Method for manufacturing optical fiber preform

Abstract

To provide a method for manufacturing an optical fiber preform having improved yield.SOLUTION: A method for manufacturing an optical fiber preform includes an accommodating step, a sealing step, a connecting step, a forming step, and a heating step. In the accommodating step, a glass rod is produced by accommodating a core rod that includes a core and an inner cladding covering an outer periphery of the core, the core rod extending along a first axis, inside a first glass tube extending along the first axis, to which fluorine is added, the first glass tube having an inner diameter larger than an outer diameter of the core rod. In the sealing step, a first end face of the first glass tube is sealed. In the connecting step, a second end face of the first glass tube located opposite to the first end face is connected to an end face of a second glass tube extending along the first axis and having an inner diameter smaller than the outer diameter of the core rod. In the forming step, a soot body covering an outer periphery of the glass rod is formed. In the heating step, the glass rod and the soot body are simultaneously heated to vitrify the soot body.SELECTED DRAWING: Figure 11

Description

[Technical Field]

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

[0002] Patent Document 1 discloses a method for manufacturing an optical fiber preform. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent No. 9,776,907 Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Document 1 describes a method for manufacturing an optical fiber preform, which includes forming a trench region around a core rod, forming a barrier layer with a high bulk density around the trench region, forming an outer cladding region around the barrier layer, withdrawing the core rod, and inserting a core rod including a core and an inner cladding into the hollow portion left after the core rod is withdrawn. With this method, there is a risk that the optical fiber preform may be damaged when the core rod is withdrawn, resulting in a decrease in yield.

[0005] An object of the present disclosure is to provide a method for manufacturing an optical fiber preform with improved yield. [Means for solving the problem]

[0006] A method for manufacturing an optical fiber preform according to a first aspect of the present disclosure includes an accommodating step, a first sealing step, a connecting step, a forming step, and a heating step. In the accommodating step, a glass rod is fabricated by accommodating a core rod inside a first glass tube. The core rod includes a core and an inner cladding covering the outer periphery of the core, and extends along a first axis. The first glass tube extends along the first axis, is doped with fluorine, and has an inner diameter larger than the outer diameter of the core rod. In the first sealing step, a first end face of the first glass tube is sealed. In the connecting step, a second end face of the first glass tube located opposite the first end face is connected to an end face of a second glass tube. The second glass tube extends along the first axis and has an inner diameter smaller than the outer diameter of the core rod. In the forming step, a soot body is formed to cover the outer periphery of the glass rod. In the heating step, the glass rod and the soot body are heated simultaneously to vitrify the soot body. [Effects of the Invention]

[0007] According to the present disclosure, it is possible to provide a method for manufacturing an optical fiber preform with improved yield. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a cross-sectional view of an optical fiber preform. [Figure 2] FIG. 2 is a diagram showing the refractive index distribution in the radial direction of the optical fiber preform. [Figure 3] FIG. 3 is a schematic diagram of each member used in the method for manufacturing an optical fiber preform according to the first embodiment. [Figure 4] FIG. 4 is a front view of the end face of the core rod. [Figure 5] FIG. 5 is a diagram for explaining the dimensions of the core rod, the first glass tube, the sealing member, and the second glass tube. [Figure 6] FIG. 6 is a flowchart showing a method for manufacturing an optical fiber preform according to the first embodiment. [Figure 7] FIG. 7 is a diagram for explaining the accommodation step. [Figure 8]FIG. 8 is a diagram for explaining the first sealing step and the connecting step. [Figure 9] FIG. 9 is a diagram for explaining the second sealing step. [Figure 10] FIG. 10 is a diagram for explaining the forming process. [Figure 11] FIG. 11 is a diagram for explaining the heating step. [Figure 12] FIG. 12 is a schematic diagram of each member used in the method for manufacturing an optical fiber preform according to the second embodiment. [Figure 13] FIG. 13 is a flowchart showing a method for manufacturing an optical fiber preform according to the second embodiment. [Figure 14] FIG. 14 is a diagram for explaining the accommodation step. [Figure 15] FIG. 15 is a diagram for explaining the first connecting step and the second connecting step. [Figure 16] FIG. 16 is a diagram for explaining the dehydration step. [Figure 17] FIG. 17 is a diagram for explaining the sealing step. [Figure 18] FIG. 18 is a diagram for explaining the forming process. [Figure 19] FIG. 19 is a diagram for explaining the heating step. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0010] [1] A method for manufacturing an optical fiber preform according to a first aspect of the present disclosure includes an accommodation step, a first sealing step, a connection step, a forming step, and a heating step. In the accommodation step, a glass rod is fabricated by accommodating a core rod inside a first glass tube. The core rod includes a core and an inner cladding covering the outer periphery of the core, and extends along a first axis. The first glass tube extends along the first axis, is doped with fluorine, and has an inner diameter larger than the outer diameter of the core rod. In the first sealing step, a first end face of the first glass tube is sealed. In the connection step, a second end face of the first glass tube located opposite the first end face is connected to an end face of a second glass tube. The second glass tube extends along the first axis and has an inner diameter smaller than the outer diameter of the core rod. In the forming step, a soot body is formed to cover the outer periphery of the glass rod. In the heating step, the glass rod and the soot body are heated simultaneously to vitrify the soot body.

[0011] In this method for manufacturing an optical fiber preform, a first glass tube that will become a fluorine-doped trench is prepared in advance, and a core rod including a core and an inner cladding is placed inside the first glass tube. After the core rod is placed inside the first glass tube, a soot body is formed to cover the outer periphery of the first glass tube. Therefore, a core rod is not required to form the soot body, and there is no need to remove the core rod after the soot body is formed. This facilitates the lengthening of the optical fiber preform and reduces the risk of damaging the optical fiber preform. As a result, the yield of the optical fiber preform can be improved. Furthermore, during the heating process, air expanded in the gap between the core rod and the first glass tube is discharged through the second glass tube. Therefore, the core rod and the first glass tube can be integrated and the formed soot body can be vitrified while suppressing the expansion of the optical fiber preform during the heating process.

[0012] [2] In the method for manufacturing an optical fiber preform according to [1] above, the core rod may have a notch on its end face facing the second glass tube, the notch extending from the outer periphery of the core rod toward the central axis of the core rod. In this case, even if the end face of the core rod and the end face of the second glass tube are in contact with each other, the notch spatially connects the gap between the core rod and the first glass tube with the hollow part of the second glass tube. Therefore, during the heating step, expanded air in the gap between the core rod and the first glass tube is efficiently discharged from the second glass tube.

[0013] [3] In the method for manufacturing an optical fiber preform according to [1] or [2] above, an inert gas may be flowed into the first glass tube during the splicing step. In this case, the inert gas is sent into the gap between the core rod and the first glass tube, thereby removing moisture contained in the gap. This makes it possible to prevent moisture from remaining inside the completed optical fiber preform.

[0014] [4] The method for manufacturing an optical fiber preform according to any one of [1] to [3] above may further include a second sealing step of sealing the second glass tube between the connecting step and the forming step, and an opening step of opening the second glass tube between the forming step and the heating step. In this case, it is possible to prevent moisture-containing air from being sent to the gap between the core rod and the first glass tube in the forming step.

[0015] [5] A method for manufacturing an optical fiber preform according to a second aspect of the present disclosure includes an accommodation step, a first connection step, a second connection step, a dehydration step, a forming step, and a heating step. In the accommodation step, a glass rod is fabricated by accommodating a core rod inside a first glass tube. The core rod includes a core and an inner cladding covering the outer periphery of the core, and extends along a first axis. The first glass tube extends along the first axis, is doped with fluorine, and has an inner diameter larger than the outer diameter of the core rod. In the first connection step, a first end face of the glass rod is connected to an end face of a second glass tube. The second glass tube extends along the first axis and has an inner diameter smaller than the outer diameter of the core rod. In the second connection step, a second end face of the glass rod located opposite the first end face is connected to an end face of a third glass tube. The third glass tube extends along the first axis and has an inner diameter smaller than the outer diameter of the core rod. In the dehydration process following the first and second connecting processes, a dehydrating gas is passed from one end of the second glass tube to the other end of the third glass tube to remove moisture from the gap between the core rod and the first glass tube. In the forming process, a soot body is formed to cover the outer periphery of the glass rod. In the heating process, the glass rod and the soot body are heated simultaneously to vitrify the soot body.

[0016] In this method for manufacturing an optical fiber preform, a first glass tube serving as a fluorine-doped trench is prepared in advance, and a core rod including a core and an inner cladding is placed inside the first glass tube. After the core rod is placed inside the first glass tube, a soot body is formed to cover the outer periphery of the first glass tube. Therefore, a core rod is not required to form the soot body, and there is no need to remove the core rod after the soot body is formed. This facilitates the lengthening of the optical fiber preform and reduces the risk of damaging the optical fiber preform. As a result, the yield of the optical fiber preform can be improved. Furthermore, during the heating process, air expanded in the gap between the core rod and the first glass tube is discharged from the second and third glass tubes. Therefore, the core rod and the first glass tube can be integrated and the formed soot body can be vitrified while suppressing the expansion of the optical fiber preform during the heating process. Furthermore, during the dehydration process, a dehydrating gas is supplied to the gap between the core rod and the first glass tube, thereby removing moisture contained in the gap. Therefore, it is possible to prevent moisture from remaining inside the completed optical fiber preform.

[0017] [6] In the method for manufacturing an optical fiber preform according to [5] above, the core rod may have notches extending from the outer periphery of the core rod toward the central axis of the core rod on the end face facing the second glass tube and the end face facing the third glass tube. In this case, even if the end faces on both sides of the core rod are in contact with the end faces of the second glass tube and the third glass tube, the notches spatially connect the gap between the core rod and the first glass tube with the hollow portions of the second glass tube and the third glass tube. Therefore, during the heating step, expanded air in the gap between the core rod and the first glass tube is efficiently discharged from the second glass tube and the third glass tube.

[0018] [7] The method for manufacturing an optical fiber preform according to [5] or [6] above may further include a sealing step of sealing the second glass tube and the third glass tube between the dehydration step and the forming step, and an opening step of opening the second glass tube and the third glass tube between the forming step and the heating step. In this case, it is possible to prevent moisture-containing air from being sent to the gap between the core rod and the first glass tube in the forming step.

[0019] [8] In any of the methods for manufacturing an optical fiber preform according to [1] to [7] above, the heating step may be performed in a reduced pressure atmosphere. In this case, the air pressure in the gap between the core rod and the first glass tube is higher than the air pressure around the first glass tube. Therefore, the air in the gap between the core rod and the first glass tube is efficiently discharged from the second glass tube. This makes it easier for the core rod and the first glass tube to be tightly attached and integrated.

[0020] [9] In the method for manufacturing an optical fiber preform according to any one of the above [1] to [8], the difference between the outer diameter of the core rod and the inner diameter of the first glass tube may be 0.1 mm or more and 5 mm or less. In this case, it is possible to easily accommodate the core rod inside the first glass tube while reducing the amount of air in the gap between the core rod and the first glass tube.

[0021]

[10] In the heating step of any one of the methods for manufacturing an optical fiber preform according to [1] to [9] above, the glass rod and the soot body may be heated at a temperature of 1300° C. or more and 1600° C. or less. In this case, it is possible to simultaneously shorten the heating time for making the soot body transparent and reduce the change in the outer diameter of the soot body before and after the heating step.

[0022]

[11] In the forming step of the method for manufacturing an optical fiber preform according to any one of [1] to

[10] above, the average bulk density of the soot body is 0.2 g / cm 3 More than 0.8g / cm 3In this case, it is possible to reduce both the frequency with which the soot body cracks during the forming process and the frequency with which the soot body fails to be integrated with the glass rod due to a reduced amount of shrinkage of the soot body during the heating process.

[0023]

[12] In the method for manufacturing an optical fiber preform according to any one of [1] to

[11] above, the arithmetic mean of the surface roughness of the inner peripheral surface of the first glass tube may be less than 0.50 μm. In this case, the frequency of failure in integrating the first glass tube with the core rod can be reduced. Also, an increase in the transmission loss of the optical fiber for light with a wavelength of 1.55 μm, which is caused by insufficient integration of the first glass tube with the core rod, can be suppressed.

[0024]

[13] In the method for manufacturing an optical fiber preform according to any one of [1] to

[12] above, the arithmetic mean of the surface roughness of the outer peripheral surface of the core rod may be less than 0.50 μm. In this case, the frequency of failure in integrating the first glass tube with the core rod can be reduced. Also, an increase in the transmission loss of the optical fiber for light with a wavelength of 1.55 μm, which is caused by insufficient integration of the first glass tube with the core rod, can be suppressed. [Details of the embodiments of the present disclosure]

[0025] Specific examples of the manufacturing method of an optical fiber preform according to an embodiment of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the following description, the same elements in the drawings will be designated by the same reference numerals, and duplicated descriptions will be omitted. (First embodiment)

[0026] FIG. 1 is a cross-sectional view of an optical fiber preform (preform) 1. An optical fiber is manufactured by drawing this optical fiber preform 1, and has the same configuration as the optical fiber preform 1 except for the dimensions. As shown in FIG. 1, the optical fiber preform 1 includes a core 11, an inner cladding 12, a trench 13, and an outer cladding 14. The core 11 is made of, for example, silica-based glass. The core 11 is doped with, for example, at least one of germanium (Ge) and chlorine (Cl), so that it has a higher refractive index than the inner cladding 12. The core 11 may also be doped with an alkali metal group. The inner cladding 12 is provided outside the core 11 and is configured to surround and cover the core 11.

[0027] The trench 13 is provided outside the inner cladding 12 and is configured to surround and cover the inner cladding 12. The outer cladding 14 is provided outside the trench 13 and is configured to surround and cover the trench 13. The inner cladding 12, the trench 13, and the outer cladding 14 are made of, for example, silica-based glass, which may be doped with fluorine. By providing the trench 13, an optical fiber with excellent bending loss resistance can be realized when the optical fiber preform 1 is drawn into an optical fiber.

[0028] Fig. 2 is a diagram showing the refractive index profile in the radial direction of the optical fiber preform 1. As shown in Figs. 1 and 2, the radius of the outer periphery of the core 11 is r1, the radius of the outer periphery of the inner cladding 12 is r2, the radius of the outer periphery of the trench 13 is r3, and the radius of the outer periphery of the outer cladding 14 is r4. The radius r1 is, for example, 2.0 mm. The radius r2 is, for example, 5.0 mm. The radius r3 is, for example, 7.5 mm. The radius r4 is, for example, 30.0 mm.

[0029] In FIG. 2, area E1 corresponds to the core 11, area E2 corresponds to the inner cladding 12, area E3 corresponds to the trench 13, and area E4 corresponds to the outer cladding 14. The vertical axis represents the relative refractive index difference, and the horizontal axis represents the radial position. As shown in FIG. 2, in the optical fiber preform 1, the relative refractive index differences of the core 11, the inner cladding 12, and the trench 13 with respect to the refractive index of the outer cladding 14 are Δ1, Δ2, and Δ3, respectively. Here, the relative refractive index difference Δ2 of the inner cladding 12 is smaller than the relative refractive index difference Δ1 of the core 11. In other words, the refractive index of the inner cladding 12 is smaller than the refractive index of the core 11. Furthermore, the relative refractive index difference Δ3 of the trench 13 is negative and smaller than the relative refractive index difference Δ2 of the inner cladding 12. In other words, the refractive index of the trench 13 is smaller than the refractive index of the inner cladding 12. The sign of the relative refractive index difference Δ3 of the trench 13 is negative, and the sign of the relative refractive index difference Δ1 of the core 11 is positive. A negative sign of the relative refractive index difference means that the refractive index difference is smaller than the refractive index of the outer cladding 14. In other words, the refractive index of the outer cladding 14 is higher than the refractive index of the trench 13 and lower than the refractive index of the core 11.

[0030] Next, each component used in the method for manufacturing the optical fiber preform 1 according to the first embodiment will be described with reference to FIG. 3. FIG. 3 is a schematic diagram of each component used in the method for manufacturing the optical fiber preform 1 according to the first embodiment. The method for manufacturing the optical fiber preform 1 according to the first embodiment uses a core rod 21, a glass tube 22 (first glass tube), a sealing member 3, and a glass tube 4 (second glass tube). The core rod 21 has a cylindrical shape. The core rod 21 includes a core 11 and an inner cladding 12. The core rod 21 extends along an axis L (first axis). The central axis of the cylindrical core rod 21 coincides with the axis L. The arithmetic mean of the surface roughness of the outer circumferential surface of the core rod 21 is, for example, less than 0.50 μm. The core rod 21 has an end face 21a, an end face 21b, and a notch 21c. The end face 21a is located at a first end of the core rod 21 in the direction along the axis L. The end face 21a has a circular shape.

[0031] The end face 21b will be described with reference to FIG. 4. FIG. 4 is a front view of the end face 21b. The end face 21b is located at a second end opposite the end face 21a in the direction along the axis L. The end face 21b has a substantially circular shape and has notches 21c at two positions on either side of the center of the end face 21b (i.e., the axis L). The notches 21c extend from the outer periphery of the core rod 21 toward the central axis (axis L) of the core rod 21. More precisely, the notches 21c extend from an imaginary cylindrical surface that defines a major portion of the outer periphery of the core rod 21 toward the central axis of the core rod 21. The notches 21c extend from the end face 21b along the axis L to partway along the core rod 21. The notches 21c are spaced apart from each other by a distance d. In a cross section perpendicular to the axis L, the notches 21c have, for example, an arc shape. The core rod 21 may have a varying outer diameter while having the notch 21c. That is, the core rod 21 may have a tapered shape in which the diameter is small at the end face 21b and gradually increases along the axis L from the end face 21b.

[0032] The glass tube 22 is a base component of the trench 13. The glass tube 22 has a cylindrical shape with both ends open. The glass tube 22 extends along an axis L. The central axis of the cylindrical glass tube 22 coincides with the axis L. Fluorine (F) is added to the glass tube 22 to lower its refractive index. The length of the glass tube 22 in the direction of the axis L is substantially equal to the length of the core rod 21 in the direction of the axis L. The arithmetic mean of the surface roughness of the inner circumferential surface of the glass tube 22 is, for example, less than 0.50 μm. The glass tube 22 has an end face 22a (first end face) and an end face 22b (second end face). The end face 22a is located at a first end of the glass tube 22 in the direction along the axis L. The end face 22b is located at a second end of the glass tube 22 opposite the first end in the direction along the axis L.

[0033] The sealing member 3 is a member that prevents the core rod 21 from falling out of the glass tube 22, and has, for example, a cylindrical shape. The sealing member 3 extends along the axis L. The central axis of the cylindrical sealing member 3 coincides with the axis L. The sealing member 3 is made of, for example, pure quartz. The sealing member 3 has an end face 3a. The end face 3a is located at the end of the sealing member 3 in the direction along the axis L.

[0034] The glass tube 4 is a component that prevents the core rod 21 from falling out of the glass tube 22, and has a cylindrical shape with both ends open. The glass tube 4 extends along an axis L. The central axis of the cylindrical glass tube 4 coincides with the axis L. The glass tube 4 is made of, for example, pure quartz. The glass tube 4 has an end face 4a and an end face 4b. The end face 4a is located at a first end of the glass tube 4 in the direction along the axis L. The end face 4b is located at a second end of the glass tube 4 in the direction along the axis L.

[0035] Referring to FIG. 5, the dimensions of the core rod 21, glass tube 22, sealing member 3, and glass tube 4 will be described. FIG. 5 is a diagram for explaining the dimensions of the core rod 21, glass tube 22, sealing member 3, and glass tube 4. In FIG. 5, the core rod 21, glass tube 22, sealing member 3, and glass tube 4 are shown in a cross section including the axis L. In one example, the outer diameter D11 of the glass tube 22, the outer diameter D12 of the sealing member 3, and the outer diameter D13 of the glass tube 4 may be equal to or different from one another. The inner diameter D3 of the glass tube 22 is larger than the outer diameter D2 of the core rod 21. The difference between the inner diameter D3 of the glass tube 22 and the outer diameter D2 of the core rod 21 is, for example, 0.1 mm or more and 5 mm or less. The inner diameter D4 of the glass tube 4 is smaller than the outer diameter D2 of the core rod 21. The inner diameter D4 of the glass tube 4 is larger than the separation distance d (see FIG. 4) between the notches 21c.

[0036] Here, examples of dimensions and surface roughness of each constituent material of the optical fiber preform 1 are shown in Table 1 as examples. [Table 1]

[0037] Next, a method for manufacturing the optical fiber preform 1 according to the first embodiment will be described with reference to Fig. 6. Fig. 6 is a flowchart showing a method for manufacturing the optical fiber preform 1 according to the first embodiment. As shown in Fig. 6, this manufacturing method includes an accommodation step ST11, a first sealing step ST12, a connection step ST13, a second sealing step ST14, a forming step ST15, an opening step ST16, and a heating step ST17.

[0038] Before performing each of the above steps ST11 to ST17, the glass tube 22 is heated (baked) while a dehydrating gas (e.g., chlorine gas (Cl2)) is flowed into the glass tube 22. The flow rate of Cl2 is, for example, 300 sccm or more and 2000 sccm or less. This dehydrates the inside of the glass tube 22. Next, etching gas (e.g., sulfur hexafluoride gas (SF6) and chlorine gas (Cl2)) is flowed into the glass tube 22. The flow rate of SF6 is, for example, 100 sccm or more and 500 sccm or less. The flow rate of Cl2 is, for example, 100 sccm or more and 500 sccm or less. This reduces the surface roughness of the inner circumferential surface of the glass tube 22. Note that if the flow rates are below the lower limit, the dehydration or etching effect will not be sufficient. On the other hand, if the flow rates are above the upper limit, the inner surface of the glass tube 22 will be cooled, similarly resulting in insufficient dehydration or etching effect.

[0039] 7 is a diagram illustrating the accommodation step ST11. In the accommodation step ST11, the core rod 21 is accommodated inside the glass tube 22 to produce the glass rod 2 shown in FIG. 7. In one example, in the glass rod 2, the end face 21a of the core rod 21 and the end face 22a of the glass tube 22 are flush with each other, and the end face 21b of the core rod 21 and the end face 22b of the glass tube 22 are flush with each other. In addition, since the outer diameter D2 of the core rod 21 is smaller than the inner diameter D3 of the glass tube 22, a gap is formed between the outer peripheral surface of the core rod 21 and the inner peripheral surface of the glass tube 22.

[0040] 8 is a diagram illustrating the first sealing step ST12 and the connecting step ST13. In the first sealing step ST12, the sealing member 3 is connected to the glass rod 2. Specifically, the end face 22a of the glass tube 22 and the end face 3a of the sealing member 3 are connected to each other. This seals the opening of the end face 22a of the glass tube 22 (more precisely, the gap between the end face 22a of the glass tube 22 and the end face 21a of the core rod 21). The outer diameter D12 of the sealing member 3 may be large enough to seal the end face 22a. In other words, the outer diameter D12 of the sealing member 3 is larger than the inner diameter D3 of the glass tube 22. The outer diameter D12 of the sealing member 3 may be smaller or larger than the outer diameter D11 of the glass tube 22.

[0041] In the connecting step ST13, the end face 22b of the glass tube 22 and the end face 4a of the glass tube 4 are connected to each other. During the connecting step ST13, an inert gas is flowed into the glass tube 22. Specifically, the gap between the core rod 21 and the glass tube 22 is filled with the inert gas, and the end face 22b and the end face 4a are connected in this state. An example of the inert gas is N2 (nitrogen). The flow rate of the inert gas is, for example, 1 slm or more and 50 slm or less. The outer diameter D13 of the glass tube 4 may be large enough to be connected to the end face 22b. That is, the outer diameter D13 of the glass tube 4 is larger than the inner diameter D3 of the glass tube 22. The outer diameter D13 of the glass tube 4 may be smaller or larger than the outer diameter D11 of the glass tube 22. A dehydration step may be performed after the connecting step ST13 to remove moisture from the gap between the core rod 23 and the glass tube 22.

[0042] 9 is a diagram illustrating the second sealing step ST14. In the second sealing step ST14, the glass tube 4 is sealed. In this step, a glass tube sealing member 41 is inserted into the glass tube 4 from the end face 4b, thereby sealing the glass tube 4. The glass tube 4 may also be sealed by, for example, deforming a portion of the glass tube 4 to close the hole in the glass tube 4 between the end face 4a and the end face 4b.

[0043] FIG. 10 is a diagram illustrating the forming step ST15. In the forming step ST15, a porous soot body 5 is formed to cover the outer periphery of the glass rod 2. The soot body 5 is a base material for the outer cladding 14. The soot body 5 extends in a direction along the axis L. The central axis of the soot body 5 coincides with the axis L. The soot body 5 is continuous in a direction along the axis L and covers a part of the glass tube 4, the entire glass rod 2, and a part of the sealing member 3. The outer diameter of the part of the soot body 5 that covers a part of the glass tube 4 decreases with increasing distance from the glass rod 2 in the direction of the axis L. The outer diameter of the soot body 5 may be constant between the end face 22b and the end face 22a in the direction of the axis L. The outer diameter of the part of the soot body 5 that covers a part of the sealing member 3 decreases with increasing distance from the glass rod 2 in the direction of the axis L. In the forming step ST15, the average bulk density of the soot body 5 is, for example, 0.2 g / cm. 3 More than 0.8g / cm 3 In one embodiment, the soot body 5 has an average bulk density of 0.55 g / cm 3 is formed.

[0044] Then, in the opening step ST16, the glass tube 4 is opened. In this step, the glass tube sealing member 41 is removed from the inside of the glass tube 4, thereby opening the glass tube 4. The glass tube 4 may be opened by providing a through-hole extending in the direction of the axis L in the glass tube sealing member 41.

[0045] FIG. 11 is a diagram illustrating the heating step ST17. In the heating step ST17, the glass rod 2 and the soot body 5 are heated simultaneously to vitrify the soot body 5 and fuse the core rod 21 and the glass tube 22 together. The glass rod 2 and the soot body 5 are simultaneously placed in the heating furnace H, whereby the glass rod 2 and the soot body 5 are heated simultaneously. The heating step ST17 is performed, for example, in a reduced pressure atmosphere. The reduced pressure atmosphere means a pressure atmosphere lower than 1 atmosphere. In the heating step ST17, the glass rod 2 and the soot body 5 are heated, for example, at a temperature of 1300°C or higher and 1600°C or lower. In one embodiment, the glass rod 2 and the soot body 5 are heated at a temperature of 1400°C or higher. The soot body 5 is vitrified by performing the above-described heating step ST17.

[0046] As described above, in the manufacturing method of the optical fiber preform 1 according to the present embodiment, a fluorine (F)-doped glass tube 22 to be the trench 13 is prepared in advance, and a core rod 21 including a core 11 and an inner cladding 12 is accommodated inside the glass tube 22. After the core rod 21 is accommodated inside the glass tube 22, a soot body 5 is formed to cover the outer periphery of the glass tube 22. Therefore, a core rod is not required to form the soot body 5, and there is no need to remove the core rod after the soot body 5 is formed. This makes it easy to increase the length of the optical fiber preform 1 and reduces the risk of damaging the optical fiber preform 1. As a result, the yield of manufacturing the optical fiber preform 1 can be improved. Furthermore, in the heating step ST17, air expanded in the gap between the core rod 21 and the glass tube 22 is discharged from the glass tube 22. Therefore, the core rod 21 and the glass tube 22 can be integrated and the formed soot body 5 can be vitrified while suppressing the expansion of the optical fiber preform 1 in the heating step ST17.

[0047] As described above, the core rod 21 may have a notch 21c extending from the outer periphery of the core rod 21 toward the central axis of the core rod 21 on the end face 21b facing the glass tube 4. In this case, even if the end face 21b of the core rod 21 and the end face 4a of the glass tube 4 are in contact with each other, the notch 21c spatially connects the gap between the core rod 21 and the glass tube 22 with the hollow part of the glass tube 4. Therefore, in the heating step ST17, the opening of the end face 4a of the glass tube 4 is not blocked by the end face 21b of the core rod 21, and the expanded air in the gap between the core rod 21 and the glass tube 22 is efficiently discharged from the glass tube 4.

[0048] As described above, in the splicing step ST13, an inert gas may be flowed into the glass tube 22. In this case, the inert gas is sent into the gap between the core rod 21 and the glass tube 22, thereby removing moisture contained in the gap. This makes it possible to prevent moisture from remaining inside the completed optical fiber preform 1.

[0049] As described above, the method for manufacturing the optical fiber preform 1 may include a second sealing step ST14 for sealing the glass tube 4 between the connecting step ST13 and the forming step ST15, and an opening step ST16 for opening the glass tube 4 between the forming step ST15 and the heating step ST17. In this case, it is possible to prevent moisture-containing air from being sent to the gap between the core rod 21 and the glass tube 22 in the forming step ST15.

[0050] As described above, the heating step ST17 may be performed in a reduced pressure atmosphere. In this case, the air pressure in the gap between the core rod 21 and the glass tube 22 is higher than the air pressure around the glass tube 22. Therefore, the air in the gap between the core rod 21 and the glass tube 22 is efficiently discharged from the glass tube 4. This makes it easier for the core rod 21 and the glass tube 22 to be tightly attached and integrated.

[0051] As described above, the difference between the outer diameter D2 of the core rod 21 and the inner diameter D3 of the glass tube 22 may be 0.1 mm or more and 5 mm or less. When the difference between the outer diameter D2 of the core rod 21 and the inner diameter D3 of the glass tube 22 is 0.1 mm or more, the core rod 21 can be easily accommodated inside the glass tube 22. When the difference between the outer diameter D2 of the core rod 21 and the inner diameter D3 of the glass tube 22 is 5 mm or less, the amount of air in the gap between the core rod 21 and the glass tube 22 can be reduced.

[0052] As described above, in the heating step ST17, the glass rod 2 and the soot body 5 may be heated at a temperature of 1300°C or higher and 1600°C or lower. By heating the glass rod 2 and the soot body 5 at a temperature of 1300°C or higher, the heating time for making the soot body 5 transparent can be shortened. By heating the glass rod 2 and the soot body 5 at a temperature of 1600°C or lower, the change in the outer diameter of the soot body 5 before and after the heating step ST17 can be reduced.

[0053] As described above, in the forming step ST15, the average bulk density of the soot body 5 is 0.2 g / cm 3 More than 0.8g / cm 3 The soot body 5 may be formed as follows: 3 By satisfying the above, it is possible to reduce the frequency of cracking of the soot body 5 in the forming step ST15. 3 By satisfying the above condition, it is possible to prevent the amount of shrinkage of the soot body 5 from decreasing in the heating step ST17, and to reduce the frequency of the soot body 5 not being integrated with the glass rod 2.

[0054] As described above, the arithmetic mean surface roughness of the inner peripheral surface of the glass tube 22 may be less than 0.50 μm. In this case, it is possible to reduce the frequency of failure in integrating the glass tube 22 with the core rod 21. It is also possible to suppress an increase in the transmission loss of the optical fiber for light with a wavelength of 1550 nm, which is caused by insufficient integration of the glass tube 22 with the core rod 21.

[0055] As described above, the arithmetic mean of the surface roughness of the outer circumferential surface of the core rod 21 may be less than 0.50 μm. In this case, it is possible to reduce the frequency of failure in integrating the glass tube 22 with the core rod 21. It is also possible to suppress an increase in the transmission loss of the optical fiber for light with a wavelength of 1550 nm, which is caused by insufficient integration of the glass tube 22 with the core rod 21.

[0056] Table 2 shows the relationship between the arithmetic mean of the surface roughness of the outer surface of the core rod 21 and the arithmetic mean of the surface roughness of the inner surface of the glass tube 22, and the yield at which the transmission loss of the optical fiber preform 1 at a wavelength of 1.55 μm is 0.19 dB / km or less. In the table, A indicates a yield of 100%, B indicates a yield of 90% or more and 99% or less, and C indicates a yield of less than 90%. [Table 2]

[0057] As is clear from Table 2, when the arithmetic mean of the surface roughness of the inner peripheral surface of the glass tube 22 is less than 0.50 μm and the arithmetic mean of the surface roughness of the outer peripheral surface of the core rod 21 is less than 0.50 μm, the yield of the optical fiber preform 1 having a transmission loss of 0.19 dB / km or less for a wavelength of 1.55 μm is 100%. Therefore, an increase in the transmission loss of the optical fiber preform 1 for light with a wavelength of 1550 nm can be suppressed. (Second embodiment)

[0058] Next, each component used in the method for manufacturing the optical fiber preform 1 according to the second embodiment will be described with reference to FIG. 12. FIG. 12 is a schematic diagram of each component used in the method for manufacturing the optical fiber preform 1 according to the second embodiment. The method for manufacturing the optical fiber preform 1 according to the second embodiment uses a core rod 23, a glass tube 22, a glass tube 6 (second glass tube), and a glass tube 7 (third glass tube). The configuration of the core rod 23 is the same as that of the core rod 21 according to the first embodiment, except for the following point. The core rod 23 has notches 23c extending from the outer periphery of the core rod 23 toward the central axis of the core rod 23 on both the end face 23b facing the glass tube 6 and the end face 23a facing the glass tube 7. The number, positions, and shapes of the notches 23c provided on both the end face 23b and the end face 23a are the same as those in the first embodiment.

[0059] The glass tube 6 and the glass tube 7 are components that prevent the core rod 23 from falling out of the glass tube 22 and have the same configuration as the glass tube 4 of the first embodiment. The glass tube 6 has an end face 6a and an end face 6b. The end face 6a is located at a first end of the glass tube 6 in the direction along the axis L. The end face 6b is located at a second end of the glass tube 6 opposite the first end in the direction along the axis L. The glass tube 7 has an end face 7a and an end face 7b. The end face 7a is located at the first end of the glass tube 7 in the direction along the axis L. The end face 7b is located at a second end of the glass tube 7 opposite the first end in the direction along the axis L.

[0060] Fig. 13 is a flowchart showing a method for manufacturing the optical fiber preform 1 according to the second embodiment. As shown in Fig. 13, this manufacturing method includes an accommodation step ST21, a first connection step ST22, a second connection step ST23, a dehydration step ST24, a sealing step ST25, a forming step ST26, an opening step ST27, and a heating step ST28.

[0061] Before performing each of the above steps ST21 to ST28, the glass tube 22 is heated (baked) while a dehydrating gas (e.g., chlorine gas (Cl2)) is flowed into the glass tube 22. The flow rate of Cl2 is, for example, 300 sccm or more and 2000 sccm or less. This dehydrates the inside of the glass tube 22. Next, etching gas (e.g., sulfur hexafluoride gas (SF6) and chlorine gas (Cl2)) is flowed into the glass tube 22. The flow rate of SF6 is, for example, 100 sccm or more and 500 sccm or less, and the flow rate of Cl2 is, for example, 100 sccm or more and 500 sccm or less. This reduces the surface roughness of the inner surface of the glass tube 22. Note that if the flow rates are below the lower limit, the dehydration or etching effect will not be sufficient. On the other hand, if the flow rates are above the upper limit, the inner surface of the glass tube 22 will be cooled, and similarly, the dehydration or etching effect will not be sufficient.

[0062] 14 is a diagram illustrating the accommodation step ST21. In the accommodation step ST21, the core rod 23 is accommodated inside the glass tube 22 to produce the glass rod 2A shown in FIG. 14. In one example, in the glass rod 2A, the end face 23a of the core rod 23 and the end face 22a of the glass tube 22 are flush with each other, and the end face 23b of the core rod 23 and the end face 22b of the glass tube 22 are flush with each other. In addition, since the outer diameter of the core rod 23 is smaller than the inner diameter D3 of the glass tube 22, a gap is formed between the outer peripheral surface of the core rod 23 and the inner peripheral surface of the glass tube 22.

[0063] 15 is a diagram for explaining the first connecting step ST22 and the second connecting step ST23. In the first connecting step ST22, the end face 22b of the glass tube 22 is connected to the end face 6a of the glass tube 6. In the second connecting step ST23, the end face 22a of the glass tube 22 is connected to the end face 7a of the glass tube 7.

[0064] FIG. 16 is a diagram illustrating the dehydration step ST24. In the dehydration step ST24, gas G, which has a dehydrating effect, is flowed from the glass tube 6 to the glass tube 7. By flowing gas G from the end face 6b to the end face 7b, gas G flows from the glass tube 6 to the glass tube 7. Examples of gas G include compounds of at least one element selected from the group consisting of Si (silicon), Ge (germanium), B (boron), P (phosphorus), Ga (gallium), As (arsenic), Sb (antimony), Al (aluminum), and Ti (titanium) with a halogen (F (fluorine), Cl (chlorine), Br (bromine), or I (iodine)). Gas G may consist solely of halogen. In one embodiment, gas G is chlorine gas (Cl2). By flowing gas G, moisture in the gap between the core rod 23 and the glass tube 22 is removed. The flow rate of Cl2 is, for example, 1000 sccm. This dehydration step ST24 may be performed while simultaneously heating the core rod 23 and the glass tube 22 (baking).

[0065] 17 is a diagram illustrating the sealing step ST25. In the sealing step ST25, the glass tube 6 and the glass tube 7 are sealed. In this step, a glass tube sealing member 61 is inserted into the glass tube 6 from the end face 6b, thereby sealing the glass tube 6. The glass tube 6 may be sealed by, for example, deforming a portion of the glass tube 6 to close the hole in the glass tube 6 between the end face 6a and the end face 6b. Alternatively, the glass tube 7 may be sealed by inserting a glass tube sealing member 71 into the glass tube 7 from the end face 7b. The glass tube 7 may be sealed by, for example, deforming a portion of the glass tube 7 to close the hole in the glass tube 6 between the end face 7a and the end face 7b.

[0066] FIG. 18 is a diagram illustrating the forming step ST26. In the forming step ST26, a porous soot body 5 is formed to cover the outer periphery of the glass rod 2A. The soot body 5 is continuous in the direction along the axis L and covers part of the glass tube 6, the entire glass rod 2A, and part of the glass tube 7. The outer diameter of the part of the soot body 5 that covers part of the glass tube 6 decreases with increasing distance from the glass rod 2A in the direction of the axis L. The outer diameter of the soot body 5 is constant between the end face 23b and the end face 23a in the direction of the axis L. The outer diameter of the part of the soot body 5 that covers part of the glass tube 7 decreases with increasing distance from the glass rod 2A in the direction of the axis L. In the forming step ST26, the average bulk density of the soot body 5 is, for example, 0.2 g / cm 3 More than 0.8g / cm 3 In one embodiment, the soot body 5 has an average bulk density of 0.55 g / cm 3 is formed.

[0067] Then, in the opening step ST27, the glass tube 6 and the glass tube 7 are opened. In this step, the glass tube 6 is opened by removing the glass tube sealing member 61 from the inside of the glass tube 6. The glass tube 6 may be opened by providing a through-hole extending in the direction of the axis L in the glass tube sealing member 61. Also, in this step, the glass tube 7 is opened by removing the glass tube sealing member 71 from the inside of the glass tube 7. The glass tube 7 may be opened by providing a through-hole extending in the direction of the axis L in the glass tube sealing member 71.

[0068] FIG. 19 is a diagram illustrating the heating step ST28. In the heating step ST28, the glass rod 2A and the soot body 5 are heated simultaneously to vitrify the soot body 5 and fuse the core rod 23 and the glass tube 22 together. The glass rod 2A and the soot body 5 are simultaneously placed in the heating furnace H, whereby the glass rod 2A and the soot body 5 are heated simultaneously. The heating step ST28 is performed, for example, in a reduced pressure atmosphere. In the heating step ST28, the glass rod 2A and the soot body 5 are heated, for example, at a temperature of 1300°C or higher and 1600°C or lower. In one embodiment, the glass rod 2A and the soot body 5 are heated at a temperature of 1400°C or higher. The soot body 5 is vitrified by performing the above-described heating step ST28.

[0069] In the manufacturing method of the optical fiber preform 1 according to the second embodiment, a fluorine (F)-doped glass tube 22 to be the trench 13 is prepared in advance, and a core rod 23 including a core 11 and an inner cladding 12 is accommodated inside the glass tube 22. After the core rod 23 is accommodated inside the glass tube 22, a soot body 5 is formed to cover the outer periphery of the glass tube 22. Therefore, a core rod is not required to form the soot body 5, and it is also not required to remove the core rod after the soot body 5 is formed. This makes it easy to increase the length of the optical fiber preform 1 and reduces the risk of damaging the optical fiber preform 1. As a result, the yield of manufacturing the optical fiber preform 1 can be improved. Furthermore, in the heating step ST28, air expanded in the gap between the core rod 23 and the glass tube 22 is discharged from the glass tubes 6 and 7. Therefore, the core rod 23 and the glass tube 22 can be integrated and the formed soot body 5 can be vitrified while suppressing the expansion of the optical fiber preform 1 in the heating step ST28. Furthermore, in the dehydration step ST24, a gas having a dehydrating effect is sent into the gap between the core rod 23 and the glass tube 22, thereby removing the moisture contained in the gap, thereby preventing moisture from remaining inside the completed optical fiber preform 1.

[0070] As described above, the core rod 23 may have notches 23c extending from the outer periphery of the core rod 23 toward the central axis of the core rod 23 on the end face 23b facing the glass tube 6 and the end face 23a facing the glass tube 7. In this case, even if the end face 23a of the core rod 23 contacts the end face 7a of the glass tube 7 and the end face 23b of the core rod 23 contacts the end face 6a of the glass tube 6, the notches 23c spatially connect the gap between the core rod 23 and the glass tube 22 with the hollow portions of the glass tube 6 and the glass tube 7. Therefore, in the heating step ST28, the openings of the end faces 6a and 7a of the glass tubes 6 and 7 are not blocked by the end face 21b and the end face 21a of the core rod 21, respectively, and the expanded air in the gap between the core rod 23 and the glass tube 22 is efficiently discharged from the glass tube 6 and the glass tube 7.

[0071] As described above, the method for manufacturing the optical fiber preform 1 may include a sealing step ST25 for sealing the glass tubes 6 and 7 between the dehydration step ST24 and the forming step ST26, and an opening step ST27 for opening the glass tubes 6 and 7 between the forming step ST26 and the heating step ST28. In this case, it is possible to prevent moisture-containing air from being sent to the gap between the core rod 23 and the glass tube 22 in the forming step ST26.

[0072] The method for manufacturing an optical fiber preform according to the present invention is not limited to the first and second embodiments. For example, the core rod 21 of the first embodiment has notches 21c at two positions on either side of the axis L, but the positions and number of the notches 21c are not limited to this. Furthermore, in the first embodiment, an inert gas is flowed into the glass tube 22 during the connecting step ST13, but the connecting step ST13 may be performed without flowing an inert gas. Furthermore, although the first embodiment includes the second sealing step ST14 and the opening step ST16, these steps may be omitted. Similarly, although the second embodiment includes the sealing step ST25 and the opening step ST27, these steps may be omitted. [Explanation of symbols]

[0073] 1...Optical fiber preform 11...Core 12...Inner cladding 13...Trench 14...Outer cladding 2,2A...Glass rod 3...Sealing member 3a...end face 4...Glass tube (second glass tube) 4a...End face 4b...end face 5...Susu body 6...Glass tube (second glass tube) 6a…End face 6b...end face 7...Glass tube (third glass tube) 7a…End face 7b…End face 21...Core Rod 21a...end face 21b...end face 22...Glass tube (first glass tube) 22a...end face 22b...end face 23...Core Rod 23a...end face 23b...end face 41...Glass tube sealing member 61...Glass tube sealing member 71...Glass tube sealing member G...gas H...Heating furnace L...axis (first axis) d...Separation distance r1…radius r2…radius r3…radius r4…radius E1...range E2...range E3...Range E4...Range

Claims

1. an accommodation step of preparing a glass rod by accommodating a core rod including a core and an inner cladding covering an outer periphery of the core and extending along a first axis inside a first glass tube that extends along the first axis, is doped with fluorine, and has an inner diameter larger than an outer diameter of the core rod; a first sealing step of sealing a first end face of the first glass tube; a connecting step of connecting a second end face of the first glass tube located on the opposite side to the first end face to an end face of a second glass tube extending along the first axis and having an inner diameter smaller than the outer diameter of the core rod; a forming step of forming a soot body covering the outer periphery of the glass rod; a heating step of simultaneously heating the glass rod and the soot body to vitrify the soot body; A method for manufacturing an optical fiber preform, comprising:

2. 2. The method for manufacturing an optical fiber preform according to claim 1, wherein the core rod has a notch extending from an outer periphery of the core rod toward a central axis of the core rod on an end surface facing the second glass tube.

3. 3. The method for manufacturing an optical fiber preform according to claim 1, wherein an inert gas is flowed into the first glass tube during the connecting step.

4. a second sealing step of sealing the second glass tube between the connecting step and the forming step; an opening step of opening the second glass tube between the forming step and the heating step; 3. The method for manufacturing an optical fiber preform according to claim 1, further comprising:

5. an accommodation step of preparing a glass rod by accommodating a core rod including a core and an inner cladding covering an outer periphery of the core and extending along a first axis inside a first glass tube that extends along the first axis, is doped with fluorine, and has an inner diameter larger than an outer diameter of the core rod; a first connecting step of connecting a first end surface of the glass rod to an end surface of a second glass tube extending along the first axis and having an inner diameter smaller than an outer diameter of the core rod; a second connecting step of connecting a second end face of the glass rod located opposite to the first end face to an end face of a third glass tube extending along the first axis and having an inner diameter smaller than the outer diameter of the core rod; a dehydration step of, after the first connecting step and the second connecting step, flowing a gas having a dehydrating effect from one of the second glass tube and the third glass tube to the other, thereby removing moisture from a gap between the core rod and the first glass tube; a forming step of forming a soot body covering the outer periphery of the glass rod; a heating step of simultaneously heating the glass rod and the soot body to vitrify the soot body; A method for manufacturing an optical fiber preform, comprising:

6. 6. The method for manufacturing an optical fiber preform according to claim 5, wherein the core rod has a notch extending from an outer periphery of the core rod toward a central axis of the core rod on an end face facing the second glass tube and an end face facing the third glass tube.

7. a sealing step of sealing the second glass tube and the third glass tube between the dehydration step and the forming step; an opening step of opening the second glass tube and the third glass tube between the forming step and the heating step; The method for manufacturing an optical fiber preform according to claim 5 or 6, further comprising:

8. 7. The method for manufacturing an optical fiber preform according to claim 1, wherein the heating step is performed in a reduced pressure atmosphere.

9. 7. The method for manufacturing an optical fiber preform according to claim 1, wherein a difference between an outer diameter of the core rod and an inner diameter of the first glass tube is 0.1 mm or more and 5 mm or less.

10. 7. The method for manufacturing an optical fiber preform according to claim 1, wherein in the heating step, the glass rod and the soot body are heated at a temperature of 1300° C. or more and 1600° C. or less.

11. In the forming step, the average bulk density of the soot body is 0.2 g / cm ３ Above, 0.8g / cm ３ 7. A method for manufacturing an optical fiber preform according to claim 1, 2, 5 or 6, which is formed as follows:

12. 7. The method for manufacturing an optical fiber preform according to claim 1, wherein the arithmetic mean of the surface roughness of the inner peripheral surface of the first glass tube is less than 0.50 [mu]m.

13. 7. The method for manufacturing an optical fiber preform according to claim 1, wherein the arithmetic mean of the surface roughness of the outer peripheral surface of the core rod is less than 0.50 [mu]m.

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