Optical fiber forming apparatus

Abstract

An optical fiber forming apparatus comprising: a draw furnace comprising: (i) a muffle furnace having an inner surface, (ii) an axial opening below the muffle furnace, the inner surface of the muffle furnace defining a passageway extending through the axial opening, and (iii) an upper inlet into the passageway; and a tube extending 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 furnace encloses the outer surface of the tube and has a space separating the outer surface of the tube from the inner surface of the muffle furnace, (ii) an inner surface defining a second passageway extending through the tube, (iii) an inlet into the second passageway of the tube, (iii) an outlet from the second passageway of the tube.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority under 35 U.S.C. § 119 to U.S. Provisional Application Serial No. 63 / 025,522, filed May 15, 2020, the contents of which are relied upon and incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The present disclosure relates to the field of optical fiber technology. BACKGROUND

[0004] A draw furnace can be used to draw an optical fiber from a preform. The draw furnace includes a passageway in which the preform and an initial draw of the optical fiber are located. The passageway is purged with an inert gas to prevent ambient air from flowing into the passageway, which can cause oxidation of components of the draw furnace. Heretofore, the inert gas has typically been helium. However, the price of helium is rising and it is not easily recycled. Argon and nitrogen are both potential replacements for helium because they are less expensive and more abundant than helium.

[0005] However, a problem exists in that the use of argon, as opposed to helium, increases the variability of the diameter of the drawn optical fiber, resulting in the diameter of the drawn optical fiber exceeding the limits of the design specification. SUMMARY

[0006] The present disclosure solves this problem in several ways. The present disclosure solves this problem by reducing the distance between the drawn optical fiber and the boundary of the inert gas in the draw furnace, which is accomplished by (i) inserting a tube into a muffle of the draw furnace through which the drawn optical fiber travels, where the inert gas flows both through the tube with the drawn optical fiber and around the tube, (ii) tapering the diameter of the muffle over a distance that is greater than the reduction in diameter of the muffle throughout the taper, or (iii) heating a portion of the muffle having a smaller diameter through which the drawn optical fiber extends. In either of the first two cases, the flow of the inert gas, e.g., argon, is sufficiently uniform so as not to significantly affect the variability of the diameter of the drawn optical fiber, such that the variability is within the limits of the specification and approximates the variability when using helium as described above. In the third case, the additional heat suppresses convective instability of the smaller diameter portion of the drawn optical fiber, again resulting in the variability of the diameter of the drawn optical fiber being within the design specification. These solutions allow the use of inert gases such as argon and nitrogen in place of helium.

[0007] According to a 1st aspect of the present disclosure, an optical fiber forming apparatus comprising: (a) a draw furnace comprising: (i) a muffle furnace having an inner surface, (ii) an axial opening below the muffle furnace, the inner surface of the muffle furnace defining a passageway extending through the axial opening, and (iii) an upper inlet into the passageway; and (b) a tube extending 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 furnace encloses the outer surface of the tube and has a space separating the outer surface of the tube from the inner surface of the muffle furnace, (ii) an inner surface defining a second passageway extending through the tube, (iii) an inlet into the second passageway of the tube, and (iv) an outlet from the second passageway of the tube.

[0008] According to a 2nd aspect, the 1st aspect further comprises: a first heating element heating the passageway of the draw furnace throughout a first range, the first range encompassing at least a portion of the passageway of the draw furnace above the inlet of the tube; and a second heating element heating the passageway of the draw furnace throughout a second range, the second range encompassing at least a portion of the passageway of the draw furnace above the first range.

[0009] According to a 3rd aspect, the 2nd aspect further comprises: a third heating element heating the passageway of the draw furnace throughout a third range, the third range encompassing a portion of the second passageway of the tube.

[0010] According to a 4th aspect, the 1st aspect further comprises: an optical fiber preform disposed within the passageway of the draw furnace; an optical fiber drawn from the optical fiber preform and extending through the second passageway of the tube; and a first heating element heating the passageway of the draw furnace throughout a first range, the first range encompassing a tip of the optical fiber preform.

[0011] According to a 5th aspect, the 4th aspect further comprises: a second heating element heating the passageway of the draw furnace throughout a second range, the second range encompassing a portion of the passageway above a body of the optical fiber preform.

[0012] According to a 6th aspect, any of the 4th to 5th aspects, wherein the optical fiber exits the outlet of the tube at a rate of at least 20 m / s and has a diameter after exiting the outlet of the tube having a standard deviation (σ) of less than 0.06 μm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

[0013] According to a seventh aspect, any of aspects 1-6, wherein the inert gas flows through the upper inlet and into the passage of the draw furnace, and forms separate streams, one of which flows through the passage of the draw furnace in the space between the inner surface of the muffle and the outer surface of the tube, and exits from the axial opening of the draw furnace, and another of which flows into the inlet of the tube, through the second passage of the tube, and exits from the outlet of the tube.

[0014] According to an eighth aspect, the seventh aspect, wherein the inert gas comprises one or more of argon and nitrogen, and less than 1% by volume of helium.

[0015] According to a ninth aspect, any of aspects 1-8, wherein the inlet of the tube has an inner diameter of 1.27 cm to 2.54 cm.

[0016] According to a tenth aspect of the disclosure, a draw furnace for an optical fiber forming apparatus, comprising: a muffle having an inner surface, the inner surface of the muffle defining a passage that is centered on an axis and extends through an axial opening below the muffle, the inner surface comprising: (a) a first straight portion having a radius from the axis that remains at least approximately constant along a length parallel to the axis; (b) a tapered portion disposed between the first straight portion and the axial opening, the tapered portion comprising a radius from the axis that decreases away from the first straight portion, thereby narrowing the passage, and a perpendicular length parallel to the axis that is at least twice as long as a maximum radius of the tapered portion; and (c) a second straight portion disposed between the tapered portion and the axial opening, the second straight portion having a radius from the axis that remains at least approximately constant along a length of at least 75 cm, the radius of the second straight portion being 0.635 cm to 1.27 cm.

[0017] According to an eleventh aspect, the tenth aspect, further comprising: an upper inlet into the passage, the upper inlet being disposed closer to the first straight portion than to the tapered portion of the inner surface of the muffle; wherein the inert gas (i) flows through the upper inlet and into the passage, (ii) then flows along the first straight portion of the inner surface of the muffle, (iii) then flows along the tapered portion, (iv) then flows along the second straight portion, and (v) then exits from the axial opening.

[0018] According to a twelfth aspect, the eleventh aspect, wherein the inert gas comprises one or more of argon and nitrogen, and less than 1% by volume of helium.

[0019] According to a thirteenth aspect, any of aspects 10-12, further comprising: an optical fiber preform disposed in the passage; and an optical fiber drawn from the optical fiber preform, the optical fiber extending through the passage and exiting from the axial opening.

[0020] According to a 14th aspect, the 13th aspect further comprises: a first heating element that heats the passage throughout a first range, the first range encompassing the tip of the optical fiber preform; and a second heating element that heats the second passage throughout a second range, the second range encompassing a portion of the passage above the body of the optical fiber preform.

[0021] According to a 15th aspect, the 14th aspect further comprises: a third heating element that heats a third range, the third range encompassing a portion of the passage of the second straight portion defined by the inner surface of the muffle.

[0022] According to a 16th aspect, any of aspects 13-15, wherein the optical fiber exits the axial opening at a rate of at least 20 meters per second and has a diameter after exiting the axial opening with a standard deviation of less than 0.6 pm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

[0023] According to a 17th aspect of the disclosure, a draw furnace for an optical fiber forming apparatus, comprising: (a) a muffle having an inner surface, the inner surface of the muffle defining a passage that is centered on an axis and extends through an axial opening below the muffle, the inner surface comprising: (i) a first straight portion having a radius from the axis that remains at least approximately constant along a length parallel to the axis; (ii) a constriction disposed between the first straight portion and the axial opening, the constriction comprising a radius from the axis that decreases away from the first straight portion, thereby constricting the passage; (iii) a second straight portion disposed between the constriction and the axial opening, the second straight portion having a radius from the axis that is at least approximately constant along a length parallel to the axis; (b) a first heating element that heats the passage throughout a first range, the first range encompassing a portion of the passage defined by the first straight portion; (c) a second heating element that heats the passage throughout a second range, the second range encompassing a portion of the passage defined by the first straight portion above the first range; and (d) a third heating element that heats the passage to a temperature of 100°C to 200°C throughout a third range, the third range encompassing a portion of the passage defined by the second straight portion.

[0024] According to a 18th aspect, the 17th aspect further comprises: an optical fiber preform disposed within the passageway; and an optical fiber drawn from the optical fiber preform and extending through the passageway and out the axial opening; wherein the first range heated by the first heating element encompasses a tip of the optical fiber preform; wherein the second range heated by the second heating element is at least partially over a body of the optical fiber preform; and wherein the third range heated by the third heating element encompasses a portion of the optical fiber drawn from the optical fiber preform.

[0025] According to a 19th aspect, any of the 17th to 18th aspects further comprise: an upper inlet into the passageway, the upper inlet disposed closer to the first straight portion than to the constriction; wherein the inert gas (i) flows through the upper inlet and into the passageway, (ii) then flows along the first straight portion of the inner surface of the muffle, (iii) then flows along the constriction of the inner surface of the muffle, (iv) then flows along the second straight portion of the inner surface of the muffle, and (v) then exits from the axial opening; and wherein the inert gas comprises one or more of argon and nitrogen, and less than 1 vol% helium.

[0026] According to a 20th aspect, the 18th aspect, wherein the optical fiber exits the axial opening at a rate of at least 20 meters per second and has a diameter after exiting the axial opening with a standard deviation of less than 0.06 pm at measurement frequencies of 0.1 Hz, 1 Hz, and 10 Hz. BRIEF DESCRIPTION OF DRAWINGS

[0027] In the drawings:

[0028] Figure 1 is an elevation view schematic of one embodiment of a fiber forming apparatus illustrating a draw furnace having a muffle with an inner surface defining a passageway, and a tube extending into the passageway, wherein an optical fiber drawn from an optical fiber preform extends through the tube;

[0029] Figure 2 is a view of Region II of Figure 1 illustrating the tube separated from the inner surface of the muffle, and the inert gas flowing as an inner stream through a second passageway of the tube, and as an outer stream between the tube and the inner surface of the muffle;

[0030] Figure 3 is an elevation view schematic of another embodiment of a fiber forming apparatus illustrating a first straight portion, a taper, and then a second straight portion of an inner surface of a muffle defining a passageway, and an optical fiber drawn extending through the passageway;

[0031] Figure 4 is a view of Region II of Figure 1a view of region IV of FIG. 1 illustrating a taper separated by a radius from the axis of the channel, the radius decreasing along the taper from the first straight portion to the second straight portion;

[0032] Figure 5 is a schematic elevation view of another embodiment of an optical fiber forming apparatus, the view illustrating a heating element that heats a range that encompasses a channel defined by a second straight portion of an inner surface of a muffle, through which a drawn optical fiber extends;

[0033] Figure 6A relates to Example 1, which is directed to Figure 1 is a stream function contour plot generated by a computational fluid dynamics simulation of an optical fiber forming apparatus of FIG. 1 having a tube, in which argon gas, as an inert gas, flows through a channel defined by an inner surface of a muffle and a second channel of the tube, the plot illustrating a one-way downward flow of argon gas and an absence of a convection cell that would disturb the optical fiber;

[0034] Figure 6B is Figure 6A a region VIB of the stream function contour plot of FIG. 2B;

[0035] Figure 6C is Figure 6B a plot of axial velocity of argon gas at line VIC of FIG. 2B, the plot illustrating that the axial velocity of argon gas is positive everywhere, which indicates a one-way downward flow;

[0036] Figure 7A relates to Comparative Example 1A, which is directed to Figure 1 is a stream function contour plot of an optical fiber forming apparatus of FIG. 1 but without the tube, in which argon gas, as an inert gas, forms a convection cell that disturbs the optical fiber near a constriction of the channel;

[0037] Figure 7B is Figure 7A a plot of axial velocity of argon gas at line VIIB of FIG. 3B, the plot illustrating that the axial velocity of argon gas is negative near the axis of the channel, thus having an upward flow of argon gas that disturbs the optical fiber;

[0038] Figure 8 relates to Comparative Example 1B, which is directed to Figure 1 is a stream function contour plot of an optical fiber forming apparatus of FIG. 1 but without the tube, in which helium gas, as an inert gas, does not form a convection cell and thus does not disturb the optical fiber near a constriction of the channel;

[0039] Figure 9A is a plot of temperature fluctuations of Example 1 and Comparative Example 1A, the plot illustrating that the optical fiber forming apparatus of Example 1 with the tube has smaller temperature fluctuations than the optical fiber forming apparatus of Comparative Example 1A without the tube, especially at lower measurement frequencies;

[0040] Figure 9B is a graph of pressure fluctuations for Example 1 and Comparative Example 1A, which illustrates that the fiber forming apparatus of Example 1 with a tube has less pressure fluctuations than the fiber forming apparatus of Comparative Example 1A without a tube, especially at lower measurement frequencies;

[0041] Figure 10A relating to Example 2, is a graph of the deviation from average diameter for a fiber formed using the fiber forming apparatus of Figure 1 as a function of time;

[0042] Figure 10B relating to Comparative Example 2A, is a graph of the deviation from average diameter for a fiber formed using the fiber forming apparatus of Figure 1 as a function of time;

[0043] Figure 11 is a graph of the standard deviation from average diameter as a function of measurement frequency for Example 2 and Comparative Example 2A, which illustrates that the standard deviation for Example 2 (fiber forming apparatus of Figure 1 with a tube) is less than 0.05 pm at all measurement frequencies (10 Hz, 1 Hz, and 0.1 Hz), while the standard deviation for Comparative Example 2A (fiber forming apparatus of Figure 1 without a tube) is about 0.15 pm or more at these measurement frequencies;

[0044] Figure 12 relating to Example 3, is a graph of the deviation from average diameter for a fiber formed using the fiber forming apparatus of Figure 1 as a function of the temperature of the fiber exiting the tube, which illustrates that the standard deviation increases as the exit temperature increases;

[0045] Figure 13 is a graph of the standard deviation from average diameter for a fiber formed using the fiber forming apparatus of Figure 1 with a tube (Example 4) and without a tube (Comparative Example 4A), neither of which have a second heating element activated to heat a second range within the channel generally above the fiber preform, which illustrates that the use of a tube results in a lower standard deviation;

[0046] Figure 14A relating to Example 5, is a flow function contour plot for the fiber forming apparatus of Figure 3 where the channel is defined by a taper and a second straight portion of relatively small diameter, which illustrates that the use of argon gas as the inert gas does not have a convection loop near the fiber being drawn from the fiber preform;

[0047] Figure 14B is Region XIVB of Figure 14A ;

[0048] Figure 14C is a graph of the axial velocity of argon gas as an inert gas as a function of position within the channel obtained at line XIVC, which graph shows positive values only, thus having a unidirectional downward flow that avoids the formation of a convection loop that would disturb the optical fiber; Figure 14B

[0049] Figure 15A is a graph of the temperature fluctuations of Example 5 and Comparative Example 1A, which graph illustrates that the fiber forming apparatus of Example 5, which employs a channel defined by a taper and a second straight portion of relatively smaller diameter, has smaller temperature fluctuations than the fiber forming apparatus of Comparative Example 1A, which does not employ a second straight portion of relatively smaller diameter, especially at lower measurement frequencies;

[0050] Figure 15B is a graph of the pressure fluctuations of Example 5 and Comparative Example 1A, which graph illustrates that the fiber forming apparatus of Example 5, which employs a channel defined by a taper and a second straight portion of relatively smaller diameter, has smaller pressure fluctuations than the fiber forming apparatus of Comparative Example 1A, which does not employ a second straight portion of relatively smaller diameter, especially at lower measurement frequencies;

[0051] Figure 16 relates to Example 5, which graph is a plot of the deviation from the average diameter of an optical fiber formed with the fiber forming apparatus of Example 5, which employs a channel defined by a taper and a second straight portion of relatively smaller diameter, as a function of time; Figure 3

[0052] Figure 17 is a graph of the standard deviation from the average diameter of Example 6 and Comparative Example 2A as a function of measurement frequency, which graph illustrates that the standard deviation of Example 6 (fiber forming apparatus of Example 6) is less than 0.06 pm at all measurement frequencies (10 Hz, 1 Hz, and 0.1 Hz), while the standard deviation of Comparative Example 2A (fiber forming apparatus of Comparative Example 2A without a tube) is about 0.15 pm or more at these measurement frequencies; and Figure 3 Figure 1 is a graph of the standard deviation from the average diameter of Example 7 and Comparative Example 2A as a function of measurement frequency, which graph illustrates that the standard deviation of Example 7 (fiber forming apparatus of Example 7, in which a heating element heats the second straight portion) is less than 0.06 pm at all measurement frequencies (10 Hz, 1 Hz, and 0.1 Hz), while the standard deviation of Comparative Example 2A (fiber forming apparatus of Comparative Example 2A without a tube, and without a heating element to heat the lower portion of the channel beyond the constriction) is about 0.15 pm or more at these measurement frequencies.

[0053] Figure 18 Figure 5 Figure 1 ​​​​​​ DETAILED DESCRIPTION

[0054] Now refer to Figure 1-2 , which illustrates one embodiment of an optical fiber forming apparatus 10. The optical fiber forming apparatus 10 includes a drawing furnace 12 and a tensioning station 14. The drawing furnace 12 includes a muffle furnace 16 and an axial opening 18 below the muffle furnace 16. The muffle furnace 16 has an inner surface 20. The inner surface 20 defines a passage 22 extending through the opening 18. The drawing furnace 12 also includes an upper inlet 24 that enters the passage 22. The muffle furnace 16 also includes a narrowing 26, wherein the diameter of the passage 22 narrows as the narrowing 26 progresses toward the axial opening 18.

[0055] The optical fiber forming apparatus 10 also includes a tube 28. The tube 28 extends into the passage 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 passage 22. In some 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 passage 22 and does not extend through the axial opening 18. In either case, at least a portion of the tube 28 is disposed above the axial opening 18 in the passage 22. The tube 28 extends upwardly above the narrowing 26.

[0056] Tube 28 includes an outer surface 30; an inner surface 32 defining a second passage 34 extending through tube 28; an inlet 36 into second passage 34 of tube 28; and an outlet 38 exiting second passage 34 of tube 28. Inlet 36 of tube 28 is positioned within passage 22 of draw furnace 12 above constriction 26. Exit 38 need not be positioned within passage 22 of draw furnace 12, but may be. For the portion of tube 28 positioned within passage 22 of draw furnace 12, inner surface 20 of muffle 16 surrounds outer surface 30 of tube 28. A space 40 separates outer surface 30 of tube 28 from inner surface 20 of muffle 16. That is, tube 28 does not contact muffle 16 within passage 22 of draw furnace 12.

[0057] The draw furnace 12 also includes a first heating element 42 in thermal communication with the muffle furnace 16. The first heating element 42 heats the passage 22 of the draw furnace 12 at least throughout a first range 44, which encompasses at least a portion of the passage 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 passage 22 of the draw furnace 12. The first heating element 42 heats the optical fiber preform 46 sufficiently to reduce 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 into the optical fiber 48 that is 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, for example, 1700°C, 1800°C, 1900°C, or 2000°C, or any range of temperatures having any two of these values as endpoints. The passage 22 of the draw furnace 12 in the first range 44 can have an elevated temperature relative to the remainder of the passage 22. The first range 44 can also encompass a body 52 of the optical fiber preform 46 above the tip 50 from which the tip 50 descends.

[0058] The optical fiber 48 drawn from the optical fiber preform 46 extends through the second passage 34 of the tube 28. In other words, the optical fiber 48 drawn from the optical fiber preform 46 extends into the inlet 36 of the tube 28, then through the second passage 34 of the tube 28, and then exits from the outlet 38 of the tube 28. In embodiments, the optical fiber 48 entering the inlet 36 of the tube 28 has a diameter greater than 125 pm, 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 that is less than 1.27 cm has a significant risk of the optical fiber 48 potentially contacting the inlet 36 or the inner surface 32 of the tube 36. An inner diameter of the tube 28 at the inlet 36 that is greater than 2.54 cm would likely result in a distance between the inner surface 32 of the tube 28 and the optical fiber 48 that is large enough to cause convection of the inert gas 54 and thus adversely affect the diameter variability. In embodiments, the inner diameter of the tube 28 at the inlet 36 is 100 to 200 times greater than the diameter of the optical fiber 48 entering the inlet 36 of the tube 28. The tensioning station 14 contacts and maintains the optical fiber 48 in a desired state of tension.

[0059] In embodiments, the inert gas 54 flows through the upper inlet 24 of the draw furnace 12 and into the passage 22 of the draw furnace 12. The inert gas 54 then causes 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 passage 34 of the tube 28, and out of the outlet 38 of the tube 28. The outer stream 58 flows through the passage 22 of the draw furnace 12 in the space 40 between the inner surface 20 of the muffle furnace 16 and the outer surface 30 of the tube 28, and then out of the axial opening 18 of the draw furnace 12.

[0060] In embodiments, the inert gas 54 includes argon or nitrogen, or a combination of argon and nitrogen. In embodiments, the inert gas 54 includes one or more of argon and nitrogen, and less than 1% by volume of helium. In embodiments, the inert gas 54 contains no intentionally included helium. In embodiments, the inert gas 54 includes primarily pure argon (e.g., more than 99% by volume of argon).

[0061] In embodiments, the tube 28 includes one or more of graphite, quartz, and stainless steel. In embodiments, the tube 28 is stainless steel.

[0062] 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 passage 22 of the draw furnace 12 at least throughout a second range 62, which encompasses at least a portion of the passage 22 of the draw furnace 12 above the first range 44. The second range 62 encompasses a portion of the passage 22 above the body 52 of the optical fiber preform 46. In embodiments, the second range 62 encompasses a pedestal 64 that supports the optical fiber preform 46.

[0063] 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 passage 22 of the draw furnace 12 throughout a third range 68, which encompasses a portion of the second passage 34 of the tube 28. The third range 68 is disposed vertically below the first range 44. The third heating element 66 thus heats both a portion of the passage 22 of the draw furnace 12 disposed around the tube 28 and the second passage 34 of the tube 28.

[0064] 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 passage 22 of the draw furnace 12 throughout a fourth range 72, which encompasses a portion of the second passage 34 of the tube 28. The fourth range 72 is disposed vertically below the first range 44. As the optical fiber 48 passes through the second passage 34 of the tube 28 toward the tensioning station 14, the cooling element 70 cools the optical fiber 48 being drawn from the optical fiber preform 46.

[0065] As further demonstrated by the examples below, the optical fiber forming apparatus 10, which includes the tube 28 extending through a portion of the passage 22 of the draw furnace 12, produces an optical fiber 48 having a diameter with a standard deviation that is within an improved and acceptable tolerance range. 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 with a standard deviation that is less than 0.1 μιη at frequencies of 0.1 Hz, 1 Hz, and 10 Hz after exiting the outlet 38 of the tube 28. 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 with a standard deviation that is less than 0.1 μιη at frequencies of 0.06 Hz, 1 Hz, and 10 Hz after exiting the outlet 38 of the tube 28.

[0066] The position of the tube 28 within the passage 22 of the draw furnace 12 is adjustable. This aspect provides a number of advantages. The inlet 36 of the tube 28 can extend relatively close to the tip 50 of the optical fiber preform 46 and thus protect the optical fiber 48 from flow disturbances of the inert gas 54 for a majority of the 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 needed to protect the optical fiber 48 from disturbances of the inert gas 54 or ambient air while the optical fiber 48 is cooling. In some cases, it can be desirable to adjust the length dimension of the tube 28 so that the tube 28 extends out of the passage 22 through the axial opening 18, allowing additional distance and time for the optical fiber 48 to cool before being exposed to flow instability conditions due to temperature differences between the optical fiber 48 and ambient air.

[0067] Reference is now made to Figure 3-4 which illustrates another embodiment of the optical fiber forming apparatus 10A. Figure 3 and 4 appear in Figure 1 and 2Like-numbered items appearing in more than one figure are denoted with the same number in all figures in which they appear. The optical fiber forming apparatus 10A includes a draw furnace 12A having a muffle furnace 16A and an axial opening 18 below the muffle furnace 16A. The muffle furnace 12A has an inner surface 20A. The inner surface 20A defines a passage 22 centered on an axis 74. The passage 22 extends through the axial opening 18 into an ambient environment 76.

[0068] The inner surface 20A includes a first straight portion 78, a tapered portion 80 below the first straight portion 78, and a second straight portion 82 below the tapered portion 80. In other words, the tapered portion 80 is vertically disposed between the first straight portion 78 and the second straight portion 82, and is vertically disposed between the first straight portion 78 and the axial opening 18. In embodiments, the first straight portion 78 encloses the optical fiber preform 46. The first straight portion 78 has a radius 84 from the axis 74. The radius 84 remains constant (or at least approximately constant, e.g., constant as permitted by manufacturing tolerances) along a length 86 parallel to the axis 74.

[0069] The tapered portion 80 includes a radius 88 from the axis 74 that decreases toward the axial opening 18, thus narrowing the passage 22. The tapered portion 80 has a vertical length 90 parallel to the axis 74 that is at least twice as long as the maximum radius 88 of the tapered portion 80. In embodiments, the tapered portion 80 has a constant slope (i.e., a rate of change of the radius 88 as a function of position along the vertical length 90). In other embodiments, the slope of the tapered portion 80 is irregular, i.e., not constant. In embodiments, the vertical length 90 is at least 40 cm, e.g., 40 cm to 80 cm.

[0070] The second straight portion 82 is vertically disposed between the tapered portion 80 and the axial opening 18. The second straight portion 82 includes a radius 92 from the axis 74. The radius 92 remains constant (or at least approximately constant) along a length 94. The length 94 is at least 75 cm. In embodiments, the length 94 is 75 cm to 200 cm, e.g., 75 cm to 150 cm, 100 cm to 150 cm, and 125 cm to 150 cm. In embodiments, the diameter of the second straight portion 82 (i.e., twice the radius 92) is 1.27 cm to 2.54 cm. That is, in embodiments, the radius 92 of the second straight portion 82 is 0.635 cm to 1.27 cm. Again, diameters less than 1.27 cm risk contacting the optical fiber 48.

[0071] The draw furnace 12A also includes an upper inlet 24 into the passage 22. The upper inlet 24 is disposed closer to the first straight portion 78 than the tapered portion 80 of the inner surface 20A of the muffle furnace 16A, e.g., above or through the first straight portion 78.

[0072] The draw furnace 12A also includes a first heating element 42. The first heating element 42 heats the passage 22 throughout the first extent 44 above the tapered portion 80. In embodiments, the draw furnace 12A also includes a second heating element 60. The second heating element 60 heats the passage 22 through a second extent above the first extent 44. In embodiments, the draw furnace 12A also includes a third heating element 66. The third heating element 66 heats a third extent 68 encompassing a portion of the passage 22 defined by the second straight portion 82 of the inner surface 20A of the muffle 16A.

[0073] During operation of the draw furnace 12A, the inert gas 54 (i) flows through the upper inlet 24 and into the passage 22, (ii) then flows along the first straight portion 78 of the inner surface 20A of the muffle 16A, (iii) then flows along the tapered portion 80, (iv) then flows along the second straight portion 82, and (v) then exits from the axial opening 18. In embodiments, the inert gas 54 is argon. In embodiments, the inert gas 54 is nitrogen. In embodiments, the inert gas 54 includes argon and nitrogen. In embodiments, the inert gas 54 includes less than 1 vol% helium, e.g., non-intentionally added helium, and the only helium in the inert gas 54 is unintentional trace amounts.

[0074] The optical fiber preform 46 is disposed within the passage 22. The first extent 44 heated by the first heating element 42 encompasses the tip 50 of the optical fiber preform 46. If included, the second extent 62 heated by the second heating element 60 encompasses a portion of the passage 22 above the body 52 of the optical fiber preform 46. The optical fiber 48 is drawn from the optical fiber preform 46, extending downward from the tip 50. The optical fiber 48 extends through the passage 22 and exits from the axial opening 18 to the tensioning station 14. In embodiments, the optical fiber 48 entering the passage 22 defined by the second straight portion 82 of the inner surface 20A of the muffle 16A has a diameter greater than 125 pm.

[0075] As further demonstrated by the examples below, the draw furnace 12A including the passage 22 defined by the tapered portion 80 and the second straight portion 82 produces an optical fiber 48 having a diameter with a standard deviation within an improved and acceptable tolerance range. In embodiments, the optical fiber 48 exits the axial opening 18 at a rate of at least 20 m / s. In embodiments, the optical fiber 48 has a diameter after exiting the axial opening 18 with a standard deviation less than 0.6 pm measured at frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

[0076] Reference is now made to Figure 5 which illustrates another embodiment of the optical fiber forming apparatus 10B. Figure 5 appear in Figure 1 and4 Like numerals refer to like elements throughout the specification. The fiber forming apparatus 10B 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 of the muffle 16 defines a passageway 22 centered on an axis 74 and extending through the axial opening 18.

[0077] The inner surface 20 of the muffle 16 includes a first straight portion 78, a constriction 96, and a second straight portion 98. The first straight portion 78 has a radius 84 from the axis 74 that remains at least approximately constant along a length 86 parallel to the axis 74. The constriction 96 is disposed vertically between the first straight portion 78 and the second straight portion 98, and is disposed vertically between the first straight portion 78 and the axial opening 18. The constriction 96 includes a radius 100 from the axis 74 that decreases away from the first straight portion 78, thus narrowing the passageway 22. The second straight portion 98 is disposed vertically below the constriction 96. The second straight portion 98 is disposed between the constriction 96 and the axial opening 18. The second straight portion 98 has a radius 102 from the axis 74 that remains at least approximately constant along a length 103 parallel to the axis 74.

[0078] The draw furnace 12 also includes a first heating element 42, a second heating element 60, and a third heating element 66. The first heating element 42 heats the passageway 22 throughout a first range 44, which encompasses a portion of the passageway 22 defined by the first straight portion 78. The second heating element 60 heats the passageway 22 throughout a second range 62, which encompasses a portion of the passageway 22 defined by the first straight portion 78 above the first range 44. That is, the second heating element 60 is disposed vertically above the first heating element 42. The third heating element 66 heats the passageway 22 throughout a third range 68, which encompasses a portion of the passageway 22 defined by the second straight portion 98. That is, the third heating element 66 is disposed vertically below the first heating element 42.

[0079] In use, the draw furnace 12 also includes an optical fiber preform 46 in the passageway 22. The first range 44 heated by the first heating element 42 encompasses the tip 50 of the optical fiber preform 46. The second range 62 heated by the second heating element 60 is at least partially above the body 52 of the optical fiber preform 46. An optical fiber 48 is drawn from the optical fiber preform 46 and extends through the passageway 22 and out of the axial opening 18. The third range 68 heated by the third heating element 66 encompasses a portion of the optical fiber 48 drawn from the optical fiber preform 46. In embodiments, the third heating element 66 heats the third range 68 to a temperature of 100°C to 200°C, for example, 125°C to 175°C, about 150°C, or 150°C.

[0080] The draw furnace 12 also includes an upper inlet 24 into the passage 22. The upper inlet 24 is disposed closer to the first straight portion 78 of the inner surface 20 of the muffle 16 than the constriction 96 of the inner surface 20 of the muffle 16. In embodiments, the upper inlet 24 is disposed vertically above the body 52 of the optical fiber preform 46. The inert gas 54 (i) flows through the upper inlet 24 and into the passage 22, (ii) then flows along the first straight portion 78 of the inner surface 20 of the muffle 16, (iii) then flows along the constriction 96 of the inner surface 20 of the muffle 16, (iv) then flows along the second straight portion 98 of the inner surface 20 of the muffle 16, and (v) then exits from the axial opening 18. In embodiments, the inert gas 54 is argon. In embodiments, the inert gas 54 is nitrogen. In embodiments, the inert gas 54 is one or more of argon and nitrogen. In embodiments, the inert gas 54 includes one or more of argon and nitrogen, and less than 1% by volume of helium, e.g., non-intentionally added helium.

[0081] As will become more apparent in the examples below, when using a non-helium inert gas, the third heating element 66 is used to heat the third range 68, and the third range 68 encompasses the passage 22 defined by the second straight portion 98, whereby the diameter of the optical fiber 48 resulting has an acceptable variability. In embodiments, the optical fiber 48 exits the axial opening 18 at a rate of at least 20 m / s, and has a diameter after exiting the axial opening 18 that has a standard deviation of less than 0.06 pm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

[0082] Examples

[0083] Examples 1 and Comparative Examples 1A and 1B: For these examples, computational fluid dynamics simulations (ANSYS Fluent v 17.2, ANSYS, Inc., Canonsburg, PA, USA) were used to generate stream function contour plots for various scenarios. For Example 1, a stream function contour plot was generated for argon as the inert gas 54 flowing through the optical fiber forming apparatus 10 having the tube 28. The tube 28 was assumed to have an inner diameter of 1.905 cm (0.75 inches) and to be formed of graphite. This stream function contour plot is reproduced in FIG. 1. and FIG. 2. The axial velocity profile for argon as the inert gas 54 is reproduced in FIG. 3. Figure 6A and 6B and the axial velocity profile for argon as the inert gas 54 is reproduced in FIG. 3. Figure 6C For Comparative Example 1A, a stream function contour plot was again generated for argon as the inert gas 54, but this time, the inert gas 54 flowed through a variant of the optical fiber forming apparatus 10 that did not have the tube 28. This stream function contour plot is reproduced in FIG. 4. Figure 7A The axial velocity profile for argon of Comparative Example 1B is reproduced in FIG. 5. Figure 7B For Comparative Example 1B, the same stream function contour plot was generated, but this time using helium as the inert gas 54 and flowing through a variant of the fiber forming apparatus 10 that did not have the tube 28. The stream function contour plot is reproduced in Figure 8 In all cases, the first heating element 42 and the second heating element 60 are activated to increase the temperature in the first range 44 and the second range 62 of the channel 22 of the draw furnace 12 .

[0084] In Comparative Example 1B, Figure 8 The stream function contour plot reproduced in shows that when the channel 22 is filled with helium as the inert gas 54, a unidirectional uniform flow of helium is obtained. In other words, when helium is used as the inert gas 54, no well-defined convection rings 104 exist. However, in Comparative Example 1A, when the channel 22 is filled with argon as the inert gas 54, convection rings 104 are generated near the inner surface of the muffle furnace 16 near the tip 50 of the optical fiber preform 46 and above the narrowing portion 26 of the channel 22 surrounding the optical fiber 48. These convection rings 104 are shown in FIG. Figure 7A In particular, the closed line illustrates the circulation of the inert gas 54 . Figure 7B The diagram illustrates the Figure 7A The axial velocity distribution of the argon gas at the dashed line VIIB is shown at . Positive values ​​indicate downward flow toward the axial opening 18, while negative values ​​indicate upward flow. The presence of both positive and negative values ​​corresponds to the circulation of the argon gas, resulting in the formation of a convection ring 104.

[0085] Without being bound by theory, it is believed that the convection ring 104 (e.g. Figure 7A The convection rings 104 are obtained when argon is used as the inert gas 54. However, as with Comparative Examples 1B and 1B, the convection rings 104 are obtained when argon is used as the inert gas 54. Figure 8 As shown in the stream function contour plot of , when helium is used as the inert gas 54, such convection rings 104 do not appear, and the variability of the diameter of the optical fiber 48 is avoided. Without being bound by theory, it is believed that the relatively high kinematic viscosity of helium compared to argon suppresses the generation of these convection rings 104. The so-called Grashof number (Gr) is a dimensionless value of natural convection that conceptualizes the relationship between the kinematic viscosity of the inert gas 54 and natural convection. The Grashof number (Gr) is defined as follows:

[0086]

[0087] Where g is the acceleration due to gravity, β is the coefficient of thermal expansion, and L cL is the characteristic length (cubed), ΔT is the temperature difference, and v is the kinematic viscosity of the gas. As is apparent from this equation, when the kinematic viscosity of the inert gas 54 is high, as with helium gas relative to argon gas, the Grashof number is low, which means that the inert gas 54 has relatively low convection. Other things being equal, the difference in kinematic viscosity (v) of helium gas versus argon gas (or nitrogen gas) results in a 70-fold difference in Grashof number (Gr).

[0088] However, when the tube 28 is used as in Example 1, the argon gas as the inert gas 54 does not form a convection loop 104 adjacent the optical fiber 48 immediately above the constriction 26. Figure 6A and 6B Example 1 A is absent immediately above the constriction 26. Figure 7A the convection loop 104 apparent in Comparative Example 1 A. Figure 6C the axial velocity plot obtained at the line VIC of Figure 6B the axial velocity of the argon gas is positive in the second passageway 34 of the tube 28, as well as in the first passageway 22 of the draw furnace 12 between the inner surface 20 of the muffle 16 and the outer surface 30 of the tube 28, which indicates unidirectional downward flow. The use of the tube 28 results in a uniform unidirectional gas flow around the optical fiber 48 being drawn from the optical fiber preform 46, which results in reduced diameter variability. Without being bound by theory, and referring back to the equation for Grashof number (Gr), the tube 28 reduces the value of the characteristic length L c which results in a smaller Grashof number (Gr), indicating lower convection. Because the tube 28 divides the inert gas 54 into two separate streams 56, 58, two separate characteristic lengths L c can be analyzed. The first characteristic length L c value that can be analyzed is the distance of the space 40 between the inner surface 20 of the muffle 16 and the outer surface 30 of the tube 28. Even though this first characteristic length L c is large, the tube 28 isolates the optical fiber 48 from any convection loop 104 that is created between the inner surface 20 of the muffle 16 and the outer surface 30 of the tube 28. The second characteristic length L c value that can be analyzed is the distance between the inner surface 32 of the tube 28 and the optical fiber 48 within the tube 28. This distance is intentionally small (e.g., when the inner diameter of the tube is less than or equal to 2.54 cm), thus limiting the value of the second characteristic length L c . In other words, this distance is too small to create a convection loop, even when the argon gas is the inert gas 54. In either case, the argon gas as the inert gas 54 does not create a convection loop 104 that adversely affects the diameter variability of the optical fiber 48.

[0089] The computational fluid dynamics model also produced temperature fluctuation data and pressure fluctuation data for Example 1 and Comparative Example 1A. This data is reproduced in graphical form in Figure 9A (Temperature Fluctuation) and Figure 9B (Pressure Fluctuation). Example 1, which employed tube 28, resulted in significantly less temperature and pressure fluctuation for the argon gas that was the inert gas 54 as compared to Comparative Example 1A, which did not employ tube 28. The reduced temperature and pressure fluctuation resulting from the use of tube 28 resulted in reduced variability in the diameter of optical fiber 48.

[0090] Example 2 and Comparative Example 2A: In Example 2, actual optical fiber 48 was drawn from optical fiber preform 46 using optical fiber forming apparatus 10 that employed tube 28 in passageway 22. Second heating element 60 was set at 8000C. The inner diameter of tube 28 was 3 / 4 inch (1.905 cm). Optical fiber 48 was drawn at a rate of 20 m / s. Inert gas 54 was substantially pure argon (-100% by volume). In Comparative Example 2A, tube 28 was not employed, but otherwise all conditions were the same as in Example 2.

[0091] For Example 2 and Comparative Example 2A, the deviation of the diameter of optical fiber 48 from the average diameter was measured as a function of time. The results are graphically illustrated in Figure 10A (Example 2) and Figure 10B (Comparative Example 2A). In both cases, the average diameter was 125 μm. For Example 2, which employed tube 28, the deviation of the diameter of optical fiber 48 from the average diameter of optical fiber 48 varied by less than 0.2 μm in either direction from the average diameter throughout the entire time period during which the diameter was measured. In contrast, for Comparative Example 2A, which did not employ tube 28, the deviation of the diameter of optical fiber 48 from the average diameter of optical fiber 48 often varied by more than 0.2 μm, and sometimes by more than 0.6 μm. Thus, Example 2, which employed tube 28, resulted in optical fiber 48 that was more uniform in diameter (i.e., less variability in diameter) than Example 2A, which did not employ tube 28.

[0092] The standard deviation from the average diameter was calculated for Example 2 and Comparative Example 2A as a function of measurement frequency and is graphically illustrated in Figure 11 . At a draw rate of 20 m / s, the standard deviation from the average diameter of optical fiber 48 of Example 2 was less than 0.06 μm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz. In contrast, the standard deviation from the average diameter of optical fiber 48 of Example 2A was about 0.15 μm or more at the same frequency range.

[0093] Example 3: For Example 3, optical fiber 48 was drawn from optical fiber preform 46 using optical fiber forming apparatus 10 with tube 28 in passageway 22 at various draw rates. Inert gas 54 was about 100% nitrogen. Second heating element 60 was activated along with first heating element 42. The deviation of the diameter of optical fiber 48 from the average diameter was measured at frequencies of 0.1 Hz, 1 Hz, and 10 Hz. The standard deviation from the average diameter was then calculated. In addition, the temperature of optical fiber 48 at exit 38 of tube 28 was measured. The results are shown in FIG. 3. Figure 12 Note that as the temperature of optical fiber 48 at exit 38 of tube 28 increased, the standard deviation of optical fiber 48 from the average diameter increased. Nonetheless, when the temperature of optical fiber 48 at exit 38 was less than or equal to 1650°C, the standard deviation of optical fiber 48 from the average diameter was typically 0.06 μ or less. All else being equal, decreasing the draw rate or increasing tube 28 so that the temperature of optical fiber 48 at exit 38 decreased, thus decreased the standard deviation of optical fiber 48 from the average diameter.

[0094] Example 4 and Comparative Example 4A: For Example 4, optical fiber 48 was drawn from optical fiber preform 46 using optical fiber forming apparatus 10 with tube 28. Substantially pure argon was used as inert gas 24. First heating element 42 was activated but second heating element 60 was not activated. For Comparative Example 4A, the same setup was used but without tube 28. In both cases, optical fiber 48 was drawn at a rate of 20 m / s. The deviation of the diameter of optical fiber 48 from the average diameter was measured at frequencies of 0.1 Hz, 1 Hz, and 10 Hz. The standard deviation from the average diameter was calculated and is shown in FIG. 4. Figure 13 At all measured frequencies, Example 4 with tube 28 resulted in a smaller standard deviation from the average diameter for the drawn optical fiber 48 compared to Example 4A without tube 28. In addition, the data for Example 4 in FIG. 4 was compared to the data for Example 2 in FIG. 2. Figure 13 Figure 11 At all measured frequencies, Example 4 with tube 28 resulted in a smaller standard deviation from the average diameter for the drawn optical fiber 48 compared to Example 4A without tube 28. In addition, the data for Example 4 in FIG. 4 was compared to the data for Example 2 in FIG. 2.

[0095] Example 5: For Example 5, a computational fluid dynamics simulation was again used to generate stream function contour plots for argon as inert gas 54 flowing through optical fiber forming apparatus 10A having passageway 22 defined by taper 80 and then flowing through second straight portion 82 of inner surface 20A of muffle furnace 16A. The diameter of second straight portion 82 (twice the radius 92) was assumed to be 0.5 inches. ​Inches (1.905 cm). Assume that both the first heating element 42 and the second heating element 60 are activated to increase the temperature in the first range 44 and the second range 62 of the channel 22 .

[0096] The stream function contour plot is reproduced in Figure 14A and 14B As shown, there is no convection ring 104 at the transition from the tapered portion 80 to the second straight portion 82. Instead, the argon gas flows in a consistent downward manner. Figure 14B The axial velocity distribution of the argon flow at line XIVC is reproduced in Figure 14C The axial velocity profile shows positive values, thus unidirectional downward flow. The axial velocity values ​​of Example 5 are greater than those of Example 1 using tube 28 (see Figure 6C ), because in Example 5, all of the inert gas 54 flows through the channel 22, while in Example 1, the flow rate of the inert gas 54 is divided into an inner flow 56 in the tube 28 and an outer flow 58 outside the tube 28. In Comparative Example 1A, a convection ring 104 is generated above the narrowing portion 26 of the channel 22, compared to the optical fiber 48 ( Figures 7A-7B ), the optical fiber 48 produced by the optical fiber forming apparatus 10A will have improved diameter variability.

[0097] The computational fluid dynamics model also generated additional temperature fluctuation data and pressure fluctuation data for Example 5 and Comparative Example 1A. The data are reproduced in the form of graphs. Figure 15A (temperature fluctuations) and Figure 15B (Pressure Fluctuation). Example 5 employs a channel 22 defined by the tapered portion 80 and the second straight portion 82 of the inner surface 20A of the muffle furnace 16A. Compared to Comparative Example 1A, Example 5 achieves significantly smaller temperature and pressure fluctuations in the argon gas. Reduced temperature and pressure fluctuations reduce the variability in the diameter of the optical fiber 48.

[0098] Without being bound by theory, it is again believed that for the optical fiber forming apparatus 10A of this embodiment, the relatively small diameter of the inner surface 20A defined by the second straight portion 82 sufficiently reduces the distance between the optical fiber 48 and the inner surface 20A (i.e., the characteristic length L c ) to minimize convection of the inert gas 54. Thus, the relatively small diameter of the inner surface 20A defined by the second straight portion 82 allows the use of inert gases 54 other than helium, such as argon or nitrogen, without significantly adversely affecting the diameter variability of the optical fiber 48.

[0099] Example 6: In Example 6, an optical fiber forming apparatus 10A was used to draw an actual optical fiber 48 from an optical fiber preform 46, the optical fiber forming apparatus 10A employing an inner surface 20A of relatively small diameter defined by a second straight portion 82. The inner surface 20A of the second straight portion 82 was 3 / 4 inch (1.905 cm) in diameter. The optical fiber 48 was drawn at a rate of 20 m / s. The inert gas 54 was substantially pure argon (-100% by volume). The second heating element 60 was activated.

[0100] For Example 6, the deviation of the diameter of the optical fiber 48 from the average diameter was measured as a function of time. The results are shown in FIG. 6. Figure 16 During the entire time period in which the diameter was measured, the deviation of the diameter of the optical fiber 48 from the average diameter of the optical fiber changed by less than 0.2 μm in either direction from the average diameter. This can be compared to Figure 10B Example 2A, which did not employ an inner surface 20A of relatively small diameter defined by a second straight portion 82. In Example 2A, the deviation of the diameter of the optical fiber 48 from the average diameter of the optical fiber 48 often changed by more than 0.2 μm, and sometimes changed by more than 0.6 μm. Thus, the optical fiber 48 of Example 6, which employed an inner surface 20A of relatively small diameter defined by a second straight portion 82, had a more consistent diameter (i.e., less variability in diameter) than the optical fiber 48 of Comparative Example 2A.

[0101] The standard deviation of the deviation of the optical fiber 48 of Example 6 from the average diameter of the optical fiber 48 was calculated. The results are shown as a function of the measurement frequency in FIG. 7. Figure 17 The results of Example 6 were compared to Comparative Example 2A. As shown in the graph, the standard deviation of the deviation of the optical fiber 48 of Example 6 from the average diameter was less than 0.06 μm for all of the measurement frequencies (10 Hz, 1 Hz, and 0.1 Hz). In contrast, the standard deviation of the deviation of the optical fiber 48 of Comparative Example 2A from the average diameter was about 0.15 μm or more at these frequencies.

[0102] Example 7: In Example 7, an optical fiber forming apparatus 10 having a third heating element 66 was used to draw an optical fiber 48, the third heating element 66 heating the passage 22 throughout a third range 68, the third range 68 encompassing a portion of the passage 22 defined by a second straight portion 98 of the inner surface 20 of the muffle 16. The third heating element 66 was set at a temperature of 150°C. The optical fiber 48 was drawn at a rate of 20 m / s. The inert gas 54 was 100% argon. The first heating element 42 and the second heating element 60 were additionally activated.

[0103] The deviation of the diameter from the average diameter of the optical fiber 48 was measured at various time frequencies (10 Hz, 1 Hz, and 0.1 Hz). The standard deviation of the deviation from the average diameter was calculated. The results are shown in FIG. 8. Figure 18The results of Example 7 were compared to the results of Comparative Example 2A on a graph. In the case of Comparative Example 2A, the third heating element 66 was not used, but otherwise the setup was the same as Example 7. As Figure 18 As illustrated by the graph, the use of the third heating element 66 resulted in a significantly lower standard deviation from the average diameter as compared to not using the third heating element 66. For Example 7, the standard deviation from the average diameter was less than 0.06 μιη at all of the measurement frequency times. For Comparative Example 2A, the standard deviation from the average diameter was about 0.15 μιη or more for the same measurement frequency times. Without being bound by theory, it is believed that the use of the third heating element 66 increased the kinematic viscosity of the inert gas 54 to suppress convective instability within the portion of the channel 22 defined by the second straight portion 98 of the inner surface 20 of the muffle 16. Referring again to the equation for the Grashof number (Gr) above, the higher the kinematic viscosity (v) of the gas, the lower the Grashof number (Gr).

Claims

1. An optical fiber forming apparatus comprising: a drawing furnace comprising: (i) a muffle furnace having an inner surface, (ii) an axial opening below the muffle furnace, the inner surface of the muffle furnace defining a passage extending through the axial opening, and (iii) an upper inlet into the passage; and a tube extending into the channel of the drawing furnace above the axial opening, the tube having (i) an outer surface, with the inner surface of the muffle furnace surrounding the outer surface of the tube and having a space separating the outer surface of the tube from the inner surface of the muffle furnace, (ii) an inner surface defining a second channel extending through the tube, (iii) an inlet into the second channel of the tube, and (iv) an outlet from the second channel of the tube, wherein the inert gas flows through the upper inlet and into the passage of the drawing furnace and forms separate streams, one of which flows through the passage of the drawing furnace in the space between the inner surface of the muffle and the outer surface of the tube and exits from the axial opening of the drawing furnace, and another of which flows into the inlet of the tube, through the second passage of the tube, and exits from the outlet of the tube; wherein the inert gas comprises one or more of argon or nitrogen, and less than 1% by volume of helium; and The inlet of the tube has an inner diameter of 1.27 cm to 2.54 cm, The optical fiber forming apparatus further comprises a third heating element that heats the channel of the drawing furnace throughout a third range, the third range covering a portion of the second channel of the tube.

2. The optical fiber forming apparatus of claim 1 , further comprising: an optical fiber preform disposed in a channel of a drawing furnace; an optical fiber drawn from the optical fiber preform and extending through the second passage of the tube; and A first heating element heats the channel of the draw furnace throughout a first range encompassing the tip of the optical fiber preform.

3. The optical fiber forming apparatus of claim 2 , further comprising: A second heating element heats the channel of the draw furnace throughout a second range encompassing a portion of the channel above the body of the optical fiber preform.

4. The optical fiber forming apparatus according to claim 2, wherein The optical fiber exits the outlet of the tube at a rate of at least 20 m / s and has a diameter after exiting the outlet of the tube with a standard deviation (σ) of less than 0.06 µm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

5. The optical fiber forming apparatus of claim 1 , further comprising: a first heating element that heats the channel of the draw furnace throughout a first range encompassing at least a portion of the channel of the draw furnace above an inlet for the tube; as well as A second heating element heats the channel of the draw furnace throughout a second range encompassing at least a portion of the channel of the draw furnace above the first range.

6. A drawing furnace for an optical fiber forming apparatus, comprising: A muffle furnace having an inner surface and an axial opening below the muffle furnace, the inner surface of the muffle furnace defining a passage centered about an axis and extending through the axial opening, the inner surface comprising: a first straight portion having a radius from the axis that is at least constant along a length parallel to the axis; a tapered portion disposed between the first straight portion and the axial opening, the tapered portion including a radius from the axis that decreases away from the first straight portion, thereby narrowing the passageway; and a perpendicular length parallel to the axis that is at least twice longer than a maximum radius of the tapered portion; and A second straight portion disposed between the tapered portion and the axial opening has a radius from the axis that remains at least constant along a length of at least 75 cm, the radius of the second straight portion being between 0.635 cm and 1.27 cm.

7. The drawing furnace of claim 6, further comprising: an optical fiber preform disposed in the channel; and an optical fiber drawn from the optical fiber preform, the optical fiber extending through the passage and exiting from the axial opening; An upper inlet into the channel is disposed closer to the first straight portion than to the tapered portion of the inner surface of the muffle furnace; a first heating element that heats the channel throughout a first range encompassing the tip of the optical fiber preform; and a second heating element that heats the channel throughout a second range encompassing a portion of the channel above the body of the optical fiber preform, wherein the inert gas (i) flows through the upper inlet and into the passage, (ii) then flows along a first straight portion of the inner surface of the muffle furnace, (iii) then flows along the tapered portion, (iv) then flows along a second straight portion, and (v) then exits from the axial opening; and in, The inert gas includes one or more of argon and nitrogen, and less than 1% by volume of helium.

8. The drawing furnace of claim 7, further comprising: A third heating element heats a third range encompassing a portion of the channel of the second straight portion defined by the inner surface of the muffle furnace.

9. A drawing furnace for an optical fiber forming apparatus, comprising: A muffle furnace having an inner surface and an axial opening below the muffle furnace, the inner surface of the muffle furnace defining a passage centered about an axis and extending through the axial opening, the inner surface comprising: a first straight portion having a radius from the axis that is at least constant along a length parallel to the axis; a constriction disposed between the first straight portion and the axial opening, the constriction including a radius from the axis that decreases away from the first straight portion, thereby narrowing the passage; and a second straight portion disposed between the constriction and the axial opening, the second straight portion having a radius from the axis that is at least constant along a length parallel to the axis; a first heating element that heats the channel throughout a first range encompassing a portion of the channel defined by the first straight portion; a second heating element that heats the channel throughout a second range encompassing a portion of the channel defined by the first straight portion above the first range; and a third heating element that heats the channel to a temperature of 100°C to 200°C throughout a third range encompassing a portion of the channel defined by the second straight portion, Wherein, the drawing furnace further comprises: an optical fiber preform disposed within the channel; and an optical fiber drawn from the optical fiber preform, the optical fiber extending through the passage and exiting from the axial opening; wherein the first range heated by the first heating element covers the tip of the optical fiber preform; The drawing furnace further includes an upper entrance into the channel, and the upper entrance is arranged closer to the first straight portion than the narrowed portion; wherein the inert gas (i) flows through the upper inlet and into the passage, (ii) then flows along a first straight portion of the inner surface of the muffle furnace, (iii) then flows along a narrowed portion of the inner surface of the muffle furnace, (iv) then flows along a second straight portion of the inner surface of the muffle furnace, and (v) then exits from the axial opening; and The inert gas includes one or more of argon and nitrogen, and less than 1% by volume of helium.

10. The drawing furnace according to claim 9, wherein: The second range heated by the second heating element is at least partially above the main body of the optical fiber preform; and The third range heated by the third heating element covers a portion of the optical fiber drawn from the optical fiber preform.

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