Rethinking alignment for systems that incorporate freeform optics

The alignment of optical systems that incorporate components with freeform surfaces provides a particular challenge. Conventional methods that use axisymmetry are not adequate, and there are too many degrees of freedom to efficiently adjust for building production systems. The obvious solution is to make the systems and the components accurately with well-defined mating interfaces, so no adjustment is required. But in the absence of alignment feedback that references these datum features, the location of an optical surface is degenerate with its form — they have equivalent effects on the optical system. For example, a decenter of a spherical surface is equivalent with a shape change that looks like wedge in the surface. Freeform optics have complicated degeneracies for all degrees of freedom that require the part to be made with simultaneous control of the optical surface AND its location. Such parts can be manufactured using feedback from a measurement system that uses a metrology platform with mating interfaces that accurately match those for the assembly. Scanning systems can measure features on the metrology platform and tie them to the surface form. The use of interferometry with computer-generated holograms can make a snapshot measurement of both the surface form and the six degree of freedom location of the metrology platform.