Additive manufacturing has taken many industries by storm, bringing a revolution to prototyping and manufacturing lines. For the most part, Glass and Optical Materials have been left on the wayside during this insurgence of additive manufacturing. Initially, the processes had very significant barriers to create practical uses for glass and optical materials. The emergence of UV curing or crosslinking polymers from a bath has proven useful for some optical elements and component applications for organic polymers. The majority of inorganic glasses are already fully cross-linked and cannot be used in curing/crosslinking methods.
Rochester Precision Optics (RPO) is constantly developing and designing optical elements and assemblies, having a cheap and quick process to prototype optics for demonstrators and initial prototype systems would be extremely valuable. RPO has investigated a number of methods to incorporate additive manufacturing of glasses into the optics and photonics fields. Laser based approaches of additive manufacturing of chalcogenide glasses will be presented. Optical properties and performance is compared across various additive manufacturing approaches and compared against bulk traditional material and processes.
This talk will address the ideas of intelligent material design when applied to infrared-transparent glasses. By using literature data as a jumping-off point, glass properties can be reduced to mathematical equations whose simultaneous solution results in a material with specifically targeted optical properties such as index and dispersion, as well as targeted mechanical properties such as glass transition temperature and coefficient of thermal expansion. Using this design approach eliminates the need for the scattershot trial-and-error method by which novel chalcogenide glass compositions are currently designed. Examples relevant to current infrared optical systems and their SWaP reduction will be discussed.
With new detectors that are capable of imaging across multiple wavelength bands, new methods need to be developed to reduce the lens count and improve performance across these multiple bands while minimizing the SWAP-c (Size, Weight, power and cost) of the system. One method that was proposed was using an update to the classical γν-ν diagram. This method which, uses instantaneous Abbe number and minimum dispersion wavelength to select materials that minimize the chromatic and thermal focal shift over the desired spectral region. A MWIR/LWIR lens was designed using this method to minimize the lens count. The lens has a continuous 3x zoom range. The lens was manufactured to determine the validity of the method that was used and to evaluate the new materials that are being developed. A comparison of the nominal design to the manufactured design is discussed. This includes a comparison of MTF performance.
Chalcogenide glasses have been steadily advancing infrared imaging capabilities and systems since the mid 1900s. Rochester Precision Optics has recently invested in bolstering their infrared glass manufacturing capabilities. While vertically integrating to reduce costs and to support the current and expanding demand for their precision glass molding, diamond turning, and assemblies that use the classic chalcogenide glasses; the optical design team has been able to capitalize on the new infrared materials further expanding the infrared optical glass map for S.W.A.P. enhancement in their designs.
The focus of this work is to highlight some of the capabilities and recent innovations in chalcogenide glass manufacturing leading to low cost methods of producing optical materials, elements, unique or previously difficult geometries.
Due to changes in the fictive temperature as a result of the precision glass molding process there is an induced change in the index of refraction. This can be on the order of 0.001 in oxide glasses and as high as 0.02 in the chalcogenide glasses. It is important to accurately define the expected index of refraction and the tolerance of it after molding as there may be an impact on the optical design tolerances and system performance. We report on the measured change in index of refraction in common chalcogenide glasses due to the Rochester Precision Optics (RPO) precision glass molding process. We will compare the change in index of refraction between as advertised, as measured, as molded, and we will look at post mold annealing recovery. Utilizing an upgraded M3 refractometer we will be able to measure the index from the visible to the LWIR.
One of the difficulties in designing infrared optical systems is the comparative lack of glasses from which to design lenses. In visible optical systems, the designer has a palette of hundreds of glass options with varying dispersions and mechanical properties. In contrast, the designer of infrared optical systems has perhaps a dozen materials options from which to choose.
Instead, what if the infrared transparent materials were designed specifically for various applications? Using a material with a targeted index dispersion profile, the designer can complete a system using fewer lens surfaces and in many cases with increased functionality such as athermalization.
Next comes the question of how to obtain such a material. One approach is somewhat scattershot: to melt series of glasses, measure each of their properties, and settle on one composition for scale-up to production volumes. This approach is both time- and resource-consuming, as the measurements for many properties require specialized equipment and sample preparation.
In contrast to this scattershot method, the principle of intelligent material design allows glass scientists to design glasses with intentionally chosen mechanical and optical properties, and greatly reduces the number of test melts required to obtain a final production solution. Intelligent material design consists of leveraging the existing literature data to make informed decisions about which glass compositions are likely to exhibit the desired properties. By describing the variation of the properties over the glass family with mathematical functions, the material design problem is reduced to the simultaneous solution of a set of equations.
We show successful printing of chalcogenide glass using two different techniques. Additive manufacturing is still a fairly new field, but is increasing rapidly. We compare some of the first tests of selective laser melting and direct laser processing techniques to chalcogenide glass.
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.
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.
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.
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.
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.
Metrology of a gradient index (GRIN) material is non-trivial, especially in the realm of infrared and large refractive index. Traditional methods rely on index matching fluids which are not available for indexes as high as those found in the chalcogenide glasses (2.4-3.2). By diffusing chalcogenide glasses of similar composition one can blend the properties in a continuous way. In an effort to measure this we will present data from both x-ray computed tomography scans (CT scans) and Raman mapping scans of the diffusion profiles. Proof of concept measurements on undiffused bonded sheets of chalcogenide glasses were presented previously. The profiles measured will be of axially stacked sheets of chalcogenide glasses diffused to create a linear GRIN profile and nested tubes of chalcogenide glasses diffused to create a radial parabolic GRIN profile. We will show that the x-ray absorption in the CT scan and the intensity of select Raman peaks spatially measured through the material are indicators of the concentration of the diffusion ions and correlate to the spatial change in refractive index. We will also present finite element modeling (FEM) results and compare them to post precision glass molded (PGM) elements that have undergone CT and Raman mapping.
As the desire to have compact multispectral imagers in various DoD platforms is growing, the dearth of multispectral
optics is widely felt. With the limited number of material choices for optics, these multispectral imagers are often very
bulky and impractical on several weight sensitive platforms. To address this issue, NRL has developed a large set of
unique infrared glasses that transmit from 0.9 to > 14 μm in wavelength and expand the glass map for multispectral
optics with refractive indices from 2.38 to 3.17. They show a large spread in dispersion (Abbe number) and offer some
unique solutions for multispectral optics designs. The new NRL glasses can be easily molded and also fused together to
make bonded doublets. A Zemax compatible glass file has been created and is available upon request. In this paper we
present some designs, optics fabrication and imaging, all using NRL materials.
Gradient index (GRIN) optics have been an up-and-coming tool in the world of optics. By combining an index gradient with a surface curvature the number of optical components for a lens system can often be greatly reduced. Their use in the realm of infra-red is only becoming realized as new efforts are being developed to create materials that are suitable and mutually compatible for these optical components. The materials being pursued are the chalcogenide based glasses. Small changes in elemental concentrations in these glasses can have significant effects on physical and optical properties. The commonality between these glasses and their widely different optical properties make them prime candidates for GRIN applications. Traditional methods of metrology are complicated by the combination of the GRIN and the curvature of the element. We will present preliminary data on both destructive and non-destructive means of measuring the GRIN profile. Non-destructive methods may require inference of index through material properties, by careful measurement of the individual materials going into the GRIN optic, followed by, mapping measurements of the GRIN surface. Methods to be pursued are micro Raman mapping and CT scanning. By knowing the properties of the layers and accurately mapping the interfaces between the layers we should be able to back out the index profile of the GRIN optic and then confirm the profile by destructive means.
With the increase in demand for infrared optics for thermal applications and the use of glass molding of chalcogenide materials to support these higher volume optical designs, an investigation of changes to the optical properties of these materials is required. Typical precision glass molding requires specific thermal conditions for proper lens molding of any type of optical glass. With these conditions a change (reduction) of optical index occurs after molding of all oxide glass types and it is presumed that a similar behavior will happen with chalcogenide based materials. We will discuss the effects of a typical molding thermal cycle for use with commercially and newly developed chalcogenide materials and show results of index variation from nominally established material data.
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