e statistical uncertainty of the timing link is reported monthly in BIPM’s Circular T [22]. In the following, we will adopt a representative value of (NICT)=0.35 ns .Since TAI and UTC dier only by a constant time oset, the satellite link allows the maser frequency to be evaluated in terms of the TAI scale interval. is fre-quency is typically expressed as the mean fractional devia-tion from nominal frequency (HM) over the eval u a ted period . e uncertainty of the link to TAI is given as [23]=√() (1)with an exponent in consideration of a mea sure-ment instability that largely results from (icker) phase noise. =5 d represents the reporting interval of the Circular T. For , the link uncertainty results in a fractional frequency uncertainty =2.0×10 . e uncertainty contribution from the DMTD system is insig-nicant in comparison [20]. RF-to-optical comparisonAn additional 100 MHz signal is transferred from the hydrogen maser to the optical clock laboratory, where it is converted to 9.2 GHz by a phase-locked dielectric resona-tor oscillator (DRO) with additive noise of －114 dBc/Hz at 1 kHz separation from the carrier.NICT-Sr1 provides an optical signal at that is con-tinuously steered to represent the Sr clock transition fre-quen cy of ≈429 THz . It diers from this only through a xed frequency oset =80 MHz that allows an acousto-optic modulator to probe the atomic spec trum without aecting and by the carefully eval u ated sys-tematic shi of the transition fre quen cy through physical eects (discussed in Section 4-3). Although NICT-Sr1 only interrogates the atomic transition for 80 ms every 1.75 s, the ultra-stable cavity used for the optical local oscillator allows for the generation of a phase-continuous signal with a sta bility characterized by an Allan deviation of ()=7×10 () [24].An optical frequency comb based on ytterbium-doped optical ber (Menlo Systems Orange Comb) is used for down-conversion from the optical to the radio-frequency do main. e carrier-envelope oset frequency is phase-locked to an external reference, and the repetition rate is stabilized to maintain a constant phase of the beat 3FiF1Overview of the TAI calibration scheme A hydrogen maser forms the junction point that is simultaneously evaluated in relation to the strontium optical lattice clock NICT-Sr1 and UTC(NICT). Although the presently evaluated maser (HM JST#15) also serves as source clock for UTC(NICT), these roles can be assigned to any combination of masers as long as DMTD comparison data is available. The link to UTC, and thus TAI, is realized through a GPS receiver referenced to UTC(NICT). NICT-Sr1 provides a laser at ≈429 THz that is continuously steered to the atomic clock transition. A fixed frequency offset of =80 MHz allows the shape of the atomic spectrum to be probed by an acousto-optic modulator (AOM) without disturbing . An optical frequency comb based on Yb-doped fiber is stabilized to the clock laser. A multiple of the comb repetition rate is then down-mixed with a multiple of the reference maser frequency generated by a phase-locked dielectric resonator oscillator (DRO). The resulting frequency of approximately 50 MHz is counted on a zero-dead-time multichannel frequency counter, together with the beat note Δ of the comb with the clock laser, and the beat signal used to stabilize the carrier-envelope offset frequency . The latter two provide diagnostic information.92 情報通信研究機構研究報告 Vol. 65 No. 2 （2019）4 原⼦周波数標準
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