We had a quick look at the data we collected yesterday, especially about the decay time of the arms due to induced fast unlocks with only one arm held on resonance at a time.

In the attached plots, the upper graph is about the overall behavior of the signals when we intentionally unlocked the IMC so to cut off the injected power. The IMC decay is quite fast  as expected, then the power measured by the PD on the end bench (transmitted power) has some rebound while decaying, and the PDs looking at the reflected power (B4 and B1p) show the predicted overshoot [1] and then the decay whose time constant is linked to the finesse of the cavity as:

Finesse = pi * tau * FSR *pi/2

We chose to analyse the response measured by B1p_PD1 that is the fastest sampled channel we can use, and also the sensor with a fair amount of power impinging on it. The outcomes are shown in the bottom plots of the attached pictures, semilogy scale to enhance any discrepancy with respect to the simple exponential model. The data stretch for each fit was chosen to start from the overshoot peak and last for 10msec (bottom axis can be misleading: t0 is the peak time of B1p).

Overall, the fit is not bad and gives some 1.77msec for the North arm and 1.80msec for the West. This, in turn, gives a Finesse of 437 for the Nort arm, and 444 for the West arm.

To be noticed that a more refined analysis is needed to take into account the IMC decay time that is possibly non-negligible with respect to the arms. Also, from the same measurements the overshoot can give an indication of matching and losses parameters [1], to be compared with the numbers obtained by the usual measurement of the arm reflectivities on-off resonance.

Work ongoing.

[1] T. Isogai, J. Miller, P. Kwee, L. Barsotti, and M. Evans, "Loss in long-storage-time optical cavities," Opt. Express 21, 30114-30125 (2013)