Generation and characterization of ultra-precision compound freeform surfaces

Two-photon grayscale lithography for free-form micro-optical arrays

Compared to standard rotationally symmetric macroscopic optical components, free-form micro-optical arrays (FMOAs), sometimes termed microstructured optical surfaces, offer greater design freedom and a smaller footprint. Hence, they are used in optical devices to deliver new functionalities, enhanced device performance, and/or a greater degree of miniaturization. But their more complex surface shape is a challenge for traditional manufacturing technologies, and this has triggered a substantial effort by research institutes and industry to develop alternative fabrication solutions. Two-photon polymerization (2PP) is a promising additive manufacturing technology to manufacture 3D optical (micro)structures. The manufacturing times involved are, however, often impractically long, especially for the excellent surface quality required for optical applications. Recently, Nanoscribe GmbH has reduced manufacturing times substantially with the introduction of so-called two-photon grayscale lithography (2GL). However, its acceleration potential and consequent impact on surface quality have, to the best of our knowledge, yet to be reported. A direct comparison between 2PP and 2GL indicates that, for the investigated FMOA, 2GL is around five times faster than 2PP and also delivers better surface quality. This study therefore confirms the potential of 2GL to manufacture complexly shaped FMOAs.

Surface topology and functionality of freeform microlens arrays

Nonsymmetric (also known as freeform) optical components have attracted a great deal of academic and industrial attention due to the substantial benefits they have demonstrated in imaging and nonimaging optical systems. Additionally, freeform microlens arrays (FMLAs) are very promising with regard to the growing demand for device miniaturization and cost reduction. As a flip side, FMLAs entail specific challenges in design, manufacturing, and characterization. Here we report on the latter and present an innovative characterization strategy that makes it possible to assess the quality of FMLAs quickly and accurately. The precisely measured surface topology of FMLAs was accurately represented using nonuniform rational basis-spline (NURBS) and its optical response was predicted by means of ray-tracing simulations. We show that for reliably measured surface topology, the results are in excellent agreement with the experimental measurements. We also show that, compared to previous studies, illuminance levels displayed in a logarithmic scale are more adequate for low light levels and represent a closer match to nonlinear human visual perception. We believe that the method presented here will contribute to speeding up the FMLA manufacturing process, one of the current downsides of this promising technology.