Geometrical ray tracing is an incomplete description of light propagation. Strictly speaking, the propagation of light is a coherent process. As a wavefront travels through free space or optical medium, the wavefront coherently interferes with itself. Modeling this coherent propagation comprises the domain of physical optics. Physical Optics Propagation (POP) is the capability of ZEMAX which uses diffraction calculations to propagate a wavefront through an optical system surface by surface. The coherent nature of light is fully accounted for by this capability.

When using POP, the wavefront is modeled using an array of points. Each point in the array stores complex amplitude information about the beam. The array is user-definable in terms of its dimension, sampling and aspect ratio.

To propagate the beam from one surface to another, either a Fresnel diffraction propagation or an angular spectrum propagation algorithm is used. ZEMAX automatically chooses the algorithm that yields the highest numerical accuracy. The diffraction propagation algorithms yield correct results for any propagation distance, for any arbitrary beam and can account for any surface aperture, including user defined apertures (UDAs).

POP applications include fiber coupling (single and multimode), diffraction propagation through any type of optical space, computing shifts in best waist focus position due to aberrations, and computing beam flux and irradiance on optical surfaces. Physical Optics Propagation can also be used for detailed analysis of arbitrary laser beam propagation through complex optics including M-squared calculations.