Bragg gratings in optical fibers in multimode propagation are investigated theoretically by using the coupled-mode theory and considering two modes coupling. Bragg gratings in multimode fibers (MMFBGs) show multiple reflection peaks due to the coupling between all bounded modes. The reflection spectra depend on the excitation condition of the bounded modes. A numerical method based on the fourth-order Runge-Kutta formula is developed to solve the coupled equations. The reflection spectra of Bragg gratings with sinusoidal and square refractive index modulations were simulated. The asymmetric change of refractive index in the cross section is also considered here. The spectra characteristics and potential applications of these gratings are discussed.
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