New moldable, infrared (IR) transmitting glasses and diffusion-based gradient index (GRIN) optical glasses enable simultaneous imaging across multiple wavebands including short-wave infrared, midwave infrared, and long-wave infrared, and offer potential for both weight savings and increased performance in optical sensors. Lens designs show potential for significant reduction in size and weight and improved performance using these materials in homogeneous and GRIN lens elements in multiband sensors. An IR-GRIN lens with Δn = 0.2 is demonstrated.
NRL is developing new glasses that transmit within the visible to LWIR range for use in multispectral imaging. We had previously developed NRL glasses which transmit in 0.9 to > 12 µm in wavelength, with refractive index ranging from 2.38 to 3.17, to expand the glass map and provide compact solutions to multispectral imaging systems. These glasses were specifically designed to have comparable glass molding temperatures and thermal properties so that they can be laminated and co-molded into optics with reduced number of air-glass interfaces (lower Fresnel reflection losses). These NRL glasses also have negative or very low dn/dT, making it easier to athermalize the optical system. A set of NRL glasses can be diffused to make infrared graded index (IR-GRIN) optics. Our multispectral optics designs using these new materials demonstrate reduced size, complexity and improved performance. The glass database compatible with Zemax and CodeV is available for distribution. We are adding n
A common challenge in metastructure fabrication is precisely tuning of the frequency of a device’s resonance. Slight variations in device dimensions or material properties can lead to a deviation in resonance frequency in comparison to design. We present a method of tuning a dielectric metastructure’s resonance by thermally adjusting the refractive index of a chalcogenide glass (ChG) material. Several characteristics of ChGs make them good candidates for use in dielectric metastructures. They exhibit high linear refractive indices, enabling high index contrast devices; they have large optical nonlinearities, making them useful for tunable devices and nonlinear frequency conversion; and they have wide transmission windows extending from the visible through the long-wave infrared. Recently, we have carried out extensive characterization of the index tuning of arsenic selenide (As2Se3) ChG thin films and observed refractive index changes larger than 0.1 in some cases. We use this refractive index change to permanently shift the resonance of a Fabry-Perot filter and the cutoff wavelength for a Bragg reflector. We then demonstrate thermal tuning to shift resonance positions for a metasurface (MS). We compare finite element modeling results with measurement results and show good agreement. This tuning mechanism has potential for use in MS devices where a precise resonance frequency is required.
New, moldable IR glasses from NRL and Graded index (GRIN) optics enable simultaneous imaging across multiple wavebands including SWIR/MWIR/LWIR and offer potential for both weight savings and increased performance. Optical properties databases compatible with Zemax and CodeV will be presented and made available for users. Lens designs show the potential for significant SWaP reduction benefits and improved performance using NRL materials and IR-GRIN lens elements in multiband sensors. The SWaP and performance advantages of these materials will be presented.
ORganically MOdified CHALCogenide (ORMOCHALC) polymers are novel materials that can be synthesized through the recently discovered inverse vulcanization process. Inverse vulcanization requires the heating of chalcogenide comonomers along with compounds that contain available pi electrons. The composition of the polymers presented includes the use of previously unexplored multi-vinyl branching agents, as well as polymer backbones that contain selenium. The crosslinking by unique comonomer species, and the use of selenium as a backbone component are significant in that they have a direct and pronounced effect on the optical properties of the polymers produced. Specific optical benefits of ORMOCHALC polymers include the extensive infrared transmission profile and the unusually high refractive indices these polymers possess. We present the synthesis and optical characterization of unique ORMOCHALC polymers.
New moldable, infrared (IR) transmitting glasses from NRL and graded index (GRIN) optical materials enable simultaneous imaging across multiple wavebands including SWIR, MWIR and LWIR and offer potential for both weight savings and increased performance in optical sensors. Lens designs show the potential for significant SWaP reduction benefits and improved performance using NRL materials and IR-GRIN lens elements in multiband sensors.
Arsenic triselenide (As2Se3) is of interest for use in dielectric metasurfaces for several reasons: It has a high linear refractive index, n=2.8, enabling high index contrast with its surrounding medium; it has an exceptionally high optical nonlinearity (n2 < 900 × that of silica) making it a good candidate for nonlinear metasurfaces; and its band gap of 1.8 eV and wide transmission window, spanning from approximately 1.5-14 μm, make it useful for applications in the shortwave infrared into the long-wave infrared. We discuss recent results in which we showed that unpassivated As2Se3 films degrade significantly under ambient conditions in the presence of light. We discuss the mechanism of this degradation and show that deposition of a thin (~10 nm) Al2O3 passivation layer, deposited via atomic layer deposition, together with preventing exposure to below-band gap light inhibits degradation. Finally, we fabricate initial As2Se3-based dielectric metasurfaces by writing features via a laser direct write system. For mid-wave to long-wave infrared applications, features with relevant sizes can be written using this technique. We measure resonances for these structures and compare to theoretical results.
Graded index (GRIN) optics offer potential for both weight savings and increased performance but have until recently been limited to visible and NIR bands (wavelengths shorter than about 0.9 µm). NRL has developed glass-based IR-GRIN lenses compatible with SWIR-LWIR wavebands. Recent designs show the potential for significant SWaP reduction benefits and improved performance using IR-GRIN lens elements in dual-band, MWIR-LWIR sensors. The SWaP and performance advantages of IR-GRIN lenses in platform-relevant dual-band imagers will be presented.
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. NRL is developing new IR glasses that transmit from 0.9 to < 12 µm in wavelength, with refractive index ranging from 2.38 to 3.17, to expand the glass map and provide compact solutions to multispectral imaging systems. These glasses were specifically designed to have comparable glass molding temperatures and thermal properties so that they can be laminated and co-molded into optics with reduced number of air-glass interfaces (lower Fresnel reflection losses). These new NRL glasses also have negative or very low dn/dT, making it easier to athermalize the optical system. Our multispectral optics designs using these new materials demonstrate reduced size, complexity and improved performance. The glass database is now available for distribution. Some of the NRL glasses are also available commercially. This presentation will cover discussions on the new optical materials, multispectral designs, as well fabrication and characterization of new optics.
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.
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. NRL is developing new IR glasses that transmit from 0.9 to > 12 µm in wavelength, with refractive index ranging from 2.38 to 3.17, to expand the glass map and provide compact solutions to multispectral imaging systems. These glasses were specifically designed to have comparable glass molding temperatures and thermal properties so that they can be laminated and co-molded into optics with reduced number of air-glass interfaces (lower Fresnel reflection losses). These new NRL glasses also have negative or very low dn/dT, making it easier to athermalize the optical system. Our multispectral optics designs using these new materials demonstrate reduced size, complexity and improved performance. The glass database is now available for distribution. Some of the NRL glasses are also available commercially. This presentation will cover discussions on the new optical materials, multispectral designs, as well fabrication and characterization of new optics.
Graded index (GRIN) optics offer potential for both weight savings and increased performance but have until recently been limited to visible and NIR bands (wavelengths shorter than about 0.9 µm). NRL has developed glass-based IR-GRIN lenses compatible with SWIR-LWIR wavebands. Recent designs show the potential for significant SWaP reduction benefits and improved performance using IR-GRIN lens elements in dual-band, MWIR-LWIR sensors. The SWaP and performance advantages of IR-GRIN lenses in platform-relevant dual-band imagers will be presented.
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. NRL is working on developing new IR glasses that transmit from 0.9 to > 12 µm in wavelength, with refractive index ranging from 2.38 to 3.17, to expand the glass map and provide compact solutions to multispectral imaging systems. These glasses were specifically designed to have comparable glass molding temperatures and thermal properties so that they can be laminated and co-molded into optics with reduced number of air-glass interfaces (lower Fresnel reflection losses). These new NRL glasses also have negative or very low dn/dT, making it easier to athermalize the optical system. Our multispectral optics designs using these new materials demonstrate reduced size, complexity and improved performance. This presentation will cover discussions on the new optical materials, multispectral designs, as well fabrication and characterization of new optics.
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.
There is a strong desire to reduce size and weight of single and multiband IR imaging systems in Intelligence, Surveillance and Reconnaissance (ISR) operations on hand-held, helmet mounted or airborne platforms. NRL is developing new IR glasses that expand the glass map and provide compact solutions to multispectral imaging systems. These glasses were specifically designed to have comparable glass molding temperatures and thermal properties to enable lamination and co-molding of the optics which leads to a reduction in the number of air-glass interfaces (lower Fresnel reflection losses). Our multispectral optics designs using these new materials demonstrate reduced size, complexity and improved performance. This presentation will cover discussions on the new optical materials, multispectral designs, as well fabrication and characterization of new optics.
Additionally, graded index (GRIN) optics offer further potential for both weight savings and increased performance but have so far been limited to visible and NIR bands (wavelengths shorter than about 0.9 µm). NRL is developing a capability to extend GRIN optics to longer wavelengths in the infrared by exploiting diffused IR transmitting chalcogenide glasses. These IR-GRIN lenses are compatible with all IR wavebands (SWIR, MWIR and LWIR) and can be used alongside conventional materials. The IR-GRIN lens technology, design space and anti-reflection considerations will be presented in this talk.
This paper presents new multispectral IR glasses with transmission from 0.9 to > 14 μm in wavelength and refractive
index from 2.38 to 2.17. These new glasses are designed to have comparable glass softening temperatures and
compatible coefficients of thermal expansion to allow bonding and co-molding of multilayer optics. With large variation
in their Abbe numbers and negative to near-zero dn/dT, optics made from these new glasses can significantly reduce the
size/weight or complexity of the multispectral imaging systems for weight sensitive platforms.
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 methodologies are discussed.
Modeling and optimization of the diffusion process is exploited to minimize the deviation of the index profile from the
design profile.
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.
Graded index (GRIN) optics offer potential for both weight savings and increased performance but have so far been
limited to visible and NIR bands (wavelengths shorter than about 0.9 μm). NRL is developing a capability to extend
GRIN optics to longer wavelengths in the infrared by exploiting diffused IR transmitting chalcogenide glasses. These
IR-GRIN lenses are compatible with all IR wavebands (SWIR, MWIR and LWIR) and can be used alongside
conventional wideband materials. Traditional multiband IR imagers require many elements for correction of chromatic
aberrations, making them large and heavy and not well-suited for weight sensitive platforms. IR-GRIN optical elements
designed with simultaneous optical power and chromatic correction can reduce the number of elements in wideband
systems, making multi-band IR imaging practical for platforms including small UAVs and soldier handheld, helmet or
weapon mounted cameras. The IR-GRIN lens technology, design space and anti-reflection considerations are presented
in this paper.
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.
While Cu(In,Ga)Se2 (CIGS) has established itself as the thin film photovoltaic material of choice with current record efficiencies in excess of 20%, current high-efficiency laboratory-scale fabrication techniques, such as multi-stage evaporation, are ill suited to mass production. Quaternary-sputtering is a promising alternative technique for CIGS deposition, where a single sputtering target made from CIGS itself in the desired stoichiometry is used as the sole deposition source. Devices made using this technique do not require any additional post deposition selenium treatment and have demonstrated peak efficiency in our laboratory in excess of 10%, showing the potential of quaternary sputtering. In an effort to reduce deposition times, we have fabricated films using pulsed DC sputtering, which substantially reduces the substrate time-at-temperature during absorber formation. DC-sputtered films are observed to have reduced surface roughness and different internal morphology from RF-sputtered films, but show increased crystallographic alignment along the (112) plane. DC-sputtered CIGS is thickness-limited to less than 600 nm due to excessive target damage and exhibits power conversion efficiencies of 5-6%.
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.
We report new materials that transmit from 0.9 to > 14 μm in wavelength and fill up the glass map for multispectral optics having 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. The new IR glasses can be easily molded and also fused together to make bonded doublets. We present the benefits of these new materials through dual-band optics designs and compare to designs using currently available crystalline materials.
We report on the study of adding metal dopants in chalcogenide glasses to enhance the photosensitivity of the fiber core
versus cladding compositions, with goal to enable high reflectivity gratings in infrared-transmitting fibers. Results for
the optical and thermal properties of these glasses will be presented, as well as for gratings formation in the glasses using
various writing wavelengths for the different doped compositions.
We report on development and characterization of square registered infrared imaging bundles fabricated from As2S3fiber for HWIL applications. Bundle properties and cross-talk measurements are presented.
Arsenic sulfide (As-S) and arsenic selenide (As-Se) glass optical fibers typically possess extrinsic absorption bands in the infrared wavelength region associated with residual hydrogen and oxygen related impurities, despite using purified precursors. We report a purification process based on the addition of 0.1 wt%tellurium tetrachloride (TeCl4) to the glass. During melting, the chlorine from TeCl4 reacts with the hydrogen impurities to produce volatile products (e.g. HCl) that can be removed by subsequent dynamic distillation. The processing conditions have been modified accordingly to give low H-S (1.5 dB/m) and low H-Se (0.2 dB/m) impurity content.
We have demonstrated Raman amplification in small core As-Se fiber. We observed over 20 dB of gain in a 1.1-meter length of fiber pumped by a nanosecond pulse of approximately 10.8 W peak power at 1.50 micrometers . The peak of the Raman gain was shifted by approximately 230 cm-1 to 1.56 micrometers . The Raman gain coefficient is estimated to be about 2.3 x 10-11 m/W, over 300 times greater than that of silica. The large Raman gain coefficient coupled with the large IR transparency window of these fibers shows promise for development of As-Se Raman fiber lasers and amplifiers in the near, mid and long IR spectral regions.
The change in the absorption loss relative to room temperature of the IR-transmitting sulfur-based (As-S-Se) and tellurium-based (Ge-As-Se-Te) glass fibers in the temperature range of-110°C? T? 110°C was investigated. For the sulfur-based (As-S-Se) glass fibers, the change in loss relative to room temperature was slightly affected by temperature in the wavelength region of 1-5 ?m. For ? ? 6 ?m, the change in loss was mainly due to multiphonon absorption. For the tellurium-based (Ge-As-Se-Te) glass fibers, the attenuation increased significantly at T 40°C. This is mainly attributed to thermally activated free carriers associated with the semi-metallic character ofthe Te atom. For ? ? 4.2 ?m, the loss due to electronic and free carrier absorption was strongly affected by temperature. In the wavelength region of 5 - 11 ?m, the loss was mainly due to free carrier absorption. Beyond ?? 1 1 ?m, multiphonon absorption dominated the loss spectrum at T ? 60°C while free carrier absorption contributed mainly to the total loss at T 80°C.
Chalcogenide glass fibers based on sulphide, selenide, telluride and their rare earth doped compositions are being actively pursued at the Naval Research Laboratory (NRL) as well as world-wide. Great strides have been made in reducing optical losses using improved chemical purification techniques, but further improvements are needed in both purification and fiberization technology to attain the theoretical optical losses. Despite this, current singlemode and multimode chalcogenide glass fibers are enabling numerous applications. Some of these applications include laser power delivery, chemical sensing, scanning near field microscopy/spectroscopy, and fiber IR sources/lasers and amplifiers.
We have fabricated long lengths of low loss sulphide and telluride glass fibers for the 1 - 6 and 3 - 12 micrometers regions, respectively. Minimum losses for core/clad fibers are approximately 0.6 and 0.7 dB/m, respectively, while core-only fibers have exhibited losses of about 0.1 dB/m. The measurements have been performed on long lengths, typically 7 - 50 meters. Fiber strengths are reasonable for many short length applications, but improved processing will lead to stronger fibers for long length applications. These fibers are candidates for chemical sensors and for IR laser power delivery.
We have fabricated stable chalcogenide glasses containing up to 35 at. % tellurium and these glasses do not exhibit crystallization upon reheating up to the fiber draw temperature. The physical properties such as Tg, packing density, and Vickers Hardness decreases while the mass density and CTE increase with Te content and these are attributed to the weaker delocalized metallic-bonding character introduced with Te. We have drawn unclad fibers with a minimum attenuation of 0.11 dB/m at 6.6 micrometers which represents the lowest loss reported for a chalcogenide glass containing high levels of Te. Preliminary core/clad fibers have been drawn with a minimum loss of 0.7 dB/m at 6.6 micrometers . Improvements in glass quality and processing will lead to lower losses. We also present data demonstrating the use of unclad fibers for evanescent sensing of numerous organic and inorganic liquids and their mixtures in the 3-12 micrometers region.
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