Structural stability of a nonparaxial singular mode beam

The exact explicit solution of the Maxwell equations for nonparaxial singular beams propagating in free space or in a homogeneous isotropic medium is considered. It is shown that, in the paraxial approximation, such solutions for mode beams of both lower and higher orders may turn into the solutions for guided modes or vortices of optical fibers. It is found that a variation of the Rayleigh length for a mode beam does not change the structure of phase and polarization singularities; it merely transforms their coordinates. In the paraxial limit, the singularities are shifted off the axis to regions with negligible light fluxes.