Last night I measured the modulation depth of the MC incident beam.
The beam is taken from one of the PO beam at the wedge plate before the IMC.
After removing the knife edge to dump this beam, the beam is sent to the west side
of the PSL table and put into the OSA cavity.
[The beam dump was returned after the measurement.]
I had some confusion and after all I use the OSA labeled as AS OSA rather than the one on the PSL table.
[The AS OSA was returned to the AP table.]
The transmission was detected by PDA255 and filtered by ITHACO 1201 preamp with G=10, no HPF, 30kHz LPF.
It was confirmed that the peak amplitudes are not reduced by the LPF filter. The resulting time series
was recorded by an oscilloscope.
Three measurements have been taken. The 11MHz peaks are offset by the carrier peak. They appropriately
removed. The ratio of the sideband and carrier peaks is converted to the modulation depth using the following formula.
P_sb / P_ca = [J1(m)/J0(m)]^2
The modulation depth for the 11MHz: 0.190 +/- 0.003
The modulation depth for the 55MHz: 0.2564 +/- 0.0003
The three scans showed very similar numbers. That's why the statistical error is such small.
I don't think the systematic error is not such good.
This number is much different form the previous meaurement by Mirko.
http://nodus.ligo.caltech.edu:8080/40m/5519 m=0.14 (11MHz) & 0.17 (55MHz)
but the measured voltages and the modulatio depths are inconsistent.
http://nodus.ligo.caltech.edu:8080/40m/5462 m=0.17 (11MHz) & 0.19 (55MHz)
Probably the modulation depths should be checked by the IMC again.
However, it is certain that the 55MHz modulation exists, and even larger than the 11MHz one.
The next is to confirm that the modulation frequency is matched with the IMC FSR.
It is to make sure that the modulation is transmitted to the main IFO without attenuation.