Study Of Distributed Vibration Detection System Based On Weak Fiber Bragg Grating Array

With the rapid development of optical fiber sensing technology,the optical fiber sensors network tend to have large capacity,long sensing-distance and high precision.The emergence of weak-FBG makes it possible to achieve large scale sensing network based on FBGs.In this paper,We propose and experimentally demonstrate a distributed sensing system using an identical weak fiber Bragg grating array.By combinating the advantages of OTDR technology and coherent detection technology,high sensitivity,high frequency response and high precision vibration measurement is achieved.Phase,frequency and location information of vibration can be demodulated by using a path-match interferometry method.The main research results of this paper are listed as follows:(1)The effects of cross-talk and noise in TDM based FBG sensing system was studied.Two kinds of cross-talk,the shadow effect and the multi-reflection,are the most important factors to limit the multiplexing capability of the FBG sensing system.To achieve high spatial resolution and high frequency response measurement,we propose a distributed vibration sensing system based on weak FBG array.(2)The relationship between the wavelength mismatch and the fringe visibility of interference signals was analyzed.In order to eliminate the wavelength mismatch which caused by the environment,the ultra-short-wFBG is introduced into the distributed vibration system.They can effectively avoid the wavelength mismatch and improve the fringe visibility of interference signal.(3)The advantages and disadvantages between the PGC demodulation algorithm and 3×3 coupler demodulation algorithm were studied.The arc tangent demodulation algorithm is studied to improve the performance of the system.The influence of the DC component of the three interference signal(the average light intensity),the AC component(the peak intensity of the interference signal)and the phase difference between the three signals is analyzed.(4)The effect of key components of the system are analyzed in detail.In order to improve the sensitivity,a laser source with good coherence stability is adopted.The driving current and temperature control in the pulse modulation circuit are carefully introduced.The performance of the system is verified by the ultra-short-wFBG array with 520 FBGs and 2.5 meters equidistance between neighbor FBGs.Experimental results show that the system can accurately collect all the interference signals of the weak FBG array;The demodulation frequency can be up to 2000Hz;The proposed system has better frequency response,which can accurately distinguish weak signal with frequency from 30 Hz to 700Hz;The distributed vibration system has higher sensitivity in contrast to the standard of seismic geophone and can measure different vibrations.