Gallium sulfide glasses

Abstract: This invention is directed broadly to transparent glasses exhibiting excellent transmission far into the infrared region of the electromagnetic radiation spectrum, those glasses consisting essentially, expressed in terms of mole percent, of 40-80% Ga₂S₃, 0-35% RS x , wherein R is at least one network forming cation selected from the group consisting of aluminum, antimony, arsenic, germanium, and indium, 1-50% Ln₂S₃, wherein Ln is at least one cation selected from the group consisting of a rare earth metal cation and yttrium, 1-45% MS x , wherein M is at least one modifying metal cation selected from the group consisting of barium, cadmium, calcium, lead, lithium, mercury, potassium, silver, sodium, strontium, thallium, and tin, and 0-10% total chloride and/or fluoride. Glass compositions consisting essentially, expressed in terms of mole percent, of 5-30% Ga₂S₃, 0-10% R₂S₃, wherein R is at least one network forming cation selected from the group consisting of aluminum, antimony, arsenic, and indium, 55-94.5% GeS₂, 0.5-25% MS x , wherein M is at least one modifying metal cation selected from the group consisting of barium, cadmium, calcium, lead, lithium, potassium, silver, sodium, strontium, tin, yttrium, and a rare earth metal of the lanthanide series, 0-10% total selenide, 0-25% total chloride and/or fluoride, and wherein the sulfur and/or selenium content can vary between 85-125% of the stoichiometric value when doped with Pr demonstrate exceptionally high values of τ.

Description: Background of the Invention U. S. Patent No. 5,240,885 (Aitken et al.) describes the preparation of rare earth metal-doped cadmium halide glasses, which glasses transmit radiation well into the infrared portion of the electromagnetic radiation spectrum due to their low phonon energy. That capability commended their utility for the fabrication of efficient lasers, amplifiers, and upconverters when doped with the appropriate rare earth metals. Because metal-sulfur bonds are generally weaker than metal-oxygen bonds, sulfide glasses exhibit much lower phonon energies than oxide glasses and, therefore, transmit radiation much further into the infrared region of the electromagnetic radiation spectrum. Accordingly, sulfide glasses were seen to have the potential of being excellent host materials of rare earth metals for applications such as those listed above requiring efficient radiative emission. Unfortunately, however, many sulfide glasses are black and, consequently, are unsuitable for some of the above applications inasmuch as such a host glass would tend to absorb the pump radiation instead of the rare earth metal dopant. One of the best known sulfide glasses, viz., arsenic sulfide, is transparent to radiation in the long wavelength range of the visible portion of the radiation spectrum as well as far into the infrared region and, hence, would appear to be a suitable host glass for rare earth metals. Nevertheless, rare earth metals have been found to be relatively insoluble in arsenic sulfide glasses, and it has proven to be difficult to dope those glasses with enough rare earth metal for sufficient pump power absorption. Rare earth metals are known to be very soluble in most oxide glasses and their apparent insolubility in arsenic sulfide glasses has been conjectured to be due to the gross structural dissimilarity existing between the latter and oxide glasses. Arsenic sulfide glasses are believed to consist of long chains and layers of covalently bonded pyramidal AsS 3 groups, whereas oxide glasses typically comprise a three-dimensional network of relatively ionically bonded MO 4 tetrahedra, where M is a so-called network-forming metal such as silicon, phosphorus, aluminum, boron, etc. Rare earth metals are readily accommodated in these ionic network structures where they can compensate charge imbalances that arise from the presence of two or more network-forming metals, e.g., aluminum and silicon in aluminosilicate glasses -- energetically similar sites may not exist in the two-dimensional covalent structures typical of arsenic sulfide and related glasses. One system of sulfide glasses which exhibit good transparency in both the visible and infrared portions of the radiation spectrum, and which possess a relatively ionic three-dimensional structure that would be expected to be more accommodating of rare earth metals, comprises gallium sulfide glasses. In contrast to arsenic sulfide glasses, the structure of these glasses is based upon a three-dimensional linkage of corner sharing GaS 4 tetrahedra. Rare earth metals are readily soluble in these glasses. In fact, some of the most stable gallium sulfide glasses contain a rare earth metal as a major constituent. Of general interest are U.S. Patents 4,612,294, 4,704,371 and 4,942,144, and the following Literature Articles: ["Verres Formes Par Les Sulfures L 2 S 3 Des Terres Rares Avec Le Sulfure De Gallium Ga 2 S 3 ", Loireau-Lozac'h et al., Mat. Res. Bull., 11 , 1489-1496 (1976)]. Other academic studies of gallium sulfide-containing glasses have included the following publications: ["Systeme GeS 2 -Ga 2 S 3 Diagramme De Phases Obtention Et Proprietes Des Verres", Loireau-Lozac'h et al. , Ann. Chim., 10 , 101-104 (1975)]; ["Etude Du Systeme Ga 2 S 3 -Na 2 S", Palazzi, C. R. Acad. Sc. Paris, 229 , Serie II, No. 9, 529-532 (1984)]; ["Study on Ge-Ga-x(X=S,Se) Glass Systems", Xilai et al. , Collected Papers, XIV Intl. Congr. on Glass , 118-127 (1986)]; ["Le Systeme Ga 2 S 3 -Ag 2 S", Guittard et al. , Ann. Chim., 8 , 215-225 (1983)]; ["An EXAFS Structural Approach of the Lanthanum-Gallium-Sulfur Glasses", Benazeth et al. , J. Non-Cryst. Solids, 110 , 89-100 (1989)]; ["Glass Formation and Structural Studies of Chalcogenide Glasses in the CdS-Ga 2 S 3 -GeS 2 System", Barnier et al. , Materials Science and Engineering, B7 , 209-214 (1990)]; {"F NMR Study of [(Ga 2 S 3 ) 0.25 (GeS 2 ) 0.75 ] 0.75 (NAF) 0.25 Glass", Baidakov et al. , Soviet Journal of Glass Physics and Chemistry, 18 , No. 4, 322-324 (1992)}; ["Chalcogenide Glasses in Ga 2 S 3 -GeS 2 -MeF n Systems", Orkina et al. , Glass Physics and Chemistry, 19 , No. 3 (1993)]; ["Active Fiber Research Highlights", Snitzer et al. , Fiber Optics Materials Research Program, Rutgers University , page 32 (April 13, 1993)]; and ["Pr 3+ :La-Ga-S Glass: A Promising Material for 1.3 µm Fiber Amplification", Becker et al. , Optical Amp. and Their Appl. , PD5, 19-23 (1992)]. Carcaly et al , Mat. Res. Bull., 15 , 545-550 (1980) relates to glass formation in the ternary system La 2 S 3 -Ag 2 S-Ga 2 S 3 and concludes that electrical conductivity is essentially dependent on the silver content of the glass. Specific glasses are disclosed with the following compositions in mole%: La 2 S 3 Ag 2 S Ga 2 S 3 35 5 60 30 10 60 25 15 60 20 20 60 Cervelle et al , Mat. Res. Bull., 15 , 159-164 (1980) relates to the determination of refractive index of glasses made from La 2 S 3 and Ga 2 S 3 by microscopic measurements of reflectance and concludes that the values vary within the range 2.3 to 2.6 as a function of the relative proportions of La and Ga. Flahaut, Verres Réfrac., 35(4) , 671-674 (1981) relates to the properties, particularly fluorescence, of La 2 S 3 /Ga 2 S 3 glasses optionally doped with rare earth metals such as Nd, Ho and Er. Summary of the Invention The above listing of literature references is indicative of the extensive research which has been conducted in recent years in the general field of gallium sulfide-containing glasses. That research disclosed properties exhibited by such glasses that suggested studies be undertaken to modify glasses having base compositions within the gallium sulfide system such that, when doped with rare earth metals, particularly neodymium, erbium and praseodymium, they could be fabricated into very efficient lasers, amplifiers, and upconverters. Therefore, our invention was directed to developing glass compositions which are not only eminently suitable for those applications, but also which can be melted and formed into desired configurations utilizing standard glass melting and forming techniques. General Description of the Invention In view of the description in the last two citations in the above Literature Articles , we began our research by investigating the utility of gallium sulfide-based glasses as hosts of Pr 3+ ions, principally for the purpose of fabricating a fiber amplifier capable of exhibiting gain at a wavelength of 1.3 µm. The measured lifetime of the 1.3µm fluorescence from Pr 3+ (τ) is large in these glasses and, due to their large refractive index, the radiative emission process is about four times more efficient than in a Pr-doped halide glass with the same τ. In order to produce a fiber amplifier, one must be able to control the refractive index of the material in such a manner as to form a waveguide structure typically consisting of a core glass demonstrating a high refractive index surrounded by a cladding glass of lower refractive index. The refractive index of lanthanum gallium sulfide glass is about 2.5 and our experiments indicated that the index appears to be rather insensitive to variations in the La:Ga ratio, as will be seen in Table I infra. We have discovered, however, that the refractive index thereof can be lowered substantially by partially replacing lanthanum with at least one modifier selected from the group calcium, sodium, and potassium. Such replacements can, in principle, allow one to achieve a core/clad structure with a numerical aperture well in excess of 0.4. From a practical point of view, calcium-substituted glasses are preferred for the cladding where the core glass is a Pr-doped lanthanum gallium sulfide glass, inasmuch as the other relevant physical properties of the calcium-substituted glasses, e.g., thermal expansion and viscosity, more closely match those of the core glass. Thus, as is reported in Table I infra, a calcium-substituted glass exhibits a linear coefficient of thermal expansion over the temperature range of 25°-300°C of about 95 X 10 -7 /°C which closely matches the linear coefficient of thermal expansion of about 90-100 X 10 -7 /°C exhibited by lanthanum gallium sulfide glasses. We have found that the compositional region over which gallium sulfide glasses can be formed is quite extensive. To illustrate, not only is there a broad glass forming region in the La 2 S 3 -Ga 2 S 3 system from about 50-80 mole percent Ga 2 S 3 , but also lanthanum can be replaced, in some instances completely, by other modifying cations including Sr, Li, Cd, Na, Hg, K, Pb, Ca, Tl, Ba, Sn, Y, and the other rare earth metals of the lanthanide series. The use of substantial amounts of La and/or Gd as modifiers in Pr-doped glasses has been theorized to suppress the tendency of Pr to cluster, thereby permitting higher dopant levels of Pr without compromising τ. In addition, the network forming component Ga can be replaced in part by other tetrahedrally coordinated metals such as Al, Ge, and In, or by pyramidally coordinated metals such as As and Sb. Examples of Pr 3+ -doped and Eu 2+ -doped glasses illustrating that compositional flexibility are recorded in Table I infra. Finally, sulfide can be replaced in part by chloride without degrading the infrared radiation transmission of these glasses. Fluoridation of the glasses may lead to blue-shifting the spectral lines of rare earth metal dopants and, in particular, centering the 1 G 4 - 3 H 5 emission of Pr 3+ at about 1.3 µm, in like manner to the effect which the replacement of oxide by fluoride has in rare earth metal-doped oxide glasses. In summary, a transparent gallium sulfide-based glass exhibiting excellent transmission far into the infrared region of the electromagnetic spectrum can be prepared from compositions consisting essentially, expressed in terms of mole percent on the sulfide basis, of 40-80% Ga 2 S 3 , 0-35% RS x , wherein R is at least one network forming cation selected from the group consisting of aluminum, antimony, arsenic, germanium, and indium, 1-50% Ln 2 S 3 , wherein Ln is at least one cation selected from the group consisting of a rare earth metal and yttrium, and 1-45% MS x , wherein M is at least one modifying cation selected from the group consisting of barium, cadmium, calcium, lead, lithium, mercury, potassium, sodium, strontium, thallium, and tin, and 0-10% total chloride and/or fluoride. When glasses having compositions encompassed within the above ranges are doped with Pr +3 ions in an amount equivalent to at least 0.005 mole percent Pr 2 S 3 , they exhibit a τ value greater than 200 µsec. Pr +3 ions in much larger amounts are operable, but a level equivalent to about 0.5% Pr 2 S 3 has been deemed to constitute a practical maximum. It is also of interest to note that, in view of the large optical non-linearity of these Ga 2 S 3 glasses (χ 3 = ∼ 45 X 10 -14 esu at 1.06µm), they possess the necessary properties for making high χ 3 waveguides. An effective waveguide structure comprises a core glass demonstrating a high refractive index surrounded by a cladding glass exhibiting a lower refractive index, the difference in those refractive indices being selected to achieve a desired numerical aperture. We have discovered a composition area particularly suitable for the fabrication of waveguide structures. This area comprises a core glass consisting of a lanthanum gallium sulfide glass and a cladding glass consisting of a lanthanum gallium sulfide glass where calcium has replaced a sufficient proportion of the lanthanum to lower the refractive index thereof to a value appropriate to achieve the desired numerical aperture. Description of Preferred Embodiments Table I records a group of glass compositions, expressed in terms of mole percent, illustrating glasses in the basic gallium sulfide system. Most of the glasses were doped with Pr 3+ ions to determine the level of τ. Because the glasses were prepared in the laboratory, a sulfide was used for each component. Such is not necessary, however. Thus, sulfur-containing batch materials other than sulfides can be utilized so long as the chosen materials, upon melting together with the other batch ingredients, are converted into the desired sulfide in the proper proportions. The glasses were prepared by compounding the batch constituents, thoroughly mixing the constituents together to aid in securing a homogeneous glass, and then dispensing the batch mixtures into vitreous carbon or alumina crucibles. The crucibles were moved into a furnace operating at about 1000-1100°C, maintained at that temperature for about 15-60 minutes, the melts thereafter poured into steel molds to form discs having a diameter of 4 cm and a thickness of 5 mm, and those discs transferred immediately to an annealer operating at about 500°-550°C. Table I also recites the density (Den.), expressed in terms of g/cm 3 , the transition temperature (T g ) and the temperature of the onset of crystallization (T x ), expressed in terms of °C, the refractive index (n D ), the linear coefficient of thermal expansion (α) expressed in terms of X 10 -7 /°C, and the τ values, expressed in terms of µsec, of each glass where measured. 1 2 3 4 5 6 7 Ga 2 S 3 65.00 70.00 65.00 65.00 70.00 70.00 70.00 La 2 S 3 24.95 19.97 24.95 14.95 19.97 9.97 19.97 Li 2 S 10.00 --- --- --- --- --- --- Na 2 S --- 10.00 10.00 20.00 --- --- --- K 2 S --- --- --- --- 10.00 20.00 --- CaS --- --- --- --- --- --- 10.00 GeS --- --- --- --- --- --- --- Pr 2 S 3 0.05 0.03 0.05 0.05 0.03 0.03 0.03 Den. --- 3.74 3.80 3.51 3.68 3.30 3.84 T g --- 528 534 520 540 524 536 T x --- 683 627 634 691 632 667 n D --- 2.31 --- --- 2.29 2.22 2.38 τ 208 --- 242 240 --- --- --- 8 9 10 Ga 2 S 3 65.00 65.00 65.00 La 2 S 3 24.95 14.95 29.95 CaS 10.00 20.00 --- BaS --- --- 5.00 In 2 S 3 --- --- --- EuS --- --- --- Gd 2 S 3 --- --- --- Pr 2 S 3 0.05 0.05 0.05 Den. 3.85 --- --- T g 536 --- --- T x 638 --- --- n D --- --- --- τ 224 --- --- α 94.5 --- --- It will be appreciated that the above procedures reflect laboratory practice only. That is, the batches for the inventive glasses can be melted in large commercial melting units and the melts formed into desired glass shapes employing commercial glass forming techniques and equipment. It is only necessary that the batches be heated to a sufficiently high temperature for a sufficient length of time to obtain a homogeneous melt, and the melt then cooled and simultaneously shaped at a sufficiently rapid rate to avoid the development of devitrification.

Claims: A transparent glass exhibiting excellent transmission far into the infrared region of the electromagnetic radiation spectrum consisting essentially, expressed in terms of mole percent on the sulfide basis, of 40-80% Ga 2 S 3 , 0-35% RS x , wherein R is at least one network forming cation selected from the group consisting of aluminum, antimony, arsenic, germanium, and indium, 1-50% Ln 2 S 3 , wherein Ln is at least one cation selected from the group consisting of a rare earth metal and yttrium, and 1-45% MS x , wherein M is at least one modifying cation selected from the group consisting of barium, cadmium, calcium, lead, lithium, mercury, potassium, sodium, strontium, thallium, and tin, and 0-10% total chloride and/or fluoride. A transparent glass according to claim 1 which, when doped with praseodymium in an amount equivalent to at least 0.005% Pr 2 S 3 , exhibits a τ value greater than 200 µsec. A waveguide structure exhibiting a desired numerical aperture comprising a core glass demonstrating a high refractive index surrounded by a cladding glass demonstrating a lower refractive index, characterised in that the core glass consists of a lanthanum gallium sulfide glass and the cladding glass consists of a lanthanum gallium sulfide glass wherein calcium has replaced a sufficient proportion of the lanthanum to lower the refractive index thereof to impart a sufficient difference between the refractive index of the core glass and the cladding glass to achieve an appropriate numerical aperture in said waveguide structure.

Inventors: AITKEN BRUCE GARDINER, NEWHOUSE MARK ANDREW, AITKEN, BRUCE GARDINER, NEWHOUSE, MARK ANDREW
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1995
Main classification: C03C3
IPC codes: C03C3, G02B6, C03C4, C03C13