A plug-and-play solution for characterizing two-way optical frequency transfer over free-space

TL;DR

This paper introduces a portable, plug-and-play two-way free-space optical link system capable of high-precision frequency transfer over urban distances, demonstrating high stability and reliability for scientific applications.

Contribution

The authors developed a compact, robust, and easy-to-deploy system for free-space optical frequency transfer, enabling practical high-precision measurements in field conditions.

Findings

Achieved fractional frequency instability of 2.0 x 10^-19 at 10 s averaging time.

Maintained 94% uptime over 15 hours in field experiments.

Successfully characterized a 3.4 km intra-city free-space link.

Abstract

Optical clock networks connected by phase-coherent links offer significant potential for advancing fundamental research and diverse scientific applications. Free-space optical frequency transfer extends fiber-based connectivity to remote areas and holds the potential for global coverage via satellite links. Here we present a compact and robust portable, rack-integrated two-way free-space link characterization system. Equipped with plug-and-play capabilities, the system enables straightforward interfacing with various optical systems and facilitates quick deployment for field experiments. In this work, we achieve a fractional frequency instability of $2.0 \times 10^{-19}$ for an averaging time of 10 s over a 3.4 km horizontal fully folded intra-city free-space link. Moreover, the system maintains an uptime of $94\%$ over 15 hours, illustrating its reliability and effectiveness for high-precision optical frequency comparisons over free-space.