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Detector Characterisation (Spectral lines)
mwas - 14:47 Wednesday 05 February 2020 (48360) Print this report
250Hz line and 50Hz from PSTAB

The power stabilization system might be the origin of the 250Hz line visible in h(t), and also a non negligible contributor to the 50Hz (and to a lesser degree 150Hz). With the photodiode signal being spoiled by 50Hz harmonics and the being impressed onto the laser amplitude.

Figure 1 shows the relative intensity noise as measured by the PSTAB out-of-loop photodiode, the B2 photodiode and the RFC TRA photodiode. The spectra are very different, but the 50Hz and 250Hz lines have very similar height in the three signals.

Figure 2 shows a zoom around 50Hz, and figure 3 a zoom around 250Hz.

Figure 4 at that time (Jan 16) there was also an intentional line injected into the PSTAB loop at 2961Hz. The amplitude of that line seen in the 3 signal is not exactly the same, but it is within +/-20%, which confirms that the 3 signals RIN is indeed comparable.


When doing a projection of the PSTAB noise to h(t) using a measured TF from July 2019 the projected 50Hz and 250Hz are at a similar level as what is actually seen in h(t).

Figure 5 shows the broadband projection.

Figure 6 is a zoom at 50Hz, where the projected noise is slightly higher than what is measured, this could be due to 50Hz feed-forward technique removing some of the 50Hz coupled by the PSTAB by acting on the arm mirrors.

Figure 7 is a zoom at 250Hz, where the projected line matches well with the measured noise in h(t).

Figure 8 is a zoom at 150Hz, where the projected line is a factor 10 smaller than the measured noise in h(t), so for the 150Hz something else than the PSTAB is the main culprit for the coupling.

I wonder if there is any improvement that could be implemented after O3 to reduce the 50Hz odd-harmonics in the PSTAB photodiodes, and if these harmonics are due to the PD itself or to the signal propagation along long cables inside the IB tower. These harmonics are a factor 100 above the PD broad band sensing noise for FFTs of 10s duration (see figure 1).

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mwas - 10:44 Monday 22 February 2021 (50846) Print this report

I have matched the in-loop error signal of the PSTAB to the out-of-loop signal using the calibration line injected at 2961Hz on Jan 16 2020.

Figure 1 shows the in-loop and out-of-loop signals on Jan 16, where there is excess 1/sqrt(f) on the out-of-loop signal.

Figure 2 shows data from Feb 16 2020. In that case most of the excess noise on the out-of-loop signal is gone, there is a flat noise with some remaining excess below 30Hz. Note that in-loop noise is a factor 10 lower. So the loop gain is high, and sensing noise is limiting the power stabilization, and not the power stabilization loop gain.

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