We observed some improvement with larger PD current yesterday. It was not clear to us why. There is some chance that a calibration issue is fooling us, but at the low frequencies we were looking at the calibration should only depend on the actuator.

Either way, to get to the bottom of this, we revisited the whitening of those PDs.

Currently, we are using 10mAmp per diode in regular operation. If the violins are smaller (as the were yesterday) we can increase the DARM offset to operate at about 15mAmp per diode, but that becomes a problem when the violin modes are high.

Attached is a plot of the whitening situation of out OMC DCPDs in full lock, with about 14mAmp per photo diode.

ndsope inset top: ADC signed of the photo diode
ndsope inset bottom: photo current on the photo diode
dtt red: PD signal in cts ADC (which is loop gain suppressed, hence the dip at 30Hz amd the peak at 100Hz)
dtt blue: dark noise
dtt green: possible option with current whitening filters: additionally engage a 2nd whitening stage plus the low-pass. Reduce the DARM offset by 3.2dB to get the violin mode peaks back down where they were.

It would be worth testing the green configuration, just to exclude that we have some load-dependent dark noise after the whitening stages.