  We have explored the performance of two "dark fibers" of a commercial
telecommunication fiber link for a remote comparison of optical clocks. The two
fibers, linking the Leibniz University of Hanover (LUH) with the
Physi-kalisch-Technische Bundesanstalt (PTB) in Braunschweig, are connected in
Hanover to form a total fiber length of 146 km. At PTB the performance of an
optical frequency standard operating at 456 THz was imprinted to a cw trans-fer
laser at 194 THz, and its frequency was transmitted over the fiber. In order to
detect and compensate phase noise related to the optical fiber link we have
built a low-noise optical fiber interferometer and investigated noise sources
that affect the overall performance of the optical link. The frequency
stability at the remote end has been measured using the clock laser of PTB's
Yb+ frequency standard operating at 344 THz. We show that the frequency of a
frequency-stabilized fiber laser can be transmitted over a total fiber length
of 146 km with a relative frequency uncertainty below 1E-19, and short term
frequency instability given by the fractional Allan deviation of
sy(t)=3.3E-15/(t/s).
