Last night we moved the SR3 heater, and while doing this we found that kappa C (which is supposed to be proportional to DARM optical gain) went the opposite way of the range. In particular, when Georgia moved the SRC alignment in yaw, we saw kappa C increase by about 1%. This made us think we were winning with DARM response. However, when we looked at the DARM spectrum we found that the response to the 331.7 Hz PCAL line had gone down. Since the PCAL line signal had not changed, this means that the DARM optical gain actually decreased. This means that kappa C is going like inverse DARM optical gain. According to documentation (T1700106), kappa C ought to be proportional to optical gain.
Lili, Danny, Sheila
Last night we looked through the front end calculation of this one step at a time, and indeed the front end does estimate that the DARM response to PCAL at 331Hz is increasing at this time. Lili has now convinced Danny and I that this is all right, that the reported change in cavity pole frequency and kappa_c are together correctly explaining the change in the magnitude of the sensing function as measured by the 331Hz line.
Ignoring the optical spring, the expression for the (normalized) time dependent part of the sensing function is kappa_c/(1+if/f_c), which I think is what the quantity called S1 in the front end is representing. The magnitude of S1 increased from 0.765 to 0.775 from 2019/04/11 03:48:47 UTC to 2019/04/11 04:07:59 UTC during the SRC alignment test show in the attached screenshot (48406) This is pretty well explained by plugging the reported cavity pole increase from 377 to 407 Hz and the decrease in kappa_c from 1.02 to 1, this would result in an increase in the magnitude of the sensing function at 331Hz even though it is a decrease in kappa_c.