Structural stability of a nonparaxial singular mode beam

The exact explicit solution of the Maxwell equations for nonparaxial singul ar 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 variati on of the Rayleigh length for a mode beam does not change the structure of phase and polarization singularities; it merely transforms their coordinate s. In the paraxial limit, the singularities are shifted off the axis to reg ions with negligible light fluxes. (C) 2001 MAIK "Nauka/ Interperiodica".