Remote Laser-Microwave Synchronization Over Kilometer-Scale Fiber Link With Few-Femtosecond Drift

We show the remote synchronization between a mode-locked laser and a microwave source, separated by a hundreds meter- to kilometer-scale fiber link, with few-femtosecond rms timing drift maintained over several hours. In a laboratory test, the measured timing drift between a mode-locked laser and a 2.856-GHz microwave source, separated by a 610-m fiber link, is 2.7 fs rms (0.048 mrad phase drift) over 7 h. The corresponding relative instability in remote laser-microwave synchronization is 7.2 × 10 ^-19 in 6300 s averaging time, in terms of overlapping Allan deviation. We further installed 1.15-km long fiber links in an accelerator building and measured the relative phase drift at the link outputs in a klystron gallery, which resulted in 6.6-fs and 31-fs rms timing drift maintained over 7 and 62 h, respectively. To achieve this performance, we combined a balanced optical cross-correlator (BOC)-based stabilized fiber link for remote timing transfer and an optical-microwave phase detector (OM-PD) for local optical-to-microwave synchronization. We identified the impact of power and polarization-state drift in the fiber link and amplitude-to-phase conversion in the OM-PD on the link stability. Based on this analysis, possible technical improvements enabling even higher timing precision and stability are identified.