I checked the amplitude of CC2 error signal and the squeezing level with different homodyne angle. During the measurement, CCFC was closed. The CCFC amplitude was 126mVpp with p pol PLL frequency of 195MHz. Note that the positive (negative) squeezing level means squeezing (anti-squeezing).
| CC2 demod phase (deg) | CC2 amplitude (mVpp) | squeezing level (dB) |
| 110 | 84.8 | -1.8 |
| 100 | 89.6 | 2.4 |
| 93 | 100 | 0 |
| 84 | 115 | -2.4 |
| 60 | 162 | -5.3 |
| 20 | 198 | -6.8 |
| 350 | 178 | -7.2 |
The CC2 amplitude changes by factor of 2 from squeezing to anti-squeezing.
From the ratio of CC2 max/min amplitude, the nonlinear gain can be obtained. According to Aritomi's PhD thesis P.55, CC2 max/min = (1+x)/(1-x) = 198/84.8. From this equation, x = 0.4 and the nonlinear gain is g = 1/(1-x)^2 = 2.8. However, this is not consistent with the measured nonlinear gain of 4.5.
Another concern is that the minimum(maximum) CC2 amplitude does not correspond to squeezing(anti-squeezing) quadrature.