Long-term stable remote laser synchronization over a 3.5-km fiber link with one-femtosecond residual timing jitter

Long-term stable timing distribution over a 3.5-km polarization maintaining (PM) fiber link using balanced optical cross-correlators (BOC) for link stabilization and optical-to-optical synchronization is demonstrated. Link operation over 80 hours showed only 0.36 fs RMS timing jitter and drift integrated from 100 μHz to 1 MHz, and remote laser synchronization over 36 hours showed a residual timing jitter and drift of only 0.93 fs integrated from 100 μHz to 1MHz. This level of timing precision can closely meet the timing requirements of the next generation of free-electron lasers (FELs).