Tunable optoelectronic oscillator with higher-order harmonic suppression based on a silicon nitride dual-ring structure

Optoelectronic oscillators (OEOs) using on-chip optical resonators have shown great potential for broadband-tunable local-oscillator generation. However, these OEOs are often constrained by limited spectral purity, as higher-order harmonics can be generated due to nonlinear gain compression and limited optical rejection. In this Letter, a broadband-tunable chip-based OEO with highly suppressed higher-order harmonics is demonstrated using a silicon nitride dual-ring structure. The proposed OEO provides higher-order harmonic suppression of up to 62 dB across 2-25 GHz and achieves a phase noise of -115 dBc/Hz at a 10-kHz offset for a 20-GHz oscillation with a 1-km fiber delay. Furthermore, the OEO demonstrates oscillation at a maximum frequency of 110 GHz. This work provides a new route toward broadband-tunable chip-based OEOs with improved spectral purity.