Low-Spurious Multiformat Microwave Signal Generation Based on an Actively Mode-Locked Dual-Loop Optoelectronic Oscillator

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A novel photonic approach for the generation of multiformat microwave signals with low-spurious components is proposed and experimentally demonstrated, based on an actively mode-locked dual-loop optoelectronic oscillator (OEO). In this scheme, a low-frequency electrical signal is injected into the Mach–Zehnder modulator (MZM) to dynamically control the net gain of the OEO cavity. When the modulation frequency is set to an integer multiple of the cavity’s free spectral range (FSR), harmonic mode locking is realized, enabling flexible waveform generation. By tailoring the waveform format of the injected signal, the OEO is capable of generating a diverse set of microwave signal types, including single-tone carriers, pulse trains, rectangular waveforms, staggered pulses, and pulse-coded signals. To suppress undesired longitudinal modes and mitigate supermode noise, a dual-loop architecture exploiting the Vernier effect is employed, significantly enhancing spectral purity. In proof-of-concept experiments, a single-tone signal at 10.8 GHz with a side-mode suppression ratio of 82 dB is achieved, corresponding to a 55-dB improvement compared to a single-loop configuration. Microwave pulse trains with a 603-kHz repetition rate are also generated under 13th-order harmonic mode locking, exhibiting phase noise levels better than −140 dBc/Hz at a 10-kHz offset and a supermode suppression ratio exceeding 80 dB. This represents an enhancement of more than 70 dB in supermode noise suppression relative to the single-loop case. The proposed architecture provides a compact, low noise, and adjustable solution for high-purity microwave waveform generation, with potential applications in radar, wireless communications, and electronic warfare systems.