Tuning the SDB1 Faraday isolator waveplate by simply looking at the transmitted power on B1p does not allow to precisely minimise the reflected power on ESQB. The residual power on ESQB can be measured on the SQZ_B1_Cam cameras, which receive the light reflected from the first polariser of the triple Faraday isolator on ESQB. However, due to a small mistuning between the axes of the two FIs (on ESQB and on SDB1) the minimum power on ESQB does not correspond to the minimum power on SQZ_B1_Cam cameras. The optimal position of the SDB1 waveplate can be determined with a calibration of total power on ESQB vs power on cameras. After the recent cleaning intervention on the ESQB Faraday isolator polarisers, the fraction of reflected power towards the SQZ_B1 cameras has slightly changed. On Jan 25 I performed a new calibration, see attached plots, with the ITF in recombined configuration.
In short, to minimise the power reflected from SDB1 to ESQB one must first find the minimum power on cameras, then rotate the waveplate to increase it by a factor 1.45. This requires adding about +5000 picomotor steps (using the command "relative movement" in VPM server SDB1_rot, and setting channel 1, axis 2, speed = 50). Best SNR is with CamFar_FitAmpX.
