Distributed Raman Optical Amplification in Phase Coherent Transfer of Optical Frequencies

We describe the application of Raman optical-fiber amplification (ROA) for the phase coherent transfer of optical frequencies in an optical fiber link. ROA uses the transmission fiber itself as a gain medium for bi-directional coherent amplification. In a test setup, we evaluated the ROA in terms of ON/OFF gain, signal-to-noise ratio, and phase noise added to the carrier. We transferred a laser frequency in a 200 km optical fiber link, and evaluated both co-propagating and counter-propagating amplification pump schemes. Then, we identified the occurrence of nonlinearities on the signal for non-optimized ROA gain. The frequency at the remote end has a fractional frequency instability of 3·10^-19 over 1000 s with the optical fiber link noise compensation.