Sheila, Kiwamu,

Tonight, we continued working on searching for the missing recycling gain. We opened up PRC1 (which uses POP_A_QPD and feeds it back to PRM) and moved PRM manually to study the effect of the spot position on PR2. This seems to work well and we were able to get back to a high recycling gain of about 34 when PSL was 25 W. The attached shows the time series of us moving PRM in pitch toward the negative side by about 90 urad. The first drop in the power recycling gain is caused by the power-up from 2 to 25 W. We had to move SRM by hand in order not to loose too much sideband build up in SRC. Our alignment action seems to pull the common soft (or something similar) to one direction in which all the test masses goes to negative pitch values (see the second attached). Therefore, as we were in O1, at this power level, the recycling gain can be still found under some alignment land. We did not try this test at a higher PSL power.

Also, we have noticed at some point that some segments of the POP QPDs already used up the half of the ADC range at 25 W. So we decreased the whitening gains to 0 dB (which were 6 dB before). Sheila changed the guardian accordingly so that it maintains the 0 dB gain as a nominal setting. Also when we had a good recycling gain with this alignment, the beams on both POP QPDs were almost falling off of the photodiodes