阅读说明：本技术 光纤拉丝炉用发热体、光纤拉丝炉以及光纤的制造方法 (Heating element for optical fiber drawing furnace, and method for manufacturing optical fiber ) 是由 北村隆之 大关伸男 竹永胜宏 于 2020-03-27 设计创作，主要内容包括：本发明提供一种光纤拉丝炉用发热体(110),上述光纤拉丝炉用发热体(110)具备发热部(110F),上述发热部(110F)由在贯通孔(110H)内配置有光纤用母材(1P)的至少局部的筒状的电阻发热体构成,发热部(110F)包括从一端部起沿着长度方向的规定的区间的第一部分(111)和位于比第一部分(111)靠另一端部侧的位置的第二部分(112),对于第二部分(112)而言,一端部侧的壁厚为第一部分(111)的壁厚以上,壁厚从一端部侧朝向另一端部侧变大。(The present invention provides a heat-generating body (110) for an optical fiber drawing furnace, the heat-generating body (110) for an optical fiber drawing furnace comprising a heat-generating part (110F), the heat-generating part (110F) being formed of an at least partially cylindrical resistance heat-generating body in which an optical fiber preform (1P) is disposed in a through-hole (110H), the heat-generating part (110F) comprising a first portion (111) in a predetermined section along a longitudinal direction from one end and a second portion (112) located on the other end side of the first portion (111), wherein the second portion (112) has a thickness on one end side equal to or greater than the thickness of the first portion (111), and the thickness increases from the one end side toward the other end side.)

1. A heating element for an optical fiber drawing furnace is characterized in that,

the heating element for an optical fiber drawing furnace comprises a heating part composed of a cylindrical resistance heating element having an optical fiber base material disposed in a through hole,

the heat generating portion includes a first portion in a predetermined section along a longitudinal direction from one end of the heat generating portion and a second portion located on the other end side of the heat generating portion with respect to the first portion,

the second portion has a wall thickness on the one end portion side that is greater than or equal to a wall thickness of the first portion, and the wall thickness increases from the one end portion side toward the other end portion side.

2. The heating element for an optical fiber drawing furnace according to claim 1,

the wall thickness of the second portion continuously varies from the one end side toward the other end side.

3. The heating element for an optical fiber drawing furnace according to claim 2,

the closer the second portion is to the other end portion side, the smaller the rate of change in wall thickness.

4. A heating element for an optical fiber drawing furnace according to any one of claims 1 to 3,

the inner diameter of the second portion is constant.

5. A heating element for an optical fiber drawing furnace according to any one of claims 1 to 3,

the second portion has an inner diameter smaller on the other end side than on the one end side.

6. The heating element for an optical fiber drawing furnace according to claim 5,

the outer diameter of the second portion is constant.

7. The heating element for an optical fiber drawing furnace according to claim 5,

the inner diameter of the second portion is a constant size starting from the one end portion side halfway toward the other end portion side.

8. A heating element for an optical fiber drawing furnace according to any one of claims 1 to 7,

the wall thickness of the first portion is constant along the length.

9. A heating element for an optical fiber drawing furnace according to any one of claims 1 to 7,

the heat generating portion includes a third portion that is provided between the first portion and the second portion and has a wall thickness larger than a wall thickness of the second portion on the one end side.

10. The heating element for an optical fiber drawing furnace according to claim 9,

the third portion has an inner diameter smaller on the other end side than on the one end side.

11. A heating element for an optical fiber drawing furnace according to any one of claims 1 to 10,

the heating element for an optical fiber drawing furnace further comprises a pair of power feeding portions which are formed by the resistance heating element and are provided at both ends of the heating portion in the longitudinal direction of the cylindrical shape,

the thickness of the power supply portion located on the other end portion side of the heat generating portion is equal to or greater than the maximum thickness of the second portion.

12. An optical fiber drawing furnace is characterized in that,

the heating element for an optical fiber drawing furnace according to any one of claims 1 to 11.

13. A method of manufacturing an optical fiber,

the method for manufacturing the optical fiber comprises the following steps: a drawing step of drawing an optical fiber preform disposed in the through hole of the first portion in the heating element for an optical fiber drawing furnace according to claim 12; and

and a slow cooling step of slowly cooling the bare optical fiber drawn in the drawing step in the through hole of the second portion of the heating element for an optical fiber drawing furnace.

14. The method of manufacturing an optical fiber according to claim 13,

the temperature of the bare optical fiber wire entering the second part is 1300-1650 ℃, and the temperature of the bare optical fiber wire output from the second part is more than 1150 ℃ and less than 1400 ℃.

15. The method of manufacturing an optical fiber according to claim 13 or 14,

the time for cooling the bare optical fiber in the second section is 0.05 seconds or more.

16. The method for manufacturing an optical fiber according to any one of claims 13 to 15,

the time for cooling the bare optical fiber in the second section is 1 second or less.

17. The method for manufacturing an optical fiber according to any one of claims 13 to 16,

the heat generating portion includes a third portion that is provided between the first portion and the second portion and has a wall thickness larger than a wall thickness of the second portion on the one end portion side,

the optical fiber manufacturing method includes a pre-cooling step of cooling the optical fiber by the third section so that the optical fiber has a temperature suitable for entering the second section.

Technical Field

The present invention relates to a heating element for an optical fiber drawing furnace, and a method for manufacturing an optical fiber.

Background

An optical fiber is manufactured by drawing an optical fiber preform having a cross-sectional structure substantially the same as that of the optical fiber. Patent document 1 describes a heating element used in a spinning furnace for drawing an optical fiber. The heating element is of a resistance heating type, is formed in a zigzag manner of graphite having a constant thickness, and has a substantially cylindrical shape as a whole. The temperature of the region surrounded by the heating element along the drawing direction is substantially constant.

Patent document 1: japanese patent No. 5557866

However, in order to achieve a long distance of optical transmission distance and a high speed of optical transmission speed in an optical fiber communication system, it is necessary to increase the optical signal-to-noise ratio, and it is required to reduce the transmission loss of an optical fiber. At present, the manufacturing method of the optical fiber has been very refined, and it is considered that the transmission loss caused by impurities contained in the optical fiber is almost reduced to the limit. The remaining transmission loss is mainly caused by scattering loss accompanying fluctuations in the structure and composition of the glass constituting the optical fiber. This phenomenon cannot be avoided because the optical fiber is made of glass.

As a method for reducing the fluctuation of the structure of the glass, there is known a method of cooling the molten glass slowly. As a method of gradually cooling the molten glass in this way, there has been attempted a method of gradually cooling an optical fiber drawn from a drawing furnace. A method of reducing the cooling rate of an optical fiber by heating the optical fiber drawn from a drawing furnace in a slow cooling furnace or the like has been studied. Even in the drawing furnace using the heating element of patent document 1, in order to reduce the fluctuation of the structure of the glass, it is necessary to heat the bare optical fiber obtained by drawing with heat from the heating element in a slow cooling furnace.

However, in an optical fiber manufacturing facility including a drawing furnace and a slow cooling furnace, a heating element of the drawing furnace and a heating element of the slow cooling furnace are required, and there is a fear that the structure of the manufacturing facility becomes complicated.

Disclosure of Invention

Accordingly, the present invention provides a heating element for an optical fiber drawing furnace, and a method for manufacturing an optical fiber, which can realize an optical fiber drawing furnace capable of manufacturing an optical fiber with reduced transmission loss with a simple configuration.

In order to solve the above-described problems, a heat generating body for an optical fiber drawing furnace according to the present invention includes a heat generating portion including an at least partially cylindrical resistance heat generating body in which an optical fiber base material is disposed in a through hole, the heat generating portion including a first portion in a predetermined section along a longitudinal direction from one end portion of the heat generating portion and a second portion located on a side of the other end portion of the heat generating portion from the first portion, the second portion having a thickness on the side of the one end portion that is greater than or equal to a thickness of the first portion, the thickness increasing from the side of the one end portion toward the side of the other end portion.

According to such a heating element for an optical fiber drawing furnace, when currents of the same magnitude flow in the first portion and the second portion, the current density becomes equal to or higher than the current density of the second portion in the first portion having a thickness equal to or smaller than the minimum thickness of the second portion, and heat is generated at a temperature equal to or higher than the second portion. Therefore, even when a voltage is applied to the heating element for an optical fiber drawing furnace so that the first portion generates heat to the temperature of the base material for drawing an optical fiber, the temperature of the second portion on the one end side is equal to or lower than the temperature of the first portion. Further, the second portion generates heat so that the temperature decreases from the one end side toward the other end side because the current density decreases from the one end side toward the other end side. Therefore, the temperature of the bare optical fiber drawn in the first section can be gradually lowered in the second section. That is, the bare optical fiber can be slowly cooled in the second portion. The heating element for an optical fiber drawing furnace according to the present invention includes a first portion capable of drawing a bare optical fiber in this manner and a second portion capable of slowly cooling the drawn bare optical fiber. Therefore, when the heating element for an optical fiber drawing furnace according to the present invention is applied to an optical fiber drawing furnace, it is possible to realize an optical fiber drawing furnace capable of manufacturing an optical fiber with reduced transmission loss with a simple structure, as compared with a case where a drawing furnace and a slow cooling furnace are provided separately.

Preferably, the thickness of the second portion continuously changes from the one end portion side toward the other end portion side.

According to such a configuration, compared to a case where the thickness of the second portion is changed stepwise from one end portion side to the other end portion side, it is possible to suppress a local rapid change in the temperature of the second portion.

Preferably, the rate of change in the wall thickness decreases as the second portion approaches the other end portion side.

According to such a configuration, the current density can be gradually reduced as the current density is closer to the other end of the second portion, and the temperature can be gradually reduced as the current density is closer to the other end of the second portion. Therefore, the temperature of the bare optical fiber can be gradually lowered as the bare optical fiber is gradually cooled at the final stage. Therefore, the temperature of the bare optical fiber can be smoothly reduced so as to minimize the virtual temperature that is an index of disorder of the structure of the glass, in accordance with the rate of structural relaxation of the glass, which decreases as the temperature of the bare optical fiber decreases.

Further, the inner diameter of the second portion may be constant.

In this case, the through hole of the second portion may be formed by a general drill or the like, and thus the inner peripheral surface of the second portion is easily manufactured. Therefore, the heating element for an optical fiber drawing furnace of the present invention can be easily realized.

Preferably, the second portion has an inner diameter smaller on the other end side than on the one end side.

There is a tendency that inert gas flows in the through hole of the heating element for the optical fiber drawing furnace. Therefore, with the above-described configuration, the inert gas flowing in the through hole can be rectified, and unnecessary movement of the bare optical fiber obtained by drawing can be suppressed. Therefore, a heating element for an optical fiber drawing furnace capable of manufacturing an optical fiber having stable characteristics can be realized.

Further, the outer diameter of the second portion may be constant.

In this case, the outer peripheral surface of the second portion can be easily produced. Therefore, the heating element for an optical fiber drawing furnace of the present invention can be easily realized.

Further, the inner diameter of the second portion may be a constant size starting from the one end portion side toward the other end portion side.

With this configuration, the inner diameter of the second portion can be reduced in accordance with a so-called waisted shape in which the optical fiber base material is drawn and reduced in diameter to a bare optical fiber. Therefore, with the above-described configuration, the inert gas flowing in the through hole can be further rectified, and unnecessary movement of the bare optical fiber obtained by drawing can be further suppressed. Therefore, a heating element for an optical fiber drawing furnace can be realized, and an optical fiber having more stable characteristics can be manufactured by the heating element for an optical fiber drawing furnace.

Preferably, the thickness of the first portion is constant along the longitudinal direction.

With this configuration, the first portion of the bare optical fiber drawn from the optical fiber base material can generate heat at a constant temperature along the longitudinal direction. Therefore, the shape called the so-called waisted shape is determined by the viscosity of the glass and the spinning tension of the portion, and therefore, it is necessary to maintain a constant temperature distribution. Therefore, in the above-described configuration, by reducing the parameter that needs to be controlled to make the temperature of the heating element constant by one, the waisted shape can be easily kept constant, and unnecessary variation in the outer diameter of the bare optical fiber can be suppressed.

Preferably, the heat generating portion includes a third portion having a thickness equal to or greater than a maximum thickness of the second portion between the first portion and the second portion.

With this configuration, the temperature of the third portion can be made lower than that of the first portion and the second portion. Therefore, in the third section, the bare optical fiber obtained by drawing is precooled, and the bare optical fiber can be introduced into the second section at an appropriate temperature.

In this case, the inner diameter of the third portion is preferably smaller on the other end side than on the one end side.

As described above, there is a tendency that inert gas flows in the through hole of the heating element for an optical fiber drawing furnace. Therefore, with the above-described configuration, the inert gas flowing in the through hole can be rectified, and unnecessary movement of the bare optical fiber obtained by drawing can be suppressed. Therefore, a heating element for an optical fiber drawing furnace capable of manufacturing an optical fiber having stable characteristics can be realized.

Preferably, the pair of power feeding portions are formed of the cylindrical resistive heating element and provided at both ends of the heating portion in the longitudinal direction, and the thickness of the power feeding portion located on the other end side of the heating portion is larger than or equal to the maximum thickness of the second portion.

With this configuration, the optical fiber reaching the lower virtual temperature in the second portion can be suppressed from being reheated at the power supply portion at the lower end, and the virtual temperature can be suppressed from increasing.

The optical fiber drawing furnace of the present invention is characterized by comprising any of the above-described heating elements for an optical fiber drawing furnace.

As described above, the heating element for an optical fiber drawing furnace according to the present invention can realize an optical fiber drawing furnace capable of manufacturing an optical fiber with reduced transmission loss with a simple structure, as compared with a case where a drawing furnace and a slow cooling furnace are provided separately from each other. Therefore, the optical fiber drawing furnace including the heating element for an optical fiber drawing furnace can perform drawing and slow cooling with a simple configuration, as compared with a case where the drawing furnace and the slow cooling furnace are provided separately from each other.

Further, the method for manufacturing an optical fiber according to the present invention includes: a drawing step of drawing the optical fiber preform disposed in the through hole of the first portion in the heating element for an optical fiber drawing furnace of the optical fiber drawing furnace; and a slow cooling step of slowly cooling the bare optical fiber drawn in the drawing step in the through hole of the second portion of the heating element for an optical fiber drawing furnace.

As described above, the optical fiber drawing furnace of the present invention can perform drawing and slow cooling with a simple configuration, as compared with a case where the drawing furnace and the slow cooling furnace are provided separately from each other. Therefore, the method for manufacturing an optical fiber of the present invention can perform the drawing step and the slow cooling step with a simple configuration.

Preferably, the bare optical fiber entering the second section has a temperature of 1300 to 1650 ℃, and the bare optical fiber exiting the second section has a temperature of 1150 ℃ or more and less than 1400 ℃.

In this way, by appropriately controlling the temperature of the optical fiber entering the second section and the temperature of the optical fiber output from the second section, structural relaxation of the glass constituting the optical fiber in the second section can be promoted. As a result, scattering loss due to fluctuations in the structure of the glass during light transmission can be suppressed, and an optical fiber with reduced transmission loss can be obtained.

Preferably, the time for cooling the bare optical fiber in the second section is 0.05 seconds or more.

Therefore, structural relaxation of the glass constituting the optical fiber in the second portion is easily promoted.

The time for cooling the bare optical fiber in the second section is preferably 1 second or less.

As the slow cooling time is longer, the glass structure is relaxed, and the transmission loss can be reduced, but the effect thereof is drastically reduced. Therefore, by setting the time for slow cooling the optical fiber to 1 second or less, the length of the second portion can be shortened, and the cost required for equipment investment can be suppressed. Further, since the drawing speed can be increased by setting the time for which the optical fiber stays in the second portion to a short time of 1 second or less, structural relaxation of the glass constituting the bare optical fiber can be promoted without lowering productivity.

Preferably, the heat generating portion includes a third portion provided between the first portion and the second portion and having a wall thickness larger than that of the second portion on the one end side, and the method of manufacturing an optical fiber according to the present invention includes a pre-cooling step of cooling the optical fiber by the third portion so that the optical fiber has a temperature suitable for entering the second portion.

It is preferable to limit the temperature of the bare optical fiber entering the second portion to a prescribed range. Therefore, by further providing such a pre-cooling step, the temperature of the optical fiber entering the second section can be easily adjusted to an appropriate range.

As described above, according to the present invention, there are provided a heating element for an optical fiber drawing furnace capable of realizing an optical fiber drawing furnace capable of manufacturing an optical fiber with reduced transmission loss with a simple configuration, an optical fiber drawing furnace, and a method for manufacturing an optical fiber.

Drawings

Fig. 1 is a diagram schematically showing the configuration of an optical fiber manufacturing apparatus.

Fig. 2 is a sectional view showing the structure of the optical fiber drawing furnace of fig. 1.

FIG. 3 is a perspective view showing the structure of a heating element for an optical fiber drawing furnace.

Fig. 4 is a flowchart showing steps of the method for manufacturing an optical fiber according to the present invention.

Fig. 5 is a graph showing a relationship between the temperature of the optical fiber and the virtual temperature of the glass constituting the optical fiber and the cooling time.

Fig. 6 is a graph showing a relationship between a change in the outer diameter of the waist portion, a change in the temperature of the optical fiber, and a change in the virtual temperature of the glass constituting the optical fiber.

FIG. 7 is a cross-sectional view showing a first modification of the heating element for an optical fiber drawing furnace.

FIG. 8 is a sectional view showing a second modification of the heating element for an optical fiber drawing furnace.

FIG. 9 is a sectional view showing a third modification of the heating element for an optical fiber drawing furnace.

FIG. 10 is a sectional view showing a fourth modification of the heating element for an optical fiber drawing furnace.

FIG. 11 is a cross-sectional view showing a fifth modification of the heating element for an optical fiber drawing furnace.

Detailed Description

Hereinafter, preferred embodiments of the method for manufacturing an optical fiber according to the present invention will be described in detail with reference to the drawings.

Fig. 1 is a diagram schematically showing the configuration of an optical fiber manufacturing apparatus used in the optical fiber manufacturing method according to the present embodiment. As shown in fig. 1, the optical fiber manufacturing apparatus includes an optical fiber drawing furnace 100. Fig. 2 is a sectional view showing the structure of the optical fiber drawing furnace 100 of fig. 1. As shown in fig. 1 and 2, the optical fiber drawing furnace 100 includes a heating element 110 for the optical fiber drawing furnace and a heat insulating part 120.

FIG. 3 is a perspective view showing a heating element 110 for an optical fiber drawing furnace. As shown in fig. 2 and 3, the heating element 110 for an optical fiber drawing furnace includes a heating member 110F and a pair of power feeding portions 114a and 114b provided at both ends of the heating member 110F. The heat generating portion 110F includes a first portion 111, a second portion 112, and a third portion 113. The heating element 110 for an optical fiber drawing furnace is composed of a resistance heating element that generates heat by resistance when current flows, and is formed by integrally molding a pair of power feeding portions 114a and 114b, a first portion 111, a second portion 112, and a third portion 113. Examples of such a resistance heating element include a ceramic heater such as graphite, silicon carbide, silicon nitride, zirconia, and alumina, which are nonmetallic heating elements. Among them, graphite is preferable from the viewpoint of excellent workability which enables easy cutting.

One end of the heating element 110 for an optical fiber drawing furnace is provided with a power feeding portion 114 a. The power feeding portion 114a has a through hole 110H formed in the center thereof and is formed in an annular shape having a constant thickness. The power supply unit 114a is connected to a power supply not shown.

The first portion 111 is provided beside the power supply portion 114 a. Therefore, the end of first portion 111 on the side of power feeding portion 114a is one end of heat generating portion 110F. The first portion 111 of the present embodiment is formed in a cylindrical shape having a constant wall thickness, and occupies a predetermined section along the longitudinal direction from one end of the heat generating portion 110F. The inner diameter of the first portion 111 is the same as the inner diameter of the power supply portion 114a, and the through hole 110H also extends toward the first portion 111. The wall thickness of the first portion 111 in the radial direction is smaller than the wall thickness of the feeding portion 114a in the radial direction. Therefore, the outer diameter of the first portion 111 is also smaller than the outer diameter of the power feeding portion 114 a. The first portion 111 generates heat based on the current to such an extent that the bare optical fiber 1R is drawn from the optical fiber preform 1P disposed in the through hole 110H. That is, the first portion 111 is a portion where the bare optical fiber 1R is drawn from the optical fiber preform 1P disposed in the through hole 110H. Further, the inner diameter of a certain part of the heating element 110 for an optical fiber drawing furnace is the diameter of the inner wall surface of the part, the outer diameter of a certain part of the heating element 110 for an optical fiber drawing furnace is the diameter of the outer wall surface of the part, and the thickness of a certain part of the heating element 110 for an optical fiber drawing furnace is the difference between the outer diameter and the inner diameter of the part.

A third portion 113 is provided beside the first portion 111 on the side opposite to the power feeding portion 114a side. The third portion 113 has an annular shape, and the through hole 110H also extends toward the third portion 113. The inner diameter of the third portion 113 is constant along the length direction and is the same size as the inner diameter of the first portion 111. The third portion 113 has a radially constant wall thickness along the longitudinal direction, which is greater than the radially thick wall thickness of the first portion 111. Therefore, the outer diameter of the third portion 113 is constant along the length direction and is larger than the outer diameter of the first portion 111. In the case where a current of the same magnitude as that of the first portion 111 flows in the third portion 113 of such a structure, the current density at the third portion 113 is lower than that at the first portion 111. Therefore, when the same magnitude of current flows in the first portion 111 and the third portion 113, the third portion 113 generates heat at a lower temperature than the first portion 111, and the temperature in the through hole 110H at the third portion 113 is lower than the temperature in the through hole 110H at the first portion 111. Therefore, the third portion 113 can cool the bare optical fiber 1R drawn in the first portion 111. That is, the third portion 113 is a portion for precooling the bare optical fiber 1R obtained by drawing.

A second portion 112 is provided beside the third portion 113 on the side opposite to the first portion 111. The second portion 112 is formed in a cylindrical shape having a thickness varying in the longitudinal direction, and the through hole 110H also extends toward the second portion 112. The inner diameter of the second portion 112 is constant along the length direction at the same size as the inner diameter of the third portion 113. The wall thickness in the radial direction of the one end portion side of the second portion 112 is equal to or greater than the wall thickness in the radial direction of the first portion 111, and is smaller than the wall thickness in the radial direction of the third portion 113. In addition, the thickness of the second portion 112 increases from one end portion side toward the other end portion side. Therefore, the outer diameter at one end side of the second portion 112 is equal to or larger than the outer diameter of the first portion 111 and smaller than the outer diameter of the third portion 113, and the outer diameter at the other end side of the second portion 112 is larger than the outer diameter of the first portion 111. In the present embodiment, the rate of change in the wall thickness of the second portion 112 from one end portion side toward the other end portion side is constant. That is, in the second portion 112, the wall thickness monotonically increases from one end portion side toward the other end portion side. In the case where a current of the same magnitude as that of the first portion 111 flows in the second portion 112 of such a structure, the current density at one end portion side of the second portion 112 becomes equal to or less than the current density of the first portion 111, and the current density of the second portion 112 monotonically decreases from the one end portion side toward the other end portion side. Therefore, when the first portion 111 and the second portion 112 flow the same magnitude of current, the one end side of the second portion 112 generates heat at a temperature equal to or lower than the first portion 111, and the second portion 112 generates heat so that the temperature monotonically decreases from the one end side toward the other end side. Therefore, the second portion 112 can gradually cool the bare optical fiber 1R passing through the second portion 112. That is, the second portion 112 is a portion where the bare optical fiber 1R obtained by drawing is gradually cooled.

A power supply portion 114b is provided beside the other end portion side of the second portion 112. The power feeding portion 114b has the same configuration as the power feeding portion 114 a. Therefore, the through-hole 110H extends from the feeding portion 114a to the feeding portion 114 b. The thickness of the feeding portion 114b in the radial direction is equal to or greater than the thickness of the other end portion of the second portion 112 in the radial direction. The power supply unit 114b is connected to a power supply not shown.

In the heating element 110 for an optical fiber drawing furnace having the above-described configuration, the power feeding portion 114a, the first portion 111, the third portion 113, the second portion 112, and the power feeding portion 114b are electrically connected in series. Therefore, by applying a voltage to the power feeding portions 114a and 114b, currents of the same magnitude flow in the first portion 111, the third portion 113, and the second portion 112.

The heating element 110 for the optical fiber drawing furnace is surrounded by the heat insulating part 120. The heat insulating portion 120 is made of, for example, ceramics.

Next, a method for manufacturing an optical fiber using the optical fiber manufacturing apparatus shown in fig. 1 will be described.

Fig. 4 is a flowchart showing steps of the method for manufacturing an optical fiber according to the present embodiment. As shown in fig. 4, the method for manufacturing an optical fiber according to the present embodiment includes a drawing step P1, a pre-cooling step P2, a slow cooling step P3, and a rapid cooling step P4. These steps will be explained below.

< drawing Process P1 >

This step is a step of drawing one end of the optical fiber preform 1P in the first portion 111. First, an optical fiber preform 1P is prepared, and the optical fiber preform 1P is made of glass having a refractive index distribution identical to that of glass constituting an optical fiber as a final product. The optical fiber has one or more cores and a cladding surrounding the outer peripheral surface of the core without a gap, and the refractive index of the core is higher than that of the cladding. For example, when the core is formed of silica glass to which a dopant such as germanium for increasing the refractive index is added, the cladding is formed of pure silica glass. For example, when the core is made of pure silica glass, the cladding is made of silica glass to which a dopant such as fluorine for lowering the refractive index is added.

Next, the optical fiber preform 1P is suspended so as to be vertical in the longitudinal direction. Then, the optical fiber preform 1P is placed in the optical fiber drawing furnace 100. Specifically, as shown in fig. 2, the optical fiber preform 1P is disposed such that the distal end of the optical fiber preform 1P is positioned in the through hole 110H of the first portion 111 of the heating element 110 for an optical fiber drawing furnace. Further, an inert gas such as nitrogen gas flows through the through hole of the heating element 110 for an optical fiber drawing furnace.

Next, a voltage is applied from a power supply not shown so that a current flows between the pair of power feeding portions 114a and 114 b. Then, the heating element 110 for the optical fiber drawing furnace, which is composed of a resistance heating element, generates heat based on resistance. At this time, since the first portion 111, the third portion 113, and the second portion 112 are connected in series as described above, currents of the same magnitude flow through the first portion 111, the third portion 113, and the second portion 112, and the first portion 111, the third portion 113, and the second portion 112 generate heat, respectively. The lower end of the optical fiber preform 1P is heated by heat from the first portion 111. At this time, the lower end of the optical fiber preform 1P is heated to, for example, 2000 ℃. That is, a voltage is applied between the pair of feeding portions 114a and 114b so that a current having the temperature at the lower end portion of the optical fiber preform 1P flows in the first portion 111. Then, the molten glass is drawn from the lower end of the heated optical fiber preform 1P at a predetermined drawing speed. The glass thus drawn becomes the bare optical fiber 1R.

< Pre-Cooling Process P2 >

This step is a step of cooling the bare optical fiber 1R so that the bare optical fiber 1R drawn from the optical fiber preform 1P in the first portion 111 in the drawing step P1 has a predetermined temperature suitable for entering the second portion 112. This process is carried out in the third section 113. As described above, since the same amount of current flows in the first portion 111 and the third portion 113, the third portion 113 generates heat at a lower temperature than the first portion 111. Therefore, the bare optical fiber 1R led out from the first portion 111 in the through hole 110H is cooled when passing through the through hole 110H of the third portion 113. Since the third portion 113 is connected to the first portion 111, the atmosphere in the through hole 110H is substantially the same as the first portion 111 and the third portion 113. Therefore, a rapid change in temperature around the bare optical fiber 1R immediately after being drawn is suppressed.

By performing this step, the cooling rate of the bare optical fiber 1R is adjusted, and the temperature of the bare optical fiber 1R entering the second portion 112 can be easily adjusted to an appropriate range. As will be described later, the temperature of the bare optical fiber 1R drawn from the first portion 111 can be estimated from the shape of the constricted portion. Further, in this way, the length of the third section 113 can be appropriately designed according to the estimated temperature of the bare optical fiber 1R and the temperature of the bare optical fiber 1R suitable for entering the second section 112.

< Slow Cooling Process P3 >

In this step, the bare optical fiber 1R drawn from the first section 111 in the drawing step P1 and adjusted to a predetermined temperature in the third section 113 in the pre-cooling step P2 is gradually cooled in the second section 112. A temperature different from that of the entered bare optical fiber 1R is formed in the second portion 112. As described above, the one end side of the second portion 112 generates heat at a temperature equal to or lower than the temperature of the first portion 111 and higher than the temperature of the third portion 113, and the second portion 112 generates heat so that the temperature monotonically decreases from the one end side toward the other end side. Therefore, the temperature of the bare optical fiber 1R passing through the second portion 112 becomes gradually lower. Therefore, the structure of the glass constituting the bare optical fiber 1R is relaxed, and an optical fiber with reduced scattering loss can be obtained.

Here, the temperature at which the bare optical fiber 1R enters the second section 112, the temperature at which it is output from the second section 112, and the time during which it stays in the second section 112 will be described.

If the temperature at the start of slow cooling of the bare optical fiber 1R is too high, the structure of the glass constituting the bare optical fiber 1R relaxes very quickly, and thus the effect of the slow cooling of the bare optical fiber 1R is weakened. On the other hand, if the temperature at the start of slow cooling of the bare optical fiber 1R is too low, the rate of structural relaxation of the glass constituting the bare optical fiber 1R becomes slow, and therefore, there is a possibility that the bare optical fiber 1R needs to be reheated during slow cooling. Therefore, it is preferable to control the temperature of the bare optical fiber 1R entering the second portion 112 and the temperature of the bare optical fiber 1R output from the second portion 112 within appropriate ranges in order to promote structural relaxation of the glass constituting the bare optical fiber 1R in the second portion 112.

In the case of silica glasses classified as so-called strengthened glasses, the time constant τ (T) of the structural relaxation, which is considered to be dependent on the viscous flow of the glass, follows the Arrhenius formula. Thus, the time constant τ (T) uses a constant A and an activation energy E determined by the composition of the glass_actThe glass is expressed by the following formula (1) as a function of the temperature T of the glass. Furthermore, k_bIs the Boltzmann constant (Boltzmann constant) and T is the absolute temperature of the glass.

1/τ(T)＝A·exp(-E_act/k_bT)···(1)

From the above equation (1), it is understood that the higher the temperature of the glass is, the faster the structure of the glass relaxes, and the more quickly the glass reaches the equilibrium state at that temperature. That is, the higher the temperature of the glass is, the faster the virtual temperature of the glass approaches the temperature of the glass.

Fig. 5 schematically shows a state of a decrease in the virtual temperature of the glass constituting the bare optical fiber when the bare optical fiber is gradually cooled. In fig. 5, the horizontal axis represents time, and the vertical axis represents temperature. In fig. 5, the solid line shows the transition of the temperature of the bare optical fiber under a certain slow cooling condition, and the broken line shows the transition of the virtual temperature of the glass constituting the bare optical fiber at that time. The dotted line represents a temperature transition of the bare optical fiber when the cooling rate is slower than the slow cooling condition shown by the solid line, and the alternate long and short dash line represents a virtual temperature transition of the glass constituting the bare optical fiber at this time.

In fig. 5, as shown by the solid line, when the temperature of the bare optical fiber is simultaneously decreased as time elapses, as shown by the dotted line, the fictive temperature is decreased in the same manner as the decrease in the temperature of the bare optical fiber. As described above, in a state where the temperature of the bare optical fiber is sufficiently high, the structural relaxation speed of the glass constituting the bare optical fiber is increased. However, as the temperature of the bare optical fiber decreases, the rate of structural relaxation of the glass decreases, and eventually the virtual temperature cannot follow the decrease in the temperature of the bare optical fiber. Here, if the cooling rate of the bare optical fiber is made slow, the bare optical fiber is kept at a relatively high temperature for a long time as compared with the case where the cooling rate is made fast, and therefore, as shown by the broken line and the one-dot chain line in fig. 5, the deviation between the temperature of the bare optical fiber and the virtual temperature becomes small, and the virtual temperature becomes lower. I.e. to promote structural relaxation of the glass. How the structural relaxation of the glass constituting the bare optical fiber can be promoted in this way depends on the temperature history of the bare optical fiber. Therefore, how the slow cooling condition is suitable for reducing the transmission loss of the bare optical fiber will be described below.

The bare optical fiber immediately after being drawn from the optical fiber base material has an extremely high temperature of approximately 1800 to 2000 ℃. In this case, as the time constant τ (T) of structural relaxation of the glass constituting the bare optical fiber, for example, a constant A and an activation energy E shown in non-patent literature (K.Saito, et al., Journal of the American Ceramic Society, Vol.89, pp.65-69(2006)) are used_actIt was calculated that the temperature of the bare optical fiber was as very short as about 0.00003 seconds when the temperature was 2000 ℃, and 0.0003 seconds when the temperature was 1800 ℃. In such a high temperature state, it is considered that the virtual temperature of the glass constituting the bare optical fiber substantially coincides with the temperature of the bare optical fiber. Thus, in this wayThe high temperature region of (2) immediately relaxes the structure of the glass even when the bare optical fiber is gradually cooled, and thus the effect of the gradual cooling is small. Therefore, as shown in this embodiment, it is preferable to precool the bare optical fiber from drawing to start slow cooling, and to set the temperature of the bare optical fiber to an appropriate temperature at the start of slow cooling.

The outer diameter of the bare optical fiber drawn from the optical fiber base material is continuously reduced from the outer diameter of the optical fiber base material to a predetermined size. In the case of a general optical fiber, the predetermined size is, for example, 125 μm. The portion where the outer diameter of the bare optical fiber drawn from the optical fiber base material changes is called a constricted portion. The temperature T of the bare optical fiber is determined from the balance of forces at the constricted portion and the mass balance. Specifically, the rate of change in the cross-sectional area S of the constricted portion of the optical fiber base material in a stable state at the speed v at which the bare optical fiber is drawn has a relationship expressed by the following equation (2) with respect to the tension F applied to the bare optical fiber being drawn, assuming that the drawing direction is x.

v·ds/dx＝V·S₀/s₀·dS/dx＝-F/β(T)···(2)

Here, S₀Is a cross-sectional area, s, of a base material for an optical fiber₀The nominal cross-sectional area of the bare optical fiber, V is the feeding speed of the optical fiber preform, β (T) is the tensile viscosity coefficient at the temperature T of the glass, 3 times the viscosity η, that is, the following formula (3) holds.

β(T)＝3η(T)···(3)

The viscosity η of the silica glass was determined by the following formula (4).

log₁₀{η(T)}＝B+C/T···(4)

In the presence of [ Pa s]When the unit (B) is-6.37 and the unit (C) is 2.32 × 10, respectively, the viscosity is η⁴[K^-1]The temperature T of the glass can be determined from the viscosity η determined by the above equation (3) by the above equation (4).

Fig. 6 shows a relationship among a change in the outer diameter (●) of the constricted portion of the bare optical fiber under certain drawing conditions, a change in the temperature (□) of the bare optical fiber obtained from the change in the outer diameter of the constricted portion, and a change in the virtual temperature (a) of the glass constituting the bare optical fiber obtained from the change in the temperature of the bare optical fiber. It is known that as the temperature of the bare optical fiber decreases and the viscosity of the glass constituting the bare optical fiber increases, the change in the outer diameter of the bare optical fiber becomes gradual. When the temperature of the bare optical fiber is lower than about 1650 ℃, the decrease in the virtual temperature of the glass constituting the bare optical fiber does not follow the decrease in the temperature of the bare optical fiber, and the temperature difference between the two increases. That is, even if slow cooling is not performed until the temperature of the bare optical fiber becomes about 1650 ℃, the virtual temperature of the glass constituting the optical fiber substantially matches the temperature of the bare optical fiber, and therefore, the effect of slow cooling performed until the temperature of the bare optical fiber becomes 1650 ℃ or lower is small. Therefore, the temperature at the start of slow cooling is preferably 1650 ℃ or lower. That is, the temperature of the bare optical fiber 1R entering the second portion 112 is preferably 1650 ℃ or lower.

As the time for gradually cooling the bare optical fiber is longer, structural relaxation of glass constituting the bare optical fiber can be promoted, and an optical fiber with reduced transmission loss can be manufactured. However, under economic conditions in view of productivity and equipment investment, the time for gradually cooling the bare optical fiber is preferably 1 second or less. When the time constant τ (T) of structural relaxation of the glass is calculated using a predetermined constant in the above equation (1), τ (T) of 0.1 second or less appears when the glass is at approximately 1420 ℃, τ (T) of 1 second appears when the glass is at approximately 1310 ℃, and τ (T) of 10 seconds appears when the glass is at approximately 1210 ℃. Therefore, even when the time for slow cooling the bare optical fiber is about 1 second, the temperature at the start of slow cooling of the bare optical fiber is preferably 1300 ℃ or more, and more preferably 1400 ℃ or more, in order to sufficiently obtain the effect of slow cooling. That is, the temperature at which the bare optical fiber 1R enters the second portion 112 is preferably 1300 ℃ or higher, and more preferably 1400 ℃ or higher.

As described above, as the temperature of the bare optical fiber decreases, the time required for the structure of the glass constituting the bare optical fiber to relax becomes longer. Specifically, when the temperature of the bare optical fiber is lower than 1150 ℃, the structure of the glass is less likely to relax in a short time of slow cooling. Therefore, the temperature of the bare optical fiber at the end of slow cooling is preferably 1150 ℃ or more and less than 1400 ℃ or more, and more preferably 1300 ℃ or more. That is, the temperature of the bare optical fiber 1R when it is output from the second portion 112 is preferably 1150 ℃ or more and less than 1400 ℃, and more preferably 1300 ℃ or more.

The time for which the bare optical fiber 1R stays in the second portion 112 is preferably 0.01 seconds or more, and more preferably 0.05 seconds or more. The longer the bare optical fiber 1R stays in the second portion 112, the easier the structure of the glass constituting the bare optical fiber 1R relaxes. The time for which the bare optical fiber 1R stays in the second portion 112 is preferably 1 second or less, and more preferably 0.5 second or less. The shorter the time for which the bare optical fiber 1R stays in the second section 112, the shorter the length of the second section 112 can be made, and therefore, the excessive equipment investment can be suppressed. In addition, the shorter the time for which the bare optical fiber 1R stays in the second portion 112, the faster the drawing speed can be obtained, and therefore the productivity of the optical fiber can be improved.

Therefore, it is preferable to determine the drawing speed of the bare optical fiber 1R, the thickness and length of the third portion 113, the thickness and length of the second portion 112, the magnitude of the voltage applied to the power feeding portions 114a and 114b, and the like so that the temperature when the bare optical fiber 1R enters the second portion 112 falls within the above range, the temperature when the bare optical fiber 1R is output from the second portion 112 falls within the above range, and the time during which the bare optical fiber 1R stays in the second portion 112 falls within the above range.

Further, the length of the second portion 112 can be set as follows. Since the glass constituting the bare optical fiber 1R has the lowest temperature history and depends only on the annealing time t, the time t required for annealing the manufactured optical fiber from a virtual temperature at which the transmission loss to be achieved can be achieved is obtained, and the drawing speed v in consideration of productivity is determined, whereby the required length L of the second portion 112 is obtained from the following equation (5).

T＝L/v···(5)

< quenching Process P4 >

After the slow cooling step P3, the bare optical fiber 1R is covered with a cladding layer to improve the resistance of the optical fiber to external damage and the like, thereby forming the optical fiber 1. The coating layer is usually made of an ultraviolet curable resin. In order to form such a coating layer, it is necessary to cool the bare optical fiber 1R to a sufficiently low temperature so as not to cause burning of the coating layer and the like. The temperature of the bare optical fiber 1R affects the viscosity of the resin applied, and as a result, affects the thickness of the clad layer. The temperature of the bare optical fiber 1R suitable for forming the coating layer is appropriately determined depending on the properties of the resin constituting the coating layer.

In the present embodiment, the bare optical fiber 1R output from the second section 112 is quenched by the cooling device 130. In this step, the bare optical fiber 1R is cooled more rapidly than in the slow cooling step P3. By providing such a step, the temperature of the bare optical fiber can be sufficiently lowered in a short section, and therefore, the clad layer can be easily formed. The temperature of the optical fiber at the time of output from the cooling device 130 is, for example, 40 to 50 ℃.

The bare optical fiber 1R cooled to the predetermined temperature by the cooling device 130 as described above is coated with the ultraviolet curable resin by the coating device 141 in which the ultraviolet curable resin is put as a coating layer covering the bare optical fiber 1R. Then, ultraviolet rays are irradiated by the ultraviolet irradiation device 142, thereby curing the ultraviolet curable resin to form a coating layer, and the bare optical fiber 1R becomes the optical fiber 1. Further, the clad layer is generally composed of a double layer. In the case of forming a double-layer coating layer, the bare optical fiber 1R is coated with the ultraviolet curable resin constituting each layer, and then the ultraviolet curable resin is cured at a time, whereby a double-layer coating layer can be formed. Alternatively, the clad layer of the second layer may be formed after the clad layer of the first layer is formed. Then, the optical fiber 1 is turned by the turning wheel 151 and wound on the reel 152.

As described above, the heat generating element 110 for an optical fiber drawing furnace according to the present embodiment includes the heat generating member 110F, and the heat generating member 110F is a cylindrical resistance heat generating element in which at least a part of the optical fiber preform 1P is disposed in the through hole 110H. The heat generating member 110F includes a first portion 111 in a predetermined section along the longitudinal direction from one end portion and a second portion 112 located on the other end portion side of the first portion 111, and the second portion 112 has a thickness larger than or equal to that of the first portion 111 on one end portion side and increases in thickness from the one end portion side toward the other end portion side.

According to the heating element 110 for an optical fiber drawing furnace, even when a current is applied to the heating element 110 for an optical fiber drawing furnace so that the first portion 111 generates heat to the temperature of the drawn optical fiber base material, the temperature of the one end side of the second portion 112 is equal to or lower than the temperature of the first portion 111, and the second portion 112 generates heat so that the temperature decreases from the one end side toward the other end side. Therefore, the temperature of the bare optical fiber 1R drawn in the first section 111 can be gradually lowered in the second section 112. That is, the bare optical fiber can be slowly cooled in the second portion 112. Since the heating element 110 for an optical fiber drawing furnace according to the present embodiment includes the first portion 111 capable of drawing a bare optical fiber and the second portion 112 capable of gradually cooling the drawn bare optical fiber, the optical fiber drawing furnace 100 capable of manufacturing the optical fiber 1 with a reduced transmission loss with a simple configuration can be realized as compared with a case where a drawing furnace and a gradual cooling furnace are provided separately.

Therefore, according to the optical fiber drawing furnace 100 of the present embodiment including the heating element 110 for an optical fiber drawing furnace, drawing and slow cooling can be performed with a simple configuration, and the optical fiber 1 with reduced transmission loss can be manufactured with a simple configuration, as compared with a case where the drawing furnace and the slow cooling furnace are provided separately.

The method for manufacturing the optical fiber 1 of the present embodiment includes: a drawing step P1 of drawing the optical fiber preform 1P disposed in the through hole 110H of the first portion 111; and a slow cooling step P3 of slowly cooling the bare optical fiber 1R drawn in the drawing step P1 in the through hole 110H of the second portion 112. Therefore, compared to the case where the drawing furnace and the slow cooling furnace are provided separately, the method of manufacturing the optical fiber 1 according to the present embodiment can perform drawing and slow cooling with a simple configuration, and can perform the drawing step P1 and the slow cooling step P3 with a simple configuration.

In the present embodiment, the thickness of the second portion 112 continuously changes from one end side to the other end side. Therefore, compared to the case where the wall thickness of the second portion 112 is changed stepwise from one end portion side to the other end portion side, local and abrupt changes in the temperature of the second portion 112 can be suppressed. In addition, the temperature of the bare optical fiber can be controlled to the optimum temperature so that the virtual temperature, which is an index of disorder of the structure of the glass, can be minimized in response to the rate of structural relaxation that decreases as the temperature of the bare optical fiber decreases.

In addition, in the present embodiment, since the inner diameter of the second portion 112 is constant, the inner circumferential surface of the second portion 112 can be easily manufactured by forming the heating element 110 for an optical fiber drawing furnace by, for example, a through hole penetrating the second portion 112 with a general drill or the like.

In the present embodiment, the thickness of the first portion 111 is constant along the longitudinal direction. Therefore, the first portion 111 of the bare optical fiber 1R drawn from the optical fiber preform 1P can generate heat at a constant temperature along the longitudinal direction. Therefore, the diameter of the optical fiber preform can be reduced to a constant shape called a constricted portion of the bare optical fiber. The shape of the waist is determined by the viscosity of the glass and the spinning tension of the portion, and therefore, with the above-described configuration, unnecessary variation in the outer diameter of the bare optical fiber can be suppressed.

In the present embodiment, the heat generating portion 110F includes the third portion 113 having a wall thickness equal to or greater than the maximum wall thickness of the second portion 112 between the first portion 111 and the second portion 112. Therefore, the temperature of the third portion 113 can be made lower than that of the first and second portions 111 and 112, and the bare optical fiber 1R drawn at the third portion 113 can be precooled, so that the bare optical fiber 1R can be made to enter the second portion 112 at an appropriate temperature.

In the present embodiment, the pair of feeding portions 114a and 114b are formed of the same resistive heating element as the heat generating portion 110F, and are provided at both ends in the longitudinal direction, and the thickness of the feeding portions 114a and 114b is equal to or larger than the maximum thickness of the second portion 112. Therefore, as in the present embodiment, the power supply portion can be formed by being integrally molded with the heat generating portion 110F, and heat generation at the power supply portions 114a and 114b can be suppressed. Therefore, it is possible to suppress the temperature control by the resistance heating in the first portion 111 and the second portion 112 from becoming difficult, and to suppress the bare optical fiber 1R and the optical fiber preform 1P from being unnecessarily heated. In the present embodiment, as described above, the thickness of each of the feeding portions 114a and 114b is equal to or greater than the maximum thickness of the second portion 112. However, at least one of the power feeding portions 114a and 114b may not have a thickness equal to or larger than the maximum thickness of the second portion 112. However, it is preferable that the thickness of power feeding portion 114b located on the other end portion side of heat generating portion 110F is not less than the maximum thickness of second portion 112. This can suppress the bare optical fiber 1R reaching a lower virtual temperature at the second portion 112 from being reheated by the lower power supply portion 114b, thereby suppressing the virtual temperature from increasing. Further, if the thickness of the feeding portion 114b is equal to or greater than the thickness of the second portion 112, the portion higher in temperature than the second portion 112 is located upward, and the temperature of the heat generating portion 110F can be monotonically decreased, and the temperature of the bare optical fiber 1R can be further monotonically decreased. It is preferable that the thickness of power feeding portion 114a located on one end side of heat generating portion 110F is equal to or greater than the maximum thickness of second portion 112. In this case, the optical fiber preform 1P can be prevented from being heated before the first portion 111 is heated to be constant. Therefore, the parameter for keeping the temperature of the first portion 111 constant is reduced by one, the shape of the waist can be easily kept constant, and unnecessary variation in the outer diameter of the bare optical fiber 1R can be further suppressed. The power feeding portions 114a and 114b may have other shapes than those of the above embodiments.

The present invention has been described above by taking preferred embodiments as examples, but the present invention is not limited to these embodiments.

The structure of the heat generating element 110 for an optical fiber drawing furnace is not limited to the above embodiment as long as the heat generating element 110F includes the first portion 111 in a predetermined section along the longitudinal direction from one end and the second portion 112 located on the other end side of the first portion 111, and the thickness of the second portion 112 on the one end side is greater than or equal to the thickness of the first portion 111 and the thickness increases from the one end side toward the other end side. Hereinafter, a modification of the heating element 110 for an optical fiber drawing furnace will be described. In the following description of the modifications, the same configurations as those of the above-described embodiment will be omitted except for those specifically mentioned.

FIG. 7 is a cross-sectional view showing a first modification of the heating element 110 for an optical fiber drawing furnace. As shown in fig. 7, the heating element 110 for an optical fiber drawing furnace of the present modification differs from the heating element 110 for an optical fiber drawing furnace of the above embodiment in that the inner diameter of the third portion 113 is smaller on the other end side than on the one end side. Therefore, the inner diameter of the second portion 112 meets the inner diameter of the other end portion side of the third portion 113, and is smaller than the inner diameter of the second portion 112 of the above-described embodiment. As in the above-described embodiment, the inert gas tends not to flow through the through-hole 110H of the heating element 110 for an optical fiber drawing furnace. Therefore, since the heating element 110 for an optical fiber drawing furnace has the configuration as in the present modification, the inert gas flowing in the through hole 110H can be rectified, and unnecessary movement of the drawn bare optical fiber 1R can be suppressed.

FIG. 8 is a sectional view showing a second modification of the heating element 110 for an optical fiber drawing furnace. As shown in fig. 8, the heating element 110 for an optical fiber drawing furnace of the present modification is different from the heating element 110 for an optical fiber drawing furnace of the above embodiment in that the third portion 113 is not formed. Since the heating element 110 for an optical fiber drawing furnace has the configuration as in the present modification, the configuration of the heating element 110 for an optical fiber drawing furnace can be simplified. In particular, when the pre-cooling process P2 is not required, the third portion 113 is not required, and therefore, it is preferable that the third portion 113 is not formed as in the present modification.

FIG. 9 is a cross-sectional view showing a third modification of the heating element 110 for an optical fiber drawing furnace. As shown in fig. 9, the heating element 110 for an optical fiber drawing furnace of the present modification differs from the heating element 110 for an optical fiber drawing furnace of the second modification in that the rate of change in the wall thickness is smaller as the second portion 112 is closer to the other end portion side. Since the heating element 110 for an optical fiber drawing furnace has the configuration as in the present modification, the current density can be gradually decreased toward the other end of the second portion 112, and the temperature can be gradually decreased toward the other end of the second portion 112. Therefore, the temperature of the bare optical fiber 1R can be gradually lowered as the bare optical fiber 1R is gradually cooled at the final stage. Therefore, the temperature of the bare optical fiber can be smoothly reduced so as to minimize the virtual temperature that is an index of disorder of the structure of the glass, in accordance with the rate of structural relaxation of the glass, which decreases as the temperature of the bare optical fiber decreases.

FIG. 10 is a cross-sectional view showing a fourth modification of the heating element 110 for an optical fiber drawing furnace. As shown in fig. 10, the heating element 110 for an optical fiber drawing furnace of the present modification differs from the heating element 110 for an optical fiber drawing furnace of the second modification in that the inner diameter of the second portion 112 is smaller on the other end side than on the one end side, and the outer diameter of the second portion 112 is constant. According to this modification, as in the first modification, even when the inert gas flows through the through-hole 110H of the heating element 110 for an optical fiber drawing furnace, the inert gas flowing through the through-hole 110H can be rectified, and unnecessary movement of the drawn bare optical fiber 1R can be suppressed.

FIG. 11 is a cross-sectional view showing a fifth modification of the heating element 110 for an optical fiber drawing furnace. As shown in fig. 11, the heating element 110 for an optical fiber drawing furnace according to the present modification differs from the heating element 110 for an optical fiber drawing furnace according to the fourth modification mainly in that the inner diameter of the second portion 112 is constant from one end side to the middle of the other end side. In the present modification, the outer diameter of the second portion 112 is not constant. Specifically, in a portion where the inner diameter of the second portion 112 becomes smaller from one end portion side toward the other end portion side, the outer diameter of the second portion 112 becomes smaller from the one end portion side toward the other end portion side. In the portion where the inner diameter of the second portion 112 is constant, the outer diameter of the second portion 112 increases from one end portion side toward the other end portion side. According to this modification, the inner diameter of the second portion 112 is smaller on the other end side than on the one end side, and the inner diameter is constant from the one end side toward the middle of the other end side, so that the inner diameter of the second portion can be made smaller in conformity with a so-called waisted shape in which the optical fiber base material is drawn and waisted into the bare optical fiber. Therefore, the inert gas flowing in the through hole can be further rectified, and unnecessary movement of the drawn bare optical fiber can be further suppressed.

Although not particularly illustrated, for example, the wall thickness of the second portion 112 may be gradually increased from one end side to the other end side. The thickness of the second portion 112 may be increased at a larger rate as it approaches the other end. In these modifications, and the third to seventh modifications, the third portion 113 described in the above embodiment and the first modification may be provided between the first portion 111 and the second portion 112.

In the above-described embodiment and modification, the thickness of the first portion 111 may not be constant in the longitudinal direction. For example, the thickness of the first portion 111 may be reduced from one end side to the other end side.

According to the present invention, there are provided a heating element for an optical fiber drawing furnace, which can realize an optical fiber drawing furnace capable of manufacturing an optical fiber with reduced transmission loss with a simple configuration, an optical fiber drawing furnace, and a method for manufacturing an optical fiber, and which can be used in the field of manufacturing an optical fiber for optical fiber communication. The present invention can also be used in the field of manufacturing fiber laser devices and other devices using optical fibers.