Design and fabrication of a lightweight AR headset demonstrator using a buried Fresnel mirror combiner

A compact lightweight AR headset architecture is presented employing a semi-transparent Fresnel mirror combiner. As compared to a grating combiner this design approach minimizes diffractive color aberrations by working in a high order blazed grating regime. Two different technologies, gray scale lithography as well as anisotropic wet-chemical etching of silicon wafers were evaluated for manufacturing of the Fresnel element. A first monocular demonstrator was assembled and analyzed analytically, numerically as well as experimentally. A crucial point in the manufacturing of the Fresnel combiner element was the high degree of surface shape accuracy of the working Fresnel edges as well as the embedding polymer layer to ensure optical imaging quality. Currently, only the wet-chemical etching technology could provide sufficiently good results. Two main reasons for aberrations could be traced down: grating effects of the Fresnel structure and asymmetric refraction at the embedding layer. As a result, different kinds of color aberration will be generated. Additionally, the asymmetric refraction causes different types of distortion. Two approaches for compensating color aberrations could be derived: optical pre-compensation of the lateral color caused by the embedding layer as well as optimizing the Fresnel structure to minimize grating effects.