Abstract:
A method for the pregobbing of an optical fiber preform to provide pre-optimized tip taper and system for drawing optical fiber therefrom. The downtime of an optical fiber drawing apparatus can be considerably shortened, by providing preforms that have a pre-optimized tip shape. Pre-optimized tips are provided which are melted off at the tip by an induction heater of a heating furnace. Preferably, the pregobbing furnace has substantially the same temperature profile as the draw furnace. Therefore, because the tip of the preform is optimized and unusable glass has been removed, throughput of the draw apparatus is advantageously increased. Moreover, the shape of the tip of the preform is optimized in that it has been exposed to the same temperature profile as it would have seen had the draw tip been formed in the draw furnace.

Description:
FIELD OF THE INVENTION 
     The invention relates to a system and method for manufacture of an optical fiber preform and optical fiber, and more particularly to a method and apparatus for preparing a preform&#39;s tip to expedite fiber draw. 
     BACKGROUND 
     The starting point in the method for manufacturing optical fiber is the preparation of an elongated, cylindrical quartz glass body, doped accordingly to provide the desired refractive index profile in the optical fiber. This so-called “preform” is introduced very slowly, by means of a downfeed device, into the high-temperature heating furnace of a draw tower. The preform is heated at its terminal end to temperatures sufficiently high to cause the tip of the preform to be gradually transformed to a honey-like consistency. After reaching the appropriate temperature, a glass gob melts off the preform by the influence of heat and gravity. Then an operator hand strips additional trash glass from the preform until a portion of the preform being drawn from contains good quality glass. At the point where good quality glass is readily strippable, the so-called “draw tip” has been formed on the preform. The draw tip consists of a generally tapering transition area located between the quartz glass cylinder portion of the preform and the thin fiber strand being drawn from the lowermost tip of the preform. The dimensions of the drawn fiber depend on the high-temperature furnace and the individual drawing conditions. However, a preferred glass strand of approximately 125 μm in diameter is continuously drawn off from the preform. It then passes through the cooling tube and passes through the coating system, which applies the protective sheath (coating). The coating system is located in the drawing tower below a diameter testing device. At the end of the draw tower, the fiber is wound onto a spool. 
     After a significant length of optical fiber is drawn, the length being dependent on the size of the preform, the preform is consumed. Production must be interrupted and the consumed preform must be replaced by a new one. Frequently, more than one hour elapses until the production of fiber with the required characteristics resumes. During this start-up time, the gobbing off of the quartz glass mass that cannot be used takes place as well as stripping and shaping of the draw tip. The shaping of the draw tip alone takes approximately 30 to 40 minutes. This is valuable time spent on the draw apparatus that, in some cases, is the bottleneck in the production process. 
     SUMMARY OF THE INVENTION 
     According to the invention, a system and method are provided. In particular a pregobbing system and method are provided for the pregobbing treatment of an optical fiber preform to provide a desired pre-optimized tip geometry prior to insertion into a draw apparatus. The method and system provide a preform having an optimized shape draw tip and permits, for example, the glass mass of an optical fiber preform that is not suitable for fiber drawing to melt off very rapidly and efficiently. The pregobbing operation is performed in a pregobbing apparatus separate from the draw furnace of the draw tower. Thus, since the pregobbing is accomplished in an “off line” fashion, i.e., in another apparatus, the fiber throughput from the draw apparatus is increased. Thus, it should be recognized that preforms including pre-optimized leading end tips (which are already appropriately melted off) result in considerably shortened downtime for the optical fiber draw apparatus. 
     In accordance with an embodiment of the invention, a system is provided for manufacturing an optical fiber preform comprising a pregobbing furnace adapted to heat the preform and cause glass to be removed, the pregobbing furnace having a temperature profile that is substantially the same as a temperature profile of a draw furnace utilized in a subsequent process to draw fiber from the preform. By having substantially equal temperature profiles, the tip is of the exact shape it would have been had the tip preparation been performed in the draw apparatus. 
     In accordance with another embodiment of the invention, a system is provided for manufacturing an optical fiber preform. The system comprises a pregobbing apparatus having a heating furnace with a first temperature profile, the pregobbing apparatus adapted to provide a pre-optimized tip shape on the optical fiber preform, and a draw furnace having a second temperature profile which is substantially equal to the first temperature profile, the draw furnace adapted to draw fiber from the preform having the pre-optimized tip shape. Preferably, the pregobbing apparatus includes an induction heater. Most preferably, both the pregobbing and the draw apparatus each include an induction heater. 
     In accordance with another embodiment of the invention, a method for manufacturing an optical fiber preform is provided comprising heating a consolidated optical fiber preform with an induction heating apparatus having a first temperature profile to allow a gob to drop under the influence of heat and gravity, removing glass from the preform until a draw tip having a pre-optimized shape is formed, and transferring the preform to a draw furnace of a draw apparatus. Preferably, the pre-optimized shape includes a tip taper having a tip length to change in radius ratio of between about 5 to about 12, and most preferably between about 6 to about 9. The “pre-optimized” tip shape as the term is used herein means a tip that has been pre-shaped such that it has a taper approximately equal to what it will have when fiber is being drawn therefrom in the draw apparatus. The ratios mentioned above represent a pre-optimized tip taper in accordance with the invention. 
     In accordance with another embodiment of the invention, a method for manufacturing an optical fiber is provided comprising the steps of heating a plurality of consolidated optical fiber preforms within a plurality of pregobbing apparatus, each apparatus including a furnace having a first temperature profile to form pre-optimized shape preform tip on each of the plurality of preforms, and transferring the plurality of preforms to a plurality of draw apparatus, each draw apparatus including a furnace and drawing optical fiber therefrom, the plurality of draw furnaces each having a second temperature profile substantially equal to the first temperature profile. Accordingly, a large number of draw apparatus may be supplied with pre-optimized preforms by a small number of pregobbing apparatus. 
     From the foregoing, it should be apparent that by using appropriately pregobbed preforms, that is, preforms that have pre-optimized draw tip shape, it is possible to advantageously reduce the down time of a glass fiber drawing unit by up to 60 minutes per replaced preform. In particular, since the high temperature draw furnace of the drawing tower needs to melt less quartz glass mass, this results in shorter heat-up times. Further, the transition phase between the initial melting and fiber draw takes considerably less time. Thus, the draw tower is in operation and producing production quality fiber more of the time. This is particularly true when the draw tip is optimized off-line in a separate pregobbing apparatus. In addition, it is not necessary to move the preform as deep into the draw furnace (time savings because of the generally low feed rate). Furthermore, stripping time at the draw tower for stripping off inadequate quality fiber is reduced. Thus, it should be recognized that off-line pregobbing represents a significant time savings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further explained below with reference to the accompanying drawings. 
     FIG. 1 illustrates a side view of an embodiment of an apparatus for pregobbing treatment of a preform in accordance with the invention. 
     FIG. 2 illustrates a side view representative shape of a consolidated preform prior to pregobbing. 
     FIG. 3 is a graphical depiction of the shape of a Non-pregobbed and Pregobbed leading end of a preform. 
     FIG. 4 illustrates a partial cross-sectional side view drawing of a pregobbing furnace in accordance with the invention. 
     FIG. 5 illustrates a flow diagram of the method steps in accordance with the invention. 
     FIG. 6 illustrates a partial cross-sectional side view drawing of a draw apparatus. 
     FIG. 7 illustrates a schematic view of a plurality of pregobbing apparatus supplying pre-optimized preforms to a plurality of draw apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An apparatus for performing the pregobbing operation off-line (in another apparatus other than the draw furnace) is shown schematically in FIG.  1 . The pregobbing apparatus  20  is used for melting off the initial trash quartz glass mass from the preform  22 , i.e., that glass at the ends of the preform that is not suitable for drawing adequate production quality fiber therefrom. In accordance with another embodiment, the apparatus optimizes the shape of the leading end (tip) of the preform  22  such that when transferred to the draw tower, drawing of fiber may readily commence. The apparatus  20  preferably includes an induction furnace  24  which preferably has a temperature profile identical to the furnace  42  utilized in the draw tower  40  (FIG.  6 ). The pregobbing furnace  24  produces a tapered draw tip  32  on the preform  22  (FIG.  4 ), such that when the preform is transferred to the draw tower (FIG.  6 ), the time needed to commence drawing fiber is dramatically reduced. Thus, the draw tower is capable of being utilized more of the time to produce production quality tele- or data-communications optical fiber. This afore-mentioned time reduction is because the draw tip has already been formed to the proper shape (pre-optimized) in the previous pregobbing furnace, and all or most of the trash glass has been removed. FIG. 3 illustrates a non-pregobbed and pregobbed tips and their relative dimensions. A pre-optimized tip preferably has a tip length to radius change ratio (as measured from the start of the tip to the end of the tip) of between about 5 to about 12, and most preferably between about 6 to about 9. The tip length is measure from the start of the taper intersecting the cylindrical part of the preform to the terminal leading end of the preform. The change in radius is measured from the start of the taper as just defined to the radius at the terminal leading end. Depending on where the glass break occurs on the preform after the pregobbing is completed, i.e., at what position along the tip the operator breaks off the solidified strand, the tip diameter may be very small, on the order of a few mm to about 15 mm. 
     The pregobbing heating furnace  24 , as best illustrated in FIG. 4, has an induction heater with an induction coil  62  and a susceptor  64 . The furnace  24  further includes a cylindrical heating chamber  28 , heat insulation  30  surrounding the heating chamber, and and the induction coil  62  surrounding the susceptor  64 , chamber and preform  22  and providing a hot zone  60  for heating the leading tip  32  of the preform  22 . The induction coil, size and shape of the chamber, and insulation provided are such that the temperature profile of the pregobbing furnace  24  adjacent the tip is substantially identical to that of the draw furnace  42  (FIG.  6 ). This is accomplished in practice by having the internal components of the furnace be identical and adjusting the temperature controls for any slight differences. 
     As best illustrated in FIG.  1  and FIG. 5 the consolidated preform  22  is provided as indicated in step  35  by traversing the preform  22  roughly into position by an overhead crane apparatus or track  34 . The preform  22  may have come directly from a consolidation process or from an intermediate holding oven. Preferably, the preform  22  is housed within a carrier box  36  which surrounds the preform such that it is protected from contamination and collisions during transit to the pregobbing apparatus  20 . An operator  52  disconnects the preform  22  from the track  34  and mounts the preform to a handle  38  extending from a motorized down feed apparatus  44 . The down feed apparatus  44  is preferably mounted to a vertically moveable carriage  46  or other moveable structure. The down feed apparatus  44  extends downwardly from the lower end of the carriage  46  and the carriage operatively slides vertically along rails  48  to allow the preform to be raised and lowered as commanded. 
     The gross vertical motion of the preform  22  and carriage  46  relative to the apparatus frame  54  are controlled by a drive motor  50  mounted to the frame  54 . Upon initiation by the operator  52  or  53 , the motor  50  begins to rotate a lead screw  56  that parallels the rails  48 . Lead screw  56  cooperates with a threaded portion (not shown) mounted in the carriage  46 . Rotation of the lead screw  56  causes movement of the vertical position of carriage  46  thereby causing the carriage to move along the rails  48  and, thus, change the vertical position the preform  22 . When loading the preform  22 , the carriage  46  would be positioned at the location shown dotted and labeled “A.” Once the preform  22  is suitably loaded onto the handle  38 , the carrying case  36  is removed and put aside, and the preform  22  is then transferred into the pregobbing furnace  24  as indicated in step  37  (FIG. 5) by lowering the preform  22  into the entrance  29  (FIG. 4) of the pregobbing furnace  24 . This is accomplished by rotating lead screw  56  such that carriage  46  slides downward along the slide rails  48 . 
     The glass preform  22  is moved downward until the lowest portion of the tip  32  is positioned roughly within the hot zone  60  of the furnace  24  (FIG.  4 ). The lower preform  22  shown in FIG. 1 is shown being lowered into the chamber  28  of furnace  24  The preform  22  is allowed to remain at this position until the tip  32  softens to the point where a small gob  27  of glass drops off from the preform  22  by the action of heat and gravity alone. The gob drops through the exit  31  and into the trash. The rough position of the preform  22  within the furnace  24  is determined by a mirror. 
     Once a small gob drops, the preform  22  is driven deeper into the hot zone  60  by down feed  44  such that the whole tip  32  (FIG. 2) is placed directly in the hot zone  60  of furnace  24 . As the preform  22  continues to heat, a second larger gob drops by the influence of heat and gravity. The operator  53  pulls on the larger gob with scissors or other like tool and continues to strip trash glass with seeds or other defects from the preform  22  and appropriately scores and discards the trash glass as it drops and solidify. Both of these gob removal steps are accomplished in step labeled  39  of FIG.  5 . However, it should be understood that there may be one or multiple gobs that drop depending on the initial conditions and the rate of stripping provided by the operator. After a predetermined amount of glass has been removed such that the operator  53  is sure that production quality glass is present in the preform from which acceptable production-quality optical fiber may be drawn, the process is stopped and the preform  22  is removed from the pregobbing furnace  24 . 
     During the pregobbing process, the hot zone of the furnace is maintained at a temperature of between about 1800° C. and 2000° C., and more preferably between 1900° C. and 1950° C. Upon exposure of the terminal end of the tip to the hot zone, a small gob  27  will generally drop within about 25 min. The entire pregobbing process (including stripping) takes about 45-50 min. 
     After the pregobbing process is completed, the preform  22  is preferably placed back in the protective case  36 . Preform  22  is then transferred in accordance with step  41  of FIG. 5 to a draw tower  40  (FIG. 6) where fiber is to be drawn therefrom. The preform  22  is preferably transferred by an overhead crane or monorail system. The preform  22  is removed from the carrier  36 , connected to a draw furnace handle  158  and lowered into the draw furnace  42 . The preform  22  with optimized draw tip  32  is driven into the hot zone  160  of the chamber  128  maintained with substantially the same temperature profile as that of the pregobbing furnace  24 . Induction coil(s)  162  surrounding the preform  22  induce heat into the susceptor  64  to provide the hot zone  60  of approximately 1800° C.-2200° C. At step  43  (FIG.  5 ), sufficient tension is provided by a tractor or other tension providing means to draw optical fiber  65  from the preform  22 . The optical fiber  65  preferably has a diameter of approximately 0.125 μm. The fiber  65  may then be cooled, coated and wound onto a spool as is conventional practice. 
     In accordance with another embodiment of the invention, as best illustrated in FIG. 7, a plurality of pregobbing apparatus  220 ,  320  may be utilized to supply preforms with pre-optimized shape tips to a plurality of draw apparatus  240 ,  340 ,  440 ,  540 . Because the pregobbing step generally takes less time than drawing fiber from a preform, there are a lesser number of pregobbing apparatus than draw apparatus. According to the method, a plurality of consolidated optical fiber preforms are heated within a plurality of pregobbing apparatus  220 ,  320 . Each apparatus  220 ,  230  includes an induction furnace as shown in FIG. 4 having a first temperature profile to form pre-optimized shape preform tips on each of a plurality of preforms. Similarly, each of the plurality of draw apparatus  240 - 540  includes an induction furnace as heretofore described. 
     The preforms with pre-optimized tips are transferred to a plurality of draw apparatus  240 - 540  by any of the paths (a-h) shown. Thus, it should be recognized that any pregobbing apparatus may supply any draw apparatus. According to the invention, the plurality of draw furnaces  240 - 540  each have a second temperature profile substantially equal to the first temperature profile. Thus, a small number of pregobbing apparatus may be utilized to supply pre-optimized preforms to a much larger number of draw apparatus. It should be recognized that although two pregobbing apparatus are shown and four draw apparatus, a smaller number or larger number of draw apparatus may be utilized as dictated by production requirements. Further, a larger number of pregobbing apparatus may be employed. 
     It will be apparent to those of ordinary skill in the art that various modifications and variations can be made to the present invention without departing from the scope of the invention. Thus, it is intended that the present invention cover the modifications and variations provided they come within the scope of the appended claims and their equivalents.