fiber optic

Abstract

An optical fiber (1) according to one embodiment of the present invention is provided with: a core part (1a); and a cladding part (1b) which surrounds the outer periphery of the core part, while having a refractive index that is lower than the maximum refractive index of the core part. The core part comprises: a center core to which germanium is added; and a low refractive index layer which surrounds the outer periphery of the center core, while having a negative -∆max that is the minimum value of the relative refractive index difference with respect to the average refractive index of the cladding part. The residual compressive stress in the center core is from 40 MPa to 150 MPa; and ∆1 that is the average maximum relative refractive index difference of the center core with respect to the average refractive index of the cladding part is 0.8% or less.

Description

[Technical Field]

[0001] The present invention relates to an optical fiber. [Background Art]

[0002] Toward the realization of next-generation communication infrastructure, low bending loss optical fibers that enable stable communication in various environments have been attracting increasing attention. As an approach for achieving low bending loss, control of the refractive index profile is mainly mentioned. For example, in the case of a trench-type profile, providing a trench layer in the cladding region can easily reduce bending loss, and it is also possible to achieve favorable properties for other characteristics such as MFD (mode field diameter).

[0003] On the other hand, optical fibers having residual stress have been disclosed to improve characteristics such as fracture characteristics of optical fibers (Patent Documents 1 to 5). [Prior Art Document] [Patent Document]

[0004] [Patent Document 1] Japanese Unexamined Patent Publication No. 2007-297254 [Patent Document 2] Japanese Patent No. 4663277 [Patent Document 3] Japanese Unexamined Patent Publication No. 2003-207675 [Patent Document 4] Japanese Patent No. 5831189 [Patent Document 5] Japanese Patent No. 6020045 [Summary of the Invention] [Problem to be Solved by the Invention]

[0005] Refractive index profile control technology primarily relied on dopant-based control. For example, to reduce bending loss by increasing the refractive index of the core, methods were employed to increase the relative refractive index difference between the core and the cladding by lowering the refractive index of the cladding or increasing the amount of dopant in the core. However, this raised concerns such as the deterioration of transmission loss due to increased Rayleigh scattering associated with dopant addition, and the need to establish dopant addition technology.

[0006] On the other hand, the relationship between residual stress in optical fibers and optical fiber properties, particularly bending resistance, had not been sufficiently studied.

[0007] The present invention has been made in view of the above, and its object is to provide an optical fiber that has low bending loss while suppressing an increase in transmission loss. [Means for solving the problem]

[0008] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention comprises a core portion and a cladding portion that surrounds the outer periphery of the core portion and has a refractive index lower than the maximum refractive index of the core portion, wherein the core portion comprises a germanium-doped center core and a cladding portion that surrounds the outer periphery of the center core and has a minimum value of -Δmax, which is the difference in specific refractive index with respect to the average refractive index of the cladding portion, and An optical fiber comprising a low refractive index layer, wherein the residual compressive stress in the center core is 40 MPa or more and 150 MPa or less, and the difference Δ1, which is the average maximum relative refractive index of the center core with respect to the average refractive index of the cladding portion, is 0.8% or less.

[0009] The aforementioned -Δmax may be -0.5% or more and -0.1% or less.

[0010] The low refractive index layer may be a trench layer separated from the center core.

[0011] The diameter of the center core may be 3 μm or more and 8 μm or less, and the outer diameter of the trench layer may be 9 μm or more and 20 μm or less.

[0012] The diameter of the center core may be 3.5 μm or more and 7.5 μm or less, and the outer diameter of the trench layer may be 9.5 μm or more and 19 μm or less.

[0013] The diameter of the center core may be 3.5 μm or more and 5 μm or less, and the outer diameter of the trench layer may be 12 μm or more and 16 μm or less.

[0014] The low refractive index layer may be a depressed layer adjacent to the center core.

[0015] The diameter of the center core may be 3 μm or more and 8 μm or less, and the outer diameter of the depressed layer may be 9 μm or more and 20 μm or less.

[0016] The diameter of the center core may be 3.5 μm or more and 7.5 μm or less, and the outer diameter of the depressed layer may be 9.5 μm or more and 19 μm or less.

[0017] The diameter of the center core may be 3.5 μm or more and 5 μm or less, and the outer diameter of the depressed layer may be 12 μm or more and 16 μm or less.

[0018] In the radial direction, the difference in specific refractive index from the center core to the low refractive index layer may change continuously.

[0019] The continuous change in the relative refractive index difference may also mean that the refractive index profile representing the relative refractive index difference changes smoothly without abrupt bending, and that the amount of change is 0.1% or less per μm in the radial direction.

[0020] The residual compressive stress in the low refractive index layer may be 20 MPa or more and 200 MPa or less.

[0021] The low refractive index layer may contain fluorine.

[0022] The optical fiber may have a transmission loss of 0.190 dB / km or less at a wavelength of 1550 nm, a mode field diameter of 8.6 μm or more at a wavelength of 1310 nm, a cable cut-off wavelength of 1260 nm or less, and a zero-dispersion wavelength of 1300 nm or more.

[0023] The macrobend loss when wound with a diameter of 20 mm at a wavelength of 1550 nm may be 0.1 dB / turn or less.

[0024] The cladding diameter may be within the range of 125 μm±10 μm.

[0025] The cladding diameter may be within the range of 115 μm±10 μm.

[0026] The cladding diameter may be within the range of 105 μm±10 μm.

[0027] The cladding diameter may be within the range of 95 μm±10 μm.

[0028] The cladding diameter may be within the range of 85 μm±10 μm.

[0029] The optical fiber may further comprise a coating portion including a primary layer surrounding an outer circumference of the cladding portion and a secondary layer surrounding an outer circumference of the primary layer, wherein an outer diameter of the secondary layer is 260 μm or less, the primary layer is formed of a resin having a Young's modulus of 0.2 MPa or more and 3 MPa or less, and the secondary layer is formed of a resin having a Young's modulus of 5 MPa or more and 2000 MPa or less.

[0030] A maximum value of a residual stress difference between the core portion and the cladding portion may be 40 MPa or more and 220 MPa or less.

[0031] A maximum value of a residual stress difference between the core portion and the cladding portion may be 70 MPa or more and 220 MPa or less. [Effects of the Invention]

[0032] The present invention offers the advantage of realizing an optical fiber with low bending loss while suppressing an increase in transmission loss. [Brief explanation of the drawing]

[0033] [Figure 1] Figure 1 is a schematic cross-sectional view of an optical fiber according to Embodiment 1 in a plane perpendicular to the longitudinal direction. [Figure 2] Figure 2 is a schematic diagram of the refractive index profile and corresponding residual stress of an optical fiber according to Embodiment 1. [Figure 3] Figure 3 is a schematic diagram of the refractive index profile of the optical fiber according to Embodiment 2. [Figure 4] Figure 4 is a schematic diagram of the refractive index profile of an optical fiber in a comparative example. [Modes for carrying out the invention]

[0034] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to the embodiments described below. In each drawing, the same or corresponding components are appropriately denoted by the same reference numerals. In this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength (λcc) as defined in ITU-T G.650.1 of the International Telecommunication Union (ITU). Furthermore, terms not specifically defined in this specification shall be defined and measured according to the definitions and measurement methods in G.650.1 and G.650.2.

[0035] (Embodiment 1) Figure 1 is a schematic cross-sectional view of an optical fiber according to Embodiment 1 in a plane perpendicular to the longitudinal direction. The optical fiber 1 is made of silica-based glass and comprises a core portion 1a and a cladding portion 1b that surrounds the outer circumference of the core portion 1a and has a refractive index lower than the maximum refractive index of the core portion 1a. The portion of the optical fiber 1 comprising the core portion 1a and the cladding portion 1b is the portion of the optical fiber made of glass and may be described as a glass optical fiber. The optical fiber 1 also comprises a coating portion 1c that surrounds the outer circumference of the cladding portion 1b. The coating portion 1c has a primary layer 1ca that surrounds the outer circumference of the cladding portion 1b and a secondary layer 1cb that surrounds the outer circumference of the primary layer 1ca. The optical fiber comprising the coating portion 1c may be described as an optical fiber core.

[0036] The outer diameter (cladding diameter) of the cladding portion 1b is not particularly limited, but is preferably within the range of 125 μm ± 10 μm. Furthermore, from the viewpoint of miniaturizing the optical fiber 1, the cladding diameter may be within the range of 115 μm ± 10 μm, 105 μm ± 10 μm, 95 μm ± 10 μm, or 85 μm ± 10 μm.

[0037] The outer diameter of the secondary layer 1cb is, for example, 260 μm or less, but it may be made smaller in accordance with the reduction in cladding diameter. The primary layer 1ca is made of a resin with a Young's modulus of 0.2 MPa to 3 MPa, for example. The secondary layer 1cb is made of a resin with a Young's modulus of 5 MPa to 2000 MPa, for example.

[0038] The resin constituting the primary layer 1ca and the resin constituting the secondary layer 1cb are, for example, UV-curable resins. UV-curable resins are compounded with various resin materials and additives, such as oligomers, diluent monomers, photopolymerization initiators, silane coupling agents, sensitizers, and lubricants. Conventionally known materials such as polyether-based urethane acrylates, epoxy acrylates, polyester acrylates, and silicone acrylates can be used as oligomers. Conventionally known materials such as monofunctional monomers and polyfunctional monomers can be used as diluent monomers. Furthermore, the additives are not limited to those mentioned above, and a wide range of conventionally known additives used for UV-curable resins can be used.

[0039] Figure 2 is a schematic diagram of the refractive index profile and corresponding residual stress of an optical fiber according to Embodiment 1. The refractive index profile shown in Figure 2 is a so-called trench-type refractive index profile and represents the specific refractive index difference. In Figure 2, profile P11 shows the refractive index profile of the core portion 1a, and profile P12 shows the refractive index profile of the cladding portion 1b. In the trench-type refractive index profile, the core portion 1a consists of a center core with a diameter of 2a, an intermediate layer formed to surround the outer circumference of the center core, with a refractive index smaller than the maximum refractive index of the center core, an inner diameter of 2a, and an outer diameter of 2b, and a trench layer formed to surround the outer circumference of the intermediate layer, with a refractive index smaller than the refractive index of the cladding portion, an inner diameter of 2b, and an outer diameter of 2c. Profile P11a is the refractive index profile of the center core, profile P11b is the refractive index profile of the intermediate layer, and profile P11c is the refractive index profile of the trench layer.

[0040] The center core is the portion of the core 1a with the highest average refractive index. The difference in the average maximum relative refractive index of the center core compared to the average refractive index of the cladding 1b is Δ1. The trench layer surrounds the outer periphery of the center core, and its minimum relative refractive index difference with respect to the average refractive index of the cladding 1b, -Δmax, is negative, making it an example of a low refractive index layer. In Figure 2, -Δmax is written as Δ2. The trench layer is separated from the center core by the interposition of an intermediate layer.

[0041] In the intermediate layer, the relative refractive index difference changes continuously from the inside to the outside in the radial direction. As a result, in optical fiber 1, the relative refractive index difference changes continuously from the center core to the trench layer in the radial direction. Here, a continuous change in the relative refractive index difference means that the refractive index profile representing the relative refractive index difference changes smoothly without abrupt bends, and the amount of change is 0.1% or less per μm in the radial direction. In contrast, the boundary between the intermediate layer and the trench layer can be defined by the amount of change in the relative refractive index difference. For example, if we look at the amount of change in the relative refractive index difference from the center side, if it is 0.1% or less per μm in the radial direction, it is the intermediate layer, and if it exceeds 0.1%, it can be said that we have crossed the boundary and moved to the trench layer.

[0042] Furthermore, the refractive index profile of the center core of the core section 1a may not always be a geometrically ideal step shape, but may also have irregularities at the top due to manufacturing characteristics, or a shape that extends from the top to the bottom. In this case, the refractive index of the region where the refractive index profile is approximately flat at the top, within the range of the core diameter 2a (center core diameter) of the core section 1a in the manufacturing design, serves as an indicator for determining Δ1. Even if the approximately flat region appears to be divided into multiple locations, or if a continuous change occurs making it difficult to define the approximately flat region, we have confirmed that it is possible to obtain characteristics close to the desired ones as long as at least one part of the core section other than the part where the refractive index changes abruptly toward the adjacent layer falls within the range of Δ1 described below, and the difference between the maximum and minimum values ​​of Δ is within a certain value of ±30%, and there are no particular problems.

[0043] Next, the constituent materials of the core portion 1a and cladding portion 1b of the optical fiber 1 will be described. The center core of the core portion 1a is made of quartz glass doped with germanium (Ge). Ge is a dopant that increases the refractive index of the quartz glass. The trench layer is made of quartz glass in which at least a portion contains a dopant that decreases the refractive index, such as fluorine (F) or boron (B). The intermediate layer is made of quartz glass with the same or similar composition as the cladding portion 1b.

[0044] The cladding 1b is made of, for example, pure quartz glass. Pure quartz glass is extremely high-purity quartz glass that substantially does not contain dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm. However, dopants may be added to the cladding 1b.

[0045] As shown in Figure 2, the core 1a of optical fiber 1 has residual compressive stress, and in particular, the residual compressive stress in the center core is between 40 MPa and 150 MPa. As a result of this large residual compressive stress in the center core, the photoelastic effect acts on the center core, and in addition to the refractive index-increasing effect of dopants such as Ge that increase the refractive index, the refractive index is increased by the photoelastic effect. Therefore, the amount of dopant added to achieve the same Δ1 can be reduced. This reduces the occurrence of Rayleigh scattering due to the dopant, thereby reducing transmission loss. Furthermore, by increasing Δ1 through the photoelastic effect, low bending loss characteristics can be achieved.

[0046] If the residual compressive stress in the center core is 40 MPa or higher, the effect of the refractive index increase due to the photoelastic effect is sufficient, and in practical terms, 150 MPa or less is preferable. If the residual compressive stress is greater than 150 MPa, the temperature during the wire drawing process described later is low, and there is a possibility that transmission loss will worsen due to the generation of glass structural defects.

[0047] Furthermore, in optical fiber 1, the cladding portion 1b has residual tensile stress. As a result, the difference in residual stress between the core portion 1a and the cladding portion 1b becomes large, which also increases the difference in specific refractive index, making it preferable to achieve low bending loss while suppressing an increase in transmission loss.

[0048] Therefore, the optical fiber 1 according to Embodiment 1 has the characteristic of having low bending loss while suppressing an increase in transmission loss.

[0049] Furthermore, a Δ1 of the center core of 0.8% or less is preferable because it allows for a reduction in the amount of dopants such as Ge, thereby suppressing an increase in transmission loss. Additionally, a Δ1 of 0.8% or less is preferable because it suppresses foaming during manufacturing.

[0050] Furthermore, it is preferable that the Δ1 of the center core be greater than 0.25%. This is because if the Δ1 of the center core is 0.25% or less, the light confinement effect weakens, which can lead to deterioration of bending loss and a shift of the zero-dispersion wavelength to a shorter wavelength.

[0051] Furthermore, if the -Δmax(Δ2) of the trench layer is between -0.5% and -0.1%, then low curvature This is preferable in that it achieves loss reduction.

[0052] Furthermore, in the core portion 1a of the optical fiber 1, it is preferable that the core diameter 2a (see Figure 2), which is the diameter of the center core, is 3 μm or more and 8 μm or less, and the outer diameter 2c (see Figure 2) of the trench layer is 9 μm or more and 20 μm or less. More preferably, the core diameter 2a is 3.5 μm or more and 7.5 μm or more, and the outer diameter 2c of the trench layer is 9.5 μm or more and 19 μm or less. In particular, it is even more preferable that the core diameter 2a is 3.5 μm or more and 5 μm or less, and the outer diameter 2c of the trench layer is 12 μm or more and 16 μm or less.

[0053] Keeping the core diameter 2a within the above range is preferable because it allows for a good balance of characteristics of the optical fiber 1, such as low bending loss, mode field diameter, cutoff wavelength, and zero-dispersion wavelength. For example, it is possible to obtain a good balance of desirable characteristics for the optical fiber 1, such as a transmission loss of 0.190 dB / km or less at a wavelength of 1550 nm, a mode field diameter of 8.6 μm or more at a wavelength of 1310 nm, a cable cutoff wavelength of 1260 nm or less, and a zero-dispersion wavelength of 1300 nm or more. In particular, controlling the outer diameter 2c of the trench layer is effective in reducing bending loss.

[0054] Furthermore, it is preferable that the maximum difference in residual stress between the core portion 1a and the cladding portion 1b is 40 MPa or more and 220 MPa or less, in order to achieve low bending loss while suppressing an increase in transmission loss, and it is even more preferable that the maximum difference is 70 MPa or more.

[0055] Furthermore, if the residual compressive stress in the trench layer is between 20 MPa and 200 MPa, the flexibility of the trench layer allows for reduced transmission loss due to structural relaxation of the core 1a. Moreover, since the entire core 1a can be subjected to a compressive atmosphere, an effect that facilitates the increase in refractive index due to the photoelastic effect is obtained.

[0056] An optical fiber 1 having residual stress as shown in Figure 2 can be realized by appropriately adjusting the linear velocity, tension, and temperature when drawing the optical fiber 1 from an optical fiber preform fabricated using various preform fabrication methods, such as the VAD (Vapor-phase Axial Deposition) method.

[0057] (Embodiment 2) Next, the optical fiber according to Embodiment 2 will be described. The optical fiber according to Embodiment 2 differs from Embodiment 1 in that its refractive index profile is a so-called W type. Therefore, the refractive index profile of the optical fiber according to Embodiment 2 will be described below, and other explanations will be omitted as appropriate.

[0058] Figure 3 is a schematic diagram of the refractive index profile of an optical fiber according to Embodiment 2. The refractive index profile shown in Figure 3 is a W-type refractive index profile and represents the specific refractive index difference. In Figure 3, profile P21 shows the refractive index profile of the core portion, and profile P22 shows the refractive index profile of the cladding portion. In the W-type refractive index profile, the core portion consists of a center core with a diameter of 2a and a depressed layer formed to surround the outer circumference of the center core, with a refractive index smaller than that of the cladding portion, an inner diameter of 2a, and an outer diameter of 2b. Profile P21a is the refractive index profile of the center core, and profile P21b is the refractive index profile of the depressed layer.

[0059] The center core is the part of the core with the highest average refractive index. The difference in the average maximum relative refractive index of the center core compared to the average refractive index of the cladding is Δ1. The depressed layer surrounds the outer periphery of the center core and has a negative minimum relative refractive index difference of -Δmax compared to the average refractive index of the cladding, making it an example of a low refractive index layer. Note that in Figure 3, -Δmax is written as Δ2. The depressed layer is adjacent to the center core.

[0060] In the depressed layer, the relative refractive index difference changes continuously from the inside to the outside in the radial direction. As a result, in the optical fiber according to Embodiment 2, the relative refractive index difference changes continuously from the center core to the depressed layer in the radial direction. Here, a continuous change in the relative refractive index difference means that the refractive index profile representing the relative refractive index difference changes smoothly without abrupt bending, and the amount of change is 0.1% or less per 1 μm in the radial direction.

[0061] In the optical fiber according to Embodiment 2, the core also has residual compressive stress, and in particular, the residual compressive stress in the center core is between 40 MPa and 150 MPa. As a result of this large residual compressive stress in the center core, the amount of dopant added to achieve the same Δ1 can be reduced, as in Embodiment 1, thus reducing transmission loss. Furthermore, Δ1 can be increased by the photoelastic effect to achieve low bending loss characteristics.

[0062] Furthermore, in the optical fiber according to Embodiment 2, the cladding portion also has residual tensile stress. As a result, similar to Embodiment 1, it is preferable for achieving low bending loss while suppressing an increase in transmission loss.

[0063] Therefore, the optical fiber according to Embodiment 2 has the characteristic of having low bending loss while suppressing an increase in transmission loss.

[0064] Furthermore, a center core Δ1 of 0.8% or less is preferable in that it suppresses an increase in transmission loss and prevents foaming during manufacturing. Also, a center core Δ1 greater than 0.25% is preferable. This is because if the center core Δ1 is 0.25% or less, the light confinement effect weakens, potentially leading to deterioration of bending loss and a shift of the zero-dispersion wavelength to a shorter wavelength.

[0065] Furthermore, if the -Δmax(Δ2) of the depressed layer is between -0.5% and -0.1%, This is preferable in that it achieves low bending loss.

[0066] Furthermore, in the core portion of the optical fiber according to Embodiment 2, it is preferable that the core diameter 2a (see Figure 3), which is the diameter of the center core, is 3 μm or more and 8 μm or less, and the outer diameter 2b (see Figure 3) of the depressed layer is 9 μm or more and 20 μm or less. More preferably, the core diameter 2a is 3.5 μm or more and 7.5 μm or more, and the outer diameter 2b of the depressed layer is 9.5 μm or more and 19 μm or less. In particular, it is even more preferable that the core diameter 2a is 3.5 μm or more and 5 μm or less, and the outer diameter 2b of the depressed layer is 12 μm or more and 16 μm or less.

[0067] Setting the core diameter 2a within the above range is preferable because it allows for a good balance of the optical fiber characteristics according to Embodiment 2, such as low bending loss, mode field diameter, cutoff wavelength, and zero-dispersion wavelength. For example, it is possible to obtain a good balance of the excellent characteristics of the optical fiber according to Embodiment 2, such as a transmission loss of 0.190 dB / km or less at a wavelength of 1550 nm, a mode field diameter of 8.6 μm or more at a wavelength of 1310 nm, a cable cutoff wavelength of 1260 nm or less, and a zero-dispersion wavelength of 1300 nm or more. In particular, controlling the outer diameter 2b of the depressed layer is effective in reducing bending loss.

[0068] Furthermore, it is preferable that the maximum difference in residual stress between the core and cladding portions is 40 MPa or more and 220 MPa or less, as this suppresses an increase in transmission loss while achieving low bending loss, and it is even more preferable that the maximum difference is 70 MPa or more.

[0069] Furthermore, a residual compressive stress of 20 MPa to 200 MPa in the depressed layer is preferable in terms of achieving low bending loss.

[0070] Furthermore, the optical fibers according to Embodiments 1 and 2 are preferable if their transmission loss at a wavelength of 1550 nm is 0.190 dB / km or less, as this indicates low transmission loss.

[0071] Furthermore, if the mode field diameter at a wavelength of 1310 nm is 8.6 μm or more, the cable cutoff wavelength is 1260 nm or less, and the zero-dispersion wavelength is 1300 nm or more, then it is preferable as it satisfies at least part of the ITU-T G652.D standard.

[0072] Furthermore, if the macrobend loss when wound to a diameter of 20 mm at a wavelength of 1550 nm is 0.1 dB / turn or less, it is preferable because it has low bending loss and satisfies at least part of the ITU-T G657.A2 standard.

[0073] (Examples) As embodiments of the present invention, optical fibers of Examples 1 to 6 having the refractive index profiles shown in Figure 2 were manufactured. When manufacturing the optical fibers of Examples 1 to 6, residual stress was adjusted by appropriately controlling the linear speed, tension, and temperature during drawing.

[0074] On the other hand, as a comparative example, an optical fiber having the refractive index profile shown in Figure 4 was manufactured. When manufacturing the optical fiber of the comparative example, the residual compressive stress of the center core was adjusted to zero by appropriately adjusting the linear speed, tension, and temperature during drawing.

[0075] Figure 4 is a schematic diagram of the refractive index profile of an optical fiber in a comparative example. In Figure 4, profile P31 shows the refractive index profile of the core, and profile P32 shows the refractive index profile of the cladding 1b. Profile P31a is the refractive index profile of the center core, profile P31b is the refractive index profile of the intermediate layer, and profile P31c is the refractive index profile of the trench layer.

[0076] In the refractive index profile of Figure 4, unlike the refractive index profile of Figure 2, the specific refractive index difference changes discontinuously in the radial direction from the center core to the trench layer. That is, the refractive index profile bends sharply near the boundary between the intermediate layer and the trench layer, and the amount of change is greater than 0.1% per μm in the radial direction.

[0077] Table 1 shows the characteristics of the optical fibers of the comparative example and Examples 1-6. "Core compressive stress" is the maximum value of the residual compressive stress of the center core. "Cladding tensile stress" is the maximum value of the residual tensile stress of the cladding. "Δ1 before compressive stress application" is the amount of Δ1 due to the increase in refractive index caused by Ge. "Δ1 after compressive stress application" is the amount of Δ1 after compressive stress is applied by drawing the wire. "Increase in Δ1 due to compressive stress" is the amount of increase in Δ1 due to the photoelastic effect caused by the compressive stress. "MFD" is the value at a wavelength of 1310 nm. "Macrobend loss" is the value at a wavelength of 1550 nm, and "φ30" and "φ20" are the diameters when the optical fiber is wound.

[0078] As shown in Table 1, all of Examples 1 to 6 exhibited low bending loss while suppressing an increase in transmission loss. In particular, the characteristics of "φ20" all of Examples 1 to 6 met the ITU-T G657.A2 standard. Specifically, in Example 1, the core compressive stress was 80 MPa and the cladding tensile stress was 30 MPa, and the compressive stress increased Δ1 from 0.33% to 0.42%, an increase of only 0.09%. In this case, the transmission loss at a wavelength of 1550 nm was 0.176 dB / km, the MFD at a wavelength of 1310 nm was 9.02 μm, the cable cutoff wavelength was 1236 nm, and the zero-dispersion wavelength was 1311 nm. Furthermore, the macro-bend loss at a wavelength of 1550 nm was 0.048 dB / turn when wound with a diameter of 20 mm and 0.014 dB / turn when wound with a diameter of 30 mm.

[0079] [Table 1]

[0080] It should be noted that the present invention is not limited to the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible. [Industrial applicability]

[0081] As described above, the optical fiber according to the present invention is suitable for realizing an optical fiber with low bending loss while suppressing an increase in transmission loss. [Explanation of symbols]

[0082] 1: Optical fiber 1a: Core 1b: Clad section 1c: Coating part 1ca: Primary layer 1cb: Secondary layer P11, P11a, P11b, P11c, P12, P21, P21a, P21b, P22, P31, P31a, P31b, P31c, P32: Profile

Claims

1. The core part, The cladding portion has a refractive index lower than the maximum refractive index of the core portion and surrounds the outer circumference of the core portion, Equipped with, The core portion includes a germanium-doped center core and a low refractive index layer surrounding the outer circumference of the center core, wherein the minimum value of the difference in specific refractive index with respect to the average refractive index of the cladding portion, -Δmax, is negative. The residual compressive stress in the center core is 40 MPa or more and 150 MPa or less. The difference Δ1, which is the average maximum relative refractive index difference between the center core and the average refractive index of the cladding portion, is 0.8% or less. The aforementioned -Δmax is -0.5% or more and -0.1% or less. The diameter of the center core is 3 μm or more and 8 μm or less. The low refractive index layer is a trench layer separated from the center core, and the outer diameter of the trench layer is 9 μm or more and 20 μm or less, or the low refractive index layer is a depressed layer adjacent to the center core, and the outer diameter of the depressed layer is 9 μm or more and 20 μm or less. Optical fiber.

2. The low refractive index layer is the trench layer, the diameter of the center core is 3.5 μm or more and 7.5 μm or less, and the outer diameter of the trench layer is 9.5 μm or more and 19 μm or less. The optical fiber according to claim 1.

3. The diameter of the center core is 3.5 μm or more and 5 μm or less, and the outer diameter of the trench layer is 12 μm or more and 16 μm or less. The optical fiber according to claim 2.

4. The low refractive index layer is the depressed layer, the diameter of the center core is 3.5 μm or more and 7.5 μm or less, and the outer diameter of the depressed layer is 9.5 μm or more and 19 μm or less. The optical fiber according to claim 1.

5. The diameter of the center core is 3.5 μm or more and 5 μm or less, and the outer diameter of the depressed layer is 12 μm or more and 16 μm or less. The optical fiber according to claim 4.

6. In the radial direction, the difference in relative refractive index changes continuously from the center core to the low refractive index layer. The optical fiber according to claim 1.

7. The statement that the relative refractive index difference changes continuously means that the refractive index profile representing the relative refractive index difference changes smoothly without abrupt bends, and the amount of change is 0.1% or less per μm in the radial direction. The optical fiber according to claim 6.

8. The residual compressive stress in the low refractive index layer is 20 MPa or more and 200 MPa or less. The optical fiber according to claim 1.

9. The low refractive index layer contains fluorine. The optical fiber according to claim 1.

10. The transmission loss at a wavelength of 1550 nm is 0.190 dB / km or less, the mode field diameter at a wavelength of 1310 nm is 8.6 μm or more, the cable cutoff wavelength is 1260 nm or less, and the zero-dispersion wavelength is 1300 nm or more. The optical fiber according to claim 1.

11. At a wavelength of 1550 nm, the macrobend loss when wound to a diameter of 20 mm is 0.1 dB / turn or less. The optical fiber according to claim 1.

12. The cladding diameter is within the range of 125 μm ± 10 μm. The optical fiber according to claim 1.

13. The cladding diameter is within the range of 115 μm ± 10 μm. The optical fiber according to claim 1.

14. The cladding diameter is within the range of 105 μm ± 10 μm. The optical fiber according to claim 1.

15. The cladding diameter is within the range of 95 μm ± 10 μm. The optical fiber according to claim 1.

16. The cladding diameter is within the range of 85 μm ± 10 μm. The optical fiber according to claim 1.

17. The coating portion further includes a primary layer surrounding the outer periphery of the cladding portion and a secondary layer surrounding the outer periphery of the primary layer, The outer diameter of the secondary layer is 260 μm or less. The primary layer is made of a resin with a Young's modulus of 0.2 MPa or more and 3 MPa or less. The secondary layer is made of a resin with a Young's modulus of 5 MPa to 2000 MPa. The optical fiber according to claim 1.

18. The maximum difference in residual stress between the core portion and the cladding portion is 40 MPa or more and 220 MPa or less. The optical fiber according to claim 1.

19. The maximum difference in residual stress between the core portion and the cladding portion is 70 MPa or more and 220 MPa or less. The optical fiber according to claim 1.

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