Femtosecond mode-locked lasers have revolutionized many fields of science and engineering1,2,3,4. Because of their ultralow noise5, it has been anticipated that mode-locked lasers would synchronize large-scale scientific facilities6,7,8,9 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.
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The authors appreciate many discussions with D. Moncton, W. Graves, M. Ferianis, H. Schlarb, S. Milton and J. Bisognano on the timing requirements for future FELs that motivated this work. We also thank F. Wong for the PPKTP design and M. Perrott for joint work on optoelectronic phase-locked loops. We acknowledge T. Schibli for his early work on laser synchronization and A. Winter, F. Ö. Ilday, Z. Zhang, F. Loehl and F. Ludwig for their early-stage contributions to fibre link stabilization and microwave extraction. This work was supported by the following agencies: the European Union, under the EuroFEL program; the US Office of Naval Research (ONR), under the Multidisciplinary University Research Initiative (MURI) program; the US Air Force Office of Scientific Research (AFOSR); the US Defense Advanced Research Projects Agency (DARPA); and the University of Wisconsin. J.K. acknowledges a doctoral fellowship from the Samsung Scholarship Foundation.
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Kim, J., Cox, J., Chen, J. et al. Drift-free femtosecond timing synchronization of remote optical and microwave sources. Nature Photon 2, 733–736 (2008). https://doi.org/10.1038/nphoton.2008.225
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