Distributed Optical Fiber Sensing Based On Stimulated Brillouin Scattering In Special Optical Fibers

Brillouin optical time domain analysis(BOTDA)has always been the research focus in the field of structural health monitoring since it was first proposed by T.Horiguchi in 1989.The development of BOTDA mainly depends on the new optical modulation technique in single mode fiber(SMF),which limits BOTDA based on the property of SMF.To address this problem,this dissertation studies the applications of BOTDA in special optical fibers in the view of sensor,proposes detailed innovative solutions,and performs theoretical and experimental studies.We theoretically study the SBS process in multimode fiber based on the SBS coupled wave equations.We numerically calculate the optical and acoustic field distribution in a grade index multimode fiber and calculate the intermodal SBS and intramodal SBS.In the case that both pump and probe are fundamental mode,BGS can maintain Lorentz shape;when higher order modes are introduced in SBS process,the Brillouin gain coefficient will redistribute so that higher order modes will acquire higher gain,which will broaden or distort the BGS.In order to overcome the limitation of strain range of standard SMF,we study how to realize BOTDA in polymer optical fiber.A narrow bandwidth fiber grating filter is used to extract the Stokes,suppress Anti-stokes and Fresnel reflection of the pump pulse.In order to avoid burning at the end face of polymer optical fiber,multimode fiber assissted coupling technique is used to reduce power density of pump pulse.Polymer optical fiber shows strong mode scrambling by comparing the variation of Brillouin frequency shift of polymer optical fiber and multimode fiber and their speckle pattern,which will produce sensing error.In order to overcome the big bending loss of standard SMF,we use singlemode-multimode-singlemode fiber structure to achieve bend-insensitive fiber sensing,of which the minimum bending radius is 1.25 mm.The coefficients of strain and temperature of the fundamental mode is 0.42MHz/?? and 0.83MHz/°C,respectively.Meanwhile,we demonstrate that the higher-order modes excited in multimode fiber can be easily influenced by bending,so that exciting t he fundamental mode is necessary for bend-insensitive distributed sensing.In order to overcome the high temperature sensing range of standard SMF,we use photonics crystal fiber(PCF)to measure 1200°C high temperature After annealed for 1 time,the BFS of PCF increases with temperature as a parabolic function with a measurement accuracy of ±3.6°C;for temperature <200°C,the temperature coefficient of PCF has been improved by ~20.3%;for temperature >1000°C,PCF can be directly used without annealing.In order to ovecome the difficulty of standard SMF to measure strain at high temperature,we use gold coated fiber to achieve high-temperature strain distributed fiber sensing,where ~2200?? is effectively tested at 700?.First,the high temperature property of gold coated fiber is studied.After annealed at 1000 ? for 2 times,gold coated fiber is under stable condition;the BFS increases with temperature nonlinearly,which agrees with that of SMF.Second,the influenc e of gold coating on strain measurement is studied.Due to the residual stress introduced during the coating process,it is necessary to pre-pulling gold coated fiber before strain measurement to eliminate it.Last,the annealed and pre-pulled gold coated fiber is used to achieve distributed high-temperature strain measurement under 700?.The strain coefficient of gold coated fiber increases with temperature gradually,which is due to the decrease of Young's modulus with temperature.