Study On Special Bragg Grating Based Mode Converters And Filters

The applications of Bragg gratings have rapidly developed in the fields of optical communications, optical sensors and all-optical signal processings, due to their advantages including compactness, large fabrication tolerance and flexibility of filtering through direct design of grating structure parameters. In this dissertation, by utilizing the filtering characteristics and the coupling between different counter-propagating modes of Bragg gratings, we focus on the theoretical and experimental investigations on active fiber Bragg grating based narrow band filter and all-optical tunable multi-channel dispersion compensation, tilted few-mode fiber Bragg grating and Bragg grating coupler based mode converter and (de)multiplexer. Besides, we theoretically design the apodization of Bragg gratings for the applications of spectrum filtering based format conversion, clock recovery and etc. The major reseach contents can be summarized as follows:(1) The basic theory of Bragg grating analysis and structure design methods are introduced and discussed.(2) The fiber grating fabrication platform is built up and the manufacturing processes are automatically controlled by a computer. We have successfully fabricated some kinds of special fiber gratings by using this platform, such as active fiber Bragg grating, polarization maintaining fiber Bragg grating, few-mode and multi-mode fiber Bragg grating, tilted fiber Bragg grating (T-FBG), phase shifted fiber Bragg grating, and etc. And the spectra of the gratings are analyzed.(3) The Erbium-doped fiber Bragg grating (EDFBG) fabricated by our platform is employed as a narrow filter in a hybrid Brillouin/Erbium fiber laser (BEFL), which realizes single-longitudinal-mode, low pump power threshold, tunable, stable lasing output with narrow linewidth. Moreover, according to the G-T etalon theory, we propose an all-pass filter based on EDFBGs, which accomplishes all-optical tunable multi-channel dispersion compensation.(4) Starting with the Coupled-Mode Theory, the coupling conditions between two different contradirectional modes in multi-mode Bragg grating is studied. We experimently demonstrate a mode converter based on weakly tilted few-mode fiber Bragg grating (T-FMFBG) with large fabrication error tolerance and high conversion efficiency. The application of this device is theoretically extended in large bandwith mode conversion and multi-mode/multi-wavelength demultiplexing. We also design and fabricate a two-mode multiplexing/demultiplexing device based on on-chip silicon asymmetrical multimode grating-assisted contradirectional coupler. And the device performance is evaluated by injecting OFDM/OQAM 256-QAM signal.(5) The apodization function of uniform and linear chirped Bragg grating are theoretically designed from presupposed filter transfer functions, by using Discrete Layer Peeling (DLP) method and Space-Frequency Mapping theory. From the point of amplitude filtering, phase filtering and complex filtering, we theoretically demonstrate high speed RZ-OOK to NRZ-OOK format conversion, RZ-OOK clock recovery, and THz triangle pulse generation based on nonuniform Bragg gratings.