Research On Heterodyne Detection Applied To Brillouin Temperature Sensing

The optical fiber sensing technology in bridges, dams and highway monitoring and many other fields have important applications. Distributed optical fiber sensing technology based on Brillouin scattering can realize real-time and accurate monitoring, can achieve early fire warning, and obtain temperature and strain information. So the signal processing of distributed optical fiber sensing technology is particularly important. This paper focused on the signal extracting of the Brillouin scattering, and also investigated theoretically and experimentally. The specific content of this paper are as follows:(1)The principle and characteristics of the spontaneous Brillouin scattering and stimulated Brillouin scattering are studied and contrasted. The relations between frequency shift and amplitude of Brillouin scattering and temperature are deduced. The process of continuous light Brillouin scattering is simulated with shooting method, and the pulse light Brillouin scattering is simulated with Finite-Difference Time-Domain method. Our sensing scheme based on reference fiber is proposed by analyzing the existing sensing scheme based on spontaneous Brillouin scattering.(2)The beat frequency process is investigated and deduced by analyzing three characteristics of coherent detection: (a) the beat frequency of two single frequency, (b) the beat frequency between a single frequency and a spectrum signal with Lorenz spectrum, (c) the beat frequency of two spectrum signals with Lorenz spectrum. The analyzing results show that the peak power of beat signal is in proportion with the product of two signals. Therefore, when we employing the reference fiber, it will be better to get strong local light with stimulated Brillouin scattering in reference fiber and it would improve the peak power of beat signal.(3)The down-converter feasibility is simulated with ADS software. This scheme could realize the down converter from 185MHz to 15MHz and largely restrain the harmonic wave from the mixer. 20-40MHz sine's signals are obtained with data acquisition card and the frequency components of the signals are analyzed by the Fast Fourier Transform.(4)The down-converter scheme based on reference fiber is proposed, this scheme could reduce 10.655GHz of beat frequency. The signal processing is more convenient by utilizing the stimulated Brillouin scattering light in the reference fiber as local oscillate light. This scheme is based on data acquisition card to get the signal and obtain the whole frequency spectrum information with Fast Fourier Transform only once. So the requirements of the local signal source are reduced. Moreover, the 20-400C temperature changes is detected successfully by analyzing the frequency spectrum of the signal, and the error range is among 10C.