Research On The Key Techniques Of Multiple Parameters Of The Distributed Optical Sensing System

Distributed optical fiber sensing is a technology using optical fiber as the transmission medium and sensing unit, which can obtain the distribution of disturbance along the whole fiber. Distributed optical fiber sensing system has been widely used in structural health diagnosis of large-scale infrastructure owing to its high measurement accuracy, long sensing length and high reliability. However, the behaviour of failure always manifests as strain, vibration or the change of temperature. The alert and location information cannot be detected by means of one parameter, so the multiple parameters detection gradually becomes the developing trend of distributed optical fiber sensor due to its importance. The researches on multiple parameters measurement are concentrating on strain and temperature. It has been realized by the temperature effect of Raman scattering and the frequency shift of Brillouin. Unfortunately, there is little report on simultaneous measurement of vibration and temperature. This article utilizes OTDR system based on Rayleigh scattering and spontaneous Raman scattering to measure the distributed vibration and temperature. The pulse modulation has been adopted to avoid nonlinear effect and achieve the Raman scattering signal with high signal-to-noise ratio(SNR) simultaneously. The main contents of this thesis are as following:① Theories on vibration sensing technology based on Rayleigh backscattering and temperature sensing technology based on Raman backscattering are described briefly, which lay a theoretical foundation for the feasibility of combining two technologies. Firstly, the principle of phase-sensitive OTDR(φ-OTDR) system is analyzed by building the discrete model, and the theory of temperature measurement based on ROTDR is explained in the field of quantum mechanics. The feasibility of simultaneous measurement of distributed vibration and temperature sensing system is analyzed by discussion of technical specifications.② The experimental research on key techniques and principal device of distributed optical fiber sensing system on multiple parameters detection is implemented. With the analysis of vibration and temperature experiment injected by pulse with different parameters, the spontaneous Raman scattering signal with high SNR can be achieved by high power pulse, meanwhile the vibration detection could be failed due to the nonlinear effect, but the temperature measurement accuracy will be worse if we decrease the pulse power. So the appropriate pulse modulation is crucial to realize the simultaneous measurement of vibration and temperature.③ In order to realize the simultaneous measurement of distributed vibration and temperature, the acoustic optical modulator has been used to generate high and low power pulse continuously. The Raman scattering generated by high power pulse is used to temperature demodulation while the Rayleigh scattering generated by low power pulse is used to vibration demodulation. Experimental results show, the vibration detection is realized with spatial resolution of 5.8m and vibration frequency of 1k Hz and 10 k Hz, and the temperature accuracy about ±3℃, spatial resolution about 4.8m is achieved simultaneously in the sensing fiber with 1.2km length.④ The frequency response range of vibration will decrease because of the pulse modulation in experiment. Considering that the vibration is fast varying signal while the temperature is slowly varying signal, we propose an improved method of pulse modulation to increase the repetition frequency of probe pulse in vibration detection at the cost of decreasing repetition frequency of probe pulse in temperature detection, which can not only protect the frequency response range of vibration detection from reduce but also meet requirements for temperature detection.The experimental research indicates that the method of pulse modulation can effectively avoid the generation of nonlinear effect and improve the SNR of the system at the same time, so as to realize the multiple parameters distributed sensing system based on combination of Rayleigh scattering and Raman scattering. The system has simple structure and low cost, and it will play a more significant role in the practical application in the future by means of further improving the system property.