[Shruti, Rana]

I made that the ITC 502 current driver was in constant current mode where the feedback coeff is 20 mA / V 5% and bandwidth 500 kHz; with the laser coefficient being 100 MHz the net feedback coefficient would be 2 GHz/V.Then, I looked at the level of the PDH signal which was roughly 1V.

In order to attenuate the signal going in to the MOD IN of the ITC 502 current driver, Rana suggested using the 50 Ohm output of the SR560 with two series 20 dB attenuators and a BNC T with one port going to the MOD IN and the other port terminated with 50 Ohm.

With the SR 560 set with 1 Hz low pass and gain of 10, the cavity locked for 3-10 seconds when it came close to resonance as seen on the monitor.

The three attachments are the PDH signal (purple), cavity reflection (blue), and cavity transmission (yellow) when the cavity seemed to lock.