Polarization Fading Suppression Technique Based On Time-delayed Orthogonal Pumping Structure And Its Application

Distributed optical fiber sensing is a technology that utilizes optical fiber as the sensing medium and transmission medium.It can make simultaneous measurements of both the spatial and temporal information of a physical field.It is widely applied in various fields,such as oil and gas pipeline monitoring,bridge and building monitoring,etc.Polarization fading is easy to occur in distributed optical fiber sensing system,which leads to the deterioration of working performance such as spatial resolution and measurement accuracy.This thesis proposes a timedelayed orthogonal pumping structure for suppressing polarization fading.Meanwhile,it applies the distributed optical fiber sensing to the Brillouin optical time domain analysis(BOTDA).The specific research contents include:(1)The physical process of the Stimulated Brillouin scattering(SBS)effect in optical fiber is studied.Moreover,the coupled amplitude equations describing SBS effect is studied.The corresponding steady-state and transient numerical solutions are presented.Combined with the polarization characteristics of the SBS effect in a random birefringence fiber,the expressions of coupled amplitude equations in vector form are derived.(2)A polarization fading suppression technique based on the time-delayed orthogonal pumping structure is proposed.Furthermore,the influences of structural parameters on the optical polarization degree at the output end are simulated and analyzed.The structural parameters include the deflection angle of the optical polarization in upper and lower branches,the optical power difference in the upper and lower branches,and the delay difference in the upper and lower branches.A polarization degree testing device of the time-delayed orthogonal pumping structure is established.According to the results,when the input light is linearly polarized light with polarization degree ranging from 0.7 to 1,the deflection angle of the optical polarization in upper and lower branches is ?/ 2,and the optical power difference in the upper and lower branches is 0 d B.Meanwhile,the delay difference in the upper and lower branches is more than 1500 m,and the polarization degree of the output light is less than 0.1,thus achieving a good polarization fading suppression effect.The experimental results are consistent with the simulation results.(3)An evaluation method for evaluating the fluctuation of Brillouin gain curve and the polarization fading effect of BOTDA system is given.The evaluation method is carried out by employing the value of R,the variable coefficient.A BOTDA system based on time-delayed orthogonal pumping structure is built,and its polarization fading suppression effect is studied experimentally.The experimental results show that the R value can be decreased from 0.50520 to 0.06979 by using the time-delayed orthogonal pumping structure proposed in this thesis.In addition,the polarization fading effect is obviously weakened when the pump power and pulse width of BOTDA system are 14 d Bm and 100 ns,the probe light power is-10 d Bm,and the sensing fiber length is 1000 m.Thus,the polarization fading effect in the system can be effectively suppressed.