Study On Mode Analysis And Spectrum Analysis Based On Multimode Fiber Specklegram Detection

With the development of digital signal processing and the imaging technology,the complexity and cost of techniques based on fiber specklegram detection is significantly reduced.As a result,the sensing and the spectrum analysis as well as other related applications based on fiber specklegram detection technique have been widely investigated.In terms of the current fiber mode analysis methods,the analytical method has strict requirements on fiber structure while the numerical method has an intrinsic contradiction between accuracy and computational complexity.Thus,in this paper,we demonstrate that a specklegram detection technique based on multimode fiber can be used for fiber mode analysis.At the same time,aiming at resolving the contradiction between the fiber length and the performance of spectrum reconstruction in the spectrum analysis based on multimode fiber specklegram detection,we present two improved schemes for the spectrometer based on modified fiber structures.The main works are as follows:Firstly,we make a theoretical analysis for coupling and multimode interference theory based on specklegram detection,and classify the existing algorithms of specklegram pattern processing into two types,including intensity based algorithm and image content based algorithm.Secondly,the relationship between mode excitation and the off-axis degree of fibers in single mode-multimode fiber structure is analyzed theoretically;and the specklegrams at the facet of multimode fiber is obtained using beam propagation method.Then we adopt an image processing technique named gray-level co-occurrence matrix to investigate the relationship between specklegrams derived from the single mode-multimode fiber structure under different off-axis condition and the gray-level co-occurrence matrix eigenvalues,and it's found that the number of excitation mode vary quasi-linearly with energy/homogeneity/entropy of gray-level co-occurrence matrix,and the conclusions are also validated by experiment.Then the conclusions are used to analyze the mode excitation inside D shaped fiber.The method based on the image processing of fiber specklegram offers a new way to characterize the fiber mode excitation qualitatively.Finally,based on the research on the working mechanism of the spectrometer realized by multimode fiber specklegram detection,we implement the reconstruction algorithm for probe signal under noise environment.We also realized the calculation of the spectrum resolution based on spectral correlation.Using off-axis single mode-step index multimode fiber and single mode-D shaped multimode-multimode fiber structure to replace the existing core-to-core welding structure,we make an improvement on the spectrometer.Comparisons are made for the performances of spectrum analysis between the spectrometers with and without modified structure.Several important performance evaluation parameters including the spectrum reconstruction error,full width at half maximum and broadband signal reconstruction are compared and an improved spectrum reconstruction performances of the two modified schemes are convinced.