I measured the cross-calibration of the two PDs on the PSL table.
I used the existing flip mounted BS that routes the beam into a PDA255, the same as in the IMC transmission.
I placed a PDA520, the same as the one measuring TRY_OUT on the ETMY table, on the transmission of the BS (Attachment 1).
I used the SR785 to measure the frequency response of PDA520 with reference to PDA255 (Attachment 2). Indeed, calibration is quite significant.
I calibrated the Y arm frequency response measurement.
However, the data seem to fit well to 1/sqrt(f^2+fp^2) - electric field response - but not to 1/(f^2+fp^2) - intensity response. (Attachment 3).
Also, the extracted fp is 3.8KHz (Finesse ~ 500) in the good fit -> too small.
When I did this measurement for the IMC in the past I fitted the response to 1/sqrt(f^2+fp^2) by mistake but I didn't notice it because I got a pole frequency that was consistent with ringdown measurements.
I also cross calibrated the PDs participating in the IMC measurement but found that the calibration amounted for distortions no bigger than 1db. |