Naoki, Andrei, Camilla, Vicky

To investigate the effect of vacuum pressure incident in HAM6 reported in 78346, Peter suggested to measure the FC loss since the FC should be the most sensitive cavity. We tried the FC ringdown measurement, but the result does not make any sense. We will take the data again.

Previous ringdown measurement: 66403, 67273

For ringdown measurement, we need the cabling as written in 67409 to lock the FC green with SQZ laser VCO since the SQZ laser frequency noise is too much for ringdown measurement with only TTFSS. We did only the first and second cabling in 67409. The third and fourth cabling is necessary if you want to do homodyne measurement.

After the cabling, we requested OPO guardian to LOCKED_SEED_DITHER, but the dither lock did not work well. The seed stayed around 25% of maximum seed trans. We confirmed that the OPO CMB EXC is connected to some DAC output which should be dither signal. Although the seed is not on resonance of OPO, the seed power seems stable so we moved on.

Then we manually locked the FC green. We also engaged one boost of FC green CMB. We adjusted green VCO tune to get the seed on resonance of FC. 

We increased the seed power to 2.7 mW. More than 2.7 mW seed saturated the FC trans IR PD.

We looked at H1:SQZ-FC_TRANS_D_LF_OUT and H1:SQZ-OPO_IR_PD_LF_OUT for FC TRANS and REFL since they are 16k channel. We maximized the seed FC trans by adjusting green VCO tune and requested OPO guardian to DOWN.

The attached figure shows the ringdown measurement. The decay time of FC TRANS and REFL are quite different and both of them are much larger than the expected 4 ms decay time. Also the decay start timing is different for TRANS and REFL. The decay of FC TRANS is not going to 0, which means there would be residual seed. The requesting OPO guardian to DOWN might not block the seed enough and this might be related to the dither locking, which is not working well.

EDIT: There is an elliptic low pass filter LP100 in FC trans IR PD, which would explain the different behavior of TRANS and REFL. We should turn off LP100 for ringdown measurement.