Yuhang, Aritomi, Michael

From 2024-01-09

We optimized IR transmission from the OPO. The BAB transmission through the OPO when locked for IR is 320 uW. However, the ppol frequency went to 245 MHz - previously, the "IR only" ppol frequency for OPO was 220 MHz, so it has changed quite a lot in a short time, so this OPO/ppol frequency issue might be independent of green.

To align the filter cavity for IR, we do 1) align to reference targets on PR window, 2) align injection beam with vacuum tube targets, 3) check reflection on squeezer bench, 4) lock FC for green and adjust detuning of IR (green AOM) to find IR resonance, 5) align IR to maximize IR FC_TRA. This constitutes locking of the filter cavity ready to measure FDS. The infrared reflection signal is obtained through a 0.5% transmission polarizer, so quite difficult to see. Anyway after just a small adjustment of input steering yaw I could see IR reflection on a sensor card on the squeezer bench already, so we really didn't need to do much intermediate alignment. It was moving quite a lot (there was an M5 earthquake in Niigata at the time). The reflected IR power at first glance was about 240 um, 75% reflection. 

Eventually we could see flashing of the IR at about 109.037 135 615 000 MHz on the AOM detuning. On second inspection of the targets in the vacuum tube the GR/IR overlap seems quite bad though, so it will have to be fixed.