Research On High Precision Distributed Brillouin Sensing System Based On Differential Amplification

In recent years, as a long-range and high-precision distributed fiber optic sensing technology, Brillouin Optical Time Domain Analysis (BOTDA) has been used to measure the temperature and strain along sensing fiber. Due to its unique advantages, BOTDA technology is widely used in bridge, dam, oil and gas pipelines and railway structural health .For the purpose of signal-to-noise ratio (SNR) enhancement of BOTDA in long-distance measurement, a method based on differential amplification is proposed. Ultimately, the performance of sensing system has been improved.Firstly, the theory of Brillouin scattering and the sensing mechanism of BOTDA technique is introduced. The SNR of direct detection and coherent detection is discussed respectively. Besides, the noise sources of direct detection and coherent detection of BOTDA system are analyzed. Moreover, due to the logarithmic detector used in the single sideband modulation coherent detection BOTDA system, the performance of data acquisition card is limited during the process of prototype integration. A method of differential amplification is proposed to eliminate the problem of large quantization noise and enhance SNR of the sensing system. In this method, a microwave signal with tunable frequencies and another equal-power microwave signal at a fixed frequency are used to modulate the laser source light and to generate the probe light and reference light, respectively. Only the probe light can sweep the Brillouin gain region, while the reference light is out of the Brillouin gain region. Subsequently, the beat note signal between the local light and probe light can carry Brillouin gain information. However, the beat note signal between the local light and the reference light has no Brillouin gain. Then, the powers of the local light and probe light are detected by two LogDs, respectively, after frequency downconversion. The detected signals are sent to a differential amplifier, and the excess DC component can be removed to obtain the "pure" Brillouin gain signal, which is further amplified by the SA circuit. Meanwhile, in order to verify the effectiveness of this differential amplification method, a contrast experiment is employed.In addtion, to improve measurement efficiency and realize commercialization of the BOTDA system, the related theoretical and technical problems of automatic control based on the BOTDA system are researched. Furthermore, a host computer software is programmed in the LabVIEW environment to achieve the above functions. Not only integrated control of each module of BOTDA system but also storage and processing of the collected data are realized by this software. In addition, we added the function of displaying measurement results and alarms.