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Thread: Making 488 from 976 using nonlinear crystals

  1. #21
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    Juntronix- Does Alphalas make PPKTP waveguides? As far as I know they only make 'bulk' PPKTP crystals, which is not really what you want for doubling extracavity CW pulses.

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    Planters - that's impressive. Most I ever saw was about 10W @ 635nm. Sadly the dye lasers are gone and the last of the laserscope equipment is going away soon.

    Krazer - They apparently offer a kit of parts to make 488nm from the DFB diodes. Still don't have details though.

  3. #23
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    10 W @ 635 is about where my stock dye laser came in. This was fiber fed from the LS and this decreases the output. Then, the COT has to be added SLOWLY because the Q switched pulse duration is about 300nsec (to my measurements). This is around the cross over where the advantageous triplet quenching of COT is balanced against its fluorescence inhibition. It definitely helps though and will increase the output by about 40%. Also, even if the dye packs include COT (I do not know what is in them), COT slowly deteriorates in air and so after a few weeks you might want to top it off. My lamps were nearly new and this sure doesn't hurt.

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    Back on topic - I've finally got the right connectors and tested one of the 976nm DFB stabilized diodes. It's working and making rated 300mW at 450mA, well under the 700mA max spec. The spectrum is weird though. Most of the power is concentrated at 976.2nm, but there is some leakage at ~970nm. If I adjust the temperature (there's a TEC in the package), there are a number of points where the leakage goes away. I don't think that's normal and is likely why the diodes ended up here. The peak wavelength does not move at all over temperature which is good. I'll check the other 3 ASAP.

    Also, I heard back from Alphalas and the PP-KTP crystal costs 1000 Euro. I was hoping they'd have a kit including the fiber collimator and lenses, but they suggested getting that stuff from Thorlabs. Does anyone have a pointer to a patent or tech paper that would describe the doubling scheme? I'm wondering if this is a simple as fiber -> fiber collimator -> focusing lens -> PP-KTP -> collimating lens -> anamorphic correction. As opposed to some sort of aligned cavity or specially coated mirrors.

  5. #25
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    Junktronix,

    You'll have one in three or four days to inspect, I'm sending you the 488 module.. In most cases Extracavity PPLN needs a actively steered resonant cavity to build up the energy for threshold of doubling. In this case they appear to be steering the diode to match the cavity, which is backwards but should work. I'll toss some papers in the box.

    Most European Crystal exotic crystals come out of plants in a single town in the former USSR. They have a network of resellers and its very profitable for the Russians and the Resellers, hence the markup. We'll talk via phone.

    There was a company in Canada offering Blue PPLN modules cheap. I'll look them up.

    Steve
    Last edited by mixedgas; 10-16-2014 at 05:44.
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    Well, this is getting interesting!


    I don't know if this is helpful but I googled 976/488 SHG and found these pics of Lazeerer's Melles Griot 488:

    Click image for larger version. 

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    Last edited by steve-o; 10-16-2014 at 07:45. Reason: added pics

  7. #27
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    That is pretty awesome. I am guessing the fiber is for beam shaping?
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    No clue. Maybe between Mixedgas and Junktronix we'll get a clue ..

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    The pump diode is fiber coupled. SO the laser can be more compact, it eases alignment, and helps with mode scrambling.

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  10. #30
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    Some interesting things in this thread, directly doubling diode lasers has been around for quite some time (see the second post in this thread for a brief history) but historically it has been extremely expensive due to the tolerances required to make it work. Check out the Coherent D3 laser linked in the aforementioned post for an idea of how tricky it is to make an extracavity doubled diode laser work reliably.

    Luckily, technology has improved a lot since the D3 was made. DFB lasers, (http://www.rp-photonics.com/distribu...ck_lasers.html ), have improved in quality and performance, so instead of the 100mw diode that was available back with the D3 was made, now we have 300mw ones which are commercially available, like the ones that Junktronix has found. Just be careful, a lot of 'frequency stabilized' lasers are not actually singlemode lasers (SLM), but rather a multimode laser which has its center frequency stabilized (like http://www.thorlabs.com/newgrouppage...tgroup_id=8042 ) intended for pumping erbium/ytterbium doped lasers, which would be worthless for this application. PS - the fiber coupling eliminates eliminates the need for careful mode matching between the diode and the enhancement cavity, greatly reduces the alignment sensitivity between the coupling into the enhancement cavity (since you just need to couple the fiber output into the cavity now), it removes the thermal issues of having the diode in the same substrate as the cavity, etc.

    Another improvement made since the D3 was created was improvements in optical crystals.
    First - a brief summary of the limiting factors of second harmonic generation: First, (if you have not already) read about how SHG works at http://en.wikipedia.org/wiki/Second-harmonic_generation we are using ), you can ignore the sections about surface second harmonic generation and the section which consideres depletion for now. Note that the generated second harmonic I(2w,l), goes as a function proportional to the effective second order nonlinearity Deff, the length of the crystal l, and the input intensity I(w), each squared. This means that to improve the conversion efficiency, you need to have the highest possible Deff, the longest possible crystal length, and the highest possible input intensity. Further note, there is a fundamental limitation that even a perfect (singlemode, gaussian profile, see http://www.rp-photonics.com/gaussian_beams.html) laser can only be focused so tightly, limited by diffraction, and that the length over which the beam stays focused (rayleigh range) is inversly prorptional to the focused spot size. Because of this, simply making the crystal longer does not help you with the conversion efficiency, because you will need to make the beam larger to keep it focused through the crystal and everything ends up canceling out (at least to first order, for a well optimized initial design). In fact the issue is actually even worse than that, because of other parasitic effects associated with second harmonic process (will touch on it later). So, the main factors in determining the conversion efficiency are the input beam intensity, and the effective second order nonlinearity, and doubling either of them gives you a 4x (squared) increase in the second harmonic conversion efficiency.

    The D3 uses a LiNbO3 crystal (or at least that is what we think it uses) which has a pretty high nonlinear optical constant of about 1pm/v, but PPKTP has a value about 10 times higher. Furthermore, the LiNbO3 uses birefringent phase matching (http://www.rp-photonics.com/birefrin...hing.html?s=ak), which is limited by spatial walkoof (http://www.rp-photonics.com/spatial_walk_off.html?s=ak), which further limits how long of a crystal you can use, where as the PPKTP uses quasi-phase-matching (QPM, http://www.rp-photonics.com/quasi_ph...hing.html?s=ak) so that the beams propagate directly along the crystal axis, and thus has no spatial walkoff.

    So, right off the bat, by switching to PPKTP over LiNbO3 you get about a 100x better performance than a D3.

    But, you still need an enhancement cavity. Why? Well, lets go back to to the second harmonic conversion efficiency calculations:
    Assume, we have:
    300mW input power
    1cm long crystal (this is an arbitrary length, but will give us a starting point), made of PPKTP (Deff = 10, n1~=n2~=n3~=1.8)
    If we want a rayleigh range of 1cm, Zr=pi*r0^2/lambda = 1cm, lambda = 860nm -> r0 = 52um, so the radius of the beam is 52um. This is a pretty rough approximation, but should be fairly close to the optimum beam size.
    So, the intensity is simply the power divided by the area, P=300mw/(pi*r0^2) = 3500w/m^2

    Now, going back to the second harmonic generation formulai, we know the second harmonic conversion efficiency, I(2w,l)/I(w,0) = (2*w^2*Deff^2*l^2 )/(n^3*c^3*e0)*I(w) for perfect phase matching (deltaN = 0), which works out to about 0.0000003 (here is a link to google calculator for the calculation ( https://www.google.com/webhp?sourcei...watts%2Fm%5E2) )

    So - no matter how hard you try to extracavity double a diode with a free space beam you are not going to get anywhere. This leaves you 3 options:

    1. Put the second harmonic generator crystal inside the laser cavity. This helps you 2 ways, first the intracavity power is much higher than the output power (for a set of cavity mirrors with a modest 99.9% reflectivity, you get a Q=~1000, and get a 1000x boost in your intracavity power), boosting your conversion efficiency up to 0.0002, and second the required conversion efficiency is reduced by the intracavity power, since you are starting with a 1000x stronger beam, so that .0002% conversion efficiency is corresponds to a 20% 'effective' conversion efficiency compared to your extracavity power. Looks a lot better now, huh? This is basically how the Novalux directly doubled diode lasers work.

    2. Put your harmonic generator in an external cavity. This has more or less the same effect as putting it in the laser cavity, but allows you to get away from the inherent issues associated with intracavity doubling (the fact that half of your power goes out the wrong end of the cavity, tricky cavity design needed to keep the generated second harmonic out of the laser medium, getting a good cavity Q with the gain medium in the cavity, etc). This is what the D3 did, the only downside is that you need to lock the length of the cavity to the frequency of the laser, and this requires sub-nanometer locking precision on the cavity length (ex, for a cavity with an enhancement factor Q=1000, you need roughly 860nm/1000 = 0.86nm precision on the length of the cavity to keep it running!), not to mention the engineering nightmare of keeping the whole thing stable under operation.

    3. Use a waveguide based second harmonic generator (ex, https://physik.uni-paderborn.de/ag/a...cation-in-ktp/ http://www.advr-inc.com/waveguide.html etc). You can get around the limitation on beam size vs crystal length by adding a waveguide in the nonlinear medium, which serves to guide the light down the length of the waveguide. Using this technique, it is possible to keep the spot size at roughly 1um radius over an arbitrarily long crystal (up to a few cm long, determined by practical issues), giving you that last factor of 1000 boost in intensity needed to get the conversion up to reasonable levels. This is how the melles griot BDD series (linked in the second post, and a few posts up) works, they use a roughly 2cm long crystal with a few um wide waveguide in the crystal, which works out to just about 100% theoritical conversion efficiency--no cavity needed. Also, the optics are dead simple for this design, you simply take your fiber coupled pump, and use a microlens (or lensed fiber) to couple the output of the laser into the waveguide, and then take the single mode output of the waveguide and collimate it like any other fiber coupled device.

    Not quite as simple as a normal dpss laser

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