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Thread: Mounting KTP and Nd:YVO4 crystals -- some questions

  1. #11
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    Lets take some of the emphasis of this thread off thermal control and move it to where it needs to be, having a TEM00 (Gaussian) transverse mode and a limiting the gain bandwidth of a laser to a single longitudinal mode. Going to a modest thermal mass makes the laser easier to stabilize, and lessens the need to hit some un-practical number like 0.001'C. Russian saying " Best is the Enemy of Good Enough!" Also, "Do not look gift pig in the snout" I used to work with ex Soviet Block laser guys, who often achieved very good lasers with very simple technology, by spending a little longer on the design, so they could use modest, obtainable parts.
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    When you look at those pictures of a stacked TEC, your looking at a unidirectional ring laser cavity with course mode selection by intra-cavity thin etalons and final mode selection by a temperature tuned birefringent mode selection in the doubling crystal. It has an optical isolator in the cavity so the beam is unidirectional. That design is based on a ring cavity (math based design) first developed for dye lasers by Kolgonik (spelling?) and Lee. The cavity is designed to compensate for astigmatism in the beam from the folds and come out circular, while having a small diameter , but long, active region in the lasing crystal and in the KTP. Its the result of say thirty or forty years of intense laser research and development, Its designed by a bunch of Skilled PhDs in physics and optics. The tooling to make it probably costs upwards of a Million Dollars.
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    No matter how much you heat or cool the crystals, you can only reduce thermal lensing somewhat, so lets USE IT TO OUR ADVANTAGE when designing the laser. The Same goes for Polarization, its there, its needed, its inherent to the KTP's functioning as a doubler, so lets live with it and use it, especially if we can use the Brewster polarizing plate as a thin etalon to also select a single mode.
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    The above RING LASER is Not easily cloned. Great for a design where a replacement laser needs to drop in with zero adjustment to system alignment, but NOT a beginner's build.
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    Lets move off of that design to something with three mirrors or less and much more practical with a small ND:YVO4 and a short doubler, shall we?
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    Some tasks:
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    First task, Have a short cavity that has the correct intra-cavity polarization for Type II SHG and lases TEM00. Shorter cavities can have fewer longitudinal modes, making it easier to select just one mode. Lets use the KTP as the selective element if we can.
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    Second task, Have just enough gain bandwidth that we can place a loss element in the cavity to select a single mode.
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    Third task: Have a long, thin, beam waist located in the Vandate and another short , thin diameter beam waist located in the KTP. As conversion to green is driven by a very high electric field in the crystal, we need a short, tiny diameter waist , but not so high a field that we "gray-track" the KTP crystal with permanent damage.
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    Forth task, Match the 808 nm light from the pump diode to a beam waist that is roughly the same length and diameter as the 1064 nm beam waist in the Vandate. Vandate lases with self polarized 1064 nm light, it is also sensitive to the polarization of the pump light, so you not only need to focus the pump light in the crystal, but you may need a waveplate to rotate it to match the alignment of the Vandate crystal. You may find a fiber coupled pump laser makes this easier at first.
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    Fifth task, Minimize "Green Noise" effects generated by non-linear effects in the KTP. The KTP is birefringent, meaning it has a different index of refraction for each of the two possible incoming 1064 nm polarization orientations. Hence the supplier asking about desired crystal cut with respect to the optical axis of the raw crystal and Phi/Theta angles.. (Not going to cover circular polarization in this thread if I don't have to, I may have to...) Really don't like to TRY TO explain "green noise" in a beginning thread, but suffice to say the KTP crystal can make the laser very unstable in amplitude and mode, two things you do not want in a holography laser.
    There are ways around it, but ask the laser show guys around here how they like cheap DPSS greens that mode hop for no reason and change divergence on the fly.
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    Sixth task, stabilize the whole thing optically, and thermally. Temperature tune the pump diode so that its lasing bandwidth is centered on the vandate's maximum adsorption range. Heat / cool (depends on design and ultimate power) the KTP so that it is temperature tuned, but do not adjust the KTP angle/temperature so that the tuning is so critically sensitive to temperature that it will only lase in a very narrow range after warming up. Note this is way down on the list. It is not that difficult.
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    Your looking for a copy, preferably second edition of "Laser Fundamentals" by Silfvast. It is a very readable book with the mathematics simplified mostly to high school algebra. It shows up cheap on Abebooks as a used book, because it is often used as a classroom text. If you get the right printing, PL's own Laserman532 is on the cover, or a picture of a whitelight Helium Selenium laser hitting a grating is on the cover. Dr. Silvast discovered a class of metal vapor lasers, including the HE-CAD, so he's writing from a experience perspective. It has really good explanations of the basics. If you want to spend an informative few months reading, "Lasers" by the late Tony Siegman really covers cavity design.
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    There is a reason I suggested that paper, its very similar to the easily stabilized JDSU/Lumentum Microgreen aka "uGreen" commercial laser, a laser I know that you can make a version of with decent tools, and without breaking the bank. It also has the advantage of needing a modest TEC or two without super thermal control.
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    If you want the legendary performance and power of a Compass or Verdi, buy a Compass or Verdi. If you want to learn, build a two or three mirror, two crystal laser first.
    Because the setup needed to make sure it is SLM is something you need to plan for as well. With what you have already, design a 200 mW TEM00 laser and be very happy if it does 75-100 mW of SLM. Be happy if it does 50, because most skilled hobbyists barely break 50 with their first DPSS greenie.
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    Now tell me about yourself, and do you have access to a lathe or milling machine?, so I can customize this thread for you. BTW, I'm the one who did some of the library research for Planters, so I know roughly where he started off with the design of Big Green...
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    The exclamation points are because my browser does not play well with the forum software, sorry about that.
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    Steve
    Last edited by mixedgas; 05-10-2017 at 14:10.
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    Mobile right now so I won't go into detail, but yes I have a lathe and manual milling machine.

  3. #13
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    Design and characterization of a diode-pumped, single longitudinal and transverse mode, intracavity-doubled cw Nd:YAG laser
    A. Agnesi, S. Dell’Acqua, G. C. Reali, P. G. Gobbi, and D. Ragazzi
    Applied Optics > Volume 36 > Issue 3 > Page 597
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    Does what you want, however does not discuss maintaining stability with the birefringent Lyot Filter formed by the Brewster plate and the KTP.. Would be fairly easy to lock the laser using a piezo on the rear mirror.
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    Probably need to ditch the vandate, and go Nd:YAG, unless you want to build the unidirectional ring and pay for an expensive optical isolator for one directional operation.
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    I'll take a picture tonight of the insides of a similar monolithic cavity that is very stable , but mine is q-switched, not SLM.
    !

    Steve
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    Quote Originally Posted by mixedgas View Post
    Design and characterization of a diode-pumped, single longitudinal and transverse mode, intracavity-doubled cw Nd:YAG laser
    A. Agnesi, S. Dell’Acqua, G. C. Reali, P. G. Gobbi, and D. Ragazzi
    Applied Optics > Volume 36 > Issue 3 > Page 597
    !
    Does what you want, however does not discuss maintaining stability with the birefringent Lyot Filter formed by the Brewster plate and the KTP.. Would be fairly easy to lock the laser using a piezo on the rear mirror.
    !
    Probably need to ditch the vandate, and go Nd:YAG, unless you want to build the unidirectional ring and pay for an expensive optical isolator for one directional operation.
    !
    I'll take a picture tonight of the insides of a similar monolithic cavity that is very stable , but mine is q-switched, not SLM.
    !

    Steve
    That is a compelling design, here's the paper for those curious:

    http://sci-hub.bz/10.1364/ao.36.000597

    So here's where I'm at with the CAD so far:



    The spacing is intentionally wide to provide a better view of the components. It's not visible in the rendering, but the groove in the vanadate mount is only on the bottom piece, the top is a featureless rectangular prism.

    Currently I'm using a c-mount diode that produces a long elliptical beam, so I will require some beam correction optics between the pump diode and vanadate.

    A fibre-coupled 808nm diode seems very tempting.... Focusing and aligning the beam is much more difficult before it goes through the KTP, with the 808nm pump beam being only 1/250 as bright as 532nm per photon. Wearing safety glasses of the appropriate power/wavelength tolerance totally obliterate the tiny amount of pump light that is actually visible.

    I'm a little bit less worried about the output optics as I plan on using this laser for holography, where we actually want a beam with a fair bit of divergence.

    I have a brewster assembly that I got from a disassembled JDSU 4601 10mW uGreen laser -- using this would be very convenient, except that it has the vanadate crystal glued directly behind it.

    Anyway, please do let me know what you think of the design

    P.S. Autodesk Fusion 360 is an absolute dream to use after trying SketchUp. It's infinitely more intuitive and free for 3 years if you're a student, or have the audacity click "I'm a student" checkbox without actually being one

    I generated this blueprint from my 3D mockup within minutes:
    Click image for larger version. 

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    Last edited by lasersareuseful; 05-18-2017 at 08:39.

  5. #15
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    You need the ability to adjust the tilt angle of the Brewster window during adjustment, otherwise you will not be able to select a single mode during alignment.

    Steve
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    Does it have to be adjustable in all three axes?

    Seems to me that the KTP's pitch and yaw will need to be adjustable as well. I can machine horizontal and vertical slots instead of holes for mounting, this would at least allow forward/back and left/right alignment... pitch alignment will be tricky though.

    I'll use shims to do the initial alignment, then I'll figure out the rest later. I may be able to machine the part afterward such that it doesn't require shims.

    This seemed a lot easier on paper :P

  7. #17
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    Some pics of a laser with one KTP adjustment, a couple of mounts with "loose mounting screw in oversize hole" adjustment. Diode lens is adjustable in two axis. The fold mirror is the final adjustment using 100 pitch screws in x_y.. Relies on really good cnc and a jig is used to align the one mirror before gluing. Were it not for the Q-switch being there, this laser would be quickly adapted for single mode. The first thing after the rod is two back to back Brewster plates. Pics attached. More pictures coming, hitting bandwidth limit on my phone
    Attached Thumbnails Attached Thumbnails 20170520_103402.jpg  

    Last edited by mixedgas; 05-20-2017 at 06:58.
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    Quote Originally Posted by mixedgas View Post
    Some pics of a laser with one KTP adjustment, a couple of mounts with "loose mounting screw in oversize hole" adjustment. Diode lens is adjustable in two axis. The fold mirror is the final adjustment using 100 pitch screws in x_y.. Relies on really good cnc and a jig is used to align the one mirror before gluing. Were it not for the Q-switch being there, this laser would be quickly adapted for single mode. The first thing after the rod is two back to back Brewster plates. Pics attached. More pictures coming, hitting bandwidth limit on my phone
    That is a really clean cavity! Looks like you're using a fibre-coupled laser? I read on Sam's Laser FAQ that you should have at least a foot of fibre before the beam exits to encourage a nice beam profile... is this true? If not I will seriously consider getting a fibre-coupled pump diode.

    If it wouldn't be too much trouble for you, I would love to see some pictures with the components labeled.

  9. #19
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    Pictures, and the diode is lens coupled, first with a cylindrical lens to correct it's fast axis.
    !
    When sealed up, the high reflector assembly with its two brass adjust screw holders has its own sealed cover plate (not shown) to make the whole thing isothermal.
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    If this had tip/tilt etalon after the Brewster windows, and no Q-Switch, it would probably lase single mode. It currently does TEM00, with 800 mw of Q-Switched green at 5 amps pump, and a really sweet output beam quality.
    !
    An Adjustable cylindrical lens in removable diode box sets initial beam path thru the lasing crystal. Whomever designed this wanted to be able to exchange pump diodes, hence the diode is in a separate box, detachable box, with precision alignment guides using metal pins and a alignment shim. The focusing lens for the lasing crystal is mounted in the laser head wall, just past the cylindrical lens. The diode is mounted in a mount on a thermally isolated pad with a TEC, not shown, on the outside of the unit, with the TEC heat sinked for cooling with a fan. (
    !
    This laser was designed for some incredibly special application and is grossly overbuilt, exceptionally well engineered, and designed to be interchangeable with other lasers with no beam alignment required. There are pads, constrained slots, and a conical hole on the underside to allow it to be precisely aimed with kinematic screws when it is installed.
    I wish I knew what is was designed for.
    !
    Steve
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    Last edited by mixedgas; 05-23-2017 at 09:38.
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  10. #20
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    More Pics for Lasersareuseful. Unknown maker and unknown model, very nice, ultra high quality custom q-switched green laser head I bought. It did not come with a Diode Driver, Cooler electronics, or PSU. I do heat the doubling crystal when I use it, the correct resistance value for the thermistor is labeled on the lid of the head, with the correct diode current.
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    Last PIC in this post is annotated or labeled, so zoom in.
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    I note they went out of their way to order the cut the KTP/LBO (whatever) crystal so it does not need to be rotated about its optical (roll) axis for peak power
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    Most of the optical mounts have oversize holes, so there is a slight amount of adjustment available. I had to adjust the Q-Switch angle for best hold-off with my bench driver.
    I do note that most of them have no pitch adjustment or shims, but there is some yaw. A few mounts have long slots for adjustment along the laser path, such as diode focus, which is done with the cylindrical lens.
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    Steve
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    Last edited by mixedgas; 05-23-2017 at 09:54.
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