Optical fiber preform, optical fiber, and method for producing optical fiber preform

Abstract

This optical fiber preform is provided with: a core part that has pure silica glass; and a cladding part that contains fluorine. With respect to a cross-sectional surface of the cladding part, the thickness of the cladding part is defined along the radial direction. The innermost layer of the cladding part is defined as a region that comprises the inner peripheral surface of the cladding part and has a thickness equal to 10% of the thickness of the cladding part. The outermost layer of the cladding part is defined as a region that comprises the outer peripheral surface of the cladding part and has a thickness equal to 10% of the thickness of the cladding part. The difference between the minimum relative refractive index difference of the entire cladding part and the respective maximum relative refractive index differences of the innermost layer and the outermost layer is 0.03% or more.

Description

Optical fiber preform, optical fiber, and method for manufacturing optical fiber preform

[0001] This disclosure relates to an optical fiber preform, an optical fiber, and a method for manufacturing an optical fiber preform. This application claims priority to Japanese Application No. 2024-006702, filed on January 19, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] In Patent Document 1, an optical fiber having a core that does not contain germanium (Ge) and a cladding that is doped with fluorine (F) is considered as an optical fiber with low transmission loss.

[0003] Patent Publication No. 2019-526073 International Publication No. 2019-026356 Japanese Patent Application Laid-Open No. 2013-018669

[0004] The optical fiber preform disclosed herein is an optical fiber preform including at least a core and a cladding. The core is the portion that will become the core after drawing, extends along the central axis, and is made of pure silica glass. The cladding is the portion that will become the cladding after drawing, surrounds the core, and contains fluorine (F). In a cross section of the cladding perpendicular to the central axis, the thickness of the cladding is defined along a radial direction extending from the central axis toward the outer circumferential surface of the cladding. The innermost layer of the cladding is defined as a region that includes the inner circumferential surface of the cladding and has a thickness that is 10% of the thickness of the cladding. The outermost layer of the cladding is defined as a region that includes the outer circumferential surface of the cladding and has a thickness that is 10% of the thickness of the cladding. Furthermore, a first difference (expressed as a positive value) between the minimum relative refractive index difference of the entire cladding and the maximum relative refractive index difference of the innermost layer is 0.03% or more. Similarly, a second difference (expressed as a positive value) between the minimum relative refractive index difference of the entire cladding portion and the maximum relative refractive index difference of the outermost layer is 0.03% or more.

[0005] FIG. 1 is a diagram illustrating an example of an optical fiber preform according to the present disclosure and its manufacturing process (the manufacturing method of the optical fiber preform according to the present disclosure). FIG. 2 is a diagram illustrating a modified example of an optical fiber preform according to the present disclosure and its manufacturing process (the manufacturing method of the optical fiber preform according to the present disclosure). FIG. 3 is a diagram illustrating the cross-sectional structure and refractive index profile of various examples of the optical fiber preform according to the present disclosure. FIG. 4 is a diagram illustrating the evaluation of the optical fiber preform according to the present disclosure, the manufacturing method of the optical fiber according to the present disclosure, and the evaluation of the optical fiber according to the present disclosure. FIG. 5 is a diagram illustrating the cross-sectional structure and refractive index profile of various examples of the optical fiber according to the present disclosure. FIG. 6 is a diagram illustrating the cross-sectional structure and refractive index profile of the cladding portion of the optical fiber preform, the cladding of the optical fiber, and the cladding pipe of the optical fiber. FIG. 7 is a diagram illustrating the measurement results of the dynamic fatigue coefficient Nd of optical fibers obtained from optical fiber preforms using cladding pipes with various refractive index profiles. FIG. 8 is a table illustrating the preferred F concentration difference of the optical fiber according to the present disclosure and the preferred glass viscosity of the optical fiber preform according to the present disclosure. FIG. 9 is a diagram illustrating a first manufacturing method (OVD method) of a cladding pipe applicable to the manufacture of the optical fiber preform according to the present disclosure. 10 is a diagram illustrating a second cladding pipe manufacturing method (VAD method) applicable to the manufacture of an optical fiber preform according to the present disclosure, and FIG. 11 is a diagram illustrating a third cladding pipe manufacturing method and a fourth cladding pipe manufacturing method applicable to the manufacture of an optical fiber preform according to the present disclosure.

[0006] [Problem to be Solved by the Present Disclosure] As a result of examining the above-mentioned conventional techniques, the inventors have discovered the following problem: In manufacturing an optical fiber preform for obtaining an optical fiber, F doping into the cladding portion (the portion to become the cladding of the optical fiber) of the optical fiber preform is carried out by using SiCl ４ The aggregate of glass particles (soot body) obtained by burning the glass in an oxyhydrogen flame or by burning siloxane is called CF ４ , SiF ４ , SF ６The core of an optical fiber preform can be obtained by sintering a glass-synthesized soot body in a small amount of halogen gas, such as helium gas (He gas), which does not change the refractive index or does not significantly change the refractive index. However, although both the cladding and core are obtained from a glass-synthesized soot body, it is difficult to simultaneously manufacture the cladding and core (manufacture and sinter a glass-synthesized soot body). This is because the relative refractive index difference between the core and cladding is large, making it difficult to create layers with different refractive indices by, for example, changing the halogen gas concentration along the radial direction of the soot body. Therefore, to obtain an optical fiber having a core made of pure silica glass and an F-doped cladding, a manufacturing method using a pre-glass-synthesized core rod (a glass rod to become the core of the optical fiber preform) and a cladding pipe (a glass pipe to become the cladding of the optical fiber preform) is sometimes used. In this manufacturing method, the core rod is inserted into the cladding pipe, and the core rod and cladding pipe are heated using an external heat source to integrate the core rod and cladding (rod-in-collapse method). In this rod-in-collapse method, if there is a large difference in glass viscosity between the core rod and the cladding pipe during integration by heating, the cladding pipe, which has a lower glass viscosity, may be deformed. Deformation of the cladding pipe poses problems such as a deterioration in the core-to-cladding outer diameter ratio (= cladding outer diameter / core outer diameter) of the final optical fiber product, bending of the optical fiber preform (bending of the internal core rod), and deterioration in the non-circularity of the core part of the optical fiber preform and the core of the optical fiber.

[0007] In addition, optical fibers obtained by drawing an optical glass preform in which a core rod and a clad pipe having a large difference in glass viscosity are integrated have sometimes had a poor dynamic fatigue coefficient (Nd) compared to optical fibers having a pure silica clad.

[0008] The present disclosure has been made to solve the above-mentioned problems, and aims to provide an optical fiber preform having a structure for obtaining a pure silica core optical fiber with improved Nd, a pure silica core optical fiber, and a method for manufacturing the optical fiber preform.

[0009] [Advantages of the Present Disclosure] According to the optical fiber preform etc. of the present disclosure, a pure silica core optical fiber with improved Nd can be obtained.

[0010] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be individually listed and described.

[0011] The optical fiber preform disclosed herein is an optical fiber preform comprising: (1) a core portion and a cladding portion. The core portion is a portion that will become the core after drawing, extends along the central axis, and is made of pure silica glass. The cladding portion is a portion that will become the cladding after drawing, surrounds the core portion, and contains fluorine (F). In a cross section of the cladding portion perpendicular to the central axis, the thickness of the cladding portion is defined along a radial direction extending from the central axis toward the outer peripheral surface of the cladding portion. The innermost layer of the cladding portion is defined as a region that includes the inner peripheral surface of the cladding portion and has a thickness that is 10% of the thickness of the cladding portion. The outermost layer of the cladding portion is defined as a region that includes the outer peripheral surface of the cladding portion and has a thickness that is 10% of the thickness of the cladding portion. Furthermore, a first difference (expressed as a positive value) between the minimum relative refractive index difference of the entire cladding portion and the maximum relative refractive index difference of the innermost layer is 0.03% or more. Similarly, a second difference (expressed as a positive value) between the minimum relative refractive index difference of the entire cladding portion and the maximum relative refractive index difference of the outermost layer is 0.03% or more.

[0012] The above-described change in the relative refractive index difference within the cladding is due to a change in the F concentration within the cladding. This means that the difference in glass viscosity between the innermost and outermost layers and the central portion of the cladding (the region sandwiched between the innermost and outermost layers) due to the addition of F increases. For example, if the difference in glass viscosity between the core and cladding is reduced near the interface between the core and cladding, the deformation of the resulting optical fiber preform and the noncircularity of the core are improved (reduced noncircularity). In this case, it is possible to prevent deformation of the optical fiber itself obtained by drawing the optical fiber preform and to prevent deterioration of the core noncircularity, thereby improving the dynamic fatigue coefficient Nd of the resulting optical fiber. In other words, it is possible to avoid breakage of the resulting optical fiber and extend its break life.

[0013] In this specification, the term "relative refractive index difference" refers to the relative refractive index difference between each portion of the glass and the refractive index n0 of pure silica glass. Specifically, it is an index obtained by dividing the difference (n1-n0) between the refractive index n1 of each portion of the glass measured with a refractive index measuring instrument and the refractive index n0 of pure silica glass by the refractive index n0 of pure silica glass (=(n1-n0) / n0), expressed as a percentage. Hereinafter, the relative refractive index difference will be referred to as Δ. In addition, in this specification, the term "core containing pure silica glass (pure silica core)" refers to a glass core in which the concentration of a dopant that changes the refractive index, such as germanium (Ge), chlorine (Cl), or fluorine (F), is 1 mass % or less, and in which the change in refractive index due to the dopant is substantially negligible. Furthermore, in this specification, the term "non-circularity" refers to the index obtained by dividing the difference between the major axis and minor axis of the core in the cross section of the optical fiber by the average outer diameter of the core, expressed as a percentage. In the case of an optical fiber preform, which is an optical fiber precursor, the non-circularity is an index that expresses the difference between the major axis and the minor axis of the core portion that will become the core after drawing in the cross section of the optical fiber preform, divided by the average outer diameter of the core portion, and expressed as a percentage. In either case, the non-circularity is preferably 1% or less. In this case, the optical fiber has a good polarization mode dispersion (hereinafter referred to as "PMD"), specifically, 0.15 ps / km. １／２ The following PMD is obtained: Furthermore, the improvement in non-circularity prevents bending of the optical fiber preform due to non-uniform heat distribution in the circumferential direction.

[0014] (2) In (1), the first difference and the second difference may both be 0.15% or less. If the first difference and the second difference exceed 0.15%, the difference in glass viscosity between the core and the cladding becomes large, increasing the possibility of deformation of the optical fiber preform and deterioration of the noncircularity of the core. In particular, if the first difference and the second difference exceed 0.2%, the dynamic fatigue coefficient Nd will clearly deteriorate.

[0015] (3) In (1) or (2), the maximum fluorine concentration of the innermost layer (hereinafter referred to as "maximum F concentration") and the maximum F concentration of the outermost layer may both be lower than the maximum F concentration of the entire cladding portion. Specifically, the difference (expressed as a positive value) between the maximum F concentration of the innermost layer and the maximum F concentration of the entire cladding portion is 500 ppm or more. Similarly, the difference (expressed as a positive value) between the maximum F concentration of the outermost layer and the maximum F concentration of the entire cladding portion is 500 ppm or more. In this way, an increase in the difference in glass viscosity between the innermost and outermost layers and the central portion of the cladding portion (the region sandwiched between the innermost and outermost layers) due to F addition within the cladding portion means, for example, a reduction in the difference in glass viscosity between the core portion and the cladding portion near the interface between the core portion and the cladding portion. This improves the deformation of the resulting optical fiber preform and the noncircularity of the core portion. This also means that the dynamic fatigue coefficient Nd of the optical fiber obtained after drawing can be further improved.

[0016] (4) In any of (1) to (3), the glass viscosity log at 1400 ° C. １０With respect to (η) (unit: Pa s), the difference (expressed as a positive value) between the minimum glass viscosity of the innermost layer and the minimum glass viscosity of the entire cladding portion may be 0.1 or more. Similarly, the difference (expressed as a positive value) between the minimum glass viscosity of the outermost layer and the minimum glass viscosity of the entire cladding portion may be 0.1 or more. In this way, the glass viscosity of the peripheral region of the cladding portion is higher than the glass viscosity of the central region sandwiched between the peripheral regions, thereby reducing the difference in glass viscosity between the core portion and the cladding portion near the interface between the core portion and the cladding portion. This improves the deformation of the obtained optical fiber preform and the noncircularity of the core portion. This also means that the dynamic fatigue coefficient Nd of the optical fiber obtained after drawing can be further improved.

[0017] (5) In any of (1) to (4) above, the bending of the optical fiber preform of the present disclosure may be 0 mm or more and 1 mm or less. By limiting the bending of the optical fiber preform in this manner, the non-circularity of the core portion is also improved, and as a result, an optical fiber obtained by drawing the optical fiber preform can have a dynamic fatigue coefficient Nd sufficient for practical use.

[0018] (6) In any one of (1) to (5), the optical fiber preform of the present disclosure may have an inner cladding portion provided between the core portion and the cladding portion, and in this case, an optical fiber obtained by drawing the optical fiber preform may have a depressed cladding structure that has bending loss resistance and excellent light confinement efficiency.

[0019] (7) In the above (6), the inner cladding may have two or more regions arranged concentrically, and the two or more regions may include a region having a relative refractive index difference smaller than the minimum relative refractive index difference of the cladding. In this case, too, an optical fiber obtained by drawing the optical fiber preform has bending loss resistance and also has a high degree of freedom in designing optical properties such as mode field diameter (hereinafter referred to as "MFD").

[0020] The optical fiber of the present disclosure is (8) an optical fiber including a core and a cladding. The core extends along a central axis and is made of pure silica glass. The cladding surrounds the core and contains fluorine (F). In a cross section of the cladding perpendicular to the central axis, the thickness of the cladding is defined along a radial direction extending from the central axis toward the outer circumferential surface of the cladding. The innermost layer of the cladding is defined as a region including the inner circumferential surface of the cladding and having a thickness equal to 10% of the thickness of the cladding. The outermost layer of the cladding is defined as a region including the outer circumferential surface of the cladding and having a thickness equal to 10% of the thickness of the cladding. A first difference (expressed as a positive value) between the minimum relative refractive index difference of the entire cladding and the maximum relative refractive index difference of the innermost layer is 0.03% or more. Similarly, a second difference (expressed as a positive value) between the minimum relative refractive index difference of the entire cladding and the maximum relative refractive index difference of the outermost layer is 0.03% or more. In this case, the non-circularity of the core is improved, the dynamic fatigue coefficient Nd of the optical fiber is improved, and it becomes possible to avoid breakage of the optical fiber and extend the breakage life.

[0021] (9) In (8), the first difference and the second difference may both be 0.15% or less. If the first difference and the second difference exceed 0.15%, the possibility of core deformation increases. In particular, if the first difference and the second difference exceed 0.2%, it becomes difficult to prevent deterioration of the dynamic fatigue coefficient Nd of the optical fiber.

[0022] (10) In the above (8) or (9), the optical fiber of the present disclosure may have a dynamic fatigue coefficient Nd of 19 or greater. In this case, it is possible to avoid breakage of the optical fiber and extend its breakage life.

[0023] (11) In any one of (8) to (10) above, the optical fiber of the present disclosure may have an inner cladding layer provided between the core and the cladding, in which case a depressed cladding structure having bending loss resistance and excellent light confinement efficiency is obtained.

[0024] (12) In the above (11), the inner cladding layer may have two or more layers, and the two or more layers may include a layer having a relative refractive index difference smaller than the minimum relative refractive index difference of the cladding. In this case, bending loss resistance is also obtained, and a high degree of freedom is obtained in designing optical properties such as MFD.

[0025] The optical fiber manufacturing method of the present disclosure is (13) a manufacturing method including a preparation step, an insertion step, and a collapsing step. In the preparation step, a core rod and a clad pipe are prepared. The core rod is a component to become the core portion of the optical fiber preform, extends along the central axis, and contains pure silica glass. The clad pipe is a component to become the clad portion of the optical fiber preform, has a through hole for accommodating the core rod, and contains fluorine (F). In the insertion step, the prepared core rod is inserted into the through hole of the clad pipe. In the collapsing step, the core rod and the clad pipe are integrated by heating. Specifically, the core rod and the clad pipe are integrated with the core rod inserted into the through hole of the clad pipe. In a cross section of the clad pipe perpendicular to the extension direction of the through hole, the thickness of the clad pipe is defined along the radial direction extending from the center of the through hole toward the outer circumferential surface of the clad pipe. The innermost layer of the clad pipe is defined as a region including the inner peripheral surface of the clad pipe and having a thickness of 10% of the thickness of the clad pipe. The outermost layer of the clad pipe is defined as a region including the outer peripheral surface of the clad pipe and having a thickness of 10% of the thickness of the clad pipe. A first difference (expressed as a positive value) between the minimum relative refractive index difference of the entire clad pipe and the maximum relative refractive index difference of the innermost layer is 0.03% or more. Similarly, a second difference (expressed as a positive value) between the minimum relative refractive index difference of the entire clad pipe and the maximum relative refractive index difference of the outermost layer is 0.03% or more. In this case, deformation of the obtained optical fiber preform and deterioration of the noncircularity of the core are improved. Furthermore, deformation of the optical fiber itself obtained by drawing the obtained optical fiber preform and deterioration of the noncircularity of the core can be prevented. As a result, the dynamic fatigue coefficient Nd of the obtained optical fiber can be improved.

[0026] (14) In the above (13), the preparing step may include preparing an inner cladding pipe having a through hole for accommodating the core rod and being inserted between the core rod and the inner wall of the through hole of the cladding pipe. Additionally, the inserting step may include inserting the inner cladding pipe between the core rod and the inner wall of the through hole of the cladding pipe, and the collapsing step may include integrating the core rod, the inner cladding pipe, and the cladding pipe. With this configuration, an optical fiber preform for an optical fiber having a depressed cladding structure with excellent light confinement efficiency and resistance to bending loss can be obtained.

[0027] (15) In the above (14), the inner cladding pipe may be formed by two or more pipes arranged concentrically. In particular, the two or more pipes may include a pipe having a relative refractive index difference smaller than the minimum relative refractive index difference of the cladding pipe. In this case, too, an optical fiber preform for an optical fiber can be obtained that has bending loss resistance and a high degree of freedom in designing optical properties such as MFD.

[0028] (16) In any of (13) to (15), the first difference and the second difference may both be 0.15% or less. If the first difference and the second difference exceed 0.15%, the difference in glass viscosity between the core rod and the cladding pipe becomes large, increasing the possibility of deformation of the obtained optical fiber preform and deterioration of the core noncircularity. In particular, if the first difference and the second difference exceed 0.2%, it becomes difficult to prevent deterioration of the dynamic fatigue coefficient Nd of the optical fiber obtained by drawing the optical fiber preform.

[0029] (17) In any of (13) to (15) above, the maximum F concentration in the innermost layer and the maximum F concentration in the outermost layer may both be lower than the maximum F concentration in the entire cladding portion. Specifically, the difference between the maximum F concentration in the innermost layer and the maximum F concentration in the entire cladding portion may be 500 ppm or more. Similarly, the difference between the maximum F concentration in the outermost layer and the maximum F concentration in the entire cladding portion may be 500 ppm or more. This improves the deformation of the resulting optical fiber preform and the noncircularity of the core. This also means that the dynamic fatigue coefficient Nd of the optical fiber obtained after drawing can be further improved.

[0030] (18) In any one of (13) to (17), the glass viscosity log at 1400 ° C. １０ With respect to (η) (unit: Pa s), the difference between the minimum glass viscosity of the innermost layer and the minimum glass viscosity of the entire cladding portion may be 0.1 or more. Similarly, the difference between the minimum glass viscosity of the outermost layer and the minimum glass viscosity of the entire cladding portion may be 0.1 or more. This improves the deformation of the obtained optical fiber preform and the noncircularity of the core portion. This also means that the dynamic fatigue coefficient Nd of the optical fiber obtained after drawing can be further improved.

[0031] (19) In any of (13) to (18) above, the bending of the optical fiber preform obtained by the manufacturing method of the optical fiber preform of the present disclosure may be 0 mm or more and 1 mm or less. By limiting the bending of the optical fiber preform in this manner, the non-circularity of the core portion is also improved, and as a result, an optical fiber obtained by drawing the optical fiber preform can have a dynamic fatigue coefficient Nd sufficient for practical use.

[0032] [Details of the embodiments of the present disclosure] Specific examples of the optical fiber preform, optical fiber, and method for manufacturing the optical fiber preform according to the present disclosure will be described in detail below with reference to the accompanying 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 addition, in the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted.

[0033]

[0013] Figure 1 is a diagram illustrating an example of an optical fiber preform according to the present disclosure and its manufacturing process (the manufacturing method of the optical fiber preform according to the present disclosure) (marked "preform manufacturing process" in Figure 1). The upper part of Figure 1 (marked "components" in Figure 1) shows the components for obtaining the basic structure of the optical fiber preform according to the present disclosure. The middle part of Figure 1 (marked "collapse" in Figure 1) shows the collapsing process for integrating the components of the optical fiber preform according to the present disclosure by heating. The lower part of Figure 1 (marked "processing" in Figure 1) shows the processing process for obtaining the optical fiber preform according to the present disclosure from the components obtained through the collapsing process.

[0034] As shown in the upper part (components) of Fig. 1 , a core rod 10 to become the core portion of the optical fiber preform of the present disclosure and a cladding pipe 20 to become the cladding portion are prepared (preparation step). The core rod 10 is a glass rod made of pure silica glass that extends along the central axis. The cladding pipe 20 is a glass pipe that has a through hole 21 to accommodate the core rod 10 and is doped with fluorine (F). In the insertion step, the core rod 10 is inserted into the through hole 21 of the cladding pipe 20.

[0035] Next, as shown in the middle (collapse) section of FIG. 1 , the collapsing process is performed with exhaust pipes 300 attached to both ends of the clad pipe 20 with the core rod 10 inserted into the through hole 21, and with the through hole 21 with the core rod 10 inserted being depressurized. That is, with the through hole 21 being depressurized, the core rod 10 and the clad pipe 20 are heated by an oxyhydrogen burner 400 while rotating in the direction indicated by arrow S21. The oxyhydrogen burner 400 moves along the direction indicated by arrow S11, and the core rod 10 and the clad pipe 20 are integrated by the oxyhydrogen burner 400 to obtain a transparent glass body. By cutting both ends of this transparent glass body at cutting positions C1 and C2, the optical fiber preform 100A of the present disclosure shown in the lower (processing) section of FIG. 1 is obtained. Note that, before cutting both ends of the transparent glass body, a drawing process may be performed to adjust the outer diameter of the obtained optical fiber preform 100A.

[0036] As shown in the lower part (processing) of Figure 1, the optical fiber preform 100A according to the present disclosure obtained through the above-described preparation, insertion, and collapsing processes includes at least a core 110A and a cladding 120A disposed around the core 110A. The core 110A is the portion that will become the core after drawing, extends along the central axis, and contains pure silica glass. The cladding 120A is the portion that will become the cladding after drawing, surrounds the core 110A, and contains fluorine (F).

[0037] 2A and 2B are diagrams for explaining a modified example of the optical fiber preform according to the present disclosure and its manufacturing process (the manufacturing method of the optical fiber preform according to the present disclosure) (denoted as "Modified Preform Structure" in FIG. 2A). The upper part of FIG. 2A (denoted as "First Modification" in FIG. 2A) shows the main manufacturing process of the optical fiber preform 100B according to the first modification. The lower part of FIG. 2A (denoted as "Second Modification" in FIG. 2A) shows the main manufacturing process of the optical fiber preform 100C according to the second modification.

[0038] The upper part of Fig. 2 (first modified example) shows an optical fiber preform 100B processed through the main manufacturing steps of an insertion step and a collapsing step (denoted as "collapse (and drawing)" in the upper part of Fig. 2). In the manufacturing of this optical fiber preform 100B, a core rod 10, a first inner clad pipe 30 having a through hole 31 into which the core rod 10 is inserted, and a clad pipe 20 having a through hole 21 into which the first inner clad pipe 30 is inserted are prepared in a preparation step. In the insertion step, the first inner clad pipe 30 is delivered into the through hole 21 of the clad pipe 20, and the core rod 10 is inserted into the through hole 31 of the first inner clad pipe 30. The core rod 10, the first inner clad pipe 30, and the clad pipe 20 are integrated in the collapsing step. Further, the obtained transparent glass body is drawn to a desired outer diameter, and then an optical fiber preform 100B having both ends processed is obtained.

[0039] The obtained optical fiber preform 100B includes a core 110B, a first inner cladding 130B disposed around the core 110B, and a cladding 120B disposed around the first inner cladding 130B. The core 110B is the portion that will become the core after drawing, extends along the central axis, and contains pure silica glass. The cladding 120B is the portion that will become the cladding after drawing, surrounds the core 110B, and contains fluorine (F). The first inner cladding 130B becomes a depressed portion after drawing, having a relative refractive index difference that is lower than that of the cladding.

[0040] The lower part of Fig. 2 (second modified example) shows an optical fiber preform 100C that has been processed through the main manufacturing steps of an insertion step and a collapsing step (also referred to as "collapse (and drawing)" in the lower part of Fig. 2). In manufacturing the optical fiber preform 100C, a preparation step includes preparing a core rod 10, a second inner clad pipe 40 having a through hole 41 into which the core rod 10 is inserted, a first inner clad pipe 30 having a through hole 31 into which the second inner clad pipe 40 is inserted, and a clad pipe 20 having a through hole 21 into which the first inner clad pipe 30 is inserted. In the insertion step, the first inner clad pipe 30 is delivered into the through hole 21 of the clad pipe 20, the second inner clad pipe 40 is inserted into the through hole 31 of the first inner clad pipe 30, and the core rod 10 is inserted into the through hole 41 of the second inner clad pipe 40. The core rod 10, the second inner clad pipe 40, the first inner clad pipe 30, and the clad pipe 20 are integrated in a collapsing process, and the resulting transparent glass body is then stretched to the desired outer diameter, resulting in an optical fiber preform 100C with both ends processed.

[0041] The obtained optical fiber preform 100C includes a core 110C, a second inner cladding 140C disposed around the core 110C, a first inner cladding 130C disposed around the second inner cladding 140C, and a cladding 120C disposed around the first inner cladding 130C. The core 110C is the portion that will become the core after drawing, extends along the central axis, and contains pure silica glass. The cladding 120C is the portion that will become the cladding after drawing, surrounds the core 110C, and contains fluorine (F). The first inner cladding 130C becomes a depressed portion having a relative refractive index difference lower than that of the cladding after drawing. The second inner cladding 140C becomes a portion having a relative refractive index difference lower than that of the core and higher than that of the cladding after drawing.

[0042] 3 is a diagram showing the cross-sectional structure and refractive index profile of various examples of optical fiber preforms according to the present disclosure (denoted as "preform structure" in FIG. 3). The upper part of FIG. 3 (denoted as "Type 1" in FIG. 3) shows the I type shown in the lower part of FIG. １ -I １ The cross-sectional structure and refractive index profile of the optical fiber preform 100A along the line are shown in the middle of Fig. 3 (denoted as "Type 2" in Fig. 3). ２ -I ２ The cross-sectional structure and refractive index profile of the optical fiber preform 100B along the line are shown in the lower part of Fig. 3 (denoted as "Type 3" in Fig. 3). ３ -I ３ The cross-sectional structure and refractive index profile of the optical fiber preform 100C along the line are shown.

[0043] The optical fiber preform 100A shown in the upper part (Type 1) of Fig. 3 comprises a core 110A extending along a central axis AX1 and a cladding 120A surrounding the core 110A. The refractive index profile of the optical fiber preform 100A is defined by the relative refractive index difference of each portion along a line L1 passing through the central axis AX1 in this cross section. That is, the relative refractive index difference of the core 110A is at the pure silica level, and the relative refractive index difference Δ of the cladding 120A relative to the pure silica level is set lower than the relative refractive index difference of the core 110A. Note that although the relative refractive index difference of the cladding 120A shown in the upper part of Fig. 3 is shown as constant in the drawing, it actually varies within the cladding 120A as described above (see the lower part of Fig. 6). Also, the major diameter D shown in the upper part of Fig. 3 Ｐ１ and minor diameter D Ｐ２ The cross-sectional shape (non-circularity) of the core portion 110A is defined by the above.

[0044] The optical fiber preform 100B shown in the middle (Type 2) of Figure 3 includes a core 110B extending along a central axis AX1, a first inner cladding 130B surrounding the core 110B, and a cladding 120B surrounding the first inner cladding 130B. The refractive index profile of the optical fiber preform 100B is defined by the relative refractive index differences of each portion along a line L1 passing through the central axis AX1 in this cross section. That is, the relative refractive index difference of the core 110B is at the pure silica level, and the relative refractive index difference Δ of the cladding 120B relative to the pure silica level is set lower than the relative refractive index difference of the core 110B. Furthermore, the relative refractive index difference of the first inner cladding 130B is set lower than the relative refractive index difference Δ of the cladding 120B. Although the relative refractive index difference of the cladding portion 120B shown in the middle of FIG. 3 is shown as constant in the drawing, it actually varies within the cladding portion 120B as described above (see the bottom of FIG. 6).

[0045] The optical fiber preform 100C shown in the lower part of Figure 3 (Type 3) includes a core 110C extending along a central axis AX1, a second inner cladding 140C surrounding the core 110C, a first inner cladding 130C surrounding the second inner cladding 140C, and a cladding 120C surrounding the first inner cladding 130C. The refractive index profile of the optical fiber preform 100C is defined by the relative refractive index differences of each portion along a line L1 passing through the central axis AX1 in this cross section. That is, the relative refractive index difference of the core 110C is at the pure silica level, and the relative refractive index difference Δ of the cladding 120C relative to the pure silica level is set lower than the relative refractive index difference of the core 110C. Furthermore, the relative refractive index difference of the first inner cladding 130C is set lower than the relative refractive index difference Δ of the cladding 120C. Conversely, the relative refractive index difference of the second inner cladding portion 140C is set to be lower than the relative refractive index difference of the core portion 110C but higher than the relative refractive index difference Δ of the cladding portion 120C. Note that although the relative refractive index difference of the cladding portion 120C shown in the lower part of Figure 3 is shown as constant in the drawing, as described above, it actually varies within the cladding portion 120C (see the lower part of Figure 6).

[0046] As an example, in the case of the optical fiber preform 100A, the prepared cladding pipe 20 has a relative refractive index difference Δ that is lower than −0.15% with respect to the core rod 10 (pure silica level). Furthermore, the viscosity reduction of the cladding pipe 20 due to the inclusion of F is significantly greater than that of a core rod 10 made of pure silica glass. In the collapsing process, heat is applied from the outside using an oxyhydrogen burner 400 with the core rod 10 inserted into the through-hole 21 of the cladding pipe 20. This collapses the cladding pipe 20, and the core rod 10 and the cladding pipe 20 are integrated.

[0047] In the above-mentioned collapsing process, if sufficient heat is applied so that bubbles of 0.3 mm or more in diameter do not remain at the interface between the core 110A and the cladding 120A of the obtained optical fiber preform 100A, the cladding pipe 20 that will become the cladding 120A may stretch, resulting in a thin cladding pipe 20. If the outer diameter ratio, defined as the outer diameter of the cladding pipe / the outer diameter of the core rod after collapsing, is smaller by 0.2 or more than the design before collapsing, the core diameter of the optical fiber, which is the final product, may become smaller, and the optical characteristics of the optical fiber may deteriorate. Specifically, the cutoff wavelength may become longer by 100 nm or more, and the bending loss may become 0.2 dB / m or more. In the obtained optical fiber, if the deformation of the core is not uniform in the circumferential direction, the PMD may become 0.15 ps / km or more due to the deterioration of non-circularity. １／２ (PMD deterioration) The PMD is measured by interferometry in a state where the optical fibers to be measured are bundled together to have a diameter of 280 mm.

[0048]

[0033] Figure 4 is a diagram for explaining the evaluation of the optical fiber preform according to the present disclosure, the manufacturing method of the optical fiber according to the present disclosure, and the evaluation of the optical fiber according to the present disclosure (denoted as "Evaluation and Drawing" in Figure 4). The upper part of Figure 4 (denoted as "Evaluation of Preform Curvature" in Figure 4) shows a diagram for explaining a method for evaluating the preform curvature of the optical fiber preforms 100A to 100C. The middle part of Figure 4 (denoted as "Drawing Apparatus" in Figure 4) shows the configuration of a drawing apparatus for obtaining optical fibers 200A to 200C from the optical fiber preforms 100A to 100C. The lower part of Figure 4 (denoted as "Evaluation of Optical Fiber Curvature" in Figure 4) shows a diagram for explaining a method for evaluating the optical fiber curvature of the optical fibers 200A to 200C obtained by the drawing apparatus shown in the middle part of Figure 4.

[0049] As shown in the upper part of Figure 4 (evaluation of preform bending), the optical fiber preforms to be evaluated (e.g., optical fiber preforms 100A to 100C) have a preform length BL of approximately 200 mm to 1000 mm. The preform bending is defined as the maximum value of the gap length BD between the center of the body of the evaluation target and the base when the evaluation target is placed on a flat base. Specifically, the maximum value of the gap length BD may be between 0 mm and 1 mm, i.e., the center of the body of the evaluation target may not be elevated more than 1 mm above the base.

[0050] As a method for preventing deterioration of optical properties due to deformation of the optical fiber preform, for example, in a cladding portion doped with fluorine (F), the difference between the maximum relative refractive index difference of the innermost layer and the minimum relative refractive index difference of the entire cladding portion may be set to 0.03% or more, and the difference between the maximum relative refractive index difference of the outermost layer and the minimum relative refractive index difference of the entire cladding portion may be set to 0.03% or more. That is, in the refractive index profile of the cladding portion defined on the cross section of the cladding portion, the slope of the line passing through the position indicating the maximum relative refractive index difference of the innermost layer and the position indicating the minimum relative refractive index difference of the cladding portion (hereinafter simply referred to as the "slope of the relative refractive index difference Δ") may be between 0.0004% and 0.1%. When the slope of the relative refractive index difference Δ is large, a sudden change in the relative refractive index difference Δ occurs near the interface between the core portion and the cladding portion, and stress-strain occurs in a narrow region where the relative refractive index difference Δ varies. This causes deformation of the resulting optical fiber preform, which deteriorates the dynamic fatigue coefficient Nd of the optical fiber obtained by drawing the optical fiber preform. On the other hand, if the slope of the relative refractive index difference Δ is small, the region in the cladding where the relative refractive index difference Δ fluctuates will extend to the vicinity of the center of the cladding. When the slope of the relative refractive index difference Δ is small like this, the reproducibility of the slope of the relative refractive index difference Δ will be poor, and the controllability of the optical properties such as the cutoff wavelength and bending loss of the final optical fiber product will be poor.

[0051] Regarding the gradient of the relative refractive index difference Δ, between the innermost layer (a region having a thickness of 10% of the cladding thickness) including the inner peripheral surface (inner interface) within the cladding portion and a first adjacent layer adjacent to the innermost layer and having the same thickness as the innermost layer, the difference between the maximum relative refractive index difference of the innermost layer and the maximum relative refractive index difference of the first adjacent layer may also be within a range of 0.01% to 0.05%. Note that this first adjacent layer is an annular region extending from a position 10% of the cladding thickness toward the outer peripheral surface of the cladding portion to a position 20% of the cladding thickness. Similarly, between the outermost layer (a region having a thickness of 10% of the cladding thickness) including the outer peripheral surface (outer interface) within the cladding portion and a second adjacent layer adjacent to the outermost layer and having the same thickness as the outermost layer, the difference between the maximum relative refractive index difference of the outermost layer and the maximum relative refractive index difference of the second adjacent layer may be within a range of 0.01% to 0.05%. The second adjacent layer is an annular region extending from the outer peripheral surface of the cladding toward the inner peripheral surface thereof at a distance of 10% of the cladding thickness to a position at a distance of 20% of the cladding thickness. If this difference exceeds 0.05%, stress-strain will occur in a narrow region, similar to the slope of the relative refractive index difference Δ. Conversely, if this difference is less than 0.01%, the dynamic fatigue coefficient Nd of the final optical fiber will deteriorate.

[0052] The optical fiber preforms 100A to 100C having a preform bend of 1 mm or less are drawn by a drawing apparatus shown in the middle of FIG. 4 to produce optical fibers 200A to 200C. Specifically, the drawing apparatus shown in the middle of FIG. 4 includes a heater 510 for heating the end of the preform and a drum 610 (winding drum) for winding the optical fiber drawn from the preform via rollers 620. For example, when drawing the optical fiber preform 100A, one end of the optical fiber preform 100A heated and melted by the heater 510 is drawn by being wound around the drum 610, which rotates in the direction indicated by arrow S41. As a result, the optical fiber 200A advances in the direction indicated by arrow S42 via the rollers 620 and is wound around the drum 610. Similarly, the optical fibers 200B and 200C are produced from the optical fiber preforms 100B and 100C by the drawing apparatus shown in the middle of FIG. 4.

[0053] The curvature of each of the optical fibers 200A to 200C manufactured by the drawing apparatus shown in the middle of Figure 4 is evaluated as shown in the bottom of Figure 4. Specifically, a sample having an optical fiber length FL of approximately 15 cm to 25 cm is cut from each of the manufactured optical fibers 200A to 200C. One end of each of these samples is mounted on a rotary drive device 700 and rotated in the direction indicated by arrow S43. The radius of curvature is measured from the curvature of the tip of the sample at this time. Note that the fiber curvature of the optical fiber disclosed herein may be 4.2 m or more.

[0054] Fig. 5 shows the cross-sectional structures and refractive index profiles of optical fibers 200A to 200C according to the present disclosure, which were manufactured using the drawing apparatus shown in the middle of Fig. 4 (labeled "fiber structures" in Fig. 5). The upper part of Fig. 5 (labeled "Type 1" in Fig. 5) shows the cross-sectional structure and refractive index profile of optical fiber 200A taken along line II-II in Fig. 4. The middle part of Fig. 5 (labeled "Type 2" in Fig. 5) shows the cross-sectional structure and refractive index profile of optical fiber 200B taken along line II-II in Fig. 4. The lower part of Fig. 5 (labeled "Type 3" in Fig. 5) shows the cross-sectional structure and refractive index profile of optical fiber 200C taken along line II-II in Fig. 4.

[0055] The optical fiber 200A shown in the upper part (Type 1) of Fig. 5 comprises a core 210A extending along a central axis AX2 and a cladding 220A surrounding the core 210A. The refractive index profile of the optical fiber 200A is defined in this cross section by the relative refractive index difference of each portion along a line L2 passing through the central axis AX2. That is, the relative refractive index difference of the core 210A is at the pure silica level, and the relative refractive index difference Δ of the cladding 220A relative to the pure silica level is set lower than the relative refractive index difference of the core 210A. Note that although the relative refractive index difference of the cladding 220A shown in the upper part of Fig. 5 is shown as constant in the drawing, it actually varies within the cladding 220A as described above. Furthermore, the major diameter D shown in the upper part of Fig. 5 Ｆ１ and minor diameter DＦ２ The cross-sectional shape of the core 210A is defined by:

[0056] The optical fiber 200B shown in the middle (Type 2) of Figure 5 includes a core 210B extending along a central axis AX2, a first inner cladding 230B surrounding the core 210B, and a cladding 220B surrounding the first inner cladding 230B. The refractive index profile of the optical fiber 200B is defined by the relative refractive index differences of each portion along a line L2 passing through the central axis AX2 in this cross section. That is, the relative refractive index difference of the core 210B is at the pure silica level, and the relative refractive index difference Δ of the cladding 220B relative to the pure silica level is set lower than the relative refractive index difference of the core 210B. Furthermore, the relative refractive index difference of the first inner cladding 230B is set lower than the relative refractive index difference Δ of the cladding 220B. Note that although the relative refractive index difference of the cladding 220B shown in the middle of Figure 5 is shown as constant in the drawing, it actually varies within the cladding 220B, as described above.

[0057] The optical fiber 200C shown in the lower part of FIG. 5 (Type 3) includes a core 210C extending along a central axis AX2, a second inner cladding 240C surrounding the core 210C, a first inner cladding 230C surrounding the second inner cladding 240C, and a cladding 220C surrounding the first inner cladding 230C. The refractive index profile of the optical fiber 200C is defined by the relative refractive index differences of each portion along a line L2 passing through the central axis AX2 in this cross section. That is, the relative refractive index difference of the core 210C is at the pure silica level, and the relative refractive index difference Δ of the cladding 220C relative to the pure silica level is set lower than the relative refractive index difference of the core 210C. Furthermore, the relative refractive index difference of the first inner cladding 230C is set lower than the relative refractive index difference Δ of the cladding 220C. Conversely, the relative refractive index difference of the second inner cladding 240C is set to be lower than the relative refractive index difference of the core 210C, but higher than the relative refractive index difference Δ of the cladding 220C. Note that although the relative refractive index difference of the cladding 220C shown in the lower part of Figure 5 is shown as constant in the drawing, as described above, it actually varies within the cladding 220C.

[0058] 6 shows the cross-sectional structure and refractive index profile of the cladding portion of an optical fiber preform, the cladding of an optical fiber, and the cladding pipe (denoted as "Cross-sectional structure (cladding portion, cladding, cladding pipe)" in FIG. 6). The upper part of FIG. 6 (denoted as "Constituent region" in FIG. 6) shows a diagram for explaining the constituent region common to the cladding portions 120A to 120C of the optical fiber preforms 100A to 100C, the claddings 220A to 220C of the optical fibers 200A to 200C, and the cladding pipe 20. The lower part of FIG. 6 (denoted as "Refractive index profile" in FIG. 6) shows the refractive index profile along the radial direction of the cladding pipe 20 shown in the upper part of FIG. 6.

[0059] In the optical fiber preforms (optical fiber preforms 100A to 100C) disclosed herein, the halogen concentration in the innermost layer (including the inner peripheral surface) and the outermost layer (including the outer peripheral surface) of the cladding is reduced, thereby hardening the peripheral region of the cladding formed by the innermost and outermost layers. This applies compressive stress to the interface between the cladding and the adjacent glass portion. Furthermore, in the optical fiber preforms disclosed herein, the F concentration near the interface is reduced, preventing severance of Si-O bonds in the glass. This prevents deformation of the optical fiber preform and improves the dynamic fatigue coefficient Nd of the optical fiber obtained by drawing the optical fiber preform.

[0060] 6 , the thickness Wc of the clad pipe 20 is defined along the radial direction extending from the center of the through hole 21 toward the outer circumferential surface of the clad pipe 20. The innermost layer 20A of the clad pipe 20 is defined as a region that includes the inner circumferential surface of the clad pipe 20 and has a thickness that is 10% of the thickness of the clad pipe 20. The outermost layer 20B of the clad pipe 20 is defined as a region that includes the outer circumferential surface of the clad pipe 20 and has a thickness that is 10% of the thickness of the clad pipe 20.

[0061] In the cladding portion 120A of the optical fiber preform 100A of the present disclosure, the innermost layer 20A and the outermost layer 20B are also defined as regions having a thickness that is 10% of the thickness of the cladding portion 120A. This relationship in the optical fiber preform 100A is similarly defined for the optical fiber preforms 100B and 100C. Furthermore, in the cladding 220A of the optical fiber 200A of the present disclosure, the innermost layer 20A and the outermost layer 20B are also defined as regions having a thickness that is 10% of the thickness of the cladding 220A. This relationship in the optical fiber 200A is similarly defined for the optical fibers 200B and 200C.

[0062] The minimum relative refractive index difference Δ of the entire clad pipe 20 ｍｉｎ and the maximum relative refractive index difference Δ ｍａｘ The first difference between the refractive indexes of the clad pipe 20 and the refractive index of the clad pipe 20 is 0.03% or more, and the minimum relative refractive index difference Δ ｍｉｎ and the maximum relative refractive index difference Δ ｍａｘ The second difference between the refractive index difference and the refractive index difference of the cladding portion 120A of the optical fiber preform 100A of the present disclosure is 0.03% or more. ｍｉｎ and the maximum relative refractive index difference Δ ｍａｘ The first difference between the refractive index difference and the refractive index difference of the cladding portion 120A is 0.03% or more, and the minimum relative refractive index difference Δ ｍｉｎ and the maximum relative refractive index difference Δ of the outermost layer 20B ｍａｘ The second difference between the refractive index difference Δ and the refractive index difference Δ of the cladding 220A of the optical fiber 200A of the present disclosure is 0.03% or more. This relationship in the optical fiber preform 100A is similarly defined for the optical fiber preforms 100B and 100C. Furthermore, in the cladding 220A of the optical fiber 200A of the present disclosure, the minimum relative refractive index difference Δ ｍｉｎ and the maximum relative refractive index difference Δ ｍａｘ The first difference between the refractive index difference and the refractive index difference of the cladding 220A is 0.03% or more, and the minimum relative refractive index difference Δ ｍｉｎ and the maximum relative refractive index difference Δ of the outermost layer 20B ｍａｘThe second difference between the cladding pipe 20 and the cladding portion 120A is 0.03% or more. This relationship for the optical fiber 200A is similarly defined for the optical fibers 200B and 200C. The relative refractive index difference Δ of the cladding pipe 20, the cladding portions 120A to 120C, and the cladding portions 220A to 220C depends on the amount of F doping. As the amount of F doping increases, the refractive index of the F-doped region decreases, and the relative refractive index difference Δ (negative value) of each portion relative to the pure silica level decreases.

[0063] The relationship between the position and the relative refractive index difference at each part of the cladding pipe 20 (which may be the cladding portion 120A of the optical fiber preform 100A or the like, or the cladding 220A of the optical fiber 200A or the like) is defined by the straight lines SL1 to SL4 on the refractive index profile shown in the lower part of Fig. 6. That is, the straight line SL1 is a straight line defined on the refractive index profile shown in the lower part of Fig. 6, and the maximum relative refractive index difference Δ ｍａｘ and the minimum relative refractive index difference Δ ｍｉｎ The line SL2 is a line passing through the maximum relative refractive index difference Δ ｍａｘ and the minimum relative refractive index difference Δ ｍｉｎ The line SL3 is a line passing through the maximum relative refractive index difference Δ ｍａｘ and the minimum relative refractive index difference Δ in the first adjacent layer that is in direct contact with the innermost layer 20A and has a thickness that is 10% of the thickness Wc of the clad pipe 20. ｍｉｎ The line SL4 is a line passing through the maximum relative refractive index difference Δ ｍａｘ and the minimum relative refractive index difference Δ in the second adjacent layer that is in direct contact with the outermost layer 20B and has a thickness that is 10% of the thickness Wc of the clad pipe 20. ｍｉｎ It should be noted that the line passing through "ΔD ＩＮ " is the minimum relative refractive index difference Δ ｍｉｎ and the maximum relative refractive index difference Δ ｍａｘ It shows the difference between ＯＵＴ " is the minimum relative refractive index difference Δ ｍｉｎ and the maximum relative refractive index difference Δ of the outermost layer 20B ｍａｘ Shows the difference between.

[0064] The slopes of the lines SL1 to SL4 are given as absolute values, and the maximum relative refractive index difference Δ ｍａｘ and the maximum relative refractive index difference Δ ｍａｘ The distance R is calculated by converting the distance from the center of the cross section of the position where the distance is 125 μm into an optical fiber having an outer diameter of 125 μm. ｍａｘ (μm), and the minimum relative refractive index difference Δ of another region that is newly set without being affected by the presence of the specific region. ｍｉｎ and the minimum relative refractive index difference Δ ｍｉｎ The distance R is calculated by converting the distance from the center of the cross section of the position where the distance is 125 μm into an optical fiber having an outer diameter of 125 μm. ｍｉｎ (μm) and the formula: (Δ ｍａｘ -Δ ｍｉｎ ) / |R ｍａｘ -R ｍｉｎ This definition also applies to the claddings 120A to 120C of the optical fiber preforms 100A to 100C, respectively, and to the claddings 220A to 220C of the optical fibers 200A to 200C, respectively.

[0065] 7 is a graph showing the results of measuring the dynamic fatigue coefficient Nd of optical fibers obtained from optical fiber preforms using cladding pipes having various refractive index profiles. ＩＮ " is the minimum relative refractive index difference Δ ｍｉｎ (center position of thickness) and the maximum relative refractive index difference Δ ｍａｘ It shows the difference between ＯＵＴ " is the minimum relative refractive index difference Δ ｍｉｎ and the maximum relative refractive index difference Δ of the outermost layer 20B ｍａｘ 6. The "outer diameter ratio" is the ratio of the outer diameters of the core and cladding in optical fiber preforms using cladding pipes with various refractive index profiles (=outer diameter of cladding / outer diameter of core), and the "dynamic fatigue coefficient Nd" is the dynamic fatigue coefficient of optical fibers obtained from these optical fiber preforms. The "slope of line SL1," "slope of line SL2," "slope of line SL3," and "slope of line SL4" are the slopes of the lines defined as shown in the lower part of Figure 6 for cladding pipes with various refractive index profiles.

[0066] Here, the optical fiber, which is the final product, may be subjected to a relatively small stress continuously over a long period of time. Even such a relatively small stress may cause the optical fiber to suddenly break if the stress load remains for a certain period of time. It is known that improving the dynamic fatigue coefficient Nd can avoid such optical fiber breakage and extend the breakage life. The dynamic fatigue coefficient Nd is used as an index representing the growth rate of scratches on the glass fiber surface. It is believed that the larger the dynamic fatigue coefficient Nd, the slower the growth of scratches on the glass surface and the less likely the optical fiber will break over the long term. The dynamic fatigue coefficient Nd is a value measured using a tensile tester in accordance with the "Telcordia GR-20-CORE" standard, and the optical fiber disclosed herein has a dynamic fatigue coefficient Nd of 19 or greater.

[0067] As a method for improving the dynamic fatigue coefficient Nd, various studies have been conducted on coating layers to protect the glass (see Patent Documents 2 and 3). However, even when the coating conditions considered to be good in these prior documents are met, there are cases in which the dynamic fatigue coefficient Nd is not met when studying optical fibers having a core made of pure silica glass and an F-doped cladding. This is thought to be because the effects of strain on the glass surface and severance of glass bonds caused by the F added to the glass cause scratches on the glass surface, which ultimately weaken the strength of the optical fiber itself and cause a decrease in the dynamic fatigue coefficient Nd.

[0068] The preform samples used to obtain the parameters listed in the table in Figure 7 were fabricated from the following cladding pipes. Specifically, the cladding pipes had an average relative refractive index difference between -0.15% and -0.25%, and the differences between the maximum relative refractive index difference of the innermost layer and the minimum relative refractive index difference within the cladding pipe, and between the maximum relative refractive index difference of the outermost layer and the minimum relative refractive index difference within the cladding pipe, were varied to ≦0.02, 0.03, 0.05, and 0.1. Each of these cladding pipes, with a core rod containing pure silica glass inserted, was heated externally with an oxyhydrogen flame (collapse of the core rod and cladding pipe) to obtain multiple preform samples. The designed outer diameter ratio (i.e., the ratio of the cladding outer diameter to the core outer diameter) was 4 times the designed value. The measured outer diameter ratio after collapse and the dynamic fatigue coefficient of the optical fiber obtained after drawing are shown in the table in Figure 7. The heat source used in the collapsing process does not have to be an oxyhydrogen burner; it can also be a resistance furnace, induction heating furnace, or plasma heating furnace. These heat sources have a wider heating area and are more uniform around the preform than an oxyhydrogen burner, so it is believed that the resulting optical fiber preform will have less deformation. In particular, if the preform deformation during the collapsing process using an oxyhydrogen burner is within a favorable range, it can be assumed that other heat sources will also produce favorable results. Furthermore, when furnace heating is used as a heat source, it is unlikely that a reduction reaction of the surface glass bond caused by hydrogen will occur, so it can be assumed that the dynamic fatigue coefficient Nd of the resulting optical fiber will be less likely to deteriorate. Therefore, even when these heat sources are used during the collapsing process, preform deformation can be effectively prevented and a favorable dynamic fatigue coefficient Nd can be obtained, as long as it is within the favorable range achieved using an oxyhydrogen burner.

[0069] As can be seen from the table in FIG. ＩＮ and ΔD ＯＵＴIf either of these does not satisfy 0.03%, sufficient effect (appropriate value of dynamic fatigue coefficient Nd) cannot be obtained. This is thought to be due to the fact that the outermost layer comes into contact with the outside air during cooling of the optical fiber preform or optical fiber manufacturing process, resulting in rapid cooling. In other words, stress strain is easily generated on the glass surface, which is prone to strain and severance of Si-O bonds, easily affecting the deformation of the optical fiber preform and deterioration of the dynamic fatigue coefficient Nd of the optical fiber. On the other hand, stress is also easily generated in the innermost layer at the interface between the core and cladding in the preform due to the difference in glass viscosity between the core and cladding. For this reason, in order to make the glass viscosity of the core and cladding, which are made of pure silica glass, closer to that of the core, the relative refractive index difference Δ of the cladding may be increased (by reducing the amount of F added) to bring the glass viscosity of the cladding closer to that of the core. Therefore, the difference ΔD between the minimum relative refractive index difference in the cladding and the relative refractive index difference of the innermost layer located near the interface between the core and cladding ＩＮ Similarly, the difference ΔD between the minimum relative refractive index difference in the cladding portion and the maximum relative refractive index difference in the outermost layer including the outer peripheral surface of the cladding portion ＯＵＴ The difference ΔD must be 0.03% or more. ＩＮ and ΔD ＯＵＴ If the difference ΔD exceeds 0.15%, the optical fiber preform may be significantly deformed. ＩＮ and ΔD ＯＵＴ If the coefficient of elasticity exceeds 0.2%, it becomes difficult to prevent the deterioration of the dynamic fatigue coefficient Nd of the optical fiber obtained.

[0070] When the average relative refractive index difference of the cladding pipe is between -0.15% and -0.25%, both the outer diameter ratio and dynamic fatigue coefficient Nd of the optical fiber as the final product will be the same regardless of the average relative refractive index difference of the cladding pipe, as long as the difference between the maximum relative refractive index differences of the innermost and outermost layers and the minimum relative refractive index difference of the cladding is the same. On the other hand, when the average relative refractive index difference of the cladding pipe is higher than -0.15%, that is, when the F concentration of the cladding pipe as a whole is low and the glass is hard, regardless of the difference between the maximum relative refractive index differences of the innermost and outermost layers and the minimum relative refractive index difference of the cladding, the dynamic fatigue coefficient Nd of the final optical fiber, the cladding / core outer diameter ratio, and the noncircularity will not deteriorate. Furthermore, if the average relative refractive index difference of the clad pipe is lower than −0.25%, the glass viscosity of the entire clad pipe becomes low, making it difficult to prevent deformation of the base material even if the difference between the minimum relative refractive index difference of the clad pipe and the maximum relative refractive index differences of the innermost and outermost layers is greater than 0.1%.

[0071] Methods for increasing the relative refractive index difference between the outermost and innermost layers include, for example, reducing the concentration of fluorine (F), a dopant that reduces the relative refractive index difference, and adding an up-dopant such as Cl or Ge, which increases the relative refractive index difference. In this case, the amount of F added must be reduced to improve the dynamic fatigue coefficient Nd and prevent deformation of the base material (to make the glass harder and stronger). Adding an up-dopant further reduces the glass viscosity of the innermost and outermost glass layers, and accelerates the cleavage of Si—O bonds. As a result, the glass strength decreases.

[0072] The following describes the appropriate ranges for the F doping amount in the optical fiber state and the glass viscosity in the optical fiber preform. Figure 8 is a table illustrating the preferred F concentration difference for the optical fiber of the present disclosure and the preferred glass viscosity for the optical fiber preform of the present disclosure (labeled "Structural Characteristics" in Figure 8). The upper part of Figure 8 (labeled "Fiber Structure" in Figure 8) shows a table illustrating the non-circularity and dynamic fatigue coefficient Nd versus the F concentration difference (the difference between the maximum F concentration in the entire cladding and the minimum F concentration in the peripheral region including the innermost and outermost layers). The lower part of Figure 8 (labeled "Preform Structure" in Figure 8) shows a table illustrating the preform bending and non-circularity versus the viscosity difference (the difference between the minimum glass viscosity in the entire cladding and the minimum glass viscosity in the peripheral region including the innermost and outermost layers).

[0073] The F concentration difference shown in the upper part of Figure 8 (fiber structure) is defined as the difference between the maximum F concentration in the surrounding area including the innermost and outermost layers and the maximum F concentration in the entire cladding of the optical fiber or the cladding portion of the optical fiber preform, when the F concentration distributions in the innermost and outermost layers (which are 1 μm-wide regions in the state of the optical fiber and 5 mm-wide regions in the state of the optical fiber preform) are measured using an EPMA (Electron Probe Micro Analyzer).

[0074] The table shown in the upper part of Figure 8 shows the core noncircularity and dynamic fatigue coefficient Nd for samples (optical fibers) with an "F concentration difference" (the difference between the maximum F concentration in the innermost and outermost layers and the maximum F concentration in the entire cladding) of 300 ppm, 500 ppm, 700 ppm, and 1000 ppm. This table shows that when the F concentration difference is 500 ppm or more, the glass strength is sufficient and there is no deterioration in the dynamic fatigue coefficient Nd or increase in noncircularity. The noncircularity criterion was determined to be "good" when it was 1% or less.

[0075] In the lower part of Figure 8 (base material structure), the glass viscosity log １０The coefficient of viscosity (η) (unit: Pa s) is calculated from the relationship between the force and temperature when an alumina rod with a diameter of 1 mm is pressed against heated glass and penetrates the glass. This table shows the bending of the preform and the noncircularity of the core of an optical fiber preform obtained when a core rod made of pure silica glass is collapsed using an F-doped cladding pipe with a viscosity difference of 0.05, 0.1, 0.2, 0.5, 1, 2, or 5, which is the difference between the minimum glass viscosity of the peripheral region including the innermost and outermost layers and the minimum glass viscosity of the entire cladding.

[0076] As can be seen from the table shown in the lower part of Figure 8, when the viscosity difference is less than 0.1, if the core rod and clad pipe are heated at a temperature sufficient to prevent the formation of bubbles of 0.3 mm or less at the interface between the core and clad of the resulting optical fiber preform, the surface of the clad pipe will sag, and the entire core rod will also sag, resulting in the noncircularity of the core becoming higher (worsening) than 1%.

[0077] On the other hand, when the viscosity difference is greater than 1 (when sufficient heat is applied to melt and integrate the core rod and clad pipe), the surface of the clad pipe becomes soft, and the viscosity of the entire clad pipe drops too much before it can be integrated with the core rod. Therefore, in this case too, the melt-integrated optical fiber preform sags at the center of its body, causing the preform bending to worsen, and the gap length BD becomes greater than 1 mm.

[0078] In the collapse process, a core rod containing pure silica glass is inserted into an F-doped cladding pipe and heated and integrated while reducing the pressure inside the through-hole of the cladding pipe. When the minimum glass viscosity of each of the innermost and outermost layers is 0.1 or more higher than the minimum glass viscosity of the entire cladding pipe, the core noncircularity of the optical fiber preform obtained by collapse (heat integration) was 1% or less. Furthermore, the bending of the optical fiber preform after collapse was 1 mm or less (gap length BD was 1 mm or less). The optical fiber obtained by drawing this optical fiber preform also had good fiber bending (see the lower panel of Figure 4) and PMD. As shown in the table in the lower panel of Figure 8, the viscosity difference may be 0.1 or more and 1 or less.

[0079] Next, various examples of methods for manufacturing a clad pipe 20 for obtaining the optical fiber preforms 100A to 100C and the optical fibers 200A to 200C of the present disclosure will be described. In this clad pipe 20, a first difference between the minimum relative refractive index difference of the entire clad pipe 20 and the maximum relative refractive index difference of the innermost layer 20A is set to 0.03% or more, and a second difference between the minimum relative refractive index difference of the entire clad pipe 20 and the maximum relative refractive index difference of the outermost layer 20B is set to 0.03% or more. Note that Fig. 9 is a diagram illustrating a first clad pipe manufacturing method (OVD method) applicable to the manufacturing of the optical fiber preform of the present disclosure (denoted "Manufacturing of clad pipe (OVD method)" in Fig. 9). Fig. 10 is a diagram illustrating a second clad pipe manufacturing method (VAD method) applicable to the manufacturing of the optical fiber preform of the present disclosure (denoted "Manufacturing of clad pipe (VAD method)" in Fig. 10). FIG. 11 is a diagram for explaining various fluorine addition methods as a third and fourth manufacturing method of a cladding pipe applicable to the manufacturing of the optical fiber preform of the present disclosure (in FIG. 11, marked as "Variations of F addition").

[0080] First, the upper part of Fig. 9 (labeled "glass synthesis" in Fig. 9) shows a glass synthesis process using an outside vapor deposition (OVD) method. The middle part of Fig. 9 (labeled "soot body" in Fig. 9) shows a soot body 900A obtained by the glass synthesis process shown in the upper part of Fig. 9. The lower part of Fig. 9 (labeled "sintering" in Fig. 9) shows an apparatus for performing a sintering process of the soot body 900A shown in the middle part of Fig. 9 and a clad pipe 20 obtained by this sintering process.

[0081] As shown in the upper part of Figure 9, glass particles are deposited on the outer periphery of the central rod 800A, which rotates in the direction indicated by arrow S91 (glass synthesis). The central rod 800A contains glass, SiC, alumina, etc. At this time, silicon tetrachloride, a raw material for glass, is hydrolyzed in the flame of an oxyhydrogen burner 810A (which can move along the central rod 800A), synthesizing glass particles. The central rod 800A is then withdrawn, yielding a soot body 900A (porous glass body), which is an aggregate of glass particles, as shown in the middle part of Figure 9. Note that, since the central rod 800A has been withdrawn, a through-hole 910 is formed in the soot body 900A, and this through-hole 910 serves as an insertion space for the core rod 10.

[0082] Next, CF is used as a source of fluorine (F) to be added to the clad pipe 20. ４ , SiF ４ , SF ６ The soot body 900A is heated and sintered at 1250°C or higher while flowing an F-containing gas such as fluorine (F). As a result, a clad pipe 20 doped with fluorine (F) is obtained, as shown in the lower part of Fig. 9. The sintering apparatus shown in Fig. 9 includes a heating furnace 820, a flow rate control device 830 for controlling the flow rate of the F-containing gas supplied into the heating furnace 820, and a heater 840.

[0083] There is a negative correlation between the bulk density of the glass particles and the F doping concentration. Therefore, in the heat sintering of the soot body 900A, the soot body 900A is fabricated so that the bulk density of each of the inner region (glass particle deposition layer near the central rod 800A) and the outer region (glass particle deposition layer at the outer peripheral portion of the soot body 900A) of the soot body 900A is higher than that of the glass particle deposition layer at other radial positions, and sintering is performed while adding fluorine (F), thereby obtaining an F-doped clad pipe 20 with a desired refractive index distribution. As a method for partially increasing the bulk density of the soot body 900A, in the glass synthesis shown in the upper part of FIG. 9, the ratio of glass raw material to hydrogen and oxygen in the oxyhydrogen burner 810A can be changed to increase the flame temperature at the timing of depositing soot in the vicinity where the bulk density is desired to be increased, thereby partially increasing the bulk density. At this time, the oxyhydrogen burner 810B including an auxiliary burner for adjusting the bulk density may be used. The amount of change in bulk density for the purpose of changing the relative refractive index difference in this way can be set so that the bulk density of the portion where the relative refractive index difference is desired to be increased is 0.03 g / cm ３ It is desirable to make it higher than this.

[0084] Furthermore, in the above-described manufacturing method using the OVD method, fluorine (F) is added to the pipe-shaped soot body 900A. However, it is also possible to obtain a clad pipe 20 with a partially increased relative refractive index difference Δ by using a soot body that is not pipe-shaped but is filled to the center. Figure 10 is a diagram for explaining a second manufacturing method (VAD method) for the clad pipe 20 applicable to the optical fiber preform of the present disclosure (labeled "Manufacturing of Clad Pipe (VAD Method)" in Figure 10). The top row of Figure 10 (labeled "Glass Synthesis" in Figure 10) shows a glass synthesis process using the VAD (Vapor-Phase Axial Deposition) method. The second row of Figure 10 (labeled "Soot Body" in Figure 10) shows a soot body 900B obtained by the glass synthesis process shown in the top row of Figure 10. The third row of Fig. 10 (marked "Sintering" in Fig. 10) shows the sintering step of obtaining a transparent glass body 900C from the soot body 900B shown in the second row of Fig. 10. The bottom row of Fig. 10 (marked "Processing" in Fig. 10) shows the clad pipe 20 obtained by processing the transparent glass body 900C obtained by the third sintering step in Fig. 10.

[0085] In this VAD manufacturing method, glass particles synthesized in the flame of an oxyhydrogen burner 810A are sprayed onto a starting rod 800B, as shown in the top row of Fig. 10. The starting rod 800B is rotated in the direction indicated by arrow S101 and pulled up in the direction indicated by arrow S102, thereby producing a solid soot body 900B, as shown in the second row of Fig. 10. Note that the oxyhydrogen burner 810B shown in the top row of Fig. 10 may be applied to partially change the bulk density of the soot body 900B.

[0086] The resulting soot body 900B is then placed in a sintering apparatus shown in the third row of Fig. 10 (having the same structure as the sintering apparatus shown in the bottom row of Fig. 9) and sintered to convert it into a solid transparent glass body 900C. Both ends of this transparent glass body 900C are cut and through-holes are formed, thereby obtaining a clad pipe 20 having through-holes 21, as shown in the bottom row of Fig. 10.

[0087] 11 is a diagram illustrating a third and fourth manufacturing method of a cladding pipe applicable to the manufacturing of an optical fiber preform according to the present disclosure (denoted as "F-doping variation" in FIG. 11). The upper part of FIG. 11 (denoted as "first variation" in FIG. 11) shows a part of the manufacturing process of a cladding pipe 20 to explain a variation of F-doping by adjusting the flow rate of an F-containing gas. The lower part of FIG. 11 (denoted as "second variation" in FIG. 11) shows a part of the manufacturing process of a cladding pipe 20 to explain a variation of F-doping using a transparent glass body uniformly doped with F.

[0088] In a first modification shown in the upper part of FIG. 11 , a soot body 900A manufactured by the OVD method (soot body 900A having a through-hole 910 shown in the middle part of FIG. 9 ) is prepared, and this soot body 900A is placed in the sintering apparatus shown in the lower part of FIG. 9 and the third part of FIG. 10 . As described above, this sintering apparatus includes a heating furnace 820, a flow control device 830, and a heater 840. Even if the prepared soot body 900A has no radial variation in bulk density, in the process of sintering while flowing an F-containing gas supplied from the flow control device 830, fluorine (F) is first impregnated into the entire soot 900A at a low temperature of 1250°C or less, at which the glass particles do not become transparent glass, and then the temperature is raised to a temperature at which the soot body 900A becomes a transparent glass body while the concentration of the F-containing gas is reduced to 60% or less compared to the concentration at the time of impregnation. Note that sintering may be performed without flowing fluorine (F). At this time, it is desirable to raise the temperature to the temperature for impregnating fluorine (F) and the sintering temperature at a rate faster than 0.2°C / min and slower than 5°C / min. If the heating rate is slow, the fluorine (F) will escape from the entire soot body 900A, the refractive index profile of the clad pipe 20 will become flat, and the desired refractive index profile will not be formed. If the heating rate is fast, the glass will solidify before the fluorine (F) escapes, preventing the fluorine (F) from escaping, and the desired refractive index profile will not be formed.

[0089] Next, in a second modified example shown in the lower part of Fig. 11 , a clad pipe 20 is manufactured by adjusting the F content of a uniformly F-doped glass pipe 900D (having a through hole 920). In this second modified example, the uniformly F-doped glass pipe 900D is placed in a heating furnace 850 and continuously heated to a temperature close to the sintering temperature while being evacuated under vacuum. This allows the fluorine (F) once added to escape from the glass pipe 900D. By utilizing this principle, fluorine (F) escapes only from the inner and outer circumferential surfaces of the glass pipe 900D that define the through hole 920, which are in contact with the vacuum space, thereby obtaining a desired clad pipe 20. The apparatus used for this fluorine removal includes a heating furnace 850 capable of vacuum evacuation and a heater 860 that heats the glass pipe 900D to remove fluorine (F) from the surface of the glass pipe 900D.

[0090] DESCRIPTION OF SYMBOLS 10...Core rod 20...Clad pipe 20A...Innermost layer 20B...Outermost layer 21...Through hole 30...First inner clad pipe 31...Through hole 40...Second inner clad pipe 41...Through hole 100A to 100C...Optical fiber preform 110A to 110C...Core portion 120A to 120C...Clad portion 130B, 130C...First inner clad portion 140C...Second inner clad portion 200A to 200C...Optical fiber 210A to 200C...Core 220A to 220C...Clad 230B, 230C...First inner clad 240C...Second inner clad 300...Exhaust pipe 400...Oxyhydrogen burner 510...Heater 610...Drum 620...Roller 700...Rotational drive device 800A...Center rod 800B... Starting rod 810A, 810B... Oxyhydrogen burner 820, 850... Heating furnace 830... Flow rate control device 840, 860... Heater 900A, 900B... Soot body 900C... Transparent glass body 900D... Glass pipe 910, 920... Through hole AX1, AX2... Center axis BL... Base material length BD... Gap length C1, C2... Cutting position D Ｐ１ , D Ｆ１ …Long diameter D Ｐ２ , D Ｆ２...minor diameter FL...optical fiber length L1, L2...lines S11, S21, S41, S42, S43, S91, S101, S102...direction SL1 to SL4...straight line Wc...thickness

Claims

1. A core part that should become a core after drawing, the core part extending along a central axis and having pure silica glass, and a clad part that should become a clad after drawing, the clad part surrounding the core part and containing fluorine. In the cross-section of the clad part orthogonal to the central axis, the thickness of the clad part is defined along a radial direction extending from the central axis toward the outer peripheral surface of the clad part. The innermost layer of the clad part is defined as a region including the inner peripheral surface of the clad part and having a thickness of 10% of the thickness of the clad part. The outermost layer of the clad part is defined as a region including the outer peripheral surface of the clad part and having a thickness of 10% of the thickness of the clad part. A first difference between the minimum relative refractive index difference as seen in the whole clad part and the maximum relative refractive index difference of the innermost layer is 0.03% or more, and a second difference between the minimum relative refractive index difference as seen in the whole clad part and the maximum relative refractive index difference of the outermost layer is 0.03% or more. An optical fiber preform.

2. Both the first difference and the second difference are 0.15% or less. The optical fiber preform according to claim 1.

3. The maximum fluorine concentration of the innermost layer and the maximum fluorine concentration of the outermost layer are both lower than the maximum fluorine concentration as seen in the whole clad part. The difference between the maximum fluorine concentration of the innermost layer and the maximum fluorine concentration as seen in the whole clad part is 500 ppm or more, and the difference between the maximum fluorine concentration of the outermost layer and the maximum fluorine concentration as seen in the whole clad part is 500 ppm or more. The optical fiber preform according to claim 1 or claim 2.

4. Regarding the glass viscosity log １０ (η) (unit: Pa·s), the difference between the lowest glass viscosity of the innermost layer and the lowest glass viscosity of the clad portion as a whole is 0.1 or more, and the difference between the lowest glass viscosity of the outermost layer and the lowest glass viscosity of the clad portion as a whole is 0.1 or more. The optical fiber preform according to any one of claims 1 to 3.

5. The bend of the optical fiber preform is 0 mm or more and 1 mm or less. The optical fiber preform according to any one of claims 1 to 4.

6. Having an inner clad part provided between the core part and the clad part. The optical fiber preform according to any one of claims 1 to 5.

7. The inner clad part has two or more concentrically arranged regions, and the two or more layers include a layer having a relative refractive index difference smaller than the minimum relative refractive index difference of the clad part. The optical fiber preform according to claim 6.

8. An optical fiber comprising: a core extending along a central axis and having pure silica glass; and a cladding portion surrounding the core and containing fluorine. In a cross-section of the cladding orthogonal to the central axis, the thickness of the cladding is defined along a radial direction extending from the central axis toward the outer peripheral surface of the cladding. The innermost layer of the cladding is defined as a region including the inner peripheral surface of the cladding and having a thickness of 10% of the thickness of the cladding. The outermost layer of the cladding is defined as a region including the outer peripheral surface of the cladding and having a thickness of 10% of the thickness of the cladding. A first difference between the minimum relative refractive index difference of the entire cladding and the maximum relative refractive index difference of the innermost layer is 0.03% or more, and a second difference between the minimum relative refractive index difference of the entire cladding and the maximum relative refractive index difference of the outermost layer is 0.03% or more.

9. The optical fiber according to claim 8, wherein both the first difference and the second difference are 0.15% or less.

10. The optical fiber according to claim 8 or 9, having a dynamic fatigue coefficient Nd of 19 or more.

11. The optical fiber according to any one of claims 8 to 10, having an inner cladding layer provided between the core and the cladding.

12. The optical fiber according to claim 11, wherein the inner cladding layer has two or more layers arranged concentrically, and the two or more layers include a layer having a relative refractive index difference smaller than the minimum relative refractive index difference of the cladding.

13. A core rod to be a core portion of an optical fiber preform, the core rod extending along a central axis and having pure silica glass, and a clad pipe to be a clad portion of the optical fiber preform, the clad pipe having a through hole for accommodating the core rod and containing fluorine. A preparation step of preparing the core rod and the clad pipe, an insertion step of inserting the core rod into the through hole of the clad pipe, and a collapse step of integrating the core rod and the clad pipe with the core rod inserted into the through hole of the clad pipe. In the cross section of the clad pipe perpendicular to the extending direction of the through hole, the thickness of the clad pipe is defined along the radial direction extending from the center of the through hole toward the outer peripheral surface of the clad pipe. The innermost layer of the clad pipe is defined as a region including the inner peripheral surface of the clad pipe and having a thickness of 10% of the thickness of the clad pipe. The outermost layer of the clad pipe is defined as a region including the outer peripheral surface of the clad pipe and having a thickness of 10% of the thickness of the clad pipe. A first difference between the minimum relative refractive index difference of the entire clad pipe and the maximum relative refractive index difference of the innermost layer is 0.03% or more, and a second difference between the minimum relative refractive index difference of the entire clad pipe and the maximum relative refractive index difference of the outermost layer is 0.03% or more. A method for manufacturing an optical fiber preform.

14. In the preparation step, an inner clad pipe having a through hole for accommodating the core rod and inserted between the core rod and the inner wall of the through hole of the clad pipe is prepared. In the insertion step, the inner clad pipe is inserted between the core rod and the inner wall of the through hole of the clad pipe. In the collapse step, the core rod, the inner clad pipe, and the clad pipe are integrated. The method for manufacturing an optical fiber preform according to claim 13.

15. The inner clad pipe is formed of two or more pipes, and the two or more pipes include a pipe having a relative refractive index difference smaller than the minimum relative refractive index difference of the clad pipe. The method for manufacturing an optical fiber preform according to claim 14.

16. The manufacturing method of an optical fiber preform according to any one of claims 13 to 15, wherein both the first difference and the second difference are 0.15% or less.

17. The manufacturing method of an optical fiber preform according to any one of claims 13 to 15, wherein the maximum fluorine concentration of the innermost layer and the maximum fluorine concentration of the outermost layer are both lower than the maximum fluorine concentration as seen over the entire clad portion, the difference between the maximum fluorine concentration of the innermost layer and the maximum fluorine concentration as seen over the entire clad portion is 500 ppm or more, and the difference between the maximum fluorine concentration of the outermost layer and the maximum fluorine concentration as seen over the entire clad portion is 500 ppm or more.

18. Regarding the glass viscosity log １０ １０ (η) (unit: Pa·s), the difference between the lowest glass viscosity of the innermost layer and the lowest glass viscosity of the clad portion as a whole is 0.1 or more, and the difference between the lowest glass viscosity of the outermost layer and the lowest glass viscosity of the clad portion as a whole is 0.1 or more. The method for manufacturing an optical fiber preform according to any one of claims 13 to 17.

19. An optical fiber preform obtained by the manufacturing method of an optical fiber preform according to any one of claims 13 to 18, the optical fiber preform having a bend of 0 mm or more and 1 mm or less.

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