Optical frequency transfer via a 660 km underground fiber link using a remote Brillouin amplifier

TL;DR

This paper demonstrates ultrastable optical frequency transfer over 660 km of underground fiber using a remote Brillouin amplifier, achieving extremely low instability and high accuracy suitable for precision applications.

Contribution

First implementation of ultrastable optical frequency transfer with a remote fiber Brillouin amplifier over a long underground fiber link.

Findings

Frequency transfer instability around 1×10⁻¹⁹ at 3000 s

Modified Allan deviation reaches 3×10⁻¹⁹ at 100 s

Overall accuracy estimated at 1×10⁻¹⁹

Abstract

In long-haul optical continuous-wave frequency transfer via fiber, remote bidirectional Er$^+$-doped fiber amplifiers are commonly used to mitigate signal attenuation. We demonstrate for the first time the ultrastable transfer of an optical frequency using a remote fiber Brillouin amplifier, placed in a server room along the link. Using it as the only means of remote amplification, on a 660 km loop of installed underground fiber we bridge distances of 250 km and 160 km between amplifications. Over several days of uninterrupted measurement we find an instability of the frequency transfer (Allan deviation of $\Lambda$-weighted data with 1 s gate time) of around $1\times10^{-19}$ and less for averaging times longer than 3000 s. The modified Allan deviation reaches $3\times10^{-19}$ at an averaging time of 100 s, corresponding to the current noise floor at this averaging time. For averaging times longer than 1000 s the modified Allan deviation is in the $10^{-20}$ range. A conservative value of the overall accuracy is $1\times10^{-19}$.