JP2016108192A - Method and apparatus for manufacturing optical fiber - Google Patents

Abstract

Even if an optical fiber preform has a large diameter variation, drawing is performed while easily and reliably sealing a gap formed between an upper end opening in an optical fiber drawing furnace and the optical fiber preform. An optical fiber manufacturing apparatus according to the present invention includes a drawing furnace (drawing furnace body 10) having an upper end opening for inserting an optical fiber preform 1, and light inserted into the upper end opening. And a seal portion 20 for sealing a gap between the fiber preform 1 and the fiber preform 1. The seal portion 20 includes two or more carbon felt rings 21 and 22 having an inner diameter equal to or smaller than the maximum outer diameter of the effective portion of the optical fiber preform 1 and the maximum outer portion of the effective portion provided between the carbon felt rings 21 and 22. And at least one of the two or more carbon felt rings 21 and 22 has an inner diameter different from that of the other carbon felt rings and the minimum outer diameter of the effective portion. It has the following inner diameter. [Selection] Figure 2A

Description

  The present invention relates to an optical fiber manufacturing method and an optical fiber manufacturing apparatus for drawing an optical fiber while sealing a gap between an upper end opening and an optical fiber preform in an optical fiber drawing furnace.

  Optical fiber drawing by an optical fiber drawing furnace (hereinafter referred to as a drawing furnace) is performed by, for example, heating and melting an optical fiber base material (glass base material) mainly composed of quartz with a heater or the like. Since the temperature in the drawing furnace at this time is as high as about 2000 ° C., carbon having excellent heat resistance is used for the parts in the drawing furnace. This carbon has the property of being oxidized and consumed in a high-temperature oxygen-containing atmosphere. For this reason, it is necessary to keep the inside of a drawing furnace in the atmosphere of noble gas, such as argon gas and helium gas, or nitrogen gas (henceforth inert gas etc.).

  Further, by making the pressure inside the furnace positive, air outside the furnace (oxygen) is prevented from entering the furnace, but the gap of the inlet of the optical fiber preform at the upper end of the drawing furnace, In other words, if the air gap is not well removed (not sealed) in the gap between the upper end opening of the drawing furnace and the optical fiber preform, air outside the furnace will be entrained.

  Therefore, a sealing mechanism that seals the gap at the upper end of the drawing furnace is necessary so that outside air is not caught in the furnace. If this portion can be well sealed, the amount of inert gas used can be reduced, which can lead to cost reduction.

  As such a sealing mechanism, an upper chamber system may be adopted. The upper chamber is a space that is installed in the upper part of the furnace body, surrounds the entire optical fiber preform, and is sealed by a dummy rod having a uniform outer diameter installed in the upper portion of the optical fiber preform. However, when drawing using a relatively inexpensive argon gas instead of an expensive helium gas, if the upper chamber method is adopted, the outer diameter fluctuation of the optical fiber becomes worse than when helium gas is used. This is due to the turbulence of the gas flow generated inside the upper chamber. For this reason, it can be said that it is particularly desirable for drawing at a low cost to adopt a contact sealing method in which the upper chamber portion is removed and the upper fiber portion is removed and the optical fiber preform is sealed.

  Regarding the contact seal method, in Patent Document 1, in order to prevent air outside the furnace from entering the furnace in the drawing furnace, a sheet-like heat-resistant sealing material is provided at the introduction port (insertion port) of the optical fiber preform, A sheet-like sealing material is formed which forms a small chamber covering the sheet-like heat-resistant sealing material at the upper part of the introduction port, and seals a gap between the introduction port formed at the upper part of the chamber on the dummy rod side outside the chamber. Is disclosed.

  Patent Document 2 includes an upper seal ring installed at the upper end of the drawing furnace so as to surround the optical fiber preform, and an expansion / contraction mechanism that applies a force to the outer periphery of the upper seal ring in the center direction of the upper seal ring, A seal structure is disclosed that seals the gap at the upper end of the drawing furnace so that the upper seal ring is always in close contact with the optical fiber preform. Here, the upper seal ring includes an inner seal ring configured by connecting a plurality of inner seal ring pieces, and an outer seal ring configured by connecting a plurality of outer seal ring pieces disposed on the outer periphery thereof. Further, the connecting portion of the inner seal ring piece and the connecting portion of the outer seal ring piece are arranged so as not to overlap each other.

  Patent Document 3 discloses an optical fiber manufacturing apparatus including a seal portion having three or more ring-shaped slit-attached carbon sheets having an inner diameter slightly smaller than a straight body portion of an optical fiber preform.

JP 2003-171139 A JP 2006-342030 A JP 2011-230978 A

  When maintaining the airtightness by bringing the sealing member into contact with the optical fiber preform as in the above-described seal structure of the upper end opening of the drawing furnace, if the variation in the optical fiber preform diameter is small, the optical fiber preform is adjusted to the preform diameter. A sufficient sealing effect can be obtained by simply closing the gap between the upper end opening of the drawing furnace and the optical fiber preform. However, in practice, the optical fiber preform diameter may vary in the longitudinal direction by, for example, ± 2 mm or more, and in some cases may vary by approximately ± 5 mm. In such a case, since the gap interval fluctuates greatly, a seal structure that can be sealed while taking into account the fluctuation amount of the gap is required.

  However, in the seal structure described in Patent Document 1, a carbon felt ring is used as the sheet-like heat-resistant sealing material, but the seal structure is complicated. Further, since the seal structure described in Patent Document 2 is a structure in which the seal ring pieces are connected, when the diameter variation of the optical fiber preform is large in the longitudinal direction, it is difficult to follow the diameter variation. Patent Document 2 also describes that in the background art, it is not preferable to use carbon felt as a seal member.

  Further, in the seal structure described in Patent Document 3, in order to follow such a change in the optical fiber preform diameter, a plurality of carbon sheets or carbon felts with slits are used to obtain the inner diameter (optical fiber preform). The contact surface is made variable. However, in this seal structure, the sealing performance may be deteriorated due to a gap generated in the slit portion.

  Furthermore, since the carbon sheet has a limit in stretchability, there is a problem that the carbon sheet breaks during stretching. On the other hand, the carbon felt is a felt formed of carbon fiber, which is like a cloth and has elasticity compared to the carbon sheet, but the outer diameter variation of the optical fiber preform that can be sealed with one sheet is ± There is a problem that when it is stretched when the large diameter portion of the optical fiber preform is sealed up to about 2 mm or less, the reduced diameter cannot catch up and the sealing performance is locally lowered when the small diameter portion passes. is there.

  As described above, in the seal structure described in Patent Document 3, even if the optical fiber preform diameter variation is large, it is only about ± 2 mm, and it is difficult to seal an optical fiber preform having a larger outer diameter variation. Met.

  The present invention has been made in view of the above circumstances, and its purpose is to provide an upper end opening and an optical fiber preform in an optical fiber drawing furnace even if the optical fiber preform has a large diameter variation. It is an object of the present invention to provide an optical fiber manufacturing method and an optical fiber manufacturing apparatus that can perform drawing while easily and reliably sealing a gap generated between the optical fiber and the optical fiber.

  An optical fiber manufacturing method according to the present invention is an optical fiber manufacturing method in which an optical fiber is drawn while sealing a gap between an upper end opening and an optical fiber preform in a drawing furnace with a seal portion. Two or more carbon felt rings having an inner diameter less than or equal to the maximum outer diameter of the effective portion of the optical fiber preform as a seal portion, and an inner diameter greater than or equal to the maximum outer diameter of the effective portion provided between the carbon felt rings The drawing is performed using a sheet ring, and at least one of the two or more carbon felt rings has an inner diameter that is different from the other carbon felt rings and has an inner diameter that is less than or equal to the minimum outer diameter of the effective portion. It is what I did.

  An optical fiber manufacturing apparatus according to the present invention seals a gap between a drawing furnace having an upper end opening for inserting an optical fiber preform and the inserted upper end of the optical fiber preform. An optical fiber manufacturing apparatus comprising: two or more carbon felt rings having an inner diameter equal to or less than a maximum outer diameter of an effective portion of the optical fiber preform; and the carbon A seat ring having an inner diameter equal to or larger than the maximum outer diameter of the effective portion provided between the felt rings, and at least one of the two or more carbon felt rings has an inner diameter different from that of the other carbon felt rings. In addition, the inner diameter is equal to or smaller than the minimum outer diameter of the effective portion.

  According to the present invention, even with an optical fiber preform having a large variation in diameter, the gap formed between the upper end opening in the optical fiber drawing furnace and the optical fiber preform is easily and reliably sealed while drawing. It becomes possible to do.

It is a figure which shows one structural example of the optical fiber manufacturing apparatus which concerns on one Embodiment of this invention. It is a figure which shows an example of the seal | sticker part in the optical fiber manufacturing apparatus of FIG. It is a figure which shows an example of the carbon felt ring used for the seal part of FIG. 2A. It is a figure which shows an example of the carbon sheet ring used for the seal part of FIG. 2A. It is a figure which shows the fluctuation | variation result of the outer diameter of the optical fiber preform | base_material with respect to an optical fiber preform | base_material position, and the internal pressure in a drawing furnace at the time of drawing using the optical fiber manufacturing apparatus of FIG.

[Description of Embodiment of the Present Invention]
First, the contents of the embodiments of the present invention will be listed and described.
(1) An optical fiber manufacturing method according to the present invention is an optical fiber manufacturing method in which an optical fiber is drawn while a gap between an upper end opening and an optical fiber preform in a drawing furnace is sealed with a seal portion. In addition, as the seal portion, two or more carbon felt rings having an inner diameter equal to or less than the maximum outer diameter of the effective portion of the optical fiber preform, and greater than or equal to the maximum outer diameter of the effective portion provided between the carbon felt rings. The drawing is performed using a sheet ring having an inner diameter, and at least one of the two or more carbon felt rings has an inner diameter different from that of the other carbon felt rings and is equal to or less than the minimum outer diameter of the effective portion. It has an inner diameter. As a result, even with an optical fiber preform with large diameter fluctuations, drawing can be performed while easily and reliably sealing the gap formed between the upper end opening in the optical fiber drawing furnace and the optical fiber preform. Is possible.

  (2) In the optical fiber manufacturing method of (1), each of the two or more carbon felt rings has an inner diameter equal to or smaller than the minimum outer diameter of the effective portion. Accordingly, it is possible to always seal with all the carbon felt rings, and to follow the reduced diameter after the diameter expansion of the optical fiber preform with the carbon felt ring having a large inner diameter.

  (3) An optical fiber manufacturing apparatus according to the present invention includes a drawing furnace having an upper end opening for inserting an optical fiber preform, and a gap between the upper end opening and the inserted optical fiber preform. An optical fiber manufacturing apparatus comprising: two or more carbon felt rings having an inner diameter equal to or less than a maximum outer diameter of an effective portion of the optical fiber preform; A seat ring having an inner diameter greater than or equal to the maximum outer diameter of the effective portion provided between the carbon felt rings, and at least one of the two or more carbon felt rings is composed of another carbon felt ring. The inner diameter is different and the inner diameter is equal to or smaller than the minimum outer diameter of the effective portion. As a result, even with an optical fiber preform with large diameter fluctuations, drawing can be performed while easily and reliably sealing the gap formed between the upper end opening in the optical fiber drawing furnace and the optical fiber preform. Is possible.

[Details of the embodiment of the present invention]
Specific examples of an optical fiber manufacturing method and an optical fiber manufacturing apparatus according to an embodiment of the present invention will be described below with reference to FIGS. In the following, a resistance furnace that heats the core tube with a heating source (heater) will be described as a drawing furnace, but an induction furnace that applies high-frequency power to the coil and induction-heats the core tube is also applicable. is there.

FIG. 1 is a diagram showing a configuration example of an optical fiber manufacturing apparatus according to an embodiment of the present invention.
As shown in FIG. 1, the optical fiber manufacturing apparatus according to the present embodiment includes an optical fiber drawing furnace main body (hereinafter simply referred to as a drawing furnace) 10, a seal portion 20, and a lid body 40. The optical fiber preform 1 is inserted into 10 and drawing is performed.

  The drawing furnace 10 includes a furnace casing 11, a core tube 12 provided therein, a cylindrical heating source (heater) 13 provided on the outer periphery of the core tube 12, and heat insulation provided on the outer periphery of the heater 13. A material 14 is provided.

  The core tube 12 accommodates the optical fiber preform 1 inserted from the upper end opening of the drawing furnace 10 therein. The heater 13 heats and melts the optical fiber preform 1 accommodated in the core tube 12. Further, the drawing furnace 10 is provided with an inert gas supply mechanism (not shown) to supply an inert gas or the like in the core tube 12 or around the heater 13 to prevent oxidation or deterioration. ing.

  In the drawing furnace 10, the optical fiber preform 1 can be moved in the drawing direction (downward direction) by a separate moving mechanism. A support bar (dummy bar) 2 for suspending and supporting the optical fiber preform 1 from above is connected. A gripping portion provided in the moving mechanism grips the support rod 2. The support rod 2 is a glass rod of the same type as the optical fiber preform 1, and may be manufactured and fused separately from the optical fiber preform 1, or may be connected by a separately provided connection member.

  1 shows an example in which the optical fiber preform 1 is cut flat at the upper end portion of the straight body portion, but there is a taper portion (reduced diameter portion) at the upper end portion of the straight body portion. Alternatively, the thin straight body after the diameter reduction may be connected by a connecting member or directly gripped.

  In addition, although the upper end part of the inner wall of the core tube 12 forms the upper end opening part in the upper end part 11a of the drawing furnace 10 as it is in FIG. 1, the example is not restricted to this. For example, an upper lid that is a narrower upper end opening than the inner diameter d of the core tube 12 may be provided on the upper side of the core tube 12, and in this case, the gap to be sealed is the narrow upper end opening and the optical fiber preform 1. It becomes a gap generated between the two. In addition, the cross-sectional shape of the optical fiber preform 1 is basically generated with the aim of a perfect circle, but there may be some unevenness regardless of the accuracy, and it may be elliptical. May be. The upper end opening may have a circular cross section, but this accuracy does not matter.

  The seal part 20 is a part for sealing a gap between the upper end opening of the drawing furnace 10 and the inserted optical fiber preform, and is a main characteristic part of the present embodiment. This will be described later with reference.

  The optical fiber drawing process in the drawing furnace 10 will be schematically described. In the drawing furnace 10, the lower part of the optical fiber preform 1 in the furnace is heated by the heater 13 in the furnace core tube 12 while being sealed so as to prevent outside air from being caught by a seal part 20 described later provided at the upper end part 11 a. To do. In the drawing furnace 10, the optical fiber 3 is melted and drooped from the lower end of the optical fiber preform 1 that has been heated and melted to have a small diameter, and the light is emitted from the discharge hole 16 provided in the lower end portion of the furnace casing 11. Pull out the fiber 3. Then, as the drawing progresses, the optical fiber preform 1 together with the support rod 2 is gradually lowered by the moving mechanism.

  Here, the lid 40 will be described. In the configuration in which the support rod 2 is provided on the optical fiber preform 1 as shown in FIG. 1, the support rod 2 is lowered to the position of the core tube 12 by the progress of the drawing process, that is, the support rod 2 is drawn. There is a scene located below the upper end portion 11 a of the furnace 10.

  In order to keep the inside of the furnace sealed even in such a situation, it is preferable to have a lid 40 in addition to the seal portion 20. The lid body 40 is a lid that passes through the support rod 2 and is placed on the upper side of the optical fiber preform 1. As shown in the figure, the lid body 40 includes a through hole 40 a and a shoulder portion 40 b that match the cross-sectional shape of the support rod 2. It is formed to have. Examples of the material of the lid body 40 include quartz and metal.

  By providing the lid 40, even if the drawing of the optical fiber 3 progresses and the optical fiber preform 1 and the support rod 2 are lowered, the lid is removed before the optical fiber preform 1 is detached from the seal portion 20. The state where the lower end surface of 40 is brought into contact with the seal portion 20 can be maintained in the sealed state. In addition, although it demonstrated on the assumption that the cover body 40 has the shoulder part 40b, the cover body 40 may be a shape which just opened the through-hole 40a of the support bar 2 in the disk.

  Next, the seal part 20 which is the main feature of this embodiment will be described with reference to FIGS. 2A to 2C. 2A is a view showing an example of a seal portion in the optical fiber manufacturing apparatus of FIG. 1, FIG. 2B is a view showing an example of a carbon felt ring used in the seal portion of FIG. 2A, and FIG. 2C is a seal portion of FIG. It is a figure which shows an example of the carbon sheet ring used for.

  2A includes two or more carbon felt rings having an inner diameter equal to or smaller than the maximum outer diameter of a region where the optical fiber core of the optical fiber preform 1 is located (hereinafter referred to as an effective portion). At least one of the two or more carbon felt rings has an inner diameter that is different from the other carbon felt rings and has an inner diameter that is equal to or smaller than the minimum outer diameter of the effective portion. Moreover, it is preferable that all the inner diameters of the provided carbon felt rings are different from each other.

  These carbon felt rings are ring-shaped carbon felts without slits. The carbon felt is a felt formed of carbon fibers, is airtight, and is a stretchable material that can be bent as compared with a carbon sheet. As the carbon material, it is preferable to use what is called high purity carbon from the viewpoint of contamination with impurities.

  FIG. 2A shows an example in which two carbon felt rings 21 and 22 are adopted as the two or more carbon felt rings. The carbon felt rings 21 and 22 are ring-shaped carbon felts as shown in the top view of FIG. 2B. The inner diameters D1 of the two are different, and the inner diameter D1 of the carbon felt ring 21 is more than that of the carbon felt ring 22. It is small and has an inner diameter equal to or smaller than the minimum value of the outer diameter (the outer diameter of the effective portion) φ of the optical fiber preform 1.

  The seal portion 20 has a seat ring 24 between the carbon felt rings 21 and 22 having an inner diameter equal to or larger than the maximum outer diameter of the effective portion (that is, the inner diameter is larger than that of the carbon felt rings 21 and 22). FIG. 2A shows an example in which, in addition to the seat ring 24, a similar seat ring 23 is provided below the carbon felt ring 21 and a similar seat ring 25 is provided above the carbon felt ring 22.

  The seat rings 23 to 25 are ring-shaped sheets as shown in the top view of FIG. 2C, and are preferably carbon, particularly high-purity carbon. A material may be used. The inner diameters D2 of the seat rings 23 to 25 may be different or the same. However, when it is assumed that the seat rings 23 to 25 have a high density, no contraction force, and no slits, It must be greater than the maximum outer diameter of the effective part.

  The carbon felt rings 21 and 22 and the seat rings 23 to 25 are placed on the upper end portion 11a of the furnace casing 11 in the order shown in the figure, and on the upper side of the seat ring 25, the carbon felt rings 21 and 22 and A quartz ring 26 is provided so that the carbon sheet rings 23 to 25 do not float and impair airtightness. When the lid 40 is provided, the lower end of the lid 40 comes into contact with the upper end of the quartz ring 26 as the drawing is progressed. The material of the quartz ring 26 is not limited to quartz, and may be a metal as long as it has heat resistance. Further, the seal portion 20 is provided with a cylindrical casing 27 made of a heat-resistant material as an outer frame of the rings 21 to 26.

  In addition, although not shown about the gas inlet and gas supply mechanism which supply gas into the furnace of the drawing furnace 10, for example, inert gas etc. are supplied between the lower side of the seat ring 23 and the upper end part 11a. A gas inlet may be provided, and gas may be supplied from the gas inlet into the furnace by a gas supply mechanism (not shown). Examples of the gas to be supplied include helium gas, argon gas, nitrogen gas, and a mixed gas thereof.

  In the optical fiber manufacturing method according to the present embodiment, drawing is performed using such a seal portion 20. And since the inner side of the stretchable carbon felt ring is brought into contact with the effective portion of the optical fiber preform 1 and sealed, it is possible to follow the fluctuation of the outer diameter φ of the optical fiber preform 1, and the seal Can be maintained.

  Furthermore, in this embodiment, not only can the air tightness be improved by sealing with two or more carbon felt rings, but also at least one of the two or more felt rings has a different inner diameter D1, Even when the optical fiber preform 1 varies in the longitudinal direction of the outer diameter φ, it is easier to maintain the hermeticity of the felt. Specifically, even if the felt is stretchable, if it is stretched more than a certain level, the sealing performance is reduced until the inner diameter D1 is restored. However, since the carbon felt ring having a large inner diameter D1 is disposed, the possibility that the sealing performance is similarly deteriorated is low, the risk of deterioration of the sealing performance can be reduced, and the seal is more easily maintained. In addition, by drawing a number of optical fiber preforms 1, even if the flexibility of the smaller inner diameter D1 is impaired, only the damaged carbon sheet ring is replaced, and the seal is obtained. Can be maintained.

  Furthermore, in this embodiment, since the seat ring 24 is installed between the carbon felt rings 21 and 22 (because there is a space), the carbon felt ring 21 when the thick portion of the optical fiber preform 1 passes through. , 22 can be bent differently. In addition, the seat ring 24 can reduce the friction between the carbon felt rings and suppress the generation of dust due to the friction between the carbon felt rings.

  As described above, according to the present embodiment, even if the optical fiber preform 1 has a large variation in the outer diameter φ, the gap generated between the upper end opening in the drawing furnace and the optical fiber preform 1 can be simplified. It becomes possible to draw the wire while securely sealing with.

  Moreover, it is preferable that all of the two or more carbon felt rings have an inner diameter equal to or smaller than the minimum outer diameter of the effective portion. As a result, all the carbon felt rings are always sealed, and even if the effective portion of the optical fiber preform 1 is expanded after the effective diameter of the optical fiber preform 1 is increased by the carbon felt ring having a large inner diameter D1, the sealing is performed. Can be made.

  Finally, referring to FIG. 3, the outer diameter (variation value from the average diameter) and the drawing of the optical fiber preform (glass preform) 1 when drawing is performed using the above-described optical fiber manufacturing apparatus. The fluctuation result of the internal pressure in the furnace will be described. The variation result shown in FIG. 3 is a result of evaluating the sealing performance of the two seals as illustrated in FIG. Here, a carbon sheet ring having an inner diameter of the glass base material maximum diameter of +14 mm and a thickness of 1 mm is used as the sheet rings 23 to 25, and a carbon felt ring having an inner diameter of the glass base material maximum diameter of −11 mm and a thickness of 5 mm is used as the carbon felt ring 21. As a carbon felt ring 22, a carbon felt ring having an inner diameter of -6 mm and a thickness of 5 mm was used. Note that at least one of the carbon felt rings 21 and 22 having a smaller inner diameter is set to be equal to or smaller than the minimum outer diameter of the effective portion of the glass base material.

  As a result, the glass base material 1 has an effective portion of the glass base material 1 as the position from the bottom of the glass base material 1 (position where the initial lowest point is 0 mm) increases as the drawing is progressed. The outer diameter φ of the steel was once reduced, then increased, and finally decreased again, but the fluctuation amount (decrease amount) of the internal pressure in the drawing furnace 10 with respect to the reference pressure (maximum pressure) at that time Was about 0.2 Pa, and it was confirmed that the airtightness of the furnace could be maintained.

  As described above, in the result of FIG. 3, it was confirmed that the pressure drop was about 0.2 Pa and that the airtightness of the furnace could be maintained. On the other hand, even when the carbon felt ring 22 having the larger inner diameter was not provided (one seal), the pressure drop was larger than this and became about 0.4 Pa.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined not by the above-mentioned meaning but by the scope of claims for patent, and is intended to include all modifications within the scope and meaning equivalent to the scope of claims for patent.

DESCRIPTION OF SYMBOLS 1 ... Optical fiber preform, 2 ... Support rod, 3 ... Optical fiber, 10 ... Drawing furnace, 11 ... Furnace housing, 11a ... Upper end part, 12 ... Core tube, 13 ... Heater, 14 ... Thermal insulation, 15 ... Clearance, 16 ... discharge hole, 20 ... seal part, 21, 22 ... carbon felt ring, 23, 24, 25 ... sheet ring, 26 ... quartz ring, 27 ... housing, 40 ... lid, 40a ... through hole, 40b ... shoulder.

Claims (3)

An optical fiber manufacturing method for drawing an optical fiber while sealing a gap between an upper end opening and an optical fiber preform in a drawing furnace with a seal portion,
As the seal portion, two or more carbon felt rings having an inner diameter equal to or smaller than the maximum outer diameter of the effective portion of the optical fiber preform, and an inner diameter equal to or larger than the maximum outer diameter of the effective portion provided between the carbon felt rings. The sheet ring is used to perform the drawing,
An optical fiber manufacturing method, wherein at least one of the two or more carbon felt rings has an inner diameter that is different from the other carbon felt rings and has an inner diameter equal to or smaller than a minimum outer diameter of the effective portion.   The optical fiber manufacturing method according to claim 1, wherein each of the two or more carbon felt rings has an inner diameter equal to or smaller than a minimum outer diameter of the effective portion. A drawing furnace having a drawing furnace having an upper end opening for inserting an optical fiber preform, and a seal portion for sealing a gap between the upper end opening and the inserted optical fiber preform. A fiber manufacturing device,
The seal portion has two or more carbon felt rings having an inner diameter equal to or smaller than the maximum outer diameter of the effective portion of the optical fiber preform, and an inner diameter equal to or larger than the maximum outer diameter of the effective portion provided between the carbon felt rings. A seat ring having
An optical fiber manufacturing apparatus, wherein at least one of the two or more carbon felt rings has an inner diameter different from that of the other carbon felt rings and having an inner diameter equal to or smaller than a minimum outer diameter of the effective portion.

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