fiber optic

Abstract

To provide an optical fiber that is suitable for reducing transmission loss and is highly manufacturable. [Solution] The optical fiber comprises a core made of silica glass and a cladding portion made of silica glass, surrounding the outer circumference of the core and having a refractive index lower than the maximum refractive index of the core. The core has a center core which has the highest average refractive index within the optical fiber, and the center core does not contain germanium as a dopant to increase the refractive index of the silica glass, and the relative refractive index difference Δclad between the refractive index of the cladding portion and the refractive index of the pure silica glass is 0% or more.

Description

Technical Field

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

Background Art

[0002] Optical fibers are known in which the relative refractive index difference of the center core is set in the range of 0.13% to 0.35% in order to expand the effective core cross-sectional area (Aeff) at a wavelength of 1550 nm (Patent Documents 1 to 5). Further, Patent Document 6 discloses an optical fiber in which the relative refractive index difference of the center core is reduced to 0.1% by adopting a hole structure.

[0003] On the other hand, Patent Document 7 discloses an optical fiber in which the relative refractive index difference of the cladding portion is made negative and the center core does not contain germanium in order to reduce transmission loss.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the optical fiber disclosed in Patent Document 7 requires a large amount of dopant, such as fluorine, to lower the refractive index of the silica glass in order to make the specific refractive index difference of the cladding portion negative, making it difficult to improve manufacturability.

[0006] The present invention has been made in view of the above, and its purpose is to provide an optical fiber that is suitable for reducing transmission loss and is highly manufacturable. [Means for solving the problem]

[0007] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention provides an optical fiber comprising a core made of silica glass and a cladding portion made of silica glass, surrounding the outer periphery of the core and having a refractive index lower than the maximum refractive index of the core, wherein the core has a center core which has the highest average refractive index in the optical fiber, the center core does not contain germanium as a dopant to increase the refractive index of the silica glass, and the relative refractive index difference Δclad of the cladding portion to the refractive index of pure silica glass is 0% or more.

[0008] The aforementioned dopant may contain chlorine.

[0009] The dopant may include chlorine and alkali metals.

[0010] The alkali metal may also be potassium.

[0011] The relative refractive index difference Δ1 between the refractive index of the cladding portion and the maximum refractive index of the center core may be 0.06% or more and 0.16% or less.

[0012] The relative refractive index difference Δ1 between the refractive index of the cladding portion and the maximum refractive index of the center core may be 0.07% or more and 0.13% or less.

[0013] The relative refractive index difference Δ1 between the refractive index of the cladding portion and the maximum refractive index of the center core may be 0.08% or more and 0.12% or less.

[0014] The optical fiber may transmit light with a wavelength of 1550 nm in single mode, and the transmission loss of the light may be 0.180 dB / km or less.

[0015] The core portion may surround the outer periphery of the center core and have a negative Δ core layer in which the relative refractive index difference with respect to the average refractive index of the clad portion is negative.

[0016] The clad portion may not contain fluorine.

[0017] Holes may be provided in the clad portion and arranged around the core portion.

[0018] The outer diameter of the clad portion may be greater than 125 μm and 250 μm or less.

[0019] The optical fiber includes a coating layer surrounding the outer periphery of the clad portion, and the outer diameter of the coating layer may be greater than 250 μm.

[0020] The optical fiber may have an effective core cross-sectional area of 160 μm 2 or more at a wavelength of 1550 nm.

[0021] The optical fiber may have an effective core cross-sectional area of 170 μm 2 or more at a wavelength of 1550 nm.

[0022] The optical fiber may have an effective core cross-sectional area of 200 μm 2 or more at a wavelength of 1550 nm.

[0023] The concentration of the chlorine may be 3000 ppm or more and 16000 ppm or less, and the concentration of the potassium may be 5 ppm or more and 1000 ppm or less.

[0024] The core portion has a depressed layer as the negative Δ core layer, the optical fiber has a W-type refractive index profile, the relative refractive index difference Δ2 of the negative Δ core layer is -0.65% or more and -0.05% or less, and when the core diameter of the center core is 2a [μm] and the outer diameter of the depressed layer is 2b [μm], 2a is 12 or more and 40 or less, and b / a may be 2 or more and 4.5 or less.

[0025] The core portion comprises a trench layer as the negative Δ core layer and an intermediate layer located between the center core and the trench layer. The optical fiber has a trench-type refractive index profile. If the relative refractive index difference of the intermediate layer with respect to the cladding portion is Δ2, then Δ2 is -0.25% or more and 0.00% or less. If the relative refractive index difference of the trench layer with respect to the cladding portion is Δ3, then Δ3 is -0.70% or more and -0.20% or less. When the core diameter of the center core is 2a [μm], the inner diameter of the trench layer is 2b [μm], and the outer diameter is 2c [μm], then 2a may be 12 or more and 38 or less, b / a may be 1.2 or more and 6.0 or less, and c / a may be 4.0 or more and 8.0 or less.

[0026] The number of pores may be 10 or more and 12 or less, the distance from the center of the cladding to the center of the pores may be 45 μm or more and 50 μm or less, and the diameter of the pores may be 8 μm or more and 12 μm or less. [Effects of the Invention]

[0027] According to the present invention, it is possible to realize an optical fiber that is suitable for reducing transmission loss and has high manufacturability. [Brief explanation of the drawing]

[0028] [Figure 1] Figure 1 is a schematic cross-sectional view of an optical fiber according to Embodiment 1. [Figure 2] Figure 2 is a schematic diagram of an example of the refractive index profile of an optical fiber according to the embodiment. [Figure 3] Figure 3 shows an example of the relationship between Δ1 and the transmission loss at a wavelength of 1550 nm. [Figure 4] Figure 4 shows an example of the relationship between b / a and transmission loss at a wavelength of 1550 nm. [Figure 5] Figure 5 shows an example of a refractive index profile. [Figure 6] Figure 6 shows an example of the relationship between fiber diameter and transmission loss at a wavelength of 1550 nm. [Figure 7] Figure 7 is a schematic cross-sectional view of an optical fiber according to Embodiment 2. [Figure 8] Figure 8 shows an example of the relationship between pore size and transmission loss. [Modes for carrying out the invention]

[0029] 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 denoted by the same reference numerals as appropriate. Furthermore, in this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength as defined in ITU-T G.650.1 of the International Telecommunication Union (ITU). In addition, 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.

[0030] (Embodiment 1) Figure 1 is a schematic cross-sectional view of an optical fiber according to Embodiment 1. The optical fiber 1 comprises a core portion 1a made of silica glass, a cladding portion 1b made of silica glass having a refractive index lower than the maximum refractive index of the core portion 1a and surrounding the outer periphery of the core portion 1a, and a coating layer 1c surrounding the outer periphery of the cladding portion 1b. The coating layer 1c has a primary layer 1ca surrounding the outer periphery of the cladding portion 1b and a secondary layer 1cb surrounding the outer periphery of the primary layer 1ca.

[0031] The optical fiber 1 has a refractive index profile as shown in Figure 2, for example. Figures 2(a) and 2(b) both show the refractive index profile in the radial direction from the central axis of the core portion 1a of the optical fiber 1. The refractive index profile is shown as the difference in relative refractive index with respect to pure silica glass. Here, pure silica 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.

[0032] Figure 2(a) shows a so-called W-type refractive index profile. In Figure 2(a), 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 W-type refractive index profile, the core portion 1a 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. The center core is the part of the core portion 1a with the highest average refractive index. The maximum relative refractive index difference of the center core with respect to the refractive index of the cladding portion 1b is Δ1. The relative refractive index difference of the average refractive index of the depressed layer with respect to the refractive index of the cladding portion 1b is Δ2. Also, the relative refractive index difference of the cladding portion 1b with respect to the refractive index of pure silica glass is Δclad. The depressed layer is an example of a negative Δ core layer, where the relative refractive index difference with respect to the refractive index of the cladding portion 1b is negative. In this specification, so-called step-type refractive index profiles are included in the W-type refractive index profile when a=b (a-b=0).

[0033] Figure 2(b) shows a so-called trench-type refractive index profile. In Figure 2(b), profile P21 shows the refractive index profile of the core portion 1a, and profile P22 shows the refractive index profile of the cladding portion 1b. In a 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. The center core is the part of the core portion 1a with the maximum average refractive index. The difference in the maximum relative refractive index of the center core with respect to the refractive index of the cladding portion 1b is Δ1. The difference in relative refractive index of the intermediate layer with respect to the refractive index of the cladding portion 1b is Δ2. Note that Δ2 is usually set to 0% or near 0%. The range of Δ2 is, for example, -0.25% to 0.00%. The relative refractive index difference between the refractive index of the cladding 1b and the refractive index of the trench layer is Δ3. Furthermore, the relative refractive index difference between the refractive index of the cladding 1b and the refractive index of the pure silica glass is Δclad. The trench layer is an example of a negative Δ core layer.

[0034] Here, the refractive index profile of the core 1a may not only 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 at the top of the refractive index profile that is approximately flat within the range of the core diameter 2a of the core 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 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.

[0035] Furthermore, the refractive index of the depressed layer, intermediate layer, trench layer, and cladding 1b is the average value of the refractive index in the radial direction of the refractive index profile.

[0036] Furthermore, it is preferable that the refractive index profile of optical fiber 1 is set to transmit light with a wavelength of 1550 nm in single mode.

[0037] Next, the constituent materials of the optical fiber 1 will be described. The center core of the core portion 1a is made of silica glass that does not contain germanium as a dopant to increase the refractive index of the silica glass, but rather contains, for example, chlorine or an alkali metal. The alkali metal is, for example, potassium or sodium.

[0038] On the other hand, the cladding portion 1b is composed of components such that the relative refractive index difference Δclad with respect to the refractive index of pure silica glass is 0% or more. The cladding portion 1b is made of, for example, pure silica glass or silica glass containing chlorine or alkali metals.

[0039] Furthermore, the depressed layer and trench layer are made of silica glass containing fluorine or boron, which are dopants that reduce the refractive index. The intermediate layer is made of silica glass with the same or similar composition as cladding 1b. Compared to boron, fluorine is preferred from the viewpoint of manufacturability.

[0040] The primary layer 1ca and the secondary layer 1cb are made of resin. This resin is, for example, an ultraviolet-curable resin. The ultraviolet-curable resin is a mixture of various resin materials and additives, such as oligomers, diluent monomers, photopolymerization initiators, silane coupling agents, sensitizers, and lubricants. As the oligomer, conventionally known materials such as polyether-based urethane acrylate, epoxy acrylate, polyester acrylate, and silicone acrylate can be used. As the diluent monomer, conventionally known materials such as monofunctional monomers and polyfunctional monomers can be used. Furthermore, the additives are not limited to those mentioned above, and a wide range of conventionally known additives used for ultraviolet-curable resins can be used.

[0041] The elastic modulus of the primary layer 1ca (primary modulus) is, for example, 0.2 MPa to 3.0 MPa, and more preferably 1.0 MPa or less. The elastic modulus of the secondary layer 1cb (secondary modulus) is, for example, 2000 MPa or less, which allows the rigidity of the secondary layer 1cb to be within an appropriate range. Furthermore, the secondary modulus is preferably 5.0 MPa or more, and more preferably 500 MPa or more.

[0042] In this optical fiber 1, the center core does not contain germanium as a dopant to increase the refractive index of silica glass, so no transmission loss occurs due to germanium scattering loss. As a result, optical fiber 1 is suitable for reducing transmission loss. Furthermore, in optical fiber 1, the components are such that the specific refractive index difference Δclad of the cladding portion 1b is 0% or more, and Δclad is not a negative value. Therefore, in optical fiber 1, the cladding portion 1b does not contain fluorine, or if it does, it can contain only a relatively small amount. As a result, optical fiber 1 has high manufacturability.

[0043] The optical fiber 1 will be explained in more detail below. Figure 3 shows an example of the relationship between Δ1 and the transmission loss at a wavelength of 1550 nm in optical fiber 1. Figure 3 shows the case where the center core contains chlorine as a dopant and the case where it contains chlorine and potassium as dopants. Figure 3 also shows the transmission loss value that does not include the effects of microbend loss and macdo bend loss. Furthermore, in Figure 3, the type of refractive index profile and structural parameters other than Δ1 are varied in various ways to optimize the transmission loss to the minimum.

[0044] According to the inventors' research, when the center core contains chlorine as a dopant, the transmission loss can be reduced to a low value of 0.180 dB / km or less when Δ1 is between 0.06% and 0.16%. Furthermore, the transmission loss can be reduced to an even lower value when Δ1 is between 0.07% and 0.13%, and to an even lower value when Δ1 is between 0.08% and 0.12%.

[0045] Furthermore, it was confirmed that if the center core contains chlorine and potassium as dopants, the transmission loss can be further reduced within the range of Δ1 described above. The preferred chlorine concentration is, for example, 3000 ppm to 16000 ppm, and the preferred potassium concentration is, for example, 5 ppm to 1000 ppm.

[0046] Next, we will explain negative Δ core layers such as depressed layers and trench layers. Negative Δ core layers contribute to achieving low transmission loss values ​​such as 0.180 dB / km even in the low range of Δ1 between 0.06% and 0.16%. Furthermore, since such negative Δ layers have a significantly smaller volume than the cladding portion 1b, the resulting decrease in manufacturability is suppressed.

[0047] The inventors investigated the reduction of transmission loss by a negative Δ core layer as follows. Specifically, in optical fiber 1, the center core contained only chlorine, fixing Δ1 to 0.1%, and the width of the negative Δ core layer was fixed to a. The relationship between b / a in the trench type and the transmission loss at a wavelength of 1550 nm (excluding the effect of microbend loss) was investigated. Fixing the width of the negative Δ core layer to a means setting c such that (cb)=a holds true. Furthermore, Δ2 was fixed to 0%.

[0048] The results are shown in Figure 4. In Figure 4, when b / a = 1, the refractive index profile corresponds to a W-type profile. The legend also shows the relative refractive index difference of the negative Δ core layer (i.e., Δ2 or Δ3). As shown in Figure 4, it was confirmed that when b / a is the same value, a larger absolute value of the relative refractive index difference of the negative Δ core layer tends to be more suitable for reducing transmission loss. However, it should be noted that increasing the absolute value of the relative refractive index difference of the negative Δ core layer requires increasing the fluorine concentration.

[0049] According to the inventors' research, when the refractive index profile is W-type, an example of preferred structural parameters is Δ2 being between -0.65% and -0.05%, 2a being between 12 μm and 40 μm, and b / a being between 2 and 4.5. When the refractive index profile is trench-type, an example of preferred structural parameters is Δ2 being between -0.25% and 0.00%, Δ3 being between -0.70% and -0.20%, 2a being between 12 μm and 38 μm, b / a being between 1.2 and 6.0, and c / a being between 4.0 and 8.0.

[0050] In Figure 2, the refractive index changes in a step-like manner at the boundaries between the center core, intermediate layer, trench layer, and cladding; however, in reality, the refractive index may change more smoothly.

[0051] Figure 5 shows examples of refractive index profiles when the refractive index changes smoothly at the boundary. Figures 5(a) and (b) are examples of trench-type refractive index profiles, while Figures 5(c) and (d) are examples of W-type refractive index profiles.

[0052] (Outer diameter of the cladding portion and outer diameter of the covering portion) Incidentally, Figures 3 and 4 show transmission loss values ​​that do not include the effects of microbend loss and Macdo-bend loss. However, from the viewpoint of practical application of optical fiber 1, it is preferable that the transmission loss, especially considering the effects of microbend loss, is reduced.

[0053] In particular, in the optical fiber 1 according to Embodiment 1, the effective core cross-sectional area at a wavelength of 1550 nm is, for example, 160 μm². 2 That is all, and furthermore, 170 μm 2 That is all, and furthermore, 200 μm 2 As explained above, and because it is relatively large, it is relatively susceptible to the effects of microbend losses.

[0054] According to the inventor's research, it was confirmed that the effect of microbend loss on transmission characteristics can be reduced by increasing the outer diameter of the cladding portion 1b or the outer diameter of the coating layer 1c in the optical fiber 1.

[0055] Specifically, the inventors used an optical fiber having a trench-type refractive index profile and the structural parameter values ​​shown in Table 1 as a reference optical fiber, and investigated its optical properties, obtaining the values ​​shown in Table 2. In Table 1, "glass diameter" refers to the outer diameter of the cladding. "First resin diameter" refers to the outer diameter of the primary layer. "Second resin diameter (fiber diameter)" refers to the outer diameter of the secondary layer, which is the outer diameter of the optical fiber. In Table 2, "λcc" refers to the cutoff wavelength. "MFD" refers to the mode field diameter at a wavelength of 1550 nm. "Aeff" is the effective core cross-sectional area at a wavelength of 1550 nm. "Wavelength dispersion" is the value at a wavelength of 1550 nm. "Dispersion gradient" is the value at the zero-dispersion wavelength. The transmission loss is the value at a wavelength of 1550 nm, and is the value when the optical fiber is wound on a general-purpose bobbin with an inner diameter of 180 mmφ under a tension of 20 gf so that a large microbend loss occurs.

[0056] As shown in Table 2, the transmission loss of the reference optical fiber was high at 0.396 dB / km. This is thought to be due to the effect of microbend loss.

[0057] [Table 1] [Table 2]

[0058] Therefore, the inventors investigated the characteristics of multiple optical fibers in which only the fiber diameter (coating thickness) was increased while the glass diameter remained fixed relative to a reference optical fiber, or in which both the glass diameter and the fiber diameter (coating thickness) were increased. Here, when increasing both the glass diameter and the fiber diameter, the glass diameter and fiber diameter were increased in the same ratio. That is, for example, when the glass diameter was increased by 1.2 times, the coating diameter was also increased by 1.2 times, and when the glass diameter was doubled, the coating diameter was also doubled.

[0059] Figure 6 shows an example of the relationship between the fiber diameter of the investigated optical fiber and the transmission loss at a wavelength of 1550 nm. Note that the data for a fiber diameter of 250 μm is from a reference optical fiber. As shown in Figure 6, it was confirmed that increasing only the fiber diameter, or increasing both the glass diameter and the fiber diameter, could reduce the transmission loss compared to the reference optical fiber. Therefore, it is preferable that the outer diameter of the cladding (glass diameter) be greater than 125 μm, and the outer diameter of the coating layer (fiber diameter) be greater than 250 μm. In particular, if the glass diameter is 125 μm, increasing the fiber diameter to more than 400 μm can reduce the transmission loss, including the effect of microbend loss, to 0.180 dB / km or less. Furthermore, if the glass diameter is 175 μm or more and the fiber diameter is 350 μm or more, the transmission loss, including the effect of microbend loss, can be reduced to 0.180 dB / km or less. However, for the glass diameter, 250 μm or less is preferable to ensure the mechanical reliability of the core and cladding.

[0060] The inventors investigated the optical properties of an optical fiber having a trench-type refractive index profile and structural parameter values ​​as shown in Table 3, and obtained the values ​​shown in Table 4. As shown in Table 4, the transmission loss of the investigated optical fiber was small, at 0.171 dB / km, despite the influence of microbend loss.

[0061] [Table 3] [Table 4]

[0062] (Embodiment 2) Figure 7 is a schematic cross-sectional view of an optical fiber according to Embodiment 2. The optical fiber 2 has a configuration in which 12 voids 2d are provided in the cladding portion 1b of the optical fiber 1. The voids 2d are arranged in a ring shape around the core portion 1a, forming a 30-degree angle with each other when viewed from the core portion 1a. The distance between the center of the cladding portion 1b and the center of the voids 2d (center-to-center distance) is R.

[0063] In optical fiber 2, the same effects as those of optical fiber 1 according to Embodiment 1 can be obtained, and the microbend loss can be reduced by the presence of voids.

[0064] The inventors investigated the transmission loss, including the effect of microbend loss, of optical fibers in which 12 voids were added to the structure of the reference optical fiber described above, similar to optical fiber 2, and in which the void diameter was varied. The results are shown in Figure 8. In Figure 8, the center-to-center distance was set to 50 μm. As shown in Figure 8, it was confirmed that the transmission loss, including the effect of microbend loss, can be reduced more by increasing the void diameter. In optical fiber 2, since the effect of microbend loss is suppressed by the void structure, the glass diameter may be 125 μm and the fiber diameter may be 250 μm.

[0065] Furthermore, according to the inventors' research, an example of a preferred pore structure characteristic for reducing the transmission loss, including the effects of microbend loss, to 0.180 dB / km or less is that the number of pores is 10 to 12, the distance between centers is 45 μm to 50 μm, and the diameter of the pores is 8 μm to 12 μm.

[0066] (Examples) Optical fibers were manufactured according to Embodiment 1 or Embodiment 2 using known VAD (Vapor Axial Deposition) and wire drawing methods. Chlorine and potassium were doped in the gas phase during the VAD matrix manufacturing process. The wire drawing speed was set to 150 m / min or higher to ensure high manufacturability. Table 5 shows the structural parameters and optical properties of the manufactured samples No. 1 to 10. In Table 5, samples No. 1 to 4 have a W-type refractive index profile (sample No. 1 has a step-type refractive index profile), and samples No. 5 to 10 have a trench-type refractive index profile. Samples No. 3 and 8 do not have a void structure. In Table 5, "d" in "void structure" refers to the diameter of the void. As shown in Table 5, the effective core cross-sectional area of ​​all samples is 200 μm². 2Despite the above, the transmission loss was less than 0.180 dB / km, even including the effect of microbend loss.

[0067] [Table 5]

[0068] In the above embodiments or examples, W-type and trench-type refractive index profiles are shown, but the refractive index profiles are not limited to these.

[0069] Furthermore, the present invention is not limited by the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Moreover, 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. [Explanation of symbols]

[0070] 1,2: Optical fiber 1a: Core 1b: Clad section 1c: Covering layer 1ca: Primary layer 1cb: Secondary layer

Claims

1. A core made of silica glass, A cladding portion made of silica glass surrounds the outer periphery of the core portion and has a refractive index lower than the maximum refractive index of the core portion, Equipped with, The core portion has a center core which has the highest average refractive index within the optical fiber. The aforementioned center core does not contain germanium as a dopant to increase the refractive index of silica glass. The relative refractive index difference Δclad between the refractive index of the cladding portion and the refractive index of the pure silica glass is 0% or more. Optical fiber.

2. The preceding dopant contains chlorine. The optical fiber according to claim 1.

3. The dopant includes chlorine and alkali metals. The optical fiber according to claim 1.

4. The alkali metal is potassium. The optical fiber according to claim 3.

5. The relative refractive index difference Δ1 between the refractive index of the cladding portion and the maximum refractive index of the center core is 0.06% or more and 0.16% or less. The optical fiber according to claim 1.

6. The relative refractive index difference Δ1 between the refractive index of the cladding portion and the maximum refractive index of the center core is 0.07% or more and 0.13% or less. The optical fiber according to claim 1.

7. The relative refractive index difference Δ1 between the refractive index of the cladding portion and the maximum refractive index of the center core is 0.08% or more and 0.12% or less. The optical fiber according to claim 1.

8. The system transmits light with a wavelength of 1550 nm in single mode, and the transmission loss of the light is 0.180 dB / km or less. The optical fiber according to claim 1.

9. The core portion surrounds the outer circumference of the center core and has a negative Δ core layer whose relative refractive index difference with respect to the average refractive index of the cladding portion is negative. The optical fiber according to claim 1.

10. The cladding portion does not contain fluorine. The optical fiber according to claim 1.

11. The cladding portion is provided with voids arranged around the core portion. The optical fiber according to claim 1.

12. The outer diameter of the cladding portion is greater than 125 μm and less than or equal to 250 μm. The optical fiber according to claim 1.

13. The cladding portion is provided with a covering layer surrounding its outer periphery, The outer diameter of the coating layer is greater than 250 μm The optical fiber according to claim 1.

14. The effective core cross-sectional area at a wavelength of 1550 nm is 160 μm². ２ That's all. The optical fiber according to claim 1.

15. The effective core cross-sectional area at a wavelength of 1550 nm is 170 μm². ２ That's all. The optical fiber according to claim 1.

16. The effective core cross-sectional area at a wavelength of 1550 nm is 200 μm². ２ That's all. The optical fiber according to claim 1.

17. The concentration of chlorine is 3000 ppm or more and 16000 ppm or less. The concentration of potassium is 5 ppm or more and 1000 ppm or less. The optical fiber according to claim 4.

18. The core portion has a depressed layer as the negative Δ core layer, It has a W-type refractive index profile, The relative refractive index difference Δ2 of the negative Δ core layer is -0.65% or more and -0.05% or less. When the core diameter of the center core is 2a [μm] and the outer diameter of the depressed layer is 2b [μm], then 2a is 12 or more and 40 or less, and b / a is 2 or more and 4.5 or less. The optical fiber according to claim 9.

19. The core portion comprises a trench layer as the negative Δ core layer and an intermediate layer located between the center core and the trench layer. It has a trench-type refractive index profile, If the difference in relative refractive index between the cladding portion and the intermediate layer is Δ2, then Δ2 is -0.25% or more and 0.00% or less. If the difference in specific refractive index between the cladding portion and the trench layer is Δ3, then Δ3 is -0.70% or more and -0.20% or less. When the core diameter of the center core is 2a [μm], the inner diameter of the trench layer is 2b [μm], and the outer diameter is 2c [μm], then 2a is between 12 and 38, b / a is between 1.2 and 6.0, and c / a is between 4.0 and 8.

0. The optical fiber according to claim 9.

20. The number of voids is 10 or more and 12 or less. The distance from the center of the cladding portion to the center of the void is 45 μm or more and 50 μm or less. The diameter of the aforementioned pore is 8 μm or more and 12 μm or less. The optical fiber according to claim 11.

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