Blair L. Unger

Layered chalcogenide glass structures for IR lenses

A technique for fabricating novel infrared (IR) lenses can enable a reduction in the size and weight of IR imaging optics through the use of layered glass structures. These structures can range from having a few thick glass layers, mimicking cemented doublets and triplets, to having many thin glass layers approximating graded index (GRIN) lenses. The effectiveness of these structures relies on having materials with diversity in refractive index (large Δn) and dispersion and similar thermo-viscous behavior (common glass transition temperature, ΔTg = 10°C). A library of 13 chalcogenide glasses with broad IR transmission (NIR through LWIR bands) was developed to satisfy these criteria. The lens fabrication methodology, including glass design and synthesis, sheet fabrication, preform making, lens molding and surface finishing are presented.

Measurement and Characterization of Dimpled Planar Light Guide Prototypes

A dimpled light guide solar concentrator has been manufactured, and measurements of this prototype have been successfully modeled to predict performance. A second generation prototype was manufactured with 70% optical efficiency at 60x geometric concentration.

Design study of a MWIR/LWIR Mulitple FOV lens

Single aperture multispectral systems are becoming prevalent thanks to advances in multispectral detectors, new optical materials, and new methods for selecting materials that minimize chromatic and thermal focal shift. This design study focuses on design of a three field-of-view, multispectral lens operating across the MWIR and LWIR spectral regions. The lens in question will have an f-number of f/3 with a 3X zoom ratio. The narrow full field-ofview of the lens is 3.33° with a wide full field-of view of 9.99°, the length of the system is 163 mm. The performance goal for the lens is diffraction limited over the thermal region. The study will provide an overview of material selection using an updated γv-v diagram, to provide achromatic and athermal characteristics. The study will then step through first order layout, optimization with key constraints, and tolerancing for manufacturability. Finally, the study will provide detailed analysis of system performance including as-built MTF over temperature, aberration analysis, and NETD contributions from narcissus.

Raman and CT scan mapping of chalcogenide glass diffusion generated gradient index profiles

Gradient index (GRIN) lenses have been created for imaging in the infrared regime by diffusion of chalcogenide glasses. The GRIN lenses are shaped using a combination of precision glass molding and single point diamond turning. The precision glass molding step, is known to cause a drop in the index of refraction in both oxide and chalcogenide glasses. This drop is a direct result of the cooling rate during the molding process. Since the GRIN lenses have an index of refraction profile created by diffusion of multiple chalcogenide glasses, we would expect that the index drop would vary as a function of position. In this paper we investigate the expected profile change due to the index drop of the constituent chalcogenide glasses, as well as report performance data on the GRIN lenses.

New IR materials for multispectral optics (Conference Presentation)

There is a strong desire to reduce size and weight of single and multiband IR imaging systems in ISR operations on hand-held, helmet mounted or airborne platforms. Current systems are limited by bulky optics. We have recently developed a large number of new optical materials based on chalcogenide glasses which transmit in SWIR to LWIR wavelength region that fill up the glass map for multispectral optics and vary in refractive index from 2.38 to 3.17. They show a large spread in dispersion (Abbe number) and offer some unique solutions for multispectral optics designs. These glasses were specifically designed to have comparable glass molding temperatures and thermal properties to be able to laminate and co-mold the optics and reduce the number of air-glass interfaces (lower Fresnel reflection losses). These new NRL glasses also have negative or very low positive dn/dT making it easier to athermalize the optical system. This presentation will cover discussions on the new optical materials, multispectral designs, fabrication and characterization of new optics.

Design of multiband optics using updated athermal/achromatic glass map

Recently, optical materials have been developed by Schott and NRL to improve material selection in the SWIR, MWIR, and LWIR wavelength regions. In addition, new multiband detectors are reaching maturity, leading to a natural push for common aperture lens systems. Detectors that can span the SWIR/MWIR, MWIR/LWIR or SWIR/MWIR/LWIR wavelengths regions will require complex optical systems to effectively utilize their full potential. Designing common aperture wide-band systems that are both achromatized and passively athermal, especially while maintaining SWAP-c (size, weight, power and cost), poses significant challenges. Through use of the updated γν-ν diagram, which provides guidance on material combinations that both achromatize and athermalize, part of that challenge is reduced. This updated γν-ν diagram uses instantaneous Abbe number and peak wavelength. The instantaneous Abbe number is a function of wavelength and is the scaled reciprocal of the instantaneous dispersion. The instantaneous Abbe number is defined at the peak wavelength, which occurs when the second derivative of the index of refraction goes to zero. Three examples will be presented using this updated athermal/achromatic glass map to demonstrate its effectiveness. These design examples will include a SWIR/MWIR design, a MWIR/LWIR design and, a SWIR/MWIR/LWIR design.

IR GRIN optics: design and fabrication

Infrared (IR) transmitting gradient index (GRIN) materials have been developed for broad-band IR imaging. This material is derived from the diffusion of homogeneous chalcogenide glasses has good transmission for all IR wavebands. The optical properties of the IR-GRIN materials are presented and the fabrication and design methodologies are discussed. Modeling and optimization of the diffusion process is exploited to minimize the deviation of the index profile from the design profile. Fully diffused IR-GRIN blanks with Δn of ~0.2 are demonstrated with deviation errors of ±0.01 refractive index units.

IR-transmitting GRIN chalcogenide optics (Conference Presentation)

Graded index (GRIN) optical materials and novel lens offer numerous benefits for infrared applications, where selection of conventional materials is limited. For optical systems that must perform over wide spectral regions, the reduction of size weight and complexity can be achieved through the use of GRIN elements. At the Naval Research Laboratory (NRL) we are developing new technologies for IR gradient index (IR-GRIN) optical materials. This paper will present the latest progress in the development of these materials including their design space guidelines, fabrication, metrology, optics characterization, and preliminary imaging demonstration.

Athermalization and achromatization of multiband optics using instantaneous Abbe number

Over the past few years, new detector technologies have enabled multiband detection through a single aperture. This creates significant SWAP advantages (size, weight and power) and has spurred significant interest in multiband optics (for instance SWIR/MWIR, MWIR/LWIR, etc.). However, due to the small number of materials available in the infrared regions, passive optical athermalization and achromatization can be challenging even over single waveband. This becomes even more challenging in the case of multiband optics. One method for determining appropriate material combinations for athermalization and achromatization is use of a y ∗ v vs. v diagram. We examine an updated form of the y ∗ v vs. v diagram using instantaneous Abbe number. While Abbe number is an effective metric for dispersion within single bands, it becomes less reliable when extended to wider wavelength ranges. Instantaneous Abbe number allows for a wider waveband to be defined, without a loss of generality; and this allows for an updated definition of the y ∗ v vs. v diagram for the development of multiband optics. We present an example of a multiband lens as well as compare the typical definition of Abbe number with instantaneous Abbe number to determine the validity of the updated model.

Optical Design Methods for Spectrographic Systems

Hamiltonian methods are applied to spectrometer design using linearly spaced gratings. The aberration function is expanded in terms of system construction parameters, constraints are then derived on certain parameters which ensure some low-order image properties.