# On the theory of coupled modes in optical cavity-waveguide structures

## TL;DR

This paper derives an alternative coupled mode theory for optical cavity-waveguide systems by treating cavity modes as quasinormal modes, providing a physically intuitive and accurate description of light propagation.

## Contribution

It introduces a new derivation of coupled mode theory based on quasinormal modes, enhancing understanding of cavity-waveguide interactions.

## Key findings

- The theory accurately predicts light behavior in coupled systems.
- Application examples demonstrate practical utility in 1D and 2D.
- The approach offers a physically appealing framework for analyzing optical cavities.

## Abstract

Light propagation in systems of optical cavities coupled to waveguides can be conveniently described by a general rate equation model known as (temporal) coupled mode theory (CMT). We present an alternative derivation of the CMT for optical cavity-waveguide structures, which explicitly relies on the treatment of the cavity modes as quasinormal modes with properties that are distinctly different from those of the modes in the waveguides. The two families of modes are coupled via the field equivalence principle to provide a physically appealing yet surprisingly accurate description of light propagation in the coupled systems. Practical application of the theory is illustrated using example calculations in one and two dimensions.