Background of the Invention Processing of consolidated high-purity glass preforms into optical fiber is well known in the art. A glass soot, which may include suitable doping, is first deposited, for example, by a vapor deposition process upon a rotating substrate, such as an alumina mandrel, to form a physical core portion.

Various vapor deposition methods are described in US Pat. Nos. 3,737,292, 3,823,995 and 3,884,550. The core portion of the soot preform is formed by introducing various gasses in predetermined amounts into a burner flame. This produces oxides that may include, for example, silicon oxide and germanium oxide. These oxides deposit on the rotating mandrel until the appropriate dimension of the core portion is reached. The oxides may be introduced in various amounts, as desired, to produce various core refractive index profiles as, for example, described in US Pat. No. 3,823,995. The core portion, once formed, is then overclad with Si02 (the near clad) until a final preform diameter is reached thereby forming a soot preform.

This soot preform is then inserted into, dried and consolidated in a consolidation furnace. Consolidation occurs by heating the preform at a high temperature (generally in the range of between about 1400°C to about 1800°C,

depending upon preform composition) until the deposited soot transforms into a solid, high-purity glass having superior optical properties. In some cases, the consolidated preform is drawn into a small diameter core cane which serves as the mandrel or substrate for further application of Si02 soot via an Outer Vapor Deposition (OVD) process. It should be understood that the core cane comprises a core and a near clad portion. If a core cane is used, the uniform, small-diameter core cane is once again clad with Si02 and again consolidated to form a final consolidated preform. This final preform is then drawn into an optical fiber.

Prior art methods of consolidation of preforms include the"gradient consolidation method"as taught in US Pat. No. 3,933,454 whereby the bottom tip of the preform is consolidated first; the consolidation continuing up the preform until completed. Another known method of consolidation consolidation referred referred as the"bulk consolidation method"where the entire preform is consolidated at one time, i. e., all the preform is subjected to the same temperature profile along its axial length. As should be understood, such consolidated preforms are used in, for example, producing core cane and optical waveguide fibers.

In the Inner Vapor Deposition (IVD) process, it is known to sometimes provide P205 at the core clad interface, the P205 extending only a very slight distance outward into the clad region (about 10-25 microns), presumably to accommodate thermal expansion differences between the clad and the core portion.

One of the problems with the afore-mentioned"gradient consolidation method"is that it may, under some circumstances, cause undesirably high shear and tensile stresses during consolidation. Such undesirably high shear and tensile stresses along the core-soot interface may cause defects such as haze, spirals, holes, or puddles at the core cane interface, i. e., at the interface of the outer surface of the core cane and the clad portion. Such defects can lead to fiber rejection for failing to meet optical quality specifications.

Additionally, because sintering in the gradient method takes place along an axial direction, this may lead to axially non-uniform sintering that may cause

non-uniform blank-to-core diameter ratios along the preform as well as dispersion problems.

Some of these problems can be attributed to the fact that normal thermal gradients cause the outermost radial portions of the preform to consolidate first, i. e., the sintering takes place in a radially inward fashion (from outside to inside). This forces outgassing, undesirably, to take place along the axial extent of the preform because the outermost consolidated glass functions as a barrier. Outgassing is particularly problematic when employing the"bulk consolidation method"because the outermost portions along the entire length of the preform consolidate first, thereby minimizing outgassing paths. This makes bulk sintering of large diameter preforms impractical.

Therefore, providing sufficient outgassing avenues is a significant problem in optical fiber preform manufacture. In particular, considerable effort has been spent on methods of improving outgassing in such glass preforms.

These efforts, although successful (e. g., the gradient method) can result in defects in some preforms. In general, bulk consolidation would be preferable to gradient consolidation, if practicable, because the process-induced stresses are less. Thus, there is a need for an improved method that minimizes the aforementioned problems resulting from insufficient outgassing paths and/or non-uniform bulk sintering of the preform.

Summarv of the Invention In accordance with the invention, a method is provided for manufacturing a consolidated preform. The method improves the ability to outgas the preform and minimizes the afore-mentioned problems by causing the soot preform to consolidate from the"inside out,"as opposed to the prior art"outside in."Desirably, because of this reversal in consolidation sequence, outgassing may occur in a radial direction. It should be recognized that radial outgassing is allowed to occur because the outgassing path is not blocked by any previously consolidated glass in the radial path, as was the case in prior art methods.

The method in accordance with the invention is accomplished by doping the soot clad region of the preform with a suitable dopant that impacts (lowers) the viscosity of the consolidating soot as a function of temperature. An example of a suitable dopant is phosphorus. More preferably, the dopant comprises P205. Preferably, the dopant is provided in the soot region in amounts that vary as a function of radial dimension of the preform. Most preferably, the dopant is provided in sufficient amounts such that the glass at an inner portion of the soot preform consolidates before the glass at an outermost portion thereof. More preferably yet, higher amounts of dopant are provided in the inner portions of the soot region and lesser amounts radially outward therefrom.

According to the invention, it is preferable that the amount of dopant drops gradually, and most preferably, linearly as a function of radially outward dimension. By way of example, for a typical preform, the amount of dopant present at the innermost portion of the soot clad region is approximately equal to or greater than about 0.1% per centimeter of soot and decreases therefrom as a function of radial dimension. Therefore, for a soot blank having 10 cm of soot, the dopant is preferably about 1% by weight of soot at the innermost portion of the soot preform and preferably about zero at an outermost portion thereof. Although, it should be recognized that other amounts may work equally well. The amounts needed will depend upon, for example, the soot composition and density, its thickness, diffusivity and temperatures in the consolidation furnace. Moreover, although P205 is described, other materials which impact the viscosity but do not significantly impact optical properties may be employed as well. For example, B203 or C2F6 may be included in the soot clad region in graduated amounts.

According to another aspect, the consolidation step is preferably accomplished in an axially uniform fashion, i. e., by bulk consolidation where the temperatures along the axial length of the preform are held constant within the furnace. The bulk consolidation method has utility for providing higher quality optical fibers with better stress distributions, minimized breaks due to the aforementioned problems and less PMD. Moreover, the method eliminates

the need to impose axial temperature gradients (as in the gradient consolidation method) to the preform that may cause large shear or tensile forces at the core cane-soot interface. As a side result, the method minimizes axial variations in optical properties. Heretofore, bulk consolidation problems made its use undesirable. It should be understood that the radially outward consolidation sequence in accordance with the invention advantageously allows for the use of bulk consolidation method while minimizing the previous problems associated with it.

Other aspects and features of the invention will be understood with reference to the following detailed description, claims and appended drawings.

Brief Description of the Figures Fig. 1 is a perspective view of a core portion in the form of a core cane including core and clad regions.

Fig. 2 illustrates a cross-sectional view of the core cane taken along line 2-2 of Fig. 1.

Fig. 3 is a partial perspective view of a soot preform including a core cane and an outside clad region in accordance with the invention.

Fig. 4 is a schematic illustration of an apparatus for drawing an optical fiber from the consolidated preform in accordance with the invention.

Fig. 5 is a cross-sectional view of the preform taken along line 5-5 of Fig. 3.

Fig. 6 is a graphical illustration of a preferred embodiment wherein the amount of the dopant varies as a function of radius of the preform.

Fig. 7 is a graphical illustration of density isocontours of the preform during consolidation.

Detailed Description of the Invention Reference will now be made in detail to the present preferred embodiment of the invention with reference to the drawings. Wherever possible, the same reference numerals shall be use throughout to refer to the same or like parts. The drawings are meant be illustrative and some dimensions have been scaled for clarity.

In accordance with the invention as shown in Fig. 1, is provided a core cane 10 having a length much greater than its diameter. The core cane 10 functions as the substrate upon which the soot clad region 16 (see Fig. 3) is applied. The purpose for using a core cane 10 is that more uniform optical properties may be achieved in the optical fiber. The core cane 10 is comprised of a core portion 14 and a near clad portion 12. The core portion 14 is generally comprised of combinations of silicon oxide and other oxide materials such as germanium oxide. The core portion is the portion which, when drawn into fiber, carries light therethrough. The clad portion 12 is preferably pure Si02.

The core cane 10 is preferably produced using an Outer Vapor Deposition (OVD) method known to those of skill in the art. A bait rod, manufactured, for example, from alumina, is mounted in and rotated in a lathe.

A core portion of soot is first applied which may include suitable dopants (e. g., germanium oxide) to provide any desired core index of refraction profile. The core portion is then overclad with pure Si02. Next, the mandrel is removed and the preform is consolidated. During consolidation, the preform is subjected to an appropriate temperature to transform the preform into solid, transparent glass. The preform is then placed in a draw furnace, where the centerline aperture is closed under a vacuum. Next, the preform is drawn into the appropriate diameter core cane 10 which is smaller in diameter than the consolidated preform.

Once the consolidated core cane 10 is produced, it includes a consolidated core 14 and a near clad portion 12 as shown in Figs. 1 and 2. As shown in Figs. 3 and 5, the core cane 10 is next overclad with a soot clad region 16 including Si02 and a suitable dopant. According to the invention, the soot clad region 16 includes a dopant that causes consolidation of the soot clad region to progress in a radially outward direction as indicated by arrow 20.

The terms"radial"or"radially"as used herein mean any direction orthogonal to an axial axis which passes through the center of the overclad preform 18.

Preferably, the soot clad region 16 comprises a phosphorus containing soot. Most preferably, the soot is comprised of P205. Although it is believed

that other dopants that lower the viscosity of the glass as a function of temperature may work as well. In accordance with another aspect of the invention, the sintering (consolidation) process preferably takes place in such a manner that the temperature during sintering is uniform along the axial direction of the preform 18, i. e., by axial bulk consolidation. In order to achieve the method in accordance with the invention where sintering progresses radially outward, the dopant is of the type whose viscosity is sensitive to temperature, i. e., its soot viscosity is lowered as a function of amount of dopant present. In order to achieve the radially outward sintering, the amount of dopant present is preferably greater adjacent to the inner portion and lesser adjacent to the outermost extent 22 of the clad region of the preform 18, i. e., the amount decreases as a function of radial dimension.

According to a preferred embodiment, to achieve radial outgassing, the amount of dopant present in the soot region decreases gradually from an inner portion 15 adjacent to the core-cane/soot clad region interface to the outside 22 of the clad region 16, as is best illustrated in Fig. 6. Fig. 5 illustrates a preferred embodiment of overclad preform 18 including : 1) a core cane 10, comprising a core region 14, a near clad portion 12, and 2) a clad region 16 surrounding the core cane 10. In a preferred embodiment as described in Fig.

6, the clad region 16 includes a dopant that decreases linearly and continuously from a maximum amount at the inner portion 15, for example, at the core cane interface, to a minimum at the outside surface 22 of the clad region 16.

As shown in Fig. 5 and Fig. 6, the dopant preferably starts at a point indicated as R2 (at the core cane-soot clad region interface) and decreases in a preferably linear and continuous fashion to the point R3 at the outermost extent 22 of the preform 18. Although a continuous gradient with a linear variation is preferred, a quadratic or exponential variation may work as well.

The precise amount of dopant will depend upon the dimensions of the core cane 10 and clad region 16, the sintering temperatures, economics and affects of the dopant on optical properties. However, the amounts should be chosen to ensure a radial outward consolidation sequence.

Example As shown in Fig. 7, a preform having an initial length of about 220 cm, a core cane dimension R2 of about 0.8 cm, a soot clad region 16 having an initial thickness (R3-R2) of about 10 cm, a soot density of about 0.46 gm/cc and a consolidation temperature of about 1450°C, the dopant in the soot clad region 16 is preferably present in about one percent (1 %) by weight of soot at the inner most portion R2, i. e., in about 0.1% per centimeter of soot in the soot clad region 16 and about zero (0%) percent at the outermost extent R3.

However, depending on the size of the soot clad region 16, soot density, diffusivity and sintering (consolidation) temperatures more or less may work. In any case, it should be understood that the amount of dopant preferably varies between a maximum amount at the inner most portion R2 of the soot clad region 16, to a minimum amount at the outermost portion 22 of the clad 16.

The preform 18 is consolidated by conventional methods in a consolidation furnace. The hot zone of the furnace may be uniform throughout the furnace in the region of the preform if the bulk consolidation method is utilized, or localized if the gradient consolidation method is utilized. In the case of the gradient method, the preform 18 is fed downward by a suitable mechanism, the speed of which is varied by a control system to accomplis full consolidation. As should be appreciated, the invention provides improved outgassing regardless of the consolidation method utilized. This is because, during the consolidation process the viscosity of the glass at the core cane interface is a minimum and the viscosity rises with radially outward dimension during the process of sintering thereby allowing viable avenues within the unconsolidated clad region 16 for radial outgassing.

As is illustrated in Fig. 7, various computerized density isocontours are shown for the preform 18 part way through during consolidation. As sintering progresses, the amount of dopant present causes the inner portions or layers of soot to consolidate first (e. g., segment B). By way of further example, Table 1 illustrates various isocontours for the above-mentioned example preform and the radial densities thereof part way through the consolidation sequence. lsocontour Segment Density (gm/cc) A 2.2-2.25 2.2-2.25 2.1-2.2 D 2.0-2.1 E 1.9-2.0 F 1.8-1.9

As is illustrated, the density of the consolidating soot in soot clad region 16 varies throughout segments B-F, from a high at segment B adjacent to the core-cane/soot clad region interface, to a minimum at segment F at the outermost extent 22 for the preform 18. The numbers indicate that the density is less at the outer most portions (e. g., segment F), i. e., that the soot is not yet fully consolidated. Contrarily, the portion of the soot clad region 16 adjacent to the core cane 10 (e. g., segment B) is fully consolidated. Thus, since there are more, and greater size interstices in the soot closer to the outer radial extent 22, outgassing may progress outwardly in a radial direction. It should be understood that the soot clad region 16 preferably comprises at least about 90% of the overall clad dimension and is positioned outward of the near clad.

Fig. 4 illustrates a method of manufacturing an optical fiber wherein the consolidated preform 24, including radially varying clad dopant in accordance with the present invention method, is inserted into a furnace 26. The furnace includes a hot zone 28 which begins to melt the preform 24 at its lower end 30 until a thin strand of molten glass drops from the lower end 30 of the preform 24. This molten strand passes through a cooling zone 32 below the furnace, and then through a non-contact diameter sensor 34. Subsequently, the strand passes through a coater 36 and subsequently through a set of tractor wheels 38. A fiber, upon reaching approximately it's final end diameter, is fed through a feed head 40 and wound onto a spool 42, as shown. A suitable control 44 determines the draw-down rate of the tractor wheels 38 responsive to input provided in line 46 from the diameter sensor 34. Control 44 maintains the appropriate diameter of the resulting optical fiber 48. It should be understood

that the presence of the viscosity-altering dopant in the clad region 16 of the preform 18 (e. g., Fig. 5) does not significantly impact the optical properties of the resulting optical fiber 48. Optionally, the preform 24 may include a suitable mechanism mounted thereto for lowering it into the hot zone 28 at a controlled rate.

It will be apparent to those of ordinary skill in the art that various modifications and variations can be made to the present invention without departing from the scope of the invention. Thus, it is intended that the present invention cover the modifications and variations provided they come within the scope of the appended claims and their equivalents.