JPH0891862A - Optical fiber drawing method and drawing furnace - Google Patents

Abstract

PURPOSE: To obtain an optical fiber drawing method capable of decreasing the fluctuations in the outside diameter of an optical fiber and an optical fiber drawing furnace capable of realizing this drawing method. CONSTITUTION: This optical fiber drawing furnace includes a furnace core tube 14 into which a preform 13 for the optical fiber is fed from an opening 23 at the top end and from the bottom end of which the optical fiber 22 is led out, an inert gas supplying means which passes an inert gas from the opening 23 at the top end of this furnace core tube 14 toward the bottom end, a heater 15 which encloses the bottom end of the preform 13 for the optical fiber across the furnace core tube 14 and melts this preform by heating, and a furnace body 12 which holds this heater 15 and the furnace core tube 14. The furnace core tube 14 has a cylindrical part 24 which exists upper than the position opposite to the part fused by heating of the preform 13 for the optical fiber and has a specified bore corresponding to the outside diameter size of the perform 13 for the optical fiber and tapered cylindrical parts 25, 26 which exist on the lower side of this cylindrical part 24 and have the bore made smaller nearer the lower side.

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 method capable of reducing fluctuations in the outer diameter of an optical fiber and an optical fiber drawing furnace capable of realizing the drawing method.

[0002]

2. Description of the Related Art In a general optical fiber for information transmission, a lower end portion of a base material for an optical fiber having a diameter of about several centimeters, which has a cross section similar to that of the optical fiber, is heated and melted, and the melted portion is melted. Obtained by continuously pulling down. As an optical fiber drawing furnace for heating the lower end portion of such an optical fiber preform, those disclosed in Japanese Examined Patent Publication No. 3-24421 and Japanese Patent Laid-Open No. 59-88336 are well known.

For example, FIG. 3 showing a cross-sectional structure of a conventional optical fiber drawing furnace disclosed in Japanese Patent Publication No. 3-24421.
As shown in, the furnace body 10 with the heat insulating material 101 incorporated therein is shown.
2 is a core tube 10 into which the optical fiber preform 103 is fed.
4 and an annular heater 105 that surrounds the central portion of the furnace tube 104 and heats and melts the lower end portion of the optical fiber preform 103. Further, at the upper end portion of the inlet pipe portion 106 projecting upward at the center of the upper end of the furnace body 102, an inert gas such as helium or nitrogen is downwardly introduced into the core tube 101 through the inlet pipe portion 106. An inert gas supply device (not shown) for supplying the gas is installed. Furthermore, the furnace body 10
At the center of the lower end of 2, there is a cylindrical mouthpiece 107 projecting downward.
The optical fiber 109 is pulled out from the eight.

The core tube 104 comprises a large cylindrical portion 110 into which the optical fiber preform 103 is fed via the inlet tube portion 106,
The upper end is connected to this large cylindrical portion 110, and the inner diameter becomes smaller toward the lower side, and the tapered cylindrical portion 111 surrounds the lower end portion of the optical fiber preform 103, and the upper end is connected to the lower end of this tapered cylindrical portion 111. At the same time, the lower end is composed of a small cylindrical portion 112 connected to the upper end of the base 107.

Therefore, the inside of the furnace body 102 is kept warm by the heat insulating material 101, and the lower end portion of the optical fiber preform 103 fed into the furnace core tube 104 is heated and melted by the heater 105 in the furnace body 102 to produce the optical fiber. It becomes 109 and is pulled out from the lower end opening 108 of the base 107. In addition, the inert gas supplied from above the core tube 104 keeps the inside of the core tube 104 in an inert gas atmosphere, prevents oxidation of the core tube 104, and keeps the inside of the core tube 104 clean. . This inert gas flows downward along the gap between the inner peripheral surface of the core tube 104 and the outer peripheral surfaces of the optical fiber preform 103 and the optical fiber 109, and is discharged from the lower end opening 108 of the mouthpiece 107 to the outside of the furnace. To be done.

As described above, by molding the core tube 104 along the contour of the optical fiber preform 103, the flow of the inert gas along the lower end of the optical fiber preform 103 in the heating and melting state. Is stable, and it is possible to suppress fluctuations in the outer diameter of the drawn optical fiber, bending of the optical fiber, and deterioration of strength.

[0007]

[Problems to be Solved by the Invention] Japanese Patent Publication No. 3-24421
In the optical fiber drawing furnace disclosed in Japanese Laid-Open Patent Publication No. 59-88336, etc., an inert gas is caused to flow along the optical fiber preform and the optical fiber in a laminar flow state, so that the outer diameter of the inert gas is reduced. Variations, uneven bending due to non-uniform residual stress, and deterioration of strength of the optical fiber due to adhesion of dust floating in the furnace are suppressed.

By the way, when connecting a plurality of optical fibers to each other via an optical connector, it is necessary to form a hole or groove for passing the optical fiber in the ferrule of the optical connector. In this case, the dimensions of these holes and grooves are set to dimensions corresponding to the optical fiber having the maximum allowable outer diameter in consideration of variations in the outer diameter of the optical fiber. Therefore, when the permissible dimension width is set to be wide, when the optical fiber having the outer diameter of the minimum permissible value is attached to the ferrule, the amount of misalignment correspondingly increases, resulting in an increase in connection loss.

[0009] Japanese Patent Publication No. 3-24421 mentioned above,
In the conventional optical fiber drawing furnace disclosed in Japanese Patent Laid-Open No. 59-88336, it is possible to suppress the fluctuation of the outer diameter of the optical fiber within ± 0.5 μm with respect to the reference outer diameter. For example, it is difficult to control within ± 0.2 μm. The reason for this is that the flow of the inert gas along the optical fiber preform and the optical fiber is still unstable even with the optical fiber drawing furnace having the above-mentioned structure, and this unstable flow of the inert gas Is considered to affect the outer diameter variation of the optical fiber.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical fiber drawing method capable of reducing fluctuations in the outer diameter of an optical fiber and an optical fiber drawing furnace capable of realizing the drawing method.

[0011]

According to a first aspect of the present invention, a step of heating and melting a lower end portion of a base material for an optical fiber and a step of continuously heating the optical fiber from the lower end of the base material for heating and melting the optical fiber are performed. The step of pulling out, the step of passing the lower end portion of the optical fiber preform and the optical fiber pulled out from the lower end of the optical fiber preform into a taper tube having a smaller inner diameter toward the lower end side, and the upper end portion of the taper tube And a step of flowing an inert gas toward the lower end of the optical fiber drawing method.

Here, it is desirable that the flow velocity of the inert gas flowing in the taper cylinder be higher toward the lower end side of the taper cylinder, and the portion of the optical fiber facing the upper end of the taper cylinder is of the optical fiber. It is effective that the diameter is 3 mm or more.

On the other hand, according to the second aspect of the present invention, a core tube in which an optical fiber preform is fed from the upper end and an optical fiber is pulled out from the lower end, and from the upper end to the lower end of the core tube An inert gas supply means for flowing an inert gas, a heater that surrounds the lower end portion of the optical fiber preform with the core tube sandwiched between them, and a heater that heats and melts this, and holds the heater and the core tube. A furnace body, the furnace core tube, the lower end thereof is opposed to the heating and melting portion of the optical fiber preform, or is located above the facing position of the heating and melting portion of the optical fiber preform. ,
And a cylindrical portion having a constant inner diameter corresponding to the outer diameter of the optical fiber preform, and a tapered cylindrical portion positioned below the cylindrical portion and having a smaller inner diameter toward the lower side. And in the optical fiber drawing furnace.

Here, at the lower end of the taper cylinder portion, a mouthpiece having a smaller inner diameter is projected toward the lower side. Is the change rate of the inner diameter of the mouthpiece equal to the change rate of the inner diameter of the taper cylinder portion? ,
Alternatively, it is preferable to set it higher. In this case, the mouthpiece may be separable by itself and attachable to and detachable from the tapered cylindrical portion.

Further, a first taper tube portion in which the taper tube portion mainly surrounds a heated and melted portion of the optical fiber preform, and a second taper which mainly encloses the optical fiber below the taper tube portion. The second taper cylinder portion may be made larger than the change rate of the inner diameter of the second taper cylinder portion. in this case,
It is effective that the diameter of the optical fiber in the portion facing the connecting portion between the first tapered tubular portion and the second tapered tubular portion is 3 mm or more.

According to a third aspect of the present invention, the optical fiber preform is fed from the upper end and the lower end is closed by a bottom plate, and a cylindrical core tube penetrates through the central portion of the bottom plate of the core tube. Together with the inner diameter becomes smaller toward the lower side, and the upper end is opposed to the lower end of the optical fiber preform and the optical fiber is pulled out from the lower end, and the upper end of the core tube to the lower end of the tapered cylinder And an inert gas supply means for flowing an inert gas toward a gas outlet formed at the lower end portion of the core tube, and for the optical fiber with at least the core tube being sandwiched between the core tube and the tapered tube. An optical fiber drawing furnace comprising a heater surrounding a lower end portion of a base material and heating and melting the base material, and a furnace body holding the heater, the core tube and the taper tube.

Here, at the lower end of the taper cylinder, a mouthpiece having a smaller inner diameter is projected toward the lower side, and the change rate of the inner diameter of the mouthpiece is the same as the change rate of the inner diameter of the taper tube, or It is desirable to set more than that. In this case, the mouthpiece may be separable by itself and may be detachable from the taper tube.

By the way, the average velocity U at an arbitrary position in the core tube having the radius r is proportional to T / r ² . here,
T is the absolute temperature of the inert gas at this arbitrary position. Practically, it is necessary to find the radii r and the absolute temperature T at a few places in the core tube, and to appropriately set the inner diameter 2r of the core tube so that the average velocity U increases toward the lower side. In this case, the absolute temperature T of the inert gas also has a distribution in the radial direction of the core tube, but normally it is r / 2 from the center as an average temperature.
It suffices to measure the absolute temperature of the place just away. In this case, setting the inner diameter of the taper tube portion or the upper end side of the taper tube larger than necessary means that the flow of the inert gas is disturbed in the heating and melting portion of the optical fiber preform located below this. It may not be possible to prevent this, so it is desirable to avoid it.

[0019]

When the flow of the inert gas separates from the inner peripheral surface of the core tube, vortices are generated on the outside of the core tube, and the flow of the inert gas becomes unstable. In particular, separation is likely to occur in a flow in which the average flow velocity in the core tube decreases. Specifically, the average flow rate of the inert gas flowing in the cylindrical core tube does not change at any position along the longitudinal direction of the core tube, but the temperature of the inert gas decreases as it goes to the lower side of the core tube. Heat shrinkage, the average flow velocity becomes slower toward the bottom of the core tube,
Peeling easily occurs at this portion. Further, when the inner peripheral surface of the core tube has a convex bent portion such as a throttle, a flow discontinuity surface is generated on the downstream side of the bent portion, and the inert gas is easily separated.

According to the first aspect of the present invention, the lower end portion of the optical fiber preform and the optical fiber pulled out from the lower end of the optical fiber preform are passed through a taper tube having a smaller inner diameter toward the lower end side. , By flowing the inert gas from the upper end portion to the lower end portion of the tapered cylinder, the flow velocity of the inert gas flowing in the tapered cylinder becomes faster toward the lower end side of the tapered cylinder, and the inert gas becomes It flows along the inner surface without separation.

Further, even if there is some turbulence in the flow of the inert gas in the heated and melted portion of the relatively large diameter of the optical fiber preform, the temperature change of the optical fiber preform due to this is caused by the optical fiber preform in this portion. It can be ignored because the heat capacity of the base metal is large. In fact, it is the portion thinner than approximately 3 to 5 mm of the heated and melted portion of the optical fiber preform that is affected by the outer diameter fluctuation due to the turbulence of the inert gas.

Therefore, by setting the relative position between the upper end of the taper tube and the lower end of the optical fiber preform so that the diameter of the optical fiber at the portion facing the upper end of the taper tube becomes 3 mm or more, The outer diameter dimension of the optical fiber preform is hardly affected by the turbulence of the inert gas.

On the other hand, according to the second aspect of the present invention, the optical fiber preform is fed from the upper end of the cylindrical portion of the furnace core tube and the inert gas supply means feeds the upper end portion of the core tube to the lower end of the core tube. Inert gas is flowed toward the part. The lower end of the optical fiber preform that is heated and melted by the heater becomes an optical fiber and is drawn out from the lower end of the tapered tube portion of the core tube. The portion of the core tube facing the heated and melted portion of the optical fiber preform is a tapered cylindrical portion having a smaller inner diameter toward the lower side, and the flow velocity of the inert gas flowing inside this is the maximum at the lower end.

According to the third aspect of the present invention, the optical fiber preform is fed from the upper end of the core tube and the gas exhaust formed from the upper end to the lower end of the core tube by the inert gas supply means. The inert gas is caused to flow toward the outlet and the lower end of the tapered cylinder. The lower end of the optical fiber preform, which is heated and melted by the heater, becomes an optical fiber and is pulled out from the lower end of the tapered cylinder. The lower end of the optical fiber preform and the optical fiber following it are surrounded by a taper cylinder having a smaller inner diameter toward the lower side, and the flow velocity of the inert gas flowing inside this is maximum at the lower end.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an optical fiber drawing furnace according to the present invention will be described in detail with reference to FIG. 1 showing its sectional structure.

Furnace body 12 having heat insulating material 11 incorporated therein
The core tube 14 into which the optical fiber preform 13 is fed
And an annular heater 15 that surrounds the central portion of the core tube 14 and heats and melts the lower end portion of the optical fiber preform 13. Further, a cylindrical inlet pipe portion 16 is provided so as to project upward at the center of the upper end of the furnace body 12, and the inlet pipe portion 1 is provided at the upper end portion of the inlet pipe portion 16.
An inert gas supply device (not shown) for supplying an inert gas such as helium or nitrogen downward is assembled into the core tube 11 via 6. Further, at the center of the lower end of the furnace body 12, a tapered extension tube 1 having an inner diameter smaller toward the lower side is formed.
7 and a flange portion 19 are superposed on a flange portion 18 at the lower end of the extension cylinder 17, and a tapered cap 20 having a smaller inner diameter toward the lower side is provided so as to project downward. The optical fiber 22 is drawn out from the lower end opening 21 of the 20.

The core tube 14 in the present embodiment has a cylindrical portion 24 having an upper end opening 23 into which the optical fiber preform 13 is fed through the inlet pipe portion 16 and an upper end connected to the cylindrical portion 24. The first taper portion 25 has a smaller inner diameter toward the lower side and surrounds the lower end of the optical fiber preform 13.
And a second taper portion 26 whose upper end is connected to the lower end of the first tapered portion 25 and whose lower end is connected to the upper end of the extension cylinder 17. The inner peripheral surface of the second taper portion 26 and the inner peripheral surface of the extension cylinder 17 are set such that the deviation of the inner diameters of these connecting portions is, for example, 1 mm or less so as to form one tapered surface as a whole. ing. Therefore, it is naturally possible to form the extension cylinder 17 integrally with the second taper portion 26 and configure the extension cylinder 17 as a part of the core tube 14. The inclination angle of the inner peripheral surface of the second taper portion 26 with respect to the inner peripheral surface of the cylindrical portion 24 is less than the inclination angle of the inner peripheral surface of the first taper portion 25 and the inner peripheral surface of the base 20. Is set to. The flow rate of the inert gas flowing out from the lower end opening 21 of the die 20 is determined in consideration of the flow velocity so that the atmosphere outside the furnace does not enter the core tube 14 from the lower end opening 21 of the die 20.

The first taper portion 25 and the second taper portion 2
6 and the inner peripheral surface of the extension cylinder 17 can be formed into a complicated curved surface corresponding to the contour shape of the heating and melting portion of the lower end of the optical fiber preform 13, but each is practically simple. It is sufficient if it is composed of a conical surface.

On the other hand, the base 20 in the present embodiment has a split structure along the axial direction. And
When the optical fiber preform 13 is drawn, the extension cylinder 17 is previously used.
The base 20 is detached from the base 20, and after the glass is melted from the lower end of the base material 13 for optical fiber, that is, the seeds are dropped, and the wire drawing work is started.
The flange portion 19 is connected to the flange portion 18 of the extension cylinder 17. That is, in order to prevent the drop seed from colliding with the lower end opening 21 of the base 20 when the drop seed is dropped, the base 20 has a split structure.

Therefore, the inside of the furnace body 12 is kept warm by the heat insulating material 11, and the lower end portion of the optical fiber preform 13 fed into the furnace core tube 14 is heated and melted by the heater 15 in the furnace body 12 to produce the optical fiber. It becomes 22 and is pulled out from the lower end opening 21 of the base 20. Further, the inside of the core tube 14 is kept in an inert gas atmosphere by the inert gas supplied from above the core tube 14, thereby preventing oxidation of the core tube 14 and keeping the inside of the core tube 14 clean. . The inert gas flows downward along the gap between the inner peripheral surface of the furnace core tube 14 and the outer peripheral surfaces of the optical fiber preform 13 and the optical fiber 22, and is discharged from the lower end opening 21 of the mouthpiece 20 to the outside of the furnace. To be done.

Here, in order to investigate the effect of the present invention, FIG.
In the embodiment shown in FIG. 2, the heating and melting portion of the optical fiber preform 13 having an inner diameter of 90 mm and a diameter of 4.5 mm at the upper ends of the cylindrical portion 24 and the first tapered portion 25, and the first tapered portion 2
The length of the first taper portion 25 (the height in the vertical direction in FIG. 1) is 50 mm and is extended from the upper end of the second taper portion 26 so that the upper end of the second taper portion 26, which is the lower end of 5, is opposed. Cylinder 17
600mm to the bottom of the base, the length of the base 20 is 50mm
Helium gas is used as the inert gas, and when converted to the standard condition of 0 ° C and 1 atm, it is 10 minutes per minute.
It is supplied at a rate of liter and the diameter is 72
An optical fiber 22 having a diameter of 125 μm was drawn at a rate of 600 m / min from the optical fiber preform 13 of mm. Then, the lower end of the first taper portion 25, that is, the second taper portion 26.
100mm below the upper edge of the point, and 4 from this point
The temperature of the inert gas at the point B located 80 mm below and 20 mm above the upper end of the base 20, which is the lower end of the extension cylinder 17, was 1550 ° C. and 810 ° C., respectively.
° C. Based on this result, the inner diameter of the lower end of the first tapered portion 25, that is, the upper end of the second tapered portion 26 is 46 mm, the inner diameter of the lower end of the extension cylinder 17 and the upper end of the base 20 is 30 mm,
The inner diameter of the lower end opening 21 of the base 20 was set to 10 mm. When the optical fiber 22 was drawn again under the same conditions, the outer diameter was 125 ± 0.10 to 0.15 μm, which was within a very small fluctuation range.

The second taper portion 2 at the point A in this case
The inner diameter of 6 is 43.4 mm, the inner diameter of the second taper portion 26 at the B point is 30.6 mm, and the inner diameter of the second taper portion 26 on the A point side that is the same velocity as the flow velocity of the inert gas at the B point. Is A ~
If it is considered that the distribution of the absolute temperature of the inert gas between B changes at a constant rate, it becomes 39.8 mm, so the flow velocity of the inert gas at the point B is better than at the point A. It is theoretically clear that it is a little larger.

In order to confirm the effect of claim 8 of the present invention, the length of the cylindrical portion 24 is extended by 40 mm and the connecting portion between the first taper portion 25 and the second taper portion 26 is changed from the state shown in FIG. As a result of drawing the optical fiber 22 by lowering it by 40 mm, it was found that the outer diameter was 125 ± 0.2 to 0.3 μm and the fluctuation range increased. The diameter of the portion of the optical fiber preform 13 facing the connecting portion between the first taper portion 25 and the second taper portion 26 at this time was 2.5 mm.

Further, for comparison, the conventional optical fiber drawing furnace shown in FIG. 3 was used, and the optical fiber 109 was drawn under substantially the same conditions as those of the embodiment shown in FIG. However, the inner diameter of the base 107 and the small cylindrical portion 112 was set to 20 mm, and the inner diameter of the lower end opening 108 of the base 107 was set to 10 mm. As a result, the outer diameter of the obtained optical fiber 109 is 125 ± 0.
It was found to vary within the range of 3 to 0.4 μm.

In the embodiment shown in FIG. 1, the lower part of the core tube is formed in a tapered shape, but the same effect can be obtained by using a taper tube separate from the core tube.

As shown in FIG. 2 showing the cross-sectional structure of another embodiment of the optical fiber drawing furnace according to the present invention, a furnace body 32 having a heat insulating material 31 incorporated therein is provided with an optical fiber mother board. A cylindrical core tube 34 into which the material 33 is fed, and an annular heater 35 that surrounds the central portion of the core tube 34 and heats and melts the lower end of the optical fiber preform 33 are assembled. Further, a cylindrical inlet pipe portion 36 is provided so as to project upward at the center of the upper end of the furnace body 32, and the upper end portion of the inlet pipe portion 36 is connected to the core tube through the inlet pipe portion 36. An unillustrated inert gas supply device for downwardly supplying an inert gas such as helium or nitrogen is assembled in the unit 34. Further, at the center of the lower end of the furnace body 32, a tapered tube receiver 38 having a gas discharge port 37 is provided in a protruding manner. The bottom plate 39 of the tapered tube receiver 38 has a tapered tube 40 having an upper end facing the lower end of the optical fiber preform 33 and an inner diameter gradually decreasing toward the lower side, and a tapered shape continuous with the tapered tube 40. Thus, the taper connecting pipe 41 protruding downward from the bottom plate 39 is attached. A flange portion 42 formed at the lower end of the taper connecting pipe 41 is connected to a flange portion 44 formed at the upper end of a tapered base 43 having a smaller inner diameter toward the lower side. The fiber 46 is adapted to be drawn out.

In the present embodiment, the inclination angle of the inner peripheral surface of the taper pipe 40 and the inclination angle of the inner peripheral surface of the taper connecting pipe 41 are set equal to each other, so that they form one taper surface as a whole. The deviation of the inner diameters of these connecting portions is set to, for example, 1 mm or less. Similarly, the displacement of the inner diameter of each of the connecting portions of the taper connecting pipe 41 and the base 43 is set to, for example, 1 mm or less. Then, similarly to the previous embodiment, the inclination angle of the inner peripheral surface of the mouthpiece 43 is set to be larger than the inclination angle of the inner peripheral surfaces of the taper pipe 40 and the taper connecting pipe 41.

Although the taper pipe 40 and the taper connection pipe 41 are formed as separate parts in this embodiment, they may be formed as a single part so that the bottom plate 39 can pass therethrough.
It is also possible to form the inner peripheral surface of the taper tube 40 into a complicated curved surface corresponding to the contour shape of the heating and melting portion of the lower end portion of the optical fiber preform 33, but practically a simple cone. It is enough if it is composed of planes.

Therefore, the inside of the furnace body 32 is kept warm by the heat insulating material 31, and the lower end portion of the optical fiber preform 33 fed into the furnace core tube 34 is heated and melted by the heater 35 in the furnace body 32, and the optical fiber is It becomes 46 and is pulled out from the lower end opening 45 of the base 43. In addition, the inert gas supplied from above the core tube 34 keeps the inside of the core tube 34 in an inert gas atmosphere, prevents oxidation of the core tube 34, and keeps the inside of the core tube 34 clean. . A part of this inert gas is discharged to the outside of the furnace from the gas discharge port 37 of the taper pipe receiver 38, and the rest of the inert gas is formed on the inner peripheral surfaces of the taper pipe 40 and the taper connecting pipe 41 and the optical fiber preform 33. And it flows downward along the gap between the outer peripheral surface of the optical fiber 46 and is discharged from the lower end opening 45 of the base 43 to the outside of the furnace.

Here, in order to investigate the effect of the present invention, FIG.
In the embodiment shown in FIG.
The dimension and shape of the inner peripheral surface in the assembled state are set to be equal to the dimension and shape of the inner peripheral surface in the state in which the second taper portion 26 and the extension cylinder 17 in the above-described embodiment are assembled, and the cap 43 is set first. The size and shape are set to be the same as those of the base 20 in the embodiment of FIG.
And optical fiber preform 33 with a diameter of 4.5 mm
The upper and lower positions of the taper tube 40 with respect to the furnace body 32 are set so that the heated and melted part of the taper tube and the upper end of the taper tube 40 face each other, and helium gas is used as an inert gas.
It is supplied at a rate of 20 liters per minute when converted to standard conditions of ℃ and 1 atm, and the optical fiber base material 13 with a diameter of 72 mm and the optical fiber 46 with a diameter of 125 μm are supplied at a rate of 600 m / min. I delineated.

In this case, a flow rate adjusting valve (not shown) attached to the gas outlet 37 adjusts so that about 50% of the inert gas flows through the tapered tube 40 and flows out of the furnace from the lower end opening 45 of the die 43. did. Then, a point C 100 mm below the upper end of the taper tube 40, and a point 48 from this point C
The base 4 which is 0 mm downward and is the lower end of the taper connecting pipe 41.
The temperature of the inert gas at point D located 20 mm above the upper end of 3 was measured to be 1630 ° C, 8
It was 30 ° C. Further, the outer diameter of the drawn optical fiber 46 could be kept within a very small fluctuation range of 125 ± 0.1 μm.

In order to confirm the effect of the third aspect of the present invention, when the optical fiber 46 is drawn by using the taper tube 40 shown in FIG.
It was found that the fluctuation range increased to 125 ± 0.2 to 0.3 μm in the outer diameter. In this case, the diameter of the heating and melting portion of the optical fiber preform 33 facing the upper end of the tapered tube 40 is 2 mm.

[0043]

According to the present invention, the inner diameter of the heating / melting portion and the inner peripheral surface of the taper tube are set so that the taper tube surrounding the heating / melting portion of the optical fiber preform and the optical fiber below the heating / melting portion is set smaller toward the lower side. Since the flow of the inert gas between and is stabilized, the fluctuation of the outer diameter of the drawn optical fiber can be suppressed more than the conventional one.

[Brief description of drawings]

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

FIG. 2 is a sectional view showing a schematic structure of another embodiment of the optical fiber drawing furnace according to the present invention.

FIG. 3 is a sectional view showing a schematic structure of an example of a conventional optical fiber drawing furnace.

[Explanation of symbols]

 11 Heat Insulation Material 12 Furnace Body 13 Optical Fiber Base Material 14 Core Tube 15 Heater 16 Inlet Tube Part 17 Extension Cylinder 18, 19 Flange Part 20 Base 21 Lower End Opening 22 Optical Fiber 23 Upper End Opening 24 Cylindrical Part 25 First Tapered Part 26 No. Two taper parts 31 Heat insulating material 32 Furnace body 33 Optical fiber base material 34 Core tube 35 Heater 36 Inlet tube part 37 Gas discharge port 38 Tapered tube receiver 39 Bottom plate 40 Tapered tube 41 Tapered connecting tube 42 Flange part 43 Base 44 Flange part 45 Bottom opening 46 Optical fiber

Claims (11)

[Claims] 1. A step of heating and melting the lower end of the optical fiber preform, a step of continuously drawing out the optical fiber from the lower end of the heated and melted optical fiber preform, and a lower end of the optical fiber preform. A step of passing the optical fiber drawn out from the lower end of the optical fiber base material through a taper tube having a smaller inner diameter toward the lower end side, and a step of flowing an inert gas from the upper end part to the lower end part of the taper tube. An optical fiber drawing method comprising: 2. The optical fiber drawing method according to claim 1, wherein the flow velocity of the inert gas flowing in the taper cylinder is higher toward the lower end side of the taper cylinder. 3. The diameter of the optical fiber in the portion facing the upper end of the tapered barrel is 3 mm or more.
Alternatively, the optical fiber drawing method according to claim 2. 4. A core tube in which an optical fiber preform is fed from the upper end and the optical fiber is pulled out from the lower end,
An inert gas supply means for flowing an inert gas from the upper end portion to the lower end portion of the core tube and the lower end portion of the optical fiber preform with the core tube sandwiched therebetween are heated and melted. And a furnace body that holds the heater and the core tube, wherein the lower end of the core tube faces the heated and melted portion of the optical fiber base material, or the optical fiber base material. A cylindrical portion having a constant inner diameter corresponding to the outer diameter of the optical fiber preform, and a lower inner diameter located below the cylindrical portion. An optical fiber drawing furnace having a taper tube portion having a smaller diameter toward the side. 5. A base having a smaller inner diameter is projected from the lower end of the tapered cylinder, and the rate of change of the inner diameter of the base is the same as the rate of change of the inner diameter of the tapered cylinder, or The fiber optic drawing furnace according to claim 4, wherein the fiber optic drawing furnace is set to have more than that. 6. The optical fiber drawing furnace as set forth in claim 5, wherein the die is separable by itself and is attachable to and detachable from the tapered cylindrical portion. 7. A first taper tube portion whose taper tube portion mainly surrounds a heated and melted portion of a base material for an optical fiber, and a second taper tube portion which is below the taper tube portion and mainly surrounds the optical fiber. The optical fiber drawing furnace according to claim 4, characterized in that the rate of change of the inner diameter of the first tapered tube portion is larger than the rate of change of the inner diameter of the second tapered tube portion. . 8. The optical fiber line according to claim 7, wherein the diameter of the optical fiber at the portion facing the connecting portion between the first tapered tubular portion and the second tapered tubular portion is 3 mm or more. Evacuation furnace. 9. A cylindrical core tube in which an optical fiber preform is fed from the upper end and a lower end is closed by a bottom plate, and a core tube that penetrates through the central portion of the bottom plate of the core tube and has a smaller inner diameter toward the lower side. And a taper cylinder whose upper end is opposed to the lower end of the optical fiber preform and whose optical fiber is pulled out from the lower end, and the lower end of the tapered cylinder and the lower end of the core tube from the upper end of the core tube An inert gas supply means for causing an inert gas to flow toward a gas outlet formed in, and a lower end portion of the optical fiber preform with at least the core tube sandwiched between the core tube and the tapered tube. An optical fiber drawing furnace comprising: a heater for heating and melting the same; and a furnace body for holding the heater, the core tube, and the tapered tube. 10. A lower end of the taper cylinder is provided with a mouthpiece having a smaller inner diameter toward the lower side. The rate of change of the inner diameter of the mouthpiece is equal to or higher than the rate of change of the inner diameter of the taper tube. The optical fiber drawing furnace according to claim 9, characterized in that 11. The optical fiber drawing furnace according to claim 10, wherein the base is itself separable and detachable from the tapered barrel.