Minimum total dispersion in multimode graded-index fibres

Abstract

This paper points out that, in impulse response calculations by the method of Gloge and Marcatili, the condition that the equal propagation constant lines and the equal group delay lines coincide is not satisfied except for a power law index profile. The group delay time is derived in this paper, by a perturbation method. The optimum index profile is determined from the measured refractive indices data of several doped silica glasses.

It is found that (1) the optimum value of the fourth order coefficient is not so different from the value which minimizes the modal dispersion, except in the short wavelength region, (2) the wavelength which minimizes the total dispersion is near 1.30μm and (3) a B₂O₃ doped fibre has a small profile dispersion and a small total r.m.s. width.