Optical fiber

Abstract

The average value of the refractive indexes of the core of the optical fiber is larger than the average value of the refractive indexes of the inner cladding and the minimum value of the refractive indexes of the outer cladding, and the average value of the refractive indexes of the grooves is smaller than the average value of the refractive indexes of the inner cladding and the minimum value of the refractive indexes of the outer cladding. The core contains germanium dioxide, the relative refractive index difference of the core with respect to the refractive index of the pure silica glass is 0.12% to 0.30%, the value obtained by subtracting the relative refractive index difference of the outer cladding with respect to the refractive index of the pure silica glass from the relative refractive index difference of the core is 0.25% or more, and the diameter of the core is 4.0 [mu] m to 9.2 [mu] m. The optical cable has a mode field diameter of 8.2 [mu] m or more and 9.6 [mu] m or less with respect to light having a wavelength of 1310 nm, a zero dispersion wavelength of 1300 nm or more and 1324 nm or less, a zero dispersion slope of 0.073 ps / (nm2. Km) or more and 0.092 ps / (nm2. Km) or less, an optical cable cutoff wavelength of 1260 nm or less, and a bending loss of 0.1 dB or less per 100 turns with respect to light having a wavelength of 1625 nm when wound around a core rod having a diameter of 60 nm.

Description

Technical Field

[0001] The present disclosure relates to an optical fiber. This application claims priority based on Japanese Application No. 2022-140078 filed on September 2, 2022, and all the contents described in the Japanese Application are cited. Background Art

[0002] Patent document 1 discloses a structure of a single-mode optical fiber. The optical fiber includes a core layer and a cladding. The core layer is a silica glass layer to which germanium and fluorine are added at the same time. The radius of the core layer is 3.0 μm to 3.9 μm. The relative refractive index difference of the core layer is -0.04% to 0.12%. The added contribution of germanium to the relative refractive index difference of the core layer is 0.02% to 0.10%. The cladding includes an inner cladding, a depressed inner cladding, an auxiliary outer cladding and an outer cladding. The radius of the inner cladding is 8 μm to 14 μm. The relative refractive index difference of the inner cladding is -0.35% to -0.10%. The radius of the depressed inner cladding is 14 μm to 20 μm. The relative refractive index difference of the depressed inner cladding is -0.6% to -0.2%. The radius of the auxiliary outer cladding is 35 μm to 50 μm. The relative refractive index difference of the auxiliary outer cladding is -0.4% to -0.15%.

[0003] Patent document 2 discloses a structure of an optical fiber. The optical fiber includes a core and a cladding having a refractive index lower than that of the core. The cladding contains fluorine (F). The fluorine concentration in the cladding is adjusted to be the smallest at the outermost portion of the cladding.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application No. 2018-511077

[0007] Patent Document 2: International Publication No. 2020-013297 Summary of the invention

[0008] Technical problem to be solved by the invention

[0009] In order to extend the transmission distance and expand the transmission capacity in the optical fiber transmission system, it is desired that the transmission loss of the optical fiber is small. As optical fibers that achieve low transmission loss, there are optical fibers using pure silica glass in the core and optical fibers with alkali metal elements added to the core. From the viewpoint of suppressing the manufacturing cost to be lower than these optical fibers, there are optical fibers with germanium dioxide (GeO2) added to the core.

[0010] In order to transmit optical signals over long distances, optical fibers for communication need to reduce transmission loss and bending loss, that is, meet standards related to transmission loss and bending loss (eg, ITU-T G.652.B or G.652.D).

[0011] In an optical fiber with germanium dioxide (GeO2) added to the core, the smaller the relative refractive index difference of the core relative to the refractive index of pure silica glass (in other words, the smaller the concentration of GeO2), the higher the viscosity of the core. When manufacturing optical fibers, the optical fiber base material is drawn while applying tension. Internal stress remains in the optical fiber taken out by drawing. When the viscosity of the core is greater than the viscosity of the cladding, tensile stress remains in the core. If excessive tensile stress remains in the core that guides most of the signal light, the core is stretched, the glass structure defects increase, and the transmission loss increases. The smaller the relative refractive index difference of the core, the smaller the refractive index difference between the core and the cladding. As a result, the effect of confining the light propagating in the core in the core becomes weaker, so the bending loss increases. Furthermore, it is difficult for the fundamental mode to be confined in the core, and the fundamental mode may be cut off. For example, in the structure described in Patent Document 2, the relative refractive index difference of the core layer is -0.04% to 0.12%. In such a structure, the relative refractive index difference of the core layer is considered to be too small, and there is a concern that transmission loss and bending loss will increase. In optical fibers with germanium dioxide (GeO2) added to the core, the greater the relative refractive index difference of the core (in other words, the greater the concentration of GeO2 in the core), the greater the Rayleigh scattering caused by the concentration fluctuation of GeO2, and the greater the transmission loss.

[0012] The purpose of the present disclosure is to reduce bending loss and transmission loss in an optical fiber with GeO2 added to the core.

[0013] Means for solving technical problems

[0014] An optical fiber involved in one embodiment of the present disclosure comprises a glass optical fiber including a core and a cladding. The cladding includes an inner cladding surrounding the core, a groove surrounding the inner cladding, and an outer cladding surrounding the groove. The average value of the refractive index of the inner cladding and the minimum value of the refractive index of the outer cladding are smaller than the average value of the refractive index of the core. The average value of the refractive index of the groove is smaller than the average value of the refractive index of the inner cladding and the minimum value of the refractive index of the outer cladding. The core contains germanium dioxide. The relative refractive index difference of the core relative to the refractive index of pure silica glass is greater than 0.12% and less than 0.30%. The diameter of the core is greater than 4.0 μm and less than 9.2 μm. The mode field diameter for light of a wavelength of 1310 nm is greater than 8.2 μm and less than 9.6 μm. The zero dispersion wavelength is greater than 1300 nm and less than 1324 nm. The zero dispersion slope is 0.073 ps / (nm 2 ·km) and above and 0.092ps / (nm 2·km). The cut-off wavelength of the optical cable is 1260nm or less. When wound on a core rod with a diameter of 60mm, the bending loss for light with a wavelength of 1625nm is less than 0.1dB per 100 turns.

[0015] Effects of the Invention

[0016] According to the present disclosure, in an optical fiber with GeO2 added to the core, bending loss and transmission loss can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a diagram showing a cross section perpendicular to the axial direction of an optical fiber according to one embodiment.

[0018] Figure 2 : is a diagram showing the refractive index distribution in the radial direction of the glass optical fiber.

[0019] Figure 3 It is a graph showing the relationship between the relative refractive index difference of the core and the transmission loss (dB / km) predicted for light of a wavelength of 1550 nm.

[0020] Figure 4 : is a graph showing six examples of optical fibers and one comparative example.

[0021] Figure 5 : is a graph showing six examples of optical fibers and one comparative example.

[0022] Figure 6 : is a graph showing the relationship between the relative refractive index difference and the average residual stress in the core in five samples of the optical fiber.

[0023] Figure 7 It is shown Figure 6 A graph showing the distribution of the relative refractive index difference in the radial direction of the five samples. DETAILED DESCRIPTION

[0024] [Description of Embodiments of the Present Disclosure]

[0025] First, the contents of the embodiments of the present disclosure are listed and described.

[0026] [1] An optical fiber according to one embodiment of the present disclosure comprises a glass optical fiber including a core and a cladding. The cladding includes an inner cladding surrounding the core, a groove surrounding the inner cladding, and an outer cladding surrounding the groove. The average value of the refractive index of the core is greater than the average value of the refractive index of the inner cladding and the minimum value of the refractive index of the outer cladding. The average value of the refractive index of the groove is less than the average value of the refractive index of the inner cladding and the minimum value of the refractive index of the outer cladding. The core contains germanium dioxide. The relative refractive index difference of the core with respect to the refractive index of pure silica glass is greater than 0.12% and less than 0.30%. The value obtained by subtracting the relative refractive index difference of the outer cladding with respect to the refractive index of pure silica glass from the relative refractive index difference of the core is greater than 0.25%. The radius of the core is greater than 4.0 μm and less than 9.2 μm. The mode field diameter for light of a wavelength of 1310 nm is greater than 8.2 μm and less than 9.6 μm. The zero dispersion wavelength is greater than 1300 nm and less than 1324 nm. The zero dispersion slope is 0.073ps / (nm 2 ·km) and above and 0.092ps / (nm 2 ·km). The cut-off wavelength of the optical cable is 1260nm or less. When wound on a core rod with a diameter of 60mm, the bending loss for light with a wavelength of 1625nm is less than 0.1dB per 100 turns.

[0027] With optical fibers having these parameters, the relative refractive index difference of the core doped with GeO2 can be controlled within an appropriate range, and the bending loss and transmission loss can be reduced to a level that complies with G.652.D.

[0028] [2] In the optical fiber of the above [1], the relative refractive index difference of the inner cladding with respect to the refractive index of pure silica glass may be greater than -0.12% and less than 0.10%. Furthermore, the relative refractive index difference of the groove with respect to the refractive index of pure silica glass may be greater than -0.70% and less than -0.20%. Furthermore, when the radius of the outer circle of the core is set to r1, the radius of the outer circle of the inner cladding is set to r2, and the radius of the outer circle of the groove is set to r3, r2 / r1 may be greater than 2.2 and less than 3.8, and r3-r2 may be greater than 4 μm and less than 15 μm. In this case, the optical fiber can have a refractive index distribution structure for complying with G.652.D.

[0029] [3] In the optical fiber of [1] or [2] above, when wound on a core rod with a diameter of 20 mm, the bending loss for light with a wavelength of 1550 nm may be less than 0.75 dB per turn. Furthermore, when wound on a core rod with a diameter of 20 mm, the bending loss for light with a wavelength of 1625 nm may be less than 1.5 dB per turn. Furthermore, when wound on a core rod with a diameter of 30 mm, the bending loss for light with a wavelength of 1550 nm may be less than 0.25 dB per 10 turns. Furthermore, when wound on a core rod with a diameter of 30 mm, the bending loss for light with a wavelength of 1625 nm may be less than 1.0 dB per 10 turns. In this case, the optical fiber can comply with G.652.D and G.657.A1, which is a standard related to bending loss.

[0030] [4] In the optical fiber of any one of [1] to [3] above, the outer cladding may be composed only of pure silica glass. In this case, since the tensile stress is concentrated on the outer cladding, it is possible to suppress the tensile stress from remaining in the core and reduce the transmission loss.

[0031] [5] In the optical fiber of any one of [1] to [3] above, the outer cladding may contain fluorine. In this case, since the refractive index difference between the core and the outer cladding becomes larger, even if the relative refractive index difference of the core is reduced, fundamental mode cutoff is less likely to occur.

[0032] [6] In the fluorine concentration distribution in the outer cladding of the optical fiber of the above [5], when the outer cladding is divided into a first region including the outer peripheral surface of the outer cladding and a second region further inside the first region, the average value of the fluorine concentration in the first region may be smaller than the average value of the fluorine concentration in the second region. In this case, since the tensile stress is concentrated near the outer peripheral surface of the outer cladding, even when the compressive stress remains in the core or the tensile stress remains in the core, the tensile stress can be reduced. Therefore, the transmission loss can be reduced.

[0033] [7] In the optical fiber of any one of [1] to [6] above, the stress remaining in the core may be a compressive stress or a tensile stress of 30 MPa or less on average. In this case, since the glass structural defects generated in the core due to excessive tensile stress can be reduced, the increase in transmission loss caused by the glass structural defects can be reduced.

[0034] [8] In the optical fiber of any one of [1] to [7] above, the core may contain substantially no alkali metal element. In this case, the manufacturing cost of the optical fiber can be reduced.

[0035] [9] In the optical fiber of any one of [1] to [8] above, when wound on a core rod having a diameter of 100 mm, the bending loss for light of a wavelength of 1625 nm may be 1.0×10 -5dB or less. In this case, the leakage loss caused by the fundamental mode cutoff can be reduced. In addition, when it is difficult to actually measure the bending loss, the bending loss can be obtained by extrapolation based on the dependence of the bending loss on the bending radius.

[0036]

[10] In the optical fiber of any one of [1] to [9] above, the core may contain fluorine, and the relative refractive index difference of the core due to fluorine relative to the refractive index of pure silica glass may be greater than -0.12% and less than 0%. In this case, GeO2 and F are added to the core at the same time. Thus, compared with the case where fluorine is not contained, the viscosity of the core can be reduced even if the relative refractive index difference of the core is the same. Therefore, the residual stress of the core can be easily made into compressive stress. In addition, since the fluorine concentration in the core is not too large, the increase of transmission loss caused by fluorine can be suppressed.

[0037] [Details of the embodiments of the present disclosure]

[0038] Specific examples of the optical fiber involved in this embodiment will be described with reference to the accompanying drawings as needed. The present invention is not limited to these examples, but is represented by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. In the following description, the same elements are marked with the same figure marks in the description of the drawings, and repeated descriptions are 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. 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.

[0039] Figure 1 1 is a diagram showing a cross section perpendicular to the axial direction of an optical fiber 10 according to an embodiment. The optical fiber 10 is a so-called optical fiber base, and complies with both the ITU-T G.652.D standard and the ITU-T G.657.A standard. The optical fiber 10 includes a glass optical fiber 13 including a core 11 and a cladding 12, and a resin layer 16 provided on the outer periphery of the glass optical fiber 13. The resin layer 16 includes a main resin layer 14 surrounding the glass optical fiber 13 and a secondary resin layer 15 surrounding the main resin layer 14.

[0040] The cladding 12 surrounds the core 11. The cladding 12 includes an inner cladding 121, a groove 122, and an outer cladding 123. The inner cladding 121 surrounds the core 11 and contacts the outer peripheral surface of the core 11. The groove 122 surrounds the inner cladding 121 and contacts the outer peripheral surface of the inner cladding 121. The outer cladding 123 surrounds the groove 122 and contacts the outer peripheral surface of the groove 122.

[0041] Figure 2 is a diagram showing the refractive index distribution in the radial direction of the glass optical fiber 13. Figure 2In FIG. 1 , range E1 corresponds to the core 11, range E2 corresponds to the inner cladding 121, range E3 corresponds to the groove 122, and range E4 corresponds to the outer cladding 123. The vertical axis represents the relative refractive index difference, and the horizontal axis represents the radial position. Figure 2 As shown, in the glass optical fiber 13, the relative refractive index differences of the core 11, the inner cladding 121, the groove 122, and the outer cladding 123 with respect to the refractive index of pure silica glass (also referred to as SiO2, pure quartz) are set to Δ1, Δ2, Δ3, and Δ4, respectively. Δ1, Δ2, Δ3, and Δ4 are numerical values ​​(unit: %) expressed by the following mathematical formulas.

[0042] Δ1=100·(n 1ave -n0) / n 1ave

[0043] Δ2=100·(n 2ave -n0) / n 2ave

[0044] Δ3=100·(n 3ave -n0) / n 3ave

[0045] Δ4=100·(n 4min -n0) / n 4min

[0046] Where n0 is the refractive index of pure silica glass, 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 refractive index of the groove 122, n 4min is the minimum value of the refractive index of the outer cladding 123.

[0047] The relative refractive index difference Δ1 of the core 11 is greater than the relative refractive index difference Δ2 of the inner cladding 121, the relative refractive index difference Δ3 of the trench 122, and the relative refractive index difference Δ4 of the outer cladding 123. In other words, the average refractive index n of the core 11 is 1ave Greater than the average refractive index n of the inner cladding 121 2ave , the average refractive index n of the groove 122 3ave and the minimum refractive index n of the outer cladding 123 4min The relative refractive index difference Δ3 of the trench 122 is smaller than the relative refractive index difference Δ1 of the core 11, the relative refractive index difference Δ2 of the inner cladding 121, and the relative refractive index difference Δ4 of the outer cladding 123. In other words, the average refractive index n of the trench 122 is 3ave Less than the average refractive index n of the core 11 1ave , the average refractive index n of the inner cladding 121 2ave and the minimum refractive index n of the outer cladding 1234min The relative refractive index difference Δ2 of the inner cladding 121 may be the same as the relative refractive index difference Δ4 of the outer cladding 123, or may be greater than the relative refractive index difference Δ4, or may be smaller than the relative refractive index difference Δ4. In other words, the average refractive index n of the inner cladding 121 is 2ave The minimum refractive index n of the outer cladding 123 can be 4min The same can also be compared with the minimum refractive index n 4min Smaller, or smaller than the minimum refractive index n 4min The sign of the relative refractive index difference Δ3 of the groove 122 is negative, and the sign of the relative refractive index difference Δ1 of the core 11 is positive. The sign of the relative refractive index difference being negative means that the average refractive index is less than the refractive index of pure silica glass. The sign of the relative refractive index difference being positive means that the average refractive index is greater than the refractive index of pure silica glass.

[0048] The relative refractive index difference Δ1 of the core 11 is greater than 0.12% and less than 0.30%. The relative refractive index difference Δ2 of the inner cladding 121 is, for example, greater than -0.12% and less than 0.10%. 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, for example, greater than 0.15% and less than 0.40%. In one embodiment, the value (Δ1-Δ2) is 0.34%. By making the value (Δ1-Δ2) such a relatively small value, the expansion of the mode field diameter of the optical fiber 10 is achieved. The relative refractive index difference Δ3 of the groove 122 is, for example, greater than -0.70% and less than -0.20%. By making the relative refractive index difference Δ3 of the groove 122 within such a range, it is not necessary to extremely increase the amount of fluorine added in the process of sintering the glass. The relative refractive index difference Δ3 of the groove 122 may also be smaller than -0.25%. The value obtained by subtracting the relative refractive index difference Δ4 of the outer cladding 123 from the relative refractive index difference Δ1 of the core 11 (Δ1-Δ4) is 0.25% or more and 0.70% or less. When the relative refractive index difference Δ1 of the core 11 is less than 0.25%, the relative refractive index difference Δ4 may be made negative by adding fluorine (F) to the outer cladding 123. The greater the concentration of fluorine (F), the more the relative refractive index difference Δ4 of the outer cladding 123 can be reduced (the absolute value can be increased).

[0049] like Figure 1 as well as Figure 2As shown, the radius of the outer circle of the core 11 is set to r1, the radius of the outer circle of the inner cladding 121 is set to r2, the radius of the outer circle of the groove 122 is set to r3, and the radius of the outer circle of the outer cladding 123 is set to r4. The radius r1 of the core 11 is, for example, not less than 2.0 μm and not more than 4.6 μm. The value (r2 / r1) obtained by dividing the radius r2 of the inner cladding 121 by the radius r1 of the core 11 is, for example, not less than 2.2 and not more than 3.8. The value (r3-r2) obtained by subtracting the radius r2 of the inner cladding 121 from the radius r3 of the groove 122 is not less than 4 μm and not more than 15 μm. In one embodiment, the value (r3-r2) is 6.0 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 groove 122 is 16.65 μm.

[0050] The diameter D1 of the core 11 is, for example, not less than 4.0 μm and not more than 9.2 μm. The outer diameter D2 of the glass optical fiber 13, i.e., the outer diameter of the cladding 12, is, for example, not less than 125 μm ± 0.7 μm, i.e., not less than 124.3 μm and not more than 125.7 μm. Since the outer diameter D2 of the glass optical fiber 13 is the same as the outer diameter of a general glass optical fiber, general instruments can be used in peripheral fixtures such as connectors and peripheral equipment such as fusion splicers, and already installed optical fibers can be easily replaced. For example, the optical fiber 10 can be easily applied to microduct optical cables, super multi-core optical cables for data centers, and various other optical cables.

[0051] The core 11 and the cladding 12 mainly contain silica glass (quartz glass). The core 11 is composed of, for example, silica glass containing germanium dioxide (GeO2) as an additive material. By adding germanium dioxide to the material constituting the core 11, the transmission loss of light in the optical fiber 10 is reduced. The silica glass of the core 11 may also contain fluorine (F) as an additive material. The relative refractive index difference caused by fluorine in the relative refractive index difference Δ1 of the core 11 is, for example, greater than -0.12% and less than 0%. When germanium dioxide and fluorine are added to the material constituting the core 11 at the same time, the stress remaining in the core 11 is likely to become a compressive stress. The concentration of fluorine contained in the core 11 is, for example, greater than 200 ppm. As a result, the stress remaining in the core 11 is likely to become a compressive stress. The fluorine concentration of 200 ppm is equivalent to -0.007% when converted into a relative refractive index difference. The average of the stress remaining in the core 11 is a compressive stress or a tensile stress of less than 30 MPa. The residual stress of the core 11 is measured using, for example, IFA-100 (manufactured by Interfiber Analysis, Inc., USA).

[0052] The core 11 does not substantially contain alkali metal elements such as lithium (Li), sodium (Na) or potassium (K). In other words, the average alkali metal element mass concentration of the core 11 is substantially zero. In this specification, substantially zero specifically means 50 ppm or less.

[0053] The inner cladding 121 mainly comprises silica glass, and contains, for example, chlorine (Cl) as an additive material. The average chlorine mass concentration of the inner cladding 121 is, for example, greater than 500 ppm and less than 5000 ppm, or, for example, greater than 500 ppm and less than 3000 ppm. The groove 122 mainly comprises silica glass, and contains, for example, fluorine (F) as an additive material. The outer cladding 123 is composed of pure silica glass. Pure silica glass refers to silica glass that does not substantially contain impurities (composed of pure silica). Alternatively, the outer cladding 123 mainly comprises silica glass, and may contain, for example, fluorine (F) as an additive material. The relative refractive index difference caused by fluorine in the relative refractive index difference Δ4 of the outer cladding 123 is, for example, greater than -0.40% and less than 0%.

[0054] In the optical fiber 10, the center of the value of the mode field diameter for light with a wavelength of 1310nm is 8.6μm to 9.2μm, and the error is ±0.4μm. That is, the mode field diameter is greater than 8.2μm and less than 9.6μm. The mode field diameter is defined by Petermann-II. When the optical fiber 10 is wound on a core rod with a diameter of 20mm, the bending loss for light with a wavelength of 1550nm is less than 0.75dB per turn. When the optical fiber 10 is wound on a core rod with a diameter of 20mm, the bending loss for light with a wavelength of 1625nm is less than 1.5dB per turn. When the optical fiber 10 is wound on a core rod with a diameter of 30mm, the bending loss for light with a wavelength of 1550nm is less than 0.25dB per 10 turns. When the optical fiber 10 is wound on a core rod with a diameter of 30mm, the bending loss for light with a wavelength of 1625nm is less than 1.0dB per 10 turns. When the optical fiber 10 is wound on a core rod with a diameter of 60 mm, the bending loss of the optical fiber 10 for light with a wavelength of 1625 nm is less than 0.1 dB per 100 turns. When the optical fiber 10 is wound on a core rod with a diameter of 100 mm, the bending loss of the optical fiber 10 for light with a wavelength of 1625 nm is less than 1.0 × 10 -5 dB or less.

[0055] The bending loss characteristics described above can be achieved by setting the value obtained by dividing the radius r2 of the inner cladding 121 by the radius r1 of the core 11 (r2 / r1) to a small value such as 3.8 or less. In this way, the optical fiber 10 has a mode field diameter centered at 8.9 μm, and has a larger mode field diameter than a conventional optical fiber (an optical fiber in which the refractive index distribution of the core and the cladding is only one level each), while satisfying the level of bending loss specified in G.657.A1. Therefore, the optical fiber 10 fully functions as a single-mode optical fiber. The bending loss when wound into a diameter of 100 mm is so small that it cannot be measured. Therefore, the bending loss at several bending diameters within the range of 20 mm to 60 mm is measured, and the bending loss when wound into a diameter of 100 mm is calculated by extrapolation based on the bending diameter dependence of the bending loss.

[0056] The zero dispersion wavelength of the optical fiber 10 is greater than 1300nm and less than 1324nm. That is, the zero dispersion wavelength of the optical fiber 10 complies with the provisions of G.652.D and G.657.A1. Such a zero dispersion wavelength can be achieved by making the value (r2 / r1) obtained by dividing the radius r2 of the inner cladding 121 by the radius r1 of the core 11 be greater than 2.2. The wavelength dispersion of the optical fiber 10 for light of a wavelength of 1550nm is greater than 13.3ps / (nm·km) and less than 18.6ps / (nm·km). The zero dispersion slope of the optical fiber 10 is 0.073ps / (nm·km). 2 ·km) and above and 0.092ps / (nm 2 By setting the wavelength dispersion and zero dispersion slope within these ranges, a bend-resistant optical fiber that complies with the G.652.D and G.657.A1 standards can be obtained.

[0057] 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 regulations of G.652.D and G.657.A1.

[0058] In order to extend the transmission distance and expand the transmission capacity, the transmission loss of the optical fiber 10 for light with a wavelength of 1550 nm is 0.180 dB / km or less or 0.174 dB / km or less. In other words, the average OH mass concentration of the core 11 and the cladding 12 is small enough to make the transmission loss for light with a wavelength of 1550 nm 0.180 dB / km or less or 0.174 dB / km or less. By making the transmission loss within this range, it is possible to provide an optical fiber 10 corresponding to a low loss level.

[0059] Figure 3 1 is a graph showing the relationship between the relative refractive index difference Δ1 of the core 11 and the predicted transmission loss (dB / km) for light of wavelength 1550nm. Figure 3In FIG. 1 , the horizontal axis represents the relative refractive index difference Δ1 (%) of the core 11, and the vertical axis represents the predicted loss (dB / km) at a wavelength of 1550 nm. Figure 3 The transmission loss shown is the sum of the loss due to Rayleigh scattering, the loss due to structural irregularities, the infrared absorption of the glass, and the loss due to ultraviolet absorption. Figure 3 The transmission losses shown do not include the absorption losses caused by OH groups and the losses caused by glass structural defects. Figure 3 As shown, the transmission loss at a wavelength of 1550nm has a substantially linear relationship with the relative refractive index difference Δ1 of the core 11. Reducing the relative refractive index difference Δ1 of the core 11 means reducing the concentration of GeO2. By reducing the concentration of GeO2, the Rayleigh scattering caused by the concentration fluctuation of GeO2 is reduced, and as a result, the transmission loss is reduced. In order to reduce the transmission loss to less than 0.180dB / km, the relative refractive index difference Δ1 of the core 11 can be less than 0.30%. In order to reduce the transmission loss to less than 0.174dB / km, the relative refractive index difference Δ1 of the core 11 can be less than 0.20%.

[0060] If the GeO2 concentration of the core 11 is too low, the viscosity of the core 11 becomes high, and the core 11 mainly bears the tension during drawing. As a result, tensile stress is likely to remain in the core 11. If a large tensile stress remains in the core 11 that guides most of the signal light, the core 11 is stretched, the glass structure defects increase, and the transmission loss increases. Therefore, in this embodiment, the relative refractive index difference Δ1 of the core 11 is set to be 0.12% or more. Fluorine (F) as an additive material acts in the direction of reducing the refractive index of the core 11. Therefore, by adding fluorine in addition to GeO2 to the core 11, the relative refractive index difference Δ1 of the core 11 can be maintained small, while increasing the GeO2 concentration and reducing the viscosity of the core 11. Therefore, by adding fluorine (F) to the core 11, the tensile stress of the core 11 can be reduced or converted into compressive stress. However, if fluorine is added excessively, the amount of GeO2 added increases in order to maintain the relative refractive index difference Δ1, and the transmission loss caused by Ge increases. Therefore, in the present embodiment, the relative refractive index difference due to fluorine in the core 11 is not less than -0.12% and less than 0%.

[0061] Figure 4 as well as Figure 5 Graphs showing six examples 1 to 6 and one comparative example 1 of the optical fiber 10. These graphs show structural parameters and optical characteristics of the optical fiber 10. In these examples 1 to 6 and comparative example 1, the relative refractive index difference Δ1 of the core 11 due to fluorine is -0.02%.

[0062] Reference Figure 4 as well as Figure 5In Examples 1 to 6, the mode field diameter (MFD) of light with a wavelength of 1310 nm is 8.2 μm or more and 9.6 μm or less, 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) and above and 0.092ps / (nm 2 ·km) or less, and the optical cable cutoff wavelength is less than 1260nm. In Examples 1 to 6, when the optical fiber 10 is wound into a diameter of 60mm, the bending loss of the optical fiber 10 for light with a wavelength of 1625nm is less than 0.1dB per 100 turns. When the optical fiber 10 is wound into a diameter of 20mm, the bending loss of the optical fiber 10 for light with a wavelength of 1550nm is less than 0.75dB per turn. When the optical fiber 10 is wound into a diameter of 30mm, the bending loss of the optical fiber 10 for light with a wavelength of 1550nm is less than 0.25dB per 10 turns. When the optical fiber 10 is wound into a diameter of 20mm, the bending loss of the optical fiber 10 for light with a wavelength of 1625nm is less than 1.5dB per turn. When the optical fiber 10 is wound into a diameter of 30mm, the bending loss of the optical fiber 10 for light with a wavelength of 1625nm is less than 1.0dB per 10 turns. When the optical fiber 10 is wound into a diameter of 100 mm, the bending loss of the optical fiber 10 for light of wavelength 1625 nm is 1.0×10 -5 The transmission loss of the optical fiber 10 for light having a wavelength of 1550 nm is 0.180 dB / km or less.

[0063] In Comparative Example 1, the mode field diameter, zero dispersion wavelength, zero dispersion slope, cable cutoff wavelength, and bending losses for light with a wavelength of 1550 nm and light with a wavelength of 1625 nm when wound into a diameter of 20 mm are controlled within the above ranges. However, when wound into a diameter of 60 mm, the bending loss for light with a wavelength of 1625 nm exceeds 0.1 dB per 100 turns, when wound into a diameter of 30 mm, the bending loss for light with a wavelength of 1550 nm exceeds 0.25 dB per 10 turns, and when wound into a diameter of 30 mm, the bending loss for light with a wavelength of 1625 nm exceeds 1.0 dB per 10 turns. Furthermore, when wound into a diameter of 100 mm, the bending loss for light with a wavelength of 1625 nm exceeds 1.0 × 10 -5dB, and the transmission loss for light of wavelength 1550nm exceeds 0.180dB / km. It is believed that the result is caused by the following reasons. That is, in Comparative Example 1, the difference (Δ1-Δ4) between the relative refractive index difference Δ1 of the core 11 and the relative refractive index difference Δ4 of the outer cladding 123 is too small (less than 0.25% unlike Examples 1 to 6). Therefore, the bending loss when wound into a diameter of 100mm increases, and the confinement of the light of the fundamental mode (LP01 mode) propagating in the core 11 is weakened. As a result, at relatively long wavelengths, the leakage loss caused by the fundamental mode cutoff increases. In Examples 1 to 6, the bending loss when wound into a diameter of 100mm is suppressed to 1.0×10 -5 Below dB, it is believed that the leakage loss caused by fundamental mode cutoff is extremely small.

[0064] In the optical fiber 10 of Example 6, the tensile stress of the core 11 is 30 MPa on average, and the transmission loss at a wavelength of 1550 nm is 0.175 dB / km. In contrast, in another comparative example, the tensile stress of the core 11 is 50 MPa on average, and the transmission loss at a wavelength of 1550 nm is 0.194 dB / km. Thus, when the average stress remaining in the core 11 is tensile stress, its magnitude may be 30 MPa or less.

[0065] Figure 6 It is a graph showing the relationship between the relative refractive index differences Δ1 to Δ4 and the average residual stress of the core 11 in five samples 1 to 5 of the optical fiber 10 . Figure 6 In addition to the relative refractive index differences Δ1 to Δ4, the relative refractive index difference value caused by fluorine in the relative refractive index difference Δ1 is also shown. Tensile stress is represented as a positive value, and compressive stress is represented as a negative value. The radius r1 of the core 11, the radius r2 of the inner cladding 121, the radius r3 of the groove 122, and the thickness of the groove 122 are equal to each other among samples 1 to 5. In addition, Figure 7 is a graph showing the distribution of the relative refractive index difference in the radial direction in samples 1 to 5. Figure 7 In FIG. 1 , the horizontal axis represents the radial position (μm), and the vertical axis represents the relative refractive index difference (%). Line G1 corresponds to sample 1, line G2 corresponds to sample 2, line G3 corresponds to samples 3 and 4, and line G4 corresponds to sample 5. Range E1 corresponds to the core 11, range E2 corresponds to the inner cladding 121, range E3 corresponds to the groove 122, and range E4 corresponds to the outer cladding 123.

[0066] from Figure 6It can be seen that in order to prevent the tensile stress of more than +30 MPa on average from remaining in the core 11, the relative refractive index difference Δ1 of the core 11 can be 0.12% or more. In addition, the greater the fluorine concentration of the outer cladding 123, the lower the viscosity of the outer cladding 123, so the stress applied to the core 11 during drawing increases relatively, and a large tensile stress remains in the core 11. Therefore, if Figure 7 As shown in FIG. 1 , the outer cladding 123 may be divided into a first region C1 including the outer peripheral surface and a second region C2 that is further inside the first region C1, so that the average fluorine concentration of the first region C1 is smaller than the average fluorine concentration of the second region C2. In this case, the relative refractive index difference Δ4 of the first region C1 is greater than the relative refractive index difference Δ4 of the second region C2. Figure 7 In the example, the fluorine concentration of the first region C1 decreases as it approaches the outer peripheral surface of the outer cladding 123. Thus, the relative refractive index difference Δ4 increases as it approaches the outer peripheral surface of the outer cladding 123. In this way, by reducing the fluorine concentration of the first region C1 close to the outer peripheral surface of the outer cladding 123, the viscosity near the outer peripheral surface of the outer cladding 123 increases, and the tensile stress is concentrated near the outer peripheral surface of the outer cladding 123. Therefore, the drawing tension can be borne near the outer peripheral surface of the outer cladding 123, and the compressive stress can remain in the core 11, or even if the tensile stress remains in the core 11, the tensile stress can be reduced. Therefore, the transmission loss can be reduced.

[0067] The optical fiber 10 of this embodiment is manufactured, for example, in the following manner. First, in order to manufacture an optical fiber mother material, a glass rod having three layers corresponding to the core 11, the inner cladding 121, and the groove 122 is prepared. Then, fluorine is added to the periphery of the glass rod while glass particles are deposited to form a glass particle layer covering the periphery of the glass rod. This glass particle layer is the basis of the outer cladding 123. Figure 7 In the case of samples 2 to 4 shown, a treatment is further performed to remove a portion of the fluorine near the outer peripheral surface (first region C1) of the glass particle layer. Next, a gas having a dehydrating effect (e.g., chlorine (Cl2)) is flowed while the glass particle layer is heated (air firing). As a result, the glass particle layer is dehydrated. Thereafter, the glass rod and the glass particle layer are heated simultaneously to sinter the glass particle layer. The glass rod and the glass particle layer are heated simultaneously by placing the glass rod and the glass particle layer in a heating furnace at the same time. The heating is carried out, for example, under a reduced pressure atmosphere. A reduced pressure atmosphere refers to an atmosphere with a pressure lower than one atmosphere. The glass rod and the glass particle layer are heated, for example, at a temperature of more than 1300°C and less than 1600°C. In one embodiment, the glass rod and the glass particle layer are heated at a temperature of 1400°C. By sintering the glass particle layer, an outer cladding 123 is formed.

[0068] The effects obtained by the optical fiber 10 of the present embodiment described above are described. As described above, the optical fiber 10 of the present embodiment includes a glass optical fiber 13 including a core 11 and a cladding 12. The cladding 12 includes an inner cladding 121 surrounding the core 11, a groove 122 surrounding the inner cladding 121, and an outer cladding 123 surrounding the groove 122. The average refractive index n of the core 11 is 1ave Greater than the average refractive index n of the inner cladding 121 2ave and the minimum refractive index n of the outer cladding 123 4min The average refractive index n of the groove 122 is 3ave is smaller than the average refractive index n of the inner cladding 121 2ave and the minimum refractive index n of the outer cladding 123 4min . The core 11 contains GeO2. The relative refractive index difference Δ1 of the core 11 is greater than 0.12% and less than 0.30%. The value obtained by subtracting the relative refractive index difference Δ4 of the outer cladding 123 from the relative refractive index difference Δ1 of the core 11 is greater than 0.25%. The radius r1 of the core 11 is greater than 4.0μm and less than 9.2μm. The mode field diameter of the optical fiber 10 for light of a wavelength of 1310nm is greater than 8.2μm and less than 9.6μm. The zero dispersion wavelength of the optical fiber 10 is greater than 1300nm and less than 1324nm. The zero dispersion slope of the optical fiber 10 is 0.073ps / (nm 2 ·km) and above and 0.092ps / (nm 2 ·km). The cable cutoff wavelength of the optical fiber 10 is below 1260nm. When wound on a core rod with a diameter of 60mm, the bending loss of the optical fiber 10 for light with a wavelength of 1625nm is below 0.1dB per 100 turns. With the optical fiber 10 having these parameters, the relative refractive index difference Δ1 of the core 11 doped with GeO2 can be controlled within an appropriate range, and the bending loss and transmission loss can be reduced to a level that meets G.652.D.

[0069] As in the present embodiment, the relative refractive index difference Δ2 of the inner cladding 121 may be greater than -0.12% and less than 0.10%. Furthermore, the relative refractive index difference Δ3 of the groove 122 may be greater than -0.70% and less than -0.20%. Furthermore, the ratio (r2 / r1) of the radius r1 of the core 11 to the radius r2 of the inner cladding 121 may be greater than 2.2 and less than 3.8, and the difference (r3-r2) between the radius r3 of the groove 122 and the radius r2 of the inner cladding 121 may be greater than 4 μm and less than 15 μm. In this case, the optical fiber can have a refractive index distribution structure for complying with G.652.D.

[0070] As in the present embodiment, when the optical fiber 10 is wound on a core rod with a diameter of 20 mm, the bending loss of the optical fiber 10 for light with a wavelength of 1550 nm may be less than 0.75 dB per turn. Furthermore, when the optical fiber 10 is wound on a core rod with a diameter of 20 mm, the bending loss of the optical fiber 10 for light with a wavelength of 1625 nm may be less than 1.5 dB per turn. Furthermore, when the optical fiber 10 is wound on a core rod with a diameter of 30 mm, the bending loss of the optical fiber 10 for light with a wavelength of 1550 nm may be less than 0.25 dB per 10 turns. Furthermore, when the optical fiber 10 is wound on a core rod with a diameter of 30 mm, the bending loss of the optical fiber 10 for light with a wavelength of 1625 nm may be less than 1.0 dB per 10 turns. In this case, the optical fiber 10 can comply with G.652.D and G.657.A1, which is a standard related to bending loss.

[0071] As described above, the outer cladding 123 may also be composed of pure silica glass only. In this case, since the tensile stress is concentrated in the outer cladding 123, it is possible to suppress the tensile stress from remaining in the core 11 and reduce the transmission loss. Alternatively, the outer cladding 123 may also contain fluorine. In this case, since the refractive index difference between the core 11 and the outer cladding 123 becomes larger, it is not easy to produce the cutoff of the fundamental mode even if the relative refractive index difference Δ1 of the core 11 is reduced. In addition, the pure silica glass in the present disclosure is silica glass without intentional addition of additives, and also includes silica glass containing trace amounts of impurities.

[0072] As described above, the stress remaining in the core 11 may be a compressive stress or a tensile stress of 30 MPa or less on average. In this case, since glass structural defects generated in the core 11 due to excessive tensile stress can be reduced, the increase in transmission loss caused by glass structural defects can be reduced.

[0073] As described above, the core 11 may not substantially contain an alkali metal element. In this case, the manufacturing cost of the optical fiber 10 can be reduced.

[0074] When the optical fiber 10 is wound on a core rod with a diameter of 100 mm as in the present embodiment, the bending loss of the optical fiber 10 for light of a wavelength of 1625 nm may be 1.0×10 -5 dB or less. In this case, the leakage loss caused by the fundamental mode cutoff can be reduced. If it is difficult to actually measure the bending loss, the bending loss can be obtained by extrapolation based on the dependence of the bending loss on the bending radius.

[0075] As in the present embodiment, the core 11 may further contain fluorine. Furthermore, the relative refractive index difference Δ1 of the core 11 caused by fluorine may be greater than -0.12% and less than 0%. In this case, GeO2 and fluorine are added to the core 11 at the same time. Thus, compared with the case where fluorine is not contained, even if the relative refractive index difference Δ1 of the core 11 is the same, the viscosity of the core 11 can be reduced. Therefore, the residual stress of the core 11 can be easily made into a compressive stress. In addition, since the fluorine concentration in the core 11 is not too large, the increase in transmission loss caused by fluorine can be suppressed.

[0076] Description of Reference Numerals

[0077] 10: optical fiber; 11: core; 12: cladding; 13: glass optical fiber; 14: primary resin layer; 15: secondary resin layer; 16: resin layer; 121: inner cladding; 122: groove; 123: outer cladding; C1: first region; C2: second region; D1: diameter; D2: outer diameter; E1, E2, E3, E4: range; G1, G2, G3, G4: line; r1, r2, r3, r4: radius; Δ1, Δ2, Δ3, Δ4: relative refractive index difference.

Claims

1. An optical fiber, A glass optical fiber having a core and a cladding, The cladding includes an inner cladding surrounding the core, a trench surrounding the inner cladding, and an outer cladding surrounding the trench, The average value of the refractive index of the core is greater than the average value of the refractive index of the inner cladding and the minimum value of the refractive index of the outer cladding, The average value of the refractive index of the trench is smaller than the average value of the refractive index of the inner cladding and the minimum value of the refractive index of the outer cladding, The core comprises germanium dioxide, The relative refractive index difference of the core with respect to the refractive index of pure silica glass is 0.12% or more and 0.30% or less, The value obtained by subtracting the relative refractive index difference of the outer cladding with respect to the refractive index of pure silica glass from the relative refractive index difference of the core is 0.25% or more, The diameter of the core is greater than or equal to 4.0 μm and less than or equal to 9.2 μm. The mode field diameter for light with a wavelength of 1310 nm is 8.2 μm to 9.6 μm, and the zero dispersion wavelength is 1300 nm to 1324 nm. The zero dispersion slope is 0.073ps / (nm 2 ·km) and above and 0.092ps / (nm 2 km) or less, The cut-off wavelength of the optical cable is below 1260nm. When the fiber is wound around a core rod with a diameter of 60 mm, the bending loss for light with a wavelength of 1625 nm is less than 0.1 dB per 100 turns.

2. The optical fiber according to claim 1, wherein The relative refractive index difference of the inner cladding layer with respect to the refractive index of pure silica glass is not less than -0.12% and not more than 0.10%, The relative refractive index difference of the groove with respect to the refractive index of pure silica glass is not less than -0.70% and not more than -0.20%, When the radius of the outer circle of the core is r1, the radius of the outer circle of the inner cladding is r2, and the radius of the outer circle of the groove is r3, r2 / r1 is greater than 2.2 and less than 3.8, and r3-r2 is greater than 4 μm and less than 15 μm.

3. The optical fiber according to claim 1 or 2, wherein: When wound on a core rod with a diameter of 20 mm, the bending loss for light with a wavelength of 1550 nm is less than 0.75 dB per turn. When wound on a core rod with a diameter of 20 mm, the bending loss for light with a wavelength of 1625 nm is less than 1.5 dB per turn. When wound on a core rod with a diameter of 30 mm, the bending loss for light with a wavelength of 1550 nm is less than 0.25 dB per 10 turns. When the wire is wound around a core rod with a diameter of 30 mm, the bending loss for light of a wavelength of 1625 nm is less than 1.0 dB per 10 turns.

4. The optical fiber according to any one of claims 1 to 3, wherein The outer cladding consists only of pure silica glass.

5. The optical fiber according to any one of claims 1 to 3, wherein The outer cladding comprises fluorine.

6. The optical fiber according to claim 5, wherein: In the fluorine concentration distribution in the outer cladding, when the outer cladding is divided into a first region including an outer peripheral surface of the outer cladding and a second region inside the first region, an average fluorine concentration in the first region is smaller than an average fluorine concentration in the second region.

7. The optical fiber according to any one of claims 1 to 6, wherein The stress remaining in the core is, on average, a compressive stress or a tensile stress of 30 MPa or less.

8. The optical fiber according to any one of claims 1 to 7, wherein The core contains substantially no alkali metal elements.

9. The optical fiber according to any one of claims 1 to 8, wherein When wound on a core rod with a diameter of 100 mm, the bending loss for light with a wavelength of 1625 nm is 1.0×10 -5 dB or less.

10. The optical fiber according to any one of claims 1 to 9, wherein The core comprises fluorine, The relative refractive index difference of the core due to fluorine with respect to the refractive index of pure silica glass is not less than -0.12% and less than 0%.

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