Yesterday, before the HPO developed some problem, Rick and I made some ISS measurements. There was no surprise in that frontend signal was good with our without HPO, and HPO output was noisy.

The first image shows the analog DC output of HPO monitor before AOM (sorry for cell phone pictures). Accounting for the DC output level (-5.9V), RIN is about -56dB/sqrtHz at 10Hz, -116dB/sqrtHz at 10kHz. (This is about 16dB of so smaller than Peter's plot in alog 26832 which is labeled as "relative power noise". It seems to me that his plot agrees very well with my raw data, not RIN, so that's probably what that was.)

The second picture shows a temporary PD we set up on the table to measure the frontend output, stealing the light coming to the diagnostic bread board. (Though we were quite sure that the cable labeled as  "AMP DC" was indeed the DC output of "PD_AMP" diode on D0902114, since that diode is in the HPO box and there was no way to make sure except to open the HPO enclosure, we set this temporary thing up there.)

The third picture shows the output of the temporary PD when HPO was off. Accounting for 2.5V DC, RIN is about -74dB/sqrtHz at 10Hz, -135dB/sqrtHz at 10kHz. We did not check the numbers but the spectra didn't seem to change with or without HPO.

The fourth picture shows the 500kHz burst with 37kHz repetition rate in either HPO monitor DC or PDA (1st loop out of loop diode) DC. This went away when there was no light but we didn't check the AC out.

We certainly looked at 1st loop out of loop sensor spectrum with and without light, but I couldn't find the pictures. But we thought there was no surprise.