Friday night Rana and I re-measured the relative intensity noise (RIN) in the carrier and 9MHz sideband at the OMC DCPD.  We believe we got a lower noise measurement (first attachment) than what Craig and Koji measured previously (LHO 46586) by:

• increasing the OMC length dither amplitude by x300 (the dither amplitude had been tiny, we should probably address this permanently)
• using PRMI lock for the 9MHz measurement

For the carrier measurement used single bounce off of ITMX with 25W input, and for the 9MHz used PRMI (where the 9MHz sideband is largest) with 35W input.

We don't see any excess RIN in the 9MHz above 300 Hz.

1233733816 carrier measurement
1233738050 9MHz measurement
1233738400 dark noise measurement

1) the carrier RIN in your plot at 30 Hz is in the 1e-7 region. Do we believe this excess above the ISS second loop measurement and above the noise floor? Or is it OMC length noise?

I’m asking because a RIN of 2e-7 is what we need to be limiting DARM, see https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=46759

If we believe the carrier single bounce RIN measured by Jamie and Rana, and use the radiation pressure intensity noise coupling (based on 8% power imbalance and matching Craig's measurement in 46817), we get something pretty close to the DARM noise:

2) same question for the excess SB RIN? Is that PRMI control noise?

For 35W input power, the ISS inner PD shows 27.3mA, whereas the outer shows 29.7mA.

Note I think it's possible that we were looking at the 45MHz sideband in the PRMI lock, rather than the 9MHz sideband.  We will attempt to retake this measurement once the IFO is recovered.

it seems that the OMC length lock is not strong enough to measure low RIN - this is why the increase of the OMC length dither decreases the RIN.

Most of the low frequency peaks are due to acoustics - we were using the QPDs for OMC-ASC instead of angular dither.