Phase-shifted Bragg grating filters with symmetrical structures

A comprehensive theoretical analysis of the compound phase-shifted uniform fiber Bragg grating (FBG) filter is presented. The transmission characteristics are examined by a transfer matrix formulation. It is shown that the transmission spectrum of such a filter is significantly controlled by the ratio of the grating lengths. By suitably choosing the ratio, a quasi-flat-top spectrum may be obtained. The general condition for the optimum case is derived. This characteristic makes the compound phase-shifted uniform FBG filter particularly advantageous for applications in wavelength-multiplexed optic-fiber communication systems.