Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – With measuring – controlling – sensing – programming – timing,...
Reexamination Certificate
1999-11-30
2002-11-05
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
With measuring, controlling, sensing, programming, timing,...
C065S390000, C065S421000, C065S404000
Reexamination Certificate
active
06474106
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to improved methods for forming rare earth and alumina (Al
2
O
3
) doped glass preforms from which optical fibers can be made. More particularly, this invention discloses methods for producing high alumina-doped, yet inclusion-free, optical waveguide preforms which can be drawn into optical waveguide components, such as couplers and amplifiers.
BACKGROUND OF THE INVENTION
Rare earth-doped optical fibers, such as erbium-doped glass, are suitable for many uses, particularly as optical amplifiers. These amplifiers make it possible to amplify an optical signal without first converting it into an electronic signal.
Erbium-doped fibers (“EDF”) are typically used in wavelength division multiplexing (“WDM”) systems. WDM are high data rate systems that allow simultaneous transmission of several signals in an optical waveguide at differing wavelengths. These systems usually include a source that can send signals at multiple wavelengths or input channels, a multiplexer, an optical fiber cable, a demultiplexer, and multiple output sources or output channels.
An EDF allows the amplification of an optical signal having a wavelength range of about 1530 to about 1610 nanometers (“nm”). The erbium-doped fiber acts as an amplifier when a continuous source of pump light, at a wavelength of either 980 or 1480 nm, is propagated through its length. When the optical signal is also sent through the erbium-doped fiber, the erbium ions, excited by the pump light, amplify the signal through the stimulated emission of photons from the excited state. Other rare earth dopants, such as praseodymium and neodymium, are possible candidates to amplify optical signals around the 1300 nm wavelength range.
An important parameter for an EDF is bandwidth. This allows the EDF to handle more channels, or accommodate more data, in a WDM system and similar applications.
An optical fiber preform is generally comprised of a central core and an outer cladding layer. The core 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 two layers allows the propagation of the optical signal within the core. A typical optical fiber glass core composition is comprised mainly of high purity SiO
2
glass with lesser amounts of GeO
2
and/or other dopants, depending upon the desired optical properties. Fibers whose cores are doped with GeO
2
exhibit low loss characteristics which extend to wavelengths around 1600 nm.
Alumina is well known in the art as a co-dopant in optical fiber preform glass compositions which also contain erbium or other rare earth dopants. Alumina, when used in these compositions, enhances the processing of the fiber preform by increasing the solubility of erbium or other rare earth dopants in the resultant fiber. Optical fibers drawn from alumina-containing preforms exhibit enhanced optical properties, such as a lower ripple value and an expanded bandwidth. Higher concentrations of alumina within an EFA improves its performance by flattening the Er gain spectrum over a given bandwidth.
Despite its many advantages, higher concentrations of alumina can present difficulties during the fabrication of the glass preform and optical fiber. One of the main difficulties encountered in fabricating alumina-containing glass preforms is the formation of inclusions, seeds (gas bubbles) or crystals. The presence of inclusions within the preform can render the subsequent fiber drawing process difficult or even impossible. Inclusions can decrease the length of fiber drawn from the preform and, at worst, prohibit fiber drawing entirely. FIG.
1
(
a
) is a photograph taken at a magnification of approximately 10 times actual size that illustrates the seeds present in a Type 3 (see supra Table I in Detailed Description) glass preform made from methods known in the art. Preforms containing many inclusions are usually scrapped, thereby increasing manufacturing costs. If the preform can be drawn into a fiber, inclusions can be a major source of attenuation loss due to scattering and decreased strength in the resulting fiber. For the manufacture of high quality preforms, particularly core preforms known as rods or canes, the reduction, and preferably, the elimination of inclusions is of critical importance.
There are many proposed mechanisms of bubble formation in alumina-containing glass preforms. Glass preforms are usually formed by a chemical vapor deposition process. Oftentimes, when the dopant system includes solids rather than liquids, such as AlCl
3
, 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. For example, during the consolidation step, solid particles of the aluminum-containing precursor such as AlCl
3
can react with oxygen at temperatures which range from about 1400 C to about 1500 C to form alumina Al
2
O
3
. The decomposition of these solid particles causes gas bubbles of Cl
2
to form in the resultant preform or preform core.
In preforms that contain both GeO
2
and Al
2
O
3
as dopants, gas bubble formation can result from, or be aggravated by, GeO
2
thermal decomposition. During the drawing or re-drawing step, the GeO
2
particles within the soot blank are driven to convert into their gaseous phase and form gas bubbles within the body of the preform or preform core. These further contribute to seed formation.
Subsequent anneal processing may be employed to reduce or eliminate the gas bubbles that are formed. For alumina doped preforms, additional heat processing may cause the unwanted formation of Al
2
O
3
crystals, namely cristobalite or mullite, within the glass. Lower temperatures and Al
2
O
3
concentrations within the composition tend to form cristobalite, whereas higher temperatures and higher Al
2
O
3
concentrations tend to form mullite and cristobalite. The likelihood of crystallite formation increases as the concentration of alumina dopant increases. Crystallites within the preform, like gas bubbles or seeds, can inhibit or prevent the preform from being drawn into fiber. Further, crystallites can become a scattering site and degrade the resulting optical properties of the fiber.
SUMMARY OF THE PRESENT INVENTION
The present invention improves the performance of an erbium doped fiber by increasing the concentration of alumina dopant within the optical fiber waveguide preform core. The present invention also provides methods for producing a rare earth and high concentration alumina doped optical fiber preforms which are substantially inclusion free. The methods of the present invention are particularly useful for making optical waveguide fibers drawn from a glass comprising Al
2
O
3
, GeO
2
, and SiO
2
, which can be used as host glass containing dopants such as erbium or other rare earths for making optical amplifiers.
The method of the present invention allows for high alumina doping yet substantially eliminates inclusions in the glass preform or cane by modifying certain process parameters during the soot lay-down step. During this step, a glass particulate, or soot blank, is created by reacting glass constituents in vapor form with oxygen via a heat source, such as a flame burner, in the reaction zone. The soot particles are deposited onto the surface of a rotating, removable mandrel. The burner or torch traverses across the length of the rotating mandrel, thus allowing a uniform deposit of soot along its surface. Unlike many other glass precursors or dopants, the aluminum-containing glass precursor, such as AlCl
3
, is in solid, rather than liquid, form. The present method increases the temperature range at which the aluminum-
Crossland Carl E.
Qi Gang
Corning Incorporated
Hoffmann John
Woodcock & Washburn LLP
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