“…Several methods exist that in certain respects might be viewed as improvements of the EIM; we mention the (Film) Mode Matching Method (MMM) (Peng and Oliner 1981;Sudbø 1993), in which the total field is expanded in the terms of O. V. Ivanova (B) · M. Hammer · R. Stoffer · E. van Groesen MESA + Institute for Nanotechnology, AAMP Group, University of Twente, P. O. Box 217, Enschede 7500 AE, The Netherlands e-mail: o.v.ivanova@math.utwente.nl the local slab modes and later matched across the interfaces, the Spectral Index Method (Vassallo and Wang 1990), in which the total field is put to zero at certain distances from the interfaces with strong refractive index contrast and later expanded in Fourier series, the Weighted Index Method (Robertson et al 1989), which using a variational procedure finds the modal field profile as the best separable solution of the wave equation and finally the Rigorous Effective Index Method (Benson et al 1992), which for rib waveguides with outer slices above cut-off, by variational means finds a simple transcendental equation for the propagation constant.…”