Method for manufacturing optical fiber preform

Abstract

To provide a method for manufacturing an optical fiber preform, capable of reducing the used amount of helium gas.SOLUTION: A method for manufacturing an optical fiber preform comprises: a dehydration step of heating a porous glass preform while supplying inert gas being nitrogen, argon or mixed gas of nitrogen and argon and dehydration gas to a furnace core tube; a first transparency step of heating the porous glass preform while stopping to supply the dehydration gas to supply the inert gas to the furnace core tube; and a second transparency step of causing to fall and heating the porous glass preform while stopping to supply the inert gas to supply helium gas to the furnace core tube.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 manufacturing a porous preform for optical fiber, which includes a step of ventilating the inside of a furnace tube with helium gas before a transparent vitrification step. Patent Document 2 discloses the use of nitrogen gas or argon gas when sintering the ineffective portion on the end side (upper end side) of the porous preform. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2022-65621 [Patent Document 2] JP 2014-214066 A Summary of the Invention [Problem to be solved by the invention]

[0004] Helium gas is usually used to make porous glass preform transparent. However, since helium gas is expensive, it is desirable to reduce the amount of helium gas used. In the method described in Patent Document 1, helium gas is used to ventilate the inside of the furnace tube before the transparentization step, so a large amount of helium gas is used. Patent Document 2 discloses a method in which the upper end side of the porous preform is sintered with nitrogen gas, but the lower end side needs to be sintered with helium gas even in the ineffective part.

[0005] An object of the present disclosure is to provide a method for manufacturing an optical fiber preform that can reduce the amount of helium gas used. [Means for solving the problem]

[0006] The present disclosure relates to a method for producing an optical fiber preform, the method comprising the steps of: heat-treating, in a furnace tube, a porous glass preform having an effective portion to be a product and a non-effective portion located below the effective portion and not to be a product; a dehydration step of heating the porous glass preform while supplying an inert gas, which is nitrogen, argon, or a mixed gas of nitrogen and argon, and a dehydration gas to the furnace tube; a first transparentization step of heating the porous glass preform while stopping the supply of the dehydration gas and supplying the inert gas to the furnace tube after the dehydration step; and a second transparentizing step of lowering and heating the porous glass base material while stopping the supply of the inert gas and supplying helium gas to the furnace tube after the first transparentizing step. Effect of the Invention

[0007] According to the method for manufacturing an optical fiber preform of the present disclosure, the amount of helium gas used can be reduced. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a cross-sectional view showing an example of a heating furnace for an optical fiber preform. [Diagram 2] FIG. 2 is an enlarged view of the lower end of the porous glass preform. 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: heat treating, in a furnace tube, a porous glass preform having an effective portion to be a product and a non-effective portion located below the effective portion and not to be a product, the method comprising the steps of: a dehydration step of heating the porous glass preform while supplying an inert gas, which is nitrogen, argon, or a mixed gas of nitrogen and argon, and a dehydration gas to the furnace tube; a first transparentization step of heating the porous glass preform while stopping the supply of the dehydration gas and supplying the inert gas to the furnace tube after the dehydration step; a second transparentization step in which, after the first transparentization step, the supply of the inert gas is stopped and the porous glass preform is lowered and heated while supplying helium gas to the furnace tube; Equipped with.

[0010] According to this embodiment, the lower end of the porous glass preform is heated without using helium gas, so that the amount of helium gas used can be reduced.

[0011] (2) In the above (1), the dehydration gas may be chlorine gas.

[0012] According to this embodiment, the porous glass base material can be effectively dehydrated.

[0013] (3) In the above (1) or (2), at least a part of the non-effective portion may be made transparent in the first transparentizing step.

[0014] According to this embodiment, since helium gas is not used to make transparent the portions that do not become products, the amount of helium gas used can be further reduced.

[0015] (4) In the above (3), the first transparentizing step may be switched to the second transparentizing step during a period from when the porous glass base material starts to descend until when the boundary between the non-effective portion and the effective portion enters a heat zone.

[0016] According to this embodiment, since the heating is switched to helium gas before the effective portion enters the heat zone, it is possible to prevent heating by nitrogen gas or the like from affecting the transparency of the effective portion.

[0017] (5) In the above (4), the first transparentizing step may be switched to the second transparentizing step after the porous glass preform starts to descend and before a part of the non-effective portion at a height half the height of the boundary between the non-effective portion and the effective portion, based on a lower end of the non-effective portion, enters a heat zone.

[0018] According to this embodiment, it is possible to more reliably prevent heating by nitrogen gas or the like from affecting the transparency of the effective portion.

[0019] (6) In any one of the above (1) to (5), the helium gas discharged from the core tube in the second transparentizing step may be recovered.

[0020] According to this embodiment, since the dehydrated gas is discharged in the first transparentization step and then the helium gas is recovered, damage to the helium recovery equipment due to the dehydrated gas can be prevented.

[0021] (7) In the above (6), the helium gas may be recovered at least 30 minutes after the dehydration step is completed.

[0022] According to this embodiment, since the helium gas is recovered after the concentration of the dehydrated gas has sufficiently decreased, deterioration of the equipment can be prevented more reliably.

[0023] [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.

[0024] (heating furnace) 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.

[0025] 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).

[0026] 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.

[0027] 1, the portion surrounded by the heater 13 in the furnace tube 11 is referred to in this disclosure as the heat zone 16. More specifically, the heat zone 16 is a region located between the upper end and the lower end of the heater 13.

[0028] A porous glass preform 2 is suspended from above inside the heating furnace 1. The porous glass preform 2 is a long preform obtained by depositing glass particles on a starting rod. Both ends of the porous glass preform 2 are tapered. The porous glass preform 2 is supported by a support rod 3 connected to the starting rod, and can be moved up and down via the support rod 3. The porous glass preform 2 has an effective portion that is made transparent and then drawn to become a product (optical fiber), and a non-effective portion that is not made into a product. Specifically, in FIG. 1, the central portion of the porous glass preform 2 between the dashed lines is the effective portion 21, the tapered portion below the effective portion 21 is the non-effective portion 22, and the tapered portion above the effective portion 21 is the non-effective portion 23.

[0029] (Method of manufacturing optical fiber base material) Next, a method for manufacturing an optical fiber preform according to this embodiment will be described. The optical fiber preform is obtained by dehydrating and sintering a porous glass preform 2 produced by a VAD method, an OVD method, or the like to make it transparent. The method for manufacturing an optical fiber preform according to this embodiment includes a dehydration step, a first transparentizing step, and a second transparentizing step.

[0030] (Dehydration process) In the dehydration process, the porous glass preform 2 is first inserted from above into the furnace tube 11, and the lower end of the porous glass preform 2 is adjusted so as to enter the heat zone 16. Next, an inert gas such as nitrogen gas, argon gas, or a mixture of these gases (hereinafter referred to as "inert gas such as nitrogen gas") and a gas having a dehydrating effect (dehydration gas) are supplied from the gas supply port 15 to the furnace tube 11, and heating by the heater 13 is started. After that, the porous glass preform 2 is gradually moved downward and passed through the heat zone 16 in sequence, thereby dehydrating the entire porous glass preform 2. The dehydration gas may be a halogen gas or a halogen compound gas, and may be chlorine gas in particular. After the dehydration of the entire porous glass preform 2 is completed, the porous glass preform 2 is moved upward to return to the position at the start of the dehydration process.

[0031] Next, the dehydrated porous glass preform 2 is sintered at a high temperature to make it transparent. The manufacturing method of the optical fiber preform according to the present embodiment includes a first transparent process and a second transparent process as processes for making the porous glass preform 2 transparent.

[0032] (First transparency process) The first transparentization step is a step performed after the dehydration step. In the first transparentization step, the supply of the dehydration gas is stopped, and heating is performed by the heater 13 while supplying an inert gas such as nitrogen gas to the furnace tube 11.

[0033] In the first transparentization step, the position of the lower end of the porous glass preform 2 is adjusted so that it is in the heat zone, and the lower end of the porous glass preform 2 is heated for a predetermined time. The lower end of the porous glass preform 2 is a portion that is likely to be insufficiently sintered, and sufficient sintering can be achieved, for example, by extending the heating time. The time for heating the lower end of the porous glass preform 2 in the first transparentization step is, for example, 15 minutes or more and 60 minutes or less, and preferably 20 minutes or more and 40 minutes or less. If the heating time is 15 minutes or more, unsintered portions are unlikely to occur. If the heating time is 60 minutes or less, the transparentization step time is not excessively long.

[0034] Usually, the porous glass preform 2 needs to be heated while supplying helium gas to make it transparent, and problems such as the generation of bubbles occur if helium gas is not used. Therefore, it was conventionally thought that helium gas also needed to be used to heat the lower end of the porous glass preform 2. However, according to the present disclosure, it has been found that the problem of bubbles does not occur even if the non-useful portion 22 at the lower end of the porous glass preform 2 is heated with an inert gas such as nitrogen gas instead of helium gas by appropriately adjusting the heating time, etc.

[0035] The reason why the problem of air bubbles does not occur even in an inert gas atmosphere such as nitrogen gas is believed to be as follows: The non-effective portion 22 at the lower end of the porous glass preform 2 is tapered and has a larger surface area than the effective portion 21. Therefore, even if air bubbles are generated inside the non-effective portion 22, they tend to escape to the outside through the surface, which is believed to be one of the reasons why the problem of air bubbles does not occur.

[0036] In the first transparentizing step, it is preferable to transparentize at least a part of the non-effective portion 22 at the lower end of the porous glass preform 2. The transparentization of the porous glass preform 2 is usually performed using helium gas, but the non-effective portion 22 can be at least partially transparentized with an inert gas such as nitrogen gas by, for example, extending the heating time.

[0037] (Second transparency process) The second transparentizing step is a step performed after the first transparentizing step. In the second transparentizing step, the supply of the inert gas such as nitrogen gas that was supplied in the first transparentizing step is stopped, and helium gas is supplied into the furnace tube 11. In the second transparentizing step, the porous glass preform 2 is lowered, and the entire porous glass preform 2 is heated to make it transparent.

[0038] According to this embodiment, since an inert gas such as nitrogen gas is used instead of helium gas in the first transparentization step, which is the initial stage of the transparentization step, the amount of helium gas used can be reduced.

[0039] 2 is an enlarged view of the non-effective portion 22 at the lower end of the porous glass preform 2. The first transparentizing process is preferably switched to the second transparentizing process between the start of descent of the porous glass preform 2 and the boundary L1 between the non-effective portion 22 and the effective portion 21 entering the heat zone 16, and more preferably between the lower end L0 of the non-effective portion 22 being taken as the height reference and the portion Lh located at half the height of the boundary L1 between the non-effective portion 22 and the effective portion 21 entering the heat zone 16. By switching to the second transparentizing process at the above timing, air bubbles are less likely to be generated in the transparentized effective portion 21.

[0040] Furthermore, since helium gas is expensive, the helium gas discharged from the core tube 11 may be collected and refined for reuse. When recovering the helium gas supplied into the core tube 11 in the transparentization step, if dehydrated gas such as chlorine gas remains in the core tube 11, it may damage the helium recovery equipment. Therefore, if helium gas is supplied immediately after the dehydration step to perform the transparentization step, the helium gas cannot be recovered until the dehydrated gas is discharged from the core tube 11. It may be possible to provide a separate step of discharging the dehydrated gas before the transparentization step, but in that case, the time required for the entire process will be longer.

[0041] In this embodiment, it is preferable to recover the helium gas discharged from the furnace core tube 11 in the second transparentization step. In this embodiment, the supply of the dehydration gas is stopped in the first transparentization step and only an inert gas such as nitrogen gas is supplied, so that the dehydration gas concentration in the furnace core tube 11 decreases at the stage of the first transparentization step. Therefore, by recovering the helium gas in the second transparentization step, the helium gas can be recovered without damaging the helium recovery equipment. In addition, since the non-useful portion 22 at the lower end of the porous glass base material 2 is heated in the first transparentization step, the time required for the entire process is not excessively long.

[0042] When recovering helium gas in the second clearing step, it is preferable to start recovering helium gas at least 30 minutes after the supply of the dehydration gas is stopped. Alternatively, it is preferable to start recovering helium gas after the total amount of gas supplied into the furnace core tube 11 after the supply of the dehydration gas is stopped becomes at least twice the volume of the furnace core tube 11. According to this configuration, helium gas can be recovered in a state where the concentration of the dehydration gas is sufficiently reduced. [Explanation of symbols]

[0043] 1 Furnace 11. Core tube 12 Furnace body 13 Heater 14 Lid 15 Gas supply port 16 Heat Zone 2. Porous glass matrix 21 Valid Part 22,23 Invalid part 3 Support rod

Claims

1. A method for manufacturing an optical fiber preform, comprising the steps of: heat-treating a porous glass preform in a furnace tube, the porous glass preform having an effective portion to be a product and a non-effective portion located below the effective portion and not to be a product; a dehydration step of heating the porous glass preform while supplying an inert gas, which is nitrogen, argon, or a mixed gas of nitrogen and argon, and a dehydration gas to the furnace tube; a first transparentization step of heating the porous glass preform while stopping the supply of the dehydration gas and supplying the inert gas to the furnace tube after the dehydration step; a second transparentization step in which, after the first transparentization step, the supply of the inert gas is stopped and the porous glass preform is lowered and heated while supplying helium gas to the furnace tube; A method for manufacturing an optical fiber preform comprising the steps of:

2. 2. The method for producing an optical fiber preform according to claim 1, wherein the dehydration gas is chlorine gas.

3. 3. The method for producing an optical fiber preform according to claim 1, wherein at least a part of the non-effective portion is made transparent in the first transparentizing step.

4. The switching from the first transparentizing step to the second transparentizing step is performed during a period from when the porous glass base material starts to descend until a boundary between the non-effective portion and the effective portion enters a heat zone. The method for producing the optical fiber preform according to claim 3 .

5. the switching from the first transparentizing step to the second transparentizing step is performed during a period from when the descent of the porous glass base material starts until a portion of the non-effective portion, which is located at a height equal to half the height of the boundary between the non-effective portion and the effective portion, based on a lower end of the non-effective portion, enters a heat zone. The method for producing the optical fiber preform according to claim 4 .

6. 2. The method for producing an optical fiber preform according to claim 1, wherein the helium gas discharged from the furnace tube is recovered in the second transparentizing step.

7. 7. The method for producing an optical fiber preform according to claim 6, wherein the helium gas is collected at least 30 minutes after the supply of the dehydration gas is stopped.

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