Excimer laser patterned dielectric masks for the fabrication of diffractive optical elements by laser ablation

Masks for laser processing are generated by laser ablation patterning of di electric layer systems. The application of these masks for the rapid fabric ation of diffractive optical elements (DOEs) is presented. The diffractive optical elements are designed as phase-only elements, assum ing an illumination with a plane wave. A continuous phase function is calcu lated using an iterative Fourier transform algorithm (IFTA). This continuou s phase function is reduced to two or four levels by an iterative Fourier q uantisation algorithm (IFQA) that is able to include focal power. The fabrication of the DOE is performed in a two-step process. First, a bin ary amplitude mask (or a set of masks for multi-level DOEs) is made by stru ctured ablation of a highly reflective dielectric coating (HR 248 nm) from a fused silica substrate. This is accomplished by using an ArF excimer lase r emitting at 193 nm, a wavelength that is sufficiently absorbed in the HfO 2/SiO2-dielectric layer system, leading to precisely ablated mask structure s. In the second step, this mask is used in a 4:1 projection configuration to generate a surface profile in a polymer substrate by ablation at 248 nm. The depth modulation can be defined by adjusting laser fluence and pulse n umber. Examples of DOEs ablated in polycarbonate are shown and their performance i s characterised.