Theoretical And Experimental Study Of The Performance Improvement Methods Of BOTDA System

In this thesis, the principles of spontaneous Brillouin scattering (SPBS) and stimulated Brillouin scattering (SBS) have been discussed in detail based on the introduction to research status of distributed optical fiber sensing technology with Brillouin optical time domain analysis (BOTDA). According to the mechanism of SBS, the stable state coupled wave equation and transient state coupled wave equation have been analyzed. The temperature and strain characteristics of Brillouin frequency shift and Brillouin intensity have been studied and the relationships between them have been established. For the cross sensitivity characteristics of temperature and strain in BOTDA system, four methods to realize simultaneous measurement of temperature and strain have been summarized.A BOTDA sensing system using microwave external modulation has been introduced. The performance and influence factors of the system have been analyzed. The pulse modulation and frequency shift modulation technologies have been studied theoretically and experimentally based on the detailed introduction of operation principle of electro-optic modulator (EOM). The drift phenomenon of EOM bias point has been analyzed and a bias control system based on disturbing signal and feedback network has been designed. The experiment of pulse modulation with bias-controlled system has been designed to demonstrate the improvement of extinction ratio of the modulated pulse. The theory of the pulse pre-pump technology used to improve the performance of BOTDA sensing system has been studied and a pulse pre-pump BOTDA (PPP-BOTDA) sensing system based on single laser has been designed. A subsystem based on combining electrical pulses to generate stepped pulses has been proposed and simulated theoretically.The results indicate that the performance of BOTDA sensing system is impacted by the extinction ratio and pulse width of modulated pulse. The extinction ratio of modulated pulse with the bias-controlled system can be as high as more than28dB and keep stable in the whole measurement time of3hours. The theoretical analysis of PPP-BOTDA shows that high accuracy and spatial resolution can be achieved simultaneously with pulse pre-pump technology. The simulation experiment of stepped pulse generation achieves a stepped pulse with1ns pump pulse width and15ns pre-pump pulse width.