While I played around with locking the cavity I was not able to see the cantilever mode dominate the PDH signal at most settings. The only times I did see it was sometimes when the cavity was losing lock after being locked for a while.
Aaron and I previously saw that the 40 Hz cantilever mode was using up most of the PDH signal's linear range while in lock.
This means that the loop has just enough gain to stay locked. Increasing the gain seems to make it unstable, so we want to lower the UGF.
There were some current and temperature settings at which I was not able to lock the cavity with the same overall gain (4.40 on the knob = -6 dB which I did not change) but at the following two settings I was able to lock the cavity with the residual noise being ~30% the PDH pk-pk:
- I ~ 114 mA, T=8.193 kOhms. PDH pk-pk was 370 mV and the locked residual pk-pk was 130 mV
- I ~ 130 mA, T=8.356 kOhms. PDH pk-pk was 400 mV and the locked residual was 154 mV
I measured the current using the 'ANALOG OUT' port of the ITC 502 on an oscilloscope using the conversion factor of 20 mA/V.
At the second setting mentioned above I also quickly measured the open loop transfer function similar to elog 2761 and elog 2759 (shown in Attachment 1 and data saved here). Since the gain was lower this time, the UGF was a smaller value of 22 kHz with a larger phase margin.