Previously we have seen that moving to in-vac REFL9I for control of CARM has led to worse DARM noise at high frequencies. So as a first step in diagnosing the issue, I wanted to check the phasing of REFL9.

With DRMI locked, I drove a line in PRCL at 212 Hz and then adjusted the in-vac REFL9 LO phase shifter in order to minimize the appearance of the line in REFL9Q. (Since we don't have CARM at the moment, PRCL is the next best thing. Of course, the phasing should be rechecked for CARM once we are back to full locking.)

Originally the phase shifter had 16+4+1/4+1/8+1/16 = 20.44 ns delay, with REFL9Q/REFL9I = 0.15/0.85 = 0.18 at 212 Hz. Now the phase shifter has 26+4+2+1+1/4+1/8 = 23.38 ns delay, with REFL9Q/REFL9I = 0.005/1.08 = 0.008.