Realization of displacement fiducials with 0.2% accuracy for calibration of gravitational wave observatories.*

Gravitational wave observatories employ auxiliary laser systems referred to as  photon calibrators (Pcals) that rely on laser radiation pressure to generate displacement fiducials proportional to the amplitude of the modulated laser power. To calibrate Pcal systems, the  global detector network is implementing a new scheme with transfer standards periodically shared between the observatories and national metrology institutes in the USA and Germany. Propagation of the transfer standard calibration to the Pcal power sensors located at the interferometer end stations at the LIGO Livingston Observatory enabled generation of periodic displacement fiducials with overall (1-σ) uncertainty of less than 0.2 %.  These fiducials are the displacement references used to calibrate the interferometer output signals. They are expected to enable calibration of the detected gravitational wave signals with sufficient accuracy to optimally extract the astrophysical information they contain. The new Pcal calibration scheme and the methods employed to reduce uncertainty during the ongoing O4 observing run are described.