| Brillouin Optical Time Domain Reflectometry(BOTDR), in which fiber acts as the sensing component and the transmission medium in the same time, attracts much attention because it can measure temperature and stress along the fiber and realize the distributed measurement.In BOTDR system, the Brillouin scattering light is difficult to detect because the intensity of the Brillouin scattering light is very weak and the frequency of Brillouin scattering light can reach up to about 11 GHz. Although the intensity of the Brillouin scattering light can be enhanced by using coherent detection, the frequency of the Brillouin scattering light after coherent detection is still too high to detect. An available solution is to shift the frequency of local reference beam to the frequency of the Brillouin scattering light by an electro-optic modulator(EOM) and then using coherent detection. By this method, the frequency of the Brillouin scattering light can be reduced to several megahertz, which can be more easily detected. However the bias point of the electro-optic modulator (EOM) may change slowly due to many external factors, such as temperature, stress. These factors will lead to instability of the modulation efficiency, which will further lead to the instability of the BOTDR system and the declination of the measurement accuracy.The methods for EOM working point control are summarized in this paper. Based on the study of the characteristics of the EOM in the BOTDR system, a novel and simplified stabilization technique based on digital signal processor and step-by-step tracking algorithm is proposed. The design of both the hardware and the software is discussed in detail in this paper, and then a comparison is made between our method and the tradition one. Experimental results show that the EOM's bias point can be locked quite well. Using this method in the BOTDR system, the local reference beam is stabilized, which improves the stabilization and the measurement accuracy of the BOTDR system. |
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