Manufacturing method for optical fiber preforms

Abstract

This reduces the increase in transmission loss of optical fibers obtained by fiberizing the upper end of a porous glass matrix. [Solution] A method for manufacturing a fiber optic preform by heat-treating a porous glass preform in a furnace tube, comprising: a dehydration step in which the porous glass preform is lowered and passed through a heat zone while maintaining a negative pressure inside the furnace tube relative to atmospheric pressure and supplying an inert gas and a dehydrating gas to the furnace tube, thereby sequentially heating it; a positive pressure step in which the pressure inside the furnace tube is increased relative to atmospheric pressure after the dehydration step; a porous glass preform raising step in which the porous glass preform is raised after the positive pressure step; and a transparency step in which the porous glass preform is lowered and passed through a heat zone while supplying an inert gas to the furnace tube, thereby sequentially heating and making it transparent after the porous glass preform raising step.

Description

Technical Field

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

Background Art

[0002] Patent Document 1 discloses a method for manufacturing a porous preform for an optical fiber, including a step of ventilating the inside of a core tube with helium gas before a transparent vitrification step.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] A preform for an optical fiber is obtained by dehydrating and sintering a porous glass preform to make it transparent. An optical fiber is obtained by drawing the preform for an optical fiber. However, when the portion corresponding to the upper end portion of the porous glass preform is made into a fiber, the transmission loss of the obtained optical fiber may increase.

[0005] An object of the present disclosure is to reduce an increase in transmission loss of an optical fiber obtained by making a fiber from the upper end portion of a porous glass preform.

Means for Solving the Problems

[0006] The present disclosure is a method for manufacturing a preform for optical fibers, comprising: a dehydration step in which a porous glass preform is heat-treated in a furnace tube, the porous glass preform is sequentially heated by lowering it and passing it through a heat zone while maintaining a negative pressure inside the furnace tube relative to atmospheric pressure and supplying an inert gas and a dehydrating gas to the furnace tube; a positive pressure step in which, after the dehydration step, the pressure inside the furnace tube is made positive pressure relative to atmospheric pressure; a porous glass preform raising step in which the porous glass preform is raised after the positive pressure step; and a transparency step in which, after the porous glass preform raising step, the porous glass preform is sequentially heated and made transparent by lowering it and passing it through a heat zone while supplying an inert gas to the furnace tube. [Effects of the Invention]

[0007] According to the method for manufacturing optical fiber preforms of this disclosure, the increase in transmission loss of optical fibers obtained by fiberizing the upper end of a porous glass preform can be reduced. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a cross-sectional view showing an example of a heating furnace for optical fiber preforms. [Figure 2] Figure 2 shows the position of the porous glass base material when the positive pressure process is carried out. [Modes for carrying out the invention]

[0009] [Description of Embodiments in this Disclosure] First, embodiments of this disclosure will be listed and described. (1) A method for manufacturing a preform for optical fibers according to one embodiment of the present disclosure is a method for manufacturing a preform for optical fibers in which a porous glass preform is heat-treated in a furnace tube, comprising: a dehydration step in which the porous glass preform is sequentially heated by lowering it and passing it through a heat zone while maintaining a negative pressure inside the furnace tube with respect to atmospheric pressure and supplying an inert gas and a dehydrating gas to the furnace tube; a positive pressure step in which the pressure inside the furnace tube is made positive with respect to atmospheric pressure after the dehydration step; a porous glass preform raising step in which the porous glass preform is raised after the positive pressure step; and a transparency step in which the porous glass preform is sequentially heated and made transparent by lowering it and passing it through a heat zone while supplying an inert gas to the furnace tube after the porous glass preform raising step.

[0010] According to this embodiment, even if outside air is drawn into the upper part of the core tube during the positive pressure process, OH groups are less likely to form at the upper end of the porous glass base material. As a result, the increase in transmission loss of the optical fiber obtained by fiberizing the upper end of the porous glass base material can be reduced.

[0011] (2) In (1) above, the porous glass base material raising step may be started after the position of the porous glass base material has been maintained for 5 minutes to 30 minutes after the pressure inside the reactor core tube has been made positive relative to atmospheric pressure in the positive pressure step. With this configuration, outside air that has entered the reactor core tube during the switch to positive pressure can be sufficiently exhausted, the increase in transmission loss due to OH groups can be reduced more reliably, and the total process time can be avoided to be excessively long.

[0012] (3) In (2) above, while the position of the porous glass base material is maintained until the porous glass base material raising process is started, the temperature of the heat zone may be lower than the temperature of the heat zone in the dewatering process. Lowering the temperature inside the reactor tube reduces convection, making it easier for outside air trapped in the upper part of the reactor tube to be discharged.

[0013] (4) In any of (1) to (3) above, after the dehydration step and before the transparency step, the dehydration gas may not be supplied for a predetermined time, and an inert gas other than helium gas may be supplied to dilute the dehydration gas in the reactor core tube, and in the transparency step, helium gas may be supplied as the inert gas, and the helium gas discharged from the reactor core tube may be recovered. By combining the dilution of the dehydration gas due to the recovery of helium gas with the configuration of the present disclosure, the increase in transmission loss of the optical fiber can be effectively reduced.

[0014] [Details of the embodiments of this disclosure] A specific example of the manufacturing method for the optical fiber preform of this disclosure will be described below with reference to the drawings. However, the present invention is not limited to these examples, and is intended to include all modifications within the meaning and scope of the claims as shown, and equivalents thereof.

[0015] (heating furnace) First, the configuration of a heating furnace for carrying out the manufacturing method of optical fiber preforms according to this disclosure will be described. Figure 1 is a cross-sectional view showing an example of a heating furnace 1 for manufacturing optical fiber preforms. The heating furnace 1 has a cylindrical furnace core tube 11 made of quartz glass, a heater 13 arranged to surround the furnace core tube 11, and a furnace body 12 formed to cover the heater 13.

[0016] A gas supply port 14 is provided at the bottom of the reactor core tube 11, and is configured to supply helium gas, chlorine gas, etc., used in the manufacturing process of optical fiber preforms into the reactor core tube 11 in the direction of arrow A1 in the figure. In addition, a gas outlet 15 is provided at the top of the reactor core tube 11, and is configured to discharge the gas inside the reactor core tube 11 in the direction of arrow A2 in the figure.

[0017] The furnace body 12 is also provided with gas supply and gas discharge ports (not shown), and is configured to supply inert gases such as nitrogen gas into the furnace body 12. Inside the furnace body 12, in addition to the heater 13, an insulating material (not shown) made of carbon or other materials is arranged.

[0018] In FIG. 1, the portion surrounded by the dashed line inside the core tube 11 and surrounded by the heater 13 is referred to as the heat zone 16 in the present disclosure. More specifically, the heat zone 16 is a region located between the upper end and the lower end of the heater 13 inside the core tube 11.

[0019] Inside the heating furnace 1, the porous glass base material 2 is suspended from above. The porous glass base material 2 is a long base material obtained by depositing glass fine particles on a starting rod. Both ends of the porous glass base material 2 are tapered. The porous glass base material 2 is supported by a support rod 3 connected to the starting rod and is movable upward and downward via the support rod 3.

[0020] (Method for manufacturing a base material for an optical fiber) Next, a method for manufacturing a base material for an optical fiber according to the present embodiment will be described. The base material for an optical fiber is obtained by dehydrating, sintering, and rendering transparent a porous glass base material 2 produced by a method such as the VAD method or the OVD method. The method for manufacturing a base material for an optical fiber according to the present embodiment includes a dehydration step, a positive pressure step, a porous glass base material lifting step, and a transparency step.

[0021] In the dehydration process, first, the porous glass base material 2 is inserted into the core tube 11 from above, and the porous glass base material 2 is arranged above the heating zone 16. The position of the porous glass base material 2 at this time may be a position where the lower end of the porous glass base material 2 does not enter the heating zone 16 as shown in FIG. 1, or may be a position where the lower end of the porous glass base material 2 enters the heating zone 16. Next, an inert gas which is nitrogen gas, argon gas, helium gas or a mixed gas of two or more of these, and a gas having a dehydration effect (dehydration gas) are supplied from the gas supply port 14 to the core tube 11, and heating by the heater 13 is started. Thereafter, the porous glass base material 2 is gradually moved downward and sequentially passed through the heating zone 16 to dehydrate the entire porous glass base material 2. The dehydration gas may be a halogen gas or a gas of a halogen compound, and particularly may be chlorine gas. FIG. 2 shows a state where the upper end of the porous glass base material 2 has reached the heating zone 16, and shows the position of the porous glass base material 2 when the dehydration process is completed. Note that FIG. 2 is the same as FIG. 1 except for the position of the porous glass base material 2, and duplicate explanations are omitted.

[0022] Since the dehydration gas such as chlorine gas used in the dehydration process is harmful, it is necessary to prevent the leakage of the dehydration gas to the outside of the heating furnace 1. Therefore, in the dehydration process, the porous glass base material 2 is heated while maintaining the inside of the core tube 11 at a negative pressure with respect to the atmospheric pressure.

[0023] After the entire porous glass matrix 2 has been dehydrated, a positive pressure process is performed to increase the pressure inside the core tube 11 to a positive pressure relative to atmospheric pressure. By creating a positive pressure inside the core tube 11, it is possible to prevent air from entering the core tube 11, mainly through the gap between the support rod 3 and the core tube 11, during the subsequent transparency process. Specific examples of actions to perform the positive pressure process include increasing the amount of gas supplied from the gas supply port 14, and decreasing or stopping the amount of gas discharged from the gas outlet 15. The gas supplied from the gas supply port 14 during the positive pressure process may be an inert gas such as nitrogen gas or argon gas. During the positive pressure process, the porous glass matrix 2 is still located at the bottom of the core tube 11, as shown in Figure 2. After creating a positive pressure inside the core tube 11, the porous glass matrix 2 may be held for a predetermined time before performing the porous glass matrix raising process described later.

[0024] After the positive pressure process, a porous glass base material raising process is performed to raise the porous glass base material 2. In this process, the porous glass base material 2 is moved from the position shown in Figure 2 to, for example, the position shown in Figure 1.

[0025] After the porous glass matrix raising process, a transparency process is carried out to make the entire porous glass matrix 2 transparent. In the transparency process, an inert gas such as helium gas is supplied into the furnace tube 11, and the porous glass matrix 2 is gradually lowered and heated by sequentially passing through the heat zone 16, thereby making the entire porous glass matrix 2 transparent. As already mentioned, the transparency process is carried out under positive pressure inside the furnace tube 11, thereby preventing air from entering the furnace tube 11. Through the above process, a fully transparent optical fiber matrix is ​​obtained.

[0026] According to the method for manufacturing optical fiber preforms of this embodiment described above, the increase in transmission loss of optical fibers obtained by fiberizing the upper end of the porous glass preform 2 can be reduced. The reason for this is explained below.

[0027] In the positive pressure induction process of this embodiment, when switching the pressure inside the reactor tube 11 from negative to positive, outside air is easily unintentionally drawn into the reactor tube 11 through gaps between the support rod 3 and the reactor tube 11, resulting in a temporary state where outside air is contained inside the reactor tube 11. At this time, if the porous glass base material 2 is located at the top of the reactor tube 11 as shown in Figure 1, the upper end of the porous glass base material 2 comes into contact with the outside air contained inside the reactor tube 11, and OH groups are formed on the upper end of the porous glass base material 2. The OH groups formed in this way cause an increase in the transmission loss of the manufactured optical fiber.

[0028] In the manufacturing method for optical fiber preforms of this embodiment, the positive pressure step is performed after the dewatering step and before the porous glass preform is raised, i.e., in the state shown in Figure 2. Therefore, even if outside air enters through the gap between the support rod 3 and the furnace tube 11 when the pressure is switched to positive, the upper end of the porous glass preform 2 can be prevented from coming into contact with the outside air. This prevents the formation of OH groups and reduces the increase in transmission loss of the optical fiber manufactured from the upper end of the porous glass preform 2.

[0029] In this embodiment, after the pressure inside the reactor core tube 11 is made positive relative to atmospheric pressure in the positive pressure induction process, the position of the porous glass base material 2 may be maintained for 5 minutes to 30 minutes before starting the porous glass base material raising process. By holding for 5 minutes or more, outside air that has entered the reactor core tube 11 during the switch to positive pressure can be sufficiently exhausted, and the increase in transmission loss due to OH groups can be more reliably reduced. By holding for 30 minutes or less, the total process time will not become excessively long, and a decrease in productivity can be avoided. A holding time of 5 minutes to 10 minutes is more preferable for the porous glass base material 2.

[0030] Furthermore, in the above embodiment, the temperature of the heat zone 16 may be lower than the temperature of the heat zone 16 in the dewatering process while the porous glass base material 2 is held in place until the porous glass base material rising process is started. Lowering the temperature inside the reactor core tube 11 reduces convection, making it easier for outside air trapped in the upper part of the reactor core tube 11 to be discharged.

[0031] In this embodiment, after the dehydration process and before the transparency process, the dehydration gas may be not supplied for a predetermined time, and an inert gas other than helium gas may be supplied to dilute the dehydration gas in the core tube 11. Then, in the transparency process, helium gas may be supplied as the inert gas, and the helium gas discharged from the core tube 11 may be recovered. Since helium gas is scarce, when helium gas is used in the transparency process, it is usually recovered and reused. However, if the helium gas recovered by the helium recovery device contains dehydration gas such as chlorine gas, the helium recovery device may deteriorate due to the dehydration gas. Therefore, time is required after the dehydration process and before the transparency process to dilute the dehydration gas in the core tube 11. If, after the dehydration process, the porous glass base material is raised (i.e., to the position of the porous glass base material 2 shown in Figure 1) and the inside of the core tube 11 is made positively pressurized, the porous glass base material will be in constant contact with the outside air during the time required to dilute the dehydration gas, making it more likely that transmission loss due to the formation of OH groups will increase. From this perspective, when helium recovery is performed, combining this configuration with the configuration of this embodiment, in which the porous glass base material 2 is held downwards as shown in Figure 2 and the inside of the reactor tube 11 is pressurized, allows the porous glass base material 2 to be retracted to the position shown in Figure 2 while the dewatered gas is being diluted. As a result, the formation of OH groups on the upper end of the porous glass base material 2 can be avoided, and the increase in transmission loss of the optical fiber can be effectively reduced.

[0032] In the above embodiment, the predetermined time for diluting the dewatering gas typically corresponds to the time from when the supply of the dewatering gas is stopped until the descent of the porous glass base material begins in the clarification process. The predetermined time is preferably 30 minutes or more and 60 minutes or less. In this embodiment, the dilution of the dewatering gas in the furnace tube 11 may be performed in parallel with at least one of the positive pressure process, the positioning of the porous glass base material, and the rising process of the porous glass base material. According to this embodiment, the total process time can be shortened.

[0033] The transparency process is usually carried out by heating the porous glass base material 2 while supplying helium gas. However, for example, the transparency of the lower end of the porous glass base material 2 may be carried out using an inert gas other than helium gas (e.g., nitrogen gas or argon gas). More specifically, the transparency process may include a first transparency step in which, after the dehydration step, the supply of dehydration gas is stopped and the porous glass base material is heated while supplying an inert gas, such as nitrogen, argon, or a mixture of nitrogen and argon, to the furnace tube 11; and a second transparency step in which, after the first transparency step, the supply of the inert gas is stopped and the porous glass base material is lowered and heated while supplying helium gas to the furnace tube 11. By performing the transparency of a portion of the porous glass base material 2 with an inert gas other than helium gas, the amount of helium gas used can be reduced. [Explanation of Symbols]

[0034] 1 Furnace 2. Porous glass base material 3 Support rod 11 core tubes 12 Furnace body 13 Heater 14 Gas supply port 15 Gas outlet 16 Heat Zones

Claims

1. A method for manufacturing a preform for optical fibers, comprising heat-treating a porous glass preform in a furnace tube, A dehydration step is performed in which the porous glass base material is sequentially heated by lowering it and passing it through a heat zone while maintaining a negative pressure inside the reactor core tube relative to atmospheric pressure and supplying an inert gas and a dehydrating gas to the reactor core tube, After the dewatering step, a positive pressure step is performed to increase the pressure inside the reactor core tube to a positive pressure relative to atmospheric pressure. After the positive pressure step, a porous glass base material raising step is performed to raise the porous glass base material, After the porous glass matrix raising step, a transparency step is performed in which the porous glass matrix is ​​lowered while supplying an inert gas to the furnace tube and passing through the heat zone, thereby sequentially heating and making the porous glass matrix transparent. A method for manufacturing a preform for optical fibers, comprising the above.

2. A method for manufacturing a fiber optic preform according to claim 1, wherein, in the positive pressure generation step, the pressure inside the furnace tube is made positive relative to atmospheric pressure, and the position of the porous glass preform is maintained for 5 minutes to 30 minutes before starting the porous glass preform raising step.

3. The method for manufacturing an optical fiber preform according to claim 2, wherein the temperature of the heat zone is lower than the temperature of the heat zone in the dehydration step while the position of the porous glass preform is maintained until the porous glass preform raising step is started.

4. After the dehydration step and before the transparency step, the dehydration gas in the reactor core tube is diluted by supplying an inert gas other than helium gas for a predetermined time without supplying the dehydration gas. A method for manufacturing a preform for optical fibers according to any one of claims 1 to 3, wherein in the transparency step, helium gas is supplied as the inert gas, and the helium gas discharged from the furnace tube is recovered.

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