Investigation On Phase Demodulation Methods Of Brillouin Optical Time Domain Reflectometer

Distributed optical fiber sensing technology has been widely studied in the field of optical fiber sensing because of its strong anti-interference ability,convenient maintenance and so on.The distributed optical fiber sensing system based on Brillouin Optical Time Domain Reflectometry（Brillouin optical time domain reflectometry,BOTDR）has become an important technology in the field of optical fiber sensing because of its superior strain and temperature measurement accuracy.In this thesis,based on the traditional BOTDR experimental system,the change of Brillouin frequency shift is transformed into the phase change of the output light by introducing a non-equilibrium M-Z interferometer structure.The phase information was extracted by phase demodulation method to obtain the change of Brillouin frequency shift,and the distributed sensing was realized by combining the relationship between brillouin frequency shift and strain and temperature.In order to solve the disadvantages of existing signal demodulation methods,three phase demodulation algorithms are proposed in this thesis.The main research contents include:（1）An improved cross-phase multiplication with two-path differential is introduced.The phase information obtained by traditional DCM（Differential and Cross Multiplying）demodulation method is distorted.Normalization and other mathematical operation are added on the basis of traditional DCM algorithm to eliminate the influence of AC component,initial phase and other factors on the demodulation result.The traditional DCM method and the improved DCM method are used for demodulation simulation experiments.the results show that the improved algorithm has better demodulation accuracy and superior anti-noise performance.（2）A new phase demodulation algorithm is designed.A set of orthogonal signals can be obtained by adding and subtracting the signals,and the phase signals to be demodulated can be directly obtained by mathematical operation.The new algorithm improves the accuracy of phase demodulation.Phase demodulation simulation experiments were carried out using the new method,and the results show that the proposed algorithm has good noise suppression effect and can accurately obtain the phase demodulation information,which can be effectively applied to Brillouin optical fiber vibration sensing system.（3）A new phase demodulation algorithm is introduced.A new demodulation algorithm based on 2×2 coupler and traditional PGC（Phase Generated Carrier）algorithm is proposed.The new demodulation algorithm is applied to Brillouin optical fiber vibration sensing system to realize phase demodulation.The principle of the traditional PGC demodulation algorithm and the new structural phase demodulation algorithm are analyzed theoretically,and the demodulation simulation experiments are carried out with the two algorithms,respectively.The results show that the new demodulation algorithm is effective,and the method improves the application value of the traditional PGC algorithm in other engineering systems.The modulation depth is analyzed based on the traditional PGC algorithm and the new structure demodulation algorithm.the results show that the new structure algorithm eliminates the influence of the modulation depth on the demodulation results.