Modeling Analysis And Simulation Study Of BOTDR System Based On Multi Longitudinal Mode FP Laser

In this thesis, the research status of Brillouin distributed optical fiber sensing technology is summarized and the basic theory of Brillouin scattering is discussed, the relationship between Brillouin frequency shift / Brillouin intensity and temperature /strain are analyzed, and BOTDR sensing scheme for distributed temperature and strain are analyzed, and the subsystems of light emission, the down-converter subsystem and signal acquisition and processing subsystem are studied and designed.To study the sensing performance of BOTDR system based on multi longitudinal mode FP laser, the characteristics of multi longitudinal mode FP laser is analyzed; the nonlinear effect of the fiber are analyzed, including single longitudinal mode laser and multi longitudinal mode laser of SBS threshold, SPM and FWM; wavelength dependence of the Brillouin scattering spectrum for multiple longitudinal mode are researched and analyzed, and through the modeling and simulation analysis frequency shift of Brillouin scattering spectrum and superposition of multi longitudinal mode FP laser; superposition of Brillouin scattering spectrum of spectrum peak power and spectral width produced by multi longitudinal mode FP laser are analyzed and researched.BOTDR system based on external modulation and local heterodyne of multi-longitudinal mode FP laser is analyzed on this thesis, and BOTDR system using external modulation of multi longitudinal mode FP laser and self-heterodyne detection of Rayleigh and Brillouin scattering based on coherent detection theory is studied.The local heterodyne detection and the self-heterodyne detection are analyzed, and signal-to-noise ratio of the two systems are deduced, and the two kinds of BOTDR system are modeled and simulated, and the system signal-to-noise ratio and strain and temperature measurement precision with the number of longitudinal mode are concluded.The results show that with the increase of longitudinal mode number, the signal to noise ratio, temperature and strain measurement accuracy of the system at the end of 25 km long fiber are improved significantly for a multi-longitudinal mode Fabry-Perot laser with a mode interval of 0.141 nm. In BOTDR system of local heterodyne detection, when the number of longitudinal mode is 19, the optimal values of temperature and strain measurement accuracy of 2.04 ? and 46.23 ?? are achieved,respectively, when the longitudinal mode number takes the number of 19, the systemsignal-to-noise ratio is 44.07 dB. When using the heterodyne BOTDR systems, the optimal values of temperature and strain measurement accuracy of 3.81 ? and 86.69?? are achieved, respectively, when the longitudinal mode number takes the number of 19, the system signal-to-noise ratio is 33.15 dB.