Focusing and correlation properties of white-light optical vortices
  We generate double-charge white-light optical vortices by sending a circularly polarized partially incoherent light through an uniaxial crystal. We show that the generated polichromatic vortices are structurally stable, and their correlation properties can be altered by the beam focusing, resulting in changes of the vortex core visibility. 
Vladlen Shvedov, Wieslaw Krolikowski, Alexander Volyar, Dragomir N.   Neshev, Anton S. Desyatnikov, and Yuri S. Kivshar