Breaking the phase noise-mode purity trade-off in long-loop OEOs via cascaded zero-dispersion recirculating loops

Optoelectronic oscillators (OEOs) generate microwave signals with ultra-low phase noise by incorporating long optical fiber loops as high-Q energy storage elements. However, long fiber loops reduce the free spectral range, leading to dense oscillation modes and requiring ultra-narrowband filters for single-mode operation, posing a long-standing trade-off between phase noise and mode purity. Here, we present a low-cost and scalable solution that breaks this trade-off. By inserting three cascaded zero-dispersion recirculating loops (ZD-RLs) into an amplified spontaneous emission (ASE)-based OEO, robust single-mode operation is achieved without using ultra-narrowband filters. The longest ZD-RL enhances the Q-factor of the microwave photonic filter (MPF) 4100 times, while the shorter loops increase the peak spacing within the MPF's passband to enable single-mode operation. This yields a sidemode suppression ratio of 72.7 dB and phase noise of -122 dBc Hz^-1 at 10 kHz offset at 10 GHz, matching commercial generators. This work shows that even low-cost broadband ASE sources can generate high-quality microwave signals, bridging the gap between laboratory OEO demonstrations and practical signal generators.