Optical fiber glass base material and method for manufacturing optical fiber glass base material

Abstract

To provide a glass base material for optical fiber in which a starting rod and a dummy glass are not easily separated, and a method for manufacturing the same. The glass preform for optical fiber has a structure in which a dummy glass is fitted into one end of a starting rod, and a part of the dummy glass and the starting rod are surrounded by cladding glass. When connecting the starting rod and the dummy glass, a manufacturing method is used in which a trowel is brought into contact with the connection portion and moved from the starting rod side toward the dummy glass side while applying a load to adjust the shape.

Description

[Technical Field]

[0001] The present invention relates to a glass preform for an optical fiber and a method for manufacturing the glass preform for an optical fiber. [Background technology]

[0002] As a method for manufacturing a glass preform for an optical fiber, 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 preform for optical fiber by depositing glass particles for cladding on the outer periphery of a starting member in which dummy glass is fused to both ends of a starting rod, i.e., on the outer periphery of the starting rod, the joint between the starting rod and the dummy glass, and part of the dummy glass, and then heating the deposited porous glass in a high-temperature furnace to form transparent cladding glass.

[0004] Regarding the method of connecting a starting rod and dummy glass, Patent Document 2 describes a method of smoothing the connection by pressing an iron against the connection and performing a "reciprocating motion." Also, Patent Document 3 describes a method of pressing the dummy glass and the starting rod together while heating the connection, and then repeatedly pulling them apart to make the outer diameter of the nodular part the same as the outer diameter of the dummy glass. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-189428 [Patent Document 2] Japanese Patent Application Publication No. 6-199533 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-80299 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when the glass preform for optical fiber produced by the above method is suspended by connecting the dummy glass 1 to a hanging shaft or the like in a later process (Fig. 1(a)), and heated under a load, separation 5 tends to occur near the joint (Fig. 1(b)). This separation can cause damage 6 to the glass preform for optical fiber, which has been a problem (Fig. 1(c)).

[0007] The starting rod is doped with germanium to increase the refractive index of the core, and the cladding is sometimes doped with an appropriate amount of fluorine to form a depressed section or trench section with a lower refractive index. In particular, when the part of the starting rod that is joined to the dummy rod is doped with fluorine, separation is likely to occur near the joint.

[0008] The present invention has been made in view of the above, and has an object to suppress separation of the starting rod and the dummy glass. [Means for solving the problem]

[0009] In order to solve the above problems, the glass preform for optical fiber of the present invention is characterized by comprising a starting rod, dummy glass fitted into one end of the starting rod and integrally joined, and cladding glass surrounding a portion of the dummy glass and the starting rod. [Effects of the Invention]

[0010] According to the present invention, the starting rod is fitted into one end of the dummy glass and joined together, so that separation near the joint is unlikely to occur even when the dummy glass is suspended and heated under load. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram showing an example of a conventional starting rod, a dummy glass splice, and an optical fiber glass preform. [Figure 2] 1 is a schematic diagram showing an example of a starting rod, a dummy glass splice, and an optical fiber glass base material of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an example of a method for manufacturing an optical fiber glass preform according to the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the following embodiments.

[0013] The glass preform for optical fiber in this embodiment is produced by joining dummy glass 1 and a starting rod 2 ((a) to (d) of Figure 2), and forming clad glass 4 to surround a portion of the dummy glass 1 and the starting rod 2 ((e) of Figure 2).

[0014] In the glass preform for optical fiber of the present invention, the dummy glass 1 is fitted into the starting rod 2 at a joint 3 between the dummy glass 1 and the starting rod 2 and joined together. That is, the dummy glass 1 is joined in a state where a part of the dummy glass 1 is inserted closer to the starting rod 2 than the end face of the starting rod 2. In a subsequent process, the glass preform for optical fiber is suspended while supporting the dummy glass 1 joined in this manner, thereby making separation near the joint 3 less likely to occur.

[0015] Such joining to the dummy glass 1 may be performed at both ends of the starting rod 2, in which case there is an advantage that separation near the joint is unlikely to occur regardless of which end the dummy glass 1 is supported and hung from.

[0016] The dummy glass 1 and the starting rod 2 are preferably airtightly joined at the joint 3. If air bubbles remain and are trapped in the joint 3, separation of the starting rod 2 and the dummy glass 1 may occur, starting from the air bubbles.

[0017] The starting rod 2 is the component that will become the center of the glass preform for the optical fiber to be manufactured, and is preferably doped with a dopant that corresponds to the refractive index profile of the desired optical fiber. For example, it may be a hollow round rod with a center made of silica glass doped with germanium (Ge) and surrounded by pure silica glass or silica glass doped with fluorine (F). (The starting rod may also be a hollow round rod.)

[0018] The shape of the joint 3 where the joining end of the dummy glass 1 is fitted into the joining end of the starting rod 2 is not particularly limited, but it is preferable that the dummy glass 1 and the starting rod 2 are joined in a state where the central axes of their cross-sectional circles coincide. This results in a uniform stress distribution in the circumferential direction of the cross section at the joint 3. When the dummy glass 1 is a solid round rod, it is preferable that the center of the joining end of the dummy glass 1 is fitted into the joining end of the starting rod 2 (Figure 2 (d)). This is particularly suitable when the outer diameters 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 processed so that the center of the joining end of the starting rod fits into the joining end of the dummy glass. This type of processing can also achieve the same effect as this embodiment.

[0020] 2, when the outer diameter of the dummy glass 1 is smaller than that of the starting rod 2, it is also preferable to fit the entire joining end of the dummy glass 1 into the joining end of the starting rod. On the other hand, when the outer diameter of the dummy glass is larger than that of the starting rod, the entire joining end of the starting rod may be fitted into the joining end of the dummy glass. The dummy glass may be tubular (hollow round rod), which reduces the weight of the dummy glass.

[0021] The present invention is particularly effective when the dummy glass 1, the starting rod 2, and the clad glass 4 contain glasses with different dopant compositions. In this case, the joint 3 at the connection portion can be visually observed.

[0022] For example, the starting rod for optical fiber is synthetic silica glass doped with dopants such as germanium (Ge) or fluorine (F) to adjust the refractive index, while the cladding glass is synthetic silica glass that contains almost no of these dopants. To reduce light absorption, these synthetic silica glasses have had their OH groups removed to reduce the OH group content to 1 ppm or less, and contain chlorine (Cl) at 100 ppm or more. Meanwhile, inexpensive natural silica glass is often used as the dummy glass.

[0023] When a glass fiber is formed by assembling three different types of glass (dummy glass, starting rod, and cladding glass), stress tends to concentrate near the joint. In a subsequent process, thermal energy is applied to the joint, and further, almost the entire load of the glass preform body for optical fiber is applied, which facilitates separation of the joint. The glass preform for optical fiber of the present invention is effective in suppressing separation of the joint between such different glasses, and is particularly effective when the starting rod is doped with fluorine. In this case, it is effective when the fluorine doping amount is 0.7 wt % or less, and even more effective when it is 0.5 wt % or less.

[0024] In the method for manufacturing a glass preform for optical fiber of the present invention, the dummy glass 1 and the starting rod 1 are aligned at their ends in a processing device such as a glass lathe (FIG. 2(a)), softened by heating with a flame (not shown) or the like, and then pressed and fused together to form a large-diameter portion 7 at the joint, the outer diameter of which is larger than that of the dummy glass 1 or the starting rod 2 (FIG. 2(b)). When a processing iron 8 is brought into contact with the large-diameter portion 7 to level it, the processing iron 8 is brought into contact with the large-diameter portion and moved from the starting rod 2 side toward the dummy glass 1 side while applying a load, thereby leveling the bulge of the large-diameter portion 7 (FIG. 2(c)), and a joint 3 is formed in which the joint end of the dummy glass 1 is fitted into the joint end of the starting rod 2 (FIG. 2(d)). Here, "leveling out" the bulge of the thick-diameter portion 7 means aligning the outer diameter of the thick-diameter portion 7 so that the difference between it and the outer diameter of the starting rod 2 is small, and the difference between them is preferably 10% or less, and more preferably 5% or less, of the outer diameter of the starting rod 2. If the thick-diameter portion 7 is not leveled out, the flow of glass particles blown onto the starting rod 2 in the subsequent glass particle deposition process will be impaired, and the formation of the glass particle deposition layer will likely be disrupted.

[0025] By using this method, it is possible to easily shape the large diameter portion 7 and complete the processing of the dummy glass 1 and the starting rod 2. By fitting the end of the dummy glass 1 into the starting rod 2 (Fig. 2(d)), localized stress and load concentration are less likely to occur, making it less likely for the glass to separate. The joint 3 at the connection point, indicated by the dotted line in the figure, can be visually observed.

[0026] The depth of insertion of the dummy glass into the starting rod can be adjusted, for example, by changing the diameter of the thick-diameter portion formed by fusing the end faces of the softened dummy glass and the starting rod together. A thick-diameter portion can be made larger to achieve a greater insertion depth, while a thin-diameter portion can be made smaller to achieve a smaller insertion depth. This can also be adjusted by adjusting the heating temperature during molding of the thick-diameter portion to change the degree of softening of the glass, or by adjusting the load applied by the trowel or the speed at which the trowel is moved during molding of the thick-diameter portion.

[0027] Furthermore, in the present invention, it is desirable that at least one of the end surfaces of the starting rod and the dummy glass be convex before welding. If both end surfaces of the starting rod and the dummy glass are flat, air bubbles may remain when the end surfaces are welded and become trapped in the joint. By making at least one of the end surfaces convex, the joint is formed as the convex portion is deformed during welding, so no air bubbles remain. Furthermore, when shaping the large diameter portion created during welding with a processing trowel, the large diameter portion formed by connecting the convex portions to each other or the convex portion to a flat surface has the advantage of being smoother and easier to shape than the large diameter portion formed by connecting flat surfaces to each other.

[0028] The tip is preferably ground in advance using a grinder, etc. The protrusion of the convex portion may be such that the center is raised, or the convex portion may be offset from the center toward the outside.

[0029] The protrusion amount is preferably between 0.5 and 2 times the radius. If the protrusion amount is less than 0.5 times, air bubbles are likely to remain, and if it exceeds 2 times, the protrusion will be easily deformed and droopy during heating by the flame.

[0030] In the present invention, the outer diameter of the dummy glass is preferably smaller than that of the starting rod. Since the main purpose of the dummy glass is to support the starting rod during the glass particle deposition and sintering processes, there is no need to make the dummy glass thicker than necessary. Furthermore, as described above, by placing a trowel on the starting rod side of the thickened portion (the portion 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, the thickened portion can be leveled, making it easy to fit the spliced ​​end of the dummy glass into the spliced ​​end of the starting rod. (Although the size of the starting rod is determined by the size and refractive index profile design of the desired optical fiber glass preform,) using as thin a dummy glass as possible reduces the heat, time, and load required for heating and joining, making processing easier. [Example]

[0031] A starting rod with an outer diameter of 50 mm, a length of 1500 mm, and flat surfaces on both ends was prepared, made of synthetic silica glass including a core manufactured using the VAD method. This starting rod was distinguished between the start side of VAD manufacturing and the end side of manufacturing. The starting rod prepared here was not doped with fluorine.

[0032] One chuck of the glass lathe was used to hold the end face of the starting rod on the VAD manufacturing side facing the other chuck, and the other chuck was used to hold a dummy glass made of natural silica glass with an outer diameter of 50 mm and a length of 500 mm. While the chuck was rotating, the end face of the starting rod on the manufacturing side and the end face of the dummy glass were heated to about 2000°C with an oxyhydrogen flame from a burner to soften them. This state is designated (α).

[0033] [Example 1] For (α), the chucks were brought close together to weld the end faces together, expanding the connection (forming a thicker diameter section). The maximum diameter of the expanded connection (thicker diameter section) was 54 mm. While continuing to heat the connection with the oxyhydrogen flame, a processing trowel made of high-purity carbon was placed against the starting rod side of the expanded connection (thicker diameter section) and a load was applied while moving it toward the dummy glass, thereby smoothing out the bulge (thicker diameter section) of the connection. The maximum diameter of the connection after this was 51 mm. The dummy glass was embedded 1 mm into the starting rod at the joint.

[0034] [Example 2] For (α), the chucks were brought close together to weld the end faces together, expanding the connection (forming a thicker diameter section). The maximum diameter of the expanded connection (thicker diameter section) was 56 mm. While continuing to heat the connection with the oxyhydrogen flame, a processing trowel made of high-purity carbon was placed against the starting rod side of the expanded connection (thicker diameter section) and a load was applied while moving it toward the dummy glass, thereby smoothing out the bulge (thicker diameter section) of the connection. The maximum diameter of the connection after this was 52 mm. The dummy glass was embedded 3 mm into the starting rod at the joint.

[0035] [Example 3] For (α), the chucks were brought close together to weld the end faces together, expanding the connection (forming a thicker diameter section). The maximum diameter of the expanded connection (thicker diameter section) was 57 mm. While continuing to heat the connection with the oxyhydrogen flame, a processing trowel made of high-purity carbon was placed against the starting rod side of the expanded connection (thicker diameter section) and a load was applied while moving it toward the dummy glass, thereby smoothing out the bulge (thicker diameter section) of the connection. The maximum diameter of the connection after this was 52 mm. The dummy glass was embedded 5 mm into the starting rod at the joint.

[0036] For the connections between the starting rod and the dummy glass in Examples 1, 2, and 3, the dummy glass was held in place by the chuck on the starting rod, and the chuck on the released side held another dummy glass made of natural quartz glass with an outer diameter of 50 mm and a length of 500 mm. While rotating the chuck, the respective end faces were heated to approximately 2000 °C with an oxyhydrogen flame from a burner to soften them. The chucks were then brought closer together, welding the end faces of the dummy glass and the starting rod at the end of the VAD process to form a thickened portion by expanding the connection. The maximum diameter of the expanded connection (thickened portion) was 54 mm. While continuing to heat the connection (thickened portion) with the oxyhydrogen flame, a processing iron was applied to the expanded connection (thickened portion) and moved back and forth between the starting rod side and the dummy glass side of the joint, thereby smoothing out the bulge (thickened portion) of the connection. This resulted in the formation of a target having dummy glass on both ends of the starting rod.

[0037] The dummy glass at both ends of the target formed in Examples 1, 2, and 3 above was held by chucks located at both ends in the chamber, and glass particles were deposited on the outer periphery of the target by the OVD method to produce a porous glass preform with an outer diameter of 300 mm. The dummy glass joined to the starting rod of this porous glass preform at the production start side of the VAD method was connected to the tip of the shaft of the lifting mechanism of the dehydration sintering device, and the starting rod was inserted into the furnace tube of the dehydration sintering device in a vertically suspended state with the production start side of the VAD method facing upward, and was moved downward while being heated at 1500°C, and the porous glass was sintered to form a transparent glass, thereby producing a glass preform for optical fiber.

[0038] Ten optical fiber glass preforms were manufactured using the configurations of Examples 1, 2, and 3. The dummy glass at the start of VAD manufacturing was held and hung vertically, and a downward load of 50 kgf was applied. The joint between the starting rod at the start of VAD manufacturing and the dummy glass was visually observed to check the incidence of separation. The results are shown in Table 1.

[0039] [Table 1]

[0040] In all of Examples 1, 2, and 3, a high separation suppression effect was confirmed, with a separation occurrence rate of 30% or less. A comparison of Examples 1, 2, and 3 shows that the effect of suppressing separation tends to be greater as the insertion depth increases. In Example 1, the insertion depth of the dummy glass into the starting rod was 1 mm, which was 2% of the outer diameter of the starting rod, 50 mm, and therefore it is believed that an insertion depth of the dummy glass into the starting rod of 2% or more of the outer diameter of the starting rod will produce a particularly excellent effect.

[0041] [Examples 4 to 7] In a separate study, starting rods with an outer diameter of 50 mm and a length of 1500 mm were prepared, each consisting of a synthetic silica glass core manufactured using the VAD method. These starting rods were doped with five different concentrations of fluorine: 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) to form a depressed region around the core. The fluorine concentrations of the starting rods were determined by pulverizing the starting rods, melting a portion of the resulting glass fragments, and analyzing them using ion chromatography. Ten optical fiber glass preforms were manufactured from each of these starting rods using the same procedure as in Example 1.

[0042] [Comparative Examples 1 and 2] Additionally, a starting rod with an outer diameter of 50 mm and a length of 1500 mm was prepared, made of synthetic silica glass including a core manufactured using the VAD method. This starting rod was doped with two types of fluorine, 0.5 wt% (Comparative Example 1) and 0.7 wt% (Comparative Example 2), to form a depressed type around the outer periphery of the core, thereby reducing the transmission loss of the optical fiber obtained by drawing. The fluorine concentration of the starting rod was determined by pulverizing the starting rod, dissolving some of the glass fragments obtained, and analyzing them using ion chromatography.

[0043] For the starting rods of Comparative Examples 1 and 2, one chuck of a glass lathe was used to hold the end face of the starting rod at the production start side using the VAD method, facing the other chuck. The other chuck held a dummy glass made of natural quartz glass with an outer diameter of 50 mm and a length of 500 mm. While the chuck was rotating, the end face of the starting rod at the production start side and the end face of the dummy glass were heated to approximately 2000 °C with an oxyhydrogen flame emitted from a burner and softened. The chucks were then brought close to each other to weld the end faces together and expand the joint (forming a thickened portion). The maximum diameter of the expanded joint (thickened portion) was 54 mm. Subsequently, while continuing to heat with the oxyhydrogen flame, a processing iron was applied to the expanded joint (thickened portion) and moved back and forth between the starting rod side and the dummy glass side of the joint to smooth out the expansion of the joint. The maximum diameter of the joint after this work was 53 mm. However, unlike Examples 1 to 7, the joint between the joining end of the dummy glass and the joining end of the starting rod was joined in a flat manner.

[0044] For the connections between the starting rod and the dummy glass in Comparative Examples 1 and 2, the dummy glass was held in place by the chuck, but the grip of the starting rod was released. Another dummy glass piece made of natural quartz glass, measuring 50 mm in outer diameter and 500 mm in length, was then held in the released chuck. While rotating the chuck, the respective end faces were heated to approximately 2000°C with an oxyhydrogen flame from a burner to soften them. The chucks were then brought closer together, welding the end faces of the dummy glass and the starting rod at the end of the VAD process to form a thickened portion by expanding the connection. The maximum diameter of the expanded connection (thickened portion) was 54 mm. While continuing to heat the connection (thickened portion) with the oxyhydrogen flame, a processing iron was applied to the expanded connection (thickened portion) and moved back and forth between the starting rod side and the dummy glass side of the joint to smooth out the bulge (thickened portion). This resulted in the formation of a target having dummy glass on both ends of the starting rod.

[0045] The dummy glass on both ends of the targets formed in Comparative Examples 1 and 2 above was held by chucks located at both ends in the chamber, and glass particles were deposited on the outer periphery of the target by the OVD method to produce a porous glass preform with an outer diameter of 300 mm. The dummy glass joined to the starting rod of this porous glass preform at the production start side using the VAD method was connected to the tip of the shaft of the lifting mechanism of the dehydration sintering device, and the starting rod was inserted into the furnace tube of the dehydration sintering device while being hung vertically with the production start side using the VAD method facing upward, and was moved downward while being heated at 1500°C to sinter the porous glass to form a transparent glass, thereby producing 10 glass preforms for optical fiber for each.

[0046] For the above-mentioned optical fiber glass preform, the joint between the starting rod and the dummy glass at the start of production was visually observed to check the incidence of separation. The results are shown in Table 2.

[0047] [Table 2]

[0048] Comparisons between Example 5 and Comparative Example 1, and Example 7 and Comparative Example 2 confirmed that even if the fluorine doping concentration of the starting rod is the same, by setting the insertion depth / outer diameter of the starting rod to 2% or more, separation at the joint can be significantly suppressed. The effects of the present invention were confirmed when the fluorine doping amount of the starting rod was 0.7 wt% or less. A particularly high effect was confirmed when the fluorine doping concentration of the starting rod was 0.5 wt% or less.

[0049] [Example 8] In a separate study, a starting rod with an outer diameter of 50 mm and a length of 1500 mm was prepared, which was made of synthetic quartz glass including a core and manufactured using the VAD method. The tip of the starting rod was ground using a disc grinder into a convex cone shape with a height of 30 mm.

[0050] [Example 9] In addition, both ends of a starting rod made of synthetic quartz glass including a core and manufactured using the VAD method, with an outer diameter of 50 mm and a length of 1,800 mm, were held in chucks on both sides of a glass lathe, and a position 300 mm from the starting side of production was heated with an oxyhydrogen flame to soften it, while one of the chucks was moved and melted, thereby forming the tip of the 1,500 mm long starting rod at the starting side of production into a convex parabolic taper shape with a height of 30 mm.

[0051] Ten optical fiber glass preforms were manufactured from the starting rods of Examples 8 and 9 using the same procedure as in Example 1, and the joint between the starting rod and the dummy glass at the start of manufacturing was visually inspected to determine the incidence of separation. The results are shown in Table 3.

[0052] [Table 3]

[0053] In both Examples 8 and 9, the incidence of separation at the joint was 0%. This is thought to be due to the fact that the convex tip prevented bubbles from getting into the joint, eliminating the starting point for separation. In Examples 8 and 9, the end face shape of the starting rod was changed, but the same effect could be obtained by changing the end face shape of the dummy glass.

[0054] [Example 10] As a separate study, a starting rod with an outer diameter of 50 mm and a length of 1500 mm was prepared, which was made of synthetic silica glass including a core and manufactured using the VAD method. One chuck of the glass lathe held the end of the starting rod facing the other chuck, while the other chuck held a dummy glass piece made of natural quartz glass with an outer diameter of 40 mm and a length of 500 mm. While the chuck was rotating, the end of the starting rod and the end of the dummy glass were heated to approximately 2000°C with an oxyhydrogen flame from a burner to soften them. The chucks were then brought close together to fuse the end faces together and expand the joint. The maximum diameter of the expanded joint was 48 mm. While continuing to heat the joint with the oxyhydrogen flame, a high-purity carbon processing trowel was applied from the starting rod side to the dummy glass side to smooth out the bulge. The maximum diameter of the joint after this process was 45 mm.

[0055] [Example 11] One chuck of the glass lathe held the end of the starting rod facing the other chuck, while the other chuck held a dummy glass piece made of natural quartz glass with an outer diameter of 60 mm and a length of 500 mm. While the chuck was rotating, the end of the starting rod and the end of the dummy glass were heated to approximately 2000°C with an oxyhydrogen flame from a burner to soften them. The chucks were then brought close together to fuse the end faces together and expand the joint. The maximum diameter of the expanded joint was 59 mm. While continuing to heat the joint with the oxyhydrogen flame, a high-purity carbon processing trowel was applied from the starting rod side to the dummy glass side to smooth out the bulge. The maximum diameter of the joint after this process was 54 mm.

[0056] Ten optical fiber glass preforms were manufactured for each of the joining bodies of the starting rod and dummy glass in Examples 10 and 11 using the same procedure as in Example 1. The joint between the starting rod and the dummy glass at the start of manufacturing was visually inspected to determine the incidence of separation. The results are shown in Table 4.

[0057] [Table 4]

[0058] In Example 10, in which the outer diameter of the starting rod was larger than the outer diameter of the dummy glass, the incidence of separation at the joint was 0%. Considering the cost of the dummy glass as well, it is considered preferable that the outer diameter of the starting rod is larger than the outer diameter of the dummy glass.

[0059] The present invention is not limited to the above-described embodiment, but can be freely modified and improved as appropriate. [Explanation of symbols]

[0060] 1: Dummy glass 2: Starting rod 3: Seams 4: Clad glass 5: Separation 6: Breakage of the optical fiber glass base material 7: Thick diameter part 8:Processing trowel

Claims

1. A starting rod; a dummy glass fitted into one end of the starting rod and integrally joined; a cladding glass surrounding a portion of the dummy glass and the starting rod; It consists of The outer diameter of the dummy glass is equal to or smaller than the outer diameter of the starting rod.

2. 2. The optical fiber glass preform according to claim 1, wherein the starting rod and the dummy glass are hermetically bonded together.

3. 3. The glass preform for an optical fiber according to claim 1, wherein a central portion of a joining end of said dummy glass is fitted into the joining end of said starting rod.

4. 4. The glass preform for an optical fiber according to claim 1, wherein the joining end of the dummy glass is fitted into the joining end of the starting rod so as to be enveloped therein.

5. The optical fiber glass preform according to claim 1 , wherein the starting rod contains germanium.

6. The optical fiber glass preform according to claim 1 , wherein the starting rod contains fluorine.

7. 7. The optical fiber glass preform according to claim 6, wherein the fluorine content is 0.7% by weight or less.

8. 7. The optical fiber glass preform according to claim 6, wherein the fluorine content is 0.5% by weight or less.

9. 9. The optical fiber glass preform according to claim 1, wherein the depth of the dummy glass inserted into the starting rod is 2% or more of the outer diameter of the starting rod.

10. 10. The optical fiber glass preform according to claim 1, wherein the dummy glass is in the form of a solid rod or a hollow cylinder.

11. 10. The optical fiber glass preform according to claim 1, wherein the starting rod is a solid rod.

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