Controlling the optical angular momentum by elliptical anisotropic fibres

We study theoretically the possibility to control the optical angular momentum (AM) by anisotropic and elliptical weakly guiding optical fibres. We study the transformation of spin and orbital AM of an incident beam presented as an arbitrary superposition of optical vortices (OVs) with l = 1 orbital number. For fibres with a simultaneous presence of form and material anisotropy we show that the orbital AM of the outgoing beam is changed by form birefringence, whereas its spin constituent changes in fibres with embedded material anisotropy. We demonstrate that the AM of the outgoing beam can be controlled by the fibre's rotation about its axis. In this case the total AM in the cross section consists of two parts, one of which is invariant under rotation and the other harmonically depends on the rotation angle. As an example we show that the adimensional orbital AM of the transformed beam by a rotated elliptical fibre input Hermite-Gaussian HG(01) beam can vary from +1 to -1. This process is accompanied by the inversion of the topological charge of the generated OV.