Patent Publication Number: US-2005120752-A1

Title: Substantially dry, silica-containing soot, fused silica and optical fiber soot preforms, apparatus, methods and burners for manufacturing same

Description:
RELATED APPLICATIONS  
      This application claims priority to, and the benefit of, U.S. Provisional Patent Application 60/200,405 filed Apr. 24, 2000 entitled “Water-Free Fused Silica And Method Therefor,” and U.S. Provisional Patent Application 60/258,132 filed Dec. 22, 2000 entitled “Substantially Dry, Silica-Containing Soot, Fused Silica And Optical Fiber Soot Preforms, Apparatus, Methods And Burners For Manufacturing Same And Method Therefor,” the disclosures of which are hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD  
      This invention relates to methods and apparatus for producing optical fiber soot preforms, high purity fused silica and silica-containing soot. More specifically, the methods and apparatus relate to producing optical fiber preforms, fused silica and silica-containing soot that is substantially free of water.  
     BACKGROUND OF THE INVENTION  
      Photomasks are used in microlithography in printing miniature circuit patterns on silicon wafers and carry an enlarged version of the circuit to be printed thereon. To reduce the size of the circuits on the silicon wafers to get more circuits on the same wafer, light with lower wavelengths are used. For laser light with a low wavelength (less than 248 nm), the photomask substrate can be made with fused silica glass that has high transmitivity. To display high transmitivity, fused silica glass needs to be very pure and contain extremely low levels of water (preferably less than about 10 ppb). The presence of large amounts of water in fused silica product makes the glass not suitable for certain low wavelength applications. Current systems operate at the 248 nm window. Lower wavelength systems heretofore have been largely unsuccessful because of water levels being too high in the silica photomask material. Thus, it would be desirable to produce a glass material that could be used at lower wavelengths.  
      One process that delivers glass with lower levels of water is the process used to make preforms for optical fiber waveguides (hereinafter the “preform manufacture process”). This preform manufacturing process utilizes several manufacturing steps. First silica containing soot is deposited onto, for example, an alumina bait rod by an Outside Vapor Deposition (OVD) method, for example. The bait rod is removed leaving a tubular soot member with a centerline aperture. This soot member may include the appropriate dopants, for example germania, such that a desired refractive index profile is achieved. The soot preform is then consolidated in a furnace with a vacuum generally applied to close the centerline aperture. Next, the consolidated preform is drawn into core cane; wherein the core cane preferably comprises part or all of the physical core of the optical fiber when finally drawn into fiber. This core cane is cut into lengths and again overclad with silica-containing soot to form the clad portion or another segment of the core if a multi-segment profile is desired. The preform is again consolidated. Chlorine gas, for example, in the atmosphere of the consolidation furnace is used to dry the preform and remove water prior to vitrification into glass in both the above-mentioned consolidation steps. The resulting final consolidated preform, is then placed in a draw furnace and drawn into a fiber in an inert gas atmosphere.  
      Unfortunately, because of the process currently used to form the soot, water is inevitably formed into the preform. Therefore, it is necessary to employ a drying step before consolidation. Specifically, the water is formed, as will be hereinafter described, because the chemical reaction of the silica precursors and fuels currently used in the process of forming soot form water as a reaction by-product. Moreover, it was discovered by the inventors herein that exposure to atmospheric conditions during standard processing techniques causes the soot preform to pick up further water. In optical telecommunications systems, one factor that determines the distance between amplification stages is the optical fiber attenuation. A significant contributor to poor attenuation is water (OH) present in the preform. Water present causes a peak in the transmission curve at about 1383 nm. This peak has a detrimental effect on the attenuation at 1550 nm, a primary transmission wavelength in optical fiber communications. Thus, it is desirable to reduce the water peak by reducing the water content of the consolidated glass as much as possible.  
      Furthermore, in fluorine doped optical fibers, fluorine doping at acceptable levels is a considerable problem. Moreover, once fluorine is present in the soot preform, fluorine migration is a significant problem because of fluorine&#39;s high mobility and small molecular size. Fluorine is utilized as a refractive index depressant, thus desirably enabling negative indices of refraction where desired. Migration dramatically reduces the amount of fluorine that may be incorporated in the soot. Moreover, migration smoothes out the refractive index profiles desired for optimal signal transmission. Thus, rather than achieving sharp transitions between profile regions, migration causes rounded transitions. Moreover, migration lowers the delta % value (a measure of the refractive index difference relative to the cladding). Thus, since fluorine is extremely mobile, it is very desirable to achieve a method and/or apparatus to prevent migration of such dopants throughout the soot preform during processing.  
      Equation 3 illustrates the forming high purity fused silica or silica soot in accordance one process used in the prior art. SiCl 4  (a silica precursor), oxygen and methane are combined and ignited in a burner to produce glass or soot which is deposited on a substrate surface. In the case of high purity fused silica, the soot is substantially simultaneously consolidated (vitrified) within the furnace when methane is utilized. The by-products of such reactions are carbon dioxide, water vapor and chlorine. In particular, large amounts of water vapor are produced. 
 
CH 4 +3O 2 +SiCl 4 →CO 2 +2H 2 O+SiO 2 +2Cl 2    (Prior Art 1) 
 
      Another currently employed process for manufacture of silica soot uses octa-methyl-cyclo-tetra-siloxane (OMCTS) as the raw material for silica soot and natural gas (predominantly methane along with other hydrocarbons) as the fuel. Natural gas is utilized as the fuel to maintain the furnace at high temperatures for manufacture of high purity fused silica. The products of combustion of the natural gas are also water vapor and carbon dioxide. The products of combustion of the OMCTS are silica, water and carbon dioxide as shown in equation 2. 
 
C 8 H 24 O 8 Si 8 +16O 2 →8CO 2 +12H 2 O+8SiO 2    (Prior Art 2) 
 
      Thus, it should be recognized that a significant by-product of the reaction in both processes outlined in equations 1 and 2 is water vapor generated as a result of combustion. Undesirably, this water gets incorporated in soot, and, once present, is very difficult to remove. To attempt to remove the water from soot articles, such as soot preforms, extensive drying step utilizing chlorine are employed. Detrimentally, however, some water remains captured in the consolidated glass produced. The presence of water is detrimental to optical properties of the glass produced. Thus, it is an industry-wide goal to further reduce the water content present in high purity fused silica and also in silica-soot articles such as soot preforms for optical fiber manufacture.  
     BRIEF SUMMARY OF THE INVENTION  
      The process and apparatus in accordance with one embodiment of the invention manufactures substantially water-free silica soot, preforms or glass. The process and apparatus to make such water-free fused silica soot, preforms or glass does so by eliminating the possibility of water ever forming in the combustion atmosphere. This is achieved in a first embodiment thereof by utilizing a substantially hydrogen-free fuel, such as carbon monoxide (CO), carbon suboxide (C 3 O 2 ), carbonyl sulfide (COS), and the like. Use of such substantially H-free fuels minimizes water formation in the combustion reaction. According to a preferred embodiment, it is desired to use a substantially hydrogen-free raw material as a glass precursor for silica also. Most preferably, a combination of substantially hydrogen-free raw material and substantially hydrogen-free fuel is utilized. Typical examples of substantially H-free glass precursors include silicon carbide (SiC), silicon monoxide (SiO), silicon nitride (Si 3 N 4 ), silicon tetrabromide (SiBr 4 ), silicon tetrachloride (SiCl 4 ), silicon tetraiodide (SiT 4 ) and silica (SiO 2 ). Si(NCO) 4  may also be utilized.  
      In accordance with the invention, when carbon monoxide, for example, is used as the fuel and combined with oxygen, the only by-product is carbon dioxide. This by-product is easily disposed of and, advantageously, no water is formed from the process reaction. This reaction is illustrated by the following equation (3). 
 
CO+½O 2 →CO 2    (3) 
 
      It was recognized by the inventors herein that the available heat from carbon monoxide is about one-fourth the heat available from natural gas (methane). Therefore, four times the fuel would be required to produce the same amount of heat. However, only one-half mole of combustion supporting oxygen is required to combust one mole of CO. Thus, the total volume of oxygen required is the same for either fuel to produce the same amount of heat. The following equation (4) shows the required carbon monoxide fuel needed to match the available heat of combusting one mole of methane (CH 4 ) used in one prior art process. 
 
4CO+2O 2 →4CO 2    (4) 
 
 Equation (5) below shows the by-products and combustion supporting oxygen needed for combustion of one mole of methane in the prior art. 
 
CH 4 +2O 2 →2H 2 O+CO 2    (5) 
 
      Thus, from the foregoing, it should be recognized that the production of substantially water-free silica soot, preforms and glass is obtainable, provided the burners are properly designed.  
      According to one embodiment of the invention, a method of manufacturing an optical fiber preform is provided. The method comprises the steps of generating heat from a combustion burner having a flame produced by igniting a substantially hydrogen-free fuel, flowing a glass precursor into the flame to produce silica-containing soot, and then depositing the silica-containing soot onto a rotating substrate. To further minimize the inclusion of water in the preform, the preform is preferably included within a substantially water-free atmosphere during the step of depositing. The substantially water-free atmosphere may be a shroud or supply of dry air, dry nitrogen, dry oxygen, dry argon, dry helium, dry carbon dioxide, and combinations thereof.  
      In another embodiment of the invention, a method of manufacturing a silica-containing soot preform is provided comprising a step of heating at least one end of the preform with at least one end burner wherein the at least one end burner combusts a substantially hydrogen-free fuel. It should be recognized that utilizing end burners that combust a substantially hydrogen-free fuel also minimizes incorporation of water into the preform. Advantageously, these end burners may be utilized in combination with the substantially H-free fuel provided to the soot-producing burner as well as in combination with providing a substantially water-free environment.  
      According to another embodiment of the invention, a method of manufacturing an optical fiber preform is provided wherein a combination of conventional deposition methods (using hydrogen-containing fuels) and substantially dry deposition methods are employed. In particular, a first combustion burner generates heat from a first flame produced by igniting a hydrogen-containing fuel or a substantially hydrogen-free fuel. A first glass precursor is flowed into the first flame to lay down a first segment of silica-containing soot within the preform. Next, heat from a second combustion burner having a second flame produced by igniting the other one of the hydrogen-containing fuel, and the substantially hydrogen-free fuel is produced. A second glass precursor is flowed into the second flame to lay down a second segment of silica-containing soot. In this way, a multiple-segment preform may be efficiently manufactured in a single lathe without having an intermediate consolidation step. This so-called single step method of forming a preform has long been sought after in the OVD arts. Glassy barrier layers, as will be described in detail herein, are preferably employed to minimize migration of water or a dopant between the segments. Moreover, glassy barrier layers may aid in the loss of F during consolidation.  
      In accordance with another embodiment of the invention, a method of manufacturing an optical fiber preform having at least one glassy barrier layer is provided. The glassy barrier layer is a thin layer of vitrified or partially vitrified glass that minimizes migration of a dopant or water within the preform. In a preferred embodiment, a first soot segment is formed. A first portion of the first soot segment is then vitrified to form the at least one glassy barrier layer. Additionally, prior to consolidation of a remaining portion of the first soot segment, a second soot segment may be deposited on the at least one glassy barrier layer. Multiple glassy barrier layers may also be utilized within a preform.  
      Another embodiment provides an optical fiber soot preform comprising first and second soot segments and a vitrified barrier layer therebetween. In all cases, the barrier layer preferably has a thickness of less than about 200 μm, more preferably less than about 100 μm, more preferably yet of less than about 30 μm, and most preferably in a range between 10 μm and about 200 μm. The barrier layers may be formed by a variety of methods as described herein. Laser and Induction heater methods and apparatus for forming the glassy barrier layer are described herein.  
      A method for producing an optical fiber preform of another embodiment of the invention comprises generating a flame from a combustion burner by igniting a substantially hydrogen-free fuel, flowing into the flame either a silicon-and-fluorine containing precursor or a silicon precursor and a separate fluorine or fluorine-containing compound thereby producing fluorine-doped, silica-containing soot. The soot is then deposited onto a substrate to form an optical fiber preform. Preferably, the silicon-and-fluorine containing precursor is selected from a group consisting of SiF 4  and chlorofluorosilanes. The separate fluorine or fluorine-containing compound is preferably selected from a group consisting of F, F 2 , CF 4 , C 2 F 6 , SF 6 , NF 3 , and combinations thereof. The substantially hydrogen-free fuel may be any of those described before.  
      In accordance with another embodiment of the invention, a method of forming a silica-containing soot is provided wherein a chlorine, fluorine, and silica containing glass precursor is reacted. According to the method, the silica-containing soot is generated by reacting, preferably in a flame, a chlorine, fluorine, and silica containing compound in a deposition (laydown) process. The reaction results in generation of a fluorinated silica-containing soot. Most preferably, the chlorine, fluorine, and silica-containing compound comprises a chlorofluorosilane. Exemplary embodiments include SiCl 3 F, SiCl 2 F 2 , or SiClF 3 . In a preferred embodiment, the chlorine, fluorine, and silica containing compound is mixed in gaseous form with a dilutent gas prior to the step of reacting thereby readily enabling the control of the amount of fluorine contained in the soot.  
      A method for producing a vitrified glass article is provided in another embodiment of the invention. The inventive method comprising several steps. First, heat is generated from a combustion burner having a flame produced by igniting a substantially hydrogen-free fuel. According to the invention, the flame is the only source of heat. Next, a glass precursor is flowed into the flame to produce silica-containing soot. Finally, the silica-containing soot is deposited onto a substrate and substantially simultaneously converted (by the heat of the flame) to form the vitrified glass article by the heat of the flame. In a preferred embodiment, soot is deposited onto a silica-containing glass member, such as a High Purity Fused Silica (HPFS) puck. According to this method, the vitrified glass article contains very low amounts of water. The step of depositing preferably takes place within a chamber that may include a purge gas, such as nitrogen provided thereto. This method is adapted for producing HPFS glass, for example, that may be used in photomask applications.  
      According to another embodiment, a combustion burner is provided. The burner is adapted for forming silica-containing soot, vitrified glass, and optical fiber soot preforms. The burner comprises a fume passage adapted to supply, at a first flow rate, a glass precursor, and a fuel passage surrounding the fume passage, the fuel passage adapted to supply a substantially hydrogen-free fuel at a flow rate at least 20 times the first flow rate. The burner may also include an inner shield passage between the fuel passage and the fume passage adapted to supply at least oxygen. The burner may further comprise an outer shield passage surrounding the fuel passage for introduction of additional gasses.  
      In accordance with another embodiment of the invention, a method of producing a fluorine-doped article is provided. The method comprises the step of depositing fluorinated, silica-containing soot containing greater than 0.5% by weight of fluorine by supplying into a flame, in an amount less than 0.5 liters/minute, a fluorine or a fluorine-containing compound. According to this embodiment, efficient incorporation of fluorine onto the soot preform is accomplished. Preferably, the fluorine is included in the silica-containing soot in an amount greater than 1% by weight. The fluorine or fluorine-containing compound may be supplied from an expelling element into the flame or incorporated directly in a fluorine-containing glass precursor, for example, chlorofluorosilane. Most preferably, the flame combusts a substantially hydrogen-free fuel, such as carbon monoxide or the other substantially-hydrogen free fuels, and the fluorine-doped soot is formed within a substantially water-free atmosphere.  
      According to another embodiment of the invention, a method of manufacturing an optical fiber preform is provided. The method includes a step of depositing soot onto a substrate within a substantially water-free atmosphere. The substantially water-free atmosphere preferably is dried air containing less than about 1000 ppm water vapor, more preferably less than 100 ppm water vapor, more preferably less than 10 ppm water vapor, more preferably yet less than 3 ppm water vapor, and most preferably less than 1 ppm water vapor. The substantially water-free atmosphere may be dry nitrogen, dry argon, dry helium, or combinations thereof or dry oxygen, dry carbon dioxide, or combinations thereof. In accordance with another measure, the substantially water-free environment preferably comprises less than 1% relative humidity at a temperature range between about −67° C. and about 125° C. It should be recognized that any significant reduction in the atmospheric water supplied to the preform may advantageously reduce the length of later drying steps.  
      In another method of manufacturing an optical fiber preform in accordance with the invention, a soot preform is transferred while subjecting the soot preform to a substantially water-free atmosphere. Thus, the preform is not contaminated with water in the transfer step while in transit to additional production operations such as from deposition to a consolidation furnace or holding furnace. In accordance with the invention, a soot preform is formed at a first location. The preform is then transferred to a second location for further processing and during such transfer, subject to a substantially water-free atmosphere. During the transfer, the preform is preferably inserted into a carrier container. Most preferably, the carrier container is subjected to a purge of substantially dry gas, such as dried air, dry nitrogen, dry oxygen, dry argon, dry helium, dry carbon dioxide, and combinations thereof.  
      A method of producing silica soot in accordance with another embodiment of the invention comprises a step of supplying a combination of combustion-enhancing fuel additive and substantially hydrogen-free fuel to a burner. Preferably, the substantially hydrogen-free fuel is selected from a group consisting of carbon monoxide (CO), carbon suboxide (C 3 O 2 ), and carbonyl sulfide (COS). The combustion-enhancing fuel additive is preferably a catalyst, an energetic fuel, or an energetic oxidizer. These additives either increase the burning speed of the substantially hydrogen-free fuel or increase the flame temperature. This advantageously improves the flame&#39;s burning rate, heat, and structure. The combustion-enhancing fuel additive is preferably supplied in an amount of less than about 50% by volume of the substantially hydrogen-free fuel, more preferably less than about 20%, more preferably yet less than 5%, and most preferably less than about 1% by volume of the substantially hydrogen-free fuel. It was discovered by the inventors hereof that small amounts of such additives (less than 5% of substantially hydrogen-free fuel) are needed to increase the burning speed of the substantially hydrogen-free fuel such that the flame remains properly attached to the burner. Larger amounts may be needed when depositing certain dopants, such as germania, to achieve the desired dopant concentrations in the soot.  
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       FIG. 1  illustrates a schematic depiction of an apparatus and method in accordance with the invention for forming substantially water-free optical fiber soot preforms.  
       FIG. 2  illustrates an end view of a soot preform mounted within a carrier container.  
       FIG. 3  is a partially cross-sectioned side view of the soot preform positioned within the carrier container of  FIG. 2  and being purged with a dry gas.  
       FIG. 4-7  illustrate various examples of refractive index profiles that may be manufactured in accordance with the present invention.  
       FIG. 8-11  illustrate various cross-sectional end views of preforms manufactured in accordance with the invention that include barrier layers.  
       FIG. 12  illustrates a top view of a ring for dispensing generally radially directed streams of fluorine into the burner flame.  
       FIG. 13  illustrates a cross-sectioned isometric view of the ring taken along section line  13 - 13  of  FIG. 12 .  
       FIGS. 14 and 15  illustrate partially cross-sectioned views of the soot preform mounted in the carrier container with the bait rod inserted and removed, respectively.  
       FIGS. 16 and 17  illustrate partially cross-sectioned views of the soot preform inserted within consolidation furnaces.  
       FIG. 18  illustrates a schematic view of the soot preform wherein soot is being deposited by a conventional method wherein the conventional burner is mounted alongside a carbon monoxide burner.  
       FIG. 19  illustrates a schematic view of multi-segment soot preform being formed in a one step method in accordance with the invention.  
       FIG. 20  illustrates a schematic view of an apparatus and method of forming a soot preform wherein the exhaust gasses are recycled.  
       FIG. 21  illustrates a cross-sectional side view of a combustion burner apparatus in accordance with an embodiment of the invention.  
       FIG. 22  illustrates a partially cross-sectional side view of a core cane drawing apparatus in accordance with an embodiment of the invention.  
       FIG. 23  illustrates a partially cross-sectional side view of a optical fiber draw apparatus in accordance with an embodiment of the invention.  
       FIGS. 24 and 25  illustrate cross-sectional side views of optical fiber preform in accordance with an embodiment of the invention.  
       FIG. 26  illustrates a partially cross-sectional side view of a optical fiber preform having a glassy barrier layer formed thereon in accordance with an embodiment of the invention.  
       FIG. 27-29  illustrate various top views of an apparatus and method for forming a glassy barrier layer on an optical fiber preform in accordance with an embodiment of the invention.  
       FIG. 30  illustrates a partially cross-sectional side view of a method and apparatus for forming substantially water free high purity fused silica in accordance with an embodiment of the invention.  
       FIG. 31  illustrates a detailed cross-sectional side view of the combustion burner apparatus of  FIG. 30  in accordance with an embodiment of the invention.  
       FIG. 32  illustrates a cross-sectional side view of a preform including a barrier layer in accordance with an embodiment of the invention.  
       FIG. 33  illustrates a perspective view of an end burner in accordance with an embodiment of the invention.  
       FIG. 34  illustrates a side cross-sectional view of an end burner along line  34 - 34  of  FIG. 33 .  
       FIG. 35  illustrates a schematic view of burner and supply system utilizing a combination of combustion-enhancing additive and substantially hydrogen-free fuels.  
       FIG. 36  illustrates a partially cross-sectioned top view of a lathe assembly with an attached induction heater assembly in accordance with an embodiment of the invention.  
       FIG. 37  illustrates a graphical plot of thickness versus density of a glassy barrier layer in accordance with an embodiment of the present invention.  
       FIGS. 38 and 39  illustrate partially cross-sectional views of a deflector assembly in accordance with an embodiment of the invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Reference will now be made in detail to the present preferred embodiments of the invention with reference to the attached drawings. Wherever possible, the same or similar reference numerals shall be used throughout to refer to the same or like parts.  
      According to a first embodiment of the present invention, a method and apparatus of manufacturing a substantially water-free optical fiber soot preform  20  is provided. As best illustrated in  FIG. 1 , the method for forming the soot preform  20  comprises the steps of generating heat from a combustion burner  25  (the details of several desirable burners are described with reference to  FIGS. 21 and 31 ) having a flame  28  produced by igniting a substantially hydrogen-free fuel  26 , flowing a glass precursor  24  into the flame  28  to produce a silica-containing soot  30 , and depositing the soot  30  onto a rotating substrate  32 . The substantially hydrogen-free fuel  26  may vary widely. Examples include carbon monoxide (CO), carbon suboxide (C 3 O 2 ), carbonyl sulfide (COS), and the like.  
      The glass precursor  24  is provided to the burner  25  and is oxidized in the flame to form the soot. The glass precursor  24  is preferably also substantially hydrogen-free and may vary widely. Typical examples include silicon carbide (SiC), silicon monoxide (SiO), silicon nitride (Si 3 N 4 ), silicon tetrabromide (SiBr 4 ), silicon tetrachloride (SiCl 4 ), silicon tetraiodide (SiT 4 ), silica (SiO 2 ), and Si(NCO) 4 .  
      The glass precursor  24  for producing the core soot region of the preform  20  may preferably include an additional compound selected from a group consisting of a germanium-containing compound such as GeCl 4 , a fluorine-containing compound such as silicon tetrafluoride (SiF 4 ), or other suitable up or down dopants to enable obtaining the desired refractive index profile. Most preferably, the glass precursor  24  utilized to form fluorine doped soot  30  in the preform  20  is a compound that is selected from a group consisting of fluorohalocarbons, chlorofluorosilanes, CF 4 , or SiF 4 , NF 3 , SF 6 , and combinations thereof. Preferably, the precursor  24  is provided in gaseous form to the burner  25 . Although if a liquid burner is utilized, the fuel may be supplied in liquid form. An optional gasifier apparatus  24   a,  gasifies the liquid precursor, if stored in liquid form. For example, SiCl 4 , GeCl 4 , Si(NCO) 4 , and Ge(NCO) 4  precursors are provided in liquid form at room temperature and therefore require gasification. Any known method of gasification may be employed, such as heating the precursor and bubbling a carrier gas (such as N 2 , O 2 , Ar, or He) through it. Although, the burner illustrated in  FIG. 21  is preferred, optionally a liquid atomizing burner, such as described in WO 99/32410 filed Dec. 3, 1998 and entitled “Burner And Method For Producing Metal Oxide Soot” may be employed. The substrate  32  onto which the soot is deposited comprises, during the startup of deposition, a rotating alumina bait rod or a glass core cane, and thereafter when some soot has been deposited, the substrate becomes the soot already deposited.  
      The substrate is preferably rotated by a motor  22  and is supported at the other end by any suitable end support member  23 , such as a V-block, which allows rotation and provides radial motion restraint. The end support  23  and motor  22  are mounted to a common frame member which preferably traverses back and forth relative to the housing  36  as indicated by double ended arrows “a” and “b” thereby moving the bait rod and soot preform  20  transversely along its axial axis relative to the soot generating burner  25 . Optionally, the burner  25  may be traversed back and forth as indicated by dashed arrow “c” while soot preform  20  is held stationary along the transverse axis and is simply rotated. The preferred rotation speed in accordance with the invention is about 180 rpm. In a preferred embodiment, preferably at least a portion of the combustion burner  25  is mounted within chamber  36   a,  of the housing  36 .  
      Fluorine and Substantially H-Free Fuel Combination  
      According to another embodiment of the invention, a method for producing an optical fiber preform is provided comprising the combination of fluorine doping and utilizing a burner that combusts a substantially H-free fuel. In particular, the use of a dry combustion source minimizes the water (H and OH) within the soot and, thus, the propensity of the highly mobile fluorine molecule from moving around through the soot matrix. In particular, as shown in  FIG. 1 , the method includes generating a flame  28  from a combustion burner  25  by igniting a substantially hydrogen-free fuel  26 , flowing into the flame, a silicon-and-fluorine containing precursor  24  or a silicon precursor  24  and a F source  37  such as fluorine or fluorine-containing compound whereby a fluorine-doped, silica-containing soot is produced, and depositing the soot onto a substrate  32  to form a fluorine-doped optical fiber preform. In one species, the silicon-and-fluorine containing glass precursor  24  further comprises chlorine. Preferably, the substantially hydrogen-free fuel  26  comprises carbon monoxide. However, any of the other aforementioned substantially hydrogen-free fuels may be utilized. Exemplary embodiments of the separate fluorine or fluorine-containing compound  37  are preferably F, F 2 , CF 4 , C 2 F 6 , SF 6 , NF 3 , and combinations thereof. However, other suitable fluorine-containing compounds may be utilized as well. Most preferably, the silicon-and-fluorine containing precursor  24  is selected from a group consisting of SiF 4  and chlorofluorosilanes. Notably, other silicon-and-fluorine containing precursors may be utilized also.  
      Chlorofluorosilane Precursor  
      One particularly promising class of glass precursor compounds for the formation of silica-and-fluorine containing soot within an optical soot preform  20  are chlorine, fluorine, and silica containing compounds. According to one method and apparatus for producing silica-containing soot, as shown in  FIG. 19 , in a deposition lathe  18   c,  a chlorine, fluorine, and silica containing compound  24   b,  is chemically reacted and oxidized, preferably in a flame  28  of a combustion burner  25 , during deposition; the reaction resulting in the generation of a fluorinated silica-containing soot  30 . The fluorinated soot is utilized to form a fluorinated segment within an optical fiber preform  20 , such as a core portion or a clad segment.  
      Preferably, the soot  30  is formed by reacting the precursor  24   b,  in a flame  28  formed by igniting a substantially hydrogen-free fuel, such as CO, in the presence of a combustion supporting gas  21 . This, as should now be recognized, forms substantially water-free fluorine-doped soot. As before, any of the afore-mentioned substantially H-free fuel alternatives may also be employed. Suitable valves such as Mass Flow Controllers (MFCs) control the flows of fuels and compounds herein. Most preferably, forming the fluorinated silica-containing soot  30  occurs within a substantially water-free atmosphere  34  provided within a housing  36 , such as has been shown and described with respect to  FIG. 1 , for example.  
      As in the previously mentioned embodiments, the fluorinated silica-containing soot  30  is deposited onto a preferably rotating substrate to form a soot preform  20 . The soot preform  20  is then dried and sintered in accordance with conventional methods or the methods described herein with reference to  FIGS. 16 and 17 . After being vitrified into a glass preform  76  ( FIG. 23 ), the preform is connected to a handle  78 , lowered into a draw furnace  80  and heated by the heat source  82  causing the lower end of the preform  76  to soften. The softened glass falls from the preform  76  as a gob and is threaded through the various components of the draw apparatus (only a portion of which is shown). Once threaded, down feed mechanisms (not shown) draw the glass fiber  84  from the preform  76  at precisely the desired rate to produce the proper diameter fiber. More details of fiber drawing may be found in U.S. Pat. No. 5,284,499.  
      Again referring to  FIG. 19 , the chlorine, fluorine, and silica containing compound illustrated as the glass precursor  24   b,  in gaseous form is preferably mixed with a dilutent gas  24   c,  prior to the step of reacting. The step of mixing the chlorine, fluorine, and silica containing compound  24   b,  with a silica-and-chlorine containing compound  24   c,  is performed to adjust and/or achieve a desired level of fluorine in the soot. The mixing is performed by suitable valves or mass flow controllers  85 . In a preferred embodiment, the dilutent gas is a glass precursor  24   d,  such as a silica-and-chlorine compound, for example SiCl 4  that has been gasified by gasifier. Preferably, the chlorine, fluorine, and silica containing compound  24   b,  comprises a chlorofluorosilane. Exemplary embodiments include SiCl 3 F, SiCl 2 F 2 , and SiClF 3 . Other embodiments may be utilized as well. By utilizing the chlorine, fluorine, and silica containing compound, the fluorinated silica-containing soot may be made to contain greater than about 0.5% by weight of fluorine, and more preferably greater than 1% by weight.  
      Dry End Burner  
      End burners  39  as illustrated in  FIGS. 1 and 33 - 34  are also preferably included within the chamber  36   a.  The flames thereof are directed at the unusable ends of the soot preform  20  and function to prevent crazing and thermal shock in the preform  20 . At least one end burner  39  in accordance with another embodiment of the invention (as shown in  FIGS. 33 and 34 ) is preferably supplied with, and operates on, at least a substantially hydrogen-free fuel  26 , such as carbon monoxide. Flow rates of about 15-20 liters per minute of CO have been utilized, for example. The substantially hydrogen-free fuel  26  is preferably mixed and used in combination with a combustion supporting gas, such as oxygen. Flow rates of oxygen to the burner are preferably 5-7 liters per minute. As shown in  FIGS. 33 and 34 , the end burners  39  preferably comprise a hollow box-like burner housing  81  with a closed end tube  83  inserted and sealed therein. Gas flows pass into the tube  83  and out through a plurality of distribution ports  83   a,  approximately seven having diameters of between about 0.040 inch (1.02 mm) and 0.080 inch (2.03 mm) that are spaced equally along the tube. The gas then flows out through the face ports  87   a,  formed through the face  87  of the burner housing  81 . The face ports  87   a,  preferably have a diameter of about 0.046 inch (1.17 mm) and there are 50-100 ports depending upon the size of the preform and which end of the preform. The gas flowing out of the ports  83   a,  is ignited to form the end burner flames.  
      As is illustrated in  FIG. 1 , most preferably the burners  39  are supplied at both ends of the preform and both operate on a substantially hydrogen-free fuel. The supply system to the end burners is not illustrated for clarity, but is preferably of conventional construction. The end burners  39  preferably are mounted to the aforementioned frame member such that they are stationary relative thereto. Thus, the burners  39  are continuously substantially aligned with the respective ends of the preform  20 . The use of end burners  39  that operate on substantially hydrogen-free fuel is another mechanism to minimize generation of, and thus exposure of the preform  20  to water vapor thereby resulting in drier soot. In a variant system, where the burner reciprocates, the end burners would be stationary.  
      Substantially Water-Free Atmosphere  
      In accordance with another embodiment of the invention, in order to prevent infiltration of water (H, OH, H 2 0) into the soot preform  20  from moisture in the atmosphere, the preform  20  is preferably included in or otherwise exposed to a substantially water-free atmosphere  34  during the step of depositing. The exposure is preferably accomplished by including the preform within a chamber  36   a,  of a housing  36  or to a shroud of the substantially water-free atmosphere. The substantially water-free atmosphere is then supplied to the housing by a substantially dry environment supply system  47 . The substantially water-free atmosphere  34  is preferably selected from a group of dry gasses such as dried air, dry nitrogen, dry oxygen, dry argon, dry helium, dry carbon dioxide, or combinations thereof. However, any suitable substantially dry environment may be employed such that low levels of water vapor are present therein.  
      In particular, one preferred method and apparatus (shown in  FIG. 1 ) of providing a substantially water-free environment in accordance with the invention is accomplished by passing a supply gas  49 , such as outside or room air, through a chilling apparatus  50 . The chiller  50  reduces the temperature of the supply gas  49  to approximately −40° C. This apparatus  47  can readily reduce water content in the supply gas to a first humidity level of less than about 1000 ppm water vapor. Optionally, the chilled supply gas  51  may be passed through a molecular sieve apparatus  52  if it is desirable to remove further water vapor. Upon exiting the sieve  52 , the substantially water-free atmosphere having a second humidity level is provided through an input duct  53  into the chamber  36   a,  of housing  36 .  
      The flow of the shroud of substantially water-free gas flows over the burner  25  and preferably over the entire length of the preform  20 . The velocity of the flow is preferably sufficiently low such that a uniform and laminar flow is supplied to the preform  20 . A flow rate of between about 150-500 cfm, and more preferably between 200-350 cfm is desirable. The flow of dry gas continues across the preform  20  and exits through exhaust  55  carrying with it any non-deposited by-products of the soot formation reaction and fuel ignition, such as CO 2 , SiO 2 , and GeO 2 . An optional scrubber  58  may be employed to reclaim or remove any particulate matter or remove any undesirable reaction by-products. This may comprise one or more pieces of equipment.  
      It should be recognized that this embodiment of the invention whereby a substantially water-free environment  34  is provided to the preform  20  during deposition may be utilized in combination with utilizing substantially hydrogen-free fuel. This combination will have the excellent benefits in terms of minimizing water (H, OH) within the soot preform  20 . Moreover, in this combination, advantageously neither the atmosphere nor the combustion process contributes to any appreciable inclusion of water into the soot preform  20 .  
      In more detail, it is preferred that the supply system  47  conditions the supply gas  49  containing water vapor to the point where a significant portion of the water is removed, hence the terms “substantially water-free atmosphere” or “substantially dry atmosphere.” Preferably, according to one measure, the water vapor content is less than 1% relative humidity at a temperature range between about −67° C. and about 125° C., and more preferably less than 0.1%. In terms of ppm water vapor, the substantially water-free atmosphere preferably exhibits less than 100 ppm, more preferably less than about 100 ppm water vapor, more preferably, less than about 10 ppm water vapor, even more preferably less than about 3 ppm, and most preferably less than about 1 ppm water vapor. Notably, it should be recognized that any significant reduction in water vapor in comparison to the prior art methane/oxygen technology operating in room humidity levels will reduce the time required for any drying step later in the preform treatment process. Therefore, it should be recognized that the present invention reduces production costs and time to produce a preform from which fiber may be drawn.  
       FIG. 20  illustrates a lather deposition apparatus  18   d,  wherein a soot preform  20   e,  is being formed in a chamber within a housing  36   d.  Flowing from a supply system  47   d,  is a substantially dry environment  34   d.  The system  47   d,  includes a chiller  50   d,  and an optional molecular sieve  52   d  as heretofore mentioned. Also included is a remover  58   d,  which functions to remove various unwanted compounds, gasses and soot materials from the exhaust flow  55   d,  such as, for example, GeO 2 , SiO 2 , Cl 2 , CO 2 , COF2, F 2 , SiF 4 , or CF 4 . The remover unit  58   d,  may comprise one or more pieces of equipment for removing such unwanted exhaust contaminants. For example, particulate matter may be removed by a particulate separator system.  
      Various wet scrubber systems or thermal reactors are available for removing the other compounds and gasses. The main difference in this embodiment as compared to that previously described in  FIG. 1  is that the exhaust flow  55   d,  is recycled through the supply system  47   d,  and then redelivered to the supply input  53   d.  This may have the advantage of allowing the chiller  50   d,  to work more efficiently, as the water vapor levels may already be very low. A vent may be required in the system because of the addition of combustion gasses.  
      According to another embodiment of the invention, the soot preform  20  is transferred from the soot deposition chamber  36   a,  to another location for further processing and is preferably included within a substantially water-free atmosphere  34   a,  during this transfer. The transfer may be, for example, to a holding oven or furnace or consolidation or draw furnace or the like. As best illustrated in  FIGS. 1, 2  and  3 , the preferred method for exposing the preform to substantially dry gas during transfer is illustrated. The method and apparatus comprises inserting the preform  20  into a carrier container  38  that includes a substantially water-free environment  34   a.  As shown in  FIG. 1 , the carrier container  38  may be inserted or housed within or otherwise directly connected to the chamber  36   a,  such that the preform may be readily inserted therein. In the illustrated embodiment, the preform  20  is inserted in the carrier  38 , then the carrier  38  is closed and removed through an exit door  36   b.  The carrier  38  may be manufactured from an inert material, for example, a high purity fused silica. The preform  20  is preferably transferred into the carrier container  38  by a robot or manually by an operator utilizing gloves (not shown) sealed to the housing  36  and which traverse into the chamber  36   a,  of the housing. In this way, the preform  20  is always exposed to the substantially water-free environment as it is being loaded into the carrier  38 .  
      Preferably, as illustrated in  FIG. 2 , the container  38  includes a clam shell construction with opposing halves  38   a,    38   b.  When closed, the preform  20  is housed within and preferably suitably secured between the halves  38   a,    38   b.  The halves, when closed, form a cavity  38   c,  into which the substantially water-free environment  34   a,  is placed. Initially, upon placement of the preform into the cavity  38   c,  the environment is the same as that provided by the supply system  47  ( FIG. 1 ). Prior to removal of the container from the chamber  36   a,  a substantially water-free environment  34   a,  is formed by subjecting the preform  20  to a purge of substantially dry gas ( FIG. 3 ). The substantially dry gas purge is continued, preferably substantially continuously, during the step of transferring to the next process. As shown in  FIG. 3 , the substantially dry gas  43  is provide into one end of the container  38 , thus filling the cavity  38   c,  with the substantially dry gas. The substantially dry gas preferably exits through a purge hole  38   d,  on the other end of the container.  
      The substantially dry gas  43  utilized in the purge is preferably selected from a group consisting of dry air, dry oxygen, dry carbon dioxide, and combinations thereof Further, dry argon, dry nitrogen or dry helium may be utilized. However, any suitable substantially dry environment may be employed for the purge step. By the term substantially dry gas or substantially dry environment, it is desired that the gas have less than about 1000 ppm water vapor, more preferably less than 100 ppm water, more preferably less than 10 ppm water vapor, more preferably yet less than about 3 ppm water vapor, and most preferably less than about 1 ppm water vapor.  
      As shown in  FIG. 3 , during transfer, a canister  43  of substantially dry gas is connected to the container  38  by a suitable fitting  38   e.  Flow is initiated and controlled by a suitable valve  43   a,  such that a slight positive pressure in the chamber  38   c,  of container  38  is maintained. This minimizes flow of atmosphere into the chamber  38   c.    
      As shown in  FIGS. 14 and 15 , upon removal or disconnection from the housing chamber  36   a,  the container  38  and preform  20  are preferably moved to an upright position ( FIG. 14 ) and the bait rod is removed. This leaves the soot preform  20  suspended by handle  20   a,  within the container  38 , all the while being subjected to the substantially dry environment  34   a,  provided by dry gas supply  43 . Upon removal of the bait rod, it may be desirable to supply dry atmosphere  34   a,  down the centerline aperture formed by attachment of a suitable fitting  20   b,  to handle  20   a,  as shown in  FIG. 15 .  
      Next, as illustrated in  FIG. 16 , and in accordance with another embodiment of the invention, the container  38  with preform  20  mounted therein may be lowered into the muffle tube  57   a,  of consolidation furnace  54  by a hollow furnace handle  54   a,  preferably with the supply of dry gas  43  still attached. The furnace preferably includes a susceptor  57   b,  insulation  57   c,  and heat inducing coil  56  as is conventional practice. In accordance with a preferred embodiment of  FIG. 17 , the aforementioned muffle tube may be entirely replaced with the container  38  made of high purity fused silica, thus eliminating one expensive component from the furnace  54 . Thus, it should be recognized that in accordance with this embodiment of  FIG. 17 , the muffle tube of the consolidation furnace  54  is formed from the wall  38   f,  of the carrier container  38  used to transport the soot preform  20  from the deposition step. However, it should be recognized that the consolidation process of the preform  20  preferably takes place within the container  38  in both of the aforementioned embodiments.  
      Once the container  38  and preform  20  are properly positioned in the furnace  54 , a supply of drying and/or consolidation gasses are then supplied to the furnace  54  preferably through the hollow handle  54   a.  After this, the supply  43  may be removed. If the aforementioned dry processes have been employed, it may be possible to eliminate the drying step altogether or at least substantially shorten it, as the soot preform has remained substantially dry throughout the steps of deposition and transport. The preform  20  is then consolidated in an atmosphere of helium at between about 1200° C. and 1600° C. to transform the soot preform into a vitrified article. Although, the container  38  is shown in the previous two embodiments as being inserted in the furnace, alternatively, the preform may be quickly removed under, for example a shroud of substantially dry gas and quickly inserted into the consolidation furnace  54 . It should also be recognized that when the various inventive features described herein are employed either individually or in combination, the length of the drying time is substantially reduced and may be eliminated. This reduces the total process time for producing consolidated preforms and the amount of chlorine that needs to be utilized. In addition, better attenuation characteristics are conceivable.  
      According to the invention, and again referring to  FIG. 1 , the substantially hydrogen-free fuel  26  utilized and provided to the burner  25  preferably comprises carbon monoxide. By the term, substantially hydrogen-free fuel, what is meant is that no hydrogen is present in the general chemical structure and that low amounts of contaminants (less than about 0.5%) are present, such as water. A combustion supporting gas  21 , such as oxygen, is preferably mixed with the hydrogen-free fuel in gaseous form by a mixing apparatus or other suitable flow controls  26   a.  Oxygen is preferably mixed with CO in a ratio of about 2:1, for example.  
      Dry fluorine-doped soot may be generated in accordance with another embodiment of the invention. Fluorine doped, silica-containing soot  30  is preferably generated by flowing fluorine or a fluorine-containing compound into the flame  28  produced by igniting a substantially-hydrogen free fuel  26 . The step of flowing may be accomplished by flowing fluorine or a fluorine-containing compound directly into the flame  28  from a fluorine source  37  or by incorporating the fluorine or fluorine-containing compound into or as an integral part of the chemical structure of the glass precursor  24 . The fluorine-containing compound is preferably selected from a group consisting of F 2 , CF 4 , C 2 F 6 , SF 6 , NF 3 , SiF 4 , chlorofluorosilanes, chlorofluorocarbons, and combinations thereof.  
      In one embodiment, within the lather deposition apparatus  18   a,  the fluorine or fluorine-containing compound is flowed into the flame  28 , from an expelling element  48  at least partly surrounding the flame  28  as shown in  FIGS. 1, 12  and  13 . The fluorine, or fluorine-containing compound is provided by suitable tubing (not shown) to an input connection  33   a,  on the hollow annular ring  33 . The fluorine or fluorine-containing compound is distributed around a hollow annular channel  33   b,  in the ring  33  and is expelled from a plurality of radially inward directed ports  33   c,  formed in the ring  33 . The ports  33   c,  may include a sight upward component as well.  
      Optionally, the fluorine or fluorine containing-compound may be emitted from an outer shield included within the combustion burner  25  as will be described with reference to  FIG. 31 .  
      It should be understood that the use of a substantially hydrogen-free fuel  26 , such as carbon monoxide, enables manufacture of fiber profiles that heretofore had to be manufactured with detailed multi-step processes. Therefore, use of the process in accordance with the invention reduces manufacturing cost and process time. According to an exemplary embodiment of the invention whose refractive index profile is shown in  FIG. 4 , a first segment  40  including silica-containing soot is laid down onto the rotating substrate  32  as shown in  FIG. 1 . The first soot segment  40 ′, as shown in  FIG. 8 , preferably includes a dopant such as a germania dopant to produce the desired refractive index profile in the consolidated preform and, thus, in the end product optical fiber.  
      Next, a second soot segment  42 ′ of preform  20   a,  is laid down by a deposition adjacent to the first soot segment  40 ′. Preferably, the second soot segment  42 ′ includes a fluorine dopant thereby depressing the refractive index of segment  42  ( FIG. 4 ) below that of pure silica and creating a moat region in the profile. It should be recognized that these two segments may be deposited one after another, because the water introduced in the deposition process is minimized. Thus, the dopants have a tendency to migrate less within the soot preform  20   a.  In the prior art, the first segment was formed as a core cane and then silica soot was deposited to form the second soot segment followed by doping of the second segment soot in consolidation furnace.  
      Glassy Barrier Layer  
      In accordance with another embodiment of the invention, at least one glassy barrier layer (e.g.,  35   a ) is formed in the soot preform during the deposition step. As best illustrated in  FIG. 8 , the glassy barrier layer  35   a,  is preferably a thin layer of vitrified glass. The barrier layer  35   a,  functions to substantially minimize the migration of any dopant (as well as water (H, OH)) present between segments of the soot preform, for example between a first and second annular segments  40 ′,  42 ′. The term “glassy” as used herein encompasses both fully vitrified glass as well as a partially vitrified glass. The layer only needs to be sufficiently vitrified (glassy) to substantially minimize migration of the dopant and/or water.  
      In one embodiment, the glassy barrier layer  35   a,  is formed by subjecting the thin layer of soot to sufficient heat to fully vitrifying it into a consolidated glass. First, a first soot segment  40 ′ is formed. A first portion of the first soot segment is then vitrified to form the at least one glassy barrier layer  35   a.  Finally, prior to consolidation of a remaining portion of the first soot segment  40 ′, a second soot segment  42 ′ is deposited onto the at least one glassy barrier layer  35   a.  The glassy barrier layer  35   a,  is effective at reducing the migration of any dopant, such as fluorine, from one segment to the other segment adjacent to the barrier. It should be understood that not only may the glassy barrier layer be formed on an outer radial periphery of the first soot segment, but it also may be formed on an inner radial periphery of the second soot segment. Barriers are particularly important when fluorine is present in an amount greater than 1.0% by weight in at least a portion of the segment.  
      The glassy barrier layer has the distinct advantage of allowing the manufacture of sharp segment transitions in the consolidated preform. Sharp, non-rounded, transitions resultantly improve both fiber attenuation and bend performance.  
      Preferably, the glassy barrier layer  35   a,  has a thickness “t” ( FIG. 24 ) of less than about 200 μm, more preferably less than about 100 μm, more preferably yet, less than about 30 μm, and most preferably between about 10 μm and 200 μm. In the embodiment shown in  FIG. 8 , the glassy barrier layer  35   a,  is formed within the soot preform  20   a,  and includes soot on both the inner and outer radial sides thereof. Preferably, the barrier layer  35   a,  is formed along the entire length of the preform, thus forming a tubular shaped structure. Optionally the barrier  35  may even be formed over the unusable end portions of the preform  20   d,  at the ends as shown in  FIG. 32 . This functions to seal the soot segment  44 ′ which contains the dry soot manufactured in accordance with one or more of the aforementioned substantially dry processes.  
      Barrier layers are particularly effective at minimizing the migration of fluorine, which is generally very mobile because of its small molecular size and activity. Thus, if, for example, in  FIG. 8 , the second soot segment  42 ′ includes a fluorine dopant, then the barrier layer  35   a,  will minimize the migration of fluorine from the second segment  42 ′ into the first soot segment  40 ′.  
      In accordance with another embodiment of the invention, multiple barrier layers may be employed in optical fiber soot preforms. Such multi-barrier layers are useful in the manufacture of multi-segment core preforms, for example. In  FIGS. 9 and 10 , a third soot segment  44 ′ is laid down adjacent to the second soot segment  42 ′ and over a second barrier layer  35   b.  The second barrier layer  35   b,  prevents any dopants from migrating out of the soot layer  42 ′ and into the soot layer  44 ′ of the preforms  20   b,    20   c,  and visa versa. Likewise, as shown in  FIG. 10 , a fourth layer  46 ′ and a third barrier layer  35   c,  may be formed. In accordance with one embodiment, the fourth layer  46 ′ is also fluorine doped, whereas the third layer is preferably germania doped. Additional glassy barrier layers may be employed as needed. Because the layers (e.g.,  42 ′,  44 ′) in-between the barrier layers ( 35   a;    35   b,    35   c ) are substantially dry, the water removal in consolidation of these multi-segment preforms is not seen as an issue.  
       FIG. 8-10  illustrate soot preforms  20   a - c  that are formed on a mandrel (thus forming the centerline aperture upon its removal) in one deposition step, i.e., without any intermediate consolidation step of the first formed segments. The  FIG. 9  soot preform will produce a core cane having a refractive index profile like that shown in  FIG. 5 . Likewise, the soot preform of  FIG. 10  will produce a core cane having a refractive index profile as shown in  FIG. 6 . Additional silica soot may be deposited on the formed core canes once formed from the soot preforms of  FIG. 8-10 . The deposition process for the additional silica soot may be the substantially dry process described herein or by conventional deposition methods.  
       FIGS. 11 and 32  illustrate a preform  20   d,  that is manufactured in accordance with a two or more step process. The core cane  32  (which preferably includes some or all of the physical core portion of the final preform) is manufactured in a first step from a core preform in accordance with the process shown in  FIG. 22 . The core cane  32  is manufactured by inserting the consolidated preform  90  into a furnace  91 . The preform  90  is then heated and melted at between about 1800° C. 2200° C. and drawn into a slender rod of diameter d c . A tractor assembly  92  applies tension while the preform  90  is lowered at a preferably constant downfeed rate into the furnace  91 . A diameter sensor  93  senses the diameter and sends a signal to a control system  94  that controls the speed of the tractor downfeed rate to maintain the desired set diameter. Optionally, the control system  94  may variably control the downfeed rate. Once an appropriate length of cane  32  is been drawn, a cutter  95 , such as a flame cutter or scoring apparatus, is activated to cut the core cane  32  into the desired length. The core cane  32  may include one or more distinct segments within it (generally corresponding to refractive index profile segments in the fiber) manufactured in accordance with the dry deposition process described herein or conventional (wet) processes.  
      In accordance with one embodiment, as shown in  FIGS. 7 and 11 , the core cane  32  includes a silica core and fluorine doped region corresponding to segments  40  and  42 , respectively, and a soot region  44 ′ which is also fluorine doped corresponding to segment  44 . A barrier layer  35   d,  may be employed on the preform  20   d,  to help minimize escape of fluorine during consolidation. In  FIG. 7 , the segment  44 ′ is preferably deposited by the substantially dry deposition method described herein. It should be recognized that silica-containing soot including germania, fluorine and other suitable dopants may be deposited in accordance with the substantially dry deposition method of the present invention in any one of the segments. For example, in  FIGS. 4-6 , a germania dopant may be added in the first soot segment during deposition resulting in an up doped segment in the refractive index profile. In  FIGS. 4-7 , fluorine is added to the second soot segment, thus down doping second segment  42 . In  FIGS. 5 and 6 , the third segments are up doped with germainia.  FIG. 6  illustrates a fourth segment having a fluorine down dopant. It should be recognized that any one, some or all of the soot segments may be manufactured by a flame hydrolysis process wherein a substantially hydrogen-free fuel is ignited to form a flame and a glass precursor is flowed (preferably in gaseous form) into a flame.  
      The layer may be vitrified by any method able to apply sufficient heat to the surface thereof. For example, one preferred method of vitrifying involves firepolishing with a flame. Preferably, the flame is produced by igniting a substantially hydrogen-free fuel (e.g., carbon monoxide) so that the vitrifying step does not add any appreciable water to the preform.  
      Barrier Layer Formed by a Laser  
      Another method for vitrifying the layer comprises exposing the surface portion to a laser beam  60  emanating from a laser device  62  as described in  FIGS. 26-29 . A laser device  62 , such as a CO 2  laser, emits a collimated beam portion  60   a,  having a spot diameter d of about 2 mm to 4 mm. The beam portion  60   a,  is passed through a focusing device  64 , such as a lens, thereby providing a focused beam  60   b.  That focused beam  60   b,  is focused on the surface  41  of the soot preform  20  such that it exhibits a exposure point  65   b,  at the surface  41  of diameter d′ of between about 0.5 mm and 2.5 mm. The laser beam  60   b,  has sufficient energy to vitrify the surface  41  and form the vitrified glassy layer  35  as the preform  20  is rotated about its axis.  
      In the illustrated embodiments of  FIGS. 26 and 27 , for each rotation, the laser or preform is moved in the axial direction by an incremental amount, for example, from exposure point  65   a,  to  65   b.  In this fashion, the laser beam  60   b,  is traversed along the axial length of the preform  20  as shown in  FIG. 28  (shown half way across) and  FIG. 29  (shown traversed the full way across the preform). The two successive positions  65   a,    65   b,  of a first and a revolution, respectively, overlap such that surface  41  is vitrified to the desired depth without any portion of the surface being missed. However, it should be noted that any axial traversal scheme may be employed such that the entire surface becomes vitrified. Preferably, deposition is suspended while the vitrified layer  35  is being formed. It should be recognized, that although the exemplary embodiments of a laser and firepolishing have been provided, that other means for vitrifying the surface may be utilizes as well, such as induction heating, and plasma torch. Any means that may generate sufficient heat may be employed.  
      Forming the Glassy Barrier Layer with an Induction Heater  
      One particularly well suited method of producing a glassy barrier layer  35  in accordance with an embodiment of the invention comprises passing an induction heater  59  along the axial length of the preform  20  as is shown in  FIGS. 36 and 38 - 39  to vitrify a thin layer of the soot and form the glassy barrier layer  35 . The induction heater  59  preferably includes a susceptor  61 , such as a annular ring of suitable susceptor material (e.g., graphite) and an induction coil  63  wound about the susceptor  61 . Four winds of water-cooled copper induction coils performed acceptably. When sufficient power (between 2.0 to 4.0 kilowatts) is supplied to the induction coil, the susceptor heats up to a sufficiently high temperature to cause the surface of the soot preform  20  to fully vitrify, i.e., consolidate. The extent (depth) of the vitrification is controlled by a control system  67  supplying power to the coil  63  and controlling the traverse speed as indicated by arrow B.  
      In operation, the method of manufacturing an optical fiber preform in accordance with an embodiment of the invention, comprising the steps of forming a first silica soot section  30  of the preform  20  by depositing silica-containing soot onto an outer surface of rotating deposition surface  41 , then exposing at least part of the length of the section  30  to heat generated by an induction heater  59  to form a glassy barrier layer  35  on only a surface of the section. Preferably, at least the entire useable length “L” of the preform  20  is exposed to the heat to form the glassy barrier layer. Even the ends are preferably heated to form a sealed end. After the glassy barrier layer  35  is formed, additional deposition of a second silica-containing soot section over top of the glassy barrier layer  35  may be performed as heretofore described herein. Preferably, at least one of the first and second silica-containing soot sections comprises a fluorine dopant As elucidated herein, the glassy barrier layer  35  substantially minimizes migration of the dopant between the sections. Moreover, it prevents re-wetting of the sections that may have been formed by a dry process as described herein.  
      In more detail, a lathe apparatus with motor  22 , chuck, and end support  23  is adapted to support the silica soot section  30  of the preform  20  within a deposition chamber  36   a.  The induction heater  59  and its drive assembly is mounted proximate the lathe and the heater is adapted to generate heat to form a glassy barrier layer  35  on the outside surface of the preform. During deposition, the entire frame may move and the burner(s) may remain stationary or visa versa. However, it should be recognized that the lathe assembly and the heater and its drive assembly are preferably coupled. The induction heater  59  is preferably stationed at a position out of the way off an end of the preform during soot deposition, as is shown by dotted lines labeled “A.” After a predetermined amount of soot has been deposited onto the preform  20 , the control system  67  initiates a command for the induction heater  59  to move axially from its stationed position. The movement may be accomplished by any suitable traverse mechanism. For example, a motor  22   a,  mounted stationary with the lathe&#39;s drive motor  22  may drive a screw drive assembly  69  or other suitable drive mechanism attached to the induction heater assembly  59 . As shown, the motor  22   a,  rotates a lead screw  71  that is threaded into a drive plate  73  extending from and rigidly secured to the heater  59 . Rotation of the lead screw  71  by motor  22   a,  moves the drive plate  73  and, thus, the induction heater  59 . The drive plate  73  is slidably received on a parallel bar  75  fixedly mounted to the frame to prevent rotation of the heater  59  while axially traversing. Other suitable anti-rotation restraint may be employed. Thus, the drive assembly  69  enables the heater  59  to be traversed axially and the susceptor and the induction coil to encircle the preform when forming the glassy barrier layer. The assembly allows the heater to be positioned in a defined relation to the center of the preform and traverse at the desired rate as commanded by the controls. The controls  67  for the induction heater  59  may preferably interface with the controls of the lathe such that the formation of the glassy layer is suitably synchronized with the deposition steps.  
      By way of example, the inventors have discovered that traversing the induction heater  59  at an axial speed in a range between about 0.5 cm/s to 3.0 cm/s, and more preferably in the range between 1.0 to 2.0 cm/s, and at a power of between 2.0 to 4.0 kilowatts, and more preferably between 2.5 and 3.0 kilowatts allows the formation of a suitable thickness barrier layer preferably made in one pass. Preferably also, rotating the preform  20  at a rotational rate of between about 80 rpm and 160 rpm during the formation of the glassy layer  35  further enhances its uniformity. During the step of exposing, the annular space between the graphite susceptor  61  of the heater  59  and the preform  20  is purged with helium.  
      The inventors herein have also determined that the method of employing an induction heater provides a smooth, radial density gradient in the barrier layer  35  that will advantageously resist cracking during the preform consolidation process performed later. For example, as shown in  FIG. 37 , a plot of density versus thickness of the barrier layer formed. In the plot, it is illustrated that the density of the layer as a function of radial dimension varies from a small value at an innermost part  77   a,  of the barrier to a high value at or near the outermost part  77   b.  The density profile illustrated is preferably constant for the useable length L of the preform. As is depicted in the diagram of  FIG. 37 , it is preferable that the thickness t of the barrier layer be preferably greater than about 10 μm. This thickness value is measured as the thickness that is preferably fully consolidated. For example, for un-doped fused silica, the fully consolidated density is about 2.2 gm/cm 3  as shown.  
      Prior to the step of exposing, a conventional or dry burner  25 ,  27  (See  FIGS. 1 and 18 ), as described herein, is used for forming the first silica-containing section  30 .  
      During the step of exposing, the burner  25 ,  27  is preferably moved aside as shown in  FIG. 36 , such that a soot stream emitted from the burner  25 ,  27  does not contact the preform  20 . If multiple burners are employed, they may be moved aside in any suitable fashion. In accordance with another embodiment shown in  FIGS. 38 and 39 , during the step of exposing wherein the glassy barrier layer  35  is formed, the flame  28 ,  29  of a soot-producing burner  25 ,  27  used for forming the first silica-containing section  30  is preferably deflected aside by a deflector  79 . The deflector is preferably moveable and moves out of the way after the barrier layer has been completed, thus enabling formation of a second silica-containing soot layer. As should be understood, the step of exposing preferably takes place within the deposition chamber  36   a.    
      As shown in  FIGS. 24 and 26 , one preferred method of manufacturing an optical fiber preform  20   a,  having a glassy barrier layer  35   a,  comprises the steps of depositing a first silica-containing soot region  40 ′ on an outside surface of a rotating substrate  32 ′ (such as the core cane shown) to a first predefined diameter dp, forming a glassy barrier layer  35   a,  adjacent to an outermost radial extent of the first soot region  40 ′ by vitrifying a surface layer of the first silica-containing soot region; and depositing a second silica-containing soot region  42 ′ on an outside radial surface of the glassy barrier layer  35   a,  to a second predefined diameter ds. The second soot region  42 ′ preferably includes a fluorine dopant. As shown in  FIG. 25 , a third soot segment  44 ′ may be deposited on an outer radial extent of the second barrier layer  35   b.  This region preferably includes a germania dopant. Although barrier layers of specific structure are illustrated herein, the shape and dimensions of the barrier layer may be modified without departing from the claims herein.  
      Combined Conventional and Substantially Water-Free Deposition Method  
      According to another illustrated embodiment of the invention, as best shown in  FIG. 1  and  FIG. 18 , a soot preform  20  is formed within a lathe apparatus  18   a,    18   b,  by forming one or more segments of the silica-containing soot by a conventional process and another part by a substantially dry process as described herein-below. In particular, the conventionally formed segments are produced by introducing a silica-containing precursor  24   a,  into a flame  29  of the burner  27  produced by igniting a hydrogen-containing fuel  31 , such as methane. The substantially dry segments comprising one or more other segments of silica-containing soot are formed by introducing a silica-containing precursor  24  into another separate flame  28  formed by igniting a substantially hydrogen-free fuel  26  ( FIG. 1 ). The conventional and dry deposition steps may occur in any order.  
      For example, a first part of the soot preform  20  may be formed by oxidizing a precursor in the substantially dry flame  28  ( FIG. 1 ). The flame  28  is formed by combusting carbon monoxide  26  in a burner  25 , and utilizing oxygen  21  as the combustion-supporting gas. A second part of the preform  20  may be formed by a conventional method by oxydizing a precursor  24   a,  in a burner  27  having a conventional flame  29  to produce soot  30  deposited on a substrate  32 , as shown in  FIG. 18   b.  For example, the flame may combust methane  31  and utilize oxygen  21  a as the combustion supporting gas. The substantially dry process may be utilized to form portions of the preform&#39;s core, for example, whereas the conventional process may be utilized to deposit the preform&#39;s cladding at a high deposition rate. Optionally, the steps may be reversed. It should be recognized that the conventional and substantially dry deposition process may be utilized in combination in any order or sequence. A glassy barrier layer, as described herein, is preferably utilized to prevent migration of water or dopants to the portion made by the dry process. The glassy barrier layer may be located at an interface between a first and second segment, for example and may be formed in either one of the first and second soot segments. Most preferably, the glassy barrier layer  35  is formed utilizing the substantially-hydrogen free fuel  26 , thus minimizing trapped water within the barrier layer. Additional barrier layers may be utilized as desired. For example, a second glassy barrier layer  35   b,  formed at an interface of the second segment  42 ′ and a third segment  44 ′. Moreover, the whole process is preferably carried out in a substantially dry atmosphere  34  supplied from an inlet  53  and exhausted by an exhaust  55 .  
      Burner  
      According to another embodiment of the invention, in order to obtain sufficient heat from the dry flame, substantially hydrogen-free fuel  26  and glass precursor  24  are preferably supplied at a predetermined flow ratio recognized by the inventors herein to be important. In particular, to generate sufficient heat, the flow of fuel to the flow of glass precursor  24  should be greater than  20 : 1 . This is accomplished, as recognized by the inventors herein, by proper sizing of the various passages within the combustion burner  25 . One preferable combustion burner is illustrated in  FIG. 21 . The burner adapted for combusting substantially hydrogen-free gas shall be referred to herein as a “dry combustion burner.” The dry combustion burner  25  includes a center fume tube  68 , formed as a slender tube, and is adapted to supply the gaseous precursor. Preferably surrounding the fume tube  68  is an inner shield passage  74  that is adapted to carry oxygen. Oxygen, a combustion supporting gas, is supplied in a ration of fuel to combustion supporting gas of about 2:1. Surrounding the fume tube  68   a,  nd inner shield  74  is the fuel passage  70  adapted to carry the large volumes of substantially hydrogen-free fuel. Although not shown exactly to scale, it is apparent that the cross-sectional area of the fuel passage  70  is much larger than of the fume tube  68 . Because, for example, carbon monoxide contains less heat when ignited, higher flows are required as compared to methane. This is designed such that the glass precursor  24  may be supplied at a first flow rate to a center fume passage  68  of the combustion burner  25  and that the substantially hydrogen-free fuel  26  may be supplied at a flow rate at least 20 times the first flow rate thereby enabling generation of sufficient heat to oxidize the precursor. The burner  25  may include multiple input ports for supplying the substantially hydrogen-free fuel  26  and the combustion supporting oxygen  21  thereby providing more uniform flow distribution in the annular shaped passages.  
      Fluorine may be incorporated into the soot in another embodiment of the invention. There are several ways that this may be accomplished in accordance with the invention. First, the fluorine may be included in the precursor, such as when a chlorofluorosilane is used for the precursor  24 . In this scenario, the precursor  24  is supplied as a gas to the fume tube  68  and oxidized by the flame  28  ( FIG. 1 ) thereby producing fluorine doped soot in the preform  20 . Alternatively, some fuel or oxygen may be supplied with the substantially hydrogen-free fuel.  
      A second way of introducing fluorine is by flowing fluorine or a fluorine-containing compound such as of F 2 , CF 4 , C 2 F 6 , SF 6 , NF 3 , SiF 4  or combinations thereof in gaseous form into a shield included within the combustion burner.  FIG. 31  illustrates a burner  25   a,  that may be utilized to incorporate fluorine-doped soot into the preform  20 . Fluorine or the fluorine-containing compound is supplied in gaseous form to outer shield passage  72  surrounding the fuel passage  70 . A water cooling jacket may be utilized surrounding the fuel passage. The rest of the design is as heretofore described.  
      A preferred embodiment of the combustion burner has a center tube  68  adapted to provide a substantially hydrogen-free glass precursor into a flame region, the center tube located along a central axis of the burner  25 ,  25   a,  (FIGS.  21 ,  31 ); an inner shield unit  74  adapted to provide oxygen into the flame region  28  ( FIG. 1 ), the inner shield unit radially displaced from the central axis of the burner, a fuel unit  70  radially displaced from the central axis of the burner and adapted to provide a substantially-hydrogen free fuel; and an outer shield unit  72  adapted to provide a fluorine containing gas enshrouding the flame region, the outer shield region radially displaced from the central axis of the burner and positioned outside the inner shield unit and the fuel unit, the burner being adapted for producing substantially water-free, fluorine doped silica.  
      A third method of incorporating fluorine into the soot is by providing the fluorine or fluorine-containing gas into an expelling element  33  surrounding or partially surrounding the flame  28 . One expelling element, i.e., an expelling ring, is described with reference to  FIGS. 12 and 13 . By the use of these methods, fluorine may be very efficiently incorporated in the soot, thereby utilizing significantly less fluorine that by the prior art method. In the prior art, fluorine was incorporated during a sintering step in the consolidation furnace. In fact, according to an embodiment of the invention, the step of achieving fluorine doping within a segment of the silica-containing soot is accomplished wherein greater than 1% by weight of fluorine is incorporated. This is accomplished by supplying to the flame, fluorine or a fluorine-containing compound in an amount less than 0.5 l/m.  
      Combination of Substantially H-Free Fuel and Combustion-Enhancing Additives  
      According to another embodiment of the invention, as best illustrated in  FIG. 35 , a method of producing silica-containing soot having very low water (H, OH) content is described. The method comprises, in one embodiment, utilizing a combination of a substantially hydrogen-free fuel  226  and a fuel additive  231 . Supplied to the burner  225  are a combination of a catalyst, for example, a hydrogen-containing fuel, an energetic fuel or an energetic oxidizer, all of which are referred to as fuel additives  231  and a substantially hydrogen-free fuel  226 . One reason it is desirable to combine the substantially H-free fuel  226  with a combustion-enhancing additive  231  is to substantially speed up the burning velocity of the substantially hydrogen-free fuel or to increase its heat of combustion. For example, the burning velocity of dry carbon monoxide is less than about 0.1 m/s. The flow rate desired for the precursor fume is on the order of 20-40 m/s. Thus, unless the burning velocity of the fuel can be substantially increased, it is difficult to keep the flame attached to the face of the burner  225  and thus, it tends to blow itself out. Further, it was discovered by the inventors herein that poor flame structure resulting from the slow burning fuels results is poor soot density, capture efficiency, and soot conversion. These resulted from problems such as insufficient heat to the bait rod, and insufficient heat to the fume.  
      As discovered by the inventors herein, adding small amounts of combustion-enhancing additives  231 , such as catalysts in combination with the substantially hydrogen-free fuel significantly increases the burning velocity of the slow burning CO from less than about 0.1 cm/s to 1 m/s or greater when used in combination. Further, flame temperature, velocity, and structure also improved. This addition in small amounts does not, however, result in detrimental amounts of water in the glass produced. The substantially hydrogen-free fuel  226  is preferably selected from a group consisting of carbon monoxide (CO), carbon suboxide (C 3 O 2 ), and carbonyl sulfide (COS). A “catalyst” as used herein is any compound or additive that forms light radicals such as H, OH and O, which increase burning velocity of the fuel combination. This improves burning velocity and flame structure. The catalysts are preferably selected from a group consisting of hydrogen (H 2 ), water (H 2 O), peroxide (H 2 O 2 ), methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), ethylene (C 2 H 4 ), acetylene (C 2 H 2 ), and their deuterated analogs D 2  (a naturally occurring isotope of hydrogen), D 2 O, D 2 O 2 , CD 4 , C 2 D 6 , C 3 D 8 , C 2 D 2  and C 2 D 4 . The catalysts may also include sources of oxygen radicals (e.g., ozone (O 3 )), HCN, and nitrous oxide (NO).  
      In another embodiment, the additive may also comprise what is termed herein an “energetic fuel” or “energetic oxidizer”. These additives exhibit exothermic properties, such that the heat contributed from their combustion is greater that the heat required to raise the temperature of the energetic fuel or energetic oxidizer to the flame temperature. In other words, they increase the flame temperature. Energetic fuels or oxidizers include low molecular weight hydrocarbons, their deuterated analogs and certain other compounds (e.g., HCN, C 2 Cl 2 , and (CN) 2 ). However, any suitable additive may be utilized that increases the flame heat in accordance with the above mentioned criteria. Although a thermodynamic analysis is needed to positively identify energetic fuels or energetic oxidizers, the presence of double or triple intra-molecular bonds, which contribute significant energy when broken, can be utilized to identify potential candidates.  
      Preferably, the additive (catalyst, energetic fuel or oxidizer) is supplied in an amount of less than about 50%, more preferably less that 20%, more preferably yet less than 5%, and most preferably less than 1%. As little as 1% or less was discovered to improve the burning velocity significantly. This allows the flame  228  to adequately seat onto the burner  225 . Larger amounts of additives  231  may be needed for deposition of germania in the desired amounts. The amount of additive  231  needed is also discovered to be dependent on the humidity of the atmosphere. Thus, it should be understood, that more combustion-enhancing additive  231  is needed when a substantially water H-free atmosphere is provided shrouding the flame (See  FIG. 1 ,  18 - 20 ). As shown in  FIG. 35 , the combination of substantially H-free fuel  226  and combustion enhancing additive  231  is ignited to form a flame  228  and a glass precursor  224  is flowed into the flame. The silica-containing soot that is formed is preferably deposited onto a rotating substrate  232  to form an optical fiber soot preform  220 .  
      The exact amount of additive desired is of course based upon how its addition affects the attenuation in the telecom window (1530-1580 nm) due to the absorption peak (water peak) at 1380. However, in one example, 0.1% CH 4  in a CO/O 2  flame resulted in less than 300 ppb of water in the glass. It is believed that the use of deuterated analogs will further lower the attenuation at the telecom window, because their absorption peak will occur at about 1870 nm, far away from the telecom window.  
      Photomask  
      In accordance with another illustrated embodiment of  FIG. 30 , the substantially hydrogen free fuel may be utilized for making a glass article, such as a disc of High Purity Fused Silica (HPFS) glass  86 . This HPFS  86  may be used for photomasks utilized in making semiconductor chips. In the case of making HPFS, silicon-containing gas molecules are reacted in a flame  128  to form SiO 2  particles. These particles are deposited on the hot surface of a body  132  where they consolidate into a very viscous fluid (deposited and virtually simultaneously vitrified) which is later cooled to the glassy (solid) state, i.e., the HPFS glass  86 .  
      According to another embodiment of the invention, a method for producing a vitrified glass article is provided. The method preferably comprises the steps of generating heat from a combustion burner  125  having a flame  128  that is produced by igniting a substantially hydrogen-free fuel  126 , the flame  128  being the only source of heat, flowing a glass precursor  124  into the flame  128  to produce silica containing soot  130 , and depositing the silica containing soot onto a substrate  132  and substantially simultaneously converting the soot to form the vitrified glass article  86 . In accordance with a preferred embodiment, the soot is deposited onto a substrate  132  that is itself a silica-containing glass member, and most preferably a HPFS glass disc. Preferably, the substrate  132  is mounted onto a bed of sand  88 . By utilizing the substantially hydrogen free fuel in accordance with the invention, the vitrified glass article  86  contains water (OH) in amount less than about several ppm. In the illustrated embodiment, the step of depositing takes place within a chamber  89 . Preferably, a purge gas, such as nitrogen, is provided into the chamber such that a substantially water free environment is provided. Generally, it is desirable to provide a pressurized atmosphere in the chamber  89  greater than an atmospheric pressure outside of the chamber.  
      In summary, the present processes in accordance with the invention can make preforms or glass boules or other glass or soot articles having extremely low water content (less than about several ppm). The method can be used where the deposition and consolidation occur separately, or in one simultaneous forming step. The substantially water-free glass is suitable for making photomask products or preforms for optical fiber manufacture as the resultant glass contains very low amounts of water.  
      The substantially water-free fused silica generally has a water content of less than 0.1% by weight. Preferably, the water content is less than 0.5% by weight. In theory, the fused silica is completely water-free. Practically, however, water contents below 0.1% by weight may be achieved.  
      To demonstrate this invention, the following experimental runs were done in a single burner fused silica laydown furnace. A standard run which produced a  6 ″ dia.×1½″ thick boule were done.  
     EXAMPLE I  
     Prior Art  
      In the OWG process, silica particles generated in flame are deposited on a colder target as amorphous or semi-sintered silica particles. The raw material for silica is OMCTS and the fuel used in natural gas. The blank thus formed is high in water-content. The blank is consolidated in the presence of fluorine to dry the fused silica glass formed. The blank after consolidation typically has a diameter of  4 ″ and has index of refraction striation that appears as annular rings. To make larger product, a piece of the blank has to be heated to softening temperature and allowed to flow out. This introduces another step to manufacturing and an opportunity to introduce contaminants in the glass.  
      The sensible heat released in the methane reaction of Prior Art Equation III is 802.4 kJ that under adiabatic conditions (no energy loss) will be used to heat up 3 moles of products formed. Prior Art Equation IV, the sensible heat released per mole of the product is 267.5 kJ.  
     EXAMPLE II  
      The sensible heat released by the combustion of carbon monoxide to carbon dioxide, Equation I, is 283 kJ that under adiabatic conditions is used to heat one mole of product of combustion. The heat released from the combustion of one mole is CH 4  is approximately three times that released from one mole of CO. The compensate for the lower heating value of CO, the flow rates of CO have to be at least three times higher to have similar heat release in the furnace. Because the heat released per unit mole of product of combustion during combustion of both methane and carbon monoxide are similar, the adiabatic flame temperatures for CO and CH 4  are expected to be similar. The adiabatic flame temperature of CO in air is 1950° C. and that for methane is 1941° C. Adiabatic flame temperatures in oxygen are much higher as the thermal load due to the excess nitrogen in air is eliminated. The adiabatic flame temperature for CH 4  in oxygen is 2643° C. The adiabatic flame temperature for CO in oxygen has been calculated to be 2705° C. Based on the similarity of the adiabatic flame temperatures, our process achieves furnace temperatures of close to 1650° C. (similar to the prior art HPFS process) for deposition and consolidation of fused silica using carbon monoxide as the fuel. The accomplish this, burners have to accommodate higher flows of CO at acceptable velocities.  
     EXAMPLE III  
      The run conditions for the glass were as follows. This glass was made using a single liquid feed burner. The SiCl 4  flow was between 5.5 and 7.5 g/min. Oxygen was used as an atomizing gas at 25 slpm. The CO gas stream was at 50 slpm. The distance at the start of the run from furnace crown to the sand was 9 inches and the crown temperature was maintained at about 1670° C.  
      As a result, this enables the manufacture of water-free fused silica glass by eliminating the use of any hydrogen containing reactants, both as raw material for silica and as fuel, the products of combustion have been made substantially water-free. Further, wet natural gas may be a source for sodium in addition to the water it carries. By eliminating the use of natural gas from the process, another possible source of contamination has been removed. The major hindrance to achieving high transmission at low wavelengths is iron contaminant. Fortunately, carbon monoxide combines readily with metals and in particular iron. Thus, using the combining power of carbon monoxide with iron adds additional purification to the chemical vapor deposition process of making high purity fused silica.  
      Still further, to extend the transmission into the extreme ultraviolet range, fluorine may be added to the silica precursor feed tube in the form of carbon tetra-fluoride. The carbon tetra-fluoride will add fluorine to satisfy broken bonds in the structure, improve transmission, and act as a scavenger for any water that may enter the process from ambient recirculated furnace air. To further improve the purity of the CO fuel gas from metal impurities, we pass the CO through a furnace at &gt;500° C. to thermally reduce any contained metals out of the CO fuel stream.  
      In addition to these embodiments, persons skilled in the art can see that numerous modifications and changes may be made to the above invention without departing from the intended scope thereof.