Theoretical And Experimental Reserch On The Use Of Optical Fiber To Generate Vortex Beam

The vortex beam carries orbital angular momentum,which has important application prospects in vortex optical multiplexing communication,particle capture,coding and decoding.Compared to structural devices that generate vortex beam,the use of optical fiber to generate vortex beam has great advantages in improving the performance of the vortex optical communication system,reducing the cost of the vortex optical communication system,and simplifying the structure of the vortex optical communication system.This paper focuses on the factors that affect the generation of vortex beams in optical fiber,the use of a few-mode fiber to generate vortex beams,and the phase detection of vortex beams.The specific work is as follows:1.The factors of influence the vortex beam generated by the fiber is analyzed.The effects of the incident beam wavelength,the difference of the inner and outer refractive index of the fiber,the core radius of the fiber,the angle of the incident fiber,and the influence of the off-axis incident fiber on the variability of the vortex beam generated by the fiber are numerically analyzed.It provides a theoretical basis for the selection of parameters for generating vortex beams using optical fiber.2.Vortex beam is generated by using a few-mode fiber.A few-mode fiber is an optical fiber with only the fundamental mode and the low-order mode.Combining the coupled mode theory and the principle of the vortex beam in the fiber,the influence of the incident angle on the excitation efficiency of the first-order mode in the few-mode fiber is analyzed.The experiment verifies that using few-mode fiber can produce first-order vortex beam.The use of vortex beam in a few-mode fiber provides a simple,low-cost method for generating vortex beam in an optical fiber.3.Phase of vortex beam is detected.The two methods of wavefront detection for vortex beams are mainly analyzed from the experimental point of view:interferometry and diffraction.The detection methods that can be selected are self-interference,triangular diffraction,and double-slit interference to determine the magnitude and the positive and negative of the topological charge number.The triangular diffraction method is used to detect the topological charge of the vortex beam produced by the few-mode fiber.The triangular diffraction experiment shows that the vortex beam topological charge generated by few-mode fiber is l=+1.The results show that:The use of optical fiber to generate vortex beam is related to the radius of the core fiber,the difference of the inner and outer refractive index,the incident wavelength,the incident angle,and the off-axis offset;A few-mode fiber can generate a first-order vector mode.After vector mode superposition,a first-order vector mode in a few-mode fiber is converted into a first-order vortex beam with a helical phase wavefront;After triangular diffraction,the vortex beam generated in the fiber is similar to the Laguerre-Gaussian beam,it is easy to draw from the triangular diffraction pattern that the vortex beam topological charge generated by the few-mode fiber is l=+1.