Bi-directional high-order orbital angular momentum mode generation using gratings in high numerical aperture fiber

Abstract

We propose a novel design and theoretically analyse it for bi-directional orbital angular momentum (OAM) mode generation in high numerical aperture (NA) optical fibers using the combination of a long period grating (LPG), a short-period (Bragg) grating and a three-port optical circulator. The proposed design consists only of standard fiber gratings rather than specially designed fibers, such as ring-shaped or other types of fibers, making our model relatively simple. Full-vector true-mode analysis has been used to study the modal characteristics of the supported modes in the high-NA fiber. High-NA optical fibers can effectively relieve the degeneracy of the linearly polarized modes. A circularly polarized (CP) light is injected into a single mode fiber, the output end of which is axially spliced into a high-NA fiber. An appropriately designed raised-cosine apodized tilted LPG (TLPG) is used for broad bandwidth coupling of the fundamental HE11 mode to the co-propagating HE21 mode. The employed TLPG converts the CP fundamental core mode (HE 11x ± iHE 11y ) to ±1 order OAM modes (HE 21even ± iHE 21odd ). Both the CP mode and the generated OAM pass through the circulator in the direction of port 1 to port 2. A second fiber is connected at the output of port 2 and a tilted fiber Bragg grating (TFBG) is used to couple the forward-propagating HE11 core mode with the counter-propagating HE21, EH11 and HE31 core modes. The TFBG reflected higher order (±1 and ±2) OAM modes are transmitted through port 3. We observe that the TFBG has a mode-conversion efficiency of more than 95 % for each mode conversion. The generated vortex beam topological charges have been verified using the coaxial interference method.