J. Kissel I've processed the DARM OLG TF measurement Evan and Sheila took last night (LHO aLOG 18269), and was dismayed to find that the DARM coupled cavity pole (DCCP) frequency has decayed back down to 270 [Hz]. This is obvious from the attached residuals, where I show two different versions of model parameters for last nights measurement compared against the two previous measurements taken during the mini-run, where the DCCP frequency was 355 [Hz] up near the expect value. I've again used a 0.99 coherence threshold, I trust this assessment of the DCCP frequency to within 2%, especially since that's the only thing I have to change in the model (besides the overall scale factor) to arrive at this conclusion (for the skeptics of my precision, see LHO aLOG 18213). What's going on here? --- Total Blind Speculation --- As Sheila mentions in the main entry (LHo aLOG 18264), the recycling gain and initial alignment had been restored to values during the mini-run. There has been quite a lot of work on SRC alignment: maybe those SRC loops which are now higher bandwidth -- though good for stable SRCL to DARM coupling (LHO aLOG 18273) are not so good for the DCCP frequency. Perhaps because they have some bad alignment offset? Perhaps we should try very slightly altering the DC alignment of these loops to see if it has an effect on the DCCP frequency, and then optimize for it. Eventually, one might imagine using the amplitude of either the PCal or DARM calibration lines as feedback to keep the pole frequency stationary and near the design value. The good news is that the overall scale factor (what we're assuming is either the optical gain or ESD driver strength variation) changed by less than 0.5%. --------------------- The measurement template has been copied over and committed to the CalSVN here: /ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/PreER7/H1/Measurements/DARMOLGTFs/2015-05-06_H1_DARM_OLGTF_LHOaLOG18269.xml The new model parameter set can be found committed here: /ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/PreER7/H1/Scripts/H1DARMparams_1114947467.m and as usual, the model is here: /ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/PreER7/H1/Scripts/H1DARMmodel_preER7.m and all residual comparisons are done here: /ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/PreER7/H1/Scripts/CompareDARMOLGTFs.m
I guess another possibility is some effect from the lower SRCL ugf. When the guardian goes into LSC_FF, the SRCL gain is reduced by 30% to reduce the amount of control noise appearing in DARM (the SRCL ugf goes from 50 Hz to 25 Hz). I'm assuming this reduction was not done during the minirun, when the good DARM pole was measured.
I suppose we should just run two DARM OLTFs in quick succession, one with the low ugf and one without.
Evan's idea seems unlikely: how could the SRCL loop, with a bandwidth of few tens of Hz, affect what happens at mich higher frequency? This would imply huge couplings of both DARM->SRCL and SRCL->DARM. Such lareg coupling should be easily visible when measuring the SRCL open loop gain.
The most likely hypothesis is that the pole frequency depends critically on the SRC alignment. One interesting test would be to inject two DARM lines, one at low frequency (50 Hz) and one at high frequency (1000 Hz) and track their variations while aligning the ITMs. We expect the low frequency to be constant (tracking the optical gain) and the high frequency to change (tracking the pole frequency).
We already inject calibration lines at 34.7 and 538.1 [Hz] which have been on since April 1 2015 (17621). They've been at a fixed amplitude that had been tuned for a 30 [Mpc] lock stretch, so the 34.7 [Hz] line may not be visible at all times, but still -- it's on the to-do list to make this comparison. Any off-site would be much appreciated!