  A circularly polarized rotationally symmetric paraxial laser beams carries
hbar angular momentum per photon as spin. Focussing the beam with a
rotationally symmetric lens cannot change this angular momentum flux, yet the
focussed beam must have spin less than hbar per photon. The remainder of the
original spin is converted to orbital angular momentum, manifesting itself as a
longitudinal optical vortex at the focus. This demonstrates that optical
orbital angular momentum can be generated by a rotationally symmetric optical
system which preserves the total angular momentum of the beam.
