HK1015889A - Optical fiber with tantalum doped clad - Google Patents

Description

Optical fiber with tantalum-doped cladding

no marking

Background

U.S. patent 4,715,679 describes an optical fiber having little or no dispersion over a wide wavelength band. The optical fiber has a central core surrounded by an inner cladding and an outer cladding. The core and cladding have one or more regions of depressed index relative to the adjacent regions. The core has a maximum index of refraction and the index of refraction decreases with distance from the center. Adjacent to the core is a first annular region of the inner cladding which has a depressed index of refraction. Adjacent to the recessed region is a second annular region having a refractive index greater than the first annular region of the recess. The index depression changes the optical energy propagation characteristics of the fiber to provide a desired relationship between waveguide dispersion and wavelength. Thus, by lowering the index of the inner cladding region adjacent to the central core, dispersion can be controlled. The index depression may be created by adding a suitable dopant that acts as a lowering, such as fluorine or boron.

However, recess regions made with fluorine and boron dopants have undesirable limitations. The depressed regions made with fluorine had a refractive index maximum depression of about 0.5% delta, but the more general result was 0.3% delta. Fluorine gas presents manufacturing problems because fluorine is corrosive and dry fluorine gas is not commercially available for use in typical Outside Vapor Deposition (OVD) processes. Boron has a great side effect on light propagating over 1200 nm. Therefore, boron is not suitable for single mode optical fibers that typically transmit light having a wavelength of about 1500 nm.

It has been proposed to use germanium to increase the refractive index of the cladding layer rather than to lower the refractive index of a region. However, germanium is not suitable for increasing the refractive index of the cladding. During drying and fusing, germanium may react with chlorine gas to form germanium monoxide. During the chlorine drying and fusing step, the monoxide is relatively volatile and can migrate out of the cladding. Therefore, it is difficult to retain germanium in the cladding and thus to increase the refractive index of the cladding relative to adjacent regions of the refractive index depression, such as fused silica.

Therefore, an unmet need is a fiber structure that fits fused silica glass and raises the refractive index of the cladding region with a dopant that does not move from its original position and does not absorb light of the wavelength transmitted through the fiber.

SUMMARY

When the cladding is doped with tantalum to raise the refractive index of the cladding above an adjacent depressed region of the core, we have found unexpected and highly desirable results. The present invention produces an optical fiber that improves dispersion using only the index increasing dopant. The present invention avoids the side effects of refractive index lowering dopants (e.g., boron and fluorine) because optical fibers made with the present invention do not require such dopants.

Tantalum has a number of technical advantages. First, tantalum does not move from its original position. Tantalum has a relatively low volatility so that it resists migration even though the fiber is subjected to high temperatures during drying and fusing. By resisting movement, the doping profile of the tantalum-doped region remains sharply defined. A second advantage is that there is less optical attenuation at the wavelengths selected for transmission. These wavelengths are around 1300 nm and 1550 nm. At these wavelengths, tantalum attenuates light considerably. Also, the rayleigh scattering by tantalum is relatively small at these wavelengths. A third advantage is that tantalum doped glasses have a smaller thermal expansion than germanium doped glasses. A fourth advantage is that tantalum has a greater influence on the refractive index than germanium by weight. Therefore, less tantalum is required to produce refraction equivalent to that produced by germanium. Attenuation is also related to quantity. Thus, less attenuation of light in optical fibers using tantalum is achieved because less tantalum is used. A fifth advantage is that tantalum is chemically stable. It is insoluble in water and most acids and bases. It is only slowly corroded by hot hydrofluoric acid. The present 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 fibers with controlled linear dispersion.

In the manufacture of optical fibers, the materials of the core and cladding (inner and outer) of the optical fiber are made of glass having minimal optical attenuation characteristics. Although optical quality glass can be used, fused silica is a particularly suitable glass. For structural and other practical considerations, the core glass and the cladding glass should have similar physical properties. Since the core glass must have a refractive index greater than that of the cladding glass, the core glass is formed of the same type of glass used for the cladding and incorporates a small amount of material to slightly increase the refractive index of the core. Therefore, the core is doped with germanium oxide. The first annular depression may be formed in the beginning of the inner cladding, adjacent portions of the core and inner cladding, or the entire outer core ring. In a preferred embodiment, the central region of the core is doped with germanium. The outer ring of the core is undoped. A cladding region adjacent to the undoped core outer ring and surrounding the core is doped with tantalum to increase its refractive index. The tantalum-doped cladding region extends from the undoped core ring to the outside of the optical fiber.

Description of the drawings

FIG. 1 is a cross-sectional view of an optical fiber made in accordance with the present invention.

FIG. 2 is a dopant profile of an optical fiber according to the present invention.

Fig. 3 and 4 show dopant profiles of other optical fibers made with the present invention.

FIG. 5 is a graph showing the dispersion results for a tantalum-doped silica overclad fiber.

FIG. 6 is a graph showing refractive index curves for tantalum and fused silica as a function of wavelength.

Detailed Description

Fig. 1 shows a cross-sectional view of a single mode optical fibre 1 made in accordance with the present invention. The optical fiber has a central core 10 defined by an outer surface 11. The inner cladding region 12 has an inner surface formed on the outer surface 11 of the core 10. The inner cladding region 12 has an outer surface 13. The inner cladding 12 is surrounded by an outer cladding 14 having an outer surface 15.

The material of the core 10 is germanium-doped fused silica. The inner cladding 12 has at least one annular region 20 made of substantially pure fused silica. The second annular region 22 comprises tantalum-doped fused silica. The dashed line 21 represents the boundary between the regions 20 and 22. The tantalum dopant extends from dashed line 21 toward outer surface 15. Although the present invention contemplates an inner cladding having undoped 20 and tantalum doped 22 regions, it may also include an optical fiber in which the entire inner cladding 12 is undoped and the outer cladding 14 is tantalum doped.

FIG. 2 shows a typical dopant profile for an optical fiber made by the present invention. The core region 10 is doped with germanium or a combination of germanium and tantalum to provide a graded index of refraction from the center up to zero at the outer surface 11 of the core 10. Adjacent to the core 10 is a first annular region 20 made of substantially pure fused silica. The second annular region 22 is doped with tantalum. The refractive index of tantalum-doped region 22 is greater than the refractive index of region 20, but less than the maximum of core 10. Thus, there is a significant change in the refractive index between regions 20 and 22. Thus, region 20 forms a recessed annular region between two adjacent regions 10 and 22, i.e., regions 10 and 22 each have a refractive index greater than that of recessed region 20. The boundary between the undoped region and the tantalum-doped region may coincide with the outer surface 13 of the inner cladding 12.

In the optical fiber 1, the maximum refractive index of the core is I₀. Adjacent the core is a first annular region 20 having a refractive index I₁. The second annular region 22 is wrapped around the first annular region 20 and has a refractive index I₂. The refractive index of the recess can be made to be I₁Is formed entirely in the outer ring of the core 10, in a contiguous annular region of the core and inner cladding, or is formed entirelyWithin the inner cladding. Therefore, I₀＞I₂＞I₁。

One feature of the present invention is that there is a cladding region ranging from the outer edge of the outermost ring a to the outer edge of the optical fiber B. The cladding region comprises SiO₂And tantalum, such that the cladding has a refractive index at least higher than that of an inner ring, typically made of pure quartz. The cladding may also contain other dopants, such as titanium for added strength. Other useful profiles are shown in figures 3 and 4.

The fiber of fig. 3 has a step-index region 30 made by doping the annular portion of the fiber with germanium. Tantalum-doped region 32 extends from step index region 30 to the outer surface of the fiber. The fiber of fig. 4 has two step index regions 30 and 31, each formed by doping an annular portion of the fiber with germanium. The doping level of region 30 is greater than that of region 31. The refractive index of tantalum-doped region 32 is greater than the refractive index of region 31 but less than the refractive index of region 30. It extends to the outside of the fiber.

FIG. 5 shows the dispersion results for a tantalum-doped silica overclad fiber. These results indicate that the dispersion of the tantalum-doped quartz material is very similar to the dispersion of the germanium-doped quartz material.

The above expectations have been confirmed by further experiments. The experimental results of 7.26 wt% tantalum doped quartz were compared with fused quartz and 5.9 wt% GeO doped quartz₂And 9.26 wt% GeO₂Comparison of quartz (c). The data shown in FIG. 6 indicate that tantalum-doped quartz has a desired refractive index along with 7.5 wt% germanium-doped quartz.

The present invention also contemplates providing optical waveguides having a core with a constant or variable index of refraction. Further variations and modifications can be made to the distribution of the core 10 and the cladding layers 5, 12 and 14 in accordance with the principles of U.S. Pat. No. 4,715,679, the disclosure of which is incorporated herein by reference. For example, the core 10 may have a step index profile, an α -type index profile, a profile that varies at a constant rate, or a profile that varies in a combination of one or more rates. It is also possible to form a depressed region in the core by ending the germanium doping before the core is completed. The remainder of the core is undoped fused silica.

The invention may be used with any suitable optical fiber where it is desirable to increase the refractive index of the cladding. Therefore, the present invention is applicable not only to single mode fibers, but also to multimode fibers, dispersion shifted fibers, fibers with larger effective areas, and high performance, ultra-long fibers with controlled linear dispersion. The dispersion of any fiber can be varied using the present invention. The present invention eliminates the undesirable side effects of index-lowering dopants (e.g., boron and fluorine) because the optical fiber made by the present invention does not require such dopants.

As mentioned above, there are many technical advantages to using tantalum. Tantalum has a relatively low volatility so that it does not migrate even when subjected to high temperatures during drying and fusing of the fiber. Thus, the doping profile of the tantalum-doped region maintains a rather steep shape. Tantalum has lower optical attenuation and lower rayleigh scattering at the wavelengths selected for transmission. These wavelengths are around 1300 nm and 1550 nm. The thermal expansion of tantalum-doped glass is smaller than that of germanium-doped glass. Tantalum, by weight, has a greater effect on light than germanium. Less tantalum is required to produce the equivalent refraction produced by germanium. Since attenuation is also proportional to the quantity, optical fibers using tantalum have less attenuation because less tantalum is used. Chemically, tantalum is stable. It is insoluble in water and most acids and bases and is only slowly attacked by hot hydrofluoric acid.

The inventive fiber 1 having a depressed index region can be made by any conventional fiber manufacturing process.

According to the invention, by introducing an appropriate concentration of a substance such as TaCl₅The process of applying the remainder of the second coating layer, which consists of the microparticles that ultimately form the cladding 14, is improved over conventional teachings. One skilled in the art will recognize that other materials may also increase the refractive index. Such other materials include zirconium, lanthanum, yttrium, cerium and germanium. In addition, miconazole nitrate (florid), zirconium, tetrachloride, hexaflorin and hexafluoroacetylacetonate (hexafluoroacetylacetonate), toAnd lanthanum, yttrium and cerium analogs are suitable for Outside Vapor Deposition (OVD) processes. Any of the above-mentioned materials in appropriate concentrations can produce a dopant in region 14 that increases the refractive index. In the preferred embodiment, Ta₂O₅Precursor in SiO₂The concentration of the micropowder component can range up to about 10 weight percent, and preferably about 3 to 5 weight percent. Here, we note that while the above description illustrates the process of the present invention, the process is entirely conventional except for the addition of tantalum in the inner cladding region 12. Accordingly, conventional process steps known to those of ordinary skill in the art may be modified. For example, various deposition processes may be used, including (but not limited to) outside vapor deposition, inside vapor deposition, vapor axial deposition, modified chemical vapor deposition, or plasma outside and inside deposition.

Conventional waveguide fiber technology is readily employed by one of ordinary skill in the art in practicing the present invention, and is therefore incorporated by reference herein in its entirety, including by way of non-limiting example, the following.

For raw materials that can be used as a precursor for micropowder (soot), see: U.S. Pat. Nos. 5,043,002 to Dobbins; and Blackwell, U.S. patent 5,152,819.

For processes for vaporizing or atomizing a micropowder precursor, see: us patent 5,078,092 to Antos; U.S. patent 5,356,451 to Cain; united states patent 4,230,744 to blankenschip; united states patent 4,314,837 to blankenschip; and Blackenship U.S. patent 4,173,305.

For burning soot precursors and depositing the core and cladding, see: U.S. patent 5,116,400 to Abbott; U.S. patent 5,211,732 to Abbott; U.S. patent 4,486,212 to Berkey; us patent 4,568,370 to Powers; us patent 4,639,079 to Powers; U.S. patent 4,684,384 to berney; us patent 4,714,488 to Powers; us patent 4,726,827 to Powers; schultz, U.S. patent 4,230,472; and U.S. patent 4,233,045 to Sarkar.

For core preform consolidation, core cane (core cane) drawing and overclad preform consolidation steps, see: lane, U.S. Pat. No. 4,906,267; lane, U.S. Pat. No. 4,906,268; lane, U.S. Pat. No. 4,950,319; united states patent 4,251,251 to blankenschip; schultz, U.S. patent 4,263,031; bailey, U.S. patent 4,286,978; us patent 4,125,388 to Powers; U.S. Pat. Nos. 4,165,223 to Powers; and Abbott, U.S. patent 5,396,323.

For optical fibers drawn from a fused overcladding preform, see: U.S. patent 5,284,499 to Harvey; koening's U.S. patent 5,314,517; amos, U.S. patent nos. 5,366,527; U.S. patent No. 4,500,043 to Brown; darcangelo, U.S. patent 4,514,205; kar, U.S. patent 4,531,959; lane, U.S. Pat. No. 4,741,748; U.S. Pat. nos. 4,792,347 to Deneka; U.S. patent 4,246,299 to Ohls; claypoole, U.S. Pat. No. 4,264,649; and U.S. patent 5,410,567 to Brundage.

Claims (16)

1. An optical fiber waveguide having a core and an inner cladding, comprising:

a central zone having a maximum refractive index I₀；

A first annular region adjacent to the central region and having a refractive index I₁Is less than I₀；

A second annular region surrounding said first annular region and having a refractive index I₂Said second annular region comprising tantalum, said region having a refractive index that is increased by a dopant that increases the refractive index by a sufficient amount to increase the refractive index of the second annular regionRefractive index I₂To above the refractive index I of the first annular region.

2. The optical fiber of claim 1 wherein the central region comprises the core and the second annular region comprises a plurality of sub-regions, wherein at least one of the sub-regions has an index of refraction I₁Is less than I₂。

3. The optical fiber of claim 1 wherein I₀＞I₂＞I₁。

4. The optical fiber of claim 1, wherein the dopant of the second annular region comprises tantalum.

5. The optical fiber of claim 1, wherein said optical fiber is one selected from the group consisting of single mode fibers, multimode fibers, dispersion shifted fibers, fibers having a large effective area, and high performance, ultra-long fibers having controlled dispersion.

6. An optical fiber, comprising:

core having maximum refractive index I₀；

An inner cladding on the core having a first annular region surrounding said core and having a refractive index I₁Is less than I₀；

A second annular region surrounding said first annular region and having a refractive index I₂Doped with tantalum in an amount sufficient to modify the refractive index I of the second annular region₂Increasing the refractive index I to the first annular region₁Above, but less than I₀。

7. The optical fiber of claim 6, wherein the first annular region comprises fused silica and the second annular region comprises tantalum-doped fused silica.

8. The optical fiber of claim 6 wherein the core comprises germanium-doped fused silica.

9. The optical fiber of claim 6, wherein said fiber is one selected from the group consisting of single mode fibers, multimode fibers, dispersion shifted fibers, fibers having a large effective area, and high performance, ultra-long fibers having controlled linear dispersion.

10. An optical fiber comprising a core region and first and second annular regions respectively surrounding said core region, wherein the refractive index of the first annular region is depressed relative to the refractive index of 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 11 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, wherein 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 said fiber is one selected from the group consisting of single mode fibers, multimode fibers, dispersion shifted fibers, fibers having a large effective area, and high performance, ultra-long fibers having controlled linear dispersion.