I was able to get the cavity to stay locked in the unmodulated (CCW) direction by increasing the gain of the slow channel. The issues we were having this morning were caused by the ~25 kHz resonance of the PZT, which must have reached unity gain when we put enough PDH gain in. The added slow gain seems to be enough at low frequency to keep the thing locked, but if I try to up the fast gain the thing starts to oscillate. We will have to modify the servo to accommodate the PZT spike to make the lock more robust.
Once having locked the original direction, I steered the double-passed beam from the AOM into the CW direction of the cavity and aligned it to isolate a 00 mode at the output. It's not quite perfect, as the REFL signal is still clipped a bit out of the faraday isolator, but it's enough to couple a decent amount of power into the cavity, and the clipping will likely disappear when we put (any) modematching optics in this path.
The AOM seems to be working very well. I was able to ramp the frequency tuning voltage into the driver and see the different modes scan through on the CCD at the output. I increased the voltage from 7.8 V (which corresponded to 45 MHz) to 8.77 V (which must be very close to the desired FSR/2 of 47.5 MHz), and saw a strong 00 at the output when the CCW beam was on resonance. Hence, the CW beam is not strictly on resonance, but it is "locked" a fixed frequency very close to one FSR from the CCW beam, and we see something close to resonance at the output as a result.
The input power in each direction is close to the same (as cursorily seen on an IR card), and the emergent power at the transmitted end of the CW path is roughly half of that on the CCW path. Given NO modematching in the CW direction, this must mean that we are fairly close to resonance in that direction.
Tomorrow, we will begin doing the following:
- Work out some at least crumby modematching for the CW beam
- Adapt the second bluebox into a tentatively working form for the other loop
- Set up the REFL_CW readout and requisite RF electronics
- Lock the CW beam
- Pilfer a Marconi
- Beat the signals at the output
- Have a working gyro
- Celebrate with champagne and whisky
Here is a picture of both directions simultaneously outputting a reasonable 00 with a macroscopic frequency shift ~1 FSR: