Ken Hagimoto

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.

Atomic fountain clock with very high frequency stability employing a pulse-tube-cryocooled sapphire oscillator

The frequency stability of an atomic fountain clock was significantly improved by employing an ultra-stable local oscillator and increasing the number of atoms detected after the Ramsey interrogation, resulting in a measured Allan deviation of 8.3 × 10-14τ-1/2. A cryogenic sapphire oscillator using an ultra-low-vibration pulse-tube cryocooler and cryostat, without the need for refilling with liquid helium, was applied as a local oscillator and a frequency reference. High atom number was achieved by the high power of the cooling laser beams and optical pumping to the Zeeman sublevel mF = 0 employed for a frequency measurement, although vapor-loaded optical molasses with the simple (001) configuration was used for the atomic fountain clock. The resulting stability is not limited by the Dick effect as it is when a BVA quartz oscillator is used as the local oscillator. The stability reached the quantum projection noise limit to within 11%. Using a combination of a cryocooled sapphire oscillator and techniques to enhance the atom number, the frequency stability of any atomic fountain clock, already established as primary frequency standard, may be improved without opening its vacuum chamber.

Mode-locked laser-type optical atomic clock with an optically pumped Cs gas cell

We propose and demonstrate a mode-locked laser-type optical atomic clock with an optically pumped cesium (Cs) gas cell. By adopting the optically pumped Cs gas cell with a double resonance method as a frequency standard, we have successfully demonstrated an ultrastable rack-mount type Cs optical atomic clock with excellent short-term stability. The obtained frequency stabilities reached as high as 1.2 x 10(-12) for tau=1 s and 8.8 x 10(-14) for tau=100 s for a 9.1926 GHz microwave output signal.

Accuracy evaluation of the optically pumped Cs frequency standard at NRLM

The optically pumped cesium (Cs) frequency standard at the National Research Laboratory of Metrology, Ibaraki, Japan (NRLM), NRLM-4, was modified in various ways in order to improve frequency stability and accuracy, We determined the frequency stability by comparing the phase with that of a hydrogen maser: the Allan deviation was /spl sigma//sub y/(/spl tau/)=5/spl times/10/sup -13///spl radic//spl tau/. We estimated the frequency corrections and uncertainties due to various sources, and the total uncertainty was found to be 2.9/spl times/10/sup -14/. The frequency difference between NRLM-4 and the international atomic time (TAI) was also measured.

An ultrastable PLL mode-locked fiber laser with a hydrogen maser clock

We have greatly improved the stability of the repetition rate of a mode-locked fiber laser by using a phase-locked loop circuit with a hydrogen maser clock. The stability reached as high as 5.9 × 10-14for an averaging time of 100 s.

Preliminary Evaluation of the Cesium Fountain Primary Frequency Standard NMIJ-F2

We describe the preliminary evaluation of the frequency corrections and their uncertainty in the cesium fountain primary frequency standard (PFS) NMIJ-F2 under development at National Metrology Institute of Japan (NMIJ). In NMIJ-F2, cold atoms generated from a vapor-loaded optical molasses in the (001) configuration are optically pumped to the Zeeman sublevels of mF = 0 to increase the number of atoms involved in the Ramsey interrogation. Moreover, a cryocooled sapphire oscillator with ultralow phase noise is employed as the local oscillator to avoid degradation of the frequency stability due to the Dick effect. As a result, we have obtained a very high fractional frequency stability of 9.7 × 10-14 τ-1/2. As for systematic frequency shifts, the fractional correction for the second-order Zeeman shift is experimentally estimated to be (-165.5 ± 0.5) × 10-15 from the first-order Zeeman shift of atoms in mF = +1 launched to various heights. The fractional frequency correction for cold-atom collisions is estimated to be (+3.3 ± 0.4) × 10-15 by extrapolating the frequency to zero density from the frequencies measured for various nonzero atom numbers. We will soon be able to make a comparison with other atomic fountain PFSs at the 1 × 10-15 level.

Analysis of Truncation Error for Dual-Mixer Time-Difference Measurement System Using Discrete Fourier Transformation

For the dual-mixer time-difference method using the discrete Fourier transformation, the formulation of the truncation error to determine the time difference between two oscillators is derived. It is found that serious error occurs due to the truncation error with a larger frequency difference between two oscillators. The condition of the discrete Fourier transformation to determine the frequency difference between two oscillators with sufficiently high accuracy for atomic clocks is obtained.

Ultrastable cesium atomic clock with a 9.1926-GHz regeneratively mode-locked fiber laser

We describe an ultrastable cesium (Cs) atomic clock with a 9.1926-GHz regeneratively mode-locked fiber laser obtained by use of an optically pumped Cs beam tube. By adopting a 1-m-long Cs beam tube with a linewidth of 110 Hz, we have successfully obtained frequency stabilities of 4.8 x 10(-12) for tau = 1 s and 6.3 x 10(-13) for tau = 50 s for a 9.1926-GHz microwave output signal. This Cs atomic clock can generate an optical pulse train with the same stability as that of the obtained microwave, which allows us to deliver a frequency standard optical signal throughout the world by means of optical fiber networks.

Two-way satellite time transfer network in Pacific Rim region

A Two Way Satellite Time Transfer (TWSTT) network in the Pacific rim region is under construction to contribute to the International Atomic Time (TAI). Four major Time and Frequency Institutes in this region have been conducting long term TWSTT experiments. In addition to these institutes, several ones in this region are planning to join this TWSTT network.

Dual-Mixer Time-Difference Measurement system using discrete Fourier transformation

Simplified Dual-Mixer Time-Difference Measurement system is proposed using discrete Fourier transformation (DFT) where no sinusoidal-pulsed converter, nor zero-cross detector are necessary. The phase meter integrating the proposed method with a dead time needed to process the time difference was demonstrated with an high resolution of σy(t=1 s)=7×10-14 and σy(t=10000 s)=1×10-16. The expected truncation error due to the usage of DFT was in good agreement with the observed one. Moreover, the multi-phase meter for five oscillator was easily demonstrated where three corner hat measurement was executed, and Allan deviation of more stable oscillator that two other ones was observed at an averaging time of approximately less than 30 s.