The results reported by Physical Optics Propagation (POP) may differ from those reported by ray-based analyses, such as the Spot Diagram, for several reasons. Namely, POP models a different type of propagation from traditional ray propagation in
that it is most commonly used to model Gaussian beams. Rays will travel in a straight line. That is, a collimated beam will never change size and a diverging beam will retain the same divergence everywhere. This is not the case for a Gaussian
beam, as it will diffract as propagates. Within the beam’s Rayleigh range, the size of a Gaussian beam changes slowly, and far beyond the Rayleigh range, the beam size changes linearly with propagation distance. Near the Rayleigh range, however,
the beam size will change nonlinearly with propagation distance. These three factors can all contribute to difference between a POP results and ray-based analysis results.
Check out this webinar for a detailed discussion on how and when (and when not) to use POP to model Gaussian beams in OpticStudio.