Analysis And Design Of Pulsed Pre-Pump Rayleigh BOTDA Sensing System

This thesis introduces the mechanism of spontaneous Brillouin scattering and stimulated Brillouin scattering, based on the domestic and abroad research about status of distributed Brillouin optical fiber sensing technique. In order to make the mechanism of stimulated Brillouin scattering(SBS) interaction, the transient coupled wave equations are analyzed emphatically.First, the principle of pulsed pre-pump Rayleigh Brillouin optical time domain analysis(PPP R-BOTDA) distributed sensing system is introduced. The transient SBS coupled wave equations in PPP R-BOTDA system are solved through finite difference time domain method and is fitted in a way of numerical solution. The amplitude waveforms of three fields are got that are sensing pulse light field,Rayleigh scattering light field and acoustic wave field.Then the transient SBS coupled wave equations in PPP R-BOTDA system are calculated by frequency domain method and is simulated by Matlab software. When sensing pulse width is less than phonon lifetime, Brillouin scattering spectrum is sharply broadened and peak power in this spectrum is reduced sharply. Simulation results show that, the linewidth of Brillouin spectrum is narrow because of introduced step pulse. Pulsed pre-pump technique can improve spatial resolution and frequency measurement accuracy of PPP R-BOTDA system to aim at reshaping of Brillouin scattering signal in spectral domain. After time-frequency transformation, the expression of gain in time domain in PPP R-BOTDA system are deduced and derived from Brillouin spectrum characteristic of spatial resolution in spectral domain. The effect of Rayleigh scattering light power gain in an entire strip of optical fiber is analyzed, so the reshaping of Brillouin scattering signal in time domain is realized and non-local effect is reduced.Finally, the experimental scheme of PPP R-BOTDA sensing system is designed.Experimental module is designed and established that two EOMs are in series to modulate step pulse light. Experimental results verify that, this experimental scheme of step pulse light subsystem is feasible. Results show that, when one EOM is biased at positive linear point of EOM transmission curve by use of microwave generator and the other is biased at null point by use of step pulse generator. Continuous Rayleighscattering light and sensing pulse light in PPP R-BOTDA system are obtained according to appropriate amplitude and modulation index of microwave-modulated signal.