Research On Brillouin Scattering In Multimode Silica Optical Fibers And Its Temperature Characteristics

In order to explore the new mechanism and new methods for distributed fiber sensing, and to evaluate the status of Brillouin scattering in multimode optical fiber in the field of distributed sensing, the current research condition of Brillouin scattering sensing technology using a multimode optical fiber is summarized in this thesis. The basic theory of multimode optical fiber and Brillouin scattering is introduced, and the relationship between Brillouin shift and Brillouin scattering power of multimode optical fiber and temperature are derived based on the analysis of Brillouin scattering in single mode optical fiber and its temperature characteristics. A method to determine the range of Brillouin scattering angle is proposed, the expressions of Brillouin shift, Brillouin linewidth, Brillouin scattering spectrum and scattering power of different mode group in step and graded multimode optical fibers are deduced based on the ray optics and wave optics theory, and then are calculated by using Matlab language. The results show that the maximum range of Brillouin scattering angle in step and graded fibers is from twice of the critical incident angle of total reflection to π, and with the increasing of mode group number, Brillouin shift, Brillouin linewidth, normalized peak gain and scattering power of step fiber change more slowly than those of graded fiber. When the input power of fiber is 5 m W, the above parameters of step fiber decrease nonlinearly in the ranges of 11.084 GHz to 10.932 GHz, 21.760 MHz to 21.168 MHz, 1 to 0.933 and 1.990×10-9 W to 1.857×10-9 W, but the correspending parameters of graded fiber decrease linearly in the ranges of 11.064 GHz to 10.969 GHz, 21.683 MHz to 21.314 MHz, 1 to 0.957 and 2.052×10-9 W to 1.965×10-9 W, respectively.Based on the theory of Brillouin scattering sensing, a measurement system of Brillouin scattering spectrum and its temperature characteristics are designed and built up. The spontaneous Brillouin scattering spectrum of 1.02 km long multimode graded fiber with a 50/125 μm diameter, and the temperature characteristics of Brillouin shift, Brillouin scattering power and Brillouin linewidth of the fiber are measured, and the effects of temperature on these three parameters are analysed and discussed. The experimental results show that Brillouin shift and Brillouin linewidth is 10.228 GHz and 89.080 MHz at room temperature of 16.8 ℃, the peak power of Brillouin scattering spectrum displayed on the spectrum analyzer is 2.046×10-6 m W. The Brillouin shift, Brillouin scattering power and Brillouin linewidth change linearly with temperature in the range of 5 ℃ to 80 ℃, and the temperature coefficients of these three parameters are 1.013±0.011 MHz per ℃, 0.399±0.072 % per ℃and-0.358±0.088 MHz per ℃, respectively.