In a manner similar to the ray-based wavefront calculations, POP uses a reference so that the sampling requirements can be reduced. Using a reference means that ZEMAX needs only carry the 'difference compared to the reference' phase rather than the absolute phase of the beam. In POP, this reference is computed surface by surface, by using a pilot beam, which is the best-fit Gaussian beam that matches the beam being launched.

The pilot beam is then propagated from surface to surface using the Skew Gaussian beam calculation. At each surface, new beam parameters, such as the new waist, phase radius, or position are computed. The properties of the pilot beam are then used to determine if the actual distribution is inside or outside the Rayleigh range, and what propagation algorithms are appropriate.

However, the pilot beam approach suffers the exact same problem as the reference sphere when the beam is split into multiple beamlets. The phase reference for one beamlet is not a good reference for its neighbor. So in this case, we turn off the use of the pilot beam reference, and use a plane (flat) wavefront reference instead (note if the beam has an existing phase radius of curvature we could choose to use that instead).

Just double-click on the Lens Array surface, go to the Physical Optics tab, select Output Pilot Radius, and set it equal to Plane (this is already done in the sample file provided):



The phase of the input beam is now measured relative to a plane, instead of the best-fit Gaussian. The sampling of the beam needs to be set adequately high, of course, and this can be tested by looking at the phase of the beam after refraction through the lenslet array surface:



The beam can then be propagated to the image surface, and an array of spots with the correct relative intensity and diffraction structure can be seen:



although the diffraction structure is easier to see when logarithmic scaling is applied: