Research On The Distributed Temperature And Strain Sensing System Based On BOTDR

Distributed optical fiber sensing technology uses optical fiber as the transmission medium and sensing unit, and this technology can monitor the information in continuous time and space. The d istributed optical fiber sensing system based on Br illouin Optical Time Domain Reflectometry(BOTDR) has good real-time performance, accurate spatial resolution and measurement accuracy, and the system structure is simple and easy to implement. The d istributed optical fiber sensing system based on BOTDR has gained wide attention at home and abroad, and it has become the main technology in the field of optical sensing. In this thesis, according to the relationship between the Brillouin frequency and the temperature and stress, a distributed sensing system to monitor the change in temperature and strain is built based on the coherent heterodyne detection. Through the pulse modulation experiment and the signal processing algorithm, the signal- noise ratio(SNR) of Brillouin signal has been improved greatly, and the spatial resolution and measurement accuracy of the system have been also optimized. The main contents of this thesis are as following:(1)The fundamental theory of BOTDR technology and the generation mechanism of spontaneous Brillouin scattering and stimulated Brillouin scattering are described briefly. The quantitative relationship between Brillouin signals' frequency and intensity and the temperature & stress, is investigated. Direct detection and coherent heterodyne detection are two main structural models of the sensing system. The system's main performance parameters and the factors that affect them are introduced in detail.(2)The distributed optical fiber sensing system based on BOTDR is designed and built. The performance parameters of the key components such as Brillouin fiber laser and Erbium-doped Fiber Amplifier(EDFA) are specially designed and tested experimently. According to the working principle of the acoustic optical modulator(AOM), we analyze the effects of modulation pulse on spontaneous Brillouin scattering in sensing optical fibers.(3)The influence of different pulse width and amplitude on the scattering signal in a 10 km sensing optical fiber is analyzed. The temperature sensing experiment is carried out with 380 m V-300 ns modulation pulse, through which the quantitative relationship between temperature variation and Brillouin frequency shift is obtained. It's obtained that the Brillouin frequency shift coefficient of temperature is 1.2MHz/? and the system's spatial resolution is 15 m by this modulation pulse. According to a 2km sensing optical fiber, the 600 m V-100 ns modulation pulse is chosen to carry out stress sensing experiments, and the qualitative relationship between the stress variation and the Brillouin frequency shift is obtained. It's shown that the spatial resolution of the system is 2m.(4)As for the signal processing part, the signal's SNR is improved by the average cumulative denoising method. Gauss window function is selected to intercept the information collected in time domain, and through short-time Fourier transform(STFT), the information points in the frequency domain are obtained. The Brillouin frequency along the sensing optical fiber can be achieved by fitting the points obtained from the STFT. By increasing the length of the Gauss window function, the spatial resolution of the temperature sensing system has been improved from 15 m to 3m or even higher.The experimental research indicates that an appropriate modulation pulse can effectively avoid the generation of nonlinear effects and improve the SNR of the system at the same time. The temperature and strain are detected, and the spatial resolution of the system is optimized. The system has simple structure and low cost, and it will play a more significant role in the practical application in the future by means of further improving the system performance.