Research On Distributed Dynamic And Static Sensing Based On Ultra-Short Fiber Bragg Grating Array

As a kind of ultra-weak fiber Bragg grating,ultra-short fiber Bragg grating(USFBG)has been widely applied to the field of distributed engineering detection because of its excellent characteristics such as low reflection loss,wide reflection spectrum,and easy construction of a large-capacity multiplexing network.In the meanwhile,various high-speed wavelength demodulation technologies have been born with the need for increasingly high-frequency dynamic signal monitoring as well.The wavelength modulation characteristic makes the grating sensing results have good stability and accuracy.However,with the further improvement of the safety and quality requirements of engineering structures,it is necessary to perform the analysis of weak high-frequency fault signals,such as perimeter security detection for human invasion and track safety inspection.These have trouble realizing by wavelength demodulation technique.Phase demodulation is proven superior to wavelength demodulation in the high sensitivity and high speed measurement of dynamic signals.The fusion of the two technologies can make up for each other,but there are few related researches.In this paper,the USFBGs serve as the sensors and reflection mirrors at the same time.Based on the dual-wavelength edge filtering technology,a dual-wavelength pulsed laser is used to realize the grating wavelength demodulation.And a fiber MachZehnder interference structure is constructed to complete the demodulation of optical phase shift caused by the change of sensing fiber,which is between the adjacent USFBG.Finally,high stability static sensing and high sensitivity dynamic sensing can be achieved.The main research contents are as follows:(1)The formation mechanism and spectral characteristics of USFBG are researched,and the temperature and strain sensing mechanism of USFBG are analyzed.The dual-wavelength edge filtering technology is researched for USFBG wavelength demodulation.The effects of grating spectral characteristics and detection wavelength spacing on the demodulation sensitivity are analyzed and compared as well.The temperature and strain sensing mechanism of the fiber phase modulation sensor are derived.According to the sensing mechanism,two common phase unwinding algorithms are compared,and the dynamic range of the demodulation algorithms are analyzed.Based on the characteristics of the wavelength and phase demodulation technologies,a model of wavelength-phase simultaneous and independent demodulation is constructed and the demodulation results are simulated.(2)A distributed dynamic and static demodulation system based on USFBG array is established.The construction of optical path and circuit design are completed.Firstly,through analyzing a mathematical model of pulse signal interference,the quasibalanced Mach-Zehnder interference structure is designed and produced,and the effect of the length error of interference arm on the phase demodulation result is analyzed.Secondly,according to the requirements for the linewidth of the light source,the pulse light source whose output wavelength is alternating is built by using the medium coherence distributed feedback laser.At last,the noise performance and the frequency response of the distributed sensing system are analyzed and simulated.(3)According to the principle of proposed system,the initialization of the parameters of the optical path and circuit is completed.The grating addressing method based on optical time domain reflection is studied,and the peak search data is extracted and stitched by the window peak finding algorithm.The simultaneous demodulation and alternating interpolation reconstruction method of time division multiplexed signals is researched.Furthermore,a distributed wavelength-phase demodulation algorithm based on optical time domain reflection is proposed.(4)An identical USFBG array of 964 gratings with 2 m spacing,1 mm length and 0.004% reflectivity is fabricated for the distributed dynamic and static experimental platform.It is used to test the dynamic and static measurement performance and crosstalk of the system respectively.The experimental results show that the system has a wide static sensing range and the detection capability of 12.5 kHz underwater acoustic signal.The standard deviation of the demodulation results of the same vibration signal is only 2.518n? at different temperatures,and the harmonic rejection ratio of the demodulated signal is similar.Thus,the dynamic and static crosstalk of the system is low,and the system has good demodulation capability of dynamic and static signal.