Multiple rings formation in cascaded conical refraction
  When a light beam passes through a cascade of biaxial crystals with aligned optic axes, the resulting transverse intensity pattern consists of multiple concentric rings. We provide a simple formulation for the pattern formation for both circularly and linearly polarized input beams, that could be applied for a cascade of an arbitrary number of biaxial crystals. We have experimentally investigated multiple ring formation with up to three cascade biaxial crystals, showing that the theoretical formulation is in full agreement with the experimental results. 
A. Turpin, Yu. V. Loiko. T. K. Kalkandjiev, and J. Mompart
physics.optics