Optical Analysis Of Segmented Aperture Averager

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Abstract
High energy lasers with a non-uniform beam distribution are used to study materials properties. When it is required that the beam intensity distribution be uniform, beam averaging schemes have to be used, such as a segmented aperture averager. It consists of a set of small mirrors located on the surface described by an equation of second order. In order to determine the optical performance of the segmented aperture averager, a unit amplitude wave is assumed in the analysis. The averaging is accomplished because the beam is spatially multiplexed, resulting in the superposition of beam segments originally at different locations. Two phenomena con-tribute to the final intensity distribution: diffraction and interference. The uniform plane is located in the Fresnel zone, producing a Fresnel diffraction pattern with the intensity distribution dependent primarily on the Fresnel number. Due to the spatial superposition of beam segments, multiple beam interference pattern results because of the interference of light directed from each segment. Multiple beam interference pattern is characterized by high spikes separated by areas of low intensity with many points of zero intensity. The intensity pattern due to the segmented aperture averager is a multiplication of these two effects. The envelope of the intensity distribution is provided by the Fresnel diffraction pattern, while the spike distribution is specified by the interference parameters. The beam uniformity is achieved as an average over the area of several intensity spikes. This beam averaging procedure can only be used if a finite resolution area for intensity uniformity is required.

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