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We describe the use of fiber Brillouin amplification (FBA) for the coherenttransmission of optical frequencies over a 480 km long optical fiber link. FBAuses the transmission fiber itself for efficient, bi-directional coherentamplification of weak signals with pump powers around 30 mW. In a test setup wemeasured the gain and the achievable signal-to-noise ratio (SNR) of FBA andcompared it to that of the widely used uni-directional Erbium doped fiberamplifiers (EDFA) and to our recently built bi-directional EDFA. We measuredalso the phase noise introduced by the FBA and used a new and simple techniqueto stabilize the frequency of the FBA pump laser. We then transferred astabilized laser frequency over a wide area network with a total fiber lengthof 480 km using only one intermediate FBA station. After compensating the noiseinduced by the fiber, the frequency is delivered to the user end with anuncertainty below 2x10-18 and an instability sigma(tau) = 2x10-14/(tau/second).

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