Beyond critical coupling: optimal design considerations for spontaneous four-wave mixing in microring resonators

TL;DR

This paper develops an analytical model for biphoton generation in microring resonators, providing design insights for optimizing photon rates and spectral properties under various coupling and pumping conditions.

Contribution

It introduces a comprehensive analytical framework that predicts biphoton correlations and generation rates for different resonator geometries and pumping schemes.

Findings

Critical coupling maximizes single-photon extraction rate.

Overcoupling enhances biphoton pair generation under CW pumping.

Tradeoffs exist between coupling, spectral factorability, and photon extraction efficiency.

Abstract

We present a self-contained analytical model for biphoton generation in microring resonators. Encompassing both all-pass and add-drop geometries, identical and distinct pump and biphoton coupling coefficients, and continuous-wave and pulsed pumping, our interaction-picture-based approach reveals time-frequency biphoton correlations while also predicting absolute generation rates. Under continuous-wave excitation, we find critical coupling of both the pump and biphoton to maximize the rate of single photons extracted from the microring, whereas critical coupling of the pump but overcoupling of the biphoton maximize the two-photon rate. Under pulsed pumping, overcoupling of both pump and biphoton (to different degrees) maximizes photon extraction probabilities, albeit under a tradeoff with spectral factorability that we quantify via parameter scans over a range of coupler pairings. As a whole, our formalism should prove valuable for the practical design of integrated photon sources, merging a flexible and intuitive biphoton-centric depiction with quantitative predictions closely tied to experimental parameters.