Fabrication of Arsenic Selenide Optical Fiber with Low Hydrogen Impurities

Arsenic selenide glass optical fibers typically possess extrinsic absorption bands in the infrared wavelength region associated with residual hydrogen and oxygen related impurities, despite using purified precursors. We report a purification process based on the addition of 0.1 wt% tellurium tetrachloride (TeCl₄) to the glass. During melting, the chlorine from TeCl₄ reacts with the hydrogen impurities to produce volatile products (e.g., HCl) that can be removed by subsequent dynamic distillation. The processing conditions have been modified accordingly to give very low H–Se impurity content. Consequently, the H–Se absorption band centered at 4.57 μm has been reduced from tens of dB/m to 0.2 dB/m.     

Reduction of Ions in Glass by Hydrogen

The reduction by hydrogen of Fe³⁺, Ce⁴⁺, and Sn⁴⁺ in soda-lime-silica glass was found to be diffusion-limited in the glass transition temperature range (500°C). The reduction was studied in fibers by chemical analysis and in bulk samples by optical absorption. Reduction to Fe²⁺, Ce³⁺, and Sn²⁺ proceeds by a reaction of the type H₂+2(ɛSi-O)^-+2Fe³⁺→2Fe²⁺+2(ɛSi–OH). Since the rate of reduction by hydrogen is quite similar for these systems and since reduction cannot be accomplished with CO, it is concluded that hydrogen is the diffusing species. A mechanism is developed in which hydrogen diffuses through the glass until it is trapped by a reducible ion.     

Partition of Hydrogen in the Modified Chemical Vapor Deposition Process

In the modified chemical vapor deposition (MCVD) process for making glass fibers, most of the hydrogen coming into the reaction from hydrogen-bearing impurities in the starting materials is not incorporated in the glass. It is instead mostly converted to HCl, which is not absorbed by the newly formed silica particles, and passes out the exhaust stack. The residual partial pressure of H₂O formed in equilibrium with HCl in the presence of O₂ and Cl₂ accounts quantitatively for the OH appearing in the glass when the known solubility of H₂O in silica is considered. The H₂O/HCl equilibrium is quenched at a temperature below that of the reaction zone at a value for which the rate of reaction approximates the transit time across the deposition zone. Quantitative agreement is obtained for published OH concentrations produced by SiHCl₃ doping.     

Optical Properties of Dysprosium-Doped Low-Phonon-Energy Glasses for a Potential 1.3 μm Optical Amplifier

Dysprosium-doped glasses were prepared in the system of gallium-based sulfide, tellurite, zirconium-baed and indium-based fluorides and their optical properties were studied. From the absorption cross sections of five f-f bands, three Judd-Ofelt parameters, ω _t ( t = 2, 4, 6), of Dy³⁺ ion were determined. The compositional variaton of the ω₂value showed the order sulfide > tellurite > fluorozirconate > fluoroindate, whereas the ω₆ value showed the opposite tendency. Compositional variaton of the fluorescence intensity ratio of the (⁴F_9/2⁶H_13/2)/(⁴F_9/2)→⁶H_15/2) is explained by the ratio of ω₂/ω₆ of doped Dy³⁺. The emission probabilities A and the branching ratio β from ⁶H_9/2 and ⁶F_11/2 levels, which are the doublet initial level of the 1.3 μm luminescence, were calculated for the glasses, and it was found that both values showed a tendency similar to that of ω₂ against the glass composition. In the sulfide glass, A was 2.6 × 10³S^-1 and β was 93%, both the highest in all of the glasses investigated. By 1.06 μm pumping of a Nd: YAG laser, the sulfide glass showed strong 1.3 μm emission and the lifetime was 25 μs, resulting in a quantum efficiency of 7%. This value is higher than that of the Pr³⁺:¹G₄ level in ZBLAN glass with β= 60%.     

Influence of As2O3 on the Optical Properties of Ultrapure Multicomponent Silicate Glasses and Optical Fibers

To obtain low transmission loss optical fibers from ultrapure multicomponent silicate glasses, it is necessary to add small quantities of As₂O₃ (or Sb₂O₃). In optical fibers prepared from glasses without these agents, a significant increase in loss is observed. To investigate this effect, the influence of As₂O₃ (added to the batch and present in the glass as As₂O₅) on the optical properties of ultrapure silicate glasses was studied. These properties are the Rayleigh scattering loss coefficient, transition metal absorption, and position of the uv absorption edge. This study showed that the increase in loss of As₂O₃-free glass cannot be assigned to any of these contributions and was attributed to absorption by electrons, trapped in relatively shallow traps in the glass network. The As⁵⁺ ions serve as deep traps and therefore remove the additional absorption. The same phenomenon, although much more pronounced, was observed in optical fibers prepared from alkali borosilicate glasses.     

Effects of the Weak Absorption Tail on the Transmission Loss of Ge–Sb–Se Optical Fibers

Selenide glass optical fibers were fabricated for Ge₃₀Sb₁₀Se₅₈S₂ and Ge₂₀Sb₁₀Se₇₀ glasses. Their transmission loss has been measured and compared with the theoretical attenuation loss that was calculated taking into account the electronic transition absorption, Rayleigh scattering, and multiphonon absorption. A low attenuation loss of the Ge₂₀Sb₁₀Se₇₀ glass composition in 1.2–1.7 μm range has been expected due to its high optical band gap energy compared with the Ge₃₀Sb₁₀Se₅₈S₂ glass. However, the measured attenuation loss of the Ge₃₀Sb₁₀Se₅₈S₂ glass fiber was ∼13 dB/m at 1.5 μm while Ge₂₀Sb₁₀Se₇₀ glass showed ∼82 dB/m. An enhanced weak absorption tail due to the localized states of the Ge₂₀Sb₁₀Se₇₀ glass was responsible for this behavior. Structural defects are related to the localized states and discussed for the present glass compositions.     

The Glass System SiO2-B2O3-Bap-Na2O for High-Numerical-Aperture Optical Fibers

The glass system SiO₂-B₂O₃-BaO-Na₂O was investigated to obtain high-refractive-index glasses for high-numerical-aperture (N.A.) optical fibers. Compositions having a mole ratio [B₂O₃]/[BaO+Na₂O]=2 have unique properties. They are both highly viscous and chemically durable despite the fact that they contain highly alkaline metals and alkaline-earth metals. Glass properties at this mole ratio are favorable for optical fibers. It is proposed that the composition for core glass of high-N. A. optical fibers be based on this mole ratio.     

Optical Absorption and Color Caused by Selected Cations in High-Density, Lead Silicate Glass

The effect of impurities on the color of dense flint glass was investigated by comparing the optical absorption caused by cations in Na₂O·3SiO₂·2PbO glass, in Na₂O·3SiO₂·2PbO:1 wt% Sb₂O₃ glass, and in Na₂O·3SiO₂ glass; all three are colorless when of high purity. Of 11 colorants, only vanadium colored both lead glasses less intensely than the Na₂O·3SiO₂ glass. Factors that make the lead glass more susceptible to coloration are higher oxidation states of polyvalent cations, larger absorption cross sections of polyvalent cations, and the tendency of prominent absorption bands to be closer to the wavelength at which eye sensitivity is a maximum.     

Use of CsCl to Enhance the Glass Stability Range of Tellurite Glasses for Er3+-Doped Optical Fiber Drawing

Tellurite glasses are important as a host of Er³⁺ ions because of their good solubility and because they present broadband optical gain compared with Er³⁺-doped silica, with the potential to increase the bandwidth of communication systems. However, the small glass stability range (GSR) of tellurite glasses compromises the quality of the optical fibers. We show that the addition of CsCl to tellurite glasses can increase their GSR, making it easier to draw good-quality optical fibers. CsCl acts like a network modifier in glass systems, weakening the network by forming Te–Cl bonds. We show that the thermal expansion coefficient mismatch is in the right direction for optical fiber fabrication purposes and that the Bi₂O₃ content can be used to control the refractive index of clad and core glasses. Single-mode and multi-mode Er³⁺-doped optical fibers were produced by the rod-in-tube method using highly homogeneous TeO₂–ZnO–Li₂O–Bi₂O₃–CsCl glasses.     

Reactions between Hot-Pressed Calcium Hexaluminate and Silicon Carbide in the Presence of Oxygen

The reactions between hot-pressed calcium hexaluminate (CaAl₁₂O₁₉, hibonite) and silicon carbide (SiC) at 1100°-1400°C in air and nominal argon atmospheres were investigated. In inert atmospheres, there was no evidence of reaction at temperatures up to at least 1400°C. In air, the oxidation of SiC produced a layer of silica or a multicomponent amorphous silicate (depending on impurities) that reacted with CaAl₁₂O₁₉. At temperatures below 1300°C, the reaction resulted in the stratification of two distinct interfacial layers: a partially devitrified CaO-Al₂O₃-SiO₂ glass adjacent to SiC and a CaAl₂Si₂O₈ (anorthite) layer adjacent to hibonite. At 1400°C, a large amount of liquid was formed, the majority of which was squeezed out from between the reaction couple. No distinct layer of anorthite was present; instead, the anorthite was replaced by a layer of alumina between the glass-rich layer and hibonite. An activation energy of 290 kJ/mol was determined for the reaction, which is consistant with oxygen diffusion through a calcium aluminosilicate glass. The reaction between rare-earth hexaluminates and SiO₂ was predicted to produce a more-viscous glass than CaAl₁₂O₁₉ and SiO₂ and, therefore, have slower reaction kinetics, because of lower mass transport in the glass.     

A New Class of Glasses for Double-Crucible Optical Fibers with High Numerical Apertures

A series of boron- and germanium-free silicate glasses was developed for the core glass of high-numerical-aperture double-crucible optical fibers. When combined with a boro-silicate cladding glass from a series also investigated in this study, fibers with a numerical aperture in the range 0.3 to 0.5 can be produced. The relatively high refractive indices in the core glasses are achieved by including large mole fractions of BaO, CaO, and ZnO, accompanied in some cases by smaller additions of ZrO₂ and/or Y₂O₃ in the glass composition. Measurements of the refractive index, density, Rayleigh scattering coefficient, and viscosities and observations of the devitrification behavior and chemical durability of the glasses are presented. Attenuation spectra of suitable core glasses and of double-crucible optical fibers comprised of different core and cladding combinations yielding high numerical apertures are given. The fibers have attenuations near 10 dB/km at 850 nm.     

Properties of As40S(60–x) Sex Glasses for IR Fiber Optics

The physical and optical properties of glasses in the As₄₀S₍₍₆₀-_x:)Se_x system where x = 0, 5,10,15, and 20 at. % Se have been investigated. The changes in the glass transition temperature, density, hardness, IR edge, and refractive index can be attributed to the presence of Se, which simply replaces S in these glasses. The glasses do not exhibit crystallization under the conditions used in this study, which is a desirable property from the viewpoint of flberization. Appropriate core and cladding glass compositions have been identified for fabrication of optical fibers with known numerical apertures. Furthermore, it has been shown that these glasses will not impart additional thermal or mechanical stresses in optical fibers made from these compositions.     

Behavior of Glass Fibers in Strong Acidic and Alkaline Media

The properties of basaltic, glass wool, quartz, and E-glass fibers were investigated under strong acid (HF:H₂O=l:3, HF:H₂O=1:1, 10% H₂SO₄+3% HF:H₂O=1:1) and alkaline (1 N NaOH + 1 N Na₂CO₃) treatment. The multiple-oxide fibers were found to be composed of several layers which exhibit different morphology and composition. These findings are in accordance with some proposals in the literature.     

Secondary Ion Mass Spectrometer Analysis of Potash–Lime–Silica Glasses Leached in Hydrochloric and Sulfuric Acids

Secondary ion mass spectrometry investigations were performed on leached K₂ O–CaO–SiO₂ glass with a chemical composition similar to that of medieval stained glass. The results reveal that the type and concentration of the acid used definitely influence the kinetics of the leaching process. In addition to hydrogen, an enrichment of Cl and S was observed in the leached layers of glass treated with HCl and H₂ SO₄, respectively.     

Preparation and Properties of Heavy-Metal Fluoride Glasses Containing Ytterbium or Lutetium

Beginning with a 19BaF₂−27ZnF₂−27YbF₃−27ThF₄ (mol%) base glass, compositional modifications were made to improve optical properties and glass-forming ability. Replacement of YbF₃ by LuF₃ removed a strong near-ir electronic absorption band, and small additions of LiF and NaF improved glass quality. The multicomponent BaF₂/ThF₄ glasses had higher refractive indices and lower expansion coefficients than fluorozirconate and fluorohafnate glasses. In the 6 to 10 μm region, these materials exhibit absorption coefficients an order of magnitude lower than those reported for other heavy-metal fluoride glasses.     

Microstructure and Optical Properties of Cd1−x ZnxS Films Sintered with Cadmium Chloride

Polycrystalline Cd_1−xZn_xS films were prepared by coating (1 - x) CdS + xZnS slurry which contained various amounts of CdCl₂ on amorphous glass substrate and sintering in nitrogen to produce films with properties suitable for fabricating all-polycrystalline Cd_1−xZn_xS/CdTe heterojunction solar cells. Optical properties of these films have been correlated with composition and microstructure. The sintered film has undulating grain boundaries due to the occurrence of chemically induced grain-boundary migration. By optimizing the amount of CdCl₂ in the slurry and sintering conditions, it is possible to produce sintered Cd_1−xZn_xS films on glass substrates with high optical transmission and to produce Cd_1−xZn_xS/CdTe solar cells with efficiency higher than 11%.     

Nucleation in ZBLAN Glasses

Nucleation rates were measured in a ZrF₄–BaF₂–NaF–LaF₃–AlF₃ glass (ZBLAN) using an optical technique. The results were compared with a similar glass having a slightly different composition. The difference in the nucleation rate is explained by classical nucleation theory using calculated free-energy differences between the ZBLAN liquid and the BaZrF₆ crystal, which is found as the precipitating phase.     

Preparation of Inorganic Compounds at Near Room Temperature by the Direct Conversion of Borate Glass in Solutions of the Corresponding Anions

The preparation of alkaline-earth chromate, selenite, and stannate compounds at near room temperature by the direct conversion of borate glass in aqueous solutions of the corresponding anions was investigated. Borate glass particles (150–300 μm) with the composition 20Na₂O·20CaO·60B₂O₃ or 20Na₂O·20BaO·60B₂O₃ (mol%) were prepared by conventional methods and immersed in dilute solutions of K₂CrO₄, K₂SeO₃, or K₂SnO₃ at 37°C. The conversion of the glasses was monitored using weight loss and pH measurements, while X-ray diffraction (XRD), X-ray fluorescence, and scanning electron microscopy were used to characterize the structure and composition of the products. After a reaction for 140–320 h, porous crystalline products identified by XRD as CaSeO₃.H₂O, CaSnO₃.3H₂O, BaCrO₄, and BaSeO₃ were obtained. The conversion of fibers (0.5–1.0 mm in diameter) of the Na₂O–BaO–B₂O₃ glass in K₂CrO₄ solution was pseudomorphic. The kinetics and mechanisms of the conversion process, as well as the structure of the products, are discussed.     

Absorption and Scattering Losses in Glasses and Fibers for Light Guidance

Fiber optics for telecommunications applications require very high purity glass. Light loss in glass results primarily from absorption and scattering. Transition metal ions and OH ions cause most of the absorption, whereas scattering is caused by microheterogeneity. Scattering losses vary from 1 to 4 dB/km at 850 nm; absorption losses are high near 630 nm, because of Cr³⁺ and Ni²⁺ impurities, but are lower near 850 nm, making this a preferred wavelength region. Flint glass with a total loss of 50 dB/km at 850 nm, prepared using pure material and special techniques, was used as the core of thin-clad optical fibers 70 μm in diameter; these fibers had an internal loss at 850 nm of ∼450 dB/km.     

Consolidation of Participate Layers in the Fabrication of Optical Fiber Preforms

Viscous sintering is experimentally identified through electron microscopy as the mechanism of consolidation of thin particulate layers in the fabrication of optical fibers. The rate of consolidation is primarily a function of the capillary number, C=[νl₀(1 -ε₀)^1/3/(σ t _s)], where ν is the glass viscosity, l ₀ the size of the initial void regions, ε₀ the initial void fraction, σ the surface tension, and t_s the sintering time. Numerical results indicate a sensitivity of the sintering rate to temperature, chemical composition of the particles, and gas thermal properties, primarily through the strong dependence of glass viscosity on these variables. These results are fully supported by the available experimental evidence. Under the conditions of the experiments, gas bubbles were occasionally produced; the causes are discussed.