Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2000-06-09
2002-10-22
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S378000, C065S390000, C065S397000, C065S398000, C065S415000, C065S416000, C065S417000, C065S418000
Reexamination Certificate
active
06467313
ABSTRACT:
FIELD OF THE INVENTION
The present invention provides a method for controlling the concentration profile of dopants incorporated into a glass article. More particularly, the present invention relates to an improved method of doping an optical fiber preform with dopants that are not readily incorporated during the initial fabrication process.
BACKGROUND OF THE INVENTION
Optical fibers are typically drawn from glass preforms. An optical fiber preform is generally comprised of a central core and an outer cladding layer. The central core, for the most part, has a higher refractive index than the cladding layer. When the preform is drawn into an optical fiber, the difference in refractive indices between the core and cladding layers allows the propagation of the optical signal within the core. Optical fiber preforms and waveguides are composed primarily of high purity silica glass.
Variations in the refractive index are obtained by adding dopants to layers within the central core or surrounding cladding layer. Certain dopants such as the oxides of titanium, germanium, aluminum, and phosphorous are added, in a weight percentage ranging from about 0.1 to 25%, to increase the refractive index of the glass. Other dopants such as fluorine and boron oxide may be added in similar amounts to decrease the refractive index of the glass. A typical optical fiber glass core composition is comprised mainly of high purity SiO
2
glass, in a weight percentage above 50%, with lesser amounts of GeO
2
, TiO
2
, and/or other dopants, depending upon the desired optical properties.
Glass optical fiber preform can be made from a variety of processes. Typical processes for making these preforms are variations of chemical vapor deposition (CVD) processes such as Outside Vapor Deposition (OVD), Modified Chemical Vapor Deposition (MCVD), or Vapor Axial Deposition (VAD). These processes typically involve the oxidation of glass precursors, such as metal chlorides, to form glass particulate. Glass precursors, such as silicon tetrachloride (SiCl
4
), germanium tetrachloride (GeCl
4
), or titanium tetrachloride (TiCl
4
) which may be liquid at room temperature, are heated within bubblers, vaporizers, or similar means to form a metal chloride vapor. Chlorides are widely used because they vaporize at relatively low temperatures prior to transportation to the reaction zone or a hot zone such as a burner flame, plasma, or heated area within a substrate tube. The chloride vapors oxidize within the reaction zone thereby forming a glass particulate. After a sufficient thickness of particulate is reached, the glass particulate eventually forms the porous soot blank. The porous soot blank is then sintered, or heated until the pores are eliminated, to form a glass preform. In the MCVD process, the formation and sintering of the glass particulate generally occurs simultaneously.
There is also a need to produce optical fibers with dopants such as rare earth and alumina, most of these dopants which are solids at room temperature sublime, rather than boil, to form a vapor. The vapor pressures of these dopants are low at the temperature ranges commonly used for conventional vapor delivery processes, i.e., about 200° C. or below. These properties make it difficult to deliver rare earth, alumina, and other dopants using conventional processes, such as chemical vapor deposition, that are amenable to dopants, such as Ge or Ti, having relatively high vapor pressure precursors.
Yet another problem that occurs in vapor deposition of dopants that sublimate rather than vaporize is the likelihood of bubble formation. Oftentimes, when the dopant system includes solids rather than liquids, solid dopant particles can be carried to the reaction zone by carrier gases during the soot lay-down step. Since the dwell time of solid particles in the heat source or flame is minimal, the solid particles cannot be completely reacted or oxidized. These unreacted solid particles may attach to the glass particulate, or soot, and become incorporated into the soot blank. The particles eventually react and decompose in subsequent processing steps involving elevated temperatures. The decomposition of these solid particles may cause gas bubbles of Cl
2
, or other gaseous by-products in the resultant preform or preform core.
To alleviate some of these above noted problems, other dopant methods such as solution doping, may be used. These methods, although effective, are also not without drawbacks. In solution doping, such as the process disclosed in U.S. Pat. No. 3,859,073, the porous glass optical fiber preform is immersed within a solution containing the dopant for a period sufficient for the dopant to be incorporated into the blank. The preform is dried for a certain time and then consolidated to form a glass article. Porosity of the preform must be tightly controlled to ensure that the dopant solution can be absorbed into the preform. If the particles within the porous preform are too loosely bound, the preform can disintegrate or crack within the solution. Further, the preform must be thoroughly dried prior to consolidation to avoid cracking or other damage. Thus, the solution doping method requires extra processing steps, adds cycle time, and potentially introduces manufacturing defects into the fiber process.
SUMMARY OF THE PRESENT INVENTION
The novel methods of the present invention control dopant profiles in glass articles comprising at least two dopants. More specifically, the present invention discloses novel methods for doping an optical fiber glass preform or glass article comprising a first dopant, such as GeO
2
, TiO
2
, P
2
O
5
, and/or B
2
O
3
, with a second dopant, such as an oxide of Al, Zr, Y, Nb, Ta, Ga, In, Sn, Sb, Bi, the 4f rare earths (atomic numbers 57-71 of the periodic table), Be, Mg, Ca, Zn, Sr, Cd, and Ba, so that the concentration of the second dopant is influenced by the concentration of the first dopant. As a result, the second dopant within the consolidated preform or glass exhibits a radial profile that correlates with, or substantially mirrors, that of the first dopant contained within the initial soot preformn. Further, the extent of radial penetration of the second dopant may be substantially equal to that of the first dopant. Moreover, the concentration of the added second dopant is higher than is expected without the presence of the first dopant.
The methods of present invention provide for the controlled incorporation of a doping chemical species not readily achieved during the initial fabrication of the glass. Accordingly, one aspect of the present invention is directed to methods for making an optical fiber glass preform having a radially uniform dopant profile and the optical fiber made therefrom. A further aspect of the present invention is directed to methods for controlling, or influencing, dopant profiles. Lastly, an additional aspect of the present invention is to effect the extent of radial penetration of the added dopant.
Yet another aspect of the present invention provides methods of improving the concentration of dopants within a glass article utilizing vapor infiltration doping techniques. Vapor infiltration processes provide a solution to incorporating dopants, such as the oxides of Al, Cd, Zn, Zr, Y, Nb, Ta, Ga, In, Sn, Sb, Bi, the 4f rare earths (atomic numbers 57-71 of the periodic table), and the alkaline earths Be, Mg, Ca, Sr, and Ba, into optical fiber glass preforms or cores, that cannot be incorporated, or incorporated with great difficulty, into the glass during the initial formation of the soot blank. The doping cycle occurs after a porous glass blank containing a first dopant, typically GeO
2
, TiO
2
, P
2
O
5
, or B
2
O
3
, is formed. The present invention provides methods for improving the dopant profiles, respective dopant concentrations, and degree of dopant penetration through careful tailoring of the amount of GeO
2
, TiO
2
, and/or other non-SiO
2
oxides present in the porous glass blank.
The vapor infiltration techniques of the present invention comprise placing a previously formed por
Chu Polly W.
Moore Lisa A.
Pierson-Stull Michelle D.
Colaianni Michael
Corning Incorporated
Krogh Timothy R.
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