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Description
The Photon calibrator (Pcal) is the system utilized to provide an absolute calibration for the advanced LIGO detectors. Fundamentally, the system injects a secondary laser source into the interferometer End Test Mass (ETM). Measurement of this external laser yields a reference of the ETM motion. Historically, the Pcal measures the light reflected by the ETM on the receiver (Rx) power sensor.
The performance of the Pcal was monitored through the Calibration Comparison Factor (CCF), deployed on the fourth observing run (O4). The Pcal force-to-displacement equation is manifested in the CCF for single actuation of the beams. Intentional actuations of the Pcal beams, done through the O4 run, demonstrate an independent measurement of the EX beam offset at the LIGO Hanford Observatory (LHO). Measurements at LHO of the interferometer beam offset show (1) the susceptibility of the Rx sensor to change from small beam actuations, and (2) the capability to monitor clipping from the ratio of two close peaks of the calibrated strain output. The IFO beam offset measurements suggest a method for measuring the Pcal center of force via beam actuation, potentially reducing the primary source of uncertainty in the Pcal system.