On Friday Terry and I adjusted the crystal position in the OPO to get phase matching and co-resonance at the same temperature. The take away messages are that the crystal translation stage is working and if we correct for mode matching the OPO threshold is between 7.4-8mW, and our phases matching temperature setting in air is just below 34C, both of these are consistent with our expectations.

Details:
We followed the procedure on page 215 of P1300006.  The main idea is to use the crystal temperature at which red and green co-resonante as a reference for crystal position, since both the position of the cavity axis along the wedge and the crystal temperature set the co-resonance.  There are a few caveats to the way we did this:

• The green power in the cavity was different by about 10% between some of these locks, so this is equivalent to about a 10% uncertainty on our measurement of nonlinear gain (if we had kept the green circulating power constant).
• It was difficult to set the co-resonance precisely because of the large noise on the cavity with the ISI locked and the purge air on. We were able to set the crystal position to within about 40 clicks on the translation stage driver.  We found that about 140 clicks are needed to compensate for the dispersion from a temperature change of 0.3C, so all of our temperature settings should be considered accurate to within about 0.1C as a reference for crystal position.
• We measure nonlinear gain by injecting a seed beam, scanning the seed phase, and measuring the de-amplification and amplification of the seed.  (x=(sqrt(max./min)-1)./(1+sqrt(max./min)); gain=1./(1-x).^2;)  In this case we were measuring small changes in the minimum seed power, and not with much accuracy.  This is why in addition to plotting the nonlinear gain, I've also plotted the maximum seed measured on the diode (uncalibrated).  This looks much more like the sinc function we are expecting. 

These caveats can explain why this measurement has large scatter.  We did this measurement with about 6mW of green injected into the OPO, and we saw our best nonlinear gain between 15-20 for this input power.  We can do a better measurement of the OPO threshold by changing the input power at a single crystal position, but this data gives us an estimate of the threshold as between 10 and 11 mW without correcting for our mode mismatch, and between 7.4-8mW correcting for the 74% mode matching from alog 40594. 

The LLO OPO was expected to have a threshold of 45mW with an input coupler that is 87.5% reflective for 532nm, while our input coupler has a reflectivity of 98%.  The threshold power is proportional to the green cavity decay rate, so our expected threshold for this OPO is 7mW (45mW*(1-sqrt(0.875)/(1-sqrt(0.98)). 

Terry and I left the crystal at the position where we see co-resonance for 33.75C, because we think this is about at the peak of the phase matching curve.  This procedure will have to be repeated once HAM6 is pumped down.