Abstract:
A deposition system for depositing a chemical vapor onto a workpiece, including a deposition chamber having a plurality of components for performing chemical vapor deposition on the workpiece. The deposition chamber includes an inner skin made of Hasteloy for sealing the plurality of components and the workpiece from the air surrounding the deposition system, and an outer skin that encloses the inner skin and is separated from the inner skin by an air gap. The outer skin includes vents that create a convection current in the air gap between the inner skin and outer skin of the deposition chamber. The deposition system also has a gas panel for regulating the flow of gases and vapors into the deposition chamber, and a computer for controlling operation of the gas panel and the components in the deposition chamber.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the present invention relates to systems and methods for depositing chemicals onto workpieces, and the products therefrom. More particularly, the present invention relates to systems and methods for depositing silica soot on a start rod for fabricating optical fiber preforms, fused silica rods, and other optical components. 
     2. Background 
     Today&#39;s communications grade optical fiber of fused silica, SiO 2 , is manufactured according to three basic steps: 1) core preform or “start rod” fabrication, 2) core-with-cladding preform fabrication, and 3) fiber drawing. The core and cladding of a preform correspond in ratios and geometry to those of the ultimate glass fiber that is drawn from the preform. The composition of the core and cladding must be such that there is a lower index of refraction in the cladding than in the core. The relatively higher index of refraction of the core to a relatively lower index of refraction of the cladding is predetermined so that when the preform is drawn into a fiber, the fiber conducts light, either in single mode or in multi-mode form. 
     The first step is to build up a start rod, forming it into a glass that will eventually become the core of the fiber, and in some cases, part of the cladding layer. The start rod is a glass rod made of silica, SiO 2 , with the portion of the start rod that comprises the core being doped with a small amount of a dopant, typically Germania, GeO 2 . The presence of the dopant in the core increases the refractive index of the glass material compared to the surrounding outer (cladding) layer. In the second step, a cladding layer is built up on the start rod. The result of this step is a preform having a core and a cladding, which is typically about 80 mm in diameter and about one meter long. The third step is fiber drawing, where the preform is heated and stretched, and typically yields about 400 km of optical fiber. 
     The primary raw ingredient to fabricating the glass preform is silicon tetrachloride, SiCl 4 , which generally comes in a liquid form. As noted above, however, SiO 2 , typically in the form of glass soot, is deposited on the start rod. The chemical reactions involved in the formation of the glass soot are complex, involving, SiCl 4 , oxygen, O 2 , and the fuel gas combustion products. In all of the techniques, the silica, SiO 2 , comprises the cladding of the preform according, generally, to the reaction: 
     
       
         SiCl 4 +O 2 =SiO 2 +2Cl 2 . 
       
     
     Generally, there are four distinct technologies for fabricating core preforms. These technologies include Modified Chemical Vapor Deposition (MCVD), Outside Vapor Deposition (OVD), Vapor Axial Deposition (VAD), and Plasma Chemical Vapor Deposition (PCVD). The resulting product for all of these technologies is generally the same: a “start rod” that is generally on the order of one meter long and 20 mm in diameter. The core is generally about 5 mm in diameter. 
     Similarly, there are generally four technologies for performing the step of adding the cladding. These technologies include tube sleeving (conceptually paralleling MCVD), OVD soot overcladding (conceptually paralleling OVD), VAD soot overcladding (conceptually paralleling VAD), and plasma (conceptually paralleling PCVD). In this step, additional cladding layers of pure or substantially pure fused silica are added to the start rod to make a final preform that can be prepared for fiber drawing. 
     In MCVD, the step of manufacturing the start rod is performed inside of a tube. Similarly, when the cladding step is performed, a larger tube is sleeved onto and fused to the start rod. Presently, the company, Heraeus, manufactures the tubes used for producing start rods and for sleeving onto and fusing with the start rods to make the preforms. 
     In OVD, when fabricating start rods, glass is deposited onto a rotating mandrel in a “soot” deposition process. The start rod is slowly built up by first depositing the germanium doped core, and then the pure silica layers. When the core deposition is completed, the mandrel is removed and then sintered into a start rod of glass. 
     In the process of OVD soot overcladding, where a cladding is deposited onto a fabricated start rod, the start rod is rotating and traversing on a lathe such that many thin layers of soot are deposited on the rod over a period of time. Although the SiO 2  is not deposited onto the start rod as a vapor, but rather as SiO 2  particles, the process is known in the art as a “chemical vapor deposition” process because the SiCl 4 , which reacts in the stream between the burner and the start rod to form SiO 2 , is input to the burner as a vapor. The porous preform that results from the OVD soot overcladding process is then sintered in a helium atmosphere at about 1500° C., into a solid, bubble-free glass blank. U.S. Pat. No. 4,599,098, issued to Sarkar, which is incorporated by reference as though fully set forth herein, provides further background on systems and techniques for OVD and OVD soot overcladding. 
     VAD is a process of depositing silica soot onto the end of a mandrel in a deposition station. Unlike the OVD process the mandrel is not removed prior to sintering. Furthermore, like the OVD soot overcladding technique, VAD soot overcladding is also used to deposit silica soot on a start rod to fabricate a preform. However, unlike OVD and OVD soot overcladding, VAD soot overcladding orients the mandrel and start rod vertically rather than horizontally and deposits the silica in one thick layer in one pass. 
     PCVD uses plasma radiation as a source of heat, and therefore is unlike the above-described processes, which use hydrogen or methane as the source of heat for the chemical reaction. 
     For the above-described technologies, typically any one of the core fabrication technologies may be combined with any one of the cladding fabrication technologies to generate a preform that may be used for drawing fiber. 
     In the OVD soot overcladding processes, one of the key measures of economic viability in comparison to the other available techniques is the deposition rate of the SiO 2  on the workpiece. For example, some companies involved in optical fiber manufacturing opt for the most cost-effective method of performing the step of overcladding the start rod in the fiber manufacturing process. With respect to this step in the process, the choice is either to purchase the cladding tubes or to perform a deposition process to add the cladding. 
     In comparing the relative costs of the two approaches, the economics often come down to whether a particular vapor deposition system that a company is considering performs at a certain minimum deposition rate. The deposition rate may be characterized, for example, by the average grams/minute of silica soot that can be deposited on the start rod until completion (i.e., an optical fiber preform ready for sintering). Thus, above a certain threshold deposition rate, performing the soot overcladding process is likely to be economically more attractive to the company than purchasing cladding tubes. Thus, companies that manufacture systems for performing soot overcladding focus on achieving the highest possible deposition rates without compromising the quality of the preform that is produced for fiber drawing. 
     The factors that determine a deposition system&#39;s deposition rate are the chemical vapor delivery rate and the efficiency of chemical vapor deposition onto the workpiece. With respect to vapor delivery, key issues generally revolve around continuously and efficiently maintaining a high (e.g., greater than 200 grams/minute) delivery rate over a prolonged period (e.g., greater than 2 hours). Several methods have been described in the prior art for supplying a hydrolyzing burner with a substantially constant flow of vaporized source material entrained in a carrier gas. For example, in U.S. Pat. No. 4,314,837 issued to Blankenship (“the Blankenship reference”), a system is described that includes several enclosed reservoirs each containing liquid for the reaction product constituent. The liquids are heated to a temperature sufficient to maintain a predetermined vapor pressure within each reservoir. Metering devices are coupled to each reservoir for delivering vapors of the liquids at a controlled flow rate. The respective vapors from each reservoir are then combined before they are delivered to the burner. 
     This device, however, is inefficient for maintaining a substantial and steady delivery rate of chemical vapor to the burner for a prolonged chemical vapor deposition process. For a substantial and steady delivery rate, the chemical reservoir described in the Blankenship reference must be vast, and significant energy expenditure is required to maintain the chemical in the reservoir in a vapor state. On the other hand, if the chemical reservoir described in the Blankenship reference is small enough to be energy-efficient, then the deposition flow must be periodically interrupted to refill the reservoir with the chemical liquid and heat the chemical until it is in a vapor state. Because maintaining a constant, high delivery flow rate, as noted above, is a critical factor in the effective deposition rate, a need exists for a system and method of chemical vapor delivery that is energy-efficient and provides high, constant and continuous delivery of chemical vapor. 
     With respect to enhancing the deposition efficiency of SiO 2  on the workpiece to improve the effective deposition rate, studies have been performed to characterize the flow of chemical vapor in the reaction chamber from the burners to the surface of the workpiece. One reference directed to this issue is Li, Tingye, Fiber Fabrication, pp. 75-77, Optical Fiber Communications, (Academic Press, Inc. 1985). As discussed in the above reference, because of the small size of the formed glass particles, momentum does not cause an impaction of the particles onto the surface of the workpiece. The small sizes of the glass particles would tend to force them to follow the gas stream around instead of at the preform surface. Rather, thermophoresis is the dominant mechanism for collection on the surface of the preform. As the hot gas stream and glass particles travel around the workpiece, a thermal gradient is established near the surface of the preform. Preferably, the thermal gradient is steep, effectively pulling the glass particles by a thermophoretic force towards the preform. 
     Various methods have been proposed to increase deposition efficiency based on establishing and maintaining the thermophoretic force, including varying the distance between the burner and the workpiece. See H. C. Tsai, R. Greif and S. Joh, “A Study of Thermophoretic Transport In a Reacting Flow With Application To External Chemical Vapor Deposition Processes,”  Int. j. Heat Mass Transfer , v. 38, pp. 1901-1910 (1995). However, even applying these methods, demand for even higher deposition rates has gone unmet. A need exists therefore, for systems and methods that offer further improvement to deposition efficiency, chemical delivery and, thereby, the overall deposition rate of chemical vapor. 
     SUMMARY OF THE INVENTION 
     The present invention generally provides, in one aspect, systems and methods for enhancing the effective deposition rate of chemicals onto a workpiece, such as the deposition of SiO 2  from a SiCl 4  vapor onto a start rod for making a preform usable for drawing into optical fiber. 
     In a second separate aspect as described herein, the present invention comprises systems and methods for manufacturing optical fiber preforms, optical fiber, silica rods, including fused silica rods, and silica wafers. 
     In a third separate aspect as described herein, the present invention comprises a deposition system that preferably includes a deposition chamber having a plurality of components for performing chemical vapor deposition on the workpiece, an inner skin made of a Nickel-Chromium-Molybdenum alloy, commonly referred to as Hasteloy, for sealing the plurality of components and the workpiece from the air surrounding the deposition system, and an outer skin enclosing the inner skin and preferably separated from the outer skin by an air gap. The deposition system also has a gas panel for regulating the flow of gases and vapors into the deposition chamber, and a computer for controlling operation of the gas panel and the components in the deposition chamber. 
     In a fourth separate aspect as described herein, the present invention comprises a deposition system that preferably includes a deposition chamber comprised of Hasteloy and including components for performing chemical vapor deposition on the workpiece, a gas panel for regulating a flow of gases and vapors into the deposition chamber, and a computer for controlling operation of the gas panel and at least one of the plurality of components. 
     In a fifth separate aspect as described herein, the present invention comprises a deposition system that preferably includes a burner for depositing particles onto the workpiece and a lathe for holding the workpiece at its two ends and for rotating and translating the workpiece relative to the burner. The deposition system preferably further includes a computer for controlling the translating and rotating of the workpiece relative to the burner. Preferably, the computer is configured to translate the workpiece relative to the burner at a rate of greater than about 1.4 meters per minute during at least a portion of the deposition process. This relative motion may be caused by translating the burner, translating the workpiece, or translating both. 
     In a sixth separate aspect as described herein, the present invention comprises an optical fiber preform formed using deposition system such as that described above in the fourth separate aspect of the invention. In a seventh separate aspect as described herein, the present invention comprises a silica rod, preferably formed of substantially pure fused silica, and formed using deposition system such as that described above in the fourth separate aspect of the invention. 
     In an eighth separate aspect as described herein, the present invention comprises a deposition system that preferably includes a deposition chamber including components for performing chemical vapor deposition on the workpiece. The deposition system preferably further includes an exhaust subsystem for exhausting constituents from the deposition chamber, and an intake subsystem for providing air into the deposition chamber. The intake subsystem preferably includes a blower for actively conveying air into the deposition chamber and a passive air intake for allowing air into the deposition chamber based on a negative pressure differential between the blower and the exhaust subsystem. 
     In a ninth separate aspect as described herein, the present invention comprises a system of chemical vapor delivery, where the system includes a reservoir, a preheater, and a vaporizer generally connected in series for transferring silicon tetrachloride (SiCl 4 ) in liquid form from the reservoir to the preheater, in which the SiCl 4  is initially heated. The SiCl 4  is then transferred from the preheater to the vaporizer where the SiCl 4  is vaporized. Preferably, the system further includes a valve interposed between the preheater and the vaporizer, and a computer electrically connected to the valve for controlling flow of the chemical into the vaporizer based on the amount of the chemical in the vaporizer. 
     Further embodiments as well as modifications, variations and enhancements of the invention are also described herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic diagram of a perspective view of a preferred embodiment of a chemical vapor deposition system. 
     FIG. 1B is a diagram of a preferred embodiment of an intake and exhaust subsystem such as may be provided by the chemical vapor deposition system depicted in FIG.  1 A. 
     FIG. 2 is a function-oriented diagram of a preferred embodiment of a chemical vapor deposition system. 
     FIG. 3A is a diagram of a functional representation of a burner, represented generally in FIG. 2, as used in a chemical vapor deposition system as depicted in FIGS. 1A and 2. 
     FIG. 3B is a side view of burner and a representative stream of emissions from the burner, which is depicted generally in FIG. 2, as is preferably provided in a chemical vapor deposition system as depicted in FIGS. 1A and 2. 
     FIG. 4 is a schematic diagram providing a perspective view of a preferred embodiment of a lathe, such as is generally depicted in FIG. 2, for holding and moving a workpiece in a chemical vapor deposition system as is depicted in FIGS. 1A and 2. 
     FIG. 5 is a diagram depicting an example of a final preform having a usable portion demonstrating a tapering effect at the ends of the preform. 
     FIG. 6 is a diagram depicting a motion profile of the translational movement of a workpiece relative to a chemical deposition burner. 
     FIG. 7 is a process flow diagram illustrating a preferred embodiment of a process of performing chemical vapor deposition such as may be performed by the chemical vapor deposition system illustrated in FIG.  1 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A depicts a preferred embodiment of a chemical vapor deposition system  100 . The chemical vapor deposition system  100  preferably includes a reaction or deposition chamber  102 , and a enclosure  104  for the deposition chamber  102 , a computer  106  and other electronic components, an enclosure  108  for the computer  106  and the other electronic components, a power distribution subsystem  110 , an enclosure  112  for the power distribution subsystem  110 , a gas panel  114 , a gas panel enclosure  116  and an intake and exhaust subsystem (see FIG.  1 B), including a main exhaust  118  and secondary exhausts  122 ,  123 . A door  105  is preferably provided through which a workpiece or start rod can be inserted into and removed from the deposition chamber  102 . Removable panels  107  are also provided to facilitate cleaning of the chamber  102 . The door  105  and panels  107  preferably include moldings to seal the chamber  102  from vapor leakage out or air leakage into the chamber  102 . Overhead lights (not shown) preferably on each side of the chamber  102  may be provided to illuminate the interior of the chamber  102 . 
     Generally, the deposition chamber  102  is structured to house a process of deposition of particles (e.g., particles of silica soot) onto a workpiece or start rod. The deposition chamber  102 , and the deposition system  100  generally, may be used to produce an optical fiber preform that, in a subsequent drawing step, may be used to manufacture optical fiber. The deposition system  100  may also be used to manufacture fused silica rods, including pure fused silica rods. For this application, the deposition system  100  generally applies silica soot to an initial start rod of substantially pure fused silica. The product of this deposition process, a pure fused silica preform, is then sintered to form the pure fused silica rod and may be used to manufacture (e.g., by drawing, slicing or otherwise reforming the pure fused silica rod) silica wafers or substrates, multi-mode optical fiber, and other optical components for a variety of applications. 
     The other subsystems and components of the deposition chamber  102  are generally provided to support the deposition process. In one embodiment, deposition material generally comprising a vapor of silicon tetrachloride (SiCl 4 ) and oxygen (O 2 ) is emitted from a chemical burner in a process of fabricating optical fiber preforms in a deposition region  146  (see FIG. 1B) of the deposition chamber  102 . In the deposition region  146  are the chemical burner, a chemical stream from the burner, and the workpiece, where the stream is directed towards the workpiece from the burner (see FIG.  2 ). The burner also preferably issues and ignites streams of hydrogen and oxygen. The resulting flame heats the chemical constituents to temperatures generally exceeding about 1000° C. A chemical reaction with the hydrogen, oxygen and SiCl 4  occurs in the stream, in which the SiCl 4  in the stream is oxidized producing particles of silicon dioxide (that are then deposited on the workpiece) and a byproduct of hydrogen chloride (HCl). 
     FIG. 1B depicts a preferred embodiment of an air intake and exhaust subsystem  150  such as may be provided in the chemical vapor deposition system  100  illustrated in FIG.  1 A. Containment and processing of the constituents of the chemical vapor deposition process preferably is, at least in part, provided by the intake and exhaust subsystem  150 . The intake and exhaust subsystem  150  preferably provides a flux of air through the deposition chamber  102  to keep the deposition chamber environment, and the thereby the workpiece, cool relative to the chemical stream that issues from the burner. 
     As illustrated in FIG. 1B, the intake and exhaust subsystem  150  preferably is configured to cool the deposition chamber and the workpiece, provide clean air and a negative pressure within the deposition chamber  102 , and provide a laminar air flow in the deposition region  146 . The intake and exhaust subsystem  150  preferably includes a blower  125 , comprising a fan  126  and a motor  128 , a prefilter  124 , a High Efficiency Particulate Air (Hepa) filter  134  and a Hepa filter interface  132 , air diffusers  140 ,  142 ,  144 , and passive air intakes  136 ,  138 . The exhaust elements preferably include an exhaust hood  130 , a main exhaust  118 , and secondary exhausts  122 ,  123 . 
     With respect to the air intake elements, air external to the deposition system  100  enters the prefilter  124  under the active power of the blower  125 . The prefilter  124  preferably has an efficiency of at least about 90% in removing contaminants from the external air. Furthermore, the prefilter  124  preferably is mounted into the intake and exhaust subsystem  150  to conveniently enable an operator to periodically replace the prefilter  124 . External air that passes through the prefilter  124  is then preferably forced through the Hepa filter  134 , via a connector  132 . The Hepa filter  134  preferably has a filtering efficiency of greater than about 95% and, due to the presence of the prefilter  124 , requires replacement infrequently. 
     The forced clean air exiting the Hepa filter  134  is preferably piped to a location beneath the chemical burner where the air is spread laterally by a central diffuser  142  into the deposition region  145 . The forced clean air may be conveyed via stainless steel tubing (e.g., six-inch diameter) or via other convenient means as are known in the art. From the central diffuser  142 , the air is directed upward through the deposition region  146  towards the workpiece and the exhaust hood  130 , and in an upward direction generally parallel to the chemical stream emitted from the burner. 
     Due to the preferably negative pressure within the deposition chamber  102 , outside air also passively enters the deposition chamber  102  through the passive air intakes  136 ,  138 . The passive air intakes  136 ,  138  preferably include filters similar in design and filtering efficiency to the prefilter  124 . From the passive air intakes  136 ,  138 , the filtered external air then enters the diffusers  140 ,  144  which laterally spread the generally upward movement of air in the deposition chamber  102 . In the embodiment shown in FIG. 1B, one passive air intake is provided on each side of the deposition region  146 . In alternative embodiments, additional intakes may be provided on each side of the deposition region  146 . Alternatively, one passive air intake may be provided which diffuses the air laterally to both sides of the deposition chamber  102 . In any event, passive intake of external air is preferably balanced on each side of the deposition region  146 . 
     A generally vertical flow of air in the deposition chamber  102  is preferably at least in part maintained by the exhaust component of the intake and exhaust subsystem  150 . The main exhaust  118  and the secondary exhausts  122 ,  123  encourage a generally upward flow of air into and out of the deposition chamber  102 . Although the exhaust hood  130  may perturb the vertical flow somewhat, the exhaust hood  130  minimizes the spread of chemicals to interior walls  104  of the deposition chamber  102  by generally capturing any chemical constituents immediately after they move past the workpiece or otherwise escape the deposition region  146 . Like the passive air intakes  136 ,  138 , a secondary exhaust  122 ,  123  is preferably provided on each side of the main exhaust  118  to encourage the generally upward flow of air in the chamber  102 . In alternative embodiments, additional secondary exhausts may be provided on each side of the main exhaust  118 . 
     In one embodiment, the main exhaust  118  is preferably an eight-inch diameter stainless steel duct and the secondary exhausts  122 ,  123  preferably comprise six-inch diameter stainless steel ducts. However, the intakes and exhausts as described herein may be of any convenient shape or diameter for maintaining the desired laminar flow of air through the chamber  102 . Preferably, however, the exhaust hood  130  and the main exhaust  118  are comprised of Hasteloy to minimize any potential for corrosion due to the exhaust of chemicals from the chamber  102 . Furthermore, optionally, the secondary exhausts  122 ,  123  also are comprised of Hasteloy. 
     The intake and exhaust subsystem  150  preferably provides a power differential between the intake and exhaust of air from the deposition chamber  102  such that a greater flux of air is forced out of the chamber  102  than is forced into the chamber  102 . For example, the intake and exhaust subsystem  150  may be configured to exhaust 2000 cubic feet per minute (CFM) of air and deposition chamber constituents from the deposition chamber  102 , while the blower  125  may force 500 CFM of filtered air into the deposition chamber  102 . The resulting pressure differential is preferably addressed by the plurality of passive air intakes  136 ,  138  that passively allow filtered external air into the deposition chamber  102 . 
     In this embodiment, the exhaust component of the subsystem  150  inherently is provided an allowance for sub-optimal performance, as long as it exhausts air and chemical constituents at a greater rate than the active air intake component of the subsystem  150  (i.e., the blower  125 ) drives air into the chamber  102 . Potentially dangerous fumes from the deposition chamber  102  are therefore controlled efficiently and at low cost compared to a subsystem that may be designed to precisely balance between intake and exhaust. Furthermore, according to this embodiment, the deposition system  100  may be compatible with a greater variety of exhaust implementations, in that such exhaust implementations only need to meet certain minimum exhaust requirements related to a relatively low active air intake specification. Such flexibility in permissible exhaust performance is particularly advantageous because the intake and exhaust subsystem  150  may be only partially design-integrated with the rest of the deposition system  100 , where an exhaust motor for the intake and exhaust subsystem  150  is supplied externally. For example, the deposition system  100  may be connected to the exhaust fixtures, including the exhaust motor, of a building or other structure that houses the deposition system  100 . 
     The intake and exhaust subsystem  150  preferably provides a laminar flow of air in the deposition region  146 . The laminar flow provided by the intake and exhaust subsystem  150  preferably assists in maintaining a focused stream of heat and chemical vapor from the burner towards the workpiece. A narrow and tight stream of flame enhances the thermophoretic effect that attracts the SiO 2  particles to the workpiece because the SiO 2  particles get hotter while the surface of the workpiece remain relatively cooler. 
     The enclosure  104  for the deposition chamber  102  preferably is comprised of preferably two “skins” or sets of walls that enclose the components that perform the deposition process. An inner skin of the two skins encloses the deposition components and is preferably entirely comprised of Hasteloy. Hasteloy is preferably used for the inner skin because of its chemically resistive properties at high temperatures. In an alternative embodiment, the inner skin of the enclosure  104  is comprised of stainless steel and has its interior walls lined with Hasteloy. In yet another alternative embodiment, the inner skin of the enclosure  104  is comprised of stainless steel with an interior lining of Teflon. Stainless steel, however, has generally been observed to rust due to the presence of HCl in the reaction chamber. Furthermore, the lining of Teflon generally may begin to peel from the steel surface at temperatures above about 420° C. 
     The outer skin of the enclosure  104  preferably is separated from the inner skin by an air gap, and preferably completely encases the inner skin. Alternatively, the outer skin encases a portion of the inner skin that is most exposed to the heat inside the deposition chamber. The outer skin is preferably formed of stainless steel, stainless steel lined with Hasteloy, stainless steel lined with Teflon, or another convenient material or combination of materials. The outer skin is preferably not substantially exposed to corrosive chemicals like the inner skin, so any of the above alternative compositions may be conveniently selected. The outer skin preferably includes numerous vents  120  to cool the inner skin of the enclosure  104  and otherwise prevent the deposition chamber  102  from overheating. In one alternative embodiment, the air gap between the outer skin and the inner skin is actively cooled. FIG. 1A depicts the vents  120  on a front side of the enclosure  104 , on the door  105  and on the removable panels  107  of the deposition system  100 . Preferably, a similarly configured array of vents  120  is provided on the opposing side of the enclosure  104  of the deposition system. 
     Large quantities of oxygen (O 2 ) and fuel gas, typically in the form of hydrogen (H 2 ) or natural gas, are passed through the deposition chamber  102  to enable the deposition process of converting SiCl 4  into SiO 2  soot that is deposited in layers onto a workpiece. During the deposition process, even with the continuous flow of air, the inner skin of the enclosure  104  rapidly heats up. Due to the preferably advantageous heat transfer properties of the inner skin, heat in the deposition chamber  102  is preferably conducted to the inner skin&#39;s outer surface. The high temperature of the inner skin&#39;s outer surface generally causes the air between the inner skin and the outer skin to heat. The vents  120  on the outer skin preferably enable cooler air outside the deposition system  100  to enter one or more vents  120 , and enable the hotter air in the air gap between the two skins to exit other vents  120 . 
     In a preferred embodiment, the vents  120  are positioned in at least two rows, including an upper and a lower row. External air preferably enters the lower row of vents  120  and hot air exits the upper row of vents  120 . By funneling hot air out of the air gap between the two skins of the enclosure  104 , heat is effectively transferred out of deposition chamber  102 , through the inner skin, and out through the vents  120  on the outer skin of the enclosure  104 . Consequently, maintaining the temperature of the workpiece in the deposition chamber  102  is furthered, so that the average temperature of the workpiece is preferably significantly lower than the temperature of the silica soot particles that are expelled from the burner. Typically, the temperature of the workpiece is greater than but follows the temperature of the deposition chamber. Thus, an increased thermal gradient between the hot silica soot particles and the cooler workpiece is provided than may exist without the vents  120 , enabling an increased thermophoretic force that attracts the silica soot particles to the surface of the cooler workpiece. 
     FIG. 2 is a diagram depicting a functional view of the chemical vapor deposition system  100  generally shown in FIG.  1 . As depicted in FIG. 2, the chemical vapor deposition system  200  preferably includes a SiCl 4  source  202 , a nitrogen (N 2 ) source  204 , an oxygen (O 2 ) source  206 , and an H 2  source  208  as raw materials for the vapor deposition system  200 . Alternatively, the N 2 , O 2 , and H 2  sources  204 ,  206 ,  208  may be piped in from an external location. The deposition system  200  preferably further includes a computer  106 ,  210 , a gas panel  114 ,  212 , a preheater  214 , and a vaporizer  216  for controlling the flow of the materials used for the deposition process. 
     The deposition system  200  preferably includes a deposition chamber or cabinet  218 , enclosing one or more, and preferably two chemical burners  220 , a lathe  222  for holding a workpiece  224  and for moving the workpiece  224  rotationally and translationally relative to the one or more burners  220 . The deposition chamber  218  preferably encloses one or more end-torches (not shown) positioned near the ends of the workpiece  224 , and which move with the workpiece  224  (in an embodiment in which the workpiece  224  moves and the chemical burner  220  remains stationary). The end torches preferably direct heat to the ends of the workpiece  224  to prevent it  224  from breaking and/or cracking. Preferably, the workpiece  224  and the end torches move so that the exhaust around the chemical burner  220  is relatively constant. Alternatively, the chemical burner  220  is moving and the workpiece  224  and end torches are stationary (except for the rotation of the workpiece  224 ). The deposition system  200  preferably further includes an air intake and exhaust subsystem  150 ,  226  including scrubbers (not shown) and other pollution control devices for removing and collecting the gasses and vapors that are expelled by the deposition system  200 . 
     The computer  106 ,  210  preferably includes electronic connections to the vaporizer  216 , the gas panel  212 , and the deposition cabinet  218  for automatically controlling functions of each component. The computer  106 ,  210  preferably further includes a connection to a user-input device such as a keyboard, touch screen, knobs, buttons, switches, mouse and/or microphone for voice activated command input for providing operational control of the deposition system  200  to a user. Moreover, the computer  106 ,  210  preferably includes a user output device, such as a display monitor or speaker for presenting a status of the system. 
     The raw deposition materials&#39; sources  202 ,  204 ,  206 ,  208  are preferably reservoirs, which may be commercially available pressurized tanks for containing each constituent material. The SiCl 4  preferably is contained in a reservoir in liquid form, preferably at room temperature. The SiCl 4  source  202  preferably is connected by a pipe or line to the preheater  214 , such that SiCl 4  may be conveyed as a liquid into the preheater  214 . Preferably, positioned above the SiCl 4  source  202  is an exhaust port  203  to convey SiCl 4  to a pollution control system (not shown) in event of a leak of SiCl 4  from its source  202 . 
     Preferably, N 2  is used to maintain a pressure of preferably about 15 PSI on the SiCl 4  source  202  to enable flow of SiCl 4  out of the SiCl 4  source  202  and into the preheater  214 . Generally, N 2  is used throughout the deposition system  200  during non-operation to purge the components of the corrosive chemicals, such as HCl and SiCl 4 , which are used or produced when the deposition system  200  is in operation. 
     The preheater  214  preferably is a commercially available device comprising a container for holding small volumes of SiCl 4  and may include a heating element such a heating blanket around the container or a coil in the container for heating the liquid form of SiCl 4 . Alternatively, the preheater  214  may be a long line wrapped in heating tape that heats the flowing SiCl 4  liquid to a desired temperature before it reaches the vaporizer  216 . Preferably, the preheater  214  receives the SiCl 4  liquid at preferably room temperature and heats the SiCl 4  liquid to a temperature of preferably about 50° C. Preferably, the preheater  214  maintains a substantially constant level of SiCl 4  in its container throughout an operation of the deposition process. The preheater  214  is connected to the vaporizer  216  for transferring the heated SiCl 4  liquid out of the preheater  214  and into the vaporizer  216 . 
     The vaporizer  216  preferably is a commercially available device that comprises a container for containing a substantial volume of SiCl 4 , a heating element to heat the SiCl 4  in the container, and numerous valves (not shown) to regulate the flow of materials into and out of the vaporizer  216 . The vaporizer  216  preferably heats the SiCl 4  to a temperature of between about 70° C. and about 80° C. Preferably, the SiCl 4  is heated until it is boiling in the vaporizer  216 . The variation in temperature between about 70° C. and about 80° C. for the vaporized SiCl 4  preferably depends on the mass flow rate of SiCl 4  vapor that is required at the burner  220  for the deposition process. Higher temperatures generally provide higher mass flow rates. 
     The vaporizer  216  is preferably electronically connected to the computer  210 . Through this electronic connection, the volume of SiCl 4  in the vaporizer  216  is preferably regulated and maintained between a minimum and maximum level. The computer  210  preferably controls the flow of SiCl 4  liquid from the SiCl 4  source  202  to the vaporizer  216  from a solenoid valve  217  preferably located at or near the pneumatic input to the vaporizer  216 . 
     The vaporizer  216  is also pneumatically connected by a line to the N 2  source  204 . As a generally inert gas, the N 2  preferably is used in the vaporizer  216  both to purge parts of the vaporizer  216  of SiCl 4  when the deposition system  100 ,  200  is not in operation and to actuate deposition system&#39;s numerous pneumatic valves. Through control from the computer  210 , the SiCl 4  source  202 , the preheater  214  and the vaporizer  216  preferably provide a constant, automatic and prolonged flow of vaporized SiCl 4  from the vaporizer  216  to the one or more burners  220  in the deposition cabinet  218 . This characteristic of constant flow only ceases when and if the SiCl 4  source reservoir  202  is emptied. 
     A constant flow of preferably 100% SiCl 4  vapor out of the vaporizer  216  to the burner  220  is provided because the vaporizer  216  is preferably automatically refilled from the preheater  214  by the computer  210  without disrupting the flow of vaporized SiCl 4  from the vaporizer  216  to the burner  220 . The preheater  214  allows the vaporizer  216  to maintain the SiCl 4  contained within the vaporizer  216  at a temperature necessary to have vaporized SiCl 4  output from the vaporizer  216  at all times during operation of the deposition system  100 ,  200 . When the level of SiCl 4  in the vaporizer  216  drops below a predetermined minimum level, the computer  210  actuates the valve  217  controlling input of SiCl 4  to allow heated SiCl 4  into the vaporizer  216  from the preheater  214 . This valve  217  is then preferably closed when a predetermined maximum level of SiCl 4  is reached in the vaporizer  216 . To maintain the temperature of the SiCl 4  from the vaporizer  216  to the burner  220 , the line between the vaporizer  216  and the burner  220  for transferring the SiCl 4  vapor is preferably heated, such as with heating tape, to maintain the SiCl 4  as a vapor as the SiCl 4  is conveyed to the burner  220 . Because the SiCl 4  is a vapor, mass flow controllers (MFCs) preferably are used to regulate the flow as controlled by the computer  210 . 
     The gas sources  204 ,  206 ,  208  are preferably pneumatically connected to the gas panel  212 . The gas panel  212  includes valves and MFCs to regulate the flow of gasses from the gas sources  204 ,  206 ,  208 . Control of the valves in the gas panel  212  is provided by the computer  210 , which is electronically connected to the gas panel  212 . Lines for O 2  and H 2  are provided to pneumatically connect the gas panel  212  and the burner  220  in the deposition cabinet  218 . Further, a separate line is preferably provided to convey O 2  to the line carrying the vaporized SiCl 4  to the burner  220 . Thus, at a “T” fitting  219 , the vaporized SiCl 4  and O 2  are mixed, and continue as a mixture in their transport to the burner  220 . Preferably, the O 2  line exiting from the gas panel  212  to the “T” fitting  219  is similarly heated using any convenient means such as heating tape. Thus, the mixture of O 2  and SiCl 4  from the “T” fitting  219  to the burner  220  is maintained at a temperature such that the SiCl 4  at least remains a vapor, and preferably may have a temperature at about or above 100° C. as it approaches the burner  220 . 
     Thus, four separate lines are input to the burner  220 : a line conveying a mixture of vaporized SiCl 4  and O 2 , a line conveying H 2  or another convenient fuel gas, a line conveying O 2  for the combustion of hydrogen, and a line conveying O 2  to shield the SiCl 4  and O 2  mixture. Preferably, a fixed ratio of H 2  to O 2  is maintained, such as two-to-one H 2  by volume. In one preferred embodiment, the fixed ratio of H 2  to O 2  is three-to-one, due to the higher effective deposition rate that is observed. FIG. 3A depicts a preferred embodiment of a burner  220 ,  300  for use in the deposition system  100 ,  200 . The burner  300  preferably receives the four streams, and emits preferably four streams from a burner face  302 , each stream being emitted from one of at least four concentric rings  304 ,  306 ,  308 ,  310  of emission holes. 
     As illustrated in the side view of the burner  300  in FIG. 3B, the innermost ring  304  of holes is provided to emit the chemical stream  314  of vaporized SiCl 4  and O 2 . The second ring  306  preferably streams O 2  alone. As shown in FIG. 3B, this inner O 2  ring  306  acts as an inner shield  312  to prevent the oxidation reaction of the SiCl 4  into SiO 2  too close to the burner face  302 , which would eventually cause a build-up of glass soot at the burner face  302 . The third concentric ring  308  of holes emits a stream  316  of the fuel gas, preferably H 2 . The fourth (outer) ring  310  of holes preferably emits a stream of O 2 , often referred to as the fuel oxygen  318 , which is used in the combustion process and to control the shape of the flame. 
     As the constituents are emitted from the burner  300 , the fuel gas and the oxygen are ignited. The SiCl 4  particles react in the flame at a controlled distance away from the face of the burner  300 . The SiCl 4  particles passing through the flame are oxidized to form silica soot that continue in a directed stream toward a workpiece  224  that may initially be in the form or a start rod. As silica soot approaches the workpiece  224 , the silica soot has a temperature on the order of about 1100° C. The chlorine is preferably separated from the other materials and combines with hydrogen to ultimately form hydrochloric fumes (HCl). These reactions generally apply to the deposition process for a cladding on an optical fiber preform. Other constituents may be used for chemical vapor deposition for other applications applying the different embodiments and aspects of the chemical vapor deposition system described herein. 
     Referring again to FIG. 2, the silica soot is deposited in layers on a continuously moving workpiece  224 . The workpiece  224  is mounted on the lathe  222 , which preferably rotates and translates the workpiece  224  relative to the burner  220 . As shown in FIG. 4, the lathe  222 ,  400  preferably includes end holders  402  into which the ends of the workpiece  224  (e.g., the start rod) are inserted. The lathe  400 ,  222  further includes at least one and preferably two motors  404  and  406  for moving the workpiece  224  relative to the burner  220  both rotationally and translationally. The motors  404 ,  406  are preferably controlled by a computer  210 , such as that depicted in FIG. 2, for controlling the speed of rotation and translation of the workpiece  224  throughout the course of the deposition process on the workpiece  224 . 
     In one preferred embodiment, a particular translation characteristic is applied to the workpiece to minimize a slow tapering effect on the resulting workpiece  224 . This tapering effect, or “footballing,” as illustrated in FIG. 5 may cause a loss of significant portion of the useful length of the preform when the preform is finally ready for drawing. Because of the need to maintain the proper diameter ratios for the core and cladding in the final preform, the tapering effect in the deposition of soot on the workpiece  224  causes a significant length at the ends of the final preform to be unusable, or if used, typically results in a degraded quality of optical fiber. Generally, the unusable portion may be more than about 20 cm at each end of the preform. 
     The tapering effect may be substantially reduced by translationally moving the workpiece  224  at a maximum speed of greater than about 1.4 meters per minute, and preferably greater than about seven (7) meters per minute during at least a part of the deposition process, and preferably after the first several deposition passes have been completed. In one embodiment, the lathe  222 ,  400  translationally moves the workpiece  224  according to a motion profile, an example of which is depicted in FIG.  6 . In FIG. 6, the example shows a motion profile where the maximum velocity is eight (8) meters per minute. Preferably, as the ends of the workpiece approach the burner, the workpiece decelerates at a constant deceleration (e.g., −250 mm/sec 2 ) and then accelerates with an opposite constant acceleration (e.g., 250 mm/sec 2 ). Preferably, the maximum speed and acceleration is limited only by the stress limitations of the workpiece caused by such motion. Applying such a motion profile, the unusable portion at each end of the final preform may be significantly reduced. 
     The positive effect on the workpiece  224  due to the increase in its translational speed may be explained by several factors. First, because of the change in direction of the motion at the ends of the workpiece  224 , the workpiece  224  at the ends tends to be hotter than at other locations. Greater heat at one location tends to cause the soot on the workpiece  224  to densify more substantially than at other locations where the temperature is lower. This densification results in a smaller diameter workpiece  224  at these locations. The smaller diameter translates into a smaller target for the streaming silica soot, and because a smaller target results in lower collecting efficiency, the deposition rate at the ends is reduced, thus increasing the tapering affect. 
     Second, the additional heat at the ends of the workpiece  224  generally decreases the temperature gradient between the silica soot particles and the workpiece  224 , and thereby, decreases the thermophoretic effect. The reduction in the thermophoretic effect causes a reduction in the deposition efficiency at the ends of the workpiece  224 . The diameter at the ends therefore increases even more slowly than at other locations on the workpiece  224 . 
     By translating the workpiece at speeds above a certain threshold, preferably the workpiece  224  at no location exceeds a predetermined temperature, and therefore the cumulative effect of the above factors is greatly reduced. The result of a reduced “footballing” effect translates into a larger useful preform and a substantially increased effective deposition rate. 
     With respect to the rotation of the workpiece, in one embodiment, the workpiece is rotated at a speed of greater than about 60 rotations per minute (RPM) to maintain a substantial thermophoretic effect between the soot particles and the workpiece. In a preferred embodiment, the speed of rotation is randomly varied between about 60 RPM and 80 RPM on each translational pass over the burner to reduce a potential for “a rippling effect” in the layering of soot on the workpiece. 
     FIG. 7 depicts a preferred embodiment of a process  700  of performing chemical vapor deposition such as may be performed by the chemical vapor deposition system  100  illustrated in FIG.  1 A. Optionally, in a first step  702 , a length for a start rod is set. In different runs of the vapor deposition system, start rods of various lengths, preferably between about 0.8 meters and about 2 meters, may be used. Preferably, a length of a start rod is input at an operator terminal and transmitted to a computer. The computer then communicates with components of the deposition system that have functions dependent on the start rod length. Specifically, the lathe may be programmed according the length of the start rod that is used for a particular run of the deposition process  700 . When the lathe receives the length value from the computer, the torch at one end of the lathe is preferably automatically repositioned to apply heat to one end of the rod. The other torch is preferably stationary. Furthermore, the motor controlling the translation of the rod executes a traverse motion profile that reflects the entered length of the start rod. 
     In a next step  704 , a first pass of depositing silica soot is performed with a high flow of fuel gas and oxygen from the chemical burner relative to the flow of SiCl 4 . As one example, the flow rate of H 2  may be about 300 standard liters per minute, with O 2  at about 100 standard liters per minute, and SiCl 4  at about 25 grams per minute. Furthermore, on this first pass the traverse speed is relatively low, preferably at about 0.5 meters/minute. The resulting high heat of the soot stream and of the workpiece on this first pass hardens the initial interfacial layers between the start rod and the cladding layers that are subsequently deposited, preferably preventing interface defects and slippage of the soot over the start rod. 
     During the first pass and throughout a run of a deposition process  700 , certain parameters are preferably fixed throughout the run. Specifically, the end torches at each end of the start rod provide a flame that preferably provides a source of heat. The end torches provide heat at the ends of the workpiece to prevent the soot from cracking and to eliminate the soot slippage over the start rod during sintering, by keeping the ends denser and tightly adhered to the handle glass. The rod should be hot enough to affix the ends of the soot to a particular point on the start rod. However, if the end burners provide too much heat, then generally the start rod bends. Furthermore, to enhance the effective deposition rate, a fixed distance between the torch and the workpiece is preferably maintained throughout the run. Thus, as the workpiece increases in diameter, the chemical burner preferably retreats from the axial center of the workpiece in conformance with the increase in diameter of the workpiece. 
     In a next step  706 , the traverse speed of the workpiece is ramped up, preferably to a maximum speed exceeding seven meters per minute. Furthermore, the flow rates of fuel gas and oxygen from the chemical burner are ramped down and the flow rate of SiCl 4  is slowly increased. 
     In a next step  708 , the flow rates of the fuel gas and oxygen are ramped up slowly. Consequently, the density of the workpiece is slowly decreased to near a threshold density, below which the workpiece may begin to crack or break. This threshold density is preferably approached to maximize the effective deposition rate. For example, in the deposition of silica soot for optical fiber preforms, the workpiece generally cracks at densities below about 0.3 grams/cm 3 . However, to maximize the effective deposition rate, the density is preferably maintained near the about 0.3 grams/cm 3  threshold. The approach of the threshold minimum density, however, is preferably approached gradually to minimize the potential for bubbles trapped in the preform. 
     While preferred embodiments of the invention have been described herein, and are further explained in the accompanying materials, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification and the drawings. The invention therefore is not to be restricted except within the spirit and scope of any appended claims.