Mizuhiko Hosokawa

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.

Frequency Measurement of the Optical Clock Transition of 40Ca+ Ions with an Uncertainty of 10-14 Level

The absolute frequency of the 4 2S1/2–3 2D5/2 optical clock transition of 40Ca+ ions has been measured for the first time with respect to the Systeme International (SI) second. A single 40Ca+ ion is laser-cooled in a small ion trap and the transition frequency is measured as the average of two symmetrical Zeeman components. The frequency is determined to be 411 042 129 776 385 (±18) Hz from 48 measurements.

Evaluation of caesium atomic fountain NICT-CsF1

In this paper, we describe the first caesium atomic fountain primary frequency standard NICT-CsF1 of National Institute of Information Communications Technology (NICT) in Tokyo, Japan. The structure of the NICT-CsF1 system and evaluation procedure of the systematic frequency shifts and their uncertainties are presented. Typically, NICT-CsF1 has a frequency stability of 4 × 10−13/τ1/2 and a frequency uncertainty of 1.9 × 10−15.

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.

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.

Direct comparison of a Ca+ single-ion clock against a Sr lattice clock to verify the absolute frequency measurement.

Optical frequency comparison of the (40)Ca(+) clock transition ν(Ca)((2)S(1/2-)(2D(5/2), 729 nm) against the (87)Sr optical lattice clock transition ν(Sr) ((1)S(0)-(3)P(0), 698 nm) has resulted in a frequency ratio ν(Ca) / ν(Sr) = 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of frequency ratio ν(Ca) / ν(Sr) to reach 1 × 10(-15) in 1000s and yielded a value consistent with that calculated from separate absolute frequency measurements of ν(Ca) using the International Atomic Time (TAI) link. The total uncertainty of the frequency ratio using optical comparison (free from microwave link uncertainties) is smaller than that obtained using absolute frequency measurement, demonstrating the advantage of optical frequency evaluation. We note that the absolute frequency of (40)Ca(+) we measure deviates from other published values by more than three times our measurement uncertainty.Optical frequency comparison of the 40Ca+ clock transition \nu_{Ca} (2S1/2-2D5/2, 729nm) against the 87Sr optical lattice clock transition \nu_{Sr}(1S0-3P0, 698nm) has resulted in a frequency ratio \nu_{Ca} / \nu_{Sr} = 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of frequency ratio \nu_{Ca} / \nu_{Sr} to reach 1x10-15 in only 1000s and yielded a value consistent with that calculated from separate absolute frequency measurements of \nu_{Ca} using the International Atomic Time (TAI) link. The total uncertainty of the frequency ratio using optical comparison (free from microwave link uncertainties) is smaller than that obtained using absolute frequency measurement, demonstrating the advantage of optical frequency evaluation. We report the absolute frequency of ^{40}Ca+ with a systematic uncertainty 14 times smaller than our previous measurement [1].

Precise frequency-drift measurement of extended-cavity diode laser stabilized with scanning transfer cavity

A simple and effective method for stabilizing laser frequencies using scanning transfer cavities and a stabilized helium–neon laser was applied to extended-cavity diode lasers near 866 nm and 397 nm. The frequency drift of the stabilized 866 nm laser measured using an optical frequency synthesizer was smaller than 200 kHz for 1 h. The square root of the Allan variance was 1×10-10 at an averaging time of 103 s.

Displacement measuring technique for satellite-to-satellite laser interferometer to determine Earth's gravity field

We present a new displacement measuring technique with simplicity, robustness, high sensitivity and wide measurement range. A set of a frequency shifter and a voltage–frequency converter is used to lock a homodyne interferometer on the half-bright fringe by eliminating the Doppler fringe resulting from mirror motion. The mirror displacement is directly retrieved from the feedback signal of a fringe control loop. By developing a table-top interferometer, we successfully demonstrated signal recovery without significant degradation. The achieved displacement sensitivity and measurement range of the interferometer were 24 nm Hz−1/2 and 1.3 mm at a Fourier frequency of 0.1 Hz, respectively. This technique was found to have a potential for application to precise displacement measurements. It is, in particular, suitable for a satellite-to-satellite laser interferometer to determine Earths gravity field.

Stable Operation of Femtosecond Laser Frequency Combs with Uncertainty at the 10 17 Level toward Optical Frequency Standards

We have developed two femtosecond laser frequency combs (FLFCs) employing different designs and no external spectral broadening devices. The FLFCs referenced to a microwave standard were directly compared and confirmed to have a relative fractional uncertainty of 8.1 ×10-17 mainly due to imperfection in our comparison technique. They have also achieved uninterrupted daily operation typically more than 8 hours. They are found to be suitable for the optical standards from the absolute frequency measurement for a clock transition of 40Ca+ ions, that was recently performed in National Institute of Information and Communications Technology.

Narrow-Line and Frequency Tunable Diode Laser System for S–D Transition of Ca+ Ions

A new type of diode laser is used for the development of an extremely narrow linewidth and frequency-tunable clock laser system for a Ca+ ion optical frequency standard. The required reduction of linewidth was achieved by locking the laser to an ultrahigh-finesse ultralow-expansion glass (ULE) reference cavity. The long-term frequency drift is reduced by stabilizing a cavity temperature at the point that the thermal expansion coefficient of the ULE is zero. As a result, the laser linewidth is decreased to lower than 25 Hz and the long-term drift is less than 0.025 Hz/s. The other laser offset locked to the stabilized laser is continuously frequency tunable in the entire free spectral range of the reference cavity without degradation of stability and linewidth. This laser system is actually applied to optical frequency standard development and used for the observation of the 2S1/2–2D5/2 transition of Ca+ ions.