Optical microcavity characterization via resonance spectra and modes

This paper describes how resonance spectra and mode profiles can be used to characterize and quantify the mode-shaping effects in open-access plano-concave optical microcavities.The presented semi-analytic theory is based on the application of perturbation theory to the roundtrip evolution of the optical field. It includes various nonparaxial effects that become prominently visible in microcavities with small radii of curvature. It extends the theory presented by van Exter et al. (2022, Phys. Rev. A 106, 013501) and verified Koks et al. (2022, Phys. Rev. A 105, 063502) to the common Gaussian mirror shape with finite mirror depth and shows how the associated spherical aberration modifies the transverse-mode splitting and the fine structure in the resonance spectrum. The presented measurements demonstrate how the different mode-shaping effects can be individually distinguished and quantified. Spin-orbit coupling, which is one of the nonparaxial effects, is prominently visible in the intriguing polarization patterns of the resonant modes. Polarization tomography yields the shape-induced birefringence and the associated polarization splitting of the fundamental modes.