  We present in this paper a method to recover the refractive index profile of
graded waveguide from the effective indices by cubic spline interpolation
function. It is proved by numerical analysis of several typical index
distributions that the refractive index profile can be reconstructed closely to
its exact profile with the presented interpolation model. This method can
reliably retrieve index profile of both more-mode (more than 4 guiding mode)
and fewer-mode (2-4) waveguides.
