Laser-to-Laser Remote Transfer and Synchronization with Sub-fs Precision over a 3.5 km Fiber Link

We present recent progress made in optical, high-precision, long-term stable timing distribution systems for synchronization of next generation X-ray free-electron lasers. Timing jitter characterization of the optical master oscillatorshows less than 170 as integrated jitter for frequencies above 10 kHz, limited by the detection noise floor. Timing stabilization of a 3.5-km polarization maintaining fiber link is successfully achieved with an unprecedented RMS drift of 0.18 fs over 80 hours of operation using balanced optical cross-correlation detection scheme. The output of the fiber link is used to remotely synchronize a second mode-locked laser, and continuous operation over 40 hours shows only 0.88 fs RMS drift between the two lasers under typical facility conditions