I again looked at the cross-correlation of POP and POPAIR 45Q during full lock, in order to find out what the vertex LSC noise looks like below the shot noise level.

This time, I added a bandstop filter to PRCL, MICH, and SRCL that gives ≥40 dB attenuation between 82 and 101 Hz. This ensures that shot noise is not impressed on the vertex dofs in this band, so that we can get a true estimate of the freerunning displacement/sensing noises.

The result of 1 hour of cross-correlation is attached. In the case of POP 9I (used for PRCL) and POP 45I (used for SRCL), there is significant coherence between the two PDs. For 9I this is not so surprising, since the same (unknown) structure can be seen in both spectra. For 45I, a similar irregular structure is seen in both PDs. The coherence in the notched portion is 0.01.

For POP 45Q (used for the Michelson), the coherence seems remarkably free of the irregular structures seen in 9I and 45I. The notched portion appears to be at the noise floor of the correlation measurement, with a coherence <0.001. The coherence of 0.01 around 130 Hz probably comes from noise being injected into the Michelson loop.

The uncontrolled quadrature (9Q) shows some evidence of irregular structure, despite the fact that POP 9 and POPAIR 9 were both phased to mimize the apperance of PRCL in Q.

Time is from 2016-02-01 04:18:00 to 05:18:00 Z.