Polymeric graded-index lenses via digital light processing vat photopolymerization additive manufacturing

Traditional methods of manufacturing graded-index (GRIN) lenses, such as chemical vapor deposition, ion exchange processes, and neutron irradiation, are time consuming, labor intensive, and cannot yield custom refractive index profiles. Additive manufacturing (AM) methods provide a potential alternative route to produce polymeric GRIN lenses easier as well as reduce costs. To date, AM approaches for GRIN lens production include inkjet 3D printing and multiphoton direct laser writing, which suffer from difficulty in material formulation and extremely large print times for macro scale lenses. This work seeks to create GRIN lenses using digital light processing (DLP) vat photopolymerization (VPP) AM. The main challenge of DLP VPP AM is related to the uncontrolled light penetration that induces additional curing in previously printed layers, which makes the control of conversion difficult. We adapted a conversion prediction model that was previously developed to predict the conversion profiles in printed parts by DLP VPP based on accumulated light dosage. By studying the index of refraction as a function of polymerization for various resin formulations, in combination with the conversion prediction model, we were able to utilize the grayscale capability of DLP to generate GRIN profiles through partial polymerization. This model was used to generate the 3D printing parameters needed to maintain proper extent of cure throughout the entirety of the print. These prints were then characterized to determine both the accuracy of the print design and the optical performance of the lens.