In this paper, a new physical model is proposed. For a uniform Bragg grating, the index modulation distribution is a cosine profile. We slice the grating into many slabs and each slab can be looked as a thin surface. A slab has higher or lower index modulation than its adjacent slabs. A slab with high index modulation and its adjacent slab with different index modulation can be taken as a Fabry-Perot cavity. Thus a grating can be looked as a system that consists of a series of a coupled Fabry-Perot cavities. A matrix method is introduced, which relies on the calculation of the reflected and transmitted fields at an interface between two slabs of dissimilar refractive indexes. The simulation results can explain the properties of fiber grating well and show good consistency with previous conclusion.
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