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

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

    I am building a DPSS laser from scratch, or trying to. There are several milestones to this project:


    1. Output 808nm from fiber array; proper current drive of diodes and proper temperature control
    2. Drive Spectra Physics ZLM head I bought off eBay
    3. Build and align simple linear end pumped cavity on optical breadboard
    4. Frequency double cavity to 532
    5. Expand cavity to Z-fold with dual diode end pumps


    That's the plan, anyway. I have #1 finished, and I've used a spectrometer to dial in the pump diode temperature so it is outputting 808nm, or very close. The pump diode is a Coherent 40W FAP.

    I'm channeling this into one end of the ZLM and I just cannot get the YV04 in that thing to produce any 1064 light. The FAP uses SMA905 connectors while the ZLM uses FC fiber connectors. I've replaced the original ZLM fiber collimator / focuser with one purchased from Edmund Optics.

    Things I've considered:


    • The collimator I bought from Edmund does not have as tight a beam waist as the original. I've adjusted it a bit though and I feel it should be plenty tight enough to generate spontaneous 1064. Waist is about 1mm.
    • The FAP is going to be random polarization. The original FC connector is keyed so I assume the FCBar diodes for the ZLM were polarized? At any rate I am assuming that there is probably a dominant polarization of the FAPs and I've tried rotating the fiber and applying more power.
    • Maybe my spectrometer is off? I have a DPSS green and the spectrometer does read about 532, so I think it's OK.


    The ZLM YV04 crystal looks pretty dark purple to me, maybe higher doping? I don't know what the %Nd is for it but comparing it to other crystals on the web I'd say at least 3%. The ZLM cavity is a Z-fold and I can put an IR detector at the opposite end of the crystal and detect 808nm light, so the crystal is not absorbing it all. What is normal here?

    I've tried pumping the crystal with up to 30 watts (all from one end though).

    I've attached a small image of the crystal while driven by the pump. Does this look normal?

    I appreciate any ideas.
    Attached Thumbnails Attached Thumbnails IMG_0211.jpeg  


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    If I had to guess I would say the waist in the crystal is probably too big or misaligned to the cavity. There is usually a pretty tightly curved optic to compensate for the thermal lensing so the alignment of the pump the cavity waist is critical. Have you set up an alignment laser for the cavity yet?

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    I hadn't even thought of pump alignment here; I've only been looking at wavelength, waist and pump power. Good tip. I haven't setup an alignment laser on this head -- I was hoping for some proof of life here and then planning to move on to using my own optics on a breadboard and spending time aligning that. But willing to try here to see how it looks. Even if the resonator is not aligned with the pump I expected to see some 1064 florescence from the crystal. Is the radiation too weak for that? Also possible I'm not seeing it because my detector cards want a beam and my IR camera (also known as my cell phone) won't reach to that long a wavelength. My spectrometer is pretty sensitive though; maybe it will pick something up.

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    A fellow who really knows lasers once said "To find a laser line look for a dark hole in the emission spectrum and start there." He was right.

    There are exceptions to that rule for some media, but Nd is not one of them.

    F MANIFOLD Nd lines have other places to put their energy if not lasing. Any flouro will be faint and is often hidden under 808 scatter.

    Do you have any faint bluish purple visible emission from the crystal?

    1064 shows up on modern black and white CMOS imagers just fine. Weak, but it is there. Apple started a trend of putting IR blocking filters on cell phone cams. This has taken root.

    One of things you can do is run a visible Hene or single mode laser diode through the YVO4 with the pump on and see how bad your lensing is.

    Moral of the story, Two curved HRs with modest radius and the correct mirror mount should be in every laser engineer's tool kit for a sanity check. A third HR with a short radius, say 6-10 cm and 1-2% transmission. Is often a good idea with DPSS.

    You may be just fine, as Kaiser says, "Align It". If it does not lase, slide the mirror along the "Z" axis and try again. Many of the DPSS lasers showing up on Ebay are prototypes. Others should be on a TV show named "When Field Service Goes Wrong" I've had more then one used laser in my career fitted with the wrong optic. Optic mounted backwards by chance?

    I once spent months working on a Ti:Saph. It would lase CW all day, but never mode lock. It was an early model and none of the part numbers penciled on the cavity optics matched the manual or the manufacturer's current parts numbering. The postdoc who worked on it mixed in spare wavelength selective optics from a newer model upstairs without documenting his changes. Professor upstairs rightfully would not let me remove the mirrors from his working laser for "musical mirrors". The postdoc left the country in a big rush one day after I started at the facility.. Gee, I wonder why... in the end we ordered a new laser, using the current laser as a trade in.


    It is rare, but there are pump polarization issues in YVO4 based lasers dependent on crystal cut. You may need to rotate the rod or pump 90 degrees. That is a last resort, try aligning first.



    Steve
    Last edited by mixedgas; 05-03-2021 at 05:55.

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    Thanks for the help guys.

    I've been working on this and here's what I've found:

    Yes, the crystal is emitting light. My spectrometer ends at 1050nm (mail out to Ocean Optics to see if I can get it recalibrated) but I do see a nice 914nm line that's coming from the crystal, so the pump diode is doing something. And yes, I can see a small bit of purple florescence from the crystal end. I suspect this is a pump alignment and / or beam waist problem.

    I tried checking the alignment with a HeNe I have (that amazingly still works: I got it used in 1987) but no success. I'm having two issues:

    1) I don't have a stable way to insert the HeNe beam. That is, I have no guarantee I'm inserting the beam perfectly into the optical axis, and even if I was, I have no way to replace it with the pump diode while keeping the same alignment.
    2) This cavity has a lot of fold mirrors. There are four bounces before it hits the HR and the beam is really dim by then. But I can definitely see how if the pump isn't inserted exactly as it was during factory alignment why it wouldn't work.

    It was very easy to get the HeNe trained perfectly at the center of the crystal and have the reflection off the first fold mirror miss its target mirror by a mile. The collimator I have is slightly smaller in diameter than the original and there is some play where it is inserted. It's probably doing the same thing.

    There are a couple of spatial filters I could remove that might make the cavity more tolerant, but I think it might be time to move onto something like an optical breadboard so I have more flexibility. I wanted to learn how to do this anyway.

    So here is my plan. Optical breadboard and simple linear cavity to start with:

    Fiber -> Collimator -> PCX focusing lens -> HR (Concave, HT @ 808) -> Nd:YVO4 -> OC (~80% reflectance)

    There is a free software package called Rezonator that can help with dimensions for a stable cavity.

    As for aligning this, I have two ideas for that HeNe:

    1. Thor Labs has a fiber port coupler that will mount into the end of my HeNe and allow me to send it through a fiber. I can keep the collimator but replace the PCX lens with an alignment target.
    2. Buy a mount for the HeNe to ensure I can send the beam in at the right optical axis and send the beam directly in. I can guarantee the beam quality more, but I can't guarantee the fiber insertion will be identical.

    What's the better way to align? #1 is a pretty expensive bit of kit but if it makes the alignment easier it would be worth it.

    Finally, I've shopped at Thor Labs, Newport and Edmund for optics and optomechanics and I can't always find what I want. For example, no concave output couplers, and none of the mirrors I've found have their backs AR coated for 808 so they're not ideal for end pumping. Edmunds Yag mirror back surface is listed as "commercial polish". Not sure what kinds of aberrations I'll get out of the pump from that. Are there better places?

    Thanks and sorry for all the newbie questions.

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    Default The Only Stupid Question is the One You Don’t Ask

    no apology required, you're doing fine.

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    Generally DPSS has enough gain that you can align it using the transmitted pump light / pre-lasing light and a B/W ccd camera. In some cases at least. Overlap the reflected spots, is what you do..

    Usually I use an old CCD camera from the security business with the short lenses used for watching cash boxes in grocery stores, a lens extender ring (5$) and a USB frame grabber on my laptop.

    That can be put together for less then 75$ with careful shopping on Fleabay / Amazon or a call to your local security camera installer who usually has no market for old 640x480 "C" mount CCD cameras... Just make sure the lens has a good iris that fully closes or gets near to fully closed. IR photons are "Bright" on camera.

    The first time you align a laser can take hours or days... I know I had quite a learning curve for two and three mirror systems when I was young.

    Can you shoot a picture of the whole resonator and post it?

    I'd skip the fiber coupling on the HENE unless I was really getting desperate. I would suggest having two mirror mounts with 80 pitch screws, a piece of 80/20 (or similar) T slotted rail to set a round hene head in. The radius of the HENE head sets in the long groove on a typical round hene. Then some optical posts to mount the mirror mounts on. Generally two mirror mounts let you get the HENE beam to almost any height and azimuth you need. One Inch T-slot makes a good mount for round HENEs, they set in in the slot and a couple of wire ties hold it in. I have many of the commercial HENE mounts, and they are not on my alignment system. I made a three point stand using the T-slot and some 1/4" x 1" aluminum bar-- its a long "tee" shape. At the long end and at the ends of the cross bar are 1/4-80 screw sets from Thorlabs. Thus I can set the beam height , level the beam, and add in some tilt if needed.

    We put orange stickers on the end of the HENE and use a sewing needle or similar to punch a hole in the sticker for the beam exit. Then you can see the dim red ghost beams come back off the optics to be aligned on the sticker.

    Start with three Thorlabs N80L6P and appropriate length screws (longer is better) if you want to build an alignment jig this way. Skip the temptation to go to a finer pitch... 1/4-80 holds up to dirt and grease in this case.
    Take a look at Thorlabs RA90s as part of the mirror mount scheme.

    Steve
    Last edited by mixedgas; 05-20-2021 at 06:06.
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    cheap webcam works too just peel the ir filter out

    Gen 1 night vision too

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    I massively edited my post below, you might want to re-read it.

    There is an Ebay Seller, 3dphotonics , that has a lot of optics right now that are HR or OC for 1064, some with HT for 532, with varying radius of curvature. At 33$ an optic, that is a deal.

    Steve
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    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).

    Click image for larger version. 

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    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:

    Click image for larger version. 

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    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.

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