Research On Performance Optimization For Long-distance BOTDR

With the acceleration of Chinese modernization process,the energy industry is booming.As an important part of China’s energy industry,the safety monitoring of high-voltage transmission cable cannot be ignored.Most of the high-voltage transmission lines and cables are set up in the field,with complex environmental conditions and large coverage,and the traditional monitoring technology has been difficult to meet the requirements.As a new security monitoring scheme,distributed optical fiber sensing technology has attracted extensive attention of researchers because of its inherent safety,corrosion resistance,anti-electromagnetic interference and long-term online monitoring advantages.Compared with other fiber optic sensing schemes,Brillouin Optical Time Domain Reflectometer(BOTDR)technology can be used for cable safety monitoring due to its advantages such as long sensing distance,single-ended injection and simple structure.In practical safety monitoring,the signal to noise ratio of the system is reduced because of the long sensing distance and the high attenuation of optical fiber.To solve the problem of long-distance BOTDR,this paper proposes a series of optimization schemes,and carries out theoretical and experimental research.The research contents are as follows:(1)The Brillouin scattering effect and sensing mechanism in optical fiber are analyzed theoretically,and the source of system noise and the threshold of nonlinear effect are analyzed.The scheme of coherence detection and microwave heterodyne sweep is determined to improve the signal-to-noise ratio of the system and reduce the cost of the system.The overall scheme of the system design,combined with the above theoretical model,the key components of the system parameters selection.(2)A BOTDR experiment platform based on Raman amplification scheme was built to collect Brillouin scattering spectrum,single-frequency curve and Brillouin frequency shift distribution curve.The backward Raman amplification scheme was selected after comparing the effects of forward and backward amplification schemes on the sensing system.According to the parameters of the backward Raman amplification scheme,the sensing range can reach more than 100 km at the spatial resolution of 20 m,and the measurement error of Brillouin frequency shift is ±3.5 MHz.The temperature measurement experiment was carried out on the100 m length of the middle segment of 100 km fiber.The temperature measurement accuracy in the variable temperature region of the fiber was ±1 MHz.The fitting coefficient of Brillouin frequency shift and the spatial resolution were consistent with the theory.(3)Clarify the principle of wavelet threshold denoising,and analyze the wavelet basis and threshold parameters.The theoretical model of the BOTDR system based on microwave frequency sweep was built,and the noise reduction was simulated for the denoised signal,and the parameters such as wavelet basis,decomposition layer number and threshold function were optimized.Taking advantage of FPGA parallel computing and based on wavelet optimal denoising parameters,the wavelet threshold denoising algorithm is implemented,and the denoising experiment is verified.