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

Abstract

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 frequencynoise power spectral density of (1-2) ' 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. © 2017 The Japan Society of Applied PhysicsO ptical frequency combs have become indispensable tools for precision measurements including atomic= molecular spectroscopy, low-phase-noise microwave generation, and ranging. 1) In optical clocks, linking different atomic clocks 2,3) or distant clocks via optical fibers 4) with a fractional uncertainty of 10 −18 is of significant concern with a future redefinition of the second in the International System of Units (SI). 5) Such endeavors, in turn, offer intriguing opportunities for testing the constancy of the fundamental constants 6) and for relativistic geodesy. 4,7) These applications necessitate ultralow-noise optical frequency combs that allow long-term and robust operation. Titanium-sapphire-based frequency combs have demonstrated outstanding stability. 8,9) However, their bulky optical setups require regular maintenance, thus hampering long-term and robust operation, which limits their potential applications. In contrast, erbium (Er) fiber combs enable all-fiber architecture for robust operation. Although a typical Er:fiber comb uses nonlinear polarization rotation (NPR) to acquire mode-locking with excellent noise performance, 9-13) the operational condition for such NPR-based Er:fiber oscillators can be sensitive to environmental conditions. For practical applications, such as the longterm operation of clocks to generate the optical second 14) and field and space applications, the all-polarization-maintaining (PM) architecture is preferred. 15,16) However, such architecture has shown relatively large phase noise. Low intrinsic phase noise with PM architecture is demonstrated by applying a nonlinear amplifying loop mirror (NALM). 17,18) In linking multiple optical frequencies, Er:fiber combs with a multibranch configuration, where each port consists of an Er-doped fiber amplifier (EDFA) and a highly nonlinear fiber (HNLF), have been employed, 3,19) as it allows sufficient output power per comb tooth optimized for the single frequency. In such a multibranch comb, the phase noise in different branches introduces the instability of ∼10 −16 at 1 s. 10,11) A record high instability of 4 × 10 −16 (τ=s) −1=2 has been demonstrated for synchronous clock comparison between strontium (Sr)-and ytterbium (Yb)-based optical lattice clocks, 3) in whic...

The unit of time: Present and future directions

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Predictable tendency of Bi NIR emission in Bi‐doped magnesium aluminosilicate laser glasses

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