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

Researchers at MIT, US, are joining forces with MenloSystems to commercialize a set of large-scale synchronization techniques that maintain sub-10-femtosecond timing accuracy over 10 hours and a distance of 300 m and more. This is said to be the first demonstration of such high-precision, robust timing synchronization (Nature Photonics 2 733).

According to Franz Kaertner, principle investigator of the project, this result will benefit the design and operation of seeded free-electron lasers, which require extremely high timing accuracy and may be applicable to the synchronization of large-scale phased-array antennas for radio astronomy.

"Just a few years ago, people thought this level of precision could not be achieved for such a long period of time," he commented. "Our result will enable scientists and engineers in different fields to really think about how to solve their problems or enhance performance by introducing the capabilities that we have shown."

Femtosecond modelocked lasers simultaneously carry extremely low jitter optical and microwave signals. Owing to their ultralow jitter properties, they have been expected to clock large-scale scientific facilities requiring extremely high timing accuracy that conventional electronic clock distribution cannot provide. However, lack of long-term stable synchronization techniques has hindered the realization of this pervasive clocking idea.

"The timing signal needs to be detected with both high timing detection sensitivity and high thermal stability," explained Kim. "Conventionally, this timing detection has been performed in the electronic domain using high-speed photodetection of optical pulse trains followed by phase-detection with microwave mixers. However, excess noise and thermal drift has seriously limited the stability that could be achieved."

To overcome this problem, Kaertner and colleagues shifted the timing detection from the electronic to the optical domain. Extensive details of the methods used can be found in the paper. In summary, the group uses ultralow-noise optical pulse trains generated by modelocked lasers as the timing signals, then distributes them by means of timing-stabilized fibre links and, finally, synchronizes the delivered timing signals with the optical and microwave sources being targeted.

The MIT team is optimistic that due to the scalable nature of its techniques, further improvements in precision and distance are possible. "The next milestone is attosecond-precision ultrafast photonics, which will open up more applications and opportunities that require even higher timing precision," concluded Kim.