Optical fiber forming apparatus

Abstract

The optical fiber forming apparatus comprises a drawing furnace having (i) a muffle having an inner surface, (ii) an axial opening below the muffle, the inner surface of the muffle defining a passage extending through the axial opening, and (iii) an upper entrance into the passage; and a tube extending into the passage of the drawing furnace above the axial opening, the tube having (i) an outer surface, the inner surface of the muffle surrounding 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 defining a second passage extending through the tube, (iii) an entrance into the second passage of the tube, and (iv) an exit from the second passage of the tube.

Description

Related Applications

[0001] This application claims the benefit of priority of U.S. Provisional Patent Application No. 63 / 025,522, filed on May 15, 2020, under 35 U.S.C. § 119, the content of which is relied upon and incorporated herein by reference in its entirety.

Technical Field

[0002] The present disclosure relates to the technical field of optical fibers.

Background Art

[0003] An optical fiber can be drawn from a base material using a drawing furnace. The drawing furnace includes a passage in which the base material and the initially drawn optical fiber are disposed. The passage is purged with an inert gas to prevent ambient air that can cause oxidation of the components of the drawing furnace from flowing into the passage. The inert gas has typically been helium heretofore. However, helium has been increasing in price and is not readily renewable. Since argon and nitrogen are less expensive and more abundant than helium, both argon and nitrogen have the potential to be alternatives to helium.

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when using argon, there is a problem that the variation in the diameter of the drawn optical fiber becomes larger than that of helium, and the diameter of the drawn optical fiber exceeds the range of the design specifications.

Means for Solving the Problems

[0005] The present disclosure addresses such problems in multiple ways. The present disclosure addresses such problems by reducing the distance between the drawn optical fiber and the boundary with the inert gas in the drawing furnace by (i) a tube inserted into the muffle of the drawing furnace through which the drawn optical fiber passes, the inert gas flowing through both the tube having the drawn optical fiber and around the tube, (ii) gradually tapering the diameter of the muffle over a distance greater than the reduction in the diameter of the muffle over the entire taper, or (iii) heating a smaller diameter portion of the muffle through which the drawn optical fiber extends. In either of the first two examples, the flow of the inert gas (such as argon) is made sufficiently uniform so as not to significantly affect the variation in the diameter of the drawn optical fiber, whereby the variation is within the specification range and can be the same as the diameter described above when using helium. In the third example, the additional heat suppresses the convective instability in the smaller diameter portion surrounding the drawn optical fiber, whereby the variation in the diameter of the drawn optical fiber can be within the design specification range. These solutions enable the use of other inert gases such as argon and nitrogen instead of helium.

[0006] According to a first aspect of the present disclosure, an optical fiber forming apparatus includes: (a) a drawing furnace including (i) a muffle having an inner surface, (ii) an axial opening below the muffle, the inner surface of the muffle defining a passage extending through the axial opening, and (iii) an upper inlet into the passage; and (b) a tube extending into the passage of the drawing furnace above the axial opening, the tube having (i) an outer surface, the inner surface of the muffle surrounding 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 defining a second passage extending through the tube, (iii) an inlet into the second passage of the tube, and (iv) an outlet from the second passage of the tube.

[0007] According to a second aspect, a first heating element that heats the passage of the drawing furnace over the entire first range surrounding at least a part of the passage of the drawing furnace above the inlet of the tube, and a second heating element that heats the passage of the drawing furnace over the entire second range surrounding at least a part of the passage of the drawing furnace above the first range. The first aspect further includes this.

[0008] According to a third aspect, the second aspect further includes a third heating element that heats the passage of the drawing furnace over the entire third range surrounding a part of the second passage of the tube.

[0009] According to a fourth aspect, the first aspect further includes an optical fiber preform disposed in the passage of the 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 that heats the passage of the drawing furnace over the entire first range surrounding the tip of the optical fiber preform.

[0010] According to a fifth aspect, the fourth aspect further includes a second heating element that heats the passage of the drawing furnace over the entire second range surrounding a part of the passage above the main body of the optical fiber preform.

[0011] According to a sixth aspect, the optical fiber exits the outlet of the tube at a speed of at least 20 m / s, and after exiting the outlet of the tube, has a diameter with a standard deviation (σ) of less than 0.06 μm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz. The fourth or fifth aspect.

[0012] According to a seventh aspect, an inert gas flows into the passage through the upper inlet of the drawing furnace, forms separate flows, one of the flows flows into the space between the inner surface of the muffle and the outer surface of the tube through the passage of the drawing furnace, and exits from the axial opening of the drawing furnace, and another of the flows flows into the inlet of the tube and exits from the outlet of the tube through the second passage of the tube. Any one of the first to sixth aspects.

[0013] According to an eighth aspect, the inert gas includes one or more of argon or nitrogen and less than 1% by volume of helium. The seventh aspect.

[0014] According to the ninth aspect, the inlet of the tube has an inner diameter of 1.27 cm to 2.54 cm and is any one of the first to eighth aspects.

[0015] According to the tenth aspect of the present disclosure, the wire drawing furnace for an optical fiber forming apparatus includes a muffle having an inner surface and an axial opening below the muffle. The inner surface of the muffle defines a passage extending through the axial opening and centered on the axis. The inner surface includes: (a) a first straight portion where the radius from the axis remains at least substantially constant along a length parallel to the axis; (b) a tapered portion disposed between the first straight portion and the axial opening, where the radius from the axis decreases away from the first straight portion to narrow the passage, and the vertical length parallel to the axis is at least twice as long as the maximum radius of the tapered portion; and (c) a second straight portion disposed between the tapered portion and the axial opening, where the radius from the axis remains at least substantially constant along a length of at least 75 cm, and the radius of the second straight portion is 0.635 cm to 1.27 cm.

[0016] According to the eleventh aspect, it further includes an upper inlet to the passage disposed closer to the first straight portion than the tapered portion of the inner surface of the muffle. The inert gas flows: (i) into the passage through the upper inlet; (ii) then along the first straight portion of the inner surface of the muffle; (iii) then along the tapered portion; (iv) then along the second straight portion; and (v) then out of the axial opening. This is the tenth aspect.

[0017] According to the twelfth aspect, the inert gas includes one or more of argon and nitrogen and less than 1% by volume of helium. This is the eleventh aspect.

[0018] According to the 13th aspect, it is any one of the 10th to 12th aspects, further comprising an optical fiber preform disposed in the passage and an optical fiber drawn from the optical fiber preform and extending outward from the axial opening through the passage.

[0019] According to the 14th aspect, it is the 13th aspect, further comprising a first heating element that heats the passage over the entire first range surrounding the tip of the optical fiber preform and a second heating element that heats the passage over the entire second range surrounding a part of the passage above the main body of the optical fiber preform.

[0020] According to the 15th aspect, it is the 14th aspect, further comprising a third heating element that heats a third range surrounding a part of the passage defined by the second straight part defined by the inner surface of the muffle.

[0021] According to the 16th aspect, it is any one of the 13th to 15th aspects, wherein the optical fiber exits the axial opening at a speed of at least 20 m / s and, after exiting the axial opening, has a diameter with a standard deviation of less than 0.6 μm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

[0022] According to a seventeenth aspect of the present disclosure, a wire drawing furnace for an optical fiber forming apparatus includes: (a) a muffle having an inner surface and an axial opening below the muffle, the inner surface of the muffle defining a passage extending through the axial opening and centered on the axis, the inner surface including: (i) a first straight portion where the radius from the axis remains at least substantially constant along a length parallel to the axis; (ii) a narrow portion disposed between the first straight portion and the axial opening, the narrow portion including a radius from the axis that decreases and narrows the passage as it moves away from the first straight portion; and (iii) a second straight portion disposed between the narrow portion and the axial opening, where the radius from the axis remains at least substantially constant along a length parallel to the axis; (b) a first heating element that heats the passage over an entire first range surrounding a portion of the passage defined by the first straight portion; (c) a second heating element that heats the passage over an entire second range surrounding 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 over an entire third range surrounding a portion of the passage defined by the second straight portion.

[0023] According to an eighteenth aspect, there is further provided an optical fiber preform disposed in the passage and an optical fiber drawn from the optical fiber preform and extending outward from the axial opening through the passage. The first range heated by the first heating element surrounds the tip of the optical fiber preform, 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 surrounds a portion of the optical fiber drawn from the optical fiber preform, according to the seventeenth aspect.

[0024] According to the 19th aspect, it further includes an upper inlet to the passage, disposed closer to the first straight portion than the narrow portion, and the inert gas (i) flows into the passage through the upper inlet, (ii) then flows along the first straight portion of the inner surface of the muffler, (iii) then flows along the narrow portion of the inner surface of the muffler, (iv) then flows along the second straight portion of the inner surface of the muffler, (v) then flows out from the axial opening, and the inert gas contains one or more of argon and nitrogen and less than 1% by volume of helium, which is the 17th or 18th aspect.

[0025] According to the 20th aspect, the optical fiber exits the axial opening at a speed of at least 20 m / s, and after exiting the axial opening, it has a diameter with a standard deviation of less than 0.06 μm at measurement frequencies of 0.1 Hz, 1 Hz, and 10 Hz, which is the 18th aspect.

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6A

Figure 6B

Figure 6C

Figure 7A

Figure 7B

Figure 8

Figure 9A

Figure 9B

Figure 10A

Figure 10B

Figure 11

Figure 12

Figure 13

Figure 14A

Figure 14B

Figure 14C

Figure 15A

Figure 15B

Figure 16

Figure 17

Figure 18

Best Mode for Carrying Out the Invention

[0027] Referring now to FIGS. 1-2, one embodiment of an optical fiber forming apparatus 10 is shown. The optical fiber forming apparatus 10 includes a drawing furnace 12 and a tension adjustment station 14. The drawing furnace 12 includes a muffle 16 and an axially directed opening 18 below the muffle 16. The muffle 16 has an inner surface 20. The inner surface 20 defines a passage 22 that extends through the axially directed opening 18. The drawing furnace 12 further includes an upper inlet 24 to the passage 22. The muffle 16 further includes a narrow portion 26, and the diameter of the passage 22 narrows as the narrow portion 26 progresses toward the axially directed opening 18.

[0028] The optical fiber forming apparatus 10 further includes a tube 28. The tube 28 extends into the passage 22 of the drawing furnace 12. Thus, the tube 28 is at least partially disposed between the axial opening 18 and the upper inlet 24 to the passage 22. In an embodiment 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 any case, at least a portion of the tube 28 is disposed above the axial opening 18 within the passage 22. The tube 28 extends above the narrow portion 26.

[0029] The tube 28 includes an outer surface 30, an inner surface 32 that defines a second passage 34 extending through the tube 28, an inlet 36 to the second passage 34 of the tube 28, and an outlet 38 from the second passage 34 of the tube 28. The inlet 36 of the tube 28 is disposed above the narrow portion 26 within the passage 22 of the drawing furnace 12. The outlet 38 need not be disposed within the passage 22 of the drawing furnace 12, but it is possible to dispose it in that way. The inner surface 20 of the muffle 16 surrounds the outer surface 30 of a portion of the tube 28 disposed within the passage 22 of the drawing furnace 12. The 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 contact the muffle 16 within the passage 22 of the drawing furnace 12.

[0030] The drawing 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 passage 22 of the drawing furnace 12 over at least a first range 44 that surrounds at least a portion of the passage 22 of the drawing furnace 12 above the inlet 36 of the tube 28. In operation of the optical fiber forming apparatus 10, the optical fiber preform 46 is disposed within the passage 22 of the drawing 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 enable the optical fiber 48 to be drawn from the optical fiber preform 46. The first range 44 heated by the first heating element 42 surrounds the tip 50 of the optical fiber preform 46, and the tip 50 is the location where the optical fiber 48 to which the optical fiber preform 46 is drawn transitions. In an embodiment, the first heating element 42 heats the first range 44 to a temperature in the range 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 passage 22 of the drawing furnace 12 within the first range 44 can have an elevated temperature compared to the remainder of the passage 22. The first range 44 can further surround the body 52 of the optical fiber preform 46, and the body 52 is above the tip 50 and from which the tip 50 descends.

[0031] 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 passes through the second passage 34 of the tube 28, and then exits from the outlet 38 of the tube 28. In an embodiment, the optical fiber 48 entering the inlet 36 of the tube 28 has a diameter greater than 125 μm, while the inlet 36 of the tube 28 has an inner diameter of 1.27 cm to 2.54 cm. If the inner diameter of the tube 28 at the inlet 36 is less than 1.27 cm, the optical fiber 48 is likely to contact the inlet 36 or the inner surface 32 of the tube 28. If the inner diameter of the tube 28 at the inlet 36 is greater than 2.54 cm, the distance between the inner surface 32 of the tube 28 and the optical fiber 48 becomes sufficiently large, causing convection of the inert gas 54, and thus may have an adverse effect on the diameter variation. In an embodiment, 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 entering the inlet 36 of the tube 28. The tension adjustment station 14 is in contact with the optical fiber 48 and maintains the optical fiber 48 at a desired tension.

[0032] In an embodiment, the inert gas 54 flows into the passage 22 of the drawing furnace 12 through the upper inlet 24 of the drawing furnace 12. Then, the inert gas 54 forms separate flows, namely, an inner flow 56 and an outer flow 58. The inner flow 56 flows into the inlet 36 of the tube 28, passes through the second passage 34 of the tube 28, and flows out from the outlet 38 of the tube 28. The outer flow 58 flows into the space 40 between the inner surface 20 of the muffle 16 and the outer surface 30 of the tube 28 through the passage 22 of the drawing furnace 12, and then flows out from the axial opening 18 of the drawing furnace 12.

[0033] In an embodiment, the inert gas 54 includes argon or nitrogen, or a combination of argon and nitrogen. In an embodiment, the inert gas 54 includes one or more of argon and nitrogen and less than 1 volume% of helium. In an embodiment, the inert gas 54 does not contain intentionally included helium. In an embodiment, the inert gas 54 includes essentially pure argon (e.g., more than 99 volume% argon).

[0034] In an embodiment, the tube 28 includes one or more graphite, quartz, and stainless steel. In an embodiment, the tube 28 is stainless steel.

[0035] In an embodiment, 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 drawing furnace 12 over at least a second range 62 that surrounds at least a part of the passage 22 of the drawing furnace 12 above the first range 44. The second range 62 surrounds a part of the passage 22 above the main body 52 of the optical fiber preform 46. In an embodiment, the second range 62 surrounds the bobbin 64 that supports the optical fiber preform 46.

[0036] In an embodiment, 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 drawing furnace 12 over a third range 68 that surrounds a part of the second passage 34 of the tube 28. The third range 68 is located vertically below the first range 44. Thus, the third heating element 66 heats both a part of the passage 22 of the drawing furnace 12 disposed around the tube 28 and the second passage 34 of the tube 28.

[0037] In an embodiment, 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 drawing furnace 12 over a fourth range 72 that surrounds a part of the second passage 34 of the tube 28. The fourth range 72 is located vertically below the first range 44. The cooling element 70 cools the optical fiber 48 drawn from the optical fiber preform 46 when the optical fiber 48 passes through the second passage 34 of the tube 28 toward the tension adjustment station 14.

[0038] As further shown in the following examples, an optical fiber forming apparatus 10 including a tube 28 extending across a portion of a passage 22 of a drawing furnace 12 manufactures an optical fiber 48 having a diameter within an acceptable tolerance with an improved standard deviation. In an embodiment, the optical fiber 48 exits the outlet 38 of the tube 28 at a speed of at least 20 m / s and, after exiting the outlet 38 of the tube 28, has a diameter with a standard deviation of less than 0.1 μm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz. In an embodiment, the optical fiber 48 exits the outlet 38 of the tube 28 at a speed of at least 20 m / s and, after exiting the outlet 38 of the tube 28, has a diameter with a standard deviation of less than 0.1 μm at frequencies of 0.06 Hz, 1 Hz, and 10 Hz.

[0039] The position of the tube 28 within the passage 22 of the drawing 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 the optical fiber 48 can be protected from disturbances in the flow of the inert gas 54 for most of the period during which the optical fiber 48 is being cooled. Similarly, the length of the tube 28 between the inlet 36 and the outlet 38 of the tube 28 can be adjusted as desired to protect the optical fiber 48 from disturbances in the inert gas 54 or ambient air while the optical fiber 48 is being cooled. In some situations, it may be desirable to size the length of the tube 28 to extend out of the passage 22 through the axial opening 18 to allow additional distance and time for cooling the optical fiber 48 before it is exposed to the unstable flow caused by the temperature difference between the optical fiber 48 and the ambient air.

[0040] Referring now to FIGS. 3-4, an optical fiber forming apparatus 10A of another embodiment is shown. The numbers appearing in FIGS. 3 and 4 that are the same as the numbers appearing in FIGS. 1 and 2 refer to like elements. The optical fiber forming apparatus 10A includes a drawing furnace 12A having a muffle 16A and an axial opening 18 below the muffle 16A. The muffle 16A has an inner surface 20A. The inner surface 20A defines a passage 22 centered about an axis 74. The passage 22 extends into the surrounding environment 76 through the axial opening 18.

[0041] 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 disposed between the first straight portion 78 and the second straight portion 82 in the vertical direction, and between the first straight portion 78 and the axial opening 18 in the vertical direction. In an embodiment, the first straight portion 78 surrounds the optical fiber base material 46. The first straight portion 78 has a radius 84 from the axis 74. The radius 84 remains constant (or at least substantially constant, for example, as long as the manufacturing tolerance allows) along a length 86 parallel to the axis 74.

[0042] The tapered portion 80 includes a radius 88 from the axis 74, and the radius 88 decreases toward the axial opening 18, thereby 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 an embodiment, the tapered portion 80 has a constant gradient (i.e., the rate of change of the radius 88 as a function of the position along the vertical length 90). In other embodiments, the gradient of the tapered portion 80 is irregular, i.e., not constant. In an embodiment, the vertical length 90 is at least 40 cm, for example, 40 cm to 80 cm.

[0043] The second straight portion 82 is disposed vertically 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 substantially constant) along a length 94. The length 94 is at least 75 cm. In embodiments, the length 94 is 75 cm to 200 cm, such as 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. Also in this case, if the diameter is less than 1.27 cm, there is a risk of contacting the optical fiber 48.

[0044] The drawing furnace 12A further includes an upper inlet 24 to the passage 22. The upper inlet 24 is disposed near the first straight portion 78 rather than the tapered portion 80 of the inner surface 20A of the muffle 16A, for example, above the first straight portion 78 or penetrating the first straight portion 78.

[0045] The drawing furnace 12A further includes a first heating element 42. The first heating element 42 heats the passage 22 over an entire first range 44 disposed above the tapered portion 80. In embodiments, the drawing furnace 12A further includes a second heating element 60. The second heating element 60 heats the passage 22 over an entire second range 62 disposed above the first range 44. In embodiments, the drawing furnace 12A further includes a third heating element 66. The third heating element 66 heats a third range 68 that surrounds a part of the passage 22 defined by the second straight portion 82 of the inner surface 20A of the muffle 16A.

[0046] During operation of the fiber drawing furnace 12A, the inert gas 54 flows (i) into the passage 22 through the upper inlet 24, (ii) then along the first straight portion 78 of the inner surface 20A of the muffle 16A, (iii) then along the tapered portion 80, (iv) then along the second straight portion 82, and (v) then exits through the axial opening 18. In an embodiment, the inert gas 54 is argon. In an embodiment, the inert gas 54 is nitrogen. In an embodiment, the inert gas 54 contains argon and nitrogen. In an embodiment, the inert gas 54 contains less than 1% by volume of helium, for example, without intentionally added helium, and the helium in the inert gas 54 is only an accidental trace amount.

[0047] The optical fiber preform 46 is disposed within the passage 22. The first range 44 heated by the first heating element 42 surrounds the tip 50 of the optical fiber preform 46. When included, the second range 62 heated by the second heating element 60 surrounds a part of the passage 22 above the main body 52 of the optical fiber preform 46. The optical fiber 48 is drawn from the optical fiber preform 46 and extends downward from the tip 50. The optical fiber 48 extends through the passage 22, exits through the axial opening 18, and reaches the tension adjustment station 14. In an embodiment, 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 μm.

[0048] As further shown in the following examples, the fiber drawing 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 within an acceptable tolerance with an improved standard deviation. In an embodiment, the optical fiber 48 exits the axial opening 18 at a speed of at least 20 m / s. In an embodiment, the optical fiber 48 after exiting the axial opening 18 has a diameter with a standard deviation of less than 0.6 μm at measurement frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

[0049] Next, referring to FIG. 5, an optical fiber forming apparatus 10B according to another embodiment is shown. The numbers that appear in FIG. 5, which are the same as the numbers that appear in FIGS. 1 and 4, refer to like elements. The optical fiber manufacturing apparatus 10B includes a drawing furnace 12. The drawing furnace 12 includes a muffle 16 and an axially extending opening 18 below the muffle 16. The muffle 16 has an inner surface 20. The inner surface 20 of the muffle 16 defines a passage 22 centered about an axis 74 and extending through the axially extending opening 18.

[0050] The inner surface 20 of the muffle 16 includes a first straight portion 78, a narrow portion 96, and a second straight portion 98. The first straight portion 78 has a radius 84 from the axis 74, and the radius 84 remains at least substantially constant along a length 86 parallel to the axis 74. The narrow portion 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 axially extending opening 18. The narrow portion 96 includes a radius 100 from the axis 74, and the radius 100 decreases as it departs from the first straight portion 78, thereby narrowing the passage 22. The second straight portion 98 is disposed vertically below the narrow portion 96. The second straight portion 98 is disposed between the narrow portion 96 and the axially extending opening 18. The second straight portion 98 has a radius 102 from the axis 74, and the radius 102 remains at least substantially constant along a length 103 parallel to the axis 74.

[0051] The drawing furnace 12 further includes a first heating element 42, a second heating element 60, and a third heating element 66. The first heating element 42 heats the passage 22 over the entire first extent 44 that surrounds a portion of the passage 22 defined by the first straight portion 78. The second heating element 60 heats the passage 22 over the entire second extent 62 that surrounds a portion of the passage 22 defined by the first straight portion 78 above the first extent 44. That is, the second heating element 60 is disposed vertically above the first heating element 42. The third heating element 66 heats the passage 22 over the entire third extent 68 that surrounds a portion of the passage 22 defined by the second straight portion 98. That is, the third heating element 66 is disposed vertically below the first heating element 42.

[0052] In use, the drawing furnace 12 further includes an optical fiber preform 46 disposed within the passage 22. The first extent 44 heated by the first heating element 42 surrounds the tip 50 of the optical fiber preform 46. The second extent 62 heated by the second heating element 60 is at least partially above the body 52 of the optical fiber preform 46. The optical fiber 48 is drawn from the optical fiber preform 46 and extends outwardly from the axial opening 18 through the passage 22. The third extent 68 heated by the third heating element 66 surrounds a portion of the optical fiber 48 drawn from the optical fiber preform 46. In an embodiment, the third heating element 66 heats the third extent 68 to a temperature of 100°C to 200°C, such as 125°C to 175°C, about 150°C, or 150°C.

[0053] The drawing furnace 12 further includes an upper inlet 24 to 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 narrow portion 96 of the inner surface 20 of the muffle 16. In an embodiment, the upper inlet 24 is disposed vertically above the main body 52 of the optical fiber preform 46. The inert gas 54 flows (i) into the passage 22 through the upper inlet 24, (ii) then flows along the first straight portion 78 of the inner surface 20 of the muffle 16, (iii) then flows along the narrow portion 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 flows out from the axial opening 18. In an embodiment, the inert gas 54 is argon. In an embodiment, the inert gas is nitrogen. In an embodiment, the inert gas 54 is one or more of argon and nitrogen. In an embodiment, the inert gas 54 includes one or more of argon and nitrogen and less than 1% by volume of helium, for example, does not include intentionally added helium.

[0054] As will be more apparent in the following examples, by heating a third range 68 surrounding the passage 22 defined by the second straight portion 98 using the third heating element 66, an optical fiber 48 having a diameter with acceptable variation when using a non-helium inert gas is obtained. In an embodiment, the optical fiber 48 exits the axial opening 18 at a speed of at least 20 m / s and, after exiting the axial opening 18, has a diameter with a standard deviation of less than 0.06 μm at measurement frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

Example

[0055] Example 1 and Comparative Examples 1A and 1B. In these examples, computational fluid dynamics simulations (ANSYS Fluent v17.2, ANSYS, Inc., Canonsburg, Pennsylvania, USA) were utilized to create streamline contour plots for various scenarios. In Example 1, a streamline contour plot was created 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 3 / 4 inch (1.905 cm) and be formed of graphite. This streamline contour plot is reproduced in FIGS. 6A and 6B, and the axial velocity profile of argon as the inert gas 54 is reproduced in FIG. 6C. In Comparative Example 1A, the same streamline contour plot was created again for argon as the inert gas 54, but here it was flowed through a modified form of the optical fiber forming apparatus 10 without the tube 28. This streamline contour plot is reproduced as FIG. 7A. The axial velocity profile of argon for Comparative Example 1B is reproduced in FIG. 7B. In Comparative Example 1B, the same streamline contour plot was generated, but here helium was utilized as the inert gas 54 and it was flowed through a modified form of the optical fiber forming apparatus 10 without the tube 28. This streamline contour plot is reproduced as FIG. 8. In all scenarios, both the first heating element 42 and the second heating element 60 were actuated to raise the temperatures within the first region 44 and the second region 62 of the passage 22 of the drawing furnace 12.

[0056] In Comparative Example 1B, the streamline contour diagram reproduced in FIG. 8 shows that when the passage 22 is filled with helium as the inert gas 54, a consistent unidirectional flow of helium occurs. In other words, when helium is used as the inert gas 54, no distinct convection cells 104 exist. However, when the passage 22 is filled with argon as the inert gas 54 in Comparative Example 1A, convection cells 104 are generated both near the inner surface 20 of the muffle 16 near the tip 50 of the optical fiber preform 46 and above the narrow portion 26 of the passage 22 around the optical fiber 48. These convection cells 104 are shown in FIG. 7A. Specifically, the closed lines indicate the circulation of the inert gas 54. The axial velocity profile of argon at the dotted line VIIB identified in FIG. 7A is shown in the graph of FIG. 7B. Positive values indicate a downward flow toward the axial opening 18, and negative values indicate an upward flow. The presence of both positive and negative values corresponds to the circulation of the argon forming the convection cells 104.

[0057] Without being bound by theory, the convection cells 104 as shown in FIG. 7A for Comparative Example 1A are considered to have a sufficient impact on heat transfer in the passage 22 between the tip 50 of the optical fiber preform 46 and the narrow portion 26 of the passage 22, and to significantly change the diameter of the optical fiber 48. When argon is used as the inert gas 54, these convection cells 104 occur. However, as shown in the streamline contour diagram of Comparative Example 1B and FIG. 8, when helium is used as the inert gas 54, such convection cells 104 do not appear, and variations in the diameter of the optical fiber 48 are avoided. Without being bound by theory, it is considered that the relatively high kinematic viscosity of helium compared to argon suppresses the generation of these convection cells 104. The so-called Grashof number (Gr), which is a dimensionless value of natural convection, conceptualizes the relationship between the kinematic viscosity of the inert gas 54 and natural convection. The Grashof number (Gr) is defined by the following formula: Gr = gβL c 3 ΔT / v 2 where g is the acceleration due to gravity, β is the coefficient of thermal expansion, and L cis the characteristic length (cubed), ΔT is the temperature difference, and v is the kinematic viscosity of the gas. As is clear from this equation, when the kinematic viscosity of the inert gas 54 is high like that of helium with respect to argon, the Grashof number is low, meaning that the convection of the inert gas 54 is relatively low. When all else is equal, the difference in the kinematic viscosity (v) between helium and argon (or nitrogen) results in a 70-fold difference in the Grashof number (Gr).

[0058] However, when the tube 28 is utilized as in Example 1, argon as the inert gas 54 does not form a convection cell 104 just above the narrow portion 26 adjacent to the optical fiber 48. FIGS. 6A and 6B show that there is no convection cell 104 just above the narrow portion 26 apparent in FIG. 7A for Comparative Example 1A. The axial velocity graph of FIG. 6C taken along line VIC of FIG. 6B shows all positive values for the axial velocity of argon both within the second passage 34 of the tube 28 and within the first passage 22 of the drawing furnace 12 between the inner surface 20 of the muffler 16 and the outer surface 30 of the tube 28, indicating a one-way downward flow. By providing the tube 28, a consistent one-way gas flow is generated around the optical fiber 48 drawn from the optical fiber base material 46, thereby reducing diameter variation. Without being bound by theory, referring back to the equation for the Grashof number (Gr), the tube 28 reduces the value of the characteristic length L c to obtain a low Grashof number (Gr), which means less convection. Since the tube 28 divides the inert gas 54 into two separate flows 56, 58, two separate characteristic lengths L c can be analyzed. The value of the first analyzable characteristic length L c is the distance of the space 40 between the inner surface 20 of the muffler 16 and the outer surface 30 of the tube 28. Even when this first characteristic length L c is large, the tube 28 isolates the optical fiber 48 from any convection cell 104 generated between the inner surface 20 of the muffler 16 and the outer surface 30 of the tube 28. The value of the second analyzable characteristic length L cThe value of c is the distance between the inner surface 32 of the tube 28 and the optical fiber 48 within the tube 28. By intentionally reducing this distance (for example, when the inner diameter of the tube is 2.54 cm or less), the value of the second characteristic length L

[0059] is limited. In other words, even when argon is the inert gas 54, this distance is small enough that no convection cells are generated. In any situation, argon as the inert gas 54 does not generate convection cells 104 that adversely affect the variation in the diameter of the optical fiber 48.

[0060] Example 2 and Comparative Example 2A. In Example 2, the actual optical fiber 48 was drawn from the optical fiber preform 46 using the optical fiber forming apparatus 10 that utilizes the tube 28 within the passage 22. The second heating element 60 was set at 800 °C. The inner diameter of the tube 28 was 3 / 4 inch (1.905 cm). The optical fiber 48 was drawn at a speed of 20 m / s. The inert gas 54 was essentially pure argon (about 100% by volume of argon). In Comparative Example 2A, the tube 28 was not utilized, but all other conditions were the same as in Example 2.

[0061] The deviation of the diameter of the optical fiber 48 from the average diameter as a function of time was measured for both Example 2 and Comparative Example 2A. The results are shown graphically in FIG. 10A (Example 2) and FIG. 10B (Comparative Example 2A). The average diameter in both cases was 125 μm. In Example 2 using the tube 28, throughout the measurement period of the diameter, the deviation of the diameter of the optical fiber 48 from the average diameter of the optical fiber 48 changed by less than 0.2 μm in either direction from the average diameter. In contrast, in Comparative Example 2A without using the tube 28, the deviation of the diameter of the optical fiber 48 from the average diameter of the optical fiber 48 often exceeded 0.2 μm and sometimes exceeded 0.6 μm. Therefore, Example 2 using the tube 28 obtained an optical fiber 48 having a more consistent diameter (i.e., less diameter variation) than Comparative Example 2A without using the tube 28.

[0062] The standard deviation from the average diameter for Example 2 and Comparative Example 2A was calculated and shown graphically in FIG. 11 as a function of the measurement frequency. At a drawing speed of 20 m / s, the standard deviation from the average diameter of the optical fiber 48 in 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 for the optical fiber 48 in Comparative Example 2A was about 0.15 μm or more at frequencies in the same range.

[0063] Example 3. In Example 3, the optical fiber 48 was drawn from the optical fiber preform 46 at various drawing speeds using the optical fiber forming apparatus 10 with the tube 28 in the passage 22. The inert gas 54 was about 100% nitrogen. The second heating element 60 and the first heating element 42 were operated. The deviation of the diameter of the optical fiber 48 from the average diameter was measured at a frequency of 1 Hz. Then, the standard deviation from the average diameter was calculated. Further, the temperature of the optical fiber 48 at the outlet 38 of the tube 28 was measured. The results are shown graphically in FIG. 12. It should be noted that as the temperature of the optical fiber 48 at the outlet 38 of the tube 28 increased, the standard deviation of the diameter of the optical fiber 48 from the average diameter increased. Nevertheless, when the temperature of the optical fiber 48 at the outlet 38 was 1650° C. or lower, the standard deviation of the diameter of the optical fiber 48 from the average diameter was typically 0.06 μm or less. When all other conditions were equal, decreasing the drawing speed or lengthening the tube 28 decreased the temperature of the optical fiber 48 at the outlet 38, resulting in a decrease in the standard deviation of the diameter of the optical fiber 48 from the average diameter.

[0064] Example 4 and Comparative Example 4A. In Example 4, the optical fiber 48 was drawn from the optical fiber preform 46 using the optical fiber forming apparatus 10 having the tube 28. Essentially pure argon was used as the inert gas 54. The first heating element 42 was operated, but the second heating element 60 was not operated. In Comparative Example 4A, the same settings were used, but the tube 28 was not used. In both cases, the optical fiber 48 was drawn at a speed of 20 m / s. The deviation of the diameter of the 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 shown graphically in FIG. 13. Over all the measurement frequencies, Example 4 using the tube 28 had a smaller standard deviation of the diameter of the drawn optical fiber 48 from the average diameter than Comparative Example 4A not using the tube 28. Further, comparing the data for Example 4 in FIG. 13 with the data for Example 2 in FIG. 11, it can be seen that operating the second heating element 60 while using the tube 28 results in a smaller standard deviation of the diameter of the optical fiber 48 from the average diameter compared to not operating the second heating element 60 while using the tube 28.

[0065] Example 5. In Example 5, again using computational fluid dynamics simulation, an argon stream function contour plot was created for the flow of argon as the inert gas 54 flowing through the optical fiber forming apparatus 10A in a state where the passage 22 is defined by the tapered portion 80 of the inner surface 20A of the muffler 16A and then the second straight portion 82. The second straight portion 82 was assumed to have a diameter of 3 / 4 inch (1.905 cm) (twice the radius 92). It was assumed that both the first heating element 42 and the second heating element 60 were operated to raise the temperature within the first range 44 and the second range 62 of the passage 22.

[0066] This stream function contour plot is reproduced in FIGS. 14A and 14B. As shown, there is no convection cell 104 at the transition from the tapered portion 80 to the second straight portion 82. Rather, the argon flows consistently downward. The axial velocity profile of the argon flow at line XIVC in FIG. 14B is reproduced in FIG. 14C. The axial velocity profile is a positive value, indicating a one-direction downward flow. In Example 5, all of the inert gas 54 flows through the passage 22, whereas in Example 1, the flow of the inert gas 54 is divided into an inner flow 56 within the tube 28 and an outer flow 58 outside the tube 28. Therefore, the value of the axial velocity in Example 5 is greater than the value in Example 1 that uses the tube 28 (see FIG. 6C). The optical fiber 48 manufactured from the optical fiber forming apparatus 10A should have an improved diameter variation compared to the optical fiber 48 manufactured from the optical fiber forming apparatus of Comparative Example 1A (FIGS. 7A - 7B) where a convection cell 104 is generated above the narrow portion 26 of the passage 22.

[0067] Furthermore, computational fluid dynamics models were used to generate the temperature fluctuation data and pressure fluctuation data for Example 5 versus Comparative Example 1A. This data is reproduced in graphical form in FIGS. 15A (temperature fluctuations) and 15B (pressure fluctuations). The results of Example 5 utilizing the passage 22 defined by the tapered portion 80 and the second straight portion 82 of the inner surface 20A of the muffler 16A show much less fluctuation in the temperature and pressure of argon compared to Comparative Example 1A. This reduction in temperature and pressure fluctuations should result in a reduction in the variation in the diameter of the optical fiber 48.

[0068] Without being bound by theory, here too, the relatively small diameter of the inner surface 20A defined by the second straight portion 82 of this embodiment of the optical fiber forming apparatus 10A results in a distance (i.e., characteristic length L c ) between the optical fiber 48 and the inner surface 20A that is reduced to a sufficient extent to minimize the convection of the inert gas 54. Thus, the relatively small diameter of the inner surface 20A defined by the second straight portion 82 enables the use of an inert gas 54 other than helium, such as argon or nitrogen, without significantly adversely affecting the variation in the diameter of the optical fiber 48.

[0069] Example 6. In Example 6, the actual optical fiber 48 was drawn from the optical fiber preform 46 using an optical fiber forming apparatus 10A that utilized an inner surface 20A of relatively small diameter defined by a straight second portion 82. The diameter of the inner surface 20A of the second straight portion 82 was 3 / 4 inch (1.905 cm). The optical fiber 48 was drawn at a speed of 20 m / s. The inert gas 54 was essentially pure argon (about 100 volume % argon). The second heating element 60 was activated.

[0070] For Example 6, the deviation of the diameter of the optical fiber 48 from the average diameter as a function of time was measured. The results are shown graphically in FIG. 16. Throughout the measurement period of the diameter, the deviation of the diameter of the optical fiber 48 from the average diameter of the optical fiber varied by less than 0.2 μm in either direction from the average diameter. This is comparable to Comparative Example 2A of FIG. 10B that did not utilize the relatively small-diameter inner surface 20A defined by the straight second portion 82. In Comparative Example 2A, the deviation of the diameter of the optical fiber 48 from the average diameter of the optical fiber 48 often exceeded 0.2 μm and sometimes exceeded 0.6 μm. Thus, Example 6 using the relatively small-diameter inner surface 20A defined by the straight second portion 82 yielded an optical fiber 48 having a more consistent diameter (i.e., less variation in diameter) compared to the optical fiber 48 of Comparative Example 2A.

[0071] The standard deviation from the average diameter of the optical fiber 48 of Example 6 was calculated. The results are shown graphically in FIG. 17 as a function of the measurement frequency. The results of Example 6 are compared with Comparative Example 2A. As shown by the graph, the standard deviation from the average diameter of the optical fiber 48 of Example 6 was less than 0.06 μm for all measurement frequencies (10 Hz, 1 Hz, and 0.1 Hz). In contrast, the standard deviation from the average diameter of the optical fiber 48 for Comparative Example 2A was about 0.15 μm or more for that measurement frequency.

[0072] Example 7. In Example 7, the optical fiber 48 was drawn using an optical fiber forming apparatus 10B having a third heating element 66 that heats the passage 22 over the entire third range 68 surrounding a portion of the passage 22 defined by the second straight portion 98 of the inner surface 20 of the muffle 16. The third heating element 66 was set to a temperature of 150°C. The optical fiber 48 was drawn at a speed of 20 m / s. The inert gas 54 was 100% argon. The first heating element 42 and the second heating element 60 were further activated.

[0073] 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 from the average diameter was calculated. The results are shown in FIG. 18. The results of Example 7 are compared with the results of Comparative Example 2A on the graph. In Comparative Example 2A, the third heating element 66 was not used, but other settings were the same as those in Example 7. As shown in the graph of FIG. 18, when the third heating element 66 is used, the standard deviation from the average diameter becomes much smaller than when the third heating element 66 is not used. In Example 7, the standard deviation from the average diameter was less than 0.06 μm over all measurement time frequencies. In Comparative Example 2A, the standard deviation from the average diameter was about 0.15 μm or more at the same measurement time frequencies. Without being bound by theory, it is considered that by using the third heating element 66, the kinematic viscosity of the inert gas 54 increases, suppressing the convection instability in a part of the passage 22 defined by the second straight portion 98 of the inner surface 20 of the muffle 16. Referring again to the above formula for the Grashof number (Gr), the higher the kinematic viscosity (v) of the gas, the lower the Grashof number (Gr).

[0074] Hereinafter, preferred embodiments of the present invention will be described item by item.

[0075] Embodiment 1 An optical fiber forming apparatus, A drawing furnace comprising: (i) a muffle having an inner surface; (ii) an axial opening below the muffle, wherein the inner surface of the muffle defines a passage extending through the axial opening; and (iii) an upper inlet into the passage. A tube extending into the passage of the drawing furnace above the axial opening, the tube having: (i) an outer surface, wherein 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 defining a second passage extending through the tube; (iii) an inlet into the second passage of the tube; and (iv) an outlet from the second passage of the tube. An optical fiber forming apparatus comprising the above.

[0076] Embodiment 2 An inert gas flows into the passage through the upper inlet of the drawing furnace, forming separate flows, one of the flows flowing into the space between the inner surface of the muffle and the outer surface of the tube through the passage of the drawing furnace, flowing out from the axial opening of the drawing furnace, and another of the flows flowing into the inlet of the tube and flowing out from the outlet of the tube through the second passage of the tube. The optical fiber forming apparatus according to Embodiment 1.

[0077] Embodiment 3 The inert gas contains one or more of argon or nitrogen and less than 1% by volume of helium. The optical fiber forming apparatus according to Embodiment 2.

[0078] Embodiment 4 A first heating element that heats the passage of the drawing furnace over the entire first range surrounding at least a part of the passage of the drawing furnace above the inlet of the tube; A second heating element that heats the passage of the drawing furnace over the entire second range surrounding at least a part of the passage of the drawing furnace above the first range The optical fiber forming apparatus according to Embodiment 1, further comprising.

[0079] Embodiment 5 An optical fiber preform disposed in the passage of the drawing furnace; An optical fiber drawn from the optical fiber preform and extending through the second passage of the tube; A first heating element that heats the passage of the drawing furnace over the entire first range surrounding the tip of the optical fiber preform The optical fiber forming apparatus according to Embodiment 1, further comprising.

[0080] Embodiment 6 The optical fiber forming apparatus according to Embodiment 5, further comprising a second heating element that heats the passage of the drawing furnace over the entire second range surrounding a part of the passage above the main body of the optical fiber preform.

[0081] Embodiment 7 The optical fiber forming apparatus according to Embodiment 6, further comprising a third heating element that heats the passage of the drawing furnace over the entire third range surrounding a part of the second passage of the tube.

[0082] Embodiment 8 The optical fiber forming apparatus according to Embodiment 5, wherein the optical fiber exits the outlet of the tube at a speed of at least 20 m / s and, after exiting the outlet of the tube, has a diameter with a standard deviation (σ) of less than 0.06 μm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

[0083] Embodiment 9 The optical fiber forming apparatus according to Embodiment 1, wherein the inlet of the tube has an inner diameter of 1.27 cm to 2.54 cm.

[0084] Embodiment 10 A drawing furnace for an optical fiber forming apparatus, a muffle having an inner surface and an axially directed opening below the muffle, the inner surface of the muffle defining a passage extending through the axially directed opening and centered about an axis, the inner surface comprising a first straight portion in which the radius from the axis remains at least substantially constant along a length parallel to the axis, and a tapered portion disposed between the first straight portion and the axially directed opening, the radius from the axis decreasing and narrowing the passage as it moves away from the first straight portion, and a vertical length parallel to the axis that is at least twice as long as the maximum radius of the tapered portion. A second straight portion disposed between the tapered portion and the axial opening, the radius from the axis remaining substantially constant along at least a length of 75 cm, the radius of the second straight portion being between 0.635 cm and 1.27 cm, and the second straight portion A wire drawing furnace comprising

[0085] Embodiment 11 Further comprising an upper inlet to the passage, disposed closer to the first straight portion than the tapered portion of the inner surface of the muffler An inert gas flows (i) into the passage through the upper inlet, (ii) then along the first straight portion of the inner surface of the muffler, (iii) then along the tapered portion, (iv) then along the second straight portion, and (v) then out of the axial opening The wire drawing furnace according to Embodiment 10

[0086] Embodiment 12 The wire drawing furnace according to Embodiment 11, wherein the inert gas comprises one or more of argon and nitrogen and less than 1% by volume of helium

[0087] Embodiment 13 An optical fiber preform disposed within the passage An optical fiber drawn from the optical fiber preform and extending outwardly from the axial opening through the passage The wire drawing furnace according to Embodiment 10, further comprising

[0088] Embodiment 14 A first heating element for heating the passage over an entire first range surrounding a tip of the optical fiber preform A second heating element for heating the passage over an entire second range surrounding a part of the passage above a main body of the optical fiber preform The wire drawing furnace according to Embodiment 13, further comprising

[0089] Embodiment 15 The wire drawing furnace according to Embodiment 14, further comprising a third heating element that heats a third range surrounding a part of the passage defined by the second straight portion defined by the inner surface of the muffler.

[0090] Embodiment 16 The wire drawing furnace according to Embodiment 13, wherein the optical fiber exits the axial opening at a speed of at least 20 m / s and, after exiting the axial opening, has a diameter with a standard deviation of less than 0.06 μm at frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

[0091] Embodiment 17 A wire drawing furnace for an optical fiber forming apparatus, comprising: A muffler having an inner surface and an axial opening below the muffler, wherein the inner surface of the muffler defines a passage extending through the axial opening and centered on an axis, and the inner surface A first straight portion in which the radius from the axis remains at least substantially constant along a length parallel to the axis; A narrow portion disposed between the first straight portion and the axial opening, the narrow portion including a radius from the axis that decreases away from the first straight portion to narrow the passage; A second straight portion disposed between the narrow portion and the axial opening, the radius from the axis remaining at least substantially constant along a length parallel to the axis; A muffler comprising: A first heating element that heats the passage over an entire first range surrounding a part of the passage defined by the first straight portion; A second heating element that heats the passage over an entire second range surrounding a part of the passage defined by the first straight portion above the first range; A third heating element that heats the passage to a temperature of 100°C to 200°C over an entire third range surrounding a part of the passage defined by the second straight portion; A fiber drawing furnace comprising

[0092] Embodiment 18 An optical fiber preform disposed in the passage, An optical fiber drawn from the optical fiber preform and extending outward from the axial opening through the passage further comprising The first range heated by the first heating element surrounds the tip of the optical fiber preform, The second range heated by the second heating element is at least partially above the main body of the optical fiber preform, The third range heated by the third heating element surrounds a part of the optical fiber drawn from the optical fiber preform, The fiber drawing furnace according to Embodiment 17.

[0093] Embodiment 19 further comprising an upper inlet to the passage, disposed closer to the first straight portion than the narrow portion, An inert gas flows into the passage through (i) the upper inlet, (ii) then along the first straight portion of the inner surface of the muffle, (iii) then along the narrow portion of the inner surface of the muffle, (iv) then along the second straight portion of the inner surface of the muffle, and (v) then out of the axial opening, The inert gas contains one or more of argon and nitrogen and less than 1% by volume of helium, The fiber drawing furnace according to Embodiment 17.

[0094] Embodiment 20 The fiber drawing furnace according to Embodiment 18, wherein the optical fiber exits the axial opening at a speed of at least 20 m / s and, after exiting the axial opening, has a diameter with a standard deviation of less than 0.06 μm at measurement frequencies of 0.1 Hz, 1 Hz, and 10 Hz.

Claims

1. An optical fiber forming apparatus, comprising: A drawing furnace, comprising: (i) a muffle having an inner surface; (ii) an axial opening below the muffle, wherein the inner surface of the muffle defines a passage extending through the axial opening; and (iii) an upper inlet into the passage. A tube extending into the passage of the drawing furnace above the axial opening, comprising: (i) an outer surface, wherein 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 defining a second passage extending through the tube; (iii) an inlet into the second passage of the tube; and (iv) an outlet from the second passage of the tube. Comprising: The muffle further comprises a narrow portion where the passage narrows toward the axial opening. The tube extends above the narrow portion and also extends below the narrow portion while maintaining the space separating the outer surface of the tube from the inner surface of the muffle. An inert gas flows into the passage through the upper inlet of the drawing furnace and forms separate flows. One of the flows flows into the space between the inner surface of the muffle and the outer surface of the tube through the passage of the drawing furnace and out of the axial opening of the drawing furnace. Another of the flows flows into the inlet of the tube and out of the outlet of the tube through the second passage of the tube. An optical fiber forming apparatus.

2. An optical fiber preform disposed in the passage of the drawing furnace; An optical fiber drawn from the optical fiber preform and extending through the second passage of the tube; A first heating element for heating the passage of the drawing furnace over the entire first range surrounding the tip of the optical fiber preform. Further comprising: The inert gas contains one or more of argon or nitrogen and less than 1% by volume of helium. The inlet of the tube has an inner diameter of 1.27 cm to 2.54 cm. The optical fiber forming apparatus according to claim 1.

3. A drawing furnace for an optical fiber forming apparatus, comprising: A muffle having an inner surface and an axial opening below the muffle, wherein the inner surface of the muffle defines a passage extending through the axial opening and centered about an axis. The inner surface is... a first straight portion in which the radius from the axis remains at least substantially constant along a length parallel to the axis; a tapered portion disposed between the first straight portion and the axial opening, the radius from the axis decreasing away from the first straight portion to narrow the passageway, and the tapered portion including a vertical length parallel to the axis that is at least twice as long as the maximum radius of the tapered portion; a second straight portion disposed between the tapered portion and the axial opening, the radius from the axis remaining at least substantially constant along a length of at least 75 cm, and the radius of the second straight portion being between 0.635 cm and 1.27 cm; A wire drawing furnace comprising the above.

4. an optical fiber preform disposed within the passageway; an optical fiber drawn from the optical fiber preform and extending outwardly from the axial opening through the passageway; an upper inlet to the passageway disposed closer to the first straight portion than the tapered portion of the inner surface of the muffle; a first heating element that heats the passageway over an entire first range surrounding a tip of the optical fiber preform; a second heating element that heats the passageway over an entire second range surrounding a portion of the passageway above a main body of the optical fiber preform further comprising an inert gas flowing (i) into the passageway through the upper inlet, (ii) then along the first straight portion of the inner surface of the muffle, (iii) then along the tapered portion, (iv) then along the second straight portion, and (v) then out of the axial opening; the inert gas including one or more of argon and nitrogen and less than 1% by volume of helium; The wire drawing furnace according to claim 3.

5. A wire drawing furnace for an optical fiber forming apparatus, comprising: a muffle having an inner surface and an axial opening below the muffle, the inner surface of the muffle defining a passageway extending through the axial opening and centered about an axis, the inner surface including: a first straight portion in which the radius from the axis remains at least substantially constant along a length parallel to the axis; A narrow portion disposed between the first straight portion and the axial opening, the narrow portion including a radius from the axis that decreases away from the first straight portion to narrow the passageway. A second straight portion disposed between the narrow portion and the axial opening, the radius from the axis remaining at least substantially constant along a length parallel to the axis. A muffler comprising: A first heating element that heats the passageway over an entire first range that surrounds a portion of the passageway defined by the first straight portion. A second heating element that heats the passageway over an entire second range that surrounds a portion of the passageway defined by the first straight portion above the first range. A third heating element that heats the passageway to a temperature of 100°C to 200°C over an entire third range that surrounds a portion of the passageway defined by the second straight portion. A wire drawing furnace comprising:

Images