We noticed wide angle scattered light behind the PBS in front of RCAV. The scattering source is probably the curved mirror behind RCAV AOM. We borrowed the similar mirror from 40m and will try to compare them.
The wide angle scattered light behind the PBS in front of RCAV might contribute to the noise in beat signal. The picture shows the scattered light with area larger than the half inch PBS cube. This picture was taken when the beam's polarization was changed to P-polarization so that most of the light was reflected from the PBS. With small transmitted light through the PBS, the scattered light can be seen clearly behind the PBS, see here.
After the inspection, it is very likely that the curve mirror behind RCAV AOM is the source. So we borrowed another R=0.3 mirror from 40m to see if it will be better or not, this will be done soon.
Note: during the inpsection, we also identified another bad PBS,pic. This is the one in front of RCAV AOM. Its center surface looks dirty, so we replaced it with a better one.