Fabrication of Functionally Graded Optical Components Using Additive Manufacturing

Rebecca Dylla-Spears (16-SI-003)

Project Description

Optical systems require proper management of thermal, optical, and mechanical properties to function as intended. Design choices must be made based on both the availability of optical components and the needs of the system. Ultimately, the final optical system design requires a balance between cost, schedule, weight, compactness, and complexity. Optics made from standard bulk materials fabricated by conventional methods often cost less. However, a single bulk material typically does not possess the desired combination of material properties such as thermal conductivity, density, stiffness, strength, transparency, or absorbance at a desired wavelength. Thus, designs using traditional components made from bulk materials may require the use of extra compensating optics, structures, or thermal management features that increase the system's complexity, footprint, and weight. In an effort to reduce system weight or improve efficiency, a designer may instead prefer to use optical components made with functionally graded materials, which contain spatially varying composition or microstructure that result in changes in properties. The availability of such materials is limited by current processing techniques. Using additive manufacturing methods, we plan to create a new class of functionally graded optical materials with spatially tailored changes in properties. We expect to fabricate three optical elements that are not achievable by conventional fabrication techniques: (1) a transparent ceramic laser gain medium with a gradient in absorption and gain coefficients via changes in dopant concentration, (2) a silica glass aberration-corrector plate with a gradient in refractive index via compositional change, and (3) a large metal aspherical mirror with a gradient in stiffness via an engineered structural change.

We intend to develop ink formulations and processing techniques needed to produce gradient-composition transparent ceramic and glass optics from additively manufactured green bodies, which are weakly bound ceramic forms prior to sintering or heating. A transparent ceramic laser gain medium with a radial gradient in gain coefficient will be printed as a green body using direct ink writing. Adjustments in the relative flow rates of a doped and an undoped ink stream will enable in-line variation of the neodymium doping concentration during printing. After printing, the green body will be sintered to transparency. A silica glass aberration corrector plate with a non-monotonic radial gradient in refractive index will also be prepared using direct ink writing technology. The refractive index gradient will be introduced during printing using titanium oxide and silicon dioxide inks blended in varying ratios. The printed form will then be consolidated to form the glass via heat treatment. Finally, a large, nickel aspherical mirror will be made lightweight by incorporating a gradient in stiffness and density. Projection microstereolithography will be used to print a microscopic truss lattice on the back side of the shaped mirror surface. The support-feature scale length will be varied and optimized to provide greatest stiffness near the mirror surface while minimizing total weight. This research project will generate intellectual property associated with optical component design, ink development, and optical component fabrication processes, and will establish LLNL as a leader in additively manufactured optical materials and components.

Mission Relevance

The ability to create new functionally graded optical materials by additive manufacturing has the potential to transform optical system design. It will enable the fabrication of optimal, robust, lightweight, and compact optics that cannot be achieved by conventional means, and therefore aligns well with LLNL's lasers and optical science and technology and advanced materials and manufacturing core competencies. Our research is expected to advance the Laboratory's interests in space and intelligence applications as well as in compact, high-average-power laser systems for inertial fusion energy, defense, and industrial applications. Technologies developed here are also expected to be of interest to the broader optics, materials science, and advanced manufacturing communities.

FY16 Accomplishments and Results

In FY16 we (1) made advances toward a transparent ceramic, gradient-doped gain media by formulating yttrium–aluminum–garnet and neodymium-doped yttrium–aluminum–garnet inks suitable for direct ink write printing and demonstrating 6-mm-diameter by 2-mm-thick transparent yttrium–aluminum–garnet by direct ink writing; (2) made progress toward gradient index glass by formulating silica and silica–titania direct ink write inks and demonstrating transparent silica glass monoliths up to 1.5-cm wide and 2-mm thick prepared by direct ink writing; and (3) made progress toward a lightweight lattice-supported mirror with development of thermal-mechanical models and tools for optimizing lattice geometry to meet requirements as well as demonstration of graded, metal-plated support lattices (see figure).