Broadband and wide-range feedback tuning scheme for phase-locked loop stabilization of tunable optoelectronic oscillators.

In this Letter, we propose and experimentally demonstrate a novel broadband and wide-range feedback tuning scheme for phase-locked loop (PLL) stabilization of tunable optoelectronic oscillators (OEO). The proposed scheme was realized in a simple and feasible way based on a dual-parallel Mach-Zehnder modulator (DPMZM) and optical bandpass filter (OBPF). The wide RF phase-shift range and broadband performance of the proposed scheme were demonstrated theoretically and experimentally, which ensured OEO's wide operating temperature range and tunability in PLL stabilization. As a result, PLL stabilization for OEO was achieved at different oscillating frequencies and the long-term stability was greatly improved without any thermal control. The measured overlapping Allan deviation of PLL-locked OEO reached lower than 10⁻¹⁰ after 1000-s averaging time, which was four orders of magnitude better than the free-running OEO. The phase noise performance was also improved at low-offset frequencies and remained the same at high-offset frequencies.