Drift-free femtosecond timing synchronization of remote optical and microwave sources

Femtosecond mode-locked lasers have revolutionized many fields of science and engineering(1-4). Because of their ultralow noise(5), it has been anticipated that mode-locked lasers would synchronize large-scale scientific facilities(6-9) requiring extremely high timing accuracy. However, the lack of long-term stable synchronization techniques has hindered the realization of pervasive synchronization with such lasers. Here we present a comprehensive set of new techniques for long-term stable synchronization of optical and microwave sources over long distances. We use ultralow-noise optical pulse trains generated from mode-locked lasers as the timing signals, then distribute them by means of timing-stabilized fibre links and, finally, synchronize the delivered timing signals with the optical and microwave sources being targeted. Using these techniques, we demonstrate, for the first time, that remotely located lasers and microwave sources can be synchronized with less than 10-fs precision over more than 10 h. This drift-free operation is an important milestone in transitioning mode-locked laser-based synchronization from the laboratory into real-world facilities.