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

  1. #1
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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  


  2. #2
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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?

  3. #3
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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.

  4. #4
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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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.

  5. #5
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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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