Brillouin amplification in phase coherent transfer of optical frequencies over 480 km fiber

TL;DR

This paper demonstrates the use of fiber Brillouin amplification for coherent optical frequency transfer over 480 km of fiber, achieving high stability and low uncertainty with a simplified stabilization technique.

Contribution

It introduces fiber Brillouin amplification for long-distance optical frequency transfer, showing improved gain, noise performance, and a new stabilization method compared to traditional amplifiers.

Findings

Achieved frequency transfer with uncertainty below 2x10^-18.

Demonstrated stable transfer over 480 km fiber link.

Compared FBA performance favorably to EDFA amplifiers.

Abstract

We describe the use of fiber Brillouin amplification (FBA) for the coherent transmission of optical frequencies over a 480 km long optical fiber link. FBA uses the transmission fiber itself for efficient, bi-directional coherent amplification of weak signals with pump powers around 30 mW. In a test setup we measured the gain and the achievable signal-to-noise ratio (SNR) of FBA and compared it to that of the widely used uni-directional Erbium doped fiber amplifiers (EDFA) and to our recently built bi-directional EDFA. We measured also the phase noise introduced by the FBA and used a new and simple technique to stabilize the frequency of the FBA pump laser. We then transferred a stabilized laser frequency over a wide area network with a total fiber length of 480 km using only one intermediate FBA station. After compensating the noise induced by the fiber, the frequency is delivered to the user end with an uncertainty below 2x10-18 and an instability sigma(tau) = 2x10-14/(tau/second).