JP2003267745A - Method and apparatus for drawing optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for drawing an optical fiber capable of spinning the optical fiber having reduced transmission loss and small variation of the outer diameter. <P>SOLUTION: A furnace bottom tube 5 is provided successively at the lower end of a furnace core tube 2. An optical fiber preform 10 is suspended inside the furnace core tube 2 and the tip of the optical fiber preform 10 is melted by heating to reduce the diameter. The diameter reduced part 11 is positioned in the furnace bottom tube 5 and drawn upto the outer diameter of an optical fiber 12 while slowly cooling the diameter reduced part 11 by flowing a temperature adjusting-gas in the furnace bottom tube 5. The drawing conditions are controlled such that the diameter reduced part 11 reaches the target outer diameter of the optical fiber 12 within the furnace bottom tube 5 and the Reynolds number R of the temperature adjusting-gas flow is 1,000 or less. <P>COPYRIGHT: (C)2003,JPO

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 and device used for drawing an optical fiber by heating and melting an optical fiber preform, and more particularly to reducing transmission loss of the optical fiber. The present invention relates to an optical fiber drawing method and apparatus capable of suppressing fluctuations in outer diameter to a small extent.

[0002]

2. Description of the Related Art Conventionally, an optical fiber is manufactured by heating and melting an optical fiber preform in a spinning furnace and drawing (spinning) the fiber. When the drawn optical fiber is exposed to cold ambient air, the glass is rapidly cooled and solidifies while retaining the hot liquid structure. Then, the density fluctuation and the density fluctuation of the glass become large, and the transmission loss due to Rayleigh scattering may increase. Therefore, the optical fiber after drawing is gradually cooled.

For example, Japanese Laid-Open Patent Publication No. 10-218635 discloses a method for reducing Rayleigh scattering by heat-treating an optical fiber and gradually cooling it in a temperature range determined based on the viscosity of glass. . Also, Japanese Patent Application 2001-
In 374320, a cylindrical lower furnace tube is provided in the lower part of the spinning furnace so as to be integrated with the spinning furnace, and an optical fiber after drawing is inserted into the lower furnace tube to adjust the temperature in the lower furnace tube. There has been proposed an optical fiber manufacturing apparatus in which the optical fiber is gradually cooled by flowing a working gas.

[0004]

However, when the drawn optical fiber is gradually cooled by using the furnace lower tube, the outer diameter of the obtained optical fiber greatly varies with respect to the target outer diameter. There is a problem that there is. The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical fiber drawing method and apparatus capable of spinning an optical fiber with reduced transmission loss and small fluctuation in outer diameter. And

[0005]

A method of manufacturing an optical fiber element wire according to the present invention for solving the above-mentioned problems comprises a lower furnace tube connected to a lower end of a core tube, and an optical fiber preform placed inside the core tube. The diameter of the tip end of the optical fiber preform is reduced by heating and melting the optical fiber preform, and the reduced diameter part is made to face the inside of the furnace lower tube, and a temperature adjusting gas is flowed in the furnace lower tube. A method of drawing an optical fiber in which the reduced diameter portion is gradually cooled and drawn until the target outer diameter of the optical fiber is achieved,
The drawing condition is controlled so that the reduced diameter portion reaches the target outer diameter of the optical fiber in the furnace lower tube and the Reynolds number of the flow of the temperature adjusting gas is 1000 or less.

As is well known, the Reynolds number is a dimensionless value representing the ratio of the inertial force and the viscous force of a fluid in a flow. It can be said that when the Reynolds number is equal to or lower than a predetermined critical value, the flow becomes a laminar flow and flows in an orderly manner, and when the Reynolds number exceeds the critical value, a turbulent flow easily occurs.

In the present invention, the Reynolds number R of the flow of the temperature adjusting gas flowing in the lower furnace tube is defined by the inner diameter of the lower furnace tube d, the flow rate of the temperature adjusting gas v, and the kinematic viscosity of the temperature adjusting gas. Is represented by R = vd / η.

Then, the reduced diameter portion is made to reach the target outer diameter of the optical fiber in the furnace lower tube, and the portion from the reduced diameter portion to the target outer diameter of the optical fiber is gradually cooled by the temperature adjusting gas. . Then, the Reynolds number R of the flow of the temperature adjusting gas is set to 1000 or less. As a result, the optical fiber can be cooled slowly and the flow of the temperature adjusting gas is stabilized, so that the transmission loss of the optical fiber can be reduced and the fluctuation of the outer diameter can be suppressed to a very small level. can do.

In order to set the Reynolds number of the flow of the temperature adjusting gas to 1000 or less, for example, the Reynolds number of the flow of the temperature adjusting gas is 1 as the furnace lower tube.
000 or less, and / or at least two kinds of gases are prepared as the temperature adjusting gas, and the mixing ratio and the temperature thereof are changed to change the flow of the temperature adjusting gas. Reynolds number is 100
It is possible to use a method in which it becomes 0 or less.

Further, according to the present invention, there is provided a core tube for accommodating the optical fiber preform, and a heater installed on the outer periphery of the core tube for heating and melting the tip portion of the optical fiber preform to reduce its diameter. An optical fiber drawing apparatus comprising: a furnace lower tube connected to a lower end of the furnace core tube; and a temperature adjusting gas supply means for supplying a temperature adjusting gas into the furnace lower tube, The Reynolds number of the temperature adjusting gas flow is 1 in the furnace lower tube according to the drawing speed of the optical fiber.
(EN) Provided is an optical fiber drawing device which can be exchanged into a predetermined shape so as to be 000 or less.

Further, according to the present invention, a core tube for accommodating the optical fiber preform, a heater for heating and melting the tip portion of the optical fiber preform to reduce the diameter, and a lower end of the core tube are connected. The furnace lower tube provided, a gas mixing means for mixing at least two kinds of gases at a predetermined ratio to obtain a temperature adjusting gas, and a mixing ratio of the gas mixed by the gas mixing means are set to the temperature adjusting means. Drawing of an optical fiber provided with gas mixing ratio control means for controlling the Reynolds number of the working gas to be 1000 or less, and temperature adjusting gas supplying means for supplying the temperature adjusting gas into the furnace lower tube. Provide a device. By using such a drawing device, the Reynolds number of the flow of the temperature adjusting gas can be easily adjusted, so that the optical fiber drawing method of the present invention can be more easily implemented.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail based on the embodiments. FIG. 1 is a schematic view showing an example of an apparatus used in the optical fiber drawing method of the present invention. In FIG. 1, reference numeral 1 is a spinning furnace main body. A core tube 2 is inserted through the spinning furnace body 1. The core tube 2 is generally a tube made of carbon.

The inner diameter of the core tube 2 is not particularly limited and is determined according to the outer diameter of the optical fiber preform 10. Generally, for example, the outer diameter of the optical fiber preform 10 is φ.
In the case of 80 to 90 mm, the range is 100 to 120 mm. The wall thickness of the core tube 2 is generally 1 to 10 m.
m. If the wall thickness of the furnace core tube 2 is less than 1 mm, the strength is lowered and the handleability is deteriorated, which is not preferable. Although the upper limit of the wall thickness of the core tube 2 is not particularly limited,
It is wasteful and uneconomical to make it thicker than necessary, so 1
It is preferably 0 mm or less.

A heater 3 is attached to the outer periphery of the core tube 2 so that the tip of the optical fiber preform 10 suspended in the core tube 2 can be heated and melted. Further, an inert gas supply means 4 for supplying an inert gas into the core tube 2 is provided on the upper portion of the spinning furnace body 1. By supplying the inert gas into the core tube 2, it is possible to suppress the deterioration of the core tube 2 due to the oxidation and the generation of dust, which is preferable. Helium, argon, nitrogen, or the like can be used as the inert gas supplied into the furnace core tube 2.

A lower furnace tube 5 is connected to the lower end of the furnace core tube 2 and is drawn out from the optical fiber preform 10 to reduce the diameter of the optical fiber 12 to a target outer diameter.
Faces the inside of the furnace lower tube 5. Lower furnace tube 5
Has a substantially cylindrical shape, and heat-resistant materials such as carbon and quartz glass are used as the material. Its dimensions are
As will be described later, the diameter-reduced portion 11 does not project from the lower end of the furnace lower tube 5 and must be set to an appropriate value so that a desired Reynolds number can be obtained. Is 20 to 150 mm, the inner diameter is 10 to 125 mm, and the length is 100 mm or more. However, the inner diameter of the lower furnace tube 5 does not necessarily have to match the inner diameter of the core tube 2. When the inner diameter of the lower furnace tube 5 does not match the inner diameter of the core tube 2, for example, a taper portion or a flange portion is formed at one end of the lower furnace tube 5, and the core tube 2 is inserted through the tapered portion or the flange portion. It can be made to connect to. The lower furnace tube 5 is preferably attached to the spinning furnace main body 1 via a screw or the like so that it can be easily removed and replaced.

Then, a temperature adjusting gas supply means 6 for supplying a temperature adjusting gas into the lower furnace tube 5 is installed. A temperature control device 7 for controlling the temperature of the temperature adjusting gas supplied into the lower furnace tube 5 is attached to the temperature adjusting gas supply means 6.

By supplying the temperature-controlled gas for temperature control to the lower furnace tube 5 to form a flow of the temperature-adjusting gas in the lower furnace tube 5, the reduced diameter portion 11 and the optical fiber 12 are formed.
Can be slowly cooled and the cooling rate can be adjusted. It also has an effect of preventing dust from adhering to the reduced diameter portion 11 and the optical fiber 12. In order to enhance the slow cooling effect, a heat insulating material, a heating device, or the like may be arranged around the furnace lower tube 5 so that the temperature does not easily decrease.

As the temperature adjusting gas, a gas having a low thermal conductivity, such as helium, argon or nitrogen, can be used. Moreover, you may use the gas obtained by mixing these 2 types or more. It is particularly preferable to use argon, which has a small thermal conductivity. The temperature of the temperature adjusting gas may be changed as necessary so as to obtain a desired cooling rate, but it is preferably room temperature to 1200 ° C.

This manufacturing apparatus is preferably provided with an exhaust means 8 for exhausting the inert gas supplied into the core tube 2 and the temperature adjusting gas supplied into the lower furnace tube 5. The mounting position of the exhaust means 8 is not particularly limited, and taking into consideration the opening positions of the inert gas supply means 4 and the temperature adjustment gas supply means 6, an appropriate position in which the flow of the inert gas or the temperature adjustment gas is less likely to be disturbed. It is preferable to provide it at, for example, above the heater 3.

In the present embodiment, firstly, by appropriately selecting the inner diameter and length of the furnace lower tube 5 and controlling the type and temperature of the temperature adjusting gas, as shown in FIG. As shown in, the reduced diameter portion 11 reaches the target outer diameter of the optical fiber 12 in the furnace lower tube 5. As shown in FIG. 2B, when the tip of the reduced diameter portion 11 comes to project from the lower end of the furnace lower tube 5, the portion of the optical fiber 12 before reaching the target outer diameter is exposed to the outside air and rapidly cooled. Therefore, the fluctuation of the outer diameter becomes large, which is not preferable.

Secondly, the optical fiber 12 is drawn by changing the drawing conditions so that the Reynolds number R of the flow of the temperature adjusting gas flowing in the furnace lower tube 5 is 1000 or less. The drawing conditions include the drawing speed, the inner diameter and length of the furnace lower tube 5, or the type of temperature adjusting gas,
Examples include temperature and flow rate.

In the present invention, the Reynolds number R of the flow of the temperature adjusting gas flowing in the lower furnace tube 5 is expressed by the equation R = v.
It is defined by d / η. Here, v is the flow velocity (m / s) of the temperature adjusting gas, d is the inner diameter (m) of the furnace lower tube 5, and η is the kinematic viscosity (m ² / m ² ) of the temperature adjusting gas.
s).

By setting the Reynolds number R of the flow of the temperature adjusting gas to 1000 or less, the flow of the temperature adjusting gas hardly becomes turbulent and is stabilized.
The fluctuation of the outer diameter of No. 2 is suppressed to be small. When the Reynolds number R becomes large, the flow of the temperature adjusting gas becomes turbulent easily,
When R exceeds 1000, the fluctuation of the outer diameter of the optical fiber 12 tends to increase, which is not preferable.

In order to set the Reynolds number R of the temperature adjusting gas flow to 1000 or less, the manufacturing conditions are changed as follows, for example. First, since the flow velocity v generally increases by increasing the drawing speed, the Reynolds number R increases. On the other hand, for example, the Reynolds number R can be reduced by narrowing the inner diameter d of the furnace lower tube 5. Therefore, before starting the drawing, a furnace lower tube 5 having an appropriate inner diameter d is selected and attached to the apparatus. Also, the length of the furnace lower tube 5 is
1 is appropriately selected so that 1 reaches the target outer diameter of the optical fiber 12 in the furnace lower tube 5.

Further, the kinematic viscosity η of the temperature adjusting gas is the viscosity μ (Pa · s) and the density ρ (kg / m of the temperature adjusting gas.
³ ) and the ratio μ / ρ. Generally, when the temperature of a gas increases, the viscosity μ increases and the density ρ decreases, so that the kinematic viscosity η increases. Therefore, the Reynolds number R can be decreased by increasing the temperature of the temperature adjusting gas. Moreover, by increasing the temperature of the temperature adjusting gas, the cooling rate of the reduced diameter portion 11 can be reduced and the gradual cooling effect can be enhanced. Therefore, it is preferable that the temperature of the temperature adjusting gas can be changed from room temperature to 1200 ° C.

Regarding the kind of the temperature adjusting gas, when helium and argon are compared, helium has a kinematic viscosity about 10 times larger than that of argon. Therefore, in order to reduce the Reynolds number R of the flow of the temperature adjusting gas, preferable. However, the thermal conductivity of helium is about 10 times that of argon.
Since it is twice as large, it is inferior to argon in terms of the slow cooling effect.

If a single gas is used as the temperature adjusting gas, the structure of the apparatus becomes simpler and the operation becomes easier. However, in order to obtain a better optical fiber, it is preferable to use a mixed gas prepared by appropriately mixing a plurality of gases in consideration of the desired transmission loss and the degree of outer diameter fluctuation. For example, when using a gas in which helium and argon are mixed at a ratio of 1: 5,
The thermal conductivity is 3.73 × 10 ⁻² W · m ⁻¹ · K ⁻¹ and the kinematic viscosity is 2.71 × 10 ⁻⁵ m ² / s.

When a mixed gas is used as the temperature adjusting gas, for example, a gas mixed in advance in a predetermined ratio is used.
A method of supplying this mixed gas to the furnace lower tube 5 via the temperature adjusting gas supply means 6 can be used. Further, as shown in FIG. 3, gas mixing means 21 for mixing at least two kinds of gases at a predetermined ratio to obtain a temperature adjusting gas,
The Reynolds number of the temperature control gas flow is 100 when the mixing ratio of the gases mixed by the gas mixing means is 100.
Gas mixing ratio control means 22 for controlling to be 0 or less
It is also possible to employ a method of preparing a mixed gas in parallel with the drawing work by using a drawing device having

In this case, as a method of controlling the gas mixing ratio, for example, a computer or the like is used to calculate the kinematic viscosity of the mixed gas based on experimental data, an approximate expression, etc. Calculate the Reynolds number R of the temperature control gas flow from the furnace lower tube dimensions, other drawing conditions such as drawing speed, etc. to find the optimum mixing ratio, and based on the found optimum value, control valve etc. It can be used to control the actual gas mixing ratio. As a result, the temperature adjusting gas having an excellent balance between the kinematic viscosity and the thermal conductivity can be constantly supplied, so that it is possible to draw an optical fiber in which both the transmission loss and the outer diameter variation are extremely suppressed.

In the manufacturing method of the present invention, the vapor phase axial method (V
AD method), external attachment method (OVD method), internal attachment method (CVD
Method, MCVD method, PCVD method), rod-in-tube method, or other optical fiber preform. The present invention can be applied to any type of optical fiber such as a single mode optical fiber, a dispersion shift optical fiber, a cutoff shift optical fiber and a dispersion compensating optical fiber.

Next, an example of a procedure for drawing the optical fiber 12 will be described. First, the lower furnace tube 5 having an appropriate inner diameter and length is selected according to the target drawing speed during operation and attached to the manufacturing apparatus. After suspending the optical fiber preform 10 inside the core tube 2, the tip of the optical fiber preform 10 is heated and melted to reduce its diameter. Next, the reduced diameter portion 11
The diameter-reduced portion 11 is gradually cooled by facing the inside of the furnace lower tube 5 and flowing a temperature adjusting gas into the furnace lower tube 5, and is drawn so as to have the outer diameter of the optical fiber 12. At this time, the cooling rate of the reduced diameter portion 11 is adjusted by controlling the temperature and flow rate of the temperature adjusting gas so that the reduced diameter portion reaches the target outer diameter of the optical fiber in the furnace lower tube, and The Reynolds number of the temperature adjusting gas flow is set to 1000 or less. The optical fiber manufactured in this manner has a reduced transmission loss and an extremely small change in outer diameter.

The drawn optical fiber is formed by a conventional method.
An optical fiber element wire can be obtained by applying a urethane acrylate resin or the like and curing it to form a two-layer coating.

Next, the present invention will be specifically described with reference to test examples. Using the optical fiber preform 10 for a single mode optical fiber having an outer diameter of φ60 mm and a length of 1000 mm,
A single mode optical fiber was manufactured by setting the target outer diameter to 125 μm. Then, in the obtained optical fiber 12,
A urethane acrylate-based UV-curable resin is applied and cured to form a primary coating layer, and then a secondary coating layer is formed in the same manner. The primary coating layer has a coat diameter of 190 μm, and the secondary coating has a coating diameter of 250 μm. Optical fiber strands were manufactured.
At this time, the Reynolds number was changed by changing the manufacturing conditions. Moreover, the outer diameter variation of each optical fiber 12 during drawing was measured using an outer diameter measuring device.

Test Example 1 The optical fiber 12 was drawn with the spinning tension set to 100 gf and the spinning linear velocity set to 300 m / min. The length of the furnace lower tube 5 was 250 mm and the inner diameter was 60 mm. Helium was used as the temperature adjusting gas, and the temperature was normal temperature. At the time of drawing, the tip of the reduced diameter portion 11 was inside the furnace lower tube 5. The Reynolds number in the furnace lower tube 5 was about 200. The outer diameter variation of the optical fiber 12 during drawing was ± 0.2 μm.

Test Example 2 The optical fiber 12 was drawn at a spinning tension of 100 gf and a spinning linear velocity of 300 m / min. The length of the furnace lower tube 5 was 250 mm and the inner diameter was 60 mm. Argon was used as the temperature adjusting gas, and the temperature thereof was room temperature. At the time of drawing, the tip of the reduced diameter portion 11 was projected to the outside of the furnace lower tube 5 because the cooling rate became slow. Reynolds number in the lower furnace tube 5 is about 1800
Met. The outer diameter variation of the optical fiber 12 during drawing is ±
It was 0.5 μm.

(Test Example 3) The optical fiber 12 was drawn at a spinning tension of 100 gf and a spinning linear velocity of 300 m / min. The length of the furnace lower tube 5 was 500 mm and the inner diameter was 60 mm. Argon was used as the temperature adjusting gas, and its temperature was controlled to 200 ° C. by using the temperature controller 7. At the time of drawing, the tip of the reduced diameter portion 11 was inside the furnace lower tube 5. The Reynolds number in the furnace lower tube 5 was about 800. The outer diameter variation of the optical fiber 12 during drawing is ± 0.2 μ
It was m.

The results of Test Examples 1 to 3 are shown in Table 1.

[0038]

[Table 1]

Test Example 4 The optical fiber 12 was drawn with the spinning tension set to 130 gf and the spinning linear velocity set to 600 m / min. The length of the furnace lower tube 5 was 250 mm and the inner diameter was 60 mm. Helium was used as the temperature adjusting gas, and the temperature was normal temperature. At the time of drawing, the tip of the reduced diameter portion 11 was inside the furnace lower tube 5. The Reynolds number in the furnace lower tube 5 was about 400. The outer diameter variation of the optical fiber 12 during drawing was ± 0.2 μm.

Test Example 5 The optical fiber 12 was drawn with the spinning tension set to 130 gf and the spinning linear velocity set to 600 m / min. The length of the furnace lower tube 5 was 500 mm and the inner diameter was 60 mm. Argon was used as the temperature adjusting gas, and its temperature was controlled to 200 ° C. by using the temperature controller 7. At the time of drawing, the tip of the reduced diameter portion 11 was projected outside the furnace lower tube 5. Reynolds number in the furnace lower tube 5 is about 1500
Met. The outer diameter variation of the optical fiber during drawing is ± 0.8
was μm.

Test Example 6 The optical fiber 12 was drawn with the spinning tension set to 130 gf and the spinning linear velocity set to 600 m / min. The length of the furnace lower tube 5 was 1000 mm, and the inner diameter was 60 mm. Argon was used as the temperature adjusting gas, and its temperature was controlled to 200 ° C. by using the temperature controller 7. At the time of drawing, the tip of the reduced diameter portion 11 was inside the furnace lower tube 5. The Reynolds number in the furnace lower tube 5 was about 1500. The fluctuation of the outer diameter of the optical fiber 12 during drawing is ± 0.4 μm.
It was m.

(Test Example 7) The optical fiber 12 was drawn with the spinning tension set to 130 gf and the spinning linear velocity set to 600 m / min. The furnace lower tube 5 had a length of 1000 mm and an inner diameter of 50 mm. Argon was used as the temperature adjusting gas, and the temperature was controlled to 400 ° C. by using the temperature controller 7. At the time of drawing, the tip of the reduced diameter portion 11 was inside the furnace lower tube 5. The Reynolds number in the furnace lower tube 5 was about 600. The outer diameter variation of the optical fiber 12 during drawing is ± 0.2 μ
It was m.

The results of Test Examples 4 to 7 are shown in Table 2.

[0044]

[Table 2]

(Test Example 8) The optical fiber 12 was drawn with the spinning tension set to 180 gf and the spinning linear velocity set to 1200 m / min. The length of the furnace lower tube 5 was 250 mm and the inner diameter was 60 mm. Helium was used as the temperature adjusting gas, and the temperature was normal temperature. At the time of drawing, the tip of the reduced diameter portion 11 was projected outside the furnace lower tube 5. The Reynolds number in the furnace lower tube 5 was about 800. Optical fiber 12 being drawn
The outer diameter variation of was 0.5 μm.

Test Example 9 The optical fiber 12 was drawn with the spinning tension set to 180 gf and the spinning linear velocity set to 1200 m / min. The length of the furnace lower tube 5 was 500 mm, and the inner diameter was 10 mm. Helium was used as the temperature adjusting gas, and the temperature was normal temperature. At the time of drawing, the tip of the reduced diameter portion 11 was inside the furnace lower tube 5. The Reynolds number in the furnace lower tube 5 was about 30. The outer diameter variation of the optical fiber 12 during drawing was ± 0.2 μm.

(Test Example 10) The spinning tension was set to 180 gf,
The spinning fiber speed is set to 1200 m / min, and the optical fiber 12
Was drawn. The furnace lower tube 5 had a length of 500 mm and an inner diameter of 50 mm. Argon was used as the temperature adjusting gas, and the temperature was controlled to 400 ° C. by using the temperature controller 7. At the time of drawing, the tip of the reduced diameter portion 11 was projected outside the furnace lower tube 5. Reynolds number in the lower furnace tube 5 is about 120
It was 0. The outer diameter variation of the optical fiber 12 during drawing is ±
It was 1.0 μm.

(Test Example 11) The spinning tension was set to 180 gf,
The spinning fiber speed is set to 1200 m / min, and the optical fiber 12
Was drawn. The length of the furnace lower tube 5 is 1500 mm,
The inner diameter was 45 mm. Argon was used as the temperature adjusting gas, and the temperature was controlled to 400 ° C. by using the temperature controller 7. At the time of drawing, the tip of the reduced diameter portion 11 was inside the furnace lower tube 5. The Reynolds number in the furnace lower tube 5 was about 1000. The fluctuation of the outer diameter of the optical fiber 12 during drawing is ± 0.
It was 2 μm.

(Test Example 12) The spinning tension was set to 180 gf,
The spinning fiber speed is set to 1200 m / min, and the optical fiber 12
Was drawn. The furnace lower tube 5 has a length of 1000 mm,
The inner diameter was 20 mm. As the temperature adjusting gas, a gas in which argon and helium were mixed at a ratio of 5: 1 was used, and the temperature thereof was room temperature. At the time of drawing, the tip of the reduced diameter portion 11 was inside the furnace lower tube 5. Reynolds number in the lower furnace tube 5 is about 50
It was 0. The outer diameter variation of the optical fiber 12 during drawing is ±
It was 0.2 μm.

The results of Test Examples 8 to 12 are shown in Table 3.

[0051]

[Table 3]

Table 4 shows the relationship between the Reynolds number and the outer diameter variation obtained in this test example. In Table 4, the Reynolds number when the tip of the reduced diameter portion 11 projects outside the furnace lower tube 5 during drawing is considered to be infinite, considering that the pipe diameter around the tip of the reduced diameter portion 11 becomes very large. Considered to be

[0053]

[Table 4]

As is clear from the results shown in Table 4, the diameter-reduced portion 11 is protected so that the tip of the diameter-reduced portion 11 is not exposed to the outside air.
Is covered with a furnace lower tube 5 and a temperature adjusting gas is flown into the furnace lower tube 5 so that the Reynolds number of the flow of the temperature adjusting gas is 1000 or less, so that the temperature does not depend on the drawing speed of the optical fiber. It is understood that the fluctuation of the outer diameter of the optical fiber 12 can be suppressed to be small.

When the transmission loss at 1.55 μm was measured for each of the obtained optical fiber strands,
When the reduced diameter portion 11 reaches the target outer diameter in the furnace lower tube 5 during drawing and the Reynolds number of the temperature adjusting gas flow is 1000 or less, whichever optical fiber 1
In No. 2 as well, the transmission loss was 0.190 dB / km or less. Therefore, by using the manufacturing method of the present invention, the reduced diameter portion 1
It can be seen that the gradual cooling of No. 1 was effectively performed, and the increase in transmission loss due to Rayleigh scattering could be suppressed.

[0056]

As described above, the method of manufacturing an optical fiber according to the present invention uses the device in which the lower part of the furnace is connected to the lower end of the core tube, and the tip of the optical fiber preform suspended in the core tube is used. When the wire is drawn to the target outer diameter of the optical fiber while gradually reducing the diameter by heating and melting the portion, and allowing the reduced diameter portion to face the inside of the furnace lower tube and gradually cooling with the temperature adjusting gas, Since the diameter portion is designed to reach the target outer diameter of the optical fiber in the furnace lower tube, and the Reynolds number of the flow of the temperature adjusting gas flowing in the furnace lower tube is 1000 or less, Since the drawn-out reduced diameter portion can be effectively cooled slowly, and turbulence of the temperature adjusting gas is less likely to occur, transmission loss of the optical fiber can be reduced regardless of the drawing speed of the optical fiber. Minimize fluctuations in outer diameter It is possible.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of an apparatus used in an optical fiber drawing method of the present invention.

FIG. 2 is a diagram illustrating an example of a state in which the optical fiber preform is reduced in diameter to the outer diameter of the optical fiber.

FIG. 3 is a schematic configuration diagram showing another example of an apparatus used in the optical fiber drawing method of the present invention.

[Explanation of symbols]

2 ... Reactor core tube, 3 ... Heater, 5 ... Furnace lower tube, 6 ... Temperature adjusting gas supply means, 7 ... Temperature control device, 10 ... Optical fiber preform, 11 ... Reduced diameter section, 12 ... Optical fiber, 21 ... Gas mixing means, 22 ... Gas mixing ratio control means.

Claims (5)

[Claims] 1. A furnace lower tube is connected to the lower end of the furnace core tube, the optical fiber preform is suspended inside the core tube, and the tip of the optical fiber preform is heated and melted to reduce its diameter. The reduced diameter portion is exposed in the lower furnace tube, and while gradually reducing the reduced diameter portion by flowing a temperature adjusting gas into the lower furnace tube, the optical fiber is drawn until the target outer diameter of the optical fiber is reached. In the drawing method, the drawing condition is changed so that the reduced diameter portion reaches the target outer diameter of the optical fiber in the furnace lower tube and the Reynolds number of the temperature adjusting gas flow is 1000 or less. An optical fiber drawing method characterized by the above. 2. The optical fiber drawing method according to claim 1, wherein the furnace lower tube has a Reynolds number of 1000 or less in the flow of the temperature adjusting gas. 3. Reynolds number of the flow of the temperature adjusting gas is set to 1000 or less by preparing at least two kinds of gas as the temperature adjusting gas and changing the mixing ratio and the temperature thereof. The optical fiber drawing method according to claim 1 or 2, characterized in that. 4. A core tube for accommodating the optical fiber preform, a heater for heating and melting the tip of the optical fiber preform to reduce its diameter, and a heater connected to the lower end of the core tube. A furnace lower tube, and an optical fiber drawing device comprising a temperature adjusting gas supply means for supplying a temperature adjusting gas into the furnace lower tube, wherein the furnace lower tube is provided according to a drawing speed of the optical fiber. The optical fiber drawing device is replaceable with a predetermined shape so that the Reynolds number of the flow of the temperature adjusting gas is 1000 or less. 5. A furnace core tube for accommodating an optical fiber base material, a heater for heating and melting the tip end portion of the optical fiber base material to reduce its diameter, and a heater connected to the lower end of the furnace core tube. The furnace lower tube, a gas mixing means for mixing at least two kinds of gases at a predetermined ratio to obtain a temperature adjusting gas, and a mixing ratio of the gas mixed by the gas mixing means are set to the flow of the temperature adjusting gas. Reynolds number is 100
An optical fiber drawing device comprising: a gas mixing ratio control means for controlling the temperature to be 0 or less; and a temperature adjusting gas supply means for supplying a temperature adjusting gas into the furnace lower tube.