We started from the first modulator (6MHz 8 MHz) where it appeared to be miscentered (see figure 1 and 2, before and after realignment). Then we went through the other modulators, the Faraday Isolator, the IPC and recentered the beam on the alignment/matching mirrors.
Then we finely aligned the beam to bring the correction of the BPC close to 0 (figure 6). The loop closed normally.
This misalignment on the first modulator must explain the beam shape of the IMC reflection that we had before (figure 3) with respect to the one that we have now (figure 4).
Now it looks like we have pure mismatch. We measured it to be 5.7% (figure 5) instead of the about 9% that we had before.
We then slightly realigned the IMC cavity and measured the new sensing matrix of the Automatic Alignment. The signals are now much cleaner. We closed the loop in drift control.
