Imaging multiple wavebands through a common aperture with transmissive optics brings new challenges for the optical system designer. Very few commonly available lens materials operate in both the visible and midwave infrared leading to complex lenses with many elements to correct for chromatic aberrations. NRL is developing new materials to fill that gap. These new materials enable greater flexibility for designers of lenses for advanced defense applications and potentially reduce the size, weight and cost of next-generation optics. This paper will discuss optical and physical properties of the new materials, progress in their development and the advantages of using NRL materials in transmissive optics designs for multiband applications.
Lasers built on crystalline fibers are capable of producing significantly higher power output per fiber compared to their glass counterparts. This paper explores the barriers for the adoption and implementation of crystal fibers in high-power laser systems and recent advances in crystal fiber fabrication and testing. An overview and update of the crystal fiber work at NRL will be presented.
There are limited fiber-based single-mode laser sources with wavelength emissions in the visible and near infrared range. Nonlinear conversion through four-wave mixing in photonic crystal fibers allows for the generation of new wavelengths far from a pump wavelength. Utilizing an all-fiber spliced configuration, we convert 1064 nm light into a W-level signal in the 750 nm – 820 nm spectral region. Out of our custom photonic crystal fiber, we demonstrate 11.3 watts in the signal band, with M2 ⪅ 1.15, at an optical-optical conversion efficiency of 37.2%.
We explore three types of advanced optical components for use in EO/IR systems. Freeform, GRIN, and meta- optics and combinations of all three used in various system designs are being studied to develop a trade space road map for future utilization. The emphasis is on SWAP benefits while maintaining or improving performance compared to existing systems.
The behavior of erbium doped fiber amplifiers (EDFA) in a master oscillator power amplifier configuration was characterized, for the first time, in-situ using a Co60 γ-ray source. The results indicate that power degradation and photo-anneal recovery processes simultaneously exist. A fiber optimized for efficiency also provides a radiation harder EDFA with reduced power degradation after a 10 kRad (100 Gy) exposure.
Rare-earth-doped fibers with single-crystal cores have the potential for 10x higher TMI threshold than their glass counterparts and are a promising candidate for use as gain media in high-power laser systems. Their utility has been limited by parasitic optical losses and difficulty in fabrication. This paper explores methods to reduce the losses in these fibers in the core, in the cladding and at the core-cladding interface and an overview of the work done at NRL in this area.
NRL is developing new materials that transmit across wide wavelength ranges. MILTRAN is a new rugged optical ceramic that transmits visible through LWIR and is 3.5 times harder than ZnS. With a negative dn/dT, it is well suited as an internal lens element. NRL-series moldable glasses transmit SWIR through LWIR and may be bonded to each other in an adhesive-free thermal process. NRL-200-series glasses transmit visible through MWIR and expand the glass map for multispectral lens designs. These new materials enable greater flexibility for designers of lenses for advanced defense applications and potentially reduce the size, weight and cost of next-generation optics. This paper will discuss new optical materials, their properties and the advantages of using NRL materials in optics designs.
Transparent magnesium aluminate spinel (MgAl2O4) has been developed as an optical ceramic for a variety of applications, including as windows. As a broadband, ultraviolet (UV) thru midwave infrared (MIR) material, it has been developed for windows and has many desirable properties compared with alternative infrared glasses and other transparent ceramics. Current efforts to advance high strength spinel manufacturing processes have demonstrated progress toward large format windows. Although low-absorption spinel, specifically in the near-infrared (NIR) has been demonstrated previously, additional processing is required for new, large-scale spinel manufacturing processes to decrease the effects of impurities near one-micron wavelengths. In this work we present recent results that show measured absorption near 1 μm is reduced by annealing, which reduces effects of trace impurities. Experimental results from photo-thermal common-path interferometer measurements are reported.
Recent progress on the procurement, purification, and sintering of a variety of high temperature transparent ceramics is presented. There are numerous applications operating in very harsh environments that require rugged windows. Oxide materials such as Y2O3 and non-oxides such as cubic β-SiC are prime candidates for these applications due to high mechanical strength, good transmission range, and isotropic structure. To fabricate a high transparency window, phase and chemically pure powder precursors must be obtained. Higher purity β-SiC powders are becoming commercially available and multiple sources are analyzed, purified, and sintered to optimize transmission. Both oxide and non-oxide materials are sintered via spark plasma sintering (SPS) and the optical and physical properties are discussed.
Rare-earth-doped fibers with single-crystal cores have the potential for 10x higher TMI threshold than their glass counterparts and are a promising candidate for use as gain media in high-power laser systems. Their utility has been limited by parasitic optical losses and difficulty in fabrication. This paper explores methods to reduce the losses in these fibers in the core, in the cladding and at the core-cladding interface. Fabrication methods are also discussed.
Rare-earth-doped fibers with single-crystal cores have the potential for 10x higher TMI threshold than their glass counterparts and are a promising candidate for use as gain media in high-power laser systems. Their utility has been limited by parasitic optical losses and difficulty in fabrication. This paper explores methods of fabrication of the fibers including core growth via LHPG and application of cladding materials.
NRL is developing new materials that transmit across wide wavelength ranges and will present recent results. MILTRAN is a new optical ceramic that transmits visible through LWIR and is well suited as an internal lens element. NRL-series moldable glasses transmit SWIR through LWIR and may be bonded to each other in an adhesive-free thermal process. NRL-200-series glasses transmit visible through MWIR and expand the glass map for multispectral lens designs. These new materials enable greater flexibility for designers of lenses for advanced defense applications and potentially reduce the size, weight and cost of next-generation optics.
Rare-earth-doped fibers with single-crystal cores have the potential for 10x higher TMI threshold than their glass counterparts and are a promising candidate for use as gain media in high-power laser systems. Their utility has been limited by parasitic optical losses and difficulty in fabrication. This paper explores methods to reduce the losses in these fibers in the core, in the cladding and at the core-cladding interface. Fabrication methods are also discussed.
In recent years, ceramic sesquioxide materials have emerged as a promising alternative to crystalline laser hosts for near- and mid-infrared laser applications. Ceramics offer a number of manufacturing advantages over crystals including lower fabrication temperatures and the amenability to forming much larger size samples. In this work, a number of RE ions, doped into multiple sesquioxide hosts, are spectroscopically characterized in order to assess their potential for near- and mid-infrared laser applications. Characterization methods included absorption and fluorescence spectroscopy as well as decay dynamics, all measured as functions of temperature. The results are analyzed in order to determine the best laser gain media in the near- and mid-infrared spectral regions.
NRL is developing new materials that transmit across wide wavelength ranges and will present recent results. MILTRAN is a new rugged optical ceramic that transmits visible through LWIR and is 3.5 times harder than ZnS and is well suited as an internal lens element. NRL-series moldable glasses transmit SWIR through LWIR and may be bonded to each other in an adhesive-free thermal process. NRL-200-series glasses transmit visible through MWIR and expand the glass map for multispectral lens designs. These new materials enable greater flexibility for designers of lenses for advanced defense applications and potentially reduce the size, weight and cost of next-generation optics.
At the Naval Research Laboratory, we have developed several rugged oxide materials for optics applications. These materials include cubic sesquioxides (Y2O3 and Lu2O3) and spinel ceramics which transmit in the UV, visible, SWIR and MWIR wavelength region. These materials have superior mechanical and optical properties compared to the currently used materials on various Navy / DoD systems. The transparent and rugged ceramics are fabricated by the Hot press/HIP method using ultra-pure powder synthesized in-house and the resulting ceramics typically have excellent optical quality with no birefringence. In this paper, we present the optical properties of these rugged transparent ceramics and lens designs using these materials will be also discussed.
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.
Rugged optics and window materials with good visible and IR transmission are needed for several sensing and imaging application. Some of the applications require very thick windows or windows in complex geometries, which impose additional limitations on material selection. Magnesium aluminate spinel (MgAl2O4), or spinel for short, is a rugged window material with excellent transmission in ultraviolet to midwave infrared (0.18 – 5.5 m) wavelengths. Spinel is comparable to sapphire and ALON in its ruggedness while having superior transmission near 5 m in wavelength. With its cubic crystal structure, spinel is isotropic unlike sapphire which is birefringent. We have been developing high optical quality spinel ceramics in different shapes and sizes. Current status of these efforts in making spinel windows in large sizes and conformal shapes for several imaging applications will be discussed.
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
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.
Transparent magnesium aluminate spinel (MgAl2O4) ceramic has excellent transmission from the UV to mid-wave IR. It is rugged with strength that is 5x that of glass. Spinel is being developed as a sensor window for numerous military platforms. At the U.S. Naval Research Laboratory (NRL), we have focused on process developments to facilitate wider acceptance of spinel for various applications. These developments include purification of spinel to reduce the absorption and scattering losses for use as an exit aperture on High Energy Laser (HEL) systems and various cost-effective densification methods to reduce manufacturing costs. In this paper, we will provide an update on some of the ongoing spinel activities at NRL.
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.
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.
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.
Single crystal fiber composed of rare earth doped YAG offers the potential for high power scaling of fiber lasers due to it lower intrinsic stimulated Brillouin cross-sections and higher thermal conductivity. The use of rare earth doped YAG fibers also mitigates issues associated with photodarkening, as well as issues associated with the silica multiphonon edge absorption and OH- quenching observed in Ho doped silica fiber lasers operating at eye safer wavelengths. To date, small diameter single crystal YAG fibers as small as 17 µm have been grown at Naval Research Laboratory to achieve a core architecture. Recent work has focused on development of cladding structures on the single crystal core material through sputtering, liquid phase epitaxy, and hydrothermal crystal growth to achieve a true core/clad all- crystalline wave guiding structure. Crystalline claddings have been grown with all three approaches with varying degree of quality and crystallinity. In this paper, we will report on our progress in fabricating crystal claddings on YAG single crystal core material.
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.
Various rare earth doped single crystal YAG and sesquioxide fibers have been drawn using a state-of-the-art Laser Heated Pedestal Growth system. All crystalline core/clad fibers where thermal and optical properties are superior over glass based fibers have been successfully fabricated using various crystal growth and deposition methods. We report on the various fabrication methods, optical characterization of these clad fibers.
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-transmitting optics used in imaging systems have high refractive indices (n=1.4 to n > 3) that require antireflective (AR) coatings. These coatings have limitations in that they can delaminate in operational environments, which is a problem particularly for broadband coatings that consist of multiple layers of dissimilar materials. In addition, residual reflections within an imaging system can cause ghost reflections, degrading performance. Recently, new methods have been developed for fabrication of anti-reflective surface structures (ARSS) on optics that significantly reduce reflection losses at the surface. The ARSS approach provides a more robust solution by using surface structures built directly into the actual surface of the optics, without the need for a coating with extraneous materials. We present recent results that demonstrate superior ARSS performance on a variety of optics for use in the infrared spectral region. These materials have been successfully patterned with ARSS using reactive ion etching (RIE) or using photolithography and etching. We report on reflection losses as low as 0.02% for fused silica at 1.06 microns, and have also demonstrated low reflection losses for ARSS on germanium, spinel ceramic, and sapphire, all of which are important for mid- to long-wave infrared imaging applications.
Transparent magnesium aluminate spinel (MgAl2O4) ceramic has excellent transmission from the UV to mid-wave IR. It is rugged with strength that is 5x that of glass. Spinel also has better IR transmission compared to sapphire and ALON. Because of its superior mechanical and optical properties, it is considered as a sensor window for numerous military platforms. At the Naval Research Laboratory (NRL), we have focused on process developments to facilitate wider acceptance of spinel for various applications. These developments include purification of spinel to reduce the absorption and scattering losses, as well as new processes to make conformal spinel windows and also to reduce manufacturing and finishing costs. In this presentation, we will provide an update on all the spinel activities at NRL
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.
Infrared (IR) transmissive moth eye-like substrates, including randomly patterned fused silica and various periodically patterned germanium substrates, were surface modified using a simple process. Goniometric analysis showed that the surface modification altered the surface wettability of each substrate, rendering them superhydrophobic. Following the surface modification, it was determined that the desirable IR transmission and antireflective properties of each substrate type were maintained. Furthermore, the hydrophobicity, IR transmission and antireflective capabilities of the substrates were shown to be significantly enhanced in comparison to native, non-patterned fused silica and germanium substrates that underwent the same processes. The results of this work provide an opportunity for the development of enhanced utility for infrared transmissive optics in wet or humid conditions.
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.
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.
Single crystal fibers are currently being developed for high power single frequency lasers in 1-2 µm region. Crystal fibers offer several advantages over traditional glass fibers such as silica fiber due to their higher thermal conductivity and higher stimulated Brillouin scattering (SBS) thresholds, along with excellent environmental stability and higher doping concentrations. Yb3+ and Ho3+ doped single crystal YAG fibers with diameters down to 17 µm and lengths >1m long have been grown using our state-of-the-art Laser Heated Pedestal Growth system. Single and double cladded rare earth doped crystal fibers have been fabricated using glasses where optical and physical properties were precisely matched to the core single crystal fiber. We also show successful fabrication of all crystalline core/clad fibers where thermal and optical properties are superior over glass based fibers. Various fabrication methods, optical characterization and gain measurements on these clad fibers will be reported.
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.
We report on the recent progress in the development of cladded single crystal fibers for high power single frequency
lasers. Various rare earth doped single crystal YAG fibers with diameters down to 17 μm with length > 1 m have been
successfully drawn using a state-of-the-art Laser Heated Pedestal Growth system. Single and double cladding on rare
earth doped YAG fibers have been developed using glasses where optical and physical properties were precisely
matched to doped YAG core single crystal fiber. The double clad Yb:YAG fiber structures have dimensions analogous
to large mode area (LMA) silica fiber. We also report successful fabrications of all crystalline core/clad fibers where
thermal and optical properties are superior over glass cladded YAG fibers. Various fabrication methods, optical
characterization and gain measurements on these cladded YAG fibers are reported.
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.
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.
Space environment is very harsh for optical systems. Currently available optical materials for space
applications are susceptible to surface and bulk damage due to high-speed impacts from dust and debris found
in the space environment. Impacts lead to surface pitting and fracturing that may compromise structural
integrity and degrade the optical performance of imaging systems. We are developing polycrystalline spinel
as a rugged optics material. With its 3x hardness and 5x strength, as compared to BK7 glass, spinel is a very
promising optical material for space imaging applications. Spinel’s broad transmission from 160 nm to 5000
nm will also enable multispectral imaging from ultraviolet to midwave infrared.
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.
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.
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.
Spinel ceramic exhibits excellent optical and mechanical properties, but its widespread use in high volume applications
has been limited primarily due to the high cost associated with hot pressing and finishing. While, we have previously
demonstrated techniques to reduce finishing costs, in this paper we report on the use of microwave sintering to make
spinel ceramic at significantly lower cost than traditional hot pressing. We also identify preferred grain growth as well as
an intra-granular fracture mode.
Ideal exit aperture windows for high-energy laser (HEL) should possess low absorption and scattering losses and be environmentally rugged and strong in order to protect the laser gain medium without compromising the light propagating through the window. Spinel is an ideal candidate for this application due to its high mechanical strength, high thermal conductivity, and excellent optical transmission between 0.2~5 μm. However, spinel ceramics fabricated with commercial powders often show inhomogeneity and suffer from absorption and scattering caused by various types of intrinsic and extrinsic impurities present in the powders. Here, we report on a convenient and economical powder purification method to significantly lower the absorption loss of transparent spinel ceramics using commercial powders. Acid washing was successfully used to reduce absorption loss in spinel ceramic fabricated using commercial powder from >20,000 ppm/cm down to 75 ppm/cm.
There are several military or commercial systems operating in very harsh environments that require rugged
windows. On some of these systems, windows become the single point of failure. These applications
include sensor or imaging systems, high-energy laser weapons systems, submarine photonic masts, IR
countermeasures and missiles. Based on the sea or land or air based platforms the window or dome on
these systems must withstand wave slap, underwater or ground based explosions, or survive flight through
heavy rain and sand storms while maintaining good optical transmission in the desired wavelength range.
Some of these applications still use softer ZnS or fused silica windows because of lack of availability of
rugged materials in shapes or sizes required. Sapphire, ALON and spinel are very rugged materials with
significantly higher strengths compared to ZnS and fused silica. There have been recent developments in
spinel, ALON and sapphire materials to fabricate in large sizes and conformal shapes. We have been
developing spinel ceramics for several of these applications. We are also developing β−SiC as a transparent
window material as it has higher hardness, strength, and toughness than sapphire, ALON and spinel. This
paper gives a summary of our recent findings.
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.
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 results for antireflective surface structures (ARSS) fabricated directly into the surface of optics and lenses which are important as high energy (multi-kW) laser components, including fused silica windows and lenses, YAG crystals and ceramics and spinel ceramics. Very low reflection losses as well as high laser damage thresholds have been measured for optics with ARSS. Progress to scale up the process for large size windows will also be presented..
The U.S. Naval Research Laboratory has pioneered the development of sintering processes for making highly transparent optical ceramics. For example, we have demonstrated the fabrication of record low absorption loss spinel as an exit window for High Energy Laser systems and rare earth doped Y2O3 and Lu2O3 for solid-state ceramic lasers. We have also developed thick spinel windows for submarine photonic masts and predicted the performance of an imaging system using testing and modeling. More recently, we have developed a novel approach of hot pressing where a transparent ceramic is produced in the net shape without requiring post polishing. This technology will result in significant cost savings associated with polishing the final optical element. We are also developing motheye structures on spinel surface to provide rugged anti-reflective solutions. We had earlier identified a Barium GalloGermanate (BGG) glass with matching index and expansion coefficient to spinel. We had demonstrated fabrication of a laminated dome for the Joint Air to Ground Missile (JAGM) program and the technology was transitioned to industry. We have pushed this technology further by developing a BGG glass – spinel ceramic transparent micro-composite, which can be processed well below spinel sintering temperatures. To address the relatively lower strength of BGG glass compared with spinel, we developed an ion-exchange process and achieved strengths up to 450 MPa. This paper gives a summary of our recent findings.
Transparent beta-SiC is of great interest because its high strength, low coefficient of thermal expansion, very high thermal conductivity, and cubic crystal structure give it a very high thermal shock resistance. A transparent, polycrystalline beta-SiC window will find applications in armor, hypersonic missiles, and thermal control for thin disc lasers. SiC is currently available as either small transparent vapor grown disks or larger opaque shapes. Neither of which are useful in window applications. We are developing sintering technology to enable transparent SiC ceramics. This involves developing procedures to make high purity powders and studying their densification behavior. We have been successful in demonstrating transparency in thin sections using Field Assisted Sintering Technology (FAST). This paper will discuss the reaction mechanisms in the formation of beta-SiC powder and its sintering behavior in producing transparent ceramics.
We report new multispectral materials that transmit from 0.9 to < 12 µm in wavelength. These materials 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. One of the glasses developed is a very good candidate to replace Ge, as it has a combination of excellent properties, including high Abbe number in the LWIR, high index of 3.2, 60% lower dn/dT, and better thermal stability at working temperatures. Our results also provide a wider selection of optical materials to enable simpler achromat designs. For example, we have developed other glasses that have relatively high Abbe number in both the MWIR and LWIR regions, while our MILTRAN ceramic has low Abbe number in both regions. This makes for a very good combination of glasses and MILTRAN ceramic (analogous to crown and flint glasses in the visible) for MWIR + LWIR dual band imaging. We have designed preliminary optics for one such imager with f/2.5, 51 mm focal length and 22 degrees FOV using a spaced doublet of NRL's glass and MILTRAN ceramic. NRL's approach reduces the number of elements, weight, complexity and cost compared with the approach using traditional optics. Another important advantage of using NRL glasses in optics design is their negative or very low positive dn/dT, that makes it easier to athermalize the optical system.
Fused silica, YAG crystals, and spinel ceramics substrates have been successfully patterned through reactive ion etching (RIE). Reflection losses as low as 0.1% have been demonstrated for fused silica at 1.06 microns. Laser damage thresholds have been measured for substrates with ARSS and compared with uncoated and/or thin-film anti-reflection (AR) coated substrates. Thresholds as high as 100 J/cm2 have been demonstrated in fused silica with ARSS at 1.06 microns, with ARSS substrates showing improved thresholds when compared with uncoated substrates.
Spinel is a rugged ceramic transparent from ultraviolet to midwave infrared (0.18 – 5.5 μm) wavelengths. It has the best transmission from 4-5 μm among the competing materials ALON and sapphire with comparable mechanical properties. We have developed low absorption loss spinel as an exit window aperture for High Energy Laser systems. We demonstrated that spinel possesses excellent thermo-optical characteristics required for the High Energy Laser systems and at the same time it can provide the necessary ruggedness desired for the realistic and harsh battlefield environment. We have demonstrated through testing that spinel can withstand very adverse conditions of rain, sand storms and salt fog conditions without any change in its optical performance. We have also developed rugged anti-reflective coatings and anti-reflective surface structures to maintain high optical transmission in adverse environment.
In this paper, we present our recent progress in the development of rare-earth (Yb3+ or Ho3+) doped Lu2O3 and Y2O3 sesquioxides for high power solid state lasers. We have fabricated high quality transparent ceramics using nano-powders synthesized by a co-precipitation method. This was accomplished by developments in high purity powder synthesis and
low temperature scalable sintering technology developed at NRL. The optical, spectral and morphological properties as
well as the lasing performance from our highly transparent ceramics are presented. In the second part of the paper, we
discuss our recent research effort in developing cladded-single crystal fibers for high power single frequency fiber lasers
has the potential to significantly exceed the capabilities of existing silica fiber based lasers. Single crystal fiber cores
with diameters as small as 35μm have been drawn using high purity rare earth doped ceramic or single crystal feed rods
by the Laser Heated Pedestal Growth (LHPG) process. Our recent results on the development of suitable claddings on
the crystal fiber core are discussed.
In this paper, we present our recent progress in developing single crystal fibers for high power single frequency fiber
lasers. The optical, spectral and morphological properties as well as the loss and gain measured from these crystal fibers drawn by Laser Heated Pedestal Growth (LHPG) system are also discussed. Results on application of various cladding materials on the crystal core and the methods of fiber end-face polishing are also presented.
In this paper, we present our recent results in developing cladded-single crystal fibers for high power single frequency fiber lasers significantly exceeding the capabilities of existing silica fiber based lasers. This fiber laser would not only exploit the advantages of crystals, namely their high temperature stability, high thermal conductivity, superior environmental ruggedness, high propensity for rare earth ion doping and low nonlinearity, but will also provide the benefits from an optical fiber geometry to enable better thermal management thereby enabling the potential for high laser power output in short lengths. Single crystal fiber cores with diameters as small as 35m have been drawn using high purity rare earth doped ceramic or single crystal feed rods by Laser Heated Pedestal Growth (LHPG) process. The mechanical, optical and morphological properties of these fibers have been characterized. The fibers are very flexible and show good overall uniformity. We also measured the optical loss as well as the non-radiative loss of the doped crystal fibers and the results show that the fibers have excellent optical and morphological quality. The gain coefficient of the crystal fiber matches the low quantum defect laser model and it is a good indication of the high quality of the fibers.
Abstract. Recent progress in high-quality transparent ceramic window materials (MgAl 2 O 4 spinel and β -SiC) and high-power solid-state laser materials (Yb 3+ ∶Y 2 O 3 , Yb 3+ ∶Lu 2 O 3 , and Ho 3+ ∶Lu 2 O 3 ) is reported. Spinel ceramic demonstrates a record low-absorption loss of 6 ppm/cm at 1.06 μm. The capability of fabricating various shapes and sizes of spinel ceramics is also demonstrated. We also report optical transparency from a β -SiC ceramic fabricated by field assisted sintering technology (FAST). We report lasing in hot-pressed Yb 3+ ∶Y 2 O 3 and Yb 3+ ∶Lu 2 O 3 ceramic made from coprecipitated powder. The highest ever reported output power and efficiency from 10% doped Yb 3+ ∶Lu 2 O 3 ceramic is also presented. Lasing oscillation from hot pressed composite of five-layered Yb 3+ ∶Y 2 O 3 ceramic is also demonstrated for the first time.
We have developed several rugged materials for multiband applications in the 0.2-12.0 μm wavelength region. These
materials include spinel ceramic and barium gallogermanate (BGG) glass which transmit in the UV, visible, SWIR and
MWIR wavelength region. They also include MILTRANTM and IR glasses which transmit in the SWIR, MWIR and
LWIR. These materials have superior mechanical and optical properties compared to the currently used materials on
various Navy / DoD systems. Spinel ceramic is comparable to sapphire in its ruggedness but it overcomes the
birefringence and poor 4-5 μm transmission of sapphire. Spinel has also been made into large size windows as well as
domes for several applications. BGG glass is very useful for systems requiring very large windows with visible -
MWIR transmission. High optical quality BGG glass windows in sizes up to 20" in diameter have been made.
MILTRANTM is > 3 times harder and stronger than ZnS and can provide a rugged window or dome for broadband
sensors. IR glass can be easily made in complex shapes. BGG glass and spinel ceramic laminated dome was developed
for tri-mode seeker. MILTRANTM and IR glasses can be combined to make achromats for multispectral imaging
systems.
This work explores the influence of head window thickness on the performance of a mid-wave infrared, panoramic
periscope imager. Our focus is on transparent spinel ceramic as the head window material. Spinel is an attractive
material for IR applications due to its good strength and transmission properties (visible through mid-wave).
However, there is some degradation in spinel transmission near the high end of the mid-wave band ( 5μm) as
the head window thickness increases. In this work we predict the relationship between head window thickness
and imager performance, as quantified by the Noise Equivalent Temperature Difference, and compare these
predictions to values estimated from experimental data. We then discuss the implications for imager design and
demonstrate a possible approach to correcting for the headwindow-induced losses. The imager used in this study
is a compact, catadioptric, camera that provides a 360o horizontal azimuth by -10o to +30o elevation field of
view and uses a 2048 x 2048, 15μm pitch InSb detector.
We report results for high quality transparent ceramic window materials (spinel and β-SiC) and high power solid state
laser materials (Yb:Y2O3 and Yb:Lu2O3). Spinel ceramic demonstrates a record low absorption loss of 6 ppm/cm at 1.06
μm. We also report optical transparency from a β-SiC ceramic fabricated by the Spark Plasma Sintering technique.
Capability of fabricating various shape and size of spinel ceramics is also demonstrated. We report lasing in hot pressed
Yb3+:Y2O3 and
Yb3+:Lu2O3 ceramic made from co-precipitated powder. Highest output power and efficiency from
heavily doped Yb3+ doped Lu2O3 ceramic are reported.
High optical quality spinel is being developed as exit aperture (windows and domes) for various electro-optic (EO) and infrared (IR) systems operating in harsh environmental conditions. These applications require windows with low absorption loss, low scattering loss, good index homogeneity, low transmitted wavefront error and high strength. Windows are also required in very large sizes and thicknesses for specialty applications. We have demonstrated
fabrication of high optical quality spinel windows with high strength and environmental ruggedness. We achieved record low absorption loss of 6 ppm/cm at 1.06 μm and transmitted wavefront error of better than λ/10 at 633 nm. We are also developing technology for making very large and very thick spinel windows.
The U.S. Naval Research Laboratory (NRL) has developed two unique materials with excellent properties for various
military and commercial applications in the UV-Vis-IR wavelength range. These materials are: an amorphous Barium
Gallo-Germanate (BGG) glass and a polycrystalline Magnesium Aluminate Spinel. The BGG glass is made using
traditional glass melting techniques, and was developed as a low cost alternative to the currently used window materials.
Large prototype windows have been fabricated for a Navy reconnaissance program. BGG windows have been
successfully tested for environmental ruggedness (MIL-F-48616) and rain erosion durability up to 300 mph. BGG glass
is currently under development and evaluation for High Energy Laser (HEL) applications. A new process has been
developed to sinter spinel to clear transparency with very high yields. This process has been used to make various sizes
and shapes (flats and domes) and is readily scalable to industrial sizes to produce large windows & domes for various
applications. NRL has also developed modified BGG glasses, which are compatible with Spinel and ALON substrates
for bonding.
BaO-Ga2O3-GeO2 (BGG) glasses have the desired properties for various window applications in the 0.5-5 μm wavelength region. These glasses are low cost alternatives to the currently used window materials. Fabrication of a high optical quality 18" diameter BGG glass window has been demonstrated with a transmitted wave front error of λ/10 at 632 nm. BGG substrates have also been successfully tested for environmental weatherability (MIL-F-48616) and rain erosion durability up to 300 mph. Preliminary EMI grids have been successfully applied on BGG glasses demonstrating
attenuation of 20dB in X and Ku bands. Although the mechanical properties of BGG glasses are acceptable for various window applications, it is demonstrated here that the properties can be further improved significantly by the glassceramization process. The ceramization process does not add any significant cost to the final window material. The crystallite size in the present glass-ceramic limits its transmission to the 2-5 μm region.
Rare earth doped chalcogenide glass fibers have been developed at NRL. Spectroscopic investigation of these glasses and fibers show that they possess strong and efficient mid-IR and long-wave IR emissions suitable for fiber lasers in these wavelength regions. Small fiber sources based on these emission lines have been developed and can be used as bright IR sources for characterizing focal planes.
BaO-Ga2O3-GeO2(BGG) glasses have the desired properties for window applications in the 0.5-5micrometers wavelength region. These glasses are low cost alternatives to the currently used window materials and are being scaled- up to large sizes for various Department of Defense (DoD) applications. Fabrication of a high optical quality 6.5' x 8.5' x 0.5' rectangular BGG glass window has already been demonstrated. A transmitted wave front error of about (lambda) /15 at 632nm has been achieved in polished BGG glasses. Recently a 20' round glass window blank has been cast. Although the mechanical properties of BGG glass are acceptable for various window applications, it is demonstrated here that they can be further improved significantly either by the glass-ceramization process or by forming a composite. Neither process adds any significant cost to the final window material. The glass composite transmits in the 0.5-5micrometers region while the crystallite size in the glass-ceramic currently limits its transmission to the 2-5micrometers region.
Naval Research Laboratory (NRL) has been developing high brightness mid-wave IR emitting fibers for HWIL testing. These fibers, based upon rare-earth doped chalcogenide glass, emit from 3.5 - 5 m and are capable of simulating very high temperatures in this band. To date, temperatures of 2400 K have been simulated. The fiber sources operate at room temperature, are environmentally tolerant, and can be formed into fiber bundles with high fill factors and low pixel to pixel cross- talk for IR scene generation. In this paper, we will present the spectral output, temporal response, temperature simulation and output uniformity of the mid-wave IR emitting fibers. The potential for long-wave IR emitting fiber sources will also be presented.
BaO-Ga2O3-GeO2 (BGG) glasses have the desired properties for window applications in the 0.5 - 5 micrometers wavelength region. BGG glasses are low cost alternatives to the currently used window materials and are being scaled-up to large sizes for various Department of Defense applications. Fabrication of a high optical quality 6.5' X 8.5' X 0.5' rectangular BGG glass window has already been demonstrated. A transmitted wave front error of about (lambda) /15 at 632 nm has been achieved in polished BGG glasses. Although the mechanical properties of BGG glasses are acceptable for various window applications, it is demonstrated here that they can be further improved significantly either by the glass-ceramization process or by forming a composite. Neither process adds any significant cost to the final window material. The glass composite transmits in the 0.5 - 5 micrometers region while the crystallite size in the glass-ceramic currently limits its transmission to the 2 - 5 micrometers region.
We have developed a spraying process for coating individual phosphor particles with electrically conducting, non- conducting, and hybrid films. The phosphor is suspended in either aqueous or non-aqueous solvent containing the appropriate coating materials precursors. Depositions via gelation or precipitation of the precursor material on the particles is prevented before spraying. The suspension is sprayed into a hot zone where the coating either gels or precipitates on the particle surface in-flight. Coatings developed in-flight are more uniform than coatings developed in solution. Typical coating thicknesses of 10 to 90 nm have been achieved in a single pass. Multiple coatings are applied using multiple passes. This process is scalable to industrial sizes and is easily adaptable for coating particles for other applications.
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