Alignment test of two sided DOEs with diffractive Lohmann lenses

Diffractive optical elements (DOEs) gained high importance in optical design due to their versatile phase profiles and easy reproduction in large numbers. A limiting factor of DOEs is the minimum feature size which defines the maximum diffraction angle of the element. To realize larger angles, e.g. for higher numerical apertures, 2 DOEs can be combined. This makes an alignment of both DOEs necessary. By structuring both surfaces of a single substrate, the number of elements is reduced and the alignment is integrated in the precise fabrication process. We analyze the combined phase function of optical test elements on the front and back surface of a wafer to test the alignment accuracy. As optical test elements Lohmann lenses are used. These freeform elements generate in combination a cylindrical lens profile with an optical power proportional to the lateral displacement of the elements. This way, the lateral displacement of front and back structures can be evaluated separately in x and y by measuring the focal length or analyzing the wave front curvature of the transmitted beam e.g. with an autofocus sensor.