Coherent optical frequency transfer at 5e-19 over a doubled 642 km fiber   link
  To significantly improve the frequency references used in radio-astronomy and precision measurements in atomic physics, we provide frequency dissemination through a 642 km coherent optical fiber link, that will be also part of a forthcoming European network of optical links. We obtained a resolution of 3e-19 at 1000 s on the frequency transfer, and an accuracy of 5e-19. The ultimate link performance has been evaluated by doubling the link to 1284 km, demonstrating a new characterization technique based on the double round-trip on a single fiber. The arming of a second fiber is avoided: this is beneficial to long hauls realizations in view of a continental fiber network for frequency and time metrology. The data analysis is based on the Allan deviation; its expression is theoretically derived for the observed noise power spectrum, which is seldom found in the literature. 
D. Calonico, E. K. Bertacco, C. E. Calosso, C. Clivati, G. A.   Costanzo, M. Frittelli, A. Godone, A. Mura, N. Poli, D. V. S...
physics.optics