Study On The Distribution Characteristics Of D-shaped Multimode Fiber Specklegrams And Their Application

With the continuous development and progress of optical fiber specklegram detection technology and image processing technology,the sensing technology based on multi-mode fiber specklegrams is becoming more and more mature,and it is well used in the fields of fiber sensing,image encryption,compressive sensing.The multimode fiber specklegrams carry a large amount of optical fiber spatial state information,and the output specklegrams can be used to obtain the interference information of the fiber,so the detection technology based on the multimode fiber output specklegrams have great development potential and broad application scenarios.This paper analyzes the distribution characteristics of D-shaped multimode fiber specklegrams in detail,and the specklegrams are applied to refractive index sensing.The main research contents and innovations are as follows:Firstly,from the three aspects of changing the eccentric excitation method,adjusting the D-shaped fiber cross-sectional parameters,and using different refractive index distributions,the mode excitation of the D-shaped multimode fiber structure is analyzed in detail.The Variations of the mode fractional power for the first 50 modes at different D-shaped fiber structure,different eccentric excitation and different refractive index distribution are studied.The results show that with the increase of eccentric displacement,grinding depth and length of the D-shaped section,more high-order modes are excited inside the multimode fiber and the mode excitation phenomenon is more obvious.Compared with the graded-index distribution,the D-shaped fiber with step-index distribution can excite more modes and the mode fractional power of different modes is more balanced.Secondly,based on the principle of multimode interference of multimode fiber and the processing algorithm of the zero mean normalized cross-correlation of specklegram,the multimode fiber output specklegrams are processed and applied to the refractive index sensing.The relationship between the refractive index and the zero mean normalized cross-correlation of the output specklegrams is analyzed in detail,when the refractive index of the external environment of the D-shaped multimode fiber structure based on step-index multimode fiber is changed.The experimental results show that with the change of the refractive index of the external environment,the zero mean normalized cross-correlation of the specklegram shows regular variations.Therefore,the refractive index can be measured by analyzing the output specklegrams of the D-shaped multimode fiber.At the same time,the influence of different parameters of the D-shaped multimode fiber structure on the refractive index measurement is analyzed.The results show that analyzing the refractive index through the D-shaped fiber output specklegrams has universal applicability in the case of different transmission wavelengths,eccentric displacement,the length and grinding depth of D-shaped fiber.