The Research On Distributed Vibration Fiber Sensing Technology Based On Self-Coherent Brillouin Scattering Dectection

Optical fiber vibration sensor has wide application prospects in the fields of security, structure health monitoring, environment monitoring and so on. Currently, the Brillouin scattering sensing system has a complex structure, and it costs too much. In this thesis, a novel method of homodyne detection based on spontaneous Brillouin scattering beat signal detecting technology is proposed, which can detect the vibration by measuring the optical fiber dynamic birefringence. Firstly, the birefringence induced by the bending of fiber and the axial tensile stress is investigated. The relationship between the birefringence and the external vibration is analyzed; Then, the relationship between birefringence of single mode fiber and spontaneous Brillouin scattering beat signal is conducted. Furthermore, some simulation analysis about spontaneous Brillouin scattering beat signal based on different code length of Simplex encoded modulation and demodulation is accomplished. Finally, an experimental platform of the BOTDR sensing system based on the detecting technology of spontaneous Brillouin scattering beat signal is built, and some related experiments is conducted. The main contents are as follows:1. The bending of single mode optical fiber and the axial tensile stress are analyzed, which induce the birefringence according to the elasto-optical effect. Besides, a vibration model of optical fiber is built, and the relationship between birefringence and vibration signal is established.2. Based on the theory of birefringence of optical fiber and Brillouin scattering self-coherent effect, the detecting method of spontaneous Brillouin scattering beat signal by homodyne detection is deduced. By the way, the detecting method of the distribution of beat length and birefringence of optical fiber is deduced. The response of the system and the birefringence using single pulse modulation are simulated.3. The performance improvement of BOTDR induced by Simplex encoded modulation and demodulation in the system is analyzed. And the enhancement of dynamic range of the Simplex encoded modulation is simulated. The response of the system with different code length of Simplex encoded modulation and demodulation is simulated. The results show that S matrix encoded modulation and demodulation can improve the signal noise ratio(SNR) of the system effectively.4. The experimental platform of the BOTDR sensing system based on the detecting technology of spontaneous Brillouin scattering beat signal is built. The spontaneous Brillouin scattering beat signal is detected. The enhancement of dynamic range with 7 order Simplex encoded modulation is 1.34 dB. Then, with the methods of moving average and 7 order S matrix encoded modulation and demodulation, the static fiber birefringence of two segments are detected, which verify the feasibility of static birefringence detecting. Finally, the vibration signal is located. Caused by vibration signal, the addition dynamic birefringence which is about-71.54?10, which verify the feasibility of vibration signal detecting by the system.Through the research of several aspects above, the detection of vibration by the BOTDR sensing system based on detecting technology of spontaneous Brillouin scattering beat signal is accomplished. The BOTDR system has a simple structure and a rapid measurement to detect vibration, and it will be of great significance in the industry of distributed optical fiber vibration sensing technology.