Ken-ichi Watabe

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.

Short Term Frequency Stability Tests of Two Cryogenic Sapphire Oscillators

Ultra-high short-term frequency stability has been realized in microwave oscillators based on liquid helium cooled sapphire resonators which operate on the same Whispering Gallery mode. Two cryogenic sapphire oscillators were built to evaluate their stability at short averaging times. These oscillators exhibited a fractional frequency stability of 1.1×10-15 at an averaging time of 1 s, which is more than 100 times better than that of a hydrogen maser. For averaging times between 2 and 640 s the measured oscillator fractional frequency instability was below 10-15 with a minimum of 5.5×10-16 at an averaging time of 20 s. The noise floors of the control servos which contribute to the short-term frequency stability are also discussed.

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.

Optical Frequency Synthesis From a Cryogenic Sapphire Oscillator Using a Fiber-Based Frequency Comb

The authors have demonstrated optical frequency synthesis using a fiber-based frequency comb system, referenced to an ultrastable microwave oscillator. The oscillator is based on a high Q-factor cryogenic sapphire resonator cooled with liquid helium. The 100-MHz signal synthesized from the 10.8-GHz oscillation frequency of the cryogenic sapphire oscillator (CSO) was used to stabilize the repetition frequency of the mode-locked fiber laser. When the synthesized optical frequency was compared with a rubidium two-photon stabilized laser at 778 nm, the measured fractional frequency stability was ~6 10-14 tau -12 for the averaging times tau between 1 and 100 s, and the best frequency stability was 3.0 10-15 for an averaging time of 1280 s

Reference Signal Synthesized from a Cryogenic Sapphire Oscillator Improved by Power Control Servo

Improvements in the frequency stability of a reference signal synthesized from a cryogenic sapphire oscillator (CSO) have been realized. This has been achieved by a power control servo with active Pound frequency stabilization in the loop oscillator based on a high Q-factor cryogenic sapphire resonator with a loaded Q-factor of 5 ×108 at 10.812 GHz and operating at a temperature of 6 K. The reference signal of 100 MHz was synthesized from the 10.812 GHz oscillation frequency. When compared with a 100 MHz hydrogen maser signal, the 100 MHz reference exhibited a fractional frequency stability of 5.6 ×10-15 at an integration time of 510 s, which is approximately two times better than that without power servo control.

Microwave Local Oscillator for a Cesium Frequency Standard Synthesized from a Cryogenic Sapphire Oscillator

A synthesized microwave local oscillator for a cesium atomic frequency standard has been implemented using an ultra-stable oscillator. The oscillator was based on a high Q-factor cryogenic-sapphire-resonator cooled with liquid helium which operates on a Whispering Gallery mode. The cesium hyperfine transition frequency of 9.192 GHz was synthesized from the 10.812 GHz oscillation frequency, chosen because the corresponding mode had the best frequency stability. When compared with a hydrogen maser reference the local oscillator exhibited a fractional frequency stability of 6×10-15 for integration times of 600 to 1200 s at 9.192 GHz, limited only by the cryogenic sapphire oscillator.

Cryogenic-Sapphire-Oscillator-Based Reference Signal at 1 GHz with

We describe a 1 GHz reference signal with 10-15 level fractional frequency stability for averaging times longer than 1 s, synthesized from a 10.8 GHz oscillation frequency of a cryogenic-sapphire-resonator-based ultra-stable oscillator and phase locked to a hydrogen maser with a time constant of about 1000 s. The degradation of the short term stability of the reference signal by phase locking to the hydrogen maser was evaluated. We also report on the implementation of the 1 GHz signal down converted to 5 MHz as a reference oscillator for a cesium atomic fountain frequency standard.

10^{-15}

A cryogenic whispering gallery sapphire resonator oscillator has been investigated using a 4 K pulse-tube cryocooler. The turnover temperature of the chosen mode in the sapphire crystal was 9.169 K with an unloaded Q-factor of 7/spl times/10/sup 8/. The prototype SLC oscillator was designed to oscillate at 9.195 GHz and exhibited a fractional frequency stability of 2/spl times/10/sup -13/ at integration times of 10 s. We project a fractional frequency stability better than 1 part in 10/sup -14/ for integration times of 1 s to 100 s with a better temperature stabilized housing and with improved vibration isolation.

Level Instability

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.

Cryogenic whispering gallery sapphire oscillator using 4 K pulse-tube cryocooler

We have realized a phase noise standard of a signal with a -100 dBc/Hz flat phase noise at 10 MHz for Fourier frequencies of 1 Hz to 100 kHz, which ensures traceability to the International System of Units (SI). The flat phase noise signal is produced using a carrier combined with white noise. To ensure traceability, both the flat phase noise signal power and the power spectral density of white noise are determined with a calibrated power meter and the noise standard, respectively. The flatness of the phase noise standard is within ±0.7 dB.