Virgo is a power-recycled Michelson interferometer with two arms of 3 km length, designed to detect gravitational waves (GWs) in a frequency range from 10 Hz to 10 kHz.
One needs to calibrate the interferometer (ITF), and to recontruct the de- tector strain signal h(t), from which GW signals are detected. The photon calibrator (PCal) is a particular mirror actuator which uses the radiation pres- sure of an auxiliary laser to drive the motion of the ITF mirror. Since the LIGO-Virgo Observing run O3 (2019/2020), the PCal is used as reference to calibrate the Virgo detector and to validate h(t).
The PCal setup is being modified to further reduce its calibration uncertain- ties for the LIGO-Virgo-KAGRA run O4 to be started mid 2022. During O3, the PCal calibration uncertainty was 1.73% and contributed for about half of the final h(t) uncertainty.
The goal for O4 is to reduce this uncertainty under 1%. To do so, we are:
• Modifying the optical setup to reduce power calibration variation cor- related with humidity as seen during O3. It has been shown that this correlation mainly came from the beam splitters used on the setup.
• Using InGaAs photodiodes instead of silicon photodiodes, in order to re- duce the photodiode sensing noise thanks to a more powerful pickoff and new electronics, and to reduce correlation between photodiode response and temperature.
• Using an integrating sphere on the PCal reflection bench to measure the reflected power directly, without additional optical elements on the beam path.
• Reducing laser power noise, which creates mirror movement noise.