Research On The Key Technology Of BOTDR System Using Pulse Code Modulation

In this thesis the research condition of Brillouin scattering optical fiber sensing technology is introduced, Brillouin scattering and its characteristic parameters in the optical fiber are analyzed, and the corresponding relations between Brillouin frequency shift, intensity and the temperature, strain in the optical fiber are founded. Besides, BOTDR optical fiber sensing system based on Golay complementary sequences modulation and Simplex code modulation are analyzed, time domain and frequency domain properties of Golay complementary sequences and its application principle and application scope in BOTDR system are analyzed. BOTDR sensing system based on Golay complementary sequences modulation is given and the signal noise ratio, dynamic range and measuring time are analyzed theoretically; meanwhile the response features of the time domain and frequency domain in the system are simulated by Matlab and the results show that BOTDR sensing system using Golay complementary sequence is improved obviously than the single-pulse BOTDR system in signal-to-noise ratio, temperature and strain resolution. In addition, in this thesis the generation method, the application principle and the improvement of the performance of Simplex code in the system are analyzed, so that the BOTDR sensing system based on Simplex code modulation is given and the time response feature of the system is simulated by Matlab. And the Golay complementary sequence, the relationship between the encoding gain and the code length of Simplex code are analyzed, and the applicable scope of the two codes is pointed out in BOTDR system. Moreover, BOTDR sensing system based on the LPR microwave heterodyne detection is introduced and the subsystem of the microwave frequency downconverter is studied emphasisly so that the design scheme of the subsystem and the selection of each component are given, the parameters of the chosen component are tested under the designed measurement system. In addition, the measurement systems of time domain and frequency domain of the back Rayleigh scattering signal and the measurement system of the frequency domain of the Brillouin scattering signal is designed and set up, the measurement of time domain and frequency domain of the back Rayleigh scattering signal and the measurement of the frequency domain of the Brillouin scattering signal is completed.