Phase- coherent comparison of two optical frequency standards over 146 km using a telecommunication fiber link

TL;DR

This study demonstrates phase-coherent transfer of optical frequency standards over 146 km of telecommunication fiber with extremely low uncertainty and high stability, enabling precise remote optical clock comparisons.

Contribution

It presents a novel implementation of a phase-coherent optical frequency transfer over long-distance fiber with low noise and high stability, including noise compensation techniques.

Findings

Frequency transfer uncertainty below 1E-19

Short-term frequency instability of 3.3E-15

Successful remote comparison of optical clocks

Abstract

We have explored the performance of two "dark fibers" of a commercial telecommunication fiber link for a remote comparison of optical clocks. The two fibers, linking the Leibniz University of Hanover (LUH) with the Physi-kalisch-Technische Bundesanstalt (PTB) in Braunschweig, are connected in Hanover to form a total fiber length of 146 km. At PTB the performance of an optical frequency standard operating at 456 THz was imprinted to a cw trans-fer laser at 194 THz, and its frequency was transmitted over the fiber. In order to detect and compensate phase noise related to the optical fiber link we have built a low-noise optical fiber interferometer and investigated noise sources that affect the overall performance of the optical link. The frequency stability at the remote end has been measured using the clock laser of PTB's Yb+ frequency standard operating at 344 THz. We show that the frequency of a frequency-stabilized fiber laser can be transmitted over a total fiber length of 146 km with a relative frequency uncertainty below 1E-19, and short term frequency instability given by the fractional Allan deviation of sy(t)=3.3E-15/(t/s).