On the validity of the effective-index method for rectangular dielectric waveguides

The effective-index method (EIM) for rectangular dielectric waveguides is derived as an approximation from an exact equivalent-network formulation. It is explained why EIM gives accurate results for the fundamental modes in near cutoff region as well as for all the guided modes in well-guiding region. As a direct improvement on EIM a simple eigenvalue equation is presented, which takes into account the shape of the predominant slab-waveguide mode. It is shown that EIM overestimates propagation constants compared to the improved results.>