Abstract:
The present invention is directed to a method and apparatus for forming soot used in making glass, and in particular, optical waveguides. A liquid precursor ( 66 ) is first fed into orifice ( 52 ) of a liquid orifice insert ( 48 ) within an injector ( 44 ) positioned within an atomizing burner assembly, and is thereafter discharged from the injector into a pressurization chamber ( 56 ). An atomization gas ( 70 ) is also fed into the pressurization chamber ( 56 ) to mix with the liquid precursor liquid stream ( 68 ) which breaks into droplets ( 76 ). The liquid precursor and atomization gas arm forced under pressure out of an atomization orifice ( 32 ) on the face of the burner ( 30 ) assembly. Flame gas ( 74 ), reaction gas ( 84 ) and shield gas ( 82 ) are ejected from burner orifices ( 40, 38, 36  and  34 ) to produce the flame. The atomized liquid precursor thus discharged is fed into the flame ( 72 ) produced at the face of the burner assembly where the atomized liquid precursor reacts with the flame to form soot ( 78 ) on a rotating mandrel ( 80 ).

Description:
This application claims the benefit of provisional application 60/095,736 filed Aug. 7, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the formation of soot used in the manufacture of glass and, more particularly, to a method and apparatus for the delivery of liquid precursors to a flame during flame hydrolysis. 
     While the invention is subject to a wide range of glass soot deposition applications, it is especially suited for making soot for use in the manufacture of optical waveguides, and will be particularly described in that connection. 
     BACKGROUND OF THE INVENTION 
     Various processes are known in the art that involve the production of metal oxides from vaporous reactants. Such processes require a feedstock solution or precursor, a means of generating and transporting vapors of the feedstock solution (hereafter called vaporous reactants) and an oxidant to a conversion reaction site (also known as a soot reaction zone to those skilled in the art), and a means of catalyzing oxidation and combustion coincidentally to produce finely divided, spherical aggregates, called soot. This soot can be collected on any deposition receptor in any number of ways ranging from a collection chamber to a rotating mandrel. The collected soot may be simultaneously or subsequently heat treated to form a non-porous, transparent, high purity glass article. This process is usually carried out with specialized equipment having a unique arrangement of nozzles and burners. 
     Much of the initial research that led to the development of such processes focused on the production of bulk silica. Selection of the appropriate feedstock was an important aspect of that work. Consequently, it was at that time determined that a material capable of generating a vapor pressure of between 200-300 millimeters of mercury (mm Hg) at temperatures below approximately 100° C. would be useful for making such bulk silica. The high vapor pressure of silicon tetrachloride (SiCl 4 ) suggested its usefulness as a convenient vapor source for soot generation and launched the discovery and use of a series of similar chloride-based feedstocks. This factor, more than any other is responsible for the presently accepted use of SiCl 4 , GeCl 4 , POCl 3 , and BCl 3  as feedstock vapor sources. 
     Use of these and other halide-based feedstocks as vapor sources, however, does have its drawbacks. The predominate drawback being the formation of hydrochloric acid (HCl) as a by-product of oxidation. HCl is not only detrimental to the deposition substrates and the reaction equipment, but to the environment as well. Overcoming this drawback, amongst others, led to the use of halide-free compounds as precursors or feedstocks for the production of soot for optical waveguides. 
     Although use of halide-free silicon compounds as feedstocks for fused silica glass production, as described in U.S. Pat. Nos. 5,043,002 and 5,152,819, avoids the formation of HCl, other problems remain, particularly when the glass is intended for the formation of optical waveguides and high purity silica soot. It has been found that, in the course of delivering a vaporized polyalkylsiloxane feedstock to the burner, high molecular weight species can be deposited as gels in the line carrying the vaporous reactants to the burner, or within the burner itself. This leads to a reduction in the deposition rate of the soot that is subsequently consolidated to a blank from which an optical waveguide fiber is drawn. It also leads to imperfections in the blank that often produce defective and/or unusable optical waveguide fiber from the affected portions of the blank. An additional problem encountered while forming silica soot using siloxane feedstocks is the deposition of particulates having high molecular weights and high boiling points on the optical waveguide fiber blank. The build-up of these particulates results in “defect” or “clustered defect” imperfections that adversely affect the optical and structural quality of optical waveguides formed using the silica soot. 
     Defects typically are in the form of small (i.e. 0.1 to 4.0 mm in diameter) bubbles in a glass body. They are often formed in fused silica by an impurity, such as uncombusted gelled polyalkylsiloxane. A very small particle of siloxane gel can be the initiation site for such a defect. Since siloxane decomposes at high temperature after being deposited on the glass body; it can give off gases that cause the formation of the defect. Clustered defects are larger glass defects found in optical waveguide fiber preforms, and often occur as a series of defects in the form of a line or a funnel-or flower-shaped cluster. Typically, a large particle of gel is the initiation site for a clustered defect. After the gel particle has struck the porous preform, it causes a raised area to stand out on the preform surface. Because the clustered defect is a raised site, more heat transfer passes to this site. Due to this increased heat transfer, more thermophoresis occurs at the site, causing the imperfection to grow and leave behind a string of defects. As a result of the clustered defect, the affected portion of the optical waveguide preform cannot be consolidated normally, and the consequent irregularity in the blank yields a defective optical waveguide. For example, in the case of a typical 100 kilometer consolidated waveguide fiber blank, which has a diameter of 70 millimeters (mm) and a length of 0.8 meter (m), the presence of one clustered defect on the surface of the blank will typically result in the loss of 5 kilometers of optical waveguide fiber during drawing. In the case of a larger consolidated blank, the negative impact of a single clustered defect is proportionately higher. In a 250 kilometer consolidated blank, which has a diameter of 90 mm and a length of 1.8 m, one clustered defect on the surface of the blank will typically result in the loss of 8 kilometers of optical waveguide fiber during drawing. 
     U.S. patent application Ser. No. 08/767,653, discloses that clustered defects can be reduced by delivering a liquid siloxane feedstock to a conversion site, atomizing the feedstock at the conversion site, and converting the atomized feedstock at the conversion site into silica. Because the precursors are delivered directly into a burner flame as a liquid rather than a vapor, the vapor pressure of the precursors is no longer a limiting factor in the formation of soot for optical waveguides. However, the external atomizers and their methods of use disclosed in application Ser. No. 08/767,653 are not without limitation. External atomizers typically have a liquid discharge orifice that is co-planer or substantially co-planar with the burner face. Accordingly, the liquid and the atomizing gases come together at the surface of the burner. Since the flame is generated adjacent the face of the burner, atomization must occur very quickly if the liquid is to be dispersed into droplets prior to reaching the flame. For this to occur, very high atomizing gas velocities are required. While these high gas velocities can disperse the liquid into small droplets, they do so by creating turbulence, which in turn adversely affects the soot deposition rate. 
     Additionally, external atomizers rely on a very small atomizing gas annulus positioned around the liquid exit orifice to provide the high velocity atomization gas that impinges on the liquid. As a result, the close proximity of the liquid exit orifice and the soot reaction zone render both the liquid exit orifice and the atomizing gas annulus of the external atomizers susceptible to soot build up and clogging. When either the annulus or the liquid exit orifice is partially clogged by this soot build up, the flame, and thus the soot stream, becomes non-uniform and the soot deposition rate suffers. Because of the small size of these openings, cleaning of the external atomizer is both time consuming and difficult. Moreover, because the burners are shut down during cleaning operations, production down time has a significant adverse economic impact on operations. 
     The external atomizer is also expensive to manufacture and limited in flexibility. Since the liquid exit orifice on the burner face of the external atomizer is generally provided with a knife-edge to facilitate rapid mixing of the atomizing gases and the liquid stream discharged from the orifice, the liquid exit orifice diameter is limited to a dimension which is greater in size than preferred. Additionally, the liquid exit orifice must be centrally positioned within the annulus to avoid the problem of non-concentricity. Any slight misalignment, and the flame will be non-concentric. Non-concentricity results in poor soot deposition and is a serious problem during laydown. Accordingly, tolerances must be tight, which in turn increases manufacturing costs. 
     There is a need therefore for alternative methods and apparatus for depositing glass precursor soot, especially for optical fiber and other waveguide related applications. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved method and apparatus for delivering a liquid precursor to a burner flame to form soot used in the manufacture of glass. The liquid precursor, capable of being converted by thermal oxidative decomposition to glass, is provided and introduced directly into the flame of a combustion burner, thereby forming finely divided amorphous soot. The amorphous soot is typically deposited on a receptor surface where, either substantially simultaneously with or subsequent to its deposition, the soot is consolidated into a body of fused glass. The body of glass may then be either used to make products directly from the fused body, or the fused body may be further treated, e.g., by forming an optical waveguide such as by drawing to make optical waveguide fiber as further described in U.S. patent application Ser. No. 08/574,961 entitled, “Method for Purifying Polyalkylsiloxanes and the Resulting Products”, the specification of which is hereby incorporated by reference. 
     The burner assembly for delivering the precursor directly to the burner flame as an atomized liquid includes a novel recessed injector orifice that delivers a precision stream of liquid into a chamber where the stream is exposed to a low velocity atomizing gas. The introduction of the gas and liquid stream into the chamber increases the pressure within the chamber causing the liquid stream to be discharged as an aerosol from an exit orifice at the burner face of the atomizing burner assembly, which, in operation, generates a conversion site flame. The small liquid droplets of the aerosol are thus fed into the flame to convert the compound by thermal oxidative decomposition to finely divided amorphous soot. Generally, a receptor surface such as a rotating mandrel is positioned in close proximity to the atomizing burner assembly to permit deposition of the soot on the receptor surface. 
     The principal advantage of the present invention is the provision of an arrangement and method which substantially reduces the clogging of the orifices on the face of the burner assembly by using lower atomizing gas velocities than other atomizing burner assemblies known in the art, while at the same time, atomizing an equivalent liquid flow rate. By using lower atomizing gas velocities, turbulence is reduced at the soot reaction zone, and thus the soot deposition rate is greatly improved. Moreover, by delivering the precursor to the flame as a liquid rather than as a vapor, gelling of the precursor is prevented in that exposure of the precursor to the high temperature environments of a vaporizer and vapor delivery system are avoided. This improves the yield and quality of the soot produced and also reduces the maintenance requirements of the production system. Additionally, because the precursor does not have to reach the vapor phase prior to its exposure to the burner flame, elements never before used to form soot for use in the manufacture of the preforms can now be converted by oxidation or flame hydrolysis to soot for use in the manufacture of glass preforms, including elements selected from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, VA, and the rare earth series of the Periodic Table of Elements. 
     In another aspect, the invention includes a burner assembly formed from a housing having a burner face with multiple gas orifices and an atomization orifice. An injector chamber resides within the housing and a plurality of gas passageways within the housing communicate with the gas orifices. An injector having an injector orifice is positioned within the injector chamber such that the injector orifice is remotely spaced from the atomization orifice. The injector, together with the housing, forms a pressurization chamber within the burner assembly. 
     In yet another aspect of the present invention, the burner assembly includes and injector constructed and arranged to deliver liquid precursors, and a housing substantially surrounding the injector. The housing has a burner face, which includes an orifice rim defining an atomization orifice. The orifice rim is shaped such that turbulence is reduced as the liquid precursor is discharged from the atomization orifice. 
    
    
     Additional features and advantages of the invention are set forth in the detailed description, which follows, and in part will be apparent from the description and claims when read in conjunction with the accompanying drawings. Other advantages of the invention will be realized and attained by the method and apparatus particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention, and are not in any way meant to limit the invention which is defined by the appended claims. Moreover, the accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention, and together with the description serve to explain the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a reactant delivery system in accordance with the present invention. 
     FIG. 2 is a top plan view of a preferred embodiment of the burner assembly of the present invention. 
     FIG. 3 is a side cross-sectional view of the burner assembly of the present invention taken along line  3 — 3  of FIG. 2 . 
     FIG. 4 is a top plan view of a preferred embodiment of the liquid tube of the present invention. 
     FIG. 5 is a side cross-sectional view of the liquid tube of the present invention taken along line  5 — 5  of FIG. 4 . 
     FIG. 6 is a top plan view of a preferred embodiment of the liquid orifice insert of the present invention. 
     FIG. 7 is a side cross sectional view of the liquid orifice insert of the present invention taken along line  7 — 7  of FIG.  6 . 
     FIG. 8 is a schematic representation of liquid precursor droplets being discharged into a flame from the exit orifice of the preferred embodiment of the atomizing burner assembly of the present invention. 
     FIG. 9 is a partial cross-sectional view of a low turbulence embodiment of the burner assembly of the present invention illustrating the rounded orifice rim. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings, in which like reference characters indicate like parts throughout the several views. The atomizing burner assembly and method for forming soot for use in the manufacture of optical waveguides of the present invention forms a portion of a broader system as shown in FIG.  1 . FIG. 1 schematically depicts an exemplary system for delivering a liquid precursor to atomizing burner assembly  10  of the present invention. It will be understood by those skilled in the optical fiber art that there are other systems and variations of the depicted system in which the present invention can be incorporated to perform the functions described and claimed herein. 
     In accordance with one aspect of the present invention, a liquid siloxane precursor such as, for example, a polymethylcyclosiloxane is stored in precursor tank  12 . Precursor tank  12  is in fluid communication with atomizing burner assembly  10  via a liquid precursor transporting conduit system that can, if desired, include metering pump  14 , filter  16 , and optional preheater  18 . The siloxane liquid precursor from tank  12  is transferred through the liquid precursor-transporting conduit by pump  14  through filter  16  to optional preheater  18 . The liquid delivered through filter  16  is under sufficient pressure to substantially prevent and inhibit its volatilization in preheater  18 , which is optionally employed to warm the liquid reactant prior to its introduction into atomizing burner assembly  10 , and avoids the high temperatures of a vaporizer which typically promotes gel formation. Burner assembly  10  preferably is provided with an inner shield gas, a reaction gas, and a mixture of methane and oxygen for the flame, as described, for example, in U.S. Pat. No. 4,165,223 to D. R. Powers, the specification of which is hereby incorporated by reference. It is to be understood, however, that other gases, such as hydrogen in addition to, or other than methane and oxygen can be and often are used to support the burner flame. 
     The liquid precursor is conveyed from filter  16  or optional preheater  18  to the atomizing burner assembly  10 , which as the name implies, atomizes the liquid precursor, provides the combustion source, and delivers the atomized liquid as an aerosol into the combustion source, which in the preferred embodiment, is a flame. Throughout the specification the precursor is described as being a “liquid” or as being in “liquid form”. What is meant by these terms is that the precursor is in a substantially liquid state. Some small portion of the reactant may be in vapor form, particularly where preheater  14  is employed in the system, or where a nitrogen blanket over the liquid is employed. A small portion of the reactant can be in vapor form as delivered to the combustion site or soot reaction zone without adversely affecting the operation of the invention. Likewise, the precursor can contain small amounts of solids provided the solids are small enough to be burned upon entering the flame produced by atomizing burner assembly  10 . The details of the present invention are described below. 
     Several types of atomizing injectors and nozzles capable of forming small droplets of liquid are known in the atomization art as disclosed in  Atomization and Spravs , by Arthur H. Lefebure, Hemisphere Publishing Co., 1989, which is incorporated herein by reference. Atomizers can be operated by various energy sources and may be categorized as, for example, internal, external, air-blast, air-assist, jet, swirl, jet-swirl, pneumatic, rotary, acoustic, ultrasonic, electrostatic, and combinations of the same. Various types of these and other atomizers are further discussed in  Liquid Atomization , by L. Bayvel and Z. Orzechowski, Taylor &amp; Francis, (1993), which also is hereby incorporated by reference. The preferred atomizing burner assembly of the present invention incorporates an internal air-blast atomizer in its design. 
     The preferred embodiment of the atomizing burner assembly of the present invention is shown in FIGS. 2 through 8 and is designated generally by reference numeral  10 . As seen in the top view depicted in FIG. 2, atomizing burner assembly  10  includes a housing  20  formed from a cover  22 , a sealing plate  24 , and a base  26  (FIG.  3 ). The cover  22  is mounted to the base  26  with fasteners  28  such as hex nuts, such that the sealing plate  24  is interposed therebetween. As shown, the cover  22  is frustoconical in shape and has a centrally positioned burner face  30 . A plurality of concentric rows of gas orifices are positioned on burner face  30  such that their common center corresponds to the location of atomization orifice  32 . Inner shield gas orifices  34  form the row closest atomization orifice  32 , followed next by a pair of rows of reaction gas orifices  36  and finally by a row of flame gas orifices  40 . It will be understood by those skilled in the art that greater or fewer rows of gas orifices can be used with and will enable the present invention. 
     As shown in the cross-sectional view of FIG. 3, the gas orifices  34 ,  36 ,  38 , and  40  communicate with gas supply lines (not shown) via a network of gas passageways  42 , some of which cannot be seen in the present view. An injector  44  also forms a part of the preferred embodiment of the atomizing burner assembly  10 . Injector  44  is centrally positioned within injector chamber  46  formed in housing  20  of atomizing burner assembly  10 , and when seated within burner assembly  10 , injector  44  and housing  20  define a pressurization chamber  56  therein. As shown clearly in FIGS. 4 through 7, injector  44  includes an elongated liquid tube  45  having a threaded bore  58  (FIGS. 4 and 5) adapted to removably receive a liquid orifice insert  48  (FIGS.  6  and  7 ). As illustrated in FIG. 4, circumferential row of atomization gas orifices  54  is positioned around the circumference of and on the head of injector  44  and surrounds threaded bore  58 . Atomization gas passageways  60  place atomization gas orifices  54  in fluid communication with injector chamber  46  which is fed atomization gas  70  from gas passageway network  42 . As illustrated in FIG. 8, atomization gas  70  is delivered from gas passageways  60 , through atomization gas orifices  54 , to pressurization chamber  56  where atomization gas  70  mixes with a stream of liquid discharged from injector  44 , as will be described in greater detail below. 
     FIGS. 6 and 7 show the detail of liquid orifice insert  48 . Liquid orifice insert  48  is preferably threaded for mating with threaded bore  58  in the head of liquid tube  45 , and is fitted with injector orifice insert  50  having a precision injector orifice  52 . Because it is removable, insert  50  can easily be changed in the event it becomes partially plugged with soot, or if a different injector orifice size is required or desired for other applications. Injector orifice insert  50  is preferably made from a material that can be cut to exacting specifications. In the preferred embodiment of the invention, it has been found that a jewel such as a ruby (Al 2  O 3 ) meets this requirement. Typically, injector orifice insert  50  is cut to provide an injector orifice  52  having a diameter of between about 0.001 inches and 0.010 inches. Preferably, injector orifice  52  has a diameter of less than or equal to 0.006 inches. Injector orifice insert Model No. RB-22012, manufactured by Bird Precision of Waltham, Massachusetts, meets these requirements, but other injector orifice inserts  50  from other manufacturers can be used. As shown in FIGS. 3 and 5, liquid tube  45  has a centrally positioned injector passageway  62  which communicates with insert channel  64  when liquid orifice insert  48  is positioned within threaded bore  58 . In this way, liquid precursor  66  from tank  12  can pass to and through precision injector orifice  52 . 
     In operation, as shown in FIG. 8, liquid precursor  66  is delivered through liquid tube  45 , liquid orifice insert  48 , subsequently through injector orifice  52 , and then into pressurization chamber  56  as a fine stream of liquid  68 . At the same time, atomization gas  70  is delivered through atomization gas passageways  60  into pressurization chamber  56 . Due in large part to the torroidal shape of chamber  56  and the reduced size of atomization orifice  32  as compared to the volume of chamber  56 , liquid stream  68  is accelerated through pressurization chamber  56  and is discharged from atomization orifice  32  as an aerosol. When atomization occurs, liquid stream  68  is ripped into numerous droplets  76  of extremely small size and is directly delivered as an aerosol to flame  72  created adjacent burner face  30  by the combustion of reaction gas  84  delivered through reaction gas orifices  36  and  38 , and flame gas  74  delivered through flame gas orifices  40 . Where flame  72  and the aerosol meet and react is known as the soot reaction zone. Thermal oxidative decomposition of the aerosol in the soot reaction zone produces finely divided amorphous soot  78 , which is deposited on rotatable mandrel  80 . 
     Droplets  76  are combusted in the soot reaction zone above burner face  30  by flame  72  fueled by, preferably, a combination of methane and oxygen. The methane and oxygen form the flame gas  74 , which is preferably conducted through flame gas orifices  40  to the soot reaction zone. A reaction gas  84  such as oxygen is delivered to the soot reaction zone through reaction gas orifices  36  and  38  to provide an oxygen rich environment for the flame  72 , and thus provide for better combustion. A shield gas  82 , such as nitrogen, argon, helium or another inert gas, but preferably nitrogen, is delivered through shield gas orifices  34  to inhibit the premature reaction of droplets  76  with flame  72 , and thus prevent soot build-up on burner face  30 . Atomization gas  70  may consist of nitrogen, or other inert gases, or mixtures thereof. It is also possible for the atomizing gas  70  to be a mixture of elements such as nitrogen and oxygen; however, it has been found that the most preferable gas is oxygen alone, as it reduces the formation of defects in the soot blank. One advantage of the preferred embodiment thus configured and operated is that atomizing burner assembly  10  of the present invention produces a narrower soot stream than external atomizing burner assemblies presently known in the art. 
     To reduce the velocity of atomizing gas  70  and prevent surface defects on the soot blank, oxygen is the most preferred atomizing gas to be used in atomizing burner assembly  10 . Using oxygen as the atomizing gas allows for better mixing of liquid precursor  66  with the oxygen before conversion to soot. Use of this atomizing gas results in quicker heating of the liquid and helps provide the oxygen needed for the reaction. Therefore, the velocity of the oxygen atomizing gas can be significantly lowered, at least by about 50%, compared to the velocity of the atomizing gas when pure nitrogen is employed. This reduction in gas velocity consequently reduces burner flame turbulence and thus soot blank defects. The construction and arrangement of pressurization chamber  56  further reduces the velocity requirements of atomization gas  70 , and thus, flame turbulence is reduced even further. 
     FIG. 9 illustrates a preferred atomizing burner assembly  10  design for reducing flame turbulence associated with the delivery of liquid precursors into a flame. This embodiment of the present invention differs from the embodiment shown in FIG. 8 in that the portion of cover  22  housing liquid orifice insert  48  has a rounded or curved orifice rim  90  bounding and defining atomization orifice  32 . Rounded orifice rim  90  reduces flame turbulence  94  produced adjacent burner face  30  and atomization orifice  32 , which in turn facilitates better atomization, and reduces soot build up (not shown) on burner face  30 . Accordingly, clogging of atomization orifice  32  and thus the need for frequent burner face cleaning are reduced. Thus, elements such as potassium and calcium present in certain optical waveguide precursors no longer solidify and deposit on the surface of burner face  30  around atomization orifice  32  as has been found with burner assembly&#39;s having a knife-edge rim, such as that shown in FIG.  8 . 
     As discussed above, orifice rim  90  is preferably a non-linear surface and is more preferably a rounded surface. During tooling, orifice rim  90  is preferably shaped to have a radius  92 , which is between about ¼ to ⅔ of the diameter of atomization orifice  32 . More preferably, radius  92  is about ½ of the diameter of atomization orifice  32 . Accordingly, if the atomization orifice  32  diameter is approximately 0.03 inches, the radius  92  of orifice rim  90  defining atomization orifice  32  will preferably be approximately 0.015 inches. It will be understood, however, that rounded surfaces other than semi-circular rounded surfaces will also reduce flame turbulence  94 , and thus, curved rounded orifice rims  90  having non-uniform radial dimensions are also intended to be a part of the invention of the burner assembly described herein. 
     The apparatus can also be provided with dopant supply tank  19 , shown in FIG. 1, which contains a compound capable of being converted by oxidation or flame hydrolysis to P 2 O 5  or to a metal oxide whose metallic component is selected from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, and the rare earth series of the Periodic Table. These oxide dopants combine with the soot generated by burner assembly  10  to provide doped soot, which can be subsequently formed into optical waveguide fibers. The dopant can be supplied to precursor tank  12  and mixed with the precursor in tank  12 , or alternatively, the dopant can be delivered from supply tank  19  to atomizing burner assembly  10  via a separate metering pump and optionally a filter (not shown) analogous to the delivery system used for the precursor stored in precursor tank  12 . 
     In accordance with the invention, the preferably halide-free, silicon-containing precursor preferably is a polyalkylsiloxane, for example, hexamethyldisiloxane. More preferably, the polyalkylsiloxane is a polymethylcyclosiloxane. Most preferably, the polymethylcyclosiloxane such as hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, and mixtures thereof. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the method and apparatus for forming soot for use in manufacturing optical waveguides of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modification and variations of this invention provided they come within the scope of the appended claims and their equivalents. In addition, the corresponding structures, materials, acts and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or acts for performing the functions in combination with other claimed elements as specifically claimed herein.