First industrial-grade coherent fiber link for optical frequency standard dissemination

We report on a fully bi-directional 680~km fiber link connecting two cities for which the equipment, the set up and the characterization are managed for the first time by an industrial consortium. The link uses an active telecommunication fiber network with parallel data traffic and is equipped with three repeater laser stations and four remote double bi-directional Erbium-doped fiber amplifiers. We report a short term stability at 1~s integration time of $5.4\times 10^{-16}$ in 0.5~Hz bandwidth and a long term stability of $1.7\times10^{-20}$ at 65\,000 s of integration time. The accuracy of the frequency transfer is evaluated as $3\times 10^{-20}$. No shift is observed within the statistical uncertainty. We show a continuous operation over 5 days with an uptime of 99.93$\%$. This performance is comparable with the state of the art coherent links established between National Metrology Institutes in Europe. It is a first step in the construction of an optical fiber network for metrology in France, which will give access to an ultra-high performance frequency standard to a wide community of scientific users.