Below is the measured OLTF of the gyro CCW loop. It is admittedly crappy, but I guess the idea is that we can first focus on the high-frequency part---which is actually somewhat clean---and then troubleshoot the lower-frequency stuff later. The main thing to notice here is that the zero we put in the PDH box at a few kHz to negate the cavity pole is not negating anything. Instead, we have a gain that is flat in frequency from there up to wherever the real cavity pole is. This is bad.
The cavity pole estimate we used before is based on the (somewhat higher) finesse we plan to operate with in the future. Since our finesse is quite lower now, our cavity pole is higher, and we must modify the PDH box gains accordingly. I looked through the elog and noticed that we had never finished taking a proper finesse measurement anyway, so I did one this evening. I first used arbcav to simulate the cavity using the most recent T measurements, which are printed and labeled on the cavity mirrors. Below is the output, indicating a finesse of 485.
To measure the finesse, I used the tip Frank had given Alastair about using the known modulation frequency as a reference for converting time to frequency in the sweep trace. In the process, I calculated an NPRO PZT actuation gain of about 17 MHz/V, which is considerably higher than we have been assuming. I used the measured FWHM of the transmission peak (see below) to calculate a finesse of about 400.
This is in the ballpark. Frank suggested that it might be better to use the width of the error response, as the apparent FWHM of the transmission peak is sensitive to the sweep frequency (though I chose the sweep frequency of 10 Hz to be high enough that displacement noise did not distort the peak, but low enough that the peak was symmetric). Using this method, for which I have no figure, I got a finesse that was closer to 500.
I think these measurements bracket the calculated finesse well enough that it can be trusted, so I will modify the PDH boxes to have their zeros at 95 MHz / (485 * 2) ~ 98 kHz.