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Thread: Laser Tattoo removers

  1. #21
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    That is a Cr:YAG passive Q-switch, aka a saturable absorber. The dark green color gives it away. Odds are the rear mirror is NOT coated on the Q-Switch.

    The laser will work best if the flashlamp is fed from a pulse forming network. If you don't use a PFN and the rod lases near it's edge, ie long pulse mode, it could crack there. If I were playing with this I would use an intracavity pinhole to keep the beam in the center of the rod until I was sure it was properly aligned.

    I used to service scientific ND:YAG. Be really careful with this wavelength and energy . Eye Damage and Tissue Damage are very easy to obtain, even with just stray light. You need safety glasses with OD6 or OD7 at 1064 nm. NOT a toy, and study up on your laser safety, and keep unprotected friends and the family pet out of the room. Nominal ocular hazard distance would easily be in the hundreds of meters for a direct beam.

    Keep your rep rate well below a few Hertz or you'll risk melting the lamp and/or shattering the rod/Q-switch.

    Buffo, treat this as a hopped up, low quality SSY1, think 5 mJ to 10s of millijoules, and say 20-50 nanosecond pulse if it goes in one pulse train, which is not always likely.

    I'm not seeing a rod, and I do not see end mirrors, in the one picture I can see. I'd start with a flat 100% HR and a 5% or 10% OC with a long radius, ie 60 or 100 cm OC or longer.. Eventually you may be talking about a transmittance of 20-30% for full power extraction. I would NOT use a flat-flat cavity with this laser. You'd need mirror mounts, a bench for the cavity, a PFN and a series injection ignitor with a very high energy blocking diode. . Something like a Thorlabs DET-10 and a 100+ Mhz oscilloscope to monitor the Q-switch performance.

    Calculating the PSU capacitor for the lamp requires removing the lamp and making measurments and some assumptions about the lamp fill pressure. The rod needs very clean cooling water held at a constant temperature for best performance. John Goncz's paper on flashlamps is needed for the calculations, but Sam Goldwasser and Don Kliepstien (Spelling?) have the math on line some place.

    You need access to a a copy of Walter Koechner's Solid State Lasers text book just to get started on understanding this beast.

    Without knowing the initial T and Doping of the Q-Switch, designing for this puppy means making a lot of assumptions.

    Lamp voltage with the right PFN may be anywhere from 250 to 750 volts, and that is a big unknown. Your lamp's explosion energy will have to be calculated as well. Overdriving it is as bad as under-driving it, if not worse. There is a region where it will like to work and a region past that where it will just heat up the rod and energy will go down.

    Assuming you use ND:YAG you want your PFN to drive the lamp for just around say 275 -350 microseconds or the laser will act weird and the Q-switch might fire multiple times with VERY low pulse energy, if it fires at all. The laser rod will be saturated in about 256 microseconds, aka 1 storage lifetime.

    Your best bet is to find what it came out of, and its resonant capacitor charging supply, rod, ignitor, and supporting parts.

    "OSR" has spoken. (pun on earlier conversation in another thread)

    PS, The lamp mounting method suggests this is a somewhat disposable module, not really designed for frequent service. See if there are lamp clips under the heat shrink, then replace the heat shrink. If no clips and its welded wire, and the lamp won't remove easily by sliding out, throw it away. Lamps get old, turn black, and become unstable, then can start misfiring or crack.
    You may be in for more of a repair job then you think. A good lamp starts at around 150$ and up.


    THIS THING IS NOT A TOY. IF THIS IS YOUR FIRST RODEO, AND YOU HAVE THE MONEY TO BUY THE PARTS, THEN GO FIND A LOW POWER CW laser to EXPERiIMENT WITH FIRST.

    This also the last way I'd remove a tatoo.. It may be for another medical purpose.

    Steve
    Last edited by mixedgas; 12-10-2018 at 14:14.
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  2. #22
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    Quote Originally Posted by buffo View Post
    I've never heard of a passively-q-switched YAG before. More to the point, I don't see a mirror behind the dark block, so even if that was the Q-switch, where is the other mirror?


    Adam
    Plenty of them out there. Ever see those Nd:YAG rods on eBay with the much darker ends? Those are passively Q-switched rods with passive Q-switches bonded onto the rod (Likely Cr:YAG).

    http://www.optocity.com/CrYAG.htm

    https://www.rp-photonics.com/q_switching.html



    Edit: I see Steve already covered that.

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    Thanks for the links and extra info. I had honestly never researched passive Q-switching. Didn't even know it was a thing! But hey, I learned something, so that's a plus at least.

    5 to 50 mjoules, even with ultra-short pulses, doesn't sound like it would be very effective at destroying tattoo ink though. And the wire leads that feed the head do *not* look like HV-rated cable, which makes me wonder how they're running a flashlamp... Are you sure it's not a direct-diode pumped YAG?

    Adam

  4. #24
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    Quote Originally Posted by buffo View Post
    Thanks for the links and extra info. I had honestly never researched passive Q-switching. Didn't even know it was a thing! But hey, I learned something, so that's a plus at least.

    5 to 50 mjoules, even with ultra-short pulses, doesn't sound like it would be very effective at destroying tattoo ink though. And the wire leads that feed the head do *not* look like HV-rated cable, which makes me wonder how they're running a flashlamp... Are you sure it's not a direct-diode pumped YAG?

    Adam
    10-50 mJ in 7 nanoseconds properly focused would form a glowing white plasma ball a millimeter or two in diameter in air at 1064, if the optics were good and there is a dust particle to ignite the plasma. Ie Snap Air. With a good lens, what that would do to tissue is amazing if used properly.

    Yes, trust me, that is a cheap lamp pumped laser cavity. Seen many of them. A few common pitfalls: If the pulse is short enough is cracking the inside of the lens at 50-100 mJ. Or a nasty burn on your hand if adjusting the mirrors and you move your hand the wrong way.


    Tattoo ink is never really destroyed during removal. Most removal I've seen takes a few to tens of treatments, either Co2 or Pulsed Laser based. They discolor the ink, destroy some other types, spread it out, bleach it, vaporize it, disrupt the area, and cause the body to reject it, and providing an opening or burn in the skin to do so.

    Evidently some of the new systems use a picosecond pulse for better effect and get the droplet size down to small enough white blood cells can carry it away.

    So since Picosecond Alexandrite is the new toy, the YAG ones like this are going to show up as scrap. (edit)

    Say Ouch:

    https://youtu.be/7BgEzOTCV0k Updated video.


    This fellow and a MD give a really good explanation of the new one.

    https://youtu.be/D0B7F5UbTOQ

    Passive Q-Switched YAGs or EO Q-Switched YAGs with very short cavities tend to have very narrow pulse widths compared to an AO Q-Switched laser.

    New tool:

    https://www.cynosure.com/product/picosure/

    110,000$ retail.

    Steve
    Last edited by mixedgas; 12-11-2018 at 10:39.
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    Wow... Pico-second pulses?!? That's a pulse width of something like .3 millimeters, which is a small fraction of the cavity size.

    I'm having a hard time wrapping my head around that. (How can the pulse width be less than the single-pass propagation length of the gain medium?)

    I mean, thinking back to the Shiva/Nova fusion research lasers, the whole point behind using the thin disc amplifiers (as opposed to a long rod) was to reduce the pulse-width, right? (OK, cooling played a role as well, but surely it wasn't the primary concern?) But even so, that laser could only deliver pulses of about 2 nanoseconds, even with a really short propagation path through each amplifier stage.

    I have to admit that even the 7 nano-second pulses from the older medical units is still pretty impressive given their relatively small size and cost. But 7 ns would give you a pulse width of a couple yards, which represents multiple passes through the gain medium - as you might expect. So while the peak power is quite high, it still follows that the population inversion in the gain medium is sustained for several passes of the beam.

    If the pulse width is *shorter* than the length of the gain medium, wouldn't that mean that the population inversion was exhausted in *less* than one pass? How is that even possible?

    Come on Steve: physics lesson time!

    Adam

  6. #26
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    Picosecond is third generation technology and been around a long time. Quick one sentence explanation. You switch from the time domain along the cavity length to the spectral domain to generate the pulse after mode locking the laser. So a mode locked laser can do a pulse far less then a cavity length by "bunching" the photons into one group with a relatively slow intracavity shutter, yet still match the "fill" of the longitudinal mode. That gets you into the picoseconds. Once you do that, the ultra short pulse opens doors for making even shorter pulses.

    Bullshit quick explanations:

    Having aligned and serviced two models of picosecond cavities, I was shocked at how easy it was to do this with a solid state rod that has lots of storage time and a very slow acousto-optic crystal that is intracavity. Tune the cavity length for around 80 MHz mode spacing and put in 160 Mhz AO intracavity would be one example of the math. Its counter intuitive, in YAG you build a long four mirror cavity to do it, by getting the mode spacing longer.

    Then it gets even freakier... You can create a region in some crystals that will change index of refraction violently in a non-linear way if the electric field from the focused light is intense enough. Alexandrite is one of the materials that can stand up to this intense light at a focal point. Place some lenses intracavity, focus the light down into that small spot inside the rod, pump the snot out of the rod, add a weak external mode locker /mode shifter /Q-switcher (in my case the laser I used had a Gm20 actuator and a AR coated glass arm intracavity) as a "Starter". Kick the starter to suppress lasing for a short period of time so the population inversion builds up inside the crystal, and physics happens... This is Kerr Lens Modelocking….. After the Kerr effect... Its far faster then anything you can do with an external mode locking setup.

    Take its pulse and then you can run it into a pulse compressor...

    That pico pulse will have blue and red edges because as you shorten the pulse, the broader it's spectral width must be for conservation of energy. There is no free lunch, conservation of energy is always around. So when it comes off the grating the Reddish edge edge will lead and blue will follow. Take a grating in grazing edge mode and spread out the pulse into its spectrum. You just created a longer pulse. Now run that into an amplifier rod. Take that off into another set of optics and invert the beam direction. Now fold the "blue" edge of the pulse, which is now leading after the flip, into another grating and recombine in time. If you choose the optics , pinhole masks, and number of lines on the grating pairs carefully, you did two things.. You slowed the pulse down enough that the amplifier did not self destruct from internal stresses, Kerr lensing, and shockwaves from dumping its energy too fast, and when you chose the right ratio of grating lines, you just time compressed the pulse when you put it back together, at a loss of some pulse energy. Now you can get down into the femtoseconds.

    Please understand that when you run a multiline argon off a grating, your not changing the pulse time domain. This is only in a special case of nonlinear optics with ultrafast pulses. After all, I'm leaving out a week's worth of math and details that I no longer come even close to understanding because I'm not working on this right now.
    Not that I ever really fully understood the physics. I needed to know enough to make the hardware work, not to design it.

    I could design an modulator based mode locked laser, but doing the Kerr Lensing and Grating Compression takes teams of pretty good Physicists and Engineers. Which is why you buy the damn things off the shelf. Learning how to tune the faster stuff is an art form. The pulse compression system had to have its room temperature stabilized to +/- 2 degrees or I had to tune it again, which was done with a laptop that moved the mirrors and gratings. They now brag on the revised version that you only have to hold the room temperature to +/- 10' C now.


    Steve
    Last edited by mixedgas; 12-12-2018 at 05:18.
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  7. #27
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    Fuck dude... You missed your calling, you know that? Can't believe you posted that off the top of your head! Should have been a teacher. Should have been a teacher!

    If only my brain didn't lock up ever time I hear the phrase "non-linear optical process". But OK - I'll push the "I believe" button and accept that crystals can perform spooky magic that appear to defy the laws of physics. Even so, "Kerr Lens Modelocking" is right at the limits of what I can wrap my head around. I was able to follow your grating example, however.

    And holy crap, I never knew they had come this far with regard to short chirped laser pulses...

    I'm going to spend a few days going down the Wiki rabbit hole and see what else I can learn.

    Adam

  8. #28
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    Oh wait till you see my favorite nonlinear toy, Stimulated Raman Scattering Pulse Compression. . You take a Q-switched Yag or a Ruby and shoot it through a thin film polarizing beam splitter, then a waveplate, and use a long focal length lens to focus it into a long tube of room temperature Carbon Disulfide liquid that has an AR coated window. Did I mention the lens has a second lens on a slide to very the focal distance a few centimeters, by slightly changing the collimation coming in before focusing?

    So when the say 50-100 mJ is focused into the tube, SRS some how sets up a fast virtual Fabry-Perot reflector in the liquid, moving backwards towards the beam splitter and chops the reflected pulse short, compressing it. It goes back through the waveplate to the angled beam splitter which picks off the shortened pulse and flings it off to an amplifier rod or two via some mirrors. You ever so slightly delay the amplifier lamp starting time for peak power. It looses very little energy in the process, the Joule meter on the output only changes about 10% as you change the time.

    As you slide the lens back and forth it changes the shortened pulse width about 50 to 1. AKA Dial-A-Pulse. Customers love it. If the cell is misadjusted it clicks loudly. If its working it is nearly silent. There is a rod sticking outside the laser with a handle and graduated scale bar on on it. You slide it to dial in what you need.

    At one time I studied for weeks in my spare time before traveling overseas to the factory to see the Ancient Physics Genus who designed this stuff. He said for a beginner, your explanation holds up, except where your getting your "mirror" moving close to being faster then light on the shorter pulses. The man did not believe in laser safety glasses however, and had more retinal damage then anyone else I had ever seen.

    Mind blowing, but thank God I don't have to try to explain that one daily any more. I'm not so sure I will ever understand it, but I had to come up with something that would work when explaining it to customers when installing the unit. I can see myself now at the Pearly Gates, "Dear God, I don't care anymore who killed Kennedy, I'm not even asking to finally understand women, but can you gently explain SRS to me?"

    Steve
    Last edited by mixedgas; 12-12-2018 at 13:30.
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  9. #29
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    Wow! That is some creepy non-linear magic! I googled carbon disulfide raman scattering and found a bunch of abstracts that flew right over my head, so once again I'll just push the "I Believe" button on the whole virtual Fabry-Perot reflector concept. Still, being able to adjust the pulse width so easily has got to be a huge selling point.

    As for your request when you're at the pearly gates, that reminds me of the 2010 ILDA cruise when I had a chance to sit down with Greg Makhov for about an hour or so before the Lase-Off started. I foolishly asked him to explain the physics behind frequency-summation lasers (because he had one of the new yellow laser pointers with him at the time). That's where I first learned to hate the phrase "non-linear optical process", and I've never gotten over it.

    Adam

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