Ultra-Compact Multimode Microring Resonator Based on Cubic Spline Curves

Microring resonators have become one of the key components for realizing signal processing, generation, and integration in microwave photonics, owing to their high Q factor, compact footprint, and tunability. However, most of the reported microring resonators are confined to the single-mode regime. In this paper, we design an ultra-compact multimode microring resonator (MMRR) based on shape-optimized multimode waveguide bends (MWBs). Cubic spline curves are used to represent the MWB boundary and the adjoint methods are utilized for inverse optimization, achieving an effective radius of 8 μm. Asymmetric directional couplers (ADCs) are designed to independently couple three modes into multimode microring, according to phase-matching conditions and transmission analysis. The MMRR is successfully fabricated on a commercial platform using 193-nm dry lithography process. The device exhibits high loaded Q factors of 2.3 × 105, 4.1 × 104 and 2.9 × 104, and large free spectral ranges (FSRs) of 5.4, 4.7 and 4.2 nm for TE0, TE1 and TE2 modes, with about 19 × 55 μm² footprint.