Didier has processed h(t) with individual B1 photodiodes. This allows to cross correlate the two photodiodes to remove shot noise and see more clearly what is underneath. Unfortunately there are the ~25min glitches which spoil the results, I have tried to work around this by adding by hand gating, with a mixed result.

Figure 1 and Figure 2 show the result one where the gating worked better than in the other case. The cross-correlated spectrum is higher than the normal one at low frequency, because the cross-correlated uses gating, while the normal one can use the median-mean average spectrum that is not affected by glitches. What is interesting is that starting from 60Hz up to 800Hz the correlated noise spectrum floor follows very well the expectation of mystery noise plus coating thermal noise. This means that the actual slope of the mystery noise is very close to the -0.66 slope currently used.

Figure 3 shows the cross-correlated spectrum in pink, the noise budget with -0.66 slope in light blue and with -0.75 slope in dark blue. The -0.75 is a much worse fit at high frequency when constrained to have the same level at 100Hz.

This confirms that the mystery noise slope is -0.66, with error bars that slope is in between -0.65 and -0.70. This measured can still be biased by other noises, as the analysis above assumes that other noises are negligible in the 80Hz-800Hz band.

This also shows that squeezing has the potential of improving the sensitivity at all frequencies above 60Hz. But in practice this will be very difficult due to the high losses in the intentionally misaligned SRC cavity.

/users/mwas/calib/Hoft_PD1vsPD2_20240205/hPD1vsPD2.m