Ion beam manufacturing of a graded-phase mirror for the generation of square “top hat” laser beams
Abstract
Spatially-engineered “top-hat” laser beams are used in solid-state high-energy lasers in order to increase the energy extraction efficiency in the amplifiers. To shape the laser beam, an efficient alternative to serrated apertures is to modify a laser cavity so that it naturally generates this “top-hat” beam, replacing a mirror of the laser cavity by a graded phase mirror. Its complex shape can be approached by microlithographic techniques based on an iterative mask and etch technique, but many steps are required to avoid large phase steps. The broad-beam ion-etching technique is well suited to manufacture such surfaces, with a good precision and a perfectly smooth surface. We shall present the technique we used for square top-hat beam generation. We shall detail the mask optimisation, combining simultaneous simulation of the ion etching and the beam build-up in the front-end laser. We shall present the results of the surface testing and the final test of the component in the laser.