Yuhang, Michael

From 2023-12-30

Don't accidentally send BAB and LO to homodyne at the same time otherwise you will get a large DC signal :O

Replace the power supply of the CC fiber PD

We realigned the LO and BAB to alignment mode cleaner and rebalanced the homodyne in order to reduce the optical loss for the mass data taking. We did some other checks. The CC PLL fiber PD for whatever reason is using a hard-to-find battery (A23 12V). We really should replace it with a power supply. While checking the green path we found that the SHG has some ridiculous 3rd order mode mismatch. It's about 30%. It cannot be reduced by beam steering, laser line filter or adjustment of mode matching lens position, and also doesn't appear to be induced by clipping anywhere. Still, it generates enough green power so it's workable for now. We saw that GRMC could not lock, and it turns out it wasn't even aligned. This is seemingly due to the reason mentioned previously, that the GRPS PZT had some hysterisis from sending the too large wrong signal. In fact, it could be realigned just by adjusting yaw from the steering mirror just past Mach-Zender. This mirror normally shouldn't be used for alignment to GRMC since it is common alignment for arms in the Mach-Zender, but in this case it worked because the PZT introduced a common misalignment. While the SHG has some weird issue, the GRMC still sends 25 mW to OPO at the appropriate MZ offset setting.

We checked CC alignment to OPO (send CC laser to OPO, block LO, check homodyne). It seems fine. So we decided to check again the ppol PLL frequency for 25 mW green and found that the CC1 error signal becomes quite large. The optimal ppol frequency, representing the OPO co-resonance condition between p-pol and s-pol, changed from 160 MHz to 250 MHz to 75 MHz in the space of about 10 minutes. This is some very strange behaviour, later we asked Matteo but he didn't have a suggestion other than multimode laser mode hop, which we are quite sure we do not have at the relevant laser current/temperature settings. Anyway we just decided to keep going on the basis that we had to take data before Hsien-Yi left. So the 25 mW green gives ppol frequency 75 MHz, for now. CC1 error (indicating squeezing level) was 384 mV, a bit larger than last time. 

Anyway, the appropriate signals were sent to DGS:

CC2 eps1 (representing local oscillator phase -> homodyne angle): K1:FDS-FC_GR_CORR
Ramp signal from function generator: K1:FDS-FC_GR_ERR
Homodyne (from SR560 with gain 1000): K1:FDS-ADCspare_1

We ended up taking data at MZ offset 4.1, 4.2, 4.3, 4.4, 4.45. For MZ offset 4.3 and 4.4 I had to reduce the SR560 gain from 1000 to 500 since it was overloading, and then again to 200 for the highest green injection power. This will also increase the proportion of dark and ADC noise. Also, when changing from 4.2 -> 4.3, the ppol frequency went from 75 MHz to 240 MHz - normally it is 160 MHz and doesn't change very much when the green injection power is changed slightly. But we kept on with taking data. During each green injection value I also made sure to check that the scan still covered squeezing and antisqueezing. The scan range also had to be reduced slightly for higher green injection due to the CC2 loop unlocking more easily.

After 30 minutes for each data point, I then took 5 minutes each for homodyne shot noise and homodyne dark noise at each SR560 gain level used in the measurement.