Polarization Control For Dual Mach-Zehnder Fiber Oil And Gas Pipeline Security System

The pipeline is an important means of transporting oil and gas resources, the length of which is growing rapidly due to the development of national economy and people pay more and more attention to pipeline security issues. The distributed optical fiber oil and gas pipeline leak detection and pre-warning system based on dual Mach-Zehnder optic fiber interferometer studied by our research group, can pre-warn of the abnormal dangerous events that happens nearby the oil and gas pipeline and locate those potential sites so as to avoid environmental pollution and economic losses. This will bring great social and economic benefits.The system locating principle is based on the cross-correlation function method, which requires two detection signals with strong correlations. In the practical application, however, the system detection signals’correlation sometimes can’t keep stable, resulting in the gross locating error and even positioning failure. In this paper, the cause of the discrepance between two detection signals and the polarization fading control method are further studied. The main contents are as follows:(1) The conversion process from the external excitation signal to the photocurrent signal in the fiber optic vibration sensing system is analyzed. The location principle of the distributed optical fiber pipeline security system is illustrated.(2) The Jones matrix polarization model is established to analyze the signal transmission process in single Mach-Zehnder fiber interferometer and the detection signals’correlation in dual Mach-Zehnder fiber interferometer, and find the reasons for phase shift and polarization fading. It comes to the conclusion that the basic reason for the degeneracy of the detection signals’correlation is the input polarization affecting the detection signals due to the inconsistency of the polarization characteristics between two sensing fibers. Based on the analysis, the solution of controlling the input polarization to eliminate polarization fading is more feasible and the requirements of polarization control are determined.(3) Experiments to obtain the control characteristics of Polarization Controller are designed. The Simulated Annealing (SA) based on the correlation coefficient of two signals is studied and chosen as the polarization control algorithm. Some key parameters in SA, such as the initial temperature, the maximum step length and the temperature update constant, are determined through the experiments. Field experiment results show that, the algorithm can effectively eliminate the adverse impact of phase distortion on the location and maintain a steady signal correlation. The system’s stability is significantly improved.