Michito Imae

Measuring the frequency of a Sr optical lattice clock using a 120 km coherent optical transfer.

We demonstrate a precision frequency measurement using a phase-stabilized 120 km optical fiber link over a physical distance of 50 km. The transition frequency of the (87)Sr optical lattice clock at the University of Tokyo is measured to be 429228004229874.1(2.4) Hz referenced to international atomic time. The results demonstrate the excellent functions of the intercity optical fiber link and the great potential of optical lattice clocks for use in the redefinition of the second.

Improved Frequency Measurement of a One-Dimensional Optical Lattice Clock with a Spin-Polarized Fermionic 87Sr Isotope

We demonstrate a one-dimensional optical lattice clock with a spin-polarized fermionic isotope designed to realize a collision-shift-free atomic clock with neutral atom ensembles. To reduce systema...

Two-way satellite time and frequency transfer networks in Pacific Rim region

A two-way satellite time and frequency transfer (TWSTFT) network in the Pacific Rim region is under construction to contribute to the calculation of the international atomic time (TAI). Four major time and frequency institutes in this region have been conducting long-term TWSTFT experiments. In addition to these institutes, several others in the region are planning to join the network. A new type of time transfer modem for TWSTFT is also described.

Two-way time transfer through 2.4 Gb/s optical SDH system

Abstract : An experiment to transfer time and frequency over 2.488 Gbits SDH (Synchronous Digital Hierarchy) systems using 175-km commercial optical fibers has been set up by CRL and NTT. We confirm that the frequency stability of the time comparison data is 10(12)/square root of tau at averaging times above 10 s. This equals that of the Cs frequency standard (H5071A) used in this experiment. The time comparison resolution is of the order of 10(-11) s (square root of time variance). The long-term stability of this system is expected to be better than 1 ns. The time comparison results of this experiment agree well the GPS common-view results.

Precise Frequency Comparison System Using Bidirectional Optical Amplifiers

Precise frequency comparisons are becoming more urgent given the recent rapid progress in next-generation frequency standards. This paper describes a new type of bidirectional optical amplifier that overcomes the fiber loss limits that have prevented accurate frequency comparisons between widely separated places; such comparison is realized by bidirectionally transmitting wavelength-division-multiplexed (WDM) signals along a single fiber. The proposed optical amplifier has an optical isolator in each two-way channel divided by wavelength filters to suppress the optical reflection that causes amplification instability. The additional insertion optical loss due to this method is about 1.5 dB. The optical gain greater than 30 dB is obtained for both signals with good optical isolation of 65 dB. A radio-frequency reference signal can directly be sent by simple intensity modulation and direct detection (IM-DD) devices in the 1550-nm region widely used in telecommunication networks. Phase comparisons of the received signals and the frequency standards at each terminal are used for frequency comparisons. The amplifier is tested in the field using two hydrogen masers. A 120-km fiber with loss of 52.5 dB is used to connect the National Metrology Institute of Japan (NMIJ) to the University of Tokyo. Because of this loss, an amplifier is needed to realize sufficient receiving power. The frequency stability of the system with a 10-MHz direct transmission is evaluated by returning the optical signal from a halfway point [55 km from the NMIJ] where the amplifier is installed. The result is 2.6 × 10-16 (Allan deviation) with the averaging time of 7 × 104 s. The laboratory result is 8.7 × 10-17 (¿ = 4 × 104 s). The amplifiers long-term stability is promising for stable frequency dissemination in addition to precise frequency comparisons.

Time and frequency transfer and dissemination methods using optical fiber network

In the time and frequency transfer and dissemination field, it is important to provide cost effective remote frequency calibration services with an uncertainty of around 10-12 for end users. It is also required to develop ultra precise transfer methods with an order of 10-15 or better uncertainty for the comparison between ultra stable frequency standards which are under developing. This study shows two methods using optical fiber networks to satisfy these demands. First, it is an economical remote calibration method using existing synchronous optical fiber communication networks. The measured frequency stability (the Allan deviation) of the transmission clock is 2times10-13 for an averaging time of one day. The result indicates the method is promising for the simple remote frequency calibration service. Second, it is an ultra precise two-way optical fiber time and frequency transfer method using a newly proposed bi-directional optical amplifier. In this method, wavelength division multiplexing (WDM) signals are transmitted along a single optical fiber. The preliminary measured frequency stability is less than 1015 (tau =104 s) for a 100-km-long fiber with the bi-directional amplifier. It suggests that the method has capability for improving TAI (International Atomic Time) and UTC (Coordinated Universal Time)

Development of multichannel dual-mixer time difference system to generate UTC (NICT)

A multichannel dual-mixer time difference (DMTD) system has been developed by the National Institute of Information and Communications Technology (NICT) for a measurement system to generate the UTC(NICT) time scale based on Coordinated Universal Time (UTC). This system measures time differences between a reference signal and 24 device under test (DUT) signals, simultaneously. We have confirmed that this system has enough accuracy to measure hydrogen maser and cesium clocks at an averaging time of 1 s.

Simple Time and Frequency Dissemination Method Using Optical Fiber Network

This paper describes a simple and cost-effective method of frequency dissemination. In current digital communication networks, node clocks are hierarchically synchronized to the atomic master clock through fiber links. This synchronized network is used as an intermediate link for remote calibration services like the global positioning system common-view method. A prototype reference signal generator has been developed for recovering the communication clock signal and synthesizing a 10-MHz signal from it. The generator output frequency at the client site can be traced to coordinated universal time (UTC) National Metrology Institute of Japan (NMIJ) with some uncertainty, depending on the stability of the node clocks and the distance from the master clock. The stability performance of the generated reference signal has been tested at Okinawa-the farthest prefecture from Tokyo, where the master clock is located (baseline distance of 1500 km). The primary rate (1.544 MHz) for telecommunication services was chosen for the 10-MHz signal generation in the experiment. A sinusoidal phase fluctuation within a one-day period is dominantly observed. This fluctuation is mainly caused by fiber expansion and contraction due to normal daily temperature changes. It degrades the stability (Allan deviation) to the level of 5 X 10-13 (t = 40 000 s, which is almost half a day). However, the major part of the phase fluctuation can be canceled by averaging a full days data. In this case, the Allan deviation becomes 1 X 10-13, which is obtained at Okinawa over ten consecutive days of measurement. The worst average frequency offset relative to UTC (NMIJ) (one-day averaging) is -6.3 X 10-13. The results indicate that this method promises to be suitable for most applications, providing an uncertainty of less than 1 X 10-12 at an averaging time of one day.

Time Comparison Experiments with Small K-Band Antennas and SSRA Equipments via a Domestic Geostationary Satellite

Time comparison experiments were made for the development of a domestic accurate time-comparison system via the K-band (30/20-GHz) link of a Japanese geostationary satellite (CS). In the experiments, SSRA communication equipment and K-band earth stations such as a 13-m antenna main station, a 2-m antenna headquarters station, and a 1-m antenna automobile station were used. Time fluctuation of about 1 ns (rms) and frequency stability of less than 1 X 10-13 (for averaging time of 100-200 min) were achieved in the two-way time transfer method. The accuracies of the experiments were estimated as about 13 ns for the time comparison between the main station and the headquarters one, referred to a two-way time transfer through a ground microwave link, and about 0.74 ns for that between the two small (mobile) stations by method of a two-way time transfer experiment at a single site with a common clock. We measured the station time delays by insertion of a pulse-modulated signal and its detection at each of the up-link path and the down-link path of the station, and obtained the results with accuracies of 3.6 ns for the mobile stations and 7.7 ns for the main station.

Millisecond pulsar observation system using acousto-optic spectrometer

We have developed a unique system using an acoustooptic spectrometer for precise timing of millisecond pulsars. This system can handle a signal of 50 MHz bandwidth with a frequency resolution of 200 kHz and a time resolution of 13 /spl mu/s, and it can average 2/sup 24/ (7 h) pulses without any dead time. >