Segmented Fresnel zone lens elements with several primary foci

The use of holographic elements containing segmented Fresnel zone lenses to generate more than one primary focus is investigated. Elements are designed to provide tolerance to misalignment in free space optical interconnection architectures where this is a key consideration. The kinoform profile is approximated in multi-level form in quartz glass using micro-structuring technologies. Elements are of 2 mm diameter, f/5 performance and intended for use at 1.52 mu m wavelength. To optimise their efficiency they are provided with anti-reflection coatings by ion beam sputter deposition. Diffraction efficiencies are 70% for 8-level structures divided between two focal spots, and 36% for 2-level structures generating four foci. The measured spot diameter is 21 mu m between first minima. Tolerance to misalignment is demonstrated.