Optical materials capable of advanced functionality in the infrared will enable optical designs that can offer lightweight or small footprint solutions in both planar and bulk optical systems. The University of Central Florida’s Glass Processing and Characterization Laboratory, together with our collaborators, have been evaluating compositional design and processing protocols for both bulk and film strategies employing multicomponent chalcogenide glasses (ChGs). These materials can be processed with broad compositional flexibility that allows tailoring of their transmission window, physical and optical properties, which allows them to be engineered for compatibility with other homogeneous amorphous or crystalline optical components. We review progress in forming ChG-based gradient refractive index (GRIN) materials from diverse processing methodologies, including solution-derived ChG layers, poled ChGs with gradient compositional and surface reactivity behavior, nanocomposite bulk ChGs and glass ceramics, and metalens structures realized through multiphoton lithography. We discussed current design and metrology tools that lend critical information to material design efforts to realize next-generation IR GRIN media for bulk or film applications.
In this paper, two freeform prism combiner designs with different geometries were studied. The first design, whose geometry is driven by the need for total-internal-refraction, achieves optical performance suitable for use in AR/VR applications, but involves highly complex surfaces and highly non-uniform performance. The second design, which removes the total-internal-refraction requirement, adopts a modified geometry which enables significantly improved aberration correction potential. The nodal-aberration-theory based design process is shown for both prism designs, and the optical performance of each design was analyzed. Performance exceeds 10% MTF at 50lp/mm over centered and decentered 3mm effective subpupils, evaluated at nine different positions within an 8mm diameter eyebox.
Freeform prism systems are commonly used for head mounted display systems for augmented, virtual, and mixed reality. They have a wide variety of applications from scientific uses for medical visualization to defense for flight helmet information. The advantage of the freeform prism design over other designs is their ability to have a large field of view and low f-number while maintaining a small and light weight form factor. Current designs typically employ a homogeneous material such as polymethyl methacrylate (PMMA). Using a GRIN material gives the designer extra degrees-of-freedom by allowing a variable material refractive index within the prism. The addition of the GRIN material allows for light to bend within the material instead of only reflecting off the surfaces. This work looks at implementing a freeform gradient-index (GRIN) into a freeform prism design to improve performance, increase field of view (FOV), and decrease form factor by the use of 3D printable polymers. A prism design with freeform GRIN is designed with a FOV of 45°, eye relief of 18.25 mm, eyebox of 8 mm, and performance greater than 10% at 50 lp/mm.
An extensive design study was conducted to find the best optimal power distribution and stop location for a 7.5x afocal zoom lens that controls the pupil walk and pupil location through zoom. This afocal zoom lens is one of the three components in a VIS-SWIR high-resolution microscope for inspection of photonic chips. The microscope consists of an afocal zoom, a nine-element objective and a tube lens and has diffraction limited performance with zero vignetting. In this case, the required change in object (sample) size and resolution is achieved by the magnification change of the afocal component. This creates strict requirements for both the entrance and exit pupil locations of the afocal zoom to couple the two sides successfully. The first phase of the design study looked at conventional four group zoom lenses with positive groups in the front and back and the stop at a fixed location outside the lens but resulted in significant pupil walk. The second phase of the design study focused on several promising unconventional four-group power distribution designs with moving stops that minimized pupil walk and had an acceptable pupil location (as determined by the objective and tube lens).
A design study is conducted in the 3-12 μm wavelength band for an F/2.5, 15.2 degree full field of view, 66 mm focal length imaging system. A survey of preferred homogeneous materials and their chromatic properties which transmit over the full wavelength band is explored. A survey of GRIN materials is also explored, which includes zinc selenide zinc sulfide GRIN, and chalcogenide GRIN from NRL and Schott. GRIN is able to expand the infrared glass map which is otherwise relatively small. The design space is explored over three and four homogeneous elements as well as one GRIN and two homogeneous elements. The three element design with the addition of GRIN results in a better balance between the two bands and is nearly diffraction-limited. The three element design can be reduced to two GRIN elements which has comparable performance to the homogeneous three element design.
The availability of more affordable, high resolution, infrared (IR) detector arrays has opened up the need for a larger set of IR materials. This has created a renewed interest in chalcogenide glasses as their transmission spans from 1 to 14 μm. The Naval Research Laboratory has shown that the chalcogenide glasses can be diffused to make gradient index (GRIN) lenses. GRIN materials are interesting because they can have unique dispersion properties that do not exist in the currently discovered homogeneous materials, a key parameter for optical designers. When fabricating a GRIN material, the ability to test and characterize its properties is essential. This has prompted the need and development of an IR Mach–Zehnder interferometer for relative index of refraction measurements of GRIN materials. These same, more affordable IR detector arrays have also allowed for what was commonly done in the visible to be developed in the IR. Now, two-dimensional change in refractive index can be measured without scanning, and the fringe density becomes less of an issue in the IR, allowing for larger changes in refractive index over a small area to be measured. An axial chalcogenide GRIN with Δn of 0.13 was measured at a wavelength of 3.39 μm.
Discrete zoom systems are commonly used as laser beam expanders and infrared zoom lenses. The reason to design a
discrete zoom lens is that they are often a desirable compromise between fixed-focal length lenses and continuous zoom
lenses, offering many advantages to imaging systems of all types. They have the advantage over continuous zoom systems
for containing fewer elements, thus reducing the weight of the system, and having one mechanical motion instead of two.
In literature there is little information on the first order parameters and starting requirements for discrete systems. This
work derives the first order equations for two different discrete zoom systems. The equations are derived from the
requirements of first order parameters which define the starting group focal lengths. The two design configurations studied
are: one zoom group flipping in and out of the system; one zoom group moving laterally along the optical axis. This work
analyzes the first order equations for both configurations and discusses the starting point for the designs taking into
consideration system limitations. Final designs for both configurations are then compared over several parameters: group
focal lengths, lens diameters, overall length, number of elements, materials, and performance.
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.
We developed software design tools in MATLAB that are compatible with Code V for supporting the process of designing zoom lenses. These tools simplify the process of finding paraxial solutions and evaluating intermediary design steps. Paraxial solutions are found through a partially random search for four group zoom systems with moving second and third groups. It requires several user-specified system parameters and then randomly assigns powers to each group. This process of randomly assigning powers is done a set number of times and only the valid solutions where no lenses crash are considered for further use. The valid designs are plotted over different design criteria and can then be selected to retrieve the first order design parameters. For the intermediate design process, the software displays lens specifications and diagnostic results across zoom for the entire lens as well as the individual groups. Systematic evaluation of the intermediate design steps is useful in determining how to proceed and improve the design. The design process is described for two different zoom lenses to show the efficiency and utility of these tools. The two zoom lenses are a 16x surveillance camera zoom lens working in the visible and a 3X zoom lens working in the visible and short wave infrared. The design procedure for these lenses covers finding the paraxial solutions to evaluating the lens for further improvement.
A design study is conducted in the 1-5μm wavelength band for an F/3, 15 degree full field of view, 38mm focal length imaging system. A survey of preferred materials shows the chromatic properties of homogeneous materials in different regions of this spectrum. A survey of GRIN materials, including zinc selenide zinc sulfide GRIN, aluminum oxynitride GRIN, and chalcogenide GRIN, expands the available chromatic properties in this spectral band. Baseline homogeneous triplet designs are explored and compared to previous studies in the literature. The inclusion of a GRIN material in the three element design improves the chromatic correction and results in a system that is nearly diffraction-limited. The three element design is reduced to two elements, where both elements are GRIN, while maintaining comparable performance to the homogeneous triplet.
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