Research On Polarization Control Technology In Distributed Optical Fiber Fence Protection System

Optical fiber sensing shows many superiorities, such as stable performance, long distance distributed measurement, anti-electromagnetic interference, etc, making it be a hot topic in the field of perimeter protection. There are three normal techniques used in optical fiber perimeter protection system, i.e. interference technology, fiber grating technology and optical time domain reflection technology according to the different principle. Nevertheless, perimeter protection system based on dual M-Z interference is widely concerned because of its high sensitivity, simple demodulation algorithm, and low cost.The method of cross correlation algorithm is used in this system to estimate the two-way signal time delay, which requires the two detection signals with strong correlation. However, in the practical application, the inconsistency of the two polarization properties causes polarization phase shift and polarization fading. Consequently, the correlation of two signals becomes poor, leading to non-ignorable positioning error, or even be unable to locate. In this thesis, the method of polarization control is in depth research, the main contents are as follows:First of all, a distributed fiber optic perimeter protection system is built, including dual M-Z interferometer, optical detection module, data acquisition module, digital output module, a polarization controller, PZT modulator, and so forth.Secondly, the design of the hardware circuit and the software programming of the system are completed. Hardware circuit design includes:system power supply design, digital output card circuit design; Software programming includes:digital output card control program, acquisition card control program, polarization control algorithm program and positioning algorithm. Through the construction of software and hardware, the manual adjustment and automatic adjustment of the polarization states are realized.Finally, the system polarization is analyzed in this thesis, the peak-peak value of the two signals as feedback control conditions is used and the different situations under the two the signals with different peak-peak value are analyzed and compared. The experimental results show that the larger the two signals'peak-peak value are, the better signal correlation and the more accurate position will be obtained. At last, three polarization control algorithms, i.e. random search, traversal search and adaptive search polarization control algorithm are analyzed and compared. The experimental results indicate that, adaptive search algorithm makes the two signals' peak-peak value maintain an optimal SNR range in a short period of time and keep the system in good polarization states in 10s, which solves the polarization phase shift problem and improves the positioning accuracy of the system.