Spontaneous polarization symmetry breaking of light in a microresonator

Birefringence - the difference in refractive index for othogonally polarized light - can manifest in the linear coupling between orthogonal modes in a given polarization basis. Inside of a resonator, birefringence leads to different resonance frequencies depending on the light’s polarization. Generally, at high intracavity powers the Kerr nonlinearity induces an intensity-dependent refractive index. In the case of circular polarized light in a cavity, the light can experience two types of Kerr effects: self-phase modulation (SPM) of the light wave with itself and cross-phase modulation (XPM) with a circular polarized lightwave of opposite handedness. In general, the effects of SPM and XPM have different magnitudes, with XPM being twice as strong in silica [1] .