James A. Corsetti

Color correction in the infrared using gradient-index materials

Abstract. The ability of gradient index (GRIN) materials to correct color in the infrared is explored. An overview of GRIN materials is provided, including a review of the most common index profiles (axial, radial, and spherical). Due to the high potential for color correction, a design study is carried out to compare a single ZnS/ZnSe GRIN element’s imaging performance to that of homogeneous singlets and doublets. A radial GRIN element is able to correct axial color and provide superior performance over an aspheric homogeneous singlet and offer a reduction in weight when compared to a homogeneous doublet. The ZnS/ZnSe GRIN is also shown to be superior to other infrared GRIN materials between 3 and 5 μm.

Design and fabrication of a polymer gradient-index optical element for a high-performance eyepiece

Abstract. A 40-deg full field-of-view high-performance eyepiece design utilizing a polymer spherical gradient-index (GRIN) optical element is presented. In the design process, the GRIN lens material is constrained to current manufacturing capabilities. Several spherical GRIN lens blanks are fabricated from a thermoformable axial GRIN polymethyl methacrylate polystyrene copolymer material. One is diamond turned into a lens for the eyepiece, and the additional blanks are used to characterize the fabrication process. The spherical GRIN profile is evaluated in the original design, and a tolerance analysis is provided.

Simultaneous interferometric measurement of linear coefficient of thermal expansion and temperature-dependent refractive index coefficient of optical materials

Characterizing the thermal properties of optical materials is necessary for understanding how to design an optical system for changing environmental conditions. A method is presented for simultaneously measuring both the linear coefficient of thermal expansion and the temperature-dependent refractive index coefficient of a sample interferometrically in air. Both the design and fabrication of the interferometer is presented as well as a discussion of the results of measuring both a steel and a CaF2 sample.

Optical design study of a VIS-SWIR 3X zoom lens

A design study is compiled for a VIS-SWIR dual band 3X zoom lens. The initial first order design study investigated zoom motion, power in each lens group, and aperture stop location. All designs were constrained to have both the first and last lens groups fixed, with two middle moving groups. The first order solutions were filtered based on zoom motion, performance, and size constraints, and were then modified to thick lens solutions for the SWIR spectrum. Successful solutions in the SWIR were next extended to the VIS-SWIR. The resulting nine solutions are all nearly diffraction limited using either PNNP or PNPZ (“Z” indicating the fourth group has a near-zero power) design forms with two moving groups. Solutions were found with the aperture stop in each of the four lens groups. Fixed f-number solutions exist when the aperture stop is located at the first and last lens groups, while varying f-number solutions occur when it is placed at either of the middle moving groups. Design exploration included trade-offs between parameters such as diameter, overall length, back focal length, number of elements, materials, and performance.

Design, fabrication, and metrology of polymer gradient-index lenses for high-performance eyepieces

High-performance eyepiece designs have been carried out using both spherical and radial gradient-index (GRIN) elements. Eyepiece designs of both geometries are shown to offer superior imaging performance with fewer elements when compared to purely homogeneous systems. These GRIN lenses are formed from monomer diffusion between polymethyl methacrylate (PMMA) and polystyrene (PSTY) during the polymerization process, resulting in a copolymer of the two homogeneous materials. A process for fabricating spherical GRIN elements is discussed where copolymer axial GRIN blanks are thermally compressed using spherical surface molds. This process curves the nominally-straight isoindicial surfaces of the axial GRIN rod to be consistent with the shape found during optimization of the design. Once compressed, the spherical blanks are diamond-turned for final surface figure and finish. Measurement of the GRIN profile is carried out using the Schmidt immersion technique in a Mach-Zehnder interferometer. Tolerances specific to GRIN elements are identified and determined to be readily achievable using the aforementioned manufacturing process.

Athermalization of polymer radial gradient-index singlets

Abstract. The article explores the possibility of athermalizing a gradient-index (GRIN) lens so that the effective focal length (EFL) of the element remains constant over a change in temperature. This is accomplished by designing the lens so that the surface curvatures and index profile compensate for one another over a change in temperature to maintain constant optical power. The means to determine how the lens geometry and index profile change with temperature for both a homogeneous and radial GRIN are explained. An analytic model for the purpose of identifying athermalized GRIN singlets is described and validated against the previous work in this field. The model is used to identify an athermalized polymer radial GRIN element and compare it with four other polymer elements of the same focal length but different index profiles, including a homogeneous one. Comparison of these singlets in CODE V® optical design software shows that the athermalized GRIN element maintains its nominal EFL over a temperature change the best of the five in the group while the homogeneous element (having no GRIN profile to counteract the effect of temperature on the surface curvatures) has the poorest performance. A numerical model to analyze more complicated GRIN systems is discussed.

Eyepiece designs with radial and spherical polymer gradient-index optical elements

Abstract. Radial and spherical polymer gradient-index (GRIN) eyepiece designs are presented. The chromatic behavior of GRIN profiles is constrained to real material properties of a polymethyl methacrylate polystyrene copolymer gradient-index system. Single-element, two-element, and multielement eyepiece design configurations each demonstrate significant spot diameter and modulation transfer function performance improvements with the use of a GRIN element. A high-performance spherical GRIN eyepiece design, with 48-deg full field-of-view and 3% distortion, is compared to a similar homogeneous glass solution.

Measurement of linear coefficient of thermal expansion and temperature-dependent refractive index using interferometric system

A system combining an interferometer with an environmental chamber for measuring both coefficient of thermal expansion (CTE) and temperature-dependent refractive index (dn/dT) simultaneously is presented. The operation and measurement results of this instrument are discussed.

Design and characterization of a copolymer radial gradient index zoom lens

Gradient-index (GRIN) zoom lenses are shown to offer superior imaging performance to homogenous designs over the visible spectrum. For a given element count, copolymer GRIN designs are better corrected for axial and lateral color than homogeneous aspheric designs. A macro was developed in CODE V® to calculate the surface contributions to both axial and lateral color for a radial GRIN lens. This macro confirms the improved color correction of the GRIN systems over the homogeneous ones.

Axial and lateral color correction in zoom lenses utilizing gradient-index copolymer elements

Gradient-index (GRIN) zoom lenses are shown to offer superior imaging performance to homogenous designs over the visible spectrum. For a given element count, copolymer GRIN designs are better corrected for axial and lateral color than homogeneous aspheric designs.