Frequency splitting of polarization eigenmodes in microscopic Fabry-Perot cavities

TL;DR

This study investigates how the geometry of elliptical mirrors in microscopic Fabry-Perot cavities causes polarization mode frequency splitting, and demonstrates how to tune or eliminate this splitting through mirror rotation.

Contribution

It provides a model linking mirror eccentricity to polarization mode splitting and shows how to control this effect in microscopic cavities.

Findings

Eccentricity lifts polarization degeneracy.

Mirror rotation can tune or eliminate splitting.

Geometric effects dominate frequency splitting.

Abstract

We study the frequency splitting of the polarization eigenmodes of the fundamental transverse mode in CO2 laser-machined, high-finesse optical Fabry-Perot cavities and investigate the influence of the geometry of the cavity mirrors. Their highly reflective surfaces are typically not rotationally symmetric but have slightly different radii of curvature along two principal axes. We observe that the eccentricity of such elliptical mirrors lifts the degeneracy of the polarization eigenmodes. The impact of the eccentricity increases for smaller radii of curvature. A model derived from corrections to the paraxial resonator theory is in excellent agreement with the measurements, showing that geometric effects are the main source of the frequency splitting of polarization modes for the type of microscopic cavity studied here. By rotating one of the mirrors around the cavity axis, the splitting can be tuned. In the case of an identical differential phase shift per mirror, it can even be eliminated, despite a nonvanishing eccentricity of each mirror. We expect our results to have important implications for many experiments in cavity quantum electrodynamics, where Fabry-Perot cavities with small mode volumes are required.