Optical angular momentum and mode conversion in optical fibres with competing form and material anisotropy

We study the mode conversion and the transformation of the optical angular momentum (AM) in strongly elliptical strongly anisotropic weakly guiding fibres with either parallel or orthogonal orientation of the axis of material anisotropy and the major axis of the deformation ellipse. The structure of l = 1 modes and the polarization corrections to the scalar propagation constant are obtained. The cases of comparable by order and equal values of form and material anisotropies are studied. It is shown that these two types of anisotropy even if applied simultaneously exert a well-distinguished separable effect on the AM of the field. The form anisotropy changes the orbital AM of the transformed beam while the material anisotropy changes its spin AM. Conversion of circularly and linearly polarized optical vortices is studied. It is found that each type of anisotropy independently affects the beam conversion in such fibres: the form anisotropy changes the angular distribution of the field, whereas the material anisotropy changes its polarization state. In the case of equal form and material anisotropy constants the fibre is found to operate as an 'omniconverter' of the AM: it changes on the same conversion length the sign of both the spin and orbital AM of the incident beam.