Research On Key Technologies Of Multimode Fiber BOTDR Sensing System

The gradually increasing scale and voltage level of the power grid has put forward higher requirements for the safe and stable operation of power equipment and cables.It is of great significance to ensure the safety of human life and equipment property by comprehensively monitoring operational status of power equipment and cables,timely and accurately predicting and detecting potential issues,and performing preventive maintenance.The Brillouin optical time domain reflectometer(BOTDR)with a single-ended optical injection structure not only has a high measurement accuracy but also will not be completely paralyzed even if there is optical fiber fracture,which can realize the distributed non-destructive monitoring of power equipment and cables.The Brillouin scattering mechanism of optical fiber was introduced in the dissertation,and the stimulated Brillouin scattering threshold and Brillouin scattering characteristics of multimode fiber(MMF)were explored.The shaping optimization method of MMF Brillouin scattering spectrum was proposed,and the signal-to-noise ratio of the BOTDR system was improved.The influence factors of fiber bend loss were theoretically studied,and a bend-tolerant fiber structure based on single-mode fiber(SMF)combined with MMF was designed.A mode demultiplexing temperature and strain simultaneous measurement method based on the single pump mode was proposed,which realizes the differential measurement between temperature and strain.The research aims to solve the problems of BOTDR existing in the practical application and provide theoretical basis and technical support for real-time,high-precision,and all-round monitoring of power cables and equipment.The main research achievements are presented as follows:1.The mechanism of spontaneous Brillouin scattering in optical fiber and the characteristics of Brillouin scattering in MMF were studied,and different coherent detection acquisition methods of spontaneous Brillouin scattering signals were compared experimentally.Aiming at the problem of low stimulated Brillouin scattering threshold and low signal-to-noise ratio of SMF BOTDR system,the influencing factors on stimulated Brillouin scattering threshold of optical fiber were analyzed,and the thresholds characteristic of different optical fibers were compared.The signal-to-noise ratio improvement ability of MMF BOTDR compared with SMF BOTDR was deduced,and a MMF BOTDR sensing system with a high signal-to-noise ratio was proposed.A shaping optimization method for Brillouin scattering spectrum of MMF based on SMF fundamental mode excitation and high-order mode filtering was proposed to improve the frequency shift measurement accuracy of the MMF sensing system.A coupling efficiency evaluation system between SMF and MMF was designed to determine the bidirectional transmission loss between fibers.2.To address the problem of large bend loss of SMF,the fiber bend loss model was constructed,and the influencing factors on fiber bending resistance were analyzed.The method for suppressing the Fresnel reflection signal at the end of the MMF was explored,and a bend-tolerant fiber structure based on the combination of input SMF and sensing MMF was proposed.The good bending tolerance of SMF alignment fusion to graded index MMF was verified by experiments.The bending tolerance performance of different MMFs with the same cladding and core refractive indexes was studied for comparison,and the better bending tolerance of the stepped index MMF structure with the same numerical aperture than graded index MMF structure was determined both theoretically and experimentally.The sensing reliability was proved by simulating the temperature sensing ability of the SMF alignment fusion to 62.5μm stepped index MMF structure in the complex bending environment.3.To ensure the good bending resistance and sensing characteristics of the SMF combined with MMF sensing structure,the effects of different optical coupling methods on the bending tolerance,Brillouin frequency shift stability,and Brillouin scattering spectrum width of the fiber structure was compared,and the alignment fusion method was identified as a more stable and reliable optical coupling method.The influence of higher-order modes on the bending resistance of the sensing structure was studied,which provides a reference for ensuring the high reliability of the sensing structure.4.In addressing the cross-sensitivity problem of the SMF Brillouin sensing system,the feasibility of utilizing MMF to distinguish temperature and strain was theoretically investigated,and the separation of Brillouin scattering spectra of different modes in MMF was realized by utilizing a mode selective photonic lantern.A temperature and strain simultaneous measurement system based on different pump modes was proposed.The effects of the combination method,coupling method,and pump modes of mode selective photonic lantern and MMF on the Brillouin scattering spectrum and Brillouin frequency shift temperature/strain sensitivity were evaluated.The combination structures that are beneficial to the improvement of sensing performance were determined.In addressing the shortcoming of stepwise mode switching of the temperature and strain measurement system with different pump modes,a mode demultiplexing temperature and strain simultaneous measurement system based on the single pump mode was proposed,which further improves the measurement speed and accuracy.The reliability of the sensing system was verified by experiments,which provides a theoretical and experimental basis for the accurate differentiation of temperature and strain in the MMF Brillouin sensing system.