Craig, Corey, Nutsinee, Terry

Earlier today Hang tried turning on a res-G in PRC2 P to stabilize our IFO alignment.  This did not succeed.
During this time he thought about the PRCL error signal depending on both the sideband and carrier gain in the PRC, and shared with me an idea about the error signal potentially changing sign with low enough sideband gain (Eq 19, T1500325)


I monitored the optical gain drop of PRCL during the last power-up by injecting a line into PRM at 138 Hz and watching the line response during the power-up.  It dropped by about 60% before we lost lock.  

Our PRCL UGF is ~60 Hz nominally.  I adjusted the PRCL1 digital gain to compensate for the drop in PRCL optical gain to keep the UGF at 60 Hz.  The PRCL1 digital gain started at 11.0, I moved it to 14.3 then 17.2.

I did the same for SRCL.  It's nominal UGF was 20 Hz, I adjusted it up by 25% to keep it there, from -8.8 to -11.0.  The total SRCL optical gain fell by 30% from IFO thermalization from INCREASE_POWER to end of lock.

I also measured MICH, whose nominal UGF is 9 Hz.  MICH optical gain did not seem to change during power-up.  

All this corroborates the POPAIR B 18 and 90 MHz story: The 9 MHz falls significantly due to IFO self-heating, but the 45 MHz does not change at all.

Holding the LSC gains steady by hand during the last lock did not improve IFO angular stability.

Posted are the before and after IFO thermalization of the lines I injected into PRCL and SRCL during power-up.