Physical Optics Propagation is a powerful tool for studying diffraction propagation of a laser beam. Often, we see users ask questions on how to increase the size of the display area of the POP beam, or why the width of the display area differs from the X and Y width entered in the Beam Definition tab. The X-width and Y-width entered in the Beam Definition tab in POP analysis window is only the initial array size, or the starting grid size. As the beam propagates, its size may change, and the grid size must be able to adjust itself to accommodate the beam.
There are two algorithms used for propagating the beam. The grid size adjustment depends on which propagation algorithm is used. POP uses Pilot beam to determine the beam position, inside or outside the Rayleigh range. Depending on the beam position, POP chooses the proper algorithm. By default, Angular Spectrum is used for propagation inside Rayleigh range, where beam size does not change significantly so grid size also remains constant. Fresnel diffraction propagation is used for propagating from inside to outside or outside to inside Rayleigh range. In this case beam size may change significantly so grid size also needs to be adjusted to avoid beam “outgrowing” the grid. One thing to pay attention to is the inverse relationship between the grid width at previous surface and current surface. When a beam propagates a long distance from focus to far from focus, it’s essentially a Fourier transform of the beam. In Fourier transforms, the resolution at a surface is inversely proportional to the grid width at the previous surface. In other words, the grid width in one space determines the pixel size in the following space, so a wider grid size at the first surface results in a higher resolution at the second surface.
If you go to Surface Properties dropdown and click Physical Optics tab, you can choose to resample the beam at a given surface by adjusting the grid size and resolution. However, keep in mind that changing grid size and point spacing on current surface may have an impact on sampling at subsequent surfaces. It’s recommended to check the Propagation Report for any warnings of low sampling, as well as check beam phase profile surface by surface to eliminate phase aliasing due to insufficient sampling.
If you are interested in learning more about POP sampling, which is the key to proper POP usage, we recommend reading this series of Knowledgebase articles written by Erin Elliott. They work great as an introduction to new POP users.
Using Physical Optics Propagation (POP), Part 1: Inspecting the beams – Knowledgebase (zemax.com)
