Wide-Range Tunable Coherent Dual-Frequency Microwave Signal Generation With Low Spurious Components in Optoelectronic Oscillator

We propose and demonstrate an approach to generating wide-range tunable coherent dual-frequency microwave signals with high side-mode suppression ratios (SMSRs), ultra-low phase noise and high stability, based on a dual-passband optoelectronic oscillator (OEO). The narrowband passband with a relatively high gain is used to generate the initial oscillation at a fixed frequency. Through injecting a single-tone signal into the OEO cavity via a microwave mixer, frequency-tunable oscillation in the other broadband passband with a relatively low gain is initiated through frequency mixing injection locking. Then, the two oscillation signals are phase-locked through frequency mixing mutual injection locking. In addition, a dual-loop configuration with the assistance of balanced photodetection is utilized to suppress the side modes and improve the phase noise performance, which also enhances the stability of the generated signal. In the experiment, coherent dual-frequency microwave signals with a fixed frequency at 20.0056 GHz and a tunable frequency in the range of 13.9705 GHz to 18.0075 GHz are generated, where the frequency tuning step is 625.6 kHz. The power fluctuation during frequency tuning is smaller than 1.5 dB, and the SMSRs are larger than 77 dB. In addition, the Allan deviation and the phase noise are in the order of 10^–11@1 s and better than –138 dBc/Hz@10 kHz, respectively.