The results of an investigation into beam shaping techniques for generating near field Gaussian intensity patterns from uniform intensity ("flat top") laser beams are presented. The motivation for this study was the desire to produce realistic High Energy Laser (HEL) far field intensity profiles for laser effects testing, without propagating the large distances necessary to obtain the true far field pattern and beam size typical of HEL target engagement scenarios. To minimize cost, maximize wavelength diversity, and provide a high laser damage threshold capability, an all reflective optical system was preferred.
Though beam shaping systems are commonly used to convert Gaussian beams to flat tops beams, the reverse problem, that of converting flat tops to Gaussians, appears to be new territory. Most beam shaping approaches, particularly those that do not preserve phase, are not reversible. Two simple approaches that use segmented mirrors for converting flat tops to Gaussians are described here. While beam integrators, commonly used to convert Gaussians to flat tops do not work in reverse, the approaches presented use segmented mirrors resembling beam integrators, and have some similar benefits.
Geometric and physical optics analyses are presented to illustrate the performance characteristics of the different approaches at wavelengths of 1.315 and 3.8 microns. A simple method to reduce interference effects in the reshaped beam, that are present when a coherent source is used, are discussed.