craig.cahillane@LIGO.ORG - posted 04:31, Saturday 02 March 2019 - last comment - 08:36, Monday 04 March 2019(47243)
9 MHz RIN to DARM as a function of DARM offset
A while ago Georgia and I moved the DARM offset and found that the 9 MHz RIN to DARM coupling decreases with increasing DARM offset. Two days ago, I swept the DARM offset from 4 pm to 14.7 pm while a 70 Hz 9 MHz RIN line was on and present in DARM. I then demodulated the line in software, plotted it as a function of DARM offset, and compared a few models to the data. It seems that 9 MHz RIN to DARM coupling falls like 1/DARM offset. This rules out couplings like those explained by Kiwamu in attachment 4, which increase with increased DARM offset.
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The 1/DARM_offset of 1/sqrt(DCPD) coupling can be explained by considering a CONSTANT amount of 9MHzHOM coupling to the DCPDs, while the optical gain scales with the offset, such that the same amount of 9MHz RIN at the DCPD is interpreted as a different noise level in m/rHz.