Self-mode-locking optoelectronic oscillator with ultrashort time delay

The optoelectronic oscillator (OEO) is a typical time-delay system with rich nonlinear dynamical characteristics. Most of the previous research on OEOs has been focused on analyzing the properties of OEOs with a long time delay, which makes it difficult to realize mode locking without additional phase-locking mechanisms. We have achieved, for the first time to our knowledge, a self-mode-locking OEO and generated stable microwave frequency combs by analyzing the characteristics of OEOs with an ultrashort time scale. In the experiment, the self-mode-locking OEOs with fundamental mode, second-order harmonic, and sixth-order harmonic were realized by adjusting the system parameters, all of which produced uniform square wave signals with tunable duty cycles, steep rising and falling edges, and periods of less than 20 ns. The self-fundamental-mode-locking OEOs with different time delays were also implemented and experimentally realized. Furthermore, the experiment revealed the self-hybrid mode-locking OEO, which is the coexistence and synchronization of the three measured self-locking modes in one OEO cavity, demonstrating the complex nonlinear dynamical behaviors of the OEO system and enabling the generation of periodic nonuniform hybrid square wave signals. The realization of the self-mode-locking OEO and the generation of flexible and stable square wave signals at ultrashort time scales enrich the study of OEO nonlinear dynamics in the realm of complex microwave waveform generation, offering promising applications in areas such as atomic clocks, radars, communications, and optoelectronic neural networks.