Zonal diffraction efficiencies and imaging of micro-Fresnel lenses

Abstract

The dependence of the zonal efficiency of a transmission Fresnel lens on the number of quantization levels and on the polarization of the light, without and with an antireflection (AR) overcoating is investigated. The height of the grooves with a blaze structure is locally optimized for maximum efficiency. A decreasing structure height over the zonal radius from the centre to the outer zones improves the efficiency because of a better-adapted blaze. An additional increase in the efficiency can be obtained by increasing the AR coating thickness over the zonal radius from the centre outwards. Furthermore, the point spread functions (PSFs) for several quantization levels are compared for both polarization cases. While the AR overcoating has nearly no effect on the shape of the x polarization PSF, the y polarization PSFs show differences between both AR coating and no coating and is reduced with an increasing number of levels. Finally, the comparison between the measured efficiency of coated silicon Fresnel lenses and a calculation lead to acceptable results. All calculations are performed with the integral equation system method with parametrization of the grating profile, an exact electromagnetic grating diffraction method for arbitrary profile shapes.