On Friday morning the AEI source was switched to squeezing mode. We engaged the automatic alignment on the homodyne detector to optimize the relative alignment of CC and LO beams, then we optimized the centering of HD photodiodes by translating the phododiodes to minimize the signal at M4 angular dither lines; we then engaged the coherent control with the usual procedure. We did not manage to increase the gain beyond 4000 with filter 1 without inducing an high frequency oscillation, and the UGF was limited to about 100 Hz in such conditions. We also maximized the LO power by acting on the waveplate before the PBS on LO path.
With the CC engaged we scanned the 4 MHz phase; we modified the proportional gain along the phase scan according to the change of error signal slope. Looking at the spectrum of HD audio channel, we found a maximum squeezing at 2.3 rad CC phase, and maximum anti-squeezing around 0.9 rad CC phase. We then tuned the LO telesope looking at the 4 MHz beat magnitude to slightly improve the mode matching. The measured squeezing level was about 1.7 dB and anti-squeezing was about 3.2 dB, see attached plot. We will need to search for extra losses on the SQZ path and to understand how the TF for CC loop has changed.