Yuhang, Michael
From Thursday last week. We made many fixes to just about all parts of the frequency independent squeezer.
SHG
The RFPD that detects the SHG error signal was moved to the reflection of the first dichroic in the green path, aided by a lens and a green block filter. The error signal becomes like the reference on the wiki. We took the open loop transfer function eps1/eps2 on the spectrum analyzer but it was strange compared to the reference version (Aritomi thesis). We tried on the old spectrum analyzer and it seemed fine, with the correct low frequency response and first mechanical mode at 21 kHz. So maybe we just missed some settings. We changed the servo gain to give 2.2 kHz unity gain frequency and 45 degree phase margin. Coherence was close to one apart from a spike at 500 Hz.
OPO optimisation
The OPO was optimized using a fast method: scan green phase (ramp signal sent to green phase shifter high voltage driver) without locking OPO and measure amplification/deamplification of BAB. We saw that for some reason the green beam was greatly misaligned in pitch on both the filter cavity and OPO paths, so we adjusted with the first steering mirror after the green FI. Then green to OPO was maximized using the second 45 degree incidence mirror after GRMC transmission to maximize nonlinear gain. We saw:
74 mV without green
268 mV with 25 mW green, before optimization
496 mV after optimization
giving a nonlinear gain of 6.7. This is really quite high compared with what we saw before. We did not see the need to adjust temperature (though maybe we should).
IRMC
We had issues with mutual lock of SHG and IRMC ever since switching to the one EOM scheme. Currently SHG and IRMC are both modulated with one DDS channel and demodulated with one DDS channel. We took some cables to add phase delay to the IRMC and got the good PDH signal. So now it locks properly, although the threshold needs to be optimized a bit.
Homodyne alignment
The homodyne was realigned and balanced. The coarse alignment steps are as usual:
1) Block all OPO side infrared signals from reaching the homodyne
2) Lock IRMC to send local oscillator to homodyne
3) Adjust pitch and yaw of lens closer to IRMC to bring the homodyne photocurrent close to zero (will go far down if misaligned)
4) Adjust pitch and yaw of lens closer to edge of table to bring homodyne photocurrent close to zero (will go far upif misaligned)
5) Adjust balancing beam splitter to finish bringing the HD signal to zero
Then local oscillator is aligned to alignment mode cleaner for fine alignment.
The spectrum of the homodyne output was seen to be flat to about 40 Hz, with 100 Hz peak from ceiling light. We turned off the ceiling light but then it somehow turned back on. We decided to investigate using the security camera system but could not login.
After LO is aligned, the squeezed path is aligned by sending BAB to the alignment mode cleaner, but first we have to lock OPO with ppol.
GRMC
For whatever reason GRMC misaligned a lot as well. Maybe had something to do with the green misalignment in pitch above. During this process we could see the filter cavity reflection moving a lot on the green injection mirror though (there was an earthquake), so the green beam shouldn't be too far misaligned to FC.
PPol
The ppol frequency was changed to optimize BAB transmission (without green). It goes to 220 MHz, down from 260 MHz some time ago.
CC
On this day we saw that the CC PLL had erratic locking. The sidebands would come close to lock and then eventually quickly be ejected away from the resonance point.