Researchers have investigated how the deformation of end test masses (ETMs) in gravitational wave detectors, such as LIGO, KAGRA, and Virgo, introduces calibration errors. This deformation is caused by Photon Calibrator (Pcal) devices and induces a non-linear displacement that deviates from the ideal pendulum motion of the ETMs. This effect is particularly significant at frequencies above 1500 Hz, a crucial range for analyzing neutron star mergers.

The displacement induced by the bulk deformation of the mirrors also depends on the beam offsets of the main interferometer and Pcal beams. To quantify this effect, finite-element analysis (FEA) software was used to evaluate the deformation in several beam offset scenarios. The ETM models analyzed include those of Advanced LIGO (aLIGO), KAGRA, Advanced Virgo (AdVirgo), and LIGO A#.

This work is important for future gravitational wave projects, as it helps to understand and mitigate a dominant source of systematic error in calibration. Characterizing the frequency-dependent effect of this deformation will improve the precision of measurements, especially in the study of astrophysical events that produce high-frequency signals, such as neutron star mergers, where calibration accuracy is fundamental for extracting reliable information.