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A method is presented to design a layer structure, consisting of a few different one-dimensional photonic crystals merged together, having an extended band gap. Hereby, the thicknesses of the two layers of each unit cell, corresponding to the involved photonic crystals, are optimized using a genetic algorithm. As numerical examples, two structures are constructed by this method, one structure is composed of four one-dimensional photonic crystals by using TiO2 and SiO2, the other is composed of two one-dimensional photonic crystals based on two materials with the refractive index of 4.6 and 1.6. Their theoretical omnidirectional relative bandwidths (defined as the gap to midgap ratio) are 51.36% and 91.72% respectively. The reflection spectra calculated by the transfer matrix method agree with the correspondent band gap.