Research On Brillouin Optical Time Domain Reflectometry Based On Multi-mode Acoustic Waveguide Fiber

Brillouin optical time domain reflectometry (BOTDR) is a distributed optical fiber sensing technology whose working principle is that the Brillouin frequency shift and power of the Brillouin scattering light are associated with strain and temperature. By measuring the frequency shift and power of the backward Brillouin scattering light generated by the pulse propagated in optical fiber, the strain and temperature can be monitored along the fiber.Multi-mode acoustic waveguide fibers are fibers with a complex refractive index profile and have more than one acoustic excitation modes. These characteristics determine the forward pump lightwave can stimulate multi-mode backward Brillouin scattering in the multi-mode acoustic waveguide fibers and the corresponding Brillouin scattering spectrum (BGS) of the fiber shows multiple peaks. The multi peaks of Brillouin scattering lightwave, representing different acoustic modes, beat with each other in the process of backward propagation, forming the Brillouin Beat Spectrum (BBS) which has only hundreds MHz peak frequency. One work in the dissertation is to report a theoretical calculation method for obtaining strain and temperature coefficients of fibers with any multimode acoustic waveguide structure used in BOTDR based on BBS detection. The calculation approach can be used to search for high sensitive fibers and provides a guide for designing high strain and temperature sensitive fibers applied in the BBS-based OTDR sensing system. In another work of the dissertation, we combine Raman amplification with BOTDR to design the homodyne BOTDR system based on Raman amplification. A homodyne BOTDR based on Raman amplification is proposed to achieve the fast and single-end measurement of temperature and strain while enhancing the sensing range or measurement accuracy. In this method, Raman amplification can be used to enhance multi-Brillouin peaks power to improve the power of the Brillouin beat peaks, which can effectively improve the signal-to-noise ratio (SNR) and sensing distance of the sensing system. The simulation results show that by using the proposed method, under the certain measurement condition the fiber sensing distance can be increased by 11 times without degrading the measurement accuracy and speed.