Sheila, Alexa, Kiwamu

Even though we swapped the oplev laser for BS recently. The BS optical level laser power started showing another power modulation, which resulted in kicks on BS through the damping loops.

For now, we recentered the beam on the QPD to reduce the intensity coupling.

The power modulation looks more like a step which randomly shows up as a function time as shown in the attached screenshot. The modulation depth is roughly 1%. This was actually big enough to cause BS angle agitation which was visible at the dark port digital camera. The coupling seemed bigger in the horizontal direction presumably due to a bigger off-centering in the horizontal direction on the QPD. After the re-center, the coupling seemed to have become lesser, but it is still kicking BS in its angle. This needs to be fixed.