Optical vortices and the higher order modes of twisted strongly elliptical optical fibres

Abstract

We investigate the structure of the higher order modes in uniformly twisted weakly guiding strongly elliptical optical fibres. An analytical solution of the vector wave equation for this problem is presented based on the effective reduction of a twisted fibre equation to a straight fibre one. On the basis of the perturbation theory the structure of the higher order elliptical screw modes and polarization corrections to the scalar propagation constant are found as functions of the pitch value and the fibre parameters. It is demonstrated that at a certain rate of twisting the l = 1 modes of such fibres are presented by two pairs of almost pure arbitrary polarized optical vortices. The orbital angular momentum of the l = 1 modes is studied as a function of pitch.