JP2002068773A - Furnace for drawing optical fiber and method of drawing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a furnace for drawing an optical fiber which is capable of stabilizing stream therein, and also to provide a method of drawing the optical fiber. SOLUTION: This furnace for drawing the optical fiber is provided with a dummy bar 2 vertically ascendable and descendable and a heater 11 heating the lower end of an optical fiber basic material 1 fitted to the dummy bar 2, and is further provided with a plurality of pass partitions 4 slidable along the dummy bar 2, a stepped structural part 5 forming a space which surrounds the periphery of the dummy bar 2 and extends upwardly and downwardly inside of the furnace, the plurality of pass partitions 4 in which one placed at lower position has an external diameter lower than that placed at an upper position, the stepped structural part 5 in which each external diameter of each stage decreases with moving downward, and the plurality of pass partitions 4 are constituted to be engaged individually in series with the stepped structural part 5 with descending the dummy bar 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber drawing furnace for drawing an optical fiber after heating and melting an optical fiber preform, and a method for drawing an optical fiber.

[0002]

2. Description of the Related Art The outer diameter of an optical fiber is about one hundred and several tens of micrometers, but the optical fiber is drawn (drawn) from the lower end of a heated optical fiber preform having an outer diameter of several tens to 120 mm.
Manufactured. The heating of the optical fiber preform is performed inside the drawing furnace. Convection may occur inside the drawing furnace due to heat generated by heating. If the air flow in the drawing furnace is disturbed by convection, adverse effects such as a change in the outer diameter of the optical fiber to be drawn occur, and the performance of the optical fiber deteriorates.

[0003] As a drawing furnace considering such a problem,
The one described in JP-A-11-343137 is known. As shown in FIG. 7, the drawing furnace described in the above-mentioned publication is devised to make the volume of the upper space of the optical fiber preform 1 attached to the lower end of the dummy rod 2 substantially constant. This suppresses and stabilizes the airflow turbulence due to convection inside the drawing furnace. If the airflow in the drawing furnace is stable, the performance of the drawn optical fiber becomes stable. Hereinafter, the drawing furnace described in the above publication will be briefly described.

In the drawing furnace shown in FIG. 7, similarly to a general drawing furnace, the furnace tube 10
In order to prevent the furnace tube 10 from deteriorating due to accelerated oxidation, an inert gas is supplied from the gas supply port 7 into the drawing furnace. It is preferable that the inert gas keeps the downward flow stably as it is, but if the drawing proceeds and the volume of the space above the optical fiber preform 1 increases, convection occurs inside the drawing furnace. Turbulence occurs in the airflow. When turbulence occurs in the airflow inside the drawing furnace, the outer diameter of the optical fiber 1a to be drawn is changed, and the performance is adversely affected.

Therefore, the volume of the space above the optical fiber preform 1 inside the drawing furnace is kept substantially constant (as small as possible) to suppress convection inside the drawing furnace, and the gas flows from the gas supply port 7 downward. In order to stably form an air flow, a plurality of partition plates 4
Is arranged. Also, an inner tube 5 'is installed inside the drawing furnace so that the partition plate 4 is arranged at a predetermined position in the drawing furnace in accordance with the descent of the optical fiber preform 1 which has been drawn and shortened. ing. The inner tube 5 'has a tapered shape as a whole, with the inner diameter continuously reduced as it goes downward. Since the plurality of partition plates 4 also have a larger diameter at the upper side, they are caught by the inner tube 5 ′ from the upper side and partition the space above the optical fiber preform 1.

[0006]

However, in the wire drawing furnace described in the above-mentioned publication, the partition plate 4 and the inner tube 5 'are thermally expanded and contracted. 'They could get stuck and become obstacles when pulling up the dummy bar 2 after drawing. Therefore, the partition plate 4 does not fit into the inner tube 5 ', and the space above the optical fiber preform 1 can be maintained substantially constant to stabilize the airflow in the drawing furnace. Improvement was desired. An object of the present invention is to provide an optical fiber drawing furnace capable of stabilizing an airflow in a drawing furnace and a method for drawing an optical fiber.

[0007]

According to a first aspect of the present invention, there is provided an optical fiber drawing furnace, wherein a dummy rod for mounting an optical fiber preform which can be moved up and down and a lower end of the optical fiber preform attached to the dummy rod are provided. And a plurality of partition plates that are vertically inserted into the dummy bar and are slidable along the dummy bar, and are formed so as to surround the periphery of the dummy bar that is vertically moved up and down. And a stepped structure that forms a space extending in the up-down direction inside, and a plurality of partition plates,
Although the outer diameter of the lower part is smaller than the outer diameter of the upper part, and the stepped structure part is reduced as the inner diameter of each step goes downward, and a plurality of the Are characterized in that they are configured so as to be sequentially locked one by one on the stepped structure portion.

According to a second aspect of the present invention, in the first aspect of the present invention, the stepped structure portion has a contact portion with the outer peripheral edge portion of the partition plate located between each step facing downward. It is characterized in that it has a tapered surface that reduces its diameter.

The invention described in claim 3 is the first or second invention.
In the invention described in (1), the partition plate has a tapered surface which is reduced in diameter at an outer peripheral edge portion in contact with the stepped structure portion.

According to a fourth aspect of the present invention, in the first aspect of the present invention, the gas in the space above the stepped structure or in the space above the stepped structure is heated. It is characterized by having an auxiliary heater.

According to a fifth aspect of the present invention, in the method of drawing an optical fiber, an optical fiber preform is attached to a lower end of a dummy rod which can be moved up and down inside a drawing furnace and attached to the dummy rod. The lower end of the optical fiber preform is heated by a heater, and the optical fiber is drawn from the lower end of the heated and melted optical fiber preform. A plurality of slidable partition plates, and a stepped structure portion formed so as to surround the periphery of the dummy rod vertically moved up and down to form a space extending vertically therein, and a plurality of partition plates. The outer diameter of the lower part is smaller than the outer diameter of the upper one, and the stepped structure part is reduced as the inner diameter of each step goes downward, and as the dummy rod descends, Stepped structure with multiple partitions It is characterized by causing locked sequentially one by one engaged.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for drawing an optical fiber according to the present invention will be described with reference to the drawings. FIG. 1 is a longitudinal sectional view showing a main part of an embodiment of an optical fiber drawing furnace of the present invention.

In this drawing furnace, the optical fiber preform 1 is attached to the lower end of the dummy rod 2 via the connecting portion 3, and the lower end of the optical fiber preform 1 is heated and melted by the heater 11. Further, an optical fiber 1a is drawn from the lower end of the optical fiber preform 1 that has been heated and melted. The optical fiber preform 1 becomes shorter as the drawing of the optical fiber 1a proceeds. In such a case, the dummy rod 2 is lowered so that the lower end of the optical fiber preform 1 is always heated by the heater 11. .

The drawing furnace includes an outer tube 6 whose upper portion is closed by a lid portion 9, a furnace tube 10 disposed below the tube 6, and a lower extension portion of the furnace tube disposed further below the tube. 12 are provided. The heater 11 is located outside the furnace tube 10. The optical fiber 1a to be drawn is drawn from the furnace tube lower extension 12. During the drawing, the heater 11
The inert gas is supplied to the inside of the drawing furnace in order to suppress the oxidative deterioration of the furnace tube 10 and the like by the heating of the furnace. This inert gas is supplied from a gas supply port 7 and is supplied to a gas passage 7.
a, the outer tube 6 and the furnace tube 10
Is supplied between

The supplied inert gas is preferably discharged from the lower core tube lower extension 12 in a stable airflow. For this reason, in the present embodiment, the volume of the space above the optical fiber preform 1 is made substantially constant by the plurality of partition plates 4 and the stepped structure portion 5 irrespective of the lowered position of the optical fiber preform 1. Like that. The overall configuration has been briefly described above. Hereinafter, each component will be described in detail.

The stepped structure 5 is integrally connected to the inner surface of the outer tube 6. The stepped structure 5 is formed in a cylindrical shape. The outer tube 6 and the stepped structure 5 may have a double wall structure. The internal space of the stepped structure portion 5 has a circular cross section in a cross section perpendicular to the central axis of the dummy bar 2. The stepped structure portion 5 is configured such that the inner diameter of the above-described circular cross section becomes smaller for each step as it goes from above to below. In the present embodiment, the stepped structure portion 5 is reduced in diameter in five stages downward.

The inner diameter of the uppermost portion (immediately below the lid 9) inside the drawing furnace is 170 mm, and the inner diameter of the lowermost portion of the stepped structure 5 is 150 mm. Inside the stepped structure portion 5, the upper portion of the optical fiber preform 1 attached to the dummy rod 2 via the connecting portion 3 is located. The outer diameter of the optical fiber preform 1 in this embodiment is 125 mm, and the length in the initial state is 1600 mm. The heater 11 heats and melts the vicinity of the lower end of the optical fiber preform 1 from the outside of the furnace tube 10, and the optical fiber 1 a is drawn out from the lower end of the optical fiber preform 1.

The gas supply port 7 provided at the lower part of the outer tube 6 passes through a gas passage 7a and the gas inlet 8 provided at the lower end of the stepped structure 5 into the drawing furnace. An inert gas is poured. Further, the upper part of the drawing furnace is covered with a lid 9 provided with a hole through which the dummy rod 2 can move so as to prevent the inside inert gas from flowing out.

In the section perpendicular to the center axis of the dummy rod 2, a large number of gas inlets 8 are provided at ten or more locations at substantially equal intervals in the circumferential direction so that the injected inert gas is The flow should be as even as possible in the circumferential direction. The gas inlet 8 may be formed over the entire circumference. Also, about four gas supply ports 7 are provided in the circumferential direction of the outer tube 6. The number of gas supply ports 7 may be larger,
Any number may be used as long as the flow of the inert gas in the drawing furnace is as uniform as possible in the circumferential direction.

Each partition plate 4 of the present embodiment comprises an outer member 4a and an inner member 4b. Outer member 4
The inner member 4a and the inner member 4b are both disks having a hole in the center, and are made of a heat-resistant material such as quartz, carbon, silicon carbide or the like, and have a thickness of about several mm to several tens of mm. In this embodiment, since the stepped structure portion 5 has a circular cross section, the partition plate 4 has a disk shape. However, when the cross section shape of the stepped structure portion 5 is rectangular, for example, It is a square plate according to the shape. When the connecting portion 3 is located at the uppermost position, the plurality of partition plates 4 are placed on the upper surface of the connecting portion 3 in a stacked state.

The outer diameter of each of the outer members 4a of the plurality of sets of partition plates 4 corresponds to the inner diameter of the stepped structure 5 described above. That is, the uppermost outer member 4a has an outer diameter that can be placed on the uppermost step of the stepped structure 5, and the second outer member 4a from the top has the outer diameter of the stepped structure 5. It has an outer diameter (small diameter) that cannot be placed on the uppermost step and an outer diameter that can be placed on the second step from the top of the stepped structure portion 5. Hereinafter, the outer diameter of the outer member 4a is a size that can be sequentially placed on the lower step.

When the drawing of the optical fiber 1a progresses and the optical fiber preform 1 becomes shorter, the connecting portion 3 descends together with the dummy rod 2. At this time, a plurality of sets of partition plates 4 Is prevented from moving downward at the stepped portions smaller than the outer diameter of each partition plate 4 (outer member 4a). As a result,
A plurality of sets of the partition plates 4 are separated from the upper partition plate 4 by the stepped structure portion 5.
Are sequentially locked to each step.

If such a partition plate 4 is not provided, a reversal of the temperature occurs in the vertical direction in the upper chimney, thereby causing a natural body flow. As a result, a change occurs at the neckdown of the gas flow (a portion where the outer diameter of the optical fiber preform 1 is reduced), and the outer diameter of the drawn optical fiber 1a fluctuates due to this change. Here, by providing the plurality of partition plates 4, generation of convection can be suppressed, and the outer diameter of the optical fiber 1a does not fluctuate. Also, since the volume of each space is small, even if convection occurs in the space, the convection is weak and does not become a pressure fluctuation and is not transmitted to the neck down.

Each partition plate 4 is constituted by the outer member 4a and the inner member 4b as described above. The outer diameter of the outer member 4a is an outer diameter that can be locked to the corresponding step of the stepped structure 5, and the inner diameter of the hole formed in the center of the outer member 4a is larger than the outer diameter of the dummy rod 2. Is also slightly larger. The outer diameter of the inner member 4b is sufficiently larger than the inner diameter of the hole formed in the center of the paired outer member 4a, and smaller than the outer diameter of the paired outer member 4a. Have been. The inner diameter of the hole formed in the center of the inner member 4b is
(But slidable with respect to the dummy bar 2).

Therefore, even if the dummy bar 2 is slightly eccentric inside the stepped structure portion 5 and is no longer concentric, the outer member 4
Since a can be moved with respect to the inner member 4b, it is possible to prevent the partition plate 4 (the outer member 4a) from damaging the stepped structure portion 5. The dummy bar 2 is eccentric inside the stepped structure portion 5 even when the optical fiber preform 1 swings, and the like. Even in this case, the gap between the dummy bar 2 and the partition plate 4 is almost completely eliminated by the inner member 4b. The exchange of gas in the air can be completely or minimally suppressed.

Further, the drawing furnace according to the present embodiment has an outer tube 6
Has an auxiliary heater 13 at the top. Unlike the heater 11 for heating and melting the lower end of the optical fiber preform 1, the auxiliary heater 13 is provided to further stabilize the airflow in the drawing furnace. Auxiliary heater 13
Heats the gas (inert gas) in the upper portion of the stepped structure portion 5 and in the space above the stepped structure portion 5.

The interior space of the drawing furnace is partitioned by each partition plate 4, and the exchange of gas between the spaces is suppressed.
However, gas may be exchanged through a small gap. In such a case, if the temperature of each space is different, gas exchange becomes more remarkable. This is because the hotter gas tends to move upward. Since the partition plate 4 is provided, the movement of the gas is suppressed. However, in order to further suppress the movement of the gas, the auxiliary heater 13 is provided in the space above the stepped structure portion 5 and above the stepped structure portion 5. By heating the gas (inert gas), the temperature difference in each space partitioned by the partition plate 4 is reduced.

Optical fiber preform 1 whose lower end is heated
Radiates heat conducted from below where the outer diameter of the upper portion becomes smaller. Therefore, the surrounding gas is heated by the radiant heat. At this time, if the temperature of the gas in the upper part of the stepped structure portion 5 and the space above the stepped structure portion 5 is low, the heated gas is likely to move upward by convection. When the optical fiber preform 1 is shortened, a distance is generated between the upper portion of the stepped structure portion 5 and the space above the stepped structure portion 5, so that the gas temperature in this portion tends to be lower. Therefore, the gas in the space above the stepped structure portion 5 and in the space above the stepped structure portion 5 is heated by using the auxiliary heater 13, and the temperature is made uniform so that the gas is less likely to move. As a result, the airflow in the drawing furnace is further stabilized.

In the case where the chimney is particularly long, the space between the partition plates 4 is widened (it is conceivable to increase the number of the partition plates. It is not realistic because it increases the size). When this space is widened, the temperature difference between the upper side and the lower side in this space is also increased, and natural convection in each space is increased. When natural convection increases, periodic pressure fluctuations occur in each space.
The pressure fluctuation is transmitted downward even from a small gap, and finally changes the outer diameter of the optical fiber 1a. In such a case, if the auxiliary heater 13 is provided as described above, the entire space above the optical fiber preform 1 has a more uniform temperature distribution, and the above-described adverse effects due to the pressure fluctuation can be prevented.

When the length of the optical fiber preform 1 is still sufficiently long, that is, the gas in the space above the stepped structure 5 and in the space above the stepped structure 5 is generated from the upper part of the optical fiber preform 1. While heating by radiant heat, the auxiliary heater 13
, The auxiliary heater 13 does not need to generate heat. In particular, when a long base material is used, the space is large, so that heat is more stable. When the length of the optical fiber preform 1 becomes short and a difference occurs in the temperature distribution in the drawing furnace, heat generation by the auxiliary heater 13 is started.

In the drawing furnace according to the present embodiment, a plurality of partition plates 4 are arranged around the dummy rod 2, and the gas inlet 8 for the inert gas is provided below the moving range of the partition plate 4. Therefore, even if the drawing of the optical fiber 1a from the optical fiber preform 1 progresses and the optical fiber preform 1 is shortened, the distance between the locked partition plate 4 and the optical fiber preform 1 is reduced. The volume of the space can always be made substantially constant. As a result,
The upward flow from the gas inlet 8 is hardly formed in the drawing furnace, the turbulence of the air flow between the optical fiber preform 1 and the partition plate 4 does not occur, and the air flow is stabilized. Even if a part of the inert gas enters the space above the partition plate 4, the flow of the inert gas in the space is small, so that the flow is the optical fiber preform 1. It does not affect the space below.

The inert gas entered from the gas inlet 8 is
The airflow flowing downward and discharged from the furnace tube lower extension 12 is stably formed, and the airflow around the drawn optical fiber 1a is maintained in a rectified state. Since the volume of the space above the optical fiber preform 1 is maintained substantially constant, the air flow hardly fluctuates due to the descent of the optical fiber preform 1 and is drawn from the lower end of the heated optical fiber preform 1. The variation in the diameter of the optical fiber 1a can be reduced.

Further, in the present embodiment, each partition plate 4 is
The partition plate 4 is locked by each step of the stepped structure 5.
Does not fit into the stepped structure portion 5 and hinders the dummy bar 2 from being pulled up. Further, the stepped structure portion 5 is easier to manufacture and realizes higher dimensional accuracy than the inner cylindrical tube 5 ′ having a tapered shape as a whole in a drawing furnace as shown in FIG. Can be.

In the above-described embodiment, the outer peripheral surface of the partition plate 4 and each step of the step structure 5 are formed at right angles to the center axis of the dummy rod 2. Although there is no problem in this case, these portions may be formed as shown in FIG. 2 or FIG. FIG. 2 or FIG. 3 is an enlarged cross-sectional view showing the locked state of the partition plate 4 and the stepped structure portion 5 at one place.

In FIG. 2, the contact portion of the stepped structure portion 5 with the outer peripheral edge of the partition plate 4 is a tapered surface 5a whose diameter is reduced downward. The angle α of the tapered surface 5a with respect to the vertical axis X can be larger than when the entire inner tube 5 'is tapered as shown in FIG. For this reason,
The partition plate 4 (the outer member 4a) does not get stuck in the stepped structure portion 5 and is fixed. When such a tapered surface 5a is formed, the outer member 4a
Comes to coincide with the center of itself and the center of the stepped structure 5, and a so-called self-centering action occurs. When such a self-centering action occurs, no more gap is formed between the outer peripheral edge of the outer member 4a and the tapered surface 5a, and the flow of gas between the upper side and the lower side of the partition plate is improved. It can be further deterred.

On the other hand, in FIG. 3, a tapered surface 4c whose diameter is reduced downward is formed on the outer peripheral edge of the partition plate 4 which comes into contact with the stepped structure portion 5. In FIG. 2, the tapered surface 5a is formed on the stepped structure portion 5 side, but here, the same effect is obtained by forming the tapered surface 4c on the partition plate 4 side. Even in this case, the angle β of the tapered surface 4c with respect to the vertical axis X can be made larger than when the entire inner tube 5 ′ is tapered as shown in FIG. For this reason, there is no possibility that the partition plate 4 (the outer member 4a) gets stuck in the stepped structure portion 5 and is fixed. Further, even if such a tapered surface 4c is formed, the above-described self-centering action occurs, so that the flow of gas between the upper side and the lower side of the partition plate can be further suppressed.

Each of the angles α and β described above is in the range of 1 ° to 80 ° from the viewpoint of preventing the partition plate 4 from biting into the stepped structure portion 5 and effectively utilizing the self-centering action. Preferably. If these angles are less than 1 °,
It becomes easy for the partition plate 4 to bite into the stepped structure portion 5. On the other hand, if these angles are more than 80 °, effective self-centering cannot be obtained. Furthermore, even within such a range, it is particularly preferable to set these angles in the range of 5 ° to 60 ° in order to achieve prevention of biting and effective use of the self-centering effect in a well-balanced manner.

As shown in FIGS. 4 to 6, the contact portion of the stepped structure portion 5 with the outer peripheral edge of the partition plate 4 is formed as a tapered surface 5a whose diameter is reduced downward. A tapered surface 4c whose diameter is reduced downward may be formed on an outer peripheral edge portion of the partition plate 4 that contacts the stepped structure portion 5. Even in this case, the partition plate 4 (the outer member 4a) is
The self-centering effect described above is generated while being stuck in and fixed, so that the flow of gas between the upper side and the lower side of the partition plate 4 can be further suppressed. Note that, in such a case, the angles α and β are determined as follows: α = β as shown in FIG. 4, α> β as shown in FIG. 5, α <β as shown in FIG. May be.

The present invention is not limited to the embodiment described above. For example, in the embodiment described above,
Although five sets of the partition plates 4 are provided, the number of the partition plates 4 may be plural. Further, in the above-described embodiment, each partition plate 4 is composed of two members, the outer member 4a and the inner member 4b, but each partition plate may be locked by one plate-like member.

Further, in the present invention, the size of the partition plate and the step structure is described using the terms inner diameter and outer diameter. The cross-sectional shape of the portion (cross-sectional shape by a plane perpendicular to the central axis of the dummy rod) is not limited to a disk shape or a circular cross-section. It can be thought of as a word that expresses

[0041]

According to the first aspect of the present invention, the volume of the space above the optical fiber preform attached to the dummy rod by the partition plate can be maintained substantially constant. The airflow can be stabilized. Since the airflow in the drawing furnace can be stabilized, an optical fiber of stable quality can be drawn. And
A step structure is formed to arrange the partition plate at a predetermined position. Since the stepped structure portion is in a form in which the partition plate does not bite, when raising the dummy bar, there is no occurrence of an obstacle such as the partition plate being unable to move and the dummy bar being unable to be raised. .

According to the second or third aspect of the present invention, by forming a tapered portion on either the partition plate or the stepped structure portion, the positioning between the partition plate and the stepped structure portion can be performed more accurately. To be done. As a result, the movement of gas between the upper side and the lower side of the partition plate can be more reliably prevented, and the airflow in the drawing furnace can be further stabilized.

According to the fourth aspect of the present invention, by heating the gas above the stepped structure or above the stepped structure, the movement of the gas in the portion where the partition plate is provided can be more reliably performed. It can be suppressed, and the airflow in the drawing furnace can be further stabilized.

According to the fifth aspect of the present invention, the volume of the space above the optical fiber preform attached to the dummy rod can be maintained substantially constant by the partition plate, thereby reducing the air flow in the drawing furnace. Can be stabilized. Since the airflow in the drawing furnace can be stabilized, an optical fiber of stable quality can be drawn. Then, a stepped structure is formed to arrange the partition plate at a predetermined position. Since the stepped structure portion is in a form in which the partition plate does not bite, when raising the dummy bar, there is no occurrence of an obstacle such as the partition plate being unable to move and the dummy bar being unable to be raised. .

[Brief description of the drawings]

FIG. 1 is a sectional view showing an embodiment of an optical fiber drawing furnace of the present invention.

FIG. 2 is a partially enlarged cross-sectional view showing another embodiment of the optical fiber drawing furnace of the present invention.

FIG. 3 is a partially enlarged sectional view showing another embodiment of the optical fiber drawing furnace of the present invention.

FIG. 4 is a partially enlarged sectional view showing another embodiment (α = β) in the optical fiber drawing furnace of the present invention.

FIG. 5 is a partially enlarged sectional view showing another embodiment (α> β) of the optical fiber drawing furnace of the present invention.

FIG. 6 is a partially enlarged sectional view showing another embodiment (α <β) of the optical fiber drawing furnace of the present invention.

FIG. 7 is a sectional view showing a conventional optical fiber drawing furnace.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical fiber preform, 1a ... Optical fiber, 2 ... Dummy rod, 4 ... Partition plate, 5 ... Stepped structure part, 10 ... Core tube, 11
... heater, 13 ... auxiliary heater.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazuya Kuwahara 1-chome, Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa F-term in Sumitomo Electric Industries, Ltd. Yokohama Works 4G021 HA03

Claims (5)

[Claims] 1. An optical fiber drawing furnace comprising: an optical fiber preform mounting dummy rod that can be moved up and down; and a heater that heats a lower end of the optical fiber preform attached to the dummy rod. A plurality of partition plates which are vertically inserted into the dummy bar and are slidable along the dummy bar, and a space formed so as to surround the periphery of the dummy bar which is raised and lowered and which extends vertically therein. A plurality of partition plates, wherein the plurality of partition plates are located below, but the outer diameter of the plurality of partition plates is made smaller than the outer diameter of those located above, and the stepped structure is The inner diameter of the partition is reduced as it goes downward, and a plurality of the partition plates are configured to be sequentially locked one by one to the stepped structure portion as the dummy bar descends. Optical fiber drawing furnace. 2. The stepped structure portion is characterized in that an abutting portion between the step and the outer peripheral edge of the partition plate has a tapered surface that is reduced in diameter downward. The optical fiber drawing furnace according to claim 1. 3. The partition plate according to claim 1, wherein the partition plate has a tapered surface which is reduced in diameter at an outer peripheral edge portion in contact with the stepped structure portion. Optical fiber drawing furnace. 4. An apparatus according to claim 1, further comprising an auxiliary heater for heating gas in an upper portion of the stepped structure portion or in a space above the stepped structure portion. An optical fiber drawing furnace according to any one of the above. 5. An optical fiber preform is attached to a lower end of a dummy rod disposed vertically up and down inside a drawing furnace, and a lower end of the optical fiber preform attached to the dummy rod is heated by a heater. An optical fiber drawing method for drawing an optical fiber from a lower end of the heat-fused optical fiber preform, wherein the drawing furnace is vertically inserted into the dummy rod and slides along the dummy rod. A plurality of possible partition plates, and a stepped structure portion formed to surround the periphery of the dummy bar vertically moved up and down to form a space extending vertically therein, and the plurality of partition plates The outer diameter of the lower part is smaller than the outer diameter of the upper part, and the stepped structure part is reduced as the inner diameter of each step goes downward, and the dummy rod is lowered. According to multiple pieces The method according to claim 1, further comprising the step of locking the partition plates one by one on the stepped structure portion.