Today we made the first attempt at the recovery of the DRMI after the commissioning of the CHRoCC. The purpose of the activity was to relock the DRMI with the CHRoCC at nominal power and study the usual figures of merit of the ITF. Here is the summary of the activity:

• after a full pre-alignment, we took some free-swinging data of the DRMI, starting at 12:26:15 UTC (+285 s, due to an unlock of the arms);
• given that the flashes on the usual figures of merit (B4 DC and sidebands, B1p_DC) were reasonably similar to what we were used to, we immediately tried to lock the DRMI; looking more closely, though, we could see that, while the sidebands magnitudes had lower peaks than usual, B4_DC had higher flashes; some of them were high enough to have B4_PD1 Vbias to trigger and disable the PD (example at 12:44:57 UTC, B4_DC ~= 74 mW);
• the lock was actually achieved pretty quickly, without any major tuning of demodulation phases, while loop gains were impacted slightly more;
• we debugged a BS local control issue which was causing additional noise, then we did the full recovery of the DRMI_1F configuration, PR and BS alignment loops included;
• we have a long stretch of data starting from 16:05:00 UTC:
  • up until 16:29:53 UTC we were in the nominal configuration, untouched (we put an offset on PR_TX = -0.025); we can observe that the sidebands and DC level are lower than usual (e.g., B4_12 ~ 0.08 instead of 0.1); also, control signals are generally noisier;
  • then it was decided to change already the CHRoCC power, by setting the voltage to 0.5 (from the initial 0.43); this happened at 16:29:53 UTC; this had the (rather) quick effect of degrading all the figures of merit, especially the sidebands;
  • at 16:59:56 we decided to revert the CHRoCC setting back to 0.43 V, but we unfortunately unlocked shortly after, at 17:00:51 UTC;
  • we could relock the DRMI at 17:23:58 UTC, when the figures of merit were already in an intermediate state due to the thermal transient;
  • then, at the end of the transient, I started to look at the ASC working point; clearly BS_TY needed an offset, which improved the situation; later, we unlocked at 18:10:10 UTC;
  • at the next relock, at 18:22 UTC, the figures of merit were already better with respect to when we started; we set again offsets on both BS TX and TY, however in this case the behaviour of the fringe with respect to the offsets is less clear, and we will need to investigate; an unexpected outcome is that, in this lock, the PRCL optical gain was quite higher than before (more similar to the old one), and the loop gain had to be lowered;
• we unlocked manually right before 18:36 UTC; a trend of the activity is in Figure 1.