The few-mode fiber can be used to transmit limited orthogonal modes,which has the advantages of small modular interference and easily controlled modes.The Brillouin scattering sensor based on the few-mode fiber can effectively reduce the cross sensitivity of multi parameter measurement,and realize the measurement of multi physical quantity.In this paper,based on the wave optics theory,the Brillouin scattering spectrum parameters of the step-index few-mode fiber are analyzed,such as frequency shift,line width and peak gain and so on.Firstly,the transmission modes of the few-mode fiber are analyzed.The finite element analysis result shows that there are 5 kinds of transmission modes:LP01,LP11,LP21,LP02 and LP31,and their effective refractive indexes are 1.4664,1.4652,1.4637,1.4630 and 1.4616,respectively.Secondly,the mathematical models of the Brillouin frequency shift,line width and peak gain of different modes in the few-mode fiber are analyzed.Finally,the parameters of Brillouin scattering spectrum with different modes' superposition are also discussed.In the few-mode fiber,due to the different effective refractive index,the light of each mode is propagated along its respective path and interacts with the particles in the fiber,thus producing different Brillouin scattering spectrum.The simulation results show that the frequency shift of the Brillouin scattering spectrum of each mode is in a range of 10.19-10.23 GHz,and the frequency shift increases with the decrease of the mode order.The Brillouin line width of each mode is in a range of 32-34 MHz,and the line width also increaseswith the decrease of the mode order.Moreover,the relative amplitude of the Brillouin scattering gain spectrum increases with the decrease of the mode order.The mathematical models of this paper are used respectively to analyze the Brillouin scattering spectra of other types of step-index few-mode fibers.It is shown that the Brillouin frequency shift,Brillouin line width and peak gain of other types of step-index few-mode fibers also increase with the decrease of the mode order.In a step-index few-mode fiber,intramodal Brillouin scattering spectrum and the intermodal Brillouin scattering spectrum are both in line with the distribution of Lorenz curve.However,the intermodal Brillouin scattering spectrum of modes' superposition leads to the line width broadening of the Brillouin scattering spectrum,and the relative amplitude of the intermodal Brillouin scattering spectrum of modes' superposition being generally smaller than that of intramodal.%少模光纤可以传输有限的正交模式,具有模间干涉小、模式易于控制等优点.基于少模光纤的布里渊散射传感器能够有效地减小多参量测量过程中的交叉敏感,实现多物理量的测量.本文基于波动光学理论对阶跃型少模光纤各个模式布里渊散射谱的频移、线宽、峰值增益等参数进行了分析.首先对少模光纤进行了模式分析,其次分析了少模光纤不同模式的频移、线宽、增益的数学模型,以及不同模式叠加的布里渊散射谱频移、线宽、增益.结果表明:少模光纤各模式布里渊散射谱的频移在10.19-10.23 GHz范围内,且随模式阶数的减小而增加;各模式布里渊散射谱的线宽在32-34 MHz,且随模式阶数的减小而增加;各模式布里渊散射增益谱相对幅度因为模式阶数的减小而增大.少模光纤中各个模式布里渊增益谱和多模式联运布里渊增益谱均符合洛伦兹曲线分布,多模式联运导致布里渊增益谱线宽展宽,且多模式联运布里渊增益谱相对振幅一般小于单个模式的布里渊相对振幅.
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