Characteristics Of Long Period Fiber Grating Coupler Analysis And Optimization Design

A long-period fiber grating (LPFG) coupler can be obtained by placing several long-period fiber gratings in parallel. Since LPFG are transmission grating, it exhibits outstanding characteristics such as no back reflection, high channel isolation, low insertion loss, wavelength selectivity and shows band-pass and band-rejection characteristics simultaneously. Therefore, it can be operated as all-fiber broadband add/drop multiplexer and has potential applications in density–wavelength–division- -multiplexed (DWDM) systems. In this paper, a 2×2 LPFG coupler composed by two uniform UV-written LPFGs is taken into theoretical and experimental study, which is vital for the further study of the transmission characteristics and optimization of mechanically induced microbending LPFG couplers and the hybrid couplers.The analysis of LPFG coupler is based on the models of fiber and gratings including two-layer scalar model, three-layer scalar model and three vector model of the fiber and coupled-mode theory of the grating depicted by Turan Edogan. By considering the evanescent-field coupling, the characteristics of LPFG coupler was investigated numerically utilizing the existing models. The results showed that the self-coupling coefficient of fundamental mode in the core has considerable influence on the resonant wavelength but can be negligible on the efficiency of the LPFG coupler. Consequently, the analytical solution of coupled-mode equations without self-coupling coefficient is significant for the design of LPFG coupler when we focus on its efficiency. However, the numerical analysis of coupled-mode equation with self-coupling coefficient is demanded for the insight of the spectral of LPFG coupler. The analysis suggested that the 100% coupling efficiency or specific power assignment of LPFG coupler can achieved by elaborate design of the modulation of refractive index, the period of LPFG grating and length, suitable external medium and offset distance.The output spectrums of LPFG couplers with different external mediums and offset distances were executed by experiments, which indicated that the higher efficiency of LPFG coupler can be obtained with higher refractive index of external medium and appropriate offset distance. Nevertheless, the resonant wavelength moved to shorter wavelength gradually when increasing refractive index of external medium but remained at different offset distances. The numerical results of the transmission spectrum and coupled spectrum fit well with the experiments, and the simulated efficiencies of LPFG coupler were almost the same as the experiments under all the situations.