Multimode AlGaAs-on-Insulator Microring Resonators for Nonlinear Photonics

The aluminum gallium arsenide-on-insulator (AlGaAsOI) platform exhibiting high nonlinearity, strong light confinement, and negligible two-photon absorption (TPA) has motivated research in realizing highly efficient nonlinear devices. Resonator structures can push nonlinear efficiency to unprecedented levels by significantly enhancing the cavity field. In recent years, multimode microring resonators, with lower scattering loss than single-mode designs, have garnered increasing attention for chip-based nonlinear photonics applications. However, the intermodal coupling will degrade the Q and introduce avoided mode crossing (AMX), causing changes in local dispersion. The perturbed dispersion significantly influences the phase-matching condition, making AMX management crucial for multimode resonator design. This paper introduces a fabrication-tolerant tapered waveguide design to achieve selective-mode coupling, enabling single-mode excitation in the bus-ring coupling region and eliminating the unwanted higher-order mode of the multimode waveguide. Leveraging the multimode microring resonators in the AlGaAsOI platform, we demonstrate dark-pulse comb generation through TE-TE mode coupling, and we obtain a large dark pulse existence range of 89 GHz by backward tuning. Moreover, we achieve a widely separated (45 THz) optical parametric oscillation (OPO) with the assistance of higher-order dispersion.