Optical fiber

Abstract

An optical fiber (10) comprises a glass fiber (13) including a core (11) and a cladding (12). The cladding (12) includes an inner cladding (121) covering an outer circumference of the core (11), a trench (122) covering an outer circumference of an inner cladding (121), and an outer cladding (123) covering an outer circumference of the trench (122). The absolute value of the inclination of an interface region between the core (11) and the inner cladding (121) is 0.15 % / µm to 2.00 % / µm. The absolute value of the inclination of an interface region between the inner cladding (121) and the trench (122) is 0.15 % / μm to 2.00 % / μm. The mode field diameter with respect to light having a wavelength of 1310 nm is 8.8 μm to 9.6 μm.

Description

optical fiber

[0001] This application claims priority to Japanese Patent Application No. 2024-108717, filed on July 5, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] Patent Documents 1 to 3 disclose optical fibers. The optical fiber described in Patent Document 1 comprises a glass fiber including a core and a cladding. The cladding includes an inner cladding covering the outer periphery of the core, a trench covering the outer periphery of the inner cladding, and an outer cladding covering the outer periphery of the trench. The refractive index of the inner cladding is lower than that of the core. The refractive index of the trench is lower than that of the inner cladding. The refractive index of the outer cladding is higher than that of the trench and lower than that of the core. Patent Document 2 describes an optical fiber comprising a centrally disposed core, a first cladding layer disposed around the core, a second cladding layer disposed around the first cladding layer, and a third cladding layer disposed around the second cladding layer. The maximum refractive index of the core is higher than that of the first cladding layer, the second cladding layer, and the third cladding layer. The maximum refractive index of the second cladding layer is lower than that of the first cladding layer and the third cladding layer.

[0003] International Publication No. WO 2023 / 032999 International Publication No. WO 2004 / 092794 International Publication No. WO 2000 / 026709

[0004] An optical fiber according to one aspect of the present disclosure comprises a glass fiber including a core and a cladding. The cladding includes an inner cladding covering the outer periphery of the core, a trench covering the outer periphery of the inner cladding, and an outer cladding covering the outer periphery of the trench. The refractive index of the inner cladding is lower than that of the core. The refractive index of the trench is lower than that of the inner cladding. The refractive index of the outer cladding is higher than that of the trench and lower than that of the core. The absolute value of the slope of the interface region between the core and the inner cladding is 0.15% / μm or more and 2.00% / μm or less. The absolute value of the slope of the interface region between the inner cladding and the trench is 0.15% / μm or more and 2.00% / μm or less. The mode field diameter for light with a wavelength of 1310 nm is 8.8 μm or more and 9.6 μm or less. When wound around a mandrel with a diameter of 15 mm, the bending loss for light with a wavelength of 1625 nm is 1.0 dB or less per turn. When wound around a mandrel with a diameter of 30 mm, the bending loss for light with a wavelength of 1625 nm is 0.1 dB or less per 10 turns. The zero dispersion wavelength is 1300 nm or more and 1324 nm or less. The zero dispersion slope is 0.073 ps / (nm ２ ・km) or more 0.092 ps / (nm ２ The cable cutoff wavelength is 1260 nm or less.

[0005] Fig. 1 is a diagram showing a cross section perpendicular to the axial direction of an optical fiber 10 according to an embodiment. Fig. 2 is a diagram showing the refractive index profile in the radial direction of a glass fiber 13. Fig. 3 is a diagram showing in detail a portion of the refractive index profile shown in Fig. 2.

[0006] [Problem to be Solved by the Present Disclosure] The optical fibers described in Patent Documents 1 and 2 have a core, an inner cladding, a trench, and an outer cladding. In such optical fibers, the refractive index does not typically change discontinuously at the interface region between the core and the inner cladding, but has a gradient in the radial refractive index profile. According to the inventor's findings, the absolute value of the gradient of the change in the relative refractive index difference in the radial direction at the interface region between the core and the inner cladding is greater than 2.00% / μm. Thus, when the refractive index profile in the radial direction at the interface region between the core and the inner cladding is gentle, chromatic dispersion due to the structure increases. Accordingly, the zero-dispersion wavelength shifts to the shorter wavelength side, and the zero-dispersion slope tends to increase. This narrows the allowable range of the core diameter design value and manufacturing error required to achieve desired optical fiber characteristics, such as bending loss and zero-dispersion slope. This leads to problems such as difficulty in optical fiber design and reduced manufacturing yield.

[0007] An object of the present disclosure is to provide an optical fiber that can expand the design value of the core diameter and the tolerance range for manufacturing errors.

[0008] Effect of the Present Disclosure According to the present disclosure, it is possible to provide an optical fiber that can expand the design value of the core diameter and the tolerance range for manufacturing errors.

[0009] [Description of Embodiments of the Present Disclosure] First, the contents of embodiments of the present disclosure will be described. [1] An optical fiber according to one aspect of the present disclosure comprises a glass fiber including a core and a cladding. The cladding includes an inner cladding covering the outer periphery of the core, a trench covering the outer periphery of the inner cladding, and an outer cladding covering the outer periphery of the trench. The refractive index of the inner cladding is lower than the refractive index of the core. The refractive index of the trench is lower than the refractive index of the inner cladding. The refractive index of the outer cladding is higher than the refractive index of the trench and lower than the refractive index of the core. The absolute value of the gradient of the change in the relative refractive index difference in the radial direction at the interface region between the core and the inner cladding is 0.15% / μm or more and 2.00% / μm or less. The absolute value of the gradient of the change in the relative refractive index difference in the radial direction at the interface region between the inner cladding and the trench is 0.15% / μm or more and 2.00% / μm or less. The mode field diameter for light with a wavelength of 1310 nm is 8.8 μm or more and 9.6 μm or less. When wound around a mandrel with a diameter of 15 mm, the bending loss for light with a wavelength of 1625 nm is 1.0 dB or less per turn. When wound around a mandrel with a diameter of 30 mm, the bending loss for light with a wavelength of 1625 nm is 0.1 dB or less per 10 turns. The zero dispersion wavelength is 1300 nm or more and 1324 nm or less. The zero dispersion slope is 0.073 ps / (nm ２ ・km) or more 0.092 ps / (nm ２ The cable cutoff wavelength is 1260 nm or less.

[0010] In the optical fiber of [1] above, the absolute value of the gradient of the change in the relative refractive index difference in the radial direction at the interface region between the core and the inner cladding and at the interface region between the inner cladding and the trench is 0.15% / μm or more. Because the gradient of the change in the relative refractive index difference in the radial direction at the interface region between the core and the inner cladding and at the interface region between the inner cladding and the trench is thus steep, an increase in chromatic dispersion due to the structure is avoided. Accordingly, the shift of the zero-dispersion wavelength to the shorter wavelength side is reduced, and the tendency for the zero-dispersion slope to increase can be avoided. Therefore, the design value of the core diameter and the tolerance for manufacturing error can be expanded to achieve the desired characteristics of the optical fiber, such as bending loss and zero-dispersion slope.

[0011] Additionally, in the optical fiber [1] described above, the absolute value of the gradient of the change in the relative refractive index difference in the radial direction in the interface region between the core and the inner cladding and in the interface region between the inner cladding and the trench is 2.00% / μm or less. Thus, the gradient of the change in the relative refractive index difference in the radial direction in the interface region between the core and the inner cladding and in the interface region between the inner cladding and the trench is not too steep. This reduces the stress difference that occurs when the soot is made transparent during optical fiber manufacturing. Therefore, a decrease in yield due to cracking of the preform can be avoided.

[0012] [2] In the optical fiber according to [1] above, the absolute value of the gradient of the change in the relative refractive index difference in the radial direction in the inner cladding may be 0.0009% / μm or more and 0.0100% / μm or less. Furthermore, the ratio (|s1| / |s2|) of the absolute value of the gradient s1 of the change in the relative refractive index difference in the radial direction at the interface region between the core and the inner cladding to the absolute value of the gradient s2 of the change in the relative refractive index difference in the radial direction in the inner cladding may be 15 or more. When the absolute value of the gradient of the change in the relative refractive index difference in the radial direction in the inner cladding is 0.0100% / μm or less, the bending loss for light with a wavelength of 1625 nm when wound around a mandrel having a diameter of 30 mm can be further reduced. Additionally, when the absolute value of the gradient of the change in the relative refractive index difference in the radial direction in the inner cladding is 0.0009% / μm or more, precise refractive index adjustment by co-doping with germanium (Ge) and fluorine (F) is not required. Therefore, it is possible to avoid an increase in the difficulty of manufacturing the optical fiber. Furthermore, it is possible to avoid deterioration in transmission loss and chromatic dispersion. In addition, since the ratio (|s1| / |s2|) is 15 or more, higher-order modes are more likely to escape, which makes it easier to reduce bending loss. This makes it possible to simultaneously improve the zero-dispersion slope and the bending loss for light with a wavelength of 1625 nm when wound around a mandrel with a diameter of 30 mm.

[0013] In the optical fiber of [1] or [2] above, the core may be doped with germanium and the trench may be doped with fluorine. When the core is doped with germanium, the gradient of the change in the relative refractive index difference in the radial direction at the interface region between the core and the inner cladding tends to be gentle. By providing such an optical fiber with the configuration of [1] or [2] above, the design value of the core diameter and the tolerance for manufacturing errors can be expanded, thereby effectively increasing the yield of the optical fiber.

[0014] [Details of the Embodiments of the Present Disclosure] Specific examples of the present embodiment will be described with reference to the drawings as necessary. 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 of the claims. In the following description, the same elements in the description of the drawings will be given the same reference numerals, and redundant explanations will be omitted. In the following description, the "outer diameter" of a certain element refers to the average value of the outer diameter of the element at each position in the axial direction of the optical fiber.

[0015] 1 is a diagram showing a cross section perpendicular to the axial direction of an optical fiber 10 according to one embodiment. The optical fiber 10 is a so-called optical fiber core and complies with at least one of the ITU-T G.652 standard and the ITU-T G.657 standard. Compliant with the ITU-T G.652 standard means compliance with at least one of G.652.A, G.652.B, G.652.C, and G.652.D. Compliant with the ITU-T G.657 standard means compliance with at least one of G.657.A and G.657.B. The optical fiber 10 includes a glass fiber 13 and a coating resin layer 16. The glass fiber 13 includes a core 11 and a cladding 12. The coating resin layer 16 includes a primary resin layer 14 and a secondary resin layer 15 provided around the outer periphery of the glass fiber 13.

[0016] The cladding 12 surrounds the core 11. The core 11 and the cladding 12 mainly contain glass such as silica glass. The core 11 is made of a material in which pure silica glass is doped with germanium (Ge). Here, pure silica glass refers to glass that contains substantially no impurities. The outer diameter of the glass fiber 13, i.e., the outer diameter of the cladding 12, is 124.5 μm or more and 125.5 μm or less.

[0017] The cladding 12 includes an inner cladding 121, a trench 122, and an outer cladding 123. The inner cladding 121 covers the outer periphery of the core 11 and is in contact with the outer peripheral surface of the core 11. The refractive index of the inner cladding 121 is smaller than that of the core 11. The trench 122 covers the outer periphery of the inner cladding 121 and is in contact with the outer peripheral surface of the inner cladding 121. The refractive index of the trench 122 is smaller than that of the inner cladding 121. The outer cladding 123 covers the outer periphery of the trench 122 and is in contact with the outer peripheral surface of the trench 122. The refractive index of the outer cladding 123 is smaller than that of the core 11 and larger than that of the trench 122. The inner cladding 121 can be made of silica glass doped with chlorine (Cl). The trench 122 can be made of silica glass doped with fluorine. The outer cladding 123 can be made of pure silica glass.

[0018] 1 , the radius of the outer periphery of the core 11 is r1, the radius of the outer periphery of the inner cladding 121 is r2, the radius of the outer periphery of the trench 122 is r3, and the radius of the outer periphery of the outer cladding 123 is r4. The radius r1 of the core 11 is, for example, 3.0 μm or more and 6.0 μm or less. The value (r2 / r1) obtained by dividing the radius r2 of the inner cladding 121 by the radius r1 of the core 11 is 2.2 or more and 3.6 or less. The value (r3−r2) obtained by subtracting the radius r2 of the inner cladding 121 from the radius r3 of the trench 122 is, for example, 3 μm or more and 10 μm or less.

[0019] FIG. 2 is a diagram showing the refractive index distribution in the radial direction of the glass fiber 13. In FIG. 2, range E1 corresponds to the core 11, range E2 corresponds to the inner cladding 121, range E3 corresponds to the trench 122, and range E4 corresponds to the outer cladding 123. The vertical axis indicates the relative refractive index difference, and the horizontal axis indicates the radial position. As shown in FIG. 2, in the glass fiber 13, the relative refractive index differences of the core 11, inner cladding 121, trench 122, and outer cladding 123 with respect to the refractive index of pure silica glass are defined as Δ1, Δ2, Δ3, and Δ4, respectively. Specifically, the relative refractive index differences Δ1, Δ2, Δ3, and Δ4 are defined by the following formula: Δ1(%)=((n1 ２ -n0 ２ ) / (2 × n1 ２ ))×100 Δ2(%)=((n2 ２ -n0 ２ ) / (2 × n2 ２ ))×100 Δ3(%)=((n3 ２ -n0 ２ ) / (2 × n3 ２ ))×100 Δ4(%)=((n4 ２ -n0 ２ ) / (2 × n4 ２ )) × 100, where n0 is the refractive index of pure silica glass. n1 is the maximum refractive index of the core 11. n2 is the average refractive index in the range from 0.6 to 0.8 times the thickness of the inner cladding 121, i.e., the radius ranges from r1 + 0.6 × (r2 - r1) to r1 + 0.8 × (r2 - r1). n3 is the minimum refractive index of the trench 122. n4 is the average refractive index of the outer cladding 123. n2 is defined as above because the refractive index of the inner cladding 121 continuously decreases with increasing distance from the core 11 and approaching the trench 122. In the following description, the relative refractive index difference is evaluated using a refractive index distribution measurement device (IFA-100 manufactured by Interfiber Analysis) with a measurement interval of 0.2 μm or less.

[0020] Figure 3 is a detailed diagram of a portion of the refractive index profile shown in Figure 2. The core 11 includes an interface region A adjacent to the inner cladding 121. A range E11 shown in Figure 3 corresponds to the interface region A between the core 11 and the inner cladding 121. The trench 122 includes an interface region B adjacent to the inner cladding 121. A range E31 shown in Figure 3 corresponds to the interface region B between the trench 122 and the inner cladding 121.

[0021] In the interface region A, the relative refractive index difference continuously decreases from Δ1 to Δ2 as the distance to the inner cladding 121 increases. Here, the gradient s1 of the change in the relative refractive index difference in the radial direction in the interface region A is defined as follows: That is, the target region is the region where the difference (Δ1-Δ2) between the relative refractive index difference Δ1 of the core 11 and the relative refractive index difference Δ2 of the inner cladding 121 changes from 0.7 to 0.3 times. In other words, the target region is the range from 70% to 30% when the difference (Δ1-Δ2) between the relative refractive index difference Δ1 of the core 11 and the relative refractive index difference Δ2 of the inner cladding 121 is taken as 100%, i.e., the range where the relative refractive index difference is from 0.7 × (Δ1-Δ2) + Δ2 to 0.3 × (Δ1-Δ2) + Δ2. The gradient s1 of the change in the relative refractive index difference in the interface region A is defined as the gradient of a linear function obtained by least-squares approximation of the change in the relative refractive index difference in the target region. This definition is based on actual measurements on a variety of optical fibers.

[0022] In the interface region B, the relative refractive index difference continuously decreases from Δ2 to Δ3 with increasing distance from the inner cladding 121. The gradient s3 of the change in the relative refractive index difference in the radial direction in the interface region B is defined as follows, similar to the gradient s1 in the interface region A. That is, the target region is the region where the difference (Δ2 - Δ3) between the relative refractive index difference Δ2 of the inner cladding 121 and the relative refractive index difference Δ3 of the trench 122 changes from 0.7 to 0.3 times. In other words, the target region is within a range from 70% to 30% when the difference (Δ2 - Δ3) between the relative refractive index difference Δ2 of the inner cladding 121 and the relative refractive index difference Δ3 of the trench 122 is taken as 100%. That is, the target region is within a range where the relative refractive index difference is from 0.7 × (Δ2 - Δ3) + Δ3 to 0.3 × (Δ2 - Δ3) + Δ3. The gradient s3 of the change in the relative refractive index difference in the interface region B is defined as the gradient of a linear function obtained by least-squares approximation of the change in the relative refractive index difference in the target region. This definition is also based on actual measurement results for various optical fibers.

[0023] In this embodiment, the absolute value of the gradient s1 of the change in the relative refractive index difference in the radial direction in the interface region A is 0.15% / μm or more, 0.20% / μm or more, or 0.25% / μm or more. The absolute value of the gradient s1 is 1.00% / μm or less, 1.50% / μm or less, or 2.00% / μm or less. The absolute value of the gradient s3 of the change in the relative refractive index difference in the radial direction in the interface region B is 0.15% / μm or more, 0.20% / μm or more, or 0.25% / μm or more. The absolute value of the gradient s3 is 1.00% / μm or less, 1.50% / μm or less, or 2.00% / μm or less.

[0024] As described above, the refractive index of the inner cladding 121 continuously decreases with increasing distance from the core 11 and increasing distance from the trench 122. Therefore, the slope s2 of the relative refractive index difference of the inner cladding 121 is defined as follows. Specifically, the slope s2 is defined as the slope of a linear function obtained by least-squares approximation of the change in the relative refractive index difference in the radial direction within a range from 0.6 to 0.8 times the thickness of the inner cladding 121. In other words, the slope s2 is defined as the slope of a linear function obtained by least-squares approximation of the change in the relative refractive index difference in the radial direction within a radius range from r1 + 0.6 × (r2 - r1) to r1 + 0.8 × (r2 - r1). In this embodiment, the absolute value of the slope s2 of the change in the relative refractive index difference in the radial direction of the inner cladding 121 is 0.0009% / μm or more, 0.0012% / μm or more, or 0.0015% / μm or more. The absolute value of the slope s2 is 0.0060% / μm or less, 0.0080% / μm or less, or 0.0100% / μm or less. The slope of the relative refractive index difference in the inner cladding 121 can also be expressed as the flatness of the relative refractive index difference in the inner cladding 121.

[0025] Furthermore, in this embodiment, a gradient s1 of the change in the relative refractive index difference in the radial direction in the interface region A and a gradient s2 of the change in the relative refractive index difference in the radial direction in the inner cladding 121 are defined. In this case, the ratio (|s1| / |s2|) of the absolute value of the gradient s1 to the absolute value of the gradient s2 is 15 or more, 30 or more, 50 or more, or 100 or more. The ratio (|s1| / |s2|) is 500 or less, 1000 or less, or 2000 or less.

[0026] Such an optical fiber 10 is fabricated, for example, as follows. When forming a preform by OVD (Outside Vapor Deposition), when deposition of the cladding soot begins after depositing the Ge-doped soot that will become the core 11, the temperature of the deposition surface of the Ge-doped soot is raised to deposit the cladding soot. This increases the bulk density of the cladding portion relative to the core portion, creating a bulk density difference between the core portion and the cladding portion. This suppresses diffusion of Ge from the core 11 to the inner cladding 121 during vitrification (transparency). Therefore, it is possible to fabricate an optical fiber 10 having a large value for the ratio (|s1| / |s2|).

[0027] The optical fiber 10 has a mode field diameter of 9.2 μm ± 0.4 μm for light with a wavelength of 1310 nm, i.e., 8.8 μm or more and 9.6 μm or less. The mode field diameter is defined by Petermann-II. When the optical fiber 10 is wound around a mandrel with a diameter of 15 mm, the bending loss for light with a wavelength of 1625 nm is 1.0 dB or less per turn. When the optical fiber 10 is wound around a mandrel with a diameter of 30 mm, the bending loss for light with a wavelength of 1625 nm is 0.1 dB or less per 10 turns. Thus, with the center of the mode field diameter at 9.2 μm, the optical fiber 10 has a larger mode field diameter than a typical optical fiber having a single-step refractive index profile in the core and cladding, while still satisfying the bending loss level specified in G.657.A2.

[0028] The zero dispersion wavelength of the optical fiber 10 is 1300 nm or more and 1324 nm or less. That is, the zero dispersion wavelength of the optical fiber 10 complies with the standard of G.657.A2. The zero dispersion slope of the optical fiber 10 is 0.073 ps / (nm ２ ・km) or more 0.092 ps / (nm ２ When the zero dispersion slope is within this range, a bend-resistant optical fiber conforming to the G.657.A2 standard can be obtained.

[0029] The cable cutoff wavelength of the optical fiber 10 is 1260 nm or less. That is, the cable cutoff wavelength of the optical fiber 10 complies with the standard of G.657.A2.

[0030] The effects obtained by the optical fiber 10 of this embodiment described above will be described. In the optical fiber 10 of this embodiment, the absolute values ​​of the gradients s1 and s3 of the change in the relative refractive index difference in the radial direction in the interface region A between the core 11 and the inner cladding 121 and the interface region B between the inner cladding 121 and the trench 122 are 0.15% / μm or more. Since the gradients s1 and s3 of the change in the relative refractive index difference in the radial direction in the interface region A and the interface region B are thus steep, an increase in chromatic dispersion due to the structure is avoided. Accordingly, the shift of the zero-dispersion wavelength toward shorter wavelengths is reduced, and the tendency for the zero-dispersion slope to increase can be avoided. Therefore, the design value of the radius r1 of the core 11 and the tolerance for manufacturing errors can be expanded to achieve the desired characteristics of the optical fiber 10, such as bending loss and zero-dispersion slope.

[0031] Additionally, in the optical fiber 10 of this embodiment, the absolute values ​​of the gradients s1 and s3 of the change in the relative refractive index difference in the radial direction in the interface regions A and B are 2.00% / μm or less. Thus, the gradients s1 and s3 of the change in the relative refractive index difference in the radial direction in the interface regions A and B are not too steep. This reduces the stress difference that occurs when vitrifying (transparentizing) soot during the manufacture of the optical fiber 10. Therefore, it is possible to avoid a decrease in yield due to cracking of the base material (preform).

[0032] As in this embodiment, the absolute value of the gradient s2 of the change in the relative refractive index difference in the radial direction of the inner cladding 121 may be 0.0009% / μm or more and 0.0100% / μm or less. When the absolute value of the gradient s2 of the change in the relative refractive index difference in the radial direction of the inner cladding 121 is 0.0100% / μm or less, the bending loss for light with a wavelength of 1625 nm when wound around a mandrel with a diameter of 30 mm can be further reduced. In addition, when the absolute value of the gradient s2 of the change in the relative refractive index difference in the radial direction of the inner cladding 121 is 0.0009% / μm or more, precise refractive index adjustment by co-doping with germanium (Ge) and fluorine (F) is not required. Therefore, an increase in the difficulty of fabricating the optical fiber 10 can be avoided. Furthermore, deterioration in transmission loss and chromatic dispersion can be avoided.

[0033] As in the present embodiment, the ratio (|s1| / |s2|) of the absolute value of the gradient s1 of the change in the relative refractive index difference in the radial direction in the interface region A to the absolute value of the gradient s2 of the change in the relative refractive index difference in the radial direction in the inner cladding 121 may be 15 or more. When the ratio (|s1| / |s2|) is 15 or more, higher-order modes are more likely to escape, making it easier to reduce bending loss. This makes it possible to simultaneously improve the zero-dispersion slope and the bending loss for light with a wavelength of 1625 nm when wound around a mandrel with a diameter of 30 mm.

[0034] As in this embodiment, the core 11 may be doped with germanium, and the trench 122 may be doped with fluorine. When the core 11 is doped with germanium, the gradient of the change in the relative refractive index difference in the radial direction at the interface region between the core 11 and the inner cladding 121 tends to be gentle. When such an optical fiber 10 has the configuration of this embodiment, the design value of the radius of the core 11 and the tolerance for manufacturing errors can be expanded. Therefore, the yield of the optical fiber 10 can be effectively increased.

[0035] DESCRIPTION OF SYMBOLS 10... Optical fiber 11... Core 12... Cladding 13... Glass fiber 14... Primary resin layer 15... Secondary resin layer 16... Coating resin layer 121... Inner cladding 122... Trench 123... Outer cladding A, B... Interface region E1, E2, E3, E4, E11, E31... Range r1, r2, r3, r4... Radius Δ1, Δ2, Δ3, Δ4... Relative refractive index difference

Claims

1. A glass fiber comprising a core and a clad, wherein the clad comprises an inner clad covering the outer periphery of the core, a trench covering the outer periphery of the inner clad, and an outer clad covering the outer periphery of the trench, wherein the refractive index of the inner clad is lower than that of the core, the refractive index of the trench is lower than that of the inner clad, and the refractive index of the outer clad is higher than that of the trench and lower than that of the core, the absolute value of the gradient of the change in relative refractive index difference in the radial direction at the interface region between the core and the inner clad is 0.15% / μm or more and 2.00% / μm or less, the absolute value of the gradient of the change in relative refractive index difference in the radial direction at the interface region between the inner clad and the trench is 0.15% / μm or more and 2.00% / μm or less, and the mode field diameter for light with a wavelength of 1310 nm is 8.8 μm or more and 9.6 μm or less, When wound around a mandrel with a diameter of 15 mm, the bending loss for light with a wavelength of 1625 nm is 1.0 dB or less per turn, when wound around a mandrel with a diameter of 30 mm, the bending loss for light with a wavelength of 1625 nm is 0.1 dB or less per 10 turns, the zero dispersion wavelength is 1300 nm or more and 1324 nm or less, and the zero dispersion slope is 0.073 ps / (nm ２ ・km) or more 0.092 ps / (nm ２ 1. An optical fiber having a cable cutoff wavelength of 1260 nm or less.

2. The optical fiber according to claim 1, wherein the absolute value of the gradient of the change in the relative refractive index difference in the radial direction in the inner cladding is 0.0009% / μm or more and 0.0100% / μm or less, and the ratio (|s1| / |s2|) of the absolute value of the gradient s1 of the change in the relative refractive index difference in the radial direction in the interface region between the core and the inner cladding to the absolute value of the gradient s2 of the change in the relative refractive index difference in the radial direction in the inner cladding is 15 or more.

3. The optical fiber according to claim 1 or claim 2, wherein the core is doped with germanium and the trench is doped with fluorine.

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