Photo-birefringent effects in crystalline AlGaAs mirror coatings

High-reflective crystalline GaAs/Al_0.92Ga_0.08As coatings show reduced Brownian noise compared to conventional dielectric coatings. However, several ultra-stable laser systems observed additional noise sources that hinder the realization of the expected improvements in frequency stability. These additional noise sources are related to the birefringence of the coatings, which can also be modified by intracavity light. While the origin of the birefringence is not yet well understood, its modification via illumination remains also unexplained. Here, we present an extensive study on the steady-state and transient modification of the birefringence by intracavity light and by uniform illumination at various wavelengths using an optical cavity at room temperature. We find a unified description that suggests a primary two-photon process for photon energies below the bandgap of GaAs, or a single-photon process at higher energies. Adding external illumination allows us to reduce noise induced by laser power fluctuations by balancing the photo-thermo-optic response of the mirrors and the photo-birefringent effect at more favorable low intracavity power levels.