I switched the post to V-block for Faraday Isolator mount, for better stability, and adjusted the Faraday isolator to minimize back reflection to the laser.
I also measure the TF from ACAV path, The UGF is ~65 kHz.
The faraday isolator was installed on a standard pillar post, so I use a V block to mount it instead.
After adjusting the FI, I remeasured the beat note frequency, and the signal did not change from yesterday measurement.
(no differences between red and green plots)
blue: beat signal before fixing the ACAV opening
red: beat signal after fixing the ACAV opening
green : beat signal after re installing the FI
ACAV TF: I connect the signal output after the PDH servo box to SR560 A and SR785 B (resp)
Then source out from SR785 is connected to SR560 B
The output of SR560 is connected to SR785 B (ref) and to the VCO
The setup for SR560 is DC coupling for A and B, select A - B, gain 1, no filter.