Patent Publication Number: US-2023159372-A1

Title: Optical fiber forming apparatus

Description:
This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 63/281,976 filed on Nov. 22, 2021, the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to the technical field of optical fibers. 
     BACKGROUND 
     Conventional manufacturing processes for producing optical fibers generally include drawing an optical fiber downward from a draw furnace and along a linear pathway through multiple stages of production in an optical fiber draw tower. Once drawn from the draw furnace, the optical fiber may be cooled in a regulated manner to achieve desired fiber properties. 
     To meet consumer demand for optical fiber, it is desirable to increase optical fiber production within existing optical fiber draw towers. To increase optical fiber production, the rate at which the optical fiber is drawn is generally increased. However, increased draw rates may lead to flow instabilities that arise due to natural convection in the ambient air between the exit of draw furnace and the entrance of a downstream Slow Cooling Device (SCD), which may lead to decreased quality of the optical fiber. 
     SUMMARY 
     According to a first embodiment of the present disclosure, an optical fiber forming apparatus comprises: optical fiber forming apparatus comprises: a draw furnace comprising: (i) a muffle with an inner surface, (ii) an axial opening below the muffle, the inner surface of the muffle defining a passageway extending through the axial opening, and (iii) an upper inlet into the passageway; and a tube that extends into the passageway of the draw furnace above the axial opening, the tube having (i) an outer surface and the inner surface of the muffle surrounds the outer surface of the tube with a space separating the outer surface of the tube from the inner surface of the muffle, (ii) an inner surface that defines a second passageway extending through the tube, (iii) an inlet into the second passageway of the tube, (iii) an outlet out of the second passageway of the tube; and a cooling device at an outlet out of the second passageway of the tube the cooling device comprising: one or more bodies having a top surface and an opposing bottom surface, an opening within the body extending from the top surface through the body to the bottom surface, wherein the opening is configured to pass an optical fiber through the body, and one or more gas outlets within the body configured to direct gas to contact the optical fiber as it passes through the opening. 
     A second embodiment of the present disclosure may include the first embodiment, wherein inert gas flows through the upper inlet and into the passageway of the draw furnace and forms separate streams, one of which flows through the passageway of the draw furnace in the space between the inner surface of the muffle and the outer surface of the tube and out the axial opening of the draw furnace, and the other of which flows into the inlet of the tube, through the second passageway of the tube, and out the outlet of the tube. 
     A third embodiment of the present disclosure may include the second embodiment, wherein the inert gas comprises one or more of argon, nitrogen or helium. 
     A fourth embodiment of the present disclosure may include the first embodiment, further comprising: a first heating element that heats the passageway of the draw furnace throughout a first range that encompasses at least a portion of the passageway of the draw furnace above the inlet of the tube; and a second heating element that heats the passageway of the draw furnace throughout a second range that encompasses at least a portion of the passageway of the draw furnace above the first range. 
     A fifth embodiment of the present disclosure may include the first embodiment, further comprising: an optical fiber preform disposed within the passageway of the draw furnace; optical fiber drawn from the optical fiber preform that extends through the second passageway of the tube; and a first heating element that heats the passageway of the draw furnace throughout a first range that encompasses a tip of the optical fiber preform. 
     A sixth embodiment of the present disclosure may include the fifth embodiment, further comprising: a second heating element that heats the passageway of the draw furnace throughout a second range that encompasses a portion of the passageway above a main body of the optical fiber preform. 
     A seventh embodiment of the present disclosure may include the sixth embodiment, further comprising: a third heating element that heats the passageway of the draw furnace throughout a third range that encompasses a portion of the second passageway of the tube. 
     An eighth embodiment of the present disclosure may include the fifth embodiment, wherein the optical fiber exits the outlet of the tube at a rate of at least 20 meters per second and has a diameter after exiting the outlet of the tube, the standard deviation (σ) of which diameter is less than 0.06 μm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz. 
     A ninth embodiment of the present disclosure may include the first embodiment, wherein the inlet of the tube has an inner diameter of 1.27 cm to 2.54 cm. 
     A tenth embodiment of the present disclosure may include the first embodiment, wherein the cooling device further comprises one or more gas inlets fluidly coupled to the gas outlets. 
     An eleventh embodiment of the present disclosure may include the first embodiment, wherein the cooling device opening has a diameter of about 2 mm to about 100 mm 
     A twelfth embodiment of the present disclosure may include the first embodiment, wherein the one or more gas outlets is a plurality of nozzles. 
     A thirteenth embodiment of the present disclosure may include the twelfth embodiment, wherein a volumetric flow rate of gas from each nozzle is about 5 slpm to about 100 slpm. 
     A fourteenth embodiment of the present disclosure may include the first embodiment, wherein the one or more gas outlets is a singular slot, wherein the slot has a width of about 50 microns to about 2 mm. 
     A fifteenth embodiment of the present disclosure may include the first embodiment, wherein the gas outlets direct gas toward the optical fiber at an angle of about 15 degrees to about 90 degrees from a vertical axis running in a direction of fiber conveyance. 
     A sixteenth embodiment of the present disclosure may include the first embodiment, wherein the gas outlets direct gas toward the optical fiber at an angle of about 15 degrees to about 90 degrees from a vertical axis running in a direction of fiber counter-conveyance. 
     A seventeenth embodiment of the present disclosure may include an optical fiber forming apparatus comprising: a draw furnace comprising: (i) a muffle with an inner surface, (ii) an axial opening below the muffle, the inner surface of the muffle defining a passageway extending through the axial opening, and (iii) an upper inlet into the passageway; and a tube that extends into the passageway of the draw furnace above the axial opening, the tube having (i) an outer surface and the inner surface of the muffle surrounds the outer surface of the tube with a space separating the outer surface of the tube from the inner surface of the muffle, (ii) an inner surface that defines a second passageway extending through the tube, (iii) an inlet into the second passageway of the tube, (iv) an outlet out of the second passageway of the tube; a cooling device at an outlet out of the second passageway of the tube the cooling device comprising: one or more bodies having a top surface and an opposing bottom surface, an opening within the body extending from the top surface through the body to the bottom surface, wherein the opening is configured to pass an optical fiber through the body, and one or more gas outlets within the body configured to direct gas to contact the optical fiber as it passes through the opening; a flame reheating device downstream from the draw furnace, wherein the flame reheating device is configured to heat the optical fiber by at least 100 degrees Celsius at a heating rate greater than 10,000 degrees Celsius/second; and a slow cooling device downstream of the draw furnace. 
     An eighteenth embodiment of the present disclosure may include the seventeenth embodiment, wherein inert gas flows through the upper inlet and into the passageway of the draw furnace and forms separate streams, one of which flows through the passageway of the draw furnace in the space between the inner surface of the muffle and the outer surface of the tube and out the axial opening of the draw furnace, and the other of which flows into the inlet of the tube, through the second passageway of the tube, and out the outlet of the tube. 
     A nineteenth embodiment of the present disclosure may include the eighteenth, wherein the inert gas comprises one or more of argon, nitrogen or helium. 
     A twentieth embodiment of the present disclosure may include the eighteenth embodiment, wherein the inert gas comprises no helium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the Figures: 
         FIG.  1    is a schematic elevational view of an embodiment of an optical fiber forming apparatus, illustrating a draw furnace with a muffle with an inner surface that defines a passageway, and a tube extending into the passageway, with optical fiber drawn from an optical fiber preform extending through the tube; 
         FIG.  2    is a view of area II of  FIG.  1   , illustrating the tube separated from the inner surface of the muffle and inert gas flowing through a second passageway of the tube as an inner stream and between the tube and the inner surface of the muffle as an outer steam; 
         FIG.  3    schematically depicts a cooling device of the optical fiber forming apparatus, according to one or more embodiments shown and described herein; 
         FIG.  4 A- 4 B  schematically depicts a cross-sectional view of the cooling device of the optical fiber forming apparatus, according to one or more embodiments described herein; 
         FIG.  5    schematically depicts an enlarged perspective view of an optical fiber within the optical fiber forming apparatus, according to one or more embodiments described herein; 
         FIG.  6    depicts the stream function plot of gas flow below the forming tube with a cooling device, according to one or more embodiments described herein; 
         FIG.  7    is a schematic elevational view of an embodiment of an optical fiber forming apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIGS.  1 - 2   , an embodiment of an optical fiber forming apparatus  10  is illustrated. The optical fiber forming apparatus  10  includes a draw furnace  12 . The draw furnace  12  includes a muffle  16  and an axial opening  18  below the muffle  16 . The muffle  16  has an inner surface  20 . The inner surface  20  defines a passageway  22  that extends through the axial opening  18 . The draw furnace  12  further includes an upper inlet  24  into the passageway  22 . The muffle  16  further includes a narrowing  26  where the diameter of the passageway  22  narrows as the narrowing  26  progresses towards the axial opening  18 . 
     The optical fiber forming apparatus  10  further includes a tube  28 . The tube  28  extends into the passageway  22  of the draw furnace  12 . The tube  28  is thus at least partially disposed between the axial opening  18  and the upper inlet  24  into the passageway  22 . In embodiments such as the illustrated embodiment, the tube  28  extends through the axial opening  18 . In other embodiments, the tube  28  is entirely within the passageway  22  and does not extend through the axial opening  18 . In any event, at least a portion of the tube  28  is disposed above the axial opening  18  within the passageway  22 . The tube  28  extends upwards above the narrowing  26 . 
     The tube  28  includes an outer surface  30 , an inner surface  32  that defines a second passageway  34  extending through the tube  28 , an inlet  36  into the second passageway  34  of the tube  28 , and an outlet  38  out of the second passageway  34  of the tube  28 . The inlet  36  of the tube  28  is disposed within the passageway  22  of the draw furnace  12 , above the narrowing  26 . The outlet  38  need not be disposed within the passageway  22  of the draw furnace  12  but can be so disposed. The inner surface  20  of the muffle  16  surrounds the outer surface  30  of the tube  28  for the portion of the tube  28  that is disposed within the passageway  22  of the draw furnace  12 . A space  40  separates the outer surface  30  of the tube  28  from the inner surface  20  of the muffle  16 . That is, the tube  28  does not touch the muffle  16  within the passageway  22  of the draw furnace  12 . 
     The draw furnace  12  further includes a first heating element  42  that is in thermal communication with the muffle  16 . The first heating element  42  heats the passageway  22  of the draw furnace  12  throughout at least a first range  44  that encompasses at least a portion of the passageway  22  of the draw furnace  12  above the inlet  36  of the tube  28 . In operation of the optical fiber forming apparatus  10 , an optical fiber preform  46  is disposed within the passageway  22  of the draw furnace  12 . The first heating element  42  heats the optical preform  46  sufficiently to decrease the viscosity of the optical fiber preform  46  and allow an optical fiber  48  to be drawn from the optical fiber preform  46 . The first range  44  that the first heating element  42  heats encompasses a tip  50  of the optical fiber preform  46 , which is where the optical fiber preform  46  transitions to the optical fiber  48  drawn therefrom. In embodiments, the first heating element  42  heats the first range  44  to a temperature of 1700° C. to 2000° C., such as 1700° C., 1800° C., 1900° C., or 2000° C., or any range having any two of these values as endpoints. The passageway  22  of the draw furnace  12  within the first range  44  may have a temperature which is elevated relative to the rest of the passageway  22 . The first range  44  can further encompass a main body  52  of the optical fiber preform  46 , which is above the tip  50  and from which the tip  50  descends. 
     The optical fiber  48  drawn from the optical fiber preform  46  extends through the second passageway  34  of the tube  28 . In other words, the optical fiber  48  drawn from the optical preform  46  extends into the inlet  36  of the tube  28 , then through the second passageway  34  of the tube  28 , and then out of the outlet  38  of the tube  28 . In embodiments, the optical fiber  48  that enters into the inlet  36  of the tube  28  has a diameter that is greater than 125 μm, while the inlet  36  of the tube  28  has an inner diameter of 1.27 cm to 2.54 cm. An inner diameter of the tube  28  at the inlet  36  smaller than 1.27 cm poses an appreciable risk that the optical fiber  48  could contact the inlet  36  or the inner surface  32  of the tube  36 . An inner diameter of the tube  28  at the inlet  36  larger than 2.54 cm would likely result in a sufficiently large distance between the inner surface  32  of the tube  28  and the optical fiber  48  that causes convection of inert gas  54  thus negatively affecting diameter variability. In embodiments, the inner diameter of the tube  28  at the inlet  36  is 100 to 200 times larger than the diameter of the optical fiber  48  that enters the inlet  36  of the tube  28 . A tensioning station (not shown) is in contact with the optical fiber  48  and maintains the optical fiber  48  at a desired tension. 
     In embodiments, inert gas  54  flows through the upper inlet  24  of the draw furnace  12  and into the passageway  22  of the draw furnace  12 . The inert gas  54  then forms separate streams—an inner stream  56  and an outer stream  58 . The inner stream  56  flows into the inlet  36  of the tube  28 , through the second passageway  34  of the tube  28 , and out the outlet  38  of the tube  28 . The outer stream  58  flows though the passageway  22  of the draw furnace  12  in the space  40  between the inner surface  20  of the muffle  16  and the outer surface  30  of the tube  28  and then out the axial opening  18  of the draw furnace  12 . 
     In embodiments, the inert gas  54  comprises argon, nitrogen, or helium. In embodiments, the inert gas  54  comprises argon or nitrogen, or a combination of argon and nitrogen. In embodiments, the inert gas  54  comprises one or more of argon and nitrogen, and less than 1 percent by volume helium. In embodiments, the inert gas  54  comprises no intentionally included helium. In embodiments, the inert gas  54  comprises essentially pure argon (e.g., more than 99 percent by volume argon). 
     In embodiments, the tube  28  comprises one or more graphite, quartz, and stainless steel. In embodiments, the tube  28  is stainless steel. 
     In embodiments, the optical fiber forming apparatus  10  further includes a second heating element  60 . The second heating element  60  is disposed vertically above the first heating element  42 . The second heating element  60  heats the passageway  22  of the draw furnace  12  throughout at least a second range  62  that encompasses at least a portion of the passageway  22  of the draw furnace  12  above the first range  44 . The second range  62  encompasses a portion of the passageway  22  above the main body  52  of the optical fiber preform  46 . In embodiments, the second range  62  encompasses a boule  64  that supports the optical fiber preform  46 . 
     In embodiments, the optical fiber forming apparatus  10  further includes a third heating element  66 . The third heating element  66  is disposed vertically below the first heating element  42 . The third heating element  66  heats the passageway  22  of the draw furnace  12  throughout a third range  68  that encompasses a portion of the second passageway  34  of the tube  28 . The third range  68  is vertically below the first range  44 . The third heating element  66  thus heats both a portion of the passageway  22  of the draw furnace  12  disposed around the tube  28  as well as the second passageway  34  of the tube  28 . 
     In embodiments, the optical fiber forming apparatus  10  further includes a cooling element  70 . The cooling element  70  is disposed vertically below the first heating element  42 . The cooling element  70  cools the passageway  22  of the draw furnace  12  throughout a fourth range  72  that encompasses a portion of the second passageway  34  of the tube  28 . The fourth range  72  is vertically below the first range  44 . The cooling element  70  cools the optical fiber  48  drawn from the optical fiber preform  46  as the optical fiber  48  passes through the second passageway  34  of the tube  28  toward a tensioning station (not shown). 
     As will be further demonstrated in the examples below, the optical fiber forming apparatus  10  that includes the tube  28  extending throughout a portion of the passageway  22  of the draw furnace  12  produces optical fiber  48  that has a diameter, the standard deviation of which is within an improved and acceptable tolerance. In embodiments, the optical fiber  48  exits the outlet  38  of the tube  28  at a rate of at least 20 m/s and has a diameter after exiting the outlet  38  of the tube  28 , the standard deviation of which diameter is less than 0.1 μm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz. In embodiments, the optical fiber  48  exits the outlet  38  of the tube  28  at a rate of at least 20 m/s and has a diameter after exiting the outlet  38  of the tube  28 , the standard deviation of which diameter is less than 0.1 μm at frequencies of 0.06 Hz, 1 Hz, and 10 Hz. 
     The position of the tube  28  within the passageway  22  of the draw furnace  12  is adjustable. This aspect provides many advantages. The inlet  36  of the tube  28  can be extended relatively close to the tip  50  of the optical fiber preform  46  and, thus, protect the optical fiber  48  from disturbances in flow of the inert gas  54  during much of the period of time while the optical fiber  48  is cooling. In the same manner, the length of the tube  28  between the inlet  36  of the tube  28  and the outlet  38  of the tube  28  can be adjusted as desired to protect the optical fiber  48  from disturbances from the inert gas  54  or ambient air while the optical fiber  48  is cooling. In some circumstances, it may be desirable to size the length of the tube  28  to extend out of the passageway  22  through the axial opening  18 , to allow additional distance and time for the optical fiber  48  to cool before becoming exposed to flow instabilities caused by the temperature difference between the optical fiber  48  and the ambient air. 
     Downstream from the tube  28 , the optical fiber enters a cooling device  130 . As depicted in  FIG.  3   , the cooling device  130  comprises one or more bodies  202 . In some embodiments, the cooling device has a length of about 10 inches to about 60 inches. In some embodiments, the cooling device has a length of about 20 inches to about 60 inches, or about 30 inches to about 60 inches, or about 40 inches to about 60 inches, or about 50 inches to about 60 inches. 
     In some embodiments, as depicted in  FIG.  3   , the cooling device  130  comprises 4 bodies  202 . The cooling device  130  may contain more or less bodies  202  than depicted in the exemplary embodiment, for example 1, 2, 3, 5, or 6 bodies  202 . Each body  202  has a top surface  210  and an opposing bottom surface  212 . The bottom surface faces the fiber conveyance direction  101 . The top surface  210  faces the counter-conveyance direction  103 . In some embodiments, a distance  214  from a bottom surface  210  of a body  202  to a top surface  210  of an adjacent body  202  is about 2 inches to about 10 inches. In some embodiments, the distance  214  is about 4 inches to about 10 inches, or about 6 inches to about 10 inches, or about 8 inches to about 10 inches. In some embodiments, the distance  214  is about 2 inches to about 8 inches, or about 2 inches to about 6 inches, or about 2 inches to about 4 inches. 
     Each body  202  has an opening  204  extending from the top surface  210  through the body  202  to the bottom surface  212 . The optical fiber  12  passes through the opening  204 . In some embodiments, the opening  204  has a diameter of about 2 mm to about 100 mm. In some embodiments, the opening  204  has a diameter of about 10 mm to about 100 mm, or about 20 mm to about 100 mm, or about 30 mm to about 100 mm, or about 40 mm to about 100 mm, or about 50 mm to about 100 mm, or about 60 mm to about 100 mm, or about 70 mm to about 100 mm, or about 80 mm to about 100 mm, or about 90 mm to about 100 mm. In some embodiments, the opening  204  has a diameter of about 2 mm to about 90 mm. In some embodiments, the opening  204  has a diameter of about 2 mm to about 80 mm. In some embodiments, the opening  204  has a diameter of about 2 mm to about 70 mm. In some embodiments, the opening  204  has a diameter of about 2 mm to about 60 mm. In some embodiments, the opening  204  has a diameter of about 2 mm to about 40 mm. In some embodiments, the opening  204  has a diameter of about 2 mm to about 20 mm. In some embodiments, the opening  204  has a diameter of about 2 mm to about 10 mm. 
     One or more gas outlets  208  within the body  202  direct gas toward the optical fiber  10  passing through the opening  204  to cool the optical fiber  10 . In embodiments, the gas outlets  208  direct gas toward the optical fiber at an angle of 90 degrees from the vertical axis. In embodiments, the gas outlets  208  direct gas toward the optical fiber at an angle of 30 degrees from the vertical axis. In embodiments, the gas outlets  208  direct gas toward the optical fiber at an angle of about 15 degrees to about 90 degrees from a vertical axis running in a direction of fiber conveyance. In embodiments, the gas outlets  208  direct gas toward the optical fiber at an angle of about 15 degrees to about 90 degrees from a vertical axis running in a direction of fiber counter-conveyance. In embodiments, the vertical axis is in the direction of the fiber conveyance  101 . The one or more gas outlets  208  direct gas toward the optical fiber  10  at an average velocity of about 20 m/s to about 350 m/s. In some embodiments, gas is directed toward the optical fiber  10  at an average velocity of about 50 m/s to about 350 m/s, or about 50 m/s to about 350 m/s, or about 100 m/s to about 350 m/s, or about 150 m/s to about 350 m/s, or about 200 m/s to about 350 m/s, or about 250 m/s to about 350 m/s, or about 300 m/s to about 350 m/s. One or more gas inlet tubes are fluidly couple to the gas outlets  208  to supply gas. In some embodiments, the gas directed toward the optical fiber is at room temperature (i.e. about 25 degrees Celsius). In some embodiments, the gas directed toward the optical fiber is cooled to less than room temperature prior to directing the gas toward the optical fiber. The gas may be cooled by passing the gas through a heat exchanger or through a vortex cooler tube. In embodiments, the gas is one or more of atmospheric air, helium, argon, nitrogen, or carbon dioxide. 
     In some embodiments, as depicted in  FIG.  4 A , the one or more gas outlets  208  are a plurality of nozzles  302  directing gas toward the optical fiber  10 . In some embodiments, the plurality of nozzles is 2 to 50 nozzles, preferably 3 to 20 nozzles, more preferably 3 to 12 nozzles. In some embodiments, each nozzle is positioned equidistant from an adjacent nozzle as measured from a center of one nozzle to a center of an adjacent nozzle. In some embodiments, each of the plurality of nozzles has a diameter  306  of about 100 micron to about 5 mm. In some embodiments, each nozzle  302  provides a volumetric flow rate of gas from about 5 slpm to about 8 slpm. 
     In some embodiments, as depicted in  FIG.  4 B , the one or more gas outlets  208  is single slot  308  directing gas toward the optical fiber  10 . In some embodiments, the slot  308  has a width  310  of about 50 microns to about 2 mm. In some embodiments, the slot  308  has a width  310  of about 100 microns to about 2 mm, or about 500 microns to about 2 mm, or about 1 mm microns to about 2 mm. 
     Referring to  FIG.  5   , the cooling device  130  directs the gas  16  toward the optical fiber  12  such that the gas  16  reduces a portion of a gas boundary layer  14  surrounding the optical fiber  12 . As the optical fiber  12  moves along the fiber conveyance pathway  102 , the gas boundary layer  14  is generated around the optical fiber  12  and comprises gas flowing primarily in the fiber conveyance direction  101 . The gas boundary layer  14  extends radially from the optical fiber  12 , terminating at a gas layer interface  18  and defining a gas boundary layer span Sb. Without being bound by theory, the gas boundary layer  14  is formed from drag generated by motion of the optical fiber  12  in the fiber conveyance direction  101 . In embodiments, the gas boundary layer  14  generally provides thermal insulation to the optical fiber  12 , thereby maintaining the optical fiber  12  at a relatively high temperature. 
     The gas  16  separates at least a portion of the gas boundary layer  14  from the optical fiber  12 . By separating at least a portion of the gas boundary layer  14  from the optical fiber  12 , the gas  16  may assist in dissipating heat from the optical fiber  12 . For example, by separating at least a portion of the gas boundary layer  14  from the optical fiber  12 , the thermal insulation provided by the gas boundary layer  14  may be reduced or removed, such that thermal energy of the optical fiber  12  may be dissipated more readily as compared to optical fiber  12  including an undisturbed gas boundary layer  14 . 
     In some embodiments, as the gas  16  is directed toward the optical fiber  12 , the gas  16  compresses the gas boundary layer, reducing the gas boundary layer span Sb. By reducing the gas boundary layer span Sb, the thermal insulation provided by the gas boundary layer  14  may be reduced, such that thermal energy of the optical fiber  12  may be dissipated more readily as compared to optical fiber  12  including an undisturbed gas boundary layer  14 . 
     Referring to  FIG.  5   , in embodiments, the optical fiber  12  includes a cladding  11  positioned around a core  13  of the optical fiber  12 . In embodiments, the cladding  11  comprises a refractive index that is different than the core of the optical fiber. For example, in embodiments, the core  13  may have a higher refractive index than the cladding  11 , and may assist in restricting light from passing out of the core  13 , for example, when the optical fiber  12  is used as an optical waveguide. 
     In  FIG.  6   , the stream function plot shows that with the forced flow of gas from the cooling device  130 , the flow below the tube  28  becomes unidirectional and steady. The forced flow from the cooling device  130  overpowers natural convection, thus suppressing flow instabilities. 
     In embodiments, an optical fiber forming apparatus  10  further includes a reheating device  140 . The reheating device  140  is configured to heat the optical fiber  10  to a temperature within a glass transformation temperature range of the optical fiber. By rapidly heating the optical fiber temperature to the glass transformation temperature range, the fictive temperature of the optical fiber can be reduced. As a consequence, Rayleigh scattering from the fiber core may also be reduced. 
     The reheating device  140  is spaced apart from the draw furnace  10  along the fiber conveyance pathway  102 . Embodiments of the reheating device  140  heat the optical fiber from a first temperature at entering the fiber reheating device to a target peak temperature, which is higher than the first temperature. In some embodiments, the first temperature of the optical fiber at entering the fiber reheating device  140  is about 20 degrees Celsius to about 1500 degrees Celsius, for example about 350 degrees Celsius to 500 degrees Celsius. In some embodiments, the target peak temperature of the optical fiber within the fiber reheating device  140  is about 900 degrees Celsius to about 1600 degrees Celsius, for example about 900 degrees Celsius to about 1400 degrees Celsius. Embodiments of the reheating device  140  described herein heat the optical fiber to a target peak temperature greater than 1100 degrees Celsius, or to a target peak temperature greater than 1200 degrees Celsius, or to a target peak temperature greater than 1250 degrees Celsius, or to a target peak temperature greater than 1300 degrees Celsius, or to a target peak temperature greater than 1400 degrees Celsius. Embodiments of the reheating device  140  described herein heat the optical fiber by at least 100 degrees Celsius, or by at least 200 degrees Celsius, or by at least 500 degrees Celsius. Embodiments of the reheating device  130  described herein heat the optical fiber by 300 degrees Celsius to 1400 degrees Celsius. Embodiments of the reheating device  140  described herein heat the optical fiber at a heating rate of greater than about 10,000 degrees Celsius/second, or at a rate of greater than about 20,000 degrees Celsius/second, or at a rate of greater than about 30,000 degrees Celsius/second, or at a rate of greater than about 40,000 degrees Celsius/second, or at a rate of greater than 50,000 degrees Celsius/second. Embodiments of the reheating device  140  described herein heat the optical fiber at a heating rate of 50,000 degrees Celsius/second to 60,000 degrees Celsius/second. The optical fiber is subsequently cooled from the target peak temperature to a second temperature such that a target fictive temperature is obtained in the optical fiber. In some embodiments, the second temperature of the optical fiber is about 700 degrees Celsius to about 1400 degrees Celsius. In some embodiments, the target fictive temperature of the optical fiber is about 800 degrees Celsius to about 1500 degrees Celsius. 
     In some embodiments, the reheating device  140  is a flame reheating device that comprises one or more flame burners. In some embodiments, each burner is capable of a heating rate of about 1,000 degrees Celsius/second to about 20,000 degrees Celsius/second. In some embodiments, each burner is capable of a heating rate of about 5,000 degrees Celsius/second to about 20,000 degrees Celsius/second. In some embodiments, each burner is capable of a heating rate of about 10,000 degrees Celsius/second to about 20,000 degrees Celsius/second. In some embodiments, each burner is capable of a heating rate of about 15,000 degrees Celsius/second to about 20,000 degrees Celsius/second. 
     In embodiments as depicted in  FIG.  7   , downstream from the reheating device  130 , the optical fiber  12  enters a first slow cooling device  150 . The cooling device  150  includes one or more cooling device heating elements that apply heat to the optical fiber as it passes through the cooling device  150 . In some embodiments, the one or more heating elements generally include any elements suitable for generating thermal energy, for example and without limitation, induction coils or the like. The cooling device  150  may assist in reducing the cooling rate of the optical fiber while the optical fiber is in a glass transition region. Reducing the cooling rate of the optical fiber in the glass transition region may generally assist in allowing the glass network of the optical fiber to rearrange in a manner that reduces attenuation resulting from Rayleigh scattering when the optical fiber is utilized as an optical waveguide.