Multicore fiber

Abstract

This multicore fiber (100) comprises: a plurality of center core parts (111, 112); a plurality of low refractive index layers (131, 132) that surround the outer circumference of each of the center core parts; and a cladding part (140) that surrounds the plurality of low refractive index layers. The maximum refractive index of the center core parts is higher than the average refractive index of the cladding part. The minimum refractive index of the low refractive index layers is lower than the average refractive index of the cladding part. Among the plurality of center core parts, the absolute value of the difference in the relative refractive index difference of the maximum refractive index of the center core parts with respect to the average refractive index of the cladding part, and the absolute value of the difference in the center core diameters are 10% or less. In two low refractive index layers among the plurality of low refractive index layers, the absolute value of the difference in the relative refractive index of the minimum refractive index of the low refractive index layers with respect to the average refractive index of the cladding part, or the absolute value of the difference in the ratio of the outer diameters of the low refractive index layers with respect to the center core diameters is 10% or more.

Description

Multicore Fiber

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

[0002] In a multicore fiber having multiple cores, the most common method for identifying the cores in a cross section is to provide a portion called a marker in the cladding. The marker has a refractive index different from that of the cladding and does not contribute to light propagation. When the cross section of the multicore fiber is observed under a microscope, such a marker is observed as a portion with a different contrast from the cladding. However, this method has a problem in that a glass preform for forming the marker must be prepared.

[0003] In response to this, a method has been proposed in which, in a multicore fiber, cores are arranged asymmetrically with respect to the axial center of the cladding, making it possible to identify the cores (Patent Document 1, Non-Patent Document 1). Also, a method has been proposed in which, for the purpose of identifying cores, multiple cores are configured with step-type cores and trench-type cores (Non-Patent Document 2). In the method described in Non-Patent Document 2, the multiple trench-type cores are further configured with cores having center cores with different diameters and refractive indices, thereby enabling the cores to be identified.

[0004] International Publication No. 2022 / 210786

[0005] Y. Sasaki et al., IEICE EXAT Nov. 2020-13 (2020)Y. Amma et al., OFC2015, Th4C.4, (2015)

[0006] However, in the method using an asymmetric arrangement such as those in Patent Document 1 and Non-Patent Document 1, it is difficult to fabricate optical components that are optically coupled to the core portion, such as an optical component called a FIFO. On the other hand, in the method in Non-Patent Document 2, there is a problem in that if there is a large difference between the diameter and refractive index of the center core portion in multiple trench-type core portions, the difference in optical properties of the center core portion will be large, and if there is a small difference between the diameter and refractive index of the center core portion, the discrimination of the core portion will be poor.

[0007] The present invention has been made in view of the above, and an object of the present invention is to provide a multi-core fiber in which the difference in optical properties of the center core portions is small and the center core portions are easy to identify.

[0008] In order to solve the above-described problems and achieve the object, one aspect of the present invention is a multicore fiber comprising a plurality of center core portions, a plurality of low-refractive-index layers surrounding the outer peripheries of the center core portions, and a cladding portion surrounding the plurality of low-refractive-index layers, wherein the maximum refractive index of the center core portions is higher than the average refractive index of the cladding portion, the minimum refractive index of the low-refractive-index layers is lower than the average refractive index of the cladding portion, the absolute value of the difference in relative refractive index difference of the maximum refractive index of the center core portion with respect to the average refractive index of the cladding portion and the absolute value of the difference in center core diameter between the plurality of center core portions are 10% or less, and the absolute value of the difference in relative refractive index difference of the minimum refractive index of the low-refractive-index layers with respect to the average refractive index of the cladding portion, or the absolute value of the difference in the ratio of the outer diameter to the center core diameter, between two of the plurality of low-refractive-index layers, is 10% or more.

[0009] The absolute value of the difference in effective core cross-sectional area between the plurality of center core portions may be 10% or less.

[0010] The plurality of center core portions may include a first center core portion and a second center core portion, the plurality of low refractive index layers may include a first low refractive index layer surrounding the outer periphery of the first center core portion and a second low refractive index layer surrounding the outer periphery of the second center core portion, the ratio of the outer diameter of the second low refractive index layer to the center core diameter of the second center core portion may be 10% or more larger than the ratio of the outer diameter of the first low refractive index layer to the center core diameter of the first center core portion, and the absolute value of the relative refractive index difference of the minimum refractive index of the second low refractive index layer with respect to the average refractive index of the cladding portion may be smaller than the absolute value of the relative refractive index difference of the minimum refractive index of the first low refractive index layer with respect to the average refractive index of the cladding portion.

[0011] The absolute value of the difference in effective core area and the absolute value of the difference in cutoff wavelength between the plurality of center core portions may be 6% or less.

[0012] The cladding portion may be free of any markers that have a refractive index different from that of the cladding portion and that do not contribute to light propagation.

[0013] At least a part of the low refractive index layer may be made of fluorine-containing silica glass.

[0014] The center core portion may be made of silica glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium.

[0015] The center core portion propagates input light in a single mode, and has an effective core area of ​​60 μm at the wavelength of the light. ２ 180 μm or more ２ The following is also acceptable.

[0016] The number of the plurality of center core portions may be 2 or more and 19 or less.

[0017] The plurality of center core portions may be arranged to have rotational symmetry with respect to the central axis of the cladding portion.

[0018] According to the present invention, a multi-core fiber can be realized in which the difference in optical properties of the center core portions is small and the center core portions are easy to identify.

[0019] FIG. 1 is a schematic cross-sectional view of a multi-core fiber according to Embodiment 1. FIG. 2A is a view showing a refractive index profile of the multi-core fiber shown in FIG. 1. FIG. 2B is a view showing a refractive index profile of the multi-core fiber shown in FIG. 1. FIG. 3A is a view showing a microscope photograph of a core portion in the multi-core fiber of Example 1. FIG. 3B is a view showing a microscope photograph of a core portion in the multi-core fiber of Example 1. FIG. 4 is a schematic cross-sectional view of a multi-core fiber according to Embodiment 2. FIG. 5A is a view showing a refractive index profile of the multi-core fiber shown in FIG. 3. FIG. 5B is a view showing a refractive index profile of the multi-core fiber shown in FIG. 3. FIG. 6 is a schematic cross-sectional view of a multi-core fiber according to Embodiment 3. FIG. 7A is a view showing a refractive index profile of the multi-core fiber shown in FIG. 6. FIG. 7B is a view showing a refractive index profile of the multi-core fiber shown in FIG. 6. FIG. 8 is a schematic cross-sectional view of a multi-core fiber according to Embodiment 4. FIG. 9A is a view showing a refractive index profile of the multi-core fiber shown in FIG. 8. FIG. 9B is a view showing a refractive index profile of the multi-core fiber shown in FIG. 8. FIG. 10A is a view showing an example of a refractive index profile. FIG. 10B is a diagram showing an example of a refractive index profile. FIG. 10C is a diagram showing an example of a refractive index profile. FIG. 10D is a diagram showing an example of a refractive index profile. FIG. 11 is a schematic cross-sectional view of a multi-core fiber according to embodiment 5. FIG. 12 is a schematic cross-sectional view of a multi-core fiber according to embodiment 6. FIG. 13 is a schematic cross-sectional view of a multi-core fiber according to embodiment 7. FIG. 14 is a schematic cross-sectional view of a multi-core fiber according to embodiment 8. FIG. 15 is a schematic cross-sectional view of a multi-core fiber according to embodiment 9. FIG. 16 is a schematic cross-sectional view of a multi-core fiber according to embodiment 10. FIG. 17 is a schematic cross-sectional view of a multi-core fiber according to embodiment 11. FIG. 18 is a schematic cross-sectional view of a multi-core fiber according to embodiment 12. FIG. 19 is a schematic cross-sectional view of a multi-core fiber according to embodiment 13. FIG. 20 is a schematic cross-sectional view of a multi-core fiber according to embodiment 14. FIG. 21 is a schematic cross-sectional view of the multi-core fiber of Example 1-1.Fig. 22 is a schematic cross-sectional view of a multi-core fiber of Example 1-2. Fig. 23 is a schematic cross-sectional view of a multi-core fiber of Example 2-1. Fig. 24 is a schematic cross-sectional view of a multi-core fiber of Example 2-2. Fig. 25 is a schematic cross-sectional view of a multi-core fiber of Example 3-1. Fig. 26 is a schematic cross-sectional view of a multi-core fiber of Example 3-2. Fig. 27 is a diagram showing 20 examples of refractive index profiles.

[0020] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments described below. Furthermore, in each drawing, identical or corresponding components are appropriately designated by the same reference numerals, and duplicate explanations are appropriately omitted. Furthermore, in this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength (λcc) defined in ITU-T G.650.1 of the International Telecommunications Union (ITU). Furthermore, other terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0021] 1 is a schematic cross-sectional view of a multicore fiber according to embodiment 1. The multicore fiber 100 is made of silica glass and includes center core portions 111 and 112, intermediate layers 121 and 122, trench layers 131 and 132, and a cladding portion 140. The center core portions 111 and 112 are an example of a plurality of center core portions, and the trench layers 131 and 132 are an example of a plurality of low-refractive-index layers.

[0022] In this specification, the portion consisting of the center core portion, intermediate layer, and trench layer may be referred to as a core portion.

[0023] The center core portions 111 and 112 have a substantially circular cross section and are arranged to have two-fold rotational symmetry with respect to the central axis of the cladding portion 140, which also has a substantially circular cross section. The maximum refractive index of the center core portion 111 is higher than the average refractive index of the cladding portion 140. The average refractive index of the cladding portion 140 is the average refractive index of the cladding portion 140 in the radial direction. The maximum refractive index of the center core portion 112 is higher than the average refractive index of the cladding portion 140. The center core portions 111 and 112 may contain at least one of germanium, fluorine, chlorine, potassium, and sodium.

[0024] The intermediate layer 121 has a substantially circular cross section and surrounds the outer periphery of the center core portion 111. The intermediate layer 122 has a substantially circular cross section and surrounds the outer periphery of the center core portion 112. The average refractive index of the intermediate layer 121 is lower than the maximum refractive index of the center core portion 111 and substantially equal to the average refractive index of the cladding portion 140. The average refractive index of the intermediate layer 122 is lower than the maximum refractive index of the center core portion 112 and substantially equal to the average refractive index of the cladding portion 140. The average refractive index of the intermediate layers 121 and 122 refers to the average refractive index in the radial direction of the intermediate layers 121 and 122. The intermediate layers 121 and 122 may be made of silica glass containing at least a portion of fluorine, or may be made of pure silica glass. Pure silica glass is extremely high-purity silica glass that substantially does not contain dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm.

[0025] The trench layer 131 has a substantially circular cross section and surrounds the outer peripheries of the center core portion 111 and the intermediate layer 121. The trench layer 132 has a substantially circular cross section and surrounds the outer peripheries of the center core portion 112 and the intermediate layer 122. The minimum refractive index of the trench layer 131 is lower than the average refractive index in the radial direction of the cladding portion 140. The minimum refractive index of the trench layer 132 is lower than the average refractive index in the radial direction of the cladding portion 140. The trench layers 131 and 132 may be made of silica glass at least partially containing fluorine.

[0026] The cladding portion 140 surrounds the outer periphery of the trench layers 131 and 132. The cladding portion 140 may be made of silica glass containing fluorine at least in part, or may be made of pure silica glass.

[0027] 2A and 2B are diagrams showing the refractive index profile of the multi-core fiber 100. Fig. 2A shows the refractive index profiles of the center core portion 111, the intermediate layer 121, the trench layer 131, and the cladding portion 140. Specifically, a profile P111 is the refractive index profile of the center core portion 111, a profile P121 is the refractive index profile of the intermediate layer 121, a profile P131 is the refractive index profile of the trench layer 131, and a profile P140 is the refractive index profile of the cladding portion 140. Similarly, in Fig. 2B, a profile P112 is the refractive index profile of the center core portion 112, a profile P122 is the refractive index profile of the intermediate layer 122, a profile P132 is the refractive index profile of the trench layer 132, and a profile P140 is the refractive index profile of the cladding portion 140. As shown in Figs. 2A and 2B, the multi-core fiber 100 has a trench-type refractive index profile.

[0028] 2A and 2B, the center core portion 111 and the center core portion 112 have the same diameter (center core diameter) of 2a. The intermediate layer 121 and the intermediate layer 122 have the same outer diameter of 2b. The trench layer 131 has an outer diameter of 2c, while the trench layer 132 has an outer diameter of 2c', which are different from each other. Here, the difference in the ratio of the outer diameter to the center core diameter, i.e., the absolute value of the difference between (c' / a) and (c / a), is 10% or more of the smaller of (c / a) and (c' / a).

[0029] Furthermore, if the relative refractive index difference of the maximum refractive index of the center core portion 111 with respect to the average refractive index of the cladding portion 140 is Δ1, and the relative refractive index difference of the maximum refractive index of the center core portion 112 with respect to the average refractive index of the cladding portion 140 is Δ1', Δ1 and Δ1' are equal. Furthermore, the relative refractive index difference of the minimum refractive index of the trench layer 131 with respect to the average refractive index of the cladding portion 140 is equal to the relative refractive index difference of the minimum refractive index of the trench layer 132 with respect to the average refractive index of the cladding portion 140. Note that hereinafter, Δ1 and Δ1' may be collectively referred to as the center core Δ. Also, they may be referred to as the relative refractive index difference trench Δ of the minimum refractive index of the trench layer with respect to the average refractive index of the cladding portion.

[0030] In the multicore fiber 100 configured as described above, the absolute value of the difference in the center core Δ and the center core diameter 2a between the center core portion 111 and the center core portion 112 is 10% or less of the center core Δ and the center core diameter 2a, specifically 0%. Furthermore, the absolute value of the difference between (c' / a) and (c / a) between the trench layer 131 and the trench layer 132 is 10% or more of the smaller of (c / a) and (c' / a). As a result, in the multicore fiber 100, the optical characteristics of the center core portion 111 and the center core portion 112 are roughly determined by their respective center core Δ and center core diameter 2a, and therefore there is little difference in their optical characteristics. Furthermore, since the absolute value of the difference between (c' / a) and (c / a) is 10% or more of the smaller value of (c / a) or (c' / a), it is easy to distinguish between the trench layer 131 and the trench layer 132 when observing the cross section of the multi-core fiber 100 with a microscope or the like. As a result, it is also easy to distinguish between the center core portion 111 and the center core portion 112. In particular, in the multi-core fiber 100, it is not necessary to provide a marker in the cladding portion 140 that has a refractive index different from that of the cladding portion 140 and does not contribute to light propagation, and it is also not necessary to arrange the center core portions 111 and 112 asymmetrically so as not to have rotational symmetry with respect to the central axis of the cladding portion 140.

[0031] The multi-core fiber according to the first embodiment can be manufactured using a well-known method for manufacturing a glass preform, such as a VAD (Vapor Phase Axial Deposition) method or an MCVD (Modified Chemical Vapor Deposition) method, a well-known drilling method in manufacturing a multi-core fiber, and a well-known fiber drawing method.

[0032] The present inventors manufactured a multi-core fiber as shown in FIG. 1 as Example 1. In Example 1, the structural parameters of core #1 (corresponding to the core portion including the center core portion 111 in FIG. 1 ) are as follows: the center core Δ is 0.27%, and the trench Δ is −0.3%. The average refractive index of the intermediate layer is the same as the average refractive index of the cladding portion. The center core diameter 2a is 11.0 μm. The ratio (b / a) is 2.5, and the ratio (c / a) is 3.0. The structural parameters of core #2 (corresponding to the core portion including the center core portion 112 in FIG. 1 ) are the same as those of core #1, except that the ratio (c′ / a) is 4.5. In this case, the absolute value of the difference between (c / a) and (c′ / a) is 50% of (c / a).

[0033] 3A and 3B are diagrams showing microscope photographs of the core portion in the multicore fiber of Example 1. Fig. 3A is a photograph of core #1, and Fig. 3B is a photograph of core #2. As can be seen from Fig. 3A and Fig. 3B, it was extremely easy to distinguish between core #1 and core #2.

[0034] The present inventors manufactured multi-core fibers similar to those in Example 1, in which the absolute value of the difference between c / a and c' / a was 0%, 10%, 20%, and 30% of c / a, and observed them under a microscope, and confirmed that the differences of 10%, 20%, and 30% were distinguishable.

[0035] (Embodiment 2) Fig. 4 is a schematic cross-sectional view of a multi-core fiber according to embodiment 2. The multi-core fiber 100A has a configuration in which the trench layer 132 of the multi-core fiber 100 shown in Fig. 1 is replaced with a trench layer 132A.

[0036] 5A and 5B show refractive index profiles of the multicore fiber 100A. Fig. 5A shows the refractive index profile of the center core portion 111, the intermediate layer 121, the trench layer 131, and the cladding portion 140, and is the same as Fig. 2A. In Fig. 5B, a profile P112 is the refractive index profile of the center core portion 112, a profile P122 is the refractive index profile of the intermediate layer 122, a profile P132A is the refractive index profile of the trench layer 132A, and a profile P140 is the refractive index profile of the cladding portion 140.

[0037] In the multicore fiber 100A, the trench layer 131 and the trench layer 132A have the same outer diameter of 2c. On the other hand, if the relative refractive index difference of the minimum refractive index of the trench layer 131 with respect to the average refractive index of the cladding portion 140 is Δ3 and the relative refractive index difference of the minimum refractive index of the trench layer 132A with respect to the average refractive index of the cladding portion 140 is Δ3', Δ3 and Δ3' are different, and the absolute value of the difference between the two is 10% or more of Δ3 or Δ3'.

[0038] In the multi-core fiber 100A configured as described above, the absolute value of the difference in the center core diameter Δ and the center core diameter 2a between the center core portion 111 and the center core portion 112 is 10% or less of the center core diameter Δ and the center core diameter 2a, specifically 0%. Furthermore, the absolute value of the difference between Δ3 and Δ3' between the trench layer 131 and the trench layer 132 is 10% or more of the smaller value of Δ3 or Δ3'. As a result, in the multi-core fiber 100A, similar to the multi-core fiber 100, there is little difference in optical properties between the center core portion 111 and the center core portion 112. Furthermore, since the absolute value of the difference between Δ3 and Δ3' is 10% or more of the smaller value of Δ3 or Δ3', it is easy to distinguish between the trench layer 131 and the trench layer 132A when observing the cross section of the multi-core fiber 100A with a microscope or the like. As a result, it is easy to distinguish between the center core portions 111 and 112, and no markers or asymmetrical arrangement are required. The difference in trench Δ between the trench layer 131 and the trench layer 132A is observed as a difference in contrast.

[0039] The present inventors manufactured multi-core fibers such as the multi-core fiber according to embodiment 2, in which the absolute value of the difference between Δ3 and Δ3′ was 0%, 10%, 20%, and 30% of Δ3, and observed them under a microscope, and confirmed that the differences of 10%, 20%, and 30% were distinguishable.

[0040] (Embodiment 3) Fig. 6 is a schematic cross-sectional view of a multi-core fiber according to embodiment 3. The multi-core fiber 100B has a configuration in which the trench layer 132 of the multi-core fiber 100 shown in Fig. 1 is replaced with a trench layer 132B.

[0041] The center core portion 111 is an example of a first center core portion. The center core portion 112 is an example of a second center core portion. The trench layer 131 is an example of a first low-refractive index layer. The trench layer 132B is an example of a second low-refractive index layer.

[0042] 7A and 7B are diagrams showing the refractive index profile of the multi-core fiber 100B. Fig. 7A shows the refractive index profile of the center core portion 111, the intermediate layer 121, the trench layer 131, and the cladding portion 140, and is the same as Fig. 2A. In Fig. 7B, a profile P112 is the refractive index profile of the center core portion 112, a profile P122 is the refractive index profile of the intermediate layer 122, a profile P132B is the refractive index profile of the trench layer 132B, and a profile P140 is the refractive index profile of the cladding portion 140.

[0043] 7A and 7B , the trench layer 131 has an outer diameter of 2c, while the trench layer 132B has an outer diameter of 2c', which are different from each other; specifically, 2c' is larger than 2c. Here, the difference in the ratio of the outer diameter to the center core diameter, i.e., the absolute value of the difference between (c' / a) and (c / a), is 10% or more of the smaller of (c / a) and (c' / a). (c' / a) is an example of the ratio of the outer diameter of the second low-refractive index layer to the center core diameter of the second center core portion. Furthermore, (c / a) is an example of the ratio of the outer diameter of the first low-refractive index layer to the center core diameter of the first center core portion.

[0044] Furthermore, if the relative refractive index difference of the minimum refractive index of the trench layer 131 with respect to the average refractive index of the cladding portion 140 is Δ3 and the relative refractive index difference of the minimum refractive index of the trench layer 132 with respect to the average refractive index of the cladding portion 140 is Δ3', Δ3 and Δ3' are different, and specifically, the absolute value of Δ3' is smaller than the absolute value of Δ3. Δ3 is an example of the relative refractive index difference of the minimum refractive index of the first low-refractive-index layer with respect to the average refractive index of the cladding portion. Δ3' is also an example of the relative refractive index difference of the minimum refractive index of the second low-refractive-index layer with respect to the average refractive index of the cladding portion.

[0045] In the multi-core fiber 100B configured as described above, the absolute value of the difference in center core Δ and the absolute value of the difference in center core diameter 2 a between the center core portion 111 and the center core portion 112 are 10% or less of the center core Δ and the center core diameter 2 a, specifically 0%. As a result, in the multi-core fiber 100, the optical characteristics of the center core portion 111 and the center core portion 112 are roughly determined by their respective center core Δ and center core diameter 2 a, so there is little difference in their optical characteristics. Examples of optical characteristics with little difference include the effective core area and chromatic dispersion characteristics.

[0046] Furthermore, since the absolute value of the difference between (c' / a) and (c / a) is 10% or more of the smaller value of (c / a) or (c' / a), it is easy to distinguish between the trench layer 131 and the trench layer 132 when observing the cross section of the multi-core fiber 100 with a microscope or the like. As a result, it is also easy to distinguish between the center core portion 111 and the center core portion 112. In particular, in the multi-core fiber 100, it is not necessary to provide a marker in the cladding portion 140 that has a refractive index different from that of the cladding portion 140 and does not contribute to light propagation, and it is also not necessary to arrange the center core portions 111 and 112 asymmetrically so as not to have rotational symmetry with respect to the central axis of the cladding portion 140.

[0047] Generally, a low-refractive-index layer such as a trench layer acts to shift the cutoff wavelength toward longer wavelengths as its thickness or refractive index decreases. In the multi-core fiber 100B, (c' / a) of the trench layer 132B is greater than (c / a) of the trench layer 131, so that in terms of thickness, the trench layer 132B acts to shift the cutoff wavelength of the center core portion 112 toward longer wavelengths. However, in the multi-core fiber 100, the absolute value of Δ3 is greater than the absolute value of Δ3'. Therefore, in terms of refractive index, the trench layer 131 acts to shift the cutoff wavelength of the center core portion 111 toward longer wavelengths. As a result, the effect of the thickness-dependent cutoff wavelength shift and the effect of the refractive index-dependent cutoff wavelength shift are offset, and the difference between the cutoff wavelengths of the center core portions 111 and 112 decreases.

[0048] Since the cutoff wavelength corresponds to the lower limit of the wavelength at which light can propagate in a single mode in an optical fiber, it is preferable for the difference in cutoff wavelength between core portions to be small in terms of management and practical use of multiple core portions. In particular, when managing the wavelength band in use or making changes such as expansion, it is more preferable for the difference in cutoff wavelength between center core portions to be, for example, 6% or less of the smaller cutoff wavelength.

[0049] The behavior of the cutoff wavelength of the two center core portions due to the low refractive index layer will be explained more specifically using the results of simulation calculations. Table 1 shows the optical characteristics of a multicore fiber having two core portions as in embodiment 3, when one core portion is set to the structural parameters No. 1 and the other core portion is set to any one of Nos. 2-1 to 2-7. In the table, "MFD" means the mode field diameter, and the macrobend loss is the macrobend loss when wound with a diameter of 20 mm. In addition, the zero-dispersion wavelength, dispersion slope, MFD, λcc, and macrobend loss in the table are also listed with the values ​​of the ITU-T G.657.A1 standard.

[0050]

[0051] In Table 1, the trench Δ of the core portion No. 1 and the core portion No. 2-1 are equal, but the c / a of the core portion No. 2-1 is 10% or more, specifically 25%, larger than that of the core portion No. 1, so the cutoff wavelengths are 1180 nm and 1253 nm, respectively. The cutoff wavelength of the core portion No. 2-1 is 73 nm (approximately 6.2% of the smaller value of 1180 nm) larger. In contrast, the c / a of the core portion No. 2-2 is 25% larger than that of the core portion No. 1, but the absolute value of the trench Δ of the core portion No. 2-2 is 0.02% smaller than that of the core portion No. 1, so the cutoff wavelengths are 1180 nm and 1240 nm. In this case, the difference in cutoff wavelength between the two is 60 nm, which is smaller than the difference in cutoff wavelength between the core portion of No. 1 and the core portion of No. 2-1. Similarly, between the core portion of No. 1 and the core portion of No. 2-7, the c / a of the core portion of No. 2-7 is 25% larger than that of the core portion of No. 1, but the absolute value of the trench Δ of the core portion of No. 2-7 is 0.12% smaller than that of the core portion of No. 1, so the cutoff wavelengths are 1180 nm and 1183 nm. In this case, the difference in cutoff wavelength between the two is only 3 nm.

[0052] Table 1 also shows that the macrobend loss of the combination of core portion No. 1 with any one of core portions No. 2-2 to No. 2-7 is smaller than the difference between core portion No. 1 and core portion No. 2-1.

[0053] Furthermore, Table 1 shows that there is almost no difference in the zero-dispersion wavelength, dispersion slope, and MFD between core section No. 1 and core sections No. 2-2 to 2-7. Furthermore, it can be seen that the zero-dispersion wavelength, dispersion slope, MFD, λcc, and macrobend loss of core section No. 1 and core sections No. 2-2 to 2-7 all satisfy the ITU-T G.657.A1 standard.

[0054] As described above, in the multi-core fiber 100B according to the third embodiment, the difference in optical properties (including cutoff wavelength and macrobending loss) between the center core portions 111 and 112 is small, and the center core portions 111 and 112 are easy to distinguish.

[0055] 8 is a schematic cross-sectional view of a multicore fiber according to embodiment 4. The multicore fiber 100C has a configuration in which the intermediate layer 121 and the trench layer 131 of the multicore fiber 100B shown in FIG. 6 are replaced with a depressed layer 151, and the intermediate layer 122 and the trench layer 132B are replaced with a depressed layer 152.

[0056] The depressed layer 151 has a substantially circular cross section and surrounds the outer periphery of the center core portion 111. The depressed layer 152 has a substantially circular cross section and surrounds the outer periphery of the center core portion 112. The average refractive index in the radial direction of the depressed layer 151 is lower than the average refractive index in the radial direction of the cladding portion 140. The average refractive index in the radial direction of the depressed layer 152 is lower than the average refractive index in the radial direction of the cladding portion 140. The depressed layers 151 and 152 may be made of silica glass at least partially containing fluorine.

[0057] The depressed layers 151 and 152 are an example of a plurality of low refractive index layers. The depressed layer 151 is an example of a first low refractive index layer included in the plurality of low refractive index layers. The depressed layer 152 is an example of a second low refractive index layer included in the plurality of low refractive index layers.

[0058] In this specification, the center core portion and the portion consisting of the depressed layer may be referred to as a core portion.

[0059] 9A and 9B are diagrams showing the refractive index profile of the multi-core fiber 100C. Fig. 9A shows the refractive index profiles of the center core region 111, the depressed layer 151, and the cladding region 140. Specifically, a profile P151 is the refractive index profile of the depressed layer 151. Similarly, in Fig. 9B, a profile P152 is the refractive index profile of the depressed layer 152. As shown in Figs. 9A and 9B, the multi-core fiber 100C has a W-shaped refractive index profile.

[0060] 9A and 9B , the depressed layer 151 has an outer diameter of 2b, while the depressed layer 152 has an outer diameter of 2b', which are different from each other; specifically, 2b' is larger than 2b. Here, the difference in the ratio of the outer diameter to the center core diameter, i.e., the absolute value of the difference between (b' / a) and (b / a), is 10% or more of the smaller of (b / a) and (b' / a). (b' / a) is an example of the ratio of the outer diameter of the second low-refractive index layer to the center core diameter of the second center core portion. (b / a) is also an example of the ratio of the outer diameter of the first low-refractive index layer to the center core diameter of the first center core portion.

[0061] Furthermore, if the relative refractive index difference of the minimum refractive index of the depressed layer 151 with respect to the average refractive index of the cladding portion 140 is Δ2 and the relative refractive index difference of the minimum refractive index of the depressed layer 152 with respect to the average refractive index of the cladding portion 140 is Δ2', Δ2 and Δ2' are different, and specifically, the absolute value of Δ2' is smaller than the absolute value of Δ2. Δ2 is an example of the absolute value of the relative refractive index difference of the minimum refractive index of the first low refractive index layer with respect to the average refractive index of the cladding portion. Δ2' is also an example of the absolute value of the relative refractive index difference of the minimum refractive index of the second low refractive index layer with respect to the average refractive index of the cladding portion.

[0062] In the following description, the relative refractive index difference between the minimum refractive index of the depressed layer and the average refractive index of the cladding portion, such as Δ2 and Δ2', may be referred to as depressed Δ.

[0063] In the multi-core fiber 100C configured as above, similarly to the multi-core fiber 100B according to the third embodiment, there is little difference in optical properties between the center core portion 111 and the center core portion 112. Examples of optical properties with little difference include the effective core area and chromatic dispersion characteristics.

[0064] Furthermore, since the absolute value of the difference between (b' / a) and (b / a) is 10% or more of the smaller of (b / a) or (b' / a), it is easy to distinguish between the depressed layer 151 and the depressed layer 152 when observing the cross section of the multi-core fiber 100A with a microscope or the like. As a result, it is also easy to distinguish between the center core portion 111 and the center core portion 112. In the multi-core fiber 100A as well, it is not necessary to provide a marker in the cladding portion 140, and it is not necessary to arrange the center core portions 111 and 112 asymmetrically.

[0065] Also, in the multi-core fiber 100C, the depressed layer 151 and the depressed layer 152 cancel out the effect of the shift in cutoff wavelength due to the thickness and the effect of the shift in cutoff wavelength due to the refractive index, and the difference between the cutoff wavelength of the center core portion 111 and the cutoff wavelength of the center core portion 112 becomes smaller.

[0066] As described above, also in the multi-core fiber 100C according to the fourth embodiment, the difference in optical properties (including cutoff wavelength and macrobending loss) between the center core portions 111 and 112 is small, and the center core portions 111 and 112 are easy to distinguish.

[0067] In Figures 2A, 2B, 5A, 5B, 7A, 7B, 9A, and 9B, the refractive index changes stepwise at each boundary between the center core portion, the intermediate layer and trench layer or depressed portion, and the cladding portion, but in reality the refractive index may change gradually.

[0068] 10A, 10B, 10C, and 10D are diagrams showing examples of refractive index profiles in which the refractive index changes gradually at the boundary. Note that FIGS. 10A and 10B show examples of trench-type refractive index profiles as in embodiments 1 and 2. Also, FIGS. 10C and 10D show examples of W-type refractive index profiles. When the refractive index profile is W-type, there is no intermediate layer, and there is a depressed layer adjacent to the center core portion and surrounding the outer periphery of the center core portion. This depressed layer corresponds to a low refractive index layer.

[0069] 11 is a schematic cross-sectional view of a multi-core fiber according to embodiment 5. The multi-core fiber 100D is made of silica glass, and includes center core portions 111, 112, 113, and 114, intermediate layers 121, 122, 123, and 124, trench layers 131, 132, 133, and 134, and a cladding portion 140.

[0070] The center core portions 111 to 114 have a substantially circular cross section and are arranged in a square lattice pattern. The center core portions 111 to 114 are arranged to have four-fold rotational symmetry with respect to the central axis of the cladding portion 140, which has a substantially circular cross section. The intermediate layers 121 to 124 have a substantially circular cross section and surround the outer peripheries of the center core portions 111 to 114, respectively. The trench layers 131 to 134 have a substantially circular cross section and surround the outer peripheries of the center core portion 111 and intermediate layer 121, the center core portion 112 and intermediate layer 122, the center core portion 113 and intermediate layer 123, and the center core portion 114 and intermediate layer 124, respectively.

[0071] The center core portions 111, 113, and 114, the intermediate layers 121, 123, and 124, and the trench layers 131, 133, and 134 have the same refractive index profile as the center core portion 111, the intermediate layer 121, and the trench layer 131 of the multicore fiber 100 according to embodiment 1. Therefore, the center core diameters and center core Δ of the center core portions 111, 113, and 114 are equal, and the trench Δ and b and c of the trench layers 131, 133, and 134 are also equal. Furthermore, the center core portion 112, the intermediate layer 122, and the trench layer 132 have the same refractive index profile as the center core portion 112, the intermediate layer 122, and the trench layer 132 of the multicore fiber 100 according to embodiment 1.

[0072] In the multi-core fiber 100D configured as described above, the absolute value of the difference in the center core diameter Δ and the center core diameter 2a between the center core portions 111, 113, and 114 and the center core portion 112 is 10% or less of the center core diameter Δ and the center core diameter 2a, specifically 0%. Furthermore, the absolute value of the difference in the ratio of the outer diameter to the center core diameter between the trench layers 131, 133, and 134 and the trench layer 132 is 10% or more of the smaller of (c / a) and (c' / a). Thus, in the multi-core fiber 100D, similar to the other embodiments, the difference in optical properties between the center core portions 111, 112, 113, and 114 is small. Furthermore, when a cross section of the multi-core fiber 100D is observed with a microscope or the like, the trench layers 131, 133, and 134 and the trench layer 132 are easily distinguishable. As a result, it is easy to distinguish between the center core portions 111, 113, and 114 and the center core portion 112, and markers or asymmetric arrangement are not required. That is, for example, any one of the trench layers 131, 133, and 134 and the trench layer 132 are an example of two low refractive index layers in which the absolute value of the difference in the ratio of the outer diameter to the center core diameter is 10% or more.

[0073] 12 is a schematic cross-sectional view of a multi-core fiber according to embodiment 6. The multi-core fiber 100E has a configuration in which the trench layers 133 and 134 of the multi-core fiber 100D shown in Fig. 11 are replaced with trench layers 133E and 134E, respectively.

[0074] The outer diameter of the trench layer 132 and the outer diameter of the trench layer 133E are different from each other, and the absolute value of the difference in the ratio of the outer diameter to the center core diameter is 10% or more of either ratio. Furthermore, the outer diameter of the trench layer 133E and the outer diameter of the trench layer 134E are different from each other, and the absolute value of the difference in the ratio of the outer diameter to the center core diameter is 10% or more of the smaller of either ratio. However, the trenches Δ of the trench layers 131, 132, 133E, and 134E are equal.

[0075] In the multi-core fiber 100E configured as described above, similarly to the other embodiments, there is little difference in optical properties between the center core portions 111 to 114. Furthermore, when the cross section of the multi-core fiber 100E is observed with a microscope or the like, the trench layers 131, 132, 133E, and 134E are easily distinguishable. As a result, the center core portions 111 to 114 are easily distinguishable, and markers or asymmetric arrangements are not required.

[0076] 13 is a schematic cross-sectional view of a multi-core fiber according to embodiment 7. The multi-core fiber 100F has a configuration in which the trench layer 132 of the multi-core fiber 100D shown in Fig. 11 is replaced with the trench layer 132A of the multi-core fiber 100A shown in Fig. 2 .

[0077] In the multi-core fiber 100F configured as described above, similarly to the other embodiments, there is little difference in optical properties between the center core portions 111 to 114. Furthermore, when the cross section of the multi-core fiber 100F is observed with a microscope or the like, it is easy to distinguish between the trench layers 131, 133, 134 and the trench layer 132A. As a result, it is also easy to distinguish between the center core portions 111, 113, 114 and the center core portion 112, and markers or asymmetric arrangement are not required.

[0078] (Embodiment 8) Fig. 14 is a schematic cross-sectional view of a multi-core fiber according to embodiment 8. The multi-core fiber 100G has a configuration in which the trench layers 133 and 134 of the multi-core fiber 100F shown in Fig. 13 are replaced with trench layers 133G and 134G, respectively.

[0079] The trench Δ of trench layer 132A and the trench Δ of trench layer 133G are different from each other, and the absolute value of the difference between them is 10% or more of the smaller trench Δ. Furthermore, the trench Δ of trench layer 133G and the trench Δ of trench layer 134G are different from each other, and the absolute value of the difference between them is 10% or more of the smaller trench Δ. Furthermore, the trench Δ of trench layer 134G and the trench Δ of trench layer 131 are different from each other, and the absolute value of the difference between them is 10% or more of the smaller trench Δ. However, the outer diameters of trench layers 131, 132A, 133G, and 134G are the same.

[0080] In the multi-core fiber 100G configured as described above, similarly to the other embodiments, there is little difference in the optical properties of the center core portions 111 to 114. Furthermore, when the cross section of the multi-core fiber 100G is observed with a microscope or the like, the trench layers 131, 132A, 133G, and 134G are easily distinguishable. As a result, the center core portions 111 to 114 are easily distinguishable, and markers or asymmetric arrangements are not required.

[0081] 15 is a schematic cross-sectional view of a multi-core fiber according to embodiment 9. The multi-core fiber 100H has a configuration in which the trench layer 133 of the multi-core fiber 100D shown in FIG. 11 is replaced with a trench layer 133H.

[0082] The outer diameter of the trench layer 132 is equal to the outer diameter of the trench layer 133H. Therefore, the outer diameter of the trench layer 133H is different from the outer diameters of the trench layers 131 and 134, and the absolute value of the difference in the ratio of the outer diameter to the center core diameter is 10% or more of the smaller ratio. However, the trenches Δ of the trench layers 131, 132, 133H, and 134 are equal to each other.

[0083] In the multi-core fiber 100H configured as described above, similarly to the other embodiments, there is little difference in optical properties between the center core portions 111 to 114. Furthermore, when the cross section of the multi-core fiber 100H is observed with a microscope or the like, the trench layers 131, 132, 133H, and 134 are easily distinguishable. As a result, the center core portions 111 to 114 are easily distinguishable, and markers or asymmetric arrangements are not required.

[0084] (Embodiment 10) Fig. 16 is a schematic cross-sectional view of a multi-core fiber according to embodiment 10. The multi-core fiber 100I has a configuration in which the trench layer 133 of the multi-core fiber 100F shown in Fig. 13 is replaced with a trench layer 133I.

[0085] The trench Δ of trench layer 132A is equal to the trench Δ of trench layer 133I. Therefore, the trench Δ of trench layer 133I is different from the trench Δ of trench layers 131 and 134, and the absolute value of the difference between the two is 10% or more of the smaller trench Δ. However, the outer diameters of trench layers 131, 132A, 133I, and 134 are equal.

[0086] In the multi-core fiber 100I configured as described above, similarly to the other embodiments, there is little difference in optical properties between the center core portions 111 to 114. Furthermore, when the cross section of the multi-core fiber 100G is observed with a microscope or the like, the trench layers 131, 132A, 133I, and 134 are easily distinguishable. As a result, the center core portions 111 to 114 are easily distinguishable, and markers or asymmetric arrangements are not required.

[0087] 17 is a schematic cross-sectional view of a multi-core fiber according to embodiment 11. The multi-core fiber 100J has a configuration in which the trench layer 132 of the multi-core fiber 100D shown in Fig. 11 is replaced with the trench layer 132B of the multi-core fiber 100B shown in Fig. 6 .

[0088] The center core portions 111, 113, and 114 are examples of first center core portions, and the center core portion 112 is an example of a second center core portion. The trench layers 131, 133, and 134 are examples of first low-refractive index layers, and the trench layer 132B is an example of a second low-refractive index layer.

[0089] In the multi-core fiber 100J configured as described above, similarly to the other embodiments, there is little difference in the optical properties (including cutoff wavelength and macrobending loss) of the center core portions 111, 112, 113, and 114. Furthermore, when the cross section of the multi-core fiber 100J is observed with a microscope or the like, it is easy to distinguish between the trench layers 131, 133, and 134 and the trench layer 132B. As a result, it is also easy to distinguish between the center core portions 111, 113, and 114 and the center core portion 112, and markers or asymmetric arrangement are not required.

[0090] (Embodiment 12) Fig. 18 is a schematic cross-sectional view of a multi-core fiber according to embodiment 12. The multi-core fiber 100K has a configuration in which the trench layers 133 and 134 of the multi-core fiber 100J shown in Fig. 17 are replaced with trench layers 133K and 134K, respectively.

[0091] The outer diameter of the trench layer 133K is larger than the outer diameter of the trench layer 132B, and the absolute value of the difference in the ratio of the outer diameter to the center core diameter is 10% or more of the smaller of either ratio. Furthermore, the outer diameter of the trench layer 134K is larger than the outer diameter of the trench layer 133K, and the absolute value of the difference in the ratio of the outer diameter to the center core diameter is 10% or more of the smaller of either ratio. Furthermore, the minimum refractive index of the trench layer 133K is larger than the minimum refractive index of the trench layer 132B, and the absolute value of the trench Δ is smaller. Furthermore, the minimum refractive index of the trench layer 134K is larger than the minimum refractive index of the trench layer 133K, and the absolute value of the trench Δ is smaller.

[0092] If trench layer 131 is defined as a first low-refractive index layer, trench layers 132B, 133K, and 134K correspond to a second low-refractive index layer. If trench layer 132B is defined as a first low-refractive index layer, trench layers 133K and 134K correspond to a second low-refractive index layer. If trench layer 133K is defined as a first low-refractive index layer, trench layer 134K corresponds to a second low-refractive index layer.

[0093] In the multi-core fiber 100K configured as described above, similarly to the other embodiments, there is little difference in optical properties (including cutoff wavelength and macrobending loss) between the center core portions 111 to 114. Furthermore, when the cross section of the multi-core fiber 100K is observed with a microscope or the like, the trench layers 131, 132B, 133K, and 134K are easily distinguishable. As a result, the center core portions 111 to 114 are easily distinguishable, and markers or asymmetric arrangements are not required.

[0094] 19 is a schematic cross-sectional view of a multi-core fiber according to embodiment 13. The multi-core fiber 100L is made of silica glass, and includes center core portions 111, 112, 113, 114, 115, 116, and 117, intermediate layers 121, 122, 123, 124, 125, 126, and 127, trench layers 131, 132, 133, 134, 135, 136, and 137, and a cladding portion 140.

[0095] The center core portions 111 to 117 have a substantially circular cross section and are arranged in a triangular lattice pattern. The center core portions 111 to 117 are arranged to have six-fold rotational symmetry with respect to the central axis of the cladding portion 140, which has a substantially circular cross section. The intermediate layers 121 to 127 have a substantially circular cross section and surround the outer peripheries of the center core portions 111 to 117, respectively. The trench layers 131 to 137 have a substantially circular cross section and surround the outer peripheries of the center core portion 111 and intermediate layer 121, the center core portion 112 and intermediate layer 122, the center core portion 113 and intermediate layer 123, the center core portion 114 and intermediate layer 124, the center core portion 115 and intermediate layer 125, the center core portion 116 and intermediate layer 126, and the center core portion 117 and intermediate layer 127, respectively.

[0096] The center core portions 111, 113 to 117, the intermediate layers 121, 123 to 127, and the trench layers 131, 133 to 137 have the same refractive index profile as the center core portion 111, the intermediate layer 121, and the trench layer 131 of the multicore fiber 100 according to embodiment 1. Therefore, the center core diameters and center core Δ of the center core portions 111, 113 to 117 are equal, and the trench Δ of the trench layers 131, 133 to 137 are also equal. In addition, the center core portion 112, the intermediate layer 122, and the trench layer 132 have the same refractive index profile as the center core portion 112, the intermediate layer 122, and the trench layer 132 of the multicore fiber 100 according to embodiment 1.

[0097] In the multi-core fiber 100L configured as described above, the absolute value of the difference in the center core diameter Δ and the center core diameter 2a between the center core portions 111, 113 to 117 and the center core portion 112 is 10% or less of the center core diameter Δ and the center core diameter 2a, specifically 0%. Furthermore, the absolute value of the difference in the ratio of the outer diameter to the center core diameter between the trench layers 131, 133 to 137 and the trench layer 132 is 10% or more of the smaller trench diameter Δ. As a result, in the multi-core fiber 100L, similar to the other embodiments, there is little difference in the optical properties of the center core portions 111 to 117. Furthermore, when the cross section of the multi-core fiber 100L is observed with a microscope or the like, it is easy to distinguish between the trench layers 131, 133 to 137 and the trench layer 132. As a result, it is easy to distinguish between the center core portions 111, 113 to 117 and the center core portion 112, and markers or asymmetric arrangement are not required.

[0098] (Embodiment 14) Fig. 20 is a schematic cross-sectional view of a multi-core fiber according to embodiment 14. The multi-core fiber 100M has a configuration in which the trench layer 132 of the multi-core fiber 100L shown in Fig. 19 is replaced with the trench layer 132A of the multi-core fiber 100A shown in Fig. 2.

[0099] In the multi-core fiber 100L configured as described above, similarly to the other embodiments, there is little difference in optical properties between the center core portions 111 to 117. Furthermore, when the cross section of the multi-core fiber 100D is observed with a microscope or the like, it is easy to distinguish between the trench layers 131, 133 to 137 and the trench layer 132A. As a result, it is also easy to distinguish between the center core portions 111, 113 to 117 and the center core portion 112, and markers or asymmetric arrangement are not required.

[0100] In any of the multicore fibers according to the above embodiments, the center core preferably propagates input light in a single mode. Here, the input wavelength is a wavelength used as signal light in optical fiber communications, for example, light in the 1.55 μm band. The larger the ratio of the trench Δ of the trench layer and the outer diameter to the center core diameter, the higher the discrimination ability. However, care must be taken because if these ratios are too large, the propagation state is likely to become multimode. Specifically, the trench Δ may be configured to be −0.8% or more, and the ratio of the outer diameter to the center core diameter of the trench layer to be 6 or less.

[0101] In addition, from the viewpoint of compatibility with many conventional optical fibers, the center core portion propagates the input light in a single mode, and the effective core area (Aeff) at the wavelength of the input light is set to 60 μm. ２ 180 μm or more ２ Furthermore, the present inventors have conducted systematic investigations using W-shaped and trench-shaped refractive index profiles and have found that if the absolute value of the difference in the center core Δ and center core diameter between multiple center core portions is set to 10% or less of the center core Δ and center core diameter, the absolute value of the difference in the effective core cross-sectional area can be set to the smaller of 10% or less, or even 6% or less, even if the relative refractive index difference or outer diameter of the low refractive index layer is changed.

[0102] Furthermore, like the multicore fiber of Embodiment 1, any of the multicore fibers of Embodiments 2 to 14 can be manufactured using a well-known method for manufacturing a glass preform, a well-known drilling method for manufacturing a multicore fiber, and a well-known fiber drawing method.

[0103] Example 1-1 As Example 1-1, a multi-core fiber as shown in FIG. 21 was manufactured. The structural parameters and optical characteristics of Example 1-1 are shown in Table 2-1. Example 1-1 is a two-core multi-core fiber like that of Embodiment 1. In core #1 and core #2, the center core Δ, center core diameter 2a, and trench Δ were equal, and the ratio c / a of the outer diameter to the center core diameter was set to a value that differed from either ratio by 10% or more. Note that the cladding ratio for the center core Δ in the table is the relative refractive index difference of the maximum refractive index of the center core portion with respect to the average refractive index of the cladding portion, and the pure silica ratio is the relative refractive index difference of the maximum refractive index of the center core portion with respect to the refractive index of pure silica glass. Similarly, the cladding ratio for the trench Δ is the relative refractive index difference of the minimum refractive index of the trench layer with respect to the average refractive index of the cladding portion, and the pure silica ratio is the relative refractive index difference of the minimum refractive index of the trench layer with respect to the refractive index of pure silica glass. Furthermore, the cladding Δ is the relative refractive index difference of the average refractive index of the cladding portion with respect to the refractive index of pure silica glass. In addition, the bending loss in the table is the bending loss when the multicore fiber is wound with a diameter of 30 mm. As shown in Table 2-1, the optical properties such as the effective core area, dispersion, dispersion slope, and transmission loss of core #1 and core #2 were approximately the same. Furthermore, it was confirmed that the cutoff wavelength was 1530 nm or less, and that light with a wavelength in the 1.55 μm band could be propagated in single mode.

[0104]

[0105] Furthermore, when an operator observed the end face of the multi-core fiber of Example 1-1 under a microscope and conducted a number of tests to identify the core portion, the accuracy rate was 99.9%, confirming that the identification was possible with almost no problems.

[0106] (Example 1-2) As Example 1-2, a multicore fiber as shown in FIG. 22 was manufactured. The structural parameters and optical characteristics of Example 1-2 are shown in Table 2-2. Example 1-2 is a two-core multicore fiber like those of Embodiments 3 to 8, and in cores #1 to #4, the center core Δ and the center core diameter 2a were made equal, and the ratio c / a of the outer diameter to the center core diameter and / or the trench Δ was set to a value that differed from either ratio or the trench Δ by 10% or more. As shown in Table 2-2, the optical characteristics such as the effective core area, dispersion, dispersion slope, and transmission loss were approximately the same for cores #1 to #4. Furthermore, it was confirmed that the cutoff wavelength was 1530 nm or less, and that light with a wavelength in the 1.55 μm band could be propagated in single mode.

[0107]

[0108] Furthermore, when an operator observed the end face of the multi-core fiber of Example 1-2 under a microscope and performed a number of tests to identify the core portion, the accuracy rate was 99.4%, confirming that the identification was possible with almost no problems.

[0109] Example 2-1 As Example 2-1, a multicore fiber as shown in FIG. 23 was manufactured. The structural parameters and optical characteristics of Example 2-1 are shown in Table 3-1. Example 2-1 is a two-core multicore fiber, but the refractive index profile is W-shaped, and the low refractive index layer is a depressed layer. In core #1 and core #2, the center core Δ, center core diameter 2a, and depressed Δ are equal, and the ratio b / a of the outer diameter of the depressed layer to the center core diameter is set to a value that differs from either ratio by 10% or more. In the table, the cladding ratio in the depressed Δ is the relative refractive index difference of the minimum refractive index of the depressed layer with respect to the average refractive index of the cladding portion, and the pure silica ratio is the relative refractive index difference of the minimum refractive index of the depressed layer with respect to the refractive index of pure silica glass. As shown in Table 3-1, the optical characteristics such as the effective core area, dispersion, dispersion slope, and transmission loss were approximately identical in core #1 and core #2. It was also confirmed that the cutoff wavelength was 1530 nm or less and that light in the 1.55 μm wavelength band could be propagated in single mode.

[0110]

[0111] Furthermore, when an operator observed the end face of the multi-core fiber of Example 2-1 under a microscope and performed a number of tests to identify the core portion, the accuracy rate was 99.9%, confirming that the identification was possible with almost no problems.

[0112] (Example 2-2) As Example 2-2, a multicore fiber as shown in FIG. 24 was manufactured. The structural parameters and optical characteristics of Example 2-1 are shown in Table 3-2. Example 1-2 is a four-core multicore fiber, but like Example 2-1, the refractive index profile was W-shaped and the low refractive index layer was a depressed layer. For cores #1 to #4, the center core Δ and the center core diameter 2a were made equal, and the ratio b / a of the outer diameter of the depressed layer to the center core diameter and / or the depression Δ were set to a value that differed from either ratio or the depression Δ by 10% or more. As shown in Table 3-2, for cores #1 to #4, the optical characteristics such as the effective core area, dispersion, dispersion slope, and transmission loss were substantially the same. Furthermore, it was confirmed that the cutoff wavelength was 1530 nm or less and that light with a wavelength in the 1.55 μm band could be propagated in single mode.

[0113]

[0114] Furthermore, when an operator observed the end face of the multi-core fiber of Example 2-2 under a microscope and conducted a number of tests to identify the core portion, the accuracy rate was 99.2%, confirming that the identification was possible with almost no problems.

[0115] (Example 3-1) As Example 3-1, a multicore fiber as shown in Fig. 25 was manufactured. The structural parameters and optical characteristics of Example 3-1 are shown in Table 4-1. Example 3-1 is a two-core type multicore fiber, and the center core Δ, center core diameter 2a, and trench Δ are equal between core #1 and core #2, and the ratio c / a of the outer diameter to the center core diameter is set to a value that differs from either ratio by 10% or more. However, when the effective core area is about 80 µm ２These structural parameters were set so that: As shown in Table 4-1, core #1 and core #2 had approximately the same optical properties, such as effective core area, dispersion, dispersion slope, and transmission loss. It was also confirmed that the cutoff wavelength was 1530 nm or less, and that light in the 1.55 μm band could be propagated in single mode.

[0116]

[0117] Furthermore, when an operator observed the end face of the multi-core fiber of Example 3-1 under a microscope and performed a number of tests to identify the core portion, the accuracy rate was 99.8%, confirming that the identification was possible with almost no problems.

[0118] (Example 3-2) As Example 3-2, a multicore fiber as shown in FIG. 26 was manufactured. The structural parameters and optical characteristics of Example 3-2 are shown in Table 4-2. Example 3-2 is a four-core multicore fiber, and in cores #1 to #4, the center core Δ and the center core diameter 2a are equal, and the ratio c / a of the outer diameter to the center core diameter and / or the trench Δ are set to a value that differs from either ratio or the trench Δ by 10% or more. However, when the effective core area is about 80 μm ２ These structural parameters were set so that: As shown in Table 4-2, cores #1 to #4 had approximately the same optical properties, such as effective core area, dispersion, dispersion slope, and transmission loss. It was also confirmed that the cutoff wavelength was 1530 nm or less, and that light in the 1.55 μm band could be propagated in single mode.

[0119]

[0120] Furthermore, when an operator observed the end face of the multi-core fiber of Example 3-2 under a microscope and conducted a number of tests to identify the core portion, the accuracy rate was 99.5%, confirming that the identification was possible with almost no problems.

[0121] (Example 4) A multi-core fiber of Example 4 was manufactured. The structural parameters and optical characteristics of Example 4 are shown in Table 5. Example 4 is a trench-type, two-core multi-core fiber like that of Embodiment 3. The center core Δ and center core diameter 2a of core #1 and core #2 were set equal. In addition, the effective core area was 125 μm ２ The center core portion was designed so that the ratio c / a of the outer diameter to the center core diameter was set to be 10% or more larger for core #2 than for core #1. The absolute value of the trench Δ was set to be smaller for core #2 than for core #1. Note that the macrobending loss in the table is the macrobending loss when the multicore fiber is wound with a diameter of 30 mm. As shown in Table 5, core #1 and core #2 had substantially the same optical properties, such as the effective core area, dispersion, dispersion slope, transmission loss, cutoff wavelength, and macrobending loss. It was also confirmed that the cutoff wavelength was 1530 nm or less, and that light in the 1.55 μm band could be propagated in single mode.

[0122]

[0123] Furthermore, when an operator observed the end face of the multi-core fiber of Example 4 under a microscope and performed a number of tests to identify the core portion, the accuracy rate was 99.7%, confirming that the identification was possible with almost no problems.

[0124] (Example 5) A multicore fiber of Example 5 was manufactured. The structural parameters and optical characteristics of Example 5 are shown in Table 6. Example 2 is a W-type, two-core multicore fiber like that of Embodiment 4. The center core Δ and center core diameter 2a of core #1 and core #2 were set equal. In addition, the effective core area was 110 μm ２The center core section was designed so that the ratio b / a of the outer diameter of the depressed layer to the center core diameter was set to be 10% or more larger for core #2 than for core #1. In the table, the cladding ratio at the depressed Δ is the relative refractive index difference of the minimum refractive index of the depressed layer with respect to the average refractive index of the cladding section, and the pure silica ratio is the relative refractive index difference of the minimum refractive index of the depressed layer with respect to the refractive index of pure silica glass. As shown in Table 6, core #1 and core #2 had substantially the same optical properties, such as effective core area, dispersion, dispersion slope, transmission loss, and cutoff wavelength, and the difference in macrobending loss was also small. It was also confirmed that the cutoff wavelength was 1530 nm or less, and light with a wavelength in the 1.55 μm band could be propagated in single mode.

[0125]

[0126] Furthermore, when an operator observed the end face of the multi-core fiber of Example 5 under a microscope and performed a number of tests to identify the core portion, the accuracy rate was 99.2%, confirming that the identification was possible with almost no problems.

[0127] (Example 6) A multicore fiber of Example 6 was manufactured. The structural parameters and optical characteristics of Example 6 are shown in Table 7. Example 6 is a trench-type, two-core multicore fiber. The center core Δ and center core diameter 2a of core #1 and core #2 were set equal. In addition, the effective core area was 80 μm ２ The center core portion was designed so that the ratio c / a of the outer diameter to the center core diameter was set to be 10% or more larger for core #2 than for core #1. As shown in Table 7, core #1 and core #2 had substantially the same optical properties, such as effective core area, dispersion, dispersion slope, transmission loss, cutoff wavelength, and macrobending loss. It was also confirmed that the cutoff wavelength was 1530 nm or less, and that light in the 1.55 μm band could be propagated in single mode.

[0128]

[0129] Furthermore, when an operator observed the end face of the multi-core fiber of Example 6 under a microscope and performed a number of tests to identify the core portion, the accuracy rate was 99.1%, confirming that the identification was possible with almost no problems.

[0130] In the above embodiment, the center core Δ and center core diameter are equal among the multiple center core portions, but if the absolute value of the difference in center core Δ and the absolute value of the difference in center core diameter are 10% or less of the smaller of the values ​​of either center core Δ or center core diameter, the optical properties such as the effective core cross-sectional area, dispersion, dispersion slope, and transmission loss will be approximately equal, and for example, the absolute value of the difference will be 10% or less of the smaller value, which is preferable.

[0131] In the above embodiments, the number of center core portions is 2, 4, or 7, but is not particularly limited thereto. For example, the number of center core portions is 2 or more and 19 or less. Furthermore, in the above embodiments, the center core portions are arranged so as to have 2-fold, 4-fold, or 6-fold rotational symmetry with respect to the central axis of the cladding portion, but this is not particularly limited thereto, and they may be arranged so as to have n-fold rotational symmetry, where n is any integer equal to or greater than 2.

[0132] Furthermore, when the refractive index profile is trench-type, the center core Δ may be 0.12% or more and 0.75% or less. The relative refractive index difference Δ2 of the average refractive index of the intermediate layer with respect to the average refractive index of the cladding portion may be -0.25% or more and 0.25% or less. The trench Δ may be -1.5% or more and -0.05% or less. The ratio b / a of the outer diameter 2b of the intermediate layer to the center core diameter 2a may be 1.2 or more and 4.5 or less. The ratio c / a of the outer diameter 2c of the trench layer to the center core diameter 2a may be 1.5 or more and 8.0 or less. The center core diameter 2a may be 6.0 μm or more and 27.0 μm or less.

[0133] Furthermore, when the refractive index profile is W-shaped, the center core Δ may be 0.11% or more and 0.82% or less. The depression Δ may be -1.2% or more and -0.02% or less. The ratio b / a of the outer diameter of the depressed layer to the center core diameter may be 1.3 or more and 7.5 or less. The center core diameter 2a may be 7.0 μm or more and 35.0 μm or less.

[0134] Furthermore, as long as the core portion can be identified, the low refractive index layers such as trench layers and depressed layers may have a substantially two-layer structure, a three-layer structure, or even a multi-layer structure. Fig. 27 is a diagram showing 20 examples of refractive index profiles in a multi-core fiber according to the present invention. Note that the "periphery" refers to the portion surrounding the center core and including intermediate layers and low refractive index layers.

[0135] Furthermore, the present invention is not limited to the above-described embodiments. The present invention also includes configurations in which the above-described components are appropriately combined. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible.

[0136] The present invention can be used in a multi-core fiber.

[0137] 100, 100A, 100B, 100C, 100D, 100E, 100F, 100G, 100H, 100I, 100J, 100K, 100L, 100M: multi-core fiber 111, 112, 113, 114, 115, 116, 117: center core portion 121, 122, 123, 124, 125, 126, 127: intermediate layer 131, 132, 132A, 132B, 133, 133E, 133G, 133H, 133I, 133K, 134, 134E, 134G, 134K, 135, 136, 137: trench layer 140: cladding portion 151, 152: depressed layer P111, P112, P121, P122, P131, P132, P132A, P132B, P140, P151, P152: Profile

Claims

1. A multicore fiber comprising: a plurality of center core portions; a plurality of low refractive index layers surrounding the outer periphery of each of the center core portions; and a cladding portion surrounding the plurality of low refractive index layers, wherein the maximum refractive index of the center core portions is higher than the average refractive index of the cladding portion, the minimum refractive index of the low refractive index layers is lower than the average refractive index of the cladding portion, the absolute value of the difference in relative refractive index difference of the maximum refractive index of the center core portions from the average refractive index of the cladding portion and the absolute value of the difference in center core diameters between the plurality of center core portions are 10% or less, and the absolute value of the difference in relative refractive index difference of the minimum refractive index of the low refractive index layers from the average refractive index of the cladding portion or the absolute value of the difference in the ratio of the outer diameter to the center core diameter between two low refractive index layers among the plurality of low refractive index layers is 10% or more.

2. The multicore fiber according to claim 1, wherein the absolute value of the difference in effective core area between the plurality of center core portions is 10% or less.

3. The multi-core fiber according to claim 1, wherein the plurality of center core portions include a first center core portion and a second center core portion, the plurality of low refractive index layers include a first low refractive index layer surrounding an outer periphery of the first center core portion and a second low refractive index layer surrounding an outer periphery of the second center core portion, a ratio of an outer diameter of the second low refractive index layer to a center core diameter of the second center core portion is 10% or more larger than a ratio of an outer diameter of the first low refractive index layer to a center core diameter of the first center core portion, and an absolute value of a relative refractive index difference of a minimum refractive index of the second low refractive index layer with respect to an average refractive index of the cladding portion is smaller than an absolute value of a relative refractive index difference of a minimum refractive index of the first low refractive index layer with respect to the average refractive index of the cladding portion.

4. The multicore fiber according to claim 3, wherein the absolute value of the difference in effective core area and the absolute value of the difference in cutoff wavelength between the plurality of center core portions are 6% or less.

5. The multicore fiber according to claim 1, wherein the cladding portion does not include any marker that has a refractive index different from that of the cladding portion and does not contribute to optical propagation.

6. The multi-core fiber according to claim 1, wherein at least a portion of the low refractive index layer is made of fluorine-containing silica glass.

7. The multi-core fiber according to claim 1, wherein the center core portion is made of silica glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium.

8. The center core portion propagates input light in a single mode, and has an effective core area of ​​60 μm at the wavelength of the light. ２ 180 μm or more ２ The multicore fiber according to claim 1 , wherein:

9. The multicore fiber according to claim 1, wherein the number of the plurality of center core portions is 2 or more and 19 or less.

10. The multi-core fiber according to claim 1, wherein the plurality of center core portions are arranged so as to have rotational symmetry with respect to the central axis of the cladding portion.

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