Research On The Influence Of Polarization And Mode Coupling On The Performance Of Multimode Fiber Rayleigh BOTDA System

In recent years,with the rapid development of distributed sensing technology and the rise of related online services,single-mode fiber has been difficult to meet people's needs for multi-parameter distributed sensing,and the research on expanding high-quality new distributed optical fiber sensing is imminent.The multimode fiber has aroused wide attention and exploration of scientific researchers due to its multi-mode transmission advantage.However,the polarization and mode coupling characteristics of the multi-mode fiber inevitably affect the performance of the sensing system.Therefore,it is urgent to explore the mode characteristics of multimode fiber and its influence on the performance of the sensing system.This thesis first introduces the research background,purpose and significance of the subject,and summarizes the research progress of multimode fiber Brillouin distributed fiber sensing technology and its polarization and mode coupling characteristics.Secondly,on the basis of considering the mode characteristics of the multimode fiber,the generation mechanism and influencing factors of the mode coupling of the multimode fiber are numerically analyzed.The results show that the mode coupling of multimode fiber mainly occurs between modes with similar phase propagation constants,and the power coupling efficiency of the multimode fiber gradually decreases due to lateral misalignment and angular deviation.Thirdly,the self-disturbance characteristics of polarization state of multimode fiber and the influence of mode coupling on the degree of linear polarization of the output light are explored.The results show that the polarization characteristics of the output light of the multimode fiber depend on the light exit angle.With the increase of the light exit angle and the degree of mode coupling,the linear polarization degree of the output light of the multimode fiber gradually decreases,and under the same conditions,the low-order mode has a stronger ability to maintain the polarization.Finally,based on the analysis of multimode fiber polarization and mode coupling,the multimode stimulated Brillouin scattering(SBS)effect is explored.The results show that the polarization and mode coupling effects of multimode fibers cause the Brillouin gain peaks of step and graded multi-mode fibers to increase,and their frequency shifts and line-widths decrease.In addition,the SBS thresholds of the two multimode fibers are much higher than that of the single-mode fiber,which is expected to improve the fiber input power of the sensing system.However,the polarization and mode coupling effects of multimode fiber cause its SBS threshold to be lower than the ideal value.Based on the multimode fiber SBS sensing theory,a multimode fiber Rayleigh Brillouin optical time domain analysis(BOTDA)system was designed and built,which realized the distributed temperature measurement of the multimode sensing fiber,then the influence of the mode characteristics of the multimode fiber on the performance of the Rayleigh BOTDA system is analyzed.The results show that the use of the high SBS threshold and polarization self-disturbance characteristics of multi-mode fiber can increase the fiber input power and reduce the coherent Rayleigh noise of the Rayleigh BOTDA system,thereby further improving the signal-to-noise ratio and the performance of the sensing system.