Material dispersion in optical fiber waveguides

In weakly guiding dielectric (glass) fiber waveguides the factors governing the envelope delay distortion of optical signals can be separated conveniently into material and waveguide contributions. By making use of the fact that the normal dispersion of the index-of-refraction of glasses accurately fits a single oscillator Sellmeier equation, simple relations of general validity are obtained for the material contribution to the group delay and frequency dependence of the group delay in dielectric fiber waveguides. Using these results expressions are obtained for the material contribution to the information bandwidth of fiber optic waveguides excited by wideband sources (light emitting diodes) and narrow band sources (lasers). It is shown that in single mode guides the bandwidth limitation imposed by material dispersion is negligible, while in multimode guides the difference in group delay of higher order modes imposes the most severe bandwidth limitation.