The feedback signal going to the laser PZT at the X end station was measured in the daytime and the nighttime.
It's been measured while the laser frequency was locked to the arm cavity with the green light.
The red curve was measured at 3pm of 8/July Friday. And the blue curve was measured at 12am of 9/July Saturday.
As we can see on the plot, the peak-peak values are followers
daytime: ~ 4Vpp
nighttime: ~1.8Vpp
It is obvious that the arm cavity is louder in the daytime by a factor of about 2.
Note: the feedback signal is sent to the PZT only above 1Hz because the low frequency part is stabilized mostly by the crystal temperature (see this entry)
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What we care about is the peak-peak value of the PZT feedback signal measured on a scope for ~30 seconds.
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