fiber optic

Abstract

An optical fiber 10 comprises a core part 11, a side core layer 12, and a cladding part 13, is configured such that ∆1 > ∆Clad > ∆2 and 0 > ∆2 hold true, where ∆1 is 0.35%-0.45%, and ∆2 is -0.2% to less than 0%, 2a is 8-10 µm, 2b is 35-45 µm, and, in a refractive index profile of the side core layer 12, there exists a bottom part 14 having a lower refractive index than the other regions of the side core layer 12, and the distance of the position of lowest refractive index of the bottom part 14 from the center of the core part 11 is a + (b – a) / 1.55 or less.

Description

Technical Field

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

Background Art

[0002] In a single-mode optical fiber, a technique of doping a clad portion with a dopant such as fluorine to reduce the refractive index of glass has been disclosed (Patent Document 1). In an 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, an optical fiber with ultra-low transmission loss can be realized by reducing the Rayleigh scattering loss caused by the concentration distribution of the dopant in the core portion.

[0003] In addition, optical fibers employing a W-shaped refractive index profile have been actively studied (Patent Document 2). The W-shaped refractive index profile is employed, for example, to expand the effective core cross-sectional area of an optical fiber. In an optical fiber with a large effective core cross-sectional area, the occurrence of non-linear optical effects in the optical fiber is suppressed, so it can be suitably used, for example, as a long-distance optical transmission line. When realizing a W-shaped refractive index profile, for example, a region called a side core layer adjacent to the core portion is doped with a dopant that reduces the refractive index of glass such as fluorine.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] In optical fibers used as optical transmission lines, it is increasingly desirable to reduce both transmission loss and macrobend loss.

[0006] The present invention has been made in view of the above, and its object is to provide an optical fiber in which transmission loss and macrobend loss are reduced. [Means for solving the problem]

[0007] 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, wherein the difference in the average maximum specific refractive index of the core portion with respect to the average refractive index of the cladding portion is Δ1, the difference in the specific refractive index of the average refractive index of the side core layer is Δ2, and the difference in the specific refractive index of the average refractive index of the cladding portion with respect to pure quartz glass is ΔClad, then Δ1 > ΔClad > Δ2 and 0 > Δ2 hold, and Δ1 is 0.35 The optical fiber has a refractive index of % or more and 0.45% or less, a Δ2 of -0.2% or more and less than 0%, and when the core diameter of the core portion is 2a and the outer diameter of the side core layer is 2b, 2a is 8 μm or more and 10 μm or less, 2b is 35 μm or more and 45 μm or less, and in the refractive index profile of the side core layer, there is a bottom portion where the refractive index is lower than in other regions of the side core layer, and the distance from the center of the core portion to the position with the lowest refractive index in the bottom portion is a + (ba) / 1.55 [μm] or less.

[0008] The bottom portion may be a portion in the refractive index profile of the side core layer where the refractive index changes abruptly, and where the refractive index profile has an inflection point.

[0009] If ΔB is the difference in relative refractive index between the average refractive index of the side core layer and the lowest refractive index of the bottom portion, then the absolute value of ΔB may be 0.03% or more and 0.07% or less.

[0010] The distance from the center of the core to the position with the lowest refractive index at the bottom portion may be a + (ba) / 1.8 [μm] or greater.

[0011] The mode field diameter at a wavelength of 1310 nm may be between 8.6 μm and 9.5 μm.

[0012] The MAC value, which is the mode field diameter at a wavelength of 1310 nm divided by the cable cutoff wavelength, may be between 7.0 and 7.2.

[0013] The core portion may contain at least one of chlorine, potassium, and sodium.

[0014] It is also acceptable if the transmission loss at a wavelength of 1550 nm is 0.35 dB / km or less.

[0015] It is also acceptable if the macrobend loss at a wavelength of 1550 nm is 0.03 dB or less when wound 10 turns with a diameter of 30 mm. [Effects of the Invention]

[0016] The present invention has the effect of realizing optical fibers with reduced transmission loss and macrobend loss. [Brief explanation of the drawing]

[0017] [Figure 1] Figure 1 is a schematic cross-sectional view of an optical fiber according to an embodiment. [Figure 2] Figure 2 is a schematic diagram of the refractive index profile of an optical fiber according to the embodiment. [Figure 3] Figure 3 is a schematic diagram of the essential parts of the refractive index profile of an optical fiber in a comparative configuration. [Figure 4] Figure 4 is a schematic diagram of the main part of the refractive index profile of the optical fiber according to the embodiment. [Figure 5]FIG. 5 is a diagram showing an example of a bulk density distribution profile of a porous base material used in the production of an optical fiber according to a comparative form. [Figure 6] FIG. 6 is a diagram showing an example of a bulk density distribution profile of a base material used in the production of an optical fiber according to an embodiment.

Embodiments of the Invention

[0018] 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 labeled with the same reference numerals. Further, in this specification, the cutoff wavelength or the effective cutoff wavelength refers to the cable cutoff wavelength defined in ITU-T G.650.1 of the International Telecommunication Union (ITU). In addition, for terms not specifically defined in this specification, the definitions and measurement methods in G.650.1 and G.650.2 shall be followed.

[0019] (Embodiment) FIG. 1 is a schematic cross-sectional view of an optical fiber according to an embodiment. The optical fiber 10 is made of silica glass and includes 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. Note that the optical fiber 10 may include a coating layer surrounding the outer periphery of the cladding portion 13.

[0020] FIG. 2 is a diagram showing the refractive index profile of the optical fiber 10. Profile P1 is the refractive index profile of the core portion 11 and has a so-called step type. 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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 with respect to the average refractive index of the cladding portion 13. In Figure 2, the zero line representing the relative refractive index difference of the pure quartz glass is shown as a dashed line.

[0025] 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.

[0026] The constituent materials of the optical fiber 10 will now be described. The core portion 11 is made of silica-based glass containing dopants for adjusting the refractive index. For example, the core portion 11 contains at least one of chlorine (Cl), potassium (K), and sodium (Na) as dopants. The core portion 11 may also contain fluorine (F) for adjusting the refractive index and glass viscosity. F is a dopant that lowers the refractive index of the silica glass, while Cl, K, and Na are dopants that increase the refractive index of the silica glass. The core portion 11 may also contain germanium (Ge), which is a dopant that increases the refractive index of the silica glass.

[0027] 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 are added. By adjusting the refractive index with these dopants, Δ1 > ΔClad > Δ2 and 0 > Δ2 are satisfied, and furthermore, the preferred exemplary ranges of Δ1, Δ2, and ΔClad described later are realized.

[0028] The structural parameters of the optical fiber 10 according to this embodiment are, for example, Δ1 is 0.35% or more and 0.45% or less, Δ2 is -0.2% or less and less than 0%, 2a is 8 μm or more and 10 μm or less, and 2b is 35 μm or more and 45 μm or less. As a result, the optical fiber can satisfy at least some of the optical properties specified in, for example, ITU-T G.652 or G.657. Note that ΔClad is, for example, -0.01%.

[0029] For example, the optical fiber 10 has a mode field diameter (MFD) of 8.6 μm or more and 9.5 μm or less at a wavelength of 1310 nm. Also, for example, the optical fiber 10 has a macrobend loss of 0.03 dB or less at a wavelength of 1550 nm when wound 10 turns with a diameter of 30 mm. Also, for example, the optical fiber 10 has a transmission loss of 0.35 dB / km or less at a wavelength of 1550 nm, in accordance with the provisions of G.652. Furthermore, it is more preferable that the transmission loss of the optical fiber 10 at a wavelength of 1550 nm is 0.2 dB / km or less.

[0030] Furthermore, in the optical fiber 10, the MAC value, which is the value obtained by dividing the mode field diameter (MFD) at a wavelength of 1310 nm by the cable cutoff wavelength (λcc), is, for example, between 7.0 and 7.2. The MAC value is one of the indicators used to evaluate the macrobend loss characteristics, and optical fibers with similar MAC values ​​will have similar macrobend loss characteristics.

[0031] Furthermore, in the optical fiber 10, the refractive index profile of the side core layer 12 includes a bottom region where the refractive index is lower than in other regions of the side core layer 12. Also, the distance from the center of the core portion 11 to the position with the lowest refractive index in the bottom region is a + (ba) / 1.55 [μm] or less.

[0032] The bottom portion will now be explained. Figure 4 is a schematic diagram of the main part of the refractive index profile of an optical fiber according to the embodiment. In Figure 4, the refractive index profiles of the core portion 11 and the side core layer 12 of the optical fiber 10 are shown by solid lines. In the refractive index profile of the side core layer 12 of the optical fiber 10, there is a bottom portion 14 in which the refractive index is lower than in other regions of the side core layer 12. The bottom portion 14 is, for example, a portion in the refractive index profile of the side core layer 12 in which the refractive index changes abruptly, and is a portion in which the refractive index profile has an inflection point.

[0033] According to our diligent research, when manufacturing an optical fiber employing a W-type refractive index profile, we fabricated an optical fiber matrix using known methods such as VAD (Vapor Axial Deposition) and OVD (Outside Vapor Deposition). When we drew an optical fiber from this matrix, we confirmed that in the refractive index profile of the side core layer 12, there exists a bottom portion 14 with a lower refractive index than other regions of the side core layer 12. If ΔB is the relative refractive index difference between the average refractive index of the side core layer 12 and the lowest refractive index of the bottom portion 14, then the absolute value of ΔB is, for example, 0.03% to 0.07%.

[0034] Furthermore, through diligent research by the inventors, it was found that transmission loss and macrobend loss are reduced when the distance from the center of the core portion 11 to the position with the lowest refractive index in the bottom portion 14 (r in Figure 4) is a + (ba) / 1.55 [μm] or less.

[0035] Furthermore, a distance r of a+(ba) / 1.8 [μm] or greater is preferable from the standpoint of ease of manufacturing.

[0036] On the other hand, Figure 3 is a schematic diagram of the essential parts of the refractive index profile of the optical fiber in the comparative configuration. The optical fiber in the comparative configuration is an optical fiber in which only the refractive index profile of the side core layer differs from that of the optical fiber 10 in the embodiment.

[0037] Figure 3 shows the refractive index profiles of the core portion 11A and side core layer 12A of the optical fiber 10A in the comparative configuration, indicated by dashed lines. The relative refractive index difference between the average refractive index of the side core layer 12A and the average refractive index of the cladding portion is denoted as Δ2'. The refractive index profile of the side core layer 12A of the optical fiber 10A also includes a bottom portion 14A. The absolute value of the relative refractive index difference ΔB' between the average refractive index of the side core layer 12A and the lowest refractive index of the bottom portion 14A is, for example, between 0.03% and 0.07%.

[0038] However, the distance from the center of the core 11A to the point where the refractive index of the bottom portion 14A is lowest (r' in Figure 4) is greater than a + (ba) / 1.55 [μm]. In other words, the bottom portion 14A is located relatively close to the cladding portion relative to the core portion 11A. As a result, optical fiber 10A is an optical fiber in which the transmission loss and macrobend loss are not reduced compared to optical fiber 10.

[0039] The present inventors will now further explain the results of their diligent research. When manufacturing an optical fiber 10A as shown in the comparative form, the inventors simultaneously formed the core portion and the portion that will become the side core layer of the optical fiber using the VAD method, added fluorine to the side core layer to create a porous matrix, and then performed vitrification sintering to produce the core matrix. Alternatively, this core matrix may be produced by the following method: a porous matrix having the core portion and the portion that will become the side core layer of the optical fiber is produced using the VAD method. Then, in order to dope the portion that will become the side core layer of the matrix with F, fluorine gas is flowed during the vitrification sintering process to produce the core matrix. Furthermore, a porous layer that will become the cladding portion is formed on the produced core matrix using the OVD method, and this is vitrified sintered to produce an optical fiber matrix. Then, an optical fiber is drawn from this optical fiber matrix.

[0040] Figure 5 shows an example of the bulk density distribution profile of a porous matrix used in the manufacture of optical fibers in a comparative configuration. Reference numeral 110A indicates the core portion 11A, and reference numeral 120A indicates the side core layer 12A. Figure 5 also shows the bulk density distribution profile at three locations in the longitudinal direction of the porous matrix: the upper, middle, and lower parts.

[0041] In the porous matrix shown in Figure 5, as circled, there is a localized increase in bulk density in section 120A, and the bulk density varies greatly in the upper, middle, and lower parts. Such variations and localized fluctuations in bulk density are thought to cause variations in the amount of doping when doping with F. Furthermore, when such variations and localized fluctuations in bulk density are large, the bottom part 14A within the side core layer 12A is located relatively close to the cladding.

[0042] Therefore, in order to suppress variations in bulk density and localized increases, the inventors made improvements to enhance the flame directionality of the burner used for depositing glass nanoparticles in the VAD method. Specifically, they installed a rectifier plate in the furnace of the VAD device to suppress airflow that disrupts the flame directionality, and increased the flow rate of combustion-supporting gas (e.g., oxygen) supplied to the burner to strengthen the flame directionality.

[0043] Figure 6 shows an example of the bulk density distribution profile of the matrix material used in the manufacture of the optical fiber according to the embodiment. Reference numeral 110 denotes the core portion 11, and reference numeral 120 denotes the side core layer 12. In Figure 6, the bulk density distribution profile is shown at three locations in the longitudinal direction of the porous matrix material: the upper, middle, and lower parts.

[0044] In the porous matrix shown in Figure 6, as circled, variations in bulk density and localized increases in portion 120 were suppressed. As a result, in the optical fiber 10, the bottom portion 14 is located closer to the core portion 11 than the bottom portion 14A in the optical fiber 10A, and r becomes a + (ba) / 1.55 [μm] or less.

[0045] (Examples, Comparative Examples) Optical fibers of Examples 1 and 2 and Comparative Examples 1 and 2, corresponding to the above embodiments and comparative examples, were manufactured, and their characteristics were measured and evaluated. The structural parameters and measurement or evaluation results of the manufactured optical fibers are shown in Table 1. In Examples 1 and 2 and Comparative Examples 1 and 2, the Q value (MAC value) was the same for all optical fibers, ranging from 7.0 to 7.2. As macrobend loss, the macrobend loss at a wavelength of 1550 nm was measured when the fiber was wound 10 turns with a diameter of 30 mm. The bottom position refers to the distance from the center of the core to the position with the lowest refractive index at the bottom (distance from the core center to the bottom position). "Evaluation 1" is an evaluation of the transmission loss at a wavelength of 1550 nm, where "○" indicates a value of 0.2 dB / km or less, and "×" indicates a value exceeding 0.2 dB / km. Furthermore, "Evaluation 2" refers to the evaluation of macrobend loss. For Example 1 and Comparative Example 1, a value of 0.03 dB / km or less was marked "○" and a value exceeding 0.03 dB / km was marked "×". For Example 2 and Comparative Example 2, a value of 0.5 dB / km or less was marked "○" and a value exceeding 0.5 dB / km was marked "×". As can be seen from Table 1, in Examples 1 and 2, transmission loss and macrobend loss were reduced, and good results were obtained.

[0046] [Table 1]

[0047] The specific numerical values ​​for each example are explained below. In Example 1, the core diameter 2a was 8 μm. The outer diameter 2b of the side core layer was 38 μm. The difference Δ1 between the average maximum relative refractive index of the core and the average refractive index of the cladding was 0.4%. The difference Δ2 between the average refractive index of the side core layer and the average refractive index of the cladding was -0.15%. The difference ΔClad between the average refractive index of the cladding and the refractive index of the pure quartz glass was -0.01%. The distance from the core center to the bottom was 12.3 μm (a + (ba) / 1.55 [μm] or less). The difference ΔB between the lowest refractive index of the bottom and the average refractive index of the side core layer was -0.07%. The mode field diameter at a wavelength of 1310 nm was 8.64 μm, and the Q value was 7.0.

[0048] In Comparative Example 1, the core diameter 2a was 8 μm. The outer diameter 2b of the side core layer was 38 μm. The difference Δ1 between the average maximum specific refractive index of the core and the average refractive index of the cladding was 0.4%. The difference Δ2 between the average refractive index of the side core layer and the average refractive index of the cladding was -0.15%. The difference ΔClad between the average refractive index of the cladding and the refractive index of the pure quartz glass was -0.01%. The distance from the core center to the bottom was 14.0 μm (greater than a + (ba) / 1.55 [μm]). The difference ΔB (ΔB') between the average refractive index of the side core layer and the lowest refractive index of the bottom was -0.07%. The mode field diameter at a wavelength of 1310 nm was 8.58 μm, and the Q value was 7.0.

[0049] In Example 2, the core diameter 2a was 8.5 μm. The outer diameter 2b of the side core layer was 40 μm. The difference Δ1 between the average maximum relative refractive index of the core and the average refractive index of the cladding was 0.37%. The difference Δ2 between the average refractive index of the side core layer and the average refractive index of the cladding was -0.19%. The difference ΔClad between the average refractive index of the cladding and the refractive index of the pure quartz glass was -0.01%. The distance from the core center to the bottom was 14.4 μm (a + (ba) / 1.55 [μm] or less). The difference ΔB between the lowest refractive index of the bottom and the average refractive index of the side core layer was -0.03%. The mode field diameter at a wavelength of 1310 nm was 8.97 μm, and the Q value was 7.1.

[0050] In Comparative Example 2, the core diameter 2a was 8.5 μm. The outer diameter 2b of the side core layer was 40 μm. The difference Δ1 between the average maximum specific refractive index of the core and the average refractive index of the cladding was 0.37%. The difference Δ2 between the average refractive index of the side core layer and the average refractive index of the cladding was -0.19%. The difference ΔClad between the average refractive index of the cladding and the refractive index of the pure quartz glass was -0.01%. The distance from the core center to the bottom was 18.4 μm (greater than a + (ba) / 1.55 [μm]). The difference ΔB(ΔB') between the average refractive index of the side core layer and the lowest refractive index of the bottom was -0.03%. The mode field diameter at a wavelength of 1310 nm was 9.22 μm, and the Q value was 7.2.

[0051] 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]

[0052] This invention is particularly suitable for application to optical fibers used as optical transmission lines. [Explanation of Symbols]

[0053] 10: Optical fiber 11: Core section 12: Side Core Layer 13: Clad section 14: Bottom section 120 :part P1, P2, P3: Profiles

Claims

1. The core part, The outer periphery of the aforementioned core portion is surrounded by a fluorine-added side core layer, The cladding portion surrounding the outer periphery of the side core layer, Equipped with, Let Δ1 be 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 be the difference in the relative refractive index of the average refractive index of the side core layer, and ΔClad be 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. Δ1 is between 0.35% and 0.45%, Δ2 is -0.2% or more and less than 0%, When the core diameter of the core portion is 2a and the outer diameter of the side core layer is 2b, 2a is 8 μm or more and 10 μm or less, and 2b is 35 μm or more and 45 μm or less. In the refractive index profile of the side core layer, there is a bottom portion where the refractive index is lower than in other regions of the side core layer. The distance from the center of the core portion to the position where the refractive index is lowest in the bottom portion is a + (b - a) / 1.8 [μm] or more and a + (b - a) / 1.55 [μm] or less. The mode field diameter at a wavelength of 1310 nm is between 8.64 μm and 8.97 μm. Optical fiber.

2. If ΔB is the difference in relative refractive index between the average refractive index of the side core layer and the lowest refractive index of the bottom portion, then the absolute value of ΔB is 0.03% or more and 0.07% or less. The optical fiber according to claim 1.

3. The MAC value, which is the mode field diameter at a wavelength of 1310 nm divided by the cable cutoff wavelength, is between 7.0 and 7.

2. The optical fiber according to claim 1.

4. The core portion contains at least one of chlorine, potassium, and sodium. The optical fiber according to claim 1.

5. The transmission loss at a wavelength of 1550 nm is 0.35 dB / km or less. The optical fiber according to claim 1.

6. The macrobend loss at a wavelength of 1550 nm is 0.03 dB or less when wound 10 turns with a diameter of 30 mm. The optical fiber according to claim 1.

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