Research On Distributed Optical Fiber Sensing Technology Based On Brillouin Scattering

The stimulated Brillouin scattering(SBS)in a distributed optical fiber sensing system based on Brillouin scattering is studied theoretically and simulated numerically in this paper.And a distributed optical fiber temperature sensing system based on Brillouin scattering is investigated experimentally.On the theoretical study and numerical simulation,a physical model of the SBS process in the distributed optical fiber temperature sensing system based on Brillouin scattering is established.Corresponding simplified coupled wave equations in its optical fiber are given.Discrete expressions of these equations are obtained using a finite difference time domain method.At the same time,nonlinear relations between Brillouin frequency shift and temperature/strain are deduced in detail.Their specific theoretical expressions are provided.A program for numerical solutions of the light fields in the Brillouin scattering process is written using Matlab based on those discrete formula and corresponding boundary conditions and initial conditions in order to calculate the changes in time and space of optical wave fields in the optical fiber in the system.And influences of the incident energy of pumped lights on Brillouin scattering waves are studied as well.Using steady state coupling wave equations under the approximate condition,a program on the changes of Brillouin scattering gain with fiber temperature is written using Matlab in order to consider the influence of fiber temperature on its Brillouin frequency shift.Changes of pump light intensities and the signal light intensities in the Brillouin distributed optical fiber sensing system with transmission distance are calculated.Spatial distributions of Brillouin gain spectrum of sensing fiber at different temperatures are simulated.Numerical calculations and simulation results show that the intensities of pumpedlight decrease with the increasing of the transmission distance along the optical fiber.And the intensities of Stokes scattering light are gradually strengthened with the increasing of the transmission distance along the optical fiber.In terms of the energy conversion relationship between the incident pump light and the Stokes scattered light,the absolute increasing of the energy of the Stokes scattered light increases rapidly when the energy of the incident pump light increases,and the width of its peak pulse becomes narrow.In influences of various fiber performance parameters on the Brillouin frequency shift,temperature/strain characteristics of the Young's modulus make the most contribution to the Brillouin frequency shift.The relation coefficient of the changes of Brillouin frequency shift with temperature/strain was determined by experimental data of these performance parameters.When the sensing fiber is at different temperatures,the distribution of the optical power of the Stokes scattered light along the optical fiber is given with the increase of the transmission distance and the increase of Brillouin frequency shift.On the experimental study,a distributed optical fiber temperature sensor system based on Brillouin scattering was set up.Temperature sensing calibration experiments in the system were performed in order to obtain the relationship between Brillouin frequency shifts and fiber temperatures in the system.This relationship in the system is calibrated for optical fibers under different heating conditions,and corrected calibration curves according to certain method are provided.On this basis,distributed temperature measurement experiments in the system are carried out.Temperature resolution and temperature measurement accuracy for the experimental system were determined.Experimental results show that system absolute errors on temperature measurement are less than 1?,and relative errors on temperature measurement are between 0.3% and 1.3%.System temperature measurement accuracy is 0.91?,system temperature resolution is 2?.