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) x 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. (C) 2017 The Japan Society of Applied Physics