Synthesis and analysis of composite long period fiber gratings using multiport lattice filter model

Abstract

Long period fiber gratings(LPFGs) is a structure of the optical fiber taking advantage of an optical scattering phenomenon. When the mode traveling the fiber core comes across a fiber core gratings, the scattering occurs into the fiber cladding by the diffraction and the mode passing the fiber gratings has some losses in certain wavelength bands. This characteristic has been utilized for fabricating the fiber filters such as band rejection filter, band pass filter, EDFA gain equalizer, WDM multi-band isolation filter and strain and temperature sensors. In this thesis, we discuss both the analysis of LPFGs calculating the transmission spectrum from the composite LPFGs and synthesis of LPFGs finding the grating profiles of LPFGs for the desired transmission spectrum. We show accurate analysis results of composite type LPFGs by multiport lattice filter model, such as phase-shifted and cascaded LPFGs as well as concatenated LPFGs having the characteristics of phase-shifted and cascaded LPFGs. To analyze the transmission spectra of the concatenated LPFGs with arbitrary wavelength spacing and line width, the coupling between the core mode and multiple cladding modes was considered simultaneously. In order to obtain an analytical matrix model for the concatenated LPFGs, a multiport lattice filter structure was used. To verify the validity of the proposed model experimentally, we have observed that the transmission spectra calculated using our multiport lattice filter model are close to the corresponding measured spectra in the wavelength band of interest. Also, We propose the synthesis method based on both the genetic algorithm and multiport lattice filter model and have obtained the optimal LPFGs grating parameters profiles such as period and induced index changes for the desired transmission spectrum by the proposed method. In order to design EDFA gain flattening filter, we have used the phased shifted LPFGs ,and have obtained more closer transmission spectrum ...