Femtosecond precision timing distribution over km level stabilized fiber link for application in XFEL

This paper demonstrates a fiber link based timing distribution with femtosecond precision. The output femtosecond laser pulse train from a passively mode-locked fiber laser serves as master clock, which is distributed over a 2 km single mode fiber link. A balanced optical cross-correlator (BOC) is used to monitor the accumulated timing error, which is actively compensated by a local fiber stretcher and a step motor. The standard deviation of the residual timing error is less than 1.1 fs over 2 h observation, and the time stability is better than 10^-18. (authors)