Optical modeling of light scattering from core optics in large-scale gravitational-wave detectors

Gravitational-wave detectors, such as the Laser Interferometer Gravitational-wave Observatory (LIGO), are long-arm Michaelson interferometers that use precision interferometry to measure miniature displacements in the position of their core optics produced by gravitational waves. Scattering is one major source of transient noise, and limiting factor when the laser power is increased. It requires critical control and good understanding of its origins. Visualizing and accurately quantifying the scattered field in the interferometer is essential in designing and effectively positioning various layers of the baffling systems, such as near-optic baffles, or far-field apertures. Zemax OpticsStudio is a standard tool for developing optical layouts in LIGO. Stray-light models based on general Lambertian scattering properties for the surfaces allow to trace possible scattering paths. Using measured bidirectional reflectance distribution function (BRDF) proprieties for baffles surfaces, as well as measured BRDF data and surface maps of the test masses, allows to also model power distribution of stray light in the system. In this work we present the modeling methods for wide angle scatter from test masses, and optimization of baffling design for LIGO and Cosmic Explorer.