A year ago I darkened Photolexicon's doorway with a lot of questions about building a frequency doubled DPSS laser. I built it, but the frequency doubling never performed well. "I will fix it when I learn to do the math to understand what's wrong." I learned to do the math. Built a nice library in Matlab that does a pretty good job of predicting both IR and frequency doubled outputs. I knew the intensity in the KTP was too low but I need a predictive model so I didn't just randomly change the cavity.
I got my answer and my answer is I'm probably not going to be able to get the KTP intensity high enough just running CW. I'm now looking at either mode-locking or q-switching (as was pointed out on this site last year but I wanted to learn the math).
I'm considering either a saturable absorber or an AOM. I have both and have been experimenting. And now I have questions. So many questions.
Mode Locking
My understanding of mode locking is that you're aligning the different cavity axial modes so they're all in phase. This way you get a regular pulse train due to constructive interference. Average power is about the same but the pulses are very intense.
To do mode locking with a saturable absorber you choose an absorber that "encourages" phase alignment by becoming more transparent when the phases do align. This feels like it needs some very careful calculation, and it also seems like the pump power needs to stay pretty well locked to one value. If you turned up the pump more doesn't this just bleach the absorber and you lose mode locking?
I bought an AOM of eBay that is designed for mode locking and the way I understand it they've set it up so the operating frequency causes then AOM to resonate and this is what is creating the modulation -- IE it's not driven by an external modulation like a Q-switch. This means that the AOM frequency has to match the resonant frequency of the cavity or you won't get mode locking. That's tricky in my case. My AOM requires a cavity that's about 2.2m long. I don't have that much bench space, and I imagine you get CW all day until the cavity length is precisely tuned. How well will that stay with temperature changes? Seems hard to maintain without some element guiding the frequency of the AOM.
Q-Switching
For Q-switching you need an absorber that has enough loss to prevent lasing. Cr:YAG has a florescence lifetime of 3ms so this essentially becomes a capacitor and continues charging until it reaches enough to bleach. Stronger pumping means the pulses happen more often. But with lasing prevented there is very little flux in the cavity. Does the absorber need to be right next to the laser crystal for this to work?
AOM Q-switching is all up to the AOM. The AOM I have I can trigger it manually and see that it diffracts the beam. But when I put it in cavity the diffracted beam still lases, just weaker. Is this a case of needing an AOM that has more diffraction? My AOM is only 2W and has a diffraction efficiency of 50% (this is the AOM I have). Or is this a case where I haven't configured it right?
Test Setup
I built this test rig to try things out. I can swap either the absorber or AOM into the upper area of the cavity. The lower area is currently configured for extracavity SHG -- this way I have a pretty strong signal that either mode locking or Q switching is working since CW will yield really poor harmonic generation:
With a .8 initial transmission saturable absorber and a focusing lens to get the intracavity beam down to 60µm I can get the laser to lase with just a little more pump power then when the absorber isn't there. IR output measures the same on my power meter but the 532 output is pretty different.
With no saturable absorber the green output is almost unnoticeable in this photo:
With the saturable absorber I measure 4mw:
I seem to be getting either mode locking or Q switching but I don't know which. What tools do I need to detect this? I have a pyroelectric joule meter I also bought off eBay hooked to my oscilloscope. It does output a signal when the laser is on but it seems DC. My cavity resonance is about 265Mhz which is just under my scope's 300Mhz bandwidth limit so I would expect to see something. Are these not fast enough?
I also tried .5 and .2 T0 Cr:YAG crystals but could get no output. My assumption here is that lower T0 causes more build-up so a larger Q-switch pulse, but you wouldn't want this for mode locking. I've turned the pump power up pretty high and neither of these would lase. The crux here is I don't know how to calculate the fleuence on the absorber when the cavity is not lasing.
Also, any good papers or books that would help with computing the best initial absorber transmission for my setup and help me model this? I have books by Siegman, Koechner and Silfvast and what looks like an excellent paper here that I haven't totally gone through yet. I want to be able to calculate the best T0 and the intracavity pulse intensity so I can figure out the best waist into the KTP.
Sorry for the insanely long post and so many Q's. Complicated subject and I don't know what I'm doing so I don't know what to ask.
Thanks for any info! -- Brian