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Thread: Building DPSS from scratch. Trouble getting YV04 to produce any spontaneous emission

  1. #21
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    Default eBay / Avia to the rescue

    I found a crystal on eBay that looked pretty good. I was dreading removing it from its mount but the crystal I bought is held in with what looks like thermal silicone and the mount was in two parts. It was really easy to separate the crystal holder from the TEC that it was glued to, and then the crystal came out very easily.

    A week. That's how long it took me to get it aligned. This crystal is only .27% Nd and I can bring the pump current way up with no thermal lensing issues (the one I cracked is .5% Nd). I am working with CASIX for some new crystals - they can do both Nd:GdO4 and Nd:YVO4 with diffusion bonded undoped endcaps. I'm interested in trying those if they're not crazy expensive. But this will work great for now.

    Alignment. I have to get better at this but now that it's aligned I'm looking at the spots from my alignment laser and I have no idea WHY it's aligned. I'm hoping someone can share some thoughts. My current cavity setup is an L-fold configuration, with the OC in line with the crystal optical axis and the HR on the arm of the "L". Here are some observations / rules I have when aligning:


    1. I should see three spots from the HR mirror due to back reflections from the pump fold mirror. The center spot should be the right one.
    2. The crystal has to be in place. My mount does not allow me to precisely control pitch / yaw on the crystal so it's never perfectly parallel with the cavity. This is always going to cause some small amount of displacement of the beam due to refraction.
    3. The OC should not have to be in place. It has to be plane parallel with the optical axis when aligned, so therefore aligning the HR separately should be fine. I say "should" here because I have seen the spots on the HR mirror shift when I've installed the OC. So I've tried to align both with and without the OC.


    The crystal does some interesting things to the HeNe beam. Without it in place, the spots all make sense and it's easy to line them up. With it in place I see two sets of spots. On this crystal they are separated horizontally. On my prior crystal they were separated vertically. I assume the orientations of the crystals are different and what I'm using as "top" is a different side.

    Here's my dilemma: my cavity is aligned and lasing, but when I look at the HeNe spots NEITHER set of spots is lined up. Here's a photo. The spots are circled in yellow and I added a diagram depicting what I see in case the photo isn't clear enough:

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    I got it aligned by sheer luck. I also bought a C-mount camera with a lens I can stop down and I removed the IR filter, but all I can see with it is just "light". Even when lasing I can't see any spots on any mirrors.

    Any advice here from folks who've done a lot of alignments would be welcome. This is something I need to be better at as I start exploring more complex cavity designs.

  2. #22
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    Default Further Fiddling. Spatial Walk Off in Vanadate?

    I've spent some more time fiddling with the cavity, trying to simplify to a linear cavity to make sure I understand how to align the resonator to give myself a better chance of aligning a Z-Fold configuration. I'm still very confused about what I'm seeing when I pass my HeNe through the vanadate. Here's a beam shot with just two mirrors in a linear configuration (Ignore the mirror at the top right - it's just picking up some stray light).

    In the below photo I started with the crystal removed and aligned the cavity in a linear configuration. The HeNe enters from the right. When I insert the crystal there is a pretty severe deflection of the beam and it looks like it has actually spit the beam in two.

    I haven't got the cavity to lase in a linear configuration but I have when I've configured it as an L-fold. To align, I align the cavity with the crystal in place, start the pump and then offset the X angle of one mirror by several turns to get lasing. When I remove the crystal I find that the cavity isn't aligned that way either, so I'm not sure what it's possibly aligned to. Is this normal or is it something odd with this crystal?

    Click image for larger version. 

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  3. #23
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    Lightbulb

    There is a significant difference in index of refraction from red to IR, plus your mirrors will almost always have some wedge, either intentional or otherwise. Toss in birefringence in the laying crystal, plus a non-polarized here, and you will have errors in overlap at
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  4. #24
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    Default

    Quote Originally Posted by mixedgas View Post
    There is a significant difference in index of refraction from red to IR, plus your mirrors will almost always have some wedge, either intentional or otherwise. Toss in birefringence in the laying crystal, plus a non-polarized here, and you will have errors in overlap at
    Oh my, thanks. And I forgot about the wavelength difference causing a change in refractive index. 632 to 1064 is going to make a difference. I started to look up how to calculate the angle change...then I remembered I'm doing this as a hobby and not writing a dissertation so I just measured.

    I have to tweak the mirror alignment about 1.5 turns of an alignment screw to get lasing. 80 TPI screws and about a 2" mirror mount is only about .01 degree of difference, so it is not a huge angle change but ends up about .5" off center. And since it's a refractive index change to a consistently longer wavelength the direction will always be the same.

    And boom, just like that, the linear cavity is lasing now too.

    Next up: double the mirrors to a Z-fold.

  5. #25
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    Laser Warning Mike Drop

    Got the Z-fold lasing today. Took me about seven hours to get it aligned and I used a much brighter 532nm alignment laser. Still only one pump running but it's time for a beer. Thanks a million to everyone who provided help here. Next up SHG.

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  6. #26
    mixedgas's Avatar
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    Default

    Your crystal is splitting the he-ne beam into it's component polarization states.

    Have you looked at your pump diode polarization, for
    some crystal cuts it really does matter.

    Steve
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  7. #27
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    Default

    Quote Originally Posted by mixedgas View Post
    Your crystal is splitting the he-ne beam into it's component polarization states.

    Have you looked at your pump diode polarization, for
    some crystal cuts it really does matter.

    Steve
    A little. I'm using Coherent FAPs which output to a multimode fiber and don't have a specific polarization. I originally assumed all the diodes would be mounted consistently inside the FAP so there would be an overall dominant polarization to the output. My coupling optics had a 1/2 wave rotator I could rotate to adjust the polarization into the crystal. I rotated the wave plate and measured the output beam, but it didn't change so I removed the wave plate from the setup. This was with the prior crystal I fractured, however, so I should try again.

    This last time around I aligned with a 532nm DPSS laser. Wouldn't that have a linear polarization output? Maybe this is splitting up the X and Y contribution?

  8. #28
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    Default

    This weekend I added back in the 1/2 wave plate and rotated while measuring output. Yes, I can make a difference here so there is some dominant polarization out of the FAPs. It's not huge but I can measure a 5% difference in output power from the best to worst rotation angle, and the best case is better than when no wave plate is installed. I don't know if that will be consistent with other FAPs or fibers, but it's a good find and I have just enough space in my current collimator setup to fit in the wave plate.

  9. #29
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    Default Efficient SHG?

    Next up for me is to try to get the output converted to green. This is the annotated cavity design I'm intending to use:

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    in this design green output comes out of mirror 2 instead of an end mirror. This way I don't leak green light into the crystal. I'm using a 100mm PCX lens to focus the intracavity beam into the KTP. Normally the rightmost mirror 3 is HR to both1064 and 532 but while tweaking things I've substituted an output coupler with 80% reflectance. This provides an "escape hatch" to keep intracavity power in check. My goal is to get the SHG conversion efficiency high enough that it matches the output of the OC, and when I'm in the ballpark I can replace the OC with a HR.

    I have this cavity lasing and producing green light. It doesn't produce very much green light: while the infrared end produces about 2w the green end is producing under 2mw. I think it is because I am not getting a tight enough focus into the KTP. I calculate a beam waist into the KTP of about 150um. I can optimize this a bit more but the best I've been able to simulate is about 120um. I can go down to about 100um, but the trade-off is the cavity becomes unstable for stronger thermal lensing.

    Papers I've seen online seem to have the KTP beam waist much smaller -- in the 30 to 50um range. I haven't found a way to get anywhere near that.

    I have optimized the KTP rotation angle and Z position for best conversion. I haven't spent much time on any tilt adjustments -- they didn't seem to make a huge difference. Is the problem here that the waist is just too large? Or could it be something else? The KTP crystal I'm using was purchased new from Eksma Optics and intended for type 2 phase matching. It is 3x3x10mm.

    Thanks for any ideas.

  10. #30
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    Default

    I assume you are heating the ktp properly but thought I’d mention it. That would almost be like forgetting to plug it in!
    I assume you angle and phased matched it as well. If the energy is there and the crystal is receiving it correctly, depth,temp, angle, phase it should work. Will it laze extra cavity on the output on a single pass? Is there a protective coating on the ends blocking the ir? Is the moon in Aquarius and gibbous?

    The temp range is really picky. Like 0.1C picky. If that tec under the mount is the heater I doubt you can control that much mass that well. Did you indium foil wrap the crystal?

    ok did a quick look. Seems you can tune angle or temp so maybe it doesn’t matter. Temp is used to fine tune angle.
    Last edited by kecked; 03-06-2022 at 04:12.

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