When we tried to study the MC finesse we did a mistake.

As we can see on the picture, it seems that the two TEM00 peaks are on the same part of the ramp (meaning same sign of cavity change length). But there was an offset as the TEM00 peaks show some small peaks symetric with respect to TEM00 peak.

So we took new data (second plot).

Finally we could extract the following parameters using the following equations : T0 / ( 1 + 4r/(1-r²) * sin ( PI (x - x0)/ FSR ) ² )

T0 = 0.98

FSR = 8.2072 ms

bg = 0.0205

r = 0.9902

x0 = -1.617 ms

with a R² factor = 0.9929

We could mesure the FMWH of the peaks at 0.026 ms which give us a finesse  = 315.66