SiO2 phase gratings fabricated by UV laser ablation patterning

Patterning of SiO 2 materials like fused silica, crystalline quartz, or SiO 2 -thin films by laser ablation is very challenging based on their high transparency in the whole spectral range from the deep UV to the near IR. Therefore laser microfabrication of UV-optical elements, e.g. silica micro lenses is rather difficult. In those cases where only a shallow surface relief on a silica slab is required, like for instance in the case of phase gratings, a new approach provides a flexible solution. The novel fabrication process consists of three steps. First, a silicon suboxide coating (SiO x with x < 2) with a predefined thickness is deposited on a fused silica substrate. Second, utilizing its strong UV-absorption, this coating is removed in defined areas by excimer laser ablation at 193 nm or 248 nm leading to the desired phase pattern in form of a binary depth profile. Third, by applying a thermal annealing process, the remaining SiO x -coating is oxidized to UV-transparent SiO 2 . This results in a SiO 2 -surface relief element with excellent UV-transmission. The precisely defined interface between substrate and layer allows for ablation with exact depth control and perfect optical surface quality. Various irradiation concepts for the fabrication of low line density gratings (period ~ 10 microns) and high line density gratings (period < 1 micron) are demonstrated. The fabricated phase masks can be used in various laser applications like fabrication of Bragg gratings in optical fibers or micro- and nano-patterning by high power laser ablation.