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The idea of implementing optical filters by coupling evanescent waves from several diffracted orders into multiple leaky waveguide modes is studied theoretically. Using a dielectric thin-film structure consisting of a coupling grating placed between adjacent waveguides, guided-mode resonance filters exhibiting multiple reflection peaks within a specified wavelength range can be obtained. These peaks originate in the resonant waveguide modes that are excited by the diffracted waves dispersed by the grating. It is shown that this device can be used to realize multiwavelength as well as wide-band spectral filters.