Abstract:
A torch and mount assembly wherein the torch, which has a plurality of gas receiving nipples is carried in a housing through which it extends. The housing has a plurality of walls forming a substantially air tight enclosure. In at least one wall of the housing are one or more leakproof fittings which are connected to similar leakproof fittings for the nipples by means of preferably flexible gas conduits within the housing. In the system, gaseous mixtures are applied from a source or sources to the fittings in the housing wall remote from the high temperatures of the torch, while the conduits within the housing are protected from mechanical and thermal stresses. An inert gas from a suitable source is introduced under pressure into the interior of the housing to reduce leakage from the torch into the housing. The atmosphere within the housing is heated, and the constituents of the housing atmosphere are monitored to aid in detecting leakage.

Description:
RELATED APPLICATIONS 
     This application is related to and deals with similar subject matter to U.S. patent application Ser. No. 10/215,837 of Tadeusz Olewicz, filed Aug. 9, 2002, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a torch assembly for use in a vapor axial deposition (VAD) process in forming optical fiber glass preforms. 
     BACKGROUND OF THE INVENTION 
     In the fabrication of optical fibers, it is customary to create a preform, which is a long glass rod having a central core and which is, in effect, a magnified or enlarged version of the optical fiber to be drawn therefrom. The preform consists of an inner core and an external cladding having an index of refraction profile that reproduces the index profile of the drawn fiber. 
     There are three major processes for making preforms, the modified chemical vapor deposition (MCVD) process, the outside vapor deposition (OVD) process, and the vapor axial deposition (VAD) process. All of these processes utilize vapor deposition, in which a “soot” is deposited on the surface of a starting object such as the interior surface of a starter tube or a starting or target rod to form a glassy layer on the surface. 
     It is with the VAD process that the present invention is primarily concerned, although it is to be understood that the principles and features of the invention are adaptable to other processes as well. 
     In the VAD process, silica vapors flowing through a heating torch deposit the desired silica particle layers on the rod. As the sooty particles build up to a desired diameter, the target rod is moved upward to make room for further growth, while the torch or torches are fixed in position. When the build-up or deposition is complete, the rod is removed and the resulting preform is sintered or consolidated into a solid preform. 
     Although the VAD process is widely used, in a production milieu certain problems arise which prevent complete satisfaction with the method, and more particularly, the apparatus. Thus, the glass torches, which must be precisely set for satisfactory and reproducible results, tend, over an extended period of use to become leaky, unstable, and not very consistent in producing repeatable results. For example, the various gases and vapors fed to the torch are generally transmitted through plastic tubing to Teflon fittings which, in turn, connect to the glass torch nipples. These Teflon fittings tend to loosen up over extended periods of use, which can, and often does, give rise to leakage, thereby destroying the calibrated delivery of the gases and vapors to the torch. The tendency to loosen is due, at least in part, to temperature fluctuations and also to lack of strain relief in the plastic delivery tubing. Lack of strain relief is a very important problem, since even small mechanical stress on the tubing can loosen the Teflon to glass interface and cause leaks. For example, during routine machine maintenance, the mechanic can accidentally push the tubing and thereby loosen the interface. A further problem, which can actually be more serious in that it can shut down the production line, is that heretofore the mounting of the tubular glass torch to the adjustable mounts, made necessary for precise positioning of the torch, can cause cracking or breaking of the glass of the torch, necessitating replacement thereof, which can cause as much as a week&#39;s delay in production. The glass tube of the torch is, generally, clamped to the adjustment stages using a V-block type clamp. By nature, the glass torch is extremely delicate as well as expensive, and in use, this type of clamping technique, if too tight a clamp, can cause cracking or breakage of the glass and, too loose a clamp results in process instability, thus a certain amount of operator skill is required in positioning and clamping the torch. 
     U.S. patent application Ser. No. 10/215,837 (hereinafter Olewicz), discloses and claims a torch assembly which is designed to and does obviate many of the aforementioned drawbacks of prior art torch assemblies. The invention of that application comprises a non-fragile mounting system having a cylindrical main glass tube surrounded by a tough and rigid tube of metal. The metallic tube is threaded at each end for receiving compression nuts, each having a ferrule such as an O-ring or a tapered ring inside. Each ferrule is a slip fit over the main glass tube of the torch and has an angled surface that butts against the end of the metallic tube. When the nuts are tightened, the sloped surfaces of the ferrules force the ferrules against the main glass tube, thereby fixing it in place within the metal tube. The metal tube in turn is clamped by a mounting clamp which is, in turn, mounted to an adjustment stage for optimum positioning of the torch. A support rail has adjustably mounted thereon one or more strain relieving clamps for holding gas or vapor delivery tubes in place to insure proper delivery of gases and vapors to the torch. More specifically, the glass torch comprises an outer main tubular body within which may be a plurality of coaxial glass tubular bodies of diminishing size for creating a plurality of gas and/or vapor delivery passages. Such nested nozzles are shown, for example, in U.S. Pat. No. 4,627,866 of Kanamori et al., and in accordance with the Olewicz invention each of the concentric delivery passages is attached to Teflon fittings connected to glass nipples formed on the nested nozzles. The stress induced in the glass main nozzle of the torch is evenly distributed over an area around the entire torch periphery and is well below critical temperature levels for glass. It has been found that hand tightening the compression nuts is more than adequate to secure the torch firmly within the glass clamp tube. Thus, the danger of too much stress being applied to the torch main tube even during temperature fluctuations, which heretofore could produce cracking or breaking, is no longer a consideration. The torch clamp tube, and not the thin glass tube, is used to mount the torch to the adjustment stages. The mounting arrangement of the invention, therefore, is more robust and safe, and provides several other benefits in addition to those just discussed. For example, the ferrule is made of elastic and resilient material such as Viton or Teflon, or other high temperature plastic and acts as a temperature compensating member during expansion and contraction of the glass and the metal. It also acts as a heat isolator allowing the gases within the torch to stay warm; acts as a vibration and shock isolator/damper, thereby protecting the glass torch; and it provides easy rotary and linear (in/out) coarse torch alignment adjustment. 
     The aforementioned Olewicz torch assembly overcomes many of the problems discussed in the foregoing, but it has been found that the Teflon fittings, under extremes of heat, tend to expand more than the glass of the torch, giving rise to leaks and, further, when the torch is moved, there can be relative movement between the glass ports and the fittings, giving rise to further leakage. For the most part, these leaks are small, at least to begin with, and thus can remain undetected while, at the same time, reducing the quality of the glass rods produced by the system. Thus, in order to insure quality production by the torch assembly, it is desirable that the heat effects on the fittings and movement thereof be reduced to a minimum, and early detection or leaks be achieved by the torch system. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a torch housing assembly that isolates the torch from stresses originating outside the housing, e.g., mechanical stresses associated with assembly and disassembly, and stresses resulting from movement of the gas feed lines relative to the torch, e.g., movement of the torch to desired heating areas of the rod. The housing further makes possible fixed, relatively stress free, connections to the gas feed lines and allows a measure of observation of the torch and gas supply while in use, thereby facilitating detection of cracks and leaks in the torch assembly. The housing itself is substantially air tight when the torch is mounted therein 
     In greater detail a torch such as that shown in Olewicz is mounted in the housing, with all of the gas feeds to the individual glass tubes being contained in the housing. Each of the several glass nipples connected to the individual torch tubular members is connected by a temperature compensating Teflon fitting to one end of a gas delivery tube, which may be, for example, corrugated or accordion pleated Teflon tubing. The fitting itself is similar to those used in the Olewicz patent and comprises a stainless steel tubular threaded member having Teflon ferrules or gaskets therein for clamping the glass nipple to the Teflon gas feed tube. The other end of the gas feed tube is connected, within the housing, to a similar fitting rigidly mounted in a wall of the housing, as by welding, and the external gas delivery tube for the particular nipple is mounted on the external end of the wall mounted fitting. With such an arrangement, the gas delivery tube connected to the nipple is isolated from any exterior stresses, and there is no relative movement of the nipple and gas tube that can occur within the housing. The housing is filled with an inert pressurized gas which, if a leak within the housing occurs, prevents the chemicals and gases from leaking out of the torch. Instead, the inert gas will tend to leak into the torch, which does not alter the proper application of chemicals to the rod. An external gas mixture sensor with an internal probe is connected to a wall of the housing, and the gas within the housing can be monitored for the presence of any of the gases, e.g., hydrogen, oxygen, and HCL, used in the torch. Presence of the component gas or gases will indicate a leak sufficiently large to overcome the presence of the insert gas. Thus, the pressurized gas can be seen to perform two functions as noted. 
     It is desirable that the interior of the housing be heated, primarily to prevent gas condensation. To this end, the housing has a heating member, such as heating rod, projecting into the interior thereof A temperature sensor is also placed or mounted inside the housing in a location remote from the heating element. This sensor can be in the form of a thermocouple which controls the heating elements. All of these accessory elements are coupled into the interior of the housing through leak-proof coupling members which are basically the same as those for the gas delivery tubes. 
     The top of the housing is a transparent cover of suitable material such as a transparent, heat resistant, plastic. This enables the operator to observe the interior of the housing to monitor for leaks, cracks, or gas condensation. Even though some leaks or cracks may be small, there will, over a short time, be an observable soot build up at the location of the crack or leak, which tells the operator that, for example, one of the tubes of the torch, or the torch itself, needs immediate replacement. 
     These and other features and principles of the present invention will be readily apparent from the following detailed description, read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic depiction of a VAD system. 
     FIG. 2 is a perspective view of an Olewicz type torch for the VAD process of the type for which the present invention is designed. 
     FIG. 3 is a perspective view of the torch and torch housing of the present invention; 
     FIG. 4 is a perspective, partially exploded view of the torch housing of the present invention; 
     FIG. 5 is a cross-sectional view of the temperature compensating gas delivery fitting of the present invention; and 
     FIG. 6 is a view of a ferrule for use in the fitting of FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic view of a VAD system  11  comprising a starting rod  12 , which may be of silica glass, one end of which is mounted in a chuck (not shown) and rotated as indicated by the arrow. Initially the end of the rod is directly in the flame of one or more oxygen-hydrogen torches  13  and  14 . Vapors from a chemical delivery system  16 , shown in dashed lines, flow into and through the glass torches  13  and  14  where they react via flame hydrolysis to form sub-microscopic particles which are deposited on the starting rod  12 . As the sooty particles build up, the starting rod  12  is moved upward to make room for new growth and continuously rotated to maintain cylindrical symmetry. A position control comprising, for example, a laser  18  and detector  19  may be used to insure proper location of the flame from the torches relative to the starting rod, and, where the torch and rod combination is contained in a housing  21 , a housing exhaust  22  may be, and usually is, included. After a soot preform  17  of proper size is built, it is removed from the chuck, dehydrated, and the preform  17  is consolidated, which removes trapped gases and water vapor, to produce a solid glass preform  17  ready for subsequent operations, such as, for example, rod in tube formation and then drawing into fiber. 
     As discussed hereinbefore, during production runs over extended periods of time, numerous problems arise, and it is to the reduction or elimination of these problems that the Olewicz invention is addressed. FIG. 2 is a perspective view of the VAD torch  23  of the Olewicz invention, shown mounted to an adjusting stage  24  for proper positioning of the torch  23 . Except for the mounting. and gas delivery system, the torch of the present invention is substantially the same. Torch  23  comprises a main glass tubular member  26  which, as will be discussed in greater detail hereinafter, is contained in a clamping tube or sleeve  27  of suitable signed and non-fragile material, preferably aluminum, which does not tend to contaminate glass, even at elevated temperatures. The clamping sleeve  27 , which is mounted to the adjusting stage  24  by a two piece clamp having a bottom portion  28  and a top portion  29 , effectively isolates the main glass tubular member  26  from high mounting clamping pressures, thereby protecting it from possible breakage. Each end of the clamping sleeve or the tube is threaded to receive nuts  31  and  32 , the function of which was discussed hereinbefore. 
     An elongated support rail  33  is mounted to the clamping sleeve  27  by a two part support clamp comprising a bottom portion  34  and a top portion  36  and functions to support a plurality of strain relieving side tube clamps  37 , which hold the numerous gas and vapor delivery tubes, such as tube  38 , without stressing or other undue distortion, with both transverse and longitudinal adjustment. 
     In the VAD process, it is usual that several gases, such as oxygen and hydrogen (for the burner flame), and several soot or vapor mixtures are applied through the torch  23  to the starting rod  12  and the preform  17  during the deposition operation. To this end, there is a plurality of nested secondary glass tubes  39 ,  41 ,  42 ,  43 ,  44 ,  46 , and  47 . Main glass tubular member  26  and each of the secondary tubes has a vapor (or gas) delivery nipple,  48 ,  49 ,  51 ,  52 ,  53 ,  54 ,  56 , and  57  respectively, to which is mounted a temperature compensating fitting  58  to which are attached the gas or vapor delivery tubes  38 . The fittings form substantially leak proof connections to the nipples. This nesting of the secondary tubes as shown in FIG. 2 is shown and explained in U.S. Pat. No. 4,627,866 of Kanamari et al., as well as the Olewicz application. 
     The torch arrangement  23  of FIG. 2 reduces leakage and misalignment of the components to a considerable extent. However, it can be seen that the gas delivery tubes  38 , the fittings  58  and the nipples  48 - 57  are all exposed, and, despite the stress relieving side tube clamps  37 , stresses can still be introduced by accident because of the exposure of the parts, as previously discussed. 
     In FIG. 3 there is shown a preferred embodiment  61  of the invention, whereby, as will be apparent hereinafter, accidental movement and stresses on the torch are substantially completely prevented. The preferred embodiment  61 , as shown in FIGS. 3 and 4, comprises a housing member  62  of a suitable material such as, for example, aluminum, having first and second side walls  63 ,  64 , a base plate  66 , and front and rear walls  67  and  68 . The front wall  67  has a central aperture  69  therein for receiving the torch  71 , which is, preferably, basically the same torch configuration shown in the Olewicz patent. As can best be seen in FIG. 4, first and second slots  72  and  73  which extend from either side of aperture  69  insertion of the torch with its extending side nipples. Plates  74  and  76 , separated by an O-ring (not shown) in which torch  71  is precisely positioned and which is guarded by means of a flanged aluminum torch guard  77 . The torch guard  77  may be replaced by the torch clamping tube  27  of Olewicz, if desired. 
     In order for the gas or vapor to be delivered to the torch  71 , side wall  64  has a plurality of apertures  78  bored therethrough for receiving leakproof temperature compensating gas delivery fittings  79  which will be discussed more fully hereinafter, and which are preferably welded to side wall  64 . The gas delivery fittings  79  receive gas from a source  85  through preferably flexible tubes or conduits, as shown in the Olewicz application. It is to be understood that the apertures  78  and fittings  79  might be in one of the other walls  62  or  63 , but the arrangement shown in FIGS. 3 and 4 is to be preferred. Each of the nipples  81  extending from the sides of the torch  71  from each of the tubular glass members  82  has fitted on the distal end thereof a leakproof temperature compensating connector  83  which is substantially identical to fittings  79 , to be discussed more fully hereinafter, and which is connected to a corresponding fitting  79  by, for example, a corrugated or accordion pleated Teflon tube  84 . It can be seen that in order for the bottom row of fittings  79  to be connected to the corresponding fittings  83 , the tubes  84  are folded under the torch  71 . The smallest diameter tubular member  86  of torch  71  is connected to a separate leakproof temperature compensating gas delivery fitting  87  which extends through the rear wall  68  of housing  62 , mounted in a bore  88 . 
     In addition to the bore  88 , there are three other ports or bores  89 ,  91 , and  92 , which contain temperature compensating leak proof fittings  95 ,  96 ,  97 , respectively. An additional bore or port  93  may also be formed in wall  68  for another fitting (not shown). It is to be understood that the location of the various bores may be other than what is shown in FIGS. 3 and 4, so long as they serve the following functions. 
     Bore  91  and its associated fitting  96  are for mounting a heater cartridge (not shown) which functions to heat the interior of housing  62 , and bore  89  and its associated fitting  95  are for mounting a thermocouple or other temperature sensing device (not shown), which is spaced from the heater, to control the temperature within housing  62 . Heating the interior of the housing makes it unnecessary to cover the heating lines of the torch with heat tape, which could obscure from direct view the various failure indications due to condensation, or torch fracture. The heater and temperature sensing device are of standard commercially available types and are not shown to avoid crowding FIG.  3 . 
     It is desirable to pressurize the interior of housing  62  with an inert gas. Thus, if a leak path such as a crack in the torch or a loosened fitting occurs, the pressure prevents the gases inside the torch from leaking to the exterior thereof. On the contrary, the inert gas will leak into the torch interior, which is not detrimental to the proper operation of the torch and the heating process. To this end, bore or port  92  and its fitting  97  form a leak proof means for introducing the inert gas from a source  100  via a flexible conduit  105  into the interior of housing  62 . 
     Bore  93  and its fitting (not shown) provide access of the housing interior for allowing gas from inside housing  62  to flow out at a small rate through sensors  90 , one of which is shown substantially for monitoring for leakage. Sensor  90  analyzes the interior gases for hydrogen, oxygen, and HCL which will indicate when any of these materials has leaked into the interior of the housing. It is to be understood that other chemical components may also be involved and detected. 
     Housing  62  has a transparent cap  94  fitted over the top thereof so that the interior of housing  62  may be observed. When a crack in the torch or other leak occurs, there results a buildup of soot at the location of the leak, which can be observed by the operator through the transparent cap  94 , thereby indicating a leak or even the very start thereof to serve as a warning that the incipient fault should be corrected before it becomes great enough to interfere seriously with the proper functioning of the apparatus. 
     The numerous temperature compensating fittings, such as  79 ,  83 , et al. may take the form of such fittings disclosed in the Olewicz application, or such other forms that serve the function of forming substantially leak-proof connections between dissimilar materials. The Olewicz type fitting for connecting a glass nipple  81  to a gas feed tube  84 , for example, is shown in FIGS. 5 and 6. FIG. 5 is a cross-sectional view of the fitting  83 . Inasmuch as both ends of the fitting  83  are substantially identical, corresponding parts at each end bear the same reference numerals. Fitting  83  is shown mounted on a nipple  81  of the torch  71 , and comprises member  98 , preferably of stainless steel, for example, having a longitudinal bore  99  extending therethrough. At each end of member  98  is a reduced diameter portion  101  having external threads  102  at the ends thereof, and preferably having an internal chamfered end  103 . A circular mounting ferrule  104  preferably of Teflon and having a slope or angled surface  106 , as shown in FIG. 6, is mounted in a slip fit on the nipple  81  and, at the other end on gas delivery tube  84  such that the slope  106  bears against the chamfered end  103 . A compression nut  107  is threaded onto the threads  102  at each end and bears against the rear surface  108  of ferrule  104 . When the nuts  107  are tightened, the ferrules  104  are cammed by the slopes  106  and chamfers  103  into tight engagement with both nipple  81  and gas delivery tube  84 , forming a leakproof coupling between nipple  81  and tube  84 , without damage to either the nipple  81  or the tube  84 . During operation, the fitting  83  is subject to wide swings in temperature which will cause it, over time, to loosen slightly on nipple  81 , thereby creating possible leakage. Teflon expands with heat to a greater extent than glass, and this differential expansion can cause leakage, as well as altering the grip of ferrule  104  on nipple  81 , which can be observed through transparent cap  94 . When this occurs, the nuts  107  can be tightened to restore the grip of the ferrule  104  on nipple  81  with only a momentary interruption in the process. Because member  98  is preferably of stainless steel, or other suitable material, it has a lesser coefficient of expansion with heat than does Teflon, hence it functions to maintain the grip of the Teflon ferrule on nipple  81 . The same leakage prevention scheme may be used with the other fittings  79 ,  95 ,  96 , and  97 , and, with some modification, fitting  87 , however, these fittings are not subject to the temperature fluctuations to the same extent as fittings  83 . The monitoring line fitting in bore  43  (not shown) may also be of the same configuration as that shown in FIG.  5 . 
     A principle feature of the present invention, as is readily apparent from the foregoing, is the stress isolation of the torch. The isolation as shown and described makes possible what has been virtually impossible heretofore, and that is that the torch can be oscillated about a mean position to increase flame coverage or to have the flame cover an area of specific geometry without fear of creating leaks or causing other damage to the torch. 
     It is to be understood that the various features of the present invention might be incorporated into other types of torch mechanisms and that other modifications or adaptations might occur to workers in the art. All such variations and/or modifications are intended to be included herein as being with the scope of the present invention as set forth in the claims hereinafter. Further, in the claims, the corresponding structures, materials, acts, and equivalents of all means or step-plus-function elements are intended to include any structure, material, or acts for performing the functions in combination with other elements as specifically claimed.