Theoretical And Experimental Research On Pulse Pre-Pump Multi-wavelength BOTDR System

Based on the introduction of the background of distributed optical fiber sensing technology,in this thesis,the Brillouin Optical Time Domain Reflectometry(BOTDR)technology is first analysed,the linear relationship between the brillouin scattering properties with temperature and strain were discussed,the principle of simultaneous measurement of temperature and strain in BOTDR system were analyzed.The theoretical research of the sensing performance of pulse pre-pump multiwavelength in BOTDR system.Based on the multi-wavelength light source with external modulation,the superposition characteristics of scattering spectrum and stimulated brillouin(SBS)threshold of multi-wavelength BOTDR system were analyzed.The generation of the step pulse in the pulse pre-pump technique and the sensing principle of pulse pre-pump in the fiber were analyzed,the transient analysis of coupling wave equation of multi-wavelength step pulse in optical fiber was analyzed,the brillouin scattering properties of different pre-pump pulse parameters were simulated.The experimental system of down-conversion signal acquisition subsystem was studied,the experimental analysis shows that in down-conversion signal acquisition subsystem,the results when used high local oscillator is batter then low local oscillator.In the experimental research of pre-pump pulse of multi-wavelength in BOTDR system,two systems of local heterodyne detection and self-heterodyne detection were designed,the detection principle and SNR of these two heterodyne detection systems are respectively analyzed.The optimal pre-pump pulse width and power are obtained through the experiment data processing and analysis in the pre-pump pulse optimization design experiment.In the temperature measurement experiment,two temperature points of 50°C and 70°C were measured within 100 m of the 9.7 km fiber end under the conditions of a peak power of 150 mW per wavelength and a room temperature of 28.3°C,three-dimensional power spectrum and sensing characteristics of brillouin scattering signals measured by two heterodyne detection systems.The experiment results show that in three wavelengths which was modulated by frequency interval of 500 MHz microwave source for heterodyne detection of step pulse optimization experiments,when the sensing pulse is 8 ns,the pre-pump pulse width 150 ns,the sensing performance when the sensing pulse and the pre-pump pulse peak power ratio of 5.7 was the best performance of the system.In the experiment of pre-pump pulse multi-wavelength BOTDR temperature measurement systems,the brillouin frequency shift of the local heterodyne detection system at the temperature of 50°C and 70°C are 10.8459 GHz and 10.8463 GHz,respectively,frequency shift fluctuations are respectively 2.59 MHz and 4.83 MHz,the heating section frequency shift mean value are 10.8632 GHz and 10.8897 GHz,frequency shift fluctuations are 1.38 MHz and 2.71 MHz,the demodulated temperatures are 41.9°C and 63.5°C.The brillouin frequency shift mean of the self-heterodyne detection system at the temperature 50°C and 70°C are 10.8441 GHz and 10.8431 GHz,respectively,frequency shift fluctuations are respectively 3.26 MHz and 5.43 MHz,the heating section frequency shift mean value are 10.8633 GHz and 10.8896 GHz,frequency shift fluctuations are 1.46 MHz and 3.12 MHz,the demodulated temperatures are 43.7°C and 64.5°C.Due to the signal is passed through the bandpass filter in the down converter signal acquisition subsystem,the system's spatial resolution is limited,so that the spatial resolution of the experiment is about 18 metres.