Jenne, Sheila, Hang

We worked on engaging the 90 MHz centering loop tonight.

We first locked the ifo in the DRMI_LOCKED stage using the old DC centering loop. Then we opened the centering loop and the src1 loops and adjust the phases for the 90 MHz WFS. The procedure was that we first  adjusted each seg. individually such that the length signal showed in I phase of each seg., to fix the relative phases between quadrants, then we drove a line by exciting one of the OM mirrors pitch (yaw) and rotated the overall phases of 4 segments together so that the pitch (yaw) signal showed up in I phases. By doing so we were able to switch to the 90 loop and lock the ifo stablely. We could also directly lock the ifo to DRMI_LOCKED stage with this 90 MHz loop.

The new phases of the 90 WFS are:

However, we somehow kept lossing lock at the DRMI_ON_POP stage with the 90 loop.

We then measured the sensing matrix at DRMI_LOCKED stage, and the results were

So the gain increased by ~20%-40% by using 90 instead of DC centering.

We thought this was the reason of lock loss, but after decreasing the gain digitally by the same amout, we still could not proceed beyond DRMI_ON_POP. The problem remains unknown.