Deterministic Parity‐Time Symmetry Single‐Mode Oscillation in Filterless Multimode Resonators

Abstract Parity‐time symmetry has great potential for mode selection in multimode resonators. However, in a PT‐symmetry system with saturable absorption mechanisms, the random background noise can initiate single‐mode oscillation at any of the maxima within the gain spectrum, that is, potential PT frequencies. Such randomness impedes the acquisition of high‐quality signals at desired frequencies. Here, this work proposes a method to obtain deterministic PT‐symmetry single‐mode oscillation in a filterless multimode resonator through one‐shot injection. With this technique, this work changes the system's gain spectrum and enhances the gain discrepancy. Utilizing the frequency domain saturable absorption of the resonator, oscillation at desired mode can maintain its frequency after the withdrawal of the injection signal. To validate the concept, this work establishes a polarization‐division multiplexed dual‐loop optoelectronic oscillator (OEO) with a 1‐km long cavity operated under PT‐symmetry conditions. By one‐shot injecting the PT‐OEO, this work effectively eliminates the randomness arising from the relatively flat gain spectrum, facilitating oscillation at any desired potential PT frequencies from 1.8 to 9 GHz without requiring elaborate frequency tuning structures. Moreover, the one‐shot injection technique produces ultra‐low phase noise performance, achieving a remarkable −158.6 dBc Hz −1 @10 kHz. This performance level stands in close comparison with the best phase noise values recorded for OEOs.