Sheila and I have been investigating the source of the PRCL/DARM coupling that we observe. As a part of this investigation, Sheila has already run two sets of projections that investigate the PRCL coupling to DARM through the other LSC degrees of freedom, and through CHARD P and Y. Those can be found here: alog 80212 and alog 78969.

Today, I took the frequency noise injection from the noise budget, and added that projection to the total. To the point: PRCL coupling to DARM through frequency noise is smaller than PRCL coupling through the other LSC degrees of freedom, therefore it is not a significant source of coupling of PRCL to DARM. Figure 1 shows the projection of PRCL noise to DARM directly, as well as the projection through other degrees of freedom. Figure 2 shows all DOFs projected to DARM.

Some details:

The PRCL and frequency noise injections were rerun as a part of noise budget injections in 80596. The PRCL injection was run while CARM was only on REFL B, to better estimate the PRCL coupling through frequency noise.

The frequency noise injection is run by using REFL A as an out-of-loop sensor with CARM only on REFL B, therefore the frequency noise projection on one sensor will be higher by sqrt(2) in regions where CARM is limited by shot noise. In the past, we've estimated this to be below 4kHz, although we haven't checked recently. The frequency noise projection to DARM in these plots is divided by sqrt(2).

The notebook used to make these plots is located in /ligo/gitcommon/NoiseBudget/simplepyNB/PRCL_frequency_projections.ipynb