Abstract:
An apparatus and method for sealing the top of an optical waveguide draw furnace is disclosed. The apparatus includes an assembly constructed and arranged to removably cover the top of the draw furnace while mating with the downfeed handle. The apparatus includes an elongated sleeve having a base and a sealing mechanism positioned on the sleeve at a location remote from the base. The sleeve defines a chamber for receiving the downfeed handle, and the sealing mechanism is arranged with respect to the sleeve to mate with the downfeed handle received in the chamber. In one aspect of the invention, the apparatus further includes an inert gas purge for providing an inert atmosphere within the chamber. A method of sealing the top of an optical waveguide draw furnace is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to a method and assembly for sealing a draw furnace, and more particularly, to a sealing assembly and method for sealing the top of an optical waveguide draw furnace.  
           [0003]    2. Technical Background  
           [0004]    Relatively high temperature heat sources are required for drawing high strength, low loss optical waveguide fibers from high silica-content optical waveguide fiber preforms or blanks. Two of the predominant heat sources utilized for drawing such fibers are zirconia furnaces and graphite furnaces. Fiber draw furnaces generally operate at temperatures greater than about 1900° C., typically as high as about 2050° C.  
           [0005]    A disadvantage associated with zirconia induction furnaces is that extended use and thermomechanical stresses due to poor thermal shock resistance cause cracks in the muffle and susceptor. This cracking causes zirconia particles to migrate from the inner surface of the furnace onto the preform and/or fiber being drawn from the preform resulting in substantially weakened fiber and unacceptable product losses. Moreover, zirconia induction furnaces are sensitive to rapid changes in temperature. Accordingly, significant amounts of time are necessary for increasing and decreasing the temperatures within the furnace. Rapid heating and cooling of the furnace results in fracturing of the zirconia muffle, which necessitates replacement of the muffle and results in significant furnace down time.  
           [0006]    Such shortcomings lead to the development of graphite induction furnaces. Graphite induction furnaces typically include a graphite muffle which is insensitive to rapid changes in temperature and thus is less susceptible to cracking. It has been found, however, that graphite furnaces are susceptible to oxidation at temperatures approaching about 450° C. and that oxidation is particularly problematic when the graphite is exposed to the high temperatures used to draw optical waveguide fibers. For this reason, among others, it is preferable to maintain an inert atmosphere within the draw furnace. Oxidation occurs when gasses from ambient atmosphere react with the solid carbon muffle at high temperatures according to the following reactions:  
           C+O 2 →CO 2 ;  (1)  
           and  
           C+CO 2 →2CO.  (2)  
           [0007]    A typical onset temperature for reaction (1) for a graphite grade used in a draw furnace is about 700° C. Reaction (2) becomes significant above 900° C. These reactions of the furnace muffle with oxygen and carbon dioxide cause the furnace muffle to be consumed, especially at elevated fiber drawing temperatures, and are referred to generally as CO events.  
           [0008]    The graphite muffle material is a composite of graphite grains bonded together by a carbon binder matrix. It is believed that the binder material is more susceptible to oxidation than the graphite grains. Therefore, when the composite of the two materials is exposed to air, and thus oxygen at temperatures above the oxidation onset temperatures, the matrix binder material preferentially oxidizes. The graphite grains, having no binder left to hold them in place, are then free to fall away from the composite structure. It is believed that this mechanism causes graphite particulate to migrate from the muffle wall to the fiber preform and/or fiber during drawing.  
           [0009]    Graphite particulate that becomes incorporated into the fiber during drawing causes unacceptable product losses due to point defects. Point defects manifest themselves as sharp attenuation increases in the signal transmitted through the fiber. Point defect product losses due to graphite particulate from draw furnaces can be greater than about 5%, which is an unacceptably high loss. Graphite particulate that has adhered to the fiber during the draw process also contributes to fiber breaks.  
           [0010]    In order to reduce graphite particulation produced by oxidation of the graphite muffle material, and thus the number of resulting point defects, an inert gas is typically supplied to the interior of the furnace to prevent ambient air and other gases from entering the furnace. Unfortunately, there are a number of operations that occur during optical waveguide draw operations which present ambient air and other gases with the opportunity to enter the furnace despite delivery of high inert gas flow rates into the furnace. When a mistake is made during these draw operations, either by human error, by mechanical failure, or otherwise, ambient air and/or other gases are often permitted to enter the furnace and a CO event occurs.  
           [0011]    Many of these CO events occur during movement or operation of the top hat assemblies presently used to close the opening in the top of a draw furnace during blank load, unload, and idle periods. Typical top hat assemblies include a viton/rulon seal that is positioned adjacent the top of the optical waveguide draw furnace when the top hat assembly is positioned over the downfeed handle and lowered onto the draw furnace. In addition, these top hat assemblies incorporate a plurality of hooks used in manually lowering and raising the top hat assembly. The hooks are specifically employed to raise the top hat assembly while the downfeed handle is being removed from the draw furnace. Currently, an operator must engage the hooks prior to removal of the downfeed handle from the draw furnace to ensure that the top hat assembly is being withdrawn from the top of the draw furnace as the downfeed handle and the remainder of the waveguide blank are removed from the hot zone of the draw furnace. Otherwise, that portion of the downfeed handle exiting the hot zone of the draw furnace will approach the top hat assembly, and the extreme heat retained by the downfeed handle will melt the viton/rulon or other sealing member of the top hat assembly. Often times, however, an operator fails to engage the hooks, which ultimately results in the melting and thus the premature replacement of the top hat sealing member.  
           [0012]    Moreover, if an operator fails to disengage the hooks after the top hat assembly has been positioned on the top of the draw furnace, certain aspects of the fiber drawing operation itself can create CO events. Generally, once a seal is formed between the top hat and the top of the draw furnace, the bottom door assembly of the draw furnace is opened and fiber drawing commences. During the start of optical waveguide fiber draw, the optical fiber blank is typically partially raised back out of the hot zone of the draw furnace to stretch and thin the fiber in order to obtain a desired fiber diameter. If the hooks are still engaged with the top hat assembly during this time, the top hat raises off of the top of the draw furnace and the seal is broken. Since the bottom door assembly of the draw furnace is open during fiber draw, the broken top hat seal creates what is known in the art as a chimney effect. The chimney effect is the rapid intake of air through the bottom opening in the draw furnace caused by, inter alia, the opening of the top of the draw furnace. Ambient air is thus forced into the muffle of the draw furnace and the oxygen contained within the air reacts with the carbon muffle causing a CO event. The graphite particulate is then free to migrate to the fiber which results in fiber being formed with an unacceptable number of point defects. In addition, the entrance of air into the draw furnace during fiber draw also results in diameter upsets which also creates unusable fiber. Moreover, each CO event degrades the draw furnace muffle which in turn reduces draw furnace efficiency and necessitates muffle replacement.  
           [0013]    In view of these and other shortcomings, an improved assembly and method of sealing an optical waveguide draw furnace is needed to prevent the entrance of ambient air and other unwanted gases into the draw furnace.  
         SUMMARY OF THE INVENTION  
         [0014]    One aspect of the present invention relates to an apparatus for cooperating with a downfeed handle to seal the top of an optical waveguide draw furnace. The apparatus includes an assembly that is constructed and arranged to removably cover the top of the draw furnace and mate with the downfeed handle. The assembly includes an elongated sleeve having a base and defining a chamber for receiving the downfeed handle. The apparatus further includes an inert gas supply which communicates with the assembly to selectively deliver inert gas into the chamber.  
           [0015]    In another aspect, the invention relates to an apparatus for cooperating with a downfeed handle to seal the top of an optical waveguide draw furnace. The apparatus includes an elongated sleeve having a top and a base, and defining a chamber for receiving the downfeed handle. The apparatus further includes a sealing mechanism positioned on the sleeve at a location remote from the base. The sealing mechanism is arranged with respect to the sleeve to mate with the downfeed handle received in the chamber.  
           [0016]    In yet another aspect the present invention is directed to a method of sealing the top of an optical waveguide draw furnace. The method includes the steps of sliding an assembly onto a downfeed handle such that the assembly is supported on the downfeed handle. The assembly defines a chamber for receiving the downfeed handle, and has a base and a sealing mechanism remote from the base. The sealing mechanism cooperates with the downfeed handle to close one end of the chamber. The chamber is purged with an inert gas, and the downfeed handle is lowered into the optical waveguide draw furnace to position the assembly on the top of the draw furnace.  
           [0017]    The top hat assembly of the present invention results in a number of advantages over other top hat assemblies presently available. Principally, the top hat assembly of the present invention significantly reduces the number of CO events resulting from air entering the top of the draw furnace, and/or air entering through the bottom of the draw furnace due to unintentional removal of the top hat assembly when the bottom door assembly is in an open position, particularly during fiber draw. The reduction of CO events significantly reduces the number and frequency of point defect losses in optical waveguide fibers drawn in furnaces, particularly those having graphite muffles, and further, prolongs the life of the graphite muffles employed in draw furnaces. In addition, because the top hat assembly of the present invention obviates the use of hooks, operator error is substantially eliminated during fiber draw operations. Moreover, the construction and arrangement of the sealing mechanism of the present invention significantly extends the seal life, which in turn reduces draw tower down time associated with the replacement of spent seals.  
           [0018]    Additional advantages are provided by the sealing assembly and method of the present invention. The inert gas purge cooperating with the top hat assembly of the present invention displaces air trapped within the top hat assembly, which in turn prevents air from entering the top of the draw furnace during optical waveguide blank loading. Without the purge, air otherwise trapped in the top hat assembly would be drawn into the draw furnace when the bottom door assembly is opened to begin fiber draw. That air, once inside the furnace, causes diameter upsets and CO events. Thus, this embodiment of the present invention further reduces the number of diameter upsets and CO events during drawing operations.  
           [0019]    A single CO event in a graphite draw furnace during fiber draw typically results in a relatively large percentage of the optical fiber having unacceptable levels of point defects. Accordingly, cleaning of the draw furnace is essential following a CO event. Generally, such cleaning operations result in the draw furnace being down for approximately 6 hours to facilitate such cleaning and servicing operations. A further advantage of the present invention is the reduction in the amount of draw tower down time due to the substantial reduction in the frequency of CO events. Less down time results in increased production of optical fiber, and particularly optical fiber having an insignificant number of point defects.  
           [0020]    The top hat assembly and method of sealing the top on an optical waveguide draw furnace of the present invention significantly reduces both mechanical and human error during draw operations. Moreover, the assembly and method of the present invention eliminates the use of hooks with the top hat assembly, and thus minimizes operator interaction with the top hat assembly during drawing operations. Such an assembly and method will reduce the frequency with which sealing members in the assembly are currently replaced, reduce the number of diameter upsets, and will significantly reduce the number of CO events during drawing operations, particularly those resulting from the chimney effect.  
           [0021]    Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.  
           [0022]    It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.  
           [0023]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a side elevation view of a preferred embodiment of the top hat assembly shown positioned on a downfeed handle in accordance with the present invention;  
         [0025]    [0025]FIG. 2 is an exploded view of the top hat assembly of FIG. 1 shown in elevation;  
         [0026]    [0026]FIG. 3 is a partial perspective view of the top hat assembly of FIG. 1 showing the sealing mechanism mating with a downfeed handle in accordance with the present invention; and  
         [0027]    [0027]FIG. 4 is a side elevation view of the top hat assembly of FIG. 1 shown adjacent the top of an optical waveguide draw furnace and resting on the blank support portion of a downfeed handle in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. An exemplary embodiment of a top hat assembly of the of the present invention is shown in FIG. 1, and is designated generally throughout by reference numeral  10 .  
         [0029]    In accordance with the invention, the present invention for sealing the top of an optical waveguide draw furnace includes an assembly that is constructed and arranged to removably cover the top of a draw furnace while mating with a downfeed handle received in the draw furnace. A downfeed handle is an elongated tubular member, which supports the preform or blank, and facilitates the insertion of the blank into and the removal of the blank from the draw furnace. The downfeed handle also facilitates the movement and positioning of the blank within the draw furnace. Preferably, an inert gas supply communicates with the assembly to selectively deliver an inert gas to the assembly, thereby displacing any air residing within the assembly.  
         [0030]    As embodied herein, and depicted in FIG. 1, a preferred embodiment of the top hat assembly  10  includes a generally cylindrical sleeve  12  having an upper sleeve flange  14  and a base  16 . Top hat assembly  10  is preferably constructed from aluminum, but it could easily be fabricated using some other material, such as, but not limited to, steel. Shown attached to upper sleeve flange  14  is a sealing mechanism  18 , which will be described, in greater detail below with reference to the FIG. 2. Shown attached to base  16  is a bumper assembly  20  which will also be described in greater detail below with reference to FIG. 2. Although described herein as being attached to sleeve  12 , it will be understood by those skilled in the art that sealing mechanism  18  and bumper assembly  20  can be formed as part of sleeve  12  so as to form a unitary top hat assembly  10 .  
         [0031]    A purge adapter  22  preferably depends from the sidewall of sleeve  12  beneath upper sleeve flange  14 . Purge adapter  22  is sized and shaped to mate with a hose coil  24  extending from an inert gas supply reservoir  26 . Top hat assembly  10  is shown positioned on the top portion of a downfeed handle  28 . The lower portion of downfeed handle  28  includes a blank support portion  30 , which is used to engage the ball of an optical waveguide fiber blank (not shown) so that the blank is suspended beneath downfeed handle  28 .  
         [0032]    As shown more clearly in the exploded elevational view of FIG. 2, sealing mechanism  18  preferably includes a retaining ring  32  having a recess  34  for supporting an oil lip seal structure  36 . Oil lip seal structure  36  is preferably sized to tightly fit within recess  34  of retaining ring  32 . More preferably, oil lip seal structure  36  is press fitted within recess  34 , as with an arbor press, so that oil lip seal structure  36  is fixedly secured within retaining ring  32 . Sealing mechanism  18  is attached to upper sleeve flange  14  with any of a number of fasteners, such as screws  38 , and can easily be removed and disassembled for cleaning and seal replacement.  
         [0033]    Bumper assembly  20  includes a bottom plate  40  interposed between a bumper  42  and bumper assembly retaining ring  44 . Bottom plate  40  is slidably received onto bumper assembly retaining ring  44  and bumper  42  is attached to bumper assembly retaining ring  44  with fasteners such as retaining screws  46 . Bumper assembly  20  is mounted to base  16  of top hat assembly  10  with fasteners such as elongated screws  48  which are circumferentially spaced from each other and passed through base  16  and bottom plate  40 . In this way, bottom plate  40 , and thus bumper assembly  20 , is secured to base  16  of top hat assembly  10 .  
         [0034]    Thus constructed, top hat assembly  10  defines a tubular chamber  50 , which is open at its ends to receive a standard optical waveguide downfeed handle  28 , as shown in FIG. 1. Chamber  50  is sized to freely receive downfeed handle  28  such that an annular channel is formed between the inner surface of sleeve  12  and the downfeed handle  28 . A purge port  52  passes through sleeve  12  to place chamber  50 , and specifically, the annular channel formed around downfeed handle  28 , in fluid communication with inert gas  54  housed within inert gas supply reservoir  26 .  
         [0035]    The details of the oil lip seal structure  36  are more clearly shown in FIG. 3. Oil lip seal structure  36  supports an annular gasket  56 , which is sized to conform to the exterior circumference of downfeed handle  28  when downfeed handle  28  is received within chamber  50  of top hat assembly  10 . An annular spring  58  is interposed between gasket  56  and oil lip seal structure  36  to bias gasket  56  against the downfeed handle  28 . Accordingly, as downfeed handle  28  shifts within chamber  50 , continuous contact between gasket  56  and downfeed handle  28  is maintained.  
         [0036]    The operation and method of sealing the top of an optical waveguide draw furnace is more clearly understood with reference to FIGS. 1 through 4. As indicted in FIG. 1, downfeed handle  28  is received in chamber  50  (FIG. 2) of top hat assembly  10  such that gasket  56  (FIG. 3) conforms to the circumference of downfeed handle  28 . As depicted in FIG. 3, continuous pressure applied by spring  58  to gasket  56  forms a seal between gasket  56  and downfeed handle  28 . Inert gas  54 , preferably argon, is delivered from inert gas supply reservoir  26 , through hose coil  24  and purge adapter  22 , and into chamber  50  through purge port  52  (FIG. 2) to displace the air within chamber  50  with inert gas  54 . Because chamber  50  is closed at the top of top hat assembly  10  by the seal created between downfeed handle  28  and gasket  56 , air and inert gas  54  are discharged from chamber  50  through the annular gap between bumper  42  (FIG. 2) and downfeed handle  28 . Thus, only inert gas resides within top hat assembly  10  as top hat assembly  10  is lowered toward the draw furnace.  
         [0037]    During fiber blank loading, top hat assembly  10  is lowered as indicated by reference arrow A (FIG. 1) until bumper  42  rests on a lip  66  of blank support portion  30  of downfeed handle  28 . As a result, and as shown in FIG. 4, top hat assembly  10  rides on blank support portion  30  of downfeed handle  28  as downfeed handle  28  is lowered into the draw furnace prior to fiber draw. The waveguide blank (not shown) depending downwardly from blank support portion  30  of downfeed handle  28  is lowered into an optical waveguide draw furnace  60  until the blank reaches the hot zone (not shown) of draw furnace  60 . As downfeed handle  28  is received within draw furnace  60 , bottom surface  62  of bottom plate  40  contacts the upper surface of a gland seal  64  positioned on the top of draw furnace  60 . The downward progress of top hat assembly  10  is thus stopped while downfeed handle  28 , and thus blank support portion  30 , continue moving downward into draw furnace  60 . When the blank (not shown) is properly positioned within the hot zone (not shown) of draw furnace  60 , the bottom door assembly (not shown) of draw furnace  60  can be opened to begin the fiber draw.  
         [0038]    Because inert gas  54  is delivered into chamber  50 , only inert gas is available at the top of draw furnace  60  to be pulled into the muffle region of draw furnace  60 . Accordingly, oxygen is not present to react with the graphite muffle (not shown) to create a CO event. Preferably, an inert gas such as helium is delivered between gland seal  64  and bottom plate  40  through upper surface  66  of gland seal  64 . The inert gas provides a blanket of inert gas between the surfaces so that top hat assembly  10  essentially floats on gland seal  64 . In this way, an operator can make the necessary x-y directional adjustments to downfeed handle  28  required to properly position the blank (not shown) during fiber draw. The blanket of inert gas permits top hat assembly  10  to move laterally across gland seal  64  as downfeed handle  28  is moved. Moveover, the inert gas delivered between bottom plate  40  and gland seal  64  provides an entry barrier to ambient air residing above the top of the draw furnace  60 . Accordingly, the delivery of inert gas  54  into chamber  50  can be terminated once the bottom door assembly is opened, and fiber is being drawn from the blank.  
         [0039]    Once drawing operations are complete, the bottom door assembly (not shown) is closed, and downfeed handle  28  and any remaining fiber blank (not shown) can be safely removed from draw furnace  60  by raising downfeed handle  28  out of draw furnace  60 . In the preferred embodiment of the method of the present invention, inert gas  54  is again delivered into chamber  50  of top hat assembly  10  once the bottom door assembly (not shown) is closed. Downfeed handle  28  is preferably mechanically, hydraulically, or pneumatically withdrawn from draw furnace  60  in the direction of reference arrow B (FIG. 4). As downfeed handle  28  proceeds vertically, lip  66  of blank support portion  30  eventually engages bumper  42 . As downfeed handle  28  is elevated further, top hat assembly  10  again rides blank support portion  30 , and is removed from the top of draw furnace  60 . Inert gas  54  is preferably continuously delivered to chamber  50  throughout the duration of the withdrawal operation. Inert gas  54 , preferably argon, directs the hot air rising from the hot zone (not shown) of draw furnace  60  away from gasket  56  and cools the downfeed handle  28  as the heated portion of downfeed handle  28  approaches gasket  56 . Accordingly, downfeed handle  28  is cooled before it reaches gasket  56 , and gasket  56  is itself cooled by inert gas  54 , thereby minimizing heat damage to gasket  56  and reducing the frequency of gasket  56  replacement.  
         [0040]    It will be apparent to those skilled in the art that various other modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.