I have done a cross correlation analysis of the two OMC DCPDs in order to study the spectral shape of the broad band noise that has been limiting the sensitivity in 200-1000 Hz band. Here are some results.
The red curve is cross-correlated noise of the two DCPDs in terms of the DCPD current in mA (with DARM loop effect removed).The data I used is a lock stretch from Sep. 26th which lasted for 6 hours in undisturbed. The frequency resolution or bandwidth is set to 0.1 Hz. The data length is 3 hours starting at 9:00:00 UTC of the day. The DARM suppression is taken out from OMC DCPD SUM. I have used the latest PreER10 DARM model (29931) to obtain a reasonable DARM suppression function. As we all know, there is high noise in 200-1000 Hz whose level is as high as shot noise (or null stream shown in green) in this frequency band.
I also made another plot, which is an overlay of 11 cross correlated spectra each of which is from different time but from the same lock stretch. See below.
This time, the frequency resolution is set to 1 Hz, so that we can get several FFTs out of the same lock stretch. Each spectrum is produced by an 18 minutes integration. As shown in the plot, there was one spectrum which is polluted by some kind of glitch. Otherwise, the shape of noise stayed almost unchanged over 3 hours. Also fig files are available.
There was a question of whether the cross-spectrum noise was from readout electronics or not. I have checked it by looking at dark noise from this past weekend in which laser happened to be off.
I conclude that the excess noise we see at around 400 Hz is not from dark noise or similar readout noise. In fact the dark noise level above 100 Hz was found to be consistent with a measurement a year ago (20526). Things look good to me. The attached is a plot of the dark noise spectra.
DCPD settings:
- First stage of the whitenings is engaged.
- High-Z (400 Ohms)