Transformation of optical vortices in elliptical and anisotropic optical fibres

We study theoretically the process of conversion of the topological charge of optical vortices upon their propagation in anisotropic and elliptical weakly guiding optical fibres. We demonstrate that in addition to the well-known mechanism of topological charge conversion due to the elliptical form of the fibre's cross-section such a conversion can also be caused by the spin-orbit interaction. We show that in strongly anisotropic fibres, in which the other mechanism is prohibited, this interaction leads to charge conversion of both linearly and circularly polarized optical vortices. We also apply our theory to the description of experiments on mode conversion in fibres (McGloin et al 1998 Appl. Opt. 37 469, Carpentier et al 2006 J. Eur. Opt. Soc. Rap. Publ. 1 06031) and demonstrate that at the initial stage of fibre stressing the most probable reason of mode conversion is stress-induced photoelastic anisotropy of the fibre's core.