Because we found the unexpected peak while we are scanning mode cleaner, I tried to realign it.

The mothod is to block the beam going to mirror without PZT in Mach-Zehnder. Then use the steering mirror to align and make the peak of mode-cleaner as high as possible. Then remove the block, and adjust the mirror without PZT in Mach-Zehnder.

After alignment, the unexpected peak becoms very small. So I tried to take the data and calculate Finesse. Firstly, I tried to fit with airy function. But I found the FSR here is very strange, see attached Fig.1. It's obvious that the software cannot tell us this is a good fit. So this fit is done by my hand. I don't know why this FSR can be this unstable. But maybe this can be interesting.

But I tried to fit only one peak, then I get the fit result of F=15100. See attached Fig.2. But note here, we use PD with amplification, it is 40dB means 100 times amplification. So the measurement of Finesse is only 152. If we consider R=0.992, the Finesse should be 248. So there is this discrepancy.

Beside, I checked the polarization again. This time I put a half-wave plate infront of mode cleaner, and change s to p polarization. I saw a increase of larger than 10 times of transmission on oscillscope. I talked with Matteo. From Fresnel law, p-pol has more transmissvity for the mirror now we use to dump beam. This is the main reason for this increase.