Fiber-optical delivery of 10 Gb/s data with picosecond timing accuracy over 75 km distance

The transfer of very accurate time information over long distances has numerous applications in modern society, including navigation, synchronization of critical infrastructure and state-of-the-art scientific research. Satellite-based links have typically been used so far, but their accuracy is limited to the nanosecond level and their availability can be compromised by natural or man-made factors. Fiber-optical signals can transfer time with increased precision and resiliency and such systems are being widely researched, mostly in the context of stand-alone time and frequency transfer. In this paper, a novel method that combines picosecond-level time transfer with conventional optical data transmission is proposed and its key functionality is demonstrated. The method is based on cross-correlating two-way high-speed data streams and determining the time delay in the link between the clocks that need to be synchronized. The cross correlation technique itself is here demonstrated to be capable of determining (instrument) delays with a resolution of a few hundred femtoseconds. Techniques that estimate and remove the impact of accuracy-limiting effects such as the presence of unequal path lengths and asymmetric propagation times of different wavelengths in the fiber link have been developed. The accuracy of the method is limited by systematic uncertainties and has been calculated to be 4 ps. An experimental test-bed to evaluate the proposed method has been built, consisting of a bidirectional optically amplified fiber link operating at 10 Gb/s bit rates. The measured timed delays over different link lengths (25, 50 and 75 km) are in very good agreement with the estimated ones, corroborating the calculated accuracy of the proposed technique. As the method is fast (4 ps accuracy is achieved using a single burst o