MULTICORE OPTICAL FIBER

Abstract

A multi-core optical fiber includes a plurality of cores along a fiber axis and a cladding surrounding the plurality of cores. The plurality of cores and the cladding contain a silica glass as a main component. The refractive index of each of the plurality of cores is greater than the refractive index of the cladding. The coefficient of thermal expansion of each of the plurality of cores is greater than the coefficient of thermal expansion of the cladding. The plurality of cores includes a first core having an elliptical shape in a cross-section orthogonal 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 greater.In the cross-section orthogonal to the 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 including one or more second cores and a straight line along the major axis of the elliptical shape is 30 degrees or less. The dispersion of the polarization modes is 0.2 ps / rtkm or ps.km-1 / 2 or less.

Description

Title of the invention: MULTICORE OPTICAL FIBER Technical field

[0001] The present disclosure relates to a multi-core optical fiber. Background

[0002] A technique for reducing polarization mode dispersion (PMD) in a multi-core optical fiber comprising a plurality of cores and a cladding surrounding the plurality of cores is known. For example, in a method for manufacturing a multi-core optical fiber disclosed in Patent Literature 1 A1, a plurality of core rods having no center hole are inserted into the holes of a soot blank to constitute the cladding, and the core rods and the soot blank are sintered. This provides low non-axial symmetry near the center of the core rod, and less polarization mode dispersion. Patent Literature 2 discloses that a plurality of multi-core units included in a multi-core optical fiber has at least threefold rotational symmetry, so that structural asymmetry is reduced and an increase in polarization mode dispersion is prevented. List of Citations Patent literature

[0003] Patent Literature 1: US 2015 / 0307387 Al Patent Literature 2: US 2011 / 0206330 Al Summary of the Invention

[0004] Even in a multi-core optical fiber in which low non-axial symmetry is achieved near the center of a core as disclosed in patent literature 1, polarization mode dispersion may be high in the case where the arrangement of the cores does not have at least threefold rotational symmetry. There is therefore a demand for a multi-core optical fiber capable of reducing polarization mode dispersion even in the case where the arrangement of the cores does not have at least threefold rotational symmetry.

[0005] The present disclosure provides a multi-core optical fiber capable of reducing polarization mode dispersion.

[0006] (1) A multi-core optical fiber according to one aspect of the present disclosure comprises a plurality of cores along the fiber axis and a cladding surrounding the plurality of cores, wherein the plurality of cores and the cladding contain a silica glass as a main component. The refractive index of each of the plurality of cores is greater than the refractive index of the cladding. The coefficient of thermal expansion of each of the plurality of cores is greater than the coefficient thermal expansion of the cladding. The plurality of cores includes a first core having an elliptical shape in a cross-section orthogonal 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. 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 including the one or more second cores and a straight line along the major axis of the elliptical shape is 30 degrees or less in a cross-section orthogonal to the fiber axis, and the polarization mode dispersion is 0.2 ps / rtkm or ps.km 1 / 2 or less.

[0007] In this multi-core optical fiber, the plurality of cores includes the first core having an elliptical shape in a cross-section orthogonal to the fiber axis and the at least one second core different from the first core. In addition, the angle formed by a straight line connecting the center of gravity of the first core and the center of gravity of the core group including the at least one second core and a straight line along the major axis of the elliptical shape of the first core is 30 degrees or less. In this case, birefringence generated by stress caused by the arrangement of the cores and birefringence generated by the shape of the core are canceled, and birefringence of a waveguide mode propagating through the first core is reduced. This reduces dispersion of polarization modes.

[0008] (2) In point (1) above, the non-circularity of the elliptical shape can be 10% or less. In this case, it is possible to reduce the connection loss when the multi-core optical fiber is connected to a normal optical fiber with a small non-circularity.

[0009] (3) In items (1) or (2) above, the plurality of cores and the cladding may contain fluorine, and the fluorine concentration in the cladding may be greater than the fluorine concentration in each of the plurality of cores. In this case, the core has a coefficient of thermal expansion greater than the coefficient of thermal expansion of the cladding, and the core contracts more strongly than the cladding due to cooling when the multi-core optical fiber is pulled, so that the compressive stress remains in the cladding close to the core in the circumferential direction of the core. Thus, although birefringence is generated by the stress caused by the arrangement of the cores, the birefringence is canceled by the birefringence generated by the shape of the core, so that the dispersion of the polarization modes can be reduced.

[0010] (4) In any of (1) to (3) above, the sheath 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 may contain fluorine, the concentration of fluorine in the first cladding may be greater than the concentration of fluorine in the second cladding, and the plurality of cores may contain at least one type of alkali element from the alkali element group including an elemental alkali metal and an elemental alkaline earth metal. In this case, for example, the tensile stress caused by drawing can be localized in the second cladding, the glass defect caused by the tensile stress can be prevented from appearing in the core and the first cladding, and the loss in the multi-core optical fiber can be reduced.

[0011] (5) In any one of (1) to (4) above, the plurality of cores may contain at least one type of element from the alkali element group including an elemental alkali metal and an elemental alkaline earth metal, and the alkali element concentration in the plurality of cores may be 1 ppm or mg / kg by weight or more and 3000 ppm or mg / kg by weight or less. In this case, the viscosity of the core is sufficiently reduced, and excessive loss caused by the high concentration of the alkali element can be reduced. Therefore, polarization mode dispersion and loss in the multi-core optical fiber can be reduced.

[0012] (6) In any of the points (1) to (5) above, the angle formed by the line straight line connecting the center of gravity of the first core and the center of gravity of the core group including the second core(s) and the straight line along the major axis of the elliptical shape may be 10 degrees or less. In this case, the dispersion of the polarization modes can be further reduced.

[0013] (7) In any of the points (1) to (5) above, the angle formed by the line straight line connecting the center of gravity of the first core and the center of gravity of the core group including the second core(s) and the straight line along the major axis of the elliptical shape may be 5 degrees or less. In this case, the dispersion of the polarization modes can be further reduced. Brief description of the drawings

[0014] [Fig-1] [Fig.l] is a perspective view of an optical fiber according to a first embodiment;

[0015] [Fig.2] [Fig.2] is a cross-sectional view of the orthogonal optical fiber at the fiber axis;

[0016] [Fig.3] [Fig.3] is a graph schematically illustrating the distribution of the coefficient of thermal expansion in optical fiber;

[0017] [Fig.4] [Fig.4] is a graph schematically illustrating the distribution of viscosity in the optical fiber;

[0018] [Fig.5] [Fig.5] is a graph schematically illustrating the distribution of stress in the optical fiber;

[0019] [Fig.6] [Fig.6] is a view schematically illustrating the stress in the plane in a cross section orthogonal to the fiber axis;

[0020] [Fig.7] [Fig.7] is a view schematically illustrating the slow axis direction of birefringence of a fundamental mode in a core;

[0021] [Fig.8] [Fig.8] is a cross-sectional view of an optical fiber according to a second embodiment;

[0022] [Fig.9] [Fig.9] is a cross-sectional view of an optical fiber according to a third embodiment;

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

[0024] [Fig. 11] [Fig. 11] is a cross-sectional view of an optical fiber along a modification;

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

[0026] [Fig. 13] [Fig. 13] is a view schematically illustrating an internal structure of the optical fiber according to the modification. Detailed description

[0027] [Details of 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, the same reference symbols will be used for the same elements or elements having the same functions, and the description will not be repeated to avoid redundancy. It is noted that the present disclosure is not limited to these examples, is indicated by the claims, and is intended to encompass all modifications within the meaning and scope equivalent to the claims.

[0028] With reference to Figures 1 and 2, a configuration of an optical fiber 1 according to a first embodiment will be described. [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 orthogonal to the axis of the fiber 2. In [Fig. 2], the illustration of a cladding layer 30 is omitted. The optical fiber 1 comprises a plurality of cores 10, a cladding 20, and the cladding layer 30. The optical fiber 1 may be, for example, an optical fiber used for large-capacity and long-distance transmission. The optical fiber 1 is a multi-core optical fiber (MCF) comprising the plurality of cores 10 and comprises two cores 10 in the present embodiment. Hereinafter, each of the two cores 10 may be described while being distinguished as core 11 or core 12.

[0029] The core 10 is along the axis of the fiber 2 which is the central axis of the optical fiber 1. The core 10 contains a silica glass as a main component. In such a way More specifically, each of the core 11 and the core 12 contains a silica glass as a main component. In the present application, the fact that a certain element contains a silica glass as a main component means that 95% by mass or more of the element is formed of silica glass. In other words, 95% by mass or more of the core 10 is formed of silica glass.

[0030] The core 10 (core 11 and core 12) contains at least one type of alkali element from a group of alkali elements including an elemental alkali metal and an elemental alkaline earth metal. The alkali element is an expression for an elemental alkali metal and an elemental alkaline earth metal in general. The elemental alkali metal is lithium (Li), sodium (Na), potassium (K), rubidium (Rb), or the like. The elemental alkaline earth metal is magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), or the like. The concentration of the alkali element in the core 10 is 1 ppm or mg / kg by weight or more and 3,000 ppm or mg / kg by weight or less. The concentration of the alkali element in the core 10 may be 10 ppm or mg / kg by weight or more and 300 ppm or mg / kg by weight or less.

[0031] The core 10 contains fluorine (F). The concentration of fluorine in the core 10 may be 1,000 ppm or mg / kg by weight or more and 5,000 ppm or mg / kg by weight or less. The composition of the core 10 and the cladding 20 may be calculated from the known weight per atom, by measuring the proportion of the number of atoms using a known method such as an electron probe micrometer analyzer (EPMA).

[0032] The refractive index of the core 10 is greater than the refractive index of the cladding 20. The light entering the optical fiber 1 is thus guided by the core 10. The relative refractive index difference between the core 10 and 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 a refracted near-field (RNF) method. In the optical fiber 1, the refractive index of a certain region (portion) is defined as the average value of the refractive indices in the region. The diameter (core diameter) of the core 10 is, for example, 7 pm or more and 14 pm or less. A single LP mode including two embodiments is guided to the core 10.

[0033] The cladding 20 surrounds the core 10. As illustrated 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 a silica glass as a main component. More specifically, each of the first cladding 21 and the second cladding 22 contains a silica glass as a main component. In addition, the cladding 20 contains fluorine. The concentration of fluorine in the cladding 20 is higher than the concentrations of fluorine in the core 11 and the core 12. More specifically, each of the first cladding 21 and the second cladding 22 contains fluorine. The fluorine concentration in the first cladding 21 and the second cladding 22 is greater than the fluorine concentration in the core 11 and the core 12. The fluorine concentration in the first cladding 21 is greater than the fluorine concentration in the second cladding 22. The fluorine concentration in the first cladding 21 may be 8,000 ppm or mg / kg by weight or more and 16,000 ppm or mg / kg by weight 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.

[0034] The outer diameter (diameter) of the second sheath 22 may be, for example, 124 μm or more and 126 μm or less. In the case where the diameter of the second sheath 22 is 124 μm or more and 126 μm or less, a standard fusion splicer or a connecting means such as a connector may be used when the optical fiber 1, which is a first optical fiber, is connected to a second optical fiber. The ratio of the outer diameter (diameter) of the first sheath 21 to the outer diameter of the second sheath 22 may be 0.4 or more and 0.7 or less.

[0035] The coating layer 30 surrounds the sheath 20 and is in close contact with the outer peripheral surface of the sheath 20. The coating layer 30 is formed for example from a resin such as an acrylate.

[0036] As illustrated in [Fig.2], the core 10 has an elliptical shape in a cross-section orthogonal to the axis of the fiber 2. The fact that the core 10 has an elliptical shape means that the non-circularity of an ellipse when the boundary line between the core 10 and the cladding 20 is fitted to the ellipse is non-zero. In the case where the long diameter of the ellipse is Rmax and the short diameter is Rmin, the non-circularity is 100 x 2 x (Rmax - Rmin) / (Rmax + Rmin) %.

[0037] In the cross-section of the optical fiber 1, the boundary line between regions having different refractive indices, such as the core 10 and the cladding 20, is defined as a curve including a point where the gradient of the refractive index is steepest (in the case where there are multiple points, the center of gravity of the points). For example, the boundary line between the core 10 and the cladding 20 is defined as a closed curve in which the gradient of the refractive index averaged along the nearest curve is maximized when the closed curve is established between the core 10 and the cladding 20. In the present embodiment, the non-circularity of the elliptical shape of the core 10 is 0.1% or more and 10% or less. The non-circularity of the elliptical shape may be 0.3% or more or may be 1% or more.The non-circularity of the elliptical shape of the core 10 is defined as the non-circularity of an ellipse when the boundary line between the core 10 and the cladding 20 is fitted to the ellipse.

[0038] The core 10 has a center of gravity in the cross-section orthogonal to the axis of the fiber 2. The center of gravity of the core 10 is the center of gravity of a cross-sectional shape transverse of core 10 (a pattern enclosed by the boundary line between core 10 and cladding 20). As illustrated in [Fig. 2], core 11 has a center of gravity PI, and core 12 has a center of gravity P2. [Fig. 2] illustrates a straight line L1 and a straight line L2. Straight line L1 is a straight line connecting center of gravity PI to core 11 and center of gravity P2 to 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, for example, as the major axis of the ellipse when the boundary line between core 10 and cladding 20 is fitted to the ellipse.

[0039] The arrangement of the cores 10 in the optical fiber 1 will be described. The plurality of cores 10 includes at least a first core and one or more second cores different from the first core. In the optical fiber 1, in the cross-section orthogonal to the axis of the fiber 2, the first core is arranged such that the angle formed by a straight line connecting the center of gravity of the first core and the center of gravity of a group of cores including 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.Here, the "core group including one or more second cores" represents all the cores 10 excluding the first core among the plurality of cores 10 and, in the case where the number of cores 10 included in the optical fiber 1 is two as in the present embodiment, the core group includes only one core 10, and in the case where the number of cores 10 is three or more, the core group includes two or more cores 10.

[0040] In the optical fiber 1, each of core 11 and core 12 is the first core. In other words, in the case where core 11 is the first core, then core 12 is the second core, and in the case where core 12 is the first core, then core 11 is the second core. Therefore, in the cross-section orthogonal to the axis of the fiber 2, core 11 is arranged so that the angle θ1 formed by the straight line L1 connecting the center of gravity PI of core 11 (first core) and the center of gravity P2 of a core group G1 of the second core (only core 12 in this example) and the straight line L2 along the major axis of the elliptical shape of core 11 is 30 degrees or less.In addition, the core 12 is arranged such that the angle 02 formed by the straight line L1 connecting the center of gravity P2 of the core 11 (first core) and the center of gravity PI of a core group G2 of the second core (only the core 11 in this example) and the straight line L3 along the major axis of the elliptical shape of the core 12 is 30 degrees or less. The angle formed by the straight line connecting the center of gravity of the first core and the center of gravity of the core group including one or more second cores and the straight line along the major axis of the shape . elliptical of the first core may be 10 degrees or less or may be 5 degrees or less.

[0041] In the optical fiber 1, the interval between the core 11 and the core 12 may be, for example, 15 pm or more and 60 pm or less or may be 25 pm or more and 50 pm or less. The interval between the core 11 and the core 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 interval between the core 11 and the core 12 may be increased, and from the viewpoint of facilitating the connection of the optical fiber 1, the interval may be decreased. In the case where the interval between core 11 and core 12 is 15 pm or more and 60 pm or less, or 25 pm or more and 50 pm or less, it is possible to facilitate the connection of optical fiber 1 while reducing crosstalk.

[0042] [Fig.3] is a graph schematically illustrating the distribution of the coefficient of thermal expansion in the optical fiber 1. [Fig. 3] illustrates the coefficient of thermal expansion a of a portion located on the straight line L1 in the optical fiber 1 of [Fig. 2]. The horizontal axis in [Fig. 3] illustrates the position r on the straight line LL. The center position r of a line segment connecting the center of gravity PI and the center of gravity P2 is set to 0. The vertical axis in [Fig. 3] illustrates the coefficient of thermal expansion a. Specifically, the coefficient of thermal expansion a10 of the core 11 and the core 12, the coefficient of thermal expansion a21 of the first cladding 21, and the coefficient of thermal expansion a22 of the second cladding 22 are illustrated. The coefficient of thermal expansion can be measured as the elongation or shrinkage due to a change in temperature.The coefficient of thermal expansion in this booklet is a value at 1000°C, but may be a value at a temperature of 700°C or lower, at which measurement is easier.

[0043] As described above, each of the core 11, the core 12, the first cladding 21, and the second cladding 22 contains fluorine. The concentration of fluorine in the second cladding 22 is greater than the concentrations of fluorine in the core 11 and the core 12, resulting in a second cladding 22 having a coefficient of thermal expansion a10 that is less than the coefficient of thermal expansion a22 of the core 11 and the core 12. In addition, the concentration of fluorine in the first cladding 21 is greater than the concentration of fluorine in the second cladding 22, resulting in a first cladding 21 having a coefficient of thermal expansion a21 that is less than the coefficient of thermal expansion a22 of the second cladding 22.

[0044] [Fig.4] is a graph schematically illustrating the viscosity distribution in optical fiber 1. [Fig.4] illustrates the viscosity q of a portion located on the straight line L1 in optical fiber 1 of [Fig.2]. The horizontal axis in [Fig.4] illustrates the position r on the straight line L1, and the vertical axis in [Fig.4] illustrates the viscosity q. Specifically, the viscosity p 10 of the core 11 and the core 12, the viscosity q21 of the first sheath 21 and the viscosity q22 of the second sheath 22 are illustrated.

[0045] As described above, the core 11 and the core 12 contain at least one type of element from the group of alkali elements including an elemental alkali metal and an elemental alkaline earth metal. Thus, the core 11 and the core 12 have a viscosity q 10 lower than the viscosity q21 of the first sheath 21 and the viscosity q22 of the second sheath 22. In addition, the viscosity q21 of the first sheath 21 is lower than the viscosity q22 of the second sheath 22.

[0046] [Fig. 5] is a graph schematically illustrating the stress distribution in the optical fiber 1. [Fig. 5] illustrates the stress ozz generated in a portion located on the straight line L1 in the optical fiber 1 of [Fig. 2]. The horizontal axis in [Fig. 5] illustrates the position r on the straight line L1, and the vertical axis in [Fig. 5] illustrates the stress ozz. Specifically, the stress ozz10 generated in the core 11 and the core 12, the stress ozz21 generated in the first cladding 21, and the stress ozz22 generated in the second cladding 22 are illustrated. In the direction along the axis of the fiber 2, tension is illustrated as positive and compression is illustrated as negative.

[0047] As described above, since the second cladding 22 has a relatively high viscosity q22, the second cladding has a positive axial stress (stress ozz22) due to tension when the optical fiber 1 is pulled. Furthermore, since the viscosity q10 of the cores 11 and 12 and the viscosity q21 of the first cladding 21 are relatively low, the cores 11 and 12 and the first cladding 21 have a negative axial stress (stresses ozz10 and ozz21). The thermal expansion coefficient a21 of the first cladding 21 is lower than the thermal expansion coefficient a10 of the core 11 and the core 12. Therefore, the first cladding 21 has a negative axial stress (stress ozz21) higher than that of the core 11 and the core 12 due to contraction caused by cooling when the optical fiber 1 is pulled.

[0048] With reference to Figures 6 and 7, the birefringence generated in the core 10 will be described. Specifically, with reference to [Fig. 6], the birefringence generated by the stress caused by the arrangement of the cores 10 will be described, and with reference to [Fig. 7], the birefringence generated by the shape of the core 10 will be described. [Fig. 6] is a view schematically illustrating the in-plane stress in a cross-section orthogonal to the axis of the fiber 2. [Fig. 7] is a view schematically illustrating the slow axis direction of the fundamental mode birefringence in the core 10. Figures 6 and 7 schematically illustrate cross-sections of the core 10 and the first cladding 21, and the hatching is omitted for the sake of convenience of description. In Figures 6 and 7, the direction along the straight line Ll (see [Fig.2]) connecting the center of gravity PI of the core 11 and the center of gravity P2 of the core 12 is defined as the X direction, and the direction orthogonal to the X direction and the direction along the axis of the fiber 2 is defined as the Y direction. In an example illustrated in Figures 6 and 7, in the cross-section orthogonal to the axis of the fiber 2, the cross-sectional shape of the core 10 has an elliptical shape, and the core 10 is arranged so that the major axis of the elliptical shape is along the X direction.

[0049] As described above, the first cladding 21 has a thermal expansion coefficient a 10 smaller than the thermal expansion coefficient a21 of the core 10. In other words, since the thermal expansion coefficient a10 of the core 10 is larger than the thermal expansion coefficient a21 of the first cladding 21, the core 10 contracts more strongly than the first cladding 21 due to cooling when the optical fiber 1 is pulled, and the compressive stress remains in the circumferential direction of the core 10 in the first cladding 21 close to the core 10. [Fig. 6] illustrates the residual compressive stress A1 and A2. Specifically, among the residual compressive stress, the compressive stress along the X direction is illustrated as the compressive stress A1, and the compressive stress along the Y direction is illustrated as the compressive stress A2.

[0050] When the compressive stress remaining in the circumferential direction of the first core (e.g., core 11) included in the plurality of cores 10 is examined, in the direction (X direction) in which the 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 by the influence of the contraction of the second core. Therefore, anisotropy occurs in the compressive stress along the circumferential direction of the core, and when the compressive stress is averaged, the compressive stress along the X direction (direction in which the cores are arranged) becomes relatively large.Thus, the refractive index with respect to the polarization in the X direction becomes lower than the refractive index with respect to the polarization in the Y direction, and birefringence is generated. Therefore, in this example, birefringence is generated in the core 10 by the stress caused by the arrangement of the cores 10, and the slow axis of birefringence (polarization direction in which the refractive index is high) is along the Y direction.

[0051] In the case where the core 10 has an elliptical shape, as illustrated in [Fig.7], the birefringence is also generated by the shape of the core 10. [Fig.7] schematically illustrates the slow axis B1 and the fast axis B2 of the birefringence generated by the shape of the core 10. The slow axis B1 of the birefringence is along the major axis of the elliptical shape, and 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 the birefringence generated by the heart shape 10 is along the X direction.

[0052] As described above, the birefringence generated by the stress caused by the arrangement of the cores 10 and the birefringence generated by the shape of the core 10 have opposite signs, and the birefringence generated by the stress caused by the arrangement of the cores 10 and the birefringence generated by the shape of the core 10 are combined, whereby the birefringence as a whole is reduced. The birefringence is reduced, whereby the dispersion of the polarization modes is reduced.

[0053] In the examples of Figures 6 and 7, the major axis of the elliptical shape of the core 10 coincides with the direction X (i.e. the direction along the straight line L1 connecting the center of gravity PI of the core 11 and the center of gravity P2 of the core 12). However, just 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 straight line L1, if the major axis of the elliptical shape is approximately along the straight line L1 (e.g., when the angle formed by the straight line L1 and the straight lines L2 and L3 along the major axis of the elliptical shape is 30 degrees or less), the birefringence generated by the stress caused by the arrangement of the cores 10 and the birefringence generated by the shape of the core 10 are canceled, and the birefringence is reduced in its entirety. This results in reduced dispersion of the polarization modes.

[0054] In the optical fiber 1, the birefringence obtained by combining the birefringence generated by the stress caused by the arrangement of the cores 10 and the birefringence generated by the shape of the core 10 is 10 6 or less. The combined birefringence may be 10 7 or less. Unless otherwise stated, the birefringence refers to the value, at a wavelength of 1550 nm, of the fundamental mode guided in the core 10 (for example, the core 11). In addition, the dispersion of the polarization modes is 0.2 ps / rtkm or ps.km 1 / 2 or less. The dispersion of the polarization modes may be 0.1 ps / rtkm or ps.km 1 / 2 or less. Unless otherwise stated, polarization mode dispersion refers to the value, at a wavelength of 1550 nm, of the fundamental mode guided to core 10 (e.g., core 11). Polarization mode dispersion can be measured using a known method such as the Jones matrix eigenvalue and eigenvector analysis (JME) method.At the time of measurement, the measurement light entered a core which is an object subject to measurement of the multi-core optical fiber to be subjected to a . measurement of multi-core optical fiber, and the measurement light output from the core is captured and measured. Although a known input / output device such as a fan-out device can be used to selectively input and output the measurement light into and from the core to be measured, it is assumed that the polarization mode dispersion of the input / output device is negligibly small or appropriately corrected from a measurement result.

[0055] As described above, in the optical fiber 1 according to the present embodiment, the plurality of cores 10 includes the core 11 and the core 12 having elliptical shapes in the cross-section orthogonal to the axis of the fiber 2. The angle θ1 formed by the straight line L1 connecting the center of gravity P1 of the core 11 and the center of gravity P2 of the core 12 and the straight line L2 along the major axis of the elliptical shape of the core 11 is 30 degrees or less. In addition, the angle θ2 formed by the straight line L1 and the straight line L3 along the major axis of the elliptical shape of the core 12 is 30 degrees or less. Therefore, the birefringence generated by the stress caused by the arrangement of the cores 10 and the birefringence generated by the shape of the core 10 are canceled, and the birefringence of a waveguide mode propagating through the core 10 is reduced. The dispersion of the polarization modes can thus be reduced.In addition, the angle 01 and the angle 02 can be 10 degrees or less or can be 5 degrees or less. In this case, the dispersion of the polarization modes can be further reduced.

[0056] The non-circularity of the elliptical shape of the core 10 is 10% or less. It is therefore possible to reduce the connection loss when the optical fiber 1 is connected to a normal optical fiber having a small non-circularity.

[0057] The core 10 and the cladding 20 contain fluorine, and the concentration of fluorine in the cladding 20 is higher than the concentration of fluorine in each of the plurality of cores 10. Therefore, the core 10 has a thermal expansion coefficient a10 higher than the thermal expansion coefficient a21 of the first cladding, and the core 10 contracts more strongly than the first cladding 21 due to cooling when the optical fiber 1 is pulled, so that the compressive stress remains in the circumferential direction of the core 10 in the first cladding 21 close to the core 10. Therefore, although birefringence is generated by the stress caused by the arrangement of the cores 10, the birefringence is canceled by the birefringence generated by the shape of the core 10, so that the dispersion of the polarization modes can be reduced.

[0058] The cladding 20 has the first cladding 21 surrounding the core 10 and the second cladding 22 surrounding the first cladding 21. The first cladding 21 and the second cladding 22 contain fluorine. The concentration of fluorine in the first cladding 21 is greater than the concentration of fluorine in the second cladding 22. The core 10 contains at least one type of alkali element from the group of alkali elements including an elemental alkali metal and an elemental alkaline earth metal. In this case, for example, the tensile stress caused by drawing can be localized in the second cladding 22, a defect in the glass caused by the tensile stress can be prevented from occurring in the core 10 and the first cladding 21, and the loss of the optical fiber 1 can be reduced.

[0059] The core 10 contains at least one type of element from the group of alkali elements including an elemental alkali metal and an elemental alkaline earth metal. The concentration of the alkali element in the core 10 is 1 ppm or mg / kg by weight or more and 3,000 ppm or mg / kg by weight or less. Thus, the viscosity of the core 10 is sufficiently reduced, and excessive loss caused by the high concentration of the alkali element can be reduced. Therefore, polarization mode dispersion and loss in the optical fiber 1 can be reduced.

[0060] With reference to [Fig. 8], an optical fiber IA according to a second embodiment will be described. [Fig. 8] is a cross-sectional view of the optical fiber IA according to the second embodiment. [Fig. 8] illustrates a cross-section of the optical fiber IA orthogonal to the axis of the fiber 2, and the illustration of the cladding layer 30 is omitted. The optical fiber IA according to the present embodiment is different from the optical fiber 1 according to the first embodiment in that the optical fiber IA according to the present embodiment includes a plurality of (two) first cladding pieces 21A and 21B.Except for the points to be described below, the core 10 according to the second embodiment has a configuration similar to that of the core 10 according to the first embodiment, and the first sheaths 21A and 21B according to the second embodiment have configurations similar to that of the first sheath 21 according to the first embodiment.

[0061] The first sheath 21A surrounds the core 11, and the first sheath 21B surrounds the core 12. The first sheath 21A and the first sheath 21B are separated from each other by the second sheath 22 interposed between them. The second sheath 22 surrounds the first sheath 21A and the first sheath 21B. In other words, the first sheaths 21A and 21B are surrounded by the common second sheath 22. The ratio of the outer diameter of each piece of the first sheath 21A and 21B to the outer diameter of the core 10 may be 2 or more and 5 or less.

[0062] In the optical fiber 1A, the interval between the core 11 and the core 12 may be, for example, 25 pm or more and 70 pm or less or may be 35 pm or more and 60 pm or less. From the viewpoint of reducing crosstalk, the interval between the core 11 and the core 12 may be increased, and from the viewpoint of facilitating the connection of the optical fiber 1A, the interval may be decreased. In the case where the interval between the core 11 and the core 12 is 25 pm or more and 70 pm or less, or 35 pm or more and 60 pm or less, it is possible to facilitate the connection of IA optical fiber while reducing crosstalk.

[0063] Also in the optical fiber 1A, similarly to the optical fiber 1, the plurality of cores 10 includes the core 11 and the core 12 having elliptical shapes in the cross-section orthogonal to the axis of the fiber 2. The angle θ1 formed by the straight line L1 connecting the center of gravity P1 of the core 11 and the center of gravity P2 of the core 12 and the straight line L2 along the major axis of the elliptical shape of the core 11 is 30 degrees or less. In addition, the angle θ2 formed by the straight line L1 and the straight line L3 along the major axis of the elliptical shape of the core 12 is 30 degrees or less. Thus, the birefringence generated by the stress caused by the arrangement of the cores 10 and the birefringence generated by the shape of the core 10 are canceled, and the birefringence of a waveguide mode propagating through the core 10 is reduced. Therefore, the dispersion of polarization modes can also be reduced in the optical fiber IA.

[0064] With reference to [Fig. 9], an optical fiber IB according to a third embodiment will be described. [Fig. 9] is a cross-sectional view of the optical fiber IB according to the third embodiment. [Fig. 9] illustrates a cross-section of the optical fiber IB orthogonal to the axis of the fiber 2, and the illustration of the cladding layer 30 is omitted. The optical fiber IB according to the present embodiment is different from the optical fiber 1 according to the first embodiment in that the optical fiber IB according to the present embodiment includes four cores 10 and four first cladding pieces 21A, 21B, 21C and 21D. Hereinafter, the respective four cores 10 can be described while being distinguished as cores 11, 12, 13 and 14.In addition, except for the points to be described below, the core 10 according to the third embodiment has a configuration similar to that of the core 10 according to the first embodiment, and the first sheaths 21A, 21B, 21C and 21D according to the third embodiment have configurations similar to that of the first sheath 21 according to the first embodiment.

[0065] The first sheath 21A surrounds the core 11. The first sheath 21B surrounds the core 12. The first sheath 21C surrounds the core 13. The first sheath 21D surrounds the core 14. The first sheaths 21A, 21B, 21C and 21D are separated from each other by the second sheath 22 interposed between them. The second sheath 22 surrounds the first sheaths 21A, 21B, 21C and 21D. In other words, the first sheaths 21A, 21B, 21C and 21D are surrounded by the common second sheath 22. The ratio of the outer diameter of each piece of the first sheath 21A, 21B, 21C and 21D to the outer diameter of the core 10 may be 2 or more and 5 or less.

[0066] In the optical fiber IB, the interval between the core 11 and the core 12 may be for example 25 pm or more and 70 pm or less or may be 35 pm or more and 60 pm or less. From the viewpoint of reducing crosstalk, the interval between core 11 and core 12 may be increased, and from the viewpoint of facilitating the connection of optical fiber IB, the interval may be decreased. In the case where the interval between core 11 and core 12 is 25 pm or more and 70 pm or less, or 35 pm or more and 60 pm or less, it is possible to facilitate the connection of optical fiber IA while reducing crosstalk. For a similar reason, the interval between core 12 and core 13, the interval between core 13 and core 14, and the interval between core 14 and core 11 may be, for example, 25 pm or more and 70 pm or less or may be 35 pm or more and 60 pm or less.

[0067] In a similar manner to the core 10 according to the first embodiment, the core 10 (cores 11, 12, 13 and 14) according to the third embodiment has an elliptical shape in the cross-section orthogonal to the axis of the fiber 2. In the present embodiment, the non-circularity of the elliptical shape of the core 10 is 0.1% or more and 10% or less. The non-circularity of the elliptical shape may be 0.3% or more or may be 1% or more. [Fig. 9] illustrates a straight line L2 and a straight line L4. 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 L4 connecting the center of gravity PI of core 11 and the center of gravity P3 of the core group G3 including core 10 (cores 12, 13 and 14) different from core 11.In the optical fiber IB, in the cross section orthogonal to the axis of the fiber 2, the core 11 is arranged so that the angle 03 formed by the straight line L4 connecting the center of gravity PI of the core 11 and the center of gravity P3 of the core group G3 including the core 10 different from the core 11 and the straight line L2 along the major axis of the elliptical shape of the core 11 is 30 degrees or less.

[0068] In optical fiber IB, cores 12, 13, and 14 are arranged in accordance with conditions similar to those of core 11. In other words, core 12 is arranged such that the angle formed by a straight line connecting the center of gravity of core 12 and the center of gravity of a core group including core 10 (cores 11, 13, and 14) different from core 12 and a straight line along the major axis of the elliptical shape of core 12 is 30 degrees or less. The core 13 is arranged such that the angle formed by a straight line connecting the center of gravity of the core 13 and the center of gravity of a group of cores including the core 10 (cores 11, 12 and 14) different from the core 13 and a straight line along the major axis of the elliptical shape of the core 13 is 30 degrees or less.The core 14 is arranged such that the angle formed by a straight line connecting the center of gravity of the core 14 and the center of gravity of a group of cores including the core 10 (cores 11, 12 and 13) different from the core 14 and a straight line along the major axis of the elliptical shape of the core 14 is 30 degrees or less.

[0069] Also in optical fiber 1B, similarly to optical fiber 1, the plurality of cores 10 includes cores 11, 12, 13, and 14 having elliptical shapes in the cross-section orthogonal to the axis of fiber 2. The angle 03 formed by the straight line L4 connecting the center of gravity PI of core 11 and the center of gravity P3 of the core group G3 including core 10 (cores 12, 13, and 14) different from core 11 and the straight line L2 along the major axis of the elliptical shape of core 11 is 30 degrees or less. The cores 12, 13 and 14 are arranged in accordance with conditions similar to those of the core 11. Thus, the birefringence generated by the stress caused by the arrangement of the cores 10 and the birefringence generated by the shape of the core 10 are canceled, and the birefringence of a waveguide mode propagating through the core 10 is reduced.Therefore, the dispersion of polarization modes can also be reduced in the IB optical fiber.

[0070] Although embodiments have been described above, the present disclosure is not necessarily limited to the embodiments described above, and various modifications are possible without departing from the spirit of the present disclosure. In addition, the embodiments described above may be appropriately combined.

[0071] As illustrated in [Fig. 10], the optical fiber IB may have a marker 50 at a position where the symmetry of the arrangement of the cores 10 is broken. Thus, each core 10 can be identified in the optical fiber IB. The marker 50 may be formed for example in the cladding 20 (the second cladding 22). The marker 50 may be formed of a material having a refractive index different from the refractive index of the cladding 20 (the second cladding 22). Also in the optical fibers 1 and IB, a marker for identifying each core 10 may be formed.

[0072] As illustrated in [Fig. 11], in optical fiber IB, a core 10 may be arranged so as to have no rotational symmetry with respect to the axis of fiber 2 (the center of the cladding). In this example, a core 11 is arranged close to the outer edge of the cladding 20, compared to cores 12, 13 and 14. Thus, each core 10 can be identified in optical fiber IB. Also in optical fibers 1 and 1A, a core 10 may be arranged so as to have no rotational symmetry with respect to the axis of fiber 2 (the center of the cladding).

[0073] In the cross-section orthogonal to the axis of the fiber 2, the non-circularity of the elliptical shape of the core 10 may be 10% or more. The concentration of alkali element in the core 10 may be 1 ppm or mg / kg by weight or less, or may be 3,000 ppm or mg / kg by weight or more. The core 10 may contain, in addition to the alkali element, at least one of fluorine at 1,000 ppm or mg / kg or more and 5,000 ppm or mg / kg or less and chlorine (Cl) at 100 ppm or mg / kg by weight or more and 3,000 ppm or mg / kg by weight or less. In this case, the viscosity of the core 10 is reduced, and the loss caused by density fluctuation or the like is reduced. The core 10 need not contain fluorine.

[0074] As illustrated in Figures 12 and 13, the optical fiber of the present embodiment described above may have a plurality of twists 61 arranged in the longitudinal direction. Figures 12 and 13 illustrate a 1D optical fiber having twists 61 as a modification of the 1B optical fiber illustrated in [Fig. 9]. [Fig. 12] is a view schematically illustrating the cross-section orthogonal to the longitudinal direction of the 1D optical fiber. [Fig. 13] is a view schematically illustrating the internal structure of the 1D optical fiber in the case where the 1D optical fiber is viewed along the direction orthogonal to the longitudinal direction of the 1D optical fiber. In Figures 12 and 13, the illustration of the first cladding 21 and the coating layer 30 is omitted. Furthermore, in [Fig.13], the illustration of cores 12 and 14 is further omitted, the second cladding 22 is illustrated by a dashed line, and cores 11 and 13 located inside the second cladding 22 are illustrated by a solid line. The 1D optical fiber has four cores (cores 11, 12, 13, and 14). Each core 10 has an elliptical shape in a cross-section orthogonal to the fiber axis (longitudinal direction of the 1D optical fiber).

[0075] In the 1D optical fiber, the positions of a plurality of cores 11, 12, 13, and 14 change so as to rotate when movement is performed along the longitudinal direction, whereby a plurality of twists 61 are formed so as to be positioned along the longitudinal direction of the 1D optical fiber. The twist 61 is a position (structure) in which a plurality of cores 10 intersect each other in the case where the 1D optical fiber is viewed along the direction orthogonal to the fiber axis. Since the 1D optical fiber has the twists 61, mode coupling occurs between two polarization modes in each core 10, and the polarization mode dispersion caused by stress and shape anisotropy in the cross-section of the 1D optical fiber is randomized by the mode coupling, and the accumulation of the polarization mode dispersion is reduced.From this point of view, the number of twists (the number of twists 61) can be 0.1 rotation / m or more. In addition, the number of twists can be 1 rotation / m or more. It is noted that the number of twists is a value obtained by averaging the absolute value of the rotation angle per unit length in the longitudinal direction. The direction of the twist 61 can be two left and right directions (two directions along the circumferential direction of the 1D optical fiber), but twists 61 in two directions in the longitudinal direction can be mixed.

[0076] Although the twist 61 has the effect of reducing the dispersion of the polarization modes, the twist 61 comes with the side effect that the arrangement of the plurality of cores 10 rotates in the longitudinal direction and, therefore, if an end portion of the 1D optical fiber is cut when light is coupled to an end surface of the 1D optical fiber, the arrangement of the cores changes, which may cause a problem in that it is difficult to couple light. In order to reduce the influence of this problem, the number of twists 61 may be 3 rotations / m or less. In addition, the number of twists 61 may be 0.3 rotations / m or less. List of alphanumeric references

[0077] 1, IA, IB, 1D optical fiber 2nd fiber axis 10, 11, 12, 13, 14 heart 20 sheath 21, 21A, 21B, 21C, 21D first sheath 22 second sheath 30 layer coating 50 marker 61 twist Al, A2 compressive stress B1 slow axis B 2 fast axis Ll, L2, L3, L4 straight line PI, P2, P3 center of gravity a 10, a21, a22 coefficient of thermal expansion qlO, q21, q22 viscosity 01, 02, 03 angle ozzlO, ozz22, ozz22 constraint

Claims

Claims

1. Multi-core optical fiber comprising: - a plurality of cores along the axis of the fiber; and - a cladding surrounding the plurality of cores, wherein: - the plurality of cores and the cladding contain a silica glass as a main component, - the refractive index of each of the plurality of cores is greater than the refractive index of the cladding, - the coefficient of thermal expansion of each of the plurality of cores is greater than the coefficient of thermal expansion of the cladding, - the plurality of cores includes a first core having an elliptical shape in a cross-section orthogonal to the fiber axis and one or more second cores different from the first core, - the non-circularity of the elliptical shape is 100 x 2 x (Rmax -Rmin) / (Rmax + Rmin) % = 0.1% or more, with Rmax, the long diameter of the ellipse formed by the first core and Rmin, the short diameter of the ellipse formed by the first core,- the angle formed by a straight line connecting the center of gravity of the first core and the center of gravity of a group of cores including the second core(s) and a straight line along the major axis of the elliptical shape is 30 degrees or less in a cross-section orthogonal to the fiber axis, and - the dispersion of the polarization modes is 0.2 ps / rtkm or ps.km1 / 2 or less.,

2. A multi-core optical fiber according to claim 1, wherein the non-circularity of the elliptical shape is 100 x 2 x (Rmax -Rmin) / (Rmax + Rmin) % = 10% or less, with Rmax being the long diameter of the ellipse formed by the first core and Rmin being the short diameter of the ellipse formed by the first core.

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

4. A multi-core optical fiber according to any one of claims 1 to 3, wherein: - the sheath includes a first sheath surrounding the plurality of cores and a second sheath surrounding the first sheath, - the first sheath and the second sheath contain fluorine, the concentration of fluorine in the first sheath is greater than the concentration of fluorine in the second sheath, and - the plurality of cores contains at least one type of alkali element from the group of alkali elements including an elemental alkali metal and an elemental alkaline earth metal.

5. A multi-core optical fiber according to any one of claims 1 to 4, wherein: - the plurality of cores contains at least one type of element from the group of alkali elements including an elemental alkali metal and an elemental alkaline earth metal, and - the concentration of alkali element in the plurality of cores is 1 ppm or mg / kg by weight or more and 3000 ppm or mg / kg by weight or less.

6. A multi-core optical fiber according to any one of claims 1 to 5, wherein the angle is 10 degrees or less.

7. A multi-core optical fiber according to any one of claims 1 to 5, wherein the angle is 5 degrees or less.

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