Research on the Theory of Nonlinear Directional Coupler and its Application to Optical Devices

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1. As the powerful numerical approches for the nonlinear optical waveguides, we have proposed the orthogonal collocation method and the beam propagatin method based on the finite difference scheme. It was shown from the practical applications that those methods are very effective for the analysis of NLDC.2. A rigorous coupled-mode theory based on the singular perturbation technique has been developed. It was shown that the global features of power transfer along various NLDC structures can be reasonably described by the coupled-mode equations which include only the linear coupling coefficient and the nonlinear self-phase modulation coefficient. The closed forms of those two coefficients were derived for the planar NLDC structure.3. The coupling length and the critical powers for optical switching in a symmetric planar NLDC has veen brought out.4. A novel asymmetric NLDC structure has been proposed, which consists of a nonlinear waveguide and a linear waveguide separated by a common linear cladding layr. It was shown that the proposed NLDC is useful for constructing a power band-pass filter, a power band-reject filter, and an optical pulse modulator, by setting the input and output ends properly.5. The power transfer characteristics in the NLDC composed of three parallel nonlinear waveguides have been analyzed. It was shown that the input/output characteristics for optical switching are superior to those of the conventional NLDC.6. The characteristics of optical waves propagating along a nonlinear waveguide with a surface corrugation have been discussed as the basic research for developing the wavelength selective NLDC structure.