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25 August 2015 Beam shaping concepts with aspheric surfaces
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Employing aspherical lenses to reduce aberrations to improve focusing qualities is a well-known concept. But the potpourri of aspheric surfaces offers way more possibilities, even the chance for flexible beam shaping setups. Since these are refractive optical elements the beam shaping is robust with respect to wavelength changes. Being able to generate ring shaped light distributions is interesting for various applications. Here the most uncommon aspheric surface – an axicon – is utilized. Axicons are glass cones, which convert the incoming light into Bessel beams. Those are characterized by their concentric ring structure and long depth of focus, which makes them very interesting for material processing. Above that, combining such an axicon with a focusing lens leads to a ring focus. Its size is not only determined by the choice of the axicon angle and the lens properties, but also through the diameter of the incoming beam. Thus, for optimal usage a flexible beam expander, which leaves the beam quality unaltered is mandatory. The remarkable properties of these beam shaping set-ups are shown in theory and experimentally. Furthermore, a configuration consisting of two axicons and interchangeable focusing lenses is presented. For one thing, this set-up allows for tuning of the working distance in any required way for optimal material processing. For another thing, the induced chromatic focal shift of the focusing lens that occurs by a change of wavelength can be compensated for by adapting the distance of the axicons and leaving the working distance unchanged.
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U. Fuchs, D. Braun, and S. Wickenhagen "Beam shaping concepts with aspheric surfaces", Proc. SPIE 9581, Laser Beam Shaping XVI, 95810L (25 August 2015);

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