Fabrication of diffractive optical elements by ArF-laser ablation of fused silica

Excimer laser ablation at 193 nm is used for the generation of a surface relief structure on fused silica with the aim to fabricate diffractive optical phase elements to be applied for efficient laser materials processing. Though there are several problems to achieve high aspect ratio material removal at this laser wavelength (193 nm) in a scarcely absorbing material (fused silica), ablation of sub-mum depth, as it is needed to generate the appropriate phase delays of diffractive phase elements (DPE) for UV-applications, is possible without cracking. Applying a machining process corresponding to a bitmapped DPE design generated by an Iterative Fourier Transform Algorithm (IFTA), the fused silica surface is patterned on a 128 x 128 pixel(2) area with a pixel size of 12.5 x 12.5 mum(2). The step height of this two level DPE has to be adapted to the combination of the wavelength, at which the element will be applied, and the refractive indices of the DPE-material and the environment. The example DPE is designed as a 10 x 10 focus raster generator for a UV-femtosecond laser operating at 248 nm. Using this DPE in combination with a 25 x Schwarzschild objective, the parallel drilling of micron sized holes in stainless steel and other metals is demonstrated.