TELLURITE GLASSES AND OPTICAL COMPONENTS

Abstract: A family of alkali-tungsten-tellurite glasses that consist essentially of, as calculated in mole percent, 10-90 % TeO2, at least 5 % WO3 and at least 0.5 % R2O where R is Li, Na, K, Cs, Tl and mixtures, that may contain a lanthanide oxide as a dopant, in particular erbium oxide, and that, when so doped, is characterized by a fluorescent emission spectrum having a relatively broad FWHM value.

Description: TELLURITE GLASSES AND OPTICAL COMPONENTS FIELD OF THE INVENTION Alkali-tungsten-tellurite glasses and their use in telecommunication system components. BACKGROUND OF THE INVENTION Optical components, in particular components employing optical fibers doped with rare earth metal ions, are widely used in telecommunication systems. A major application is in signal amplifiers which employ fluorescent ion emission for amplification of a signal. The ion emission occurs within the same operating wavelength region as the signals. Pump energy excites the rare earth metal ion causing it to fluoresce and thereby provide optical gain. Glasses, doped with a rare earth metal ion and pumped with appropriate energy, exhibit a characteristic, fluorescence intensity peak. The evolution of telecommunication systems, with their increasing demands for bandwidth, has created a need for a rare-earth-doped, amplifier material having the broadest possible emission spectrum in the wavelength region of interest. It is a purpose of the present invention to meet this need. The bandwidth of a fluorescent intensity curve is, rather arbitrarily, taken as the full width half maximum (FWHM) of the curve in nanometers wavelength. This value is the lateral width of the curve at one half the maximum intensity, that is, at one half the vertical height of the peak of the curve. Unfortunately, many glasses, that exhibit a fluorescence in an appropriate region, exhibit a rather narrow bandwidth. It is a further purpose of the invention to provide a family of glasses that exhibit a relatively broad bandwidth. It is well known that glasses doped with erbium can be caused to emit fluorescence in the 1520-1560 nm. region. This enables a signal operating in this wavelength range to be amplified. The significance of the 1550 wavelength in optical communication has led to extensive studies regarding the behavior of erbium as a rare earth metal dopant in glasses. It has also led to the study of a variety of glasses as the host for the erbium ion. The low phonon energy of tellurite glasses can lead to long emission lifetimes at certain pump wavelengths. As an example, erbium in tellurite glasses exhibits long tau-32 (980 emission) lifetimes relative to silicates. The long emission lifetimes at the pump wavelength can reduce the efficiency of an amplifier or laser because of excited state absorption. A practical 980 pumping scheme can be obtained by co-doping glasses with components having phonon overtones that are resonant with the energy difference between 980 and 1530 nm. Such components include H 2 0, B 2 0 3 , P 2 0 5 . For certain applications, long, erbium, tau-32 emission lifetime is desirable. These include long-band amplifiers, where pumping directly into the ground-state absorption does not impact noise figure; also tilt-free amplifiers, in which 980 and 1480 pump lasers are combined to dynamically adjust the gain without affecting the shape of the gain spectrum. It is also known that glasses with moderately low maximum phonon energies, when doped with thulium, can display fluorescence in the vicinity of 1450 nm. Although this wavelength lies outside of the currently used telecommunications band, it still lies within the transparency window of most commercial optical fiber. With the ever-increasing demand for useful bandwidth, there will be a need for additional amplifier devices that operate over the remaining portions of this window that are not covered by erbium. It is known, for example, that, as the concentration of a rare earth metal ion, such as erbium, is increased, the optical gain increases up to a certain point. Beyond this point, the fluorescent signal is quenched, and the optical gain decreases. This phenomenon is considered to result from the dopant, rare earth metal ions interacting with each other in a manner commonly referred to as clustering. It is another purpose of the invention to provide a family of glasses which is readily capable of dissolving erbium ions, and which exhibits a broad bandwidth indicating that clustering is inhibited. It has been reported that certain tellurite glasses, doped with erbium ions, provide a very broad, erbium emission band in the 1540 nm. region of the spectrum. Glass compositions were not reported, but other studies indicate that the glasses are alkali-alkaline earth-zinc-tellurite glasses. Most tellurite glass families require the presence of at least about 50% Te0 2 in the glass compositions to permit glass formation. It has been thought that a broader range of the glass forming oxide, Te0 2 , should provide a variety of structural motifs of the network-forming TeO x species. Such species include polymerized, bipyramidal Te0 4 groups at high Te0 2 content and isolated pyramidal Te0 3 groups at low Te0 2 content. The diversity of species should yield a greater diversity of structural sites for the incorporation of dopant ions, such as erbium ions. This avoids the ions clustering and becoming ineffective for fluorescent emission and consequent amplification. Also, the diversity of dopant sites should give rise to broadened emission spectra. It is, then, a basic purpose of the present invention to provide a family of tellurite glasses that can be melted over an extensive compositional region, and in which compounds of rare earth metals, such as erbium, are readily soluble. SUMMARY OF THE INVENTION The invention resides, in part, in a family of alkali-tungsten-tellurite glasses that consist essentially of, as calculated in mole percent, 10-90% Te0 2 , at least 5% W0 3 , and at least 5% R 2 0, where R is Li, Na, K, Rb, Cs, Tl and mixtures. The invention further resides in an optical component for a telecommunication system that is composed of an alkali-tungsten-tellurite glass that has a high thermal stability (T x -T g ), that readily dissolves rare earth metal oxides and that has a composition consisting essentially of, as calculated in mole percent, 15-85% Te0 2 , 5-55% W0 3 , 0.5-40% R 2 0, where R is Li, Na, K, Rb, Cs, Tl and mixtures, and 0.005-10% of an oxide in the lanthanide series. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, FIGURE 1 is a graphical representation comparing the emission spectrum of a glass of the present invention and that of a corresponding prior glass, FIGURE 2 is a graphical representation illustrating the thermal stability of a glass in accordance with the invention. DESCRIPTION OF THE INVENTION It has been indicated earlier that known tellurite glass families generally require at least 50 mole percent Te0 2 in their composition. The present invention is based on discovery of an alkali-tungsten-tellurite glass family that has a much greater compositional region. Further, the glasses in this family are readily capable of dissolving rare earth metals as dopants. In particular, when these glasses are doped with an erbium compound, they exhibit broad emission features, that is, they exhibit a relatively large FWHM. This is especially significant for the commercially important 1.54 H-m band. In addition, they provide other features that enhance their ability in producing optical materials, particularly fibers. The alkali-tungsten-tellurite glasses are unique in that their Te0 2 content may range broadly from 10-90 mole percent. This broad range of possible Te0 2 contents offers the possibility of a concomitant variety of structural motifs of the network-forming Te0 2 species. As indicated earlier, these include polymerized, bipyramidal, Te0 4 groups when the glass has a high Te0 2 content. With a lower Te0 2 content, isolated pyramidal Te0 3 groups become possible. This variety of structural motifs yields a greater diversity of structural sites for incorporating dopant ions such as erbium ions (Er +3 ). The consequence of this is an enhanced solution of effective erbium ions and broadened erbium emission spectra. TABLE I sets forth the composition in mole percent of representative alkali-tungsten-tellurite glasses in accordance with the invention. Also presented are FWHM values in nanometers and thermal stability index values for glasses having these compositions. For comparison purposes, a glass composition, having no tungsten content, is shown as example 5. TABLE 1 2 3 4 5 Na-O 15 10 5 13 12 K 2 O 15 10 5 13 12 WO 3 45 30 15 39 - TeO 2 25 50 75 35 76 Er 2 O 3 0.05 0.05 0.05 0.05 0.05 FWHM 50 50 58 34 (nm) τ x -τ g cc.) 127 134 143 142 108 These glasses may be utilized in bulk form for planar devices. However, it is contemplated that they will also be fabricated in the form of optical fibers. For the latter purpose, the thermal stability of the glass is an important consideration. Accordingly, a large value for the thermal stability index (T x -T g ). is significant. The thermal stability index is the temperature interval between a temperature (T x ) and the glass transition temperature (T g ). As used here, T x represents the temperature at the onset of crystal formation. Any development of crystals in the glass can be detrimental. For drawing of fibers, an index value of at least 100° C. is desired. Such an index value, in view of the steepness of the viscosity curve for these glasses, is sufficient to permit fiberization of the glasses in a practical manner for example, by redrawing "rod-in-tube" preforms. The property features of the inventive glasses are further described with reference to FIGURES 1 and 2. FIGURE 1 is a graphical representation comparing the emission spectrum of a tungsten-containing, tellurite glass with the emission spectrum of a tungsten-free, alkali tellurite glass. Relative emission intensity is plotted on the vertical axis in arbitrary units (a.u.) for comparison purposes. Wavelength in nanometers, is plotted on the horizontal axis. Curve A represents the emission spectrum for a known alkali tellurite glass that contains no tungsten content in its composition. This glass has the composition of Example 5 in TABLE I. Curve B represents the emission spectrum for a typical, alkali-tungsten-tellurite glass of the present invention (Example 3). The dashed, lateral lines indicate the FWHM value for each glass. The flatter and broader character of Curve B, and the consequently greater FWHM value of the tungsten-containing glass, are apparent. This is a typical and significant feature of the present glasses. FIGURE 2, also a graphical representation, illustrates the thermal stability in a typical alkali-tungsten-tellurite glass. In FIGURE 2, temperature in ° C. is plotted on the vertical axis, and Te0 2 content, in mole percent, in alkali- tungsten-tellurite glasses is plotted on the horizontal axis. Curve C represents the T x temperatures in ° C. for glasses having Te0 2 content varying from 20-80 mole percent. Curve D represents the glass transition temperatures (T g ) for the same glasses over the same Te0 2 content range. It will be observed that the T g curve (Curve D), rises slowly to about 60% Te0 2 and then gradually descends a bit. The T x curve (Curve C) also rises to about 40% Te0 2 content; then levels off. The significant feature is the vertical distance between the curves at any given content of Te0 2 . It will be observed that this vertical difference (T x -T g ) is about 125° C. at about 20% Te0 2 and grows to nearly 150° C. at high Te0 2 content. As indicated earlier, this is extremely important in avoiding devitrification problems during the drawing of fibers by any preform redraw technique. In general, the basic glass family of the invention, that is, the undoped alkali-tungsten-tellurite glass family, consists essentially of, in mole percent, 10-90% Te0 2 and the remainder at least 0.005% R 2 0, where R is Li, Na, K, Rb, Cs and/or Tl, and at least 5% W0 3 . Within this broad composition range, a somewhat narrower range has characteristics that make the glasses particularly useful in the fabrication of clad fibers doped with erbium for amplification components. The base glass compositions for this purpose, consist essentially of, as calculated in mole percent, 15-85% Te0 2 , 5-55% W0 3 , and 0.5-40% R 2 0, where R indicates the elements recited above. In addition, the base glass will contain 0.005-10% of an oxide of a rare earth metal in the lanthanide series, including erbium. The base glass compositions may be modified to alter the physical properties of glasses having these compositions. In particular, they may be modified for the purpose of providing combinations of core and cladding glasses for an optical fiber. For that purpose, the glasses must exhibit a difference in refractive indices as is well known. Otherwise, however, it is desirable that the core and cladding glasses have properties as near identical as possible. The modifying oxides may include 0-30% MO where M is Mg, Ca, Sr, Ba, Zn, Cd, Pb, Y, La, Gd, Lu, Ti, Zr, Hf, Nb, Ta, Bi, H, B and/or P. More particularly, glass compositions may be modified by the addition of 0-30% MO wherein M is Mg, Ca, Sr, Ba, Zn and/or Pb, 0-20% Y 2 0 3 and/or Sb 2 0 3 , and/or 0-15% Ti0 2 , Nb 2 0 5 and/or Ta 2 0 5 . These additions are particularly useful in developing core/cladding compositions appropriate for clad fiber production. The various oxide constituents can be partially replaced by the corresponding metal halides in amounts such that the total halogen content may be up to about 25% of the total halogen plus oxygen content of the composition. Also, the ratio of R 2 0 to W0 3 is kept above 1 :3, preferably 920, in order to avoid the reduction of W +6 to a lower oxidation state that could introduce sources of attenuation in fibers. Tellurite glasses, having compositions as shown in TABLE I, were melted by first mixing a batch in customary manner. The tellurium, tungsten and lanthanide components were introduced as oxides. The alkali metal oxides were introduced as either the nitrate or the carbonate. The batch was manually mixed and placed in a 96% silica, or a gold, crucible. The crucible was introduced into an electric furnace operating at 750° C. to melt the batch. Melting time was on the order of 30-60 minutes. The glass thus produced was annealed at a temperature near the transition temperature (T g ) of the glass.

Claims: WE CLAIM: 1. A family of alkali-tungsten-tellurite glasses, the compositions of which consist essentially of, as calculated in mole percent, 10-90% Te0 2 , at least 5% W0 3 and at least 0.5% R 2 0, where R is Li, Na, K, Rb, Cs, Tl and mixtures. 2. A family of glasses in accordance with claim 1 wherein R is Na and/or K. 3. A family of glasses in accordance with claim 1 that have a high thermal stability index (T x -T g ) to avoid devitrification during glass forming, that readily dissolve rare earth metal oxides to provide fluorescing ions, that have an emission spectrum with a wide bandwidth, and that have compositions consisting essentially of, as calculated in mole percent, 15-85% Te0 2 , 5-55% W0 3 , 0.5-40% R 2 0 where R is Li, Na, K, Rb, Cs, Tl and mixtures, and 0.005- 10% of an oxide in the lanthanide series. 4. A family of glasses in accordance with claim 3 wherein the oxide in the lanthanide series is erbium oxide or thulium oxide. 5. A family of glasses in accordance with claim 3 containing one or more optional, modifying components of MO wherein M is one or more of Ca, Sr, Ba, Mg, Zn, Cd, Pb, Y, Sb, La, Gd, Lu, Ti, Zr, Hf, Nb, Ta, Bi, H, B and P. 6. A family of glasses in accordance with claim 5 containing at least one optional, modifying component selected from 0-30% MO where M is an oxide of Mg, Ca, Sr, Ba, Zn, Cd and/or Pb, 0-20% Y 2 0 3 and/or Sb 2 0 3 and 0-15% Ti0 2 , Nb 2 0 3 and/or Ta 2 0 5 . 7. A family of glasses in accordance with claim 5 wherein a portion of the metal oxides are replaced by metal halides providing that the atomic ratio of the total halide content to the total halide content plus oxygen content of the glass is not over 1 :4. 8. A family of glasses in accordance with claim 3 wherein the ratio of total R 2 0:W0 3 is at least about 1 :3. 9. A family of glasses in accordance with claim 5 containing a lightweight element selected from B, H, P and mixtures to shorten the tau-32 lifetime of a fluorescing ion. 10. An optical component for a telecommunication system composed of an alkali-tungsten-tellurite glass that has a high thermal stability (T x -T g ), that readily dissolves rare earth metal oxides, that has an emission spectrum with a wide bandwidth, and that has a composition consisting essentially of, as calculated in mole percent, 15-85% Te0 2 , 5-55% W0 3 , 0.5-40% R 2 0, where R is Li, Na, K, Rb, Cs, Tl and mixtures, and 0.005-10% of an oxide in the lanthanide series. 11. An optical component in accordance with claim 10 wherein R is Na and/or K and the oxide in the lanthanide series is erbium or thulium. 12. An optical component in accordance with claim 10 wherein the glass has a 1.5 emission band with a FWHM value of at least 40 and a thermal stability index (T x -T g ) of at least 120° C. 13. An optical component in accordance with claim 10 wherein a portion of the metal oxides in the glass composition are replaced by metal halides, the replacement being such that the ratio of the total halide content to the total halide plus oxygen content is not over 1 :4. 14. An optical component in accordance with claim 10 wherein the ratio of total R 2 0:W0 3 is at least about 1 :3. 15. An optical component in accordance with claim 10 wherein the component is a clad optical fiber. 16. An optical component in accordance with claim 15 wherein the clad optical fiber is embodied in an optical amplifier.

Inventors: AITKEN BRUCE G, ELLISON ADAM J, AITKEN, BRUCE, G., ELLISON, ADAM, J.
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 2000
Main classification: C03C3
IPC codes: C03C3, C03C4, C03C13, G02B6, H01S3