Bragg fiber with a doubly defect layer-based tunable multiband optical filter

Abstract

A Bragg fiber waveguide having double-defect layers in its periodic cladding region is investigated and optimized for inline tunable multi-bandpass filter applications. The proposed waveguide is classified in terms of a symmetric waveguide and an asymmetric waveguide depending upon the refractive index of the defect layers. The required reflectance and confinement loss equations are obtained using the boundary matching technique and the transfer matrix method. Due to the presence of the double-defect layer, two sharp passbands in the bandgap region are observed. The separation of the passbands and their intensities can be controlled by introducing the number of unit cells between the defect layers. Our analysis shows that the passband’s intensity, bandwidth, and central wavelength can also be tuned by changing the refractive index of the core material. In addition, the central wavelength for both defect modes are blue shifted with the increase of the core refractive index. In all of our considered cases, the higher tunability with a core refractive index variation in the right defect mode is 25.38 nm/RIU for the symmetric cases having defect layer refractive indices of 1.45. Similarly, the higher tunability with a core refractive index variation in the left defect mode is 16.9 nm/RIU for the asymmetric cases having a refractive index of 1.45 in the first defect layer and 3.42 in the second defect layer. Hence, our proposed waveguide can work as a tunable multi-bandpass inline filter with a narrow bandwidth.