Phase vortices from a Young's three-pinhole interferometer
  An analysis is presented of the phase vortices generated in the far field, by an arbitrary arrangement of three monochromatic point sources of complex spherical waves. In contrast with the case of three interfering plane waves, in which an infinitely-extended vortex lattice is generated, the spherical sources generate a finite number of phase vortices. Analytical expressions for the vortex core locations are developed and shown to have a convenient representation in a discrete parameter space. Our analysis may be mapped onto the case of a coherently-illuminated Young's interferometer, in which the screen is punctured by three rather than two pinholes. 
Gary Ruben & David M. Paganin (School of Physics, Monash University)