NOVEL BAND -REJECT FILTER DESIGN USING MULTILAYER BRAGG MIRROR AT 1550 NM

Novel band-reject filter is proposed using multilayer Bragg mirror structure by computing reflection coefficient at 1550 nm wavelength for optical communication. Dimension of different layers and material composition are modified to study the effect on rejection bandwidth, and no. of layers is also varied for analyzing passband characteristics. GaN/Al xGa 1-xN composiiton is taken as the choice for simulation purpose, carried out using propagation matrix method. Refractive indices of the materials are considered as function of bandgap, operating wavelength and material composition following Adachi’s model. One interesting result arises from the computation that band-reject filter may be converted into band-pass one by suitably varying ratio of thicknesses of unit cell, or by varying Al mole fraction. Simulated results can be utilised to design VCSEL mirror as optical transmitter.