Method for producing optical fiber preform, and optical fiber preform

Abstract

This method for producing an optical fiber preform is provided with: a production step for producing a glass microparticle deposition body having a glass rod including a glass body and a dummy rod connected to an end of the glass body, and a deposition part in which glass microparticles are deposited on the outer peripheral surface of the glass rod; and a sintering step for heating and sintering the glass particle deposit by means of a heater. In the manufacturing step, the glass particle accumulation body is manufactured such that the accumulation portion has a first accumulation portion on the outer peripheral surface of the glass body and a second accumulation portion on the outer peripheral surface of the portion of the dummy rod closer to the glass body, and the thickness of the second accumulation portion decreases in the second direction as the second accumulation portion moves away from the first accumulation portion. In the sintering step, heating by a heater is performed using a heat-insulating jig so as to sinter the first deposition part into transparent glass, and the second deposition part is kept in an unsintered state so as to become opaque glass.

Description

Technical Field

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

[0002] This application claims the benefit of priority based on Japanese Patent Application No. 2022-100529 filed on June 22, 2022, and incorporates by reference all the contents described in the aforementioned Japanese Application. Background Art

[0003] In the manufacturing method of optical fiber mother material described in Patent Document 1, a glass particle deposit composed of a glass rod and a tubular portion formed by depositing glass particles on the glass rod is heated and sintered to manufacture the optical fiber mother material. The glass particle deposit has an effective portion and an ineffective portion. The effective portion is the portion that is drawn into an optical fiber at the lower end of the optical fiber mother material. The ineffective portion is the upper end portion of the optical fiber mother material that is not drawn into an optical fiber. In the manufacturing method described in Patent Document 1, cracks caused by the difference in shrinkage between the unsintered portion and the sintered portion in the ineffective portion of the optical fiber mother material are suppressed by reducing the area that remains in an unsintered state in the ineffective portion of the optical fiber mother material.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2012-082127 Summary of the invention

[0007] The manufacturing method of an optical fiber base material according to one embodiment of the present disclosure comprises: a manufacturing step of manufacturing a glass particle deposit, the glass particle deposit having a glass rod and a deposited portion formed by depositing glass particles on the outer peripheral surface of the glass rod, the glass rod including a glass body extending in a first direction and a dummy rod connected to the end of the glass body; and a sintering step of heating and sintering the glass particle deposit by at least one heater. In the manufacturing step, the glass particle deposit is manufactured so that the deposited portion has a first deposited portion on the outer peripheral surface of the glass body and a second deposited portion on the outer peripheral surface of a portion of the dummy rod close to the glass body, and the thickness of the second deposited portion decreases along a second direction intersecting the first direction as it moves away from the first deposited portion. In the sintering step, a heat insulating jig is arranged near the boundary between the exposed portion of the dummy rod exposed to the outside of the glass particle deposit and the second deposited portion, and the heat insulating jig is used to perform heating by a heater, so that the first deposited portion is sintered to make it transparent glass, and the second deposited portion is kept in an unsintered state to become opaque glass.

[0008] An optical fiber mother material according to an embodiment of the present disclosure comprises: a glass rod extending in a first direction; and a tubular portion formed on the outer peripheral surface of the glass rod and composed of silica glass. The glass rod comprises a glass body extending in the first direction, and a dummy rod connected to the end of the glass body. The tubular portion comprises: a first portion formed on the outer peripheral surface of the glass body; and a second portion formed on the outer peripheral surface of the dummy rod, the thickness of which decreases along a second direction intersecting the first direction as it moves away from the first portion. The first portion comprises transparent glass. The second portion comprises opaque glass. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 This is a diagram showing a state of an optical fiber preform before sintering according to one embodiment.

[0010] Figure 2 This is a diagram showing an optical fiber preform according to one embodiment.

[0011] Figure 3 This is a diagram showing the structure of a sintering furnace used in a method for producing an optical fiber preform according to one embodiment.

[0012] Figure 4 It is shown Figure 3 A diagram of the thermal insulation fixture used in the sintering furnace is shown.

[0013] Figure 5 This is a flowchart showing a method for manufacturing an optical fiber preform according to one embodiment.

[0014] Figure 6 It is a diagram for explaining a method for manufacturing an optical fiber preform according to a comparative example.

[0015] Figure 7 It is a diagram for explaining a method for manufacturing an optical fiber preform according to one embodiment. DETAILED DESCRIPTION

[0016] [Technical Problems to be Solved by the Present Disclosure]

[0017] Optical fibers are manufactured from optical fiber base materials manufactured by various methods. In the manufacture of such optical fibers, in order to reduce the manufacturing cost of the optical fibers, the optical fiber base materials are increasingly enlarged. As a result, the weight of the optical fiber base materials increases. Under such circumstances, the inventors have discovered that in the conventional manufacturing method of the optical fiber base material, cracks are generated in the ineffective portion.

[0018] For example, an optical fiber base material has a glass rod and a tubular portion. The glass rod is made of pure silica glass, including an ineffective portion installed on a device when manufacturing the optical fiber base material. The tubular portion is made of silica glass to which fluorine is added, and is formed on the outer peripheral surface of the glass rod. In such an optical fiber base material, since the viscosity of the silica glass to which fluorine is added is lower than that of the pure silica glass, the glass rod will be deformed in the ineffective portion when the optical fiber base material is sintered and cooled. In addition, since the thermal expansion coefficients of the silica glass to which fluorine is added are different from those of the pure silica glass, deformation will be generated at the interface between the glass rod and the tubular portion in the ineffective portion when the optical fiber base material is sintered and cooled. In this way, since deformation occurs in the ineffective portion, cracks may occur in the ineffective portion that supports the weight of the optical fiber base material.

[0019] [Effects of the present disclosure]

[0020] The present disclosure can provide a method for manufacturing an optical fiber preform and an optical fiber preform that can suppress the occurrence of cracks in an ineffective portion when manufacturing the optical fiber preform.

[0021] [Description of Embodiments of the Present Disclosure]

[0022] First, the contents of the embodiments of the present disclosure are listed and described.

[0023] (1) A method for manufacturing an optical fiber preform according to one embodiment includes: a manufacturing step of manufacturing a glass particle deposit, the glass particle deposit having a glass rod and a deposited portion formed by depositing glass particles on the outer peripheral surface of the glass rod, the glass rod including a glass body extending in a first direction and a dummy rod connected to the end of the glass body; and a sintering step of heating and sintering the glass particle deposit by at least one heater. In the manufacturing step, the glass particle deposit is manufactured so that the deposited portion has a first deposited portion on the outer peripheral surface of the glass body and a second deposited portion on the outer peripheral surface of a portion of the dummy rod close to the glass body, and the thickness of the second deposited portion decreases along a second direction intersecting the first direction as it moves away from the first deposited portion. In the sintering step, a heat insulating jig is arranged near the boundary between the exposed portion of the dummy rod exposed to the outside of the glass particle deposit and the second deposited portion, and heating is performed by the heater so that the first deposited portion is sintered to become transparent glass, and the second deposited portion is kept in an unsintered state to become opaque glass.

[0024] In the manufacturing method of the optical fiber mother material, a heat-insulating fixture is arranged near the boundary between the exposed part of the dummy rod exposed to the outside of the glass particle accumulation body and the second accumulation part. The heat-insulating fixture is used to perform heating based on a heater to sinter the first accumulation part to make it transparent glass, and to keep the second accumulation part in an unsintered state to become opaque glass. According to such a method, since the second accumulation part is not sintered and remains in an unsintered state, deformation of the interface between the dummy rod and the second accumulation part after the sintering process due to the difference in viscosity or thermal expansion coefficient in the optical fiber mother material can be suppressed, and deformation in the ineffective part when manufacturing the optical fiber mother material can be suppressed. As a result, when manufacturing the optical fiber mother material, the optical fiber mother material can be prevented from falling due to cracks in the ineffective part.

[0025] (2) In the method for manufacturing an optical fiber preform according to (1) above, in the sintering step, the glass particle deposit is heated and sintered in such a manner that the temperature of a heater close to the second deposit is 1400° C. or higher and lower than 1450° C. According to such a method, for example, compared with a case where the glass particle deposit is sintered at a temperature of 1450° C. or higher, only the first deposit can be sintered more reliably without sintering the second deposit.

[0026] (3) In the method for manufacturing an optical fiber preform according to (2) above, in the sintering step, a difference between an inner diameter of at least one heater and an outer diameter of the glass particle deposit may be 500 mm or less.

[0027] (4) In the method for manufacturing an optical fiber preform according to any one of (1) to (3), at least one heater may include a first heater facing the first accumulation portion and a second heater facing the second accumulation portion. In the sintering step, the heating temperature of the second heater may be set at least 20°C lower than the heating temperature of the first heater. According to such a method, only the first accumulation portion can be sintered more reliably without sintering the second accumulation portion.

[0028] (5) In the method for manufacturing an optical fiber preform according to (4) above, the heating temperature of the first heater may be 1450° C. or higher and 1500° C. or lower. The heating temperature of the second heater may be 1400° C. or higher and lower than 1450° C.

[0029] (6) In the method for manufacturing an optical fiber preform according to any one of (1) to (5) above, the dummy rod may be made of pure silica glass, or the deposited portion may be made of silica glass to which fluorine is added. According to such a method, an optical fiber with extremely low transmission loss can be obtained.

[0030] (7) In the method for manufacturing an optical fiber preform according to any one of (1) to (6) above, the heat insulating jig may be arranged so that the distance from the boundary is 0 mm or more and 100 mm or less when viewed from the second direction.

[0031] (8) An optical fiber mother material according to an embodiment of the present disclosure comprises: a glass rod extending in a first direction; and a tubular portion formed on the outer peripheral surface of the glass rod and composed of silica glass. The glass rod comprises a glass body extending in the first direction, and a dummy rod connected to the end of the glass body. The tubular portion comprises: a first portion formed on the outer peripheral surface of the glass body; and a second portion formed on the outer peripheral surface of the dummy rod, the thickness of which decreases along a second direction intersecting the first direction as the distance from the first portion increases. The first portion comprises transparent glass. The second portion comprises opaque glass.

[0032] In the optical fiber mother material, the first part includes transparent glass, and the second part is composed of opaque glass. That is, the second part is different from the first part and remains unsintered to become an ineffective part. According to such a structure, deformation at the interface between the dummy rod and the second part due to the difference in viscosity or thermal expansion coefficient can be suppressed. As a result, in the optical fiber mother material, cracks in the ineffective part, which is the part that does not become an optical fiber during drawing, can be suppressed.

[0033] (9) In the optical fiber mother material involved in the above (8), its weight may be greater than 20 kg. According to such a structure, since the amount of optical fiber that can be produced by one drawing becomes larger, the yield rate when drawing optical fiber from the optical fiber mother material can be improved. In addition, when the weight of the optical fiber mother material is increased, since the deformation in the dummy rod becomes larger, cracks are likely to occur in the ineffective part when the optical fiber mother material is sintered. However, in the optical fiber mother material involved in one embodiment of the present disclosure, since the second part serving as the ineffective part remains in an unsintered state, the occurrence of cracks in the ineffective part can be suppressed.

[0034] (10) In the optical fiber base material involved in (8) or (9) above, the OH mass concentration of the dummy rod may be less than 10 ppm. According to such a structure, for example, the dummy rod can be made using electrically fused silica glass, so the manufacturing cost of the optical fiber base material can be suppressed. In addition, when the OH mass concentration of the dummy rod is reduced, the viscosity of the dummy rod becomes larger, and deformation is likely to remain in the dummy rod, so that cracks are likely to occur in the ineffective portion when the optical fiber base material is sintered. However, in the optical fiber base material involved in one embodiment of the present disclosure, since the second part serving as the ineffective portion remains unsintered, the occurrence of deformation in the ineffective portion can be suppressed. As a result, the occurrence of cracks in the ineffective portion can be suppressed.

[0035] (11) In the optical fiber mother material involved in any one of the above (8) to (10), the dummy rod may be composed of pure silica glass, or the tubular portion may be composed of silica glass to which fluorine is added. According to such a structure, an optical fiber with very low transmission loss can be obtained. Here, since the thermal expansion coefficient of silica glass to which fluorine is added is smaller than the thermal expansion coefficient of pure silica glass, deformation is likely to remain in the dummy rod. Therefore, when the optical fiber mother material is sintered, cracks are likely to occur in the ineffective portion. However, in the optical fiber mother material involved in one embodiment of the present disclosure, since the second part serving as the ineffective portion remains unsintered, the occurrence of deformation in the ineffective portion can be suppressed. As a result, the occurrence of cracks in the ineffective portion can be suppressed.

[0036] [Details of the embodiments of the present disclosure]

[0037] Hereinafter, a method for manufacturing an optical fiber mother material and a specific example of an optical fiber mother material according to an embodiment of the present disclosure will be described with reference to the accompanying drawings. In the following description, the same reference numerals are used for the same elements or elements having the same functions, and repeated descriptions are omitted. It should be noted that the present invention is not limited to these examples, but is represented by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. Hereinafter, the longitudinal direction of the glass particle deposit 10a and the optical fiber mother material 20 is recorded as a first direction, and the direction orthogonal (intersecting) to the longitudinal direction, i.e., the radial direction, is recorded as a second direction.

[0038] Reference Figure 1 and Figure 2 , an example of an optical fiber preform manufactured by a manufacturing method involved in one embodiment of the present disclosure is described. Figure 1 Part (a) is a cross-sectional view showing a cross section of the glass particle deposit 10 a along a first direction. Figure 1Part (b) is a cross-sectional view showing a cross section of the glass particle deposit 10a along the second direction. The glass particle deposit 10a has, for example, a circular cross section. The glass particle deposit 10a has a glass rod 11 and a deposited portion 12. The diameter of the glass particle deposit 10a is, for example, 250 mm or more. The glass rod 11 extends in the length direction (first direction) and includes a glass body 13 and a pair of dummy rods 14 and 15. The glass body 13 is composed of a central portion that becomes the core of the optical fiber and a peripheral portion that becomes a part of the cladding. The central portion is, for example, composed of pure silica glass, or silica glass to which a trace amount of fluorine or chlorine is added. The peripheral portion is, for example, composed of silica glass to which fluorine is added. One dummy rod 14 is connected to one end 13a of the glass body 13. Another dummy rod 15 is connected to the other end 13b of the glass body 13. The pair of dummy rods 14 and 15 are, for example, composed of pure silica glass. The glass rod 11 may be a rod in which the glass body 13 and the pair of dummy rods 14 and 15 are integrated.

[0039] The accumulation part 12 is provided outside the glass rod 11 and is configured to surround the glass rod 11. The accumulation part 12 is composed of silica glass to which fluorine is added. The accumulation part 12 is configured to include a first accumulation part 12a, a second accumulation part 12b, and a third accumulation part 12c. The first accumulation part 12a is provided on the outer peripheral surface 13c of the glass body 13. The first accumulation part 12a extends in the length direction, and for example, the thickness T1 along the second direction is 110 mm. The second accumulation part 12b is provided on the outer peripheral surface 14b of a portion of the outer peripheral surface 14a of the dummy rod 14 close to the glass body 13. The third accumulation part 12c is provided on the outer peripheral surface 15b of a portion of the outer peripheral surface 15a of the dummy rod 15 close to the glass body 13. The second accumulation part 12b and the third accumulation part 12c are continuous with the first accumulation part 12a. The thickness T2 of the second accumulation part 12b and the third accumulation part 12c along the second direction decreases as they are away from the first accumulation part 12a. In addition, the dummy rod 14 has an exposed surface 14 c (exposed portion) which is exposed to the outside of the glass particle deposit 10 a in the dummy rod 14 .

[0040] The optical fiber mother material 20 is Figure 1 The glass fine particle deposit 10a shown is manufactured by sintering. Figure 2 Part (a) is a diagram showing an example of the optical fiber preform 20 . Figure 2Part (b) is a cross-sectional view showing a cross section of the optical fiber base material 20. The weight of the optical fiber base material 20 is, for example, more than 20 kg. The optical fiber base material 20 has a glass rod 21 and a tubular portion 22. The glass rod 21 is formed by sintering the glass rod 11. The glass rod 21 extends in the length direction. The glass rod 21 includes a glass body 23 and a pair of dummy rods 24 and 25. The glass body 23 is composed of a central portion that becomes the core of the optical fiber when the optical fiber base material 20 is drawn, and a peripheral portion that becomes a part of the cladding. The central portion is composed of, for example, pure silica glass, or silica glass to which a trace amount of fluorine or chlorine is added. The peripheral portion is composed of, for example, silica glass to which fluorine is added. One dummy rod 24 is connected to one end 23a of the glass body 23. Another dummy rod 25 is connected to the other end 23b of the glass body 23. The dummy rods 24 and 25 are composed of, for example, pure silica glass. The OH mass concentration of the dummy rods 24 and 25 is, for example, 10 ppm or less.

[0041] The tubular portion 22 is formed on the outer peripheral surface of the glass rod 21 and surrounds the glass rod 21 in a concentric circle shape. The tubular portion 22 is composed of, for example, silica glass to which fluorine is added. The tubular portion 22 includes a first portion 22a, a second portion 22b, and a third portion 22c. The first portion 22a is formed by sintering the first accumulation portion 12a. The first portion 22a is a portion that becomes the cladding of the optical fiber when the optical fiber base material 20 is drawn. The first portion 22a surrounds the glass body 23. The first portion 22a includes, for example, transparent glass. The thickness T3 of the first portion 22a along the second direction is, for example, 40 mm. The second portion 22b corresponds to the second accumulation portion 12b before the glass particle accumulation body 10a is sintered. The second portion 22b and the third portion 22c surround the dummy rods 24 and 25, respectively. The thickness T4 of the second portion 22b and the third portion 22c along the second direction decreases as they are away from the first portion 22a. The second portion 22b is, for example, not sintered as a whole and is composed of opaque glass. The third portion 22c corresponds to the third deposited portion 12c before the glass particle deposit 10a is sintered. The third portion 22c may be sintered or unsintered. It should be noted that in this specification, "unsintered" means "a state that has not yet reached transparent glass although heated by a heater".

[0042] According to the above, the glass body 23 and the first portion 22a are the portions that become optical fibers when the optical fiber base material 20 is drawn, that is, the effective portions. In addition, the dummy rods 24, 25 and the second portion 22b and the third portion 22c are the ineffective portions that do not become optical fibers when the optical fiber base material 20 is drawn. It should be noted that the dummy rods 24 and the second portion 22b are the portions that support the optical fiber base material 20 relative to the manufacturing device when the optical fiber base material 20 is manufactured, and are the portions to which the weight of the optical fiber base material 20 is directly applied.

[0043] Next, refer to Figure 3 A manufacturing apparatus for the optical fiber preform 20 according to one embodiment of the present disclosure will be described. Figure 3 1 is a cross-sectional view showing an example of a sintering furnace 1 for manufacturing an optical fiber base material 20. The sintering furnace 1 includes a furnace core tube 2, a connecting member 3, a supply unit 4, a discharge unit 5, a heat insulating fixture 6, and a heater 7. The furnace core tube 2 is, for example, a hollow cylindrical shape, and contains a glass particle accumulation body 10a. A dummy rod 14 of the glass particle accumulation body 10a is fixed to the connecting member 3. The supply unit 4 is provided at the lower part of the furnace core tube 2, and supplies an inert gas such as He gas. The discharge unit 5 is provided at the upper part of the furnace core tube 2, and discharges the gas in the furnace core tube 2.

[0044] The heat-insulating fixture 6 is configured to be movable in the longitudinal direction of the glass particle deposit 10a. Alternatively, the heat-insulating fixture 6 cannot move. The heat-insulating fixture 6 is arranged near the boundary 16 between the exposed surface 14c of the dummy rod 14 and the second depositing portion 12b. The heat-insulating fixture 6 is arranged near the boundary 16 so as to overlap with the exposed surface 14c when viewed from the second direction. Alternatively, "arranged near the boundary 16" means that when viewed from the second direction, the distance L from the boundary 16 is, for example, greater than 0 mm and less than 100 mm. Alternatively, the distance L is more preferably greater than 5 mm and less than 50 mm, for example, 10 mm. Alternatively, the heat-insulating fixture 6 is arranged at a position lower than the boundary 16 when viewed from the second direction, and the distance L in this case is also the same as above. Figure 4 : is a diagram showing an example of the heat insulating jig 6. The heat insulating jig 6 is, for example, in the shape of a circular plate. The heat insulating jig 6 is, for example, formed of carbon. The heat insulating jig 6 is provided with a through hole 6a through which the dummy rod 14 passes. The through hole 6a is, for example, circular. In addition, the outer diameter d6 of the heat insulating jig 6 is, for example, greater than 100 mm and less than 400 mm. The inner diameter d7 is, for example, a radius of greater than 10 mm and less than 50 mm.

[0045] The heater 7 is arranged to surround the furnace core tube 2 on the outside of the furnace core tube 2. The heater 7 is, for example, in a ring shape. The inner diameter of the heater 7 is larger than the outer diameter of the furnace core tube 2. In addition, the minimum value of the distance between the heater 7 and the glass particle accumulation body 10a may be less than 250 mm, for example, less than 200 mm. In this case, the difference between the inner diameter of the heater 7 and the outer diameter of the glass particle accumulation body 10a may be less than 500 mm, for example, less than 400 mm. The heater 7 includes, for example, a first heater 71, a second heater 72, and a third heater 73. The second heater 72, the first heater 71, and the third heater 73 are arranged in sequence from the discharge portion 5 toward the supply portion 4 in the longitudinal direction of the furnace core tube 2. The heater 7 may be composed of four or more heaters, or may be composed of two heaters. When the glass particle accumulation body 10a is accommodated in the furnace core tube 2, the second heater 72 is opposite to the second accumulation portion 12b arranged near the connecting member 3, and the first heater 71 is opposite to the first accumulation portion 12a. At this time, the third heater 73 is opposite to at least one of the first stacking section 12a and the third stacking section 12c. The second heater 72 is close to the second stacking section 12b. The heating temperature of each heater 71, 72, 73 is, for example, 1400°C or more. Alternatively, the heating temperatures of each heater 71, 72, 73 are different from each other. For example, the heating temperature of the second heater 72 may be 1400°C or more and less than 1450°C. Alternatively, the heating temperatures of the first heater 71 and the third heater 73 may be 1450°C or more. Alternatively, the heating temperature of the second heater 72 may be set to be at least 20°C lower than the heating temperature of the first heater 71. In addition, the heating temperatures of the first heater 71 and the third heater 73 may be 1500°C or more. Such heaters 71, 72, 73 are, for example, carbon heaters.

[0046] Next, refer to Figure 5 A method for manufacturing the optical fiber preform 20 according to one embodiment of the present disclosure will be described. Figure 5 2 is a flowchart showing a method for manufacturing an optical fiber preform. To manufacture the optical fiber preform 20, first, the glass particle deposit 10a is produced using a burner or the like (step S01).

[0047] In the process of making the glass particle deposit 10a, the glass particles are deposited around the target by VAD (Vapor-phase Axial Deposition) method while rotating and lifting the target upward to make the glass particle deposit 10a. More specifically, glass particles are generated from glass raw material gas in the flame of the burner, and the glass particles are deposited to make the glass particle deposit 10a. The flame of the burner is obtained by burning a combustible gas (such as hydrogen) supplied to the burner.

[0048] In addition, in the process of manufacturing the glass particle deposit 10a, the glass particles are deposited in a manner to form a deposit 12 having a first deposit 12a, a second deposit 12b, and a third deposit 12c. Specifically, first, a glass rod 11 in which a glass body 13 and dummy rods 14 and 15 are integrated is prepared. Next, the glass particles are deposited on the glass rod 11. Thus, a glass particle deposit 10a having a first deposit 12a on the glass body 13, a second deposit 12b on the dummy rods 14 and 15, and a third deposit 12c is manufactured.

[0049] Next, if the process of making the glass particle deposit 10a is completed, a vitrification process is performed, which includes: arranging the glass particle deposit 10a in the furnace core tube 2, and heating and dehydrating the glass particle deposit 10a using the heater 7 (step S02); adding fluorine to the glass particle deposit 10a (step S03); and sintering the glass particle deposit 10a by the heater 7 (step S04). In the dehydration process, the glass particle deposit 10a is heated at, for example, 1200°C while supplying a dehydration gas such as chlorine or a chlorine compound into the furnace core tube 2. In this way, the OH group in the glass particle deposit 10a is removed. It should be noted that, in the dehydration process, the heating by the heater 7 can also be performed by rotating the glass particle deposit 10a, and the same is true in the fluorine addition process and the sintering process described below.

[0050] In the subsequent fluorine addition step, a fluorine source gas such as silicon tetrafluoride (SiF4), sulfur hexafluoride (SF6) or carbon tetrachloride (CCl4) is supplied into the furnace core tube 2 while heating the glass particle deposit 10a at, for example, 1300°C to perform a fluorine addition treatment. In this way, fluorine is added to the glass particle deposit 10a.

[0051] In the subsequent sintering step, first, the heat insulating jig 6 is moved in the longitudinal direction of the glass particle deposit 10a, and is arranged near the boundary 16 between the exposed surface 14c of the dummy rod 14 and the second deposited portion 12b so as to overlap with the exposed surface 14c when viewed from the second direction (see Figure 3). It should be noted that such a configuration of the heat-insulating clamp 6 may be performed before the above-mentioned dehydration step. Afterwards, the temperature of the heater 7 is further increased than that in the fluorine addition step, for example, the glass particle deposit 10a is heated in such a manner that the heating temperature of the first heater 71 and the third heater 73 is greater than 1450°C and less than 1500°C, and the heating temperature of the second heater 72 is greater than 1400°C and less than 1450°C. In this way, the glass particle deposit 10a is sintered to become transparent glass. It should be noted that in the sintering step, the heating temperature of the second heater 72 is set to be lower than the heating temperature of the first heater 71 and the third heater 73, for example, by 20°C or more (refer to Figure 3 ).

[0052] By using such a heat-insulating fixture 6 to lower the temperature of the second heater 72 than usual, the sintering process is performed to sinter the first accumulation portion 12a, which is the main part of the optical fiber preform, to make it transparent glass, and to keep the second accumulation portion 12b, which is the supporting portion when the optical fiber preform is manufactured, in an unsintered state to make it opaque glass. Figure 2 The optical fiber preform 20 shown is shown. By drawing such an optical fiber preform 20 using a known drawing device, an optical fiber can be obtained.

[0053] Hereinafter, the effects of the method for producing an optical fiber preform according to the present embodiment will be described in comparison with a comparative example. Figure 6 It is a diagram showing a method for manufacturing an optical fiber preform according to a comparative example. Figure 7 1 is a diagram showing a method for manufacturing an optical fiber mother material according to the present embodiment. In this comparative example, the heat-insulating fixture 106 is disposed at a position away from the boundary 16 between the dummy rod 14 and the second stacking portion 12b when viewed from the second direction. In addition, the heat-insulating fixture 106 is a glass disk made of quartz. In this comparative example, the reflection of heat caused by the heat-insulating fixture 106 is weak, so if the temperature of the second heater 172 is not sufficiently increased, the first stacking portion 12a cannot be sintered. On the other hand, if the temperature of the second heater 172 is increased, the second stacking portion 12b close to the dummy rod 14 is also sintered and becomes glass. As a result, the soot portion of the dummy rod 14 is fixed to the glass portion of the second stacking portion 12b. As a result, deformation occurs between the dummy rod 14 and the second stacking portion 12b as an ineffective portion, cracks occur in the dummy rod 14 supporting the optical fiber mother material, and the optical fiber mother material sometimes falls during manufacturing.

[0054] Compared with the above-mentioned comparative example, in the manufacturing method of the optical fiber preform 20 involved in this embodiment, the heat-insulating jig 6 is arranged near the boundary 16 between the exposed surface 14c exposed to the outside of the glass particle accumulation body 10a and the second accumulation part 12b in the dummy rod 14, and the heat-insulating jig 6 is used to perform heating by the heater 7 so that the first accumulation part 12a is sintered and the second accumulation part 12b is kept in an unsintered state. More specifically, as Figure 7 As shown, in the sintering process, the heating temperature of the second heater 72 arranged near the second accumulation part 12b is lower than usual so that the second accumulation part 12b is not sintered, and the heat from the second accumulation part 12b is efficiently transferred to the first accumulation part 12a by using the reflection of the heat insulating fixture 6 arranged close to it. Thus, even when the temperature of the second heater 72 is, for example, 50°C lower than the first heater 71 or the usual setting temperature, the first accumulation part 12a in the accumulation part 12 can be sintered. According to such a method, the sintering of the first accumulation part 12a used for drawing the optical fiber can be reliably performed, and since the second accumulation part 12b close to the member supporting the optical fiber mother material is not sintered, the deformation at the interface between the dummy rod 14 and the second accumulation part 12b due to the difference in viscosity or thermal expansion coefficient can be suppressed. As a result, since the occurrence of deformation in the dummy rod 14 can be suppressed, it is possible to prevent the occurrence of cracks in the dummy rod 14 supporting the optical fiber mother material and the optical fiber mother material from falling during manufacturing. Furthermore, since the second deposited portion 12 b is not sintered, the temperature drop rate of the first deposited portion 12 a after sintering is moderated by the heat insulating effect of the second deposited portion 12 b . This can suppress the occurrence of deformation in the dummy rod 14 .

[0055] Furthermore, in the method for manufacturing the optical fiber preform 20 according to the present embodiment, in the sintering step, the glass particle deposit 10a is heated and sintered in such a manner that the temperature of the second heater 72 in the heater 7 close to the second deposit 12b is 1400° C. or higher and lower than 1450° C. According to such a configuration, for example, compared with the case of sintering at a temperature of 1450° C. or higher, only the first deposit 12a can be sintered more reliably without sintering the second deposit 12b.

[0056] In addition, in the method for manufacturing the optical fiber preform 20 according to the present embodiment, in the sintering step, the temperature of the second heater 72 facing the second accumulation portion 12b may be set to be at least 20°C lower than the temperature of the first heater 71 facing the first accumulation portion 12a. With such a configuration, it is possible to more reliably sinter only the first accumulation portion 12a without sintering the second accumulation portion 12b near the dummy rod 14.

[0057] In the method for manufacturing the optical fiber preform 20 according to the present embodiment, the dummy rods 14 and 15 are made of pure silica glass. In the manufacturing process, fluorine is added to the deposition portion 12. With such a structure, an optical fiber with extremely low transmission loss can be obtained.

[0058] In the optical fiber base material 20 involved in this embodiment, the first part 22a includes transparent glass. The second part 22b is composed of opaque glass. That is, the second part 22b is different from the first part 22a and remains unsintered to become an ineffective part. According to such a structure, deformation at the interface between the dummy rod 24 and the second part 22b due to the difference in viscosity or thermal expansion coefficient can be suppressed. As a result, in the optical fiber base material 20, cracks in the ineffective part, which is the part that does not become an optical fiber during drawing, can be suppressed.

[0059] In addition, the weight of the optical fiber mother material 20 involved in this embodiment may be 20 kg or more. According to such a structure, since the amount of optical fiber that can be produced by one drawing becomes larger, the yield rate when drawing optical fiber from the optical fiber mother material can be improved. In the case where the weight of the optical fiber mother material is increased, since the deformation in the dummy rod becomes larger, cracks are likely to occur in the ineffective portion when the optical fiber mother material is sintered. However, in the optical fiber mother material 20 involved in this embodiment, since the second part as the ineffective portion remains in an unsintered state, the occurrence of deformation in the ineffective portion can be suppressed. As a result, the occurrence of cracks in the ineffective portion can be suppressed.

[0060] In addition, in the optical fiber base material 20 involved in the present embodiment, the OH mass concentration of the dummy rod 24 may be less than 10 ppm. According to such a structure, for example, the dummy rod 24 can be made of electrically fused silica glass, so the manufacturing cost of the optical fiber base material 20 can be suppressed. In addition, when the OH mass concentration of the dummy rod is reduced, the viscosity of the dummy rod becomes larger, and deformation is likely to remain in the dummy rod, so that cracks are likely to occur in the ineffective portion when the optical fiber base material is sintered. However, in the optical fiber base material 20 involved in the present embodiment, since the second portion 22b as the ineffective portion remains in an unsintered state, the occurrence of deformation in the ineffective portion can be suppressed. As a result, the occurrence of cracks in the ineffective portion can be suppressed.

[0061] In addition, in the optical fiber base material 20 involved in the present embodiment, the dummy rods 24 and 25 are made of pure silica glass, and fluorine is added to the tubular portion 22. According to such a structure, an optical fiber with very low transmission loss can be obtained. Here, since the thermal expansion coefficient of the silica glass to which fluorine is added is smaller than the thermal expansion coefficient of the pure silica glass, deformation may remain in the dummy rods 14 and 15. Therefore, when the optical fiber base material 20 is sintered, cracks are likely to occur in the ineffective portion. However, in the optical fiber base material 20 involved in one embodiment of the present disclosure, since the second part 22b as the ineffective portion remains in an unsintered state, the occurrence of deformation in the ineffective portion can be suppressed. As a result, the occurrence of cracks in the ineffective portion can be suppressed.

[0062] As mentioned above, the manufacturing method of the optical fiber preform 20 and the optical fiber preform 20 according to the present disclosure have been described in detail, but the present invention is not limited to the above-mentioned embodiment, and can be applied to various embodiments or modified examples.

[0063] In the above embodiment, the heater 7 includes three heaters, namely, the first heater 71, the second heater 72, and the third heater 73, but the present invention is not limited thereto. The heater 7 may be any heater that can heat and sinter the glass particle deposit 10a, sinter the first deposit 12a to make it transparent glass, and keep the second deposit 12b unsintered to make it opaque glass. For example, the heater 7 may include only the first heater 71 and the second heater 72 that are longer in the longitudinal direction than those in the above embodiment. In addition, for example, the heater 7 may be a heater that extends in the longitudinal direction of the furnace core tube 2 and is composed of a heater that can change the heating temperature according to the location.

[0064] Description of Reference Numerals

[0065] 1…Sintering furnace

[0066] 2…furnace core tube

[0067] 3…Connecting parts

[0068] 4…Supply Department

[0069] 5…Discharge section

[0070] 6…Insulation fixture

[0071] 6a…Through hole

[0072] 7…Heater

[0073] 10a…Glass particle accumulation

[0074] 11…Glass rod

[0075] 12…Accumulation Department

[0076] 12a: first accumulation section

[0077] 12b…Second accumulation section

[0078] 12c…third accumulation section

[0079] 13…Vitreous

[0080] 13a, 13b…end

[0081] 13c…Outer surface

[0082] 14, 15…Dummy stick

[0083] 14a, 14b, 15a, 15b...outer surface

[0084] 14c…exposed surface (exposed part)

[0085] 16…Junction

[0086] 20…Optical fiber mother material

[0087] 21…Glass rod

[0088] 22… Tubular part

[0089] 22a…Part 1

[0090] 22b…Part 2

[0091] 22c…Part 3

[0092] 23…Vitreous

[0093] 23a…end

[0094] 23b…end

[0095] 24…Dummy stick

[0096] 25…Dummy stick

[0097] 71…First heater

[0098] 72…Second heater

[0099] 73…Third heater

[0100] 106…Insulation fixture

[0101] 172…Second heater

[0102] d6…Outer diameter

[0103] d7…Inner diameter

[0104] T1…Thickness

[0105] T2…Thickness

[0106] T3…Thickness

[0107] T4…Thickness

[0108] L…distance.

Claims

1. A method for manufacturing an optical fiber mother material, comprising: A step of manufacturing a glass particle deposit, the glass particle deposit having a glass rod and a deposited portion formed by depositing glass particles on an outer peripheral surface of the glass rod, the glass rod including a glass body extending in a first direction and a dummy rod connected to an end of the glass body; and A sintering step of heating and sintering the glass fine particle accumulation body by at least one heater, In the manufacturing step, the glass particle deposit is manufactured so that the deposit has a first deposit on the outer peripheral surface of the glass body and a second deposit on the outer peripheral surface of a portion of the dummy rod close to the glass body, and the thickness of the second deposit decreases along a second direction intersecting the first direction as it moves away from the first deposit. In the sintering process, a heat-insulating clamp is arranged near the boundary between the exposed portion of the dummy rod exposed to the outside of the glass particle accumulation body and the second accumulation portion, and the heat-insulating clamp is used to perform heating based on the heater to sinter the first accumulation portion to make it transparent glass, and to keep the second accumulation portion in an unsintered state to become opaque glass.

2. The method for manufacturing an optical fiber mother material according to claim 1, In the sintering step, the glass fine particle deposit is heated and sintered so that the temperature of the heater close to the second depositing portion among the heaters is 1400° C. or higher and lower than 1450° C.

3. The method for manufacturing an optical fiber mother material according to claim 2, In the sintering step, a difference between an inner diameter of the at least one heater and an outer diameter of the glass fine particle deposit is 500 mm or less.

4. The method for manufacturing an optical fiber preform according to any one of claims 1 to 3, The at least one heater includes a first heater facing the first deposition portion and a second heater facing the second deposition portion. In the sintering step, the heating temperature of the second heater is set to be at least 20° C. lower than the heating temperature of the first heater.

5. The method for manufacturing an optical fiber mother material according to claim 4, The heating temperature of the first heater is 1450° C. or higher and 1500° C. or lower. The heating temperature of the second heater is 1400°C or higher and lower than 1450°C.

6. The method for manufacturing an optical fiber preform according to any one of claims 1 to 3, The virtual rod is made of pure silica glass. The deposition portion is made of silica glass to which fluorine is added.

7. The method for manufacturing an optical fiber preform according to any one of claims 1 to 3, The heat insulating jig is arranged so that the distance from the boundary is greater than or equal to 0 mm and less than or equal to 100 mm when viewed from the second direction.

8. An optical fiber mother material, comprising: a glass rod extending in a first direction; and The tubular portion is formed on the outer peripheral surface of the glass rod and is made of silica glass. The glass rod has a glass body extending in the first direction and a dummy rod connected to an end of the glass body. The tubular portion comprises: a first portion formed on the outer peripheral surface of the glass body; and a second portion formed on the outer peripheral surface of the dummy rod, the thickness of which decreases along a second direction intersecting the first direction as the distance from the first portion increases. The first portion comprises transparent glass, The second portion is made of opaque glass.

9. The optical fiber mother material according to claim 8, The weight of the optical fiber mother material is greater than 20 kg.

10. The optical fiber mother material according to claim 8 or 9, The OH mass concentration of the dummy rod is less than 10 ppm.

11. The optical fiber mother material according to claim 8 or 9, The virtual rod is made of pure silica glass. The tubular portion is made of silica glass to which fluorine is added.

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