Ultra-stable long distance optical frequency distribution using the Internet fiber network and application to high-precision molecular spectroscopy

We report an optical link of 540 km for ultrastable frequency distribution over the Internet fiber network. The phase-noise compensated link shows a fractional frequency instability in full bandwidth of 3?10?14 at one second measurement time and 2?10?18 at 30 000 s. This work is a significant step towards a sustainable wide area ultrastable optical frequency distribution and comparison network. Time transfer was demonstrated simultaneously on the same link and led to an absolute time accuracy (250 ps) and long-term timing stability (20 ps) which outperform the conventional satellite transfer methods by one order of magnitude. Current development addresses the question of multiple users distribution in the same metropolitan area. We demonstrate on-line extraction and first results show frequency stability at the same level as with conventional link. We also report an application to coherent frequency transfer to the mid-infrared. We demonstrate the frequency stabilisation of a mid-infrared laser to the near-infrared frequency reference transferred through the optical link. Fractional stability better than 4?10?14 at 1 s averaging time was obtained, opening the way to ultrahigh resolution spectroscopy of molecular rovibrational transitions.