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It is well known that the location of guided-mode resonance (GMR) in grating waveguide structures closely tracks the leaky mode dispersion curves. In this paper, taking Bragg reflection due to periodicity and interaction between different modes into account, we first present a schematic diagram of the dispersion relations of leaky modes in multimode grating waveguide structures, both for s-polarized (TE mode) and p-polarized (TM mode) incident waves. Due to the perturbation of the grating layer, the interaction between different resonance modes (transverse standing waves) is inevitable. This transverse interference will result in the non-Bragg nature resonance band gaps in the dispersion curves. Exploiting the characteristics of leaky mode dispersions over the full range of the first Brillouin zone, we hoped we could gain some insight into the relationship among the mode interactions, band gaps, and their benefits to optical elements utilizing the GMR effect in grating waveguide structures. Finally, a specific structure is analyzed. [ABSTRACT FROM AUTHOR]