(Sheila, Daniel)
We used the normalized PDH signal to feed back to the additive offset of the common mode servo which in turn moves the mode cleaner and the laser frequency. The loop bandwidth was 20Hz with a 1/f shape. This kept the red laser light on resonance in the arm cavity for good. We then swept the offset of the normalized PDH error signal to scan the cavity resonance. The attached plot shows the sweep. It took about 20 seconds. The FWHM as measured by the red transmitted light corresponds to 49.7 units. However, it should be 165Hz. The calibration factor was set to 0.208 (from 0.063).
This measurement was made with 2 stages of whitening on REFL_9 and 36dB of whitening gain.
In the refl DC bias path we used a gain of 50, 6dB input gain on the common mode board input 2, with negative polarity. We has the boost on the COMM PLL off, and used common compensation on the common mode board.
Attached is the measurement of the gain in the DC bias path.
The cavity pole is really 42 Hz, so the calibration should be set to 0.106.