A simple approximation method is proposed that is partly based on the conventional coupled-mode analysis and uses the coupled transmission-line equations the matrix elements of which are coupling coefficients. The coupling coefficients between adjacent cores are evaluated from the propagation constants of the even and odd modes of a dual-core structure, considering the orientation of the transverse electric field vector. The coupling coefficients between the nonadjacent are assumed to be negligible. This procedure corresponds to Huckel's approximation in the analysis of molecular orbitals governed by Schrodinger's equation. The results of numerical calculation are compared with those of a rigorous analysis. It is found that as the separation between cores is decreased and the effect of coupling between nonadjacent cores becomes stronger, the accuracy of the method is slightly reduced.
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