Research On Distributed Optical Fiber Sensing System Based On Brillouin Scattering

Recently, the distributed fiber sensor based on scattering light has many merits, such as light and soft, resistance to electromagnetic interference, easy to carry out the distributed measurement, sensing and transmission in one fiber, and so on, so that it is pretty popular in sensing field. The absolute distributed fiber sensing system mainly has two kinds: one is based on Raman scattering and the other is based on Brillouin scattering. Thereinto, the distributed fiber sensing system based on Raman has been researched deeply and is basically commercial now; on the other hand, the distributed fiber sensing system based on Brillouin scattering can measure temperature and strain simultaneously so it is more attractive compared with that based on Raman scattering, but the detection of Brillouin scattering light is difficult so that its development is slow and has not been commercial up to today.In this dissertation, Brillouin scattering mechanism was researched deeply and the previous research process and method were analyzed in detail. After that, considering the characteristics of Brillouin scattering light detection, we proposed a feasible and novel scheme of distributed optical fiber temperature and strain sensing system based on Brillouin scattering. According to this scheme, the system was built step by step and the corresponding experiments and tests were done. In consideration of related calculation and influencing factors, the measurements had been taken. Simply speaking, our method is as following: narrow-linewidth continuous light from a distributed feedback laser (DFB-laser) is modulated into pulse light by the photoelectrical modulator. The pulse light is amplified by Er-doped fiber amplifier (EDFA). After that, it enters the sensing fiber and makes Brillouin backscattering light generate. There are several keys in our work. Firstly, the Brillouin scattering light is obtained through single-input which makes the system installation convenient. Secondly isogenous heterodyne interference is put forward in the first time to demodulate Brillouin backscattering signal which avoids the disadvantage of the general interference method demanding two component lights' intensity approach, and simplifies the testing system. Thirdly, the measurement has been taken to strengthen the signal in order to simplify the later signal process. Fourthly, the Brillouin frequency shift and intensity variance can be obtained at the same time by the electrical method. Thus the temperature and strain can be gotten simultaneously. Lastly, a series of experiment research and system testing result analysis are mode. Concretely, the main text includes the following aspects.In Ch1 introduction, summary introduces basal knowledge, including fiber's transmission characteristics, scattering light classification in single-mode fiber and their definitions, two kinds fiber sensing technologies (Fiber grating sensing technology and distributed fiber sensing technology which includes those based on Rayleigh scattering, based on Raman scattering and based on Brillouin scattering). Then, fiber sensing technology based on Brillouin scattering research status quo and subject signification is displayed. Lastly, the main task in this dissertation is introduced simply.In Ch2 theory basis of Brillouin scattering and its sensing technology which is theoretical part of the research object, the Brillouin scattering characteristics are analyzed in theory. The category of the Brillouin scattering and their generation principles are introduced. And the spectrum characteristics which include the Brillouin frequency shift, spectrum shape, and intensity distribution, are analyzed. Lastly, it is described how to measure temperature and strain suffered by the sensing fiber.In Ch3 distributed fiber sensing system based on Brillouin scattering is one of the dissertation's two cores, including system structure, system methods, key components performance analysis and usage, software development and experiment platform. Firstly, in system structure, the system is introduced on the whole and the signal flow is described. Secondly, in system methods, four special methods in the system are introduced, including the basal principle and advantages of the isogenous heterodyne interference used to detect the Brillouin signal, the methods to strengthen the signal, which are double-pulse strengthening Brilliouin scattering signal and making Rayleigh component in heterodyne interference strengthening, the demodulation of temperature and strain by the electrical way including its structure, theory basis and calculation method, the normalization of the Brillouin intensity and system performance index analysis. Thirdly, in key components performance analysis and usage, the main devices in this system are displayed in detail, including performance index, selection principle and circuit controller principle, and so on. Fourthly, in software development, according to the demands of this system, it depicts the software control to the digitizer, data flow, averaging process, and so on. Fifthly, in experiment platform, the picture of the whole system is shown. Lastly, the conclusion is given.In Ch4 experiment research, which includes research on Er-doped fiber amplifier (EDFA), research on the influence of incidence light parameters to backscattering light spectrum, and research on nonlinear phenomenon. Concretely speaking, the experimental research of the EDFA is done and the relationship between backscattering light and incidence light is gotten through adjusting the parameters of the incidence light. When parameters of incidence light are suitable, the fiber nonlinearity phenomenon occurs. The corresponding experiments are done to look for its rule and influencing factors. The possible reasons are given based on the above experiments.In Ch5 system testing and result analysis, is the other core in this dissertation. In signal testing, the high frequency signal of heterodyne interference is seen from the spectrum analyzer, the signal along the sensing fiber is gotten from the oscilloscope, and the temperature and strain testing experiments for single-mode fiber are done, which results are good. The peak frequency from the spectrum analyzer is 10.8420GHz, which is consistent with the theoretical value of Brillouin frequency shift. The two signal waveforms from the oscilloscope have the information of the backscattering light along the optical fiber. The temperature coefficients in the temperature elevation and reduction process are separately 1.0843 MHz/℃and 1.0462 MHz/℃, which are near to the data in the previous article which is 1.2MHz/℃and the little difference is caused by the apparatus error. The strain coefficient under room temperature is 0.049 MHz /με, which is consistent with thedata in the previous article. The above data indicate the system's feasibility and veracity. In the calculation, the spatial resolution, light pulse power and signal to noise ratio are calculated.In Ch6 conclusion and prospect, the whole conclusion is given. The author analyzes the issues of this system and points out the research and experiment which should be gone on for this system in the future. In the end, the author prospects the future and forecasts the application foreground of the system.