Greater Tokyo Fiber Link

NICT operates the JGN ultra-high-speed network testbed, a fiber optic network with access points in Japan and abroad. Such fiber connections can also be used to compare optical frequency standards with high accuracy, not only to confirm their agreement, but also to enable a variety of applications. Several laboratories in the Greater Tokyo area are actively developing optical lattice clocks. Besides NICT, these include the University of Tokyo, RIKEN, the National Metrology Institute of Japan (NMIJ) and the NTT Basic Research Laboratory.

One such application is to use highly accurate clocks as geodetic sensors for the relativistic effects of gravity. But capturing changes in the gravity environment requires a stable reference, and for a state-of-the-art optical clock, the only usable reference is another, independent optical standard, which needs to be physically separated from the probed gravitational environment. Optical fiber links can provide the required connections between distant optical lattice clocks.

NICT conducts research and development on the delivery of precise frequency reference signals over fiber since the late 2000s. In 2011, a frequency comparison experiment conducted between NICT and the University of Tokyo clearly detected the relativistic frequency shift due to the 56m difference in the elevation of the clocks [1]. This is the first real-time detection of relativistic effects between two optical clocks located in distant laboratories.

In addition to reference frequencies, optical fibers can also transfer standard time signals. This will make it possible to supply future telecommunications networks with the highly accurate standard time enabled by measurements of optical clocks, which will be the basis for the low-latency communications expected for B5G, 6G, and other technologies.

NICT is conducting active research on fiber transmission of time signals, and has developed a technology to provide an accurate time signal at the distant end of a fiber link by modulating an optical carrier in the typical communications band with a microwave reference signal [2], and then superimposing the required time information. A link of this type has been established over the JGN network to provide the Hongo campus of the University of Tokyo with a time signal synchronized to the Japan Standard Time generated at NICT's Koganei headquarters. This signal is available 24/7 and has an uncertainty of only 13 nanoseconds.

1) A. Yamaguchi, M. Fujieda, M. Kumagai, H. Hachisu, S. Nagano, Y. Li, T. Ido, T. Takano, M. Takamoto, and H. Katori, Direct comparison of distant optical lattice clocks at the 10^-16 uncertainty, Appl. Phys. Express 4 082203 (2011)

2) M. Kumagai, M. Fujieda, S. Nagano, and M. Hosokawa, Stable radio frequency transfer in 114 km urban optical fiber link Opt. Lett. 34 2949 (2009)