All-polarization-maintaining, single-port Er:fiber comb for high-stability comparison of optical lattice clocks

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${}^{-1\mathrm{/}2}$ 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.