  We developed an all-optical link system for making remote comparisons of two
distant ultra-stable optical clocks. An optical carrier transfer system based
on a fiber interferometer was employed to compensate the phase noise
accumulated during the propagation through a fiber link. Transfer stabilities
of $2\times10^{-15}$ at 1 second and $4\times10^{-18}$ at 1000 seconds were
achieved in a 90-km link. An active polarization control system was
additionally introduced to maintain the transmitted light in an adequate
polarization, and consequently, a stable and reliable comparison was
accomplished. The instabilities of the all-optical link system, including those
of the erbium doped fiber amplifiers (EDFAs) which are free from phase-noise
compensation, were below $2\times10^{-15}$ at 1 second and $7\times10^{-17}$ at
1000 seconds. The system was available for the direct comparison of two distant
$^{87}$Sr lattice clocks via an urban fiber link of 60 km. This technique will
be essential for the measuring the reproducibility of optical frequency
standards.
