We will now look at using Physical Optics Propagation to model near field diffraction effects that occur when a uniform beam is truncated by an aperture.

Open the file "Samples > Physical Optics > Gibbs Phenomenon.zmx".

Notice the "U" next to the Semi-Diameter of the stop surface in this example. This indicates that a fixed, hard aperture has been placed on this surface. The radial size of the aperture is equal to the Semi-Diameter of the surface. Thus, in this case, the aperture is a circular aperture with a radius of 0.1 mm.

The file will open with two cross section POP windows. One window shows the POP output at surface 1 while the other window shows the results at the image surface (surface 2). Both windows have the same beam defined. The only difference is the "End Surface" defined in the "General" tab of the settings dialog. The "Beam Definition" tab shows that a uniform beam with a waist size of 0.1 mm has been defined using the "Top Hat" beam type.



The left cross section plot shows the uniform beam amplitude prior to the aperture. The right plot shows the characteristic ringing from diffraction that is seen at the edges of the beam a short distance after the aperture.

  

This ringing will not be predicted by geometrical ray tracing. Physical Optics Propagation is required to model such effects.

TIP:  Since Physical Optics Propagation propagates arrays of complex amplitude, the phase of the beam can be displayed as well. To view the phase output using POP, simply change the "Data" setting in the "Display" tab of the POP settings dialog to Phase.