COUPLING CHARACTERISTICS BETWEEN OPTICAL SLAB WAVEGUIDES WITH GRADUAL DISTRIBUTION OF REFRACTIVE INDEX.

The refractive-index distribution of the optical slab waveguide which is produced by the diffusion technique or ion exchange method is of gradual change rather than step change. This paper analyzes the relation between the refractive-index distribution and the coupling characteristics among waveguides. Both symmetrical (Gaussian and parabolic) distribution and asymmetrical (error function and linear) distribution are analyzed and the phase constant of normal modes in the coupling system is obtained. It is found that the shape of the distribution greatly influences the coupling characteristics of a multimode waveguide rather than a single-mode waveguide. The variation of the complete coupling length due to the mode number is larger in the Gaussian type than in the step type and smaller in the error function and linear types.