Research On Detection Technology Of Fiber Brillouin Scattering Signal

Brillouin optical time domain reflectometer(BOTDR) is realizedthrough combining OTDR with spontaneous Brillouin scattering forsensing, which has great application potentials in the civil facilities,environmental monitoring, aerospace industry, power monitoring andgeotechnical engineering. The detection technology of weak Brillouinscattering signal in the optical fiber is researched combining with the lab’ssub-topics of national973project in this paper. Based on the principle ofcoherent detection, a novel BOTDR system was proposed in order to solvethe problem of high cost and complexity in the traditional BOTDR, and itwill lay a foundation for designing novel Brillouin sensors. The mainresearch content and innovations include:(1) The developments of the Brillouin scattering distributed optical fibersensors, the research background and significance are introduced. The researchstatus and application situation of BOTDR technology are emphasized.Meanwhile, the characteristic of Brillouin scattering spectrum and sensingmechanism are analyzed, and the temperature and strain dependences ofBrillouin spectrum have been established. That provides important theoreticalfoundation for design of BOTDR system.(2) The simulated BOTDR system was designed based on FFT spectrumanalysis technology, and the sensing signal was simulated by Matlab software.The method of cumulative average was used to improve the signal to noise ratio(SNR). The influence of the factors of sampling rate, FFT points, averagenumber on the precision of coherent signal frequency demodulation wasanalyzed.(3) The BOTDR system with coherent detection was set up and tested. Asingle-mode Brillouin fiber ring laser was used as reference light which interferes with spontaneous Brillouin sensing signal, so the high frequencyBrillouin sensing signal of11GHz was transferred into low frequency signal of200MHz. And the performance of the whole BOTDR system was evaluated.(4) Using LabVIEW program to realize the acquisition of coherent signaland the analysis of real-time FFT spectrum. the SNR of system was improvedthrough cumulative average method. The results of the frequency demodulationof25km fiber were analyzed, and the main problems of system were discussed.