Ultrafast laser beam shaping for material processing at imaging plane by geometric masks using a spatial light modulator

•An ultrafast laser beam shaping technique for material processing is demonstrated.•Arbitrary beam intensity shapes can be created using a spatial light modulator.•The beam shape is obtained at near field by geometric masks.•The shape reconstructed at imaging plane has a size comparable to the beam waist.•The machined footprint on a metallic sample has the corresponding beam shape.

We have demonstrated an original ultrafast laser beam shaping technique for material processing using a spatial light modulator (SLM). Complicated and time-consuming diffraction far-field phase hologram calculations based on Fourier transformations are avoided, while simple and direct geometric masks are used to shape the incident beam at diffraction near-field. Various beam intensity shapes, such as square, triangle, ring and star, are obtained and then reconstructed at the imaging plane of an f-theta lens. The size of the shaped beam is approximately 20 µm, which is comparable to the beam waist at the focal plane. A polished stainless steel sample is machined by the shaped beam at the imaging plane. The shape of the ablation footprint well matches the beam shape.