Computer generated microlenses with high efficiency

Blazed Fresnel zone microlens arrays were fabricated by ion-beam-sputter deposition technique on different substrate materials. The lenses have circular and square apertures with the smallest width of 0.2 mm and different focal lengths for the wavelengths of 1.52 mu m and 0.63 mu m respectively. The kinoform profile in each zone of the Fresnel zone lenses was approximated by a sixteen level profile. Such stepped profiles were realized with several masks, written with electron-beam and with photolithographic technology. The effects of fabrication errors, such as level heights, alignment and linewidths errors, on the diffraction efficiency were discussed. The microlenses were coated with an antireflection coating. The reflection of these lenses was analyzed on the basis of an angular spectrum approach. A minimum reflectivity as low as 2*10-4 was realized using in situ controlled multilayers of TiO2 and SiO2. Our measurements reveal, that the spot-sizes of the fabricated microlenses are close to the diffraction limited values. The highest measured diffraction efficiency for the sixteen level structure is 96%.