Beam tube boundary effects in stray light modeling of long Fabry-Pérot arm cavities for third-generation gravitational-wave detectors
Phys. Rev. D 113, 102003 – Published 18 May, 2026
DOI: https://doi.org/10.1103/hgbb-h2v3
Abstract
Next-generation gravitational-wave detectors such as Cosmic Explorer and the Einstein Telescope will operate 10–40 km Fabry-Pérot arm cavities inside vacuum beam tubes. FFT-based paraxial tools treat propagation in free space and therefore do not explicitly enforce beam tube boundary conditions. We introduce a waveguidelike mode description of the optical field that incorporates an imposed beam tube boundary condition and enables an independent benchmark of free-space FFT tools. We derive the associated modal-mixing matrices for mirrors and baffles, including a closed-form series for axisymmetric circular apertures. We quantify the strain-equivalent couplings from baffle miscentering and from a localized near-wall tube defect, and show that they are suppressed as baffle density increases. In the relevant regime of densely baffled cavities and small perturbations, beam tube boundary effects are subdominant, which supports the continued use of FFT-based codes to guide the design of third generation detectors.
