Research On The BOTDR Based Temperature And Strain Sensing Technology Using Heterodyne Detection

The research condition of temperature and strain sensing based on Brillouin scattering has been introduced in this thesis. The characteristics and the sensing mechanism of temperature and strain based on Brillouin scattering in optical fiber have been analyzed, and the temperature and strain dependences of Brillouin frequency shift and intensity have been founded. The ratio of the Brillouin scattering light power and Rayleigh scattering light power is used to eliminate the impact of optical loss to compensate the Brillouin scattered light power variation, and to establish the design theory of BOTDR based temperature and strain sensor system using heterodyne detection. Sensing system and signal processing based on heterodyne detection have been studied. The microwave frequency synthesizer in down converter subsystem has been studied in detail. The method of applying Golay complementary sequences in Brillouin sensing system has been used. A high speed Golay complementary code with a variable length and adjustable width is designed based on Max+plus II according to the principle of Golay complementary sequences, and waveform simulation has been done. In the meanwhile, the performance of BOTDR sensing system applying Golay complementary sequences is analyzed from the signal to noise ratio, dynamic range and measurement time of the sensing system. The sensing system has been simulated from time domain and frequency domain, and the simulation results of the Matlab show that the signal to noise ratio, temperature and strain resolution of the system are greatly improved by the employment of Golay complementary pulse modulation and correlation detection comparing with single light pulse. Finally, the partial experimental study of transmitted light signal has been conducted based on Acousto-optic modulator.