Fabrication of diffractive phase elements for the UV-range by laser ablation patterning of dielectric layers

Ablation patterning of layered systems enables very precise ablation depth control. Within a certain process window, the ablation depth does not depend critically on the laser fluence, but is given by the layer thickness. This leads to a much better surface quality compared to bulk material ablation. Therefore, patterning of layer systems is a promising method for fabricating diffractive optical phase elements (DPE), which are given by a surface relief in a transparent optical material. The diffraction efficiency of these DPEs depends critically on the height of the relief steps. Using UV-transparent layers on fused silica substrates, the DPEs for UV applications are fabricated. The materials have to be selected in a way that high transparency at the operation wavelength, but sufficient absorption of the layers at the ablation wavelength is provided. Ta2O5 is a material transparent at 308 nm, but absorbing at 248 nm. Ablation patterning at 248 nm using a mask which contains the computer generated design leads to a DPE that can be applied at 308 nm for beam shaping or the generation of complex irradiation patterns. The fabrication of a two-level DPE and its performance in beam reconstruction is demonstrated. (C) 2002 Elsevier Science B.V. All rights reserved.