Later in the afternoon the razor blade dumper has been replaced with a larger one, to make sure the beam is always well dumped against alignment changes. In the morning we have measured the fraction of optical power reflected by the first (from southern side) polariser of the double stage Faraday isolator. This beam is sent to the SQZ_B1_CamNear and SQZ_B1_CamFar cameras on ESQZB. With the ITF in recombined configuration, we changed the optical power by rotating the SDB1 Faraday waveplate. Below the measured power (in mW) reflected by the polarizer vs the incident power in mW:
in refl
570 2.5
230 1.7
107 1.17
0.38 0.36
The large deviation from linearity at low incident power is because of the different polarisation state of the reflected beam when the SDB1 Faraday waveplate is tuned for maximum transmission.
Extrapolating for higher power, with an incident power of 1 W the reflected power is around 3 mW, and each camera will receive about 1.5 mW.
The two cameras provide a sensitive tool to optimize the position of the SDB1 waveplate, which so far was tuned by maximizing the power on B1p. Indeed this morning we found some residual mistuning, i.e. the optical power on ESQZB was four times higher than for optimal tuning.
In conclusion, the SDB1 Faraday shutter can be safely open even when the SDB1 waveplate is mistuned. Very little power will reach the squeezer after the three Faraday isolation stages, and only moderate power reaches the two cameras.
