Single-mode optical fiber, method for designing a single-mode optical fiber, and method for manufacturing a single-mode optical fiber.

Abstract

This optical fiber comprises a side core layer that surrounds the outer periphery of a core part, and a cladding part that surrounds the outer periphery of the side core layer, the expressions ∆1 > ∆Clad > ∆2 and 0 > ∆2 being true, where ∆1 is the average maximum relative refractive index difference of the core part with respect to the average refractive index of the cladding part, ∆2 is the relative refractive index difference of the average refractive index of the side core layer, and ∆Clad is the relative refractive index difference of the average refractive index of the cladding part with respect to pure quartz glass, the effective core cross-sectional area for a wavelength of 1550 nm is 100 µm2 to 160 µm2, (∆1 - ∆2) is 0.36% to 0.54%, ∆2 is -0.23% to -0.08%, b / a is 2.5-3.9, where 2a is the core diameter and 2b is the outside diameter of the side core layer, and the value of (∆1 - ∆2) is such that the cutoff wavelength is no more than 10 nm from the lowest value thereof.

Description

[Technical Field]

[0001] This invention relates to optical fibers, methods for designing optical fibers, and methods for manufacturing optical fibers. [Background technology]

[0002] Optical fibers employing a W-type refractive index profile are being actively investigated (Patent Documents 1-4). The W-type refractive index profile is used, for example, to increase the effective core cross-sectional area of ​​the optical fiber. In optical fibers with a large effective core cross-sectional area, the occurrence of nonlinear optical effects within the optical fiber is suppressed, making them suitable for use as, for example, long-distance optical transmission lines. Note that the effective core cross-sectional area is sometimes referred to as Aeff. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 6500451 [Patent Document 2] Patent No. 6527973 [Patent Document 3] Japanese Patent Publication No. 2003-66259 [Patent Document 4] Japanese Patent Publication No. 2009-122277 [Overview of the project] [Problems that the invention aims to solve]

[0004] Generally, there is a trade-off relationship between effective core cross-sectional area and cutoff wavelength; increasing the effective core cross-sectional area tends to increase the cutoff wavelength. However, in order to achieve single-mode transmission in the wavelength band used for optical transmission, it is sometimes necessary to lower the cutoff wavelength of the optical fiber. Therefore, there was room for investigation into how to achieve both an increase in effective core cross-sectional area and a decrease in cutoff wavelength.

[0005] The present invention has been made in view of the above, and its object is to provide an optical fiber with an enlarged effective core cross-sectional area, a low cutoff wavelength optical fiber, a method for designing an optical fiber, and a method for manufacturing an optical fiber. [Means for solving the problem]

[0006] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention comprises a core portion, a side core layer surrounding the outer periphery of the core portion, and a cladding portion surrounding the outer periphery of the side core layer. If Δ1 is the difference in the average maximum relative refractive index of the core portion with respect to the average refractive index of the cladding portion, Δ2 is the difference in the relative refractive index of the average refractive index of the side core layer, and ΔClad is the difference in the relative refractive index of the average refractive index of the cladding portion with respect to pure quartz glass, then Δ1 > ΔClad > Δ2 and 0 > Δ2 hold true, and the effective core cross-sectional area at a wavelength of 1550 nm is 100 μm². 2 More than 160μm 2 The optical fiber is such that (Δ1-Δ2) is 0.36% or more and 0.54% or less, Δ2 is -0.23% or more and -0.08% or less, when the core diameter of the core portion is 2a and the outer diameter of the side core layer is 2b, b / a is 2.5 or more and 3.9 or less, and (Δ1-Δ2) is such that the cutoff wavelength is within 10 nm from the lowest value.

[0007] The optical fiber has (Δ1-Δ2) -0.0946(b / a) 3 +0.9962(b / a) 2 -3.5477(b / a)+4.6605≧(Δ1-Δ2)≧-0.0840(b / a) 3 +0.8739(b / a) 2 It is also acceptable if it satisfies -3.0106(b / a)+3.7956.

[0008] The optical fiber may also have a (Δ1-Δ2) ratio of 0.37% or more and 0.44% or less.

[0009] The optical fiber may also have a b / a ratio of 2.8 or more and 3.8 or less.

[0010] One aspect of the present invention includes a core portion, a side core layer surrounding the outer periphery of the core portion, and a clad portion surrounding the outer periphery of the side core layer. Let the average maximum specific refractive index difference of the core portion with respect to the average refractive index of the clad portion be Δ1, the specific refractive index difference of the average refractive index of the side core layer be Δ2, and the specific refractive index difference of the average refractive index of the clad portion with respect to pure silica glass be ΔClad. Then, it is a method for designing an optical fiber in which Δ1 > ΔClad > Δ2 and 0 > Δ2 hold. When the core diameter of the core portion is 2a and the outer diameter of the side core layer is 2b, according to the value of b / a, (Δ1 - Δ2) is set to a value such that the cut-off wavelength is within 10 nm from the minimum value. Under the condition of the set (Δ1 - Δ2), as structural parameters, the Δ1, the Δ2, the ΔClad, the core diameter of the core portion, and the outer diameter of the side core layer are set so that the optical characteristics become desired characteristics. This is a method for designing an optical fiber.

[0011] One aspect of the present invention is a method for manufacturing an optical fiber, which manufactures an optical fiber so as to satisfy the structural parameters set in the method for designing the optical fiber.

Advantages of the Invention

[0012] According to the present invention, there is an effect that an optical fiber with a low cut-off wavelength and an enlarged effective core cross-sectional area can be realized.

Brief Description of the Drawings

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view of an optical fiber according to an embodiment. [Figure 2] FIG. 2 is a schematic diagram of the refractive index profile of an optical fiber according to an embodiment. [Figure 3] FIG. 3 is a diagram showing an example of the relationship between (Δ1 - Δ2) and λcc. [Figure 4] FIG. 4 is a diagram showing an example of the relationship between (Δ1 - Δ2) and bending loss. [Figure 5] FIG. 5 is a diagram showing an example of the relationship between (Δ1 - Δ2) and transmission loss. [Figure 6] Figure 6 shows an example of favorable conditions for (Δ1-Δ2) and b / a. [Modes for carrying out the invention]

[0014] 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.

[0015] (Embodiment) Figure 1 is a schematic cross-sectional view of an optical fiber according to an embodiment. The optical fiber 10 is made of silica-based glass and comprises a core portion 11, a side core layer 12 surrounding the outer periphery of the core portion 11, and a cladding portion 13 surrounding the outer periphery of the side core layer 12. The optical fiber 10 may also have a coating layer surrounding the outer periphery of the cladding portion 13.

[0016] Figure 2 shows the refractive index profiles of the optical fiber 10. Profile P1 is the refractive index profile of the core portion 11, which has a so-called stepped shape. Profile P2 is the refractive index profile of the side core layer 12. Profile P3 is the refractive index profile of the cladding portion 13.

[0017] Here, the refractive index profile of the core portion 11 may not only be a geometrically ideal step shape, but may also have an uneven shape 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 at the top of the refractive index profile that is approximately flat within the range of the core diameter 2a of the core portion 11 in the manufacturing design serves as an indicator for determining Δ1.

[0018] The structural parameters of the optical fiber 10 are described below. As mentioned above, the core diameter of the core portion 11 is 2a. The outer diameter of the side core layer 12 is 2b.

[0019] Furthermore, the relative refractive index difference (maximum relative refractive index difference) between the average refractive index of the cladding 13 and the average maximum refractive index of the core 11 is Δ1. The relative refractive index difference between the average refractive index of the cladding 13 and the average refractive index of the side core layer 12 is Δ2. Note that the average maximum refractive index of the core 11 is the average value in the radial direction of the refractive index in the region that is approximately flat at the top of the refractive index profile. The average refractive index of the side core layer 12 and the cladding 13 is the average value of the refractive index in the radial direction of the refractive index profile.

[0020] Furthermore, the relative refractive index difference between the average refractive index of the cladding portion 13 and the refractive index of the pure quartz glass is ΔClad. Here, pure quartz glass is an 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. In Figure 2, the dashed line shows the relative refractive index difference of the pure quartz glass and the average refractive index of the cladding portion 13.

[0021] For Δ1, Δ2, and ΔClad, the following conditions hold: Δ1 > ΔClad > Δ2 and 0 > Δ2. That is, the optical fiber 10 has a W-type refractive index profile. Also, although Figure 2 shows the case where ΔClad is less than 0%, ΔClad may be 0% or greater.

[0022] The constituent materials of the optical fiber 10 will now be described. The core portion 11 is made of silica-based glass containing a refractive index adjusting dopant to increase the refractive index. For example, the core portion 11 contains at least one, for example two or more, of germanium (Ge), chlorine (Cl), fluorine (F), potassium (K), and sodium (Na) as dopants. F is a dopant that lowers the refractive index of the silica glass, while (Ge), Cl, K, and Na are dopants that increase the refractive index of the silica glass. The core portion 11 may also be made of pure silica glass.

[0023] On the other hand, the side core layer 12 and the cladding portion 13 are made of silica-based glass to which only F and Cl, only F, or only Cl is added. By adjusting the refractive index with these dopants, Δ1 > ΔClad > Δ2 and 0 > Δ2 are satisfied, and furthermore, a suitable range for Δ1, Δ2, and ΔClad, which will be described later, is realized. The cladding portion 13 may also be made of pure silica glass.

[0024] (Suitable structural parameters) The structural parameters of the optical fiber 10 according to this embodiment, Δ1, Δ2, ΔClad, 2a, and 2b, will be described below.

[0025] In order to realize an optical fiber 10 with an enlarged effective core cross-sectional area and a low cutoff wavelength, the inventors diligently investigated the structural parameters of the W-type refractive index profile and the optical properties obtained therefrom, and as a result, found the following.

[0026] In other words, in the case of a W-type refractive index profile, the average refractive index perceived by the light differs depending on whether the light field propagating through the optical fiber is mainly confined to the core or extends to the side core layers. Therefore, we hypothesized that the behavior of light confinement, particularly in higher-order propagation modes, would also differ. Furthermore, we thought that this difference in average refractive index would also affect the behavior of λcc. In addition, we considered that there might be a value for which λcc takes its lowest value when the parameter (Δ1-Δ2) is changed. In particular, in optical fibers with a relatively large effective core cross-sectional area (Aeff), the spread of the light field to the side core layers tends to be larger, so we thought that a situation where λcc takes its lowest value when (Δ1-Δ2) is changed is more likely to occur.

[0027] Therefore, for the optical fiber 10, the change in λcc when (Δ1-Δ2) is varied under several b / a conditions was investigated using simulation calculations. In all b / a conditions, the investigation was conducted within the range of Δ1 from 0.21 to 0.29% and Δ2 from -0.23 to -0.08%. The following results were obtained.

[0028] Figure 3 shows an example of the relationship between (Δ1-Δ2) and λcc. In Figure 3, the change in λcc is shown when b / a is set to 2, 2.5, 3, 3.5, or 4, and (Δ1-Δ2) is varied from 0.3% to 0.55% or 0.56%. The horizontal axis shows (Δ1-Δ2)[%], and the vertical axis shows the increase from the minimum value of λcc. Note that when b / a is a different value, the minimum value of λcc is also a different value. Also, when b / a is 2, λcc continues to decrease even when (Δ1-Δ2) is increased to 0.55%, and the existence of a minimum value was not confirmed. Therefore, when b / a is 2, please note that the vertical axis shows the increase from the value of λcc when (Δ1-Δ2) is 0.55%, rather than the increase from the minimum value of λcc.

[0029] As can be seen from Figure 3, there is a close relationship between (Δ1-Δ2) and λcc. When (Δ1-Δ2) is changed, λcc changes in a way that causes it to follow a downward-convex curve, and it was confirmed that there is a value of (Δ1-Δ2) at which λcc is at its lowest value. Furthermore, it was confirmed that λcc is large when (Δ1-Δ2) is too large or too small relative to the value of (Δ1-Δ2) at which λcc is at its lowest value.

[0030] Setting (Δ1-Δ2) appropriately to reduce the λcc of the optical fiber means that the wavelength bandwidth for single-mode transmission can be widened accordingly, which is desirable from the viewpoint of broadening the wavelength bandwidth used in optical transmission. Furthermore, if setting (Δ1-Δ2) appropriately to reduce the λcc of the optical fiber to a value smaller than the desired value allows the core diameter to be set larger until the λcc reaches the desired value, thereby enabling expansion of Aeff and improvement of macrobend characteristics.

[0031] Also, as shown in FIG. 3, in the vicinity of (Δ1 - Δ2) where λcc takes the minimum value, the change in λcc with respect to the change in (Δ1 - Δ2) is relatively gentle. Therefore, if (Δ1 - Δ2) is set to a value at which λcc takes the minimum value or a value in its vicinity, even when the value of (Δ1 - Δ2) deviates from the set value due to an error such as a manufacturing error, the deviation of λcc from the set value can be made relatively small. From this perspective, if (Δ1 - Δ2) is set to a value such that λcc is within 10 nm from the minimum value, it becomes difficult for manufacturing errors and the like to affect λcc, and stable manufacturability can be realized.

[0032] From the above description, the optical fiber 10 according to the present embodiment is an optical fiber in which (Δ1 - Δ2) is a value such that λcc is within 10 nm from the minimum value.

[0033] Also, from the perspective of bending loss, it is preferable that (Δ1 - Δ2) is 0.36% or more. Here, the bending loss means the bending loss at a wavelength of 1550 nm when bent with a diameter of 30 mm, unless otherwise specified. Also, such bending loss may be described as macro bend loss, and the condition of bending with a diameter of 30 mm may be described together with "30 mmφ".

[0034] The bending loss will be described with reference to FIG. 4. FIG. 4 is a diagram showing an example of the relationship between (Δ1 - Δ2) and the bending loss. For Δ1, Δ2, 2a, and b / a, various combinations were set to cover all cases. As can be seen from FIG. 4, as (Δ1 - Δ2) becomes smaller than 0.36%, the bending loss increases rapidly. The solid line in FIG. 4 is a curve showing an example of a fitting function obtained by the least squares method for the data points. The fitting function is a fourth-order function of (Δ1 - Δ2), and specifically, y = 2126.5x 4 - 3638.2x 3 + 2328.4x 2The result was -660.83x + 70.245, and it can be seen that the bending loss increases sharply as (Δ1-Δ2) becomes smaller than 0.36%. Therefore, from the viewpoint of reducing bending loss, (Δ1-Δ2) of 0.36% or more is desirable. Furthermore, if (Δ1-Δ2) is 0.37% or more, the bending loss can be reduced more reliably.

[0035] Furthermore, from the viewpoint of transmission loss, the optical fiber 10 preferably has a (Δ1-Δ2) of 0.54% or less, and more preferably 0.44% or less.

[0036] The transmission loss will be explained with reference to Figure 5. Figure 5 shows an example of the relationship between (Δ1-Δ2) and the transmission loss. Note that the transmission loss is the value at a wavelength of 1550 nm. In the legend, "Ge" refers to the typical transmission loss when the core portion 11 contains germanium as a dopant, and "Si" refers to the typical transmission loss when the core portion 11 is made of pure quartz glass.

[0037] As can be seen from Figure 5, in both the case of "Ge" and "Si", when (Δ1-Δ2) is 0.45% or higher, the transmission loss increases as (Δ1-Δ2) increases, and when it exceeds 0.55%, it tends to increase sharply. Furthermore, if (Δ1-Δ2) is too large, the amount of dopant that needs to be added becomes large, making manufacturing difficult. Therefore, from the viewpoint of transmission loss, (Δ1-Δ2) is preferably 0.54% or less, and more preferably 0.44% or less.

[0038] Next, the preferred range for b / a will be explained with reference to Figure 3. As can be seen from Figure 3, when b / a is 2, the existence of the minimum value of λcc was not confirmed even when (Δ1-Δ2) was increased to 0.55%, and when b / a is 2.5, the existence of the minimum value was confirmed up to 0.54% when (Δ1-Δ2) was changed to 0.56%. Therefore, considering that a preferred value for (Δ1-Δ2) is 0.54% or less as described above, a b / a of 2.5 or higher is preferred. Also, when b / a is 3.9, the value of (Δ1-Δ2) at which λcc is within 10 nm from the minimum value is 0.36%. Therefore, considering that a preferred value for (Δ1-Δ2) is 0.36% or more as described above, a b / a of 3.9 or lower is preferred. Furthermore, considering that it is even more preferable for (Δ1-Δ2) to be between 0.37% and 0.44%, a b / a of 2.8 or higher is preferred, and a b / a of 3.8 or lower is preferred.

[0039] Furthermore, for Δ2, a value between -0.23% and -0.08% is preferable because it allows for a widened Aeff and a low cutoff wavelength, thus achieving good characteristics.

[0040] Table 1 shows the range in b / a from 2.5 to 4.0 where (Δ1-Δ2) is a value such that the cutoff wavelength is within 10 nm of the minimum value, and the bending loss does not increase abruptly.

[0041] [Table 1]

[0042] Therefore, as a design method for the optical fiber 10, it is preferable to set (Δ1-Δ2) according to the value of b / a as shown in Table 1, and then set Δ1, Δ2, ΔClad, 2a, and 2b as structural parameters so that the optical characteristics become the desired characteristics under the set conditions of (Δ1-Δ2). Note that 2a may be appropriately changed to adjust to the desired λcc according to the wavelength band used for optical transmission. Aeff also changes with changes in 2a, and specifically Aeff increases with increasing 2a. Note that the trend in Table 1 has been confirmed with good reproducibility in a range where the confinement of light to the core is neither too strong nor too weak, and specifically Aeff at a wavelength of 1550 nm is 100 μm 2 More than 160μm 2 The following conditions were confirmed with good reproducibility. Therefore, the Aeff of optical fiber 10 at a wavelength of 1550 nm is preferably, for example, 100 μm. 2 More than 160μm 2 The following applies:

[0043] Furthermore, as a method for manufacturing the optical fiber 10, it is preferable to manufacture the optical fiber using a known manufacturing method so as to satisfy the structural parameters set in the design method described above. Specifically, the optical fiber 10 can be easily manufactured by manufacturing an optical fiber matrix using a known method such as the VAD (Vapor Axial Deposition) method, OVD (Outside Vapor Deposition) method, MCVD (Modified Chemical Vapor Deposition) method, or plasma CVD method, and then drawing the optical fiber 10 from this optical fiber matrix in a drawing furnace.

[0044] For example, dopants such as Ge, F, K, and Na can be added to the optical fiber matrix by using a dopant-containing gas during soot synthesis. Cl can be added to the optical fiber matrix by leaving residual chlorine gas in the dehydration process. F can be added to the optical fiber matrix by flowing fluorine gas during the vitrification sintering process.

[0045] Furthermore, Figure 6 shows an example of favorable conditions for (Δ1-Δ2) and b / a. Specifically, Figure 6 shows the maximum value of (Δ1-Δ2) for each b / a as shown in Table 1, indicated by the black circle data points, and the minimum value of (Δ1-Δ2) for each b / a as indicated by the black square data points. In addition, in Figure 6, line L1 shows the curve fitted to the black square data points using the least squares method, and line L2 shows the curve fitted to the black circle data points using the least squares method.

[0046] The value of line L1 is (Δ1-Δ2)=-0.0840(b / a). 3 +0.8739(b / a) 2 This can be expressed as -3.0106(b / a) + 3.7956. The line L2 is (Δ1-Δ2) = -0.0946(b / a). 3 +0.9962(b / a) 2 It can be expressed as -3.5477(b / a)+4.6605. Therefore, in optical fiber 10, b / a is 2.5 or more, and (Δ1-Δ2) is -0.0946(b / a) 3 +0.9962(b / a) 2 -3.5477(b / a)+4.6605≧(Δ1-Δ2)≧-0.0840(b / a) 3 +0.8739(b / a) 2 -3.0106(b / a)+3.7956 Satisfying this condition is more preferable from the viewpoint of achieving an increased effective core cross-section and a low cutoff wavelength. In other words, for the combination of b / a and (Δ1-Δ2), it is more preferable that it be within the range enclosed by line L1 and line L2 from the viewpoint of achieving an increased effective core cross-section and a low cutoff wavelength.

[0047] Furthermore, line L3 represents b / a = 2.8. Line L4 represents b / a = 3.8. Line L5 represents (Δ1-Δ2) = 0.37%. Line L6 represents (Δ1-Δ2) = 0.44%. Therefore, regarding the combination of b / a and (Δ1-Δ2), it is preferable from the viewpoint of achieving an increased effective core cross-sectional area, a low cutoff wavelength, as well as low bending loss and low transmission loss, that the range of b / a is enclosed by lines L1 and L2, and also enclosed by lines L3, L4, L5, and L6.

[0048] Furthermore, if the refractive index of the side core layer 12 is made too low (Δ2 is made too small), not only will the amount of dopant that needs to be added increase, making manufacturing difficult, but (Δ1-Δ2) will also increase, potentially leading to increased transmission loss as shown in Figure 4. Therefore, setting (Δ1-Δ2) to 0.54% or less, and even 0.44% or less, is preferable in order to avoid making the refractive index of the side core layer 12 too low.

[0049] (Examples) As an example, optical fiber preforms manufactured using the VAD method were drawn to produce optical fibers No. 1 to 13 with a W-type refractive index profile, and their optical properties were measured. The structural parameters and optical properties of each sample are shown in Table 2. "MFD" stands for mode field diameter, and "Slope" stands for dispersion slope. Note that MFD, Aeff, bending loss, chromatic dispersion, and Slope are values ​​at a wavelength of 1550 nm.

[0050] To achieve the W-type refractive index profile, the core portion was made by adding Ge to increase the refractive index compared to pure quartz glass, while the cladding portion was made of pure quartz glass.

[0051] As shown in Table 2, for all optical fibers from samples No. 1 to 13, (Δ1-Δ2) is between 0.36% and 0.54%, Δ2 is between -0.23% and -0.08%, and b / a is between 2.5 and 4. Furthermore, for all optical fibers from samples No. 1 to 13, Aeff is 100 μm. 2 More than 160μm2 2a was set as follows, and λcc was kept within 10 nm of the minimum value (see also Table 1). As a result, as shown in Table 2, it was confirmed that all of the optical fibers from samples No. 1 to 13 had a λcc of 1530 nm or less and were relatively small, and that they could achieve good characteristics such as a bending loss of 1 dB / m or less, or even 0.5 dB / m or less.

[0052] For example, in optical fiber sample No. 3, (Δ1-Δ2) is 0.36%, Δ2 is -0.11%, b / a is 3, 2a is 13.3 μm, and Aeff is 123 μm. 2 The optical fiber of sample No. 9 has a λcc of 1430 nm and a bending loss of 0.36 dB / m. Furthermore, the optical fiber of sample No. 9 has a (Δ1-Δ2) of 0.43%, a Δ2 of -0.16%, a b / a of 3, a 2a of 13.3 μm, and an Aeff of 116 μm. 2 The optical fiber of sample No. 12 has a λcc of 1497 nm and a bending loss of 0.04 dB / m. Furthermore, the optical fiber of sample No. 12 has a (Δ1-Δ2) of 0.39%, a Δ2 of -0.14%, a b / a of 3.0, a 2a of 14.1 μm, and an Aeff of 130 μm. 2 The optical fiber of sample No. 13 has a λcc of 1518 nm and a bending loss of 0.06 dB / m. Furthermore, the optical fiber of sample No. 13 has a (Δ1-Δ2) of 0.39%, a Δ2 of -0.15%, a b / a of 3.2, a 2a of 13.6 μm, and an Aeff of 124 μm. 2 The λcc is 1468 nm and the bending loss is 0.10 dB / m.

[0053] Furthermore, the connection characteristics and cable characteristics (loss after cabling, etc.) of optical fibers No. 1 to 13 were checked, and no particular problems were found.

[0054] [Table 2]

[0055] 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]

[0056] As described above, the present invention is suitable for use in optical fibers. [Explanation of Symbols]

[0057] 10: Optical fiber 11: Core section 12: Side Core Layer 13: Clad section L1, L2, L3, L4, L5, L6: line P1, P2, P3: Profiles

Claims

1. The core part, A side core layer surrounding the outer periphery of the core portion, The cladding portion surrounding the outer periphery of the side core layer, Equipped with, Let Δ1 be the difference in the average relative refractive index of the core portion with respect to the average refractive index of the cladding portion, Δ2 be the difference in the relative refractive index of the side core layer, and ΔClad be the difference in the relative refractive index of the cladding portion with respect to pure quartz glass. Then Δ1 > ΔClad > Δ2 and 0 > Δ2 hold true. The effective core cross-sectional area at a wavelength of 1550 nm is 106 μm². ２ Above 130 μm ２ The following: (Δ1 - Δ2) is between 0.36% and 0.54%, Δ2 is between -0.23% and -0.08%, When the core diameter of the core portion is 2a and the outer diameter of the side core layer is 2b, b / a is 2.5 or more and 3.9 or less. The above (Δ1 - Δ2) is, -0.0946(b / a) ３ +0.9962(b / a) ２ -3.5477(b / a)+4.6605≧(Δ1-Δ2)≧-0.0840(b / a) ３ +0.8739(b / a) ２ -3.0106(b / a)+3.7956 satisfies Single-mode optical fiber.

2. The above (Δ1 - Δ2) is 0.37% or more and 0.44% or less. A single-mode optical fiber according to claim 1.

3. The aforementioned b / a is 2.8 or more and 3.8 or less. A single-mode optical fiber according to claim 1 or 2.

4. The mode field diameter at a wavelength of 1550 nm is between 11.3 μm and 11.9 μm. A single-mode optical fiber according to claim 1.

5. The core diameter is 12.0 μm or more and 13.8 μm or less. A single-mode optical fiber according to claim 1.

6. A method for designing an optical fiber comprising a core portion, a side core layer surrounding the outer periphery of the core portion, and a cladding portion surrounding the outer periphery of the side core layer, wherein the difference in the maximum average relative refractive index of the core portion with respect to the average refractive index of the cladding portion is Δ1, the difference in the relative refractive index of the average refractive index of the side core layer is Δ2, and the difference in the relative refractive index of the average refractive index of the cladding portion with respect to pure quartz glass is ΔCad, such that Δ1 > ΔCad > Δ2 and 0 > Δ2. When the core diameter of the core portion is 2a and the outer diameter of the side core layer is 2b, (Δ1 - Δ2) is set to a value such that the cutoff wavelength is within 10 nm from the lowest value, according to the value of b / a. Under the conditions set above (Δ1 - Δ2), the structural parameters Δ1, Δ2, ΔClad, the core diameter of the core portion, and the outer diameter of the side core layer are set so that the optical properties become the desired properties. Design methods for single-mode optical fibers.

7. A single-mode optical fiber is manufactured to satisfy the structural parameters set in the single-mode optical fiber design method described in claim 6. A method for manufacturing single-mode optical fiber.

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