Freeform Optics

Project Overview

One of the next paradigms for optics fabrication is the growing field of freeform optics (optics having at least one optical surface with no translational or rotational symmetry). The additional degrees of freedom for the shape enables the design of novel freeform optical components which can significantly reduce the optical system size & weight, reduce the number of optical components used, increase optical functionality, enable complex optical element design, improve optical performance, and/or adapt to the geometric shape of the high-level system.

In this study, we first built an on-site freeform optics fabrication capability via the purchase/commissioning of a CNC grinder/polisher (for fabrication research & prototyping) and an optical metrology system (for measuring complicated freeform optics). Then we developed a predictive toolkit to determine the tool influence function (TIF) for various tool-kinematics-workpiece process variables, with particular focus on fused silica optics polished with CeO2 slurry. The TIF can be described as the local material removal (rate and shape) at the contact between the polishing tool and the workpiece. It is a critical factor that needs to be understood quantitatively with high precision for deterministic freeform fabrication. We examined fundamental interactions that occur at the workpiece-slurry-tool interface. Various phenomena have been identified or characterized with quantitative models including kinematics, spatially dependent friction, elastic & hydrodynamic pressure distribution, workpiece curvature effect, shear removal mechanisms, and contributions to the cause & reduction of mid spatial artifacts. This improved understanding has enabled: 1) the ability to predict the TIF spots to within 20% of that measured over a large range of process conditions; and 2) a number of quantitative process design rules for increased determinism and quality of the final surface figure. Using this, a suite of novel, challenging freeform optics have been or are being fabricated both for this project (e.g., Alvarez lenses & beam shaper optics) and other LLNL projects (continuous phase plates, spiral optics, & waveplates). These optics have been fabricated with approximately an order of magnitude improvement (characterized as the rms deviation of the as-fabricated surface figure shape relative to the design surface figure) compared to that reported to date in the literature.

Mission Impact

The broader space of design and manufacturability for precision freeform optics would specifically aid the development of novel laser and imaging systems particularly useful for DoD applications, Directed Energy, and Stockpile Stewardship missions as well as enhancing the core competency of Lasers and Optical Science and Technology at LLNL.

Publications, Presentations, and Patents

Ray, N., T. Suratwala, J. Menapace, L. Wong, W. Steele, G. Tham, and B. Bauman. 2022. "Modeling the Hydrodynamic Impact on the Tool Influence Function During Hemispherical Subaperture Optical Polishing." Applied Optics 61, no. 18 2022: 5392-400.

Suratwala, T., J. Menapace, R. Steele, L. Wong, G. Tham, N. Ray, B. Bauman, M. Gregory, and T. Hordin. 2021. "Mechanisms Influencing and Prediction of Tool Influence Function Spots During Hemispherical Sub-Aperture Tool Polishing on Fused Silica." Applied Optics 60 (1), no. 1 (2021/01/01 2021): 201-14.

Suratwala, T., J. Menapace, G. Tham, R. Steele, L. Wong, N. Ray, and B. Bauman. 2022. "Understanding and Reducing Mid-Spatial Frequency Ripples During Hemispherical Sub-Aperture Tool Glass Polishing." Applied Optics 61, no. 11: 3084-95.

Suratwala, T., J. Menapace, G. Tham, R. Steele, L. Wong, N. Ray, B. Bauman, M. Gregory, and T. Hordin. 2021. "Effect of Workpiece Curvature on the Tool Influence Function During Hemispherical Sub-Aperture Tool Glass Polishing." Applied Optics 60 (4), no. 4 (2021/02/01 2021): 1041-50.