The Study On Technologies In Super-Long Distance Distributed Brilouin Optical Sensing

Distributed optical fiber sensing technology is a research hotspot in the field of optical fiber sensing. It is different from traditional Sensing technology for it has distributed, long distance, corrosion resistance, high precision, and the ability to obtain continious information on distribution of measured field in time and space along the optical fiber. Distributed brillouin optical fiber sensor is sensitive to temperature and strain, and it’s considered to be the most promising applications of long-distance distributed optical fiber sensing technology. Especially in many practical applications for monitoring, such as railway, coastline, oil pipeline, which need distributed sensing for hundreds of kilometers or more,To achieve long-distance and high-performance sensing, the following problems need to be solved:in long-distance distributed brillouin fiber optic sensors, the stimulating light and probe light and the brillouin scattering light are attenuated by fiber in the process of transmission, and for100km sensing optical fiber (loss coefficient is0.2dB/km), it need a detecting dynamic range for more than40dB, which is difficult to achieve for general distributed detection system. And because of the restriction of stimulated scattering threshold, it is difficult to simply increase the energy of the light injected into the fiber to improve the detected optical power from the end of fiber. Distributed brillouin fiber optic sensor detectes brillouin scattering signal, which is weak and has an only11GHZ frequency difference with the Rayleigh scattering signal, and it is difficult to distinguish between them. Moreover, the power and frequency shift of brillouin scattering signal are functions of temperature and strain, as a result, it need appropriate signal processing technologies to extract the amplitude, frequency and phase information of brillouin scattering signal to realize the multiple-parameter sensing.BOTDR system assisted with a optical fiber Raman amplifier (FRA) is studied in this paper. The stimulating pulse light and the spontaneous scattering light are distributed amplified during transmission, using the sensing fiber as the gain medium. Numerical simulation and experimental research shows that for different sensing distance, BOTDR can be adjusted by regulating Raman pump power and EDFA optical gain, to achieve better sensing results, and100km sensing is realized in experiments with a spatial resolution of50m and a temperature resolution of±3℃. Study using the coherent detection method to processing signals, which can amplify the small signal and extract the amplitude, frequency and phase information at the same time. The simulation results show an improvement of detection accuracy relative to the direct detection, and the realization of the multi-parameter sensing for temperature and strain.Other approaches to improve the performance of sensing are also discussed in this paper, such as the loss-mechanism BOTDA.