Ultra-stable long distance optical frequency distribution using the Internet fiber network

TL;DR

This paper demonstrates a 540 km ultrastable optical frequency distribution over the Internet fiber network, achieving high stability and robustness with a cost-effective automated station, advancing wide-area frequency comparison capabilities.

Contribution

It introduces a fully automated optoelectronic station for long-distance ultrastable frequency transfer over existing Internet fiber infrastructure.

Findings

Fractional frequency instability of 5x10^-15 at 1 second

Fractional frequency instability of 2x10^-19 at 30,000 seconds

Successful long-distance ultrastable frequency distribution over 540 km

Abstract

We report an optical link of 540 km for ultrastable frequency distribution over the Internet fiber network. The stable frequency optical signal is processed enabling uninterrupted propagation on both directions. The robustness and the performance of the link are enhanced by a cost effective fully automated optoelectronic station. This device is able to coherently regenerate the return optical signal with a heterodyne optical phase locking of a low noise laser diode. Moreover the incoming signal polarization variation are tracked and processed in order to maintain beat note amplitudes within the operation range. Stable fibered optical interferometer enables optical detection of the link round trip phase signal. The phase-noise compensated link shows a fractional frequency instability in 10 Hz bandwidth of 5x10-15 at one second measurement time and 2x10-19 at 30 000 s. This work is a significant step towards a sustainable wide area ultrastable optical frequency distribution and comparison network.