Laser has been keeping a close relation with atomic frequency standards since its origin. Recently, progress in the laser and the optical technology brought a lot of new developments in the atomic frequency standards in both microwave region and optical region. Especially, new technology on optical frequency measurements, laser stabilization and atom control technique are now opening a way to new standards in optical region whose accuracies can be beyond the limit of Cs standards. In this article we show the basics of the characterization of frequency and introduce an outline of the recent progress in atomic frequency standards and laser technology.

View full abstract

Optical frequency standards based on narrow-linewidth transitions in single laser-cooled trapped ions are developed for accuracy and stability higher than those of the existing Cesium microwave primary frequency standards. Fundamental knowledge of optical frequency standards with trapped ions and the status of optical frequency standard development in NICT and other institutes are presented. The performance of the optical frequency standards is clearly approaching that of the best microwave frequency standards.

View full abstract

In recent years, the coherent population trapping (CPT) phenomenon using a very small cell has attracted considerable attention for its potential application to ultra-compact atomic clock. For the miniaturization of CPT clock, it is important to clarify influence by the collision with atoms and cell walls. We study the effect of cell geometry on the CPT spectrum using a thin Cs vapor cell without buffer gas. When the cell length is below 1 mm and the laser intensity is sufficiently weak, the transit-time broadening is largely suppressed because only slow atoms have enough time to settle down on the dark state and contribute to the CPT signals. Furthermore, we show that introduction of buffer gas may broaden the CPT resonance in extremely thin cells. We also show that sub-Doppler signal which can be used for stabilization of laser frequency is obtained by using the CPT signal.

View full abstract

The periodic pulse train generated from a mode-locked laser can be described as a comb of modes, the frequencies of which are equally separated. This optical frequency comb has been successfully applied to optical frequency measurement. The principle and setup of the measurement, in particular, the system including anoctave-spanning optical frequency comb, are reviewed. The recent development and applications of the comb, including the optical frequency divider, are briefly described.

View full abstract

We introduce recent research activities related to iodine-stabilized lasers, including the 633-nm iodine-stabilized He-Ne laser, the 532-nm iodine-stabilized Nd: YAG laser, and the 660-nm iodine-stabilized Nd: YAG laser. Applications of these topics and their future prospects are presented.

View full abstract

We have developed accurate optical frequency standards based on acetylene absorption lines in the telecom region (C-band) and have measured their optical frequencies using several kinds of optical frequency combs. Based on these results, in 2004 we started optical frequency (wavelength) calibration service for stabilized Cband lasers at National Metrology Institute of Japan (NMIJ). We also have the preliminary results of our experiments in transferring wavelength standards via optical fiber networks.

View full abstract

Rare calcium isotope ⁴³Ca⁺ ion (0.135%) is the only naturally abundant isotope with hyperfine structure. This isotope is considered an attractive candidate both for an optical frequency standard and for quantum information processing. We report a method to load ⁴³Ca⁺ ions selectively into a linear Paul trap using a two-step, resonant photo-ionization process. Isotope selectivity is achieved by utilizing the isotope shifts for the resonant 4s^{2 1}S₀-4s4p ¹P₁ transition of neutral calcium. The second non-resonant excitation is achieved by an ultraviolet light emitting diode (LED). The use of an LED is simpler and more economical than using a laser for the second transition. Additionally the laser cooling of ⁴³Ca⁺ is demonstrated using two ultraviolet-diode lasers and two infrared-diode lasers. Due to the hyperfine structure, the laser requirements for Doppler cooling ⁴³Ca⁺ are more demanding than for the other isotopes.

View full abstract

We examined the effects of Ar⁺ laser (457, 488, 514nm; 25, 50, 100mW) irradiation on the law of polar excitation using frog sciatic nerve. The law of polar excitation is well known, as both cathode-make-excitation (CE) and anode-break-excitation (AE) are elicited when nerves are extracellularly stimulated with a longer pulse at suprathreshold intensity. Compound action potentials (CAP) were suppressed by the laser irradiation. Furthermore, the existence of irradiation conditions (power, wavelength) for greatly suppressing AE in comparison with CE was demonstrated. Ar⁺ laser irradiation did not suppress conduction but did suppress generation of CAP. It seems possible that only AE generation is suppressed by local irradiation using the Ar⁺ laser. Since the contribution of Hyperpolarization-activated channels (Ih) to AE generation had been demonstrated and Ih expression is associated with intractable epilepsy, the present results suggest a novel LLLT for intractable epilepsy as an alternative to Ih blockers.

View full abstract

A new personal identification method has been investigated which promises to be a useful technique for protecting society against the recent increase in card counterfeiting crimes. The method uses micro-cracks created in a transparent material made of acrylic resin as an ID-writer by means of a highly focused, diodepumped second-harmonic 532-nm YAG laser with an 8-ns pulse length. Various ID patterns were produced by varying the shape, the arrangement, and the number of spots using the laser-induced crack-creation technique. Using visible laser speckle patterns obtained from the micro-cracking region, reading image analysis was successfully accomplished by using a template matching method in an ID-reader configuration. The results indicate the possibility of up to 6.4 million ID discriminations. This experimental result indicated that the false acceptance rate and false rejection rate were sufficiently small to make this method a viable personal identificatio method.

View full abstract

We have proposed a compact spectroscopic sensor based on the arrayed waveguide grating (AWG) with the use of a wavelength multi/demultiplexer in a photonic network. We fabricated a new planar sensor using a visible AWG for spectroscopic sensing of absorption characteristics with multiple dicing grooves for accommodating a liquid sample. A parabola-shaped sample injection waveguide is effective for improving the sensitivity because diffraction loss in the direction of its substrate is reduced. In this research, discrimination of chlorophyll a and b as test samples is to be investigated using the AWG spectroscopic sensor with a parabolashaped sample injection waveguide.

View full abstract

In recent years, second-harmonic generation (SHG) using periodically poled nonlinear crystals has attracted considerable attention. We developed a 423-nm light source using an 846-nm tunable diode laser. To evaluate the performance of the light source, we irradiated an SHG light at a calcium atomic beam and the observed fluorescence spectrum. Isotope selective photo-ionization of calcium atoms was performed using the developed light source.

View full abstract