Brillouin amplification supports $1\times10^{-20}$ accuracy in optical frequency transfer over 1400~km of underground fibre

TL;DR

This paper demonstrates that optical frequency transfer over 1400 km of underground fiber using autonomous Brillouin amplifiers can achieve an unprecedented fractional frequency accuracy of about 1.1×10⁻²⁰, enabling ultra-precise measurements.

Contribution

The study shows that fiber Brillouin amplifiers enable phase-continuous, long-distance optical frequency transfer with record accuracy, advancing the capabilities of remote frequency comparisons.

Findings

Achieved fractional frequency offset of (1.1±0.4)×10⁻²⁰ over 1400 km.

Attained an instability of 6.9×10⁻²¹ at ~30,000 s averaging time.

Set an upper limit for uncertainty in long-distance optical frequency transfer.

Abstract

We investigate optical frequency transfer over a 1400~km loop of underground fibre connecting Braunschweig and Strasbourg. Largely autonomous fibre Brillouin amplifiers (FBA) are the only means of intermediate amplification, allowing phase-continuous measurements over periods up to several days. Over a measurement period of about three weeks we find a weighted mean of the transferred frequency's fractional offset of $(1.1\pm0.4)\times10^{-20}$. In the best case we find an instability of $6.9\times10^{-21}$ and a fractional frequency offset of $4.4\times10^{-21}$ at an averaging time of around 30~000~s. These results represent an upper limit for the achievable uncertainty over 1400 km when using a chain of remote Brillouin amplifiers, and allow us to investigate systematic effects at the $10^{-20}$-level.