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 oscillator shows 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.