Multicore fiber preform manufacturing method and multicore fiber manufacturing method

Abstract

Provided is a method for manufacturing a multicore fiber preform comprising a plurality of core parts and a cladding part which surrounds the outer peripheries of the plurality of core parts and which has a refractive index lower than the maximum refractive index of the plurality of core parts. The method comprises: a first step in which a plurality of core preforms, which respectively include the plurality of core parts, are respectively inserted into a plurality of holes provided in the longitudinal direction in a central cladding preform, the central cladding preform being a rod-shaped preform, having the plurality of holes and constituting a portion of the cladding part; a second step in which the central cladding preform and the core preforms are integrated together; and a third step in which an outer cladding part, which is a part of the cladding part, is provided on the outer periphery of the central cladding preform.

Description

Manufacturing method of multi-core fiber preform and manufacturing method of multi-core fiber

[0001] The present invention relates to a method for manufacturing a multicore fiber preform and a method for manufacturing a multicore fiber.

[0002] Various manufacturing methods have been actively investigated as methods for manufacturing a multicore fiber preform. For example, the flame deposition method (hereinafter sometimes simply referred to as the deposition method) is the most attractive method in terms of being a low-cost technology that has been established as a conventional fiber manufacturing method. However, many problems have been reported when applied to a multicore fiber. For example, because a multicore fiber preform has multiple cores, even if flame soot is deposited, it interferes with one of the cores. As a result, the soot is not deposited uniformly around a specific core, resulting in the problem of voids remaining in the multicore fiber preform. Therefore, methods in Patent Documents 1 and 2 have been proposed as methods for preventing voids from remaining.

[0003] However, the method of Patent Document 1 requires the preparation of a polygonal core rod, which complicates the process. On the other hand, Patent Document 2 proposes a method in which, when fabricating the cladding portion of a multicore fiber, a first cladding is formed near a central rod and a second cladding is formed outside it. This method is a deposition method, and is an excellent proposal because it can solve the problem of voids without making the conventional method too complicated, but it still requires complex control of the deposition method and conditions.

[0004] Along with the deposition method, another mature cladding formation method is the jacket method, in which a core preform is inserted into a cladding tube and integrated. However, when applied to a multicore fiber, multiple core preforms need to be formed between the jackets by the stacking method, which requires a thin-diameter glass rod to fill the gaps between the core preforms, resulting in a very complicated process. As a solution to this problem, a method has been proposed in which the interior is formed by the stacking method and the cladding is formed on the outside by the deposition method while adjusting the non-circularity, as in Non-Patent Document 1, but this still requires a complicated stacking process.

[0005] On the other hand, the drilling method is a very widely used method for manufacturing multi-core fiber preforms, and a great number of reports have been published on it.

[0006] Japanese Patent Application Publication No. 9-5541 Patent No. 5740065

[0007] P. Sillard et al., “Simple Multi-Core Fiber Fabrication Method”, Th1A.1, ECOC 2022

[0008] However, the hole-punching method requires a process of heating and integrating the core preform and the cladding preform. Therefore, as the cladding preform becomes larger, the amount of heat that needs to be applied during integration increases. This leads to problems such as unintended thermal deformation of the core preform and the cladding preform, insufficient heat resulting in insufficient integration and air bubbles remaining in the multi-core fiber preform, and the need for an extremely large and expensive heat source to provide the required amount of heat.

[0009] The present invention has been made in view of the above, and an object of the present invention is to provide a method for manufacturing a multi-core fiber preform and a method for manufacturing a multi-core fiber that are suitable for manufacturing a large multi-core fiber preform.

[0010] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a method for manufacturing a multicore fiber preform including a plurality of core portions and a cladding portion that surrounds the outer periphery of the plurality of core portions and has a refractive index lower than the maximum refractive index of the plurality of core portions, the method comprising: a first step of inserting a plurality of core preforms, each having a respective one of the plurality of core portions, into each of the plurality of holes in a rod-shaped preform, a central cladding preform that becomes part of the cladding portions; a second step of integrating the central cladding preform and the core preform; and a third step of providing an outer cladding portion that becomes part of the cladding portions around the outer periphery of the central cladding preform.

[0011] In the third step, the outer cladding portion may be provided by a deposition method or a jacket method.

[0012] In the second step, the central clad preform and the core preform may be heated in a state where a vacuum state is formed in a gap between the central clad preform and the core preform.

[0013] The refractive index of the central cladding base material and the outer cladding portion may be approximately equal.

[0014] In a cross section perpendicular to the longitudinal direction of the multicore fiber preform, if the radius of a circumscribing circle circumscribing the outer edges of the multiple core preforms is A and the outer radius of the central cladding preform is B, then B / A may be 1.05 or more, and furthermore B / A may be 10 or less.

[0015] In a cross section perpendicular to the longitudinal direction of the multicore fiber preform, when the outer radius of the central cladding preform is B and the outer radius of the outer cladding portion is C, C / B may be 1.10 or more.

[0016] One aspect of the present invention is a method for manufacturing a multicore fiber, including: a multicore fiber preform manufacturing step of carrying out the method for manufacturing a multicore fiber preform; and a drawing step of drawing a multicore fiber from the manufactured multicore fiber preform.

[0017] According to the present invention, it is possible to realize a method for manufacturing a multi-core fiber preform and a method for manufacturing a multi-core fiber that are suitable for manufacturing a large multi-core fiber preform.

[0018] Fig. 1 is a schematic cross-sectional view of a multicore fiber preform manufactured by a manufacturing method according to embodiment 1. Fig. 2 is a diagram showing a refractive index profile of a core portion in a multicore fiber preform. Fig. 3 is a flow diagram of a manufacturing method of a multicore fiber preform according to the embodiment shown in Fig. 1. Fig. 4 is a diagram explaining the manufacturing steps. Fig. 5 is a diagram explaining the manufacturing steps. Fig. 6 is a flow diagram of a manufacturing method of a multicore fiber according to embodiment 2. Fig. 7 is a schematic cross-sectional view of a multicore fiber preform manufactured by a manufacturing method according to embodiment 3.

[0019] 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.

[0020] 1 is a schematic cross-sectional view of a multicore fiber manufactured by a manufacturing method according to embodiment 1, taken along a cross section perpendicular to the longitudinal direction. A multicore fiber preform 10 is made of silica-based glass, and includes two core portions 11 and a cladding portion 12 surrounding the outer peripheries of the core portions 11. The core portions 11 are arranged axially symmetrically with respect to a central axis X of the cladding portion 12. The core portions 11 are an example of a plurality of core portions.

[0021] The core portion 11 includes a center core 11a, an intermediate layer 11b surrounding the outer periphery of the center core 11a, and a trench layer 11c surrounding the outer periphery of the intermediate layer 11b.

[0022] Fig. 2 is a diagram showing the refractive index profile of the core region 11 in the multicore fiber preform 10. The core region 11 has a so-called trench-type refractive index profile. In Fig. 2, the diameter of the center core 11a is 2a, the outer diameter of the intermediate layer 11b is 2b, and the outer diameter of the trench layer 11c is 2c. The refractive index profile of the cladding region 12 is shown outside the trench layer 11c. Fig. 2 shows a case where the cladding region 12 is made of substantially pure silica glass. Pure silica glass is extremely high-purity silica glass that does not substantially contain dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm.

[0023] The center core 11a is the portion of the core portion 11 having the maximum average refractive index. The maximum relative refractive index difference of the center core 11a with respect to the refractive index of pure silica glass is Δ1. The relative refractive index difference of the average refractive index of the intermediate layer 11b with respect to the refractive index of pure silica glass is Δ2. Note that Δ2 is usually set to 0% or close to 0%. The relative refractive index difference of the average refractive index of the trench layer 11c with respect to the refractive index of pure silica glass is Δ3. Δ3 is a negative value smaller than Δ2. The core portion 11 may contain dopants used in optical fibers, such as germanium (Ge), fluorine (Fe), chlorine (Cl), and alkali metals. The refractive index profile of the core portion 11 is achieved by setting the amount of these dopants added.

[0024] 1, the cladding 12 has a refractive index lower than the maximum refractive index of the core 11. The cladding 12 is made of, for example, pure silica glass or fluorine-doped silica glass.

[0025] The cladding portion 12 includes a first cladding portion 12a that surrounds the outer periphery of the core portion 11, a second cladding portion 12b that surrounds the outer periphery of the first cladding portion 12a, and a third cladding portion 12c that surrounds the outer periphery of the second cladding portion 12b. The second cladding portion 12b and the third cladding portion 12c are arranged concentrically about the central axis X and have approximately the same refractive index.

[0026] Here, the circumscribing circle CC1 indicated by the dashed line is a circumscribing circle of the two first cladding portions 12a, the center of which coincides with the central axis X. A1 is the radius of the circumscribing circle CC1. Furthermore, B1 is the outer radius of the second cladding portion 12b. Furthermore, C1 is the outer radius of the third cladding portion 12c. A1 is an example of the radius A of a circumscribing circle circumscribing the outer edges of multiple core preforms in a cross section perpendicular to the longitudinal direction of the multicore fiber preform. B1 is an example of the outer radius B of the central cladding preform. C1 is an example of the outer radius C of the outer cladding portion.

[0027] Furthermore, D1 is the distance between the central axes of the core regions 11 (core-to-core distance), E1 is the distance from the central axis of the core region 11 to the outer circumferential surface of the second cladding region 12b, and F1 is the thickness of the third cladding region 12c.

[0028] 3 is a flow diagram of a method for manufacturing a multicore fiber preform according to the first embodiment. First, in step S101, a central cladding preform and a core preform are prepared. Next, in step S102, the core preform is inserted into the hole in the central cladding preform. Next, in step S103, the core preform and the central cladding preform are integrated. Next, in step S104, an outer cladding portion is provided around the outer periphery of the central cladding preform. In this way, a multicore fiber preform is manufactured.

[0029] Each step will be specifically described below with reference to FIGS.

[0030] First, in step S101, as shown in Fig. 4, the second cladding portion 12b as shown in Fig. 1 is prepared as a central cladding base material. The second cladding portion 12b is a member that becomes part of the cladding portion 12. The second cladding portion 12b is a rod-shaped base material and has two holes 12ba that are a plurality of holes provided in the longitudinal direction.

[0031] 4, two core preforms 13 are prepared as the plurality of core preforms. Each of the two core preforms 13 has a core portion 11 and a first cladding portion 12a surrounding the outer periphery of the core portion 11.

[0032] The core preform 13 can be manufactured using a method such as a vapor axial deposition (VAD) method or a modified chemical vapor deposition (MCVD) method. The second cladding portion 12b can be formed by drilling a hole 12ba in a glass rod manufactured using a method such as a VAD method or an MCVD method. The hole 12ba has an appropriate clearance with respect to the outer diameter of the core preform 13 so that the core preform 13 can be inserted.

[0033] Next, in step S102, the core preform 13 is inserted into each of the two holes 12ba of the second cladding portion 12b, as shown in Fig. 4. Step S102 is an example of a first step.

[0034] Next, in step S103, the second cladding portion 12b is integrated with the core preform 13. This integration can be achieved by heating the second cladding portion 12b and the core preform 13 while creating a vacuum in the gap between the second cladding portion 12b and the core preform 13. Step S103 is an example of the second step.

[0035] Next, in step S104, as shown in FIG. 5, an intermediate preform formed by integrating the second cladding portion 12b and the core preform 13 is inserted into the third cladding portion 12c as shown in FIG. 1, which serves as an outer cladding portion. The third cladding portion 12c, which serves as an outer cladding portion, is also called a glass jacket. Next, with a vacuum state formed in the gap between the intermediate preform and the third cladding portion 12c, the outer periphery of the third cladding portion 12c is heated by a heat source 100. As a result, the third cladding portion 12c is fused to the outer periphery of the second cladding portion 12b, and the third cladding portion 12c can be provided on the outer periphery of the second cladding portion 12b. In this way, a multi-core fiber preform is manufactured. Step S104 described above is an example of a third step.

[0036] According to the manufacturing method of the first embodiment as described above, the intermediate preform formed by integrating the core preform 13 and the second cladding portion 12b in the second step has a smaller volume than the final multicore fiber preform manufactured through the third step, and therefore, integration can be performed with a relatively small amount of heat. As a result, integration can be performed without a large and expensive heat source, while suppressing or preventing unintended thermal deformation and residual air bubbles. Moreover, the multicore fiber preform finally manufactured is larger than the intermediate preform by providing the second cladding portion 12b as an outer cladding portion. Therefore, the manufacturing method of the first embodiment is suitable for manufacturing large multicore fiber preforms.

[0037] In addition, although A1, B1, and C1 are defined in FIG. 1 , B1 / A1 is preferably 1.05 or more. If B1 / A1 is 1.05 or more, defects such as cracks are less likely to occur when drilling the holes 12ba in the second cladding portion 12b. Furthermore, if B1 / A1 is 10 or less, heat is easily transferred to the core preform 13 when the second cladding portion 12b and the core preform 13 are heated and integrated, which is preferable. Furthermore, C1 / B1 is preferably 1.10 or more. If C1 / B1 is 1.10 or more, the advantage of providing an outer cladding portion and enlarging the preform becomes more pronounced.

[0038] Furthermore, among the optical properties of a multi-core fiber manufactured by drawing from a multi-core preform manufactured by the manufacturing method according to the embodiment 1, optical properties other than the optical property related to confinement loss are determined at the stage of the intermediate preform. That is, according to the manufacturing method according to the embodiment 1, the preform can be increased in size without changing the optical properties of the intermediate preform other than the optical property related to confinement loss. This can be confirmed by comparing the optical properties of a multi-core fiber manufactured by drawing from a multi-core preform manufactured by the manufacturing method according to the embodiment 1 with the optical properties of a multi-core fiber manufactured by drawing from an intermediate preform. In this case, however, it is necessary to draw both multi-core fibers so that their core diameters are the same. The optical properties related to confinement loss include, for example, confinement loss, cutoff wavelength, bending loss, etc.

[0039] (Embodiment 2) As embodiment 2, a method for manufacturing an optical fiber using a multi-core fiber preform 10 as shown in Fig. 1 will be described. Fig. 6 is a flow chart of the method for manufacturing a multi-core fiber according to embodiment 2.

[0040] In the second embodiment, in step S201, the method for manufacturing a multicore fiber preform according to the first embodiment is performed as a multicore fiber preform manufacturing step to manufacture a multicore fiber preform 10. Subsequently, in step S202, a drawing step is performed to draw the manufactured multicore fiber preform 10. The drawing step can be performed using a known drawing device.

[0041] According to the second embodiment as described above, it is possible to manufacture a multi-core fiber at lower cost by using a large multi-core fiber preform 10 that has been properly manufactured.

[0042] 7 is a schematic cross-sectional view of a multicore fiber manufactured by a manufacturing method according to embodiment 3, taken along a cross section perpendicular to the longitudinal direction. A multicore fiber preform 20 is made of silica-based glass, and includes four core portions 21 and a cladding portion 22 surrounding the outer peripheries of the core portions 21. The core portions 21 are arranged axially symmetrically with respect to a central axis X of the cladding portion 22. The core portions 21 are an example of a plurality of core portions.

[0043] 1, the core region 21 includes a center core 21a, an intermediate layer 21b surrounding the outer periphery of the center core 21a, and a trench layer 21c surrounding the outer periphery of the intermediate layer 21b. Similarly to the core region 11, the core region 21 has a trench-type refractive index profile as shown in FIG. 2. The refractive index profile of the core region 11 is realized by setting the amount of dopant added to the optical fiber as described above.

[0044] The cladding portion 22 has a refractive index lower than the maximum refractive index of the core portion 21. The cladding portion 22 is made of, for example, pure silica glass or fluorine-doped silica glass.

[0045] The cladding portion 22 includes a first cladding portion 22a surrounding the outer periphery of the core portion 21, a second cladding portion 22b surrounding the outer periphery of the first cladding portion 22a, and a third cladding portion 22c surrounding the outer periphery of the second cladding portion 22b. The second cladding portion 22b and the third cladding portion 12c are arranged concentrically about the central axis X and have approximately the same refractive index.

[0046] Here, the circumscribing circle CC2 indicated by the dashed line is a circumscribing circle of the four first cladding portions 22a, the center of which coincides with the central axis X. A2 is the radius of the circumscribing circle CC2. Furthermore, B2 is the outer radius of the second cladding portion 22b. Furthermore, C2 is the outer radius of the third cladding portion 22c. A2 is an example of A, which is the radius of a circumscribing circle circumscribing the outer edges of multiple core preforms in a cross section perpendicular to the longitudinal direction of the multicore fiber preform. B2 is an example of B, which is the outer radius of the central cladding preform. C2 is an example of C, which is the outer radius of the outer cladding portion.

[0047] Furthermore, D2 is the distance between the central axes of the core portions 21 (inter-core distance), E2 is the difference between the outer diameter of the second cladding portion 22b and 1 / 2 of the inter-core distance, and F2 is the thickness of the third cladding portion 12c.

[0048] Such a multicore fiber preform 20 can be manufactured by a manufacturing method similar to the manufacturing method of the multicore fiber preform according to embodiment 1. Moreover, using such a multicore fiber preform 20, a multicore fiber can be manufactured by a manufacturing method similar to the manufacturing method of the multicore fiber according to embodiment 2. The multicore fiber manufactured in this way is manufactured using a properly manufactured large-sized multicore fiber preform 20, and is therefore cheaper.

[0049] In the above embodiment, the core preform includes the first cladding portion, but the core preform does not necessarily have to include the first cladding portion. In this case, the core preform consists of only the core portions. In this case, the circumscribing circle is a circumscribing circle of multiple core portions, the center of which coincides with the central axis of the cladding portion.

[0050] Although the outer cladding portion is formed by the jacket method in the above embodiment, it may also be formed by a deposition method, in which raw material gas for the outer cladding portion is blown onto the central cladding base material with a burner to deposit soot, and the soot is then vitrified to form the outer cladding portion.

[0051] (Example 1, Comparative Example 1) As Example 1, a multicore fiber preform having the structure shown in Fig. 1 was manufactured by the manufacturing method according to the embodiment 1. The core preform and the second cladding portion were manufactured by a flame deposition method based on the VAD method. The core preform did not have a first cladding portion and had a structure consisting of only the core portion.

[0052] Regarding the core portion, the center core was made of silica glass doped with chlorine and potassium (100 wtppm). The intermediate layer and trench layer were made of silica glass doped with fluorine. Δ1, Δ2, and Δ3 were set to 0.10%, -0.17%, and -0.50%, respectively. 2a was set to 5.25 mm, 2b to 7.875 mm, and 2c to 15.75 mm. Therefore, the outer diameter of the core base material was 15.75 mm.

[0053] The second cladding was made of fluorine-doped silica glass. The diameter of the second cladding was 46 mm, and the diameter of the two holes was 16 mm. The relative refractive index difference of the second cladding to pure silica glass was -0.17%.

[0054] The second clad and the two core preforms were integrated while a dehydration gas was passed through the gap to produce an intermediate preform. The position and non-circularity of the core in the intermediate preform, as well as the non-circularity of the second clad, were checked and found to be in a good condition, indicating that good integration had been achieved.

[0055] Next, a third cladding portion was formed around the second cladding portion of the intermediate preform by a deposition method, to prepare a multi-core fiber preform. The relative refractive index difference of the third cladding portion with respect to pure silica glass was set to -0.17%. The outer diameter of the third cladding portion was set to 62.5 mm.

[0056] In addition, D1 in FIG. 1 was set to 20 mm, E1 was set to 13 mm, and F1 was set to 8.25 mm.

[0057] Thereafter, a multicore fiber having a cladding diameter of 125 μm was drawn from the multicore fiber preform by the manufacturing method of Example 1. In the drawn multicore fiber, 2a, 2b, and 2c (see FIG. 2) were 10.5 μm, 15.75 μm, and 31.5 μm, respectively. Therefore, b / a and / a were 1.5 and 3.0, respectively. Furthermore, the sizes corresponding to D1, E1, and F1 in FIG. 1 were 40 μm, 26 μm, and 16.5 μm, respectively.

[0058] In the multicore fiber manufactured in this manner, the inter-core crosstalk at a wavelength of 1550 nm was -32 dB per 100 km, which was almost as designed. Other optical properties were as shown in Table 1. In Table 1, "Aeff" means the effective core area. The transmission loss, Aeff, and bending loss are values ​​at a wavelength of 1550 nm. The bending loss is a value when the bending diameter is 30 mm. As shown in Table 1, the transmission loss, effective core area, cable cutoff wavelength, and bending loss all exhibited favorable properties as designed.

[0059]

[0060] In addition, as Comparative Example 1, a central clad base material having the same configuration as the second clad portion except for its larger outer diameter of 62.5 mm was prepared, and a process of integrating it with the core base material was carried out. However, in many cases, gaps remained between the core base material and the central clad base material, preventing sufficient integration, and even when they were integrated, the non-circularity of the core portion and the clad portion was often large.

[0061] (Example 2, Comparative Example 2) As Example 2, a multicore fiber preform having the structure shown in Fig. 7 was manufactured by a manufacturing method similar to the manufacturing method according to the embodiment 1. The core preform and the second cladding portion were manufactured by a flame deposition method based on the VAD method. The core preform did not have a first cladding portion and had a structure consisting of only the core portion.

[0062] Regarding the core portion, the center core was made of silica glass doped with chlorine and potassium (100 wtppm). The intermediate layer and trench layer were made of silica glass doped with fluorine. Δ1, Δ2, and Δ3 were set to 0.10%, -0.27%, and -0.45%, respectively. 2a was set to 4.5 mm, 2b to 8.1 mm, and 2c to 10.8 mm. Therefore, the outer diameter of the core base material was 10.8 mm.

[0063] The second cladding was made of fluorine-doped silica glass. The diameter of the second cladding was 42.5 mm, and the diameter of each of the four holes was 11 mm. The relative refractive index difference of the second cladding to pure silica glass was -0.27%.

[0064] The second clad and the four core preforms were integrated while a dehydration gas was passed through the gaps to produce an intermediate preform. The position and non-circularity of the cores in the intermediate preform, as well as the non-circularity of the second clad, were checked and found to be in a good condition, indicating that good integration had been achieved.

[0065] Next, a third cladding portion was formed on the outer periphery of the second cladding portion of the intermediate preform by a deposition method, to fabricate a multi-core fiber preform. The relative refractive index difference of the third cladding portion with respect to pure silica glass was set to -0.27%. The outer diameter of the third cladding portion was set to 62.5 mm.

[0066] In addition, D2 in FIG. 7 was set to 20 mm, E2 was set to 11.25 mm, and F2 was set to 10 mm.

[0067] Thereafter, a multicore fiber having a cladding diameter of 125 μm was drawn from the multicore fiber preform by the manufacturing method of Example 2. In the drawn multicore fiber, 2a, 2b, and 2c (see FIG. 2) were 9 μm, 16.2 μm, and 21.6 μm, respectively. Therefore, b / a and / a were 1.8 and 2.4, respectively. In addition, the sizes corresponding to D2, E2, and F2 in FIG. 7 were 40 μm, 22.5 μm, and 20 μm, respectively.

[0068] In the multicore fiber manufactured in this manner, the inter-core crosstalk at a wavelength of 1550 nm was within the range of -35±3 dBm per 100 km, which was almost as designed. Other optical properties were as shown in Table 2. As shown in Table 2, the transmission loss, effective core area, cable cutoff wavelength, and bending loss all exhibited favorable properties as designed.

[0069]

[0070] In addition, as Comparative Example 2, a central clad base material having the same configuration as the second clad portion except for its larger outer diameter of 62.5 mm was prepared, and a process of integrating it with the core base material was carried out. However, in many cases, gaps remained between the core base material and the central clad base material, preventing sufficient integration, and even when they were integrated, the non-circularity of the core portion and the clad portion was often large.

[0071] In the above embodiments, the number of cores is 2 or 4, but is not particularly limited and may be, for example, 7 or 19. In the above embodiments, the refractive indexes of the central cladding base material and the outer cladding are equal, but they do not have to be equal, and may be approximately equal. "Approximately equal" means, for example, that the difference in average relative refractive index with respect to pure silica glass is 0.03% or less.

[0072] 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.

[0073] The present invention can be used in the manufacture of multi-core fibers.

[0074] 10, 20: Multicore fiber preform 11, 21: Core portion 11a: Center core 11b: Intermediate layer 11c: Trench layer 12, 22: Cladding portion 12a, 22a: First cladding portion 12b, 22b: Second cladding portion 12ba: Hole 12c, 22c: Third cladding portion 13: Core preform 21a: Center core 21b: Intermediate layer 21c: Trench layer 100: Heat source CC1, CC2: Circumscribed circle X: Central axis

Claims

1. A method for manufacturing a multi-core fiber preform comprising a plurality of core portions and a cladding portion surrounding the outer periphery of the plurality of core portions and having a refractive index lower than the maximum refractive index of the plurality of core portions, comprising: a first step of inserting a plurality of core preforms each having a respective one of the plurality of core portions into each of the plurality of holes in a central cladding preform which is a rod-shaped preform and has a plurality of holes arranged in the longitudinal direction and which becomes a part of the cladding portion; a second step of integrating the central cladding preform and the core preform; and a third step of providing an outer cladding portion which becomes a part of the cladding portion around the outer periphery of the central cladding preform.

2. The method for manufacturing a multi-core fiber preform according to claim 1, wherein in the third step, the outer cladding portion is provided by a deposition method or a jacket method.

3. A method for manufacturing a multicore fiber preform as set forth in claim 1, wherein in the second step, the central cladding preform and the core preform are heated while a vacuum state is formed in a gap between the central cladding preform and the core preform.

4. The method for manufacturing a multicore fiber preform according to claim 1, wherein the refractive indexes of the central cladding preform and the outer cladding portion are approximately equal.

5. The method for manufacturing a multi-core fiber preform according to claim 1, wherein, in a cross section perpendicular to the longitudinal direction of the multi-core fiber preform, a radius of a circumscribing circle circumscribing the outer edges of the multiple core preforms is A and an outer radius of the central cladding preform is B, where B / A is 1.05 or more.

6. The method for manufacturing a multi-core fiber preform according to claim 5, wherein B / A is 10 or less.

7. The method for manufacturing a multicore fiber preform according to claim 1, wherein, in a cross section perpendicular to the longitudinal direction of the multicore fiber preform, C / B is 1.10 or more, where B is an outer radius of the central cladding preform and C is an outer radius of the outer cladding portion.

8. A method for manufacturing a multicore fiber, comprising: a multicore fiber preform manufacturing step of carrying out the method for manufacturing a multicore fiber preform according to any one of claims 1 to 7; and a drawing step of drawing a multicore fiber from the manufactured multicore fiber preform.

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