Frank showed me that the RIN level from PMC is too high and caused by back reflection. 

PMC's psd is reduced by blocking the beam going to ACAV and RCAV.  This can be seen

on SR785, by blocking the beam.

So I removed PBS in front of the laser, and replaced it with a Faraday isolator. 

The isolator is temporarily mounted on a post, we will use a more rigid  V-shape block holder later.

The isolator shifts the beam path a bit, and the beam is needed to be re-align to the PMC. 

I couldn't finished it by Friday night, I think Frank re-aligned it already.

I'll measure RIN  and compare them again.