Precision Interferometric Measurements of Mirror Birefringence in High-Finesse Optical Resonators

High-finesse optical resonators found in ultrasensitive laser spectrometers utilize supermirrors ideally consisting of isotropic high-reflectivity coatings. Strictly speaking, however, the optical coatings are often non-uniformly stressed during the deposition process and therefore do possess some small amount of birefringence. When physically mounted the cavity mirrors can be additionally stressed in such a way that large optical birefringence is induced. Here we report a direct measurement of optical birefringence in a two-mirror Fabry-Pérot cavity with R = 99.99 % by observing TEM₀₀ mode beating during cavity decays. Experiments were performed at a wavelength of 4.53 microm, with precision limited by both quantum and technical noise sources. We report a splitting of δ_ν = 618(1) Hz, significantly less than the intrinsic cavity linewidth of δ_cav ≈ 3 kHz. With a cavity free spectral range of 96.9 MHz, the equivalent fractional change in mirror refractive index due to birefringence is therefore Δn/n = 6.38(1) × 10^-6.