Multicore fiber

Abstract

A multicore fiber (1) comprises: two to four core elements (10) that include a core (11) and a trench layer (13) that surrounds the outer circumferential surface of the core (11); and cladding (20) that uninterruptedly surrounds the outer circumferential surface of each core element (10). The outer diameter of the core elements (10) is equal to or less than 1 μm less than the inter-core distance Λ between a core element (10) and the nearest adjacent core element (10). The mode field diameter of light propagated by the cores (11) is 8.2–8.8 μm. The trench volume TV, which is the product of the cross-sectional area of the trench layer (13) in a cross-section orthogonal to the longitudinal direction of the cladding (20) and the relative refractive index difference of the trench layer (13) relative to the cladding (20), is at least 105 μm2% but no more than 350 μm2%.

Description

Multicore Fiber

[0001] The present invention relates to a multicore fiber.

[0002] With the spread of optical fiber communication systems, the amount of information transmitted by optical fibers has increased dramatically. Against this background, multicore fibers, in which the outer peripheries of multiple cores are surrounded by a single cladding, are sometimes used. Multicore fibers can transmit multiple signals using light propagating through each of the multiple cores, thereby increasing the transmission capacity per optical fiber.

[0003] Patent Document 1 listed below describes a multicore fiber in which four core elements are arranged. The core element includes a core and a trench portion that surrounds the outer peripheral surface of the core and has a lower refractive index than the cladding. Patent Document 1 refers to this trench portion as the first cladding region, and the cladding as the second cladding region. This multicore fiber has a cladding diameter of 125 μm, and achieves crosstalk of −54 dB / km.

[0004] Japanese Patent Application Laid-Open No. 2020-86054

[0005] As described above, the cladding diameter of the multi-core fiber in Patent Document 1 is 125 μm. However, in order to meet the demands for arranging more multi-core fibers in a limited space when using the multi-core fiber in an optical fiber cable, for example, and for improving the mechanical reliability of the multi-core fiber, there is a demand for a multi-core fiber with a smaller cladding diameter. On the other hand, when the cladding diameter is reduced, the distance between the core and the outer circumferential surface of the cladding becomes smaller, which raises concerns that the confinement loss is likely to increase, and the inter-core distance becomes smaller, which is likely to increase crosstalk.

[0006] Therefore, an object of the present invention is to provide a multi-core fiber that can reduce the cladding diameter while suppressing crosstalk and confinement loss.

[0007] A first aspect of the present invention for solving the above problems comprises two to four core elements including a core and a trench layer surrounding the outer peripheral surface of the core, and a clad surrounding the outer peripheral surface of each of the core elements without any gaps, wherein each of the core elements is arranged on a circumference centered on the center of the clad, the diameter of the core element is 1 μm smaller than the inter-core distance between the core element and the closest adjacent core element, when light having a wavelength of 1.31 μm propagates through the core, the mode field diameter of the light is 8.8 μm or less, and a trench volume which is the product of the cross-sectional area of ​​the trench layer in a cross section perpendicular to the longitudinal direction of the clad and the absolute value of the relative refractive index difference of the trench layer with respect to the clad is 105 μm. ２ % or more 350μm ２ % or less.

[0008] With such a multi-core fiber, when light with a wavelength of 1.31 μm is propagated through the multi-core fiber, even if the cladding thickness is approximately 25.3 μm and the inter-core distance Λ is approximately 35 μm, the crosstalk from one adjacent core at a bending radius of 300 mm can be −35 dB / km or less, and the confinement loss at a bending radius of 76 mm can be 0.01 dB / km or less. Therefore, while maintaining sufficient optical communication quality when light with a wavelength of 1.31 μm is propagated through the multi-core fiber, the cladding diameter can be 104 μm or less when there are four core elements, and the cladding diameter can be 90 μm or less when there are two core elements. In this way, the multi-core fiber of the present invention can suppress crosstalk and confinement loss while reducing the cladding diameter.

[0009] In addition, in a second aspect of the present invention, the trench volume is 135 μm ２ % or more.

[0010] With this configuration, when light with a wavelength of 1.31 μm is propagated through the multicore fiber, the confinement loss can be further reduced to 0.001 dB / km or less even if the cladding thickness is approximately 25.3 μm.

[0011] In addition, in a third aspect of the present invention, the trench volume is 190 μm ２ % or more.

[0012] With this configuration, when light with a wavelength of 1.31 μm is propagated through the multicore fiber, the confinement loss can be kept to 0.01 dB / km or less even if the cladding thickness is approximately 21.7 μm.

[0013] In addition, in a fourth aspect of the present invention, the trench volume is 215 μm ２ % or more.

[0014] With this configuration, when light with a wavelength of 1.31 μm is propagated through the multicore fiber, the confinement loss can be reduced to 0.001 dB / km or less even if the cladding thickness is approximately 21.7 μm.

[0015] In addition, in a fifth aspect of the present invention, the trench volume is 234 μm ２ % or more.

[0016] With this configuration, when light with a wavelength of 1.625 μm is propagated through a multicore fiber, crosstalk from one adjacent core can be kept to −30 dB / km or less even if the inter-core distance is about 35 μm.

[0017] In addition, in a sixth aspect of the present invention, the trench volume is 269 μm ２ % or more.

[0018] With this configuration, when light with a wavelength of 1.625 μm is propagated, the confinement loss can be kept to 0.01 dB / km or less even if the cladding thickness is approximately 21.7 μm.

[0019] In addition, in a seventh aspect of the present invention, the trench volume is 306 μm ２ % or more.

[0020] With this configuration, when light with a wavelength of 1.625 μm is propagated through the multicore fiber, the confinement loss can be kept to 0.001 dB / km or less even if the cladding thickness is about 21.7 μm.

[0021] In addition, for aspects 3 and later, the core distance can be set to about 35 μm and the clad thickness can be set to about 21.7 μm, so when there are four core elements, the clad diameter can be set to 93 μm or less, and when there are two core elements, the clad diameter can be set to 80 μm or less.

[0022] As described above, according to the present invention, a multi-core fiber is provided that can reduce the cladding diameter while suppressing crosstalk and confinement loss.

[0023] 1 is a diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber according to an embodiment of the present invention; FIG. 2 is a diagram showing the refractive index profile of the core element of FIG. 1; FIG. 3 is a diagram showing the relationship between the bending radius and the magnitude of crosstalk when propagating light with a wavelength of 1.31 μm; FIG. 4 is a diagram showing the relationship between the bending radius and the magnitude of crosstalk when propagating light with a wavelength of 1.625 μm; FIG. 5 is a diagram showing the relationship between the cladding thickness and the magnitude of confinement loss; FIG. 6 is a diagram showing the relationship between the wavelength and the confinement loss; FIG. 7 is a diagram showing the relationship between the trench volume and the inter-core distance when propagating light with a wavelength of 1.31 μm; FIG. 8 is a diagram showing the relationship between the trench volume and the inter-core distance when propagating light with a wavelength of 1.625 μm; FIG. 9 is a diagram showing the relationship between the trench volume and the cladding thickness when propagating light with a wavelength of 1.31 μm; FIG. 10 is a diagram showing the relationship between the trench volume and the cladding thickness when propagating light with a wavelength of 1.625 μm; FIG. 11 is a diagram showing the relationship between the trench volume and the cladding thickness when propagating light with a wavelength of 1.625 μm;

[0024] Preferred embodiments of the multicore fiber according to the present invention will be described in detail below with reference to the drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention can be modified and improved from the embodiments within the scope of the claims without departing from the spirit thereof. Note that, for ease of understanding, the scales of the drawings may differ from those described in the following description.

[0025] 1 is a diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber according to this embodiment. The multicore fiber 1 of this embodiment includes four core elements 10, claddings 20 that tightly surround the outer peripheral surfaces of the core elements 10, and coating layers 30 that cover the outer peripheral surfaces of the claddings 20. Note that the cladding 20 is also called a common cladding because it surrounds the outer peripheral surfaces of the core elements 10.

[0026] Each core element 10 includes a core 11, an outer core 12 tightly surrounding the outer circumferential surface of the core 11, and a trench layer 13 tightly surrounding the outer circumferential surface of the outer core 12.

[0027] The cores 11 are positioned at equal intervals on the same circumference CR centered at the center C of the cladding 20. The diameter of the core 11 is, for example, 5.5 μm or more and 8.8 μm or less. The outer diameter of the outer core 12, i.e., the inner diameter of the trench layer 13, is 12 μm or more and 25 μm or less. The outer diameter of the trench layer 13, i.e., the diameter of the core element 10, is 1 μm or less smaller than the inter-core distance between the core element 10 and the closest adjacent core element 10, for example, 24 μm or more and 40 μm or less. In the following description, the outer diameter of the trench layer 13 may be indicated as Dtrench as shown in FIG. 1. The center-to-center distance between adjacent cores 11 may be indicated as Λ, in which case the center-to-center distance between the cores 11 positioned diagonally is 2 μm or less. １／２Λ. The distance from the center of the core 11 to the outer peripheral surface of the cladding 20 is sometimes referred to as the cladding thickness, and the cladding thickness is sometimes indicated by OCT. The diameter of the core 11 is sometimes indicated by 2×r1, the outer diameter of the outer core 12, i.e., the inner diameter of the trench layer, is sometimes indicated by 2×r2, and the outer diameter of the trench layer 13, i.e., the diameter of the core element 10, is sometimes indicated by 2×r3. Therefore, Dtrench = 2×r3. The distance from the center of the cladding 20 to the center of the core 11 is sometimes indicated by R1. The diameter of the cladding 20 is sometimes indicated by Dclad.

[0028] In this case, the diameter Dclad of the clad 20 of this embodiment can be expressed by the following formula (1): Dclad=2 １／２ Λ+2OCT (1)

[0029] The refractive index of the core 11 is higher than the refractive index of the outer core 12 and the cladding 20, and the relative refractive index difference of the core 11 with respect to the cladding 20 is 0.2% or more and 0.9% or less. The refractive index of the trench layer 13 is lower than the refractive index of the outer core 12 and the cladding 20. The relative refractive index difference of the trench layer 13 with respect to the cladding 20 is -0.9% or more and -0.1% or less. The refractive index of the outer core 12 may be the same as the refractive index of the cladding 20, or may be higher or lower than the refractive index of the cladding 20. The relative refractive index difference of the outer core 12 with respect to the cladding 20 is -0.3% or more and 0.3% or less. The relative refractive index difference of the core 11 with respect to the cladding 20 may be indicated as Δ1, the relative refractive index difference of the outer core 12 with respect to the cladding 20 may be indicated as Δ2, and the relative refractive index difference of the trench layer 13 with respect to the cladding 20 may be indicated as Δ3.

[0030] The core 11 is made of silica glass doped with a dopant such as germanium that increases the refractive index. The cladding 20 is made of silica glass doped with no dopant. The trench layer 13 is made of silica glass doped with a dopant such as fluorine that decreases the refractive index. The outer core 12 may be doped with a dopant that increases the refractive index, a dopant that decreases the refractive index, or no dopant at all, depending on its relationship with the refractive index of the cladding 20. Note that whether or not dopants are added to the core 11, outer core 12, trench layer 13, and cladding 20, and if so, the type of dopant, are not limited to those described above.

[0031] The trench volume is the product of the cross-sectional area of ​​the trench layer 13 in a cross section perpendicular to the longitudinal direction of the cladding 20 and the absolute value of the relative refractive index difference of the trench layer 13 with respect to the cladding 20. The trench volume may be denoted as TV. The cross-sectional area of ​​the trench layer 13 is expressed as (r3 ２ -r2 ２ ) × π. Therefore, the trench volume TV can be expressed by the following formula (2): TV = (r3 ２ -r2 ２ )×π×|Δ3| (2)

[0032] Note that, when the diameter of the cladding 20 is small, a marker (not shown) or the like may cause uneven residual stress in the multicore fiber 1, resulting in fiber curl. From the viewpoint of suppressing this fiber curl, it is preferable that the distance R1 from the center of the cladding 20 to the center of the core 11 is equal to or greater than the cladding thickness OCT, i.e., OCT / R1≦1. In this case, it is possible to easily position the marker closer to the center C of the cladding than the core element 10, and fiber curl can be suppressed.

[0033] The coating layer 30 includes an inner coating layer 31 that coats the outer peripheral surface of the cladding 20, and an outer coating layer 32 that coats the outer peripheral surface of the inner coating layer 31. The inner coating layer 31 and the outer coating layer 32 are each made of a resin such as an ultraviolet curable resin, and the inner coating layer 31 and the outer coating layer 32 are made of different resins.

[0034] Next, a description will be given of the characteristics of the multi-core fiber 1. In the following description, there are cases where the significant figures are three digits.

[0035] 2 is a diagram showing a predetermined refractive index profile of the core element 10 of the multicore fiber 1 of FIG. 1. In FIG. 2, the same symbols are written at positions indicating the relative refractive index differences of the core 11, outer core 12, trench layer 13, and cladding 20. In this example, the relative refractive index difference Δ2 of the outer core 12 with respect to the cladding 20 is 0.01%. The relative refractive index difference Δ1 of the core 11 with respect to the cladding 20 is 0.35%. The relative refractive index difference Δ3 of the trench layer 13 with respect to the cladding 20 is −0.4%. Furthermore, the diameter 2×r1 of the core 11 is 7 μm, the outer diameter 2×r2 of the outer core 12 is 16 μm, and the outer diameter 2×r3 of the trench layer 13, which is the diameter of the core element 10, is 32 μm. In this case, the trench volume TV is approximately 234 μm. ２ %. In this case, the optical properties of the core element 10 are as shown in Table 1 below.

[0036] Next, the crosstalk XT of the core element having the refractive index profile of Fig. 2 will be described. Fig. 3 is a diagram showing the relationship between the bending radius of the multicore fiber 1 and the crosstalk XT from one adjacent core when light with a wavelength of 1.31 µm is propagated. As shown in Fig. 3, when light with a wavelength of 1.31 µm is propagated, in the case where the inter-core distance Λ is the smallest at 35 µm, the crosstalk XT from one adjacent core is at most -75 dB / km when the bending radius of the multicore fiber 1 is 300 mm. Note that when light with a wavelength of 1.31 µm is propagated in the multicore fiber 1, sufficient optical communication quality is maintained as long as the amount of crosstalk from one adjacent core is -35 dB / km or less.

[0037] 4 is a diagram showing the relationship between the bending radius of the multicore fiber 1 and the crosstalk XT from one adjacent core when light with a wavelength of 1.625 μm is propagated. As shown in FIG. 4, when light with a wavelength of 1.625 μm is propagated, in the case where the inter-core distance Λ is the smallest at 35 μm, the crosstalk XT from one adjacent core is at most smaller than −36 dB / km when the bending radius of the multicore fiber 1 is 300 mm. Note that when light with a wavelength of 1.625 μm is propagated in the multicore fiber 1, sufficient optical communication quality is maintained as long as the amount of crosstalk between adjacent cores is −30 dB / km or less.

[0038] 2, since the outer diameter 2×r3 of the trench layer 13 is 32 μm, it is preferable from the viewpoint of manufacturing that the inter-core distance Λ is 33 μm or more. In this description, the case where the inter-core distance Λ is 35 μm or more has been considered.

[0039] Next, the confinement loss CL of a core element having the refractive index profile of FIG. 2 will be described. FIG. 5 is a diagram showing the relationship between the cladding thickness OCT and the magnitude of the confinement loss CL. Note that FIG. 5 shows a simulation of the confinement loss CL obtained by disposing a core element 10 in a single-core fiber having a cladding diameter of 100 μm and an outer diameter including the coating layer of 160 μm, and by shifting the position of the core element 10 from the center to the outer periphery of the cladding. The bending radius of the optical fiber in this case was set to 76 mm. The bending radius of the optical fiber, 76 mm, is the radius of a typical bobbin and is smaller than the bending radius of an optical fiber cable. As shown in FIG. 5, when propagating light with a wavelength of 1.625 μm, which is prone to deterioration in confinement loss, a cladding thickness OCT of 21.5 μm or more is required to achieve a confinement loss CL of 0.001 dB / km or less. This condition is also true for the multi-core fiber 1. When light having a wavelength of 1.55 μm is propagated, if the cladding thickness OCT is 19.3 μm or more, the confinement loss CL is 0.001 dB / km or less. When light having a wavelength of 1.31 μm is propagated, if the cladding thickness OCT is 17.5 μm or more, the confinement loss CL is 0.001 dB / km or less. The magnitude of this confinement loss CL is two orders of magnitude lower than the optical loss generally required in optical communications. Therefore, if the confinement loss CL is 0.001 dB / km or less, it is a sufficiently small loss for optical communications. When light having a wavelength of 1.625 μm is propagated through the multicore fiber 1, sufficient optical communication quality is maintained if the confinement loss CL is 0.01 dB / km or less.

[0040] Fig. 6 is a diagram showing the relationship between wavelength and confinement loss. Fig. 6 shows cases where the bending radius R is 76 mm, 10 mm, and 7.5 mm. Note that Fig. 6 is a simulation using a multicore fiber 1 in which the core element 10 has the refractive index profile shown in Fig. 2, the inter-core distance Λ is 40 μm, the cladding thickness OCT is 21.7 μm, the diameter of the cladding 20 is 100 μm, and the outer diameter of the coating layer 30 is 160 μm. As shown in Fig. 6, even when light of a wavelength of 1.625 μm, which is the largest confinement loss, is propagated and the multicore fiber 1 is bent at a radius of 7.5 mm, the confinement loss is approximately 0.75 dB / km. According to ITU-T G. 657 A2, the confinement loss when light of a wavelength of 1.625 μm is propagated through an optical fiber with a bending radius of 7.5 mm is 1.0 dB / turn or less. Therefore, the confinement loss shown in Fig. 6 is approximately 0.75 dB / km, even when light of a wavelength of 1.625 μm is propagated through an optical fiber with a bending radius of 7.5 mm. 657 A2 is fully satisfied.

[0041] As described above, in the case of a core element having the refractive index profile of FIG. 2 , the inter-core distance Λ can be set to 35 μm, and the cladding thickness OCT can be set to 21.5 μm. In this case, from equation (1), Dclad is approximately 92.5 μm. Therefore, in the case of a core having the refractive index profile of FIG. 2 , a multi-core fiber having crosstalk and confinement loss characteristics required by the market and a cladding 20 diameter of 95 μm or less can be realized. Note that in the case of a core element having the refractive index profile of FIG. 2 , even if the inter-core distance Λ is set to 40 μm and the cladding thickness OCT is set to 21.5 μm, Dclad is approximately 99.97 μm. Therefore, in the case of a core having the refractive index profile of FIG. 2 , by setting the inter-core distance Λ to 40 μm, crosstalk XT can be further reduced, and a multi-core fiber having crosstalk and confinement loss characteristics required by the market and a cladding 20 outer diameter of 100 μm or less can be realized.

[0042] Next, the crosstalk XT and the confinement loss CL when the refractive index profile in FIG. 2 is changed to change the trench volume TV will be described.

[0043] Fig. 7 is a diagram showing the relationship between the trench volume TV and the inter-core distance Λ when light with a wavelength of 1.31 µm is propagated through the core elements 10 of the multicore fiber 1. Fig. 7 was obtained by simulating a multicore fiber in which the diameter of the cladding 20 is 100 µm, the outer diameter of the coating layer 30 is 160 µm, the bending radius is 300 mm, and the profile shown in Table 2 below. Note that each core element 10 of the multicore fiber 1 used in the simulation has a profile as shown in Table 2. Specifically, in the multicore fiber samples A to F in Table 2, the inter-core distance Λ was set to 35 µm, 36 µm, 38 µm, and 40 µm, and the relationship between the crosstalk XT from one adjacent core and the trench volume TV for each inter-core distance Λ was obtained. Then, using this relationship, the relationship between the trench volume TV and the inter-core distance Λ was fitted when the crosstalk XT from one adjacent core was -35 dB / km, -40 dB / km, -50 dB / km, and -60 dB / km. For this reason, Figure 7 shows the relationship between the trench volume TV and the inter-core distance Λ where the crosstalk XT from one adjacent core is constant. Note that a bending radius of 300 mm is the bending radius of a typical optical fiber in an optical fiber cable.

[0044] The mode field diameters MFD of the light propagating through the cores 11 of A to F in Table 2 at a wavelength of 1.31 μm are 8.5 μm, 8.5 μm, 8.6 μm, 8.5 μm, 8.5 μm, and 8.4 μm, in that order.

[0045] In Fig. 7, the relationship between the trench volume TV and the inter-core distance Λ is as follows: When the crosstalk XT from one adjacent core is -35 dB / km: Λ = -0.0415 TV + 34.9 When the crosstalk XT from one adjacent core is -40 dB / km: Λ = -0.0413 TV + 36.3 When the crosstalk XT from one adjacent core is -50 dB / km: Λ = -0.0407 TV + 39.1 When the crosstalk XT from one adjacent core is -60 dB / km: Λ = -0.0402 TV + 41.9

[0046] As described above, when the outer diameter Dtrench of the trench layer 13 is 32 μm, it is not common to make the inter-core distance smaller than 33 μm. As shown in FIG. 7, when the inter-core distance is 35 μm or more and light with a wavelength of 1.31 μm is propagated, in order for the crosstalk XT from one adjacent core to be −35 dB / km or less, even if the trench volume TV is zero, that is, even if the trench layer 13 does not exist, the crosstalk XT from one adjacent core will be −35 dB / km or less. Furthermore, when the inter-core distance Λ is 35 μm or more, the trench volume TV must be 101 μm or less. ２ %, the crosstalk XT can be suppressed to -50 dB / km or less, and the trench volume TV is 172 μm ２ % or more, the crosstalk XT can be suppressed to -60 dB / km or less. Furthermore, when the inter-core distance Λ is 40 μm or more, even if the trench volume TV is zero, the crosstalk XT from one adjacent core is -50 dB / km or less, and when the trench volume TV is 47.3 μm ２ % or more, the crosstalk XT from one adjacent core can be suppressed to −60 dB / km or less.

[0047] FIG. 8 is a diagram showing the relationship between the trench volume TV and the inter-core distance Λ when light with a wavelength of 1.625 μm is propagated through the core elements 10 of the multicore fiber 1 under the same conditions as in FIG.

[0048] In the case of Fig. 8, the relationship between the trench volume TV and the inter-core distance Λ is as follows: When the crosstalk XT from one adjacent core is -20 dB / km: Λ = -0.0524 TV + 43.2 When the crosstalk XT from one adjacent core is -25 dB / km: Λ = -0.0522 TV + 45.3 When the crosstalk XT from one adjacent core is -30 dB / km: Λ = -0.0521 TV + 47.2 When the crosstalk XT from one adjacent core is -35 dB / km: Λ = -0.0519 TV + 49.2 When the crosstalk XT from one adjacent core is -40 dB / km: Λ = -0.0518 TV + 51.2

[0049] As shown in Figure 8, when light with a wavelength of 1.625 μm is propagated, if the inter-core distance Λ is 35 μm or more and the trench volume TV is 234 μm or more, the crosstalk XT from one adjacent core is -30 dB / km or less. ２ %, the crosstalk XT can be suppressed to -35 dB / km or less, and the trench volume TV is 313 μm ２ % or more, the crosstalk XT can be suppressed to -40 dB / km or less. Also, when the core distance Λ is 40 μm or more, the trench volume TV is 138 μm. ２ %, the crosstalk XT can be suppressed to -30 dB / km or less, and the trench volume TV is 177 μm ２ %, the crosstalk XT can be suppressed to -35 dB / km or less, and the trench volume TV is 216 μm ２ % or more, the crosstalk XT can be suppressed to -40 dB / km or less.

[0050] Fig. 9 is a diagram showing the relationship between the trench volume TV and the cladding thickness OCT when light with a wavelength of 1.31 µm is propagated through the core element 10 of the multicore fiber 1. Fig. 9 was obtained by simulating a multicore fiber in which the diameter of the cladding 20 is 100 µm, the outer diameter of the coating layer 30 is 160 µm, the bending radius is 76 mm, and the profile shown in Table 2 below is used. Specifically, for the multicore fiber samples A to F in Table 2, the cladding thickness OCT was set to 21.7 µm, 23.1 µm, 23.5 µm, 24.5 µm, and 25.3 µm, and the relationship between the confinement loss CL and the trench volume TV for each cladding thickness OCT was obtained. Using this relationship, the relationship between the trench volume TV and the cladding thickness OCT was fitted when the confinement loss CL was 0.001 dB / km, 0.01 dB / km, 0.1 dB / km, and 1 dB / km. Therefore, Fig. 9 shows the relationship between the trench volume TV and the cladding thickness OCT when the confinement loss CL is constant.

[0051] In Figure 9, the relationship between the trench volume TV and the cladding thickness OCT is as follows: When the confinement loss CL is 1 dB / km: OCT = -0.0343 TV + 26.3 When the confinement loss CL is 0.1 dB / km: OCT = -0.0380 TV + 27.9 When the confinement loss CL is 0.01 dB / km: OCT = -0.0420 TV + 29.7 When the confinement loss CL is 0.001 dB / km: OCT = -0.0452 TV + 31.4

[0052] As shown in FIG. 9, when light with a wavelength of 1.31 μm is propagated, the trench volume TV is 105 μm. ２ %, the confinement loss CL can be made 0.01 dB / km or less even if the cladding thickness OCT is approximately 25.3 μm. ２ %, the confinement loss CL can be made 0.01 dB / km or less even if the cladding thickness OCT is approximately 21.7 μm. ２ %, the confinement loss CL can be made 0.001 dB / km or less even if the cladding thickness OCT is approximately 25.3 μm. ２ %, the confinement loss CL can be made 0.001 dB / km or less even if the cladding thickness OCT is approximately 21.7 μm or less.

[0053] FIG. 10 is a diagram showing the relationship between the trench volume TV and the cladding thickness OCT when light with a wavelength of 1.625 μm is propagated through the core elements 10 of the multicore fiber 1 under the same conditions as in FIG.

[0054] In the case of FIG. 10, the relationship between the trench volume TV and the cladding thickness OCT is as follows: When the confinement loss CL is 1 dB / km: OCT = -0.0707 TV + 34.7 When the confinement loss CL is 0.1 dB / km: OCT = -0.0589 TV + 35.1 When the confinement loss CL is 0.01 dB / km: OCT = -0.0494 TV + 35.0 When the confinement loss CL is 0.001 dB / km: OCT = -0.0431 TV + 34.9

[0055] As shown in FIG. 10, when light with a wavelength of 1.625 μm is propagated, the trench volume TV is 196 μm. ２ %, the confinement loss CL can be made 0.01 dB / km or less by setting the cladding thickness OCT to approximately 25.3 μm. ２ %, the confinement loss CL can be made 0.01 dB / km or less even if the cladding thickness OCT is approximately 21.7 μm. ２ %, the confinement loss CL can be made 0.001 dB / km or less by setting the cladding thickness OCT to approximately 25.3 μm. ２ % or more, even if the cladding thickness OCT is approximately 21.7 μm, the confinement loss CL can be made 0.001 dB / km or less.

[0056] 7 and 9, when light with a wavelength of 1.31 μm is propagated through the multi-core fiber 1, the trench volume TV is 105 μm. ２ %, by setting the inter-core distance Λ to 35 μm or more and the cladding thickness OCT to approximately 25.3 μm, the crosstalk XT from one adjacent core can be kept to −35 dB / km or less and the confinement loss CL to 0.01 dB / km or less. Therefore, when the trench volume TV is 105 μm or more, ２% or more, the diameter of the cladding 20 can be made 104 μm or less while maintaining sufficient optical communication quality. In other words, the diameter of the cladding 20 can be made small while maintaining sufficient optical communication quality. ２ % or more, the confinement loss CL can be reduced to 0.001 dB / km or less under this condition.

[0057] In addition, the trench volume TV is 190 μm ２ %, the inter-core distance Λ can be set to approximately 40 μm and the cladding thickness OCT to approximately 21.7 μm or less, thereby making it possible to keep the crosstalk XT from one adjacent core to -35 dB / km or less and the confinement loss CL to 0.01 dB / km or less. ２ % or more, the diameter of the cladding 20 can be made 104 μm or less while maintaining sufficient optical communication quality. In other words, the diameter of the cladding 20 can be made small while maintaining sufficient optical communication quality. In this case, even if the inter-core distance Λ is approximately 35 μm, the crosstalk XT from one adjacent core can be made -35 dB / km or less. Therefore, by setting the inter-core distance Λ to approximately 35 μm and the cladding thickness OCT to approximately 21.7 μm, the diameter of the cladding 20 can be made 93 μm or less. In addition, when the trench volume TV is 215 μm ２ % or more, even if the cladding thickness OCT is approximately 21.7 μm, the confinement loss CL can be made 0.001 dB / km or less.

[0058] 8 and 10, when light having a wavelength of 1.625 μm is propagated through the multi-core fiber 1, the trench volume TV is 234 μm. ２ %, by setting the inter-core distance Λ to approximately 35 μm, the crosstalk XT from one adjacent core can be reduced to -30 dB / km or less, and even if the cladding thickness OCT is approximately 25.3 μm, the confinement loss CL can be reduced to 0.01 dB / km or less. Therefore, when the trench volume TV is 234 μm ２% or more, the diameter of the cladding 20 can be set to 100 μm or less while maintaining sufficient optical communication quality. ２ % or more, even if the cladding thickness OCT is approximately 25.3 μm, the confinement loss CL can be made 0.001 dB / km or less.

[0059] In addition, the trench volume TV is 269 μm ２ %, by setting the inter-core distance Λ to 40 μm, the crosstalk XT from one adjacent core can be reduced to −30 dB / km or less, and by setting the cladding thickness OCT to approximately 21.7 μm, the confinement loss CL can be reduced to 0.01 dB / km or less. Therefore, when the trench volume TV is 269 μm ２ % or more, the diameter of the cladding 20 can be made 104 μm or less while maintaining sufficient optical communication quality. In other words, the diameter of the cladding 20 can be made small while maintaining sufficient optical communication quality. In this case, even if the inter-core distance Λ is approximately 35 μm, the crosstalk XT from one adjacent core can be made -30 dB / km or less. Therefore, by setting the inter-core distance Λ to approximately 35 μm and the cladding thickness OCT to approximately 21.7 μm, the diameter of the cladding 20 can be made 93 μm or less. In addition, when the trench volume TV is 306 μm ２ % or more, the confinement loss CL can be reduced to 0.001 dB / km or less under this condition.

[0060] Next, the upper limit of the trench volume TV will be described.

[0061] FIG. 11 is a diagram showing the relationship between the trench volume TV and the 22 m cable cutoff wavelength λcc. If the wavelength is longer than λcc shown in FIG. 11, single mode light can propagate. As shown in FIG. 11, the relationship between the trench volume TV and the cable cutoff wavelength λcc is expressed by the following equation (3): λcc = 0.0011 TV + 0.9707 (3)

[0062] In G.652.D, the cable cutoff wavelength λcc when transmitting light with a wavelength of 1.31 μm is set to 1.26 μm or less. In this case, from equation (3), the upper limit of the trench volume TV is 263 μm. ２ %. However, when manufacturing an optical fiber, residual stress in the glass of the optical fiber during the drawing process reduces the cable cutoff wavelength λcc by about 30 nm. Therefore, the cable cutoff wavelength λcc can be reduced to 1.29 μm or less. Therefore, in this case, the upper limit of the trench volume TV is 290 μm. ２ %. Note that if the trench volume TV is large, the difference in viscosity between the trench and other regions can cause the residual stress in the optical fiber after spinning to be non-uniform within the cross section of the optical fiber, which can prevent the desired performance from being obtained. Therefore, the upper limit of the trench volume TV is set to 350 μm. ２ %.

[0063] Although the present invention has been described above using the embodiments as examples, the present invention is not limited to the above embodiments.

[0064] For example, in the above embodiment, an example was described in which the outer diameter Dtrench of the trench layer 13 was in the range of 25 μm to 35.04 μm based on r3 in Table 2, and the mode field diameter was in the range of 8.4 μm to 8.6 μm. However, when the outer diameter Dtrench of the trench layer is 40 μm or less and the mode field diameter is 8.8 μm or less, the trench volume TV is 106 μm. ２ % or more 350μm ２ % or less, there is almost no difference from the crosstalk described above, and there is almost no difference from the confinement loss CL described above.

[0065] Furthermore, in the above embodiment, an example in which the number of core elements 10 is four has been described. However, the number of core elements may be, for example, two. Fig. 12 is a diagram showing a multi-core fiber 1 having two core elements 10. When there are two core elements as shown in Fig. 12, the diameter Dclad of the cladding 20 is given by the following formula (4): Dclad = Λ + 2OCT (4)

[0066] Therefore, from the results of FIGS. 7 and 9, when light with a wavelength of 1.31 μm is propagated through the multi-core fiber 1, the trench volume TV is 105 μm. ２ %, by setting the inter-core distance Λ to 35 μm, the crosstalk XT from one adjacent core can be reduced to −35 dB / km or less, and by setting the cladding thickness OCT to approximately 25.3 μm, the confinement loss CL can be reduced to 0.01 dB / km or less, and the diameter of the cladding 20 can be reduced to 85.6 μm, and the trench volume TV can be reduced to 135 μm. ２ % or more, the confinement loss CL can be further reduced to 0.001 dB / km or less. ２ %, the crosstalk XT from one adjacent core is −35 dB / km or less with the above inter-core distance remaining the same, and even if the cladding thickness OCT is approximately 21.7 μm, the confinement loss CL can be 0.01 dB / km or less. The diameter of the cladding 20 can be reduced to 78.4 μm, and the trench volume TV can be reduced to 215 μm. ２ % or more, the confinement loss CL can be further reduced to 0.001 dB / km or less. In this way, when light with a wavelength of 1.31 μm is propagated through the multi-core fiber 1, the trench volume TV is set to 105 μm. ２ %, the diameter of the cladding 20 can be made 90 μm or less while maintaining sufficient optical communication quality. In other words, the diameter of the cladding 20 can be made small while maintaining sufficient optical communication quality.

[0067] 8 and 10, when light with a wavelength of 1.625 μm is propagated through the multi-core fiber 1, the trench volume TV is 196 μm. ２ %, by setting the inter-core distance Λ to 40 μm, the crosstalk XT from one adjacent core is −30 dB / km or less, and by setting the cladding thickness OCT to approximately 25.3 μm, the confinement loss CL is 0.01 dB / km or less, and the diameter of the cladding 20 can be reduced to 90.6 μm, and the trench volume TV is 223 μm. ２% or more, the confinement loss CL can be further reduced to 0.001 dB / km or less. ２ %, by setting the inter-core distance Λ to 35 μm, the crosstalk XT from one adjacent core is −30 dB / km or less, and by setting the cladding thickness OCT to approximately 21.7 μm, the confinement loss CL is 0.01 dB / km or less, and the diameter of the cladding 20 can be reduced to 78.4 μm, and the trench volume TV is 306 μm. ２ % or more, the confinement loss CL can be further reduced to 0.001 dB / km or less. In this way, when light with a wavelength of 1.625 μm is propagated through the multi-core fiber 1, the trench volume TV is set to 196 μm. ２ %, the diameter of the cladding 20 can be made 90 μm or less while maintaining sufficient optical communication quality. In other words, the diameter of the cladding 20 can be made small while maintaining sufficient optical communication quality.

[0068] In addition, when there are three core elements 10, the core elements 10 are arranged on a circumference CR centered on the center of the clad, so that the diameter of the clad 20 can be made to be a size between the diameter of the clad 20 when there are two core elements and the diameter of the clad 20 when there are four core elements.

[0069] Furthermore, in the above embodiment, the core element 10 includes the outer core 12 . However, the outer core 12 is not an essential component, and the core element 10 does not necessarily have to include the outer core 12 .

[0070] Furthermore, in the above embodiment, an example was shown in which the inter-core distances Λ were equal, but the present invention is not limited to this, and as long as the core elements 10 are arranged on the circumference CR, the inter-core distances Λ do not have to be equal.

[0071] As described above, according to the present invention, a multi-core fiber capable of suppressing crosstalk and confinement loss while reducing the cladding diameter is provided, and can be used in the field of optical communications and other devices using multi-core fibers.

Claims

1. A fiber optic optical fiber comprising: two to four core elements including a core and a trench layer surrounding the outer peripheral surface of the core; and a cladding surrounding the outer peripheral surface of each of the core elements without any gaps; each of the core elements is arranged on a circumference centered on the center of the cladding; the diameter of the core element is 1 μm smaller than the inter-core distance between the core element and the closest adjacent other core element; when light having a wavelength of 1.31 μm propagates through the core, the mode field diameter of the light is 8.8 μm or less; and a trench volume, which is the product of the cross-sectional area of ​​the trench layer in a cross section perpendicular to the longitudinal direction of the cladding and the absolute value of the relative refractive index difference of the trench layer with respect to the cladding, is 105 μm. ２ % or more 350μm ２ % or less.

2. The trench volume is 136 μm ２ % or more. The multi-core fiber according to claim 1 .

3. The trench volume is 190 μm ２ % or more. The multi-core fiber according to claim 1 .

4. The trench volume is 214 μm ２ % or more. The multi-core fiber according to claim 1 .

5. The trench volume is 234 μm ２ % or more. The multi-core fiber according to claim 1 .

6. The trench volume is 269.7 μm ２ % or more. The multi-core fiber according to claim 1 .

7. The trench volume is 306 μm ２ % or more. The multi-core fiber according to claim 1 .

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