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.
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.
Innovative mid-infrared imaging fiber bundle has been developed that is flexible, rugged and high fiber-count for use with infrared cameras for thermal imaging applications. High-quality chalcogenide fibers are used to produce coherent fiber bundle that is 2 meters in length, 4000 fibers in a 3mm diameter bundle, minimum bend radius of 10cm, and low attenuation over the spectral range of 1.5-6.5 microns. Individual fiber pixel size is 34 microns and the NA is 0.3. This paper presents the fabrication process and the optical characterization of the mid-infrared imaging fiber bundle.
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.
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.
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.
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
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.
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 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 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.
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.
An all-dielectric hollow waveguide structure consisting of radially alternating dielectric layers and exhibiting a photonic bandgap is proposed. Fabrication involves the extrusion of a stack-of-plates into a hollow structure composed of alternating high/low index pairs. Specifically, a billet consisting of alternating glass layers is forced through a die such that laminar flow forces the periodicity from an axial to a radial orientation. The concept is demonstrated using lead-borosilicate and As2Se3 glasses.
Hollow glass waveguides are an increasingly popular fiber- based delivery system for IR laser power. They are low loss and can be bent to radii less than 3 cm while maintaining an excellent modal distribution. The additional loss due to bending has been shown to scale linearly with the inverse of bending radius. For small bending radii however, this bending loss has bene shown to increase beyond this trend due to mode mixing. Oscillating modes have been observed by varying the radius of curvature for a waveguide bent into a single compete loop. Curved HGW distal tip prototypes of bore sizes 530 and 320 micrometers , were made and exhibited a 0.56 and a 0.17 dB increase in loss respectively over the straight waveguides.
IR transmitting hollow waveguides are an attractive alterative to solid-core IR fibers. Hollow waveguides are made from silica glass or plastic tubing which has highly reflective coatings deposited on the inside surface. These guides have losses as low as 0.1 dB/m at 10.6 micrometer and may be bent to radii less than 5 cm. For laser-power delivery applications the hollow glass guides have been shown to be capable of transmitting up to 1 kW of CO2 laser power. In some power delivery applications it is necessary to have a distal tip configured to bend and/or concentrate the light for more efficient ablation. Curved tips (2 cm in length) are shown to increase the loss of a 1-m long straight guide from 22 to 34%. New research is described on the fabrication of coherent hollow glass bundles for IR imaging. The number of guides in the bundle is currently less than 20 but the results indicate that the hollow bundle can be coated to achieve an identical spectral response for each individual guide.
Linearly tapered hollow-glass waveguides (HGW) were fabricated using tapered silica glass tubing and wet chemistry techniques. Attenuation constants for these tapered HGWs were found to be higher than for similarly sized non-tapered HGWs, but the tapered guides showed reduced loss on bending. HGWs with rectangular and square cross-sections were also fabricated from non-circular bore silica glass tubing using wet chemistry techniques. These guides were able to maintain linear polarization of CO2 laser light better than circular bore HGWs fabricated by the same methods, with as high as 97% of the input polarization preserved for a 227 μm X 1253 μm bore guide. The non-circular bore HGWs had higher attenuation constants than similarly sized circular bore HGWs and sacrificed some spatial purity of the output beam.
Coiled hollow waveguide gas absorption cells were designed and fabricated using polymer and glass tubing. These coiled waveguides were found to be acceptable for the detection of gases in the IR region of the spectrum. Through the in-depth investigation of straight and bent losses in hollow waveguides, an equation was developed to calculate the loss of the coiled system. By setting the straight loss to zero, it was found that additional loss on bending was approximately 1 dB per turn, independent of the bore size or bending radius.
Hollow glass waveguides are an attractive fibers delivery system for a broad range of IR wavelengths, including the 3 μm Er:YAG and 10.6 μm CO2 lasers. The losses for these waveguides are as low as 0.2 dB/m at the 10.6 wavelength for waveguides with a 700 μm bore. At the shorter wavelengths, like that of the Er:YAG laser, losses are higher than those predicted theoretically. This is shown to be a result of the increasing effect of surface roughness of the inner coatings. Variation of attenuation from waveguide to waveguide is examined, and the variation in the dielectric layer is evaluated.