Modeling Analysis And Optimization Design Of BOTDR System Based On Pulse Coding Technology And Multi Longitudinal Mode Laser

In this thesis the theory of Brillouin scattering is analyzed based on the summary of the research status of Brillouin optical time domain reflectometer(BOTDR)technology,including Spontaneous Brillouin Scattering and Stimulated Brillouin Scattering,and introduced the principle of temperature and strain measurement with Brillouin frequency shift and intensity.On the basis of encoding and decoding principle and coding gain of Golay code?Simplex code and CCPONS code,the principle and composition of pulse coding BOTDR system with heterodyne detection is analyzed.The time domain output signal of the system using different pulse code modulation are analyzed and compared by MATLAB simulation,in the meanwhile,the signal-to-noise ratio improvement of the coding length and the averaging times are analyzed.A Golay pulse coding BOTDR system is proposed,which is based on the the single wavelength DFB laser,the three wavelength and multi longitudinal mode FP laser modulation and APD heterodyne detection,respectively.The coding and decoding principle of Golay code which is applied to the system and the principle of local-heterodyne detection are analyzed in three systems.The restriction of Stimulated Brillouin Scattering threshold in optical fiber on the coding length of the system is discussed.The mathematical expression of signal-to-noise ratio is deduced and the relationship of signal-to-noise ratio with APD multiplication factor,coding length and longitudinal mode number is studied.The expressions of the optimal multiplication factor of APD,the optimal coding length and the optimal longitudinal mode number are obtained,respectively.With a bandwidth of the APD detector of 500 MHz,and the peak power and pulse width of 50 mW and 100 ns,the modeling analysis and optimization design of the three systems are presented by MATLAB.The results show that,in order to achieve high precision sensing,in addition to considering the restriction relationship of SBS threshold and the coding length of the system,the problem of saturation of APD photoelectric detector caused by signal power,and also considering the type of laser,the receiver bandwidth and other factors of practical system,weigh the various parameters to determine the optimal multiplication factor of APD,the optimal coding length and the optimal longitudinal mode number.For the single wavelength BOTDR encoding system,with the APD multiplication factor of 5 and the coding length of 128 bits,the optimal temperature and strain resolution achieved 1.60? and 35.48??;For the three wavelength BOTDR encoding system,with the APD multiplication factor of 5 and the coding length of 64 bits,the optimal temperature and strain resolution achieved 1.33? and 29.49??;For the multi longitudinal mode FP laser BOTDR encoding system,with the APD multiplication factor of 5,the coding length of 32 bits and the longitudinal mode number of 7,the optimal temperature and strain resolution achieved 1.29 ?? 29.16 ??.