Multi-core optical fiber

Abstract

The invention provides a multi-core optical fiber capable of reducing polarization mode dispersion. A multi-core optical fiber is provided with a plurality of cores along an optical fiber axis, and a cladding covering the plurality of cores. The plurality of cores and the cladding layer 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. The linear expansion coefficient of each of the plurality of cores is greater than the linear expansion coefficient of the cladding. The plurality of cores includes a first core having an elliptical shape in a cross section orthogonal to the optical fiber axis, and one or more second cores different from the first core. The out-of-roundness of the elliptical shape is 0.1% or more. In a cross section orthogonal to the optical fiber axis, 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 composed of the one or more second cores and a straight line along the long axis of the elliptical shape is 30 degrees or less. The polarization mode dispersion is 0.2 ps / rtkm or less.

Description

Technical Field

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

[0002] Technologies for reducing polarization mode dispersion (PMD) in multi-core optical fibers comprising multiple cores and a cladding surrounding the multiple cores are known. For example, in the method for manufacturing a multi-core optical fiber disclosed in Patent Document 1, multiple core canes without a central hole are inserted into a hole in a soot blank that serves as the cladding, and the core canes and the soot blank are sintered. This achieves low asymmetry near the center of the core canes and further reduces polarization mode dispersion. Patent Document 2 discloses that by having the multiple core units of a multi-core optical fiber have three or more rotational symmetries, structural asymmetry is reduced, thereby preventing an increase in polarization mode dispersion.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: U.S. Patent Application Publication No. 2015 / 0307387

[0006] Patent Document 2: U.S. Patent Application Publication No. 2011 / 0206330 Summary of the Invention

[0007] Technical problem to be solved by the invention

[0008] However, even in multi-core optical fibers that achieve low asymmetry near the core center, as disclosed in Patent Document 1, polarization mode dispersion may sometimes become high if the core arrangement does not have three or more rotational symmetry. Therefore, there is a need for multi-core optical fibers that can reduce polarization mode dispersion even when the core arrangement does not have three or more rotational symmetry.

[0009] An object of the present disclosure is to provide a multi-core optical fiber capable of reducing polarization mode dispersion.

[0010] Solutions for solving technical problems

[0011] A multi-core optical fiber according to one embodiment of the present disclosure includes multiple cores along the optical fiber axis and a cladding surrounding the multiple cores. The multiple cores and the cladding contain silica glass as a main component. The refractive index of each of the multiple cores is higher than the refractive index of the cladding. The linear expansion coefficient of each of the multiple cores is larger than the linear expansion coefficient of the cladding. The multiple cores include a first core having an elliptical shape in a cross section perpendicular to the optical fiber axis and one or more second cores different from the first core. The non-circularity of the elliptical shape is greater than 0.1%. In a cross section perpendicular to the optical fiber axis, 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 is less than 30 degrees. The polarization mode dispersion is less than 0.2 ps / rtkm.

[0012] Effects of the Invention

[0013] According to the present disclosure, a multi-core optical fiber capable of reducing polarization mode dispersion can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a perspective view of the optical fiber according to the first embodiment.

[0015] Figure 2 It is a cross-sectional view of the optical fiber perpendicular to the optical fiber axis.

[0016] Figure 3 This is a graph schematically showing the distribution of the linear expansion coefficient in an optical fiber.

[0017] Figure 4 This is a graph schematically showing the viscosity distribution in the optical fiber.

[0018] Figure 5 is a graph schematically showing the distribution of stress in an optical fiber.

[0019] Figure 6 It is a diagram schematically showing the in-plane stress in a cross section perpendicular to the optical fiber axis.

[0020] Figure 7 It is a diagram schematically showing the slow axis direction of birefringence of the fundamental mode in the core.

[0021] Figure 8 It is a cross-sectional view of an optical fiber according to the second embodiment.

[0022] Figure 9 It is a cross-sectional view of an optical fiber according to the third embodiment.

[0023] Figure 10 is a cross-sectional view of an optical fiber according to a modification.

[0024] Figure 11 is a cross-sectional view of an optical fiber according to a modification.

[0025] Figure 12 is a cross-sectional view of an optical fiber according to a modification.

[0026] Figure 13 It is a diagram schematically showing the internal structure of an optical fiber according to a modification. DETAILED DESCRIPTION

[0027] [Description of Embodiments of the Present Disclosure]

[0028] First, the contents of the embodiments of the present disclosure will be listed and described.

[0029] (1) A multi-core optical fiber according to one embodiment of the present disclosure comprises a plurality of cores along an optical 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 plurality of cores each having a higher refractive index than the cladding, the plurality of cores each having a larger linear expansion coefficient than the cladding, the plurality of cores including a first core having an elliptical shape in a cross section perpendicular to the optical fiber axis and one or more second cores different from the first core, the non-circularity 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 in a cross section perpendicular to the optical fiber axis and a straight line along the major axis of the elliptical shape being 30 degrees or less, and the polarization mode dispersion being 0.2 ps / rtkm or less.

[0030] In this multi-core optical fiber, the multiple cores include a first core having an elliptical shape in a cross-section perpendicular to the optical fiber axis, and one or more second cores different from the first core. Furthermore, the angle formed between a straight line connecting the center of gravity of the first core and the center of gravity of the core group consisting of the one or more second cores and a straight line along the major axis of the elliptical shape of the first core is 30 degrees or less. In this case, the birefringence caused by the stress caused by the core arrangement and the birefringence caused by the core shape are offset, reducing the birefringence of the waveguide mode propagating in the first core. As a result, polarization mode dispersion can be reduced.

[0031] (2) In the above (1), the non-circularity of the elliptical shape may be 10% or less. In this case, the connection loss when the multi-core optical fiber is connected to a normal optical fiber with a small non-circularity can be reduced.

[0032] (3) In (1) or (2) above, the plurality of 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 plurality of cores. In this case, since the core has a larger linear expansion coefficient than the cladding, the core contracts more strongly than the cladding due to cooling during drawing of the multi-core optical fiber, and thus compressive stress remains in the cladding near the core in the circumferential direction of the core. 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.

[0033] (4) In any of the above (1) to (3), the cladding may include a first cladding surrounding the plurality of 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 plurality of cores containing at least one alkali element from the alkali element group consisting of alkali metal elements and alkaline earth metal elements. In this case, for example, tensile stress caused by drawing can be localized in the second cladding, thereby preventing glass defects caused by tensile stress from occurring in the cores and the first cladding, and reducing the loss of the multi-core optical fiber.

[0034] (5) In any of (1) to (4) above, the plurality of cores may contain at least one element from the alkali element group consisting of alkali metal elements and alkaline earth metal elements, and the concentration of the alkali element in the plurality of cores may be greater than or equal to 1 wtppm and less than or equal to 3000 wtppm. In this case, the viscosity of the cores can be sufficiently reduced, and excessive loss caused by 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.

[0035] (6) In any of (1) to (5) above, the angle between a straight 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 straight 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.

[0036] (7) In any of (1) to (5) above, the angle between a straight 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 straight 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.

[0037] [Details of the embodiments of the present disclosure]

[0038] Specific examples of multi-core optical fibers according to embodiments of the present disclosure are described below with reference to the accompanying drawings. In the following description, identical elements or elements having identical functions are denoted by identical reference numerals, and duplicate descriptions are omitted. It should be noted that the present disclosure is not limited to these examples but is defined by the claims, which are intended to encompass all modifications within the meaning and scope equivalent to the claims.

[0039] Reference Figure 1 and Figure 2 , the structure of the optical fiber 1 according to the first embodiment will be described. Figure 1 It is a perspective view of the optical fiber 1 according to the first embodiment. Figure 2 is a cross-sectional view of the optical fiber 1 perpendicular to the optical fiber axis 2. Figure 2 The cladding 30 is omitted from the illustration. Optical fiber 1 includes multiple cores 10, a cladding 20, and a cladding 30. Optical fiber 1 may be used, for example, for long-distance, high-capacity transmission. Optical fiber 1 is a multi-core fiber (MCF) having multiple cores 10. In this embodiment, it includes two cores 10. In the following description, the two cores 10 may be separately referred to as core 11 and core 12.

[0040] Core 10 extends along the central axis of optical fiber 1, i.e., optical fiber axis 2. Core 10 comprises silica glass as a primary component. More specifically, cores 11 and 12 each comprise silica glass as a primary component. In this specification, "a component comprises silica glass as a primary component" means that at least 95% by mass of the component is composed of silica glass. Specifically, at least 95% by mass of core 10 is composed of silica glass.

[0041] Core 10 (core 11 and core 12) contains at least one alkali element from the group of alkali elements consisting of alkali metals and alkaline earth metals. Alkali elements are a general term for alkali metals and alkaline earth metals. Alkali metals include lithium (Li), sodium (Na), potassium (K), and rubidium (Rb). Alkaline earth metals include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The concentration of the alkali element in core 10 is between 1 wtppm and 3000 wtppm. Alternatively, the concentration of the alkali element in core 10 may be between 10 wtppm and 300 wtppm.

[0042] The core 10 contains fluorine (F). The fluorine concentration in the core 10 may be greater than or equal to 1000 wtppm and less than or equal to 5000 wtppm. The composition of the core 10 and the cladding 20 can be calculated based on the known weight per atom by measuring the atomic ratio using a known method such as EPMA (Electron Probe Micro Analyzer).

[0043] The refractive index of the core 10 is higher than the refractive index of the cladding 20. Thus, the light input into the optical fiber 1 is guided in the core 10. The difference in the relative refractive index of the core 10 with respect to the cladding 20 is, for example, greater than 0.3% and less than 0.6%. The refractive index of the core 10 and the cladding 20 can be measured by a well-known method such as the RNF (Refracted Near Field) method. In the optical fiber 1, the refractive index of a certain area (portion) is defined as the average value of the refractive index in the area. The diameter (core diameter) of the core 10 is, for example, greater than 7 μm and less than 14 μm. A single LP mode including two polarization modes is guided to the core 10.

[0044] The cladding 20 wraps around the core 10. Figure 2 As shown, the cladding 20 includes 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. Furthermore, the cladding 20 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 is higher than the fluorine concentration in the second cladding 22. The fluorine concentration in the first cladding 21 may be greater than 8000 wtppm and less than 16000 wtppm. The fluorine concentration in the second cladding 22 may be greater than 0.5 times and less than 0.9 times the fluorine concentration in the first cladding 21.

[0045] The outer diameter of the second cladding 22 may be, for example, 124 μm to 126 μm. When the diameter of the second cladding 22 is 124 μm to 126 μm, a standard connection unit, such as a fusion splicer or connector, can be used to connect the optical fiber 1 (the first optical fiber) to the second optical fiber. The ratio of the outer diameter of the first cladding 21 to the outer diameter of the second cladding 22 may be 0.4 to 0.7.

[0046] The coating layer 30 wraps the cladding layer 20 and is in close contact with the outer peripheral surface of the cladding layer 20. The coating layer 30 is formed of a resin such as acrylic acid.

[0047] like Figure 2 As shown, in a cross section perpendicular to the optical fiber axis 2, the core 10 has an elliptical shape. The elliptical shape of the core 10 means that when the boundary line between the core 10 and the cladding 20 is fitted into an ellipse, the non-circularity of the ellipse is non-zero. When the major axis of the ellipse is Rmax and the minor axis is Rmin, the non-circularity is 100 × 2 × (Rmax - Rmin) / (Rmax + Rmin)%.

[0048] Within the cross-section of the optical fiber 1, the boundary between regions with different refractive indices, such as the core 10 and cladding 20, is defined as a curve formed by the point where the refractive index gradient is steepest (or the centroid if there are multiple points). For example, the boundary between the core 10 and the cladding 20 is defined as the closed curve where, when a closed curve is set between the core 10 and the cladding 20, the average refractive index gradient along the closed curve is the maximum. In this embodiment, the non-circularity of the elliptical shape of the core 10 is greater than 0.1% and less than 10%. The non-circularity of the elliptical shape of the core 10 can be greater than 0.3% or greater than 1%. The non-circularity of the elliptical shape of the core 10 is defined as the non-circularity of the ellipse when the boundary between the core 10 and the cladding 20 is fitted to the ellipse.

[0049] The core 10 has a center of gravity in a cross section perpendicular to the optical fiber axis 2. The center of gravity of the core 10 is the center of gravity of the cross-sectional shape of the core 10 (the figure surrounded by the boundary line between the core 10 and the cladding 20). Figure 2 As shown, core 11 has a center of gravity P1 and core 12 has a center of gravity P2. Figure 2 , a straight line L1 and a straight line L2 are shown. Straight line L1 is a straight line connecting the center of gravity P1 of core 11 and the center of gravity P2 of core 12. Straight line L2 is a straight line along the major axis of the elliptical shape of core 11. Straight line L3 is a straight line along the major axis of the elliptical shape of core 12. The major axis of the elliptical shape of core 10 is defined as, for example, the major axis of an ellipse obtained by fitting the boundary line between core 10 and cladding 20.

[0050] The configuration of the cores 10 in the optical fiber 1 is described. The multiple cores 10 include at least one first core and one or more second cores different from the first core. In the optical fiber 1, in a cross section perpendicular to the optical fiber axis 2, the first core is configured so that the angle between the straight 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 the straight line along the major axis of the elliptical shape of the first core is 30 degrees or less. Here, "a core group consisting of one or more second cores" refers to all cores 10 other than the first core among the multiple cores 10. When the number of cores 10 in the optical fiber 1 is 2 as shown in this embodiment, the core group is composed of only one core 10. When the number of cores 10 is 3 or more, the core group is composed of two or more cores 10.

[0051] In optical fiber 1, core 11 and core 12 each serve as the first core. That is, when core 11 serves as the first core, core 12 serves as the second core, and when core 12 serves as the first core, core 11 serves as the second core. Therefore, in a cross section perpendicular to optical fiber axis 2, core 11 is positioned so that an angle θ1 formed between a straight line L1 connecting the center of gravity P1 of core 11 (the first core) and the center of gravity P2 of the core group G1 of the second cores (in this example, only core 12) and a straight line L2 along the major axis of the elliptical shape of core 11 is 30 degrees or less. Furthermore, core 12 is positioned so that an angle θ2 formed between a straight line L1 connecting the center of gravity P2 of core 12 (the first core) and the center of gravity P1 of the core group G2 of the second cores (in this example, only core 11) and a straight line L3 along the major axis of the elliptical shape of core 12 is 30 degrees or less. The angle between the straight 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 the straight line along the major axis of the elliptical shape of the first core may be 10 degrees or less, or 5 degrees or less.

[0052] In the optical fiber 1, the spacing between the cores 11 and 12 can be, for example, 15 μm to 60 μm, or 25 μm to 50 μm. The spacing 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. To reduce crosstalk, the spacing between the cores 11 and 12 can be increased, or to facilitate connection of the optical fiber 1, the spacing can be reduced. When the spacing between the cores 11 and 12 is 15 μm to 60 μm, or 25 μm to 50 μm, crosstalk can be reduced while facilitating connection of the optical fiber 1.

[0053] Figure 3 is a graph schematically showing the distribution of the linear expansion coefficient in the optical fiber 1. Figure 3 In the Figure 2 The linear expansion coefficient α of the portion of the optical fiber 1 located on the straight line L1. Figure 3 The horizontal axis of represents the position r on the straight line L1. The center position r of the line segment connecting the center of gravity P1 and the center of gravity P2 is set to 0. Figure 3 The vertical axis represents the linear expansion coefficient α. Specifically, it shows the linear expansion coefficient α10 of core 11 and core 12, the linear expansion coefficient α21 of first cladding 21, and the linear expansion coefficient α22 of second cladding 22. The linear expansion coefficient can be measured as elongation or contraction caused by temperature changes. The linear expansion coefficients in this specification are values at 1000°C, but values at temperatures below 700°C, which are easier to measure, can also be used.

[0054] As described above, the core 11, core 12, first cladding 21, and second cladding 22 each contain fluorine. The fluorine concentration in the second cladding 22 is higher than the fluorine concentration in the core 11 and core 12. Consequently, the second cladding 22 has a linear expansion coefficient α22 that is smaller than the linear expansion coefficient α10 of the core 11 and core 12. Furthermore, the fluorine concentration in the first cladding 21 is higher than the fluorine concentration in the second cladding 22. Consequently, the first cladding 21 has a linear expansion coefficient α21 that is smaller than the linear expansion coefficient α22 of the second cladding 22.

[0055] Figure 4 is a diagram schematically showing the distribution of viscosity in the optical fiber 1. Figure 4 In the Figure 2 The viscosity η of the portion of the optical fiber 1 located on the straight line L1. Figure 4 The horizontal axis represents the position r on the straight line L1, Figure 4 Specifically, the vertical axis represents the viscosity η. Specifically, the viscosity η10 of the core 11 and the core 12, the viscosity η21 of the first cladding 21, and the viscosity η22 of the second cladding 22 are shown.

[0056] As described above, cores 11 and 12 contain at least one element from the alkali element group consisting of alkali metal elements and alkaline earth metal elements. Consequently, cores 11 and 12 have a viscosity η10 that is lower than the viscosity η21 of first cladding 21 and the viscosity η22 of second cladding 22. Furthermore, the viscosity η21 of first cladding 21 is lower than the viscosity η22 of second cladding 22.

[0057] Figure 5 is a diagram schematically showing the distribution of stress in the optical fiber 1. Figure 5 In the Figure 2 The stress σzz generated in the portion of the optical fiber 1 located on the straight line L1. Figure 5 The horizontal axis represents the position r on the straight line L1, Figure 5 The vertical axis represents stress σzz. Specifically, it shows stress σzz10 generated in the cores 11 and 12, stress σzz21 generated in the first cladding 21, and stress σzz22 generated in the second cladding 22. In the direction along the optical fiber axis 2, tension is represented as positive and compression is represented as negative.

[0058] As described above, because the second cladding 22 has a relatively high viscosity η22, it experiences positive axial stress (stress σzz22) due to the tension applied during the drawing of the optical fiber 1. On the other hand, because the viscosity η10 of the cores 11 and 12 and the viscosity η21 of the first cladding 21 are relatively low, the cores 11 and 12 and the first cladding 21 experience 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 the contraction caused by cooling during the drawing of the optical fiber 1, the first cladding 21 experiences a greater negative axial stress (stress σzz21) than the cores 11 and 12.

[0059] Reference Figure 6 and Figure 7 , the birefringence generated in the core 10 will be described. Specifically, refer to Figure 6 , birefringence caused by stress due to the arrangement of the core 10 will be described, referring to Figure 7 , birefringence caused by the shape of the core 10 will be described. Figure 6 Schematically shows the in-plane stress in a cross section perpendicular to the optical fiber axis 2 . Figure 7 Schematically shows the slow axis direction of birefringence of the fundamental mode in the core 10. Figure 6 and Figure 7 In FIG, the cross section of the core 10 and the first cladding 21 is schematically shown, and the hatching is omitted for ease of explanation. Figure 6 and Figure 7 In the example, the straight line L1 (refer to Figure 2 ) is defined as the X direction, and the direction perpendicular to the X direction and the direction along the optical fiber axis 2 is defined as the Y direction. Figure 6 and Figure 7 In the example shown, the cross-sectional shape of the core 10 has an elliptical shape in a cross section perpendicular to the optical fiber axis 2 , and the core 10 is arranged so that the major axis of the elliptical shape is along the X direction.

[0060] 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, since 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 during drawing of the optical fiber 1, and compressive stress remains in the circumferential direction of the core 10 in the first cladding 21 close to the core 10. Figure 6 , the remaining compressive stresses A1 and A2 are shown. Specifically, among the remaining 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.

[0061] Considering the compressive stress remaining in the circumferential direction of a first core (e.g., core 11) included in the plurality of cores 10, the compressive stress A2 along the circumferential direction (Y direction) of the first core is weakened in the direction (X direction) where a second core (e.g., core 12) exists, due to the contraction of the second core. Consequently, the compressive stress along the circumference of the cores becomes anisotropic, and when the compressive stress is averaged, the compressive stress along the X direction (the direction in which the cores are arranged) becomes relatively greater. As a result, the refractive index for polarization in the X direction is lower than the refractive index for polarization in the Y direction, resulting in birefringence. Therefore, in this example, birefringence is generated in the cores 10 due to the stress caused by the arrangement of the cores 10, with the slow axis of this birefringence (the polarization direction in which the refractive index is higher) oriented in the Y direction.

[0062] In such Figure 7 Even when the core 10 has an elliptical shape, birefringence occurs due to the shape of the core 10. Figure 7 , schematically illustrates the slow axis B1 and fast axis B2 of birefringence caused by the shape of core 10. The slow axis B1 of birefringence is along the major axis of the elliptical shape, while the fast axis is along the minor axis. In this example, the major axis of the elliptical shape is along the X direction. Therefore, in this example, the slow axis B1 of birefringence caused by the shape of core 10 is along the X direction.

[0063] In this way, the birefringence caused by the stress caused by the core arrangement and the birefringence caused by the core shape have opposite signs, and by combining them, the overall birefringence is reduced. The reduction in birefringence reduces polarization mode dispersion.

[0064] exist Figure 6 and Figure 7 In the example, the major axis of the elliptical shape of the core 10 is aligned with the X direction (ie, the direction along the straight line L1 connecting the center of gravity P1 of the core 11 and the center of gravity P2 of the core 12). Figure 2 As shown in the optical fiber 1, even if the major axis of the ellipse does not completely coincide with the straight line L1, as long as it is substantially aligned with the straight line L1 (for example, the angle formed between the straight line L1 and the straight lines L2 and L3 along the major axis of the ellipse is 30 degrees or less), the birefringence caused by the stress caused by the arrangement of the core 10 and the birefringence caused by the shape of the core 10 are offset, thereby reducing the overall birefringence. As a result, the polarization mode dispersion is reduced.

[0065] In the optical fiber 1, the combined birefringence of the birefringence caused by the stress due to the arrangement of the core 10 and the birefringence caused by the shape of the core 10 is 10 -6 The synthesized birefringence can also be 10 -7Unless otherwise specified, birefringence refers to the value of the fundamental mode guiding the wave to core 10 (e.g., core 11) at a wavelength of 1550 nm. Furthermore, polarization-mode dispersion is 0.2 ps / rtkm or less. Polarization-mode dispersion may also be 0.1 ps / rtkm or less. Unless otherwise specified, polarization-mode dispersion refers to the value of the fundamental mode guiding the wave to core 10 (e.g., core 11) at a wavelength of 1550 nm. Polarization-mode dispersion can be measured using known methods such as the JME (Jones Matrix Eigenanalysis) method. During measurement, measurement light is input into the core to be measured in a multi-core optical fiber, and the measurement light output from this core is captured and measured. To selectively input and output measurement light to and from the core to be measured, known input / output devices such as fan-out equipment may be used. However, the polarization-mode dispersion of the input / output device is minimized to a negligible level or appropriately corrected based on the measurement results.

[0066] As described above, in the optical fiber 1 according to this embodiment, the plurality of cores 10 include cores 11 and 12 each having an elliptical shape in a cross section perpendicular to the optical fiber axis 2. The angle θ1 formed by the straight line L1 connecting the center of gravity P1 of core 11 and the center of gravity P2 of core 12 and the straight line L2 along the major axis of the elliptical shape of core 11 is 30 degrees or less. Furthermore, the angle θ2 formed by the straight line L1 and the straight 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 caused by the arrangement of the cores 10 and the birefringence caused by the shape of the cores 10 are offset, thereby reducing the birefringence of the waveguide mode propagating in the cores 10. As a result, polarization mode dispersion can be reduced. Furthermore, the angles θ1 and θ2 can be 10 degrees or less, or 5 degrees or less. In this case, polarization mode dispersion can be further reduced.

[0067] The elliptical shape of the core 10 has an out-of-roundness of 10% or less. This can reduce the connection loss when the optical fiber 1 is connected to a normal optical fiber with a small out-of-roundness.

[0068] The core 10 and cladding 20 contain fluorine, and the fluorine concentration in the cladding 20 is higher than the fluorine concentration in each of the cores 10. Consequently, the core 10 has a linear expansion coefficient α10 greater than the linear expansion coefficient α21 of the first cladding 21. Since the core 10 contracts more strongly than the first cladding 21 due to cooling during drawing of the optical fiber 1, compressive stress remains in the first cladding 21 near the core 10 in the circumferential direction of the core 10. As a result, birefringence is generated by the stress caused by the arrangement of the core 10, but this birefringence is offset by the birefringence caused by the shape of the core 10, thereby reducing polarization mode dispersion.

[0069] The cladding 20 includes 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 that in the second cladding 22. The core 10 contains at least one alkali element from the alkali element group consisting of alkali metal elements and alkaline earth metal elements. In this case, for example, tensile stress caused by drawing can be localized in the second cladding 22, preventing the occurrence of glass defects in the core 10 and the first cladding 21 due to tensile stress, thereby reducing the loss of the optical fiber 1.

[0070] The core 10 contains at least one element from the alkali element group consisting of alkali metals and alkaline earth metals. The concentration of the alkali element in the core 10 is between 1 wtppm and 3000 wtppm. This significantly reduces the viscosity of the core 10 and reduces excessive losses caused by high alkali element concentrations. Consequently, polarization mode dispersion and losses in the optical fiber 1 can be reduced.

[0071] Reference Figure 8 An optical fiber 1A according to the second embodiment will be described. Figure 8 It is a cross-sectional view of an optical fiber 1A according to the second embodiment. Figure 8 A cross section perpendicular to the fiber axis 2 of the optical fiber 1A is shown, and the cladding layer 30 is omitted from illustration. The optical fiber 1A according to this embodiment differs from the optical fiber 1 according to the first embodiment in that it includes multiple (two) first cladding layers 21A and 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 cladding layers 21A and 21B according to the second embodiment have the same configuration as the first cladding layer 21 according to the first embodiment.

[0072] 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 by the second cladding 22. The second cladding 22 surrounds the first cladding 21A and the first cladding 21B. In other words, 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 greater than 2 and less than 5.

[0073] In optical fiber 1A, the spacing between cores 11 and 12 can be, for example, 25 μm to 70 μm, or 35 μm to 60 μm. To reduce crosstalk, the spacing between cores 11 and 12 can be increased, or to facilitate connection of optical fiber 1A, the spacing can be decreased. When the spacing between cores 11 and 12 is 25 μm to 70 μm, or 35 μm to 60 μm, crosstalk can be reduced while facilitating connection of optical fiber 1A.

[0074] In optical fiber 1A, as in optical fiber 1, multiple cores 10 include cores 11 and 12, each having an elliptical shape in a cross section perpendicular to the optical fiber axis 2. The angle θ1 formed between a straight line L1 connecting the center of gravity P1 of core 11 and the center of gravity P2 of core 12 and a straight line L2 along the major axis of the elliptical shape of core 11 is 30 degrees or less. Furthermore, the angle formed between straight line L1 and a straight line L3 along the major axis of the elliptical shape of core 12 is 30 degrees or less. This reduces the birefringence of the waveguide mode propagating through core 10 by canceling out the birefringence caused by the stress of the arrangement of cores 10 and the birefringence caused by the shape of core 10. Consequently, polarization mode dispersion can be reduced in optical fiber 1A as well.

[0075] Reference Figure 9 An optical fiber 1B according to the third embodiment will be described. Figure 9 It is a cross-sectional view of an optical fiber 1B according to the third embodiment. Figure 9 A cross section perpendicular to the fiber axis 2 of the optical fiber 1B is shown, and the cladding 30 is omitted from illustration. The optical fiber 1B according to this embodiment differs from the optical fiber 1 according to the first embodiment in that it includes four cores 10 and four first claddings 21A, 21B, 21C, and 21D. Hereinafter, the four cores 10 will sometimes be described as cores 11, 12, 13, and 14, respectively. 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.

[0076] 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 each other by the second cladding 22. 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 diameters of the first claddings 21A, 21B, 21C, and 21D to the outer diameter of the core 10 may be greater than 2 and less than 5.

[0077] In the optical fiber 1B, the spacing between the cores 11 and 12 can be, for example, 25 μm to 70 μm, or 35 μm to 60 μm. From the perspective of reducing crosstalk, the spacing between the cores 11 and 12 can be increased, while from the perspective of facilitating the connection of the optical fiber 1B, the spacing can be reduced. When the spacing between the cores 11 and 12 is 25 μm to 70 μm, or 35 μm to 60 μm, crosstalk can be reduced while facilitating the connection of the optical fiber 1B. For the same reason, the spacing between the cores 12 and 13, the spacing between the cores 13 and 14, and the spacing between the cores 14 and 11 can be, for example, 25 μm to 70 μm, or 35 μm to 60 μm.

[0078] Like the core 10 according to the first embodiment, the core 10 (cores 11, 12, 13, 14) according to the third embodiment has an elliptical shape in a cross section perpendicular to the optical fiber axis 2. In this embodiment, the non-circularity of the elliptical shape of the core 10 is greater than 0.1% and less than 10%. The non-circularity of the elliptical shape may be greater than 0.3% or greater than 1%. Figure 9 , straight line L2 and straight line L4 are shown. Straight line L2 is a straight line along the major axis of the elliptical shape of core 11. Straight line L4 is a straight line connecting the center of gravity P1 of core 11 and the center of gravity P3 of core group G3 composed of cores 10 different from core 11 (cores 12, 13, 14). In optical fiber 1B, in a cross section perpendicular to optical fiber axis 2, cores 11 are arranged so that an angle θ3 formed between straight line L4 connecting the center of gravity P1 of core 11 and the center of gravity P3 of core group G3 composed of cores 10 different from core 11 and straight line L2 along the major axis of the elliptical shape of core 11 is 30 degrees or less.

[0079] In optical fiber 1B, cores 12, 13, and 14 are arranged under the same conditions as core 11. Specifically, core 12 is arranged so that the angle formed by a straight line connecting the center of gravity of core 12 and the center of gravity of the core group consisting of cores 10 (cores 11, 13, and 14) different from core 12 and the line along the major axis of the elliptical shape of core 12 is 30 degrees or less. Core 13 is arranged so that the angle formed by a straight line connecting the center of gravity of core 13 and the center of gravity of the core group consisting of cores 10 (cores 11, 12, and 14) different from core 13 and the line along the major axis of the elliptical shape of core 13 is 30 degrees or less. Core 14 is arranged so that the angle formed by a straight line connecting the center of gravity of core 14 and the center of gravity of the core group consisting of cores 10 (cores 11, 12, and 13) different from core 14 and the line along the major axis of the elliptical shape of core 14 is 30 degrees or less.

[0080] In optical fiber 1B, similar to optical fiber 1, multiple cores 10 include cores 11, 12, 13, and 14, each having an elliptical shape in a cross-section perpendicular to the optical fiber axis 2. The angle θ3 formed by a straight line L4 connecting the center of gravity P1 of core 11 and the center of gravity P3 of a core group G3 comprised of cores 10 different from core 11 (cores 12, 13, and 14) and a straight line L2 along the major axis of the elliptical shape of core 11 is 30 degrees or less. Cores 12, 13, and 14 are arranged under the same conditions as core 11. This arrangement cancels out birefringence caused by stress in the arrangement of cores 10 and birefringence caused by the shape of cores 10, reducing the birefringence of the waveguide mode propagating through core 10. Consequently, polarization mode dispersion can also be reduced in optical fiber 1B.

[0081] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above embodiments, and various modifications can be made without departing from the spirit of the present disclosure. In addition, the above embodiments can be appropriately combined.

[0082] like Figure 10 As shown, optical fiber 1B may also include a mark 50 at a position that disrupts the symmetry of the arrangement of the cores 10. This allows identification of each core 10 in optical fiber 1B. The mark 50 may be formed, for example, on the cladding 20 (second cladding 22). The mark 50 may also be formed of a material having a refractive index different from that of the cladding 20 (second cladding 22). Optical fibers 1 and 1A may also include a mark for identifying each core 10.

[0083] like Figure 11 As shown, in optical fiber 1B, core 10 may be arranged without rotational symmetry with respect to fiber axis 2 (cladding center). In this example, core 11 is arranged closer to the outer edge of cladding 20 than cores 12, 13, and 14. This allows each core 10 to be distinguished in optical fiber 1B. In optical fibers 1 and 1A, core 10 may be arranged without rotational symmetry with respect to fiber axis 2 (cladding center).

[0084] In a cross section perpendicular to the optical fiber axis 2, the elliptical shape of the core 10 may have a non-roundness of 10% or greater. The concentration of alkali elements in the core 10 may be 1 wtppm or less, or 3000 wtppm or greater. In addition to alkali elements, the core 10 may also contain at least one of 1000 wtppm to 5000 wtppm of fluorine and 100 wtppm to 3000 wtppm of chlorine (Cl). In this case, the viscosity of the core 10 is reduced, thereby reducing losses due to density fluctuations, etc. The core 10 does not need to contain fluorine.

[0085] like Figure 12 and Figure 13As shown in FIG. 1 , the optical fiber of the present embodiment may also have a plurality of twists 61 arranged in the longitudinal direction. Figure 12 and Figure 13 In, as Figure 9 A modification of the optical fiber 1B shown is an optical fiber 1D having a twist 61 . Figure 12 1D is a diagram schematically showing a cross section perpendicular to the longitudinal direction of the optical fiber 1D. Figure 13 Schematically shows the internal structure of the optical fiber 1D when the optical fiber 1D is viewed in a direction perpendicular to the longitudinal direction of the optical fiber 1D. Figure 12 and Figure 13 In FIG, the first cladding layer 21 and the cladding layer 30 are omitted. Figure 13 In the figure, cores 12 and 14 are further omitted. The second cladding 22 is indicated by a dotted line, and the cores 11 and 13 located within 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 optical fiber axis (the longitudinal direction of the optical fiber 1D), each core 10 has an elliptical shape.

[0086] In optical fiber 1D, by rotating the positions of the multiple cores 11, 12, 13, and 14 as they move along the length, multiple twists 61 are formed along the length of the optical fiber 1D. Twists 61 are locations (structures) where the multiple cores 10 intersect when viewing the optical fiber 1D from a direction perpendicular to the fiber axis. The presence of twists 61 in optical fiber 1D generates mode coupling between the two polarization modes in each core 10. This mode coupling randomizes the polarization mode dispersion caused by stress and shape anisotropy within the cross section of the optical fiber 1D, thereby reducing the accumulation of polarization mode dispersion. From this perspective, the number of twists (the number of twists 61) can be 0.1 turns / m or greater. Alternatively, the number of twists can be 1 turn / m or greater. It should be noted that the number of twists is the value obtained by averaging the absolute value of the rotation angle per unit length along the length. The direction of the twists 61 can be either left or right (in both directions along the circumference of the optical fiber 1D), but twists 61 in both directions can also coexist along the length.

[0087] While 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. Consequently, when coupling light to the end face of the optical fiber 1D, if the end of the optical fiber 1D is cut, the core arrangement changes, potentially making it difficult to couple light. To reduce the impact of this problem, the number of twists 61 can be set to 3 turns / m or less. Alternatively, the number of twists 61 can be set to 0.3 turns / m or less.

[0088] Description of Reference Numerals

[0089] 1, 1A, 1B, 1D optical fibers

[0090] 2 Fiber axis

[0091] 10, 11, 12, 13, 14 cores

[0092] 20 cladding

[0093] 21, 21A, 21B, 21C, 21D first cladding

[0094] 22 Second cladding

[0095] 30 cladding layer

[0096] 50 marks

[0097] 61 Twist

[0098] A1, A2 compressive stress

[0099] B1 slow axis

[0100] B2 Fast Axis

[0101] L1, L2, L3, L4 straight lines

[0102] P1, P2, P3 center of gravity

[0103] α10, α21, α22 linear expansion coefficients

[0104] η10, η21, η22 viscosity

[0105] θ1, θ2, θ3 angles

[0106] σzz10, σzz21, σzz22 stresses.

Claims

1. A multi-core optical fiber having: a plurality of cores along the axis of the optical fiber; and a cladding layer, wrapping 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, The linear expansion coefficient of each of the plurality of cores is larger than the linear expansion coefficient of the cladding, The plurality of cores include a first core having an elliptical shape in a cross section orthogonal to the optical fiber axis, and one or more second cores different from the first core, The out-of-roundness of the elliptical shape is greater than 0.1%, In a cross section perpendicular to the optical fiber axis, an angle formed by a straight line connecting the center of gravity of the first core and the center of gravity of the core group consisting of the one or more second cores and a straight line along the major axis of the elliptical shape is 30 degrees or less, Polarization mode dispersion is less than 0.2ps / rtkm.

2. The multi-core optical fiber according to claim 1, wherein: The out-of-roundness of the elliptical shape is 10% or less.

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

4. The multi-core optical fiber according to claim 1 or 2, wherein: The cladding includes a first cladding wrapping the plurality of cores and a second cladding wrapping the first cladding. The first cladding layer and the second cladding layer contain fluorine, The fluorine concentration in the first cladding layer is higher than the fluorine concentration in the second cladding layer, The plurality of cores include at least one alkali element selected from the group consisting of alkali metal elements and alkaline earth metal elements.

5. The multi-core optical fiber according to claim 1 or 2, wherein: The plurality of cores include at least one alkali element selected from the group consisting of alkali metal elements and alkaline earth metal elements. The concentration of the alkali element in the plurality of cores is 1 wtppm or more and 3000 wtppm or less.

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

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

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