Calculation of the effective index for nonguiding regions: comment

In computer simulations the effective-index method is often used to reduce three-dimensional waveguide structures to two dimensions by definition of an effective index. For nonguiding regions, however, an effective index cannot be calculated. Jaeger and Lai [Appl. Opt. 31, 7183-7190 (1992)] present a method in which the choice of an effective index for nonguiding regions is based on a linear extrapolation formula. We show, however, that this linear relationship does not hold in the vicinity of cutoff and therefore yields an arbitrary effective index for the waveguide under cutoff. Furthermore it is shown that errors in simulations based on the effective-index method can actually be increased by the Jaeger and Lai method because of a further overestimation of the lateral contrast.