Optical fiber wire drawing furnace and optical fiber manufacturing method

Abstract

A wiredrawing furnace comprising: a furnace core pipe into which a glass base material for optical fiber is to be inserted; a furnace body which houses therein a heater for heating said glass base material for optical fiber from the outside; a lower chamber disposed below the furnace core pipe; and a protective pipe disposed below the lower chamber, wherein an inert gas is streamed from top to bottom within the furnace core pipe. Further, the wiredrawing furnace has: a gas introduction port through which a prescribed gas is introduced into the protective pipe; and a narrowed area which is disposed on the upstream side of the gas introduction port and which is provided with an interior space that has a radially-oriented cross-sectional area smaller than that of the lower chamber.

Description

Technical Field

[0001] The present disclosure relates to an optical fiber drawing furnace and an optical fiber manufacturing method. This application claims priority based on Japanese Patent Application No. 2020-051898 filed on March 23, 2020, and incorporates all the descriptions described in the above application.

Background Art

[0002] Patent Document 1 discloses an optical fiber manufacturing apparatus that supplies helium gas into a drawing furnace to draw an optical fiber and cools the drawn optical fiber in argon gas in a cooling pipe connected to the drawing furnace.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Means for Solving the Problems

[0004] An optical fiber drawing furnace according to an aspect of the present disclosure includes a core tube into which a glass preform for an optical fiber is inserted, a heater that heats the glass preform for the optical fiber from the outside, a furnace body that houses the core tube therein, a lower chamber disposed below the core tube, a protective tube disposed below the lower chamber, and a furnace gas introduction portion that flows an inert gas downward from above in the core tube, and is an optical fiber drawing furnace having a gas introduction port for introducing a predetermined gas into the protective tube, and having a constriction smaller than the radial cross-sectional area of the lower chamber on the upper side of the gas introduction port and the pressure inside the lower chamber above the constriction is maintained at a positive pressure higher than the pressure inside the protection tube below the constriction 。

[0005] Further, an optical fiber manufacturing method according to an aspect of the present disclosure is a method for manufacturing an optical fiber using the above optical fiber drawing furnace.

Brief Description of the Drawings

[0006]

Figure 1

Figure 2A

Figure 2B

Figure 3A

Figure 3B

Mode for Carrying Out the Invention

[0007] [Problems to be Solved by the Present Disclosure] In the drawing furnace, a lower chamber (lower chimney) and a protective tube are provided below the lower chamber, and the optical fiber is protected in the lower chamber and the protective tube so that the glass diameter of the optical fiber (glass fiber part) is stabilized. On the other hand, since SiO2 gas or the like is generated from the heated glass base material, in the lower chamber, the SiO2 gas or the like cools and silica (SiO2) powder is generated. The silica powder floats in the inert gas flowing through the drawing furnace, accumulates in the drawing furnace, is discharged out of the furnace from the lower chamber or the outlet of the protective tube of the drawing furnace, or contacts or collides with the optical fiber. When the glass fiber part during drawing contacts or collides with the silica powder floating in the gas in the drawing furnace, the strength of the optical fiber decreases. When the strength of the optical fiber decreases, the frequency of disconnection of the optical fiber increases in the fiber screening test (proof test) performed after the drawing is completed, and the productivity may decrease. In order to reduce the amount of silica powder generated from the glass base material, there is a method of lowering the drawing furnace temperature. However, since the drawing furnace temperature affects the desired glass tension during drawing and also depends on the size of the base material, it is difficult to simply lower the drawing furnace temperature. In addition, it is possible to increase the gas flow rate in the drawing furnace in order to reduce the concentration per unit gas flow rate of silica powder suspended in the gas in the drawing furnace, but this may increase the manufacturing cost.

[0008] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described. The optical fiber drawing furnace according to the present disclosure is (1) an optical fiber drawing furnace including a furnace tube into which an optical fiber glass preform is inserted, a heater for heating the optical fiber glass preform from the outside, a furnace body for accommodating the furnace tube, a lower chamber disposed below the furnace tube, a protective tube disposed below the lower chamber, and an in-furnace gas inlet for flowing an inert gas from above downward into the furnace tube, the furnace having a gas inlet for introducing a predetermined gas into the protective tube, and a narrowed portion smaller than a radial cross-sectional area of ​​the lower chamber above the gas inlet. has the pressure inside the lower chamber above the constriction maintained at a positive pressure higher than the pressure inside the protection tube below the constriction . This allows the concentration of silica powder per gas flow rate inside the protective tube to be reduced, thereby reducing the probability of silica powder coming into contact with or colliding with the optical fiber, and suppressing the occurrence of a decrease in the strength of the optical fiber.

[0009] (2) The protective tube may be connected to a lower portion of the lower chamber. This prevents the optical fiber from being exposed to the outside air between the lower chamber and the protective tube, and stabilizes the glass diameter.

[0010] (3) It is desirable that the gas inlet be provided from the center to the upper side of the protective tube. This makes it possible to reduce the concentration of silica powder per gas flow rate in most parts of the protective tube, thereby reducing the probability of silica powder coming into contact with or colliding with the optical fiber and suppressing the occurrence of a decrease in the strength of the optical fiber.

[0011] (4) A gas vent hole for discharging the inert gas in the lower chamber to the outside may be provided at the lower part of the lower chamber. This further reduces the probability of silica powder contacting or colliding with the optical fiber, and can suppress the occurrence of a decrease in the strength of the optical fiber.

[0012] (5) Gas may be forcibly exhausted from the gas vent hole. This further reduces the probability of silica powder contacting or colliding with the optical fiber, and can suppress the occurrence of a decrease in the strength of the optical fiber.

[0013] (6) It is desirable that the gas inlet is provided at the upper end of the protective tube. Thereby, the concentration of silica powder can be quickly reduced in the protective tube over almost the entire length of the protective tube, and the probability of silica powder contacting or colliding with the optical fiber is further reduced.

[0014] (7) The predetermined gas may be argon, nitrogen, or air. Thereby, by using an inexpensive gas, the manufacturing cost can be suppressed while suppressing the occurrence of a decrease in the strength of the optical fiber.

[0015] (8) The gas inlets may be provided at equal intervals in the circumferential direction of the protective tube. Thereby, since the gas hits the optical fiber evenly, it is possible to prevent the optical fiber from swaying in the protective tube, and to suppress the influence on the optical fiber diameter variation and the fiber bending (fiber curl) caused by temperature non-uniformity.

[0016] (9) The gas inlet may be provided downward so as to introduce the predetermined gas downward into the protective tube. Thereby, the turbulence of the gas flow in the protective tube can be suppressed.

[0017] (10) The gas inlet may be diagonally downward so as to introduce the predetermined gas spirally into the inside of the protective tube, and may be directed in a direction along the wall surface of the protective tube. This makes it easier to separate the silica powder from around the fiber. Also, it is possible to reduce the turbulence of the gas flow around the fiber.

[0018] A method for manufacturing an optical fiber according to one aspect of the present disclosure is ( 11 ) manufacturing an optical fiber using the above-described optical fiber drawing furnace. As a result, the concentration of silica powder per gas flow rate in the protective tube can be decreased, so the probability of the silica powder contacting or colliding with the optical fiber becomes low, and the occurrence of a decrease in the strength of the optical fiber can be suppressed.

[0019] [Details of Embodiments of the Present Disclosure] Hereinafter, with reference to the drawings, preferred embodiments of an optical fiber drawing furnace and an optical fiber manufacturing method according to the present disclosure will be described. In the following description, components denoted by the same reference numerals in different drawings may be the same, and the description thereof may be omitted. Note that the present disclosure is not limited to the examples in these embodiments, and includes all modifications within the scope of the matters described in the claims and within the equivalent scope. Also, as long as combinations of a plurality of embodiments are possible, the present disclosure includes combinations of any embodiments.

[0020] (First Embodiment) FIG. 1 is a schematic diagram of an optical fiber drawing furnace according to one aspect of the present disclosure. The optical fiber drawing furnace (hereinafter referred to as the "drawing furnace") 10 has a furnace body 11, an upper chamber 12 provided above the furnace body 11, and a lower chamber 13 provided below. The upper chamber 12 and the lower chamber 13 are hollow tube-shaped. Inside the furnace body 11, a heater 15 for heating and melting the glass base material 1 is arranged, and a cylindrical furnace core tube 14 is arranged so as to be surrounded by the heater 15. Between the heater 15 and the furnace body 11, a heat insulating material 16 is provided so as to surround the heater 15 in order to prevent the heat from the heater 15 from being dissipated to the outside. Note that the heater may use induction heating. Below the lower chamber 13, a protection tube 20, which will be described in detail later, is arranged. The protection tube 20 is preferably in close contact with and connected to the lower chamber 13, but there may be a gap between the protection tube 20 and the lower chamber 13 to some extent. Further, in the present embodiment, the configuration in which there is one lower chamber 13 and one protection tube 20 is described, but each of them may be divided into a plurality of pieces or may be integrated.

[0021] The drawing of the optical fiber 2 is a process of suspending the glass base material 1 into the furnace core tube 14 by a base material hoisting mechanism (not shown), heating the lower part of the glass base material 1 with the heater 15, and melting and dropping the optical fiber (glass fiber part) 2 from the lower end of the melted glass base material 1, and is performed so that the optical fiber 2 taken out from below the drawing furnace 10 has a predetermined outer diameter. In the furnace core tube 14, nitrogen introduced from the in-furnace gas introduction part 17, or an inert gas such as helium or argon is supplied from above downward. Since the inside of the furnace core tube 14 is in an inert gas atmosphere, oxidation of the furnace core tube 14, which is a carbon component, etc. can be prevented and the inside can be kept clean.

[0022] The inert gas introduced into the furnace core tube 14 is heated to approximately 2000 °C or higher in the furnace core tube 14. Then, a part of the heated inert gas is discharged to the outside together with the optical fiber 2 through the space inside the furnace core tube 14 by downflow, passing through the lower chamber 13 and the protection tube 20.

[0023] In this embodiment, the lower chamber 13 and the protective tube 20 are connected, and at the connection part, a constriction 13a of the lower chamber 13 and a constriction 20a of the protective tube 20, each having a cross-sectional area of the internal space smaller than that of the lower chamber 13, are formed. Further, directly below the constriction 20a of the protective tube 20, a gas inlet 21 is provided. The gas inlets 21 are provided, for example, at four locations at equal intervals in the circumferential direction of the protective tube 20. The constrictions 13a and 20a are for keeping the pressure inside the core tube 14 and the lower chamber 13 above these constrictions 13a and 20a positive with respect to the pressure inside the protective tube 20 below the constrictions 13a and 20a.

[0024] The constriction is not limited to being provided at the connection part between the lower chamber 13 and the protective tube 20 as long as it is on the upper side of the gas inlet 21, and it may be provided only on either one of the lower chamber 13 and the protective tube 20. Also, the gas inlet 21 is preferably provided at the upper end part (including not only the upper end but also the vicinity of the upper end) of the protective tube 20, but it may be provided on the upper side from the longitudinal center of the protective tube 20. In the case where the lower chamber 13 and the protective tube 20 are integrated, it is desirable to provide the gas inlet 21 near the central part thereof. Then, a gas such as argon, nitrogen, or air in a clean state and less expensive than helium is introduced into the gas inlet 21 from the outside.

[0025] In this embodiment, by reducing the concentration per unit gas flow rate of the silica powder floating in the inert gas in the drawing furnace 10 inside the protective tube 20 having the gas inlet 21, the probability that the optical fiber 2 during drawing contacts or collides with the silica powder is reduced.

[0026] For example, when an inert gas with a flow rate of Q1 slm (converted to Q1 liters per minute under standard conditions) is flowing downward in the core tube 14, and assuming that N silica powders are contained per Q1 liter, the concentration of the silica powder per gas flow rate in the lower chamber 13 is N / Q1. When a gas of Q2 slm is introduced into the protective tube 20 from the gas inlet 21 of the protective tube 20, the gas flow rate flowing from above to below through the protective tube 20 becomes Q3 (=Q1 + Q2) slm. Therefore, the concentration of the silica powder per gas flow rate in the protective tube 20 becomes N / Q3, and the concentration of the silica powder floating in the inert gas in the drawing furnace 10 per unit gas flow rate can be reduced in the protective tube 20 having the gas inlet 21. By providing the constriction 13a of the lower chamber 13 and the constriction 20a of the protective tube 20, the pressure in the lower chamber 13 can be kept higher than the pressure inside the protective tube 20 below the constrictions 13a and 20a. Thereby, the gas introduced from the gas inlet 21 is suppressed from flowing toward the lower chamber 13 side.

[0027] Thereby, the probability of the silica powder coming into contact with or colliding with the optical fiber 2 in the protective tube 20 can be reduced. Further, due to the gas introduced from the gas inlet 21 of the protective tube 20, the silica powder contained in the gas flowing in the core tube 14 is quickly discharged to the outside from the protective tube 20. The optical fiber 2 moves at a constant speed through the core tube 14, the lower chamber 13, and the protective tube 20. However, since the concentration of the silica powder per unit gas flow rate when passing through the protective tube 20 is lower than that when no gas is introduced, the probability of the silica powder coming into contact with or colliding with can be reduced as a whole, and the occurrence of a decrease in the strength of the optical fiber can be suppressed.

[0028] In addition, the gas introduced into the protective tube 20 can rapidly reduce the silica powder concentration in the protective tube 20 below the gas inlet 21 by introducing it near the top of the protective tube 20. For this reason, it is desirable to reduce the silica powder concentration over the entire length of the protective tube 20 by providing the gas inlet 21 near the upper end of the protective tube 20. Also, the gas introduced into the protective tube 20 is preferably evenly flowed from the circumferential direction of the optical fiber 2 so as not to shake the optical fiber 2.

[0029] (Second Embodiment) Next, another example of the gas inlet 21 provided in the protective tube 20 will be described. FIG. 2A is a diagram showing another example of the gas inlet of the protective tube and is a cross-sectional view in the longitudinal direction. In the present embodiment, a plurality of gas inlets 21' provided in the protective tube 20 are provided so as to be inclined obliquely downward toward the protective tube 20. For this reason, the gas introduced into the protective tube 20 from the gas inlet 21' is introduced into the protective tube 20 at a downward angle. As a result, the gas introduced into the protective tube 20 smoothly flows in the traveling direction of the optical fiber 2, so that the disturbance of the gas flow is suppressed, and the influence on the diameter variation of the optical fiber 2 and the influence on the fiber curl are suppressed. Also in the present embodiment, the gas introduced into the protective tube 20 is preferably evenly flowed from the circumferential direction of the optical fiber 2.

[0030] (Third Embodiment) Next, still another example of the gas inlet 21 provided in the protective tube 20 will be described. FIG. 2B is a diagram showing still another example of the gas inlet of the protective tube and is a cross-sectional view in the radial direction. In the present embodiment, a plurality of gas inlets 21'' provided in the protective tube 20 are provided so as to introduce gas not in the direction toward the center of the protective tube 20 but obliquely downward and along the wall surface. As a result, the gas introduced into the protective tube 20 swirls along the inner wall of the protective tube 20, that is, flows in a spiral shape in the circumferential direction, and it becomes easier to separate the silica powder from around the optical fiber 2. The gas introduced into the protective tube 20 may be flowed from one gas inlet or may be evenly flowed from a plurality of gas inlets 21''.

[0031] (Fourth Embodiment) FIG. 3A is a schematic diagram of an optical fiber drawing furnace according to another aspect of the present disclosure. FIG. 3B is a cross-sectional view taken along the line 3B-3B of FIG. 3A. In the present embodiment, it is different from the first to third embodiments in that a gas vent hole 13b that opens to the outside is provided below the lower chamber 13, but the other configurations are the same, so the description of the overlapping configurations will be omitted.

[0032] In the present embodiment, as shown in FIG. 3B, a plurality of gas vent holes 13b are provided at equal intervals around the constriction 13a provided below the lower chamber 13. A part of the inert gas that has flowed through the core tube 14 and the lower chamber 13 is discharged to the outside together with the silica powder from the gas vent hole 13b. The gas vent hole 13b is configured to discharge the inert gas below the lower chamber 13, but it may be provided on the side surface of the lower part of the lower chamber so as to discharge the inert gas to the side of the lower chamber.

[0033] For example, similar to the first embodiment, when an inert gas with a flow rate of Q1 slm is flowing downward in the core tube 14 and it is assumed that N silica powders are contained per Q1 liter, the concentration of the silica powder per gas flow rate in the lower chamber 13 is N / Q1. And in the case of the present embodiment, if an inert gas of Q4 slm is released to the outside from the gas vent hole 13b, then a gas of (Q1 - Q4) slm is introduced from the lower chamber 13 into the protective tube 20. Note that the concentration of the silica powder per gas flow rate contained in the inert gas introduced into the protective tube 20 does not change from N / Q1, but the amount of silica powder entering the protective tube 20 per minute is N×(1 - Q4 / Q1).

[0034] Then, similar to the first embodiment, when a gas of Q2 slm is introduced into the protective tube 20 from the gas inlet 21 of the protective tube 20, the gas flow rate flowing from above to below in the protective tube 20 becomes Q5 (= Q1 - Q4 + Q2) slm. Therefore, the concentration of the silica powder per unit gas flow rate in the protective tube 20 is {N×(1 - Q4 / Q1)} / Q5. Thus, the concentration of the silica powder floating in the inert gas in the protective tube 20 per unit gas flow rate can be made lower than that in the first embodiment.

[0035] (Example) When no constriction was provided at the connection point and no gas was introduced, the fiber breakage frequency in the screening test (the number of fiber breaks per 1 Mm (= 1000 km)) was 2 breaks / Mm. On the other hand, when a constriction was provided and gas was introduced, the breakage frequency was reduced to 1.5 breaks / Mm. Furthermore, when a gas vent hole was provided at the bottom of the lower chamber and gas was introduced, the breakage frequency was reduced to 1 break / Mm.

[0036] In the fourth embodiment as well, similar to the second and third embodiments, it is desirable to introduce the gas introduced into the protective tube 20 at a downward angle inside the protective tube 20 or to introduce it so as to flow spirally in the circumferential direction along the inner wall of the protective tube 20. Furthermore, in order to make the flow rate of the inert gas discharged to the outside from the gas vent hole 13b larger than the flow rate of the inert gas directed toward the protective tube 20, the inert gas in the lower chamber 13 may be sucked from the gas vent hole 13b and forcibly exhausted. In this way, by discharging the inert gas to the outside from the gas vent hole 13b, it becomes possible to control the ratio between the gas flow rate pulled by the optical fiber and the gas flow rate discharged from the gas vent hole.

Description of Reference Numerals

[0037] 1... glass preform, 2... optical fiber, 10... drawing furnace, 11... furnace body, 12... upper chamber, 13... lower chamber, 13a... constriction, 13b... gas vent hole, 14... core tube, 15... heater, 16... heat insulating material, 17... in-furnace gas introduction part, 20... protective tube, 20a... constriction, 21, 21’, 21”... gas inlets.

Claims

1. A core tube into which a glass preform for an optical fiber is inserted, a heater for heating the glass preform for the optical fiber from the outside, and a furnace body for housing the core tube therein, a lower chamber disposed below the core tube, a protective tube disposed below the lower chamber, a furnace gas introduction part for flowing an inert gas downward from above in the core tube, An optical fiber drawing furnace comprising: having a gas inlet for introducing a predetermined gas into the protective tube, and having a constriction smaller than the radial cross-sectional area of the lower chamber on the upper side of the gas inlet, An optical fiber drawing furnace in which the pressure inside the lower chamber above the constriction is maintained at a positive pressure higher than the pressure inside the protective tube below the constriction.

2. The optical fiber drawing furnace according to claim 1, wherein the protective tube is connected below the lower chamber.

3. The optical fiber drawing furnace according to claim 1 or claim 2, wherein the gas inlet is provided on the upper side from the center of the protective tube.

4. The optical fiber drawing furnace according to any one of claims 1 to 3, having a gas vent hole for discharging the inert gas in the lower chamber to the outside below the lower chamber.

5. The optical fiber drawing furnace according to claim 4, wherein gas is forcibly exhausted from the gas vent hole.

6. The optical fiber drawing furnace according to any one of claims 1 to 5, wherein the gas inlet is provided at the upper end of the protective tube.

7. The optical fiber drawing furnace according to any one of claims 1 to 6, wherein the predetermined gas is argon, nitrogen, or air.

8. The optical fiber drawing furnace according to any one of claims 1 to 7, wherein the gas inlets are provided at equal intervals in the circumferential direction of the protective tube. **Claim 9** The optical fiber drawing furnace according to any one of claims 1 to 8, wherein the gas inlets are provided downward so as to introduce the predetermined gas downward into the protective tube. **Claim 10** The optical fiber drawing furnace according to any one of claims 1 to 8, wherein the gas inlets are obliquely downward and directed along the wall surface of the protective tube so as to introduce the predetermined gas spirally into the protective tube. **Claim 11** An optical fiber manufacturing method for manufacturing an optical fiber using the optical fiber drawing furnace according to any one of claims 1 to 10.

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