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To significantly improve the frequency references used in radio-astronomy andprecision measurements in atomic physics, we provide frequency disseminationthrough a 642 km coherent optical fiber link, that will be also part of aforthcoming European network of optical links. We obtained a resolution of3e-19 at 1000 s on the frequency transfer, and an accuracy of 5e-19. Theultimate link performance has been evaluated by doubling the link to 1284 km,demonstrating a new characterization technique based on the double round-tripon a single fiber. The arming of a second fiber is avoided: this is beneficialto long hauls realizations in view of a continental fiber network for frequencyand time metrology. The data analysis is based on the Allan deviation; itsexpression is theoretically derived for the observed noise power spectrum,which is seldom found in the literature.

Coherent optical frequency transfer at 5e-19 over a doubled 642 km fiber link