S. Karki, C. Biwer, R. Savage

The factor of two that was missing from the pcal inverse actuation filter installed at X-end is now retrieved from a new transfer function measurement  and a appropriate new filter is ready to install. The filters we used previously were created by using the transfer function taken from PCALY. The two Pcal should in principle be very similar and we were working on that principle. 

Today in trying to troubleshoot, we took a transfer function between RxPD and EXC at Xend and used it to create the inverse act. filter. We were able to retrieve the missing factor and possibly the sign as well. This still doesnot explain why there is this factor of 2 in the pcal response between the two end. Will continue investigating on it.

The factor of two is not unexpected and should not impact the calibration. We pick off a small part of the laser light that goes to the ETM using an uncoated optic oriented close to Brewster’s angle.  The small fraction of the light reflected from this optic (less than 1%) is a strong function of the angle of incidence. The reflected light is directed to the Optical Follower Servo photodetector . The percentage of light reflected, along with attenuators installed in front of the PD, coupled with the diffraction efficiency of the AOM, could easily account for this factor of 2. The excitation signal generates an offset in the Optical Follower Servo that is compared with the light level from the OFS PD.  The OFS ensures that they are equal and opposite (in sign). We monitor the actual light that is directed to and reflects from the ETM and the calibration is based on this.

In the meantime, we will install this new filter at the next opportunity and do a test through a full transfer function as well as waveform injection.

Attached is a plot with measured TF and an appropriate fit.