Distributed Raman amplification in phase coherent transfer of optical frequencies over long-haul and metro fiber links

This paper reports a study on distributed Raman amplification (DRA) in the phase-coherent transfer of narrow linewidth optical frequencies in single-mode fiber links. Both co-and counter-propagating amplification pump schemes have been investigated and with optimized input parameters, a coherent optical frequency at 1542 nm was transferred on a 305 km fiber. Moreover, we report on the successful use of DRA for the optical frequency transfer on a real 94 km telecom metro fiber link in presence of two ITU WDM data channels with an attained fractional frequency instability of 3·10^-19 at 1000 s.