[Aritomi, Yuhang]
First we fixed DDS AA phase to 150 deg and optimized I/Q demodulation phase again. As long as FC alignment is good, changes of the optimal demodulation phase are within a few degrees. During this measurement, we checked that WFS1 I3/Q3 coupling is less than 3%.
| segment | WFS1 1 | WFS1 2 | WFS1 3 | WFS1 4 | WFS2 1 | WFS2 2 | WFS2 3 | WFS2 4 |
| DGS demod phase | 100 | 105 | 104 | 99 | 135 | 136 | 137 | 135 |
Then we injected a 12Hz line to INPUT PIT and measured sensing matrix, but there is still large PY coupling. We found that 12Hz peak heights on each QPD1 segment are quite different (following table).
| segment | WFS1 I1 | WFS1 I2 | WFS1 I3 | WFS1 I4 |
| 12Hz peak height | 20.52 | 10.21 | 17.05 | 11.65 |
This gain unbalance may cause the coupling problem. So we compensated this gain unbalance in matrix in DGS (attached picture). Each number in the matrix is decided by 10/(12Hz peak height on the segment). In this case, there is no coupling in WFS1 I YAW for INPUT PIT driving, but there is still 16% coupling in WFS1 I PIT for INPUT YAW driving.
After that we aligned FC well and measured 12Hz peak height again. This time the gain unbalance is different from previous measurement.
| segment | WFS1 I1 | WFS1 I2 | WFS1 I3 | WFS1 I4 |
| 12Hz peak height | 16.77 | 11.06 | 19.73 | 9.53 |
To decide the gain unbalance precisely, we will check PDH signal on each segment and calibrate it by sending a 12 kHz line to PZT as we did for FC PDH signal. 12kHz is within DGS bandwidth and it is around UGF of FC loop. Since what only matters is ratio of gain of each segment, it is not a problem even if the injected line is suppressed by FC control loop.
