Glass preform for optical fiber, and method for manufacturing glass preform for optical fiber

Abstract

The present invention provides a glass preform for optical fibers that is difficult to separate from the starting rod and the dummy glass, and a method for manufacturing the same. A glass preform for optical fibers having a structure in which a dummy glass is inserted into one end of a starting rod, and a portion of the dummy glass and the starting rod are surrounded by clad glass. A manufacturing method for aligning the shape by applying a load and moving the dummy glass from the starting rod side toward the dummy glass side while placing a soldering iron on the connection part when connecting the starting rod and the dummy glass.

Description

Technology Field

[0001] The present invention relates to a glass preform for optical fibers and a method for manufacturing a glass preform for optical fibers. Background Technology

[0002] As a method for manufacturing a glass preform for optical fibers, a method is known in which a porous glass preform obtained by the VAD method or the OVD method is sintered.

[0003] Patent Document 1 describes a method for producing a glass base material for optical fibers by depositing glass fine particles for cladding on the outer circumference of a starting member, that is, on the starting rod, the joint between the starting rod and the dummy glass, and the outer circumference of a part of the dummy glass, and heating the deposited porous glass in a high-temperature furnace to make transparent clad glass.

[0004] Regarding the method of connecting a starting rod and a dummy glass, Patent Document 2 describes a method of smoothing the connection part by pressing a soldering iron firmly against the connection part and performing a "reciprocating motion." In addition, Patent Document 3 describes a method of matching the outer diameter of a bump-shaped part to the outer diameter of the dummy glass by repeatedly pushing the dummy glass and the starting rod against each other and moving them apart while heating the connection part. Prior art literature

[0005] Japanese Patent Publication No. 1999-189428 Japanese Patent Publication No. 1994-199533 Japanese Patent Publication No. 2014-80299 The problem to be solved

[0006] However, when the glass preform for optical fiber produced by the above method is suspended by attaching a dummy glass (1) to a shaft or the like in a subsequent process (Fig. 1a) and heated while under load, separation (5) is likely to occur near the joint (Fig. 1b). This was a problem because breakage (6) of the glass preform for optical fiber could occur due to the above separation (Fig. 1c).

[0007] In the starting rod, in addition to germanium doped to increase the core's refractive index, there are cases where a suitable amount of fluorine is doped into the clad to form a depressed or trench section that lowers the refractive index. In particular, when the part of the starting rod joined to the dummy rod is doped with fluorine, separation was prone to occur near the joint.

[0008] Therefore, the present invention has been made in consideration of the above, and its purpose is to suppress the separation of the starting rod and the dummy glass. means of solving the problem

[0009] To solve the above problem, the glass base material for an optical fiber according to the present invention is characterized by comprising a starting rod, a dummy glass inserted into one end of the starting rod and integrally bonded, and a clad glass surrounding a part of the dummy glass and the starting rod. Effects of the invention

[0010] According to the present invention, since the starting rod is inserted into one end of the dummy glass and integrally joined, it is difficult for separation to occur near the seam even when the dummy glass is stretched and heated under a load. Brief explanation of the drawing

[0011] FIG. 1a is a schematic diagram showing an example of a conventional starting rod, a dummy glass connection, and a glass base material for an optical fiber. FIG. 1b is a schematic diagram showing an example of a conventional starting rod, a dummy glass connection, and a glass base material for an optical fiber. FIG. 1c is a schematic diagram showing an example of a conventional starting rod, a dummy glass connection, and a glass base material for an optical fiber. FIG. 2a is a schematic diagram showing an example of a starting rod, a dummy glass connection, and a glass base material for an optical fiber according to the present invention. FIG. 2b is a schematic diagram showing an example of a starting rod, a dummy glass connection, and a glass base material for an optical fiber according to the present invention. FIG. 2c is a schematic diagram showing an example of a starting rod, a dummy glass connection, and a glass base material for an optical fiber according to the present invention. FIG. 2d is a schematic diagram showing an example of a starting rod, a dummy glass connection, and a glass base material for an optical fiber according to the present invention. FIG. 2e is a schematic diagram showing an example of a starting rod, a dummy glass connection, and a glass base material for an optical fiber according to the present invention. Specific details for implementing the invention

[0012] Hereinafter, an example of a method for manufacturing a glass preform for an optical fiber according to the present invention will be described based on the attached drawings. Furthermore, the present invention is not limited to the embodiments described below.

[0013] The glass base material for the optical fiber of the present embodiment is made by joining a dummy glass (1) and a starting rod (2) (Figs. 2a to 2d) and forming a clad glass (4) to surround a part of the dummy glass (1) and the starting rod (2) (Fig. 2e).

[0014] In the glass base material for optical fibers according to the present invention, at the joint (3) between the dummy glass (1) and the starting rod (2), the dummy glass (1) is inserted into the starting rod (2) and integrally bonded. That is, the dummy glass (1) is bonded in such a state that a portion of it extends toward the starting rod (2) rather than the cross-section of the starting rod (2). In a subsequent process, by supporting the dummy glass (1) bonded in this manner and suspending the glass base material for optical fibers, it becomes difficult for separation to occur near the joint (3).

[0015] The joining with the dummy glass (1) can be done at both ends of the starting rod (2), and in this case, there is an advantage that separation near the joint is less likely to occur even if the dummy glass (1) at either end is supported and suspended.

[0016] It is preferable that the dummy glass (1) and the starting rod (2) be airtightly joined at the joint (3). If air bubbles remain trapped within the joint (3), separation of the starting rod (2) and the dummy glass (1) may occur starting from these air bubbles.

[0017] The starting rod (2) is a component that serves as the center of the glass base material for the optical fiber being manufactured, and it is preferable that a dopant be added to correspond to the refractive index distribution of the target optical fiber. For example, it is a hollow rod shape in which the center is silica glass doped with germanium (Ge) and the surroundings are pure silica glass or silica glass doped with fluorine (F) (the starting rod may also be a hollow rod shape).

[0018] The shape of the joint (3) in which the joint end of the dummy glass (1) is fitted into the joint end of the starting rod (2) is not particularly limited, but it is preferable that the joint be formed such that the central axis of the cross-sectional circle of the dummy glass (1) and the starting rod (2) coincides. This ensures that the stress distribution in the circumferential direction of the cross-section in the joint (3) is uniform. When the dummy glass (1) is a solid round bar, it is preferable that the central part of the joint end of the dummy glass (1) be fitted into the joint end of the starting rod (2) (Fig. 2d). In particular, it is suitable for cases where the outer diameter of the starting rod and the dummy glass are the same.

[0019] However, when using dummy glass containing a large amount of dopant impurities, the viscosity of the dummy glass may be lower than that of the starting rod when heated to the same temperature. In this case, the joint may be machined so that the central part of the joint end of the starting rod is fitted into the joint end of the dummy glass. The same effect as in the present embodiment can be obtained through such machining.

[0020] Also, as shown in FIGS. 2a to 2e, when the outer diameter of the dummy glass (1) is smaller than the outer diameter of the starting rod (2), it is preferable to have the joint end of the dummy glass (1) entirely fitted into the joint end of the starting rod. On the other hand, when the outer diameter of the dummy glass is larger than the outer diameter of the starting rod, it is acceptable to have the joint end of the starting rod entirely fitted into the joint end of the dummy glass. The dummy glass may be in the shape of a tube (hollow rod), and this reduces the weight of the dummy glass.

[0021] The present invention is particularly effective when the dummy glass (1), starting rod (2), and clad glass (4) contain glass with different dopant compositions. In this case, the seam (3) of the connection part is visible to the naked eye.

[0022] For example, starting rods for optical fibers are synthetic quartz glass to which dopants such as germanium (Ge) or fluorine (F) are added to adjust the refractive index, whereas clad glass is synthetic quartz glass that does not contain most of these dopants. To reduce light absorption, OH groups are removed from these synthetic quartz glasses to keep the OH group content below 1 ppm, and they contain more than 100 ppm of chlorine (Cl). Meanwhile, inexpensive natural quartz glass is often used as the dummy glass.

[0023] When formed by a combination of three different types of glass—dummy glass, starting rod, and clad glass—stress is prone to concentration near the seam. In subsequent processes, thermal energy is applied to this seam, and as almost all of the load of the glass base material for optical fiber is applied, the separation of the seam is prone to occur. The glass base material for optical fiber according to the present invention is effective in suppressing the separation of these different glass seams, and is particularly more effective when the starting rod is doped with fluorine. In this case, it is effective when the doping amount of fluorine is 0.7% by weight or less, and more effective when it is 0.5% by weight or less.

[0024] In the method for manufacturing a glass base material for an optical fiber according to the present invention, a dummy glass (1) and a starting rod (2) are placed so that their ends face each other in a processing device such as a glass lathe (Fig. 2a), then heated by a flame (not shown) or the like to soften them, and then pressed and welded together to produce a thick section (7) with an outer diameter thicker than that of the dummy glass (1) or the starting rod (2) at the joint (Fig. 2b). When applying a processing iron (8) to the thick section (7) to make it smooth, the processing iron (8) is applied to the thick section and moved while applying a load from the starting rod (2) side toward the dummy glass (1) side, thereby smoothing out the bulging part of the thick section (7) (Fig. 2c), and a joint (3) is formed in which the joint end of the dummy glass (1) is inserted into the joint end of the starting rod (2) (Fig. 2d). Here, "making the swollen part of the thick section (7) even" means making the outer diameter of the thick section (7) equal so that the difference from the outer diameter of the starting rod (2) becomes small, and it is preferable to make the difference between the two less than 10% of the outer diameter of the starting rod (2), and more preferable to make it less than 5%. If the thick section (7) is not made even, the flow of glass fine particles emitted to the starting rod (2) during the subsequent glass fine particle deposition process is damaged, and it is easy for disturbances to occur in the formation of the glass fine particle deposition layer.

[0025] By adopting this method, the diaphragm (7) can be easily shaped, and the processing of the dummy glass (1) and the starting rod (2) can be completed. Since the end of the dummy glass (1) is fitted into the starting rod (2) (Fig. 2d), localized stress or load concentration is less likely to occur, making it less likely for the glass to separate. Additionally, the joint (3) of the connecting part, indicated by a dotted line in the figure, can be observed with the naked eye.

[0026] The insertion depth of the dummy glass into the starting rod can be controlled, for example, by the size of the diameter of the thick section formed by welding the softened dummy glass and the cross-section of the starting rod together. If the thick section is large, the insertion depth can be made larger, and if the thick section is small, the insertion depth can be made smaller. Alternatively, this can be controlled by changing the degree of glass softening by adjusting the heating temperature when forming the thick section, or by adjusting the load applied by the soldering iron or the movement speed of the soldering iron when forming the thick section.

[0027] In addition, in the present invention, it is preferable that at least one of the cross-sections of the starting rod and the dummy glass before welding is convex. If both cross-sections of the starting rod and the dummy glass are flat, air bubbles may remain and be trapped within the seam during welding of both cross-sections. By making at least one of the cross-sections convex, the seam is formed as the convex portion deforms during welding, so no air bubbles remain. Furthermore, when the thick portion produced during welding is shaped with a processing iron, the thick portion formed by connecting convex portions to each other or by connecting a convex portion to a flat surface is smoother than the thick portion formed by connecting flat surfaces to each other, so there is an advantage that it is easy to shape.

[0028] It is preferable to grind the tip with a grinder or similar tool in advance to prepare the convex shape. The protrusion of the convex part may have a raised center, but it may also be offset outward from the center.

[0029] The amount of protrusion should preferably be between 0.5 and 2 times the radius. If the amount of protrusion is less than 0.5 times, bubbles are likely to remain, and if it exceeds 2 times, the convex part is likely to deform and sag downward during heating by a flame.

[0030] In addition, in the present invention, it is preferable that the outer diameter of the dummy glass be thinner than the outer diameter of the starting rod. Since the primary purpose of the dummy glass is to support the starting rod during the processes of glass fine particle deposition or sintering, there is no need to make the dummy glass thicker than necessary. Furthermore, as described above, by applying a soldering iron to the starting rod side of the thickened portion (the part with a thickened outer diameter) formed at the joint between the starting rod and the dummy glass and applying a load toward the dummy glass side to smooth the thickened portion, the joint end of the dummy glass can be easily inserted into the joint end of the starting rod. (Although the size of the starting rod is determined by the size of the target optical fiber glass preform or the design of the refractive index distribution,) the thinner the dummy glass is, the less heat, time, and load required for heating and joining are, making processing easier.

[0031] Examples

[0032] A starting rod was prepared having an outer diameter of 50 mm and a length of 1500 mm, with both ends having flat surfaces, made of synthetic quartz glass containing a core manufactured using the VAD method. This starting rod is distinguished into a manufacturing start side and a manufacturing end side by the VAD method. In addition, the starting rod prepared here is not doped with fluorine.

[0033] One chuck of the glass lathe grips the end face of the starting rod, which is the manufacturing start side of the VAD method, toward the other chuck, and grips a dummy glass made of natural quartz glass with an outer diameter of 50 mm and a length of 500 mm with the other chuck, and while rotating the chuck, the end face of the starting rod and the end face of the dummy glass are heated to about 2000°C with an oxyhydrogen flame from a burner to soften them. This state is denoted as (α).

[0034] [Example 1]

[0035] By bringing the chucks closer together with respect to (α), the cross-sections were welded together to swell the connection (forming a thick section). The maximum diameter of the swollen connection (thick section) was 54 mm. While continuing to heat the connection with an oxyhydrogen flame, a processing iron made of high-purity carbon was placed on the starting rod side of the swollen connection (thick section) and moved by applying a load toward the dummy glass side to smooth out the swollen area (thick section) of the connection. After the operation, the maximum diameter of the connection was 51 mm. At the joint, the dummy glass was inserted 1 mm into the starting rod.

[0036] [Example 2]

[0037] By bringing the chucks closer together with respect to (α), the cross-sections were welded together to swell the connection (forming a thick section). The maximum diameter of the swollen connection (thick section) was 56 mm. While continuing to heat the connection with an oxyhydrogen flame, a processing iron made of high-purity carbon was placed on the starting rod side of the swollen connection (thick section) and moved by applying a load toward the dummy glass side to smooth out the swollen area (thick section) of the connection. After the operation, the maximum diameter of the connection was 52 mm. At the joint, the dummy glass was inserted 3 mm into the starting rod.

[0038] [Example 3]

[0039] By bringing the chucks closer together with respect to (α), the cross-sections were welded together to swell the connection (forming a thick section). The maximum diameter of the swollen connection (thick section) was 57 mm. While continuing to heat the connection with an oxyhydrogen flame, a processing iron made of high-purity carbon was placed on the starting rod side of the swollen connection (thick section) and moved by applying a load toward the dummy glass side to smooth out the swollen area (thick section) of the connection. After the operation, the maximum diameter of the connection was 52 mm. At the joint, the dummy glass was inserted 5 mm into the starting rod.

[0040] For the connection between the starting rod and the dummy glass of Examples 1, 2, and 3, the chuck of the dummy glass was held, the gripping by the chuck of the starting rod was opened, and another dummy glass made of natural quartz glass with an outer diameter of 50 mm and a length of 500 mm was held in the chuck on the opened side. While rotating the chuck, each cross-section was heated to approximately 2000°C with an oxyhydrogen flame from a burner to soften it, and then the chucks were brought close together to weld the cross-sections of the dummy glass and the starting rod on the side of the VAD method of manufacturing completion, thereby forming a swollen joint (thickened section). The maximum diameter of the swollen joint (thickened section) was 54 mm. While continuing to heat the joint (thickened section) with the oxyhydrogen flame, a processing soldering iron was applied to the swollen joint (thickened section) and moved back and forth between the starting rod side and the dummy glass side of the joint to evenly smooth the swollen area (thickened section) of the joint. By doing so, a target having dummy glass at both ends of the starting rod was formed.

[0041] A porous glass preform with an outer diameter of 300 mm was produced by gripping the dummy glass at both ends of the target formed by the above Examples 1, 2, and 3 with a chuck placed at both ends inside the chamber and depositing glass fine particles on the outer circumference of the target by the OVD method. The dummy glass, which is bonded to the manufacturing start side of the VAD method on the starting rod of this porous glass preform, was connected to the tip of the shaft of the lifting mechanism of the dehydration sintering device, and inserted into the core tube of the dehydration sintering device in a vertically suspended state with the manufacturing start side of the VAD method on the starting rod facing upward, and moved downward while heating at 1500°C to sinter the porous glass to make it transparent glass, thereby producing a glass preform for optical fibers.

[0042] Ten glass preforms for optical fibers were manufactured for each of the configurations of Examples 1, 2, and 3 above. A dummy glass on the VAD manufacturing start side was held and suspended vertically, and a downward load of 50 kgf was applied. The separation rate was investigated by visually observing the seam between the starting rod on the VAD manufacturing start side and the dummy glass. The results are shown in Table 1.

[0043]

[0044] In all of Examples 1, 2, and 3, a high separation suppression effect was confirmed, with a separation rate of 30% or less. From a comparison of Examples 1, 2, and 3, it is observed that a deeper insertion depth tends to increase the effect of suppressing separation. Since the insertion depth of the dummy glass into the starting rod in Example 1 is 1 mm, which is 2% of the outer diameter of the starting rod (50 mm), it is believed that a particularly excellent effect is achieved when the insertion depth of the dummy glass into the starting rod is 2% or more of the outer diameter of the starting rod.

[0045] [Examples 4–7]

[0046] As a separate review, a starting rod with an outer diameter of 50 mm and a length of 1500 mm, made of synthetic quartz glass containing a core manufactured using the VAD method, was prepared. To form a depressurized portion on the outer circumference of the core, this starting rod was doped with fluorine in five different concentrations: 0 wt% (…Example 1), 0.1 wt% (…Example 4), 0.3 wt% (…Example 5), 0.5 wt% (…Example 6), and 0.7 wt% (…Example 7). The fluorine concentration of the starting rod was obtained by dissolving a portion of the glass fragment obtained by crushing the starting rod and analyzing it using ion chromatography. For these starting rods, 10 glass preforms for optical fibers were prepared for each in the same order as in Example 1.

[0047] [Comparative Examples 1, 2]

[0048] In addition, a starting rod with an outer diameter of 50 mm and a length of 1500 mm, made of synthetic quartz glass containing a core manufactured using the VAD method, was prepared. To form a deflected shape on the outer circumference of the core, fluorine was doped into this starting rod in two types, 0.5 wt% (Comparative Example 1) and 0.7 wt% (Comparative Example 2), thereby reducing the transmission loss of the optical fiber obtained by drawing the wire. The fluorine concentration of the starting rod is a value obtained by dissolving a portion of the glass fragment obtained by crushing the starting rod and analyzing it using ion chromatography.

[0049] For the starting rods of Comparative Examples 1 and 2, the cross-section of the starting rod on the manufacturing start side by the VAD method was held toward the other chuck of a glass lathe, and a dummy glass made of natural quartz glass with an outer diameter of 50 mm and a length of 500 mm was held with the other chuck. While rotating the chucks, the cross-section of the starting rod on the manufacturing start side and the cross-section of the dummy glass were heated to approximately 2000°C with an oxyhydrogen flame from a burner to soften them. Then, by bringing the chucks closer together, the cross-sections were welded to the joint (forming a thick section). The maximum diameter of the swollen joint (thick section) was 54 mm. Afterward, while continuing to heat with the oxyhydrogen flame, a processing soldering iron was applied to the swollen joint (thick section) and moved back and forth between the starting rod side and the dummy glass side of the joint to smooth out the swollen area of ​​the joint. The maximum diameter of the joint after the operation was 53 mm. However, unlike Examples 1 to 7, the seam between the joint of the dummy glass and the joint of the starting rod was joined in a planar manner.

[0050] For the connection between the starting rod and the dummy glass of Comparative Examples 1 and 2, the chuck of the dummy glass was held, the gripping by the chuck of the starting rod was opened, and another dummy glass made of natural quartz glass with an outer diameter of 50 mm and a length of 500 mm was held in the chuck on the opened side. While rotating the chuck, each cross-section was heated to approximately 2000°C with an oxyhydrogen flame from a burner to soften it, and then the chucks were brought close together to weld the cross-sections of the dummy glass and the starting rod on the side of the VAD method of manufacturing completion, thereby forming a swollen thick section of the connection. The maximum diameter of the swollen connection (thick section) was 54 mm. While continuing to heat the connection (thick section) with the oxyhydrogen flame, a processing soldering iron was applied to the swollen connection (thick section) and moved back and forth between the starting rod side and the dummy glass side of the joint to perform a process of smoothing the swollen area (thick section) of the connection. By doing so, a target having dummy glass at both ends of the starting rod was formed.

[0051] A porous glass preform with an outer diameter of 300 mm was produced by gripping the dummy glass at both ends of the target formed by Comparative Examples 1 and 2 above with a chuck placed at both ends inside the chamber and depositing glass fine particles on the outer circumference of the target by the OVD method. The dummy glass, which is bonded to the manufacturing start side of the VAD method on the starting rod of this porous glass preform, was connected to the tip of the shaft of the lifting mechanism of the dehydration sintering device, and inserted into the core tube of the dehydration sintering device in a vertically suspended state with the manufacturing start side of the VAD method on the starting rod facing upward, and moved downward while heating to 1500°C, and sintering the porous glass to make it transparent glass, thereby producing 10 glass preforms for optical fibers each.

[0052] For the above-mentioned glass preform for optical fibers, the separation rate was investigated by visually inspecting the seam between the starting rod and the dummy glass on the manufacturing start side. The results are shown in Table 2.

[0053]

[0054] From the comparison between Example 5 and Comparative Example 1, and Example 7 and Comparative Example 2, it was confirmed that even if the fluorine dope concentration of the starting rod is the same, the occurrence of joint separation can be significantly suppressed by making the insertion depth / outer diameter of the starting rod 2% or more. The effect of the present invention was confirmed in the range where the amount of fluorine dope in the starting rod is 0.7 wt% or less. A particularly high effect was confirmed in the range where the fluorine dope concentration of the starting rod is 0.5 wt% or less.

[0055] [Example 8]

[0056] As a separate review, a starting rod with an outer diameter of 50 mm and a length of 1500 mm, made of synthetic quartz glass containing a core manufactured using the VAD method, was prepared. The tip of the starting side of the starting rod was ground into a convex shape with a height of 30 mm using a disc grinder.

[0057] [Example 9]

[0058] In addition, both ends of a starting rod with an outer diameter of 50 mm and a length of 1800 mm, made of synthetic quartz glass containing a core manufactured using the VAD method, were held by the chucks on both sides of a glass shelf, and by heating a position 300 mm from the starting side of manufacturing with an oxyhydrogen flame to soften it while moving one of the chucks to melt it, the tip of the starting side of the starting rod with a length of 1500 mm was formed into a convex parabolic taper shape with a height of 30 mm.

[0059] For the starting rods of Examples 8 and 9, 10 glass preforms for optical fibers were manufactured for each in the same order as in Example 1, and the separation rate was investigated by visually observing the seam between the starting rod and the dummy glass at the start of manufacturing. The results are shown in Table 3.

[0060]

[0061] In both Examples 8 and 9, the separation rate of the seam was 0%. It is believed that the fact that the starting point for separation was eliminated because the tip became convex prevented air bubbles from entering the seam. In Examples 8 and 9, the cross-sectional shape of the starting rod was changed, but an equivalent effect is obtained even if the cross-sectional shape of the dummy glass is changed.

[0062] [Example 10]

[0063] As a separate review, a starting rod with an outer diameter of 50 mm and a length of 1500 mm, made of synthetic quartz glass containing a core manufactured using the VAD method, was prepared.

[0064] One chuck of the glass lathe gripped the end face of the starting rod on the manufacturing side toward the other chuck, and the other chuck gripped a dummy glass made of natural quartz glass with an outer diameter of 40 mm and a length of 500 mm. While rotating the chucks, the end face of the starting rod on the manufacturing side and the end face of the dummy glass were heated to approximately 2000°C with an oxyhydrogen flame from a burner to soften them, and then the chucks were brought close together to weld the ends together, causing the joint to swell. The maximum diameter of the swollen joint was 48 mm. In addition, while continuing to heat the joint with the oxyhydrogen flame, a processing iron made of high-purity carbon was applied from the starting rod side of the swollen joint toward the dummy glass side to smooth out the swollen area of ​​the joint. After the operation, the maximum diameter of the joint was 45 mm.

[0065] [Example 11]

[0066] One chuck of the glass lathe gripped the end face of the starting rod on the manufacturing side toward the other chuck, and the other chuck gripped a dummy glass made of natural quartz glass with an outer diameter of 60 mm and a length of 500 mm. While rotating the chucks, the end face of the starting rod on the manufacturing side and the end face of the dummy glass were heated to approximately 2000°C with an oxyhydrogen flame from a burner to soften them, and then the chucks were brought close together to weld the ends together, causing the joint to swell. The maximum diameter of the swollen joint was 59 mm. In addition, while continuing to heat the joint with the oxyhydrogen flame, a processing iron made of high-purity carbon was applied from the starting rod side of the swollen joint toward the dummy glass side to smooth out the swollen area of ​​the joint. After the operation, the maximum diameter of the joint was 54 mm.

[0067] For the composites of the starting rod and dummy glass of Examples 10 and 11, 10 glass preforms for optical fibers were manufactured in the same order as in Example 1, and the separation rate was investigated by visually inspecting the seam between the starting rod and the dummy glass at the start of manufacturing. The results are shown in Table 4.

[0068]

[0069] In Example 10, where the outer diameter of the starting rod is thicker than the outer diameter of the dummy glass, the rate of seam separation was 0%. When considering the cost of the dummy glass together, it is considered desirable for the outer diameter of the starting rod to be thicker than the outer diameter of the dummy glass.

[0070] Furthermore, the present invention is not limited to the embodiments described above, and is freely modified, improved, etc. Explanation of the symbols

[0071] 1: Dummy glass 2: Starting Road 3: Seam 4: Clad glass 5: Separation 6: Breakage of glass preform for optical fibers 7: Taegyeongbu 8: Processing iron

Claims

Claim 1 A glass base material for an optical fiber comprising a starting rod, a dummy glass integrally bonded and inserted into one end of the starting rod, the dummy glass including an inserted portion inserted into the starting rod and an extended portion extending from the starting rod, wherein the diameter of the extended portion at the boundary with the inserted portion is smaller than the diameter of the inserted portion, and a clad glass surrounding a part of the dummy glass and the starting rod. Claim 2 A glass base material for an optical fiber in which the starting rod and the dummy glass are hermetically bonded in claim 1. Claim 3 A glass base material for an optical fiber according to claim 1 or 2, wherein the central portion of the joint end of the dummy glass is fitted into the joint end of the starting rod. Claim 4 A glass base material for an optical fiber, wherein, in claim 1 or 2, the bonding end of the dummy glass is fitted so as to be wrapped around the bonding end of the starting rod. Claim 5 In claim 1 or 2, the starting rod is a glass base material for an optical fiber containing germanium. Claim 6 In claim 1 or 2, the starting rod is a glass base material for an optical fiber containing fluorine. Claim 7 In claim 6, a glass preform for optical fiber having a fluorine content of 0.7 weight% or less. Claim 8 In claim 6, a glass substrate for optical fiber having a fluorine content of 0.5 weight% or less. Claim 9 A glass base material for an optical fiber according to claim 1 or 2, wherein the outer diameter of the dummy glass is less than or equal to the outer diameter of the starting rod. Claim 10 A glass base material for an optical fiber according to claim 1 or 2, wherein the insertion depth of the dummy glass into the starting rod is 2% or more of the outer diameter of the starting rod. Claim 11 A glass base material for optical fibers according to claim 1 or 2, wherein the dummy glass is in the shape of a solid rod or a hollow round bar. Claim 12 A glass base material for optical fibers according to claim 1 or 2, wherein the starting rod is a solid rod. Claim 13 A method for manufacturing a glass preform for optical fibers, comprising: a process of bringing one end of a starting rod and one end of a dummy glass into contact and heating to join them; a process of forming a thick portion with an outer diameter thicker than that of the dummy glass and the starting rod at the joint of the starting rod and the dummy glass; a process of placing a soldering iron on the thick portion and moving the soldering iron while applying a load from the starting rod side of the thick portion toward the dummy glass side to form a joint in which the joint end of the dummy glass is fitted into the joint end of the starting rod; a process of depositing a layer of glass fine particles on the outer circumference of the dummy glass and the starting rod; and a process of heating the layer of glass fine particles to make it transparent. Claim 14 In claim 13, a method for manufacturing a glass preform for optical fibers in which fluorine is doped into the starting rod. Claim 15 A method for manufacturing a glass preform for optical fibers according to claim 13 or 14, wherein the starting rod is doped with 0.7% by weight or less of fluorine. Claim 16 A method for manufacturing a glass preform for optical fibers according to claim 13 or 14, wherein the starting rod is doped with 0.5 weight percent or less of fluorine. Claim 17 A method for manufacturing a glass preform for an optical fiber, wherein, in claim 13 or 14, at least one of the starting rod or the dummy glass is further processed into a convex shape. Claim 18 A method for manufacturing a glass base material for an optical fiber according to claim 13 or 14, wherein the outer diameter of the dummy glass is less than or equal to the outer diameter of the starting rod.

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