Research On The Influence Of Self-phase Modulation And Modulation Instability In The BOTDA System

Besides advantages normal optical fibre sensing technologies have,such as high sensitivity,anti-electromagnetic disturb,highpressurization,corrosion resisting,little volume,easy to achieve distributed sensing,distributed optical fiber sensing system can also get continuous distributed time and space information of the measured farm at the same time.Basing on the stimulated Brillouin scattering sensing technology and taking the whole fiber as sensing medium,we can realize the real time measure of long distance continuous temperature and stress.The Brillouin optical time-domain analysis(BOTDA)system,which takes Brillouin frequency shift as the measurand,has been widely studied in the distributed optical fiber temperature or stress sensing field.With the increase of Brillouin distributed optical fiber sensing distance,In order to ensure that the signal to noise ratio of the probe light signal at the end of the sensing optical fiber high enough,high Brillouin pump power is needed.This will unavoidable aggravate self-phase modulation(SPM)and modulation instability(MI)and other nonlinear effects.These nonlinear effects will cause the depravation of system performance though different mechanization.The change of Brillouin pump power will result in frequency chirp,which lead to self-phase modulation(SPM).These all contribute to the decline of the peak Brillouin gain and the broaden of the Brillouin gain spectrum(BGS).For interference optical fiber sensing system,the main damage modulation instability(MI)caused is the broaden of the frequency spectrum.It will bring phase noise as well which is harmful to the system.The Brillouin optical time-domain analysis system(BOTDA)is a distributed optical fiber sensing system based on the measurement of the effective Brillouin gain,in which high power pumping pulse is demanded to fulfill optical time domain reflectometry(OTDR)spatial orientation.The broaden of BGS and the decline of the Brillouin gain caused by SPM and MI distinguishly will worsen the system performance seriously.In this paper,we mainly studied the regular of SPM and MI in the distributed optical fiber system.Do research on the effect of the Brillouin gain spectrum caused by detail change of the pump power.Though anlyzing the different performance of SPM and MI,we can provide reference frame to raise the measure precision and sensing distance.In the theory and simulation part of this paper,the propagation equation of the Brillouin pump pulse in the SMF is introduced,thereafter the second-order underdamping system model is also shown,with its key parameters explained detailedly.In the experiment part of this paper,rectangular-shaped and Gaussian-shaped pump pulse are taken into consideration to study the connection and difference between SPM and MI in BOTDA system.Based on theoretical analyses and experimental discussions,the influence of self-phase–modulation(SPM)and modulation instability(MI)in the BOTDA system has been inestigated.And the main content of this thesis contains as follows: Theoretically analyz on SPM and MI,formula derivation,the solution of the nonlinear Schrodinger equation(NLSE),and imaging the Brillouin gain spectrum;Found the numerical modeling of SPM,and draw the picture of SPM along the change of pump pulse power,pulse shape and noise level;Take the second-order underdamping system into the fodation of the model of true pulse.Study and draw the picture of BGS along the change of pump pulse power,noise level and pulse shape.We use FP scanner to observe self phase modulation(SPM)directly,studying and drawing the picture of BGS along the sensing fiber when the input pulses are rectangular pulse,wide Gaussian pulse and narrow Gaussian pulse distinguishly.Then set up the BOTDA experimental system,experimentally analyze self phase modulation(SPM)and modulation instability(MI),and make the comparison between the experiment result of SPM and MI.In chaper four,we build the 25 km BOTDA system and use rectangular pulse and Gaussian pulse as the input pump pulse.Observe the trace picture of the Brillouin gain along the sensing fiber.Make comparasion between SPM and MI.We Study the effect they have on the measurement of BGS and the measure precision and sensing distance of BOTDA system.At last but not least,summarize the work having been done and make the future outlook.