We roughly measured the optical gain of the Michelson.
See the attached figure.
As noted in 1423, optical gain [cnt/m] can be obtained from P2P value of the freely swinging error signal:
(Optical gain) = (Vpp/2) * 4pi / lambda.
Therefore, the current optical gain is (5 +/- 1) * 10^10 [m/cnt] (to strain: (1.5 +/- 0.3) * 10^13 [/cnt] )
Cool elliptic fitting to obtain the optical gain is attached
Super cool fitting code by T. Yamamoto-san is located in /users/CAL/PHASE1/scripts/EllipseFitting.C
Run the following command to fit:
$: cd /users/CAL/PHASE1/scripts
$: root
$: .x EllipseFitting.C("timeseries_firename")
Vpp/2 = 4930 +/- 90, accoriding to this fitting.
To do this fitting, I selected clean data. Free swinging MICH time series data are stored in /users/CAL/PHASE1/measurements/optgain
