  We report the results of studies of torsion effect on the optical
birefringence in LiNbO3 crystals. We have found that twisting of those crystals
causes a birefringence distribution revealing non-trivial peculiarities. In
particular, it has a special point at the center of cross section perpendicular
to the torsion axis where zero birefringence value occurs. It has also been
ascertained that the surface of the spatial birefringence distribution has a
conical shape, with the cone axis coinciding with the torsion axis. We have
revealed that an optical vortex, with the topological charge equal to unity,
appears under the torsion of LiNbO3 crystals. It has been shown that, contrary
to the q-plate, both the efficiency of spin-orbital coupling and the orbital
momentum of the emergent light can be operated by the torque moment.
