Transmission Mechanism Of Ultra-Short Pulses In Fiber Bragg Grating And Its Application To Distributed Sensing

Fiber Bragg grating(FBG)has been widely used in optical fiber communication and optical fiber sensing systems due to its overwhelming advantages,such as small size,anti-electromagnetic interference,low cost and multiplexing ability.In high-speed and long-distance communication systems,ultra-short pulses are often used as light sources to improve the bit rate of systems.The transmission characteristics of the ultra-pules in the FBG is the basic principle to use the FBG as the dispersion compensation element and the pulse shaper in an optical fiber communication system.In addition,one of the most important advantages of FBG sensors is distributed sensing and most fully distributed FBG sensors are based on the response to ultra-short pulses of the FBG under partial disturbance.Therefore,the research of the propagation theory mentioned above will provide an important theoretical basis for the design of demodulating system.In this thesis,the propagation characteristics of the ultra-short pulses in a partially disturbed FBG are studied in detail and applications based on these characteristics to distributed sensing are discussed.First,the theory and method to calculate the pulse response spectrum of the fiber grating are introduced and the numerical simulation of ultra-short pulses propagation properties in uniform fiber grating,apodized fiber grating,chirped fiber grating and apodized chirped fiber grating is given.The effects of the grating parameters on the reflection spectrum and the pulse response spectrum are also studied.The results show that the grating reflective index has a great influence on the pulse propagation properties.For the apodized fiber grating,the apodization property determines the grating reflection spectrum and pulse response spectrum.Secondly,we take the strain as an example to research the reflective spectrum and pulse response spectrum of the FBG under partial disturbance.The effects of strain size,strain length,strain locations and the number of strain on the simulation results are studied and summarized in specific and the regularities are summed up.For the FBG reflective spectrum,the size of the strain determines the phase shift of the initial reflection peak and the center wavelength of the strain reflection peak;the strain length determines the reflectivity of the strain reflection peak,and the strain position basically does not affect the reflective spectrum.For the FBG pulse response spectrum,the size of the strain determines the amplitude,period and number of oscillation peaks in the strain region of the pulse response spectrum;the strain length determines the size of the oscillation region and the number of oscillation peaks;the strain position determines the location of the oscillation area in the grating pulse response spectrum.Finally,the transmission matrix of the polarization-maintaining fiber Bragg grating under torsion is deduced.The reflective spectrum and pulse response spectrum of the polarizationmaintaining fiber grating are simulated with this matrix.According to the regularities of the reflective spectrum only the torsion size can be measured and the torsion directions cannot be identified,however,the torsion size and the torsion directions can be measured simultaneously by using the pulse response properties.Based on the pulse propagation properties of FBG,a fully distributed sensing demodulation system is proposed.