Cascaded optical fiber link using the Internet network for remote clocks comparison

TL;DR

This paper demonstrates a 1100 km cascaded optical fiber link over the Internet with active noise compensation, achieving ultra-stable frequency transfer suitable for wide-area optical frequency comparison.

Contribution

It introduces a fully automated, long-distance optical fiber link with active noise compensation, extending the reach to 1480 km while maintaining high stability.

Findings

Achieved fractional frequency instability of 4x10^-16 at 1 s

Extended the link to 1480 km with similar performance

Operated continuously for 3.5 days with high stability

Abstract

We report a cascaded optical link of 1100 km for ultra-stable frequency distribution over an Internet fiber network. The link is composed of four spans for which the propagation noise is actively compensated. The robustness and the performance of the link are ensured by five fully automated optoelectronic stations, two of them at the link ends, and three deployed on the field and connecting the spans. This device coherently regenerates the optical signal with the heterodyne optical phase locking of a low-noise laser diode. Optical detection of the beat-note signals for the laser lock and the link noise compensation are obtained with stable and low-noise fibered optical interferometer. We show 3.5 days of continuous operation of the noise-compensated 4-span cascaded link leading to fractional frequency instability of 4x10-16 at 1-s measurement time and 1x10-19 at 2000 s. This cascaded link was extended to 1480-km with the same performance. This work is a significant step towards a sustainable wide area ultra-stable optical frequency distribution and comparison network at a very high level of performance.