Optical fiber

Abstract

A cladding (12) includes: an inner cladding (121) that covers an outer circumference of a core (11); a trench (122) that covers an outer circumference of the inner cladding; and an outer cladding (123) that covers an outer circumference 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. When a relative refractive index difference of the core with respect to the refractive index of pure silica is Δ1, a relative refractive index difference of the inner cladding with respect to the refractive index of pure silica is Δ2, a relative refractive index difference of the trench with respect to the refractive index of pure silica is Δ3, the radius of the outer circumference of the core is r1, the radius of the outer circumference of the inner cladding is r2, and the radius of the outer circumference of the trench is r3, r2 / r1 is 2.2 to 3.6, r3-r2 is 3 μm to 10 μm, Δ1-Δ2 is 0.15% to 0.40%, and Δ3 is −0.70% to -0.10%.

Description

optical fiber

[0001] This application claims priority to Japanese Patent Application No. 2023-222849, filed December 28, 2023, and incorporates by reference the entire contents of said Japanese application.

[0002] Patent Document 1 describes an optical fiber. The optical fiber includes a central core, an optical cladding, and a jacket. The central core is made of GeO ２ The relative refractive index difference of the central core with respect to the optical cladding is 0.2% or more and 0.32% or less. The relative refractive index difference of the jacket with respect to the optical cladding is 0.03% or more and 0.20% or less. The stress remaining in the central core is compressive stress. The absolute value of the stress remaining in the central core is 30 MPa or less. The 2 m fiber cutoff wavelength is 1300 nm or more. The cutoff wavelength at a fiber length of 100 m is 1500 nm or less. The transmission loss at a wavelength of 1550 nm is 0.18 dB / km or less. In a cross section perpendicular to the fiber axis, the stress in a portion of 50% or more of the cross-sectional area of ​​the jacket is tensile stress.

[0003] Patent Document 2 describes an optical fiber. This optical fiber includes a core region, an optical cladding region, and a jacket region. The jacket region has a substantially uniform composition from the inner periphery to the outer periphery. A compressively strained layer in which compressive stress remains is formed in the outermost periphery of the jacket region. The compressive stress remaining in the compressively strained layer is 10 MPa or more. The thickness of the compressively strained layer is 30% or less of the outer diameter of the jacket region.

[0004] JP 2013-122502 A JP 2011-102964 A

[0005] 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 core is doped with germanium. When the relative refractive index difference of the core with respect to the refractive index of pure silica is Δ1, the relative refractive index difference of the inner cladding with respect to the refractive index of pure silica is Δ2, the relative refractive index difference of the trench with respect to the refractive index of pure silica is Δ3, the radius of the outer periphery of the core is r1, the radius of the outer periphery of the inner cladding is r2, and the radius of the outer periphery of the trench is r3, r2 / r1 is 2.2 or more and 3.6 or less, r3 - r2 is 3 μm or more and 10 μm or less, Δ1 - Δ2 is 0.15% or more and 0.40% or less, and Δ3 is -0.70% or more and -0.10% or less. When wound around a 15 mm diameter mandrel, the bending loss for light with a wavelength of 1625 nm is 1.0 dB or less per turn. When wound around a 30 mm diameter mandrel, the bending loss for light with a wavelength of 1625 nm is 0.1 dB or less per 10 turns. The cable cutoff wavelength is 1260 nm or less. The stress remaining in the core is compressive stress. The transmission loss of the glass fiber for light with a wavelength of 1550 nm is 0.19 dB / km or less.

[0006] FIG. 1 is a diagram showing a cross section perpendicular to the axial direction of an optical fiber according to a first embodiment. FIG. 2 is a diagram showing the refractive index profile in the radial direction of a glass fiber. FIG. 3 is a graph showing the relationship between compressive stress remaining in the core and the frequency of breakage of the optical fiber. FIG. 4 is a graph showing the relationship between residual stress in the core and the yield of low-loss grade optical fiber. FIG. 5 is a graph showing the relationship between radial position relative to the center of the optical fiber and residual stress in the optical fiber. FIG. 6 is a graph showing the relationship between the absolute value of the value obtained by differentiating the residual stress in the outer cladding with respect to radial position and the yield at which the dynamic fatigue coefficient of the outer cladding meets or exceeds the international standard. FIG. 7 is a graph showing the relationship between the variation (3σ) in the outer diameter fluctuation of the glass fiber and the percentage of optical fibers with a transmission loss of 0.32 dB / km or less at a wavelength of 1310 nm. FIG. 8 is a table showing the specifications and characteristics of optical fibers according to Samples 1 to 4 as examples and comparative examples.

[0007] [Problem to be Solved by the Present Disclosure] Single-mode optical fibers for communications are required to have reduced bending loss, i.e., to satisfy standards related to bending loss (e.g., ITU-T G.657.A2). Reducing transmission loss is also required for transmitting optical signals over long distances.

[0008] Effect of the Present Disclosure According to the present disclosure, an optical fiber capable of reducing bending loss and transmission loss can be provided.

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

[0010] [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 core is doped with germanium. When the relative refractive index difference of the core with respect to the refractive index of pure silica is Δ1, the relative refractive index difference of the inner cladding with respect to the refractive index of pure silica is Δ2, the relative refractive index difference of the trench with respect to the refractive index of pure silica is Δ3, the radius of the outer periphery of the core is r1, the radius of the outer periphery of the inner cladding is r2, and the radius of the outer periphery of the trench is r3, r2 / r1 is 2.2 or more and 3.6 or less, r3 - r2 is 3 μm or more and 10 μm or less, Δ1 - Δ2 is 0.15% or more and 0.40% or less, and Δ3 is -0.70% or more and -0.10% or less. When wound around a 15 mm diameter mandrel, the bending loss for light with a wavelength of 1625 nm is 1.0 dB or less per turn. When wound around a 30 mm diameter mandrel, the bending loss for light with a wavelength of 1625 nm is 0.1 dB or less per 10 turns. The cable cutoff wavelength is 1260 nm or less. The stress remaining in the core is compressive stress. The transmission loss of the glass fiber for light with a wavelength of 1550 nm is 0.19 dB / km or less.

[0011] An optical fiber having these parameters makes it possible to reduce bending loss and transmission loss.

[0012] [2] In the optical fiber of [1] above, when the optical fiber is wound around a mandrel having a diameter of 100 mm, the bending loss for light having a wavelength of 1625 nm is 1.0 × 10 per turn. －４ It may be less than 1 dB.

[0013] [3] In the optical fiber of [1] or [2] above, when the residual stress in the outer cladding at a position r [μm] from the center of the optical fiber is Q(r) [MPa], |dQ(r) / dr| may be 2.0 [MPa / μm] or less in the section from 10% to 90% of the thickness of the outer cladding.

[0014] [4] In the optical fiber according to any one of [1] to [3] above, the core may further be doped with fluorine.

[0015] [5] In the optical fiber of [4] above, the concentration of fluorine contained in the core may be 200 ppm or more.

[0016] [6] In the optical fiber of any one of [1] to [5] above, the average value of the compressive stress remaining in the core in the radial direction of the core may be 100 MPa or less.

[0017] [7] In the optical fiber of any one of [1] to [6] above, the transmission loss of the glass fiber for light having a wavelength of 1310 nm may be 0.32 dB / km or less. The transmission loss of the glass fiber for light having a wavelength of 1550 nm may be 0.18 dB / km or less. [Details of the embodiment of the present disclosure]

[0018] Specific examples of optical fibers according to 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 equivalent to the claims. In the following description, identical elements in 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. Similarly, the "thickness" of a certain element refers to the average value of the thickness of the element at each position in the axial direction of the optical fiber. (First embodiment)

[0019] 1 is a diagram showing a cross section perpendicular to the axial direction of an optical fiber 10A according to a first embodiment. The optical fiber 10A is a so-called optical fiber bare wire 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 10A includes a glass fiber 13 including a core 11 and a cladding 12, and a coating resin layer 16A including a primary resin layer 14 and a secondary resin layer 15 provided around the outer periphery of the glass fiber 13.

[0020] 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, for example, a material in which germanium (Ge) is added to pure silica glass. Here, pure silica glass does not substantially contain impurities. Adding germanium to the material that constitutes the core 11 increases the transmission loss of light in the optical fiber 10A.

[0021] The outer diameter D2 of the glass fiber 13, i.e., the outer diameter of the cladding 12, is 125 μm±0.5 μm, i.e., 124.5 μm or more and 125.5 μm or less, and the diameter D1 of the core 11 is 6.0 μm or more and 12.0 μm or less. Since the outer diameter D2 of the glass fiber 13 is the same as the outer diameter of a typical glass fiber, typical peripheral devices such as connectors and fusion splicers can be used, facilitating the replacement of existing optical fibers. For example, the optical fiber 10A can be easily applied to microduct cables, ultra-multicore cables for data centers, and various other cables.

[0022] The cladding 12 includes an inner cladding 121 that surrounds the outer periphery of the core 11 and is in contact with the outer peripheral surface of the core 11, a trench 122 that surrounds the outer periphery of the inner cladding 121 and is in contact with the outer peripheral surface of the inner cladding 121, and an outer cladding 123 that surrounds the outer periphery of the trench 122 and is in contact with the outer peripheral surface of the trench 122. The inner cladding 121 can be made of silica glass doped with chlorine (Cl). The average chlorine mass concentration of the inner cladding 121 is, for example, 500 ppm to 5000 ppm, and more preferably, 500 ppm to 3000 ppm. The trench 122 can be made of silica glass doped with fluorine. The outer cladding 123 can be made of pure silica glass. The average chlorine mass concentration of the outer cladding 123 is, for example, substantially zero. Here, "substantially zero" specifically means 50 ppm or less. The average OH mass concentration of the outer cladding 123 is, for example, 5 ppm to 500 ppm, more preferably, 5 ppm to 200 ppm, and is achieved by, for example, sintering the outer cladding 123 in a vacuum atmosphere.

[0023] 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 are defined as Δ1, Δ2, Δ3, and Δ4, respectively. Δ1, Δ2, Δ3, and Δ4 are numerical values ​​(unit: %) expressed by the following formula: Δ1=100·(n １ａｖｅ -n ０ ) / n １ａｖｅ Δ2 = 100 · (n ２ａｖｅ -n ０ ) / n ２ａｖｅ Δ3 = 100 · (n ３ａｖｅ -n ０ ) / n ３ａｖｅ Δ4 = 100 · (n ４ａｖｅ -n ０ ) / n ４ａｖｅHowever, n ０ is the refractive index of pure silica, n １ａｖｅ is the average refractive index of the core 11, n ２ａｖｅ is the average refractive index of the inner cladding 121, n ３ａｖｅ is the average value of the refractive index of the trench 122, n ４ａｖｅ is the average value of the refractive index of the outer cladding 123. In this case, the relative refractive index difference Δ2 of the inner cladding 121 is smaller than the relative refractive index difference Δ1 of the core 11. In other words, the refractive index of the inner cladding 121 is smaller than the refractive index of the core 11. The relative refractive index difference Δ3 of the trench 122 is negative and smaller than the relative refractive index difference Δ2 of the inner cladding 121. In other words, the refractive index of the trench 122 is smaller than the refractive index of the inner cladding 121. The sign of the relative refractive index difference Δ1 of the core 11 is positive. The negative sign of the relative refractive index difference Δ3 of the trench 122 means that the refractive index of the trench 122 is smaller than the refractive index of pure silica. The refractive index of the outer cladding 123 is larger than the refractive index of the trench 122 and smaller than the refractive index of the core 11.

[0024] The value (Δ1-Δ2) obtained by subtracting the relative refractive index difference Δ2 of the inner cladding 121 from the relative refractive index difference Δ1 of the core 11 is 0.15% or more and 0.40% or less. In one embodiment, the value (Δ1-Δ2) is 0.34%. Such a relatively small value (Δ1-Δ2) allows for an expansion of the mode field diameter of the optical fiber 10A. The absolute value |Δ2| of the relative refractive index difference Δ2 of the inner cladding 121 is 0.10% or less. The relative refractive index difference Δ3 of the trench 122 is -0.70% or more and -0.10% or less. Since the relative refractive index difference Δ3 of the trench 122 is within this range, it is not necessary to add an extremely large amount of fluorine in the glass sintering process. The relative refractive index difference Δ3 of the trench 122 may be less than -0.25%. In one embodiment, the relative refractive index difference Δ1 of the core 11 is 0.34%, the relative refractive index difference Δ2 of the inner cladding 121 is 0.00%, the relative refractive index difference Δ3 of the trench 122 is −0.30%, and the relative refractive index difference Δ4 of the outer cladding 123 is 0.00%.

[0025] As shown in FIGS. 1 and 2 , the radius of the outer periphery of the core 11 is r1, the radius of the inner cladding 121 is r2, and the radius of the outer periphery of the trench 122 is r3. In this case, 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 3 μm or more and 10 μm or less. In one embodiment, the value (r3-r2) is 6.0 μm. The value (r3-r2) may be greater than 4.5 μm. The outer diameter of the outer cladding 123, i.e., the outer diameter of the glass fiber 13, is within the range of 125 μm±0.5 μm. In one embodiment, the radius r1 of the core 11 is 3.8 μm, the radius r2 of the inner cladding 121 is 10.65 μm, and the radius r3 of the trench 122 is 16.65 μm.

[0026] The material constituting the core 11 is further doped with fluorine (F). When the material constituting the core 11 is co-doped with germanium and fluorine, the residual stress in the core 11 becomes compressive stress. When the value of the residual stress is negative, the residual stress becomes compressive stress. The concentration of fluorine contained in the core 11 is, for example, 200 ppm or more. This makes it easier for the residual stress in the core 11 to become compressive stress. A fluorine concentration of 200 ppm corresponds to a relative refractive index difference of −0.007%. The compressive stress remaining in the core 11 is, for example, greater than 10 MPa, greater than 30 MPa, or 40 MPa or more. The compressive stress remaining in the core 11 is, for example, 100 MPa or less, or 80 MPa or less. The compressive stress remaining in the core 11 is the average absolute value of the residual stress in the core 11 in the radial direction.

[0027] 3 is a graph showing an example of the relationship between the compressive stress remaining in the core 11 and the breakage frequency of the optical fiber 10A. In FIG. 3, the breakage frequency of the optical fiber 10A is a relative value when the breakage frequency of the optical fiber 10A when the compressive stress remaining in the core 11 is 0 MPa is set to 1. In this example, the relative refractive index difference Δ1 of the core 11 is 0.35%, the relative refractive index difference Δ2 of the inner cladding 121 is 0.02%, and the relative refractive index difference Δ3 of the trench 122 is −0.30%. The radius r1 of the core 11 is 4.0 μm, the radius r2 of the inner cladding 121 is 8.8 μm, and the radius r3 of the trench 122 is 14.8 μm. As is clear from FIG. 3, when the compressive stress remaining in the core 11 is 100 MPa or less, the breakage frequency of the optical fiber 10A can be significantly reduced compared to when the compressive stress remaining in the core 11 is greater than 100 MPa. If the compressive stress remaining in the core 11 is 80 MPa or less, the frequency of breakage of the optical fiber 10A can be further reduced.

[0028] FIG. 4 is a graph showing an example of the relationship between the residual stress of the core 11 and the yield of a low-loss grade optical fiber 10A. The residual stress of the core 11 is the average value of the residual stress of the core 11 in the radial direction. Specifically, a low-loss grade optical fiber 10A has a transmission loss of 0.32 dB / km or less for light with a wavelength of 1310 nm. In FIG. 4, when the residual stress value is negative, the residual stress is compressive stress. When the residual stress value is positive, the residual stress is tensile stress. In this example, the same optical fiber as in the example in FIG. 3 was used. As in the example in FIG. 3, the relative refractive index difference Δ1 of the core 11 was 0.35%, the relative refractive index difference Δ2 of the inner cladding 121 was 0.02%, and the relative refractive index difference Δ3 of the trench 122 was −0.30%. The radius r1 of the core 11 is 4.0 μm, the radius r2 of the inner cladding 121 is 8.8 μm, and the radius r3 of the trench 122 is 14.8 μm. As is clear from Fig. 4, the yield of the low-loss grade optical fiber 10A when the residual stress in the core 11 is compressive stress is significantly greater than the yield of the low-loss grade optical fiber 10A when the residual stress in the core 11 is tensile stress.

[0029] FIG. 5 is a graph showing an example of the relationship between the radial position r relative to the center of the optical fiber 10A and the residual stress of the optical fiber 10A. In FIG. 5, section s1 corresponds to the core 11, section s2 corresponds to the inner cladding 121, section s3 corresponds to the trench 122, and section s4 corresponds to the outer cladding 123. In this example, the same optical fiber as in the example of FIG. 3 was used. As in the example of FIG. 3, the relative refractive index difference Δ1 of the core 11 is 0.35%, the relative refractive index difference Δ2 of the inner cladding 121 is 0.02%, and the relative refractive index difference Δ3 of the trench 122 is −0.30%. The radius r1 of the core 11 is 4.0 μm, the radius r2 of the inner cladding 121 is 8.8 μm, and the radius r3 of the trench 122 is 14.8 μm. Here, the residual stress of the outer cladding 123 at the radial position r [μm] is defined as Q(r) [MPa]. In the section from 10% to 90% of the thickness of the outer cladding 123 (specifically, in the section between the radial position corresponding to 10% of the thickness of the outer cladding 123 and the radial position corresponding to 90% of the thickness of the outer cladding 123, assuming that the position of the interface between the outer cladding 123 and the trench 122 is the radial position corresponding to 0% of the thickness of the outer cladding 123), the absolute value of the value (dQ(r) / dr) obtained by differentiating the residual stress Q(r) with respect to the radial position r satisfies, for example, the following condition: |dQ(r) / dr|≦2.0 [MPa / μm] Here, |dQ(r) / dr| is calculated based on the moving average of the residual stress Q(r) at a plurality of points set at intervals of 0.15 μm in the radial direction (or at a predetermined number of points set at equal intervals, for example, 25 points). The residual stress Q(r) is measured using, for example, IFA-100 (manufactured by Interfiber Analysis, Inc., USA).

[0030] Figure 6 is a graph showing the relationship between |dQ(r) / dr| and the yield of optical fiber 10A with a dynamic fatigue coefficient of 18 or greater, which is the international standard. In Figure 6, the yield of optical fiber 10A with a dynamic fatigue coefficient of 18 or greater falls below 90% when |dQ(r) / dr| is greater than 2.0 MPa, and decreases significantly as |dQ(r) / dr| increases. As is clear from Figure 6, when |dQ(r) / dr| is 2.0 MPa or less, the yield of optical fiber 10A with a dynamic fatigue coefficient of 18 or greater remains high, at 95% or greater. That is, by keeping |dQ(r) / dr| at 2.0 [MPa / μm] or less, the dynamic fatigue coefficient of optical fiber 10A can fully satisfy the international standard.

[0031] In the optical fiber 10A, the mode field diameter for light with a wavelength of 1310 nm is 9.2 μm±0.4 μm, 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 10A 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 10A 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. When the optical fiber 10A is wound around a mandrel with a diameter of 100 mm, the bending loss for light with a wavelength of 1625 nm is 1.0×10 －４ dB or less. Such bending loss characteristics can be achieved by setting the value (r2 / r1) obtained by dividing the radius r2 of the inner cladding 121 by the radius r1 of the core 11 to 3.6 or less. Thus, the optical fiber 10A satisfies the bending loss level specified in G.657.A2 while having a mode field diameter centered at 9.2 μm larger than that of a typical optical fiber (one in which the core and cladding each have only one refractive index profile step). Therefore, the optical fiber 10A functions satisfactorily as a single-mode fiber. Since the bending loss when wound around a diameter of 100 mm is so small that it cannot be measured, the bending loss is measured at several bending diameters in the range of 20 mm to 60 mm, and the bending loss is calculated by extrapolation based on the bending diameter dependency of the bending loss.

[0032] The zero-dispersion wavelength of the optical fiber 10A is 1300 nm or more and 1324 nm or less. In other words, the zero-dispersion wavelength of the optical fiber 10A complies with the provisions of G.657.A2. Such a zero-dispersion wavelength can be achieved by setting the value (r2 / r1) obtained by dividing the radius r2 of the inner cladding 121 by the radius r1 of the core 11 to 2.2 or more. The chromatic dispersion of the optical fiber 10A for light with a wavelength of 1550 nm is 18.6 ps / (nm km) or less. The zero-dispersion slope of the optical fiber 10A is 0.092 ps / (nm km). ２ When the chromatic dispersion and the zero-dispersion slope are within these ranges, a bending-resistant optical fiber conforming to the G.657.A2 standard can be obtained.

[0033] The cable cutoff wavelength of the optical fiber 10A is 1260 nm or less, i.e., the cable cutoff wavelength of the optical fiber 10A complies with the standard of G.657.A2.

[0034] The optical fiber 10A has a transmission loss of 0.32 dB / km or less for light with a wavelength of 1310 nm. In other words, the average OH mass concentration of the core 11 and the cladding 12 is small enough that the transmission loss for light with a wavelength of 1310 nm is 0.32 dB / km or less. By keeping the transmission loss within this range, it is possible to expand the wavelength range that can be used for information transmission in optical communication systems, and to provide an optical fiber 10A that corresponds to a low-loss grade.

[0035] The optical fiber 10A has a transmission loss of 0.19 dB / km or less for light with a wavelength of 1550 nm, or 0.18 dB / km or less. In other words, the average OH mass concentration of the core 11 and the cladding 12 is small enough that the transmission loss for light with a wavelength of 1550 nm is 0.19 dB / km or less, or 0.18 dB / km or less. By having the transmission loss within this range, the optical fiber 10A can be provided as a low-loss grade.

[0036] When the standard deviation of the fluctuation in the outer diameter of the glass fiber 13 in the axial direction is σ, 3σ is, for example, 0.1 μm or more and 0.5 μm or less. Here, the standard deviation σ indicates the variation in the longitudinal fluctuation of the measured values ​​(i.e., outer diameter fluctuation) when measured at regular intervals (e.g., 1 m intervals) in the longitudinal direction. It is more preferable that the value 3σ falls within the range of 0.2 μm or more and 0.5 μm or less. The outer diameter fluctuation needs to be equal to or less than a predetermined value to satisfy the international standard for glass diameter.

[0037] Figure 7 is a graph showing the relationship between the variation (3σ) of the outer diameter fluctuation of the glass fiber 13 and the percentage of optical fibers with a transmission loss of 0.32 dB / km or less at a wavelength of 1310 nm. As is clear from Figure 7, when the variation (3σ) of the outer diameter fluctuation is 0.1 μm or more, the percentage of optical fibers with a transmission loss of 0.32 dB / km or less exceeds 90%, indicating that transmission loss can be sufficiently reduced. There is a correlation between the variation in outer diameter fluctuation and the transmission loss at a wavelength of 1310 nm; the smaller the variation in outer diameter fluctuation, the greater the transmission loss at a wavelength of 1310 nm. However, if the variation in outer diameter fluctuation is large, the splice loss may increase or the outer diameter may not meet the specifications. Therefore, by slightly varying the outer diameter fluctuation (3σ being 0.1 μm or more and 0.5 μm or less), transmission loss at a wavelength of 1310 nm can be reduced within a range where outer diameter fluctuation is not a problem.

[0038] 8 shows the specifications and characteristics of the optical fibers according to sample numbers 1 to 4 as examples and comparative examples. The radius of the outer cladding 123 is 62.5 μm in all cases.

[0039] As shown in Figure 8, Samples 1 and 2 achieved good values ​​for each optical characteristic. In contrast, Sample 3 exhibited excessive transmission loss at a wavelength of 1380 nm, and chromatic dispersion (λ = 1550 nm) deviated from the G.657.A2 standard. Sample 4 exhibited excessive bending loss at a wavelength of 1625 nm and a diameter of 30 mm. For this reason, the value (r2 / r1) obtained by dividing the radius r2 of the inner cladding 121 by the radius r1 of the core 11 is preferably 2.2 or more and 3.6 or less. The stress remaining in the cores of Samples 1 to 4 was compressive stress.

[0040] 10A... Optical fiber 11... Core 12... Cladding 121... Inner cladding 122... Trench 123... Outer cladding 13... Glass fiber 14... Primary resin layer 15... Secondary resin layer 16A... Coating resin layer D1... Diameter D2... Outer diameter r1... Radius r2... Radius r3... Radius E1... Range E2... Range E3... Range E4... Range Δ1... Relative refractive index difference Δ2... Relative refractive index difference Δ3... Relative refractive index difference Δ4... Relative refractive index difference

Claims

1. A glass fiber comprising a core and a cladding, wherein 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; germanium is added to the core; when the difference in specific refractive index of the core with respect to the refractive index of pure silica is Δ1, the difference in specific refractive index of the inner cladding with respect to the refractive index of pure silica is Δ2, the difference in specific refractive index of the trench with respect to the refractive index of pure silica is Δ3, the radius of the outer periphery of the core is r1, the radius of the outer periphery of the inner cladding is r2, and the radius of the outer periphery of the trench is r3, r2 / r1 is 2.2 or more and 3.6 or less, r3 - r2 is 3 μm or more and 10 μm or less, Δ1 - Δ2 is 0.15% or more and 0.40% or less, Δ3 is -0.70% or more and -0.10% or less; the bending loss for light with a wavelength of 1625 nm when wound around a mandrel with a diameter of 15 mm is 1.0 dB or less per turn, the bending loss for light with a wavelength of 1625 nm when wound around a mandrel with a diameter of 30 mm is 0.1 dB or less per 10 turns; the cable cut-off wavelength is 1260 nm or less; the stress remaining in the core is compressive stress; and the transmission loss of the glass fiber for light with a wavelength of 1550 nm is 0.19 dB / km or less.

2. When the optical fiber is wound around a mandrel with a diameter of 100 mm, the bending loss with respect to light having a wavelength of 1625 nm is 1.0×10 －４ dB or less per turn, and the optical fiber according to claim 1.

3. The optical fiber according to claim 1 or claim 2, wherein when the residual stress of the outer cladding at a position r [μm] from the center of the optical fiber is Q(r) [MPa], |dQ(r) / dr| in the section from 10% to 90% of the thickness of the outer cladding is 2.0 [MPa / μm] or less.

4. The optical fiber according to any one of claims 1 to 3, wherein fluorine is further added to the core.

5. The optical fiber according to claim 4, wherein the concentration of the fluorine contained in the core is 200 ppm or more.

6. The optical fiber according to any one of claims 1 to 5, wherein the average value of the compressive stress remaining in the core in the radial direction of the core is 100 MPa or less.

7. The optical fiber according to any one of claims 1 to 6, wherein the transmission loss of the glass fiber with respect to light having a wavelength of 1310 nm is 0.32 dB / km or less, and the transmission loss of the glass fiber with respect to light having a wavelength of 1550 nm is 0.18 dB / km or less.

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