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Thread: Making 488 from 976 using nonlinear crystals

  1. #31
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    .. damn .. my head just exploded ..


    --updated--/ edit -
    Quote Originally Posted by krazer View Post
    3.
    / .. crystal with a few um wide waveguide in the crystal, which works out to just about 100% theoritical conversion efficiency--no cavity needed. Also, the optics are dead simple for this design, you simply take your fiber coupled pump, and use a microlens (or lensed fiber) to couple the output of the laser into the waveguide, and then take the single mode output of the waveguide and collimate it like any other fiber coupled device.
    So, it looks like option #3 is the way to go .. How about not building the whole thing from the ground up/ scratch. If a person was so inclined, could he not find bits and pieces to build one of these without having to fab the whole thing? Also, we are talking about a 300mw pump diode? What would the output be? Not much power for show use I would imagine ..
    Last edited by steve-o; 10-21-2014 at 04:28.

  2. #32
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    Thanks for the detailed explanation - saved me a lot of time trying to understand all this. Alphalas says their PP-KTP is bulk crystal rather than waveguide type. They cautioned me that coupling the pump laser into the waveguide is non-trivial. I suppose it's the same as coupling a beam into a single-mode fiber which requires a precision 5-axis mechanism. Does anyone know a source for waveguide PP-KTP?

    I got the laser head from Steve and a matching controller from eBay and they are on the bench making 6-7mW at the diode max current of 650mA. The head has <900h so I'm not sure why it's not producing it's rated 20mW. Might have to try powering the head with my own drivers and see if tuning the temperature makes it any happier.

  3. #33
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    So are you actually seeing some 488nm being produced from your setup right now Kevin?

  4. #34
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    Yes coupling into the waveguide takes some doing, it is pretty similar to coupling into a single mode fiber. If the waveguide is designed well the mode should be more or less round, and with a simple lensed fiber or aspherical collimator/focusing in a precision 5 axis mount will do the trick. The mode matching between the fiber and the mode of the waveguide takes some careful thought (picking the correct lenses, lest you have to try coupling into the waveguide with 10 different lenses to find the one that works). If you look at the pictures of the MDD from melles griot it looks like they are using a lensed fiber which was aligned using a precision alignment jig then glued in place. They also used a crystal cut at a slight angle, most likely to reduce back reflections which could destabilize the pump laser, and cause etalon effects inside the waveguide. They also added a prism pair on the output to correct for the elliptical/astigmatic beam, this could just be from the angle cut crystal, but more likely it is because the waveguide is not so great at 488nm (since it has to also guide 976nm) and has an elliptical mode as a result, which luckily can be completely removed with a simple prism pair. The good news is that since it is a single mode waveguide you can get a perfect 1.0 m^2 out of the crystal with simple corrective optics (prism pair, etc).

    As far as getting ppktp waveguides, http://www.advr-inc.com/waveguide.html and http://www.hcphotonics.com/products2...at=1751&sn=938 have them, and I think there are a few other companies in europe/china that will do it although I have never purchased components from them. Prepare yourself for some sticker shock, a custom waveguide can easily run into the $10k range, more if you want one that comes packaged and fiber coupled.

    As far as the power output is concerned, I am not sure what is considered state of that art for these devices in CW operation but 10% conversion seems to be pretty par for the course for these devices, and that is not counting coupling losses (another 5-50% at each end depending on how good your are at it). In theory there is no reason you shouldn't be able to get 100% conversion to the second harmonic (ie, 100mw IR -> 100mw blue) but there are a lot of practical fabrication issues that you have to fight against and I think that if you were able to demonstrate anything more than about 25% your paper would get published in a decent optical journal.

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