Analysis of inhomogeneous multimode dielectric waveguides

A theoretical investigation is made of the characteristics of multimode dielectric fiber waveguides, prepared by deposition from the gaseous phase and characterized by M-shaped radial variation of the refractive index. The WKB method is used to analyze the influence of deviations of the refractive index profile from a parabola on the dispersive properties of the waveguides. These calculations are made for various values of the depth, width, and steepness of a dip of the refractive index which appears at the center of the waveguide because of the evaporation of the dopant. It is shown that the dip increases the spreading of a pulse and accelerates lower-order modes. The optimal shape of the outer edge of the permittivity profile, corresponding to a reduction in the width of the pulse characteristic, is found for a fixed dip of the refractive index.