Research On Signal Demodulation Technology Of Microstructured Optical Fiber Based Quasi-Distributed Sensing System

Optical fiber sensing technology is an important application branch of optical fiber communication technology.Due to the distinguished advantages of light weight,compact,passive,resistance to corrosion,resistance to electromagnetic interference,easy networking,optical fiber sensors have been widely deployed in aerospace,critical engineering facilities,national defense security,biomedicine,resource exploration,disaster monitoring etc...In recent years,distributed fiber sensing technology is of intensive research,which paves an efficient way for large-coverage and long-haul sensing applications.Oriented to the development trend of long distance,large capacity,high precision,fast response,rapid progress has been made in distributed fiber sensing technologies.The integration of new sensing mechanism,new fiber fabrication and new demodulation algorithm facilitates the innovation of the sensing performance,thus extending much more widespread applications.In this thesis,oriented to the practical applications of the fiber distributed sensing technology,we respectively investigate the optical fiber distributed sensing mechanism,the characteristics of large capacity multiplexing,high performance detection method,low cost application platform,and high precision demodulation algorithm.Subsequently,we design a quasi-distributed sensing system which is quite desirable for practical applications.The main contents of this thesis are summarized as follows:(1)Optical mechanism and preparation of microstructure optical fiber unit: the formation mechanism of the optical fiber bragg grating sensor was analyzed based on both the coupled mode theory and transfer matrix theory.Particularly,influences of fiber length,refractive index,raster cycle and optical modulation depth on the optical spectrum were simulated and investigated.Furthermore,temperature and stress sensing characteristics of fiber bragg grating sensor were studied.Meanwhile,we analyze the optical properties as well as the spectrum of fiber grating in fabry-perot sensing unit,and the influence of sensing parameters and cavity length to optical path difference were simulated.Finally we proposed the concept of fiber microstructure sensing unit and optical time-domain reflectometer(M-OTDR)technology based on the microstructure,and the continuous preparation methods of fiber microstructure unit were given in this thesis in order to provide the technical foundation for large-scale sensor network multiplexing.(2)Microstructure optical fiber multiplexing technology: sensor multiplexing can improve the scale of the quasi distributed optical fiber sensing effectively and reduce the acquisition cost of a single sensor,so we analyze the mainstream sensor multiplexing methods such as time division multiplexing,wavelength division multiplexing and frequency division multiplexing respectively.Then sensing demodulation scheme for each multiplexing mode were proposed: the limitations of time division multiplexing is high requirement of reflectivity and signal overlapping caused by multipath crosstalk;wavelength division multiplexing scale is limited by light source and system bandwidth severely;frequency division multiplexing requires high accuracy of demodulation system.Finally,multi-domain multiplexing based microstructure sensing unit was proposed.Through three-dimensional coding contains time/wavelength/frequency division on microstructure fiber,index of sensor unit number as well as optical spectral complexity are significantly enhanced,which put forward higher requirements to demodulation precision,speed and cost of the demodulation system.(3)Multi-domain parallel fast high precision demodulation technology: according to the requirement of high speed and high precision demodulation for multi-domain multiplexing system,multi-domain parallel fast demodulation method based on scanning filter was proposed.Based on analysis of sampling mechanism and control logic of fabry-perot scanning filter,high speed and high precision scanning filter was selected,and then we proposed the subsection parallel demodulation algorithm.We analyzed the system multiplexing capacity,demodulation speed and demodulation precision,the results is shown that theoretical multiplexing and demodulation volume can achieve to 8000 units at most.Finally,construction of the demodulation system was completed based on NI5781 platform,which is applied to the experiments on the wavelength division/frequency division two-dimensional multiplexing microstructure optical fiber sensing network,and the wavelength detection precision of the 3 pm and temperature detection accuracy of ± 0.4 °C were verified through the temperature experiment,meanwhile the demodulation rate of 500 Hz was illustrated by the vibration experiment results.Overall,tremendous demodulation performances of the method proposed were proved,in high speed,high precision.With upgrading of the device and updating of the demodulation algorithm,the performance index such as multiplexing capacity,demodulation speed and demodulation precision are expected to promote further.(4)Large capacity and high precision demodulation based on the Analytic Mode Decomposition(AMD): in view of high precision demand to the large capacity optical fiber microstructure quasi distributed sensing demodulation,signal demodulation method of high density frequency division multiplexing sensor network based on AMD was proposed and implemented.Firstly,through analyzing the theory of AMD,frequency division multiplexing demodulation method of the microstructure sensing unit established and simulated.Comparing the demodulation performance of three method namely Fast Fourier Transform(FFT),Wavelet Transform,AMD,it is illustrated that AMD method can achieve improvement of multiplexing capacity five times more than FFT and signal demodulation recovery degree can improve at least more than 90 times.Finally,the distributed stress analysis experiment verified capacity expansion and high precision detection ability of AMD.This method does not involve costs increasing brought by hardware upgrading,which only relies on the novel algorithm to increased capacity of the sensor multiplexing.(5)Low cost fast demodulation technology based on InGaAs detector: according to the demand of low cost demodulation for quasi distributed microstructure optical fiber sensing,the demodulation mechanism based on InGaAs detector was deeply analyzed,and through the spectral resolution of phase grating spectrometer and InGaAs detector,wavelength information of multispectral can be extracted.Hereafter,we designed the wavelength/time/frequency division multiplexing system,and temperature sensing experiment with application of wavelength demodulation based on InGaAs detector was conducted,the experimental results illstrated that the demodulation system based on InGaAscan realize accurate signal recovery with the linearity up to more than 0.999.Demodulation system based on InGaAs detector has the advantages of low cost,compact structure,which could be greatly improved with the great technology development of speed and pixels of InGaAs,then the detection accuracy and the system multiplexing capacity can be improved effectively and continuously.Finally the demodulation cost of a single sensor unit could be lowered significantly.