Generation and Conversion of Higher Order Optical Vortices in Optical Fiber with Helical Fiber Bragg Gratings

Helical fiber Bragg grating (H-FBG) induced by UV-side exposure exhibits wavelength-dependent coupling between different modes. In this paper, we propose an effective method to generate high-order optical vortices (OVs) using H-FBG written in a multimode fiber. We simulate the conversion of OVs with different topological charges: 0 → ± 1, ±1 → 0, 0 → ± 2, 0 → ± 3 with conversion efficiency as high as 97%. We prove that higher order topological charge can be obtained simply by changing the parameters of the fiber to increase the number of modes in the fiber. We also analyze the conversion of H-FBG under various conditions of the induced asymmetric index profile and the length of H-FBG.