Gradient Index Optics at DARPA

Abstract

Executive SummaryThe Defense Advanced Research Projects Agency (DARPA) asked the Institute for Defense Analyses (IDA) to assemble a summary of work on gradient index optics completed under funding from the Agency. The purpose was to archive the efforts, motivation, and accomplishments that the Agency supported and to provide a reference for any future programs that explore this new area of optics.Although optical instruments, such as lenses and mirrors, dating back thousands of years ago have been unearthed, lenses were not put into practical use until the invention of eyeglasses in the thirteenth century. The microscope and the telescope were invented in the seventeenth century, but no revolutionary changes occurred in these complex lens systems for the next 200 years. Most lens systems remained homogeneous multi-element systems with spherical glass surfaces and fixed optical properties. Recently, however, new materials and lens designs have been developed, inspired by properties of biological eyes. Materials with a gradient index (GRIN) allow the development of compact systems that have high focusing power while correcting for aberration.In 2002, DARPA initiated the Bio-Optic Synthetic Systems (BOSS) program, which aimed to synthesize the components of a biologically inspired vision system and demonstrate a level of performance beyond that of standard optical imaging systems (i.e., with reduced size and complexity). Out of more than a dozen exploratory efforts, four efforts were selected for further development and demonstration: fluidic adaptive zoom lenses, foveated imaging, photon sieves, and nanolayer lenses.As an example, Case Western Reserve University (CWRU) and the Naval Research Laboratory (NRL) collaborated on the nanolayer lens effort to create a synthetic lens that would mimic the structure and capabilities of an octopus eye. The CWRU/NRL team used a forced assembly nanolayer coextrusion process to form films that had a tailored refractive index consisting of thousands of nanolayers of two different polymers that had different refractive indices. The films of various n were stacked to create a refractive index range (Δn) and formed into hemispheres, which were combined to form the synthetic bi-convex octopus lens. A zoom lens system constructed from three of these GRIN lenses was demonstrated on a small unmanned aerial vehicle (UAV).Limited manufacturing capabilities have hindered practical applications of GRIN lenses. In 2008, DARPA initiated the Manufacturable Gradient Index Optics (M-GRIN) program to address GRIN lens manufacturing issues in the development of low-cost, iii customizable GRIN-based optics for use in a variety of military systems. This effort included a significant focus on metrology, where new tools were required for monitoring the polymer nanolayer process for GRIN optics. Performers in the M-GRIN program included the following teams:

Ultralow Dispersion Multicomponent Thin‐Film Chalcogenide Glass for Broadband Gradient‐Index Optics

“…Gradient refractive index (GRIN) optics present unique opportunities for control of the chromatic properties of an optical system . Novel GRIN materials can be engineered to provide dispersive properties which lie far outside those found in nature, providing new degrees of freedom for optical design as well as the potential for use in new applications . In this paper, a novel photothermal process is utilized to spatially modulate high‐index nanocrystals within a metastable chalcogenide glass thin film, composed of Ge‐As‐Pb‐Se (GAP‐Se) constituents, thereby achieving ultralow dispersion over an unprecedented bandwidth of 1–12 µm wavelength and enabling creation of an arbitrary index gradient required for GRIN optics.…”

“…By taking advantage of the additional degrees of freedom associated with volumetric focusing, it is possible to engineer a material system capable of dispersive properties that lie far outside what is found in nature, such as ultralow dispersion across a broadband spectrum, thereby greatly reducing chromatic aberration . GRIN materials are media in which the refractive index is varied as a function of spatial position, usually through a variation of material composition . This variation can be induced by chemical diffusion (i.e., ion exchange), lamination of homogeneous, dissimilar materials (as in the case of polymers or glasses), and ion implantation to yield either radial or axial GRIN, respectively.…”

“…By introducing an index gradient along the radial axis of the optical element, light can be refracted within the volume as opposed to along a curved surface. As the dispersive properties of the component phases which bring about the volumetric focusing are different than in the case of a homogenous spherical lens, new degrees of freedom are achieved for chromatic aberration correction . The 14%GeSe 2 –42%As 2 Se 3 –44%PbSe GAP‐Se film composition was selectively engineered to modulate the concentration of high‐index nanocrystals, thereby providing ultralow dispersion across the entire 1–12 µm broadband spectrum.…”

Monolithic Chalcogenide Optical Nanocomposites Enable Infrared System Innovation: Gradient Refractive Index Optics

“…[ 22–25 ] The opportunities GRIN flat lenses offer increase the design space and requisite volume needed for optical elements, enabling reduced size, weight, element count, and cost, thereby extending the trade space for optical performance parameters. [ 19,24–27 ] A material manufacturing paradigm that could exploit such novel functionality will modify the way optical systems are designed and one such strategy is discussed here.…”

Diffusion-based gradient index optics for infrared imaging

“…Various techniques have been utilized to fabricate GRIN optics for visible light including glass element diffusion, 14 ion exchange in glass, 15 and polymer nanolayer extrusion. 16 Recent efforts funded by the DARPA M-GRIN program 17 have reinvigorated interest in the development of GRIN materials, especially in the IR [3][4][5][18][19][20] where correction of chromatic aberration is a key design challenge. To date, GRIN optics for IR are not commercially available with the notable exception that some GRIN lenses designed for visible wavelengths also have incidental utility in the near-IR at wavelengths shorter than about 1.5 μm.…”