Photo-birefringent effects in crystalline AlGaAs mirror coatings

TL;DR

This study investigates the birefringence effects in crystalline GaAs/AlGaAs mirror coatings, revealing how intracavity light modifies birefringence and proposing methods to reduce associated noise, thereby improving laser frequency stability.

Contribution

It provides an extensive analysis of birefringence modification mechanisms in crystalline coatings and introduces a unified model involving two-photon and single-photon processes.

Findings

Birefringence can be modified by intracavity light and uniform illumination.

A unified model suggests two-photon processes below GaAs bandgap and single-photon processes above.

External illumination can reduce laser power fluctuation noise by balancing photo-thermal and birefringent effects.

Abstract

High-reflective crystalline $GaAs/Al_{0.92}Ga_{0.08}As$ 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 and its modification by intracavity light. The origin of the birefringence is not yet well understood and its modification via illumination remains 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 to reduce noise induced by laser power fluctuations by balancing the photo-thermal-optic response of the mirrors and the photo-birefringent effect at a more favorable low intracavity power.