Flat-top beams are a light beam with a relatively flat intensity profile -- in contrast to Gaussian beams, for example. They often have a super-Gaussian shape.

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flat-top beams, explained by RP; top-hat beam, supergaussian, intensity profile, beam shaper

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a) Top-hat beam profile (thick gray line) at the focal plane and the

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Saturation effects in nonlinear absorption, refraction, and frequency conversion: a review - ScienceDirect

Process Overview DEGRADATION AND SURFACE ENGINEERING

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How to design a Gaussian to Top Hat beam shaper – Knowledgebase

Understand the difference between Tophat beam and Gaussian beam in one article