Optical fiber

Abstract

To provide a relatively inexpensive optical fiber having low transmission loss.SOLUTION: An optical fiber 1 comprises a core part 1a and a cladding part 1b that surrounds an outer periphery of the core part 1a, has a refractive index lower than a maximum refractive index of the core part 1a, and contains a dopant decreasing a refractive index. An outer diameter of the cladding part 1b is 70 μm or more and 120 μm or less, a cable cutoff wavelength is 1530 nm or less, and a transmission loss is 0.18 dB / km or less at a wavelength of 1550 nm.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] An optical fiber that realizes ultra-low transmission loss is disclosed by doping a clad portion with a dopant such as fluorine that reduces the refractive index of glass (Patent Document 1). In this type of optical fiber with a reduced refractive index of the clad portion, it is possible to reduce or almost eliminate the dopant doped in the core portion. As a result, by reducing the Rayleigh scattering loss caused by the concentration distribution of the dopant in the core portion, an optical fiber with ultra-low transmission loss can be realized. In this type of optical fiber, for example, it was difficult to realize a transmission loss of 0.18 dB / km or less at a wavelength of 1550 nm in an optical fiber having a clad portion made of quartz glass that does not contain a dopant that reduces the refractive index.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when doping the clad portion with a relatively expensive and difficult-to-handle dopant such as fluorine or boron, which is another dopant that lowers the refractive index of glass, there is a problem that the material cost and manufacturing cost of the clad portion increase, and as a result, the optical fiber becomes expensive.

[0005] The present invention has been made in view of the above, and an object thereof is to provide an optical fiber that has a low transmission loss and is relatively inexpensive.

Means for Solving the Problems

[0006] In order to solve the above-described problems and achieve the object, one aspect of the present invention provides an optical fiber including: a core portion; and a cladding portion that surrounds the outer periphery of the core portion, has a refractive index lower than the maximum refractive index of the core portion, and contains a dopant that reduces the refractive index, wherein the outer diameter of the cladding portion is 70 μm or more and 120 μm or less, the cable cutoff wavelength is 1530 nm or less, and the transmission loss at a wavelength of 1550 nm is 0.18 dB / km or less.

[0007] The effective core cross-sectional area at a wavelength of 1550 nm may be 150 μm 2 or less.

[0008] The effective core cross-sectional area at a wavelength of 1550 nm may be 50 μm 2 or more.

[0009] At least a part of the cladding portion may be made of silica glass containing fluorine or boron.

[0010] The core portion may be made of pure silica glass or silica glass containing one or more of chlorine, fluorine, germanium, potassium, and sodium, and have a center core having the maximum average refractive index among the optical fibers.

[0011] The optical fiber may further include a coating layer that surrounds the outer periphery of the cladding portion, and the outer diameter of the coating layer may be 210 μm or less.

[0012] The coating layer may include a primary layer that surrounds the outer periphery of the cladding portion and a secondary layer that surrounds the outer periphery of the primary layer, and the thickness of the primary layer may be 10 μm or more.

[0013] The normalized microbend loss, normalized by the microbend loss at a wavelength of 1550 nm of a standard optical fiber having characteristics compliant with the standards defined in ITU-T G.652 and having a coating layer with a thickness of 62.5 μm on the outer periphery of a cladding portion with an outer diameter of 125 μm, may be 20 or less at a wavelength of 1550 nm.

[0014] The microbend loss may be a value measured by the sandpaper method.

[0015] The increase in transmission loss due to microbend loss may be 0.0193 dB / km or less.

[0016] The refractive index profile may be of a step type.

[0017] The refractive index profile may be of a W type.

[0018] The refractive index profile may be of a trench type.

[0019] The core portion has a center core with the maximum average refractive index among the optical fiber, and the relative refractive index difference between the average refractive index of the center core and the average refractive index of the cladding portion may be 0.20% or more and 0.50% or less.

[0020] The core portion has a center core with the maximum average refractive index among the optical fiber, and the relative refractive index difference between the average refractive index of the center core and the refractive index of pure silica glass may be -0.15% or more and 0.17% or less.

[0021] The core portion has a center core with the maximum average refractive index among the optical fiber, and the relative refractive index difference between the average refractive index of the center core and the refractive index of pure silica glass may be -0.10% or more and -0.01% or less.

[0022] The core part has a center core with the maximum average refractive index among the optical fibers, and the difference in relative refractive index of the average refractive index of the center core with respect to the refractive index of pure silica glass may be 0.02% or more and 0.13% or less.

[0023] The difference in relative refractive index of the average refractive index of the cladding part with respect to the refractive index of pure silica glass may be -0.50% or more and -0.13%.

[0024] The transmission loss may be 0.175 dB / km or less.

Advantages of the Invention

[0025] According to the present invention, there is an effect that an optical fiber with relatively low transmission loss and relatively low cost can be realized.

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Embodiment for Carrying Out the Invention

[0027] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited by the embodiments described below. Also, in each drawing, the same or corresponding components are appropriately assigned the same reference numerals. In addition, in this specification, the cut-off wavelength or the effective cut-off wavelength refers to the cable cut-off wavelength defined in ITU-T G.650.1 of the International Telecommunication Union (ITU). Also, for terms not specifically defined in this specification, the definitions and measurement methods in G.650.1 and G.650.2 shall apply.

[0028] (Embodiment) FIG. 1 is a schematic cross-sectional view of an optical fiber according to an embodiment. The optical fiber 1 includes 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 includes 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.

[0029] The optical fiber 1 has a refractive index profile as shown in FIG. 2, for example. FIGS. 2(a), (b), and (c) all show the refractive index profile in the radial direction from the central axis of the core portion 1a of the optical fiber 1. Note that the refractive index profile is shown by the differential refractive index with respect to pure silica glass. Here, pure silica glass is an extremely high-purity silica glass that substantially does not contain a dopant for changing the refractive index and has a refractive index of about 1.444 at a wavelength of 1550 nm.

[0030] FIG. 2(a) shows a step-type refractive index profile. In FIG. 2(a), profile P11 shows the refractive index profile of the core portion 1a, and profile P12 shows the refractive index profile of the clad portion 1b. In the step-type refractive index profile, the diameter (core diameter) of the core portion 1a is 2a, and the relative refractive index difference (maximum relative refractive index difference) between the maximum refractive index of the core portion 1a and the average refractive index of the clad portion 1b is Δ1. Also, the relative refractive index difference between the average refractive index of the clad portion 1b and the refractive index of pure silica glass is ΔClad. In the case of FIG. 2(a), the center core, which is the portion with the maximum average refractive index within the core portion 1a, corresponds to the entire core portion 1a.

[0031] FIG. 2(b) shows a so-called W-type refractive index profile. In FIG. 2(b), profile P21 shows the refractive index profile of the core portion 1a, and profile P22 shows the refractive index profile of the clad portion 1b. In the W-type refractive index profile, the core portion 1a is composed of a center core with a diameter of 2a and a depressed layer that surrounds the outer periphery of the center core and has an inner diameter of 2a and an outer diameter of 2b and a refractive index smaller than that of the clad portion. The center core is the portion with the maximum average refractive index within the core portion 1a. The maximum relative refractive index difference between the center core and the average refractive index of the clad portion 1b is Δ1. The relative refractive index difference between the average refractive index of the depressed layer and the average refractive index of the clad portion 1b is Δ2. Also, the relative refractive index difference between the average refractive index of the clad portion 1b and the refractive index of pure silica glass is ΔClad.

[0032] Fig. 2(c) shows a so-called trench-type refractive index profile. In Fig. 2(c), profile P31 represents the refractive index profile of the core part 1a, and profile P32 represents the refractive index profile of the clad part 1b. In the trench-type refractive index profile, the core part 1a is formed to have a center core with a diameter of 2a and an intermediate layer that surrounds the outer periphery of the center core and has a refractive index smaller than the maximum refractive index of the center core, with an inner diameter of 2a and an outer diameter of 2b, and a trench layer that surrounds the outer periphery of the intermediate layer and has a refractive index smaller than the refractive index of the clad part, with an inner diameter of 2b and an outer diameter of 2c. The center core is the part with the maximum average refractive index in the core part 1a. The maximum specific refractive index difference between the center core and the intermediate layer is Δ1. The specific refractive index difference between the intermediate layer and the clad part 1b is Δ2. Note that Δ2 is usually set to 0% or near it. The specific refractive index difference between the trench layer and the clad part 1b is Δ3. Also, the specific refractive index difference between the average refractive index of the clad part 1b and the refractive index of pure silica glass is ΔClad.

[0033] Here, the refractive index profile of the core part 1a is not only the case of a step type with a geometrically ideal shape, but also the case where the shape of the top is not flat and irregularities are formed due to manufacturing characteristics, or the shape is such that it draws a skirt from the top. In this case, within the range of the core diameter 2a of the core part 1a in the manufacturing design, the refractive index of the region that is substantially flat at the top of the refractive index profile becomes an index for determining Δ1. Note that when it is considered that there are multiple regions that are substantially flat, or when continuous changes occur and it is difficult to define the region that is substantially flat, at least some part of the core part other than the part where the refractive index changes rapidly towards the adjacent layer falls within the following range of Δ1, and if the difference Δ between the maximum value and the minimum value is within ±30% of a certain value, it has been confirmed that it is possible to obtain characteristics close to the desired ones, and there is no particular problem.

[0034] Also, the average refractive index of the depressed layer, intermediate layer, trench layer, and clad part 1b is the average value of the refractive index in the radial direction of the refractive index profile.

[0035] The constituent materials of the optical fiber 1 will be described. The center core of the core part 1a is made of, for example, pure silica glass or silica glass containing one or more of chlorine (Cl), fluorine (F), germanium (Ge), potassium (K), and sodium (Na). F is a dopant that reduces the refractive index of silica glass, and Ge, Cl, K, and Na are dopants that increase the refractive index of silica glass.

[0036] On the other hand, at least a part of the cladding part 1b is made of silica glass containing, for example, fluorine or boron (B) which is a dopant that reduces the refractive index. On the other hand, the depressed layer and the trench layer are made of silica glass containing more fluorine or boron, which are dopants that reduce the refractive index, than the cladding part. The intermediate layer is made of silica glass having the same or a component close to that of the cladding part 1b. Here, as the dopant for lowering the refractive index, fluorine is more preferable from the viewpoint of manufacturability.

[0037] 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, photoinitiators, silane coupling agents, sensitizers, lubricants, etc. 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. Further, the additives are not limited to those described above, and conventionally known additives used for ultraviolet curable resins and the like can be widely used.

[0038] In this optical fiber 1, since the cladding portion 1b contains a dopant that reduces the refractive index, low transmission loss can be achieved. For example, the transmission loss is 0.18 dB / km or less at a wavelength of 1550 nm. Although the dopants described above are exemplified as the dopants contained in the cladding portion 1b and the core portion 1a, the dopants are not limited to these as long as the transmission loss is 0.18 dB / km or less at a wavelength of 1550 nm. Further, in this optical fiber 1, the cable cut-off wavelength is 1530 nm or less, and light having a wavelength in the 1550 nm band (for example, 1530 nm to 1565 nm) or a longer wavelength can be transmitted in single mode, so it is practical.

[0039] Furthermore, in this optical fiber 1, the outer diameter (cladding diameter) of the cladding portion 1b is 70 μm or more and 120 μm or less, which is smaller than the typical cladding diameter of 125 μm. As a result, it is possible to drastically reduce the volume of the cladding portion 1b containing expensive dopants such as fluorine and boron that reduce the refractive index, resulting in a significant cost reduction.

[0040] The inventor investigated the reduction rate of the volume of the cladding portion with respect to the optical fiber having a cladding diameter of 125 μm when the cladding diameters of three types of optical fibers having different effective core cross-sectional areas and containing a dopant that reduces the refractive index in the cladding portion were changed. The effective core cross-sectional area (Aeff) was set to typical values of 80 μm 2 , 110 μm 2 , 125 μm 2 . Further, the cladding diameter was changed at intervals of 10 μm from 70 μm to 120 μm. The results are shown in Table 1. As shown in Table 1, it was confirmed that when the cladding diameter is 120 μm, the volume of the expensive cladding portion 1b can be reduced by about 8%, and when it is 80 μm, it can be reduced by about 60%. It was confirmed that reducing the diameter of the cladding portion 1b is an extremely effective means for cost reduction.

[0041]

Table 1

[0042] [Microbend Loss] Here, when reducing the cladding diameter of the cladding portion 1b in the optical fiber 1, the concern is an increase in microbend loss. Therefore, the present inventor has intensively studied microbend loss.

[0043] Generally, in an optical fiber, when the glass diameter, that is, the cladding diameter, is reduced, the microbend loss (also called side pressure loss) increases. Usually, the transmission loss of the optical fiber increases when it is made into an optical fiber cable. The increase amount of the transmission loss at this time is closely related to the microbend loss, and when the microbend loss is large, the increase amount is also large.

[0044] Here, a single-mode optical fiber having characteristics conforming to the standards defined in ITU-T G.652 is described as a standard single-mode fiber (SMF) as the standard optical fiber. Such a standard SMF usually has a coating layer made of resin with a thickness of about 62.5 μm on the outer periphery of the cladding portion. In the case of a two-layer structure, for example, the coating layer is composed of a primary layer with a thickness of about 37.5 μm and a secondary layer with a thickness of about 25 μm surrounding the outer periphery of the primary layer. Therefore, the outer diameter of the coating layer is about 250 μm.

[0045] In the optical fiber 1, if the microbend loss is set to be 20 times or less the microbend loss of the standard SMF at a wavelength of 1550 nm, a practically acceptable level of microbend loss can be achieved. In addition, if the value obtained by normalizing the microbend loss with the microbend loss in the standard SMF is defined as the normalized microbend loss, the normalized microbend loss of the optical fiber 1 according to the present embodiment is preferably 20 or less, and more preferably 10 or less. The value of 20 when the normalized microbend loss is 20 or less is a value that can suppress the microbend loss to a practically acceptable level even after cabling.

[0046] Note that the microbend loss can be the value measured by the sandpaper method similar to the fixed-diameter drum method specified in JIS C6823:2010. The sandpaper method is, for example, the transmission loss in state A where an optical fiber with a length of 500 m is wound around a fixed drum wrapped with #1000 grit sandpaper with a tension of 100 gf in a non-overlapping single layer, and the difference between the transmission loss of the optical fiber in state B where it is wound around the same bobbin as in state A without the sandpaper wrapped, with the same tension and the same length as in state A, is defined as the value of the microbend loss. Here, the transmission loss of the optical fiber in state B does not include the microbend loss and is considered the transmission loss inherent to the optical fiber itself. Also, in this measurement method, since the transmission loss is measured, for example, at a wavelength of 1550 nm, the microbend loss is also the value at a wavelength of 1550 nm. Hereinafter, unless otherwise specified, the microbend loss is the value at a wavelength of 1550 nm.

[0047] Note that the microbend loss can also be the value measured by other methods, such as the wire mesh method.

[0048] Also, generally in an optical fiber, when the cladding diameter is reduced, leakage loss may occur where the light propagating in the core part leaks out from the cladding part. The value of the leakage loss is preferably at a level that can be almost ignored, such as 0.001 dB / km or less at a wavelength of 1625 nm.

[0049] Furthermore, in an optical fiber such as optical fiber 1 that contains a dopant that reduces the refractive index in the cladding part 1b, the stress distribution between the core part and the cladding part may be different compared to an optical fiber whose cladding is mainly made of pure silica glass. Therefore, in order to achieve low transmission loss, it is necessary to suppress the increase in transmission loss based on such a difference in stress distribution. As a result of the systematic and comprehensive study by the present inventor based on simulation calculations and experiments, it was confirmed that such an increase in transmission loss can be achieved by adjusting the structural parameters in the step type, W type, and trench type, which are the refractive index profiles including the core part 1a.

[0050] The following will be described in detail. FIG. 3 is a diagram showing an example of the relationship between Δ1 and the normalized microbend loss at a wavelength of 1550 nm in the same configuration as the optical fiber 1. FIG. 3 is a result based on simulation calculations and is also a result of examining various values of the glass diameter (cladding diameter). Further, FIG. 3 fixes Aeff of the optical fiber at 80 μm 2 and shows the case where the thickness of the primary layer of the coating layer (primary thickness) is fixed at 25 μm and the thickness of the secondary layer (secondary thickness) is fixed at 17.5 μm. Also, the reason why there are multiple data points with the same glass diameter and the same Δ1 is that, as structural parameters, Δ2 and b / a in the case of the W type, and Δ3, b / a, and c / a in the case of the trench type were changed within a certain range for calculation. Specifically, in the case of the W type, Δ2 was changed in the range of -0.3% to -0.08% and b / a was changed in the range of 1.8 to 4. In the case of the trench type, Δ3 was changed in the range of -0.3% to -0.08%, b / a was changed in the range of 1.8 to 4.5, and c / a was changed in the range of 3.0 to 5.0.

[0051] As shown in FIG. 3, it was confirmed that the larger the glass diameter or the larger Δ1, the smaller the normalized microbend loss tends to be on average, and conversely, the smaller the glass diameter or the smaller Δ1, the larger the normalized microbend loss tends to be on average.

[0052] Next, FIG. 4 is a diagram showing an example of the relationship between Aeff and the normalized microbend loss at a wavelength of 1550 nm in the same configuration as the optical fiber 1. FIG. 4 is a result based on simulation calculations and is also a result of confirming that it is consistent with the experimental results, and is also a result of examining the refractive index profiles of the step type, W type, and trench type. Further, FIG. 4 shows the case where the glass diameter is fixed at 120 μm, the primary thickness of the coating part is fixed at 25 μm, the secondary thickness is fixed at 17.5 μm, and the cable cut-off wavelength is 1530 nm or less.

[0053] As shown in Fig. 4, it was confirmed that the normalized microbend loss is exponentially correlated with Aeff and has a relatively high correlation. Also, for the same Aeff, it was confirmed that the normalized microbend loss tends to be smaller for the W-type and trench-type than for the step-type. For example, for the W-type and trench-type, even when Aeff is 140μm 2 the normalized microbend loss was 5 or less.

[0054] Next, the microbend loss is considered to be affected not only by the glass diameter but also by the primary thickness and secondary thickness of the coating layer, and is particularly greatly affected by the primary thickness. Therefore, the present inventor conducted an investigation through simulation calculation results and experiments.

[0055] Fig. 5 is a diagram showing an example of the relationship between the glass diameter and the normalized microbend loss at a wavelength of 1550 nm in the same configuration as the optical fiber 1. Note that Fig. 5 is a result based on simulation calculation and experiment, and is also a result of examining values of 10 to 30 μm for the primary thickness. The refractive index profile is of the step type or W type, Aeff is fixed at 80μm 2 and the secondary thickness is fixed at 17.5 μm.

[0056] As shown in Fig. 5, the microbend loss is affected not only by the glass diameter but also by the thickness of the primary layer. For example, it was found that the thinner the primary layer thickness, the greater the microbend loss.

[0057] Next, while changing Aeff, the refractive index profile, the structural parameters of the refractive index profile (Δ1, Δ2, Δ3, 2a, b / a, c / a, etc.), the glass diameter, and the primary thickness were changed at that Aeff, and a systematic examination was conducted.

[0058] FIG. 6 is a diagram showing an example of the relationship among Aeff, the glass diameter, and the minimum Δ or the minimum primary thickness. FIG. 6(a) is a diagram showing an example of the relationship among Aeff, the glass diameter, and the minimum Δ. FIG. 6(b) is a diagram showing an example of the relationship among Aeff, the glass diameter, and the minimum primary thickness. Here, the minimum Δ is the minimum Δ1 that can simultaneously satisfy a low leakage loss of 0.001 dB / km or less at a wavelength of 1625 nm (hereinafter, may be referred to as the low leakage loss reference value) and a normalized microbend loss of 20 or less. The minimum primary thickness is the minimum primary thickness that can simultaneously satisfy the low leakage loss reference value and a normalized microbend loss of 20 or less when Aeff and the glass diameter are changed.

[0059] From FIG. 6(a), it was found that when Δ1 is less than 0.2%, there is no solution that satisfies all the characteristics. Here, all the characteristics are the low leakage loss reference value, a normalized microbend loss of 20 or less, and a cable cutoff wavelength of 1530 nm or less.

[0060] Also, from FIG. 6(a), when Δ1 is 0.5% or more, the cable cutoff wavelength exceeds 1530 nm, so it was found that there is no solution that satisfies all the characteristics unless Aeff is 150 μm 2 or less.

[0061] Also, from FIG. 6(b), it was found that there is no solution that satisfies all the characteristics unless the primary thickness is 10 μm or more. Also, when trying to set the fiber diameter, which is the outer diameter of the coating layer, to 210 μm or less, which is smaller than the typical 250 μm, to a fiber diameter preferable for high-density packing of the optical fiber in the optical fiber cable, it was found that it is difficult unless the primary thickness is 50 μm or less.

[0062] Based on the above systematic study, the inventor found that the glass diameter is 70 μm or more and 120 μm or less, the primary thickness is 10 μm or more and 50 μm or less, Δ1 is 0.20% or more and 0.50% or less, and Aeff is 150 μm 2It has been found that, within the following optimal parameter ranges, while keeping the fiber diameter at 210 μm or less, it is possible to simultaneously satisfy the low leakage loss reference value, the normalized microbend loss of 20 or less, and the cable cut-off wavelength of 1530 nm or less.

[0063] As described above, it is preferable that a small glass diameter realizes cost reduction of the optical fiber, and a small fiber diameter realizes cost reduction by reducing the resin material and high density of the optical fiber in the optical fiber cable.

[0064] Regarding Aeff, from the viewpoints of leakage loss and microbend loss, there is no particular problem even if it is small. However, considering optical non-linearity and connectivity with other optical fibers, for example, a typical optical fiber with an Aeff of about 80 μm 2 Aeff is preferably 50 μm or more, for example, 70 μm 2 or more. 2

[0065] Next, the transmission loss will be further described. Regarding the transmission loss, in addition to the various parameter combinations as described above, various parameters (process parameters) in the manufacturing process, such as the residual stress distribution generated during the fiber drawing of the optical fiber, act on it. Therefore, the inventor fabricated many optical fibers under the conditions that satisfy the above optimal parameter ranges and investigated their transmission loss characteristics. Note that the process parameters were optimized under each condition during the trial production.

[0066] Regarding the coating layer, an ultraviolet curable resin material with an elastic modulus of 0.2 MPa or more and 3.0 MPa or less was used for the primary layer, and an ultraviolet curable resin material with an elastic modulus of 5.0 MPa or more and 2000 MPa or less was used for the secondary layer. These ultraviolet resin materials are typical materials used for the coating layer of standard SMF.

[0067] ​FIG. 7 is a diagram showing an example of the relationship between the normalized microbend loss and the transmission loss, summarized based on the prototype results. Note that the transmission loss is the value at a wavelength of 1550 nm. Also, the solid line indicates a level of 0.18 dB / km.

[0068] As shown in FIG. 7, it was found that when the normalized microbend loss exceeds 20, no matter how much the refractive index of the cladding part is reduced and the material and process are optimized, the transmission loss of 0.18 dB / km can no longer be obtained. Although these relationships are the same as those of a normal-diameter optical fiber, the optimal range is specific to the thin-diameter optical fiber, which is an important finding.

[0069] Also, even for optical fibers with the same value of microbend loss, although it varies depending on the material and refractive index profile doped in the core part and the cladding part, it was experimentally confirmed that the limit within which the desired characteristics can be realized by optimization is in the range where the normalized microbend loss is 20 or less.

[0070] Note that the dashed line L1 is a curve obtained by approximating the data points by the least squares method using a fifth-order function. The dashed line L1 is represented by y = 2.8341×10 -8 x 5 -1.8284×10 -6 x 4 +4.0977×10 -5 x 3 -3.4584×10 -4 x 2 +1.7779×10 -3 x + 0.1607. Here, y is the transmission loss and x is the normalized microbend loss.

[0071] Here, the value obtained by setting x = 0 in the above fifth-order function, that is, 0.1607 dB / km, is considered to be the transmission loss when there is no influence of microbend loss, that is, when no lateral pressure is applied. Therefore, considering that 0.18 - 0.1607 = 0.0193, it is preferable to configure the optical fiber by selecting a refractive index profile and a cross-sectional structure such that the increase in transmission loss due to microbend loss in the actual use state where lateral pressure is applied, such as when the optical fiber is wound around a bobbin or in a cable state, is on average 0.0193 dB / km or less, in order to achieve a transmission loss of 0.18 dB / km or less.

[0072] Note that the broken line L2 shows an example of the relationship between the normalized microbend loss and the transmission loss of an optical fiber that does not contain a dopant for lowering the refractive index in the cladding portion and has a reduced-diameter cladding portion. Both the broken line L1 and L2 are fifth-order functions, and although their trends are the same, the ways of change are different from each other, and it can be seen that the optimal ranges and the obtained values of transmission loss are different.

[0073] FIG. 8 is a diagram showing an example of the relationship between the glass diameter and the transmission loss, summarized based on the trial production results. Note that the results shown in FIG. 8 are for the case where chlorine is doped in the core portion and fluorine is doped in the cladding portion. Also, the transmission loss is shown for the minimum value among a plurality of optical fibers having the same Aeff and refractive index profile. The solid line indicates a level of 0.18 dB / km.

[0074] As shown in FIG. 8, it was confirmed that a glass diameter of 70 μm or more is preferable from the viewpoint of reducing the transmission loss. On the other hand, when the glass diameter is less than 70 μm, it was confirmed that it is difficult to make the transmission loss 0.18 dB / km or less even if other parameters are optimized. This can also be considered to be due to the fact that when the glass diameter is less than 70 μm, it is difficult to make the normalized microbend loss 20 or less, and thus the influence of microbend loss on the transmission loss is significant. Note that the same tendency was observed even when the dopants in the core portion and the cladding portion were different from the above.

[0075] In addition, the present inventor has found from numerous experiments that in an optical fiber containing a dopant that reduces the refractive index in the cladding portion, the difference in specific refractive index of the average refractive index of the center core, where the average refractive index in the core portion is the maximum, with respect to the refractive index of pure silica glass has a great influence on the transmission loss.

[0076] FIG. 9 is a diagram showing an example of the relationship between Δ1´, which is the difference in specific refractive index with respect to the silica level of the center core, and the transmission loss at a wavelength of 1550 nm. Here, the silica level is the level of the refractive index of pure silica glass. FIG. 9 shows a relationship specific to an optical fiber with a small diameter of 120 μm or less in which fluorine is included as a dopant in the cladding portion. In the illustrated example, it can be seen that when Δ1´ is -0.15% or more and 0.17% or less, the transmission loss can be made 0.18 dB / km or less. It should be noted that Δ1 without the "´" is the difference in specific refractive index based on the cladding portion or the intermediate layer. The same applies to Δ2´ and Δ3´ described later.

[0077] Furthermore, as can also be seen from FIG. 9, if Δ1´ is -0.10% or more and -0.01% or less, or 0.02% or more and 0.13% or less, an ultra-low transmission loss of 0.18 dB / km or less can be obtained more stably, and an ultra-low transmission loss of 0.175 dB / km or less can be obtained, which is more preferable.

[0078] Thus, there is a close relationship between Δ1´ (the absolute value of the difference in specific refractive index of the center core) with respect to the silica level and the transmission loss of the small-diameter optical fiber, and it has an optimal range specific to the small diameter. Therefore, manufacturing an optical fiber within the optimal range of the present invention is extremely important for reducing the transmission loss of a small-diameter optical fiber containing a dopant that reduces the refractive index in the cladding portion.

[0079] Note that the optical fiber 1 according to the present embodiment can be easily manufactured by producing an optical fiber preform by a known method such as the VAD (Vapor Axial Deposition) method, the OVD (Outside Vapor Deposition) method, the MCVD (Modified Chemical Vapor Deposition) method, or the plasma CVD method, and then drawing the optical fiber 1 from this optical fiber preform.

[0080] For example, for dopants such as germanium, fluorine, potassium, and sodium, they can be added to the optical fiber preform by using a gas containing the dopant during the soot synthesis. Also, for potassium and sodium, taking advantage of their diffusion speed, they may be doped into the glass by a gas phase method or an immersion method, etc., not during the soot synthesis. For chlorine, it can be added to the optical fiber preform by leaving the chlorine gas used in the dehydration process. For fluorine, it can be added to the optical fiber preform by flowing fluorine gas in the vitrification sintering structure.

[0081] (Example) As an example of the present invention, optical fibers of Sample Nos. 1 to 22 were drawn from an optical fiber preform produced using the VAD method, and their optical properties were measured. A coating layer of an ultraviolet curable resin composed of a primary layer and a secondary layer was formed on the optical fiber. The Young's modulus of the primary layer was set to 0.4 MPa, and the Young's modulus of the secondary layer was set to 1000 MPa.

[0082] The configurations and optical characteristics of the optical fibers of Sample Nos. 1 to 22 are shown in Tables 2A and 2B. In Tables 2A and 2B, "Δ1´", "Δ2´", and "Δ3´" are the relative refractive index differences of the core, the depressed layer or the intermediate layer, and the trench layer with respect to the silica level, respectively. Also, "primary diameter" is the outer diameter of the primary layer, and "fiber diameter" is the outer diameter of the secondary layer. Regarding the dopant, for example, "Cl2+K+Na / F" means that the core is doped with chlorine, potassium, and sodium, and the cladding part is doped with fluorine.

[0083] For any of Sample Nos. 1 to 22, the cable cutoff wavelength was 1530 nm or less. Therefore, all of Sample Nos. 1 to 22 functioned as single-mode optical fibers at a wavelength of 1550 nm.

[0084] Also, for any of Sample Nos. 1 to 22, the cladding diameter was 70 μm or more and 120 μm or less, and the transmission loss at a wavelength of 1550 nm was 0.18 dB / km or less.

[0085] Also, for any of Sample Nos. 1 to 22, Aeff at a wavelength of 1550 nm was 150 μm 2 or less, and 50 μm 2 or more.

[0086] Also, for any of Sample Nos. 1 to 22, the fiber diameter was 210 μm or less.

[0087] Also, for any of Sample Nos. 1 to 22, the thickness of the primary layer was 10 μm or more. For example, even in Sample No. 22 with the thinnest primary thickness, it was 17.5 μm.

[0088] Also, for any of Sample Nos. 1 to 22, the normalized microbend loss was 20 or less at a wavelength of 1550 nm.

[0089] Also, for any of Samples No. 1 to 22, Δ1 was 0.20% or more and 0.50% or less, and Δ1´ was -0.15% or more and 0.17% or less.

[0090] Also, for any of Samples No. 1 to 22, ΔClad was -0.50% or more and -0.13%.

[0091] Specifically, for Sample No. 1, when the refractive index profile is step-type, Δ1 is 0.42%, ΔClad is -0.33%, 2a is 9 μm, the glass diameter is 118 μm, the primary diameter is 169 μm, the fiber diameter is 207 μm, the dopant of the center core is Cl2, and the dopant of the clad part is F, the transmission loss is 0.167 dB / km, the normalized microbend loss is 1.6, and Aeff is 75 μm 2 These are suitable values, and it was an especially good combination of core dopants from the viewpoints of manufacturability and low loss.

[0092] Also, for Sample No. 3, when the refractive index profile is step-type, Δ1 is 0.39%, ΔClad is -0.36%, 2a is 10 μm, the glass diameter is 112 μm, the primary diameter is 163 μm, the fiber diameter is 197 μm, the dopant of the center core is K, and the dopant of the clad part is F, the transmission loss is 0.156 dB / km, the normalized microbend loss is 1.9, and Aeff is 86 μm 2 These are suitable values, and it was an especially good combination of core dopants from the viewpoints of manufacturability and low loss, and it was also an especially good combination of profile structures from the viewpoints of manufacturability and good optical characteristics.

[0093] Also, for Sample No. 4, when the refractive index profile is of the W type, Δ1 is 0.36%, Δ2 is -0.4%, ΔClad is -0.23%, b / a is 2, 2a is 9 μm, the glass diameter is 110 μm, the primary diameter is 155 μm, the fiber diameter is 193 μm, the dopant of the center core is Cl2 + K, and the dopant of the clad part is F, the transmission loss is 0.153 dB / km, the normalized microbend loss is 1.1, and Aeff is 58 μm 2 These are suitable values, and it was an especially good combination of core dopants from the viewpoints of manufacturability and low loss.

[0094] Also, for Sample No. 7, when the refractive index profile is of the W type, Δ1 is 0.27%, Δ2 is -0.11%, ΔClad is -0.14%, b / a is 2.9, 2a is 12.6 μm, the glass diameter is 102 μm, the primary diameter is 146 μm, the fiber diameter is 182 μm, the dopant of the center core is Cl2 + K + Na, and the dopant of the clad part is F, the transmission loss is 0.158 dB / km, the normalized microbend loss is 3.5, and Aeff is 111 μm 2 These are suitable values, and it was an especially good combination of profile structures from the viewpoints of manufacturability and good optical characteristics.

[0095] Also, for Sample No. 9, when the refractive index profile is of the W type, Δ1 is 0.25%, Δ2 is -0.17%, ΔClad is -0.3%, b / a is 2.8, 2a is 14.1 μm, the glass diameter is 97 μm, the primary diameter is 140 μm, the fiber diameter is 174 μm, the dopant of the center core is F + Na, and the dopant of the clad part is F, the transmission loss is 0.18 dB / km, the normalized microbend loss is 8.9, and Aeff is 127 μm 2 These are suitable values, and it was an especially good combination of profile structures from the viewpoints of manufacturability and good optical characteristics.

Table 2A

Table 2B

[0096] Note that the present invention is not limited by the above embodiments. Those configured by appropriately combining the above-described components are also included in the present invention. Further, additional effects and modifications can be easily derived by those skilled in the art. Therefore, a broader aspect of the present invention is not limited to the above embodiments, and various changes are possible.

Explanation of Reference Numerals

[0097] 1: optical fiber 1a: core part 1b: cladding part 1c: coating layer 1ca: primary layer 1cb: secondary layer L1, L2: dashed lines P11, P12, P21, P22, P31, P32: profiles

Claims

1. A core part, A cladding part that surrounds the outer periphery of the core part, has a refractive index lower than the maximum refractive index of the core part, and contains a dopant that reduces the refractive index, A coating layer that surrounds the outer periphery of the cladding part, Comprising, The outer diameter of the cladding part is 70 μm or more and 120 μm or less, The cable cut-off wavelength is 1530 nm or less, The transmission loss at a wavelength of 1550 nm is 0.175 dB / km or less, The effective core cross-sectional area at a wavelength of 1550 nm is 150 μm ２ or less, The outer diameter of the coating layer is 210 μm or less, The coating layer includes a primary layer that surrounds the outer periphery of the cladding part and a secondary layer that surrounds the outer periphery of the primary layer, and the thickness of the primary layer is 10 μm or more and 50 μm or less, The core part has a center core with the maximum average refractive index among the optical fibers, and the relative refractive index difference between the average refractive index of the center core and the average refractive index of the cladding part is 0.20% or more and 0.50% or less, The relative refractive index difference between the average refractive index of the center core and the refractive index of pure silica glass is -0.10% or more and -0.01% or less, The refractive index profile is of W type or trench type Optical fiber.

2. The effective core cross-sectional area at a wavelength of 1550 nm is 50 μm ２ or more The optical fiber according to Claim 1.

3. At least a part of the cladding part is made of silica glass containing fluorine or boron The optical fiber according to any one of Claims 1 or 2.

4. The core part is made of pure silica glass or silica glass containing one or more of chlorine, fluorine, germanium, potassium, and sodium, and has a center core with the maximum average refractive index among the optical fibers The optical fiber according to any one of Claims 1 to 3.

5. The normalized microbend loss normalized by the microbend loss at a wavelength of 1550 nm of a standard optical fiber having characteristics conforming to the standards defined in ITU-T G.652 and having a coating layer with a thickness of 62.5 μm on the outer periphery of a cladding part with an outer diameter of 125 μm is 20 or less at a wavelength of 1550 nm The optical fiber according to any one of Claims 1 to 4.

6. The microbend loss is a value measured by the sandpaper method The optical fiber according to Claim 5.

7. The increase in transmission loss due to microbend loss is 0.0193 dB / km or less The optical fiber according to any one of Claims 1 to 6.

8. The specific refractive index difference of the average refractive index of the clad part with respect to the refractive index of pure silica glass is -0.50% or more and -0.13%. The optical fiber according to any one of claims 1 to 7.

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