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Hybrid micro-optical elements by laser-based fabrication of Fresnel lenses on the end face of gradient index lenses.

Fresnel lenses are fabricated directly upon the end face of gradient index (GRIN) lenses by F2-laser machining at 157 nm wavelength. The employed laser processing technique combines a mask projection configuration at 25-x demagnification with a rotation of the structured lens. The ablation characteristics of the GRIN materials require very high pulse fluences with typical values above 7 J/cm2. Topography measurements on the Fresnel lenses reveal a good contour accuracy with residual deviations from the design profile well below 100 nm. Such hybrid optical elements, combining GRIN lenses with diffractive lenses in one element, can serve as the basis for high-performance micro-optical imaging systems with diameters up to 2 mm. Examples of possible applications include imaging sensors like proximity sensors or color-corrected microscope objectives with high numerical aperture for endoscopy applications.