Wideband Frequency Synthesis Using Dual-Optical Phase-Locked Loop Synchronized to Optical Comb

This paper presents a tunable and phase-stable frequency synthesizer based on optical heterodyne mixing. Two continuous-wave lasers (CWLs) are frequency-stabilized to distinct lines of an ultra-stable optical frequency comb (OFC) of a mode-locked laser (MLL) using dual-optical phase-locked loop (OPLL) with offset locking. A compact coupler-isolator optical assembly is implemented, with the optical paths planned to reduce path mismatches and suppress fiber-induced phase fluctuations. Sub-GHz OPLL offsets (below 400 MHz) lead to relaxed reference phase noise requirements. The OPLLs effectively suppress closely spaced comb lines and mixing artifacts, with a loop bandwidth of approximately 1 MHz. Stable locking is demonstrated across the 10–30 GHz frequency range, with mid-band phase noise below −85 dBc/Hz for 2–200 kHz offset frequencies. Measurements at multiple frequencies confirm frequency-independent additive phase noise, validating the dual-OPLL approach's potential for compact, tunable and phase-stable terahertz synthesis.