In light of the recent ASC improvement, we are now curious about how stable the DARM coupled cavity pole is as a function of time.

In order to monitor it in a casual way, I set up a lockin demodulator like I did before (alog 18436), but this time with a Pcal line.

I set up a realtime demodulator in the LSC front end for a new Pcal line that Evan added last night (alog 19823). It uses demodulator 1 of osc 3 and a Pcal line at 325.1 Hz. The demodulation phase was adjusted such that the estimated cavity pole becomes roughly 350 Hz although I did not measure the actual DARM cavity pole via an open loop measurement. This will introduce some bias in the estimated cavity pole, but I think this is fine for now because we are interested in the stability rather than the absolute value. I then edited the ISC_LOCK guardian so that it does not send the OSC 3 excitation to the ETM suspensions any more. The attached is a screen shot of the demodulator setting.

Looking at the spectrum of the demodulated real and imaginary parts, I see that the fluctuation are coming from flat sensing noise (i.e. shot noise). Because of that, a longer integration time would improve the precision of the measurement. I empirically set the integration time (or the cut off frequency of the low pass filters) to 0.03 Hz or 33 sec in order to get a few percent precision. We might try a higher excitation amplitude at some point because I would like to know if there is a fast-varying component at around 1 Hz where some suspension fluctuation may matter. Also, I briefly looked at other Pcal lines of 331.9, 534 and 540 Hz in the frequency domain with a resolution of 0.003 Hz and saw no side-lobes around them, indicating that their performances are also limited by shot noise.