Deployment of a 2 x 856 km coherent optical link for high performance frequency transfer

In collaboration with our academic partners LNE-SYRTE and LPL, and in the frame of the REFIMEVE+ project, we have deployed an optical coherent link of 2 x 856 km between Paris and Grenoble using the fibers provided by RENATER. We have achieved an extremely high stability of the optical frequency at the output of the fiber link (phase stability better than 2 fs), and demonstrated an excellent robustness of the whole system (uptime > 99.9% during the 6 days duration of the measurement campaign). Propagation of optical frequencies at this level of stability over such long distances represent a world record and an important step for the REFIMEVE+ project.