Akifumi Takamizawa

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 the 1 S 0 - 3 P 0 Clock Transition in 87 Sr Using a Cs Fountain Clock as a Transfer Oscillator

We performed an absolute frequency measurement of the 1S0–3P0 transition in 87Sr with a fractional uncertainty of 1.2 × 10−15, which is less than one-third that of our previous measurement. A caesium fountain atomic clock was used as a transfer oscillator to reduce the uncertainty of the link between a strontium optical lattice clock and the SI second. The absolute value of the transition frequency is 429 228 004 229 873.56(49) Hz.We performed an absolute frequency measurement of the 1S0–3P0 transition in 87Sr with a fractional uncertainty of 1.2 × 10−15, which is less than one-third that of our previous measurement. A caesium fountain atomic clock was used as a transfer oscillator to reduce the uncertainty of the link between a strontium optical lattice clock and the SI second. The absolute value of the transition frequency is 429 228 004 229 873.56(49) Hz.

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.

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.

Recent progress of an atomic fountain frequency standard NMIJ-F1 (2006 – 2007)

We review the current status of development of NMIJ-F1 for the recent two years. We have performed frequency measurement a few times per year with a combined (Type A, Type B) uncertainty of 3.9times10-15. For more frequent contribution to keep International Atomic Time (TAI), optics and microwave system have been modified.

External cavity diode laser with frequency drift following natural variation in air pressure.

A compact and mechanically very robust external cavity diode laser was made by removing any position adjusters such as precision screws and piezo actuators, taking advantage of a cats eye retroreflector insensitive to misalignment. Under free-running operation during 150xa0h, the frequency drift followed natural variation in air pressure with a ratio of -66.6±0.1u2009u2009MHz/hPa in a range between 1001.5 and 1013.6xa0hPa. The ratio was in good agreement with that estimated from variation in the effective cavity length derived from the relation between the pressure and the refractive index of air. These results indicate that an external cavity diode laser with predictable frequency drift was successfully created.

Development of the cesium fountain frequency standard, NMIJ-F2

We have made much progress on NMIJ-F2, which is our second cesium fountain frequency standard aiming an uncertainty of <; 1×10^-15 as an immediate goal. The frequency stability is improved to 8.3×10^-14τ^-1/2 (τ: averaging time) by applying a cryogenic sapphire oscillator using a pulse-tube cryocooler as a local oscillator and optically pumping the atoms to the Zeeman sublevel mF = 0. Then, the homogeneous magnetic field in the interrogation region is obtained with magnetic shielding, a long solenoid coil, and two additional coils. The fractional frequency correction for the 2nd-order Zeeman shift is evaluated to be -165.5×10^-15. Moreover, the fractional frequency correction for the collisional shift (the frequency shift due to collisions between cold atoms) is measured to be (+3.3±0.5)×10^-15 by an extrapolation method.

Autonomous cryogenic sapphire oscillators employing low vibration pulse-tube cryocoolers at NMIJ

Two liquid-helium-cooled cryogenic sapphire-resonator oscillators (CSOs), have been modified to operate using cryo-refrigerators and low-vibration cryostats. The Allan deviation of the first CSO was evaluated to be better than 2 x 10-15 for averaging times of 1 s to 30 000 s, which is better than that of the original liquid helium cooled CSO. The Allan deviation of the second CSO is better than 4 x 10-15 from 1 s to 6 000 s averaging time.

Generation of incoherent light from a laser diode subject to external optical injection from a superluminescent diode

In this study, incoherent light with a spectral linewidth of 7 nm and 140 mW of power was generated from a laser diode (LD) into which incoherent light emitted from a superluminescent diode was injected with 2.7 mW of power. The spectral linewidth of the light from the LD was broadened to 12 nm when the diodes output power was reduced to 15 mW. In the process of transformation from single-mode laser light to incoherent light with a broad spectrum by increasing injection-light power, multimode laser oscillation and a noisy spectrum were found in the light from the LD. This optical system can be used not only for amplification of incoherent light but also as a coherence-convertible light source.

Frequency measurement of a Sr optical lattice clock using a coherent optical link over a 120-km fiber

We demonstrate a precision frequency measurement using a phase-stabilized 120-km optical fiber link over a physical distance of 50 km. The absolute frequency of the 87Sr optical lattice clock is measured to be 429228004229874.1(2.4) Hz.