Abstract

It is desirable to represent the propagation behaviour of a single-mode fibre by a few key parameters. A method currently in favour is to define an "equivalent-step-index" fibre, whose core radius a and relative refractive index difference Δ should lead to reasonably accurate calculations of cut-off wavelength, spot size (and hence of losses at joints or due to microbending) and waveguide dispersion, using standard formulae. The approaches used are either to calculate suitable average values of a and Δ from the refractive index profile, or to find the values from the cut-off wavelength and the relation between spot size and wavelength in the single-mode region. The data used would usually be experimental. In the present paper, however, they are calculated from an assumed parabolic profile, for which a spot size formula has been published by Marcuse.