JP2012148924A - Optical fiber drawing furnace, and drawing method of optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber drawing furnace, which can draw an optical glass preform exhibiting a large variation in diameter while sealing a gap between an upper end opening of an optical fiber drawing furnace body and the optical glass preform, at a low cost.SOLUTION: The optical fiber drawing furnace has the upper end opening through which an optical fiber glass preform 1 is inserted, a furnace core pipe accommodating the glass preform 1 inserted from the upper end opening inside, a heating source heating and melting the glass preform 1, and a sealing mechanism sealing a gap between the upper end opening and the glass preform 1. The sealing mechanism has an annular case 31 at a position on the above side of the upper end opening and surrounding the glass preform 1. The annular case 31 has blowout ports (blowout port 32a and the like) to blow out a sealing gas from the inner peripheral surface, wherein the maximum value of the width of a gap between the inner peripheral surface of the annular case 31 and the glass preform 1 is taken as δ, the distance L between the uppermost blowout port and the lowermost one is made L≥20δ.

Description

  The present invention relates to an optical fiber drawing furnace and a drawing method, and more particularly, an optical fiber having a sealing mechanism that seals a gap between an upper end opening of a drawing furnace main body and a glass preform for an optical fiber. And an optical fiber drawing method using the drawing furnace.

  The optical fiber is drawn out by heating, for example, a glass preform formed mainly of quartz in a drawing furnace. Carbon is mainly used as an in-furnace material for this drawing furnace, and in order to prevent oxidation of this carbon, an inert gas or nitrogen gas (hereinafter referred to as an inert gas) is filled in the furnace. ing.

  Further, by making the pressure in the furnace positive, air outside the furnace (oxygen) is prevented from entering the furnace, but the gap between the inlets of the glass base material at the upper end of the drawing furnace, that is, If the gap between the upper end opening of the wire drawing furnace and the glass base material is not well sealed, air outside the furnace will be entrained from here. Therefore, a sealing mechanism is required to seal the outside air from the gap so as not to get into the furnace. If this portion can be well sealed, the amount of inert gas used can be reduced, which can lead to cost reduction.

  A drawing furnace employing a conventional sealing mechanism will be described with reference to FIG. The drawing furnace shown in FIG. 3 has a main body (hereinafter referred to as a drawing furnace body) 10 provided with a seal mechanism 5. The drawing furnace body 10 includes a furnace casing 11, a core tube 12 provided therein, a cylindrical heater 13 provided on the outer periphery of the core tube 12, and a heat insulating material 14 provided on the outer periphery of the heater 13. With. Further, the drawing furnace body 10 is provided with an in-furnace 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. Yes.

  The sealing mechanism 5 is configured such that the same type of gas as the in-furnace gas is supplied to the annular casing 51 installed on the upper end surface 11a of the furnace casing 11 and the internal space (gas reservoir) 52 of the casing 51 from the outside. A gas supply mechanism (not shown) that supplies the sealing gas as a sealing gas, and an air outlet 53 provided on the inner peripheral surface of the casing 51 so that the sealing gas blows out from the inner space 52 to the outer peripheral surface of the glass base material 1 are provided. Yes.

  As the blowout port 53, a blowout slit or a blowout hole arranged in the entire circumferential direction so as to blow out in the horizontal direction as shown in the figure is used. Here, the seal gas blown out from the blowout port 53 along the outer peripheral surface of the glass base material 1 is directed toward the gap 15 generated as a result between the outside of the furnace and the upper end opening and the glass base material 1. The air outside the furnace is not caught in the furnace. Such a method is called a gas seal method.

In the drawing furnace body 10 of FIG. 3, with such a configuration, the lower part of the glass base material 1 in the furnace is heated by the heater 13 in the furnace core tube 12 while preventing outside air from being caught in the furnace. The optical fiber 1 f can be melted and drooped from the lower end of the glass preform 1 that has been heated and melted to have a small diameter, and the optical fiber 1 f can be drawn out from the discharge hole 16 provided in the lower end portion of the furnace casing 11. Here, as the drawing progresses, the glass base material 1 together with the support bar 2 may be gradually lowered by the moving mechanism.
As a gas seal system, there is a gas seal system in which a multi-layered inert gas reservoir is provided at the upper end opening of a drawing furnace body as in the sealing mechanism described in Patent Document 1, for example.

  A conventional sealing mechanism that employs a system different from the gas seal system will be described with reference to FIG. The drawing furnace shown in FIG. 4 is formed by installing a sealing mechanism 6 on a drawing furnace body 10. The sealing mechanism 6 is configured not to directly seal the preform portion 1a of the glass base material 1, but to seal in a state where it is in contact with the quartz dummy rod portion 1b. Therefore, the sealing mechanism 6 includes a tube body 61 and is sealed by an airtight plate 63 made of heat-resistant rubber or the like held by an annular airtight plate holder 62 installed on the tube body 61.

Such a method of providing the dummy rod portion 1b, covering the entire preform with the tube 61, and drawing the airtightly with the dummy rod portion 1b is called an upper chimney method or a contact seal method. The advantage of the upper chimney system is that the preform portion 1a can be drawn to the end because it is sealed by the dummy rod portion 1b.
As the upper chimney system, a space other than the vicinity of the surrounding wall of the drawing chamber is defined in the vertical direction in the upper space of the optical fiber preform in the drawing chamber, for example, as in the drawing furnace described in Patent Document 2. Some have a partition plate.

JP-A-60-137841 JP-A-5-147969

  If the variation of the glass base material diameter is small, even if the gas sealing method is adopted as the sealing mechanism, the gap between the upper end opening of the drawing furnace body and the glass base material is simply blocked according to the glass base material diameter. If the sealing is performed in this manner, a sufficient sealing effect can be obtained. However, when the variation of the glass base material diameter is as large as about ± 10 mm, for example, the gap interval greatly fluctuates. It is necessary to seal while taking into account, and the gas seal method requires a large amount of gas.

On the other hand, in the upper chimney system, even if the glass base material diameter fluctuates, there is no effect on the seal part because the dummy rod part takes airtightness. However, in the expensive He gas (large heat transfer coefficient) as the furnace gas However, when inexpensive N ₂ gas or Ar gas (small heat transfer coefficient) is used, there is a problem that the diameter variation of the optical fiber after drawing becomes large. This is because enclosing the entire preform increases the internal capacity of the drawing furnace body, so using a gas with a small heat transfer coefficient creates a temperature difference in the furnace, which tends to cause gas convection. It is guessed. This problem becomes particularly noticeable when the glass preform for optical fibers is enlarged. For this reason, the upper chimney system also imposes a manufacturing restriction such that it is necessary to use an expensive He gas that hardly causes convection.

  The present invention has been made in view of the actual situation as described above, and an object of the present invention is to provide a glass preform for an optical fiber having a large diameter variation, and an upper end opening portion and a glass preform in an optical fiber drawing furnace body. It is an object of the present invention to provide an optical fiber drawing furnace and a drawing method capable of drawing at low cost while sealing a gap generated between them.

  An optical fiber drawing furnace according to the present invention includes an upper end opening into which a glass preform for an optical fiber is inserted, a furnace tube that accommodates the glass preform inserted from the upper end opening, and a glass preform. And a sealing mechanism that seals a gap between the upper end opening and the glass base material with a sealing gas. The sealing mechanism has an annular housing at a position surrounding the glass base material above the upper end opening, and the annular housing has a blow-out port for blowing seal gas from the inner peripheral surface. When the maximum value of the width of the gap between the inner peripheral surface of the annular casing and the glass base material is δ, the distance L between the uppermost side and the lowermost side of the outlet is L ≧ 20δ. It shall be.

  Moreover, it is preferable that the said cyclic | annular housing | casing has a multistage blowing part of a multistage structure which has two or more blower outlets in the position spaced apart in the height direction. And it is preferable that the said multistage blowing part blows off sealing gas diagonally upwards in the blower outlet located in the uppermost stage, and diagonally downward in the blower outlet located in the lowest stage.

Further, the sealing mechanism includes an exhaust part for exhausting a seal gas between a blower outlet located at the uppermost stage and a blower outlet located at the lowermost stage in the multistage blower part, and a seal gas discharged from the exhaust part. It is preferable to have a concentration detection unit that detects the concentration of the above.
Furthermore, it is preferable that the sealing mechanism has a furnace internal pressure detection unit that is provided at a lower portion of the outlet located at the lowest stage in the multistage outlet and detects the internal pressure of the core tube.

  An optical fiber drawing method according to the present invention includes an upper end opening into which a glass preform for an optical fiber is inserted, a furnace core tube that accommodates the glass preform inserted from the upper end opening, and a glass preform. An optical fiber is drawn using a drawing furnace including a heating source that is heated and melted. In this drawing method, a seal mechanism having an annular casing is installed at a position surrounding the glass base material above the upper end opening, and the gap between the upper end opening and the glass base material is separated from the seal mechanism. Drawing is performed while sealing by blowing out seal gas. The annular housing has a blowout port for blowing out seal gas from the inner peripheral surface, and when the maximum width of the gap between the inner peripheral surface of the annular housing and the glass base material is δ, the blowout port It is assumed that the distance L between the uppermost side and the lowermost side is L ≧ 20δ.

  According to the present invention, it is possible to draw an optical fiber glass preform having a large diameter variation at a low cost while sealing a gap formed between the upper end opening of the optical fiber drawing furnace body and the glass preform. It becomes possible.

It is a figure for demonstrating the outline of the drawing furnace which concerns on this invention. It is a figure which shows an example of the sealing mechanism in the drawing furnace of FIG. It is a figure which shows an example of the drawing furnace which employ | adopted the conventional sealing mechanism. It is a figure which shows the other example of the drawing furnace which employ | adopted the conventional sealing mechanism.

  FIG. 1 is a diagram for explaining the outline of a drawing furnace according to the present invention, in which 1 is a glass preform for optical fiber, 2 is a support rod, 3 is a sealing mechanism, and 10 is an optical fiber line. A main body of a furnace (hereinafter referred to as a drawing furnace body).

As shown in FIG. 1, a draw furnace body 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 And a heat insulating material 14 provided on the outer periphery of the heater 13. The core tube 12 accommodates the glass base material 1 inserted from the upper end opening. The heater 13 heats and melts the glass base material 1 accommodated in the core tube 12.
Further, the drawing furnace body 10 is provided with an in-furnace 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. Yes.

In the drawing furnace body 10, the glass base material 1 can be moved in the drawing direction (downward direction) by a separately provided moving mechanism. A support rod 2 for suspending and supporting the glass base material 1 from above is connected.
The support bar 2 may be formed integrally with the glass base material 1 or may be separately manufactured and fused. Although the circular shape is mentioned as a cross-sectional shape of the support rod 2, it is not restricted to it. Moreover, in order to connect the support bar 2 and the glass base material 1, you may provide a connection part (fitting part) separately.

  An optical fiber drawing process in such a drawing furnace body 10 will be schematically described. In the drawing furnace body 10, the lower part of the glass base material 1 in the furnace is heated by the heater 13 in the furnace core tube 12 and is heated and melted so as to prevent the outside air from being entrained by the sealing mechanism 3 described later. The optical fiber 1 f is melted and drooped from the lower end of the glass base material 1, and the optical fiber 1 f is drawn out from the discharge hole 16 provided in the lower end portion of the furnace casing 11. Then, as the drawing progresses, the glass base material 1 is gradually lowered together with the support rod 2 by the moving mechanism.

  Hereinafter, the sealing mechanism 3 according to the present invention will be described. The sealing mechanism 3 is for sealing a gap 15 generated between the upper end opening portion into which the glass base material 1 is inserted in the upper end surface 11a of the drawing furnace body 10 and the outer peripheral surface of the glass base material 1 with a sealing gas. Mechanism.

  Although FIG. 1 shows an example in which the upper end portion of the inner wall of the core tube 12 forms the upper end opening portion of the upper end surface 11a of the drawing furnace body 10 as it is, this is not restrictive. For example, an upper lid that is an upper end opening narrower than the inner diameter d of the core tube 12 may be provided on the upper side of the core tube 12. In this case, the gap to be sealed is the narrow upper end opening and the glass base material 1. It becomes a gap generated between the two. In addition, the cross-sectional shape of the glass base material 1 is basically generated with the aim of a perfect circle, but some irregularities may exist regardless of the accuracy, and the glass base material 1 may be elliptical. Also good. The upper end opening may have a circular cross section, but this accuracy does not matter.

As a main feature of the present invention, the seal mechanism 3 has an annular casing 31 at a position above the upper end opening and surrounding the glass base material 1. In the sealing mechanism 3 shown in FIG. 1, an annular casing 31 is installed on the upper end surface 11 a of the furnace casing 11.
Further, although not shown in FIG. 1, the seal mechanism 3 includes a gas supply mechanism that supplies seal gas from the outside to the internal space (gas reservoir) of the casing 31, and the outer peripheral surface of the glass base material 1 from the internal space. A blower outlet provided on the inner peripheral surface of the housing 31 is provided so that the seal gas is blown out. Here, the gas blown out from the outlet along the outer peripheral surface of the glass base material 1 is directed to the outside of the furnace and to the gap 15 generated between the upper end opening and the glass base material 1, and the air outside the furnace from the gap 15. Is not involved.

The annular casing 31 is blown when the maximum value (that is, the maximum width) of the gap between the inner peripheral surface of the annular casing 31 described later and the outer peripheral surface of the glass base material 1 is δ. A distance L between the uppermost side and the lowermost side of the outlet is set to L ≧ 20δ. The maximum width δ of the gap will be described later with reference to FIG.
Further, the distance L between the uppermost side and the lowermost side of the air outlets refers to the distance between the upper end and the lower end when there is one air outlet, and will be described later with reference to FIG. When there are a plurality of outlets, the distance between the uppermost outlet (the upper end) and the lowermost outlet (the lower end) is indicated.

  For example, when the diameter D of the glass base material 1 is 90 mm and is formed with a diameter variation of ± 10 mm, the diameter d of the core tube 12 only needs to be about 120 mm, so the width of the gap is about 10 to 20 mm ( The maximum width δ is 20 mm). In this case, if the distance L is set to L ≧ 20δ, that is, 400 mm or more, sufficient sealing can be performed.

  Since such a sealing mechanism 3 has a height (interval) at a portion where the gas is blown out, it can be sealed even if the maximum width δ of the gap is large to some extent. That is, according to the present invention, it is possible to draw with the same variation in the outer diameter of the optical fiber 1f as in the case where the variation in the diameter of the glass base material 1 is large. Of course, since the sealing mechanism 3 in the present invention is not an upper chimney system, expensive He gas is not essential, and since there is no upper chimney, the supply device for supplying the glass base material 1 can be configured to be short, so that the equipment cost is increased. Become cheap.

Next, a preferred example of the sealing mechanism 3 in the drawing furnace of FIG. 1 will be described with reference to FIG.
The annular casing 31 preferably has a multistage structure having two or more outlets at positions spaced apart in the height direction. The gas blowing portion having a multi-stage structure and having a height as described above is hereinafter referred to as a multi-stage blowing portion.

And the sealing mechanism 3 is provided with the gas supply part 40 which supplies gas to a multistage blowing part, and the controller 39 which controls the supply amount as said gas supply mechanism.
In the example of FIG. 2, the air outlets 32, 33, 34, and 37 are provided as the multistage air outlets, and the seal gas supplied from the gas supply unit 40 is supplied to the air outlets 32 a, 33 a, and 34 a of the air outlets 32, 33, 34, and 37. 37a is blown over the inner peripheral surface of the annular casing 31. As the air outlets 32a, 33a, 34a, and 37a, air outlet slits or air outlet holes arranged in the entire circumferential direction as shown in the figure are used.

  When the multistage outlet is provided as shown in FIG. 2, the distance L corresponds to the length between the uppermost outlet 32a and the lowermost outlet 37a among the outlets, as shown. The seal mechanism 3 may be manufactured so that the distance L satisfies the condition of L ≧ 20δ using the maximum gap δ illustrated in the figure.

  In addition, the multistage outlet is configured to blow out the seal gas with the outlet located at the uppermost part of the outlet, with the outlet facing obliquely upward and with the outlet located at the lowermost part obliquely downward. It is preferable to configure. In the example of FIG. 2, the air outlet 32 a in the uppermost air outlet 32 of the air outlets 32, 33, 34, 37 is blown obliquely upward and obliquely upward, and the air outlet 37 a in the lower air outlet 37. Is blown diagonally downwards. In the example of FIG. 2, the air outlet 33 a in the air outlet 33 is formed so as to blow out in the horizontal direction, and the air outlet 34 a in the air outlet 34 is inclined downward so as to be easily discharged from the exhaust part 35 described later. However, the direction of the air outlets other than the uppermost stage and the lowermost stage is not particularly limited.

  The annular casing 31 in the seal mechanism 3 preferably has an exhaust part 35 for discharging seal gas. The exhaust part 35 is provided at a position other than the uppermost stage and other than the lowermost stage in the multistage blowing section, that is, between the uppermost blowing section and the lowermost blowing section. As the exhaust port 35a in the exhaust part 35, exhaust slits or exhaust holes arranged in the entire circumferential direction as shown in the figure are used.

Examples of the gas to be blown out include inert gases such as He, Ar, and N ₂ , but the seal gas blown out from the blow-out port may be the same type of gas as the in-furnace gas to be filled in the core tube 12. , Some different gases may be used. When a different gas is used, for example, when an expensive gas (He) is used as the furnace gas, the lower gas blowing unit 37 uses the same expensive gas (He) as the gas inside the furnace, and the upper gas blows out. An inexpensive gas (nitrogen or the like) can be used as the gas flowing through the sections 32, 33, and 34. Of course, when the gas blown out from the blowout port 37a and the gas blown out from the blowout ports 32a, 33a, 34a are of different types, the supply sources of these gases are different, and therefore the gas supply unit 40 and the controller 39 are separate supply sources. Supply from is controlled individually. When a part of the seal gas is different from the in-furnace gas, the blow-out port for blowing out the in-furnace gas in the sealing mechanism is not limited to the lowermost stage described above, and may include up to the middle part.

  When the same kind of gas is blown out from the blowout ports 32a, 33a, 34a, 37a, it means that the same kind of gas as the furnace gas is blown out from all the blowout ports. be able to. The gas blown out from the sealing mechanism may be the same as or partially different from the furnace gas as described above, but the gas blown out by the sealing mechanism is also one of the sealing gases. Are all included in “seal gas”. That is, the expression “blowing the seal gas from the seal mechanism” includes the case where the furnace gas flows from a part of the blowing portion of the seal mechanism.

  As described above, the multistage outlet illustrated in FIG. 2 is provided with the outlets 32 to 34, the exhaust part 35, and the outlet 37 from the upper stage, and the individual components play the following roles. . The blowing part 32 plays a role of preventing outside air mixing (blowing outside), and the blowing parts 33 and 34 play a role of relaxing the outside air (oxygen) mixing concentration. And the exhaust part 35 has the role which cut | disconnects the edge of external air and furnace atmosphere, and exhausts the surrounding external air. Therefore, the exhaust part 35 is particularly useful when a part of the seal gas is different from the in-furnace gas. The blowing unit 37 stabilizes the air flow in the furnace and replenishes or supplies the gas in the furnace.

  Furthermore, it is preferable that the seal mechanism 3 has a furnace pressure detector 38 that is provided at the lower stage of the lowermost blowing part 37 and detects the internal pressure of the reactor core tube 12 (actually, the pressure at the upper part of the reactor core tube 12). Note that the furnace pressure detection unit 38 may be provided in the casing 31 as shown in FIG. 2 or in the furnace outside the seal mechanism 3 as long as it is the lower stage of the lowermost blowing part 37. . In either case, a vent hole dedicated to detecting the furnace pressure may be provided separately.

  And it is preferable that the sealing mechanism 3 adjusts the flow volume of the gas of the blowing part which blows out the same gas as the in-furnace gas based on the detection result of the in-furnace pressure detection part 38. FIG. This flow rate adjustment may be performed so that the controller 39 keeps the furnace pressure constant. When the furnace pressure fluctuates, the outer diameter fluctuation of the optical fiber 1f generated by drawing increases, but by controlling the furnace pressure constant in this way, the outer diameter fluctuation of the generated optical fiber 1f is also stabilized. The quality of the optical fiber can be maintained.

  Moreover, it is preferable that the sealing mechanism 3 has a concentration detection unit 36 that detects the concentration (oxygen concentration) of the gas discharged from the annular casing 31 in order to detect external air contamination. The concentration detection unit 36 is preferably installed in the exhaust unit 35, but may be installed anywhere as long as it is in the multistage blowing unit. The seal mechanism 3 adjusts the supply flow rate of the seal gas based on the detection result of the concentration detector 36. This adjustment may be performed by the controller 39. The gas flow rate may be drawn, for example, while controlling the sealing gas to be about 20 liters / minute in total, for example, controlling the oxygen concentration to be 50 ppm or less.

  In the case where different types of gases are used as part of the seal gas blown out from the blow-out portion and the furnace gas, the controller 39 can individually control the flow rate of each seal gas based on the detected oxygen concentration. Good. In addition, when the furnace pressure is also considered as described above, the controller 39 controls the flow rate of the blowing part through which the same gas as the furnace gas flows, based on the furnace pressure (or the oxygen concentration and the furnace pressure), What is necessary is just to control the flow volume of the seal gas sent from other blowing parts based on oxygen concentration (or oxygen concentration and furnace pressure).

  Further, the present invention can take a form as a drawing method as described in the drawing procedure. Specifically, in this drawing method, an upper end opening into which the glass base material is inserted, a furnace tube that accommodates the glass base material inserted from the upper end opening, and the glass base material are heated and melted. An optical fiber is drawn using a drawing furnace equipped with a heating source. In this drawing method, a sealing mechanism having an annular housing is installed at a position surrounding the glass base material above the upper end opening, and the gap between the upper end opening and the glass base material is sealed. Drawing is performed while sealing with sealing gas blown from the mechanism.

  As described above, the annular casing has a blowout port for blowing out seal gas from the inner peripheral surface, and the maximum width of the gap between the inner peripheral surface of the annular casing and the glass base material is δ. If the distance L between the uppermost side and the lowermost side of the air outlets is L ≧ 20δ, it may be used. In addition, about the other application example of the drawing method, it is the same as that of what was demonstrated about the drawing furnace, The description is abbreviate | omitted.

DESCRIPTION OF SYMBOLS 1 ... Glass base material, 1f ... Optical fiber, 2 ... Support rod, 3 ... Sealing mechanism, 10 ... Drawing furnace body, 11 ... Furnace housing, 11a ... Upper end surface, 12 ... Core tube, 13 ... Heater, 14 ... Insulating material, 15 ... gap, 16 ... discharge hole, 31 ... annular housing, 32, 33, 34, 37 ... blowout part, 32a, 33a, 34a, 37a ... blowout port, 35 ... exhaust part, 35a ... exhaust port , 36... Concentration detector, 38. Furnace pressure detector 39, controller 40, gas supply unit.

Claims (6)

An upper end opening into which a glass base material for an optical fiber is inserted, a furnace core tube that houses the glass base material inserted from the upper end opening, and a heating source that heats and melts the glass base material An optical fiber drawing furnace comprising: a sealing mechanism that seals a gap between the upper end opening and the glass base material with a sealing gas;
The sealing mechanism has an annular casing at a position above the upper end opening and surrounding the glass base material, and the annular casing has a blowout port for blowing a sealing gas from an inner peripheral surface. When the maximum value of the width of the gap between the inner peripheral surface of the annular casing and the glass base material is δ, the distance L between the uppermost side and the lowermost side of the outlet is L. An optical fiber drawing furnace characterized in that ≧ 20δ.   2. The optical fiber drawing furnace according to claim 1, wherein the annular housing has a multistage blowout portion having a multistage structure having two or more blowout openings at positions separated in a height direction.   The said multistage blowing part blows off the said sealing gas diagonally upward at the blower outlet located in the uppermost stage among the said blower outlets, and diagonally downward at the blower outlet located in the lowest stage. Optical fiber drawing furnace.   The sealing mechanism includes an exhaust part for exhausting the seal gas between an outlet located at the uppermost stage and an outlet located at the lowermost stage in the multistage outlet, and a seal gas exhausted from the exhaust part. The optical fiber drawing furnace according to claim 2, further comprising: a concentration detection unit that detects a concentration of the optical fiber.   The said sealing mechanism is provided in the lower part of the blower outlet located in the lowest stage in the said multistage blowing part, and has a furnace internal pressure detection part which detects the internal pressure of the said core tube, The any one of Claims 2-4 characterized by the above-mentioned. 2. An optical fiber drawing furnace according to item 1. An upper end opening into which a glass base material for an optical fiber is inserted, a furnace core tube that houses the glass base material inserted from the upper end opening, and a heating source that heats and melts the glass base material An optical fiber drawing method for drawing an optical fiber using a drawing furnace equipped with
A seal mechanism having an annular housing is installed at a position above the upper end opening and surrounding the glass base material, and a gap between the upper end opening and the glass base material is separated from the seal mechanism. Drawing while sealing by blowing out seal gas,
The annular casing has a blowout port for blowing out seal gas from the inner peripheral surface, and when the maximum value of the width of the gap between the inner peripheral surface of the annular casing and the glass base material is δ, A method of drawing an optical fiber, wherein a distance L between the uppermost side and the lowermost side of the outlet is L ≧ 20δ.

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