When the effective-index method is used to reduce a three-dimensional refractive-index distribution to two dimensions, e.g., for use with the two-dimensional beam propagation method, strictly speaking, the effective index n(eff) cannot be calculated for nonguiding regions. Here a technique for obtaining values of n(eff) in such regions is described. The method begins with the index distributions in regions for which values of n(eff) are calculable and then perturbs them for those regions of interest. Using an electro-optic Y-branch optical modulator and a voltage-induced optical waveguide modulator as examples, we demonstrate this method and compare predicted results with measured results.
