Motohiro Kumagai

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.

Direct Comparison of Distant Optical Lattice Clocks at the 10-16 Uncertainty

Fiber-based remote comparison of 87Sr lattice clocks in 24 km distant laboratories is demonstrated. The instability of the comparison reaches 5×10-16 over an averaging time of 1000 s, which is two orders of magnitude shorter than that of conventional satellite links and is limited by the instabilities of the optical clocks. By correcting the systematic shifts that are predominated by the differential gravitational redshift, the residual fractional difference is found to be (1.0±7.3)×10-16, confirming the coincidence between the two clocks. The accurate and speedy comparison of distant optical clocks paves the way for a future optical redefinition of the second.

All-optical link for direct comparison of distant optical clocks

We developed an all-optical link system for making remote comparisons of two distant ultra-stable optical clocks. An optical carrier transfer system based on a fiber interferometer was employed to compensate the phase noise accumulated during the propagation through a fiber link. Transfer stabilities of 2 × 10(-15) at 1 second and 4 × 10(-18) at 1000 seconds were achieved in a 90-km link. An active polarization control system was additionally introduced to maintain the transmitted light in an adequate polarization, and consequently, a stable and reliable comparison was accomplished. The instabilities of the all-optical link system, including those of the erbium doped fiber amplifiers (EDFAs) which are free from phase-noise compensation, were below 2 × 10(-15) at 1 second and 7 × 10(-17) at 1000 seconds. The system was available for the direct comparison of two distant (87)Sr lattice clocks via an urban fiber link of 60 km. This technique will be essential for the measuring the reproducibility of optical frequency standards.

Coherent microwave transfer over a 204-km telecom fiber link by a cascaded system

We have demonstrated a microwave transfer over a 204-km noisy urban fiber link by a cascaded system with 2 stages, which connected 10-GHz and 1-GHz transfer systems in series. A diurnal phase-noise cancellation ratio of 45 dB was obtained by use of an electronic phase-noise compensation system. Additionally, the stabilities reached 6 × 10-14 at 1 s and 5 × 10-17 at one-half day, which agreed with the root-sumsquare of those of the 10-GHz and 1-GHz transfers. We verified for the first time that the transfer stability degrades only ¿(N) times in a cascaded system with N stages.

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.

Stability Transfer between Two Clock Lasers Operating at Different Wavelengths for Absolute Frequency Measurement of Clock Transition in 87Sr

We demonstrated transferring the stability of one highly stable clock laser operating at 729 nm to another less stable laser operating at 698 nm. The two different wavelengths were bridged using an optical frequency comb. The improved stability of the clock laser at 698 nm enabled us to evaluate the systematic frequency shifts of the Sr optical lattice clock with a shorter averaging time. We determined the absolute frequency of the clock transition 1S0–3P0 in 87Sr to be 429 228 004 229 873.9 (1.4) Hz referenced to the SI second on the geoid via International Atomic Time (TAI).

Precise Evaluation of a Phase-Locked THz Quantum Cascade Laser

We have demonstrated the phase-locking of a THz quantum cascade laser (THz-QCL) toward the realization of an accurate and stable local oscillator for a high-resolution receiver. The beat note between the THz-QCL and a THz reference was obtained by heterodyne mixing in a superconducting hot electron bolometer mixer (HEBM) and used for stabilizing the phase of 3.1 THz radiation from the THz-QCL. The phase-locked 3.1 THz radiation was fully evaluated with a superlattice harmonic mixer operating in the THz band, and this revealed that the THz-QCL synchronized with the microwave reference with a fractional frequency instability of 3×10-15 at an averaging time of 100 s, corresponding to a center frequency deviation within 1 mHz, and the imposed phase noises during the heterodyne mixing were negligibly small.

Accuracy evaluation of optically pumped primary frequency standard CRL-O1

In cooperation with the Time and Frequency Group of the National Institute of Standards and Technology, the Communications Research Laboratory has developed an optically pumped primary frequency standard (PFS) which is named CRL-O1. The accuracy of CRL-O1 has been evaluated and reports have been sent to the Bureau International des Poids et Mesures. The results of the evaluations have been published in Circular T. We estimate the biases of the frequency shifts and the so-called type A and type B uncertainties. The combination of type A and type B uncertainties is about 7 × 10−15. The evaluated frequency of CRL-O1 is in good agreement with that of other PFSs.

First comparison of primary frequency standards between Europe and Asia

In December 2006, PTB and NICT simultaneously operated their caesium atomic fountain primary frequency standards, PTB-CSF1 and NICT-CsF1 for 15 days starting from MJD 54079. This was the first direct comparison of primary frequency standards between Europe and Asia. The fountains were compared to local H-masers and the frequency difference between the H-masers was determined directly by TWSTFT with frequency transfer uncertainty below 10-15. PTB-CSF1 and NICT-CsF1 showed good agreement within the stated uncertainties.