[Aso, Ushiba]

Abstract:

We tweaked ASC offsets and ITMY drive align matrix for ITMY BPC.
We found relatively good configuration, so we keep this configuration today.

Detail:

To obtain better ensitivity, we tweaked IFO alignment.
First, I reverted BPC setpoint and ASC offsets, PR2 PIT and IMMT1 PIT and YAW OpLev values to the one I set the day before yesterday and checked the reproducibility.
However, the sensitivity is worse, so we started tweaking of ASC alignment.

Since the beam jitter coupling around 200-400Hz is large, we started from MICH offset tweaking because beam jitter coupling through MICH is simple and we think it is the most effective.
Then, we tweaked PRC2/INP2 offset to reduce the peak around 116Hz.
Thanks to these alignment tweaking, 116Hz pek height is almost same as the best sensitivity while keeping the peak around 200-400Hz slightly better.
Also, GRX alignment by using POP PICO motors are conducted during the work.

After that, we changed drive align matrix of ITMY to change the beam position on ITMY to reduce 140Hz peak but no effect or even worse when moving beam spot.
So, we gave up tuning of ITMY BPC.

Final setting of ASC offset and BPC are as follows:
K1:VIS-ITMY_TM_DRIVEALIGN_P2L_GAIN = -1.4
K1:VIS-ITMY_TM_DRIVEALIGN_Y2L_GAIN = 0
K1:ASC-PRC2_P_OFFSET = -0.03
K1:ASC-MICH_P_OFFSET = -0.1
K1:ASC-MICH_Y_OFFSET = -0.04
The other WFS offsets = 0

After finalizing the IFO setting, I measured OLTF of DARM loop to confirm there is no significant change (fig1).
Since the OLTF gain change from the last CAL measurement (klog34310) is less than 0.1dB, current calibration should be fine.

Note:

YAW BPC of ITMY improves transmission power without changing optical gain.
Offsets on MICH ASC seems to affect 200-400Hz peaks, which originates from the beam jitters that can be seen in IMC WFS signal.
116Hz peak height can be reduced by changing PRC2 and MICH ASC offsets.