MIT Research Laboratory of Electronics (RLE) engineers have achieved a dramatic breakthrough in drift-free synchronization based on mode-locked lasers. This achievement is an important milestone in transitioning mode-locked laser-based synchronization systems from the laboratory into real-world facilities.

Their work, reported in Nature Photonics on November 2nd, could bring unprecedented capabilities to large-scale facilities that need femtosecond accuracy for many hours of continuous operation. In particular, advanced X‑ray free-electron lasers (FELs)—some already under construction and others planned in the near future—immediately require such extremely high timing accuracy. Another potential beneficiary of the new RLE findings could be phased-array antennas for radio astronomy such as the Atacama Large Millimeter Array (ALMA) currently under construction.

The principal investigator of the team announcing these findings, Franz X.Kärtner, Professor of Electrical Engineering and senior member of the RLE Optics and Quantum Electronics Group, said, “When we started this work four years ago most people in the field thought this was impossible to do. Now, we have already demonstrated precision and stability that enables a new generation of light sources with vastly improved performance. In fact, significant funding for this work came from European facilities that are in the process of implementing these systems.”

Femtosecond (10-15 s) mode-locked lasers have revolutionized many fields of science, most recently by enabling high-precision optical frequency measurements using frequency combs. Owing to their ultra-low noise properties, mode-locked lasers have been expected to clock large-scale scientific facilities requiring extremely high timing accuracy that conventional electronic clocking cannot provide.

However, lack of long-term stable synchronization techniques has hindered the realization of this pervasive clocking idea. None of the previous work has demonstrated drift-free remote synchronization over hundreds of meters with femtosecond timing accuracy maintained over extended periods of time, which is an essential prerequisite for running such large-scale facilities.

In their Nature Photonics paper, the RLE researchers present a comprehensive set of novel large-scale synchronization techniques that achieve, for the first time, sub-10-femtosecond timing accuracy maintained over more than 10 hours and over distances of more than 300 m. The demonstrated relative timing stability in timing distribution, optical-optical synchronization, and optical-microwave synchronization represents multiple orders of magnitude improvement compared to previous work.

Said Jungwon Kim, the lead author of the paper, “It is really exciting to see the techniques developed in our lab are already tested and becoming installed in real accelerator facilities around the world.”

The equivalent accuracy of synchronization is keeping the timing with less than a second accumulated error since the birth of the universe.

As the demand for higher timing accuracy increases, the technique developed by the RLE researchers will find more applications in commercial applications such as communication and computation networks and precise tracking/positioning systems.

The lead author of the paper is Dr. Kim (PhD ’07), a postdoctoral associate in RLE. Other authors are Electrical Engineering and Computer Science graduate students Jonathan A. Cox and Jian Chen, and Professor Kärtner, all members of RLE.

The research was funded by the European Union, under the EuroFEL program; the U.S. Office of Naval Research (ONR), under the Multidisciplinary University Research Initiative (MURI) program; the U.S. Air Force Office of Scientific Research (AFOSR); the U.S. Defense Advanced Research Projects Agency (DARPA); the University of Wisconsin; and the Samsung Scholarship Foundation.

Related Links:

Drift-free femtosecond timing synchronization of remote optical andmicrowave sources

Professor Franz X. Kärtner

RLE Optics and Quantum Electronics Group

Department of Electrical Engineering and Computer Science