Miho Fujieda

Stable radio frequency transfer in 114 km urban optical fiber link

An rf dissemination system using an optical fiber link has been developed. The phase noise induced during optical fiber transmission has been successfully cancelled using what we believe to be a novel fiber-noise compensation system with a combination of electrical and optical compensations. We have performed rf transfer in a 114 km urban telecom fiber link in Tokyo with a transfer stability of 10(-18) level at an averaging time of 1 day. Additionally, a high degree of continuous operation robustness has been confirmed.

Direct Comparison of Distant Optical Lattice Clocks at the 10-16 Uncertainty

Fiber-based remote comparison of 87Sr lattice clocks in 24 km distant laboratories is demonstrated. The instability of the comparison reaches 5×10-16 over an averaging time of 1000 s, which is two orders of magnitude shorter than that of conventional satellite links and is limited by the instabilities of the optical clocks. By correcting the systematic shifts that are predominated by the differential gravitational redshift, the residual fractional difference is found to be (1.0±7.3)×10-16, confirming the coincidence between the two clocks. The accurate and speedy comparison of distant optical clocks paves the way for a future optical redefinition of the second.

All-optical link for direct comparison of distant optical clocks

We developed an all-optical link system for making remote comparisons of two distant ultra-stable optical clocks. An optical carrier transfer system based on a fiber interferometer was employed to compensate the phase noise accumulated during the propagation through a fiber link. Transfer stabilities of 2 × 10(-15) at 1 second and 4 × 10(-18) at 1000 seconds were achieved in a 90-km link. An active polarization control system was additionally introduced to maintain the transmitted light in an adequate polarization, and consequently, a stable and reliable comparison was accomplished. The instabilities of the all-optical link system, including those of the erbium doped fiber amplifiers (EDFAs) which are free from phase-noise compensation, were below 2 × 10(-15) at 1 second and 7 × 10(-17) at 1000 seconds. The system was available for the direct comparison of two distant (87)Sr lattice clocks via an urban fiber link of 60 km. This technique will be essential for the measuring the reproducibility of optical frequency standards.

Coherent microwave transfer over a 204-km telecom fiber link by a cascaded system

We have demonstrated a microwave transfer over a 204-km noisy urban fiber link by a cascaded system with 2 stages, which connected 10-GHz and 1-GHz transfer systems in series. A diurnal phase-noise cancellation ratio of 45 dB was obtained by use of an electronic phase-noise compensation system. Additionally, the stabilities reached 6 × 10-14 at 1 s and 5 × 10-17 at one-half day, which agreed with the root-sumsquare of those of the 10-GHz and 1-GHz transfers. We verified for the first time that the transfer stability degrades only ¿(N) times in a cascaded system with N stages.

Ultra-stable frequency dissemination via optical fiber at NICT

We have developed a radio-frequency (RF) dissemination system using optical fibers. The phase noise induced during the transmission is actively cancelled by the compensation system with a voltage-controlled crystal oscillator. A first proving test was conducted on an urban telecom fiber link with a length of 10 km, and a frequency stability of 1 X 10-17 was achieved at an averaging time of one day. As an application of ultrastable frequency dissemination, a 1-GHz signal based on a cryogenic sapphire oscillator was transferred through a 25-km fiber and used as a microwave reference for an optical frequency comb. A fractional frequency stability of an ultranarrow clock laser for a Ca+ ion optical frequency standard was measured by the comb as 9 X 10-15 at 1 s, which included both laser stability and transferred reference stability.

Carrier-phase two-way satellite frequency transfer over a very long baseline

In this paper we report that carrier-phase two-way satellite time and frequency transfer (TWSTFT) was successfully demonstrated over a very long baseline of 9000 km, established between the National Institute of Information and Communications Technology (NICT) and the Physikalisch-Technische Bundesanstalt (PTB). We verified that the carrier-phase TWSTFT (TWCP) result agreed with those obtained by conventional TWSTFT and GPS carrier-phase (GPSCP) techniques. Moreover, a much improved short-term instability for frequency transfer of 2 × 10−13 at 1 s was achieved, which is at the same level as previously confirmed over a shorter baseline within Japan. The precision achieved was so high that the effects of ionospheric delay became significant; they are ignored in conventional TWSTFT even over a long link. We compensated for these effects using ionospheric delays computed from regional vertical total electron content maps. The agreement between the TWCP and GPSCP results was improved because of this compensation.

Remote atomic clock synchronization via satellites and optical fibers

Abstract. In the global network of institutions engaged with the realization of International Atomic Time (TAI), atomic clocks and time scales are compared by means of the Global Positioning System (GPS) and by employing telecommunication satellites for two-way satellite time and frequency transfer (TWSTFT). The frequencies of the state-of-the-art primary caesium fountain clocks can be compared at the level of 10−15 (relative, 1 day averaging) and time scales can be synchronized with an uncertainty of one nanosecond. Future improvements of worldwide clock comparisons will require also an improvement of the local signal distribution systems. For example, the future ACES (atomic clock ensemble in space) mission shall demonstrate remote time scale comparisons at the uncertainty level of 100 ps. To ensure that the ACES ground instrument will be synchronized to the local time scale at the Physikalisch-Technische Bundesanstalt (PTB) without a significant uncertainty contribution, we have developed a means for calibrated clock comparisons through optical fibers. An uncertainty below 40 ps over a distance of 2 km has been demonstrated on the campus of PTB. This technology is thus in general a promising candidate for synchronization of enhanced time transfer equipment with the local realizations of Coordinated Universal Time UTC. Based on these experiments we estimate the uncertainty level for calibrated time transfer through optical fibers over longer distances. These findings are compared with the current status and developments of satellite based time transfer systems, with a focus on the calibration techniques for operational systems.

Development of a GPU-Based Two-Way Time Transfer Modem

We have developed a new two-way time transfer modem to improve the time transfer precision of remote clock comparison. As a timing signal, we apply a binary offset carrier, which is similar to those signals used for the next-generation Global Navigation Satellite Systems. We took advantage of versatile A/D and D/A converters, and most of the digital signal processing stages were realized by software, running on an off-the-shelf PC. This enabled us to realize the complete system with cheaper equipment, leading to an affordable low-cost modem. For the real-time digital signal processing stages implemented in software, we relied on a graphics processing unit (GPU) developed for computer game enthusiast. The developed modem can receive four channels at the same time with a single GPU card. We performed two-way satellite time transfer experiments using these modems between Japan and Taiwan. The obtained results are consistent within 200 ps with respect to the results of GPS carrier phase time transfer. As a consequence, we improved the time transfer precision by nearly one order of magnitude as compared to a conventional two-way modem without increasing the connection fees caused by commercial communication satellites.

Delay Difference Calibration of TWSTFT Earth Station Using Multichannel Modem

Delays in signal transmission and reception paths of an Earth station must be calibrated accurately, and their variation should be reduced in the two-way satellite time and frequency transfer (TWSTFT). We have developed a portable TWSTFT station using a modem built by the National Institute of Information and Communications Technology (NICT) and performed calibration between NICT and Telecommunication Laboratories. An uncertainty of about 1 ns in the differential calibration was achieved. In addition, we have monitored the delay variation in the Earth station using a measurement system installed at NICT to improve accuracy of the TWSTFT. The contributed instability was 50 ps/day

Direct comparison of optical lattice clocks with an intercontinental baseline of 9000 km

We have demonstrated a direct frequency comparison between two ⁸⁷Sr lattice clocks operated in intercontinentally separated laboratories in real time. Two-way satellite time and frequency transfer technique, based on the carrier-phase, was employed for a direct comparison, with a baseline of 9000 km between Japan and Germany. A frequency comparison was achieved for 83,640 s, resulting in a fractional difference of (1.1±1.6)×10⁻¹⁵, where the statistical part is the largest contributor to the uncertainty. This measurement directly confirms the agreement of the two optical frequency standards on an intercontinental scale.