Ultra-Low Phase Noise Frequency Synthesis Using Electro-Optic Detector-Based Comb-Microwave Synchronization

This paper presents an ultra-low phase noise mode-locked laser (MLL) referenced Mach-Zehnder modulator (MZM) based opto-electronic phase-locked loop (OEPLL). The phase noise dynamics of the OEPLL is characterized to account for both in-band and out-of-band contributions. We implement single-output modulation with unbalanced photodetection, in contrast with the state-of-the art balanced detection systems. By precise MLL synchronization to a quartz-based signal generator, coupled with optimization of the MLL pulse energy and MZM RF drive levels, we achieve significant improvements in both in-band and out-of-band OEPLL phase noise performance. For a 10 GHz carrier, the OEPLL achieves phase noise below −20 dBc / Hz at offset frequencies as low as 100 mHz. With a loop bandwidth of approximately 2 MHz, the OEPLL achieves in-band phase noise below −155 dBc / Hz across 50 kHz-2 MHz offset frequencies, with a minimum level of approximately −170 dBc / Hz.