Multiple critical coupling and sensing in a microresonator-waveguide system

TL;DR

This paper investigates multiple critical coupling configurations in a microresonator-waveguide system, revealing oscillatory coupling behavior and potential for sensitive environmental sensing and optical switching.

Contribution

It demonstrates that multiple geometrical configurations can achieve critical coupling due to oscillatory power flow, expanding understanding of microresonator-waveguide interactions.

Findings

Multiple critical coupling configurations exist due to oscillatory coupling.

Critical coupling depth is highly sensitive to environmental changes.

System can be used for sensitive sensing and optical switching.

Abstract

We study the optical transmission of a waveguide that is side-coupled to a high-$Q$ circular microresonator. The coupling is critical if the intrinsic resonator losses equate the coupling losses to the waveguide. When this happens, the transmittance of the waveguide displays resonance dips with maximal depth as the frequency is swept through the resonators resonances. We show that multiple configurations, parameterised by the minimal distance between the resonator and the waveguide, can lead to critical coupling. Indeed, for a sufficiently large resonator radius, the flow of power between the waveguide and the resonator can change sign several times within a single pass. This leads to an oscillatory coupling parameter as a function of the separation distance. As a result, multiple geometrical configurations can lead to critical coupling, even if the waveguide lies in the equatorial plane of the resonator. These results are explained using coupled-mode theory and full wave numerical simulations. In the vicinity of secondary or higher-order critical coupling, the depth of the transmittance dip is very sensitive to the environment. We discuss how this effect can be exploited for sensing purpose. Alternatively, by actively controlling the environment in the secondary critical configuration, the waveguide/resonator system can be driven as an optical switch.