On Tuesday (May 17th) was performed a FIS measurement with the squeezing beam going to the retroreflector on SQB1.
We put the retroreflector into the beam path (SQB1 pico, from 0 to 1.2 million steps) and verified the alignment on EQB1 (PD_MONI, cams & homodynes) by tuning the tilts of the retroreflector, first using the SC (which was preemptively aligned to the FC and in reflection into EQB1 with >3% misalignment, see a previous log entry this week by me (ding)), then using the CC+SQZ beams to maximize the contract on the HD when beating with LO.
Good alignment of CC-LO on HD was reached at 13:23 UTC (check EQB1_HD_DIFF_RF_4MHz for the fringes).
A first phase scan from 4 to 0 rad was performed at 14:29:00 UTC. Fast gain of CC loop was at 100, coarse at 0.5 (see the corresponding channels in datadisplay). We reached about +6.85 dB of ASQZ and -4.66 dB of SQZ.
Then the fast gain was increased and a realignment was done to optimize this measurement. At 17:13 UTC a quick scan of the demod phase was done, going from 0 to 3.14 rad in 20s. See plots attached. We reached the following :
- parametric gain g=2.42 (7.7 dB)
- measured antisqueezing A=2.29 (7.2 dB)
- masured squeezing S=0.555 (5.1 dB)
- optical efficiency (g-1)/(A-1)=91%, i.e. 9% optical losses
This nicely compares with the squeezing measurements from EQB1 (at ~6% optical losses), considering the 3% measured extra losses on SQB1 due to clipping/ HWPs in the double pass FIs.
For reference, a shot noise measurement attempt was made around 17:22:20 UTC. Beware to check the OPA_status to make sure that the OPA was locked during the measurement.
Then we started to suffer from OPA unlocks due to humidity changes and we moved away the retroreflector.
NOTE : The Retroreflector either : has a strong hysteresis, or has a problem of getting stuck in the translationnal stage, since we needed to go up to -800000 steps to bring it away (was initially away at 0 step). The mechanism has to be thoroughly checked the next time we open SQB1, since a critical failure on the retroreflector position mechanism means the beams cannot access the FC and FDS is impossible.