Multicore optical fiber, optical fiber with optical connector, and optical fiber with fan-in / fan-out device

Abstract

This multicore optical fiber (10) comprises eight or more cores (1), a cladding (2) surrounding the cores (1), and a coating layer (3) coating the cladding (2). The cores (1) include seven or more outer cores (5) disposed on a circumference centered on the central axis of the cladding (2), and one or more inner cores (6) disposed closer to the central axis of the cladding (2) than the outer cores (5). The outer cores (5) each have a core part (11) and a trench part (13) surrounding the core part (11) and having a lower refractive index than the cladding (2). Provided that Λ is the shortest distance between a first central axis of one of the outer cores (5) and a second central axis of one of the inner cores (6), the shortest distance between the central axes of the outer cores (5) and the coating layer (3) is less than or equal to Λ. The refractive index of the coating layer (3) is higher than the refractive index of the inner cores (6).

Description

Multi-core optical fiber, optical fiber with an optical connector, and optical fiber with a fan-in / fan-out device

[0001] The present invention relates to a multi-core optical fiber, an optical fiber with an optical connector, and an optical fiber with a fan-in / fan-out device. This application claims priority based on Japanese Patent Application No. 2024-190686 filed in Japan on October 30, 2024, and incorporates its content herein by reference.

[0002] Patent Document 1 discloses a multi-core optical fiber having a cladding, one core (inner core) disposed at the center of the cladding, and a plurality of cores (outer cores) surrounding this core. A trench layer having a refractive index lower than that of the cladding is formed in the core.

[0003] International Publication No. 2013 / 129232

[0004] In the multi-core optical fiber, leakage light, which is light leaking from the outer core in a bent portion or a connection portion, etc., is likely to be coupled to the inner core. When the leakage light is coupled to the inner core, noise may occur in the signal light.

[0005] One aspect of the present invention is made in consideration of such circumstances, and an object thereof is to provide a multi-core optical fiber, an optical fiber with an optical connector, and an optical fiber with a fan-in / fan-out device that can reduce noise generated in the signal light.

[0006] The multi-core optical fiber according to the first aspect of the present invention includes eight or more cores, a cladding surrounding the cores, and a coating layer covering the cladding. The cores include seven or more outer cores arranged on a circumference centered on the central axis of the cladding, and one or more inner cores arranged closer to the center of the cladding than the outer cores. The outer core has a core portion and a trench portion surrounding the core portion and having a refractive index lower than that of the cladding. When the shortest distance between the first central axis of the outer core and the second central axis of the inner core is Λ, the shortest distance between the first central axis of the outer core and the coating layer is Λ or less, and the refractive index of the coating layer is higher than the refractive index of the inner core.

[0007] With this configuration, light leaking from the outer core at bends and connections in a multicore optical fiber is more easily coupled to the coating layer. Therefore, it is possible to suppress the propagation of leaked light in the region inside the circumference passing through the central axes of the multiple outer cores. Consequently, leaked light is less likely to couple to the inner core. Thus, noise generated in the signal light propagating through the inner core can be reduced.

[0008] A second aspect of the present invention is a multicore optical fiber according to the first aspect, wherein the shortest distance from the first central axis of the outer core to the interface between the cladding and the coating layer is 22.7 μm or more.

[0009] A third aspect of the present invention is a multicore optical fiber according to the first or second aspect, wherein the shortest distance between the trenches is 2.5 times or more the wavelength of the signal light used for optical communication.

[0010] A fourth aspect of the present invention is a multicore optical fiber according to any one of the first to third aspects, wherein there is a loss region radially inward from the outer core in which the loss at the signal optical wavelength used for optical communication is greater than the loss in the cladding.

[0011] A fifth aspect of the present invention is a multicore optical fiber according to the fourth aspect, wherein the loss region is arranged in a position that allows for the identification of a plurality of cores.

[0012] A sixth aspect of the present invention is a multicore optical fiber according to any one of the first to fifth aspects, wherein the loss in the cladding layer at the signal optical wavelength for optical communication is greater than the loss in the cladding at the signal optical wavelength.

[0013] A seventh aspect of the present invention is a multicore optical fiber according to any one of the first to sixth aspects, wherein the number of inner cores is two or more, and the plurality of inner cores are arranged on a circumference centered on the central axis of the cladding.

[0014] An eighth aspect of the present invention is a multicore optical fiber according to any one of the first to seventh aspects, wherein the distance between the first central axes in one pair of adjacent outer cores is different from the distance between the first central axes in at least one pair of outer cores other than the pair of outer cores.

[0015] A ninth aspect of the present invention is a multicore optical fiber according to any one of the first to eighth aspects, wherein the thickness of the coating layer is 25 μm or more.

[0016] An optical fiber with an optical connector according to the tenth aspect of the present invention has one multicore optical fiber according to any one of the first to ninth aspects.

[0017] An optical fiber with an optical connector according to the eleventh aspect of the present invention has a plurality of multicore optical fibers of any one of the first to ninth aspects.

[0018] An optical fiber with a fan-in / fan-out device according to the twelfth aspect of the present invention has a multicore optical fiber of any one of the first to ninth aspects.

[0019] One aspect of the present invention provides a multicore optical fiber, an optical fiber with an optical connector, and an optical fiber with a fan-in / fan-out device that can reduce noise generated in signal light.

[0020] This is a cross-sectional view of a multicore optical fiber according to the first embodiment. This figure shows the relationship between the shortest distance from the center of the outer core to the interface between the cladding and the coating layer and the propagation loss of the outer core. This is a schematic diagram showing a test apparatus for investigating fracture resistance. This figure shows the relationship between the thickness of the coating layer and fracture resistance. This is a configuration diagram of an optical fiber with an optical connector according to the first embodiment. This is a configuration diagram of an optical fiber with an optical connector according to the second embodiment. This is a cross-sectional view of a multicore optical fiber according to the second embodiment. This is a perspective view of an optical fiber with a fan-in / fan-out device according to the embodiment.

[0021] Hereinafter, multicore optical fibers, optical fibers with optical connectors, and optical fibers with fan-in / fan-out devices according to the embodiment will be described with reference to the drawings.

[0022] [Multicore Optical Fiber] (First Embodiment) Figure 1 is a cross-sectional view of a multicore optical fiber 10 according to the first embodiment. Figure 1 shows a cross-section perpendicular to the longitudinal direction of the multicore optical fiber 10. In Figure 1, the left-right direction is the X direction. In Figure 1, the up-down direction is the Y direction. The X and Y directions are perpendicular to the central axis C2 of the cladding 2. The X and Y directions are perpendicular to each other. In a cross-sectional view, the direction that intersects the central axis C2 of the cladding 2 (multicore optical fiber 10) is called the radial direction. Along the radial direction, the direction approaching the central axis C2 is called the radially inward direction, and the direction moving away from the central axis C2 is called the radially outward direction. The direction that circles around the central axis C2 is called the circumferential direction.

[0023] The positions of each component of the multicore optical fiber 10 are tentatively defined in accordance with Figure 1. In the following explanation, the upper side (+Y side) in Figure 1 may be defined as the upper side. Specifically, in the inner cores 6A, 6B, 6C, and 6D, which are arranged at equal distances from the central axis C2, inner core 6A is above inner core 6B. Inner core 6D is above inner core 6C. The positional relationships defined here do not limit the orientation of the multicore optical fiber 10 when it is in use.

[0024] As shown in Figure 1, the multicore optical fiber 10 comprises a plurality of cores 1, a cladding 2 surrounding the cores 1, a coating layer 3 covering the cladding 2, and markers 4 provided on the cladding 2.

[0025] Multiple cores 1 include outer cores 5 and inner cores 6. The number of cores 1 is set to 8 or more. When the number of cores 1 is 8 or more, the amount of information transmitted by each core 1 can be increased. In this embodiment, the number of cores 1 is 16.

[0026] The outer core 5 has a core portion 11, an inner cladding portion 12 surrounding the core portion 11, and a trench portion 13 surrounding the inner cladding portion 12. The number of outer cores 5 is 7 or more. When the number of outer cores 5 is 7 or more, the amount of information transmitted by the outer cores 5 can be increased. In this embodiment, the number of outer cores 5 is 12. The cross-sectional shape of the outer core 5 perpendicular to the longitudinal direction is circular. The central axis of each outer core 5 is referred to as the central axis C5 (first central axis C5).

[0027] The core portion 11 can be formed from quartz to which a dopant (such as germanium) that increases the refractive index has been added. The refractive index of the inner cladding portion 12 is lower than that of the core portion 11. The inner cladding portion 12 may be formed from high-purity quartz (for example, quartz without additives). The inner cladding portion 12 may also be formed from quartz to which a dopant that adjusts the refractive index has been added.

[0028] The refractive index of the trench portion 13 is lower than that of the inner cladding portion 12. The refractive index of the trench portion 13 is lower than that of the cladding 2. The trench portion 13 surrounds the core portion 11 via the inner cladding portion 12. The trench portion 13 is the outermost layer of the outer core 5 (the region including the outer peripheral surface of the outer core 5). The trench portion 13 can be formed, for example, of quartz to which a dopant (such as fluorine) that lowers the refractive index has been added.

[0029] The outer core 5 is positioned on a circumference R7. The circumference R7 is the circumference centered on the central axis C2 (center of the cladding 2) of the cladding 2. The core portion 11 of the outer core 5 is positioned on the circumference R7. The central axis C5 of the outer core 5 is positioned on the circumference R7. It is sufficient that at least a portion of each part of the outer core 5 is positioned on the circumference centered on the central axis C2.

[0030] Multiple outer cores 5 can be arranged, for example, in positions that are rotationally symmetric about the central axis C2. That is, the outer cores 5 can be arranged in positions that are n times symmetric about the central axis C2 (where n is the number of outer cores 5). In this embodiment, the outer cores 5 are arranged in positions that are rotationally symmetrical 12 times about the central axis C2. The distance between adjacent outer cores 5 and their central axes C5 is equal to each other. That is, the outer cores 5 are arranged at equal pitches around the central axis C2.

[0031] When the outer core 5 is positioned on the circumference R7, connection loss due to circumferential (axial) misalignment between the multicore optical fiber 10 and optical products (multicore optical fiber, transmitting / receiving devices, etc.) can be reduced. Therefore, the generation of leaked light can be suppressed.

[0032] The shortest distance L3 between the trenches 13 of the outer core 5 is preferably 2.5 times or more the wavelength of the signal light used for optical communication. The shortest distance L3 is, for example, the distance between adjacent outer cores 5 in the circumferential direction. By making the shortest distance L3 2.5 times or more the wavelength of the signal light used for optical communication, light propagating in the region inside the circumference R7 is more likely to leak out of the circumference R7 through the gaps between adjacent outer cores 5. Therefore, light confinement inside the circumference R7 becomes less likely.

[0033] The twelve outer cores 5 are outer cores 5A to 5L. In Figure 1, the outer cores 5A to 5L are arranged in this order clockwise along the circumference R7.

[0034] The multiple outer cores 5 are arranged so as to be symmetrical with respect to the axis of symmetry A1. More specifically, the outer cores 5A to 5F and the outer cores 5G to 5L are positioned symmetrically with respect to the axis of symmetry A1. The axis of symmetry A1 is a straight line that passes through the central axis C2 of the cladding 2 and is parallel to the Y direction.

[0035] The multiple outer cores 5 are arranged so as to be symmetrical with respect to the axis of symmetry A2. Specifically, outer cores 5A to 5C, 5J to 5L and outer cores 5D to 5I are positioned symmetrically with respect to the axis of symmetry A2. The axis of symmetry A2 is a straight line that passes through the central axis C2 of the cladding 2 and is parallel to the X direction.

[0036] Multiple outer cores 5 are arranged symmetrically with respect to the axis of symmetry A3. Specifically, outer cores 5L, 5A-5D and outer cores 5F-5J are positioned symmetrically with respect to the axis of symmetry A3. The axis of symmetry A3 is a straight line passing through the central axis C5 of outer core 5E, the central axis C6 of inner core 6B, the central axis C2 of cladding 2, the central axis C6 of inner core 6D, and the central axis C5 of outer core 5K. The axis of symmetry A3 is inclined at 45° with respect to the X and Y directions.

[0037] Multiple outer cores 5 are arranged symmetrically with respect to the axis of symmetry A4. Specifically, outer cores 5C to 5G and outer cores 5I to 5L and 5A are positioned symmetrically with respect to the axis of symmetry A4. The axis of symmetry A4 is a straight line passing through the central axis C5 of outer core 5B, the central axis C6 of inner core 6A, the central axis C2 of cladding 2, the central axis C6 of inner core 6C, and the central axis C5 of outer core 5H. The axis of symmetry A4 is inclined at 45° with respect to the X and Y directions.

[0038] The inner core 6 has a core portion 21 and an inner cladding portion 22 surrounding the core portion 21. The number of inner cores 6 is one or more. In this embodiment, the number of inner cores 6 is four. The four inner cores 6 are referred to as inner cores 6A to 6D. The cross-sectional shape of the inner cores 6 perpendicular to the longitudinal direction is circular. The central axis of each inner core 6 is referred to as the central axis C6 (second central axis C6).

[0039] The core portion 21 can be formed from quartz doped with a dopant (such as germanium) that increases the refractive index. The refractive index of the inner cladding portion 22 is lower than that of the core portion 21. The refractive index of the inner cladding portion 22 is lower than that of the cladding 2. The inner cladding portion 22 is the outermost layer of the inner core 6 (the region including the outer circumferential surface of the inner core 6). The inner cladding portion 22 can be formed from quartz doped with a dopant (such as fluorine) that lowers the refractive index. Because the inner core 6 has no trenches, confinement of higher-order modes of light unsuitable for communication is less likely to occur.

[0040] The inner core 6 is positioned closer to the central axis C2 of the cladding 2 than the outer core 5. In other words, the radius of the circumference R6 is shorter than the distance obtained by subtracting the radius of the circumference R6 from the radius of the circumference R7. In particular, in this embodiment, the central axes C2, C5, and C6 of the cladding 2, outer core 5, and inner core 6 are located on the axes of symmetry A3 and A4, respectively. Therefore, the shortest distance between the central axis C2 of the cladding 2 and the central axis C6 of the inner core 6 is shorter than the shortest distance between the central axis C5 of the outer core 5 and the central axis C6 of the inner core 6. The inner core 6 is positioned radially inward from the outer core 5. In other words, the distance between the central axis C6 and the central axis C2 of the inner core 6 is shorter than the distance between the central axis C5 and the central axis C2 of the outer core 5.

[0041] The inner core 6 is positioned on a circumference R6. The circumference R6 is the circumference centered on the central axis C2 (center of the cladding 2) of the cladding 2. The core portion 21 of the inner core 6 is positioned on the circumference R6. The central axis C6 of the inner core 6 is positioned on the circumference R6. Each of the inner cores 6 only needs to have at least a portion of it positioned on the circumference centered on the central axis C2.

[0042] The plurality of inner cores 6 can be arranged, for example, at positions that are rotationally symmetric around the central axis C2. That is, the inner cores 6 can be arranged at positions that are m-fold symmetric (m is the number of inner cores 6) around the central axis C2. In the present embodiment, the inner cores 6 are arranged at rotationally symmetric positions that are 4-fold symmetric around the central axis C2. The distance between the central axes C6 of adjacent inner cores 6 is equal to each other. That is, the inner cores 6 are arranged so as to have an equal pitch around the central axis C2.

[0043] When the inner cores 6 are arranged on the circumference R6, when connecting the multi-core optical fiber 10 and an optical product (such as a multi-core optical fiber, a transceiver device, etc.), the connection loss due to the circumferential direction (axial direction) positional deviation between the multi-core optical fiber 10 and the optical product can be reduced. Therefore, the generation of leakage light can be suppressed.

[0044] The inner core 6A and the inner core 6B are arranged side by side with a gap in the Y direction. The inner core 6C and the inner core 6D are arranged side by side with a gap in the Y direction. The inner core 6A and the inner core 6D are arranged side by side with a gap in the X direction. The inner core 6B and the inner core 6C are arranged side by side with a gap in the X direction.

[0045] The plurality of inner cores 6 are arranged so as to be line-symmetric with respect to the symmetry axis A1. Specifically, the inner cores 6A, 6B and the inner cores 6C, 6D are at positions that are line-symmetric with respect to the symmetry axis A1.

[0046] The plurality of inner cores 6 are arranged so as to be line-symmetric with respect to the symmetry axis A2. Specifically, the inner cores 6A, 6D and the inner cores 6B, 6C are at positions that are line-symmetric with respect to the symmetry axis A2.

[0047] The plurality of inner cores 6 are arranged so as to be line-symmetric with respect to the symmetry axis A3. Specifically, the inner core 6A and the inner core 6C are at positions that are line-symmetric with respect to the symmetry axis A3.

[0048] The plurality of inner cores 6 are arranged to be line-symmetric with respect to the symmetry axis A4. Specifically, the inner core 6B and the inner core 6D are at positions that are line-symmetric with respect to the symmetry axis A4.

[0049] Let the shortest distance between the central axis C5 of the outer core 5 and the central axis C6 of the inner core 6 be Λ. The shortest distance L1 between the central axis C5 of the outer core 5 and the cladding layer 3 is Λ or less. Thereby, the light leaking from the outer core 5 is likely to be coupled to the cladding layer 3. Therefore, it is possible to suppress the leakage light from propagating in the region inside the outer core 5.

[0050] The cladding 2 surrounds the core 1 (the outer core 5 and the inner core 6) and the marker 4. The cladding 2 may be formed of, for example, high-purity quartz (for example, quartz containing no additives). The central axis C2 of the cladding 2 coincides with the central axis of the multi-core optical fiber 10.

[0051] The coating layer 3 is formed of a resin or the like. Examples of the resin constituting the coating layer 3 include UV curable resins (for example, urethane acrylate-based resins). The coating layer 3 covers the outer peripheral surface of the cladding 2 over the entire circumference. The coating layer 3 may have a single-layer structure or a multilayer structure. The multilayer coating layer 3 may have two layers or three or more layers.

[0052] The coating layer 3 allows light to propagate. The refractive index of the coating layer 3 is higher than the refractive index of the inner core 6. Specifically, the refractive index of the coating layer 3 is higher than the refractive index of the core portion 21 of the inner core 6. Thereby, the light leaking from the outer core 5 is more likely to be coupled to the coating layer 3 than to the inner core 6. Also, the refractive index of the coating layer 3 is higher than the refractive index of the cladding 2.

[0053] The thickness T1 of the coating layer 3 is preferably 25 μm or more. Thereby, the strength (for example, tensile strength) of the coating layer 3 can be increased. The thickness T1 of the coating layer 3 may be, for example, 60 μm or less.

[0054] It is desirable that the loss in the coating layer 3 at the signal wavelength used for optical communication is greater than the loss in the cladding 2 at the signal wavelength. This reduces the power of the leaked light coupled to the coating layer 3 compared to when it propagates through the cladding 2, thus lowering the power of the leaked light coupled from the coating layer 3 to the cladding 2.

[0055] The shortest distance L2 from the central axis C5 of the outer core 5 to the interface 7 between the cladding 2 and the coating layer 3 is preferably 22.7 μm or more. This reduces the propagation loss of signal light in optical communication. Note that if the cladding 2 and the coating layer 3 are in contact around the entire circumference, the shortest distance L1 between the central axis C5 of the outer core 5 and the coating layer 3 is equivalent to the shortest distance L2.

[0056] Marker 4 is formed, for example, on the central axis C2 side (i.e., radially inward) of the outer core 5. Marker 4 is formed, for example, inside the circumference R5 which is the inscribed circle of the outer core 5. Marker 4 is surrounded by cladding 2. The refractive index of marker 4 is different from that of cladding 2. Marker 4 is, for example, a region having a refractive index lower than that of cladding 2. Marker 4 can be formed, for example, of quartz to which a dopant (such as fluorine) that lowers the refractive index has been added. The cross-sectional shape of marker 4 perpendicular to the longitudinal direction is circular. The outer diameter of marker 4 is, for example, smaller than the outer diameters of the outer core 5 and the inner core 6.

[0057] Marker 4 is formed above (to the +Y side) the inner cores 6A and 6D in Figure 1, and is separated from the inner cores 6A and 6D. Marker 4 is formed outside a rectangular region with the four inner cores 6 as its vertices, for example. Marker 4 is formed outside the circumference R6, for example.

[0058] The central axis C4 (center) of marker 4 is located outside the symmetry axes A1 to A4. Therefore, marker 4 is formed in a position that disrupts the symmetry (line symmetry) of the outer core 5 and inner core 6. Consequently, each core 1 can be identified based on the distance between marker 4 and each core 1. It can be said that marker 4 is formed in a position that makes core 1 identifiable.

[0059] It is desirable that marker 4 is in a region where the loss at the signal optical wavelength for optical communication is greater than the loss in cladding 2 (hereinafter referred to as the loss region LR). For example, if marker 4 is formed of quartz doped with a dopant (such as fluorine), the loss at the signal optical wavelength will be greater compared to cladding 2, which is formed of quartz without the dopant.

[0060] If marker 4 is in a loss region LR where the loss at the signal light wavelength is greater than the loss in cladding 2, then even if the leaked light from core 1 leaks to a region closer to the central axis C2 than the outer core 5, the leaked light can be attenuated in the loss region LR. Therefore, the leaked light is less likely to couple to the inner core 6.

[0061] [Effects of the multicore optical fiber according to this embodiment] In the multicore optical fiber 10 according to this embodiment, if the shortest distance between the central axis C5 of the outer core 5 and the central axis C6 of the inner core 6 is Λ, then the shortest distance L1 between the central axis C5 of the outer core 5 and the coating layer 3 is Λ or less. As a result, light leaking from the outer core 5 at bends and connections of the multicore optical fiber 10 is more easily coupled to the coating layer 3. Therefore, it is possible to suppress the propagation of leaked light in the region on the central axis C2 side of the outer core 5. Consequently, leaked light is less likely to couple to the inner core 6. Thus, noise generated in the signal light propagating through the inner core 6 can be reduced.

[0062] (Example) A multicore optical fiber 10 shown in Figure 1 was fabricated. The radius of the core portion 11 of the outer core 5 is 3.9 μm. The relative refractive index difference of the core portion 11 with respect to the cladding 2 is 0.32%. The radius of the inner cladding portion 12 is 11.0 μm. The relative refractive index difference of the inner cladding portion 12 with respect to the cladding 2 is -0.1%. The radius of the trench portion 13 is 15.2 μm. The relative refractive index difference of the trench portion 13 with respect to the cladding 2 is -0.4%. There are 12 outer cores 5. The radius of the circumference R7 passing through the central axis C5 of the outer core 5 is 68.3 μm. The distance between the central axes C5 of adjacent outer cores 5 is 35.3 μm.

[0063] The radius of the core portion 21 of the inner core 6 is 3.9 μm. The relative refractive index difference of the core portion 21 with respect to the cladding 2 is 0.32%. The radius of the inner cladding portion 22 is 12.5 μm. The relative refractive index difference of the inner cladding portion 22 with respect to the cladding 2 is -0.05%. There are 4 inner cores 6. The radius of the circumference R6 is 28.3 μm. The distance between the central axes C6 of adjacent inner cores 6 is 40.0 μm.

[0064] The diameter of cladding 2 is 190 μm. The distance between the central axis C4 of marker 4 and the central axis C2 of cladding 2 is 41.8 μm. The thickness T1 of the coating layer 3 is 27 μm. The difference in refractive index of the coating layer 3 compared to cladding 2 is 0.5%. The refractive index of the coating layer 3 is higher than that of the core portion 21 of the inner core 6. Also, the refractive index of the coating layer 3 is higher than that of cladding 2. The diameter of the coating layer 3 (diameter of the multicore optical fiber 10) is 244 μm.

[0065] The shortest distance Λ between the central axis C5 of the outer core 5 and the central axis C6 of the inner core 6 is 40.0 μm. The shortest distance L3 between adjacent trench portions 13 of the outer core 5 is 5.0 μm. The shortest distance L1 between the central axis C5 of the outer core 5 and the coating layer 3 is 26.7 μm. The shortest distance L2 from the central axis C5 of the outer core 5 to the interface 7 between the cladding 2 and the coating layer 3 is 26.7 μm.

[0066] In the multicore optical fiber 10 according to the embodiment, the shortest distance L1 between the central axis C5 of the outer core 5 and the coating layer 3 is less than or equal to the shortest distance Λ between the central axes C5 and C6 of the outer core 5 and the inner core 6. The refractive index of the coating layer 3 is higher than that of the inner core 6 (more specifically, the core portion 21). As a result, light leaking from the outer core 5 is more likely to couple with the coating layer 3. Therefore, leaked light is less likely to couple with the inner core 6. Thus, noise generated in the signal light propagating through the inner core 6 can be reduced.

[0067] Figure 2 shows the relationship between the propagation loss (attenuation) (dB / km) of the outer core 5 and the shortest distance L2 (OCT) between the central axis C5 of the outer core 5 and the coating layer 3, when the signal wavelength for optical communication is 1.31 μm. From Figure 2, it can be seen that the propagation loss can be reduced if the shortest distance L2 is 22.7 μm or more.

[0068] In the multicore optical fiber 10 according to the embodiment, if the signal light wavelength for optical communication is 1.31 μm, and the shortest distance L3 between adjacent outer cores 5 trenches 13 is 3.28 μm or more, then the shortest distance L3 becomes 2.5 times or more the signal light wavelength. As a result, light from the region on the central axis C2 side of the outer core 5 is more likely to leak out through the gap between adjacent outer cores 5. Therefore, light confinement in the region on the central axis C2 side of the outer core 5 becomes less likely.

[0069] In the multicore optical fiber 10 according to the embodiment, the loss in the coating layer 3 at the signal light wavelength of 1.31 μm is greater than the loss in the cladding 2 at the signal light wavelength of 1.31 μm, which is 0.4 dB / km. In other words, the power of the leaked light coupled to the coating layer 3 is lower than when it propagates through the cladding 2. This makes it possible to reduce the power of the leaked light coupled from the coating layer 3 to the cladding 2.

[0070] Figure 3 is a schematic diagram showing a test apparatus for examining fracture resistance to trauma. The test apparatus shown in Figure 3 comprises a first capstan 31 and a second capstan 32. Sandpaper 33 (model number #800) is provided around the entire circumference of the outer surface of the first capstan 31.

[0071] The following test was conducted using multiple multicore optical fibers 10 with different coating layer thicknesses 3 as subjects. The multicore optical fibers 10 were wired so as to pass through the first capstan 31 and the second capstan 32. The multicore optical fibers 10 were in contact with sandpaper 33 on the outer surface of the first capstan 31. A load was applied to the second capstan 32 in a direction away from the first capstan 31.

[0072] Figure 4 shows the relationship between the thickness of the coating layer 3 and its resistance to fracture trauma. The horizontal axis of Figure 4 represents the thickness of the coating layer 3. The vertical axis of Figure 4 represents the tensile strength when the multicore optical fiber 10 is fractured. From Figure 4, it can be seen that when the thickness of the coating layer 3 is 25 μm or more, the multicore optical fiber 10 can be given a tensile strength of 0.5 GPa or more.

[0073] [Optical Fiber with Optical Connector] (First Embodiment) Figure 5 is a configuration diagram of an optical fiber 100 with an optical connector according to the first embodiment. As shown in Figure 5, the optical fiber 100 with an optical connector comprises an optical connector 101 and one multicore optical fiber 10 (see Figure 1).

[0074] The optical connector 101 comprises a ferrule 102 having one insertion hole 102a and a housing 103. The optical connector 101 is attached to the tip of the multicore optical fiber 10.

[0075] The tip 10a of the multicore optical fiber 10 (see Figure 1), from which the coating layer 3 has been removed, is inserted through the insertion hole 102a of the ferrule 102. The housing 103 accommodates the ferrule 102. The optical connector 101 is a single-core connector because it has a ferrule 102 with one insertion hole 102a. Specific examples of the optical connector 101 include LC connectors and SC connectors.

[0076] Since the optical fiber 100 with an optical connector has a multi-core optical fiber 10, it can reduce noise generated in the signal light propagating through the inner core 6.

[0077] [Optical Fiber with Optical Connector] (Second Embodiment) Figure 6 is a configuration diagram of an optical fiber 200 with an optical connector according to the second embodiment. As shown in Figure 6, the optical fiber 200 with an optical connector comprises an optical connector 201 and a plurality of multicore optical fibers 10 (see Figure 1).

[0078] The optical connector 201 comprises a ferrule 202 and a housing 203. The ferrule 202 has a plurality of through holes 202a and two guide pin holes 202b. The tip portion 10a of the multicore optical fiber 10 (see Figure 1), from which the coating layer 3 has been removed, is inserted through the through holes 202a. Guide pins can be inserted into the guide pin holes 202b. The housing 203 accommodates the ferrule 202. Because the optical connector 201 has a ferrule 202 with a plurality of through holes 202a, it is a multi-core connector. Specific examples of the optical connector 201 include MMC connectors and MPO connectors.

[0079] The optical fiber 200 with an optical connector shown in Figure 6 is equipped with a female optical connector 201 having guide pin holes, but the optical connector may also be a male optical connector having guide pins protruding from the tip surface of the ferrule.

[0080] Since the optical fiber 200 with an optical connector has a multi-core optical fiber 10, it can reduce noise generated in the signal light propagating through the inner core 6.

[0081] [Multicore Optical Fiber] (Second Embodiment) Figure 7 is a cross-sectional view of the multicore optical fiber 110 according to the second embodiment, perpendicular to the longitudinal direction. Common components with the multicore optical fiber 10 according to the first embodiment (see Figure 1) are denoted by the same reference numerals and their description is omitted.

[0082] As shown in Figure 7, the multicore optical fiber 110 comprises multiple cores 1 (outer cores 5 and inner cores 6), a cladding 2, a coating layer 3, and a marker 4. The multicore optical fiber 110 has the same configuration as the multicore optical fiber 10 (see Figure 1), except for the arrangement of the outer cores 5.

[0083] Multiple outer cores 5 constitute multiple core groups 8. In this embodiment, the multiple outer cores 5 are divided into six core groups 8. Each core group 8 includes two adjacent outer cores 5. The two outer cores 5 that make up the core group 8 are outer cores 5M and 5N, respectively. Outer cores 5M and 5N are arranged alternately on the circumference R7.

[0084] The distance between the central axes C5 of two outer cores 5M and 5N belonging to the same core group 8 (common core group 8) is "L4". The distance between the central axes C5 of an outer core 5N in one of two adjacent core groups 8 and an outer core 5M in the other core group 8 is "L5". Comparing the distance L5 between the central axes C5 of outer cores 5M and 5N belonging to different core groups 8 with the distance L4 between the central axes C5 of a pair of outer cores 5M and 5N belonging to the same core group 8, distance L5 is longer than distance L4.

[0085] [Effects of the multicore optical fiber according to this embodiment] In the multicore optical fiber 110 according to this embodiment, the distance L5 between the central axes C5 of the outer cores 5M and 5N included in different core groups 8 is relatively long. Therefore, it is possible to suppress the functioning of the trench portions 13 of these outer cores 5M and 5N as a photonic bandgap structure. Consequently, light confinement in the region on the central axis C2 side of the outer core 5 (for example, the region inside the circumference R5) becomes less likely. Therefore, it becomes easier to reduce noise generated in the signal light propagating through the inner core 6.

[0086] [Optical Fiber with Fan-in / Fan-out Device] Figure 8 is a perspective view showing an optical fiber 40 with a fan-in / fan-out device according to an embodiment. The fan-in / fan-out device is abbreviated as "FIFO". As shown in Figure 8, the optical fiber 40 with a FIFO comprises a FIFO 41 and a multicore optical fiber 10 (see Figure 1). The FIFO 41 comprises a plurality of single-core fibers 42 and a capillary 43.

[0087] The FIFO 41 can relay connection between the multicore optical fiber 10 and multiple single-core fibers 42 for signal transmission. The multiple single-core fibers 42 are arranged to be connectable to the core 1 (see Figure 1) of the multicore optical fiber 10.

[0088] The capillary 43 holds a plurality of single-core fibers 42. The capillary 43 has a base portion 44 and an extension portion 45 extending from the base portion 44. The extension portion 45 has a tapered portion 46 that extends from the base portion 44 while decreasing in diameter in the longitudinal direction, and a tip portion 47 that extends from the tapered portion 46. The multi-core optical fiber 10 is connected to the tip portion 47.

[0089] FIFO 41 is, for example, a melt-drawn FIFO. The capillary 43 can be manufactured, for example, as follows: A portion of the capillary 43, in which a plurality of single-core fibers 42 are interposed, is heated and melt-drawn to form an extended portion 45.

[0090] FIFOs can be of various types, including fiber bundle type, spatial optical type, and planar optical waveguide type. A fiber bundle type FIFO bundles and holds multiple small-diameter fibers and connects them to a multicore optical fiber. A spatial optical type FIFO uses a spatial optical system to connect each core of a multicore optical fiber to each core of a single-core fiber. A planar optical waveguide type FIFO connects each core of a multicore optical fiber to each core of a single-core fiber via a planar optical waveguide formed from polymer material or the like.

[0091] The technical scope of the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention. The multicore optical fiber 10 shown in Figure 1 has 12 outer cores 5 and 4 inner cores 6 as core 1. The number of cores (total number of outer and inner cores) may be 8 or more. The number of outer cores is not particularly limited, and may be 7 or more. The number of inner cores is not particularly limited, and may be 1 or multiple (any number of 2 or more).

[0092] In the multicore optical fiber 10 shown in Figure 1, the outer core 5 has a three-layer structure consisting of a core portion 11, an inner cladding portion 12, and a trench portion 13. However, the outer core 5 may also have a two-layer structure consisting only of a core portion 11 and a trench portion 13 surrounding the core portion 11.

[0093] In the multicore optical fiber 10 shown in Figure 1, marker 4 is exemplified as a region where the loss at the signal light wavelength is greater than the loss in cladding 2. However, the loss region LR, where the loss at the signal light wavelength is greater than the loss in cladding 2, is not limited to the marker. The loss region LR only needs to have a loss at the signal light wavelength greater than the loss in cladding. The refractive index of the loss region LR is not particularly limited. For example, the refractive index of the loss region LR and the refractive index of the cladding may be the same.

[0094] In the multicore optical fiber 10 shown in Figure 1, the outer core 5 and inner core 6 are positioned symmetrically with respect to the symmetry axes A1 to A4, respectively. However, the outer core and inner core do not necessarily have to be arranged symmetrically.

[0095] In the multicore optical fiber 10 shown in Figure 1, the outer core 5 and the inner core 6 are positioned symmetrically with respect to a common axis of symmetry A1 to A4. However, the axis of symmetry of the outer core and the axis of symmetry of the inner core do not necessarily coincide. Even if the axis of symmetry of the outer core and the axis of symmetry of the inner core do not coincide, if the markers 4 are formed at positions where the distance between the marker 4 and each core 1 is different, each core 1 can be identified based on the distance between the marker 4 and each core 1. Furthermore, a marker for the outer core may be provided at a position off the axis of symmetry of the outer core, and a marker for the inner core may be provided at a position off the axis of symmetry of the inner core.

[0096] The refractive index of the inner cladding portion 22 of the inner core 6 may be the same as that of the cladding 2. The inner core 6 is not limited to having a core portion 21 and an inner cladding portion 22. The inner core 6 can also have a configuration without an inner cladding portion 22 (a configuration consisting only of the core portion 21). The inner core 6 may also have a trench portion.

[0097] In the multicore optical fiber 110 shown in Figure 7, the distance L5 between the central axes C5 of the outer cores 5M and 5N belonging to different core groups 8 is different from the distance L4 between the central axes C5 of a pair of outer cores 5M and 5N belonging to the same core group 8. However, the configuration of the multicore optical fiber is not limited to this example. For example, it is sufficient that the distance between the central axes C5 of one pair of adjacent outer cores 5 is different from the distance between the central axes C5 of at least one other pair of outer cores 5.

[0098] Furthermore, without departing from the spirit of the present invention, the components in the above-described embodiments may be replaced with well-known components as appropriate, and the above-described embodiments and modifications may be combined as appropriate.

[0099] 1...Core, 2...Cladding, 3...Coating layer, 4...Marker, 5...Outer core, 6...Inner core, 7...Interface, 10, 110...Multicore optical fiber, 11...Core section, 13...Trench section, 100, 200...Optical fiber with optical connector, C2...Central axis (center of cladding), C5...Central axis (center of outer core, first central axis), C6...Central axis (center of inner core, second central axis), L1...Shortest distance between the central axis of the outer core and the coating layer (shortest distance between the center of the outer core and the coating layer), L2...Shortest distance from the central axis of the outer core to the interface between the cladding and the coating layer (shortest distance from the center of the outer core to the interface between the cladding and the coating layer), L3...Shortest distance between trench sections, L4, L5...Distance between the central axes of the outer core, LR...Loss region, R6...Circumference, R7...Circumference, T1...Thickness of the coating layer

Claims

1. A multicore optical fiber comprising eight or more cores, a cladding surrounding the cores, and a coating layer covering the cladding, wherein the cores include seven or more outer cores arranged on a circumference centered on the central axis of the cladding, and one or more inner cores arranged closer to the center of the cladding than the outer cores, the outer cores having a core portion and a trench portion surrounding the core portion and having a refractive index lower than that of the cladding, the shortest distance between the first central axis of the outer core and the second central axis of the inner cores being Λ, the shortest distance between the first central axis of the outer core and the coating layer being Λ or less, and the refractive index of the coating layer being higher than that of the inner cores.

2. The multicore optical fiber according to claim 1, wherein the shortest distance from the first central axis of the outer core to the interface between the cladding and the coating layer is 22.7 μm or more.

3. The shortest distance between the trench portions is 2.5 times or more the wavelength of the signal light used for optical communication, as described in claim 1 or 2.

4. A multicore optical fiber according to any one of claims 1 to 3, wherein the multicore optical fiber has a loss region radially inward from the outer core, in which the loss at the signal optical wavelength used for optical communication is greater than the loss in the cladding.

5. The multicore optical fiber according to claim 4, wherein the loss region is positioned to identify a plurality of cores.

6. The multicore optical fiber according to any one of claims 1 to 5, wherein the loss in the coating layer at the signal light wavelength for optical communication is greater than the loss in the cladding at the signal light wavelength.

7. The multicore optical fiber according to any one of claims 1 to 6, wherein the number of inner cores is two or more, and the plurality of inner cores are arranged on a circumference centered on the central axis of the cladding.

8. The multicore optical fiber according to any one of claims 1 to 7, wherein the distance between the first central axes in one pair of adjacent outer cores is different from the distance between the first central axes in at least one pair of outer cores other than the pair of outer cores.

9. The multicore optical fiber according to any one of claims 1 to 8, wherein the thickness of the coating layer is 25 μm or more.

10. An optical fiber with an optical connector, having one multicore optical fiber as described in any one of claims 1 to 9.

11. An optical fiber with an optical connector having a plurality of multicore optical fibers as described in any one of claims 1 to 9.

12. An optical fiber with a fan-in / fan-out device, having a multicore optical fiber as described in any one of claims 1 to 9.

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