= Trying to find the noise sources, Cont'd =

Today I tried several things in a random way.

0. recovered the coupling of green beam to the fiber coupler

1. rotate the polarization to EOM => No change observed
We thought RFAM might be a coupling path from jitter to the sensing of PDH, so rotated the polarization to EOM to see if any change in PDHX control signal occurs or not.
I put a half-wave plate and rotate it with 2 deg steps, but did not see any changes. After this test, I removed the half-wave plate.
See the photo.

2. switch off "Noise Eater" of PROMETHEUS => No change observed

3. put an iris on POP table to reject some junk light => No change observed
I put an iris in the main path and the path from the FI to RFPD, and neither made changes.

4. tap optics on POP table => confirmed that the structures around a few hundred Hz come from mirror mounts on POP table

5. try to damp the mode of a mirror mount => failed
I tried with sticky tape to damp the mode, but I could not make it. Probably we need a better tool like rubber?

6. tap fiber on POP table => broad noise floor from ~100 Hz to 1 kHz appeared
I feel that it is easier to excite the noise floor by tapping the fiber lying on the table than hung part of the fiber.
The following plot shows the behavior of the green transmission when I tapped the fiber on the table
I start suspecting that fiber phase noise might be ~1Hz/sqrt(Hz) or more in a broad frequency region.