It turned out the noise in AS55 was due to a clipping. After fixing the clipping the noise successfully went down.
I was going to briefly check the clipping and go ahead locking DRMI, but for some reason I couldn't stop myself from working on this issue.
Here is a plot of the noise spectra taken before and after fixing the clipping.
The configuration of this measurement is exactly the same as that I did before (#5595)
(what I did)
+ Locked power-recycled ITMY so that the AS beam is bright enough to work with.
+ Shook BS at 1 Hz in the YAW direction
+ Looked around the AP table with an IR viewer and searched for a clipping moving at 1Hz.
+ Found the first lens in the AS beam path has clipped the beam at the upper side. A tiny portion of the beam was clipped.
+ Corrected the beam height to 4 inch by steering the very first mirror.
+ Raised the height of the lens because it was about 3.5 inch or so.
+ Found the lens had a scratch (~1 mm size ) at 1 cm blow the center on the surface.
=> I tried finding a spare 2 inch lens with a long focul length, but I couldn't find it,
So I left the lens as it is, but we should buy some 2 inch lenses just in case like this.
+ Replaced the 1 inch beam splitter by 2-inch 99% BS so that most of the light goes into the RFPD and a little bit goes into the camera.
The AS55 signal contains more noise than the REFL signals.