Waves, rays, and the method of stationary phase.
If one employs a diffraction-integral approach to wave propagation and diffraction, the connection between waves and conventional geometrical and diffracted rays is provided by the Method of Stationary Phase (MSP). However, conventional ray methods breakdown in focal regions because of the coalescense of stationary points. Then one may use the MSP to express the focused field in terms of aperture-plane Point-Spread-Function (PSF) rays. A tutorial review of these two ray techniques is given, and a number of applications are discussed with emphasis on the physical interpretation. Examples include focusing in free space or through a plane interface, plane-wave diffraction by a circular aperture, and diffraction of a Gaussian beam by a circular aperture followed by transmission into a biaxial crystal.
Stamnes
Jakob
J