Theoretical detailed analyses for DC readout and a Fabry-Pérot gravitational-wave detector
Phys. Rev. D 113, 125006 – Published 3 June, 2026
DOI: https://doi.org/10.1103/m4mr-8bp3
Abstract
The quantum expectation value and the stationary noise spectral density for a Fabry-Pérot gravitational-wave detector with a DC (Direct Current) readout scheme are discussed in detail only through the quantum electrodynamics of lasers and the Heisenberg equations of mirrors’ motion. We demonstrate that the initial conditions of the mirrors’ motion concentrate around the fundamental frequency of the pendulum and are not related to the frequency range of our interest. Although, in the ideal case, there is consensus that the shot-noise contribution from the laser to the high-frequency range of the signal-referred noise spectral density decreases as the injected laser power increases, our derived noise spectral density shows that the shot-noise contribution does not decrease. This is due to leakage of the classical carrier field from the classical radiation pressure forces in the Fabri-P’erot cavity, and the carrier field is used as the reference in the DC readout scheme. Since classical radiation pressure acts as a constant force, it shifts the pendulum’s equilibrium point of the mirrors’ motion. To recover the ideal case, we must consider adjusting the interferometer’s tuning point to place the mirrors at their equilibrium positions. We investigate the case where the equilibrium tuning is incomplete and show that the behavior of the above shot noise is due to this incompleteness. We also discuss the maximum deviation of the mirror displacements from the equilibrium point during incomplete tuning to recover a near-ideal case.
