First I optimized the p pol PLL frequency to maximize the amplitude of CCFC error signal. The optimized p pol PLL frequency was 200 MHz for OPO temperature of 7.163 kOhm and the CCFC calibration amplitude was 134mVpp.

Then I checked the nonlinear gain. The nonlinear gain was 4.4, which corresponds to the generated squeezing of 10.1dB.

I used a red LEMO cable between DDS output and mixer for CCFC LO to have larger detuning. The figure 1 shows CCFC FDS. The 50Hz bump somehow disappeared today. The detuning fluctuation is 51-69Hz. Note that the detuning fluctuation is not bad (51-57 Hz) other than the anti-squeezing quadrature.

I found that the 50Hz and its harmonics are already very large in shot noise (figure 2). The DC balance or ground condition is not good?

Degradation parameters:

sqz_dB = 10.1;                 % generated squeezing (dB)

L_rt = 120e-6;                 % FC losses

L = 0.49;                      % Propagation losses

A0 = 0.06;                     % Squeezer/filter cavity mode mismatch

C0 = 0.02;                     % Squeezer/local oscillator mode mismatch

ERR_L =   1.5e-12;             % Lock accuracy (m)

ERR_csi = 30e-3;               % Phase noise (rad)