Dr. Dripta Bhattacharjee

Gravitational wave (GW) source parameter estimation and localization rely on accurately calibrated strain data. Calibration of the LIGO detectors is achieved using displacement fiducials generated by radiation pressure based systems called Photon Calibrators (Pcals). These systems rely on power-modulated auxiliary laser beams reflecting from suspended interferometer optics, to provide continuous calibration by generating displacement fiducials proportional to the modulated laser power. This technique has been developed in LIGO for more than a decade and, for the latest O4 run, provides absolute calibration accuracy at a sub-0.3% level. To increase our confidence in our calibration accuracy, and reduce relative calibration errors between observatories, the global network of GW observatories in collaboration with the national metrology institutes (NMIs) in the US and Germany have implemented a novel calibration scheme for the current observing run. This talk details the results of implementing the global calibration scheme till date and the improvements in the measurement techniques that facilitated the state of the art in absolute calibration of GW observatories. These developments enabled reducing the systematic uncertainty in the interferometer calibration of the LIGO interferometers to ~ 2 % level in the most sensitive frequency region. Independent studies have shown that the calibration accuracy of the current generation of GW detectors does not limit the extraction of source parameters from detected GW signals. However, as sensitivities improve and more detectors are added to the global network of GW observatories, precise and accurate calibration is becoming increasingly critical