Today we computed the calibration of the SSFS Corr signal in Hz/V which will be then used to evaluate the CMRF.

To do that, we started from a previously calibrated signal: indeed, the RFC error signal (Sc_MC_RFC_Err_Pre) was already calibrated in [um]  by Paolo.

Then, with the INJ stand alone, a noise injection was performed on the RFC in order to measure the RFC OLTF. This measurement was compared with the model, which consists of a pole for the mechanics of the IMC end mirror and the IMC filter. From this comparison, the scaling factor of 2.36 was derived (figure 1).

In order to calibrate SSFS_Corr, a noise injection from the SSFS driving has been performed, and the transfer function between SSFS_Corr and RFC_Err was computed. However, the RFC_Err is measured in closed loop, so it is necessary to compensate for the RFC CLTF, which is RFC_CLTF = 1/(1+RFC_OLTF)

Therefore, the calibration is obtained by:

(RFC_Err [um] / CLTF) / SSFS_Corr [V]

which yelds a calibration factor of 1.6e-11 [m/V] (figure 2). Using the IMC length and the laser frequency, it can be converted in 31.3066 Hz/V.