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Thread: Homemade DPSS CW TiSa Laser

  1. #1
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    Default Homemade DPSS CW TiSa Laser

    Hello people,

    I am currently building a cw DPSS Titanium-Sapphire laser at home, pumped by a homemade diode module where 2x 4W FAC 465nm diodes and 1x 1W 520nm diode are combined. I have everything here that should be needed but I can't get my setup to actually lase so I'd be very happy if you guys (especially people with TiSa experience) could post some ideas on what might be wrong The build has been going quite a while, the original thread is on the laserdiscourse forum.
    First of all, my original setup is an X-Fold resonator with 2x R100 concave HRs, one plano HR and one 5%T OC. The crystal is brewster cut, 2.7mm long and absorbs ~70% of pumplight (FOM is >150).
    Click image for larger version. 

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    With my specs, assuming that the pump beam is focussed well enough, this should work (looking at the literature), but no luck yet, despite 10s of hours of alignment. I have two main issues, pump focussing and Alignment.
    1. Pump focussing/Energy density in the crystal. One issue is the focussing inside the crystal, which of course is directly connected to the pump module, which working kind of okay right now. Either the UV glue drifted a bit or my diode alignment was not as good as I thought. Either way, two spots (1 green and 1 blue) overlap very well in the crystal, but the third one is way off and totally blurry. Is there any way to soften/remove cured UV glue so I can correct some things? But just the overlapping spot has 4W blue + 1W green which should definitely work. The other issue related to this is defocussing by the pump mirror. Remember that the beam is focussed through a plano-concave mirror, which increases the spot size a lot. With a correction lens, I get around 164um x 86um spot size (simulated with BeamXpert) which is not great but might still work. The mode volume for the X-fold is ~50x50um.
    2. Resonator alignment. Aligning the X-fold resonator has not worked out for me yet since my alignment laser (1mW HeNe) fades out quickly after so many bounces and I have not found any info on alignment strategies for X-folds. My Pr:YLF laser at 640nm (and tens of mW at the lowest pump power) works way better, but still, no luck. I suppose the difference in index of refraction between 640nm and ~800nm leads to problems since the crystal is brewster cut.
    One issue is not only making sure that the resonator is aligned but that the pumped volume also overlaps with the mode volume. I can definitely do this in principal using the 2 pump fold mirrors but again, I have found no info at all about this problem in x-folds. I align the resonator by coupling the Pr:YLF into the outcoupler (bottom left) and overlap the spots (simplifiedÖ). One could think that I should just get the pump beam to retrace the alignment beam back to the pinhole on the bottom left, but again, the index of refraction is a problem.
    So, I thought about it and sought to simplify the resonator. Although I must say that I have never seen a different TiSa Resonator other that x-Fold or more complicated. Generally its for astimatism compensation of the brewster cut crystal I think, but is that even important for cw-lasing? I donít know, maybe some experts here can tell me why this is done even for cw lasers.
    Click image for larger version. 

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    I started by eliminating one plane HR mirror by folding one concave mirror directly back into the crystal, essentially creating a V-fold (see 2). Didnít work. Then I used the simplest resonator I can do with my mirrors: a hemispheric one. This one was risky since the plane outcoupler reflects some pumplight back into the diodes but luckily I didnít destroy anything. Alignment should be no problem essentially since Iíve done that tens of times with other lasers, but still, no luck. I tried resonator 3 at first, but then thought I could drastically reduce the spot size by using shorter focal lengths for the pump lens. With 4 this is possible (remember, the resonator length for 3 and 4 is 100mm). A dichroic should then separate the pump and IR. Neither setup worked, so maybe the pump is insuffient / OC has too much outcoupling. Although in the literature, this combination has worked fine! So I am torn whether just bad alignment, bad pump quality/power density or too much outcoupling is the bottleneck here. By the way all resonators are stable at least in theory (checked with ReZonator 2.0).
    Although I can report that I have seen some IR fluorescence from the crystal, though very dim.
    Click image for larger version. 

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    This is on resonator 3, but also worked with 4.
    Hereís also a video. I recognize this spot behaviour from my other hemispheric resonators, but overlapping the spots produced no lasing. Especially the tighter spot has high divergence and fades quickly so this is no laser action. Maybe 1 and 2 produce this as well but since the distance from crystal to OC is so much larger, it may not be detectable by my camera anymore.

    So here I am, wondering if laser action is shortly around the corner (just one more hour of alignment!) or if my setup can work at all.
    I have also ordered a 808nm laser module for alignment, this should have no issues regarding different index of refraction and shouldnít fade from the reflections due the mirrors being HR for that wavelength. Having to use my camera for alignment is really annoying though. But if it works, why not.

    If there is anyone here who has some thoughts or experience regarding even one of the points mentioned above I'd be extremely happy about hearing from you them Thanks for your time!

    Nik

  2. #2
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    0. Read any A. E Siegman lately? Dr. Siegman was a cool mentor, trust me on that. Dr Kochner has some cool books on Solid State Lasers, that are a must read.
    1. Have you calculated if you have a stable, low astigmatism resonator with Kogelnik et al. (1972) Astigmatically compensated cavities for CW dye lasers
    2. Depending on crystal cut, defects, polishing quality, and orientation, anisotropy happens, Ti Saph is very sensitive to crystal orientation about its optical axis. Try rotating the crystal around the lasing axis and translate it back and forth.
    3. With Ti Saph, temperature changes from the pump adsorption can cause horrid thermal noise and thermal lensing in the crystal. Is your crystal water cooled?
    4. Quoting the Movie Real Genius, "Didn't anybody ever tell you to make sure your optics are clean?"
    5. How good is the polish and coating on your crystal?
    6. Did you verify the coatings and radius of all the optics. I spent months on a Kerr Lensing ring saph with subtle coating problems. Could get CW but no self mode locking. Turned out to be polarization rotation in the mirror coating, as well as other things.
    7. If your Happy and your Kerr Lensing, clap your hands... Its been known to "choke" itself to death with too tight or too short of a pump region, or self Lens itself right out of operation.
    8. Do you have an IR ccd camera, once your close with the alignment laser, its more about overlapping the emission spots and scanning one axis at a time. It helps to have some adsrbing IR glass or ND filters for the camera.
    9. Refractive index changes like mad, and if your cavity and pump are even remotely close to stable, you'll see bright flashes as you get close to closing the loop. If you cannot align your Ti:Saph with a business card and camera, something is very, very wrong.
    9 I know the book says no, but pump polarization may matter.
    10. Intra-cavity loss from Air and Water Vapor adsorption is wavelength dependent and can be a lossy witch.
    11. Brewster is as Brewster Be, You sure your at the right angle for the index of refraction? The Law of Malus could bite you.
    12. Pump power actually measured, or just imagined?
    13. Measure your single pass gain with an IR Laser diode while pumped?
    14. Mode matching, does the
    Lasing mode and pump mode structure overlap?

    Please wear safety glasses and be aware of stray beams.

    See attached.

    Steve
    Attached Files Attached Files
    Last edited by mixedgas; 11-28-2022 at 23:02.
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    Hi Steve,

    thank you very much for your tips! Especially the paper by Kogelnik has really helped me, I have now redeveloped the resonator to a three mirror V-shape with the correct angle for astigmatism correction, for me its 8.8į. For testing purposes, I have also replaced the OC with an HR, calculating the threshold power using the used values and one of your papers gives me 650mW, which I am definitely above. But no lasing yet.

    1. No, but I am very familiar with Dr. Koechners book(s)
    2. Have done that, no luck. Could try it again though, maybe I was not persistent enough.
    3. My crystal is seated on a small aluminium holder with a small-ish heatsink, which does not heat up at all during testing.. So no active (water)cooling. Is this so critical it can completely destroy laser action?
    4. Everything is clean, as far as I can tell.
    5. The crystal is brand new, no marks at all, perfect mirror polish. No coating since it is brewster cut.
    6. Yes, I've tested the mirrors for their specs, ROC is correct and they are highly reflective around 808nm.
    7. How could I determine whether that's a problem? Maybe try to calculate the focal length and adjust the cavity?
    8/9. Its really good to know that there's a visual indication when it lases, other than IR light! My camera does pick up IR, but I suppose I must get the bright flashes first.
    11. Yes, I am pretty sure. I used a polarizer to check the reflected light from the brewster face, its 100% polarized, though the pump is not. Also, the holder is machined in a way that seats the crystal at the right angle so I only have to align the holder to be at 90į to the optical axis.
    12. Calculated through diode current, can't actually measure powers that high. They are actively cooled though, I don't see a reason why all three would be far below spec. Although the combining into one tight spot is a big issue, which I am working on. According to my research every diode on its own should be able to threshhold the crystal though..
    13. I can't really do that with my setup and equipment :/

    14. As you told me via PM (thx btw) I agree that this might be a big issue, maybe even the dealbreaker. I am using the exact same setup as the paper in the attachment, same ROCs, same Resonator, similar crystal length and they got it to lase with >500mW of 452nm! I even got all HRs, no OC.. But how do I make sure they overlap? I am currently adjusting the resonator with the pump beam, but overlapping all the green spots as I would think I'd have to doesn't work, no flashes at all. Btw, the calculated beam radius for my 4W 465 diodes is 43*82um, the mode radius is 41*66um so it should be pretty similar, even if my cylinder lenses are not perfectly aligned within the pump module.

    Nik
    Attached Files Attached Files

  4. #4
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    I just had a friend in Cleveland call me about a 8900S CW Ti:Saph, and making it potentially mode lock. Small world. From helping him, I received a new manual. SPs manual has a few tips you may find interesting.

    Please see attached Spectra manual . The comments on doping and crystal axis are interesting.

    Please see 3-2 in the 3900 manual.

    I've worked on Mira and Chameleon. The Mira had been modified and a few of the mirrors exchanged, and I had to rotate the rod a bit for seemingly no reason to get it to tune. The Birefringent filters required multiple, multiple attempts to install correctly for the full tuning range.

    Chameleon was easy to tune, but needed re-aligned each day along it's long axis.

    Both lasers were sensitive to drafts and changes in humidity. A few degrees C change in the room and all hell could break lose on Mira, and Chameleon when ultrafast. A downdraft on the laser was disastrously evil. Yes, it can be sensitive.

    Changing the cavity lengths is critical, see critical confocal cavity in Kolgenik. Also check if you need to add angle for astigmatism correction per Liu,

    For a raw, new, ring cavity, Simply slide mirror mounts back and forth. Next up, once lasing weakly , slide crystal back and forth for peak stability and making sure the mode match overlaps.

    Try seeing if you have single pass gain. Set up a collimated 808 nm diode, simply pass it down the crystal with no mirrors. Monitor the amplified power and locate your "Sweet Spot". Background can be subtracted out by blinking or chopping the probe beam.

    Where are your "A" and "C" axis related to the rod?

    I would water cool the rod with a constant temperature bath, it is "that" sensitive.


    My point being, unlike ND:YAG, TiSa is an art form, with massive reading comprehension requirements. Give it time.

    Much more then I have suggested and I'm in over my head. Gas lasers I can design from scratch. Other then operational experience, for me, TiSa is difficult.

    I just asked a more experienced Laser Designer to step in. English is not his native language, lets see if he can help. Hopefully Arnold is monitoring his PMs.

    Steve
    Attached Files Attached Files
    Last edited by mixedgas; 11-29-2022 at 15:14.
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    Default Not Ti:Saph...but maybe helpful?

    I an by no means an expert -- I've only built one laser and Steve was instrumental in helping me to get it to work. But I have spent many backbreaking hours aligning a Z-fold Nd:YVO4 and I learned some things. I don't know if they apply to Ti:Saph....but posts are cheap.


    1. My crystal refracted the incident alignment beam -- it produced more than one spot at an angle, but only one one axis. My crystal was not Brewster cut so I aligned it so the angles it produced were in the X-axis. That way I could dial in the plane-parallel Y-axis, align to one of the spots, and sweep an end mirror until I got some light. I did this in the dark with an IR card as the initial lasing could be pretty weak if the other mirrors weren't dialed in. That refraction angle is going to be dependent on incoming wavelength, and since your alignment laser differs from the fundamental you just have to plan for it not really being aligned even when it looks perfect.
    2. Mode matching is critical and since you're going to have to sweep a mirror to get it aligned (assuming Ti:Saph refracts in a similar fashion to Nd:YVO4) you need the mode matched to reduce the number of variables. You need a way to see that the alignment laser and the pump beam match up. With Nd:YVO4, there was some faint florescence both from the pump light and from a 532 laser I used for alignment and I was able to match things up this way while wearing goggles. Your pump mode is a little oversized but this will make it easier to match up...provided there is enough intensity to get a population inversion.


    I documented the alignment technique I used for the Z-fold here. Maybe it will be of some help: https://harmlesshavoc.com/blog/2022/...z-fold-edition.

    Good luck! I'm looking forward to seeing light out of this thing.

  6. #6
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    Default Update

    Hi all,

    first of all, thanks brianpe to your suggestions, nice website and instructions! One rarely sees those with such high quality.

    Over the last few weeks, although I didn't have much time to work on the laser, I did have some correspondence with some authors of DPSS TiSa papers who gave me some valuable tips. First of all, I have abandoned the X-fold resonator, too many variables and doesn't fit well on my optics table (too short arm lengths). I did try the alignment technique mentioned in Steve's manual, I actually saw all the mentioned spots, but no luck. I also switched from a 5% OC to a 2% OC. Now that I write this, I didn't actually try the X-fold setup with the 2% OC - I'll do that tomorrow. My crystal is now water cooled, stays below 20įC.
    These are the tips from the researchers - maybe they'll be useful to someone else:
    Pretty much all of them mentioned that my pump mode radius may be too large - I think this is the root of the trouble I am having right now. I measured them using a razorblade and a translation stage (google "knife edge method", my cut-off is ~11%, not 1/e≤) and my green beam is 35x40um and the blue beam is 30x70um. Note that in the crystal one dimension is expanded by 1.7, the refraction index of TiSa. Usual values in the literature tend to be around 20um, but especially the green diode is actually similar in power and size of some published articles so it should generally work..

    Another tip (for the folded resonator, type 2 in the original post) was to use the fluorescence spots to align the two curved mirrors and then an amplified photodiode to align the flat OC. I did all that, aligned the spots and tuned the OC to the maximum value on my photodiode, but sadly no lasing.

    I also received the tip of removing one arm by folding the beam into the curved mirror (essentially replace X-fold with V-fold resonator), which is what I already did so I know this should work in theory.

    Polarization does actually matter, as Steve said. Ti:Sa absorbs light much more (factor depends on doping, but can be around 2x) in one polarization than in the other. Luckily, both my 520nm diode and one 465nm are correctly oriented so this is not an issue. The difference in color between the 465nm diodes is astounding - in the correct orientation, the crystal is a lot pinker as the more blue light is absorbed and turned into fluorescence.

    One of the researchers said that 200mm arm length may be too short although the cavity it self is stable. longer arm length - tighter mode radius between the curved mirrors. Unfortunately my breadboard is not very big.. So I used a HR mirror to extend the arm, now it should be around 550mm which is in the realm of what the literature reports (although 200mm can be found as well).

    The folding angle is important and must be calculated correctly - I checked this at least 5 times, it is correct.

    One of the resarchers also emphasized that TiSa is very sensitive to losses, I think Steve hinted at this already. I needs to be cooled, used at brewsters angle and with low OCs (1-3%). I have 2% so that should be alright.

    I have read consistently that powers as low as 1-2 W should be enough to lase (some researchers also told me this in their emails). Thats encouraging, but also quite frustrating since I feel that I am doing everything right but no lasing.

    Here are two videos of the setups and me trying to align the resonator by overlapping the fluorescence spots.

    Vid1 Here you see the setup. I taped some orange plexi infront of my phone camera, filters out all of the green pump light.

    Vid2 Changed the mirror distances a bit, so the spots look different. Interestingly, here you can see a very faint third spot which can be moved by moving the OC. Aligning all three spots (in my mind) guarantees perfect resonator alignment, but no lasing.

    To anyone with experience in this field: Are the spots supposed to be equally big like in Vid1 or can they be differently sized like in Vid2?

    My alignment process is as follows, btw: I remove the OC and use a 808nm beam (same wavelength as the TiSa output!) to align the curved mirrors by inserting a pinhole at the OCs location and moving the curved mirrors until the reflected spot goes through the pinhole. Then I insert the OC and move it to reflect the 808nm directly into the 808 source.
    For fine adjustment, I use the fluorescent spots like shown in the vids.

    After probably 100 hrs of alignment, I can get all spots aligned from scratch in about 20 minutes, I am very certain that alignment is not the issue. So, after ruling out this, polarization, wrong outcoupling, crystal cooling, resonator stability (I am using reZonator 2.0), folding angle and too low power, only four issues are possible:

    1. Pump radius too big. Most likely. Although I am using the exact same green diode as one paper, maybe it is damaged, idk. I have also tried a galileam beam expander to get smaller focus radii, but it introduced a lot of spherical aberrations.. Although the core spot is indeed smaller.
    2. Haven't found the perfect position of the mirrors spacings yet, even after so much time. In my mind unlikely, but definitely possible, I'll keep trying.
    3. The crystal or mirrors are defect/wrong specs. Steve mentioned this and I ruled it out, but at least the eBay stuff might not be what it says it should be, although both sellers are reputable. Thorlabs and Optogama I trust. I'll have to go to my uni's optics lab to check all the mirrors and the crystal.
    4. Dust flying in the air introduces high losses. My basement is not extremely dusty (still cover all the optics when not in use) but maybe clean room conditions are mandatory? Idk.

    If nothing works, I'll look for smaller power diodes with better M≤ (which is usually the way in the literature) and ask my optics faculty to measure the optics.. But since I'll be going to the US for 6 months next week, this - very much to my frustration - will have to wait.

    Sorry for the long post - As Steve said, this project is very complicated. As always thoughts on my notes are appreciated!

    ~Nik

    PS: I have attached a screenshot of the ReZonator simulation, this is the very latest iteration of the resonator.
    Attached Thumbnails Attached Thumbnails TiSa-Rezonator.PNG  


  7. #7
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    Quote Originally Posted by NiklasH View Post
    Hi all,

    first of all, thanks brianpe to your suggestions, nice website and instructions! One rarely sees those with such high quality.

    Over the last few weeks, although I didn't have much time to work on the laser, I did have some correspondence with some authors of DPSS TiSa papers who gave me some valuable tips. First of all, I have abandoned the X-fold resonator, too many variables and doesn't fit well on my optics table (too short arm lengths). I did try the alignment technique mentioned in Steve's manual, I actually saw all the mentioned spots, but no luck. I also switched from a 5% OC to a 2% OC. Now that I write this, I didn't actually try the X-fold setup with the 2% OC - I'll do that tomorrow. My crystal is now water cooled, stays below 20įC.
    These are the tips from the researchers - maybe they'll be useful to someone else:
    Pretty much all of them mentioned that my pump mode radius may be too large - I think this is the root of the trouble I am having right now. I measured them using a razorblade and a translation stage (google "knife edge method", my cut-off is ~11%, not 1/e≤) and my green beam is 35x40um and the blue beam is 30x70um. Note that in the crystal one dimension is expanded by 1.7, the refraction index of TiSa. Usual values in the literature tend to be around 20um, but especially the green diode is actually similar in power and size of some published articles so it should generally work..

    Another tip (for the folded resonator, type 2 in the original post) was to use the fluorescence spots to align the two curved mirrors and then an amplified photodiode to align the flat OC. I did all that, aligned the spots and tuned the OC to the maximum value on my photodiode, but sadly no lasing.

    I also received the tip of removing one arm by folding the beam into the curved mirror (essentially replace X-fold with V-fold resonator), which is what I already did so I know this should work in theory.

    Polarization does actually matter, as Steve said. Ti:Sa absorbs light much more (factor depends on doping, but can be around 2x) in one polarization than in the other. Luckily, both my 520nm diode and one 465nm are correctly oriented so this is not an issue. The difference in color between the 465nm diodes is astounding - in the correct orientation, the crystal is a lot pinker as the more blue light is absorbed and turned into fluorescence.

    One of the researchers said that 200mm arm length may be too short although the cavity it self is stable. longer arm length - tighter mode radius between the curved mirrors. Unfortunately my breadboard is not very big.. So I used a HR mirror to extend the arm, now it should be around 550mm which is in the realm of what the literature reports (although 200mm can be found as well).

    The folding angle is important and must be calculated correctly - I checked this at least 5 times, it is correct.

    One of the resarchers also emphasized that TiSa is very sensitive to losses, I think Steve hinted at this already. I needs to be cooled, used at brewsters angle and with low OCs (1-3%). I have 2% so that should be alright.

    I have read consistently that powers as low as 1-2 W should be enough to lase (some researchers also told me this in their emails). Thats encouraging, but also quite frustrating since I feel that I am doing everything right but no lasing.

    Here are two videos of the setups and me trying to align the resonator by overlapping the fluorescence spots.

    Vid1 Here you see the setup. I taped some orange plexi infront of my phone camera, filters out all of the green pump light.

    Vid2 Changed the mirror distances a bit, so the spots look different. Interestingly, here you can see a very faint third spot which can be moved by moving the OC. Aligning all three spots (in my mind) guarantees perfect resonator alignment, but no lasing.

    To anyone with experience in this field: Are the spots supposed to be equally big like in Vid1 or can they be differently sized like in Vid2?

    My alignment process is as follows, btw: I remove the OC and use a 808nm beam (same wavelength as the TiSa output!) to align the curved mirrors by inserting a pinhole at the OCs location and moving the curved mirrors until the reflected spot goes through the pinhole. Then I insert the OC and move it to reflect the 808nm directly into the 808 source.
    For fine adjustment, I use the fluorescent spots like shown in the vids.

    After probably 100 hrs of alignment, I can get all spots aligned from scratch in about 20 minutes, I am very certain that alignment is not the issue. So, after ruling out this, polarization, wrong outcoupling, crystal cooling, resonator stability (I am using reZonator 2.0), folding angle and too low power, only four issues are possible:

    1. Pump radius too big. Most likely. Although I am using the exact same green diode as one paper, maybe it is damaged, idk. I have also tried a galileam beam expander to get smaller focus radii, but it introduced a lot of spherical aberrations.. Although the core spot is indeed smaller.
    2. Haven't found the perfect position of the mirrors spacings yet, even after so much time. In my mind unlikely, but definitely possible, I'll keep trying.
    3. The crystal or mirrors are defect/wrong specs. Steve mentioned this and I ruled it out, but at least the eBay stuff might not be what it says it should be, although both sellers are reputable. Thorlabs and Optogama I trust. I'll have to go to my uni's optics lab to check all the mirrors and the crystal.
    4. Dust flying in the air introduces high losses. My basement is not extremely dusty (still cover all the optics when not in use) but maybe clean room conditions are mandatory? Idk.

    If nothing works, I'll look for smaller power diodes with better M≤ (which is usually the way in the literature) and ask my optics faculty to measure the optics.. But since I'll be going to the US for 6 months next week, this - very much to my frustration - will have to wait.

    Sorry for the long post - As Steve said, this project is very complicated. As always thoughts on my notes are appreciated!

    ~Nik

    PS: I have attached a screenshot of the ReZonator simulation, this is the very latest iteration of the resonator.
    if you can get to cleveland with your optics we can put the pump I use for my MIRA on it to see if itís the optics or the pump.
    im looking to threshold a 3900S with diodes this month so Iíll report back if it works. Iíll be driving with 2-3w of fac corrected 450nm laser diodes. Iíll first align with the MIRA pump so I know it works and then try the diodes. This is a simple z fold arrangement that is cw. If that works next step is qswitch. If that works next is a sesam to mode lock it. Hoping it is stable enough to mode lock while being qswitched. These seem mutually exclusive of each other but since the lock is so much faster than the switch maybe it just stops and starts and the qswitch will just be the kick to keep the mode locked after each pulse train. Thing is I donít see how you can do both since the point is to build up the population before you raise the q in the first place. In fact Iíd think this dual mode would be undesirable so if I get the q switch Iíll likely remove it and then try mode locking. My need is more short pulses than energy anyway.

    - - - Updated - - -

    Quote Originally Posted by NiklasH View Post
    Hi all,

    first of all, thanks brianpe to your suggestions, nice website and instructions! One rarely sees those with such high quality.

    Over the last few weeks, although I didn't have much time to work on the laser, I did have some correspondence with some authors of DPSS TiSa papers who gave me some valuable tips. First of all, I have abandoned the X-fold resonator, too many variables and doesn't fit well on my optics table (too short arm lengths). I did try the alignment technique mentioned in Steve's manual, I actually saw all the mentioned spots, but no luck. I also switched from a 5% OC to a 2% OC. Now that I write this, I didn't actually try the X-fold setup with the 2% OC - I'll do that tomorrow. My crystal is now water cooled, stays below 20įC.
    These are the tips from the researchers - maybe they'll be useful to someone else:
    Pretty much all of them mentioned that my pump mode radius may be too large - I think this is the root of the trouble I am having right now. I measured them using a razorblade and a translation stage (google "knife edge method", my cut-off is ~11%, not 1/e≤) and my green beam is 35x40um and the blue beam is 30x70um. Note that in the crystal one dimension is expanded by 1.7, the refraction index of TiSa. Usual values in the literature tend to be around 20um, but especially the green diode is actually similar in power and size of some published articles so it should generally work..

    Another tip (for the folded resonator, type 2 in the original post) was to use the fluorescence spots to align the two curved mirrors and then an amplified photodiode to align the flat OC. I did all that, aligned the spots and tuned the OC to the maximum value on my photodiode, but sadly no lasing.

    I also received the tip of removing one arm by folding the beam into the curved mirror (essentially replace X-fold with V-fold resonator), which is what I already did so I know this should work in theory.

    Polarization does actually matter, as Steve said. Ti:Sa absorbs light much more (factor depends on doping, but can be around 2x) in one polarization than in the other. Luckily, both my 520nm diode and one 465nm are correctly oriented so this is not an issue. The difference in color between the 465nm diodes is astounding - in the correct orientation, the crystal is a lot pinker as the more blue light is absorbed and turned into fluorescence.

    One of the researchers said that 200mm arm length may be too short although the cavity it self is stable. longer arm length - tighter mode radius between the curved mirrors. Unfortunately my breadboard is not very big.. So I used a HR mirror to extend the arm, now it should be around 550mm which is in the realm of what the literature reports (although 200mm can be found as well).

    The folding angle is important and must be calculated correctly - I checked this at least 5 times, it is correct.

    One of the resarchers also emphasized that TiSa is very sensitive to losses, I think Steve hinted at this already. I needs to be cooled, used at brewsters angle and with low OCs (1-3%). I have 2% so that should be alright.

    I have read consistently that powers as low as 1-2 W should be enough to lase (some researchers also told me this in their emails). Thats encouraging, but also quite frustrating since I feel that I am doing everything right but no lasing.

    Here are two videos of the setups and me trying to align the resonator by overlapping the fluorescence spots.

    Vid1 Here you see the setup. I taped some orange plexi infront of my phone camera, filters out all of the green pump light.

    Vid2 Changed the mirror distances a bit, so the spots look different. Interestingly, here you can see a very faint third spot which can be moved by moving the OC. Aligning all three spots (in my mind) guarantees perfect resonator alignment, but no lasing.

    To anyone with experience in this field: Are the spots supposed to be equally big like in Vid1 or can they be differently sized like in Vid2?

    My alignment process is as follows, btw: I remove the OC and use a 808nm beam (same wavelength as the TiSa output!) to align the curved mirrors by inserting a pinhole at the OCs location and moving the curved mirrors until the reflected spot goes through the pinhole. Then I insert the OC and move it to reflect the 808nm directly into the 808 source.
    For fine adjustment, I use the fluorescent spots like shown in the vids.

    After probably 100 hrs of alignment, I can get all spots aligned from scratch in about 20 minutes, I am very certain that alignment is not the issue. So, after ruling out this, polarization, wrong outcoupling, crystal cooling, resonator stability (I am using reZonator 2.0), folding angle and too low power, only four issues are possible:

    1. Pump radius too big. Most likely. Although I am using the exact same green diode as one paper, maybe it is damaged, idk. I have also tried a galileam beam expander to get smaller focus radii, but it introduced a lot of spherical aberrations.. Although the core spot is indeed smaller.
    2. Haven't found the perfect position of the mirrors spacings yet, even after so much time. In my mind unlikely, but definitely possible, I'll keep trying.
    3. The crystal or mirrors are defect/wrong specs. Steve mentioned this and I ruled it out, but at least the eBay stuff might not be what it says it should be, although both sellers are reputable. Thorlabs and Optogama I trust. I'll have to go to my uni's optics lab to check all the mirrors and the crystal.
    4. Dust flying in the air introduces high losses. My basement is not extremely dusty (still cover all the optics when not in use) but maybe clean room conditions are mandatory? Idk.

    If nothing works, I'll look for smaller power diodes with better M≤ (which is usually the way in the literature) and ask my optics faculty to measure the optics.. But since I'll be going to the US for 6 months next week, this - very much to my frustration - will have to wait.

    Sorry for the long post - As Steve said, this project is very complicated. As always thoughts on my notes are appreciated!

    ~Nik

    PS: I have attached a screenshot of the ReZonator simulation, this is the very latest iteration of the resonator.
    if you can get to cleveland with your optics we can put the pump I use for my MIRA on it to see if itís the optics or the pump.
    im looking to threshold a 3900S with diodes this month so Iíll report back if it works. Iíll be driving with 2-3w of fac corrected 450nm laser diodes. Iíll first align with the MIRA pump so I know it works and then try the diodes. This is a simple z fold arrangement that is cw. If that works next step is qswitch. If that works next is a sesam to mode lock it. Hoping it is stable enough to mode lock while being qswitched. These seem mutually exclusive of each other but since the lock is so much faster than the switch maybe it just stops and starts and the qswitch will just be the kick to keep the mode locked after each pulse train. Thing is I donít see how you can do both since the point is to build up the population before you raise the q in the first place. In fact Iíd think this dual mode would be undesirable so if I get the q switch Iíll likely remove it and then try mode locking. My need is more short pulses than energy anyway.

  8. #8
    Join Date
    Nov 2022
    Posts
    5

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    Thank you so much for your offer! However, I'll be around Long Beach, CA.. Anyways, I'd really, really love to hear as much detail about your project as possible! Information like diode type used, collimation setup and resonator arm lengths are very difficult to find. Crystal length, mirror curvature and measured pump radius would be very interesting as well. I think if you were to take a video (using an IR-sensitive camera, like on a smartphone) of the alignment - e.g. overlapping fluorescent spots - you'd probably be the first person ever to publish footage of a DPSS TiSa alignment on the free internet.
    But I ofc don't want to force anything - I'd even be happy to just hear if it worked!

    Greetings from Germany!

    PS: Q-switched mode-locked Ti:Sa Laser definitely are possible, I've seen papers in which this is reported, but haven't found any with diode pumping. Might be worth publishing if you get it to do that.

  9. #9
    Join Date
    Nov 2008
    Location
    Cleveland Ohio
    Posts
    2,600

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    Iíll let you know when I know. Was thinking of a fac corrected num44 through a collimation lens into the laser.

  10. #10
    Join Date
    Oct 2012
    Location
    Germany
    Posts
    1,479

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    ... for fac correction I'm mostly using this type of prism-corrected modules - https://www.lasertack.com/en/standard-modules

    Viktor
    Aufruf zum Projekt "MŁll-freie Meere" - https://reprap.org/forum/list.php?426
    Call for the project "garbage-free seas" - https://reprap.org/forum/list.php?425

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