S. Dwyer, R. Short

This afternoon we set out to better align the filter cavity. I started by locking the FC in green (just by taking the SQZ_FC Guardian up to 'GR_SUS_LOCKING' and switching the FC Green 'on') and trying to improve the alignment with FC1 and FC2. Ultimately I didn't get much improvement and the green transmission (FC_TRANS_C_LF) was around 8mW with the cavity locked. I then used the seed beam to dither lock the OPO (seed power was still set low at ~0.5mW) and ran a mode scan on the VCO as in alog92044 with the command z step H1:SQZ-FC_VCO_TUNEOFS '+0.01,1200' -s 0.5

The TEM00 mode during this scan was at a VCO offset of around 3.6 and an IR transmission (FC_TRANS_D_LF) of 1.4mW, which possibly saturated the diode. We can clearly see FC2 length following as the frequency was changing, which caused the green transmission to slowly drop (ndscope screenshot of the scan attached). We checked the FC2 L2P gain and found it at 6.4 with just a "1/1000" scalar in the filter bank. It's likely this isn't the completely correct configuration, so in the near future we'd like to run measurements on the suspension to figure out what the exact filters and gain should be for FC2 L2P. Once that happens, the modescan should show no drop in green transmission, which will make aligning the FC in this way easier.