Marc, Michael, Yuhang

Today we wanted to do 2 activities in parallel :

1 compare the oplev and AA snr

2 measure the OPO NL gain.

We locked the FC with all loops on green for activity 1 and locked OPO while sending 10 Hz line with 1 Vpp on GR phase shifter for activity 2.

We found out that this caused an increase of Oplev (especially end mirror) and AA noises level as shown in figure 1. The green and brown reference curves are without beam dump, the blue and red are with deam dump and have the expected noise level.

By moving a beam dump from in-between the GR phase shifter and the GRMC to just before the OPO we could remove this new noise.

Therefore it seems that the green back-scattered light is the culprit.

We'll do further check to understand the coupling path and mechanism of this noise.