EP1242326A2

EP1242326A2 - Method for drawing an optical fiber from a porous preform

Info

Publication number: EP1242326A2
Application number: EP00968303A
Authority: EP
Inventors: Randy L. Bennett; Robert A. Fanning; Daniel W. Hawtof; Ji Wang
Original assignee: Corning Inc
Current assignee: Corning Inc
Priority date: 1999-07-08
Filing date: 2000-06-16
Publication date: 2002-09-25
Also published as: CA2379153A1; AU7824100A; JP2003516919A; WO2001004063A2; WO2001004063A3; CN1360561A

Abstract

Method for manufacturing optical fiber comprises the steps of laying down core and cladding materials to form a soot blank, the soot blank including a glass modifier, loading the unconsolidated soot blank into a draw tower, providing a hot zone to heat a portion of the blank to a temperature sufficient to sinter the soot into molten glass, and directly drawing the molten glass into either a consolidated, drawn preform rod (cane) or into an optical fiber.

Description

METHOD FOR MANUFACTURING OPTICAL FIBER USING DIRECT DRAW

Related Applications The present application is a continuation-in-part application of U.S. Serial

Number 09/350,068 filed on July 8, 1999, now pending. This application claims priority to under 35 USC §120 and incorporates herein by reference U.S. Patent Application 09/350,068 filed on July 8, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to improvements to methods for manufacturing optical fiber, and more particularly to methods for manufacturing optical fiber containing dopants that are difficult to process to fiber without quality defects, for example, fibers for use in specialty components, such as optical amplifiers.

2. Technical Background

The technology relating to optical fiber has made a number of important advances. For example, it is now possible to use optical fiber technology to construct optical amplifiers, which do not require the conversion of an optical signal into an electrical signal for amplification. To implement these and other applications, the materials used to manufacture optical fiber have been modified and refined to produce the desired optical properties. Alumina and antimony oxide are examples of materials that are used as dopants in the newest generation of optical fibers used to make optical amplifiers.

Unfortunately, it has been observed that using current manufacturing techniques with these new materials does not always lead to satisfactory results. In currently used methods for manufacturing optical fiber, a sintered glass preform is created that contains the full desired core and cladding profile. The ingot of glass (preform) is then loaded into a draw tower, where it is heated and then drawn into finished optical fiber.

Fig. 1 shows, in detail, a flowchart of a currently employed method 10 for generating an optical fiber preform, known as the outside vapor deposition (OVD) process. The OVD process of manufacturing the glass preform includes the following stages:

1. Laydown of core materials, and some cladding materials, to produce a soot blank.

2. Consolidation of the soot blank into a glass blank. 3. Redraw of the glass blank into canes.

4. Testing of the canes for optical properties.

5. Laydown of cladding material onto a cane to produce a second soot blank.

6. Consolidation of the second soot blank into a glass blank having the full desired core and cladding profile of the finished fiber.

In the first stage of this process, shown in Fig. 1 as step 12, a flame hydrolyzer is used to lay down the core materials as layers of soot onto a bait rod fabricated from, for example, alumina (Al₂O₃ ) or other suitable material as the rod is being rotated by a lathe assembly. The flame hydrolyzer provides the precursor materials to the delivery burner. Core materials containing older dopants, such as GeO₂, are typically delivered to the bait rod using vapor delivery burners known in the art. The precursors and dopants determine the composition of each layer of soot deposited onto the bait rod. In addition to the core materials, a predetermined portion of cladding materials is also deposited onto the bait rod in this first stage, on top of the core materials. After these materials have been laid down, the bait rod is removed, leaving a soot blank with all of the core profile and some of the cladding. In addition, the soot blank has a centerline hole down its length resulting from the removal of the bait rod.

In the next stage of the process, shown as step 14 in Fig. 1, the core soot blank is consolidated. First, the soot blank is dried during a consolidation step within a consolidation furnace utilizing gaseous chlorine (Cl₂) and helium (He) at approximately 1000° C. This drying step draws off water and certain undesirable metal elements. In particular, water in the blank interferes with the glass matrix, and can lead to unwanted attenuation in the final fiber product. After the blank has been dried, it is then sintered to clear glass by heating it to approximately 1500° C. The consolidation process takes several hours to perform, including an extended cooling period for the consolidated blank.

In step 16, after consolidation, a so-called "redraw" process is performed on the consolidated glass blank. In the redraw process, the blank is loaded into a redraw tower, heated, and then pulled into long, thin canes of a desired diameter. The redraw process also serves to close the centerline hole in the blank. This is accomplished by the application of a vacuum simultaneously with redraw.

In step 18, after redraw, the resulting canes are preferably tested to determine their optical properties, in particular, their index of refraction. This allows the manufacturing process to be adjusted downstream, as needed, for any variations of the index of refraction from its proper level.

In step 20, a second laydown process is performed. This time, flame hydrolysis is used to lay down silica cladding materials on top of a glass cane produced in step 16. In laying down the cladding materials, a vapor delivery system is typically used. The amount of cladding materials that are laid down can be adjusted to compensate for any deviations from the desired index of refraction detected in step 18. The result of this second laydown is another soot blank. However, the glass cane is not removed from the soot blank, but is rather left in place throughout the rest of the manufacturing process. In step 22, the second soot blank is consolidated by drying and sintering, as in step 14, described above. The result of the second consolidation step is a glass blank having the full core and cladding profile of the finished fiber.

In step 24, the completed blank is loaded into a draw tower, which includes a hot zone having a temperature of approximately 2000° C.

It step 26, a portion of the blank near its bottom end is heated until the glass melts, and a gob drops off, drawing behind it a trail of molten fiber, which cools to room temperature and hardens almost immediately upon leaving the hot zone. The fiber is then coated, collected, and wound onto a spool for storage. Although the above-described method has proven highly satisfactory for the manufacture of many fibers, including standard long-haul fibers, a manufacturing issue has arisen with respect to the manufacture of certain specialty optical fibers. These include, for example, fibers used in optical amplifiers. In order to achieve the desirable optical properties (for example, a broader wavelength response), materials designers have introduced new dopants into the core.

It has been found that the use of certain new dopants, including alumina and antimony oxide, can lead to the formation of crystals (hereinafter crystalization) within the cane. These crystals at some point during the consolidation process and become apparent after consolidation and after the redraw, described above, and can lead to voids, known as "seeds," in the canes. The fibers produced from preforms that contain crystals suffer from quality defects. The crystals create voids in the fiber at the elevated temperatures experienced in standard fiber draw processing. Voids in the fiber render the fiber unusable. The fiber is generally unusable because the crystalization cause unacceptably high background losses, i.e., attenuation. One solution to this problem attempts to eliminate crystals after they have formed. However, it would be desirable to prevent crystals from forming in the first place, if possible.

Thus, there is thus a need for an optical fiber manufacturing process that avoids the crystallization problem in the manufacture of such specialty fibers. SUMMARY OF THE INVENTION

A first embodiment of the invention provides a method for manufacturing optical fiber comprising the steps of laying down core and cladding materials to form a soot blank, the soot blank, and preferably, at least the core thereof including a glass modifier, loading the unconsolidated soot blank into a draw tower, providing a hot zone to heat a portion of the blank to a temperature sufficient to sinter the soot into molten glass, and directly drawing the molten glass into fiber. This method minimizes crystalization in such specialty fibers including such glass modifiers. Minimizing crystalization improves attenuation properties. In accordance with one embodiment, the core preferably includes an optically active dopant selected from the group of Er, Yb, Nd, Tm, and Pr. The glass modifier is preferably selected from a group consisting of Al, As, Be, Ca, La, Ga, Mg, Sb, Sn, Ta, Ti, Y, Zn, and Zr. Attenuation and the water peak are reduced in accordance with another embodiment of the invention by exposing the soot blank to a halide gas during the step of drawing optical fiber. Preferably, the halide gas is a chlorine-containing gas selected from the group of Cl₂, C₂F₆, SOCL , GeCl₄, and SiCl₄.

A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a flowchart of a method according to the prior art for manufacturing optical fiber; Fig. 2 shows a flowchart of a method according to the present invention for manufacturing optical fiber using a direct draw process; and

Fig. 3 shows a flowchart of a method according to the present invention for manufacturing canes of optical fiber core material.

Fig. 4 shows a side view of an OVD process for laying down core and clad materials.

Fig. 5 shows a cross-sectional view of the soot blank of Fig. 4 taken along section line 5-5.

Fig. 6 shows a partial cross-sectional view of the soot blank loaded into a furnace section of a draw tower.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention now will be described more fully with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. However, the described invention may be embodied in various forms and should not be construed as limited to the exemplary embodiments set forth herein.

Rather, these representative embodiments are described in detail so that this disclosure will be thorough and complete, and will fully convey the structure, operation, functionality and potential scope of applicability of the invention to those skilled in the art. Fig. 2 shows a flowchart of a first embodiment of a method 28 according to the present invention. It has been discovered that it is possible to take a soot blank directly to draw without a separate consolidation step thereby avoiding crystalization when glass modifiers are employed in the soot preform. In fibers manufactured in accordance with the invention, the glass modifier is included in at least the core of the fiber. Glass modifiers include, for example, Al, As, Be, Ca, La, Ga, Mg, Sb, Sn, Ta, Ti, Y, Zn, and Zr. Glass modifiers do not include the glass formers; Si, Ge, P, and B. Glass modifiers in accordance with an aspect of the invention are used in conjunction with the rare earth metals and function to de-cluster the rare earth metals in the core or change the spectral properties of the fiber produced. Thus, in step 30 of Fig. 2 and as illustrated in Fig. 4, all of the core and cladding materials are laid down in the proper weight ratio to form an unconsolidated soot blank 60 having the full core 54 and cladding 56 profile of the finished fiber. For example, dopant containing core material soot is preferably deposited onto a bait rod 58 using a liquid delivery system, as discussed further below. The dopants preferably include an optically active dopant selected from the group of Er,

Yb, Nd, Tm, and Pr. Such optically active dopants are important in the formation of specialty fibers for use in optical amplifiers, for example. Then, without a separate consolidation step, the cladding soot 56 is laid down directly over the core soot 54 to the proper weight ratio. The bait rod 58 is then removed, leaving just the unconsolidated soot blank 60 with a centerline aperture 78 formed therein. Fig. 4 illustrates a traversing burner 52 (as indicated by the arrow -A) laying down first core soot 54 onto the slender tapered rotating bait rod 58 and then clad soot 56 to form the soot blank 60. The core soot is preferably laid down by a liquid delivery system (later described herein), whereas the cladding is preferably laid down using a conventional vapor delivery system. A cross-sectional drawing of the soot blank 60 illustrating the core 54 and clad 56 portions is shown in Fig. 5. A motor 62 imparts rotation to a chuck 64 which grasps the bait rod 58 and thereby resultantly imparts rotation to the soot blank 60 during the laydown deposition process. Optionally, the burner 52 may be stationary and the lathe assembly may traverse back and forth while rotating the bait rod 58 and preform 60

As should be recognized, in the prior art OVD process described in connection with Fig. 1, the core and cladding are laid down in separate stages, with the core soot being consolidated and drawn into a glass cane before the cladding is laid down as soot on top of the cane in a separate step and then consolidated. In contrast, in accordance with the present invention, in step 32, the unconsolidated soot blank 60 is loaded directly into the draw tower 70 (Fig. 6). As shown in Fig. 6 and described in step 34, the hot zone 68 of the draw tower 70 heats a lower portion 60a of the soot blank 60 to a temperature high enough to sinter the soot blank into molten glass, i.e., approximately between 1600°C and 2200°C. In step 36, the molten glass is drawn directly into optical fiber 72.

It has been discovered that, practicing the present invention, it is possible to produce high quality specialty fibers which include glass modifiers without the need for consolidation and while minimizing or eliminating the crystalization problem associated with the prior art method. In particular, it has been discovered that fiber produced in this manner, which includes glass modifiers in at least the core portion 54, is free from the crystallization that occurs using the prior art method. In the prior art method, it has been observed that crystallization appears to occur at some point during the consolidation and redraw of the core soot blank. In particular, it has been hypothesized that crystal growth occurs during the relatively long time period required to cool the core blank after it has been sintered into glass.

In the present method, there is no consolidation and redraw of the core soot blank. Rather, the core soot blank is sintered and then drawn directly into fiber without the slow cooling used in the prior art process. Because of the relatively small diameter of the drawn fiber (in the range of 80 to 150 microns), the temperature of the fiber drops from the hot temperature of approximately 2000° C to room temperature in a matter of a few seconds or less, compared with the hours typically required to cool a sintered core blank. Thus, because of the "quick quench" of the present method, there is no time for unacceptable crystals to form. Returning to step 30 of Fig. 2, in the present embodiment of the invention, the laydown of core and cladding materials is performed using the outside vapor deposition process described above. However, it would be within the scope of the present invention to use other methods to create an unconsolidated soot blank, such as an vapor axial deposition (VAD) technique or other soot deposition processes. As discussed above, it has been found that it is particularly desirable to use a liquid delivery system, such as that described in United States Patent Application Serial No. 08/767,653, filed on December 17, 1996, or PCT Application Serial No. PCT/US98/25608, filed on December 3, 1998, assigned to the assignee of the present application and incorporated herein by reference, to deliver the core and cladding materials to the flame hydrolyzer in this step. Optionally, a vapor delivery system may be employed for the cladding application. However, any method that would produce an acceptable quality soot blank may be employed.

After the unconsolidated soot blank 60 has been created, in order to reduce attenuation, the blank may be optionally dried by gaseous chlorine (Cl₂) and helium

(He) or other suitable gas at an elevated temperature (800°C - 1200°C), i.e., at a temperature high enough to dry the blank without sintering it. The optional preform drying pre-step may be accomplished in a separate consolidation furnace. However, it should be understood, it is possible to manufacture high-quality optical fiber without this drying substep, although such as step is generally desirable.

The unconsolidated soot blank 60 is then loaded into the draw tower 70 (Fig. 6) in step 32. The draw tower 70 currently used to practice the invention is characterized by having a hot zone 68 of approximately 12 inches in length.

Before the drawing portion of the process begins, the soot blank 60 may be purged in the draw furnace with helium 74 and chlorine 76 in a percentage of about 0.1-

5% of chlorine and 95%-99.9% of helium, and more preferably about 1% chlorine and 99% helium, to further remove any water or metals trapped within the blank's porous structure. In addition, the centerline hole 78 of the blank may be plugged at a lower end thereof prior to heating to prevent a "chimney" effect in the heated draw furnace 70, which would cause a reduction of the core materials. Further, a vacuum may optionally be applied to the centerline hole 78. Although, the porosity of the soot blank 60 prevents a perfect vacuum from being formed, a sufficient vacuum can be developed to help close up the centerline hole 78 as the optical fiber 72 is drawn, if required. In any event, it is desirable for the centerline hole 78 to be maintained with a clean, dry atmosphere during the drawing process. Preferably, during a drying step, the preform 60 is driven down and up through the hot zone 68 at a temperature between about 800°C - 1200°C for about one hour to drive out any undesirable water or metals in the soot while subjecting the preform 60 to the flow of chlorine and helium mentioned above.

In accordance with a preferred aspect of the invention, a halide gas such as a chlorine-containing gas (for example, Cl₂, C₂F₆, SOCl , GeCl₄, SiCl₄, or combinations thereof) are supplied to the soot blank 60 during the drawing (steps 34-36) also. Most preferably, the halide gas is included with an inert muffle gas, such as helium or argon. The inert muffle gas and the halide gas are preferably provided during draw in a ratio of between about 0.1%-5% : 95%-99.9%. As the gases penetrate the interstices of the soot preform 60, the gas diffuses into the centerline 78 and provides a flow out through the centerline (as shown by arrow B) to minimize entry of atmosphere and/or contaminants therein. Addition of halide gas during draw was discovered by the inventors herein to be an important feature for reducing the water peak at 1380 nm to less than 300 dB/km and providing minimum background losses of less than 100 dB/km, and more preferably less than about 30 dB/km in specialty fibers including an optically active dopant such as Er. An example attenuation spectrum for an Er doped fiber including an alumina modifier manufactured in accordance with the present invention is shown in Fig. 7. The plot illustrates lower and upper peaks 80, 82 due to the presence of Er in the fiber at 980 nm and 1530 nm. These peaks are truncated (resulting in the dip shown at the center of the peak) due to measurement saturation. The minimum background loss 84 is measured as the minimum value between the tails of these peaks and are generally a measure of the passive losses in the fiber due to scattering, material absorption, imperfections, etc.

In step 34, the peak temperature of the hot zone is slowly raised to between 1600°C and 2200°C, and more preferably to 1900°C or above. The precise temperature used will vary with the composition of the glass. The soot blank 60 is lowered through this hot zone 68 to draw fiber 72 therefrom. Only the "root" at the lower portion 60a of the blank 60 sees this temperature, as it is down-driven into the hot zone 68. It has been found that the slow heating of the relatively long hot zone to this temperature facilitates the combined sintering and drawing process. It should be noted that this temperature is significantly higher than the approximately 1525° C temperature that is currently used to consolidate a soot blank into glass in the two-stage process described above in connection with Fig. 1.

The portion of the soot blank 60 in the hot zone 68 sinters into molten glass. In step 36, similar to the prior-art process, a gob drops off of the blank, drawing behind it a trail of optical fiber 72 that cools and solidifies almost instantly as it is exposed to room temperature air or coolant gas. In the present embodiment, the fiber 72 may be drawn at the speed of 1.0 to 10.0 meters per second. The fiber 72 is then collected and rolled onto spools for storage as is conventional practice.

The above-described process has been successfully used to produce four 1.5 km Er/Sb/Si fibers, free from unacceptable crystallization effects using a draw temperature of about 1975° C, and a draw speed of about 2.0 meters per second. It has also been used to produce several Sb/Al/Er/Si fibers. The diameters of the fibers produced include 80, 100, 125, and 150 microns.

The present invention allows long lengths of generally defect-free fiber to be produced for use in amplifiers and other specialty applications. In addition, it is now possible to explore the use of other materials beyond those currently in use.

Finally, it is within the scope of the present invention to use a "direct redraw" process to manufacture glass canes of core material. As discussed above, the new generation of dopants are typically used in the fabrication of the core portion of optical fibers. Thus, the problem of crystal formation typically arises during the initial consolidation of the core soot blank and the subsequent redraw of the consolidated soot blank into canes. If a crystal-free cane of core material can be produced, this cane can be used in a two-stage process without the formation of seeds (i.e., voids) in the final fiber. A two-stage process, for example, allows optical testing of the cane prior to the laydown of the cladding materials on top of the cane. As described above, this allows the manufacturing process to be fine-tuned to allow for variations in the cane.

Fig. 3 shows an embodiment of a direct redraw process 38 according to the present invention. In step 40, the core materials and a predetermined portion of cladding materials are laid down onto a bait rod using a flame hydrolyzer to form an unconsolidated soot blank as in Fig. 4. As discussed above, it is desirable to use a liquid delivery system in conjunction with the flame hydrolyzer to deliver specially doped materials to the bait rod. In step 42, the unconsolidated core blank is not consolidated, but is rather loaded directly into a redraw tower. In step 44, the unconsolidated core blank is heated to a temperature sufficient to sinter the unconsolidated blank directly into molten glass. In step 46, the molten glass is redrawn into canes. The canes are formed by a process virtually identical to that shown in Fig. 6 except that the diameter of the resultant molten strand is much larger. In step 48, a flame hydrolyzer is used to overclad the canes with the remainder of overclad soot needed. In step 50, the direct draw method shown in Fig. 2 and described above is picked up from step 32 of Fig. 2 to manufacture optical fiber from the overcladded cane.

Again, after the unconsolidated core soot blank has been created in step 40 of Fig. 3, in order to reduce attenuation, the unconsolidated blank may be optionally dried by gaseous chlorine (Cl₂) and helium (He) or other suitable gas at an elevated temperature (800° C - 1200° C), i.e., at a temperature high enough to dry the blank without sintering it.

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

Claims

What is claimed is:

1. A method for manufacturing optical fiber, comprising the steps of: (a) laying down core and cladding materials to form a soot blank, at least a core of the soot blank including a glass modifier;

(b) loading the unconsolidated soot blank into a draw tower;

(c) heating a portion of the blank within a hot zone of the draw tower to a temperature sufficient to sinter an end portion of the soot blank into molten glass; and

(d) drawing the molten glass into an optical fiber in the draw tower.

2. The method of claim 1 wherein step (a) further comprises the steps of:

(al) laying down core and cladding materials onto a bait rod to form a soot blank; and

(a2) removing the bait rod, leaving a centerline hole in the soot blank.

3. The method of claim 2 wherein step (c) includes applying a vacuum to the centerline hole as heat is applied to the soot blank.

4. The method of claim 2 wherein in step (al) the core and cladding materials are laid onto the soot blank using a flame hydrolyzer.

5. The method of claim 2 wherein in step (a2) after the bait rod is removed, the resulting centerline hole is maintained with a clean, dry atmosphere.

6. The method of claim 4 wherein in step (al) at least the core materials are delivered to the flame hydrolyzer using a liquid delivery system.

7. The method of claim 4 wherein in step (al) the cladding materials are delivered to the flame hydrolyzer using a vapor delivery system.

8. The method of claim 1 wherein the glass modifier is selected from a group consisting of Al, As, Be, Ca, La, Ga, Mg, Sb, Sn, Ta, Ti, Y, Zn, and Zr.

9. The method of claim 1 wherein at least the core includes an optically active dopant selected from the group of Er, Yb, Nd, Tm, and Pr.

10. The method of claim 1 wherein at least the core includes an optically active dopant selected from the group of Er, Yb, Nd, Tm, and Pr and the glass modifier is selected from a group consisting of Al, As, Be, Ca, La, Ga, Mg, Sb, Sn, Ta, Ti, Y, Zn, and Zr.

11. The method of claim 1 further comprising the steps of: exposing the soot blank to a halide gas in a drying step within the draw tower at a temperature between 800°C and 1200°C, and continuing to expose the soot blank to a halide gas at a temperature between 1600°C and 2200°C during the step of drawing optical fiber.

12. The method of claim 1 further comprising a step of exposing the soot blank to a halide gas during the step of drawing optical fiber.

13. The method of claim 12 wherein the halide gas is a chlorine-containing gas.

14. The me: hod of claim 13 wherein the chlorine-containing gas is selected from the group of Cl₂, C₂F₆, SOCL₄, GeCl₄, and S1CI₄.

15. The e' hod of claim 12 wherein the chlorine-containing gas is included with an inert muf'le gas.

16. The mchod of claim 15 further comprising a step of providing the inert muffle gas and the chlorine-containing gas during the step of drawing optical fiber in a ratio of between about 95%-99.9% : 0.1%-5%.

17. The method of claim 1 further including a step performed between steps (b) and (d) of removi j trapped water or metals in the soot blank.

18. The method of claim 1 further including a step performed between steps (b) and (d) of dryint' the soot blank in the draw tower at an elevated temperature between

800°C aι . 1200°C.

19. The method of claim 18 further comprising flowing a halide gas into the draw furnace during drying.

20. The me: jd of claim 1 wherein in step (c) the hot zone reaches 1900° C or higher.

21. The method of claim 1 wherein the hot zone is approximately twelve inches in length.

22. The method of claim 1 wherein the molten glass is drawn into optical fiber at the approximate rate of between one to ten meters per second.

23. The me' d of claim 1 wherein in step (a) the core materials laid down onto the soot blai -. are doped with alumina.

24. The me: ιod of claim 23 wherein in step (a) the core materials laid down onto the soot blai are doped with erbium.

25. The method of claim 1 wherein in step (a) the core materials laid down onto the soot blanl. are doped with antimony.

26. The method of claim 25 wherein in step (a) the core materials laid down onto the soot blank are doped with erbium.

27. The method of claim 1 wherein the optical fiber drawn exhibits a minimum background loss of less than 100 dB/km.

28. The method of claim 27 wherein the optical fiber drawn exhibits a minimum background loss of less than 30 dB/km.

29. The method of claim 1 wherein the fiber drawn exhibits attenuation of less than 300 dB/km at 1380 nm.

30. A method for manufacturing optical fiber, comprising the steps:

(a) loading an unconsolidated soot blank into a draw tower, the soot blank including a glass modifier;

(b) providing a hot zone to heat a portion of the soot blank to a temperature suffici -Tit to convert the soot into molten glass; and

(c) draλving the molten glass into optical fiber.

31. The method of claim 30 wherein step of removing trapped water is performed by purging the soot blank in the draw furnace with helium and chlorine gasses.

32. The method of claim 30 further including a step between steps (a) and (c) of drying the soot blank in the draw tower at an elevated temperature between about 800°C to 1200°C in the presence of a halide gas.

33. The method of claim 30 wherein in step (b), the hot zone is between 1600°C and 2200°C and a halide gas is present during the step of drawing the optical fiber.

34. The method of claim 30 wherein the molten glass is drawn into fiber at the approximate rate of one to ten meters per second.

35. A method for manufacturing canes of optical fiber core material, comprising the steps: (a) laying down core and cladding materials to form a soot blank, at least the core including a glass modifier;

(b) loading the unconsolidated soot blank into a redraw tower;

(c) providing a hot zone to heat a portion of the blank to a temperature sufficient to sinter the soot into molten glass; and (d) redrawing the molten glass into canes.