An exact numerical method of analyzing deep dielectric gratings with multistep lamellar profiles is presented. This method is able to predict the diffraction efficiencies of dielectric gratings for both TE and TM polarizations.In this paper we use a modal expansion method in terms of modal functions which are rigorous solutions of the Maxwell's equations and fulfill the boundary conditions on the upright enclosing sides of the rectangular profiles.The coefficients of the modal expansion are determined such that the mean -square boundary residual becomes minimum, and the sequence of approximate solutions converges rapidly and monotonically with respect to the truncation size of the expansion order with help of the smoothing procedure of the modal fields.To show the validity of this algorithm in the sinusoidal surface -relief gratings, the results are compared with those obtained by other methods.
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