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

Abstract

In this paper we propose a novel design and theoretically analyse it for bi-directional orbital angular momentum (OAM) mode generation in high
numerical aperture (NA) optical fiber using combination of long period grating, short-period (Bragg) grating and a three port optical circulator. The proposed design consisting only standard fiber gratings rather than specially designed fibers, like ring-shaped fiber or other types of fiber, makings our model relatively simple. The 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 fiber can effectively relieve the degeneracy of the linearly polarised modes. A circularly polarised (CP) light is injected into a single mode fiber, the output end of which is axially spliced to a high NA fiber. An appropriately designed raised-cosine apodized tilted long period grating (TLPG) is used for broad bandwidth coupling of the fundamental HE11 mode to co-propagating HE21 mode. The employed TLPG converts the circularly polarised fundamental core mode (HE_x11 ± iHE_y11 ) to ±1-order OAM modes (HE_even 21 ± iHE_odd21 ). 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 the 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 transmit through the port 3. We have observed that the TFBG has a mode conversion efficiency of more than 95% for each mode conversion. The generated vortex beam’s topological charges have been varified using the coaxial interference method.