Mode-Locked Optoelectronic Oscillator Based on a Dual-Optical-Electrical-Loop

A mode-locked optoelectronic oscillator (OEO) that generates microwave frequency combs (MFCs) based on a dual-optical-electrical-loop without external signal injection is proposed and experimentally demonstrated. An additional feedback loop with almost the same time delay as the primary loop produces a low-frequency microwave signal, which is coupled into the primary loop to lock the OEO's mode. The experimental results show that the mode-locked fundamental and the 5th-order harmonic OEO with both repetition rates of 930 kHz are realized. Under the fundamental and 5th-order harmonic mode-locking states, the single-sideband (SSB) phase noise at 10 kHz frequency offset is measured to be -92.16 dBc/Hz and -102.22 dBc/Hz, respectively. Compared to the previously reported actively mode-locked OEO with an external injection signal, our scheme does not require external signal injection and the MFCs can be stably locked when the fiber (200m) in the loop is heated from 35 degrees C-65 degrees C, which can overcome the problems of modulated signals and mode spacing detuning in long-term operation with more flexible and universal characteristics.