Vicky, Sheila, Naoki
First we tried SRCL offset of -400, which looked good in yesterday's FIS SRCL offset measurement 79903. We took the calibration measurement with SRCL offset of -400 in 79911, but Louis reported in the mattermost that there is a large optical spring in the sensing function. Also, FDS with SRCL offset of -400 is worse than nominal. Vicky will add more plots for this.
Then we decided to change the SRCL offset to -290 and optimized FC detuning. This improved the sensitivity below 100 Hz as shown in the first attachment and improved the range by ~5Mpc. The optimal FC detuning changed from -34 Hz to -28 Hz and this could be because of SRCL offset change and arm power change.
After FC detuning improvement, we took the calibration measurement with SRCL offset of -290 in 79922, but the measurement did not make sense according to Louis so we took an another calibration measurement with SRCL offset of -290 in 79928.
Following up with some FIS SRCL measurements from today as we were navigating how to best optimize SRCL offset for squeezing, which gives best sensitivity around 100 Hz.
Blue & purple traces - When first looking at SQZ after re-calibrating at -400 counts srcl offset, SQZ looked kinda v-shaped, like as if the SRCL detuning is big. We first tried optimizing the sqz angle for the bucket (blue, CLF RF6 demod phase @ 222 deg), then tried to optimize high-freq sqz (purple, CLF RF6 @ 215deg). For this SRCL offset at -400 counts, with about (222-215=) 7 degrees difference on the CLF RF6 demod phase (bad estimate is ~3.5deg diff on sqz angle), this changed the kHz squeezing level by about 1.9dB. See the trends on this screenshot.
Yellow trace - We then tried to see if FIS + SRCL @ -400 counts was the same as yesterday lho79903 and yes it was same. But zooming into yesterday's plots, it looks like this -400 SRCL offset setting (yesterday's blue trace) was actually not a great spot (already well passed zero detuning), as there is a little bit of ballooning between 100-200Hz that we did not notice yesterday.
Given that squeezing, and also calibration, saw that this SRCL offset @ -400 cts had a bad spring effect, we backed it off to -290 counts and took another cal meausrement. We chose -290 to be ~halfway between the -475 ct (pink) and -100 ct (black) settings tried yesterday (see dtt). For -475 ct and -100 ct, we realized today that DARM between 250-500 Hz had about the same level of anti-sqz coupled in by the SRCL detuning. Unsure if this means they have the same physical detuning, this is something we will try out in quantum noise models to understand better.
Green trace - shows FIS + SRCL @ -290 counts. It is where we expected from yesterday. Leaving it here.
In summary, today we tried a few methods of "optimizing srcl detuning," to get it closer to 0, but also realized we need to think more carefully with quantum noise models like, what is the easiest / most sensitive metric for squeezing to see the srcl detuning.
- looked at mid-squeezing, where the high freq squeezing angle matches no squeezing. the hope was that at this angle, we are mostly seeing phase variations across the band, using no-sqz as the reference. -400 ct was noticeably less flat compared to no-sqz than -290 ct.
- while always maximizing 1 kHz squeezing, try to see if we can "flatten" darm in the 100s Hz band. This was a kinda confusing to evaluate because QN in the bucket is a very marginal/small effect you have to squint closely to see. We tried to do this yesterday and only in hindsight noticed that -400 counts was not good in yesterday's plots. From models, we can look for what do we even want / expect for 0 detuning.
- while maximizing bucket ~350 Hz squeezing, try to see what is the best we can do for darm noise in the bucket.
We then moved onto FDS + SRCL @ -290 counts, and optimized the filter cavity detuning, as Naoki describes above.
Adding plots of the sensing function. From these measurements, we see that the sensing function is also an indicator of bad/good SRCL offset. Additionally, *something* changed from Thursday to Saturday, as seen in the Saturday calibration measurement trace.
Figures (1) and (2) are the different sensing functions where the second figure ranges from 0-40 Hz. The uncertainties of each measutrement are plotted in figures (3) and (4), where figure (4) ranges from 0-40Hz.
