As reported in elog2801, we replaced a PD for FC green lock. The new PD has only RF output, but has better SNR. First I checked the PDH signal of FC green lock with the new PD and found that the demodulation phase was not optimized. I changed the demodulation phase of FC green lock (DDS2 CH1) from 354.99 deg to 270 deg. I saved this DDS2 setting as 20220215_dds2.stp.

The FC could be locked with gain of 2.4 for 1/f filter and 0.8 for 1/f^4 filter. The injection green power was 21.1 mW. Since the FC servo gain becomes half with the new PD compared with old PD reported in elog2770, the new PD seems to have larger gain by a factor of 2.

Then I measured OLTF and error signal (EPS1, 1Vpp fixed range) of FC green lock with 1/f and 1/f^4 filters (Fig. 1,2). Both UGF are around 12kHz. The dark noise with new PD is 3e-5 V/rtHz, while the one with old PD is 2e-5 V/rtHz as shown in elog2770. Since the signal of new PD is 2 times larger, the SNR of new PD should be better than that of old PD.

There are many harmonics of 50Hz. These harmonics are not present in dark noise, so they should come from green.

During this measurement, Z correction with gain of 10 was engaged. BS pointing and AA were not engaged since they are unstable. We should investigate these loops.