High-aperture beams

Beams of a high angle of convergence and divergence are called high-aperture beams. Different ways of defining high-aperture generalizations to paraxial beams are reviewed for both scalar beams and electromagnetic beams. The different approaches are divided into three types. The particular examples of Gaussian beams and Bessel beams are discussed. For Gaussian beams, beams that exhibit a Gaussian variation in the waist necessarily include evanescent components, which rules out their use in describing propagation over all space. Generalizations of the definitions of beam width and the beam-propagation factor M2 for high-aperture beams are described. The similarities among the three types of high-aperture beams and the different models of ultrashort-pulsed beams are discussed.