Few-mode silicon nitride elliptic microdisk resonators with a high-quality factor

Conventional resonators such as microring resonators (MRRs) or microdisk resonators (MDRs) in large sizes can achieve a high quality factor (Q factor) but also introduce the trouble of multi-mode. Here, a silicon nitride elliptic MDR is designed and experimentally verified to suppress higher order modes while achieving a high Q factor. The resonator consists of an elliptic microdisk and an optimized bent waveguide. The bending loss in the waveguide is used to break the coupling conditions of the modes, thus reducing the coupling efficiency of the higher-order modes and obtaining a few-mode MDR transmission spectrum. In the experiment, the elliptic MDR with only two radial modes is successfully obtained. The TE₁-mode 3-dB bandwidth, Q factor, and extinction ratio (ER) are 123 MHz, 1.57 × 10⁶, and 14.8 dB, respectively. The suppression ratio (SR) between modes TE₁ and TE₂ is 12 dB. The proposed few-mode elliptic MDR has a broad application prospect in integrated microwave photonic systems.