Method for manufacturing glass substrate for optical fiber

Abstract

A method for producing a glass substrate for an optical fiber, which can obtain a glass substrate for an optical fiber having reduced fluctuations in optical properties in the longitudinal direction, is provided. The method comprises: a first heat treatment step in which a porous glass substrate inserted into a container of a sintering furnace is heated by a heater mounted on the outer periphery of the container while ascending or descending in the longitudinal direction in an atmosphere containing a chlorine-based gas within the container of the sintering furnace; a second heat treatment step in which, after the first heat treatment step, the porous glass substrate is heated by the heater while ascending or descending in the longitudinal direction in an atmosphere containing an inert gas within the container to obtain a transparent glass body; and a fluorine pre-doping step prior to the second heat treatment step in which one or both ends of the porous glass substrate are heated by the heater in the atmosphere containing a fluorine-based gas within the container.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This nonprovisional application claims the benefit of priority under 35 U.S.C. §119(a) to Japanese Patent Application No. 2020-155731, filed on September 16, 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a transparent vitrification process for dehydrating and sintering a porous glass substrate, and in particular, to a method for producing a glass substrate for optical fiber having uniform properties in the longitudinal direction when doped with fluorine. Background Art

[0004] There is known a method for producing a glass substrate in which a porous glass substrate is placed in a heating furnace and dehydrated, doped with fluorine, and sintered to obtain a transparent glass substrate.

[0005] In JP 2004-307281, it is described that a porous glass substrate is placed in a heating furnace and moved through a heating zone under a chlorine-containing gas atmosphere to dehydrate it, then moved through a heating zone under a fluorine gas atmosphere to perform a fluorination treatment, and then subjected to a transparent vitrification process to obtain a transparent glass substrate uniformly doped with fluorine in the radial direction.

[0006] JP 2012-250887 describes placing a porous glass substrate in a heating furnace and moving it through a heated zone under a chlorine-containing gas atmosphere to dehydrate the substrate, and then moving it through a heated zone under an atmosphere containing helium and fluorine-containing gases to incorporate fluorine into part of the cladding and make it transparent glass.

[0007] JP 2017-154935 describes that a porous glass substrate is heated in a chlorine-containing gas atmosphere while standing in a soaking furnace to dehydrate the substrate, heated in a fluorine-containing gas atmosphere to dope fluorine, and then heated in an atmosphere containing helium to make it transparent glass. Summary of the Invention

[0008] [Technical Issues]

[0009] When fluorine is doped into a porous glass substrate during sintering, the doping distribution fluctuates in the longitudinal direction of the glass substrate. Optical fibers made from such glass substrates are likely to exhibit longitudinal fluctuations in optical properties. In particular, optical fibers made from the porous glass substrate near the upper and lower ends are likely to exhibit fluctuations in optical properties. In view of the above problems, the present invention aims to provide a method for manufacturing an optical fiber glass substrate that can produce an optical fiber glass substrate having reduced longitudinal fluctuations in optical properties.

[0010] [Technical solution]

[0011] In order to solve the above-mentioned problems, the method for manufacturing a glass substrate for an optical fiber according to the present invention includes: a first heat treatment step, in which a porous glass substrate inserted into a container of a sintering furnace is heated by a heater installed on the outer periphery of the container while rising or falling in a longitudinal direction in an atmosphere containing a chlorine-based gas within the container of the sintering furnace; a second heat treatment step, in which, after the first heat treatment step, the porous glass substrate is heated by the heater while rising or falling in a longitudinal direction in an atmosphere containing an inert gas within the container to obtain a transparent glass body; and a fluorine pre-doping step before the second heat treatment step, in which one or both ends of the porous glass substrate are heated by the heater in an atmosphere containing a fluorine-based gas within the container.

[0012] In the present invention, the pre-fluorine doping step may be performed before the first heat treatment step. Alternatively, the pre-fluorine doping step may be performed after the first heat treatment step is completed.

[0013] In the present invention, in the first heat treatment step, the interior of the container has a mixed atmosphere of a chlorine-based gas and a fluorine-based gas. In the second heat treatment step, the interior of the container has a mixed atmosphere of an inert gas and a fluorine-based gas.

[0014] In the present invention, the fluorine pre-doping step can be performed when the porous glass substrate is in a fixed position or is slightly moved. The fluorine pre-doping step can be performed at a temperature between 1000°C and 1400°C.

[0015] In the present invention, the length of the heater may be less than or equal to one quarter of the length of the porous glass substrate.

[0016] In the present invention, the fluorine-based gas introduced into the container may be any one of SiF4, CF4, SF6 and C2F6. The chlorine-based gas introduced into the container may be SiCl4 or Cl2.

[0017] [Beneficial Effects]

[0018] By stretching the optical fiber glass substrate obtained by the method for producing the optical fiber glass substrate of the present invention, an optical fiber having suppressed longitudinal deviation in optical properties can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The structure of the heating device used in the method for producing the glass substrate is shown.

[0020] Figure 2 (a) to 2(e) show the use of Figure 1 The process of manufacturing the heating device is shown.

[0021] Figure 3 The longitudinal distributions of the cutoff wavelengths of the optical fibers obtained by Example 1 and Comparative Example 1 are shown.

[0022] Figure 4 The longitudinal distributions of the cutoff wavelengths of the optical fibers obtained by Example 2 and Comparative Example 2 are shown. DETAILED DESCRIPTION

[0023] [First embodiment]

[0024] In a method for manufacturing a glass substrate for optical fiber according to one embodiment of the present invention, a porous glass substrate is first prepared using various methods, including axial vapor deposition (VAD), outside vapor deposition (OVD), and multi-burner multilayer deposition (MMD). The porous glass substrate produced by these methods consists of glass particles alone or an aggregate of glass particles deposited on the outer periphery of a transparent glass rod. The porous glass substrate thus formed is then sintered to produce a transparent glass substrate for optical fiber.

[0025] In the VAD method, a burner is positioned below a rotating starting glass rod, and a raw material gas is introduced into the oxyhydrogen flame formed by the burner, thereby generating glass particles through a flame hydrolysis reaction. The generated glass particles are deposited in the axial direction of the starting glass rod to produce a porous glass substrate. In the OVD and MMD methods, for example, a burner is positioned at the periphery of a starting glass rod rotating in a reaction vessel, and a raw material gas is introduced into the oxyhydrogen flame formed by the burner, thereby generating glass particles through a flame hydrolysis reaction. The generated glass particles are deposited around the periphery of the starting glass rod to produce a porous glass substrate.

[0026] In a conventional single-mode optical fiber substrate, a region called the "core" is formed in the center. The core is typically doped with Ge, which increases the refractive index of the silica glass. Surrounding the core is a layer with a lower refractive index than the core, called the "cladding." Cladding is typically produced in two stages, first creating a portion of the cladding and core, then adding the remaining cladding to the exterior of the core, or in multiple stages, adding the cladding multiple times. In this invention, the term "glass substrate" refers collectively to components including a portion of the cladding and core, as well as the core and the entire cladding.

[0027] The prepared porous glass substrate is dehydrated and transparently vitrified using a heating apparatus comprising a furnace tube made of a heat-resistant material such as carbon or quartz and a heater arranged around the outer circumference of the furnace tube. Transparent vitrification is achieved by moving the porous glass substrate, inserted into the furnace tube of the heating apparatus, up and down within a heated zone. A dehydration step may also be provided prior to the transparent vitrification. In this dehydration step, the interior of the furnace tube is exposed to a mixed gas atmosphere of a chlorine-based gas and an inert gas, and dehydration is achieved by raising, lowering, or oscillating the porous glass substrate relative to the heated zone.

[0028] Fluorine doping is performed by including a fluorine-based gas in the atmosphere during the dehydration process or the transparent vitrification process. Fluorine has the effect of lowering the refractive index profile of silica glass. Fluorine doping imparts a complex refractive index profile to the glass substrate, thereby enabling the adjustment of the optical properties of the optical fiber obtained from this glass substrate.

[0029] Because the properties of optical fibers are determined by the refractive index profile of the glass substrate from which they are drawn, it is important to ensure that the doping level of the glass substrate is consistent along the longitudinal direction in order to achieve the desired optical properties throughout the entire length of the fiber. To achieve this consistency, it is beneficial to ensure that the thermal history is consistent across the longitudinal direction. However, the upper and lower portions of the porous glass substrate have a tapered shape with a diameter that tapers toward the ends. Since this tapered portion has a smaller heat-receiving area than a straight body, maintaining a uniform thermal history while maintaining a constant temperature in the heated zone is extremely difficult.

[0030] Figure 1 The structure of the heating device used in the manufacturing method of the glass substrate is shown. The heating device for the glass substrate used in the manufacturing method according to the present embodiment is equipped with a sintering furnace 1. The sintering furnace has a furnace core tube 3, which is a container inside the furnace body 2. The bottom of the furnace core tube is equipped with an air inlet 4, and the top is equipped with an exhaust port 5. The top of the furnace core tube is equipped with a lid 6, which is opened and closed when inserting and removing the porous glass substrate. A lifting device 7 is installed above the furnace core tube, which supports a hanger 8 passing through the lid so that it can be lifted and rotated. The hanger 8 supports a guide rod 9. A porous glass substrate 10 is connected to one end of the guide rod 9. When the porous glass substrate is heated for transparent vitrification, the peripheral atmosphere of the porous glass substrate is a helium atmosphere. However, if a gas other than helium is mixed in, there is no problem as long as it is a small amount.

[0031] Before transparent vitrification of the porous glass substrate in a sintering furnace, the surrounding atmosphere of the porous glass substrate can be made into a mixed atmosphere of chlorine-based gas and helium and heated. This effectively removes OH groups in the porous glass substrate (dehydration). The same effect can also be achieved by using an inert gas other than helium (such as nitrogen or argon) instead of helium.

[0032] The glass substrate manufacturing equipment is configured as follows: a porous glass substrate is inserted into a furnace tube of a sintering reactor, and while the porous glass substrate is raised or lowered, the interior of the furnace tube is heated by a heater installed on the outer circumference of the furnace tube to obtain a transparent glass body (vitrification).

[0033] Fluorine doping can be achieved by mixing fluorine gas into one or both of the dehydration and vitrification processes as described above.

[0034] Figure 2 (a) to 2(e) show the use of Figure 1 The process of manufacturing the heating device is shown.

[0035] (Pre-doping with fluorine step)

[0036] First, the porous glass substrate 10 is supported by the suspension rod 8 of the lifting device 7 through the dummy bar 9 and is inserted into the furnace core tube 3. Figure 2 As shown in (a), the porous glass substrate 10 is held for a predetermined time so that the lower end portion 15 is in a position heated by the heater 13. The electric energy supplied to the heater 13 is controlled based on the temperature of a thermocouple 11 mounted on the outer surface of the furnace tube 3. Specifically, the interior of the furnace tube 3 is set to a mixed atmosphere of an inert gas such as helium and a fluorine-based gas. The heater 13 is controlled so that the temperature indicated by the thermocouple 11 is a predetermined temperature within the range of 1000 to 1400°C. During this process, the position of the porous glass substrate 10 can be fixed or the porous glass substrate 10 can be slightly moved at a low speed.

[0037] (First Heat Treatment Step)

[0038] Then, the interior of the furnace tube 3 is made into a mixed atmosphere of chlorine-based gas, inert gas, and fluorine-based gas. The heater 13 is controlled so that the temperature indicated by the thermocouple 11 is a predetermined temperature within the range of 1000-1400°C. The porous glass substrate 10 is gradually lowered through the heating zone to Figure 2 At this time, the lowering is continued until, for example, the upper end portion 14 of the straight body of the porous glass substrate 10 reaches a position below the lowermost portion of the heater 13 .

[0039] By including a chlorine-based gas in the atmosphere gas during the first heat treatment step, OH groups in the porous glass substrate 10 can be removed. Furthermore, since the atmosphere gas includes a fluorine-based gas, fluorine can be doped into the porous glass substrate 10. By setting the temperature within the range of 1000 to 1400° C., uniform doping in the radial direction is facilitated because the process can be performed while maintaining the porous state of the substrate.

[0040] (Second Heat Treatment Step)

[0041] Then, the porous glass substrate 10 is pulled up once and moved to Figure 2 At the position shown in (c) (i.e., the position where the lower end portion 15 of the porous glass substrate 10 is heated by the heater 13), the interior of the furnace tube 3 is replaced with a helium atmosphere and this process is maintained. The heater 13 is controlled so that the temperature indicated by the thermocouple 11 is higher than 1450°C, and the porous glass substrate 10 is gradually lowered. By passing the substrate through the heating zone and lowering to Figure 2 A transparent glass substrate is obtained at the position shown in (d) (i.e., the position where the upper end of the straight portion is lower than the lowermost portion of the heater 13).

[0042] Then, the glass substrate 10 is raised to Figure 2 The process is completed by moving the porous glass substrate 10 to the position shown in (e) (i.e., the position where the lower end portion 15 of the porous glass substrate 10 is lower than the uppermost end of the heater 13), and the transparent glass substrate is taken out from the heating apparatus.

[0043] If necessary, a cladding layer is attached to the exterior of the resulting glass substrate, and then an optical fiber having a diameter of 125 μm is produced from the glass substrate by a drawing method.

[0044] In this embodiment, the terminal portion is pre-doped with fluorine at the start of the first heat treatment step, and then the process is continuously switched to the first heat treatment step. This effectively suppresses fluctuations in the optical properties of the terminal portion at the start of the first heat treatment step.

[0045] [Second embodiment]

[0046] The second embodiment of the present invention is described below. The second embodiment differs from the first embodiment primarily in that a fluorine pre-doping step is performed after the first heat treatment step. The following will primarily describe the differences from the first embodiment, while any points not specifically mentioned are the same as those of the first embodiment.

[0047] (First Heat Treatment Step)

[0048] First, the porous glass substrate 10 is supported by the dummy bar 9 on the suspension rod 8 of the lifting device 7 and inserted into the furnace tube 3. The interior of the furnace tube 3 is made into a mixed atmosphere of chlorine-based gas, inert gas, and fluorine-based gas. The heater 13 is controlled so that the temperature indicated by the thermocouple 11 is a predetermined temperature within the range of 1000 to 1400°C. The porous glass substrate 10 is placed in the furnace tube 3. Figure 2 The position shown in (a) is gradually lowered. The porous glass substrate 10 passes through the heating zone and is lowered to Figure 2 (b) The position shown.

[0049] By including a chlorine-based gas in the atmosphere gas during the first heat treatment step, OH groups in the porous glass substrate 10 can be removed. Furthermore, since the atmosphere gas includes a fluorine-based gas, fluorine can be doped into the porous glass substrate 10. By setting the temperature within the range of 1000 to 1400° C., uniform doping in the radial direction is facilitated because the process can be performed while maintaining the porous state of the substrate.

[0050] (First fluorine pre-doping step)

[0051] Then, if Figure 2 As shown in (b), the upper end portion 14 of the porous glass substrate 10 is held for a predetermined time so that the porous glass substrate 10 is in a position heated by the heater 13. In this state, an inert gas or helium gas and a fluorine-based gas are introduced and maintained at a predetermined temperature range of 1000 to 1400°C.

[0052] (Second fluorine pre-doping step)

[0053] Then, if Figure 2 As shown in (c), the porous glass substrate 10 is pulled up once and placed in a position where the lower end portion 15 of the porous glass substrate 10 is heated by the heater 13, and this process is maintained. In this state, an inert gas or helium gas and a fluorine-based gas are introduced and maintained at a predetermined temperature range of 1000 to 1400°C.

[0054] It should be noted that only one of the first and second pre-fluorine doping steps may be performed. During each pre-fluorine doping step, the position of the porous glass substrate 10 may be fixed or the porous glass substrate may be slightly moved at a low speed.

[0055] (Second Heat Treatment Step)

[0056] The interior of the furnace tube 3 is replaced with a helium atmosphere, and the heater 13 is controlled so that the temperature indicated by the thermocouple 11 is higher than 1450°C. The porous glass substrate 10 is gradually lowered through the heating zone to Figure 2 (d) The transparent glass substrate is thus obtained.

[0057] Then, the glass substrate 10 is raised to Figure 2 The process is completed by moving the porous glass substrate 10 to the position shown in (e) (i.e., the position where the lower end portion 15 of the porous glass substrate 10 is lower than the uppermost end of the heater 13), and the transparent glass substrate is taken out from the heating apparatus.

[0058] If necessary, a cladding layer is attached to the exterior of the resulting glass substrate, and then an optical fiber having a diameter of 125 μm is produced from the glass substrate by a drawing method.

[0059] In this embodiment, after the first heat treatment step is completed, the process is continuously switched to the first pre-fluorine doping step for the terminal portion at the end of the first heat treatment step. This can effectively suppress fluctuations in the optical properties of the terminal portion at the end of the first heat treatment step.

[0060] Furthermore, the terminal portion at the start of the first heat treatment step and the terminal portion at the start of the subsequent second heat treatment step are positioned at the same location. After the second fluorine pre-doping step is performed on the terminal portion at the start of the second heat treatment step, the process can be continuously switched to the second heat treatment step. This method effectively suppresses fluctuations in optical properties, particularly at the terminal portion at the start of the first heat treatment step.

[0061] [Third embodiment]

[0062] The third embodiment of the present invention is described below. The third embodiment differs from the first embodiment primarily in that a fluorine-based gas is included in the atmosphere during the second heat treatment step. The following will primarily describe the differences from the first embodiment, while any points not specifically mentioned are the same as those of the first embodiment.

[0063] (Pre-doping with fluorine step)

[0064] First, the porous glass substrate 10 is supported by the suspension rod 8 of the lifting device 7 through the dummy bar 9 and is inserted into the furnace core tube 3. Figure 2 As shown in (a), the porous glass substrate 10 is held for a predetermined time so that the lower end portion 15 is in a position heated by the heater 13. The electric energy supplied to the heater 13 is controlled based on the temperature of a thermocouple 11 mounted on the outer surface of the furnace tube 3. Specifically, the interior of the furnace tube 3 is set to a mixed atmosphere of an inert gas such as helium and a fluorine-based gas. The heater 13 is controlled so that the temperature indicated by the thermocouple 11 is a predetermined temperature within the range of 1000 to 1400°C. During this process, the position of the porous glass substrate 10 can be fixed or the porous glass substrate 10 can be slightly moved at a low speed.

[0065] (First Heat Treatment Step)

[0066] Then, the interior of the furnace tube 3 is made into a mixed atmosphere of chlorine-based gas and inert gas. The heater 13 is controlled so that the temperature indicated by the thermocouple 11 is a predetermined temperature within the range of 1000-1400°C. The porous glass substrate 10 is gradually lowered through the heating zone to Figure 2 At this time, the lowering is performed until, for example, the upper end of the straight body reaches a position below the lowermost portion of the heater.

[0067] By including a chlorine-based gas in the atmosphere gas of the first heat treatment step, OH groups in the porous glass substrate 10 can be removed. Alternatively, the atmosphere gas may include a fluorine-based gas. In this case, a predetermined amount of fluorine can be uniformly doped in the radial direction of the porous glass substrate 10.

[0068] (Second Heat Treatment Step)

[0069] Then, pull up 10 at a time and move it to Figure 2 (c) The interior of the furnace tube 3 is replaced with a mixed atmosphere of helium and fluorine-based gas, and this process is maintained. The heater is controlled so that the temperature indicated by the thermocouple 11 is higher than 1450°C. The porous glass substrate 10 is gradually lowered through the heating zone to Figure 2 (d) The transparent glass substrate is thus obtained.

[0070] By including a fluorine-based gas in the atmosphere of the second heat treatment step, transparent glass can be prepared while fluorine is doped into the glass substrate 10. The fluorine doping amount distribution in the radial direction can be adjusted by adjusting the temperature, the partial pressure of the fluorine-based gas, and the lowering speed of the substrate.

[0071] Then, the glass substrate 10 is raised to Figure 2 (e) to complete the process and remove the transparent glass substrate from the heating apparatus.

[0072] If necessary, a cladding layer is attached to the exterior of the resulting glass substrate, and then an optical fiber having a diameter of 125 μm is produced from the glass substrate by a drawing method.

[0073] [Fourth embodiment]

[0074] The fourth embodiment of the present invention is described below. The fourth embodiment is similar to the second embodiment in that a fluorine pre-doping step is performed after the first heat treatment step. However, it differs from the second embodiment primarily in that the atmosphere gas during the second heat treatment step contains a fluorine-based gas. The following will primarily describe the differences from the second embodiment, while any points not specifically mentioned are the same as those of the second embodiment.

[0075] (First Heat Treatment Step)

[0076] First, the porous glass substrate 10 is supported by the dummy rod 9 on the lifting device 7 and inserted into the furnace tube 3. The interior of the furnace tube 3 is made into a mixed atmosphere of chlorine-based gas and inert gas. The heater 13 is controlled so that the temperature indicated by the thermocouple 11 is a predetermined temperature within the range of 1000 to 1400°C. The porous glass substrate 10 is placed in the furnace tube 3. Figure 2 The position shown in (a) slowly descends through the heating zone to Figure 2(b) The position shown.

[0077] By including a chlorine-based gas in the atmosphere gas during the first heat treatment step, OH groups in the porous glass substrate can be removed. Alternatively, the atmosphere gas may also include a fluorine-based gas. In this case, a predetermined amount of fluorine can be uniformly doped in the radial direction of the porous glass substrate 10.

[0078] (First fluorine pre-doping step)

[0079] Then, if Figure 2 As shown in (b), the upper end portion 14 of the porous glass substrate 10 is held for a predetermined time so that the porous glass substrate 10 is in a position heated by the heater 13. In this state, an inert gas or helium gas and a fluorine-based gas are introduced and maintained at a predetermined temperature range of 1000 to 1400°C.

[0080] (Second fluorine pre-doping step)

[0081] Then, if Figure 2 As shown in (c), the porous glass substrate 10 is pulled up once and placed in a position where the lower end portion 15 of the porous glass substrate 10 is heated by the heater 13, and this process is maintained. In this state, an inert gas or helium gas and a fluorine-based gas are introduced and maintained at a predetermined temperature range of 1000 to 1400°C.

[0082] It should be noted that only one of the first and second pre-fluorine doping steps may be performed. During each pre-fluorine doping step, the position of the porous glass substrate 10 may be fixed or the porous glass substrate may be slightly moved at a low speed.

[0083] (Second Heat Treatment Step)

[0084] Then pull up 10 at a time and move it to Figure 2 (c) The interior of the furnace tube 3 is replaced with a mixed atmosphere of helium and fluorine-based gas, and this process is maintained. The heater 13 is controlled so that the temperature indicated by the thermocouple 11 is higher than 1450°C. The porous glass substrate 10 is gradually lowered through the heating zone to Figure 2 (d) The transparent glass substrate is thus obtained.

[0085] By including a fluorine-based gas in the atmosphere of the second heat treatment step, transparent glass can be prepared while fluorine is doped into the glass substrate 10. In addition, the fluorine doping amount distribution in the radial direction can be adjusted by adjusting the temperature, the partial pressure of the fluorine-based gas, and the lowering speed of the substrate.

[0086] Then, the glass substrate 10 is raised to Figure 2(e) to complete the process and remove the transparent glass substrate from the heating apparatus.

[0087] If necessary, a cladding layer is attached to the exterior of the resulting glass substrate, and then an optical fiber having a diameter of 125 μm is produced from the glass substrate by a drawing method.

[0088] In the manufacturing method according to each embodiment described above, by adjusting the temperature indicated by the thermocouple 11, the time maintained at each position, the flow rate of the fluorine-based gas, the positions of the upper end 14 and the lower end 15 of the glass substrate 10 relative to the heater 13, and the like, uniform optical properties in the longitudinal direction of the optical fiber obtained from the glass substrate 10 by the drawing method can be achieved in the fluorine pre-doping step.

[0089] Furthermore, by setting the length of the heater 13 of the heating apparatus to be less than a quarter of the length of the porous glass substrate to be processed, a steep temperature distribution can be formed relative to the porous glass substrate 10. Thus, fine optical property changes can be adjusted by the fluorine pre-doping step.

[0090] As the fluorine-based gas for the atmosphere of the fluorine doping process, SiF 4 , CF 4 , SF 6 , or C 2 F 6 is preferable because it is easier to adjust the fluorine doping distribution in the radial direction.

[0091] In addition, SiCl4 or Cl2 is preferably used as the chlorine-based gas used in the atmosphere of the first heat treatment step. As a result, not only can the OH group be effectively removed, but the quartz glass constituting the substrate is not mixed with unexpected metal impurities.

[0092] Example

[0093] (Example 1)

[0094] A porous glass substrate having a total length of 2000 mm was produced by the VAD method and inserted into a heating apparatus having a heater length of 300 mm for heat treatment.

[0095] First, the porous glass substrate 10 is placed Figure 2 (a) The temperature is raised to 1300°C as indicated by the thermocouple 11, and a mixed gas is introduced into the furnace tube 3 at a flow rate of Cl2: 0.7 L / min and Ar: 30 L / min. Under the control and maintenance of the atmosphere gas and temperature, the porous glass substrate 10 is moved from the top to the bottom at a speed of 10 mm / min. Figure 2 Position shown in (b) (first heat treatment step).

[0096] Next, He gas was introduced into the furnace tube 3 at a flow rate of 20 L / min, and the porous glass substrate 10 was Figure 2 The position shown in (b) moves to Figure 2 (c) The position shown.

[0097] With the porous glass substrate 10 held in this position, He gas was introduced into the furnace tube 3 at a flow rate of 20 L / min, and SiF4 gas was introduced at a flow rate of 1 L / min. The temperature was maintained at 1300°C. This state was maintained for 30 minutes (pre-fluorine doping step).

[0098] Then, He gas was introduced into the furnace core tube 3 at a flow rate of 20 L / min, and SiF4 gas was introduced at a flow rate of 0.25 L / min, and the temperature was raised and maintained so that the temperature indicated by the thermocouple 11 was 1500°C. The porous glass substrate was moved from the furnace core tube 3 to the furnace core tube 3 at a speed of 5 mm / min. Figure 2 The position shown in (c) moves downward Figure 2 (d) The transparent vitrification is thus carried out (second heat treatment step).

[0099] Then the transparent glass substrate is raised to Figure 2 (e) and removed from the heating device.

[0100] After the cladding was uniformly attached to the exterior of the produced transparent glass substrate using the OVD method, the substrate was drawn into an optical fiber and the optical characteristics of the optical fiber were measured.

[0101] (Comparative Example 1)

[0102] A porous glass substrate was produced in the same manner as in Example 1 and inserted into a heating apparatus for heat treatment. After the first heat treatment process was applied in the same manner as in Example 1, He gas was introduced into the furnace tube 3 at a flow rate of 20 L / min, and the porous glass substrate 10 was removed from the furnace tube 3. Figure 2 The position shown in (b) moves to Figure 2 (c) The substrate was then subjected to transparent vitrification by applying a second heat treatment step in the same manner as in Example 1, without employing a fluorine pre-doping step. After the cladding was uniformly attached to the exterior of the transparent glass substrate removed from the heating apparatus using the OVD method, the substrate was drawn into an optical fiber, and the optical characteristics of the optical fiber were measured.

[0103] Figure 3 The longitudinal distributions of the cutoff wavelengths of the optical fibers obtained in Example 1 and Comparative Example 1 are shown. The horizontal axis corresponds to the longitudinal position from the upper end to the lower end of the transparent glass substrate removed from the heating apparatus. The porous glass substrate, having a length of 2,000 mm, contracted during the transparent vitrification process in the second heat treatment step, reducing its length by approximately half.

[0104] It can be seen that, compared with Comparative Example 1, in Example 1, the cutoff wavelength fluctuation at the lower end of the glass substrate can be suppressed by providing the fluorine pre-doping step.

[0105] (Example 2)

[0106] A porous glass substrate having a total length of 2000 mm was produced by the VAD method and inserted into a heating apparatus having a heater length of 300 mm for heat treatment.

[0107] First, the porous glass substrate 10 is placed Figure 2 (a) The temperature is raised to 1300°C as indicated by the thermocouple 11, and a mixed gas is introduced into the furnace tube 3 at a flow rate of Cl2: 0.7 L / min, Ar: 30 L / min, and SiF4: 0.1 L / min. While controlling and maintaining the atmosphere gas and temperature, the porous glass substrate 10 is moved from the top to the bottom at a speed of 10 mm / min. Figure 2 Position shown in (b) (first heat treatment step).

[0108] At this position, He gas was introduced into the furnace core tube 3 at a flow rate of 20 L / min, and SiF4 gas was introduced at a flow rate of 0.1 L / min. The temperature was maintained at 1300° C. This state was maintained for 15 minutes (pre-fluorine doping step).

[0109] Next, He gas was introduced into the furnace tube 3 at a flow rate of 20 L / min, and the porous glass substrate 10 was Figure 2 The position shown in (b) moves to Figure 2 (c) The position shown.

[0110] Then, He gas was introduced into the furnace tube at a flow rate of 20 L / min, and SiF4 gas was introduced at a flow rate of 0.25 L / min, and the temperature was raised and maintained so that the temperature indicated by the thermocouple 11 was 1550°C. The porous glass substrate 10 was moved from the furnace tube to the furnace tube at a speed of 5 mm / min. Figure 2 The position shown in (c) moves downward Figure 2 (d) The transparent vitrification is thus carried out (second heat treatment step).

[0111] Then the transparent glass substrate is raised to Figure 2 (e) and removed from the heating device.

[0112] After the cladding was uniformly attached to the exterior of the produced transparent glass substrate using the OVD method, the substrate was drawn into an optical fiber and the optical characteristics of the optical fiber were measured.

[0113] (Comparative Example 2)

[0114] A porous glass substrate was produced in the same manner as in Example 2 and inserted into a heating apparatus for heat treatment. After applying the first heat treatment process in the same manner as in Example 2, He gas was introduced into the furnace tube 3 at a flow rate of 20 L / min without the pre-fluorine doping step. Figure 2 The position shown in (b) moves to Figure 2 (c) The position shown.

[0115] Then, transparent vitrification of the substrate was performed by applying a second heat treatment step in the same manner as in Example 2. After the cladding was uniformly attached to the exterior of the transparent glass substrate taken out of the heating apparatus using the OVD method, the substrate was drawn into an optical fiber and the optical characteristics of the optical fiber were measured.

[0116] Figure 4 The longitudinal distributions of the cutoff wavelengths of the optical fibers obtained in Example 2 and Comparative Example 2 are shown. The horizontal axis corresponds to the longitudinal position from the upper end to the lower end of the transparent glass substrate removed from the heating apparatus. The porous glass substrate, having a length of 2,000 mm, contracted during the transparent vitrification process in the second heat treatment step, reducing its length by approximately half.

[0117] Compared with Comparative Example 2, in Example 2, the cutoff wavelength fluctuation at the upper end of the glass substrate can be suppressed by providing the fluorine pre-doping step.

[0118] As described above, from the optical fiber glass base material obtained by the method for producing the optical fiber glass base material according to the present invention, an optical fiber having suppressed longitudinal deviation in optical properties can be obtained.

Claims

1. A method for manufacturing a glass substrate for optical fiber, the method comprising: a first heat treatment step in which the porous glass substrate inserted into the container of the sintering furnace is heated by a heater installed on the outer periphery of the container while ascending or descending in a longitudinal direction in an atmosphere containing a chlorine-based gas within the container of the sintering furnace; a second heat treatment step, wherein, after the first heat treatment step, the porous glass substrate is heated by the heater while ascending or descending in the longitudinal direction in the atmosphere containing the inert gas within the container to obtain a transparent glass body; and a fluorine pre-doping step prior to the second heat treatment step, wherein one or both ends of the porous glass substrate are heated by the heater in an atmosphere containing a fluorine-based gas in the container, thereby pre-doping fluorine only in the vicinity of one or both ends of the porous glass substrate; Fluorine is doped over the entire range in the longitudinal direction of the porous glass substrate by including a fluorine-based gas in the atmosphere of one or both of the first heat treatment step and the second heat treatment step.

2. The manufacturing method according to claim 1, wherein The fluorine pre-doping step is performed before the first heat treatment step.

3. The manufacturing method according to claim 1, wherein The fluorine pre-doping step is performed after the first heat treatment step is completed.

4. The manufacturing method according to claim 1, wherein In the first heat treatment step, the interior of the container has a mixed atmosphere of a chlorine-based gas and a fluorine-based gas.

5. The manufacturing method according to claim 1, wherein In the second heat treatment step, the interior of the container has a mixed atmosphere of an inert gas and a fluorine-based gas.

6. The manufacturing method according to claim 1, wherein The fluorine pre-doping step is performed when the porous glass substrate is fixed or slightly moved.

7. The manufacturing method according to claim 1, wherein: The pre-fluorine doping step is performed at a temperature between 1000°C and 1400°C.

8. The manufacturing method according to claim 1, wherein: The length of the heater is less than or equal to one quarter of the length of the porous glass substrate.

9. The manufacturing method according to claim 1, wherein: The fluorine-based gas introduced into the container is any one of SiF4, CF4, SF6, and C2F6.

10. The manufacturing method according to claim 1, wherein: The chlorine-based gas introduced into the container is SiCl4 or Cl2.

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