Most of space-based observing systems make water-vapor- and temperature-related measurements, while spacebased observing systems for wind measurement is limited. The current passive space-based observing systems for wind measurement has a large coverage area and high temporal and horizontal resolutions but has a low vertical resolution. The World Meteorological Organization (WMO) wants to develop space-based wind profiling systems. A Doppler Wind Lidar (DWL) is a useful and power technology for wind measurement and it can be designed as compact mobile, airborne, and space-based systems. DWL would provide us with a wind profile having high vertical resolution, low bias, and good precision, and it is necessary to fill the gap of current observations. The National Institute of Information and Communications Technology (NICT) is developing a single-frequency high-energy Tm,Ho:YLF laser, 2-μm key technology and instrument for a future space-based coherent DWL. We demonstrated the Tm,Ho:YLF laser producing a pulse energy of 125 mJ operating at 30 Hz meeting requirements for the future spacebased coherent DWL. In the paper, we will describe recent progress at NICT.
An 87Sr-based-optical lattice clock in NICT is compared to that of The University of Tokyo using a >50 km fiber
link. In this work, we have demonstrated for the first time that two distant Sr lattice clocks generate the same
frequency with systematic uncertainty of 0.31 Hz (7.3 × 10-16 fractionally) for the 429 THz clock frequency.
We developed an optical frequency standard with the 4 2S1/2-3 2D5/2 electric quadrupole transition of 40Ca+ ions. Its
absolute transition frequency is 411 042 129 776 390(±7) Hz. The accuracy is limited by the electric quadrupole shift
and the ambient magnetic field fluctuation. To determine the absolute transition frequency with a better accuracy, we
have observed two pairs of the symmetrically-splitting Zeeman components and measured the transition frequency
corrected for the electric quadrupole shift. In addition, we are developing a magnetic-shielded ion-trap chamber to
suppress the transition-line broadening caused by the magnetic field fluctuation.
Buffer gas induced collision shift for the 88Sr 1S0-3P1 transition is investigated by precision saturation spectroscopy of
thermal gas in a heat cell. The cell was filled with rare gases of helium, neon, argon, and xenon as buffer gases. Helium
showed the largest fractional shift coefficient of 1.6x10-9 Torr-1. The disagreement between our experiments and simple impact calculations indicates effective atom losses from zero-velocity class which contributes to saturated absorption
spectroscopy. The result could be useful to evaluate the background gas collision shift of Sr lattice clocks.
Keywords: collision, saturated absorption spectroscopy, lattice clock, optical clock, density shift
Optical frequency standards are being developed worldwide to lead a new definition of the unit of time. We
are developing broadband optical frequency combs, which aims to count the laser frequency highly stabilized
to the resonance of atomic reference and generate a rf frequency standard directly converted from the optical
frequency. The developed frequency combs was successfully operated with a measurement accuracy of 3×10-14
at the averaging time of 1 s. From the primary demonstration, it was confirmed to be available for the frequency
stability measurement of a clock laser used in an optical frequency standards. The frequency combs will be key
components for the development of optical atomic clock.
The present work discusses methods of stabilizing the frequencies of commercially-available laser diodes. Laser diodes are generally compact and long-lived. The frequency stability, which makes them ideal for onboard laser interferometer light-sources, in applications such as the satellite-to-satellite tracking systems used to verify fluctuations in earth's gravity field, which, in turn, indicate other critical changes in the environment, is the key characteristic of this work. We used the devices typically operating at 780nm, and their frequencies can be stabilized using either of two systems; one, employing the Doppler-free absorption line of Rb atoms and another, sing the Faraday effect of the Rb absorption line. In both cases, the use of the proper modulation frequency and amplitude improved frequency stability, overall, attaining 2.05×10-12 and 2.73×10-11, respectively, in the square root of the Allan variance, by measuring the beat-note between two independently-stabilized laser diodes.
Currently in the initial stages of development, the endeavor aims to use satellite-to-satellite tracking laser interferometer-based optical technique, to document fluctuations in earth’s gravitational field indicating other critical changes in the environment. This system must be able to measure infinitesimal changes in the relative velocity of the two satellites, using a laser light source, which oscillates at frequency stability better than 10-13 in the square root of the Allan variance. We have stabilized the laser’s oscillation frequency using the Faraday effect of Rb absorption lines. This method modulates the reference frequency of the stabilization system by modulating the magnetic field applied to the Rb absorption cell, instead of the oscillation frequency of the laser diode. Furthermore, we have adapted the “double optical feedback” to the laser diode for narrowing its oscillation spectrum and improving its frequency stability. In recent years, a “femtosecond optical comb generator” has been developed as a new reference frequency source for absolute frequency measurement. This optical comb generator is controlled by the microwave frequency standards systems and provides stability of 4x10-13 at an averaging time of 1s and at the order of 10-15 at 1000s averaging time. We have measured the frequency stability of our system using the optical comb. We obtained the best spectrum narrowing effect using two gratings as external reflectors in the double optical feedback setup. The obtained results were 6.269x10-11 ≤ σ ≤ 1.516x10-10 (24.11kHz ≤ f ≤ 58.31kHz) from 1s to 39s in the averaging time.
Currently in the initial stages of development, the endeavor aims to use satellite-to-satellite tracking laser interferometer-based optical technique, to document fluctuations in earth's gravitational field indicating other critical changes in the environment. This system must be able to measure infinitesimal changes in the relative velocity of the two satellites, using a laser light source, which oscillates at frequency stability better than 10-13 in the square root of the Allan variance. The constraints placed upon the system will ultimately require that the interferometer's light-source be small, lightweight, extremely efficient, durable, and possess high frequency-stability. Present-day technology of a laser diode possesses all of these characteristics, except the last and most critical one; frequency stability. So, our ongoing efforts are all focused on the search for a method of stabilizing the oscillation frequency of the satellite-to-satellite laser-diode light source. We used a Rubidium absorption line as a frequency reference of the frequency stabilization system and the "double optical feedback" method to narrow the laser diode's oscillation linewidth, in order to improve its frequency stability. We have measured the frequency, to determine its degree of stability, by comparing the "femtosecond optical comb generator"- and laser diode-frequencies.
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