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Thread: Planning a Nd:glass laser-building from the parts box.

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    Laser Warning Planning a Nd:glass laser-building from the parts box.

    I used to actually do fun stuff when I was in school, but that was 17 years ago. Now, I am cleaning up the closet of shoe boxes and mailers that got collected when Ebay first opened and I got my first job at the local sandwich shop. Among them, I found my old TE N2 low-pressure laser, which I used to pump 20 different household items to lasing threshold for my Physics II class in high school, mostly laundry soaps and highlighters, some RIT dyes, etc. It was fun and I enjoyed it.

    In a few boxes next to it, I have found the following items and want to construct a new laser, with the eventual end goal of a laser engraver for 0.010" deep engravings in some metals. Multiple passes are OK, and if the laser I end up building can't do that, well, that's OK too. The actual engraving machine part is no problem-I have built CNC machines before.

    4 flashlamps, all Russian manufacture from the late 1980s looks like. INP3-7/80A, I can find a little data on them but they have some kind of UV-blocking coating on them. They are unused because I blasted the 5th and 6th ones I had into shards trying to pump laser dyes, before I found out about the UV-blocking coating.

    Several good energy storage capacitors, 35uF at 5000V Maxwell (got 1) and 115uF at 2300V Aerovox (got 5). The Maxwell was part of a xenon strobe light collision avoidance system, and the aerovox caps were removed from out-of-date AED defibrillators. All are new, just have some dust on them.

    A 6.35mm x 80mm Nd:Glass rod, marked R&G Serial 100283, Nd:Glass Laser Rod 3x1/4" Coated R100/R50%@1055nm Pack 1.4, 1pcs. I recently missed buying another one that was as far as I can tell identical to it on ebay, right down to the package and markings but it was serial number 101948.

    A handful of crudely made, but probably acceptable mirror mounts (not that I need them if the above rod is really coated with mirrors already!) These were some of my first attempts at machining. Takes me back!

    A couple of other things have come out of the boxes-a Heathkit 1mW HeNe laser, a few supermarket laser scanner tubes, and a few laser mirrors that may or may not have the matching information on the case they're in.

    I want to build the Nd:glass rod into a small flashlamp pumped laser, and try to get some SHG going, and couple this into a bit of fibre optic cable. I want to also co-beam a low power laser (like one of the HeNe's or a visible diode laser) through the fibre for alignment/aiming.

    Here's what I know:

    The integral mirror laser rod won't be easily Q-switched, limiting it's abilities for my end goal.

    My current laser safety goggles are not suitable. (they're only OD4 rated at 1055nm)

    The total energy potentially stored in my Aerovox pulse caps is as much as 1500J, which should be more than overkill for both evaporating my fingers and my flashlamps, so caution here.

    I have a full CNC machine shop at my disposal and plenty of bar material. (I teach machine tool and engineering at the local technical high school)

    Here's what I DON'T know:

    Where my copy of Kohcher's Solid State Lasers has gotten to.

    How to calculate input energy to the flashlamp required to get the rod over the threshold, without blasting it to bits.

    How to choose the right mirrors to suit whatever rod/lamp combo I replace this one with when it's time to go for engraving-level power, if this rod can't get me there.

    I'd like to start with input pump energy. Can anyone help me figure how much lamp energy I need to have on hand to get the rod over the lasing threshold, with what appears to be a 50% reflective OC?

  2. #2
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    I see no mention of a series inductor with your lamps. That is the general reason for lamp explosions.

    OK< CNC, so making a resonator is easy for you. It also gives you a really good chance of Q-switching or water cooling.

    You do machining, so basic math is easy for you. The spreadsheet linked is a bit of severe overkill.

    Winding the inductor coil will be easy for you, copper air core coils are easy to make on a lathe and then dip in varnish and bake.


    Sams Flashlamp page

    So Link: https://www.repairfaq.org/sam/lasersps.htm#spsdls1

    and https://www.repairfaq.org/sam/laser/pfncalc1.xls

    ND:Glass lases over a 20 nm spectrum but likes really, really low rep rates unless water cooled (water cooling is an advanced project).

    The UV coating on the lamp results in higher laser power, embrace it if possible.

    The EG&G lamp handbook, not the new one sanitized by Excelitas marketing is attached.

    Calculating Ko of the lamps you have will take some practice, but you can assume they are filled with 450 Torr of Xenon. The rest can be measured with a caliper. I need to check, but I remember K0 = 1.27 L/D for Xenon at 450 torr where Length and Diameter are in millimeters.

    Don's Flashlamp Page:

    https://donklipstein.com/xeguide.html#eg

    Steve
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    Last edited by mixedgas; 01-08-2023 at 09:33.
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    Yeah, I am familiar with the need for a pulse forming network and series inductance to protect the lamp-I would imagine I got to the point that the old power supply would not put out any more voltage and so I tried to decrease the pulse risetime hoping to get a "faster" pumping pulse, to get a stubborn dye to lase...only for it to be a moot point as the lamp was coated to block out the UV light I needed to excite the dye with!

    Water cooling is a challenge to seal up but other than that it's no big deal to make a water cooled setup. I've been eyeballing water cooled medical laser setups all weekend.

    The rod is indeed Nd:Glass and has a HR coating on one end and something less than 100% coating on the other end-I visited a friend at the local university and he measured the coating reflectivity for me. I've left the rod with him, he is going to do some tests to determine doping percentage, but he says it's definitely glass, has integral mirrors, and shouldn't be hard to get it going.

    I have plenty of stock to make the housing/reflector from. Aluminum, copper, brass, steel, etc. I seem to remember gold and copper being the best reflective metals for the wavelengths involved, is it worth the effort to machine a copper elliptical housing over machining one out of aluminum? Copper is a pain to machine due to it's propensity to gunk up tools as it gets hot.

    I figure it's time to dig as much data as I can get on these flashlamps. Someone somewhere knows something and I can probably get some data to work with if I keep at it.

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    mixedgas's Avatar
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    50% T in ND:Glass is good for a rod driven by multiple flashlamps. Upper state lifetime for Glass is around 350 uSec, so your adjusting your PFN for a peak flash duration between .707 and 1.1 times that. Actual lamp on time can be longer, but there is no use pumping the rod past the point that storage is full.

    No need for a super reflective pump enclosure / reflector, Aluminum or PTFE or a ellipse painted with barium carbonate white paint will do. Diffuse reflectors might require a bit more energy to pump, but work just fine, if not better then polished ellipses for flashlamp pumping.

    Sounds like your rod is a master oscillator for a pulse compression system. There were not are not many other uses for Nd:Glass. Nd:Glass has a very wide emissoon spectrum , which is used to make sure all the energy is extracted from amplifiers made of other materials. I have worked on such systems.

    You can add a bit more external HR, to a point, a perfect use for a home made resonant reflector out of a few pieces
    of highly polished glass with air gap spacing. An OC off Ebay would work as well. Just make sure it has a radius of a meter or so.

    Hint: Unexposed b&w print film turns brown when hit with modest amounts of Nd:Yag lasing, yet doesn't brown much from pump lamp light. No developer required and will see a few milliJoules and up.

    Steve
    Last edited by mixedgas; 01-09-2023 at 16:47.
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    Looking at the flashlamp data, most of which I got from Don Klipstein's site, I may have a problem.

    INP3-7/80A, the flashlamps I have, have the following characteristics (best I can figure!)

    The flashlamp has an 80mm arc length, and a 7.2mm ID, and a xenon fill of 450 torr. Ko should be 14.2 ohms-amps^.5, according to Don, plus I did the math again myself to make sure.

    Don says for a daylight-like spectrum, no series inductor, that 385 joules would be the maximum pulse energy at around 480V. More than that and the spectrum starts to shift blue. Looking in the english-translated datasheet, 580V is called out as the recommendation for operation without a series inductor, maxing out at 610v for no series inductance. Don mentions all of this.

    Putting the data in to the PFNCalc1.xls program, I get the following information:

    Rod Dimensions: Length 80mm, Diameter 6mm, Dopant Concentration 1% (estimating low, I will know better when I get the fluoroscope data, estimating 2%)

    I used the rod inputs that were recommended for Nd:YAG, 1% population inversion, 30% spectral match to the lamp, 55% electrical efficiency, 50% geometric efficiency.

    I get back Rod Threshold Energy at 2.64J, and I specified a 230J input to the lamp, which should put the rod 719% above the PI threshold. That seems like an appropriate amount of overdoing it to make it work.

    Putting the lamp data in, at 230J input energy, I'm only at 20% of the lamp's explosion energy, so that seems good.

    Now on to the pulse forming network.

    The capacitors I have are 115uF/2300VDC max. If I charge to 2000V, a single capacitor gets me the 230J I used for input. The flashlamp data suggests that the lamp is designed for flashlamp-pumped lasers, but then the "specified values" gives data for a 580V charge voltage, 2400uF of capacitance, and a flash duration of 4000uS(!!)

    Clearly I need to play with the PFN section more, before I start dictating what input energy I am working with. So, a few ieterations through, and I wound up with this:

    Lamp Input 96 Joules, 345uF capacitance, (3 caps in parallel), charged to 745V, with a 40uH series inductance, including parasitics. This will give a ~350uS flash duration, and a 0.9 damping factor.

    Putting these values into the PFN Current and Spectral calculations, I get "DIpk Good" at 2494.7Amps/cm^2, so I guess that's OK?

    In the end, I still get told that I will be ~300% over the lasing threshold, so it can't be too terrible a starting place. It does seem to point to the fact that I have a stack of extra durable flashlamps for the size and doping of laser rod that I currently have.

    I also have a pair of CTH:YAG rods but they are damaged-one end is chipped badly, one rod loosing 50% of the last 5mm, and the other rod 80% of the last 5mm. They were 4mm diameter and 130mm long, orginally, and I bought them to facet into gemstones-they are one of the best green garnets out there! Might have to try my hand at cutting them to a clean length, and re-facing the broken end at either parallel to the existing clean end, or at Brewster's angle. That's Future Me's problem to solve.

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    Two lamps.... Better deal long run...

    Steve
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    Default A mumble about krypton flashlamps.

    Having worked on and with a Laserscope in the distant past, I remember the Krypton lamps for the heads were thin walled and they utilized the pressure of the DI water in the cavity to be an opposing force on the outside of the lamp to get higher energy put into the lamp before it would explode ( you cant compress a liquid ). Having blown up two lamps when the start wire vaporized and the first pulse was monstrous without the preionization wire there to do what its supposed to do, I had the job of cleaning up the mess and relamping. Be sure to get the right flashlamp for whatever flashlamp pulsed laser project you may be brewing.
    On a side note. I have an optical rail ,carriers, precision optical mounts and maybe the housing from a Holmium YAG head floating around in my kitchen. Drop me a note, its yours, just pay shipping. This is from a full sized Holmium YAG head so the rail is like 3 feet long, made of Aluminum.

    -Nate.
    Will there be three phase!!!!

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    Mixedgas-do you mean two lamps alternating, or two lamps simultaneously? I will need to find different capacitors if I use two lamps, I only have an odd number. That's not a *huge* deal, though.

    Vidal Wolf, I will send you a PM.

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