Months-long real-time generation of a time scale based on an optical clock.

Time scales consistently provide precise time stamps and time intervals by combining atomic frequency standards with a reliable local oscillator. Optical frequency standards, however, have not been applied to the generation of time scales, although they provide superb accuracy and stability these days. Here, by steering an oscillator frequency based on the intermittent operation of a 87Sr optical lattice clock, we realized an "optically steered" time scale TA(Sr) that was continuously generated for half a year. The resultant time scale was as stable as International Atomic Time (TAI) with its accuracy at the 10-16 level. We also compared the time scale with TT(BIPM16). TT(BIPM) is computed in deferred time each January based on a weighted average of the evaluations of the frequency of TAI using primary and secondary frequency standards. The variation of the time difference TA(Sr) - TT(BIPM16) was 0.79 ns after 5 months, suggesting the compatibility of using optical clocks for time scale generation. The steady signal also demonstrated the capability to evaluate one-month mean scale intervals of TAI over all six months with comparable uncertainties to those of primary frequency standards (PFSs).

SI-traceable measurement of an optical frequency at the low 10-16 level without a local primary standard.

SI-traceable measurements of optical frequencies using International Atomic Time (TAI) do not require a local primary frequency reference, but suffer from an uncertainty in tracing to the SI second. For the measurement of the 87Sr lattice clock transition, we have reduced this uncertainty to the low 10-16 level by averaging three sets of ten-day intermittent measurements, in which we operated the lattice clock for 104 s on each day. Moreover, a combined oscillator of two hydrogen masers was employed as a local flywheel oscillator (LFO) in order to mitigate the impact of sporadic excursion of LFO frequency. The resultant absolute frequency with fractional uncertainty of 4.3 × 10-16 agrees with other measurements based on local state-of-the-art cesium fountains.

Dissemination of optical-comb-based ultra-broadband frequency reference through a fiber network.

We disseminated an ultra-broadband optical frequency reference based on a femtosecond (fs)-laser optical comb through a kilometer-scale fiber link. Its spectrum ranged from 1160 nm to 2180 nm without additional fs-laser combs at the end of the link. By employing a fiber-induced phase noise cancellation technique, the linewidth and fractional frequency instability attained for all disseminated comb modes were of order 1 Hz and 10-18 in a 5000 s averaging time. The ultra-broad optical frequency reference, for which absolute frequency is traceable to Japan Standard Time, was applied in the frequency stabilization of an injection-seeded Q-switched 2051 nm pulse laser for a coherent light detection and ranging LIDAR system.

Microwave synthesis from a continuous-wave terahertz oscillator using a photocarrier terahertz frequency comb.

We report low-noise microwave synthesis from radiation with a frequency of 0.3 THz using a photocarrier frequency comb in a photoconductive antenna. The synthesized microwave signal at 1 GHz is phase coherent to the 0.3 THz radiation and has a fractional instability of 1×10(-15) within 300 s averaging times and single-sideband phase noise of -105 dB/Hz at a 100 Hz offset from the carrier. This terahertz (THz)-to-microwave synthesizer is capable of being a THz frequency divider, which would be indispensable to not only THz metrology but also future high-speed wireless networks.