WO2002028790A1

WO2002028790A1 - Method and apparatus for manufacturing optical fiber preforms using a large diameter bait rod

Info

Publication number: WO2002028790A1
Application number: PCT/US2001/027439
Authority: WO
Inventors: Daniel W Hawtof
Original assignee: Corning Incorporated
Priority date: 2000-10-03
Filing date: 2001-09-05
Publication date: 2002-04-11
Also published as: AU2001288712A1

Abstract

A method and apparatus for manufacturing a soot preform utilizing a large, preferably hollow, bait rod having a diameter of greater than about 18 mm, and optimally between 25 mm - 35 mm. In accordance with the invention, capture rate during startup is at least about 30%. The method comprises installing a bait rod having an outer diameter greater than about 18 mm, generating a soot stream of core soot, and depositing the core soot onto an outer surface of the bait rod. According to another embodiment, the ratio of bait rod diameter (db) to soot stream dimension (ds) is between about 2-5.

Description

METHOD AND APPARATUS FOR MANUFACTURING OPTICAL FIBER PREFORMS USING A LARGE DIAMETER BAIT ROD

Field of the Invention The present invention relates generally to a method and apparatus for manufacturing an optical fiber preform, and more particularly to a method and apparatus for manufacturing such a preform with a large bait rod thereby enabling high startup capture efficiencies.

Background of the Invention

Producing a soot preform and then consolidating it into a high purity glass core blank is a desired pre-step in the manufacture of optical fiber. In one prior art method, the glass core blank, once consolidated, is generally redrawn into a slender core cane and, at the same time, the centerline aperture resulting from the manufacturing process is closed. Cladding soot is then deposited onto the core cane, and again consolidated, to produce the final consolidated preform from which optical fiber is drawn. In the prior art manufacturing process for making core cane, silica glass soot, which may include suitable doping, is deposited, for example, by an OVD flame hydrolysis process upon a rotating substrate such as an alumina bait rod. The core portion of the soot preform is formed by introducing various gaseous or liquid feedstocks in predetermined amounts into a burner flame. This produces soot oxides that include, for example silicon oxide and germanium oxide. These oxides deposit on the rotating bait rod in predetermined amounts until the appropriate diameter of the core portion is reached. The oxides are preferably introduced in various percentages, as desired, to produce the desired core refractive index profile. Once the desired diameter and composition of dopants in the core are achieved, the bait rod is removed. This forms a centerline aperture, i.e., a hole that extends lengthwise through the core soot preform.

As before mentioned, the core soot preform, once formed, is then consolidated and drawn into a core cane. During this process, closure of the centerline aperture is accomplished. Heretofore, closure has been a significant problem. Moreover, the size of the bait rod useable has been limited in the prior art because only centerline holes of a certain size could be closed sufficiently without causing manufacturing defects and other problems such as irregularities in the centerline closure shape. The bigger the hole, the flatter and less concentric the hole closure. Typically, prior art bait rods for making cane have been approximately 9 mm in diameter. Use of larger bait rods has been limited due to inability to close the centerline aperture acceptably.

Many efforts have been directed towards increasing capture efficiency as such gains would allow faster production of core cane. Thus far, however, such attempts have been largely focused on the burners and flow conditions. Therefore, there is a need for a simple and cost-effective method and apparatus that increases the startup capture efficiency of the core blank manufacturing process.

Summary of the invention

In accordance with the invention, an improved method and apparatus is provided for manufacturing an optical fiber soot preform. The method and apparatus in accordance with the invention advantageously increases the capture efficiency during initial soot deposition in the core making process. In particular, the efficiency is increased at the startup of deposition to above 30%, and more preferably above 40%. "Capture efficiency" as defined herein is the percent of soot captured (deposited ) onto the bait rod divided by overall amount of soot generated by the burner. The term "startup", as used herein, means the beginning of the deposition process where soot is first deposited onto the bait rod. The diameter grows from the starting bait diameter as soot is deposited.

In accordance with one embodiment of the invention, a method of manufacturing a soot preform is provided. The method comprises the steps of installing a bait rod having an outer diameter greater than 18 mm, generating a soot stream of core soot, and depositing the core soot onto an outer surface of the bait rod. In accordance with a preferred embodiment, the bait rod is rotated during deposition. Preferably, the bait rod is hollow thereby minimizing temperature difference from the outside to the center, which causes large internal stresses and undesirable bait cracking.

In another embodiment of the method, the step of depositing further comprises depositing both the core and the cladding in a one-step process. In the one-step process, a core portion is deposited onto the bait rod and then a cladding portion is deposited onto the core portion. This is accomplished without any intermediate consolidation steps taking place. Thus, the "one-step process" is reflective of the elimination of the formerly used intermediate consolidation step.

Most preferably in accordance with the invention, the soot is produced from an array of burners. The array, when employed, preferably includes 20 or more burners. The burners may be of the conventional variety as disclosed in

US 5,922,100 to Cain et al., of the planar variety described in WO99/32410 to Hawtof or any other suitable burner for producing soot. The soot streams of the burners are preferably oriented such that they are substantially aligned with each other and offset from the bait rod axis. In this way, the soot streams produced are directed at a longitudinal centerline of the bait rod.

According to one embodiment, the burners are preferably of the planar variety as disclosed in the aforementioned WO99/32410 and include an elongated slot formed in the burner face. Preferably, the slot in at least one, and more preferably all the slots, of the array of burners are aligned with the longitudinal axis of the bait rod. Thus, the soot streams produced simultaneously deposit along the length of the preform from the plurality of burners. In accordance with a particular preferred embodiment, the bait rod diameter is between about 18 mm and 35 mm, and more optimally between about 25 mm and 35 mm. The large diameter of the bait rod in accordance with the invention advantageously allows a soot capture rate during startup that is at least about 30% during the step of depositing, and more preferably greater than 40%. Moreover, during the step of depositing, a soot capture rate is reduced by less than 5% for lateral offsets from a bait rod centerline of the burner or burner array of up to 5 mm. Thus, it should be recognized that an advantage of the present invention is that the capture rate is less sensitive to lateral offset present between the bait rod and burner(s) as compared to the prior art small diameter bait rods.

According to another embodiment of the invention, the ratio of bait rod diameter to soot stream dimension (e.g., soot stream width) is optimally maintained between about 2 and 5. The width dimension is measured across the soot stream at a position mid way between a face of the burner and the surface of the preform being formed by soot deposition.

Another embodiment of the invention is an apparatus for manufacturing a soot preform. The apparatus comprises a bait rod having an outer diameter greater than 18 mm, and at least one burner adapted to deposit core soot onto an outer surface of the bait rod. Preferably the bait rod is hollow and has a wall dimension of between about 1.0 mm and 7.0 mm. The bait rod is assembled and held in place by at least one end plug inserted into at least one hollow end of the bait rod. Preferably, an expansion member is mounted between the bait rod and the at least one end plug to prevent cracking of the hollow bait rod. In a preferred embodiment, an end plug is inserted into each hollow end of the bait rod. Preferably, a rod interconnects the plugs and passes through the bait rod's hollow portion. This rod adds additional rigidity to the assembly and the end plugs are used to mount the bait rod into a conventional lathe apparatus. In another embodiment of the invention, the at least one burner comprises a changeable precursor supply enabling deposit of first a core portion onto the bait rod and then a cladding portion onto the core portion. By the term "core portion" as used herein, what is meant is that portion of the preform that is defined by a change in glass chemistry. For example, one boundary would be defined as a boundary where a germania doped silica core meets a pure silica cladding. Another would be where a pure silica core meets a fluorine doped cladding. Preferably, the at least one burner includes an array of burners preferably including greater than 20 burners, and more preferably greater than 30 burners. The burners are preferably substantially aligned with each other along a line offset from and parallel with a longitudinal axis of the bait rod. A slot is preferably formed in at least one burner; the length of the slot being preferably substantially aligned with a longitudinal axis of the bait rod.

More preferably yet, an array of burners is provided each including a slot, the lengths of which are substantially aligned with the longitudinal axis of the bait rod. Thus, the burner array includes a plurality of individual burners aligned with each other and which may have separate or one common precursor supply.

Of course, it should be recognized that the present invention may be utilized to form the core only, which is then consolidated and drawn into a core cane. The core cane may then have additional silica containing soot such as cladding applied to it in subsequent step. Thus, in a traditional multi-step method, the invention has the advantage of increasing efficiency in forming the core portion.

According to another embodiment, the burner, when ignited, emits a soot stream having a predefined width dimension (d_s) as measured across the soot stream at a position mid way between a face (top surface) of the burner and the bait rod. Preferably, a ratio of bait rod diameter (db) to the width dimension (d_s) is between about 2 and 5.

Thus it should be recognized that the method and apparatus in accordance with the invention has utility for enabling higher production rates of optical fiber soot preforms, and in particular core soot preforms. Advantageously, the process and apparatus of the invention allow for excellent core/clad concentricity, because of the one-step process utilized for laying down both core and cladding eliminates the intermediate consolidation and redraw step.

Furthermore, the process and apparatus of the invention allows for increased process efficiencies, allowing soot preforms to be manufactured in less time.

Additionally, alignment between soot stream and bait rod is not as critical when a large bait rod according to the invention is utilized to make the core. This advantageously results in better profile control. In other words, variations in dopant concentrations across the soot stream can be essentially ignored when utilizing the large diameter bait rod in accordance with the invention. For example, utilizing the large bait rod enables capture of approximately the desired ratios of silica to germania.

Additionally, in one embodiment, the large bait rod in accordance with the invention has better rigidity, thus reducing detrimental vibration. Other aspects of the invention will be understood with reference to the following detailed description, claims and appended drawings.

Brief Description of the Figures

Fig. 1 illustrates a perspective view of a core portion of a soot preform being formed by a process in accordance with an embodiment of the invention.

Fig. 2 illustrates a perspective view of a clad portion of a soot preform being formed by a process in accordance with an embodiment the invention.

Fig. 3 illustrates a graph of capture efficiency (%) versus bait rod target size

(mm).

Fig. 4 illustrates a graph of normalized soot blank weight versus offset (mm).

Fig. 5a illustrates a partially cross-sectioned end view illustrating a burner with zero offset from the longitudinal axis of the bait rod. Fig. 5b illustrates a partially cross-sectioned end view illustrating a burner with lateral offset from the longitudinal axis of the bait rod.

Fig. 6 illustrates a partial exploded side view of the bait rod assembly in accordance with the invention.

Fig. 7 illustrates cross-sectional view of the clamping member along section line 7-7 of Fig. 6.

Fig. 8a illustrates side view of another apparatus including a large bait rod in accordance with the present invention.

Fig. 8b illustrates an end view of another apparatus including a large bait rod in accordance with the present invention.

Fig. 9 illustrates cross-sectional view of the support member along section line 9-9 of Fig. 8a.

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 or similar reference numerals shall be used throughout to refer to the same or like parts. A first step in the method of manufacturing an optical fiber soot preform in accordance with the invention is illustrated in Fig. 1 wherein a soot preform 20 is preferably formed by an Outside Vapor

Deposition (OVD) process. In this process, silica-containing glass soot 24 is generated by oxidizing one or more glass precursors in at least one burner. More preferably, a burner array 26 is employed to produce the soot 24. One suitable burner array is described in WO 99/32410 entitled "Burner And Method For Producing Metal Oxide Soot." The soot 24 is deposited on the outside surface of a bait rod 22. In accordance with a preferred embodiment, the bait rod 22 is a rotating, substantially-hollow, cylindrical, tapered bait rod manufactured from an alumina material.

The bait rod 22 has an outside bait diameter dimension d_b of at least 18 mm. More preferably, the diameter dimension d is at least 25 mm, and most preferably, the diameter dimension is optimally between about 25 mm and 35 mm. It has been determined by the inventor that the optimum capture efficiency at startup is achieved at about 30 mm diameter dimension db. Notably, this is substantially larger than the dimensions of core-making bait rods that have been heretofore used in the prior art. Thus, dramatic improvements in capture efficiency in making the core portion of the preform

20 are achieved at startup. Startup as referred to herein means the initial deposition process used to lay down the core layer 28.

Example This example is provided for further clarification of the invention and is not to be considered limiting thereof. The bait rod 22, in one example, includes an outer diameter d_b of about 1.875 inch (48 mm) at one end, an outer diameter d_b of about 1.750 (44.5 mm) at the other end, and a length of about 48 inches (1.22 m). The hollowed out portion 50 preferably has an inner diameter dj of about 1.500 inch (38.1 mm) all the way through the center of the bait rod 22. Thus, the wall thickness of the bait rod 22 in the example is about 0.125 inch to 0.187 inch ( 3.2 mm to 4.7 mm). According to an embodiment of the invention, it is desired to have a thin wall, for example, less than about 0.25 inch (6.4 mm) such that radial expansion is minimized which might cause cracking of the bait rod 22. The longitudinal axis C-C of the bait rod 22 is defined by, and consists of a line positioned to coincide with the center of the hollowed out portion 50.

Referring now to both Figs. 1 and 2, the soot preform 20 in one embodiment, is formed in a one-step process wherein the core soot layer 28 is first deposited, and then the cladding layer 30 (Fig. 2) is deposited over top of the core layer 28, without any intermediate consolidation process being performed. The one-step refers to depositing all of the required soot to make the final preform in one deposition sequence without first consolidating the core region. The core and clad layers 28, 30 are deposited by introducing various fuels or ignition supporting gasses or liquid precursors into the burner array 26 (e.g., CH₄ O₂, SiCI , GeCI₄) to produce various oxide soot deposits (e.g., SiO₂ and GeO₂) on portions of the preform 20. The core portion 28 may be formed to include any desired index of refraction profile and the desired amount of dopants by adjusting the relative amounts of a first precursor 32 and a second precursor 34 fed to the burner array 26. For example, the first precursor 32 may be SiCI₄ and the second precursor 34 may be GeCI₄. The precursors may be supplied in liquid or gaseous form to the flames of the burner array 26.

Preferably, some mixing of the precursors 32, 34 will take place by a suitable valve mechanism 36. The flames of the burners 25 are produced by igniting a suitable fuel 27, such as methane gas in the presence of oxygen.

During the deposition steps, the bait rod 22 and the preform 20 are simultaneously rotated about a longitudinal axis C-C thereof and translated back and forth along the axis C-C to deposit uniform layers of soot as indicated by the arrows A and B, respectively. The lathe mechanisms for rotation and traversing are conventional in the art. Alternatively, the burner array 26 may be traversed back and forth while the bait rod 22 is rotated. Preferably the burners 25 in the burner array are aligned such that they are at the same distance from the surface of the bait rod 22 and move away from the bait rod in unison when the diameter of the preform grows. The soot stream of the burners are oriented preferably on one side of the bait rod and preferably substantially directed at the longitudinal axis of the bait rod. Although a one-step method is described herein, it should be recognized that the present invention may be employed to produce only the core portion. For example, in this method, the core portion would be formed first by utilizing the large diameter bait rod, depositing the silica-containing soot thereon and then consolidating and drawing the soot core preform into a core cane. The large diameter hole may be closed in accordance with the method described in

US Patent Application 09/558,770 filed April 26, 2000 and entitled "An Optical Fiber And A Method For Fabricating A Low Polarization-Mode Dispersion And Low Attenuation Optical Fiber" thereby resulting in a consolidated core cane. Next, the core cane drawn from the consolidated preform would have additional silica-containing soot deposited thereon to form a second segment. This additional soot added may be another segment of the core, if appropriate additional doping is added during consolidation thereof. Addition of fluorine may be employed during consolidation to cause a down-doped region or moat in the refractive index profile, for example. The process may be repeated again to add a third up-doped segment containing a germania dopant, for example. After again consolidating and drawing, silica cladding may be added thereupon forming a fourth segment.

According to an alternative embodiment, the core portion may comprise pure silica and be drawn into a cane as before-mentioned. A fluorine doped portion may then be added to the core by depositing further silica and doping the silica with fluorine. Additional fluorine doped portions may be added as well after consolidation and redraw into core cane.

Fig. 3 illustrates the discovery by the inventor that the capture efficiency is optimized when the target size, i.e., the diameter of the bait rod 22 is about 30 mm. Increasing the target size further above this diameter does not gain any significant additional capture efficiency. In fact, the curve becomes asymptotic after this dimension is achieved. Thus, from the graph it should be understood that good startup capture rates (>30% are achievable with a bait rod diameter of greater than 18mm is employed. Also, optimum rates of capture efficiency (40-45%) are achieved at between 25 mm and 35 mm bait rod diameter. Fig. 4 illustrates a graph of Normalized Soot Blank Weight versus Offset showing a family of curves 29a, 29b, 29c. What this graph visually illustrates is that offset of the large bait rod in accordance with the invention from the soot stream centerline as shown in Fig. 5B does not appreciably reduce the deposition efficiency. Therefore, the Normalized Soot Blank Weight is not appreciably affected. Thus, it should be recognized that a great advantage of the present invention is that the apparatus is significantly insensitive to lateral offset d_o between the soot stream 24 generated by the burner 25 and the longitudinal axis C-C (shown as dots in Figs. 5A and 5B) bait rod 22. In particular, as illustrated by line 29c, for bait rods having a diameter of greater than 18mm, less than 5% reduction in capture efficiency occurs when the soot stream is offset laterally from the longitudinal axis of the bait rod by d₀ being as much as about 5 mm. Any small offset between the bait rod longitudinal axis and the soot stream for smaller diameter bait rods, such as shown in Figs 29a and 29b, have much greater loss in capture efficiency. Accordingly, the invention has the advantage of allowing larger offsets in equipment due to assembly, manufacturing tolerances and wear thereof. The embodiments of Figs. 6 and 7 show another embodiment of the bait rod 22 in accordance with the invention. Fig. 6 (the center portion of which has been removed for clarity) illustrates an exploded view of a bait rod assembly 38 including the preferably hollow tubular bait rod 22, metal end plugs 40a, 40b, resilient members 42a, 42b, and assembly through rod 44. The end plugs 40a, 40b preferably have cylindrical pilot portions 46a, 46b that are received in the respective hollow ends of the bait rod 22. Shoulder portions 48a, 48b formed on the plugs limit the axial extent to which the plugs may be inserted into the hollow 50 of bait rod 22. Resilient expansion members 42a, 42b, which are preferably Viton o-rings, allow for differential expansion to occur between the bait rod 22 and the end plugs 40a, 40b in the radial direction. This was discovered to alleviate undesirable cracking of the bait rod. Preferably, the expansion members 42a, 42b reside in grooves (not shown) formed into the pilots to minimize axial movement of the expansion members. Other forms of suitable expansion members may be employed, such as elastomer strips wrapped around a portion or all of the pilots of the plugs, springs, etc.

Moreover, other suitable high temperature materials may be employed.

The bait rod assembly 38 is constructed by first inserting both ends plugs 40a, 40b, with resilient members installed thereon, into the respective opposing hollow ends of bait rod 22. The Inconel assembly rod 44 is then inserted through one of the end plugs 40a or 40b, through the hollow 50, and through the other one of the end plugs 40a or 40b. A sufficient amount of rod 44 is allowed to extend out of each end. These ends of rod 44 are used for mounting the bait rod assembly 38. The rod 44 is secured to end plugs via clamping members 52a, 52b, a representative one 52b being shown in Fig. 7, integral with and formed on the ends of plugs 40a, 40b.

Fig. 8a illustrates another apparatus for manufacturing a soot preform in accordance with the invention. Provided are a bait rod 22 having an outer diameter d_b greater than about 18 mm, more preferably greater than 25 mm, and most preferably between 25 and 35 mm, and at least one burner 25 adapted to deposit core soot 24 onto an outer surface 23 of the bait rod 22. The bait rod assembly 38 is preferably identical to that illustrated in Fig. 6. The assembly 38, including the bait rod 22, is installed in a lathe apparatus 55 having a motor 54, a grasping mechanism 56, such as a collet or chuck mechanism for firmly grasping the one end of assembly rod 44, and an end support 58. The motor 54 and support 58 are mounted to a common frame 57 (only portions of which are shown). At the right end of the assembly 38, the rod 44 rests in the end support 58 that functions to allow generally free rotation, yet which restricts lateral and downward motion. In a preferred embodiment, the support 58 is a v-block as is shown in Fig. 9 thereby facilitating ease of removal of the assembly from the lathe via quick release of holding member 56. In operation, the at least one burner 25 receives glass precursor from precursor supply 60 to form the soot stream 24. The precursor supply 60 is changeable in that it is made up of a source of silicon feedstock 32 and germanium feedstock 34. The amounts of each precursor supplied may be adjusted by suitable controls to adjust the flow controllers 36a, 36b as desired to meet the required profile requirements of the end fiber. A suitable fuel source 27, such as CH₄ and oxygen, is ignited and the precursor(s) are oxidized in the flames to form the soot stream 24. The soot stream 24 deposits onto the bait rod 22 as the lathe assembly 55 and bait rod assembly 38 rotate about the longitudinal axis and traverse axially as indicated by arrows A and B, respectively.

Within the process, a core portion is deposited onto the bait rod 22 by appropriate control of flow controllers 36a, 36b, and 36c to arrive at the correct composition and desired refractive index profile when drawn into a core cane or fiber. In accordance with another embodiment, a cladding portion (See Fig. 2) is deposited onto the core portion 28 (Fig. 1 ) that is preferably pure silica soot. Most preferably, the at least one burner comprises an array of burners such as shown in Fig. 1 and 2 wherein at least 20, and more preferably at least

30 aligned burners are employed. Moreover, a plurality of burners of the types shown in Fig. 8a and 8b may be aligned with each other to lay down soot more efficiently. In each case, the burners are preferably aligned along a parallel line offset from a longitudinal axis C-C of the bait rod 22. In accordance with another embodiment of the invention, it has been discovered that to achieve the desired level of soot capture efficiency during startup, that the ratio of bait rod diameter d_b to soot stream width dimension d_s should be maintained between about 2 and 5 (Fig. 8a, 8b). The width dimension d_s of the soot stream is fairly defined on a properly adjusted flame and is measured across the soot stream 24 at a position mid way between a face 62 of the burner 25 and the surface of the substrate being deposited onto. In Fig. 8a, the burner shown is conventional, thus the dimension d_s is the same in all planes. However, in Fig. 8b, the burner 25 is a planar burner as illustrated in Figs. 1 and 2, thus, the dimension d_s of the soot stream is measured only along the transverse width. Once the preform is formed, it may be dried and consolidated in accordance with commonly assigned US patent application 09/558,770 entitled An Optical Fiber and Method For Fabricating A Low Polization-Mode Dispersion And Low Attenuation Optical Fiber. 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.

Claims

Claims What is claimed is:

1. A method of manufacturing a soot preform, comprising the steps of: providing a bait rod having an outer diameter greater than 18 mm, generating a soot stream of core soot, and depositing the core soot onto an outer surface of the bait rod.

2. The method of claim 1 further comprising the step of rotating the bait rod during the step of depositing.

3. The method of claim 1 wherein the bait rod is hollow.

4. The method of claim 1 wherein the step of depositing further comprises depositing a core portion onto the bait rod and then depositing a cladding portion onto the core portion.

5. The method of claim 1 wherein the step of depositing comprises producing the soot from an array of burners.

6. The method of claim 5 wherein the step of depositing comprises arranging the array of burners to include greater than 20 burners; the soot streams of the burners being substantially aligned with a longitudinal centerline of the bait rod.

7. The method of claim 5 wherein the step of depositing comprises arranging the array of burners to comprise greater than 30 burners.

8. The method of claim 5 further comprising a step of providing a slot in at least one of the array of burners that is aligned with a longitudinal axis of the bait rod.

9. The method of claim 5 further comprising a step of providing a slot in each one of the array of burners, the slots being substantially aligned with the longitudinal axis of the bait rod.

10. The method of claim 1 wherein a bait rod diameter is between about 18 mm and 40 mm.

11. The method of claim 1 wherein a bait rod diameter is between 25 mm and 35 mm.

12. The method of claim 1 wherein during the step of depositing, a soot capture rate is reduced by less than 5% for lateral offsets from a longitudinal axis of the bait rod centerline of a burner or burner array of up to 5 mm.

13. The method of claim 1 wherein during the step of depositing, a width dimension measured across the soot stream transverse to the bait rod and mid way between a face of the burner and the bait rod, is in a ratio of bait rod diameter to width dimension between about 2 and 5.

14. The method of claim 1 wherein during the step of deposition, a deposition capture efficiency at startup is at least 30%.

15. A method of manufacturing a soot preform, comprising the steps of: rotating a bait rod having an outer diameter of greater than 18 mm, depositing a core soot portion onto an outer surface of the bait rod, and depositing a cladding soot portion onto an outer surface of the core soot portion.

16. A method of manufacturing a soot preform, comprising the steps of: rotating a bait rod having an outer diameter of 18 mm or greater, and depositing, with a plurality of burners, (a) a core soot portion onto an outer surface of the bait rod, and

(b) a cladding soot portion onto an outer surface of the core soot portion.

17. An apparatus for manufacturing a soot preform, comprising: a bait rod having an outer diameter greater than 18 mm, and at least one burner adapted to deposit core soot onto an outer surface of the bait rod.

18. The apparatus of claim 17 wherein the bait rod is hollow.

19. The apparatus of claim 18 wherein the bait rod has a wall dimension of less than about 6.4 mm.

20. The apparatus of claim 18 further comprising at least one end plug inserted in at least one hollow end of the bait rod.

21. The apparatus of claim 20 further comprising an expansion member mounted between the bait rod and the at least one end plug.

22. The apparatus of claim 18 further comprising an end plug inserted into each hollow end of the bait rod.

23. The apparatus of claim 22 further comprising a rod interconnecting the plugs; the rod passing through the hollow portion of the bait rod.

24. The apparatus of claim 17 wherein the at least one burner comprises a changeable precursor supply enabling deposit of a core portion onto the bait rod and then a cladding portion onto the core portion.

25. The apparatus of claim 17 wherein the at least one burner further comprises an array of burners.

26. The apparatus of claim 25 wherein the array of burners comprises greater than 20 burners substantially aligned with a line offset from a longitudinal axis of the bait rod.

27. The apparatus of claim 25 wherein the array of burners comprises greater than 30 burners aligned with each other.

28. The apparatus of claim 17 wherein a slot is formed in the at least one burner, a length of the slot being substantially aligned with a longitudinal axis of the bait rod.

29. The apparatus of claim 17 wherein the at least one burner comprises an array of burners, each including a slot, a length of which being substantially aligned with the longitudinal axis of the bait rod.

30. The apparatus of claim 17 wherein the burner, when ignited, emits a soot stream having a width dimension measured across the soot stream transverse to a longitudinal axis of the bait rod and mid way between a face of the burner and the bait rod; a ratio of bait rod diameter to width dimension being between about 2 and 5.

31. The apparatus of claim 17 wherein a deposition capture efficiency at startup is at least 30%.