Method for manufacturing optical fiber preform

Abstract

To provide a method for manufacturing an optical fiber preform, capable of avoiding problems of unsintering and bubbles even in a large-sized preform.SOLUTION: A method for manufacturing an optical fiber preform comprises transparently vitrifying the porous preform by heating while sequentially moving the porous preform in a downward direction of a heating furnace. In the vitrifying, the temperature of the heating furnace is controlled so as to continuously decrease the temperature from the heating temperature of a lower end part of the porous preform toward that of a central part of the porous preform and so as to continuously increase the temperature from the heating temperature of the central part of the porous preform toward that of an upper end part of the porous preform.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a method for manufacturing an optical fiber preform. [Background technology]

[0002] Patent Document 1 discloses a method for sintering a long optical fiber porous preform, which can suppress foaming and convert the porous preform into a transparent glass. Patent Document 2 discloses a method for manufacturing an optical fiber preform, which can prevent defects such as cracks and bubbles from occurring. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2003-277095 A [Patent Document 2] JP 2017-81773 A Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, large-sized porous glass preforms, for example with an outer diameter of 200 mm or more, have come to be used. Such large-sized porous glass preforms are prone to problems such as insufficient heating at the tip, resulting in unsintered parts, and excessive heating at the center, resulting in the generation of bubbles.

[0005] An object of the present disclosure is to provide a method for manufacturing an optical fiber preform that can avoid problems such as under-sintering and the generation of bubbles even in the case of a large preform. [Means for solving the problem]

[0006] A method for producing an optical fiber preform according to an embodiment of the present disclosure is a method for producing an optical fiber preform, comprising the steps of: heating a porous preform while moving the porous preform downward in a heating furnace; and subjecting the porous preform to a transparent vitrification process, In the transparent vitrification process, The temperature of the heating furnace is adjusted so that the heating temperature is continuously decreased from the heating temperature of the lower end of the porous base material to the heating temperature of the central portion of the porous base material; and The temperature of the heating furnace is adjusted so that the heating temperature increases continuously from the heating temperature of the central portion of the porous base material toward the heating temperature of the upper end portion of the porous base material. Effect of the Invention

[0007] According to the present disclosure, it is possible to provide a method for manufacturing an optical fiber preform that can avoid problems such as unsintered and air bubbles even in the case of a large preform. [Brief description of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view showing an example of a sintering furnace for carrying out a manufacturing method of an optical fiber preform according to the present disclosure. [Diagram 2] 1 is a graph showing an example of heating temperature conditions in one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Description of the embodiments of the present disclosure] First, the embodiments of the present disclosure will be described. A method for manufacturing an optical fiber preform according to an embodiment of the present disclosure includes the steps of: (1) A method for manufacturing an optical fiber preform, comprising the steps of: heating a porous preform while moving it downward in a heating furnace; and forming the porous preform into a transparent vitrified material, In the transparent vitrification process, The temperature of the heating furnace is adjusted so that the heating temperature is continuously decreased from the heating temperature of the lower end of the porous base material to the heating temperature of the central portion of the porous base material, and The temperature of the heating furnace is adjusted so that the heating temperature increases continuously from the heating temperature of the central portion of the porous base material toward the heating temperature of the upper end portion of the porous base material.

[0010] According to this embodiment, the heating temperature of the central portion, which is easiest to make transparent, is set to the lowest, so that the porous base material can be made uniformly transparent without causing problems such as unsintered portions and air bubbles.

[0011] (2) In the above (1), the section in which the temperature of the heating furnace is adjusted so as to be continuously lower from the heating temperature of the lower end portion to the heating temperature of the central portion includes a first section and a second section following the first section, The temperature decrease per unit time in the first section may be greater than the temperature decrease per unit time in the second section.

[0012] The lower end is the part where heating starts, and is particularly prone to insufficient sintering. By sintering only the vicinity of the lower end at a high temperature as in the present embodiment, it is possible to make the lower end transparent while preventing the effective portion located above the lower end from being excessively heated.

[0013] (3) In the above (1) or (2), the difference between the heating temperature of the lower end portion and the heating temperature of the central portion may be 20° C. or more and 80° C. or less.

[0014] According to this embodiment, by heating within a preferred temperature range, it is possible to more reliably prevent the lower end portion and the effective portion from being unsintered or to prevent the generation of bubbles.

[0015] [Details of the embodiment of the present disclosure] Specific examples of the method for manufacturing an optical fiber preform according to the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0016] First, the configuration of a heating furnace for carrying out the manufacturing method of the optical fiber preform of the present disclosure will be described. Fig. 1 is a cross-sectional view showing an example of a heating furnace 1 for manufacturing an optical fiber preform. The heating furnace 1 has a cylindrical quartz glass core tube 11, a heater 13 arranged to surround the core tube 11, a furnace body 12 formed to cover the heater 13, and a lid 14 covering the upper part of the core tube 11.

[0017] A gas supply port 15 is provided at the bottom of the furnace core tube 11, and is configured so that helium gas, chlorine gas, and the like used in the manufacturing process of the optical fiber preform can be supplied into the furnace core tube 11 from the direction of the arrow in the figure. In addition, the lid 14 is provided with a gas exhaust port (not shown).

[0018] The furnace body 12 is also provided with a gas supply port and a gas exhaust port (not shown) so that an inert gas such as nitrogen gas can be supplied into the furnace body 12. Inside the furnace body 12, in addition to the heater 13, a heat insulating material (not shown) made of a material such as carbon is arranged.

[0019] A long porous preform 2 is suspended from above inside the heating furnace 1. The porous preform 2 is a porous body obtained by depositing glass particles on a starting rod. The porous preform 2 is supported by a support rod 3 and can be moved in the vertical direction via the support rod 3. The outer diameter of the porous preform 2 is, for example, 200 mm or more.

[0020] Next, a manufacturing process of an optical fiber preform will be described. The optical fiber preform described below is, for example, a glass preform for an optical fiber core. The manufacturing process of an optical fiber preform includes a dehydration process and a clarification process.

[0021] In the dehydration step, heating is started after the lower end of the porous base material 2 is adjusted to a position where it can be heated by the heater 13. Thereafter, the porous base material 2 is gradually moved downward to dehydrate the entire porous base material 2.

[0022] Next, a transparency process is performed in which the dehydrated porous preform 2 is sintered at high temperature to make it transparent. In the transparency process, the porous preform is sintered using the heater 13 while helium gas is supplied into the furnace tube 11. In the transparency process, like the dehydration process, heating begins from the lower end of the porous preform 2, and the porous preform 2 is gradually moved downward to sinter and make the entire porous preform 2 transparent. The downward movement speed of the porous preform 2 is kept almost constant over the entire length of the porous preform 2. In this way, a transparent vitrified optical fiber preform is obtained.

[0023] The porous base material 2 is easily made transparent depending on the position in the longitudinal direction. Germanium is added to the radial center of the portion (constant outer diameter portion) of the porous base material 2 whose outer diameter is constant along the longitudinal direction, while germanium is not added to the radial center of both longitudinal ends. For this reason, the glass viscosity is relatively high at both longitudinal ends of the porous base material 2, and if the heating temperature is too low, the porous base material cannot be made sufficiently transparent. On the other hand, the vicinity of the longitudinal center is easily made transparent, and if the heating temperature is too high, air bubbles are generated. In addition, in a large base material with an outer diameter of 200 mm or more, the tip of the base material may be hardened to prevent cracking, and in that case, the tip of the base material is particularly likely to be insufficiently made transparent.

[0024] In a manufacturing method for an optical fiber preform according to an embodiment of the present disclosure, in the transparent vitrification process, the temperature of the heating furnace 1 is adjusted so that the heating temperature is continuously lower from the lower end 21 to the central portion 22, and is also adjusted so that the heating temperature is continuously higher from the central portion 22 to the upper end 23. That is, the heating temperature conditions are set so that the heating temperature is lowest in the central portion 22 in the transparent vitrification process. In the transparent vitrification process according to this embodiment, the heating temperatures of the lower end 21 and the upper end 23, which are difficult to make transparent, are set higher than the heating temperature of the central portion 22, thereby making it possible to avoid unsintered portions and the generation of bubbles.

[0025] Here, the lower end 21 is a tapered portion located at the lower end of the porous preform 2, and the upper end 23 is a tapered portion located at the upper end of the porous preform 2. The central portion 22 is a portion located at the center of the constant outer diameter portion in the longitudinal direction of the porous preform 2.

[0026] An example of a specific aspect of the transparent vitrification process in the method according to this embodiment is as follows. First, the tip of the lower end 21 is aligned with the height of the upper end of the heater 13, and the set temperature of the heater 13 is set to a predetermined temperature. Next, the porous base material 2 is lowered at a constant speed. The set temperature of the heater 13 is kept constant until the boundary between the lower end 21 and the constant outer diameter portion is aligned with the height of the upper end of the heater 13. Then, when the boundary between the lower end 21 and the constant outer diameter portion is aligned with the height of the upper end of the heater 13, the set temperature of the heater 13 begins to be lowered toward the heating temperature of the central portion 22. The set temperature of the heater 13 is continuously lowered, and when the central portion 22 is aligned with the height of the upper end of the heater 13, the set temperature of the heater 13 begins to be raised toward the heating temperature of the upper end 23. When sintering is completed up to the upper end 23 and the entire porous base material 2 becomes transparent, the transparent vitrification process is terminated.

[0027] In the above example, the height reference is set to the upper end of heater 13, but the height reference may be the center in the vertical direction of heater 13 or another predetermined position. In addition, the heating temperature reference is set to the set temperature of heater 13, but there is no particular limitation as long as it allows confirmation of the relative high and low of the heating temperature.

[0028] Moreover, the temperature adjustment from the heating temperature of the lower end portion 21 to the heating temperature of the central portion 22 does not have to start strictly at the boundary between the tapered portion and the constant outer diameter portion, but can be appropriately adjusted depending on the ease of heating of the lower end portion 21 and the effective portion, etc. Moreover, the central portion 22, which has the lowest heating temperature, does not have to be strictly at the longitudinal center of the constant outer diameter portion, but can be a position shifted up or down from the center of the constant outer diameter portion by up to about 10% of the total length of the constant outer diameter portion.

[0029] In the transparent vitrification process, the section in which the temperature of the heating furnace 1 is adjusted so as to be continuously lower from the heating temperature of the lower end 21 to the heating temperature of the central portion 22 includes a first section and a second section following the first section, and the temperature drop per unit time in the first section may be larger than the temperature drop per unit time in the second section. FIG. 2 is a graph showing an example of heating temperature conditions in this embodiment. The vertical axis of the graph is heating temperature, and the horizontal axis is time. Time t1 in FIG. 2 indicates the time when the temperature starts to drop from the lower end 21 to the heating temperature of the central portion 22. Time t3 indicates the time when the set temperature of the heater 13 reaches the heating temperature of the central portion 22. Time t2 indicates a point between t1 and t3. In this embodiment, the temperature drop per unit time in the first section S1 from time t1 to t2 (the absolute value of the slope of the graph in FIG. 2) is larger than the temperature drop per unit time in the second section S2 from time t2 to t3. The lower end 21 is the starting point for heating, and is a portion that is particularly prone to unsintering. By adjusting the heating temperature as in this embodiment, only the vicinity of the lower end 21 can be sintered at a high temperature. According to this embodiment, it is possible to more reliably make the lower end 21 transparent while preventing the effective portion on the lower end 21 side from being heated excessively. The effective portion refers to the portion that is intended to be made into an optical fiber that can be shipped as a product after drawing in a subsequent process.

[0030] It is more preferable that the temperature drop per unit time in the first section is 50 to 160 times the temperature drop per unit time in the second section. When the temperature drop per unit time is in this range, the heating temperature of the effective portion on the lower end 21 side can be kept within a suitable range.

[0031] In this embodiment, the difference in heating temperature between the lower end portion 21 and the central portion 22 may be 20°C or more and 80°C or less. When the difference in heating temperature is within the above range, it is possible to more reliably prevent the lower end portion 21 and the effective portion from being unsintered or the occurrence of bubbles. The difference in heating temperature between the lower end portion 21 and the central portion 22 is more preferably 30°C or more and 80°C or less, and even more preferably 40°C or more and 70°C or less. [Explanation of symbols]

[0032] 1 Furnace 11. Core tube 12 Furnace body 13 Heater 14 Lid 15 Gas supply port 2 Porous base material 21 Lower end 22 Central part 23 Upper end 3 Support rod

Claims

1. A method for manufacturing an optical fiber preform, comprising the steps of: heating a porous preform while moving it downward in a heating furnace; and forming the porous preform into a transparent vitrified material, the method comprising the steps of: In the transparent vitrification process, The temperature of the heating furnace is adjusted so that the heating temperature is continuously decreased from the heating temperature of the lower end of the porous base material to the heating temperature of the central portion of the porous base material, and adjusting the temperature of the heating furnace so that the heating temperature increases continuously from the heating temperature of the central portion of the porous base material to the heating temperature of the upper end portion of the porous base material; A method for manufacturing optical fiber base material.

2. In the transparent vitrification process, a section in which the temperature of the heating furnace is adjusted so as to be continuously lower from the heating temperature of the lower end portion to the heating temperature of the central portion includes a first section and a second section following the first section, A temperature decrease per unit time in the first section is greater than a temperature decrease per unit time in the second section. A method for producing the optical fiber preform according to claim 1.

3. 3. The method for manufacturing an optical fiber preform according to claim 1, wherein a difference between a heating temperature of the lower end portion and a heating temperature of the central portion is 20° C. or more and 80° C. or less.

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