The Big Green Thread (I'm gonna regret posting this in the morning)

Thanks Steve, I'll send you an email asking for that vandate cooker paper. I'll use that "educational purposes" method.

So.. awesome. So who's measuring these ratios on the optical table?
Eh. Working on the order of tens to hundreds of uM can't be that horrible. I suppose "best beam quality" also implies stability?

Nevermind. I just need to look it up.

Planters - So then there could be benefit from heating the vanadate - reducing the T gradient. Still have a thermal shock threshold to watch though..

MeatBall, he was trying to match 30 watts of Q-switched green into a old Ti:sa while watching pulse ringdown on a scope. I helped him do it manually at low power by moving a lens. That got him half way, and he needed to look at M^2 to get the rest of the way.

Then he cranked the power. I imagine the initial experince was a sparking sucess on the knief edges. I came in the next mornin, and them thar micro stages was computer controlled, very quickly, with Labview. I forgot to check how long the blades lasted, but not long. :o

The pumped laser was never in danger. It just was not coming up to full power until the mode matching is done.

Steve

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Planters - So then there could be benefit from heating the vanadate - reducing the T gradient. Still have a thermal shock threshold to watch though..

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I don't think so. For the same reason the temperature of the outside of the Vandate should have little effect on the GRADIENT. For a given heat flow the delta T has to remain roughly the same. I also am not too concerned about thermal shock, but thermal expansion and mechanical contact/loading is a concern. To keep the Vandate temp from running away it has to maintain good thermal contact with the heat sink and hence the indium. If the temp of the crystal rises too far there will be large mechanical stress from expansion within its mount and if it is designed too loose then thermal conduction suffers.

I have been hesitant to post in this thread, mostly because I am skeptical as to how successful it will be and really don't have any time to put into it, but I have wanted to try this for quite a while so I decided it was time to at least put in a few thoughts...

In any case, 12w of pump sounds reasonable, getting it mode matched to your green mode is going to be hard. If you insist on not doing side pumping or fiber coupling/lens ducts/etc, you are really going to need a well optimized cavity to get 2w out of a single emitter. The trick with the pbs will work with yag, but is not a good idea with vandate (it is polarization sensitive to the pump, so you will have a really funny gain cross section, and Not Good things happen).

If you decide to go with a single emitter you are going to be up against a world of hurt with the mode profile- a haven't seen a 200um emitter diode with that kind of power, so you are probably looking at a 400um emitter. You can cylindrical lens the crap out of it, but optimistically you are looking at a squarish spot .5mm on a side in the crystal with a diode like that as a pump. The intensity will be 2-4kW/cm^2 which isn't too bad actually, I have seen numbers on the order of 7-10kW/cm^2 as the damage threshold for end pumped vandate for a 0.5% doped crystal. The problem is going to be the ugly thermal lens created, which is going to give you the miserable M^2 associated with old fddpss lasers. You can play tricks with the lenses and try and get a round spot (there are tons of papers of people who do ray tracing of their beam compensation optics, and FEM analysis of the vandate to get what the thermal lensing looks like), then you can start comparing your cavity modes to the pump profile, and hope there is a spot that gives the correct waist size in the doubler and decent mode matching in the crystal.

Then once you get past the thermal lensing/modematching issues, when you start modulating it you end up in another world of hurt, because the thermal lensing starts changing depending on the average output power, so you need to start running sweeps in your simulation to see if you can find a cavity that can handle the change in thermal lensing from 1/infinity meters to 1/0.5meters without going nuts with the mode hops. You can also optimize the pump intensity in the vandate to try and minimize the energy stored in the crystal, to improve the high speed modulation. I am not 100% sure but I have a hunch that any yag based lasers will have terrible high frequency performance, since the lifetime is 1ms (10x longer than vandate) so I would expect a 3db drop in your modulation performance at 1khz, or a 90-10 time response of like 10ms :(

At the end of the day I decided that it would be fun to mess around with some optics and maybe generate a watt or 10 of green (mmm faps ), but I figured by the time I got everything optimized I would be so broke that I couldn't afford the 1w direct injected green diodes available for $50 which would be released the day I got it working *retardo


A good reference I have used before was http://wr.lib.tsinghua.edu.cn/sites/...9096544812.pdf for some general thoughts on higher power lasers.
a decent how-to for a 5w laser end pumped from a fiber, 5w out from 15w in http://iopscience.iop.org/0256-307X/...X_16_7_014.pdf

[QUOTE=krazer;245729]I have been hesitant to post in this thread, mostly because I am skeptical as to how successful it will be and really don't have any time to put into it, but I have wanted to try this for quite a while so I decided it was time to at least put in a few thoughts...

Krazer, Thanks!

Why hesitate, you just saved us from possibly making a expensive mistake!
This is why I love peer review. Krazer found the paper I was looking for. I need time to digest it.
I'm not so sure we're dead, we just may be down to 2-3 watts and no modulation. Galvo/AO blanking works for me.

We do need a domestic alternative to the Asian lasers.

I need to take care of a bunch of stuff during business hours, but this evening, watch your emails.

Steve

Krazer,

Thanks for the references. I read the first paper and I actually was encouraged. Despite your cautions, they seemed to have pretty fair performance and are using components and dimensions that are very similar to what we have been looking at. Re the diode coupling thing I was hoping that fiber coupling had been on the agenda in any case, but maybe not.

This modulation concern has me scratching my head. Throughout the articles that I have reviewed there are PO vs diode power or amp curves, but there is no mention that cavity adjustments or active temperature regulation has been fiddled with as the power is ramped up or down to maximize the output. If this is true then all bets are off, but this would seem to be a very significant fact and would undermine the experimental set-up conditions given as a single set of parameters for the system. And, these measurements are certainly bench top "adjust diode-read PO, adjust diode-read PO etc" so the effects of thermal lensing are going to be worse case; in other words, the rod will have sufficient time to reach a maximum thermal gradient unlike a situation where let's say you modulate 0-100% at 10Khz. I suspect the rod in this case will lens at somewhere between these extremes. I'm not naive, it will probably not be at 50%.

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In any case, 12w of pump sounds reasonable, getting it mode matched to your green mode is going to be hard. If you insist on not doing side pumping or fiber coupling/lens ducts/etc, you are really going to need a well optimized cavity to get 2w out of a single emitter. The trick with the pbs will work with yag, but is not a good idea with vandate (it is polarization sensitive to the pump, so you will have a really funny gain cross section, and Not Good things happen).

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That is good to know. So a wave plate to maximize diode to Vandate coupling would be a good addition even if not PBS combining. Also, The Jon pics of the Excel show PBS combining and so this must be a YAG laser.

I haven't looked at the second paper yet ( it's kidna expensive).

PRE-DESIGN

This is what we have to carefully check this weekend:

Pump Adsorption
Desired Pump Mode/Quality/Size
Pump polarization
2 ENDED PUMPING vs Side Pumping.
YAG or VANDATE
3 Mirror or 4 mirror.
OC transmission.
Stock Optics vs Custom
Damage Threshold of Asian Optics.
Location of Waists
Desired temperatures of diode, crystal, rod
ALL intracavity polarizations.
Type I vs Type II SHG.
"Green Problem" type noise and adsorption.
Size ratio of 1064 to 808.
Interaction lengths in crystal.
Available STOCK Pump diodes. (Yes, they do exist)
Using "FAP" vs raw diode. (Yes, I can order fiber coupled stock pumps)
Open block diode arrays vs fiber.
Classic Intracavity Waveplate Green with ND:YAG vs Vandate with Pump Polarization issues.
Pumping at 83X or 88X nanometers (direct upper level pump) instead of 808.

Or just use the end pumped, 3.6 watt published design and get it over with.

Others CAN AND SHOULD add to this list.

I'm going to send my suggested DOMESTIC US pump diode vendor to Planters and let him see what they can do.
The reasoning, a repeatable diode vs a one-off group buy from Asia is preferable for sustained use.
Considering Asia vendors disolve over night, one does not want to get locked into a non-standard diode package...

At work, pump diodes on 5-10 watt class greens last say two years of 24/7/365 operation. Recently suggested life on a Scientfic Grade, US Made, Unmodulated, Miata Class Starship is 15,000 Hours MTBF (2.5 years) , but those FAP type diodes are 7000$ a set of two, installed, from the laser factory. Life is defined to reduction in power of XX percent... I'd need to check what that percentage is.

IS a 1000 hour diode design life acceptable? 500 Hours?
How "cheap" can I go?
Steve

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Or just use the end pumped, 3.6 watt published design and get it over with.

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I love that. No... you've gone this far lets go a little farther. I'll call them. A 1000 hour design is OK with me. That is a 6 hour show two days a week, every week for 3 years ( 50% modulation). That's a pretty long list, but I think a lot centers around the pump.

Interesting update. I do not know if we can work with a 7W pump, but a fiber coupled diode that maintains polarization is VERY reasonable. If we could go with dual end pumping this is still very cost effective and 14W. A little more power and free space is more expensive. A rule of thumb is that for a few thousand hour lifetime expect to be able to increase OP by 10%.

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Originally Posted by planters

The pics of the Excel show PBS combining and so this must be a YAG laser.

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They expressly-said they use Vanadate and iirc, LBO, vs KTP.. I'm more certain of the YVO4-part, but, I suppose they could have been pulling my leg.. :p

That-said, Sir Peter, what do you think of the 'Excel' design, there, with the as-shown temp-regging components, and H/V pol-scheme of the pumps? You really think vanadate would present issues, seeing the power / beam-specs these pups do-produce, and at up to 30kHz? Not 'challenging', just asking for thoughts / opin on that obviously-vetted / proven design..

Also, *not* trying to 'force-feed' the Excel design, here, it's just simply an obvious 'cavity-design win', in-terms of beam-specs / power / performance-stability and ruggedness.. just fwiw, to 'learn from'.. :p

j