Coupling ideality of integrated planar high-Q microresonators

TL;DR

This study investigates the design-dependent parasitic losses in integrated high-Q microresonators, identifying higher-order mode coupling as a key loss source, and demonstrates how optimized coupler designs can significantly improve performance.

Contribution

It provides a systematic analysis of parasitic losses due to mode coupling in microresonator designs and offers optimized coupler solutions to enhance coupling ideality.

Findings

Higher-order mode coupling causes parasitic losses.

Optimized coupler design achieves near-unity ideality.

Mode exchange through the coupler affects dispersion properties.

Abstract

Chipscale microresonators with integrated planar optical waveguides are useful building blocks for linear, nonlinear and quantum optical devices. Loss reduction through improving fabrication processes has resulted in several integrated micro resonator platforms attaining quality (Q) factors of several millions. However only few studies have investigated design-dependent losses, especially with regard to the resonator coupling section. Here we investigate design-dependent parasitic losses, described by the coupling ideality, of the commonly employed microresonator design consisting of a microring resonator waveguide side-coupled to a straight bus waveguide. By systematic characterization of multi-mode high-Q silicon nitride microresonator devices, we show that this design can suffer from low coupling ideality. By performing full 3D simulations to numerically investigate the resonator to bus waveguide coupling, we identify the coupling to higher-order bus waveguide modes as the dominant origin of parasitic losses which lead to the low coupling ideality. Using suitably designed bus waveguides, parasitic losses are mitigated, and a nearly unity ideality and strong overcoupling (i.e. a ratio of external coupling to internal resonator loss rate > 9) are demonstrated. Moreover we find that different resonator modes can exchange power through the coupler, which therefore constitutes a mechanism that induces modal coupling, a phenomenon known to distort resonator dispersion properties. Our results demonstrate the potential for significant performance improvements of integrated planar microresonators, achievable by optimized coupler designs.