(Michael R., Rick S., Oliver P.)

After turning on the high power satge, the amount of light in the FSS path was about six times higher than in the low power mode. To operate the FSS properly, the amount of light at the RF photodiode had to be attenuated. Therefore, we add an additional R=80% splitter to the beam phath. Also, the lens, focussing the beam to this diode had been exchanged: the focal length is now 100 mm instead of 50 mm, which gives more space for the beamsplitter, the diode itself, a stirring mirror in front of the diode and a beam dump, blocking the light, reflected from the chip (see the attached picture).
We were able to lock the FSS. The measured parameters are as follows:

power downstream of the Faraday isolator behind the reference cavity:  39.0 mW

DC voltage at photodiode in reflection, if locked: 0.052 V

DC voltage at photodiode in transmission, if locked: 1.119 V after optimizing lambda/4 wave plate

We adjusted the unity gain frequency using RF analyzer. The common gain has been set to 16 dB. We found the unity gain frequency at 309 kHz with 44 deg of phase margin.

If the FSS remains unlocked, the powers and voltages are:

RFPD DC 0.386 V

incident on RefCav 73 mW --> 86.5% visibility (including sidebands)

total power transmitted by RefCav: 43.5 mW