Recent developments in fiber based frequency combs have made them the preferred link between optical and microwave frequencies. This is due to their robustness and cost-effectiveness. We have developed fiber based frequency combs at NMIJ (National Metrology Institute of Japan) and used them to measure a range of wavelengths, including stabilized lasers with wavelengths at 633, 1064, 1542nm. Our comb has continuously worked for over 1 and half years in our laboratory. In addition, one of our fiber combs was recently shipped to the Australian National Measurement Institute to perform a comb comparison with the NMIA (National Measurement Institute, Australia) fiber comb. In this paper, we firstly survey the octave-spanning comb generation and optical frequency measurement using mode-locked lasers. We subsequently describe a development of the fiber comb at NMIJ, and the results of the comb comparison performed in Australia. The consistency of the frequency measurements of the comb comparison reached to the 10^-16 level.

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Femtosecond-laser-based optical frequency comb has received a lot of interest as powerful metrological tools capable of covering the visible to mid-infrared region. However, frequency comb has not been investigated in the long-wavelength terahertz (THz) region. In this paper, the concept of the frequency comb is extended to the THz region. THz frequency comb is suitable for high-accuracy, high-resolution THz spectroscopy, which can be used to identify molecules of interest with fingerprints in the THz region. Multifrequency-heterodyning photoconductive detection was used; it involves using two stabilized Ti: sapphire lasers having slightly mismatched mode-locked frequencies. The detailed structure of a THz frequency comb was clearly observed with a frequency accuracy of 2.5×10^-7 and a resolution of 81.8MHz.

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High harmonic generation (HHG) by passive femtosecond enhancement cavities is reviewed with a detailed example studied at JILA. The technology is aimed towards generation of phase-coherent VUV radiation that maintains a one-second coherence time of fundamental pulses. The basic strategy to couple fs pulses into high-finesse optical cavities and the difficulties encountered in doing so are first summarized. Then, the current status, technical limitations, and future outlook of this novel HHG are described.

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In this article, a passive type optical frequency comb generator and its applications are reviewed. We have developed a module of the optical frequency comb generator. The module is small, and suitable for mass production. An optical frequency counter in which the modules are used has been developed for generalpurpose frequency measuring instrument enabling the precise measurement as high as 10^-10. In addition, I introduced an experiment of wave form synthesis of the optical frequency comb based on spectral line-by-line pulse shaping.

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Optical frequency comb technology opened up new fields not only in frequency metrology but in length metrology. Frequency-traceable length measurement using comb provides direct realization of the definition of meter, remote calibration using a GPS technology, and precise measurements of wide range of lengths by taking advantage of high dynamic range in frequency measurements. However, the potential in length metrology has not been fully explored yet. Here we report new techniques using optical combs for small displacement and long absolute-distance measurements. The displacement measurement uses Fabry-Perot cavities and wide-range frequency measurements. A reference cavity compensates the effects of the environmental fluctuation and achieves 10-pm stability for 30-minutes measurement even in air. In the long distance measurement, phase measurements of the intermode beats of a comb realize high stability and small cyclic errors of better than 2 micrometers for 240-m distance. A developed compact setup shows extremely small slope error of less than 1×10^-7 over 200-m distance.

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Recently, the delivery of frequency standards using optical combs through optical fiber networks has become an important subject. When we transmit optical combs through an optical fiber network, it is very important to investigate the distortion in the power spectrum of the longitudinal modes that occurs during the transmission. In this paper, we describe the spectrum deformation of the longitudinal mode that occurs during long-haul transmission through a dispersion-shifted fiber, when we use a 10 GHz mode-locked fiber laser with a linewidth as narrow as 1 kHz as the signal source. We found that the deformation of the longitudinal mode power spectrum occurs mainly at the tail of the spectrum, and the linewidth broadening increases in proportion to the transmission distance. The main reason for the linewidth broadening was the phase fluctuation of the optical fiber caused by temperature fluctuation.

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Experiments to transmit a 1kW COIL laser beam of 1.31μm wavelength and a 5kW fiber laser beam of 1.07μm wavelength through glass fibers 1km in length were conducted. Three types of fiber from different companies were tested, and the minimum pure loss observed was 15% per km. A Raman component was detected in the 5 kW experiment.

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We have developed a new laser material based on silica glass (Nd-doped silica glass) with high optical quality and high quantum yield, and demonstrated a high energy laser oscillation. Since the maximum output laser energy reaches up to 37.3 J and its thermal shock parameter (12.0W/cm) is 1.5 times larger than Nd: YAG (7.9 W/cm), this medium is expected to work as a high-peak-and high-average-power laser in the range of-100J/10Hz. We also discuss some applications on Nd-doped silica glass.

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