The Research On Brillouin Distributed Optical Fiber Vibration Sensing Based On Phase Detection

Distributed optical fiber sensing technique has attracted wide attention and research with the advantages in long measuring distance, strong anti-interference ability, convenient installation, easy maintenance and so on. Distributed optical fiber sensor based on Brillouin optical time domain reflectometry(BOTDR) can realize the sensing of strain or temperature along the fiber. In recent years, these distributed optical fiber sensor has been used for structure healthy or temperature monitoring in large constructions such as reservoirs, power stations, oil wells, tunnels etc. Currently, BOTDR used for temperature or strain sensing has been gradually commercialized. However, these sensors can only measure static strain or temperature. They cannot used for dynamic signal detection such as vibration signal. Distributed vibration sensor based on BOTDR has been reported rarely all over the world.After a review of literature in BOTDR and the research status of optical fiber sensing technology around the world, a scheme of distributed optical fiber vibration sensing system based of BOTDR with phase detection is presented. This system overcomes the shortcoming of traditional BOTDR, which does not have the dynamic detection capability, and improve the ability of real-time detection. This BOTDR system can be used for vibration signal sensing.Firstly, the basic theory of Brillouin scattering in the optical fiber is introduced. The relationship between frequency of Brillouin scattering light and the vibration happened in the axis direction of optical fiber is proposed. This is the theoretical foundation of vibration sensing through the system.Secondly, the application of coding technology in BOTDR is discussed. Golay complementary sequence is analyzed in detail, including the characteristics as well as coding and decoding principle. The impact of BOTDR system with coded light pulse pump is discussed, especially the variation of SNR, width of Brillouin scattering spectrum and the stimulated Brillouin scattering threshold. By using MATLAB, system response with encoded pulse light pump and decoding process is simulated.Thirdly, the basic principle of 3x3 coupler phase demodulation method is analyzed, the feasibility of 3x3 coupler phase demodulation method in BOTDR system is discussed. The length limit of delay line in the 3x3 coupler phase demodulation system is described, which lead to a limitation of strain measurement range and the spatial resolution of BOTDR system. This algorithm is theoretically derived and is simulated in MATLAB.Finally, experimental system of the BOTDR presented by this article is established. Pulse coding and filtering has been analyzed respectively. After the data acquisition and signal processing, experimental results after demodulation is given. With this experimental system, distributed vibration signal which frequency is less than 4Hz can be detected. The spatial resolution is about 25 m, and the dynamic range is about 12.06 dB.