(a) A set of CALs (dashed lines) of two interfering waves and . By adding the third wave—wave —a regular vortex lattice is generated. The arrows a, b, and c are phasors of wave , , and , respectively. The OVs appear at points where the three phasors form a triangle. The whole pattern can be divided into two subsequently appearing regions: the region with positive vortices and the region with negative vortices, respectively. The representing angle value 0 and (bold lines) are borders between these two regions. Vortex points are marked by black dots. The four neighboring vortices of the same topological charge form an equilateral basic cell (dash dotted blue line). (b) The ends of three non-collinear wave vectors define the OVI plane. The optical vortex generated by the three-plane waves moves along the straight line (dashed line) perpendicular to the OVI plane. When adding one more wavevector (blue in this Figure) the OVI plane cannot be defined, but in very specific cases.
