Towards freeform microlens arrays for near infrared astronomical instruments

A key requirement for astronomical instruments in next generation Extremely Large Telescopes (ELTs) is the development of large-aperture Integral Field Units (IFUs) that enable the efficient and spatially contiguous sampling of the telescope image plane for coupling stellar light onto a spectrometer. Current IFUs are complex to fabricate and suffer from stray light issues, which limits their application in high-contrast studies such as exoplanet imaging. In this paper, we present our work on the development of freeform microlens arrays using the rapidly maturing technique of ultrafast laser inscription and selective wet chemical etching. Using the focus spot from a femtosecond laser source as a tool with an essentially unrestricted "tool-path", we demonstrate that it is possible to directly write the surface of a lenslet in three dimensions within the volume of a transparent material. We further show that high surface quality of the lenses can be achieved by using an oxy-natural gas flame to polish the lens surface roughness that is characteristic of the post-etched structures. Using our technique, the shape and position of each lenslet can be tailored to match the spatial positioning of a two-dimensional multimode fiber array, which can be monolithically integrated with the microlens array.