Research On Fiber Sensing Based On Multimode Fiber Specklegram Detection

With the development and improvement of computer technology,digital signal processing technology and photoelectric detection devices,the complexity and cost of technology based on fiber specklegram detection have been greatly reduced,which has promoted the development of fiber specklegram detection in sensing and other fields.Multi-mode fiber has a large core diameter and supports a large number of high-order modes which are very sensitive to external changes,which makes the sppecklegram detection technology based on multimode fiber structure show great application value.In this paper,the new application scene and specklegram processing technology of multimode fiber specklegram detection technology are studied.The main achievements are as follows.According to the current fiber modal analysis method,the analytical method has strict requirements on fiber structure as well as the numerical analysis method has a limitation of computational accuracy and performance.In this paper,the gray level co-occurrence matrix in the image processing technology is introduced into the specklegram processing algorithm,and the method of analyzing the number of waveguide modes based on the gray level co-occurrence matrix of the specklegram is proposed.Then we analyzed the relationship between the eigenvalues of gray-level co-occurrence matrices and the specklegram of fiber when the number of modes in multimode fiber changed,the approximate linear relationship between the number of modes in multimode fibe and the energy and homogeneity of the gray-level co-occurrence matrices is obained,and verified by experiments.This conclusion is then applied to the qualitative analysis of the number of modes within the fiber.The relationship between the eigenvalues of the gray-level co-occurrence matrices and the displacement is studied by simulating the micro-displacement sensing using the difference of the two kinds of fiber offsets in the single-mode and multimode structure,and the output speckles of the fiber with different bending radius is also studied by this method.Based on the analysis of the number of single-mode and multi-mode optical fiber with different offsets and the number of fiber modes under different bending conditions,taking use of the conclusion that there is an approximate linear relationship between the mode amount and the energy and homogeneity of the gray-level co-occurrence matrices,we obtained the approximate linear relationship between energy/homogeneity and different offsets and different bending radius,which can be used to quantitatively analyze the offset and bending radius by the energy and homogeneity of the gray-level co-occurrence matrices of the specklegram to realize the study of fiber micro-displacement sensing and fiber bending sensing.