This alog builds off of the work done by Joshua to measure PR2 and PR3 BOSEM coupling in alogs 80732 and 80814. His injections showed that PR2 coupling to DARM occurs up to about 12 Hz, and PR3 coupling up to 25 Hz. To note: these injections were performed above 10 Hz, and the measurements were able to pick up some coupling between 7-10 Hz as well. However, I think the most significant coupling, and limitation of the BOSEMs likely occurs in the auxiliary controls below 5 Hz. There is some indication of that from these measurements.

I used his injections to investigate the pathways of this coupling. I modeled my scripts after the scripts that Sheila has written previously to measure couplings to DARM through multiple pathways, as in 80730.

I made two plots, one showing each PR2 bosem contribution to DARM and one showing each PR3 bosem contribution to DARM.

There is also a set of plots attached to this alog that show each PR2 and PR3 bosem coupling to DARM projected through MICH, PRCL, SRCL, CHARD P and Y and PRC2 P and Y (PR3 ASC, PR3 LSC, PR2 ASC, PR2 LSC).

Overall, these results indicate that PR2 and PR3 coupling to DARM through ASC and LSC loops are fairly minimal. The PR2 and PR3 noise seems to show up most strongly in RF9, so there is strong coupling to PRCL and PRC2. I predict there is probably coupling to CARM, especially for PR3. As a quick test, I compared the LSC REFL RF9 Q error signals during the injection and saw coupling of the PR3 BOSEM noise up 30 Hz (dtt screenshot, where reference trace shows quiet time, live trace is during the PR3 T2 BOSEM injection).

As an example, I projected the BOSEM noise to PRCL, PR2 projection and PR3 projection (the result is uncalibrated). I think if we injected at even lower frequencies, we would see that PRCL, is greatly limited by this noise.

If we would like to do further tests to investigate this coupling, we should: inject at lower frequencies, open the POP beam diverter to see the appearance of this noise on the POP WFS, and move to 1 CARM sensor to see the appearance of this noise on an out-of-loop CARM sensor. The POP WFS test would help us understand if, for example, this excess noise on PR3 is causing some sort of jitter that couples to DARM. This would help further understand why PR3 BOSEM seems to couple strongly to DARM up to almost 30 Hz, but only appears weakly in the auxiliary DOFs.

However, it is also important to finish investigating the contributions of all triple suspensions, so PRM, BS, and all the SRC mirrors.

I made all of these plots using the utils code from the aligoNB repository. All code, DTT templates and plots can be found in my home directory: /ligo/home/elenna.capote/BOSEM