Abstract:

It would be better to increase the OMC LSC loop gain in daytime to reduce the OMC residual motion at 8-9 Hz.

Detail:

After recovery of IFO, the line noise in DARM spectrum at high frequency has many sidebands as shown in fig1.
This sidebands seem to come from OMC length fluctuation.
Figure 2 shows the DARM spectra around 300 Hz line and OMC error spectrum (Black: 16:05 JST, Red: 16:30 JST).
Sidebands in DARM spectrum are small (or disappeared) when the peak at 9 Hz in OMC error signals are small, so it is likely that the upconversion noise of OMC error signals.

Then, I checked OLTF of OMC LSC to see if there is a room to reduce the peak at 9 Hz.
Figure 3 shows the OLTF of OMC LSC with the different gains.
Blue, red, and green lines show the measurement results when OMC LSC loop gain is nominal, 2 times larger, and 3 times larger, respectively.
Though gain peaking around UGF becomes large, suppression gain at 9 Hz can be improved by increasing overall gain.

Figure 4 shows the spectra when the OLTF gain was changed.
Black, red, and pink lines show the DARm spectrum and OMC error signals when OMC LSC loop gain is nominal, 2 times larger, and 3 times larger, respectively.
Error signals at 9 Hz can be reduced while maintaining no excess around 40 Hz apart from 300 Hz line in DARM.
So, gain peaking seems no significant impact on the sensitivity.

Since we plan to start injecion tests this week, it would be better to increase the OMC LSC gain for reducing the upconversion noise at high frequency during the injection tests.