Measurement noise floor for a long-distance optical carrier transmission   via fiber
  We investigated the measurement floor and link stability for the transfer of an ultra-stable optical frequency via an optical fiber link. We achieved a near-delay-limited instability of 3x10^(-15)/(tau x Hz) for 147 km deployed fiber, and 10^(-20) (integrations time tau = 4000 s) for the noise floor. 
G. Grosche, O. Terra, K. Predehl, T. Haensch, R. Holzwarth, B.   Lipphardt, F. Vogt, U. Sterr, H. Schnatz