There are not yet any results to report about the SRCL dither arm power measurement, but I have some notes about the attempts to run this measurement over the last few commissioning periods.

This measurement works by using the radiation pressure coupling of SRCL to induce differential radiation pressure in the arms, which depends on a couple factors, one being the arm power. This measurement is made by injecting into the SRCL control signal, and measuring the transfer function of of the relative intensity noise on the transmission QPDs of each arm to DARM.

Measurement requirements:

• know the calibration of DARM well (must be CAL DELTA L in this case)
• turn off the SRCL feedforward
• Make sure the TMS QPDs are centered
• ensure high coherence (>0.9) of DARM to TMS QPDs

However, I once caused a lockloss running Craig's measurement templates, so I wanted to be sure that I could drive hard enough to get the required coherence, but not break the lock.

Last week, I ran a couple of test measurements with the SRCL feedforward off, and the TMS QPDs as well centered as possible. I have a screenshot of the results below. You can see that the DARM/SRCL coherence is very good, but the DARM/TMS QPD coherence is very bad. This is true for the NSUM channels as well as the individual segments. When I tried to drive harder, I would get ETMX saturation warnings. The signal appears almost four orders of magnitude above the noise in DARM and about one order of magnitude above the noise in the TMS QPDs.

The centering loops we use center onto QPD B of both arms, but cause the beam to be off center on QPD A in both arms. However, the coherence is equally low on both the A and B QPDs, so I don't think the centering is the issue. Craig and I previously tried centering onto the A QPDs, but then you become miscentered on B, and also cause other problems, see 67066.

Today, I instead drove single lines using awggui. I was able to achieve >0.9 coherence at three different frequencies without causing any saturations anywhere, at about 70 Hz, 49 Hz and 34 Hz. Good coherence was observed with both the NSUM channels and the individual segments of the TMS QPDs. I also confirmed that we see the expected differential phase in the signal between the X and Y arm.

We were not thermalized today, so I plan to rerun this measurement on Thursday using these three points. This should be sufficient information to measure the arm power, and also confirm that we observe the predicted 1/f^2 coupling. I can also use the regularly scheduled Thursday calibration measurements to make sure we know the CAL DELTA L calibration.