Bloch-Floquet waves in optical ring resonators

TL;DR

This paper models optical ring resonators as one-dimensional photonic crystals using Bloch-Floquet theory, revealing that mode coupling and frequency splitting are related to photonic band formation and the Fourier components of the dielectric function.

Contribution

It reframes intracavity Rayleigh scattering in ring resonators as photonic band formation, providing a new theoretical perspective on modal coupling.

Findings

Modal coupling described as photonic frequency bands.

Bragg planes match phase matching conditions.

Frequency splitting proportional to Fourier coefficients.

Abstract

Modal coupling between frequency-degenerate resonances of an optical ring resonator is a commonly observed phenomenon that results in adverse mode splitting. Traditionally, this coupling is attributed to Rayleigh scattering of a propagating electromagnetic wave into its associated degenerate counter-propagating mode from small perturbations to the dielectric material of the resonator. We have chosen to reframe the problem of intracavity Rayleigh scattering by considering the optical ring resonator as an infinitely-long, one-dimensional photonic crystal (PhC) that possesses a lattice constant equal to the perimeter of the ring. Through application of Bloch-Floquet theory, we show that modal coupling between degenerate resonances of a ring can effectively be described as the formation of photonic frequency bands in the dispersion relation of the resonator. We additionally demonstrate that the Bragg planes of the PhC lattice coincide with the phase matching conditions for constructive interference in the ring. Finally, we show that the magnitude of frequency splitting of a particular resonance is proportional to its associated coefficient in the Fourier expansion of the ring's periodic dielectric function.