OPTICAL FIBER WITH TANTALUM DOPED CLAD

Abstract: Improved optical waveguide fiber (1) comprised of a central core (10) region surrounded by inner clad region (12). The second annular region (14) is doped with tantalum. The core (10) is doped with germanium. The first annular region (20) located between two adjacent regions of relatively high indexes of refraction. The index of refraction of both adjoining regions (10, 20) is greater than the central region (20).

Description: OPTICAL FIBER WITH TANTALUM DOPED CLAD BACKGROUND U.S. Patent No. 4,715,679 describes an optical fiber with little or no dispersion over a wide band of wavelengths. The optical fiber has a central core surrounded by an inner clad that is in turn surrounded by an outer clad. The core and clad have one or more regions with a depressed index of refraction compared to the adjacent regions. The core has a maximum refractive index and that index that may decrease with the distance from the center. Adjacent the core is a first annular region of the inner clad that has a depressed index of refraction. Adjacent the depressed region is a second annular region having an index of refraction greater than the depressed, first annular region. The index depression modifies the light energy propagation characteristics of a fiber to provide a desired relationship between waveguide dispersion and wavelength. So, dispersion is controlled by depressing the index of refraction of an inner clad region that is adjacent the central core. The index depression is created by adding a suitable depressant dopant such as fluorine or boron. However, depressed regions made with fluorine and boron dopants have undesired limitations. Depressed regions made with fluorine have a maximum amount of index depression of about 0.5 percent delta, but 0.3 percent delta is a more common result. Fluorine presents manufacturing problems because it is corrosive and a commercial source of dry fluorine for a common outside vapor deposition (OVD) process is not currently available. Boron has a large adverse effect on propagation of light with wavelengths above 1200 n . As such, boron is not useable for single mode optical fibers which generally transmit light at about 1500 nm. Instead of lowering the index of a region, others have proposed raising the index of clad with germanium. However, germanium is not suitable for raising the index of the clad. Germanium reacts with chlorine during drying and consolidation to form a germanium monoxide. The monoxide is relatively volitile and migrates out of the clad during chlorine drying and consolidation steps. So, it is difficult to keep germanium in the clad and thereby raise the index of the clad with respect to an adjacent region of depressed index, such as fused silica. Accordingly, there is an unfulfilled need for an optical fiber structure that is compatible with fused silica glass, increases the index of refraction of a clad region with a dopant that does not migrate from its initial location and does not absorb light at the wavelength transmitted through the optical fiber. SUMMARY We have discovered unexpected and highly desireable results when we dope a clad with tantalum to raise the index of the clad above an adjacent depressed region of the core. This invention results in an optical fiber that uses only index increasing dopants to modify the chromatic dispersion. The invention eliminates the unwanted side effects of index lowering dopants, such as boron and fluorine, since optical fibers made with the invention do not require such dopants. Tantalum has a number of technical advantages. For one, tantalum does not migrate from its initial position. Tantalum has low volitivity so it resists migration even when the fiber is subjected to high temperatures during drying and consolidation. By resisting migration, the doping profile of the tantalum doped region remains sharply defined. Its second advantage is a low attenuation of light at the wavelengths chosen for transmission. Those wavelengths are around 1300 nm and 1550 nm. At those wavelengths, tantalum has a relatively low attenuation of light. Also, Rayleigh scattering by tantalum is relatively low at those wavelengths. A third advantage is glass doped with tantalum has a lower thermal expansion than glass doped with germanium. A fourth advantages is tantalum has a greater effect on refractive index, by weight, than does germanium. So, less tantalum is need to produce the same refraction that is produced by a germanium. Attenuation is also related to quantity. Hence, the attenuation of light in optical fibers using tantalum is less because less tantalum is used. A fifth advantage is tantalum is chemically stable. It is insoluble in water and most acids and alkalis. It is slowly attacked by hot hydrofluoric acid. The invention is applicable to all optical fibers, including but not limited to single mode fibers, multimode fibers, dispersion shifted fibers, fibers with large effective areas, and high performance, ultra long distance fibers with controlled linear dispersion. In the manufacture of the optical fiber, materials for the core and clad (inner and outer) regions of the optical fiber are made from glass having minimum light attenuation characteristics. Although an optical quality glass may be used, fused silica is a particularly suitable glass. The glass for the core and cladding glasses should have similar physical characteristics for structural and other practical considerations. Since the core glass must have a higher index of refraction than the clad glass, the core glass is formed from the same type of glass used for the clad and is doped with a small amount of material to slightly increase the refractive index of the core. So, the core is doped with germania. A first annular depressed region may be formed in an initial portion of the inner clad, in adjacent portions of the core and inner clad, or entirely within an outer annulus of the core. In the preferred embodiment, a central region of the core is doped with germanium. An outer annulus of the core is left undoped. The clad region adjacent the undoped core annulus and surrounding the core is doped with tantalum to increase its index of refraction. The tantalum doped clad region extends from the undoped core annulus to the outside of the fiber. DESCRIPTION OF THE DRAWINGS Figure 1 is a crossectional view of an optical fiber made in accordance with the invention. Figure 2 is a doping profile of one optical fiber with the invention. Figures 3 and 4 show doping profiles of other optical fibers made with the invention. Figure 5 is a graph showing the dispersion results of tantalum-doped silica overclad fiber. Figure 6 is a graph showing the refractive index as a function of wavelength for tantalum and fused silica. DETAILED DESCRIPTION Figure 1 shows a cross-sectional view of a single mode optical fiber 1 made in accordance with the invention. The optical fiber has a central core 10 that is defined by an outer surface 11. An inner clad region 12 has an inner surface formed on the outer surface 11 of the core 10. The inner clad region 12 has an outer surface 13. Inner clad 12 is surrounded by outer clad 14 which has an outer surface 15. The material of the core 10 is fused silica doped with germanium. The inner clad layer 12 has at least one annular region 20 of substantially pure fused silica. A second annular region 22 comprises fused silica doped with tantalum. A dashed line 21 indicates the boundary between regions 20 and 22. The tantalum doping extends from dashed line 21 to the outer surface 15. While the invention contemplates on inner clad with undoped region 20 and tantalum doped region 22, it also includes a fiber where the whole inner clad 12 is undoped and the outer clad 14 is doped with tantalum. Figure 2 shows a typical doping profile for an optical fiber made with the invention. The core region 10 is doped with germanium or a combination of germanium and tantalum to provide a gradient refractive index from a maximum at the center to zero at the outer surface 11 of the core 10. Adjacent core 10 is first annular region 20 of substantially pure fused silica. A second annular region 22 is doped with tantalum. The tantalum doped region 22 has an index of refraction greater than region 20 but less than the maximum of core 10. As such, there is a significant change in the index of refraction between regions 20 and 22. Accordingly, region 20 forms a depressed annular region located between two adjoining regions 10, 22, each of which has an index of refraction greater than the depressed region 20. The boundary 21 between undoped and tantalum doped regions may coincide with outer surface 13 of inner clad 12. In optical fiber 1 the core has a maximum index of refraction I 0 . Adjacent the core is the first annular region 20 with a refractive index I,. The second annular region 2 surrounds the first annular region 20 and has a refractive index of I 2 . The first annular region 20 with the depressed index I, may be formed entirely in an outer annulus of the core 10, in adjoining annular regions of the core and the inner clad, or entirely within the inner clad. So, I 0 )I 2 )Iι- A feature of the invention is a clad region that ranges from the outside edge of the outermost annulus A to the outside edge of the optical fiber B. This clad region contains Si0 2 and tantalum that increases the clad's refractive index above at least one inner annulus, which would typically be pure silica. The clad also may contain other dopants, such as titanium for added strength. Other useful profiles are shown in Figures 3 and 4. The fiber of Figure 3 has a step index region 30 made by doping an annular portion of the fiber with germanium. The tantalum doped region 32 extends from the step index region 30 to the outside surface of the fiber. The fiber of Figure 4 has two step index regions, 30, 31, each formed by doping an annular portion of the fiber with germanium. The region 30 is more heavily doped than is the region 31. The tantalum doped region 32 has an index of refraction greater than region 31 but less than region 30. It extends to the outside of the fiber. Figure 5, shows the dispersion results of a tantalum-doped silica overclad fiber. These results indicate that tantalum doped silica material dispersion is quite similar to the material dispersion of germanium doped silica. The above expectations were confirmed by further experiment. Experimental results for 7.26 wt% tantalum-doped silica were compared to fused silica and silica doped with 5.9 wt% GeO 2 and 9.26 wt% GeO 2 . The data shown in Figure 6 indicates that tantalum doped silica follows the expected refraction of 7.5 wt% germanium-doped silica. The present invention also contemplates optical waveguides with cores having either a constant or a varied index of refraction. Further modifications, alterations and changes to the profiles of the core 10 and clad 5, 12 and 14 regions may be made in accordance with the teachings of U.S. Patent No. 4,715,679 which is incorporated herein by reference. For example, the core 10 may have a step index profile, an alpha index profile, a profile that changes at a constant rate, or a profile that changes at a combination of one or more rates. The depressed region may also be formed in the core by terminating germanium doping before the core is complete. The balance of the core would be undoped fused silica. The invention may be used in any suitable optical fiber where it is desired to raise the index of the clad. So, the invention is applicable not only to single mode fibers, but also to multimode fibers, dispersion shifted fibers, fibers with large effective areas, and high performance, ultra long distance fibers with controlled linear dispersion. With the invention, the dispersion of any fiber can be modified. The invention eliminates the unwanted side effects of index lowering dopants, such as boron and fluorine, since optical fibers made with the invention do not require such dopants. As mentioned above, there are a number of technical advantages to using tantalum. Tantalum has low volatility so it does not migrate even when the fiber is subjected to high temperatures during drying and consolidation. As such, the doping profiles of tantalum doped regions remain relatively sharp. Tantalum has a low attenuation of light and low Rayleigh scattering at the wavelengths chosen for transmission. These wavelengths are around 1300 nm and 1550 nm. Glass doped with tantalum has a lower thermal expansion than glass doped with germanium. Tantalum has a greater effect on light, by weight, than does germanium. So, less tantalum is need to produce the same refraction that is produced by a germanium. Since attenuation is also proportional to quantity, the attenuation of optical fibers using tantalum is less because less tantalum is used. Tantalum is chemically stable. It is insoluble in water and most acids and alkalis and is only slowly attacked by hot hydrofluoric acid. The inventive fiber 1 with a depressed index region is made by any conventional fiber manufacturing process. According to the invention, the process for application of the remainder of the second coating of soot ultimately forming the clad 14 is modified from conventional teachings by the introduction of suitable concentrations of a tantalum precursor such as TaCI 5 . Those skilled in the art will appreciate that other materials may also raise the index of refraction. Such other materials include zirconium, lanthanum, yttrium, cerium, and germanium. In addition, florid, zirconium, tetrachloride, and hexaflorin, hexafluoroacetylactonates, and analogous compounds of lanthanum, yttrium, and cerium are compatible with the OVD process. Anyone of the above in suitable concentrations may yield an index increasing dopant in region 14. In preferred embodiments, the concentration of Ta-.0 5 precursor in the Si0 2 soot precursor composition ranges up to about 10 weight percent and most preferably about 3 to about 5 weight percent. We note here that although the above description illustrates the processes of the invention, the process aside from the tantalum addition to inner clad region 12 is entirely conventional. Hence, modifications to the conventional process steps known to those of ordinary skilled in the art can be employed. For example, any of the various laydown processes can be used, including but not limited to outside vapor deposition, inside vapor deposition, vapor axial deposition, modified chemical vapor deposition, or plasma outside inside deposition. Conventional optical waveguide fiber technology is readily employed by those of ordinary skill in the art in practicing the invention, all of which is hereby incorporated herein by reference, and including by way of non-limiting examples the following. As to raw materials useful as soot precursors, see: Dobbins U.S. Patent No. 5,043,002; and Blackwell U.S. Patent No. 5,152,819. As to processes for the vaporization or nebulization of soot precursors, see: Antos U.S. Patent No. 5,078,092; Cain U.S. Patent No. 5,356,451; Blankenship U.S. Patent No. 4,230,744; Blankenship U.S. Patent No. 4,314,837; and Blankenship U.S. Patent No. 4,173,305. As to burning soot precursors and laydown of core and cladding, see: Abbott U.S. Patent No. 5,116,400; Abbott U.S. Patent No. 5,211,732; Berkey U.S. Patent No. 4,486,212; Powers U.S. Patent No. 4,568,370; Powers U.S. Patent No. 4,639,079; Berkey U.S. Patent No. 4,684,384; Powers U.S. Patent No. 4,714,488; Powers U.S. Patent No. 4,726,827; Schultz U.S. Patent No. 4,230,472; and Sarkar U.S. Patent No. 4,233,045. As to the steps of core preform consolidation, core cane drawing, and overclad preform consolidation, see: Lane U.S. Patent No. 4,906,267; Lane U.S. Patent No. 4,906,268; Lane U.S. Patent No. 4,950,319; Blankenship US. Patent No. 4,251,251; Schultz U.S. Patent No. 4,263,031; Bailey U.S. Patent No. 4,286,978; Powers U.S. Patent No. 4,125,388; Powers U.S. Patent No. 4,165,223; and Abbott U.S. Patent 5,396,323. As to fiber drawing from a consolidated overclad preform, see: Harvey U.S. Patent No. 5,284,499; Koening U.S. Patent No. 5,314,517; Amos U.S. Patent No. 5,366,527; Brown U.S. Patent No. 4,500,043; Darcangelo U.S. Patent No. 4,514,205; Kar U.S. Patent No. 4,531,959; Lane U.S. Patent No. 4,741,748; Deneka U.S. Patent No. 4,792,347; Ohls U.S. Patent No. 4,246,299; Claypoole U.S. Patent No. 4,264,649; and Brundage U.S. Patent No. 5,410,567.

Claims: We Claim: 1. An optical fiber waveguide with a core and inner clad comprising a central region having a maximum index of refraction, I 0 ; an first annular region adjacent the central region and having an index of refraction, I„ that is less than I 0 ; a second annular region surrounding said first annular region and having an index of refraction I 2 , said second annular region comprising tantalum for increasing the index of said region with enough index increasing dopant to raise the index of refraction of the second annular region I 2 above the index of refraction of the first annular region I. 2. The optical fiber of claim 1 wherein the central region comprises the core and the second annular region contains multiple sub-regions at least one of which has refractive index, I„ less than I 2 . 3. The optical fiber waveguide of claim 1 where I 0 >I 2 >I 1 . 4. The optical fiber of claim 1 wherein the dopant for the second annular region comprises tantalum. 5. The optical fiber of claim 1 wherein the optical fiber is one selected from the group consisting of single mode fibers, multimode fibers, dispersion shifted fibers, fibers with large effective areas, and high performance, ultra long distance fibers with controlled chromatic dispersion. 6. An optical fiber comprising: a core having a maximum index of refraction, I 0 ; an inner clad layer on the core having a first annular region surrounding said core and having an index of refraction, an index of refraction, I,, that is less than I 0 ; a second annular region surrounding said first annular region and having an index of refraction I 2 doped with enough tantalum to raise the index of refraction of the second annular region I 2 above the index of refraction of the first annular region I,, but less than L 7. The optical fiber of claim 6 wherein the first annular region comprises fused silica and the second annular region comprises fused silica doped tantalum. 8. The optical fiber of claim 6 wherein the core comprises fused silica doped with germaniu . 9. The optical fiber of claim 6 wherein the optical fiber is one selected from the group consisting of single mode fibers, multimode fibers, dispersion shifted fibers, fibers with large effective areas, and high performance, ultra long distance fibers with controlled linear dispersion. 10. An optical fiber comprising a core region and first and second annular regions, respectively surrounding said core region wherein the index of refraction of the first annular region is depressed with respect to the adjacent core and second annular regions and the second annular region comprises tantalum. 11. The optical fiber of claim 11 wherein the core region comprises fused silica and tantalum. 12. The optical fiber of claim 1 1 wherein the core region comprises fused silica, germanium, and tantalum. 13. The optical fiber of claim 10 wherein the second annular region comprises fused silica and tantalum. 14. The optical fiber of claim 10 wherein the amount of tantalum in the second annular region ranges up to about 10 weight percent. 15. The optical fiber of claim 14 where the amount of tantalum in the second annular region ranges from about 3 weight percent to about 5 weight percent. 16. The optical fiber of claim 10 wherein the optical fiber is one selected from the group consisting of single mode fibers, multimode fibers, dispersion shifted fibers, fibers with large effective areas, and high performance, ultra long distance fibers with controlled linear dispersion.

Inventors: MURPHY JAMES P, SMITH DAVID K, MURPHY, JAMES, P., SMITH, DAVID, K.
Applicants: CORNING INC, MURPHY JAMES P, SMITH DAVID K, CORNING INCORPORATED, MURPHY, JAMES, P., SMITH, DAVID, K.
Application year: 1997
Main classification: G02B6
IPC codes: C03C13, G02B6