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
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.
Fiber lasers rely on clad pump architectures where double clad designs are used. Currently, polymers are used successfully as pump claddings on Yb-doped laser fibers. In order to transition to resonantly pumped fiber lasers at eye safer wavelengths, polymer pump claddings must have low absorption at those wavelengths. There is currently no suitable low index, low absorption, thermally stable, and high thermal conductivity polymer to act as a low-index coating. This work presents a new class of polymers, termed fluorinated polymer composites (FPCs), which possess improved thermal properties. The FPCs are fabricated by incorporating ceramic nanoparticles into fluorinated polymers. Ultimately, the FPCs could enable further use of fiber lasers at commonly used wavelengths, as well as at eye-safer wavelengths.
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
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.
A nanoparticle (NP) doping technique was used for making erbium-doped fibers (EDFs) for high energy lasers. The nanoparticles were doped into the silica soot of preforms, which were drawn into fibers. The Er luminescence lifetimes of the NP-doped cores are longer than those of corresponding solution-doped silica, and substantially less Al is incorporated into the NP-doped cores. Optical-to-optical slope efficiencies of greater than 71% have been measured. Initial investigations of stimulated Brillouin scattering (SBS) have indicated that SBS suppression is achieved by NP doping, where we observed a low intrinsic Brillouin gain coefficient, of ~1× 10-11 m/W and the Brillouin bandwidth was increased by 2.5x compared to fused silica.
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.
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.
Nanoparticle (NP) doping is a new technique for making erbium-doped fibers (EDFs); the Er ions are surrounded by a
cage of aluminum and oxygen ions, substantially reducing Er3+ ion-ion energy exchange and its deleterious effects on
laser performance. Er-Al-doped NPs have been synthesized and doped in-situ into the silica soot of the preform core. We
report the first known measurements of NP-doped EDFs in a resonantly-core pumped master oscillator-power amplifier
(MOPA) configuration; the optical-to-optical slope efficiency was 80.4%, which we believe is a record for this type of
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.
Trivalent holmium has 14 laser channels from 0.55 to 3.9 μm. The laser emission of most interest is the transition 5I7→5I8 near 2 μm because of its potential for use in eye-safe systems and medical applications. In this paper, we present our recent results in the development of Ho3+ doped laser materials for eye-safe solid state lasers. We report a calorimetric study of non-radiative losses in two micron pumped holmium doped laser host materials such as silica glass, yttrium aluminum garnet (YAG) crystal and Lu2O3 ceramics. Optical, spectral and morphological properties as well as the lasing performance from highly transparent ceramics are presented.
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.
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.
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.
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 results in the development of Ho3+ doped sesquioxides for eye-safe solid state lasers. We have synthesized optical quality Lu2O3 nanopowders doped with concentrations of 0.1, 1.0, 2.0, and 5% Ho3+. The powders were synthesized by a co-precipitation method beginning with nitrates of holmium and lutetium. The nanopowders were hot pressed into optical quality ceramic discs. The optical transmission of the ceramic discs is excellent, nearly approaching the theoretical limit. The optical, spectral and morphological properties as well as the lasing performance from highly transparent ceramics are presented.
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.
Ceramic laser materials have come a long way since the first demonstration of lasing in 1964. Improvements in powder
synthesis and ceramic sintering as well as novel ideas have led to notable achievements. These include the first Nd:YAG
ceramic laser in 1995, breaking the 1 KW mark in 2002 and then the remarkable demonstration of more than 100 KW
output power from a YAG ceramic laser system in 2009. Additional developments have included highly doped
microchip lasers, ultrashort pulse lasers, novel materials such as sesquioxides, fluoride ceramic lasers, selenide ceramic
lasers in the 2 to 3 μm region, composite ceramic lasers for better thermal management, and single crystal lasers derived
from polycrystalline ceramics. This paper highlights some of these notable achievements.
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+:Lu2O3 ceramic made from co-precipitated powder. Highest output power and efficiency from
heavily doped Yb3+ doped Lu2O3 ceramic are reported.