The last twenty years have seen a dramatic improvement in the performance of infrared detectors, especially those with uncooled microbolometer arrays. These make new commercial applications possible, from the Internet of things to drones for crop monitoring. Such emerging markets are constantly pushing the requirements on the existing technology to improve cost, performance, weight and size. In this context, gradient index (GRIN) lenses can offer a new degree of freedom compared to their homogeneous counterparts. For example, recent papers have shown how GRIN lenses could help reduce the number of elements in a system, by helping correct lens chromatism and athermalization. While the interest in using GRIN is obvious, mastering their fabrication with infrared materials remains highly challenging. Encouraging progress has been made in that direction, for instance by stacking different materials or by laser writing. Very recently, it has been demonstrated that partial crystallization of chalcogenide glass allows for tuning the refractive index. In this paper, we introduce a new technique based on the controlled formation of nano-crystallites in chalcogenide glass to fabricate the first macroscopic radial GRIN in the infrared. We also propose a simple way to test the index gradient value by interferometry and demonstrate GRIN with index contrast of ▵n ~ 3⋅10-2 between the center and the edge of the disk. The process is easily repeated and opens the possibility for a rapid transfer to the industry.
High Tellurium (Te) content chalcogenide glass fibers are considered as candidates for single mode waveguides in the upper wavelength range (10 – 20 μm) of the DARWIN mission. In this paper two methods for IR optical characterization of the Te glass fibers are presented, including (1) a broadband spectral technique using an FTIR spectrophotometer and (2) a CO2 laser set-up for measuring the fiber attenuation at 10.6 μm. In both methods the optical attenuation (in dB/m) of experimental mono index Te glass fibers of around 500 μm diameter has been determined by applying the fiber cut-back technique. Some typical results of both methods for a few different types of high Te-glass fibers will be shown. Since Te-glasses are semiconducting materials, the optical properties of Te-glass fibers strongly depend on temperature. Preliminary low temperature measuring results confirm the beneficial effect of cooling on the transmission of high Te glass fibers.
The search for Earth-like exoplanets, orbiting in the habitable zone of stars other than our Sun and showing biological activity, is one of the most exciting and challenging quests of the present time. Nulling interferometry from space, in the thermal infrared, appears as a promising candidate technique for the task of directly observing extra-solar planets. It has been studied for about 10 years by ESA and NASA in the framework of the Darwin and TPF-I missions respectively .
Nevertheless, nulling interferometry in the thermal infrared remains a technological challenge at several levels. Among them, the development of the "modal filter" function is mandatory for the filtering of the wavefronts in adequacy with the objective of rejecting the central star flux to an efficiency of about 105. Modal filtering  takes benefit of the capability of single-mode waveguides to transmit a single amplitude function, to eliminate virtually any perturbation of the interfering wavefronts, thus making very high rejection ratios possible.
The modal filter may either be based on single-mode Integrated Optics (IO) and/or Fiber Optics. In this paper, we focus on IO, and more specifically on the progress of the on-going “Integrated Optics” activity of the European Space Agency.
We developed the extrusion method to prepare arsenic-free chalcogenide glass fibers with glass cladding. By using the double nested extrusion molds and the corresponding isolated stacked extrusion method, the utilization rate of glass materials was greatly improved compared with the conventional extrusion method. Fiber preforms with optimal stability of core/cladding ratio throughout the 160 mm length were prepared using the developed extrusion method. Typical fiber structure defects between the core/cladding interface, such as bubbles, cracks, and core diameter variation, were effectively eliminated. Ge-Sb-Se/S chalcogenide glasses were used to form a core/cladding pair and fibers with core/cladding structure were prepared by thermally drawing the extruded preforms. The transmission loss, fiber bending loss, and other optical characters of the fibers were also investigated.
We report the use of the low cost hot-pressing technique to produce ZnS for multispectral operation, from visible up to
12 μm. Considerable progress has been obtained by developing efficient precipitation and combustion powders synthesis
procedures. The main emphasis has been on the elaboration of ZnS precursor powders with controlled
morphology/chemical composition to reduce extrinsic scattering and impurities. We were able to produce ZnS parts with
visible transparency and transmission in the 8-12 μm range that is comparable to that of CVD ZnS. The correlation of
processing variables with powders sinterability and optical transmission of the HPed ceramics is discussed.
In this paper, a first part will present the mechanical properties of the chalcohalide glass system GeS2-Ga2S3-CsCl. The hardness, Young’s modulus, shear modulus and toughness of a series of glasses (0.8 GeS2 - 0.2 Ga2S3)100-x CsClx with x = 0, 5, 10, 15 have been investigated and compared with other glasses. Two particular compositions 75 GeS2 - 15 Ga2S3 - 10 CsCl and 65 GeS2 - 20 Ga2S3 - 15 CsCl are compared with existing industrial chalcogenide glasses. In a second part, two multispectral antireflective coatings are presented. These coatings, developed for a multispectral application, enhanced the transmissions in specific bands.
The thermal imaging market has experienced a strong growth during the recent years due to continued cost reduction of night vision devices. The development of uncooled focal plane detector arrays is the major reason for the cost reduction. Another reason is the continuous improvement of the optical solution. In this paper, we present a new multispectral material which responds to the increasing demand for optics operating simultaneously in the visible/SWIR (Short Wave InfraRed) and the thermal infrared region. The most important properties of some glasses from the GeS2-Ga2S3- CsCl system are highlighted in this study. A stable composition 15Ga2S3-75GeS2-10CsCl allowed the synthesis of a large glass without crystallization. The refractive index of this glass was precisely measured from 0.6 to 10.4μm by using the Littrow method. The chromatic dispersion was then calculated and compared with other multispectral materials.
Colorless sulfide glasses can be obtained by selecting appropriately the composition within the Ga2S3-GeS2-CsCl pseudo-ternary glass system. The addition of electronegative chlorine ions into the sulfide glassy network results in a widening of its optical bandgap without altering its infrared transparency. Glasses transparent from the near UV (380 nm) up to the middle infrared (11.5 μm) are thus achievable. Such extended infrared transmission for a colorless glass is the widest among the known heavy metal oxide and fluoride glasses, e.g. fluoroindate glasses are transparent from 350 nm up to 8-9 μm. We present in this work our recent progress on the preparation of this chloro-sulfide glass of high optical quality. Efforts have been devoted in a first step to reduce the content of extrinsic impurities such as OH, SH and H2O. In a second step, protective coatings have been deposited on polished glass samples to improve their chemical durability and assess their potential for practical applications. Large improvement of both optical quality, in terms of transmission spectrum flattening, and chemical durability were achieved. Finally, the high thermal stability against crystallization of this glass shows a high potential for lens molding and applications in multispectral imaging.
The military uncooled infrared market is driven by the continued cost reduction of the focal plane arrays whilst maintaining high standards of sensitivity and steering towards smaller pixel sizes. As a consequence, new optical solutions are called for. Two approaches can come into play: the bottom up option consists in allocating improvements to each contributor and the top down process rather relies on an overall optimization of the complete image channel. The University of Rennes I with Thales Angénieux alongside has been working over the past decade through French MOD funding’s, on low cost alternatives of infrared materials based upon chalcogenide glasses. A special care has been laid on the enhancement of their mechanical properties and their ability to be moulded according to complex shapes. New manufacturing means developments capable of better yields for the raw materials will be addressed, too. Beyond the mere lenses budget cuts, a wave front coding process can ease a global optimization. This technic gives a way of relaxing optical constraints or upgrading thermal device performances through an increase of the focus depths and desensitization against temperature drifts: it combines image processing and the use of smart optical components. Thales achievements in such topics will be enlightened and the trade-off between image quality correction levels and low consumption/ real time processing, as might be required in hand-free night vision devices, will be emphasized. It is worth mentioning that both approaches are deeply leaning on each other.
In this paper, the strong influence of alkali halide in chalcogenide glasses is reminded, leading for the
first time to highly transparent glasses from the visible range up to 11μm. The behavior of crystallization has
been demonstrated to be similar in sulfide and selenide glasses containing gallium as well. The structural
evolution of several glass compositions from the Ge-Ga-S or Ge-Ga-Se systems leading to reproducible glass ceramics
has been studied by XRD, NMR and thermal analysis. Whatever the composition, gallium plays a
fundamental role as nanosized domains appear by phase separation between Ge rich regions and Ga rich
regions. The determination of the appropriate crystallization time and temperature has permitted to obtain new
passive and active glass-ceramics with a broadened transmittance region thanks to the incorporation of
various alkali halides. In the first case, the controllable generation of nanocrystals leads to an increase of the
main thermo-mechanical properties. In the second case, the incorporation of rare-earth ions inside the glass ceramics
has exacerbated their photoluminescence properties. The possibility to combine the ceramization
process with the shaping has also been demonstrated.
In the present paper we focus on the fabrication of waveguides which will be able to work in the large infrared window
[6-20μm], compatible with the ESA requirements in the framework of the detection of Exo-solar planets by nulling
The first step in the fabrication of such components is the realization of planar waveguides being able to guide light in
this spectral range. In order to do so, telluride materials were selected: Te75Ge15Ga10 bulk glasses were chosen as
substrates and TeGe films as guiding layers. The Te75Ge15Ga10 bulk glasses were purified during their synthesis which
ensures an optimal transmission in the whole range from 6 to 20 μm. TeGe thick films with different compositions were
deposited by thermal co-evaporation. Homogeneous films with thickness up to 15 microns could be produced. The M-lines
measurement of their refractive index at λ = 10.6 μm highlighted a linear behavior versus the atomic percentage in
tellurium and confirmed their compatibility for the project.
First planar waveguides could be optically characterized after having prepared their input and output facets by an
appropriate polishing procedure. Guidance of light was demonstrated in the whole range [6-20 μm].
Modal filtering is based on the capability of single-mode waveguides to transmit only one complex amplitude function to
eliminate virtually any perturbation of the interfering wavefronts, thus making very high rejection ratios possible in a
nulling interferometer. In the present paper we focus on the progress of Integrated Optics in the thermal infrared [6-20μm] range, one of the two candidate technologies for the fabrication of Modal Filters, together with fiber optics. In
conclusion of the European Space Agency's (ESA) "Integrated Optics for Darwin" activity, etched layers of chalcogenide
material deposited on chalcogenide glass substrates was selected among four candidates as the technology with the best
potential to simultaneously meet the filtering efficiency, absolute and spectral transmission, and beam coupling
requirements. ESA's new "Integrated Optics" activity started at mid-2007 with the purpose of improving the technology
until compliant prototypes can be manufactured and validated, expectedly by the end of 2009. The present paper aims at
introducing the project and the components requirements and functions. The selected materials and preliminary designs,
as well as the experimental validation logic and test benches are presented. More details are provided on the progress of
the main technology: vacuum deposition in the co-evaporation mode and subsequent etching of chalcogenide layers. In
addition, preliminary investigations of an alternative technology based on burying a chalcogenide optical fiber core into a
chalcogenide substrate are presented. Specific developments of anti-reflective solutions designed for the mitigation of
Fresnel losses at the input and output surface of the components are also introduced.
In this paper, we present results concerning the fabrication and characterization of glass-ceramics based on chalcohalide for application as laser host materials. The objective is to develop a highly efficient host material for rare-earth doping. The studied system is Ga-Ge-S-CsCl with Er3+ ions as doping elements. Glass-ceramics have been prepared by thermal treatment of the base glass. The evolution of the optical transmission versus annealing time and temperature has been investigated. Preliminary up-conversion measurement of Er3+ were performed. Glass ceramics show higher luminescence efficiency as compared to the base glass. Nano-crystalline phases have been generated in well-controlled experimental conditions, so that crystals with reproducible size smaller than 50 nm could be achieved.
New infrared transmitting glass ceramics based on the Ge-Sb-S-CsCl system have been studied. By selecting an appropriate glass composition, glass ceramics with different quantities of micro-crystals can be reproducibly obtained with different annealing temperatures and durations. The presence of crystals induces some additional losses in the short wavelength region, namely between 0.6 and 2 μm. However, the glass ceramics keep the same transmission as the original glass after 3 μm up to 11 μm. To our best knowledge, it is the first time that highly reproducible glass-ceramics based on chalcogenide glasses are obtained. Compared to the glass, the corresponding glass-ceramics show better resistance to fracture propagation. The feasibility of shaping the glass-ceramics by molding has also been demonstrated.
IR glass optical fibers have been developed in order to optimize their response when they are used as evanescent wave chemical sensors. The diameter of the sensitive part of the fiber can be reduced by tapering the fiber during the drawing process or by chemical polishing. In using an FTIR spectrometer associated with a MCT detector, it was possible to evaluate the influence of the fiber diameter on the polymer coating IR signature as well as the sensitivity of a such sensor. The high flexibility of thin fibers allows the achievement of a detection probe which has been introduced in a microwave oven in order to follow a chemical reaction. It is verified that the chalcogen-based fiber is not sensitive to microwave radiation and gives excellent on line IR fingerprints to check kinetics and reaction mechanisms.
TeX glass fibers with monoindex structure are routinely achieved with a minimum attenuation, less than 1dB/m, in the 8-12 micrometers strategic window. Their exceptional properties in the IR allow to investigate many potential applications such as laser power delivery, temperature sensing as well as remote chemical monitoring. Chalcohalide fibers having optical losses of 0.5dB/m between 7 and 9 micrometers have been obtained by the preform drawing technique. These optical fibers are generally protected with an appropriate coating, thermal and/or UV plastic. Such materials show clearly improved mechanical properties and a high flexibility suitable for industrial manipulations. A high sensitivity of TeX glass fibers for chemical analysis by remote evanescent wave spectroscopy has been demonstrated. The detection efficiency has been studied as a function of various parameters especially the fibers' diameter. TeX glass fibers with a tapered shape have allowed to detect very low concentrations of less than 1 percent vol. in ethanol.
Tellurium Iodide based glass fiber preparation has been optimized in order to respond to several IR waveguide technological demands. Three directions have been examined: 1) realization of a two fiber systems for radiometry in the room temperature range, 2) chemical polishing of a core-clad fiber for evanescent wave remote IR spectroscopy, 3) attempts for single mode fiber operating in the 10 μm region for planetary spectroscopy.
The TeX glass fibers, with high flexibility and relatively low losses, have been developed for many applications especially carbon-dioxide laser power transmission and radiometry. The use of TeX glass fibers to transmit thermal radiation of an object to a remote detector allows temperature measurements, without contact, in inaccessible and hostile environments. The TeX glass fiber sensor can detect temperatures in a wide range [minus 20, 200 degrees Celsius] with a resolution estimated better than 0.2 degrees Celsius at high temperature (200 degrees Celsius) and close to 1 degree Celsius at room temperature. The transmission of carbon-dioxide laser beam through a TeX glass fiber has been performed. More than 2.6 W have been obtained through a 1 meter long fiber by injecting the maximum input power of 6 W at the wavelength of 9.3 micrometer. TeX glass fibers are very promising for biomedical applications such as welding which require an energy transfer through the fiber and a temperature monitoring by another fiber.
Infrared TeX fibers operating in a wide wavelength region have various potential uses in the short distance area such as laser power delivery, remote temperature monitoring and chemical analysis. TeX glass fibers with a minimum attenuation of 0.5 dB/m in the 7 - 10 micrometer range have been obtained. A plastic coating protects these fibers from external environment and improves their mechanical properties. Remote spectroscopy using mono-index fiber is one of the most promising applications. This new technology allows the identification and in situ analysis of many substances such as oils and fertilizers, which have their fingerprint in the 2 - 13 micrometer domain. The detection efficiency using evanescent wave absorption has been studied as a function of the fiber's diameter. It is found that the sensitivity increases very rapidly when the fibers' diameter decreases. The possibility of detecting very low concentrations has been tested by using TeX tapered fibers.
The TeX glass optical fibers have been developed for their broad transparency in the 3 to 13 micrometer region and their good thermal, mechanical and chemical properties. The minimum losses of these fibers are approximately 0.5 dB/m in the mid-IR domain of 7 - 9 micrometer. Owing to these properties, these fibers are useful in a wide range of applications, requiring a relatively low power level, such as temperature sensing. Temperature measurements using a TeX glass fiber have been investigated. The set-up was mainly composed of a polymer-coated fiber which transmitted the signal of a black body to a HgCdTe detector. Fibers with different lengths and different diameters have been used for this experiment and the temperature sensing has been performed in the region of minus 30 degrees Celsius up to 400 degrees Celsius. It has been found that the signal transmitted to the detector increases very rapidly when the temperature is higher than 60 degrees Celsius. However, the sensitivity of the used set-up is still high, even at temperatures as low as minus 30 degrees Celsius. The resolution of the sensor is estimated to be better than 1 degree Celsius in the region of room temperature. These fibers provide a possibility of non-contact low temperature sensing.
TeX glass fibers with a core-cladding structure are prepared by one of three methods: modified crucible method, preform method, or double crucible method. The raw elements are purified in order to eliminate some oxide impurities. They are then all distilled. The Te-Se-As- I system was chosen for the core and cladding glasses because of its stability against crystallization. The numerical aperture (N.A.) of the fiber is typically between 0.15 and 0.4. The diameter ratio of the core and cladding can be varied in the range of 0.15 - 0.9. These fibers are covered with a thermal plastic, to improve their mechanical properties. The optical losses of the fibers are measured between 2 and 13 micrometers by the cut-back method. The modified crucible method was the best to reduce the loss due to structural imperfections at the interface of the core and cladding. The lowest loss of 0.5 dB/m was achieved in the 7 - 9 micrometer region. Many applications of TeX glass fibers are actually tested in our laboratory such as thermal imaging, laser power delivery and remote spectroscopy. This last technology allows in-situ detection and quantification of several chemical compounds which have their characteristic absorptions in the 3 - 13 micrometer region.
The TeX glasses are attracting much attention as materials for low loss mid-IR optical fibers and are consequently good candidates for thermal imaging, laser power delivery, and more recently remote sensing. The TeX glass fiber, transmitting in a wide optical window, has a minimum attenuation in the 9-10 micrometers region. Fibers with an attenuation of less than 0.5 dB/m have been repeatly obtained. These fibers are coated with a UV curable or thermal plastic, in order to improve their mechanical properites. The IR remote spectroscopy using TeX fibers is one of the most promising applications. This technology allows to perform in situ, real-time, and on-line analysis of chemical and biological compounds. The study of industrial processes such as fermentations has been performed by this method, based on the use of these IR TeX fibers.
A new generation of infrared fibers, the TeX glass fibers, operating from 3 to 13 micrometers , exhibits a minimum attenuation of about 0.5 dB/m in the 7 - 9.5 micrometers range. A polymer coating on these fibers increases the mechanical properties and fibers with high flexibility have been obtained. CO2 laser power delivery has been performed using a TeX glass fiber. More than 2.6 W has been transmitted through a 1 meter long fiber by injecting the maximum output power of the laser. One of the most promising applications of these glass fibers is the remote spectroscopy using either evanescent wave or direct absorption analysis. These technologies provide an opportunity to realize in situ and on-line control of chemical and biological processes. Our prototype system using a FTIR spectrometer allows quantitative and qualitative detection of organic species such as alcohol, cosmetic products which have their fingerprints in the spectral region from 3 to 13 micrometers . The detection efficiency using evanescent wave absorption has been studied as a function of the fiber's diameter. It has been found that this efficiency increases very rapidly when the fiber's diameter decreases.
The tellurium halide based glass fibers, the TeX glass fibers, have a wide range of IR transmission. The minimum attenuation of about 0.5 dB/m is located in the wavelength region of 7-9.5 micrometers . Fibers having a core-clad structure have been developed. The diameter of the fibers can change from about 50 micrometers to 700 micrometers depending on the applications. These fibers are very stable in water and in normal air condition. The minimum bending radius for a fiber with a diameter of 200 micrometers is less than 1 cm. the maximum working temperature is 120 degree(s)C. A tunable CO2 laser with a maximum output power of 7 W is used for the power delivery experiments. The used TeX glass fibers have a diameter of about 600 micrometers and a length of about 1 meter. The two ends of the fiber are just cleaved without polishing. The relationship between output and input power has been established at different wavelengths. More than 206 W has been obtained through a TeX glass fiber at the wavelength of 9.3 micrometers corresponding to the strong absorption of dental tissues.
The new family of IR transmitting glasses, the TeX glasses, based on the association of tellurium and halide (Cl, Br, or I) are characterized by a wide optical window extending from 2 to 18 micrometers and a strong stability towards devitrification. Optical fibers drawn from these glasses exhibit low losses in the 7 - 10 micrometers range (less than 1 dB/m for single index fibers, 1 - 2 dB/m for fibers having a core-clad structure). The TeX glass fibers have been used in a remote analysis set-up which is mainly composed of a FTIR spectrometer coupled with a HgCdTe detector. This prototype system permits qualitative and quantitative analysis in a wide wavelength region lying from 3 to 13 micrometers , covering the fundamental absorption of more organic species. The evolution of a lactic and an alcoholic fermentation has been monitored by means of this set-up.
New infrared glasses based on chalcogen halides specially tellurium halide are described. These TeX glasses exhibit low optical loss in the 8 - 12 micrometers region and a strong dependence of their thermal properties on the chemical composition. The variation of the refractive index versus temperature T, wavelength (lambda) and composition leads to information on optical dispersion. These ductile, plastic glasses can be molded with an excellent duplication of optical surface and drawn into IR optical fibers with attenuation as low as 0.2 dB/m in the 8 micrometers region.
Arsenic is introduced into the tellurium halide based glasses by substituting Se or Te. Glass transition temperature, Tg, is significantly increased and Tg as high as 150 degree(s)C has been obtained. These new glasses still have a multiphonon absorption situated in the region of 18 micrometers and they still have potential low losses in the 8 - 12 micrometers region. The first optical fibers obtained from these glasses show an attenuation of about 5 dB/m in the second atmospheric window and the losses are due to defects in the fibers.
The thermal relaxation of the tellurium halide based glasses has been studied at different temperatures. It was found that some of these glasses can be appreciably relaxed at room temperature and the glass transition temperature increases significantly. It is always possible to increase Tg of all these glasses by appropriate heat treatment. The fictive temperatures as a function of annealing time have been determined for several glasses. This study allows to optimize the heat treatment.
The thermal properties of the tellurium halide glasses, the TeX glasses, are greatly improved by introducing a trivalent element As. Tg as high as 150 degree(s)C can be obtained and the glasses still transmit up to 18 micrometers . Vitreous thin films have been deposited on different substrates, using the sputtering techniques. Mono-index fibers and fibers having a core- cladding structure have been successfully drawn. The typical attenuation for the first fibers is less than 5 dB/m and for the second fibers is less than 7 dB/m.
New IR glasses transmitting from 1 to 20 im and having low loss potentiality in the 8- 12 pm region have
been obtained in the Te-Br-Se and Te-I-Se systems. Single index fibers have been drawn from rods and the
attenuation measuredin normal atmospheric conditions. The influence ofthe band gap absorption mechanism
appears to be very critical as well as the addition ofelements such as Bi which seems to improve the mechnical