Multicore optical fiber

Abstract

To provide a multicore optical fiber capable of reducing polarization mode dispersion.SOLUTION: A multicore optical fiber comprises a plurality of cores along a fiber axis, and a cladding surrounding the plurality of cores. The plurality of cores and the cladding contain silica glass as a main component. Each of the plurality of cores has a refractive index higher than that of the cladding. Each of the plurality of cores has a coefficient of linear expansion greater than that of the cladding. The plurality of cores include a first core having an elliptical shape in a cross section perpendicular to the fiber axis, and one or more second cores that are different from the first core. The elliptical shape has a non-circularity of 0.1% or more. In the cross section perpendicular to the fiber axis, an angle formed between a straight line connecting the center of gravity of the first core and the center of gravity of a group of cores composed of the one or more second cores, and a straight line along a long axis of the elliptical shape, is 30 degrees or less. A polarization mode dispersion is 0.2 ps / rtkm or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to multi-core optical fibers. [Background technology]

[0002] There are known techniques for reducing polarization mode dispersion (PMD) in a multi-core optical fiber having multiple cores and a cladding surrounding the multiple cores. For example, Patent Document 1 discloses a method for manufacturing a multi-core optical fiber in which multiple core canes without a hole in the center are inserted into holes in a soot blank that serves as the cladding, and the core canes and the soot blank are sintered. This achieves low non-axial symmetry near the center of the core cane, and also achieves lower polarization mode dispersion. Patent Document 2 discloses that the multiple multi-core units included in a multi-core optical fiber have three-fold or more rotational symmetry, thereby reducing structural asymmetry and preventing an increase in polarization mode dispersion. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2015 / 0307387 [Patent Document 2] US Patent Application Publication No. 2011 / 0206330 Summary of the Invention [Problem to be solved by the invention]

[0004] However, even in a multi-core optical fiber in which low axial non-symmetry is realized near the core center as disclosed in Patent Document 1, if the core arrangement does not have three-fold or more rotational symmetry, polarization mode dispersion may become high. Therefore, there is a demand for a multi-core optical fiber that can reduce polarization mode dispersion even if the core arrangement does not have three-fold or more rotational symmetry.

[0005] An object of the present disclosure is to provide a multi-core optical fiber capable of reducing polarization mode dispersion. [Means for solving the problem]

[0006] A multi-core optical fiber according to one embodiment of the present disclosure comprises a plurality of cores along a fiber axis and a cladding surrounding the plurality of cores, the plurality of cores and the cladding containing silica glass as a main component, the refractive index of each of the plurality of cores being higher than the refractive index of the cladding, the linear expansion coefficient of each of the plurality of cores being higher than the linear expansion coefficient of the cladding, the plurality of cores including a first core having an elliptical shape in a cross section perpendicular to the fiber axis and one or more second cores different from the first core, the noncircularity of the elliptical shape being 0.1% or more, the angle formed by a straight line connecting the center of gravity of the first core and the center of gravity of a core group consisting of the one or more second cores and the major axis of the elliptical shape being 30 degrees or less in the cross section perpendicular to the fiber axis, and the polarization mode dispersion being 0.2 ps / rtkm or less. [Effects of the Invention]

[0007] According to the present disclosure, a multi-core optical fiber capable of reducing polarization mode dispersion is provided. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view of an optical fiber according to the first embodiment. [Figure 2] FIG. 2 is a cross-sectional view of an optical fiber perpendicular to the fiber axis. [Figure 3] FIG. 3 is a graph schematically showing the distribution of the linear expansion coefficient in an optical fiber. [Figure 4] FIG. 4 is a graph showing a schematic diagram of the viscosity distribution in an optical fiber. [Figure 5] FIG. 5 is a graph showing a schematic diagram of the stress distribution in an optical fiber. [Figure 6]FIG. 6 is a diagram showing a schematic diagram of in-plane stress in a cross section perpendicular to the fiber axis. [Figure 7] FIG. 7 is a diagram schematically showing the slow axis direction of birefringence in the fundamental mode in the core. [Figure 8] FIG. 8 is a cross-sectional view of the optical fiber according to the second embodiment. [Figure 9] FIG. 9 is a cross-sectional view of the optical fiber according to the third embodiment. [Figure 10] FIG. 10 is a cross-sectional view of an optical fiber according to a modified example. [Figure 11] FIG. 11 is a cross-sectional view of an optical fiber according to a modified example. [Figure 12] FIG. 12 is a cross-sectional view of an optical fiber according to a modified example. [Figure 13] FIG. 13 is a diagram schematically showing the internal structure of an optical fiber according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be listed and described. (1) A multi-core optical fiber according to one embodiment of the present disclosure includes a plurality of cores along a fiber axis and a cladding surrounding the plurality of cores, the plurality of cores and the cladding containing silica glass as a main component, the refractive index of each of the plurality of cores being higher than the refractive index of the cladding, the linear expansion coefficient of each of the plurality of cores being higher than the linear expansion coefficient of the cladding, the plurality of cores including a first core having an elliptical shape in a cross section perpendicular to the fiber axis and one or more second cores different from the first core, the noncircularity of the elliptical shape being 0.1% or more, the angle formed by a line connecting the center of gravity of the first core and the center of gravity of a core group consisting of the one or more second cores and a line along the major axis of the elliptical shape being 30 degrees or less in the cross section perpendicular to the fiber axis, and the polarization mode dispersion being 0.2 ps / rtkm or less.

[0010] In this multi-core optical fiber, the multiple cores include a first core having an elliptical shape in a cross section perpendicular to the fiber axis, and one or more second cores different from the first core. Furthermore, the angle formed by a line connecting the center of gravity of the first core and the center of gravity of a core group consisting of one or more second cores and a line along the major axis of the elliptical shape of the first core is 30 degrees or less. In this case, birefringence caused by stress due to the core arrangement and birefringence caused by the core shape cancel each other out, reducing the birefringence of the guided mode propagating through the first core. As a result, polarization mode dispersion can be reduced.

[0011] (2) In the above (1), the non-circularity of the elliptical shape may be 10% or less. In this case, it is possible to reduce the splice loss when splicing the multi-core optical fiber to a normal optical fiber having a small non-circularity.

[0012] (3) In the above (1) or (2), the multiple cores and the cladding may contain fluorine, and the fluorine concentration in the cladding may be higher than the fluorine concentration in each of the multiple cores. In this case, the cores have a linear expansion coefficient higher than that of the cladding, and the cores contract more strongly than the cladding due to cooling during drawing of the multi-core optical fiber, so that compressive stress remains in the cladding close to the cores in the circumferential direction of the cores. As a result, birefringence occurs due to stress caused by the arrangement of the cores, but this birefringence is offset by birefringence caused by the shape of the cores, thereby reducing polarization mode dispersion.

[0013] (4) In any of the above (1) to (3), the cladding may have a first cladding surrounding the multiple cores and a second cladding surrounding the first cladding, the first cladding and the second cladding containing fluorine, the fluorine concentration in the first cladding being higher than the fluorine concentration in the second cladding, and the multiple cores may contain at least one alkali element selected from an alkali element group consisting of alkali metal elements and alkaline earth metal elements. In this case, for example, the tensile tension caused by drawing can be localized in the second cladding, and glass defects due to the tensile tension can be prevented from occurring in the cores and the first cladding, thereby reducing loss in the multi-core optical fiber.

[0014] (5) In any of the above (1) to (4), the cores may contain at least one element from an alkali element group consisting of alkali metal elements and alkaline earth metal elements, and the concentration of the alkali element in the cores may be 1 wtppm or more and 3000 wtppm or less. In this case, the viscosity of the cores can be sufficiently reduced, and excess loss due to a high concentration of the alkali element can be reduced. Therefore, polarization mode dispersion and loss in the multi-core optical fiber can be reduced.

[0015] (6) In any of the above (1) to (5), the angle formed by a line connecting the center of gravity of the first core and the center of gravity of the core group composed of one or more second cores and a line along the major axis of the elliptical shape may be 10 degrees or less. In this case, polarization mode dispersion can be further reduced.

[0016] (7) In any of the above (1) to (5), the angle formed by a line connecting the center of gravity of the first core and the center of gravity of the core group composed of one or more second cores and a line along the major axis of the elliptical shape may be 5 degrees or less. In this case, polarization mode dispersion can be further reduced.

[0017] [Details of the embodiments of the present disclosure] Specific examples of multi-core optical fibers according to embodiments of the present disclosure will be described below with reference to the drawings. In the following description, identical elements or elements having identical functions will be designated by the same reference numerals, and duplicated descriptions will be omitted. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0018] The configuration of an optical fiber 1 according to a first embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a perspective view of the optical fiber 1 according to the first embodiment. FIG. 2 is a cross-sectional view of the optical fiber 1 perpendicular to a fiber axis 2. In FIG. 2, the coating layer 30 is not shown. The optical fiber 1 includes a plurality of cores 10, a cladding 20, and the coating layer 30. The optical fiber 1 may be, for example, an optical fiber used for long-distance, large-capacity transmission. The optical fiber 1 is a multi-core optical fiber (MCF) including a plurality of cores 10, and includes two cores 10 in this embodiment. Hereinafter, the two cores 10 may be described separately as a core 11 and a core 12.

[0019] The core 10 is aligned along the fiber axis 2, which is the central axis of the optical fiber 1. The core 10 contains silica glass as a main component. More specifically, each of the cores 11 and 12 contains silica glass as a main component. In this specification, a member containing silica glass as a main component means that 95% or more by mass of the member is made of silica glass. In other words, 95% or more by mass of the core 10 is made of silica glass.

[0020] Core 10 (core 11 and core 12) contains at least one alkali element from an alkali element group consisting of alkali metal elements and alkaline earth metal elements. Alkali element is a general term for alkali metal elements and alkaline earth metal elements. Alkali metal elements include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), etc. Alkaline earth metal elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. The concentration of the alkali element in core 10 is 1 wtppm or more and 3000 wtppm or less. The concentration of the alkali element in core 10 may be 10 wtppm or more and 300 wtppm or less.

[0021] The core 10 contains fluorine (F). The concentration of fluorine in the core 10 may be 1000 wtppm or more and 5000 wtppm or less. The composition of the core 10 and the cladding 20 can be calculated from the known weight per atom by measuring the ratio of the number of atoms using a known method such as an EPMA (Electron Probe Micro Analyzer).

[0022] The refractive index of the core 10 is higher than the refractive index of the cladding 20. As a result, light input to the optical fiber 1 is guided in the core 10. The relative refractive index difference of the core 10 with respect to the cladding 20 is, for example, 0.3% or more and 0.6% or less. The refractive indices of the core 10 and the cladding 20 can be measured by a known method such as the RNF (Refracted Near Field) method. In the optical fiber 1, the refractive index of a certain region (portion) is defined as the average value of the refractive index within that region. The diameter (core diameter) of the core 10 is, for example, 7 μm or more and 14 μm or less. A single LP mode including two polarization modes is guided in the core 10.

[0023] The cladding 20 surrounds the core 10. As shown in FIG. 2 , the cladding 20 has a first cladding 21 surrounding the core 10 and a second cladding 22 surrounding the first cladding 21. The cladding 20 contains silica glass as a main component. More specifically, the first cladding 21 and the second cladding 22 each contain silica glass as a main component. The cladding 20 also contains fluorine. The fluorine concentration in the cladding 20 is higher than the fluorine concentration in the core 11 and the core 12. More specifically, the first cladding 21 and the second cladding 22 each contain fluorine. The fluorine concentration in the first cladding 21 and the second cladding 22 is higher than the fluorine concentration in the core 11 and the core 12. The fluorine concentration in the first cladding 21 is higher than the fluorine concentration in the second cladding 22. The fluorine concentration in the first cladding 21 may be 8000 wtppm or more and 16000 wtppm or less. The fluorine concentration in the second cladding 22 may be 0.5 times or more and 0.9 times or less the fluorine concentration in the first cladding 21.

[0024] The outer diameter (diameter) of the second cladding 22 may be, for example, 124 μm or more and 126 μm or less. When the diameter of the second cladding 22 is 124 μm or more and 126 μm or less, a connecting means such as a standard fusion splicer or connector can be used to connect the first optical fiber 1 to the second optical fiber. The ratio of the outer diameter (diameter) of the first cladding 21 to the outer diameter of the second cladding 22 may be 0.4 or more and 0.7 or less.

[0025] The covering layer 30 surrounds the clad 20 and is in close contact with the outer peripheral surface of the clad 20. The covering layer 30 is made of a resin such as an acrylate.

[0026] 2, the core 10 has an elliptical shape in a cross section perpendicular to the fiber axis 2. The core 10 having an elliptical shape means that when the boundary line between the core 10 and the cladding 20 is fitted to an ellipse, the non-circularity of the ellipse is non-zero. If the major axis of the ellipse is Rmax and the minor axis is Rmin, the non-circularity is 100×2×(Rmax−Rmin) / (Rmax+Rmin)%.

[0027] In the cross section of the optical fiber 1, the boundary between regions with different refractive indices, such as the core 10 and the cladding 20, is defined as a curve formed by the point where the gradient of the refractive index is steepest (or the center of gravity, if there are multiple points). For example, when a closed curve is set between the core 10 and the cladding 20, the boundary between the core 10 and the cladding 20 is defined as a closed curve where the gradient of the refractive index averaged along the closed curve is greatest. In this embodiment, the noncircularity of the elliptical shape of the core 10 is 0.1% or more and 10% or less. The noncircularity of the elliptical shape of the core 10 may be 0.3% or more, or may be 1% or more. The noncircularity of the elliptical shape of the core 10 is defined as the noncircularity of the ellipse when the boundary between the core 10 and the cladding 20 is fitted to the ellipse.

[0028] In a cross section perpendicular to the fiber axis 2, the core 10 has a center of gravity. The center of gravity of the core 10 is the center of gravity of the cross-sectional shape of the core 10 (a figure enclosed by the boundary between the core 10 and the cladding 20). As shown in FIG. 2, the core 11 has a center of gravity P1, and the core 12 has a center of gravity P2. FIG. 2 shows lines L1 and L2. The line L1 is a line connecting the center of gravity P1 of the core 11 and the center of gravity P2 of the core 12. The line L2 is a line along the major axis of the ellipse shape of the core 11. The line L3 is a line along the major axis of the ellipse shape of the core 12. The major axis of the ellipse shape of the core 10 is defined, for example, as the major axis of the ellipse when the boundary between the core 10 and the cladding 20 is fitted to the ellipse.

[0029] The arrangement of cores 10 in optical fiber 1 will be described. The multiple cores 10 include at least one first core and one or more second cores different from the first core. In optical fiber 1, the first cores are arranged so that, in a cross section perpendicular to fiber axis 2, the angle formed by a line connecting the center of gravity of the first core and the center of gravity of a core group composed of one or more second cores is 30 degrees or less with a line along the major axis of the elliptical shape of the first core. Here, the "core group composed of one or more second cores" refers to all of the multiple cores 10 excluding the first core. When the number of cores 10 in optical fiber 1 is two, as in this embodiment, the core group is composed of only one core 10, and when the number of cores 10 is three or more, the core group is composed of two or more cores 10.

[0030] In the optical fiber 1, each of the core 11 and the core 12 is a first core. That is, when the core 11 is the first core, the core 12 is the second core, and when the core 12 is the first core, the core 11 is the second core. Therefore, in a cross section perpendicular to the fiber axis 2, the core 11 is arranged so that the angle θ1 formed by the line L1 connecting the center of gravity P1 of the core 11 (the first core) and the center of gravity P2 of the core group G1 of the second cores (only the core 12 in this example) and the line L2 along the major axis of the elliptical shape of the core 11 is 30 degrees or less. The core 12 is arranged so that the angle θ2 formed by the line L1 connecting the center of gravity P2 of the core 12 (the first core) and the center of gravity P1 of the core group G2 of the second cores (only the core 11 in this example) and the line L3 along the major axis of the elliptical shape of the core 12 is 30 degrees or less. The angle formed by a line connecting the center of gravity of the first core and the center of gravity of the core group consisting of one or more second cores and a line along the major axis of the elliptical shape of the first core may be 10 degrees or less, or may be 5 degrees or less.

[0031] In the optical fiber 1, the distance between the cores 11 and 12 may be, for example, 15 μm or more and 60 μm or less, or 25 μm or more and 50 μm or less. The distance between the cores 11 and 12 is defined as the distance between the center of gravity P1 of the core 11 and the center of gravity P2 of the core 12. From the viewpoint of reducing crosstalk, the distance between the cores 11 and 12 may be increased, and from the viewpoint of facilitating the connection of the optical fiber 1, the distance may be decreased. When the distance between the cores 11 and 12 is 15 μm or more and 60 μm or less, or 25 μm or more and 50 μm or less, the connection of the optical fiber 1 can be facilitated while reducing crosstalk.

[0032] FIG. 3 is a graph schematically illustrating the distribution of linear expansion coefficients in the optical fiber 1. FIG. 3 shows the linear expansion coefficients α of the optical fiber 1 of FIG. 2 at a portion located on a line L1. The horizontal axis of FIG. 3 indicates the position r on the line L1. The position r at the center of the line segment connecting the centers P1 and P2 is set to 0. The vertical axis of FIG. 3 shows the linear expansion coefficient α. Specifically, the linear expansion coefficients α10 of the cores 11 and 12, the linear expansion coefficient α21 of the first cladding 21, and the linear expansion coefficient α22 of the second cladding 22 are shown. The linear expansion coefficients can be measured as expansion or contraction due to a change in temperature. The linear expansion coefficients in this specification are values ​​at 1000°C, but may also be values ​​at temperatures below 700°C, which are easier to measure.

[0033] As described above, the core 11, the core 12, the first cladding 21, and the second cladding 22 each contain fluorine. The fluorine concentration in the second cladding 22 is higher than the fluorine concentration in the cores 11 and 12. As a result, the second cladding 22 has a linear expansion coefficient α22 that is smaller than the linear expansion coefficient α10 of the cores 11 and 12. Furthermore, the fluorine concentration in the first cladding 21 is higher than the fluorine concentration in the second cladding 22. As a result, the first cladding 21 has a linear expansion coefficient α21 that is smaller than the linear expansion coefficient α22 of the second cladding 22.

[0034] Fig. 4 is a graph schematically showing the viscosity distribution in the optical fiber 1. Fig. 4 shows the viscosity η of the portion located on the line L1 in the optical fiber 1 of Fig. 2. The horizontal axis of Fig. 4 indicates the position r on the line L1, and the vertical axis of Fig. 4 indicates the viscosity η. Specifically, the viscosity η10 of the cores 11 and 12, the viscosity η21 of the first cladding 21, and the viscosity η22 of the second cladding 22 are shown.

[0035] As described above, the cores 11 and 12 contain at least one element from the alkali element group consisting of alkali metal elements and alkaline earth metal elements. As a result, the cores 11 and 12 have a viscosity η10 that is lower than the viscosity η21 of the first cladding 21 and the viscosity η22 of the second cladding 22. The viscosity η21 of the first cladding 21 is also lower than the viscosity η22 of the second cladding 22.

[0036] Fig. 5 is a graph schematically showing the distribution of stress in the optical fiber 1. Fig. 5 shows the stress σzz occurring in the portion of the optical fiber 1 in Fig. 2 located on the line L1. The horizontal axis of Fig. 5 represents the position r on the line L1, and the vertical axis of Fig. 5 represents the stress σzz. Specifically, the graph shows the stress σzz10 occurring in the cores 11 and 12, the stress σzz21 occurring in the first cladding 21, and the stress σzz22 occurring in the second cladding 22. In the direction along the fiber axis 2, tension is shown as positive and compression is shown as negative.

[0037] As described above, the second cladding 22 has a relatively high viscosity η22 and therefore has a positive axial stress (stress σzz22) due to the tension when drawing the optical fiber 1. On the other hand, the viscosity η10 of the cores 11 and 12 and the viscosity η21 of the first cladding 21 are relatively low, so the cores 11 and 12 and the first cladding 21 have negative axial stresses (stresses σzz10 and σzz21). The linear expansion coefficient α21 of the first cladding 21 is smaller than the linear expansion coefficient α10 of the cores 11 and 12. Therefore, due to contraction caused by cooling when drawing the optical fiber 1, the first cladding 21 has a larger negative axial stress (stress σzz21) than the cores 11 and 12.

[0038] The birefringence occurring in the core 10 will be described with reference to Fig. 6 and Fig. 7. Specifically, the birefringence occurring due to stress resulting from the arrangement of the core 10 will be described with reference to Fig. 6, and the birefringence occurring due to the shape of the core 10 will be described with reference to Fig. 7. Fig. 6 is a diagram schematically showing in-plane stress in a cross section perpendicular to the fiber axis 2. Fig. 7 is a diagram schematically showing the slow axis direction of the birefringence of the fundamental mode in the core 10. In Figs. 6 and 7, the cross sections of the core 10 and the first cladding 21 are shown schematically, and hatching has been omitted for ease of explanation. In Figs. 6 and 7, the direction along the straight line L1 (see Fig. 2) connecting the center of gravity P1 of the core 11 and the center of gravity P2 of the core 12 is defined as the X direction, and the direction perpendicular to the X direction and the direction along the fiber axis 2 is defined as the Y direction. In the examples shown in FIGS. 6 and 7, the cross section of the core 10 perpendicular to the fiber axis 2 has an elliptical shape, and the core 10 is arranged so that the major axis of the elliptical shape is along the X direction.

[0039] As described above, the first cladding 21 has a linear expansion coefficient α21 that is smaller than the linear expansion coefficient α10 of the core 10. In other words, because the linear expansion coefficient α10 of the core 10 is larger than the linear expansion coefficient α21 of the first cladding 21, the core 10 contracts more strongly than the first cladding 21 due to cooling when drawing the optical fiber 1, and compressive stress remains in the first cladding 21 that is close to the core 10 in the circumferential direction of the core 10. Figure 6 shows the residual compressive stresses A1 and A2. Specifically, of the residual compressive stresses, the compressive stress along the X direction is shown as compressive stress A1, and the compressive stress along the Y direction is shown as compressive stress A2.

[0040] Considering the compressive stress remaining in the circumferential direction of a first core (e.g., core 11) included in the multiple cores 10, in the direction (X direction) where a second core (e.g., core 12) different from the first core exists, the compressive stress A2 along the circumferential direction (Y direction) of the first core is weakened due to the influence of contraction of the second core. This causes anisotropy in the compressive stress along the circumferential direction of the core, and when the compressive stresses are averaged, the compressive stress along the X direction (the direction in which the cores are arranged) becomes relatively large. As a result, the refractive index for waves polarized in the X direction is lower than the refractive index for waves polarized in the Y direction, resulting in birefringence. Therefore, in this example, birefringence occurs in the cores 10 due to the stress caused by the arrangement of the cores 10, and the slow axis of this birefringence (the polarization direction in which the refractive index increases) is along the Y direction.

[0041] When the core 10 has an elliptical shape as shown in FIG. 7, birefringence also occurs due to the shape of the core 10. FIG. 7 schematically shows the slow axis B1 and fast axis B2 of the birefringence due to the shape of the core 10. The slow axis B1 of the birefringence is aligned with the major axis of the elliptical shape, and the fast axis is aligned with the minor axis. In this example, the major axis of the elliptical shape is aligned with the X direction. Therefore, in this example, the slow axis B1 of the birefringence caused by the shape of the core 10 is aligned with the X direction.

[0042] In this way, the birefringence caused by the stress due to the core arrangement and the birefringence caused by the core shape have opposite signs, and the combination of these two reduces the overall birefringence. The reduced birefringence reduces polarization mode dispersion.

[0043] 6 and 7, the major axis of the elliptical shape of core 10 coincides with the X direction (i.e., the direction along the line L1 connecting the center of gravity P1 of core 11 and the center of gravity P2 of core 12). However, as in the optical fiber 1 shown in FIG. 2, even if the major axis of the elliptical shape does not completely coincide with the line L1, as long as it is approximately along the line L1 (for example, if the angle formed by the line L1 and the lines L2 and L3 along the major axis of the elliptical shape is 30 degrees or less), the birefringence caused by the stress due to the arrangement of core 10 and the birefringence caused by the shape of core 10 cancel each other out, and the birefringence as a whole is reduced. As a result, polarization mode dispersion is reduced.

[0044] In the optical fiber 1, the birefringence resulting from the stress caused by the arrangement of the core 10 and the birefringence caused by the shape of the core 10 is combined. -6 The combined birefringence is 10 -7 or less. Unless otherwise specified, the birefringence refers to the value at a wavelength of 1550 nm of the fundamental mode guided to the core 10 (for example, core 11). Furthermore, the polarization mode dispersion is 0.2 ps / rtkm or less. The polarization mode dispersion may be 0.1 ps / rtkm or less. Unless otherwise specified, the polarization mode dispersion refers to the value at a wavelength of 1550 nm of the fundamental mode guided to the core 10 (for example, core 11). The polarization mode dispersion can be measured using a known method such as the Jones Matrix Eigenanalysis (JME) method. During measurement, measurement light is input to a core to be measured of the multi-core optical fiber, and the measurement light output from the core is captured and measured. A known input / output device such as a fan-out device may be used to selectively input / output measurement light to / from the core to be measured, but the polarization mode dispersion of the input / output device is assumed to be negligibly small or appropriately corrected based on the measurement results.

[0045] As described above, in the optical fiber 1 according to this embodiment, the multiple cores 10 include cores 11 and 12 each having an elliptical shape in a cross section perpendicular to the fiber axis 2. The angle θ1 formed by the line L1 connecting the center of gravity P1 of core 11 and the center of gravity P2 of core 12 and the line L2 along the major axis of the elliptical shape of core 11 is 30 degrees or less. The angle θ2 formed by the line L1 and the line L3 along the major axis of the elliptical shape of core 12 is 30 degrees or less. This allows the birefringence caused by stress due to the arrangement of cores 10 and the birefringence caused by the shape of core 10 to cancel each other out, thereby reducing the birefringence of the guided mode propagating through core 10. As a result, polarization mode dispersion can be reduced. The angles θ1 and θ2 may be 10 degrees or less or 5 degrees or less. In this case, polarization mode dispersion can be further reduced.

[0046] The non-circularity of the elliptical shape of the core 10 is 10% or less, which makes it possible to reduce the connection loss when the optical fiber 1 is connected to a normal optical fiber with a small non-circularity.

[0047] The core 10 and the cladding 20 contain fluorine, and the fluorine concentration in the cladding 20 is higher than the fluorine concentration in each of the multiple cores 10. As a result, the core 10 has a linear expansion coefficient α10 that is larger than the linear expansion coefficient α21 of the first cladding 21, and the core 10 contracts more strongly than the first cladding 21 due to cooling when drawing the optical fiber 1, so that compressive stress remains in the first cladding 21 close to the core 10 in the circumferential direction of the core 10. As a result, birefringence occurs due to stress caused by the arrangement of the core 10, but this birefringence is offset by birefringence caused by the shape of the core 10, thereby reducing polarization mode dispersion.

[0048] The cladding 20 has a first cladding 21 surrounding the core 10 and a second cladding 22 surrounding the first cladding 21. The first cladding 21 and the second cladding 22 contain fluorine. The fluorine concentration in the first cladding 21 is higher than the fluorine concentration in the second cladding 22. The core 10 contains at least one alkali element selected from an alkali element group consisting of alkali metal elements and alkaline earth metal elements. In this case, for example, the tensile tension caused by drawing can be localized in the second cladding 22, preventing glass defects caused by the tensile tension from occurring in the core 10 and the first cladding 21, thereby reducing loss in the optical fiber 1.

[0049] The core 10 contains at least one element from an alkali element group consisting of alkali metal elements and alkaline earth metal elements. The concentration of the alkali elements in the core 10 is 1 wtppm or more and 3000 wtppm or less. This sufficiently reduces the viscosity of the core 10 and reduces excess loss due to a high concentration of alkali elements. Therefore, polarization mode dispersion and loss in the optical fiber 1 can be reduced.

[0050] An optical fiber 1A according to the second embodiment will be described with reference to Fig. 8. Fig. 8 is a cross-sectional view of the optical fiber 1A according to the second embodiment. Fig. 8 shows a cross section perpendicular to the fiber axis 2 of the optical fiber 1A, and the coating layer 30 is not shown. The optical fiber 1A according to this embodiment differs from the optical fiber 1 according to the first embodiment in that it includes a plurality of (two) first claddings 21A, 21B. Except for the points described below, the core 10 according to the second embodiment has the same configuration as the core 10 according to the first embodiment, and the first claddings 21A, 21B according to the second embodiment have the same configuration as the first cladding 21 according to the first embodiment.

[0051] The first cladding 21A surrounds the core 11, and the first cladding 21B surrounds the core 12. The first cladding 21A and the first cladding 21B are separated from each other with the second cladding 22 sandwiched therebetween. The second cladding 22 surrounds the first cladding 21A and the first cladding 21B. That is, the first claddings 21A and 21B are surrounded by the common second cladding 22. The ratio of the outer diameter of the first claddings 21A and 21B to the outer diameter of the core 10 may be 2 or more and 5 or less.

[0052] In the optical fiber 1A, the distance between the cores 11 and 12 may be, for example, 25 μm or more and 70 μm or less, or 35 μm or more and 60 μm or less. From the viewpoint of reducing crosstalk, the distance between the cores 11 and 12 may be increased, and from the viewpoint of facilitating the connection of the optical fiber 1A, the distance may be decreased. When the distance between the cores 11 and 12 is 25 μm or more and 70 μm or less, or 35 μm or more and 60 μm or less, the connection of the optical fiber 1A can be facilitated while reducing crosstalk.

[0053] In the optical fiber 1A, similarly to the optical fiber 1, the multiple cores 10 include cores 11 and 12 having an elliptical shape in a cross section perpendicular to the fiber axis 2. The angle θ1 formed by the line L1 connecting the center of gravity P1 of core 11 and the center of gravity P2 of core 12 and the line L2 along the major axis of the elliptical shape of core 11 is 30 degrees or less. Furthermore, the angle formed by the line L1 and the line L3 along the major axis of the elliptical shape of core 12 is 30 degrees or less. As a result, the birefringence caused by the stress resulting from the arrangement of cores 10 and the birefringence caused by the shape of cores 10 cancel each other out, reducing the birefringence of the guided mode propagating through cores 10. As a result, the optical fiber 1A can also reduce polarization mode dispersion.

[0054] An optical fiber 1B according to the third embodiment will be described with reference to FIG. 9. FIG. 9 is a cross-sectional view of an optical fiber 1C according to the third embodiment. FIG. 9 shows a cross section perpendicular to the fiber axis 2 of the optical fiber 1B, and the coating layer 30 is not shown. The optical fiber 1A according to this embodiment differs from the optical fiber 1 according to the first embodiment in that it includes four cores 10 and two first claddings 21A, 21B, 21C, and 21D. Hereinafter, the four cores 10 may be described separately as cores 11, 12, 13, and 14. Furthermore, except for the points described below, the core 10 according to the third embodiment has the same configuration as the core 10 according to the first embodiment, and the first claddings 21A, 21B, 21C, and 21D according to the third embodiment have the same configuration as the first cladding 21 according to the first embodiment.

[0055] The first cladding 21A surrounds the core 11. The first cladding 21B surrounds the core 12. The first cladding 21C surrounds the core 13. The first cladding 21D surrounds the core 14. The first claddings 21A, 21B, 21C, and 21D are separated from one another with the second cladding 22 sandwiched therebetween. The second cladding 22 surrounds the first claddings 21A, 21B, 21C, and 21D. In other words, the first claddings 21A, 21B, 21C, and 21D are surrounded by the common second cladding 22. The ratio of the outer diameter of the first claddings 21A, 21B, 21C, and 21D to the outer diameter of the core 10 may be 2 or more and 5 or less.

[0056] In the optical fiber 1B, the distance between the cores 11 and 12 may be, for example, 25 μm or more and 70 μm or less, or 35 μm or more and 60 μm or less. From the viewpoint of reducing crosstalk, the distance between the cores 11 and 12 may be increased, and from the viewpoint of facilitating the connection of the optical fiber 1B, the distance may be decreased. When the distance between the cores 11 and 12 is 25 μm or more and 70 μm or less, or 35 μm or more and 60 μm or less, it is possible to facilitate the connection of the optical fiber 1B while reducing crosstalk. For the same reason, the distance between the cores 12 and 13, the distance between the cores 13 and 14, and the distance between the cores 14 and 11 may be, for example, 25 μm or more and 70 μm or less, or 35 μm or more and 60 μm or less.

[0057] Like the core 10 according to the first embodiment, the core 10 according to the third embodiment (cores 11, 12, 13, and 14) has an elliptical shape in a cross section perpendicular to the fiber axis 2. In this embodiment, the noncircularity of the elliptical shape of the core 10 is 0.1% or more and 10% or less. The noncircularity of the elliptical shape may be 0.3% or more, or may be 1% or more. FIG. 9 shows lines L2 and L4. The line L2 is a line along the major axis of the elliptical shape of the core 11. The line L4 is a line connecting the center of gravity P1 of the core 11 and the center of gravity P3 of the core group G3 composed of cores 10 (cores 12, 13, and 14) different from the core 11. In the optical fiber 1B, in a cross section perpendicular to the fiber axis 2, the core 11 is arranged so that the angle θ3 formed by the line L4 connecting the center of gravity P1 of the core 11 and the center of gravity P3 of the core group G3 composed of cores 10 other than the core 11 and the line L2 along the major axis of the elliptical shape of the core 11 is 30 degrees or less.

[0058] In optical fiber 1B, cores 12, 13, and 14 are arranged under the same conditions as core 11. That is, core 12 is arranged so that the angle formed by a line connecting the center of gravity of core 12 and the center of gravity of a core group composed of cores 10 other than core 12 (cores 11, 13, and 14) and a line along the major axis of the ellipse of core 12 is 30 degrees or less. Core 13 is arranged so that the angle formed by a line connecting the center of gravity of core 13 and the center of gravity of a core group composed of cores 10 other than core 13 (cores 11, 12, and 14) and a line along the major axis of the ellipse of core 13 is 30 degrees or less. Core 14 is arranged so that the angle formed by a line connecting the center of gravity of core 14 and the center of gravity of a core group composed of cores 10 other than core 14 (cores 11, 12, and 13) and a line along the major axis of the ellipse of core 14 is 30 degrees or less.

[0059] In the optical fiber 1C, similarly to the optical fiber 1, the multiple cores 10 include cores 11, 12, 13, and 14 having an elliptical shape in a cross section perpendicular to the fiber axis 2. The angle θ3 formed by the line L4 connecting the center of gravity P1 of the core 11 and the center of gravity P3 of the core group G3 composed of cores 10 (cores 12, 13, and 14) different from the core 11 and the line L2 along the major axis of the elliptical shape of the core 11 is 30 degrees or less. The cores 12, 13, and 14 are arranged according to the same conditions as the core 11. As a result, the birefringence caused by the stress resulting from the arrangement of the cores 10 and the birefringence caused by the shape of the cores 10 cancel each other out, reducing the birefringence of the guided mode propagating through the cores 10. As a result, the optical fiber 1B can also reduce polarization mode dispersion.

[0060] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure. In addition, the above-described embodiments may be combined as appropriate.

[0061] 10, the optical fiber 1B may have a marker 50 at a position that breaks the symmetry of the arrangement of the cores 10. This allows each core 10 to be identified in the optical fiber 1B. The marker 50 may be formed in the cladding 20 (second cladding 22), for example. The marker 50 may be made of a material having a refractive index different from that of the cladding 20 (second cladding 22). Markers for identifying each core 10 may also be formed in the optical fibers 1 and 1A.

[0062] 11, in optical fiber 1B, cores 10 may be arranged so as not to have rotational symmetry with respect to fiber axis 2 (center of cladding). In this example, core 11 is arranged closer to the outer edge of cladding 20 than cores 12, 13, and 14. This makes it possible to identify each core 10 in optical fiber 1B. In optical fibers 1 and 1A as well, cores 10 may be arranged so as not to have rotational symmetry with respect to fiber axis 2 (center of cladding).

[0063] In a cross section perpendicular to the fiber axis 2, the noncircularity of the elliptical shape of the core 10 may be 10% or more. The concentration of the alkali element in the core 10 may be 1 wtppm or less, or 3000 wtppm or more. In addition to the alkali element, the core 10 may contain at least one of fluorine from 1000 wtppm to 5000 wtppm and chlorine (Cl) from 100 wtppm to 3000 wtppm. In this case, the viscosity of the core 10 is reduced, and loss due to density fluctuations and the like is reduced. The core 10 does not have to contain fluorine.

[0064] The optical fiber of the present embodiment described above may have a plurality of twists 61 aligned in the longitudinal direction, as shown in FIGS. 12 and 13. FIGS. 12 and 13 show an optical fiber 1D having twists 61 as a modification of the optical fiber 1B shown in FIG. 9. FIG. 12 is a schematic diagram showing a cross section perpendicular to the longitudinal direction of the optical fiber 1D. FIG. 13 is a schematic diagram showing the internal structure of the optical fiber 1D when viewed in a direction perpendicular to the longitudinal direction of the optical fiber 1D. In FIGS. 12 and 13, the first cladding 21 and the coating layer 30 are omitted. Furthermore, in FIG. 13, the cores 12 and 14 are further omitted, the second cladding 22 is indicated by a dashed line, and the cores 11 and 13 located inside the second cladding 22 are indicated by a solid line. The optical fiber 1D has four cores 10 (cores 11, 12, 13, and 14). In a cross section perpendicular to the fiber axis (the longitudinal direction of the optical fiber 1D), each core 10 has an elliptical shape.

[0065] In the optical fiber 1D, the positions of the multiple cores 11, 12, 13, and 14 change so as to rotate as the optical fiber 1D moves along the longitudinal direction, thereby forming multiple twists 61 positioned along the longitudinal direction of the optical fiber 1D. The twists 61 are locations (structures) where the multiple cores 10 intersect with one another when the optical fiber 1D is viewed along a direction perpendicular to the fiber axis. By including the twists 61 in the optical fiber 1D, mode coupling occurs between two polarization modes in each core 10, and the polarization mode dispersion (PMD) caused by stress in the cross section of the optical fiber 1D and anisotropy of its shape is randomized by the mode coupling, thereby reducing the accumulation of PMD. From this perspective, the number of twists (the number of twists 61) may be 0.1 turns / m or more. The number of twists may also be 1 turn / m or more. The number of twists is a value obtained by averaging the absolute value of the rotation angle per unit length along the longitudinal direction. The twist 61 can have two directions, left and right (two directions along the circumferential direction of the optical fiber 1D), but the twist 61 may have two directions mixed in the longitudinal direction.

[0066] While the twist 61 has the effect of reducing polarization mode dispersion, it also has the side effect of rotating the arrangement of the multiple cores 10 in the longitudinal direction, which can cause a problem that when coupling light to the end face of the optical fiber 1D, cutting the end of the optical fiber 1D changes the core arrangement, making it difficult to couple light. To reduce the impact of this problem, the number of twists 61 may be 3 turns / m or less. Alternatively, the number of twists 61 may be 0.3 turns / m or less. [Explanation of symbols]

[0067] 1, 1A, 1B, 1C, 1D...Optical fiber 2...Fiber axis 10, 11, 12, 13, 14...Core 20...Clad 21, 21A, 21B, 21C, 21D...First cladding 22...Second cladding 30…Covering layer 50...Marker 61...Twist A1, A2...Compressive stress B1…Slow axis B2…speed axis L1, L2, L3, L4…straight line P1, P2, P3…center of gravity α10, α21, α22...Linear expansion coefficient η10, η21, η22...Viscosity θ1, θ2, θ3...Angles σzz10, σzz21, σzz22...Stress

Claims

1. a plurality of cores along the fiber axis; a cladding surrounding the plurality of cores; the plurality of cores and the cladding contain silica glass as a main component; the refractive index of each of the plurality of cores is higher than the refractive index of the cladding; a linear expansion coefficient of each of the plurality of cores is greater than a linear expansion coefficient of the cladding; the plurality of cores include a first core having an elliptical shape in a cross section perpendicular to the fiber axis, and one or more second cores different from the first core; The non-circularity of the elliptical shape is 0.1% or more, in a cross section perpendicular to the fiber axis, an angle formed by a line connecting the center of gravity of the first core and the center of gravity of a core group constituted by the one or more second cores and a line along a major axis of the elliptical shape is 30 degrees or less, Polarization mode dispersion is less than 0.2 ps / rtkm. Multicore optical fiber.

2. The non-circularity of the elliptical shape is 10% or less. The multi-core optical fiber according to claim 1 .

3. the plurality of cores and the cladding contain fluorine; The fluorine concentration in the cladding is higher than the fluorine concentration in each of the plurality of cores. The multi-core optical fiber according to claim 1 or 2.

4. the cladding includes a first cladding surrounding the plurality of cores and a second cladding surrounding the first cladding, the first cladding and the second cladding contain fluorine; the first cladding has a higher fluorine concentration than the second cladding; the plurality of cores contain at least one alkali element selected from an alkali element group consisting of alkali metal elements and alkaline earth metal elements; The multi-core optical fiber according to claim 1 or 2.

5. the plurality of cores contain at least one alkali element selected from an alkali element group consisting of alkali metal elements and alkaline earth metal elements; a concentration of the alkali element in the plurality of cores is 1 wtppm or more and 3000 wtppm or less; The multi-core optical fiber according to claim 4 .

6. The angle is 10 degrees or less. The multi-core optical fiber according to claim 1 or 2.

7. The angle is 5 degrees or less. The multi-core optical fiber according to claim 1 or 2.

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