Multicore fiber

Abstract

This multicore fiber comprises 2-4 core elements (10), and satisfies Z≤453 when Z=(r1 2πΔ1)(r3 2-r2 2)π|Δ3| / r2, wherein r1 is the radius of a core (11), r2 is the radius of an intermediate layer (12), r3 is the radius of a trench layer (13), Δ1 is the relative refractive index of the core (11), Δ2 is the relative refractive index of the intermediate layer (12), and Δ3 is the relative refractive index of the trench layer (13).

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 having two or four cores and a cladding diameter of 125 μm±1 μm. This document states that in order to reduce crosstalk between adjacent cores to −30 dB or less, it is best to set the minimum inter-core distance in the range of 40 μm to 50 μm.

[0004] Patent Document 2 listed below describes a multicore fiber having four cores and a cladding diameter of 125 μm±1 μm. This multicore fiber has trench portions that surround each core individually and have a refractive index lower than that of the cladding. The cutoff wavelength of this multicore fiber is 1.53 μm.

[0005] JP 2013-088458 A JP 2020-086054 A

[0006] Multicore fibers tend to be prone to crosstalk. In order to suppress crosstalk, it is conceivable to increase the inter-core distance or to provide a trench portion as in the multicore fiber of Patent Document 2.

[0007] However, increasing the inter-core distance can lead to an increase in the diameter of the multi-core fiber, but in optical fibers, not just multi-core fibers, there is a demand to suppress the increase in diameter from the viewpoint of avoiding a shortened lifespan. For this reason, there is a demand to make the inter-core distance 40 μm or less. Furthermore, providing a trench portion makes it difficult for light to leak from the core, which tends to lengthen the cutoff wavelength. However, it is preferable that the cutoff wavelength be 1.26 μm or less as specified in ITU-T G652.D.

[0008] Therefore, an object of the present invention is to provide a multicore fiber that has a cutoff wavelength of 1.26 μm or less and can suppress crosstalk even when the inter-core distance is 40 μm or less.

[0009] A first aspect of the present invention for solving the above problems is a core, an intermediate layer that tightly surrounds the outer peripheral surface of the core, and a trench layer that tightly surrounds the outer peripheral surface of the intermediate layer, the core elements being arranged in a rotational symmetry, and a cladding that tightly surrounds the outer peripheral surface of each of the core elements, the core having a radius of r １ and the radius of the outer periphery of the intermediate layer is r ２ The radius of the outer periphery of the trench layer is r ３ and the relative refractive index difference of the refractive index of the core to the refractive index of the cladding is Δ １ and the relative refractive index difference of the refractive index of the intermediate layer to the refractive index of the clad is Δ ２ The relative refractive index difference of the refractive index of the trench layer to the refractive index of the cladding is Δ ３ In this case, 3.5 μm≦r １ ≦4.4μm 8.0μm≦r ２ ≦10.0μm 13.0μm≦r ３ ≦18.0μm 0.33%≦Δ １ ≦0.45% −0.10%≦Δ ２ ≦0.07% −0.70%≦Δ ３ ≦-0.25%, and Z=(r １ ２ πΔ １ ) (r ３ ２ -r ２ ２ )π|Δ３ | / r ２ When the minimum center-to-center distance between adjacent cores is Λ, the relationship Z≦453 is satisfied, and 40 μm−0.040(r ３ ２ -r ２ ２ )π|Δ ３ The multi-core fiber is characterized by satisfying |≦Λ≦40 μm.

[0010] The above r １ ~r ３ , and Δ １ ~Δ ３ By satisfying the above ranges, the mode field diameter, zero dispersion wavelength, and zero dispersion slope specified in ITU-T G652.D can be satisfied. １ ２ πΔ １ ) refers to the core volume, and (r ３ ２ -r ２ ２ )π|Δ ３ | refers to the trench volume, and r ２is also the radius of the inner circumference of the trench layer. When the core volume is large, when the trench volume is large, and when the inverse of the size of the inner circumference of the trench layer is large, the light confinement force tends to be large and the cutoff wavelength tends to be longer. In other words, Z is the product of three factors that increase the light confinement force and lengthen the cutoff wavelength. Therefore, Z is an important factor that indicates the light confinement force of the core element and its influence on the cutoff wavelength. As a result of extensive research by the inventors, it was found that if Z is 453 or less, the cutoff wavelength specified in ITU-T G652.D can be 1.26 μm or less. Furthermore, when the core elements are arranged in a square lattice pattern, by having Λ satisfy the above formula, crosstalk can be -35 dB / km or less when light with a wavelength of 1.55 μm is propagated in the same direction. In a multicore fiber in which two to four core elements are arranged, crosstalk is greatest when four core elements are arranged in a square lattice pattern. Therefore, when Λ satisfies the above formula, in a multicore fiber in which two to four core elements are arranged, crosstalk can be reduced to −35 dB / km or less when light is propagated in the same direction. By transmitting light in both directions using a multicore fiber in which crosstalk is −35 dB / km or less when light is propagated in the same direction, optical communication can be performed with crosstalk sufficiently suppressed. As described above, according to this aspect, it is possible to provide a multicore fiber that can suppress crosstalk even when the cutoff wavelength is 1.26 μm or less and the inter-core distance is 40 μm or less.

[0011] In the second aspect of the present invention, ３ ２ -r ２ ２ )π|Δ ３ The multi-core fiber of Aspect 1 is characterized in that |≦Λ≦40 μm is satisfied.

[0012] According to this aspect, when the core elements are arranged in a square lattice pattern, the crosstalk can be −45 dB / km or less when light having a wavelength of 1.55 μm is propagated in one direction.

[0013] A third aspect of the present invention is the multi-core fiber according to the first or second aspect, characterized in that the diameter of the cladding is 120 μm or less.

[0014] According to this aspect, the diameter of the optical fiber can be made small.

[0015] In a fourth aspect of the present invention, when the distance from the center of the core to the outer periphery of the cladding is OCT, OCT≦51 μm−Λ / (2 １／２ 4. The multicore fiber according to any one of Aspects 1 to 3, wherein the following condition is satisfied:

[0016] According to this aspect, when two to four core elements are arranged in a rotationally symmetrical manner, the cladding diameter can be made 102 μm or less, thereby enabling the optical fiber to have a small diameter.

[0017] Aspect 5 of the present invention is a method for fabricating a fiber having a core element having two or three core elements, and a distance from the center of the core to the outer periphery of the cladding, where OCT is a distance from the center of the core to the outer periphery of the cladding, such that OCT≦51 μm−Λ / (3 １／2 4. The multicore fiber according to any one of Aspects 1 to 3, wherein the following condition is satisfied:

[0018] According to this aspect, when two or three core elements are arranged in a rotationally symmetrical manner, the cladding diameter can be made 102 μm or less, thereby enabling the optical fiber to have a small diameter.

[0019] A sixth aspect of the present invention is the multicore fiber according to any one of the first to third aspects, wherein the number of the core elements is two, and where OCT is the distance from the center of the core to the outer periphery of the cladding, OCT≦51 μm−Λ / 2 is satisfied.

[0020] According to this aspect, when two core elements are arranged in rotational symmetry, the diameter of the cladding can be made 102 μm or less, thereby making it possible to make the optical fiber thinner.

[0021] As described above, according to the present invention, a multicore fiber is provided that has a cutoff wavelength of 1.26 μm or less and can suppress crosstalk even when the inter-core distance is 40 μm or less.

[0022] Fig. 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 distribution of the relative refractive index difference of the refractive index of the core elements with respect to the refractive index of the cladding. Fig. 3 is a diagram showing the relationship between the parameter Z and the cutoff wavelength. Fig. 4 is a diagram showing the relationship between the trench volume and the magnitude of crosstalk. Fig. 5 is a diagram showing the relationship between the trench volume and the inter-core distance. Fig. 6 is a diagram showing an example of a multicore fiber having two core elements. Fig. 7 is a diagram showing an example of a multicore fiber having three core elements.

[0023] 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.

[0024] 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 circumferential surfaces of the core elements 10, and coating layers 30 that cover the outer circumferential surfaces of the claddings 20. Note that the cladding 20 is a single cladding that commonly surrounds the outer circumferential surfaces of the core elements 10, and is therefore also called a common cladding.

[0025] The clad 20 has an outer shape that is, for example, circular, and the diameter of the clad 20 is preferably 120 μm or less from the viewpoint of thinning.

[0026] Each core element 10 includes a core 11, an intermediate layer 12 that tightly surrounds the outer periphery of the core 11, and a trench layer 13 that tightly surrounds the outer periphery of the intermediate layer 12.

[0027] The cores 11 are positioned at equal intervals on the same circumference CR centered on the center C of the cladding 20. Therefore, in this embodiment, the cores 11 are arranged in four-fold rotational symmetry with the center C as the reference.

[0028] As shown in FIG. 1, the radius of the core 11 is r １ The outer radius of the intermediate layer 12, i.e., the inner radius of the trench layer 13, is represented by r ２ and the radius of the outer periphery of the trench layer 13, i.e., the radius of the core element 10, is r ３ Each radius r １ ~r ３ satisfies the following formulas (1) to (3): 3.5 μm≦r １ ≦4.4μm (1) 8.0μm≦r ２ ≦10.0μm (2) 13.0μm≦r ３ ≦18.0μm (3)

[0029] The shortest distance between the centers of adjacent cores 11 is sometimes referred to as the inter-core distance. When the inter-core distance between adjacent cores is represented by Λ, the inter-core distance between the centers of diagonally positioned cores 11 is 2 １／２ Λ. The distance between the center of the core 11 and the outer peripheral surface of the cladding 20 may be referred to as the cladding thickness. When the cladding thickness is represented by OCT and the diameter of the cladding 20 is represented by D, the diameter D of the cladding 20 in this embodiment can be expressed by the following formula (4): D=2 １／２ Λ+2OCT (4)

[0030] In order for the cladding diameter D to be 102 μm or less, the cladding thickness OCT should satisfy the following formula (5): OCT≦51 μm−Λ / (2 １／２ ) (5)

[0031] In order for the cladding diameter D to be 92 μm or less, the cladding thickness OCT must satisfy the following formula (6), and in order for the cladding diameter D to be 82 μm or less, the cladding thickness OCT must satisfy the following formula (7): OCT≦46 μm−Λ / (2 １／２ ) (6) OCT≦41μm-Λ / (2 １／２ ) (7)

[0032] 2 is a diagram showing the distribution of the relative refractive index difference of the refractive index at each portion of the core element 10 with respect to the refractive index of the cladding 20. In FIG. 2, the same symbols are written at positions indicating the relative refractive index difference of the core 11, the intermediate layer 12, the trench layer 13, and the cladding 20. The relative refractive index difference of the core 11 with respect to the cladding 20 is Δ １ and the relative refractive index difference of the intermediate layer 12 with respect to the cladding 20 is Δ ２ and the relative refractive index difference of the trench layer 13 with respect to the cladding 20 is Δ ３ The relative refractive index difference Δ １ ~Δ ３ satisfies the following formulas (8) to (10): 0.33%≦Δ １ ≦0.45% (8) −0.10%≦Δ ２ ≦0.07% (9) −0.70%≦Δ ３ ≦−0.25% (10)

[0033] In FIG. 2, the relative refractive index difference Δ ２ is greater than zero, and the relative refractive index difference Δ ２ is smaller than zero is shown by a dashed line, and the relative refractive index difference Δ ２ The case where is zero is omitted.

[0034] 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 intermediate layer 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, intermediate layer 12, trench layer 13, and cladding 20, and if so, the type of dopant, are not limited to those described above.

[0035] The product of the cross-sectional area of ​​the core 11 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 core 11 with respect to the cladding 20 is defined as the core volume V. Ｃ The cross-sectional area of ​​the core 11 is r１ ２ ×π. Therefore, the core volume V Ｃ The core volume V can be expressed by the following formula (11). Ｃ The larger V is, the stronger the light confinement force in the core 11 is, and more light can be propagated, which tends to result in a longer cutoff wavelength. Ｃ =r １ ２ ×π×Δ １ (11)

[0036] 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 is defined as the trench volume V Ｔ The cross-sectional area of ​​the trench layer 13 is (r ３ ２ -r ２ ２ ) × π. Therefore, the trench volume V Ｔ can be expressed by the following formula (12). The larger the trench volume, the more the leakage of light from the core element 10 is suppressed, and the cutoff wavelength of the propagating light tends to become longer. V Ｔ = (r ３ ２ -r ２ ２ ) × π × |Δ ３ | (12)

[0037] 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 from the center C of the cladding 20 to the center of the core 11 is equal to or greater than the cladding thickness OCT. In this case, it is possible to more easily position the marker closer to the center C of the cladding 20 than the core element 10, thereby suppressing fiber curl.

[0038] 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.

[0039] Next, the characteristics of the multi-core fiber 1 will be described.

[0040] Multi-core fibers of Samples 1 to 34 were prepared. Each multi-core fiber was the same as that shown in FIG. 1. The radius r of the core 11 of each multi-core fiber was １ , the radius r of the outer periphery of the intermediate layer 12 ２ , the radius r of the outer periphery of the trench layer 13 ３ , the relative refractive index difference Δ of the core 11 with respect to the cladding 20 １ , the relative refractive index difference of the intermediate layer 12 with respect to the cladding 20 is Δ ２ , and the relative refractive index difference Δ ３ are shown in Table 1 below.

[0041] For samples 1 to 34, the trench volume V Ｔ , mode field diameter MFD, parameter Z, cutoff wavelength λ ｃｃ , the zero dispersion wavelength λ ０ , and the zero dispersion slope S ０ The results are shown in Table 2.

[0042] The parameter Z is defined by the following equation (13): Z=(r １ ２ πΔ １ ) (r ３ ２ -r ２ ２ )π|Δ ３ | / r ２ (13)

[0043] Among the parameters Z, (r １ ２ πΔ １ ) is the core volume V as shown in equation (11). Ｃ (r ３ ２ -r ２ ２ )π|Δ ３ | is the trench volume V as shown in equation (12). Ｔ As explained above, the core volume VＣ When is large, the trench volume V Ｔ In each case where r is large, the light confinement force is large and the cutoff wavelength tends to become longer. ２ As explained above, r represents the radius of the inner periphery of the trench layer 13. ２ When the distance from the core 11 to the trench layer 13 is large, the distance from the core 11 to the trench layer 13 tends to be large. When the core 11 and the trench layer 13 are close to each other, the influence of the trench layer 13 on the light propagating through the core 11 is large, the light is easily confined in the core 11, and the cutoff wavelength tends to become longer. In other words, Z is a function of the three elements that increase the light confinement force and make the cutoff wavelength longer: the core volume VC, the trench volume VT, and the reciprocal of the radius of the inner circumference of the trench layer 13 (1 / r ２ ) Therefore, Z is an important factor that indicates the light confinement power in the core element 10 and the influence on the cutoff wavelength.

[0044] Parameter Z and cutoff wavelength λ in Table 2 ｃｃ The relationship between the parameter Z and the cutoff wavelength λ (μm) is shown in Figure 3. ｃｃ It can be seen that there is a strong positive correlation between the parameter Z and the cutoff wavelength λ (μm). ｃｃ The correlation coefficient of λ (μm) was 0.84. The approximate line of each point in FIG. 3 obtained by the least squares method is given by the following formula (14): ｃｃ =0.00053Z+1.01697 (14)

[0045] From Table 2, it can be seen that in samples 27 to 34, the cutoff wavelength λcc is greater than 1.26 μm as defined in ITU-T G652.D. Among samples 27 to 34, the smallest parameter Z is 464 for samples 27 and 32. In addition, in samples 1 to 26, the cutoff wavelength λ ｃｃ is 1.26 μm or less, which satisfies the requirements of ITU-T G652.D. Among samples 1 to 26, the parameter Z is smaller than 464 of samples 27 and 32, and the maximum parameter Z is 453 of sample 23. Therefore, if the parameter Z is 453 or less, the cutoff wavelength λ ｃｃbecomes 1.26 μm or less.

[0046] Next, crosstalk will be described.

[0047] The magnitude XT of crosstalk between adjacent cores 11 when light with a wavelength of 1.55 μm is propagated through the multicore fiber 1 was calculated for inter-core distances Λ of 34 μm, 36 μm, 38 μm, and 40 μm. The results are shown in Table 3. Table 3 also shows the trench volume V shown in Table 2. Ｔ Also shown.

[0048] As shown in Table 3, the trench volume V Ｔ 4 shows the relationship between the magnitude of crosstalk XT and the inter-core distance Λ of 34 μm, 36 μm, 38 μm, and 40 μm. When the approximate straight line is found by the least squares method for each point, the following formulas (15) to (18) are obtained. Λ=34 μm: XT=−0.0972V Ｔ -20.642 (15) Λ=36μm: XT=-0.1084V Ｔ -23.440 (16) Λ=38μm: XT=-0.1036V Ｔ -29.692 (17) Λ=40μm: XT=-0.0985V Ｔ -36.358 (18)

[0049] 400G-ZR, a transceiver standard with a transmission distance of 120 km using light with a wavelength of 1.55 μm, uses a 16-QAM modulation format. To transmit information 120 km using this format by propagating light in the same direction through adjacent cores of a multicore fiber, the crosstalk value from other cores at a wavelength of 1.55 μm must be -45 dB / km or less. Furthermore, when communication capacity is increased, for example, a 64-QAM modulation format may be used, in which case the magnitude of crosstalk must be further reduced to transmit light. When the magnitude of crosstalk XT from other cores at a wavelength of 1.55 μm is -35 dB / km or less, bidirectional transmission can be used to suppress crosstalk to cores that affect communication, and 120 km transmission can be achieved using a 64-QAM format. Therefore, using equations (15) to (18), when the magnitude of crosstalk XT is −35 dB / km and −45 dB / km, the trench volume V Ｔ The results are shown in Table 4.

[0050] FIG. 5 shows the trench volume V Ｔ 5 is a graph showing the relationship between the trench volume V and the core distance Λ. Ｔ The relationship between the core distance Λ and the core distance Λ is expressed by the following formula (19): Λ=40−0.040V Ｔ (19)

[0051] Therefore, in order for the magnitude of crosstalk XT to be −35 dB / km or less, the core-to-core distance Λ must satisfy the following formula (20): Ｔ ≦Λ (20)

[0052] In addition, the trench volume V when the crosstalk magnitude XT shown in Table 4 is −45 dB / km Ｔ The relationship between the core distance Λ and the core distance Λ is expressed by the following formula (21): Λ=43−0.037V Ｔ (21)

[0053] Therefore, in order for the magnitude of crosstalk XT to be −45 dB / km or less, the core-to-core distance Λ must satisfy the following equation (22): Ｔ ≦Λ (22)

[0054] In addition, in a general multicore fiber, the inter-core distance is set to 40 μm. Therefore, in order to prevent the diameter of the multicore fiber 1 from increasing, the inter-core distance Λ needs to be 40 μm or less. Therefore, when the magnitude of crosstalk XT is −35 dB / km or less, or −45 dB / km, the following formulas (22) and (23) need to be satisfied. 40−0.040V Ｔ ≦Λ≦40 (22) 43-0.037V Ｔ ≦Λ≦40 (23) where r ３ <Λ.

[0055] As described above, the multi-core fiber 1 of this embodiment satisfies the formulas (1) to (3) and the formulas (8) to (10). Therefore, the mode field diameter, zero-dispersion wavelength, and zero-dispersion slope defined in ITU-T G652.D can be satisfied. Furthermore, the multi-core fiber 1 of this embodiment has Z defined in formula (13) of 453 or less. Therefore, the cutoff wavelength λ ｃｃ can be set to 1.26 μm or less. Moreover, the multicore fiber 1 of this embodiment satisfies formulas (22) and (23). Therefore, the multicore fiber 1 can suppress crosstalk while setting the inter-core distance Λ to 40 μm or less.

[0056] Although the present invention has been described above using the embodiments as examples, the present invention is not limited to the above embodiments.

[0057] In the above embodiment, the case where there are four core elements 10 has been described. However, the present invention is not limited to this, and can also be applied to the case where there are two or three core elements 10.

[0058] 6 is a diagram showing an example of a multicore fiber 1 having two core elements 10. In this example, the cores 11 are arranged in two-fold rotational symmetry with respect to the center C. Therefore, in this example, the diameter D (μm) of the cladding 20, the inter-core distance Λ (μm), and the cladding thickness OCT (μm) satisfy the following formula (24): D=Λ+2OCT (24)

[0059] Therefore, in order for the cladding diameter D to be 102 μm or less, the following formula (25) must be satisfied: OCT≦51 μm−Λ / 2 (25)

[0060] Furthermore, in order for the cladding diameter D to be 92 μm or less, the following formula (26) must be satisfied, and in order for the cladding diameter D to be 82 μm or less, the following formula (27) must be satisfied: OCT≦46 μm−Λ / 2 (26) OCT≦41 μm−Λ / 2 (27)

[0061] 7 is a diagram showing an example of a multicore fiber 1 having three core elements 10. In this example, the cores 11 are arranged in a three-fold rotational symmetry with respect to the center C. Therefore, in this example, the diameter D (μm) of the cladding 20, the inter-core distance Λ (μm), and the cladding thickness OCT (μm) satisfy the following formula (28): D=(2 / (3 １／2 ))Λ+2OCT (28)

[0062] Therefore, in order for the cladding diameter D to be 102 μm or less, the following formula (29) must be satisfied: OCT≦51 μm−Λ / (3 １／2 ) (29)

[0063] Furthermore, in order for the cladding diameter D to be 92 μm or less, the following formula (30) must be satisfied, and in order for the cladding diameter D to be 82 μm or less, the following formula (31) must be satisfied: OCT≦46 μm−Λ / (3 １／2 ) (30) OCT≦41μm-Λ / (3 １／2 ) (31)

[0064] The multicore fiber 1 having two core elements 10 shown in FIG. 6 may satisfy the formulas (29) to (31).

[0065] As described above, according to the present invention, a multicore fiber is provided which has a cutoff wavelength of 1.26 μm or less and can suppress crosstalk even when the inter-core distance is 40 μm or less, and can be used in the field of optical communications and other devices using multicore fibers.

Claims

1. A semiconductor device comprising two to four core elements arranged in a rotationally symmetrical manner, each including a core, an intermediate layer tightly surrounding the outer periphery of the core, and a trench layer tightly surrounding the outer periphery of the intermediate layer, and a cladding tightly surrounding the outer periphery of each of the core elements, wherein the radius of the core is r １ and the radius of the outer periphery of the intermediate layer is r ２ The radius of the outer periphery of the trench layer is r ３ and the relative refractive index difference of the refractive index of the core to the refractive index of the cladding is Δ １ and the relative refractive index difference of the refractive index of the intermediate layer to the refractive index of the clad is Δ ２ The relative refractive index difference of the refractive index of the trench layer to the refractive index of the cladding is Δ ３ In this case, 3.5 μm≦r １ ≦4.4μm 8.0μm≦r ２ ≦10.0μm 13.0μm≦r ３ ≦18.0μm 0.33%≦Δ １ ≦0.45% −0.10%≦Δ ２ ≦0.07% −0.70%≦Δ ３ ≦-0.25%, and Z=(r １ ２ πΔ １ ) (r ３ ２ -r ２ ２ )π|Δ ３ | / r ２ When the minimum center-to-center distance between adjacent cores is Λ, the relationship Z≦453 is satisfied, and 40 μm−0.040(r ３ ２ -r ２ ２ )π|Δ ３ |≦Λ≦40 μm, where r ３ <Λ.

2. 43μm-0.037(r ３ ２ -r ２ ２ )π|Δ ３ The multicore fiber according to claim 1, wherein |≦Λ≦40 μm is satisfied.

3. The multi-core fiber according to claim 1 or 2, characterized in that the diameter of the cladding is 120 μm or less.

4. When the distance from the center of the core to the outer periphery of the cladding is OCT, OCT≦51 μm−Λ / (2 １／２ The multicore fiber according to any one of claims 1 to 3, wherein the following condition is satisfied:

5. When the number of the core elements is 2 or 3, and the distance from the center of the core to the outer periphery of the cladding is OCT, OCT≦51 μm−Λ / (3 １／2 The multicore fiber according to any one of claims 1 to 3, wherein the following condition is satisfied:

6. The multicore fiber according to any one of claims 1 to 3, characterized in that the number of core elements is 2, and when the distance from the center of the core to the outer periphery of the cladding is OCT, the following relationship is satisfied: OCT≦51 μm−Λ / 2.

Images