The first known demonstration of tunable terahertz-wave generation by difference-frequency generation of dual signal-wave quasi-phase-matched optical parametric oscillation was performed with periodically poled LiNbO 3 (PPLN) with a series of gratings. An organic ionic salt, 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST), was used as a nonlinear crystal. A compact terahertz-wave source resulted, and changing the temperature of the PPLN permitted the wavelength to be varied from 120 to 160 mm. The wavelength could be tuned from 100 to 700 mm by proper selection of combinations of periodically poled gratings.

We have demonstrated single-photon interference over 150 km using time-division interferometers for quantum cryptography, which were composed of two integrated-optic asymmetric Mach-Zehnder interferometers, and balanced gated-mode photon detectors. The observed fringe visibility was more than 80% after 150 km transmission.

We have demonstrated, for the first time to our knowledge, periodic poling of a 2-mm-thick near-stoichiometric LiTaO(3) substrate and its operation in a nanosecond optical parametric oscillator. Because the coercive field of stoichiometric LiTaO(3) is ~2 kV/mm , which is approximately one tenth that of the conventional congruent field, periodic poling of thicker stoichiometric substrates was successfully performed by means of an electric-field poling process at room temperature. The performance of a parametric oscillator with a 1-mm-thick sample was compared with that of the oscillator with the periodically poled congruent oscillator. The stoichiometric device exhibited better performance.

We constructed an optical interferometer for a Bennett-Brassard quantum key distribution system using integrated optics based on planar lightwave circuit technology, and tested its operation and stability. Experimental results show that this interferometer is useful in implementing a practical quantum key distribution system.

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