Optical fiber preform and optical fiber preform manufacturing method

Abstract

To provide an optical fiber preform manufacturing method capable of restraining variation of a characteristic of an optical fiber preform in a longitudinal direction.SOLUTION: An optical fiber preform manufacturing method according to the present invention includes: a first step of forming a core by spraying first raw material gas to a starting base material from a burner for a core, and forming a first clad by spraying second raw material gas to the core from a burner for a clad; a second step of forming a second clad by spraying third raw material gas to an end of the core from the burner for the clad, after the first step; a step of forming a core rod by heating the first clad and the second clad; and a step of forming a third clad on an outer periphery of the core rod. A flow rate of silicon tetrafluoride in the third raw material gas is larger than that of silicon tetrafluoride in the second raw material gas.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] The VAD (Vapor-phase Axial Deposition) method is widely known as a method for manufacturing an optical fiber preform (Patent Document 1). In the VAD method, glass particles are blown onto a starting substrate from a burner while the starting substrate is being pulled up, thereby forming soot, which is a deposit of glass particles. The VAD method has the advantage that it is easy to make large optical fiber preforms.

[0003] Generally, in the manufacturing method of an optical fiber preform, in order to suppress cracks in the end regions of the soot, it is common practice to advance the sintering of the end regions of the soot more than the sintering of the central region of the soot when forming the soot. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2017-226569 Summary of the Invention [Problem to be solved by the invention]

[0005] However, by sintering the end regions of the soot more rapidly than the central region of the soot, the composition of the cladding in the end regions of the soot may change, which may cause the longitudinal properties of the optical fiber preform to vary and deteriorate.

[0006] An object of the present invention is to provide a method for manufacturing an optical fiber preform that can suppress fluctuations in the characteristics of the optical fiber preform in the longitudinal direction. [Means for solving the problem]

[0007] According to one aspect of the present invention, there is provided a method for manufacturing an optical fiber preform, comprising: a first step of forming a core by spraying a first raw material gas from a core burner onto a starting substrate, and forming a first clad by spraying a second raw material gas from a clad burner onto the core; a second step of forming a second clad after the first step by spraying a third raw material gas from the clad burner onto the end of the core; a step of heating the first clad and the second clad to form a core rod; and a step of forming a third clad around the outer periphery of the core rod, wherein the flow rate of silicon tetrafluoride in the third raw material gas is greater than the flow rate of silicon tetrafluoride in the second raw material gas.

[0008] According to another aspect of the present invention, there is provided an optical fiber preform comprising a core and a cladding covering the core, wherein the variation in fluorine content in the cladding in the longitudinal direction is 0.05 wt% or less. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a method for manufacturing an optical fiber preform that can suppress fluctuations in the characteristics of the optical fiber preform in the longitudinal direction. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view of an optical fiber preform according to an embodiment of the present invention; [Figure 2] 1 is a graph showing the refractive index of an optical fiber preform according to an embodiment. [Figure 3] 1 is a schematic diagram of a VAD device according to one embodiment. [Figure 4] 1 is a flowchart of a method for manufacturing an optical fiber preform according to an embodiment. [Figure 5] 1 is a graph showing the relative refractive index difference in the longitudinal direction of an optical fiber preform. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Elements having common functions throughout the drawings will be designated by the same reference numerals, and duplicate descriptions may be omitted or simplified.

[0012] FIG. 1 is a cross-sectional view of an optical fiber preform according to this embodiment. The optical fiber preform 1 has a core 2 and a cladding 3. The optical fiber preform 1 may be made of silica-based glass. The optical fiber preform 1 is made of a porous body (soot) that is a deposit of glass particles. The soot is produced by a VAD device, which will be described later.

[0013] The core 2 is the center of the optical fiber preform 1 and extends in the longitudinal direction of the optical fiber preform 1. The core 2 is made of silica-based glass doped with a dopant such as germanium, and has a higher refractive index than pure silica-based glass. The refractive index of the core 2 is set higher than the refractive index of the cladding 3, and light transmitted through the core 2 is totally reflected at the interface between the core 2 and the cladding 3. This prevents the light transmitted through the core 2 from leaking into the cladding 3, and allows the light to be confined in the core 2.

[0014] The cladding 3 covers the outer periphery of the core 2 and extends in the longitudinal direction of the optical fiber preform 1. The cladding 3 is made of silica-based glass doped with a dopant such as fluorine, and has a lower refractive index than pure silica-based glass. The cladding 3 has an inner cladding that covers the outer periphery of the core 2, and an outer cladding that covers the inner cladding. The refractive index of the inner cladding is set lower than the refractive index of the outer cladding, allowing for stronger confinement of light transmitted through the core 2.

[0015] 2 is a graph showing the refractive index of the optical fiber preform 1 according to this embodiment. In FIG. 2, the vertical axis represents the relative refractive index difference with respect to the outer cladding, and the horizontal axis represents the radial distance of the optical fiber preform 1 from the center of the core 2.

[0016] In FIG. 2, region R1 represents the relative refractive index difference of the core 2, region R2 represents the relative refractive index difference of the inner cladding, and region R3 represents the relative refractive index difference of the outer cladding. The core 2 has a relative refractive index difference Δ1 greater than 0. The relative refractive index difference Δ1 is preferably, for example, 0.3% or more and 0.5% or less. The relative refractive index difference Δ1 varies depending on the germanium content of the core 2. The inner cladding has a relative refractive index difference Δ2 less than 0. The relative refractive index difference Δ2 is preferably, for example, -0.15% or more and -0.03% or less. The relative refractive index difference Δ2 varies depending on the fluorine content of the cladding 3.

[0017] 3 is a schematic diagram of a VAD device 100 according to this embodiment. The VAD device 100 includes a housing 101, an exhaust unit 102, a rotating unit 103, a driving unit 104, a first burner 105, a second burner 106, and a control unit 107. The VAD device 100 sequentially produces an intermediate 11 and a soot 12 from a starting substrate 10, which is the raw material of the optical fiber preform 1.

[0018] The housing 101 has a box-like shape with side walls, a ceiling, and a bottom wall. The housing 101 may be made of a sturdy metal, but the material of the housing 101 is not limited to a specific material. The housing 101 contains a starting substrate 10. The starting substrate 10 is made of, for example, silica-based glass.

[0019] An exhaust unit 102 is provided on the side wall of the housing 101. The exhaust unit 102 has a pump and a valve (not shown), and exhausts gases generated from the first burner 105 and the second burner 106 to the outside of the housing 101.

[0020] One end of the starting substrate 10 is connected to a rotating unit 103. The rotating unit 103 is equipped with a chuck, a motor, etc., and rotates the starting substrate 10 while gripping it with the chuck. The rotating unit 103 is connected to a driving unit 104. The driving unit 104 may be equipped with a motor, etc. The driving unit 104 can move the starting substrate 10 upward (in the longitudinal direction) by raising the rotating unit 103. The driving unit 104 may also be equipped with a displacement meter that measures the amount of movement of the starting substrate 10 in the longitudinal direction.

[0021] The first burner 105 is provided in the housing 101 and is arranged to face the starting substrate 10. The first burner 105 is provided in front of the second burner 106 with respect to the direction of movement of the starting substrate 10. The first burner 105 may be a multi-tube burner. The number of first burners 105 is not limited to the example in FIG. 3 and may be selected as any number.

[0022] The first burner 105 has a nozzle for supplying a flame-forming gas and a nozzle for supplying a raw material gas (first raw material gas) for the core 2. The flame-forming gas may contain a combustible gas such as hydrogen, a combustion-supporting gas such as oxygen, etc. The raw material gas for the core 2 may contain silicon tetrachloride, germanium tetrachloride, phosphoryl chloride, and boron bromide. Silicon tetrachloride is a raw material for glass. Germanium tetrachloride is a dopant for increasing the refractive index of glass. The relative refractive index difference Δ1 in the optical fiber preform 1 can be changed by adjusting the flow rate of germanium tetrachloride in the raw material gas for the core 2.

[0023] The first burner 105 emits an oxyhydrogen flame by burning hydrogen and oxygen. When the raw material gas for the core 2 is supplied to the oxyhydrogen flame, the raw material gas for the core 2 undergoes flame hydrolysis, producing glass particles that form the core 2. The glass particles that form the core 2 are blown onto the starting substrate 10, which rises and rotates inside the housing 101. The glass particles that form the core 2 are deposited on the starting substrate 10, forming an intermediate 11.

[0024] The second burner 106 is provided in the housing 101 and is provided so as to face the starting substrate 10. The second burner 106 may be a multi-tube burner. The number of the second burners 106 is not limited to the example of FIG. 3 and may be selected to any number. The second burner 106 may also take the form of a burner array in which a plurality of second burners 106 are integrated together.

[0025] The second burner 106 has a nozzle for supplying a flame-forming gas and a nozzle for supplying a raw material gas for the cladding 3. The flame-forming gas may contain a combustible gas such as hydrogen, a combustion-supporting gas such as oxygen, etc. The raw material gas for the cladding 3 may contain silicon tetrachloride and silicon tetrafluoride. Silicon tetrafluoride is a dopant for lowering the refractive index of glass. The relative refractive index difference Δ2 in the optical fiber preform 1 can be changed by adjusting the flow rate of silicon tetrafluoride in the raw material gas for the cladding 3.

[0026] The source gas for clad 3 is supplied to the oxyhydrogen flame of second burner 106, whereby the source gas for clad 3 undergoes flame hydrolysis, generating glass particles that form clad 3. The glass particles that form clad 3 are blown onto intermediate body 11, which rises and rotates inside housing 101. The glass particles that form clad 3 are deposited around intermediate body 11, forming soot 12.

[0027] The control unit 107 controls the exhaust unit 102, the rotation unit 103, the drive unit 104, the first burner 105, and the second burner 106. The control unit 107 can control, for example, the exhaust air volume of the exhaust unit 102, the rotation speed of the starting substrate 10 of the rotation unit 103, the longitudinal movement speed of the starting substrate 10 of the drive unit 104, the flow rates of silicon tetrachloride and germanium tetrachloride of the first burner 105, and the flow rates of silicon tetrachloride, germanium tetrachloride, and silicon tetrafluoride of the second burner 106.

[0028] The control unit 107 can change the manufacturing conditions for each longitudinal position of the starting substrate 10. For example, because the edge region of the soot 12 is discarded during the manufacturing process, germanium tetrachloride is not supplied to the first burner 105 for the intermediate 11 corresponding to the edge region of the soot 12. This reduces the amount of germanium used in manufacturing the soot 12. Furthermore, if the source gas for the clad 3 supplied to the second burner 106 in the central region of the soot 12 is the second source gas and the source gas for the clad 3 supplied to the second burner 106 in the edge region of the soot 12 is the third source gas, the flow rate of silicon tetrafluoride in the third source gas is controlled to be greater than the flow rate of silicon tetrafluoride in the second source gas. The edge region of the soot 12 can vary depending on the entire longitudinal length of the soot 12. For example, the edge region of the soot 12 can be a region 5 cm to 20 cm from the bottom end of the soot 12. The central region of the soot 12 may be the entire soot 12 excluding the edge regions of the soot 12 .

[0029] During the production of the soot 12, the soot 12 is heated by a burner (not shown), and the gaps between the glass particles of the soot 12 are reduced by sintering. As a result, the volume of the soot 12 is reduced, resulting in a high-density core rod.

[0030] Sintering of the edge regions of the soot 12 is more advanced than sintering of the central region of the soot 12. For example, by setting the heating temperature of the edge regions of the soot 12 higher than the heating temperature of the central region of the soot 12, or by setting the heating time of the edge regions of the soot 12 longer than the heating time of the central region of the soot 12, the sintering of the edge regions of the soot 12 can be more advanced than the sintering of the central region of the soot 12. Because the glass particles forming the core 2 have a higher density than the glass particles forming the clad 3, cracks can occur in the soot 12 during the production of the soot 12 due to the density difference between the core 2 and the clad 3. Furthermore, the glass particles forming the clad 3 are deposited in the edge regions of the soot 12 from the edge of the intermediate body 11, and more glass particles forming the clad 3 are deposited in the edge regions of the soot 12 than in the central region of the soot 12. For this reason, cracks in the soot 12 are more likely to occur in the edge regions of the soot 12. By allowing the edge regions of the soot 12 to be sintered more rapidly than the central region of the soot 12, the density of the clad 3 in the edge regions of the soot 12 can be increased, and cracks in the soot 12 can be suppressed.

[0031] The core rod is heated and drawn, and glass particles that form the cladding 3 are deposited on the core rod by the OVD (Outside Vapor Deposition) method. After the glass particles that form the cladding 3 have been deposited to a desired thickness, the core rod is heated to form the optical fiber preform 1. The optical fiber preform is drawn by a drawing device to form a bare optical fiber. A resin is coated around the bare optical fiber to obtain an optical fiber strand.

[0032] 4 is a flowchart of a method for manufacturing the optical fiber preform 1. First, a starting substrate 10 is prepared in the VAD device 100 (step S101).

[0033] Next, the first burner 105 blows glass particles that form the core 2 generated from the first raw material gas onto the starting substrate 10, and the second burner 106 blows glass particles that form the clad 3 (first clad) generated from the second raw material gas onto the starting substrate 10. This forms the central region of the soot 12 (step S102).

[0034] Next, the second burner 106 sprays glass particles that form the clad 3 (second clad) produced from the third source gas onto the end of the intermediate 11. This forms the end region of the soot 12 (step S103). Here, the flow rate of silicon tetrafluoride in the third source gas is greater than the flow rate of silicon tetrafluoride in the second source gas. In this way, the soot 12 is produced from the starting substrate 10.

[0035] Next, the soot 12 is heated to form a core rod (step S104). Next, glass particles that form the cladding 3 (third cladding) are deposited on the outer periphery of the core rod by the OVD method to a desired thickness, and then the core rod is heated (step S105). In this way, the optical fiber preform 1 is formed.

[0036] In the production of the soot 12, sintering of the end regions of the soot 12 progresses more than that of the central region of the soot 12, and the density of the end regions of the soot 12 becomes higher than that of the central region of the soot 12. Furthermore, as the density of the soot 12 increases, it becomes more difficult for silicon tetrafluoride in the soot 12 to dope into the cladding 3. As a result, silicon tetrafluoride in the end regions of the soot 12 diffuses into the central region of the soot 12. As a result, the fluorine content in the end regions of the soot 12 becomes lower than that in the central region of the soot 12, and this can cause a variation in the relative refractive index difference Δ2 in the longitudinal direction of the optical fiber preform 1.

[0037] In this embodiment, the flow rate of silicon tetrafluoride is appropriately set in the longitudinal direction of the soot 12 to suppress fluctuations in the characteristics of the optical fiber preform 1 in the longitudinal direction. Specifically, the flow rate of silicon tetrafluoride in the third raw material gas is set to be higher than the flow rate of silicon tetrafluoride in the second raw material gas. This makes the fluorine content in the end regions of the soot 12 higher than the fluorine content in the central region of the soot 12, preventing the fluorine content in the end regions of the soot 12 from becoming too low due to diffusion of silicon tetrafluoride into the central region of the soot 12.

[0038] FIG. 5 is a graph showing the relative refractive index difference Δ2 in the longitudinal direction of the optical fiber preform 1. In FIG. 5, the vertical axis represents the relative refractive index difference Δ2, and the horizontal axis represents the longitudinal position of the optical fiber preform 1 relative to the upper end of the optical fiber preform 1. The upper end of the optical fiber preform 1 is the upper end of the central region of the soot 12, and the lower end of the optical fiber preform 1 is the lower end of the end region of the soot 12. The optical fiber preform 1 in the example is manufactured by increasing the flow rate of silicon tetrafluoride in the end region of the soot 12 compared to the flow rate of silicon tetrafluoride in the central region of the soot 12. The optical fiber preform 1 in the comparative example is manufactured without changing the flow rate of silicon tetrafluoride between the end region of the soot 12 and the central region of the soot 12. As described above, due to the diffusion of silicon tetrafluoride in the end region of the soot 12, the relative refractive index difference Δ2 in the end region of the optical fiber preform 1 becomes larger than the relative refractive index difference Δ2 in the central region of the optical fiber preform 1. 5, the variation in the relative refractive index difference Δ2 in the longitudinal direction of the optical fiber preform 1 in the example is smaller than that of the optical fiber preform 1 in the comparative example. Therefore, by making the flow rate of silicon tetrafluoride in the end region of the soot 12 greater than the flow rate of silicon tetrafluoride in the central region of the soot 12, the variation in the relative refractive index difference Δ2 can be suppressed.

[0039] In this embodiment, the flow rate of silicon tetrafluoride in the third source gas is preferably 1.3 to 1.9 times the flow rate of silicon tetrafluoride in the second source gas. If the flow rate is less than 1.3 times, the influence of silicon tetrafluoride diffusion into the central region of the soot 12 cannot be sufficiently reduced. If the flow rate is more than 1.9 times, the fluorine content in the edge region of the soot 12 becomes too high, which may cause fluctuations in the relative refractive index difference Δ2.

[0040] The results of experiments on the optical fiber preform according to the embodiment of the present invention will be described below.

[0041] [Table 1]

[0042] Table 1 shows the soot density (g / cm 3 ) in Examples 1 to 16 and Comparative Examples 1 to 4. 3 ), the increase rate (%) of the silicon tetrafluoride flow rate of the raw material gas in the end region of the soot relative to the silicon tetrafluoride flow rate of the raw material gas in the central region of the soot, the fluctuation (%) of the relative refractive index difference Δ2 in the longitudinal direction of the optical fiber preform, and the evaluation of the fluctuation of the relative refractive index difference Δ2 in the longitudinal direction of the optical fiber preform. In the examples and comparative examples, the soot density was measured using X-ray CT inspection. Also, in the examples and comparative examples, the relative refractive index difference Δ2 was measured at intervals of 200 mm in the longitudinal direction of the optical fiber preform, and the fluctuation of the relative refractive index difference Δ2 was measured by subtracting the minimum value from the maximum value of the relative refractive index difference Δ2.

[0043] The evaluation in Table 1 indicates whether the variation in the relative refractive index difference Δ2 satisfies the standard (0.0021% or less). If the variation in the relative refractive index difference Δ2 satisfies the standard, the evaluation is judged as good, and if the variation in the relative refractive index difference Δ2 does not satisfy the standard, the evaluation is judged as poor.

[0044] In Examples 1 to 4, the soot density was 0.285 g / cm 3The increase rates of the silicon tetrafluoride flow rate were 30%, 60%, 80%, and 90%, respectively. The fluctuations in the relative refractive index difference Δ2 were 0.002%, 0.0015%, 0.001%, and 0.0008%, respectively. In Examples 1 to 4, the fluctuations in the relative refractive index difference Δ2 were 0.0021% or less, and all were evaluated as good (OK).

[0045] In Examples 1 to 4, the soot density was 0.285 g / cm 3 The increase rates of the silicon tetrafluoride flow rate were 30%, 60%, 80%, and 90%, respectively. The fluctuations in the relative refractive index difference Δ2 were 0.002%, 0.0015%, 0.001%, and 0.0008%, respectively. In Examples 1 to 4, the fluctuations in the relative refractive index difference Δ2 were 0.0021% or less, and all were evaluated as good (OK).

[0046] In Examples 5 to 8, the soot density was 0.25 g / cm 3 The increase rates of the silicon tetrafluoride flow rate were 30%, 60%, 80%, and 90%, respectively. The fluctuations in the relative refractive index difference Δ2 were 0.0019%, 0.0016%, 0.001%, and 0.0008%, respectively. In Examples 1 to 4, the fluctuations in the relative refractive index difference Δ2 were 0.0021% or less, and all were evaluated as good (OK).

[0047] In Examples 9 to 12, the soot density was 0.3 g / cm 3 The increase rates of the silicon tetrafluoride flow rate were 30%, 60%, 80%, and 90%, respectively. The fluctuations in the relative refractive index difference Δ2 were 0.002%, 0.0015%, 0.001%, and 0.0008%, respectively. In Examples 9 to 12, the fluctuations in the relative refractive index difference Δ2 were 0.0021% or less, and all were evaluated as good (OK).

[0048] In Examples 13 to 16, the soot density was 0.35 g / cm 3The increase rates of the silicon tetrafluoride flow rate were 30%, 60%, 80%, and 90%, respectively. The fluctuations in the relative refractive index difference Δ2 were 0.0021%, 0.0015%, 0.001%, and 0.0008%, respectively. In Examples 9 to 12, the fluctuations in the relative refractive index difference Δ2 were 0.0021% or less, and all were evaluated as good (OK).

[0049] In Comparative Example 1, the soot density was 0.285 g / cm 3 The increase rate of the silicon tetrafluoride flow rate was 20%. The variation in the relative refractive index difference Δ2 was 0.005%. In Comparative Example 1, the variation in the relative refractive index difference Δ2 was greater than 0.0021%, and the evaluation was poor (NG).

[0050] In Comparative Example 2, the soot density was 0.25 g / cm 3 The increase rate of the silicon tetrafluoride flow rate was 20%. The variation in the relative refractive index difference Δ2 was 0.005%. In Comparative Example 2, the variation in the relative refractive index difference Δ2 was greater than 0.0021%, and the evaluation was poor (NG).

[0051] In Comparative Example 3, the soot density was 0.3 g / cm 3 The increase rate of the silicon tetrafluoride flow rate was 20%. The variation in the relative refractive index difference Δ2 was 0.005%. In Comparative Example 3, the variation in the relative refractive index difference Δ2 was greater than 0.0021%, and the evaluation was poor (NG).

[0052] In Comparative Example 4, the soot density was 0.35 g / cm 3 The increase rate of the silicon tetrafluoride flow rate was 20%. The variation in the relative refractive index difference Δ2 was 0.005%. In Comparative Example 4, the variation in the relative refractive index difference Δ2 was greater than 0.0021%, and the evaluation was poor (NG).

[0053] [Table 2]

[0054] Table 2 shows the fluorine content (wt%) in the upper end region of the optical fiber preform, the fluorine content in the central region of the optical fiber preform, the fluorine content in the lower end region of the optical fiber preform, and the fluorine content variation (wt%) for the examples and comparative examples. The fluorine content variation in Table 2 is calculated by subtracting the minimum value from the maximum value of the fluorine content in each of the upper end region, central region, and lower end region of the optical fiber preform. Table 2 shows the quantitative analysis of the fluorine content in the cladding of the optical fiber preform.

[0055] For the soot of the optical fiber preform in the example, the flow rate of silicon tetrafluoride in the raw material gas forming the end region of the soot was 1.3 times or more and 1.9 times or less than the flow rate of silicon tetrafluoride in the central region of the soot. For the soot of the optical fiber preform in the comparative example, the flow rate of silicon tetrafluoride in the raw material gas forming the end region of the soot was not changed relative to the flow rate of silicon tetrafluoride in the central region of the soot.

[0056] In the example, the fluorine content in the upper end region of the optical fiber preform was 0.44 wt%, the fluorine content in the central region of the optical fiber preform was 0.43 wt%, and the fluorine content in the lower end region of the optical fiber preform was 0.43 wt%. The variation in the fluorine content was 0.01 wt%.

[0057] In the comparative example, the fluorine content in the upper end region of the optical fiber preform was 0.41 wt%, the fluorine content in the central region of the optical fiber preform was 0.39 wt%, and the fluorine content in the lower end region of the optical fiber preform was 0.30 wt%. The variation in the fluorine content was 0.11 wt%.

[0058] As described above, by increasing the flow rate of silicon tetrafluoride in the soot edge region relative to the flow rate of silicon tetrafluoride in the soot central region, the variation in the fluorine content in the longitudinal direction of the optical fiber preform can be suppressed, and the variation in the fluorine content in the longitudinal direction of the optical fiber preform can be preferably 0.05 wt% or less.

[0059] The present invention is not limited to the above-described embodiments and can be modified in various ways. For example, an example in which a part of the configuration of one embodiment is added to another embodiment, or an example in which a part of the configuration of another embodiment is replaced with another embodiment, is also an embodiment of the present invention. Furthermore, with respect to parts not specifically explained or illustrated in the embodiments, well-known or publicly known techniques in the relevant technical field can be applied as appropriate. [Explanation of symbols]

[0060] 1 Optical fiber preform 2 cores 3. Clad 10 Starting substrate 11 Intermediates 12 Suits 100 VAD device 101 Case 102 Exhaust section 103 Rotating part 104 Drive unit 105 First Burner 106 Second Burner 107 Control Unit

Claims

1. a first step of forming a core by blowing a first raw material gas from a core burner onto a starting substrate, and forming a first clad by blowing a second raw material gas from a clad burner onto the core; a second step of forming a second clad by spraying a third source gas from the clad burner onto the end of the core after the first step; heating the first cladding and the second cladding to form a core rod; and forming a third cladding on the outer periphery of the core rod, A method for manufacturing an optical fiber preform, wherein a flow rate of silicon tetrafluoride in the third source gas is greater than a flow rate of silicon tetrafluoride in the second source gas.

2. 2. The method for manufacturing an optical fiber preform according to claim 1, wherein the flow rate of silicon tetrafluoride in the third source gas is 1.3 times or more and 1.9 times or less the flow rate of silicon tetrafluoride in the second source gas.

3. 2. The method for manufacturing an optical fiber preform according to claim 1, wherein in the second step, the third source gas is sprayed onto a region that is 5 cm to 20 cm from the end of the core.

4. The density of the soot formed in the first step and the second step is 0.25 g / cm ３ 0.35g / cm or more ３ and 2. The method for manufacturing an optical fiber preform according to claim 1, wherein the flow rate of silicon tetrafluoride in the third source gas is 1.3 times or more and 1.9 times or less the flow rate of silicon tetrafluoride in the second source gas.

5. The density of the soot formed in the first step and the second step is 0.25 g / cm ３ 0.35g / cm or more ３ and 2. The method for manufacturing an optical fiber preform according to claim 1, wherein the flow rate of silicon tetrafluoride in the third source gas is 1.6 times or more and 1.9 times or less the flow rate of silicon tetrafluoride in the second source gas.

6. The density of the soot formed in the first step and the second step is 0.25 g / cm ３ 0.35g / cm or more ３ and 2. The method for manufacturing an optical fiber preform according to claim 1, wherein the flow rate of silicon tetrafluoride in the third source gas is 1.6 times or more and 1.8 times or less the flow rate of silicon tetrafluoride in the second source gas.

7. 2. The method for manufacturing an optical fiber preform according to claim 1, wherein a variation in the relative refractive index difference in the longitudinal direction of the optical fiber preform is 0.0021% or less.

8. 2. The method for manufacturing an optical fiber preform according to claim 1, wherein in the step of forming the core rod, the first cladding and the second cladding are heated so as to cause sintering of the second cladding to proceed more rapidly than sintering of the first cladding.

9. a step of drawing the optical fiber preform manufactured by the method for manufacturing an optical fiber preform according to any one of claims 1 to 8 to form a bare optical fiber; A method for manufacturing an optical fiber, comprising the step of coating the bare optical fiber with resin.

10. The core and a cladding covering the core; An optical fiber preform, wherein the variation in fluorine content in the longitudinal direction of the cladding is 0.05 wt % or less.

Images