Quadrefringence of optical vortices in a uniaxial crystal
  The splitting of a single optical vortex into four separate ones in a singular beam is theoretically and experimentally described for the propagation of light obliquely through a uniaxial crystal. Also we found the condition under which the new-born vortices in each four individual beams propagate independently without dislocation reactions and have different locations in all beams for any crystal lengths. 
T. Fadeyeva, A. Rubass, Yu. Egorov, A. Volyar, G. Swartzlander Jr