Noise Folding in Optoelectronic PLLs for Ultralow Phase Noise: Modeling and Suppression With Experimental Validation

This article presents a detailed study on the realization of an ultralow phase noise optoelectronic frequency synthesizer. The frequency synthesis architecture employs an optoelectronic phase-locked loop (OEPLL) by which a microwave tunable oscillator (TO) is locked to the optical pulse train of a mode-locked laser (MLL). In a second phase-locked loop (PLL), the MLL is synchronized to an ultralow noise electronic signal generator (SG) to improve the MLL’ s close-in phase noise while preserving its excellent phase noise at medium to high offset frequencies. In the OEPLL, an electro-optical phase detector (PD), employing a Mach–Zehnder modulator (MZM) and a pair of photodetectors, is used. It is shown that the MZM effectively undersamples the TO signal, which can cause noise folding. By appropriate filtering of the TO signal, noise folding can be avoided. The impact of noise folding on the effective noise floor at the PD is mathematically modeled for the first time, and optimal filter parameters are derived. Furthermore, the optimal selection of MZM RF drive levels is investigated, highlighting their role in balancing nonlinear distortion, PD gain, and phase noise performance. To experimentally validate the presented analysis, frequency synthesis at a 10 GHz carrier frequency is demonstrated, achieving phase noise levels below −20 dBc/Hz at offset frequencies as low as 100 mHz. Within the OEPLL loop bandwidth of approximately 2 MHz, the measured in-band phase noise reaches below −155 dBc/Hz across offset frequencies ranging from 50 kHz to 2 MHz with a minimum noise level of approximately −170 dBc/Hz at 400 kHz offset frequency. The measured phase noise at different offset frequencies is explained from the combination of quartz-locking of the MLL, suppression of undersampling artifacts, optimal filter selectivity, modulation nonlinearity, and TO phase noise.