J. Kissel

I've gather a new measurement suite for the DARM loop / Sensing Function -- Results are attach, and sensing function parameters are quoted below.
The interferometer conditions during the measurement:
   - 20 W input power
   - DARM actuator configuration: EY L1, EY L2, EX L3. EX L3 ESD Bias set to -4.65 volts at the (20 bit) DAC, or -4.65 * 40 V_ESD/V_DAC = -186 V at the ESD Bias Electrode
   - all modern (red) ASC loops were engaged including SRC (36 MHz BS control, 72 MHz SRC control). 
   - let the IFO thermalize for ~30 minutes or so before I began taking the measurement. 
   - all calibration lines were OFF (PCAL and SUS).

The optical gain is consistent with what has recently been done to match PCAL calibration lines to DELTAL EXTERNAL (i.e. adjusting the inverse sensing function gain from 1.0 to 1.1).
This is the first quality measurement of the H1 IFO's SRC detuning since we replaced ITMX and the ETMs between O2 and O3 -- and results look good -- the optical spring frequency appears to have been reduced from between ~6-8 Hz (depending on the alignment and thermal state of the IFO) to f_s = 3.6 +/- 0.1 Hz (informed by only one measurement, 68% C.I.). The Q remains unresolved (though we expect the Q to be very close to critically coupled at ~1/2).

More details below.

The processed results based on today's measurements for the sensing function parameters are as follows:

Parameter                              | Quantiles (0.15, 0.50, 0.84)
---------------------------------------------------------------------
Cavity gain, H_c (ct/m)                | 3.245e+06, 3.249e+06, 3.253e+06
Cavity pole, f_cc (Hz)                 | 423.7, 425.2, 426.8
Detuned SRC spring frequency, f_s (Hz) | 3.441, 3.603, 3.758
Detuned SRC spring quality factor, Q_s | 30.71, 23.81, 19.41
Residual time delay, tau_c (usec)      | -2.499, -1.806, -1.112

See previous values (that informed what is currently installed and calibrating DELTAL_EXTERNAL) in LHO aLOG 44364.
Specifically, I draw your attention to the optical gain ratio of (previous / current) = 3.552e6 ./ 3.249e+06 = 1.093 ~~ 1.1, which is what's installed currently in the CAL-CS GAIN field.

If we so chose to update the sensing function filters in the CAL-CS model (probably not yet), those filters should be:
Inverse Sensing FOTON values:
H1:CAL-CS_DARM_ERR Bank
SRCD2N: zpk([425.2376;3.5282;-3.6796],[0.1;0.1;7000],1,"n")gain(1298.18)
Gain: gain(3.078e-07)

Inverse Sensing without cavity pole FOTON values for CFTD path:
H1:CAL-CS_DARM_CFTD_ERR Bank
SRCD2N: zpk([3.5282;-3.6796],[0.1;0.1],1,"n")gain(1298.18)
Gain: gain(3.078e-07)

Data was taken with the templates:
/ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/O3/H1/Measurements/FullIFOSensingTFs
2018-12-05_H1DARM_OLGTF_23to800Hz_10min.xml
2018-12-05_H1_PCAL2DARM_TF_23to800Hz_2min.xml

Results were processed and produced with
/ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/O3/H1/Scripts/FullIFOSensingTFs$ python3.5 process_sensingmeas_20181205.py


And updated DARM loop model comparison to come -- but we know we at least need to make the updates described in LHO aLOG 44667.