Potential Applications of Computed Axial Lithography in Manufacturing Optical Elements

There are many applications of optical elements for which rapid fabrication of freeform geometries is desirable: among them, ophthalmic lenses, beam splitters and shapers, and components of custom diffractive displays. Additive manufacturing (AM) with photopolymers is of interest for these applications. However, the requirements of optics manufacturing are especially challenging for AM to meet. Requirements for surface roughness, dimensional tolerancing, and refractive index homogeneity are more stringent than most AM processes can offer.<br/><br/>The volumetric AM process of computed axial lithography (CAL) may offer a practical route to optical prototyping and manufacturing. CAL defines 3D geometries in photosensitive materials in a single processing step by tomographically synthesizing a light dose distribution. A projected light pattern evolves in synchronization with rotation of the light-sensitive volume to enable temporal superposition of the dose. Where the dose exceeds a material-dependent threshold, conversion to the solid component takes place. Unlike most AM processes, CAL is layer-less, and so can produce smooth surface finishes as low as sub-10 nm r.m.s.; it is also extremely rapid because of the absence of relative motion between part and resin and hence the elimination of hydrodynamic stresses during light exposure. Typical illumination times for centimeter-scale components are 30 s to several minutes. We show the results of using CAL to print plano-convex lenses, Fresnel geometries, and lenslet arrays, among other structures, and discuss the performance of the CAL process in relation to these geometries.<br/><br/>We discuss the possible role of the orientation of the target geometry within the printing volume on the quality of obtained components. We explore the factors influencing a parasitic optical waveguiding effect that is sometimes observed within the printing volume. We further consider possibilities for real-time imaging of the profile of lenses during printing to adjust the delivered dose and compensate for inhomogeneities that may emerge in fabrication. Finally, we discuss the possibilities that arise from being able to immerse pre-existing solid objects into the CAL printing volume. This “overprinting” capability may ultimately offer a path to incorporating sensors or active display elements into printed optics.