  All-polarization-maintaining, single-port Er:fiber combs offer long-term
robust operation as well as high stability. We have built two such combs and
evaluated the transfer noise for linking optical clocks. A uniformly broadened
spectrum over 135-285 THz with a high signal-to-noise ratio enables the optical
frequency measurement of the subharmonics of strontium, ytterbium, and mercury
optical lattice clocks with the fractional frequency-noise power spectral
density of $(1-2)\times 10^{-17}$ Hz$^{-1/2}$ at 1 Hz. By applying a
synchronous clock comparison, the comb enables clock ratio measurements with
$10^{-17}$ instability at 1 s, which is one order of magnitude smaller than the
best instability of the frequency ratio of optical lattice clocks.
