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

  1. #11
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    UOTE=brianpe;357140]Wow, 3D Photonics has a lot of really good stuff...with specs too so you know what you're getting. I definitely see some useful things there that could save me a bundle. And thanks for the tips for the IR camera.

    Here is a close-up of the resonator. There are a total of eight mirrors and this photo also shows the beam path. There is more off to the right but it is just steering mirrors and beam shaping optics (all outside the cavity).

    Attachment 58064

    I'm going to abandon getting this head working and just take ideas and possibly parts from it. I'm thinking of an L-Fold design on an optical breadboard like this:

    Attachment 58065
    I should have much better control of the pump waist. This diagram shows an eventual change to SHG but I plan to start with just IR and a 80% OC for 1064 and then add the KTP later.[/QUOTE]
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    Ah, one of the old spectra physics heads, we used them at work. They just discontinued them.

    You can recalibrate your spectrometer if you have a source of distinct spectral lines like an argon laser or a mercury-argon lamp. You create a spreadsheet of wavelength peaks and ccd pixel location and do some math and it gives you the numbers you need to enter into the software to calibrate it.

  3. #13
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    Default How to focus a multimode fiber into Vanadate?

    Ok, so I did a couple of things as a follow up on my last post:

    1. I tried (and failed) to remove the Nd:YV04 crystal from the Spectra Physics head.
    2. I built my proposed diagram on an optical breadboard.

    The Nd:YV04 crystal looks like it is held in with some melted Indium foil. So I heated up the assembly to just past Indium's melting point and tried to get the holder to separate. It didn't...so I heated it more....and I damaged the anti-reflection coating on the crystal. It's now cloudy. I put it back in the head..and walked away slowly. That was unfortunate as it was the only part of this laser head I planned to retain long term.

    New crystal in hand (only 8mm and now .5% Nd, but I got a bargain on it) I decided to soldier on and lay out my design on an optical breadboard. I've attached an image with the relevant components labeled:

    Click image for larger version. 

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    And now, along with some rudimentary optical equations I pulled from a paper on Newport.com, I think I can see the issue I was originally having.

    The pump diode I'm using is a Coherent FAP. This is fiber coupled and the recommended fiber is 800um multimode with an NA of .16. I'm feeding this into a collimator, and then out of the collimator I'm passing it through a PCX lens to focus it on, or just into, the Vanadate. If I do the math, I find that the output of the collimator should be about 4.88mm in diameter. That matches what I measure and is within the clear aperture reported by Edmund Optics. But the divergence is a whopping 26mRad, and when I plug this back in I find that the smallest dot I can expect from the PCX lens is 2.8mm (also, unfortunately, verified by me). I'm never going to get a population inversion with that. The spot size gets smaller if I decrease the focal length of the lens, but the shortest focal length I can use with "off the shelf" breadboard components is about 40mm, and that still yields a spot size of 2mm.

    I think the fiber size / NA was also the thing that was causing my collimator on the Spectra Physics to not produce a tiny spot. The Spectra Physics head is more closely packed, and I was able to get a smaller spot there because I could use a shorter focal length, but still not dense enough I suspect.

    I considered a beam expander / telescope here, but the divergence gets worse when the beam gets smaller, so I am not sure that will help with anything.

    Anyone have ideas on how best to do this?

    Thanks..Still learning.

  4. #14
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    Default Advice on alignment techniques?

    I've made some progress on getting a good tight beam into the Vanadate. I've gotten rid of the collimator I was using and replaced it with a 1" 75mm EFL PCX lens. This collimates the light at an expanded beam size of about 24mm, which really reduces the divergence. I'm then focusing that down with a 50mm PCX and that gets me a beam about 400um in diameter. I hope that's tight enough.

    I am also sending the fiber through a 1/2 wave rotator. The Coherent FAP I'm using as a pump source isn't polarized, but I assume all the diodes are mounted similarly inside and this results in some dominant polarization at the fiber end. I was thinking I could rotate the rotator to maximize the output of the vanadate, but I tried this tonight and I can't measure any difference with my spectrometer. Is this a bad idea?

    I used MixedGas's idea of dual mirrors to set the angle / height of a HeNe to point into the cavity. I setup two pinholes at the optical axis height and aligned these mirrors until the HeNe was clearing both. I removed the output coupler and I set a pinhole up right in front of the HeNe and aligned the HR mirror so it was pointing back into the pinhole. My cavity is an L fold, so this was a little hit and miss as there are a lot of ghost images from the front and back sides of the fold mirror. I then put the output coupler back and aligned so its back reflection also cleared the pinhole.

    So that's my starting point. Now I'm trying to get it to lase. I'm pumping at just above threshold and only one end of the vanadate. My IR card shows there is some 808 leaking out the back of the vanadate and my spectrometer shows the crystal is fluorescing. Is there any way to tell if I'm pumping it enough to get a population inversion?

    Are there any good techniques for getting a cavity aligned? I haven't tried using an old CCD camera and aligning the spots yet as MixedGas recommended, but that's on the top of my list. I have a camera just for this and will pull the filter this weekend. Are there other techniques? I believe I also need to adjust the Z axis length in order to get the cavity to resonate at 1064nm, correct? I am using mirror mounts with three adjustment screws, and by turning all three at once they will translate along the Z axis. That's going to be a bit of a pain as I'll lose alignment as I adjust. Anyone have a feel for how sensitive this is? Mount screws are 80 TPI.

    Annotated photo below of my setup. I've removed the HeNe and pinholes. Thanks in advance for any ideas / feedback!

    Click image for larger version. 

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  5. #15
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    Looks like you are making good progress

    I wouldn't worry about the z length too much, as long as the cavity is stable the exact length isn't very important.

    I find the trickiest part when getting a cavity aligned for the first time is sorting out which of the spots is which from the alignment laser, but once you get that figured out it should be easy enough to get first light from the cavity.

    Last thing to check is that the fluorescence from the pump beam is aligned with the cavity mode. But other than that I would just slowly ramp up the pump current until you see first light from the cavity. If you can't find it the cavity is probably aligned to a back reflection from one of the mirrors so you need to try harder getting the rough alignment correct.

    Did you account for any thermal lensing when you set up the cavity? I would expect that a vandate crystal like that is going to have an effective focal length of ~10-100cm depending on pump power, which can throw off the cavity stability. You should see the HeNe beam divergence change when the crystal is pumped, which can also be used to help get the cavity mode and pump mode overlapped (if the alignment beam moves when you pump it, its not aligned to the pump--you should see a pure change in divergence)

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    Thanks! I tried a bunch of things this weekend. No luck yet, but a good way to spend a Saturday.

    First, I pulled the IR filter from a cheap web cam. The good news is it works great. The bad news is there's no iris on the lens and pointing it at the cavity just washes out with white. So I'll need to solve that.

    I also realigned the cavity as carefully as I could. I sort of ray traced on paper where / what I should expect to see for ghost spots. I expect to see three spots reflecting back in a horizontal line. What I actually see is just a straight line of light -- I can't discern any beginning or ending to each of the spots. I aligned to what looked like the middle of the line.

    I brought the pump power up to about 23A. This is probably about 20W of light. The crystal glows brightly but no lasing:

    Click image for larger version. 

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    My thermal camera shows that crystal is getting pretty hot, so I connected up the TEC underneath it. The TEC current stayed pretty low though - the crystal holder is staying a nice 23C but doesn't seem to be cooling the crystal much at all. I know vanadate has terrible thermals but I'm wondering if this setup is going to help keep the crystal cool at all. I designed this crystal holder before I had the crystal, so I made it so it's kind of like a vise that can grab a variety of crystal widths. It has thermal contact on three sides and I've wrapped the crystal in indium foil:

    Click image for larger version. 

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    I'm not going to change this until I get something lasing, but any feedback along the lines of "you're going to cook your crystal" would be welcome.

    Anyway....back to getting it to lase.

    Thermal Lensing
    I didn't take thermal lensing into account when I designed the cavity. I couldn't get a decent feel (or find a reasonable formula) to figure out how much lensing to expect so I just thought I'd cross that bridge later. Anyway, I took your estimates and did a stability scan in Rezonator with my cavity design. The cavity will destabilize if thermal lensing gets worse than about 15cm EFL.

    I also ran the pump with the alignment HeNe to see if the divergence of the HeNe gets worse but I'm not sure where to look for that. There's a clear reflection on the HR but its size didn't change. But it's also only 10cm from the crystal. I guess I could remove the HR, which would allow me to project the HeNe for a distance and measure the divergence that way. Is that the way to do it?

    Mirrors
    The final thing I realized today is I'm not 100% positive the fold mirror and OC are facing the right way. The fold mirror had no markings at all, so I just guessed. The OC has some pencil markings that I took to read "reflectance is 80% in the direction of the arrow":

    Click image for larger version. 

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    I don't know if that's right. I ordered a calibration light source for my spectrometer that should arrive tomorrow. My spectrometer only reads out down to 1050nm so I plan to shift its calibration to include 1064. Earlier in this thread macona gave a nice tip about recalibrating my spectrometer and I found the instructions. Doesn't seem too bad. Once I've shifted it down I have a cheap DPSS laser pointer that probably spews out a bunch of 1064 alongside the green. I should be able to use that to verify mirror orientation.
    Viewers
    Anyone have any experience with the Find-R-Scopes available at Edmund Optics? I'm looking for a way to see the 1064 that's better than my IR webcam and hoping that if it is stopped down I can better see into the cavity. MixedGas recommended an old security CCD camera. I've looked and they're plentiful and cheap, but output is analog and cobbling together something to connect to a laptop or modern HDMI monitor sounds like more hunting than I want to do. There are a few samples of Find-R-Scopes on eBay that look decent and quite a bit less money than Edmund Optics wants for them.

    Thanks for reading!

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    Default Success!

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    Lasing! Yesterday I re-calibrated my spectrometer and shifted its wavelength sensitivity down 50nm, which allows me to detect 1064. I setup a rig where I could test the reflectance of different mirrors and used a cheap 532nm laser pointer to reflect off them into the spectrometer. My fold mirror was in backwards. This mirror had no markings on it so I guessed....poorly.

    Re-aligning after that wasn't too bad, although I did it with the crystal installed which provided more fun ghost spots. The crystal has bonded to the indium foil, which has bonded to my mount, and I didn't want to remove the whole thing. Also, for some reason my HR mirror gave me six spots. I expected three (back reflections from the fold mirror with the center spot being the true reflection). There were three more vertically above the first three. There's nothing in the cavity that should cause ghost spots vertically, so I just randomly chose the lower set of spots. I guessed right.

    Output power efficiency isn't that great -- I'm getting about 5% electrical efficiency with a threshold current of 15 amps. But this is a great starting point and now I can experiment with beam waist size / position and better matching the pump volume to the mode volume. Besides, 2 watts of output isn't too bad for a first attempt.
    Last edited by brianpe; 09-05-2021 at 09:20.

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    Thats awesome! There is nothing quite like seeing the first signs of lasing out of a fresh optical system

    Have fun tweaking the cavity -- you should be able to get decent efficiency out of an end pumped system like that but as with pretty much any bulk optic laser it will end up being a tradeoff between beam quality and power. If you haven't already, figuring out a way to get the (highly attenuated) beam on a camera will go a long way in tweaking the cavity alignment for beam quality. The usual approach is something like a OD40 attenuator looking at the leakage through a HR mirror. Not sure if you have seen it already, but there is a great piece of software called bullseye that works with pretty much any firewire camera https://github.com/jordens/bullseye

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    Post Progress..and setbacks

    I wanted to see if I could get some SHG working so I added a KTP crystal into the cavity. I left the existing IR OC mirror in place. I was hoping there would be enough density in the cavity to generate some green in addition to the IR beam (the OC is transparent to visible frequencies). That way I only had the KTP angle to adjust.

    And green! It worked on the first try, although I was a bit of an idiot because I didn't realize it was working because my goggles filter 808, 1064 and 532.

    I replaced the OC with a HT 532 HR 1064 mirror, re-aligned and got a pure green beam. The power conversion is terrible though. I was getting about 2 watts of IR out. With green only I'm seeing about 2mw, and that's close to the noise floor of my power meter. It's the most unsafe eye-safe laser ever.

    Now that SHG has proven itself I wanted to focus on getting better efficiency. Also, thermal lensing is a much bigger problem than I anticipated. I can't drive the pump past about 22 amps before it stops lasing. This weekend I swapped out the OC mirror to go back to IR so I could try some new pump optics. I calculate a mode volume diameter of about 300um, but I'm pumping at about 800um. I have new optics that should bring the pump area down to about 400um which is a much better match. But after a full day of trying to get the cavity aligned I just can't get it to lase. That's when I noticed a small shimmer of light inside the vanadate when aligning. Close inspection of the crystal shows a stress fracture inside it. So now I'm full of questions:

    Is it still possible to use this crystal if I shift it so the mode volume doesn't intersect the fracture? Or is this a non-starter without a new crystal?

    Why did it fracture? Thoughts:

    Power Density Too High?

    One thought I had here was when I switched out to the HT532 HR1064 mirror. The green conversion efficiency was terrible. Could the energy density have gotten high enough to fracture the crystal?

    Or is the pump density just too high in general? I don't really know what the limit is but maybe the amount of thermal lensing I'm seeing gives a hint.

    Insufficient Cooling?

    The vanadate is held in a big block of aluminum that is thermally regulated by a TEC. Judging by the current the controller is passing to the TEC this isn't doing much to cool the crystal. A thermal camera shows the crystal at about 300F (~150C). Given how bad a thermal conductor vanadate is I am not sure how to improve this. Looking at eBay's offering of used vanadate crystals in mounts taken from production lasers some are only touching the vanadate on two sides. Perhaps they're using a large pump volume to disperse the heat? How much of an issue is this?

    Crystal fractures are about $400 a mistake, so I'd like to make this one count.

    Thanks for reading!

  10. #20
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    That is a bummer about your crystal -- those failures do definitely sting.

    As long as the fracture is small you can just shift the crystal around and get it back up and running. One thing to watch out for is that there can be internal stresses in the crystal which will screw up your polarization and cause very weird behavior of the cavity, in particular when operating in a polarized mode.

    As to why the crystal failed -- that is hard to say but your theories do make sense. In general it is risky business lasing a cavity with 2 HRs, in particular with the thermal lensing it is possible you got a waist in the crystal which did you in. In general it is my experience that damage due to excessive circulating optical power _usually_ manifests itself as damaged coatings, and bulk damage has more to do with excessive mechanical stress in the crystal from thermal gradients or the likes.

    I doubt that your cooling is the problem, for the relatively low powers, large mode volume, and your crystal temperature measurements the crystal seems like it is doing fine. I didn't use vandate, but for the yag crystals I ran in grad school we would run 400w into a 2mm mode diameter with single side cooling (and cryogenic cooling to mitigate the thermal lensing...).

    I would suggest shifting the crystal around a bit and see if that gets you back up an running. You might also look into the scrap crystals from the avia's that are ubiquitous on ebay--a lot of them are in pretty rough shape but just for screwing around at home they should do the trick.

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