Research On The System Of Brillouin Optical Fiber Sensor Based On Multi-wavelength Light Source

Since the fully distributed optical fiber sensors (FDOFSs) based on Brillouin scattering have the ability to measure the strain and temperature continuously along long distance, they can be applied in the health monitoring of many structures such as large buildings, roads, bridges, dams, optical cables, and oil and gas pipe lines. And the FDOFSs have already been the research focus for their wide application prospectsBrillouin optical time-domain reflectometry (BOTDR) and Brillouin optical fiber time-domain analysis (BOTDA) are two main sensing technologies for FDOFSs based on Brillouin scattering, which have the advantages of long sensing range, high measurement accuracy and simple structure. Due to the restriction of the loss and the fiber nonlinear effects in optic fiber, the peak power of probe pulse in BOTDR and BOTDA are limited, thus the sensing range and the system Signal-to-Noise-Ratio (SNR) are limited too. The conventional Brillouin fiber sensing technology is based on single-frequency probe, its SNR improvement mainly depends on increasing pulse power within the nonlinear threshold and improving the measurement times. In order to further improve the SNR, we have proposed several feasible Brillouin fiber sensing system based on multi-wavelength probe light, and built BOTDR and BOTDA systems with multi-wavelength light source. The expected results are obtained in experiment. The main works of this dissertation are as follows: 1. The transmission characteristics of the multi-wavelength lightwave in the Brillouin sensing system are theoretically and experimentally studied. The nonlinear effects of the multi-wavelength lightwave are studied from the aspects of stimulated Brillouin scattering, self-modulation and the nonlinear effects among different wavelengths. The Brillouin spectrum of the multi-wavelength lightwave is studied from the frequency shift dependence on source wavelength and the spectrum shape dependence on wavelength interval. The amplification characteristic in an EDFA is studied from the aspect of amplifier noise. The feasibility of multi-wavelength Brillouin sensing system is proved.2. Three basic structures of Brillouin sensing system based on multi-wavelength probe light are proposed. The signal and noise of these systems are analyzed respectively according to the different system structures. And the potential SNR improvement of each system structure is theoretical analyzed.3. Electro-optical modulator is used to generate three-wavelength lightwave, and then a BOTDR configuration and a BOTDA based on Brillouin loss configuration are built with three-wavelength lightwave respectively. The experimental results show that the new system has the original spatial resolution. And comparing to the conventional single-wavelength system, the new systems achieve higher SNR and measurement accuracy in the same sensing range, or longer sensing range with the same measurement accuracy.