Theoretical And Experimental Research On Multi-wavelength BOTDR System Based On Golay Coding

Brillouin Optical Time Domain Reflectometry(BOTDR)has drawn wide attention from researchers at home and abroad due to its advantages of single-ended measurement,simple structure,high precision of measurement positioning.However,the traditional BOTDR system based on monopulse has low signal-to-noise ratio(SNR),and its performance of measurement accuracy and dynamic range and spatial resolution are to be further improved.The theoretical and experimental research on multi-wavelength BOTDR system based on Golay-coding were analyzed and the low SNR of BOTDR system was improved in the thesis.The research status of BOTDR technology was described in this paper,the theory of Brillouin scattering and the principle of temperature and strain measurement with Brillouin frequency shift and intensity were analyazed in the thesis.In addition to the two detection methods of BOTDR sensor system such as local heterodyne detection and self-heterodyne detection were introduced,the generation method of multi-wavelength probe light and the structure of microwave down-conversion system were descriped in this paper.Based on the above theory,the system structure of five-wavelength BOTDR sensing system based on Golay coding under two detection methods is designed respectively,the mathematical expressions of the output signal and the output noise of the two systems are deduced,and the question that the limitation of stimulated Brillouin scattering threshold limits encoding length was discussed.The performance of the BOTDR system under the two detection methods was compared in this paper,the simulation analysis was carried out including the relationship between the SNR and the encoding length,the relationship between the encoding length and the coding gain,and obtained the best code-length under different detection methods.The experimental system based on local heterodyne detection and self-heterodyne detection were respectively set up,the influence of single wavelength and five wavelength on the Brillouin scattering signal of system and the impact of NRZ and RZ Golay coding on the system were analyzed experimentally.The Experiment Result indicated that the five-wavelength probe light improved the intensity of Brillouin scattering signal in BOTDR system,and the RZ Golay coding improved the SNR of BOTDR sensing system.Using the above experimental result,the measured data were obtained by multiple experiments and processing of overlaying and averaging when the pulse width was 50 ns and the theoretical fiber peak value was 100 m W,the spectrum was analyzed and the optimal code length were given out when different detection methods were used.