Manufacturing method of glass base material for optical fiber

Abstract

(Problem) The objective is to provide a method for manufacturing a glass base material for optical fibers that can obtain a glass base material for optical fibers with reduced length variation in the optical properties of the optical fibers. (Solution) A method for manufacturing a glass substrate for optical fibers comprises: a first heat treatment process in which a porous glass substrate inserted into a container is heated by a heater installed on the outer circumference of the container while raising or lowering it in the longitudinal direction, with the inside of the container being in a chlorine-containing gas atmosphere; a second heat treatment process in which, after the first heat treatment process, a transparent glass body is obtained by heating the porous glass substrate with a heater while raising or lowering it in the longitudinal direction, with the inside of the container being in an inert gas atmosphere; and a preliminary fluorine doping process in which, prior to the second heat treatment process, one or both ends of the porous glass substrate are heated by a heater while the inside of the container is in a fluorine-containing gas atmosphere.

Description

Technology Field

[0001] The present invention relates to a transparent vitrification treatment of dehydrating and sintering a porous glass substrate, and in particular, to a method for manufacturing a glass substrate for optical fibers having uniform characteristics in the longitudinal direction when doped with fluorine. Background Technology

[0002] A method for manufacturing a glass substrate is known, wherein a porous glass substrate is placed in a heating furnace and dehydrated, fluorine-doped, and sintered to obtain a transparent glass substrate (see, for example, Patent Documents 1 to 3).

[0003] Patent Document 1 describes a method of obtaining a transparent glass substrate uniformly doped with fluorine in the diameter direction by receiving a porous glass substrate in a heating furnace, moving it through a heating zone under a chlorine-containing gas atmosphere to dehydrate it, moving it through a heating zone under a fluorine gas atmosphere to perform a fluorine addition treatment, and then vitrifying it.

[0004] Patent Document 2 describes a method of receiving a porous glass base material in a heating furnace, moving it through a heating zone under a chlorine-containing gas atmosphere to dehydrate it, and then moving it through a heating zone under a helium gas and fluorine-containing gas atmosphere to dope a part of the clad with fluorine and simultaneously make it transparent glass.

[0005] In addition, Patent Document 3 describes a porous glass matrix being placed in a cracking furnace, dehydrated by heating under a chlorine-containing gas atmosphere, doped with fluorine by heating under a fluorine-containing gas atmosphere, and then converted into transparent glass by heating under a helium-containing gas atmosphere. Prior art literature

[0006] Japanese Published Patent Application No. 2004-307281, Japanese Published Patent Application No. 2012-250887, Japanese Published Patent Application No. 2017-154935 The problem to be solved

[0007] When fluorine is doped into a porous glass substrate during sintering, there is a problem in that the distribution of the doping amount varies along the long side of the glass substrate, and variations in the optical properties of the optical fiber produced therefrom are likely to occur along the long side. In particular, variations in the optical properties of the optical fiber produced near the top and bottom of the porous glass substrate are likely to occur. Taking the above problems into account, the present invention aims to provide a method for manufacturing a glass substrate for optical fibers that can obtain a glass substrate for optical fibers with reduced variations in the optical properties of the optical fibers. means of solving the problem

[0008] To solve the above problem, a method for manufacturing a glass substrate for optical fibers according to the present invention comprises: a first heat treatment process in which a porous glass substrate inserted into a container is heated by a heater installed on the outer circumference of the container while raising or lowering it in the longitudinal direction, with the inside of the container of a sintering furnace in a chlorine-containing gas atmosphere; a second heat treatment process in which, after the first heat treatment process, a transparent glass body is obtained by heating the porous glass substrate with a heater while raising or lowering it in the longitudinal direction, with the inside of the container in an inert gas atmosphere; and a preliminary fluorine doping process in which, prior to the second heat treatment process, one or both ends of the porous glass substrate are heated by a heater while the inside of the container in a fluorine-containing gas atmosphere.

[0009] In the present invention, the preliminary fluorine doping process may be performed before the first heat treatment process. Alternatively, the preliminary fluorine doping process may be performed after the completion of the first heat treatment process.

[0010] In the present invention, in the first heat treatment process, the inside of the container may be made into a mixed atmosphere of chlorine-based gas and fluorine-based gas. Also, in the second heat treatment process, the inside of the container may be made into a mixed atmosphere of inert gas and fluorine-based gas.

[0011] In the present invention, the preliminary fluorine doping process may be carried out by fixing or slightly moving the position of the porous glass substrate. In addition, the preliminary fluorine doping process may be carried out at a temperature of 1000°C or higher and 1400°C or lower.

[0012] In the present invention, the length of the heater can be made less than or equal to one-fourth of the length of the porous glass base material.

[0013] In the present invention, the fluorine-based gas introduced into the container may be any of SiF4, CF4, SF6, and C2F6. Also, the chlorine-based gas introduced into the container may be any of SiCl4 and Cl2. Effects of the invention

[0014] By wire drawing the glass substrate for optical fiber obtained by the method for manufacturing a glass substrate for optical fiber related to the present invention, an optical fiber with suppressed length variation in optical properties can be obtained. Brief explanation of the drawing

[0015] FIG. 1 is a diagram showing the configuration of a heating device used in a method for manufacturing a glass base material related to the present invention. Figure 2 is a diagram showing the sequence of a manufacturing method using the heating device shown in Figure 1. Figure 3 shows the length distribution of the cutoff wavelengths of the optical fibers obtained from Example 1 and Comparative Example 1, respectively. Figure 4 shows the length distribution of the cutoff wavelengths of the optical fibers obtained from Example 2 and Comparative Example 2, respectively. Specific details for implementing the invention

[0016] [First Embodiment]

[0017] In the method for manufacturing a glass substrate for optical fibers related to an embodiment of the present invention, a porous glass substrate is first manufactured by various methods including vapor phase deposition (VAD), external deposition (OVD), and multi-burner multi-layer deposition (MMD). The porous glass substrates manufactured by these methods are all formed as aggregates of only glass microparticles or as glass microparticles deposited on the outer circumference of a transparent glass rod. The porous glass substrate formed in this way is then sintered to become a transparent glass substrate for optical fibers.

[0018] The VAD method involves placing a burner below a rotating starting glass rod and introducing raw material gas into an oxyhydrogen flame formed by the burner to produce glass microparticles through a flame hydrolysis reaction, and depositing the produced glass microparticles along the axial direction of the starting rod to produce a porous glass matrix. The OVD method and MMD method, for example, involve placing a burner on the outer circumference of a starting glass rod rotating inside a reaction vessel and introducing raw material gas into an oxyhydrogen flame formed by the burner to produce glass microparticles through a flame hydrolysis reaction, and depositing the produced glass microparticles on the outer circumference of the starting glass rod to produce a porous glass matrix.

[0019] In a standard substrate for a single-mode optical fiber, a region called the core is formed in the center. The core is often doped with Ge to increase the refractive index of the quartz glass. Additionally, a layer with a lower refractive index compared to the core, called the clad, is formed around the core. Regarding the manufacturing method of the clad, a two-step method is common in which a portion of the clad and a portion having the core are manufactured first, and the remaining clad is applied to the outside thereof, or a multi-step method in which the application of the clad is carried out in multiple stages. In the present invention, the glass substrate refers collectively to the portion having the clad and the core, and the core and the clad as a whole.

[0020] The dehydration and transparent vitrification of the manufactured porous glass substrate are carried out using a core tube formed of a heat-resistant material such as carbon or quartz, and a heating device having a heater placed on the outer circumference of the core tube. Transparent vitrification is performed by moving the heating zone by raising or lowering the porous glass substrate inserted into the core tube of the heating device. Additionally, prior to transparent vitrification, it is possible to form a dehydration process by placing the inside of the core tube in a mixed gas atmosphere of chlorine-based gas and inert gas, and raising, lowering, or agitating the porous glass substrate relative to the heating zone.

[0021] Fluorine doping is performed by incorporating a fluorine-based gas into the atmosphere gas of this dehydration process or transparent vitrification process. Fluorine has the effect of lowering the refractive index distribution of quartz glass, and by performing fluorine doping, a complex refractive index distribution is created in the glass substrate, thereby allowing the optical properties of the optical fiber obtained from the glass substrate to be adjusted.

[0022] Since the characteristics of an optical fiber depend on the refractive index distribution of the underlying glass substrate, it is important to equalize the doping amount along the length of the glass substrate in order to obtain desired optical characteristics over the entire length of the optical fiber. To equalize the doping amount along the length, it is effective to equalize the thermal history at each longitudinal position. However, especially at the top and bottom of the porous glass substrate, a tapered shape is formed that contracts toward the ends, and since the heat-receiving area of ​​this tapered portion is smaller compared to the straight section, it is difficult to equalize the thermal history while maintaining a constant temperature in the heating zone.

[0023] FIG. 1 is a diagram showing the configuration of a heating device used in a method for manufacturing a glass base material related to the present invention. The heating device for a glass base material used in a method for manufacturing a glass base material related to the present embodiment comprises a sintering furnace (1). The sintering furnace has a core tube (3), which is a container, inside a furnace body (2). In the core tube, a gas inlet port (4) is formed at the bottom and an exhaust port (5) is formed at the top. A cover (6) that opens and closes when the porous glass base material is loaded and unloaded is formed on the upper surface of the core tube. A lifting device (7) is installed above the core tube, and the lifting device supports a suspension rod (8) that penetrates the cover so that it can be lifted and unloaded and rotated freely. A dummy rod (9) is supported on this suspension rod. A porous glass base material (10) is attached to one end of the dummy rod (9). When heating the porous glass base material to make it transparent glass, the atmosphere around the porous glass base material is made into a helium gas atmosphere. However, if the amount is small, there is no problem even if gases other than helium are mixed in.

[0024] In addition, before the porous glass substrate is made transparent in the sintering furnace, the atmosphere around the porous glass substrate may be heated to a mixed atmosphere of chlorine-based gas and helium gas. Accordingly, OH groups within the porous glass substrate can be effectively removed (dehydrated). Also, the same effect is obtained by using an inert gas other than helium (e.g., nitrogen, argon) instead of helium gas.

[0025] A manufacturing apparatus for glass substrates is configured in this way so that a porous glass substrate is inserted into the core tube of a sintering furnace, the inside of the core tube is heated by a heater installed on the outer circumference of the core tube, and a transparent glass body is obtained while raising or lowering the porous glass substrate (vitrification).

[0026] Fluorine doping becomes possible by mixing and treating with a fluorine-based gas in either or both of the above dehydration and vitrification processes.

[0027] FIG. 2 is a process diagram showing the sequence of a manufacturing method using the heating device shown in FIG. 1, from (a) to (e).

[0028] (Preliminary Fluorine Doping Process)

[0029] First, a porous glass base material (10) is supported by a suspension rod (8) of a lifting device (7) via a dummy rod (9) and inserted into a core tube (3). As shown in FIG. 2(a), the porous glass base material (10) is maintained for a predetermined time so that the lower part (15) is heated by a heater (13). The power supplied to the heater (13) is controlled based on the temperature of a thermocouple (11) installed on the outer surface of the core tube (3). Specifically, the interior of the core tube (3) is made into a mixed atmosphere of an inert gas such as helium and a fluorine-based gas, and the heater (13) is controlled so that the temperature indicated by the thermocouple (11) is within a predetermined temperature range of 1000 to 1400°C. During the process, the position of the porous glass base material (10) may be fixed, but it may also be performed while moving it at a low speed.

[0030] (1st heat treatment process)

[0031] After that, the interior of the core tube (3) is made into a mixed atmosphere of chlorine-based gas, inert gas, and fluorine-based gas, and the heater (13) is controlled so that the temperature indicated by the thermocouple (11) is within a predetermined temperature range of 1000 to 1400 ℃. The porous glass base material (10) is slowly lowered and passed through a heating zone to the position shown in FIG. 2(b). At this time, the descent is performed, for example, until the upper part of the direct-moving portion of the porous glass base material (10) reaches a position below the lowest part of the heater (13).

[0032] By including a chlorine-based gas in the atmosphere gas of the first heat treatment process, OH groups in the porous glass substrate (10) can be removed. Also, by including a fluorine-based gas, fluorine can be doped into the porous glass substrate (10). Furthermore, by keeping the temperature within the range of 1000 to 1400 ℃, the treatment can be performed while maintaining the porous state of the substrate, making it easy to perform uniform doping in the radial direction.

[0033] (Second heat treatment process)

[0034] After that, the porous glass base material (10) is lifted up and moved to the position shown in FIG. 2(c) (i.e., the position where the lower part (15) of the porous glass base material (10) is heated by the heater (13), and the inside of the core tube (3) is gas-exchanged with a helium gas atmosphere and left to stand. The heater (13) is controlled so that the temperature indicated by the thermocouple (11) is 1450°C or higher, and the porous glass base material (10) is slowly lowered. By passing the base material through the heating zone and lowering it to the position shown in FIG. 2(d) (i.e., the position where the upper part of the direct motion section is lower than the lower part of the heater (13)), a transparent glass base material is obtained.

[0035] After that, the glass substrate (10) is raised to the position shown in Fig. 2(e) (i.e., the position where the bottom part (15) of the porous glass substrate (10) is below the top of the heater (13)), the process is terminated, and the transparent glass substrate is removed from the heating device.

[0036] The glass substrate manufactured in this way has an additional clad layer attached to the outside as needed. Afterwards, an optical fiber with a diameter of 125 μm is manufactured by performing wire drawing from the glass substrate.

[0037] In this embodiment, a preliminary fluorine dope process is performed for the processing start end of the first heat treatment process, and then the process is continuously carried out to the first heat treatment process, so the variation in optical properties at the processing start end of the first heat treatment process can be efficiently suppressed.

[0038] [Second Embodiment]

[0039] Next, a second embodiment of the present invention will be described. The second embodiment differs from the first embodiment mainly in that the preliminary fluorine doping process is performed after the first heat treatment process. Below, the differences from the first embodiment will be described in detail, and aspects not specifically mentioned will be treated as identical to the first embodiment.

[0040] (1st heat treatment process)

[0041] First, a porous glass base material (10) is supported by a suspension rod (8) of a lifting device (7) via a dummy rod (9) and inserted into a core tube (3). The interior of the core tube (3) is made into a mixed atmosphere of chlorine-based gas, inert gas, and fluorine-based gas, and a heater (13) is controlled so that the temperature indicated by the thermocouple (11) is a predetermined temperature within the range of 1000 to 1400 ℃. Starting from the position shown in FIG. 2(a), the porous glass base material (10) is slowly lowered. The porous glass base material (10) is lowered through a heating zone to the position shown in FIG. 2(b).

[0042] By including a chlorine-based gas in the atmosphere gas of the first heat treatment process, OH groups in the porous glass substrate (10) can be removed. Also, by including a fluorine-based gas, fluorine can be doped into the porous glass substrate (10). Furthermore, by keeping the temperature within the range of 1000 to 1400 ℃, the treatment can be performed while maintaining the porous state of the substrate, making it easy to perform uniform doping in the radial direction.

[0043] (1st Preliminary Fluorine Doping Process)

[0044] After that, as shown in FIG. 2(b), the upper portion (14) of the porous glass base material (10) is positioned so that it is heated by the heater (13) and maintained for a predetermined time. In this state, an inert gas or a helium and fluorine-based gas is introduced and maintained at a predetermined temperature within the range of 1000 to 1400 ℃.

[0045] (2nd Preliminary Fluorine Doping Process)

[0046] After that, as shown in FIG. 2(c), the porous glass base material (10) is lifted up and kept in a position where the lower part (15) of the porous glass base material (10) is heated by a heater (13). In this state, an inert gas or a helium and fluorine-based gas is introduced and maintained at a predetermined temperature within the range of 1000 to 1400 ℃.

[0047] In addition, the first pre-fluorine doping process and the second pre-fluorine doping process may be performed as either one. Also, during the processing of each pre-fluorine doping process, the position of the porous glass base material (10) may be fixed, but it may also be performed while moving it at a low speed.

[0048] (Second heat treatment process)

[0049] The inside of the core tube (3) is gas-exchanged with a helium gas atmosphere, and the heater (13) is controlled so that the temperature indicated by the thermocouple (11) is 1450°C or higher. The porous glass base material (10) is slowly lowered and passed through a heating zone to lower the base material to the position shown in FIG. 2(d), thereby obtaining a transparent base material.

[0050] After that, the glass substrate (10) is raised to the position shown in Fig. 2(e) (i.e., the position where the bottom part (15) of the porous glass substrate (10) is below the top of the heater (13)), the process is terminated, and the transparent glass substrate is removed from the heating device.

[0051] The glass substrate manufactured in this way has an additional clad layer attached to the outside as needed. Afterwards, an optical fiber with a diameter of 125 μm is manufactured by performing wire drawing from the glass substrate.

[0052] In this embodiment, since the process is continuously transitioned to the first pre-fluorine dope process with respect to the end of the process after the end of the first heat treatment process, the variation in optical properties of the end of the process of the first heat treatment process can be efficiently suppressed.

[0053] In addition, the treatment start point of the first heat treatment process and the treatment start point of the subsequent second heat treatment process may be set to the same position, and after performing the second preliminary fluorine dope process on the treatment start point of the second heat treatment process, the process may proceed continuously to the second heat treatment process. By doing so, variations in optical properties of the treatment start point of the first heat treatment process can be effectively suppressed, in particular.

[0054] [Third Embodiment]

[0055] Next, a third embodiment of the present invention will be described. The third embodiment differs from the first embodiment mainly in that the atmosphere gas of the second heat treatment process includes a fluorine-based gas. Below, the differences from the first embodiment will be described in detail, and any parts not specifically mentioned will be treated as identical to the first embodiment.

[0056] (Preliminary Fluorine Doping Process)

[0057] First, a porous glass base material (10) is supported by a suspension rod (8) of a lifting device (7) via a dummy rod (9) and inserted into a core tube (3). As shown in FIG. 2(a), the porous glass base material (10) is maintained for a predetermined time so that the lower part (15) is heated by a heater (13). The power supplied to the heater (13) is controlled based on the temperature of a thermocouple (11) installed on the outer surface of the core tube (3). Specifically, the interior of the core tube (3) is made into a mixed atmosphere of an inert gas such as helium and a fluorine-based gas, and the heater (13) is controlled so that the temperature indicated by the thermocouple (11) is within a predetermined temperature range of 1000 to 1400°C. During the process, the position of the porous glass base material (10) may be fixed, but it may also be performed while moving it at a low speed.

[0058] (1st heat treatment process)

[0059] After that, the interior of the core tube (3) is made into a mixed atmosphere of chlorine gas and inert gas, and the heater (13) is controlled so that the temperature indicated by the thermocouple is a predetermined temperature within the range of 1000 to 1400 ℃. The porous glass base material (10) is slowly lowered and passed through a heating zone to the position shown in FIG. 2(b). At this time, the descent is performed, for example, until the upper end of the direct motion section reaches a position below the lowest end of the heater.

[0060] By including a chlorine-based gas in the atmosphere gas of the first heat treatment process, OH groups in the porous glass base material (10) can be removed. In addition, the atmosphere gas may include a fluorine-based gas, in which case a predetermined amount of fluorine can be uniformly doped in the radial direction of the porous glass base material (10).

[0061] (Second heat treatment process)

[0062] After that, the porous glass base material (10) is lifted up and moved to the position shown in FIG. 2(c), and the inside of the core tube (3) is gas-exchanged with a mixed atmosphere of helium gas and fluorine gas and kept in a state of standby. The heater is controlled so that the temperature indicated by the thermocouple (11) is 1450°C or higher. The porous glass base material is slowly lowered and passed through a heating zone to lower the base material to the position shown in FIG. 2(d), thereby obtaining a transparent glass base material.

[0063] By including a fluorine-based gas in the atmosphere gas of the second heat treatment process, the glass substrate (10) can be doped with fluorine and made transparent glass. By adjusting the temperature, the partial pressure of the fluorine-based gas, and the descent speed of the substrate, the radial distribution of the fluorine doping amount can be controlled.

[0064] After that, the glass base material (10) is raised to the position shown in Fig. 2(e), the process is terminated, and the transparent glass base material is removed from the heating device.

[0065] The glass substrate manufactured in this way has an additional clad layer attached to the outside as needed. Afterwards, an optical fiber with a diameter of 125 μm is manufactured by performing wire drawing from the glass substrate.

[0066] [Fourth Embodiment]

[0067] Next, a fourth embodiment of the present invention will be described. The fourth embodiment is similar to the second embodiment in that a preliminary fluorine dope process is performed after the first heat treatment process, but it differs from the second embodiment mainly in that the atmosphere gas of the second heat treatment process includes a fluorine-based gas. Below, the differences from the second embodiment will be described in detail, and any parts not specifically mentioned will be treated as identical to the first embodiment.

[0068] (1st heat treatment process)

[0069] First, a porous glass base material (10) is supported by a suspension rod (8) of a lifting device (7) via a dummy rod (9) and inserted into a core tube (3). The interior of the core tube (3) is made into a mixed atmosphere of chlorine gas and inert gas, and a heater (13) is controlled so that the temperature indicated by the thermocouple (11) becomes a predetermined temperature within the range of 1000 to 1400 ℃. Starting from the position shown in FIG. 2(a), the porous glass base material (10) is slowly lowered and passed through a heating zone to the position shown in FIG. 2(b).

[0070] By including a chlorine-based gas in the atmosphere gas of the first heat treatment process, OH groups in the porous glass base material can be removed. In addition, the atmosphere gas may include a fluorine-based gas, in which case a predetermined amount of fluorine can be uniformly doped in the radial direction of the porous glass base material (10).

[0071] (1st Preliminary Fluorine Doping Process)

[0072] After that, as shown in FIG. 2(b), the upper portion (14) of the porous glass base material (10) is positioned so that it is heated by the heater (13) and maintained for a predetermined time. In this state, an inert gas or a helium and fluorine-based gas is introduced and maintained at a predetermined temperature within the range of 1000 to 1400 ℃.

[0073] (2nd Preliminary Fluorine Doping Process)

[0074] After that, as shown in FIG. 2(c), the porous glass base material is lifted up and kept in a position where the lower part (15) of the porous glass base material (10) is heated by a heater (13). In this state, an inert gas or a helium and fluorine-based gas is introduced and maintained at a predetermined temperature within the range of 1000 to 1400 ℃.

[0075] In addition, the first pre-fluorine doping process and the second pre-fluorine doping process may be performed as either one. Also, during the processing of each pre-fluorine doping process, the position of the porous glass base material (10) may be fixed, but it may also be performed while moving it at a low speed.

[0076] (Second heat treatment process)

[0077] After that, the porous glass base material (10) is lifted up and moved to the position shown in FIG. 2(c), and the inside of the core tube (3) is gas-exchanged with a mixed atmosphere of helium gas and fluorine gas and kept in a state of standby. The heater (13) is controlled so that the temperature indicated by the thermocouple (11) is 1450°C or higher. The porous glass base material (10) is slowly lowered and passed through a heating zone to lower the base material to the position shown in FIG. 2(d), thereby obtaining a transparent glass base material.

[0078] By including a fluorine-based gas in the atmosphere gas of the second heat treatment process, it is possible to dope fluorine into the glass substrate and make it transparent glass. In addition, the radial distribution of the fluorine doping amount can be controlled by adjusting the temperature, the partial pressure of the fluorine-based gas, and the descent speed of the substrate.

[0079] After that, the glass base material (10) is raised to the position shown in Fig. 2(e), the process is terminated, and the transparent glass base material is removed from the heating device.

[0080] The glass substrate manufactured in this way has an additional clad layer attached to the outside as needed. Afterwards, an optical fiber with a diameter of 125 μm is manufactured by performing wire drawing from the glass substrate.

[0081] In the manufacturing method according to each embodiment described above, by adjusting the temperature indicated by the thermocouple (11) of the preliminary fluorine doping process, the holding time at each position, the flow rate of the fluorine gas, and the position of the upper part (14) or lower part (15) of the glass base material (10) relative to the heater (13), it is possible to achieve uniformity of the optical properties in the longitudinal direction of the optical fiber obtained by wire drawing from the glass base material (10).

[0082] In addition, by making the length of the heater (13) of the heating device less than one-fourth of the length of the porous glass substrate (10) being processed, a rapid temperature distribution can be formed on the porous glass substrate (10), so that it is possible to adjust subtle variations in optical properties caused by the preliminary fluorine doping process.

[0083] For the fluorine-based gas used in the atmosphere for fluorine doping treatment, using any of SiF4, CF4, SF6, or C2F6 makes it easy to control the radial fluorine doping distribution and is preferable.

[0084] In addition, if either SiCl4 or Cl2 is used as the chlorine-based gas in the atmosphere of the first heat treatment process, not only can OH groups be effectively removed, but unintended metal impurities are less likely to be incorporated into the silica glass constituting the base material, which is desirable.

[0085] Examples

[0086] [Example 1]

[0087] A porous glass substrate with a total length of 2000 mm was manufactured using the VAD method, and heat treatment was performed by inserting it into a heating device with a heater length of 300 mm.

[0088] First, with the porous glass substrate (10) placed at the position shown in FIG. 2(a), a mixed gas was flowed into the core tube (3) at a flow rate of Cl2: 0.7 L / min and Ar: 30 L / min, and the temperature was raised to 1300 ℃ as indicated by the thermocouple (11). While controlling and maintaining this atmosphere gas and temperature, the porous glass substrate (10) was moved from the top downward at a speed of 10 mm / min to the position shown in FIG. 2(b). (First heat treatment process)

[0089] Next, He gas was introduced into the core tube (3) at a flow rate of 20 L / min, and the porous glass base material (10) was moved from the position shown in FIG. 2(b) to the position shown in FIG. 2(c).

[0090] With the porous glass substrate (10) maintained in this position, He gas was introduced into the core tube (3) at a flow rate of 20 L / min and SiF4 gas at a flow rate of 1 L / min. The temperature was maintained at 1300 ℃. This state was maintained for 30 minutes. (Preliminary fluorine doping process)

[0091] After that, He gas was introduced into the core tube (3) at a flow rate of 20 L / min and SiF4 gas at a flow rate of 0.25 L / min, and the temperature indicated by the thermocouple (11) was raised and maintained at 1500 ℃. The porous glass substrate was moved downward from the position shown in FIG. 2(c) at a speed of 5 mm / min to the position shown in FIG. 2(d), and the substrate was made transparent glass. (Second heat treatment process)

[0092] After that, the transparent glass base material was pulled up to the position shown in Fig. 2(e) and removed from the heating device.

[0093] After uniformly attaching a clad layer to the exterior of a manufactured transparent glass substrate using the OVD method, the optical properties were measured by wire drawing on an optical fiber.

[0094] [Comparative Example 1]

[0095] A porous glass substrate (10) was prepared in the same manner as in Example 1 and inserted into a heating device to perform heat treatment. After performing the first heat treatment process in the same manner as in Example 1, He gas was introduced into the core tube (3) at a flow rate of 20 L / min, and the porous glass substrate (10) was moved from the position shown in FIG. 2(b) to the position shown in FIG. 2(c). After that, without performing a preliminary fluorine doping process, the second heat treatment process in the same manner as in Example 1 was performed to make the substrate transparent glass. After uniformly attaching a clad layer to the transparent glass substrate removed from the heating device using the OVD method, the optical properties were measured by wire drawing on an optical fiber.

[0096] Figure 3 shows the length distribution of the cutoff wavelengths of the optical fibers obtained from Example 1 and Comparative Example 1, respectively. The horizontal axis corresponds to the long side position from the top to the bottom of the transparent glass substrate removed from the heating device. Additionally, the porous glass substrate with a length of 2000 mm shrank during the process of becoming transparent glass through the second heat treatment process, and its length was reduced to about half.

[0097] It can be seen that by forming a preliminary fluorine dope process, Example 1 was able to suppress the cutoff wavelength variation at the bottom of the glass substrate compared to Comparative Example 1.

[0098] [Example 2]

[0099] A porous glass substrate with a total length of 2000 mm was manufactured using the VAD method, and heat treatment was performed by inserting it into a heating device with a heater length of 300 mm.

[0100] First, with the porous glass substrate (10) in a waiting position as shown in FIG. 2(a), a mixed gas was flowed into the core tube (3) at a flow rate of Cl2: 0.7 L / min, Ar: 30 L / min, and SiF4: 0.1 L / min, and the temperature was raised so that the temperature indicated by the thermocouple (11) reached 1300 ℃. While controlling and maintaining this atmosphere gas and temperature, the porous glass substrate (10) was moved from the top downward at a speed of 10 mm / min to the position shown in FIG. 2(b). (First heat treatment process)

[0101] At this position, He gas was introduced into the core tube (3) at a flow rate of 20 L / min and SiF4 gas at a flow rate of 0.1 L / min. The temperature was maintained at 1300 ℃. This state was maintained for 15 minutes. (Preliminary fluorine doping process)

[0102] Next, He gas was introduced into the core tube (3) at a flow rate of 20 L / min, and the porous glass base material (10) was moved from the position shown in FIG. 2(b) to the position shown in FIG. 2(c).

[0103] After that, He gas was introduced into the furnace at a flow rate of 20 L / min and SiF4 gas at a flow rate of 0.25 L / min, and the temperature was raised and maintained at 1550 ℃ as indicated by the thermocouple (11). The porous glass substrate (10) was moved downward from the position shown in FIG. 2(c) at a speed of 5 mm / min to the position shown in FIG. 2(d), and the substrate was made transparent glass. (Second heat treatment process)

[0104] After that, the transparent glass base material was pulled up to the position shown in Fig. 2(e) and removed from the heating device.

[0105] After uniformly attaching a clad layer to the exterior of a manufactured transparent glass substrate using the OVD method, the optical properties were measured by wire drawing on an optical fiber.

[0106] [Comparative Example 2]

[0107] A porous glass substrate (10) was prepared in the same manner as in Example 2 and inserted into a heating device to perform heat treatment. After performing the first heat treatment process in the same manner as in Example 2, without performing the preliminary fluorine doping process, He gas was introduced into the core tube (3) at a flow rate of 20 L / min, and the porous glass substrate was moved from the position shown in FIG. 2(b) to the position shown in FIG. 2(c).

[0108] After that, the base material was made transparent glass by performing a second heat treatment process identical to that of Example 2. After uniformly attaching a clad layer to the transparent glass base material removed from the heating device using the OVD method, the optical properties were measured by wire drawing on an optical fiber.

[0109] Figure 4 shows the length distribution of the cutoff wavelengths of the optical fibers obtained from Example 2 and Comparative Example 2, respectively. The horizontal axis corresponds to the long side position from the top to the bottom of the transparent glass substrate removed from the heating device. Additionally, the porous glass substrate with a length of 2000 mm shrank during the process of becoming transparent glass through the second heat treatment process, and its length was reduced to about half.

[0110] It can be seen that by forming a preliminary fluorine dope process, the cutoff wavelength fluctuation at the top of the glass substrate in Example 2 was suppressed compared to Comparative Example 2.

[0111] As explained above, an optical fiber with suppressed length variation in optical properties can be obtained from a glass substrate for an optical fiber obtained by the method for manufacturing a glass substrate for an optical fiber related to the present invention. Explanation of the symbols

[0112] 1 : Sintering furnace 2 : Noche 3: Core tube 4: Lower gas inlet 5: Upper gas exhaust port 6 : Cover 7 : Lifting device 8 : Hyeonsubong 9 : Dummy Load 10: Porous glass matrix 11 : Heat pair 12: Temperature control unit 13 : Heater 14 : Top part 15 : Bottom part

Claims

Claim 1 A method for manufacturing a glass preform for optical fibers, comprising: a first heat treatment process in which a porous glass preform inserted into a sintering furnace is heated by a heater installed on the outer circumference of the container while raising or lowering the preform in the longitudinal direction, while making the inside of the container a chlorine-containing gas atmosphere; a second heat treatment process in which, after the first heat treatment process, a transparent glass body is obtained by heating the porous glass preform with the heater while raising or lowering the preform in the longitudinal direction, while making the inside of the container an inert gas atmosphere; and a preliminary fluorine doping process in which, prior to the second heat treatment process, one or both ends of the porous glass preform are heated by the heater while making the inside of the container a fluorine-containing gas atmosphere, wherein, in the preliminary fluorine doping process, a portion other than one or both ends of the porous glass preform is not heated by the heater, and a fluorine-containing gas is mixed to create a mixed atmosphere within the container of at least one of the first heat treatment process and the second heat treatment process. Claim 2 A manufacturing method according to claim 1, characterized in that the preliminary fluorine dope process is performed before the first heat treatment process. Claim 3 A manufacturing method according to claim 1, characterized in that the preliminary fluorine doping process is performed after the completion of the first heat treatment process. Claim 4 A manufacturing method according to any one of claims 1 to 3, wherein the preliminary fluorine doping process is performed by fixing or moving the position of the porous glass base material. Claim 5 A manufacturing method characterized by performing the preliminary fluorine dope process at 1000°C or higher and 1400°C or lower, in any one of claims 1 to 3. Claim 6 A manufacturing method according to any one of claims 1 to 3, characterized in that the length of the heater is one-fourth or less of the length of the porous glass base material. Claim 7 A manufacturing method according to any one of claims 1 to 3, characterized in that the fluorine-based gas introduced into the container is any one of SiF4, CF4, SF6, and C2F6. Claim 8 A manufacturing method according to any one of claims 1 to 3, characterized in that the chlorine-based gas introduced into the container is either SiCl4 or Cl2. Claim 9 delete Claim 10 delete

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