J. Kissel

I've analyzed the measurement data we've taken to quantify the DARM loop's sensing function (via frequency-dependent sweeps) we've been taking since the start of O3. For each data set, I've fit the data using the standard MCMC technique, and plotted the equivalent of the time-dependent correction factors as a function of time and commissioning events.

See three attachments:
(1) The collection of all processed data plotted against its MCMC fit (note, none of these are corrected for time-dependence as we're trying to *expose* the time-dependence with these plots)
(2) A time series of the resulting MCMC fit parameters for each measurement that we *would* normally use to correct the data sets for time-dependence (note, we don't use those TDCFs computed by the calibration lines because the lines must be OFF during the sweeps to preserve PCAL actuator range)
(3) A continuous time-series of (some of) the time-dependent correction factors as a function of measured PSL input power and SR3 disc heater. (note this trend was taken on 2019-06-21, so does not include the most recent sweeps from 2019-06-27, but the and no major commissioning events have happened since and IFO has been roughly stable.)

The commissioning events that I've highlighted that believe would have impact on the sensing function are as follows (all times UTC):
    2019-04-01 15:00     O3 Start (PSL:35W, SR3:5W)
        -- start of the observing run;measured PSL input power is (roughly 35 W), and the SR3 disc heater is set at 5W.
    2019-04-26 16:51     SR3 Heater 5W>4W, Pwr Osc Start 
        -- when the SR3 disc heater is first reduced from 5W to 4W requested power, a drop in measured temperature on its thermistor from 87 [deg C] to 75 [deg C]. From here until May 10, the ASC system is not tuned, which results in large (1% level, 2 minute period) power fluctuations in the arms.
    2019-05-10 05:45     ASC Tuned Up, Pwr Osc Fixed 
        -- ASC system tuned up, no more oscillations
    2019-06-04 15:45     SR3 Heater Power Fail 
        -- The disc heater power failed due to some computer problems. Upon restart, the temperature started out a bit high, and did not settle back in to "nominal" 75 deg C until a few days later.
    2019-06-10 18:17     PSL Power 35W>37W 
        -- major commissioning effort to increase the laser power from "35W" (roughly) to "37W" (roughly) succeeds and we start permanently running at "37W."
    2019-06-11 15:00     SR3 Heater Temp Stable 
        -- here's when the SR3 disc heater temperature recovered to 75 deg C and has remained there.

One can see lots of things:
(A) by looking through the processed measurements of the sensing function, one can clearly see that
    (i) the MCMC is still not doing a quality job at fitting the detuned SRC optical spring. Some argue that this is not an optical spring, but an artifact of our drive system and L2A2L coupling, but for the purposes of this entry, and for lack of a better term, we'll continue to call it the detuned SRC opical spring.
    (ii) There is no obvious change in shape between 2019-04-17 and 2019-05-02 measurements when the SR3 disc heater power was reduced from 5W to 4 W.
    (iii) The shape of spring changes significantly between 2019-05-09 and 2019-05-22, remains the same until the 2019-06-10 measurement, just after the power up from 35 to 37 W.  Perhaps this feature is much more dependent on the ASC configuration than the mode matching of the SRC? 
(B) The relative change in optical gain change matches well between calibration line continuous measurements and the MCMC fit.
(C) The cavity pole frequency also agrees between continuously measured value and the sweeps -- though it would be really nice to see these on the same plot (working on it.)

There are more things to find and interpret from this collection of plots, and I'd like to make them better, but this analysis is LONG overdue, so I want to post what I have for the time being and continue efforts otherwise.

Upcoming work:
    - acquire minute trends of continuously measured TDCFs (all of them, including detuned SRC spring frequency) from the frames and plot them against these MCMC computed values
    - process (and continue to gather) sensing function measurements using PCAL X instead of PCAL Y
    - gather data comparing sensing functions against moving spot positions

Thank you all for your patience with me.