Methods Of Disturbance Positioning Based On Dual Mach-Zehnder Interferometer

Perimeter security is of great value in the area of border control of country, government building, power plants, schools, communities, etc. Because of the inherent advantages of high sensitivity, insulation, anti-electromagnetic interference, and the ability of distributed sensing, perimeter security system based on optical fiber sensoring performs better than the traditional system does.According to measurement method, optical fiber sensor can be categorized into point sensor, quasi distributed sensor, and distributed sensor which is commonly used in the disturbance positioning system. This thesis describes several different kinds of fiber sensors, and then focuses on the disturbance positioning of perimeter security systems based on dual Mach-Zehnder(M-Z) interferometer.Firstly, the thesis analyzes the impact on the fiber optic interferometer output under different factors. Based on the coupled-mode theory, we deduce the transmission matrix of fiber coupler, which is an important optical splitting device in fiber interferometer. After we get the theoretical output form of fiber interferometer, it also shows that the change of the interferometer output light intensity is closely related to the optical path difference.Secondly, a brief introduction of signal de-noising method based on wavelet analysis and empirical mode decomposition is given. The interference output signal contains heavy high-frequency noise, which can greatly increase the delay estimation error. De-noising is a necessary process before any signal processing.Thirdly, the study on the data collected from a prototype system shows that the data from the two interferometers, which theoretically should have a fixed time delay, appear unstable situation where signal delay alternates with time back and forth, and thus leads to the error increasing. By comparing the real data with the simulation data, we propose the working point shift model for the dual M-Z interferometer positioning system.Finally, we proposed two approaches to decrease the delay estimation error, they are stabilizing working point and phase demodulation. Based on differential cross-multiplication, we proposed a new phase demodulation method for 2×2 coupler. Through the demodulations of emulated data and real data, we get better positioning results than directly estimating the delay of the interferometer output. This is also the key innovation of this thesis.