Structure of modes with greater than unity azimuthal number in twisted anisotropic optical fibers

We solved the vector wave equation to obtain modes with azimuthal number 2 of weakly guiding twisted anisotropic optical fiber. We considered the mutual action of linear anisotropy of the fiber material, twisted mechanical stress, and spin-orbital interaction. We established that modes with azimuthal number 2 are represented by linearly or circularly polarized optical vortices depending on the interplay between anisotropy of the material and the twisting/ spin-orbit-interaction effect. We established analytical expressions for the corresponding propagation constants at arbitrary fiber parameters, such as a linear anisotropy, twisting, and spin-orbital-interaction. We demonstrated the orbital birefringence and optical Zeeman effect for high-order modes with azimuthal number 2 in twisted anisotropic fibers.

I. Introduction

Since the discovering of optical vortices (OVs) [1], such unique optical beams are of great interest to many researchers due to their features and opportunities for its application. OVs appear the promising tools for data encoding, controlling and transmitting [2]–[4], for trapping and manipulating microparticles [5], for some tasks in astronomy [6], [7], etc. Nowadays, one of the main trends of OV applications is associated with the possibility of the data encoding by the values of orbital angular momentum (OAM) [8]–[10] of OVs. Such a technique gives great potential for using of OVs in the information technologies. Indeed, since OAM is determined as ℓℏ per photon, where the topological charge of OVs ℓ = 0, ±1, ±2, ...,, that allows one to encode information by the theoretically unlimited degrees of freedom of light. Such an approach gives the ability of a significant increase i n data-carrying capacity [10], especially when combined with the well-known techniques using amplitude, phase, and polarization of the beam. Moreover, the data encoding in degrees of freedom of light (values of OAM) allows one to achieve the principally new level of data protection [11].