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PHASE MASKS MADE OF BIREFRINGENT PLATES FOR SHAPING LASER BEAMS WITH ULTRASHORT PULSES FOR LASER MATERIAL PROCESSING IN THE IMAGE PLANE

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Phase masks made of birefringent CaCO3 crystal plates are developed to create laser beam with a given shape and intensity distribution close to flat-top in image construction scheme. The principle of phase masks operation is based on creating the phase shift of  or 2 (depending on the initial plate thickness) in linearly polarized radiation passing through etched areas with given shapes. The phase shift in these areas transforms into the intensity distribution at an analyzer output, which can be projected with a demagnification by a high-quality lens into its image plane aligned with the micro-processing plane (target). Phase masks in the form of a square and a square in a square are made by processing optically transparent materials with laser-induced microplasma and successfully tested in an experimental setup in an imaging scheme with a laser emitting pulses of 120 ns duration at the wavelength of 1.06 μm. Phase masks are also used in this experimental setup for laser ablation of polished steel samples. The shapes of footprints on samples well match the formed beams shapes.

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