Research On Phase Demodulation And Pulse Coding Technology In BOTDA System

In this thesis the principles of spontaneous Brillouin scattering(SPBS) and stimulated Brillouin scattering(SBS) have been discussed based on the summary of the research status of Brillouin optical time domain analysis(BOTDA) technique.According to the mechanism of Brillouin scattering, the temperature and strain characteristics of Brillouin frequency shift have been analyzed and the corresponding relations between Brillouin frequency shift and temperature/strain are studied.The principle of the traditional direct detection BOTDA system and the heterodyne detection BOTDA system are introduced. The expression of stimulated Brillouin scattering transfer function is analyzed. The comparison bewteen direction and heterodyne detection BOTDA system is conducted. The expressions of signal-to-noise ratio(SNR) of two kinds systems are derived.The Brillouin phase demodulation method is studied deeply based on the analysis of heterodyne detection BOTDA system. A new phase demodulation method for heterodyne detection BOTDA system which combines the average superposition algorithm and the FFT analysis-spectrum algorithm is designed. Theoretical analysis and experimental demonstration are conducted to verify the feasibility and effectiveness of the method. The experimental results show that spectrum algorithm have good demodulation effect for signals with different SNR. The relative phase measurement error can be reduced to 0.71% for the signal of 0.3 V amplitue with a 0.5 V noise standard deviation by spectrum algorithm with 1000 times superposition. The broadband phase demodulation scheme for heterodyne detection BOTDA system is designed, which provides a basis of temperature and strain detection along a long distance fiber.The principle of BOTDA system based on pulse coding is studied. The principles of the encoding and decoding of Simplex, Golay and CCPONS codes in BOTDA system are analyzed. The time domain output signals of BOTDA system using different pulse code modulation are simulated by Matlab, and the improvement of system SNR using three codes are compared. The simulation results show that the pulse coding technique can enhance the performance of BOTDA system effectively and the best system performance can be achieved in CCPONS code modulation system.