A comparison between vectorial and semi-vectorial finite element method (FEM) formulations for modal analysis of dielectric waveguides is reported. The influence of the index step and of the mesh distribution on the solution accuracy is investigated. In particular, besides on the effective index, attention is focused on the correctness of the field distributions whose evalaution is a crucial requirement in waveguide analysis and related applications. As many authors have been concentrated on the accuracy of the propagation constant, no useful criteria have been proposed to test the spatial distribution of the unknown field. To overcame this lack two error figures have been introduced and applied to different FEM solutions. In particular the following formulations have been compared: that one baed on the transverse magnetic field, those based on the so called edge-elements and a new formulation presented by the authors. Results show the availability of the proposed approach and the usefulness of the introduced figures for a deep waveguide analysis.
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