Generation of cylindrically symmetric modes and orbitalangular momentum modes with tilted optical gratings inscribed in high numerical aperture fibers

TL;DR

This paper demonstrates how tilted optical gratings inscribed in high NA fibers can generate cylindrically symmetric modes and orbital angular momentum modes, with a simple method to calculate grating parameters for efficient mode coupling.

Contribution

It introduces a straightforward visual method to determine tilt angles and modulation periods for gratings to generate specific fiber modes in high NA fibers.

Findings

Tilted gratings effectively generate OAM and symmetric modes.

The method simplifies the calculation of grating parameters.

High NA fibers can produce diverse mode structures.

Abstract

Optical fiber with high numerical aperture (NA) can efficiently relieve the degeneracy of higher order linearly polarized modes. The degeneracy relief is investigated in two types of high NA fibers, i.e., low index cladding fiber and high-index-core fiber. A naked core fiber, as with low index cladding, can be used theoretically to generate the orbital angular momentum mode (OAMM) HE21 and the cylindrically symmetric modes (CSMs) TM01 and TE01. A high index core fiber incorporated with high contrast index structure can be used similarly to obtain OAMM HE31. The generation of both CSMs and OAMMs required tilted optical gratings to couple the fundamental core mode HE11 into these modes. The tilt angle and modulation period of the grating fringes can be calculated simply and visually with the method proposed in this article.