Joseph D. Owen

Design, fabrication, and testing of convex reflective diffraction gratings

The convex reflective diffraction grating is an essential optical component that lends itself to various applications. In this work, we first outline the design principles of convex diffraction gratings from wavefront quality and efficiency perspectives. We then describe a unique fabrication method that allows for the machining of convex diffraction gratings with variable groove structure, which is extendable to rotationally non-symmetric convex diffraction grating substrates. Finally, we demonstrate two quantitative wavefront measurement methods and respective experimental validation.

Increased Compactness of an Imaging Spectrometer Enabled by Freeform Surfaces

A freeform imaging spectrometer in the Offner-Chrisp geometry is demonstrated to be 5x more compact than one with traditional surfaces. Performance and manufacturability was analyzed using spectral full-field displays and a demonstration prototype was realized.

Precision glass molding of freeform optics

Precision glass molding is a viable process for the cost-effective volume production of freeform optics. Process development is complex, requiring iterative trials of mold manufacture and metrology, glass mold prototyping, metrology and functional testing. This paper describes the first iteration in the development of a process for an Alvarez lens for visible light. The challenges of this optic are extremely tight band-RMS tolerances on a freeform shape over a maximum clear aperture of 45 mm, a 16:1 aspect ratio and a freeform departure of 329 micrometers. A freeform glass mold for an Alvarez lens was manufactured by coordinated-axis diamond turning in a mold substrate using a custom tool error correction method. The results of prototype precision glass molding are also reported. Mold surfaces and molded optical surfaces are analyzed with scanning white light interferometry. A surface roughness of approximately 3 nm RMS is obtained for both the mold substrate and the glass optic with high-fidelity reproduction of micro-surface structure in the glass. These measurements also identify challenging areas, particularly the presence of mid-spatial frequency errors on the optic originating from the machine thermal control system. The form of the molds was also measured with a profilometer; however, the mold surface does not agree with the expected prescription with an overall deviation in form of approximately 10 μm. The machining process is expected to have sub-micrometer error and the sources of this discrepancy are still being determined. Metrology of the glass optics is currently in progress.

Testing of a Convex Reflective Diffraction Grating

A quantitative nulling interferometric measurement technique for wavefront fidelity validation of a blazed convex reflective diffraction grating is presented. A null wavefront of λ/45 RMS at wavelength 633 nm is demonstrated.

Ultra-Precision Diamond Machining of Freeform Optics for the IR

We review the use of multi-axis milling as a method for manufacturing freeform optics in brittle materials. Implementing this technology requires understanding of material behavior and error correction/minimization. Several examples with infrared applications are discussed.

Moth's eye anti-reflection gratings on germanium freeform surfaces

Germanium is commonly used for optical components in the infrared, but the high refractive index of germanium causes significant losses due to Fresnel reflections. Anti-reflection (AR) surfaces based on subwavelength “moth’s eye” gratings provide one means to significantly increase optical transmission. As found in nature, these gratings are conformal to the curved surfaces of lenslets in the eye of the moth. Engineered optical systems inspired by biological examples offer possibilities for increased performance and system miniaturization, but also introduce significant challenges to both design and fabrication. In this paper, we consider the design and fabrication of conformal moth’s eye AR structures on germanium freeform optical surfaces, including lens arrays and Alvarez lenses. Fabrication approaches and limitations based on both lithography and multi-axis diamond machining are considered. Rigorous simulations of grating performance and approaches for simulation of conformal, multi-scale optical systems are discussed.