Tackling the Limits of Optical Fiber Links

TL;DR

This paper investigates fundamental noise limits in optical fiber links through theoretical analysis and experiments, proposing optimized interferometers and demonstrating high stability in long-distance frequency transfer.

Contribution

It introduces new models for noise in various optical link configurations and designs low-temperature sensitivity interferometers, achieving record stability over 540 km fiber links.

Findings

Derived unsuppressed delay noise for three optical link configurations.

Designed interferometers with extremely low temperature sensitivity.

Achieved a relative frequency stability of 3E-20 over 540 km fiber link.

Abstract

We theoretically and experimentally investigate relevant noise processes arising in optical fiber links, which fundamentally limit their relative stability. We derive the unsuppressed delay noise for three configurations of optical links: two-way method, Sagnac interferometry, and actively compensated link, respectively designed for frequency comparison, rotation sensing, and frequency transfer. We also consider an alternative two-way setup allowing real-time frequency comparison and demonstrate its effectiveness on a proof-of-principle experiment with a 25-km fiber spool. For these three configurations, we analyze the noise arising from uncommon fiber paths in the interferometric ensemble and design optimized interferometers. We demonstrate interferometers with very low temperature sensitivity of respectively -2.2, -0.03 and 1 fs/K. We use one of these optimized interferometers on a long haul compensated fiber link of 540km. We obtain a relative frequency stability of 3E-20 after 10,000 s of integration time.