The Study On Spectrum Characteristics And The Wavelength Tuning Techniques Of Fiber Grating

The invention of fiber Bragg grating is viewed as another landmark in the history of optical fiber communication, succeeding the invention of erbium doped fiber amplifier(EDFA). Since photosensitivity of optical fiber was first discovered by K.O.Hill et al. at the Canadian Communication Research Center in 1978, dramatic improvements have been made in both fiber Bragg grating theory and fabrication techniques. And all these improvements are achieved in only a litle more than two decades. Various kinds of fiber Bragg grating with different novel structures have been coming forth ceaselessly and their application fields expanding increasingly. Nowadays, fiber Bragg grating have been widely used in the fields such as fiber optical communication, optical sensing and laser technology et al., and revolutionized these fields.This dissertation is mainly focused on fiber Bragg grating spectrum characteristics and tuning techniques. Main contents of this thesis are classified as follows:1. Starting from the Maxwell-field equations, we deduce the wave-propagation equations, and then analyze the characteristics of guided modes in optical fiber. Based on these, we derive the coupled-mode equations that describe the interaction between the modes. Furthermore, we analyze fiber Bragg grating of a constant refractive index modulation and period, found the coupled-mode equations that describe interactions between the forward-propagating guided core mode and the backward-propagating guided core mode. Nextly, transfer matrix theory, and Fourier transform's method of fiber grating are briefly introduced.2. Simulation results, using the transfer matrix method, of several kinds of fiber Bragg grating are presented, such as fiber Bragg grating, long period fiber grating, chirped fiber grating, double-period fiber Bragg grating, phase-shifted fiber grating, super-structure fiber grating, and effects of all kinds of apodization profile functions and simulation parameters selection on the result's accuracy are analyzed.3. Stress tuning techniques of fiber Bragg grating are studied systematically. An effective Stress tuning mechanism, named elastic beam, is introduced. The basic linear tuning principle of fiber grating wavelength without chirp effect using variational section beam with uniform intensity is theoretically analyzed. Three kinds of basic design scheme of variational section beam and the condition that the elastic beam section as tuning device of the wavelength of fiber grating without chirp effect must satisfy are presented. The characteristics of the tuning technologies using cantilever beam based on identical section and variational section are discussed. It is shown that by bring lateral load to bear on simple beam and cantilever beam a linear tuning of fiber gating wavelength without chirp effect can be realized, and the setup is simple in structure and easy to design. Tuning sensitivity of this device can be designed according to practical needs, so it provides the user with greater flexibility. It can be used in the fields such as tunable fiber lasers, high sensitive optical sensors et al.4. A wavelength shift detecting technology for fiber Bragg grating based on tunable-filtering technology is analyzed theoretically and experimentally. Numerical and experimental results show that there is an optimum Bragg wavelength difference between the two fiber Bragg grating's that maximize the sensitivity for this demodulation technique.