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We have explored the performance of two "dark fibers" of a commercialtelecommunication fiber link for a remote comparison of optical clocks. The twofibers, linking the Leibniz University of Hanover (LUH) with thePhysi-kalisch-Technische Bundesanstalt (PTB) in Braunschweig, are connected inHanover to form a total fiber length of 146 km. At PTB the performance of anoptical frequency standard operating at 456 THz was imprinted to a cw trans-ferlaser at 194 THz, and its frequency was transmitted over the fiber. In order todetect and compensate phase noise related to the optical fiber link we havebuilt a low-noise optical fiber interferometer and investigated noise sourcesthat affect the overall performance of the optical link. The frequencystability at the remote end has been measured using the clock laser of PTB'sYb+ frequency standard operating at 344 THz. We show that the frequency of afrequency-stabilized fiber laser can be transmitted over a total fiber lengthof 146 km with a relative frequency uncertainty below 1E-19, and short termfrequency instability given by the fractional Allan deviation ofsy(t)=3.3E-15/(t/s).

Phase- coherent comparison of two optical frequency standards over 146 km using a telecommunication fiber link