All-optical link for direct comparison of distant optical clocks

TL;DR

This paper presents an all-optical fiber link system enabling highly stable remote comparison of distant optical clocks, crucial for assessing optical frequency standard reproducibility.

Contribution

The authors developed a fiber-based optical link with phase noise compensation and polarization control, achieving record stability for remote optical clock comparison.

Findings

Achieved transfer stability of 2×10^{-15} at 1s and 4×10^{-18} at 1000s over 90 km.

Demonstrated stable comparison of two distant ^87Sr optical clocks.

System is suitable for high-precision optical frequency standard measurements.

Abstract

We developed an all-optical link system for making remote comparisons of two distant ultra-stable optical clocks. An optical carrier transfer system based on a fiber interferometer was employed to compensate the phase noise accumulated during the propagation through a fiber link. Transfer stabilities of $2\times10^{-15}$ at 1 second and $4\times10^{-18}$ at 1000 seconds were achieved in a 90-km link. An active polarization control system was additionally introduced to maintain the transmitted light in an adequate polarization, and consequently, a stable and reliable comparison was accomplished. The instabilities of the all-optical link system, including those of the erbium doped fiber amplifiers (EDFAs) which are free from phase-noise compensation, were below $2\times10^{-15}$ at 1 second and $7\times10^{-17}$ at 1000 seconds. The system was available for the direct comparison of two distant $^{87}$Sr lattice clocks via an urban fiber link of 60 km. This technique will be essential for the measuring the reproducibility of optical frequency standards.