Multi-core optical fiber manufacturing method and multi-core optical fiber base material

Abstract

The method for manufacturing a multi-core optical fiber includes: inserting a plurality of glass rods into a plurality of holes provided in a glass tube; sealing the first end of the glass tube; and decompressing the inside of the plurality of holes from the second end of the glass tube, and drawing while integrating the glass tube with the plurality of glass rods. When S1 [m2] is the cross-sectional area of the glass tube before the wire drawing step, S2 [m2] is the sum of the cross-sectional areas of the plurality of glass rods before the wire drawing step, and S3 [m2] is the cross-sectional area of the multi-core optical fiber after the wire drawing step, S1 [m2] is the cross-sectional area of the glass tube before the wire drawing step. The insertion step is performed such that the sum of the areas of the gaps between the glass tube and the plurality of glass rods prior to the wiredrawing step in a cross-section of the glass tube orthogonal to the axial direction is equal to or less than (S1 + S2) / S3 * 5.66 * 10-4 [mm2]. The insertion step is performed in an environment having a higher cleanliness than grade 1000.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a manufacturing method of a multi-core optical fiber (hereinafter, MCF) and a MCF preform. This application claims priority based on Japanese Application No. 2023-042979 filed on March 17, 2023, and all the recitations described in the Japanese application are hereby incorporated by reference. BACKGROUND

[0002] In the MCF, by disposing a plurality of cores in one optical fiber, it is possible to increase the core density. For this reason, it is possible to increase the number of cores while keeping the outer diameter of the optical fiber cable constant. In addition, it is also possible to reduce the outer diameter of the optical fiber cable while keeping the number of cores built in the optical fiber cable constant. According to the MCF, it is possible to reduce the inter-core distance compared to arranging existing optical fibers to form an optical fiber array. Therefore, it is expected that the MCF will be effectively utilized as a high-density optical wiring.

[0003] In Patent Literature 1, a manufacturing method of an optical fiber is disclosed in which, after assembling a preform by inserting a core rod into an outer outer layer tube by a RIT (Rod-In-Tube) method, the front end of the preform is heated, and drawing is performed while integrating the outer outer layer tube with the core rod. In Patent Literature 2, a manufacturing method of an optical fiber is disclosed in which, after assembling a preform by inserting a first outer cladding tube into a second outer cladding tube and inserting a core rod segment into the first outer cladding tube by a RIT method, the lower end of the preform is heated, and drawing is performed while reducing the diameters of the first outer cladding tube and the second outer cladding tube.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Publication No. 2004-182595

[0007] Patent Literature 2: Japanese Patent Application Publication No. 2006-265095 SUMMARY

[0008] The manufacturing method of the MCF according to an aspect of the present disclosure is a manufacturing method of a MCF having a plurality of cores and a cladding that wraps the plurality of cores, including the steps of: inserting a plurality of glass rods into a plurality of holes provided in a glass tube; sealing a first end of the glass tube; and reducing the pressure inside the plurality of holes from a second end of the glass tube, while integrating the glass tube with the plurality of glass rods, and performing drawing. When the cross-sectional area of the glass tube before the step of drawing is S1 [m 2 ], the sum of the cross-sectional areas of the plurality of glass rods before the step of drawing is S2 [m 2 ], and the cross-sectional area of the MCF after the step of drawing is S3 [m 2to make the total of the areas of the gaps between the glass tubes and the plurality of glass rods before the process of drawing the glass tubes in the cross section orthogonal to the axis (S1+S2) / S3 x 5.66 x 10 -4 [mm 2 The process of inserting is performed in the following manner, and the process of inserting is performed in an environment having a cleanliness higher than the grade 1000. BRIEF DESCRIPTION OF DRAWINGS

[0009] Figure 1 is a cross-sectional view of the MCF to which the embodiment relates.

[0010] Figure 2 is a flowchart showing the manufacturing method of the MCF to which the embodiment relates.

[0011] Figure 3 is a cross-sectional view for explaining the preparation process.

[0012] Figure 4 is a cross-sectional view of the glass tube.

[0013] Figure 5 is a cross-sectional view for explaining the connecting process.

[0014] Figure 6 is a cross-sectional view for explaining the inserting process.

[0015] Figure 7 is a cross-sectional view of the MCF preform to which the embodiment relates.

[0016] Figure 8 is a cross-sectional view for explaining the sealing process.

[0017] Figure 9 is a cross-sectional view for explaining the drawing process.

[0018] Figure 10 is a cross-sectional view of the MCF preform to which the modification example relates.

[0019] Figure 11 is a cross-sectional view of the optical fiber preform.

[0020] Figure 12 is a graph showing the relationship between the cladding diameter and the length of the optical fiber obtained from one preform. DETAILED DESCRIPTION

[0021] [Technical problem to be solved by the present disclosure]

[0022] In the manufacturing method described above, after the assembly of the mother material by inserting the glass rods that become the cores into the glass tube that becomes the cladding, the glass tube is drawn without the gas phase purification process inside the glass tube. Therefore, there is a high possibility that the drawing is performed in a state where foreign matter is mixed between the glass tube and the glass rods. In general, the more the number of cores, the larger the area of the gap between the glass tube and the glass rods in the cross section of the glass tube orthogonal to the axial direction. Thus, the number of foreign matter mixed into the mother material from the surrounding environment increases. Furthermore, if the cladding diameter is increased in order to accommodate more cores, the length of the optical fiber obtained from the same size of the mother material becomes shorter. Therefore, the number of quality abnormality portions per unit length of the optical fiber easily increases.

[0023] The present disclosure provides a manufacturing method of MCF and a mother material of MCF capable of reducing the frequency of quality abnormality.

[0024] [Effects of the present disclosure]

[0025] According to the present disclosure, it is possible to provide a manufacturing method of MCF and a mother material of MCF capable of reducing the frequency of quality abnormality.

[0026] [Explanation of the implementation aspects of the present disclosure]

[0027] First, the implementation aspects of the present disclosure are listed to be explained.

[0028] (1) The manufacturing method of MCF according to an aspect of the present disclosure is a manufacturing method of MCF having a plurality of cores and a cladding that wraps the plurality of cores, including the following processes: inserting a plurality of glass rods into a plurality of holes provided in a glass tube; sealing a first end of the glass tube; and reducing the pressure inside the plurality of holes from a second end of the glass tube while integrating the glass tube with the plurality of glass rods to draw. When the cross-sectional area of the glass tube before the process of drawing is S1 [m 2 ], the sum of the cross-sectional areas of the plurality of glass rods before the process of drawing is S2 [m 2 ], and the cross-sectional area of the MCF after the process of drawing is S3 [m 2 ], the process of inserting is performed in such a way that the sum of the areas of the gaps between the glass tube before the process of drawing and the plurality of glass rods in the cross section of the glass tube orthogonal to the axial direction is (S1+S2) / S3x5.66x10 -4 [mm 2 ] or less. The process of inserting is performed in an environment having cleanliness higher than the level 1000.

[0029] In the manufacturing method of MCF described above, the frequency of quality abnormality can be reduced to 0.02 / km or less.

[0030] (2) In the above (1), it can also be that the sum of the areas of the gaps is (S1+S2) / S3x1.42x10-3 [mm 2 ]The inserting process is performed in the following manner. In this case, the frequency of quality abnormalities can be reduced to 0.05 / km or less.

[0031] (3) In the above (1), it can also be that the sum of the areas of the gaps is (S1+S2) / S3 x 2.83 x 10 -3 [mm 2 ]The inserting process is performed in the following manner. In this case, the frequency of quality abnormalities can be reduced to 0.10 / km or less.

[0032] (4) The manufacturing method of the MCF according to another aspect of the present disclosure is a manufacturing method of an MCF having a plurality of cores and a cladding that surrounds the plurality of cores, including the following processes: inserting a plurality of glass rods into a plurality of holes provided in a glass tube; sealing a first end of the glass tube; and reducing the pressure inside the plurality of holes from a second end of the glass tube while integrating the glass tube with the plurality of glass rods to perform drawing. When the cross-sectional area of the glass tube before the process of drawing is set as S1 [m 2 ], the sum of the cross-sectional areas of the plurality of glass rods before the process of drawing is set as S2 [m 2 ], and the cross-sectional area of the MCF after the process of drawing is set as S3 [m 2 ], the sum of the areas of the gaps between the glass tube before the process of drawing and the plurality of glass rods in a cross section of the glass tube orthogonal to the axial direction is (S1+S2) / S3 x 5.66 x 10 -5 [mm 2 ]The inserting process is performed in the following manner, and the inserting process is performed in an environment having cleanliness higher than the level 10000.

[0033] In the above manufacturing method of the MCF, the frequency of quality abnormalities can be reduced to 0.02 / km or less.

[0034] (5) In the above (4), it can also be that the sum of the areas of the gaps is (S1+S2) / S3 x 1.42 x 10 -4 [mm 2 ]The inserting process is performed in the following manner. In this case, the frequency of quality abnormalities can be reduced to 0.05 / km or less.

[0035] (6) In the above (4), it can also be that the sum of the areas of the gaps is (S1+S2) / S3 x 2.83 x 10 -4 [mm 2 ]The inserting process is performed in the following manner. In this case, the frequency of quality abnormalities can be reduced to 0.10 / km or less.

[0036] (7) The MCF base material involved in one aspect of this disclosure is a base material for an MCF having multiple fiber cores and a cladding covering the multiple fiber cores, comprising: a glass tube having multiple holes; and multiple glass rods inserted into the multiple holes. When the cross-sectional area of ​​the glass tube before drawing is set as S1[m 2 Let the sum of the cross-sectional areas of the multiple glass rods before drawing be S2[m 2 Let the cross-sectional area of ​​the drawn MCF be S3[m 2 When the glass tube is drawn, the total area of ​​the gaps between the glass tube and the multiple glass rods in the cross-section orthogonal to the axial direction is (S1+S2) / S3×5.66×10. -4 [mm 2 The cleanliness level of the gaps is higher than that of Class 1000.

[0037] In the aforementioned MCF base material, the frequency of quality abnormalities can be reduced to below 0.02 per km.

[0038] (8) Another aspect of this disclosure relates to an MCF base material for an MCF having multiple fiber cores and a cladding covering the multiple fiber cores, comprising: a glass tube having multiple holes; and multiple glass rods inserted into the multiple holes. When the cross-sectional area of ​​the glass tube before drawing is set as S1[m 2 Let the sum of the cross-sectional areas of the multiple glass rods before drawing be S2[m 2 Let the cross-sectional area of ​​the drawn MCF be S3[m 2 When the glass tube is drawn, the total area of ​​the gaps between the glass tube and the multiple glass rods in the cross-section orthogonal to the axial direction is (S1+S2) / S3×5.66×10. -5 [mm 2 The cleanliness level of the gaps is higher than that of Class 10000.

[0039] In the aforementioned MCF base material, the frequency of quality abnormalities can be reduced to below 0.02 per km.

[0040] [Details of the embodiments of this disclosure]

[0041] The following description, with reference to the accompanying drawings, illustrates a method for manufacturing the MCF and specific examples of the MCF base material of this disclosure. It should be noted that this disclosure is not limited to these examples, but rather, as illustrated by the claims, is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are labeled with the same reference numerals, and repeated descriptions are omitted.

[0042] Figure 1 This is a cross-sectional view of the MCF, orthogonal to the axial direction AD1, according to the implementation method. For example... Figure 1As shown, the MCF 1 is provided with a plurality of cores 2, a common cladding 3, and marks 4 for identifying the respective cores 2. The axial direction AD1 is a direction in which the MCF 1 extends, and is a direction parallel to the central axis of the MCF 1.

[0043] The plurality of cores 2 extend in the axial direction AD1 of the MCF 1. In the present embodiment, the number of the cores 2 is four. The plurality of cores 2 are arranged in positions that are rotationally symmetrical with respect to the center of the MCF 1 in a cross section orthogonal to the axial direction AD1. The cross-sectional shapes of the plurality of cores 2 are circular shapes that are identical to each other. In the present embodiment, the number of the cores 2 is four, and the MCF 1 is a four-core optical fiber, but is not limited thereto. The core 2 includes a central core 5 and an optical cladding 6. The diameter of the central core 5 is, for example, 6 μm or more and 12 μm or less.

[0044] The cladding 3 covers the plurality of cores 2 and the marks 4. The cladding 3 is provided so as to be in contact with the outer peripheral surfaces of the plurality of cores 2 and the marks 4, respectively. The diameter of the cladding 3 (cladding diameter) is, for example, 124 μm or more and 126 μm or less.

[0045] Generally, the refractive index of the central core 5 is higher than the refractive index of the common cladding 3, and the refractive index of the optical cladding 6 is lower than or equal to the refractive index of the common cladding 3. The central core 5, the optical cladding 6, and the common cladding 3 each include a silica-based glass as a main component. The silica-based glass is a glass including 50% or more of silicon dioxide. The central core 5, the optical cladding 6, and the common cladding 3 each include a dopant for adjusting the refractive index. Germanium is widely known as an additive element for increasing the refractive index. Fluorine and boron are widely known as additive elements for decreasing the refractive index.

[0046] The mark 4 is provided for identifying the respective cores 2. The mark 4 is provided at a position away from the central axis of the MCF 1. The mark 4 is provided at a position away from the respective cores 2. The diameter of the mark 4 is, for example, smaller than the diameter of the core 2, and is 2 μm or more and 10 μm or less. The mark 4 is, for example, distinguished from the core 2 by being colored. The mark 4 has, for example, a refractive index different from the refractive index of the cladding 3. The mark 4 includes a silica-based glass as a main component. The silica-based glass is a glass including 50% or more of silicon dioxide. The mark 4 includes, for example, a dopant for coloring or for adjusting the refractive index.

[0047] Figure 2 is a flowchart showing a manufacturing method of an MCF according to the present embodiment. As shown, the manufacturing method of the MCF 1 includes a preparation step S10, a connection step S20, an insertion step S30, a sealing step S40, and a drawing step S50. The MCF 1 is manufactured, for example, by sequentially performing the preparation step S10, the connection step S20, the insertion step S30, the sealing step S40, and the drawing step S50. The MCF 1 is manufactured by the RIT method. Figure 2 The manufacturing method of the MCF 1 includes the preparation step S10, the connection step S20, the insertion step S30, the sealing step S40, and the drawing step S50. The MCF 1 is manufactured, for example, by sequentially performing the preparation step S10, the connection step S20, the insertion step S30, the sealing step S40, and the drawing step S50. The MCF 1 is manufactured by the RIT method.

[0048] Figure 3 This is a cross-sectional view used to illustrate the preparation process. Preparation process S10 is the preparation of... Figure 3 The process of preparing the glass tube 10 and the dummy tube 20 is shown. In the preparation step S10, in addition to the glass tube 10 and the dummy tube 20, a glass rod 30, which will be described below (see reference), may also be prepared. Figure 6 ) and sealing part 40 (refer to) Figure 8 ).

[0049] Figure 4 This is a cross-sectional view of the glass tube that is orthogonal to the axial direction AD2. Figure 3 and Figure 4 The glass tube 10 shown is a common cladding tube that forms the cladding 3. Axial direction AD2 is the direction in which the glass tube 10 extends, and it is parallel to the central axis of the glass tube 10. Axial direction D2 is parallel to axial direction D1. The glass tube 10 has a first end 10a and a second end 10b along axial direction AD2. The first end 10a and the second end 10b are the two end faces of the glass tube 10.

[0050] A plurality of holes 11 are provided in the glass tube 10. The plurality of holes 11 penetrate the glass tube 10 along the axial direction AD2 and open at a first end 10a and a second end 10b, respectively. The plurality of holes 11 have a circular cross-sectional shape. The plurality of holes 11 include a plurality of holes 11A provided at positions corresponding to a plurality of fiber cores 2 and a plurality of holes 11B provided at positions corresponding to markings 4. For example, the inner diameter of hole 11A is larger than the inner diameter of hole 11B. The glass tube 10 is formed, for example, by forming the plurality of holes 11 in a cylindrical glass rod using a drill bit.

[0051] Figure 3 The dummy tube 20 shown is a cylindrical glass tube with a hole 21. The dummy tube 20 is used to hold the glass tube 10. The dummy tube 20 has an outer diameter that is the same as the outer diameter of the glass tube 10. The outer diameter of the dummy tube 20 may also be smaller than the outer diameter of the glass tube 10 within the range where the hole 21 and the hole 11 communicate.

[0052] Figure 5 This is a cross-sectional diagram used to illustrate the connection process. For example... Figure 5 As shown, connection step S20 is the process of connecting the dummy tube 20 to the second end 10b of the glass tube 10. The glass tube 10 and the dummy tube 20 are connected coaxially. The dummy tube 20 is fused to the second end 10b in such a way that at least a portion of each of the holes 11 overlaps with the hole 21 when viewed along the axial direction AD2.

[0053] Figure 6 This is a cross-sectional view used to illustrate the insertion process. For example... Figure 6The insertion process S30 is a process of inserting the plurality of glass rods 30 into the plurality of holes 11 provided in the glass tube 10. The plurality of glass rods 30 are inserted one by one into the plurality of holes 11. In the present embodiment, the plurality of glass rods 30 are inserted into the plurality of holes 11 from the first end 10a side. The plurality of glass rods 30 can also be inserted into the plurality of holes 11 from the second end 10b side through the holes 21 of the dummy tube 20. Thus, the MCF preform 100 (refer to Figure 7 ) is obtained.

[0054] Figure 7 is a cross-sectional view of the MCF preform related to the present embodiment, which is orthogonal to the axial direction AD2. Figure 7 The MCF preform 100 illustrated is a preform for the MCF 1. The MCF preform 100 is provided with the glass tube 10 provided with the plurality of holes 11 and the plurality of glass rods 30 inserted into the plurality of holes 11. The glass rod 30 has a cylindrical shape. The outer diameter of the glass rod 30 is set to a size that enables insertion into the corresponding hole 11. The plurality of glass rods 30 include a glass rod 30A (core rod) that becomes the core 2 and a glass rod 30B (marker rod) that becomes the marker 4. The glass rod 30A includes a central core portion 31 that becomes the central core 5 and an optical cladding portion 32 that becomes the optical cladding 6. The outer diameter of the glass rod 30A is smaller than the inner diameter of the hole 11A. The outer diameter of the glass rod 30B is smaller than the inner diameter of the hole 11B. For this reason, the plurality of gaps G are formed between the glass tube 10 and the plurality of glass rods 30. In the MCF preform 100, the inner surface of the hole 11 and the outer circumferential surface of the glass rod 30 are not engaged with each other.

[0055] Figure 8 is a cross-sectional view for explaining the sealing process. As Figure 8 illustrated, the sealing process S40 is a process of sealing the first end 10a of the glass tube 10. In the sealing process S40, the first end 10a is sealed by fusion bonding the sealing portion 40 composed of a glass block to the first end 10a. The sealing portion 40 covers substantially the entire first end 10a and seals the openings of the plurality of holes 11 at the first end 10a.

[0056] Figure 9 is a cross-sectional view for explaining the drawing process. As Figure 9 illustrated, the drawing process S50 is a process of reducing the pressure inside the plurality of holes 11 (specifically, the gaps G (refer to Figure 7 )) from the second end 10b of the glass tube 10, and drawing the glass tube 10 and the plurality of glass rods 30 integrally by heating. Thus, the MCF 1 is obtained.

[0057] The drawing process S50 is performed in a state where the coupler 50 is connected to the dummy tube 20. The vacuum pump 51 and the gas supply portion 52 are connected to the coupler 50 via valves, respectively. In the drawing process S50, the vacuum pump 51 depressurizes the inside of the plurality of holes 11 through the coupler 50 and the dummy tube 20. In a case where the vacuum pump 51 is stopped in the middle of the drawing, the gas supply portion 52 supplies clean gas to the inside of the plurality of holes 11 through the coupler 50 and the dummy tube 20. By the gas supply portion 52, it is possible to prevent air of the ambient environment from entering the inside of the plurality of holes 11 at the time of the stop of the vacuum pump 51 and to cause the MCF preform 100 to be contaminated. The gas supply portion 52 can also supply oxygen or a rare gas to the inside of the plurality of holes 11 at the same time as the vacuum pump 51 depressurizes the inside of the plurality of holes 11.

[0058] In the RIT method, it is also possible to perform vapor phase treatment on the inner surface of the glass tube 10 (the inner surface of the plurality of holes 11) after the plurality of glass rods 30 are inserted into the glass tube 10 by the insertion process S30, as in the sleeve collapsing method. However, in the drawing process S50, in order to depressurize the inside of the glass tube 10, it is necessary to open the first end 10a of the glass tube 10. Therefore, even if the foreign matter is removed, the inside of the glass tube 10 is exposed to the ambient atmosphere thereafter. For this reason, it is considered that the effect of the vapor phase treatment is limited in the RIT method, and that improving the cleanliness of the ambient atmosphere is effective in preventing the mixing of foreign matter.

[0059] Figure 10 is a cross-sectional view of the MCF preform in the modification. Figure 10 The MCF preform 101 in the modification shown is a preform for an MCF in which the number of cores is 2. Figure 11 is a cross-sectional view of the optical fiber preform in the modification. Figure 11 The optical fiber preform 102 shown is a preform for an optical fiber in which the number of cores is 1. In the case where the number of cores is 1, the mark 4 is not needed (refer to Figure 1 ). As is clear from the cross-sectional shape shown by Figure 7 , Figure 10 and Figure 11 , in the preform of the RIT method, in general, the more the number of cores increases, the larger the gap between the glass tube and the glass rod becomes. If the gap becomes large, the risk of more foreign matter mixing from the ambient environment increases. That is, in general, the more the number of cores increases, the more susceptible it is to the cleanliness of the ambient environment.

[0060] It is known that in a case where foreign matter is mixed in the preform, the cladding diameter varies at the time of drawing, or the mechanical strength decreases. Since such a quality abnormal portion needs to be removed, the number of foreign matters per unit length of optical fiber must be a certain number or less.

[0061] Figure 12 This is a graph showing the relationship between cladding diameter and fiber length obtained from a single source material. Increasing the number of cores makes it impossible to control the cladding diameter within the same range as that of standard single-mode fiber (125 μm), necessitating a thicker cladding diameter. Consequently, the fiber length obtained from a single source material becomes shorter. If we define the fiber length obtained from a single source material as 1 when the number of cores is 4 and the cladding diameter is 125 μm, then, for example, with 12 cores, the cladding diameter is 180 μm, and the fiber length obtained from a single source material is less than 0.5 μm.

[0062] The tendency for fiber length to decrease with increasing core count, as seen in this case, is particularly pronounced in uncoupled MCFs. This is because, in uncoupled MCFs, the core spacing needs to be at least a certain level to reduce crosstalk. In other words, even with the same base material size and amount of foreign matter, drawing a fiber with a larger cladding diameter will increase the number of quality defects per unit fiber length.

[0063] In summary, generally speaking, the more fiber cores there are, the larger the gap between the cladding tube and the glass rod, and the greater the amount of foreign matter mixed into the matrix from the surrounding environment. Furthermore, since a larger cladding diameter is required to accommodate more fiber cores, the fiber length obtained from the same size matrix becomes shorter. As a result, the number of quality defects per unit fiber length tends to increase. In view of these realities, the inventors of this disclosure have conducted research to reduce the frequency of fiber quality defects to below a certain level.

[0064] Let the cross-sectional area of ​​the glass tube 10 before drawing (i.e., before the drawing process S50) that is orthogonal to the axial direction AD2 be S1[m]. 2 Let S2[m] be the sum of the cross-sectional areas of the multiple glass rods 30 before drawing that are orthogonal to the axial direction AD2. 2 The cross-sectional area of ​​MCF1 after wire drawing (i.e., after wire drawing process S50) orthogonal to the axial direction AD1 is set as S3[m]. 2 Here, S1 does not include the cross-sectional area of ​​the multiple holes 11. The sum of the cross-sectional areas of the multiple holes 11 is S2[m]. 2 [The above.] Additionally, S3 includes the cross-sectional area of ​​core 2, cladding 3, and marking 4. S3 does not include the cross-sectional area of ​​the coating resin. That is, the cross-sectional area of ​​the drawn MCF1 refers to the cross-sectional area of ​​the glass portion. For simplicity, when the length of the MCF mother material 100 is set to 1m, the theoretical length of the optical fiber obtained from the MCF mother material 100 is...

[0065] (S1+S2) / S3 / 1000[km].

[0066] When the allowable frequency of the quality abnormality part is set to F [pieces / km], the number of foreign matters allowed to be mixed inside the MCF preform 100 is

[0067] (S1+S2) / S3 / 1000 x F [pieces].

[0068] (Specific Example 1)

[0069] As the specific example 1, consider assembling the MCF preform 100 in an environment of the level 1000 (the number of particles of 0.5 μm or more contained per cubic foot is 1000 or less) that can actually be managed. Since 1 foot = 0.3048 m, the foreign matter density inside the MCF preform 100 is

[0070] 1000 / 0.3048 3 ≈35315 [pieces / m 3 ].

[0071] When the MCF preform 100 is assembled by inserting the plurality of glass rods 30 into the glass tube 10 through the insertion process S30, the sum of the volumes of the gaps G in the MCF preform 100 is

[0072] (S1+S2) / S3 / 1000 x F / (1000 / 0.3048 3 )[m 3 ]

[0073] = (S1+S2) / S3 x 0.3048 3 x 10 -6 x F [m 3 ]

[0074] Hereinafter, the frequency of the quality abnormality targeted can be realized.

[0075] As described above, since the length of the MCF preform 100 is 1 m, by making the sum of the areas of the gaps G in the cross section of the glass tube 10 orthogonal to the axial direction AD2

[0076] (S1+S2) / S3 x 0.3048 3 x 10 -6 x F [m 2 ]

[0077] Hereinafter, the frequency of the quality abnormality targeted can be realized.

[0078] The allowable frequency F of the quality abnormality is, for example, 0.02 pieces / km (1 piece per 50 km on average). In this case, as long as the sum of the areas of the gaps G is

[0079] (S1+S2) / S3 x 0.3048 3 x 10-6 x 0.02 [m 2 ]

[0080] = (S1 + S2) / S3 x 5.66 x 10 -4 [mm 2 ]

[0081] The following is also acceptable.

[0082] The allowable frequency F of quality abnormalities can also be 0.05 per km (1 per 20 km on average). In this case, as long as the sum of the areas of the gaps G is

[0083] (S1 + S2) / S3 x 0.3048 3 x 10 -6 x 0.05 [m 2 ]

[0084] = (S1 + S2) / S3 x 1.42 x 10 -3 [mm 2 ]

[0085] The following is also acceptable.

[0086] The allowable frequency F of quality abnormalities can also be 0.10 per km (1 per 10 km on average). In this case, as long as the sum of the areas of the gaps G is

[0087] (S1 + S2) / S3 x 0.3048 3 x 10 -6 x 0.10 [m 2 ]

[0088] = (S1 + S2) / S3 x 2.83 x 10 -3 [mm 2 ]

[0089] The following is also acceptable.

[0090] That is, the sum of the areas of the gaps, the values of S1, S2, and S3 are set in such a manner that the sum of the areas of the gaps G in the cross section of the glass tube 10 orthogonal to the axial direction AD2 before drawing is 5.66 x 10 -4 [mm 2 ] or less, or (S1 + S2) / S3 x 1.42 x 10 -3 [mm 2 ] or less, or (S1 + S2) / S3 x 2.83 x 10 -3 [mm 2 ] or less, and the insertion process S30 of the embodiment 1 is performed. The insertion process S30 of the embodiment 1 is performed in an environment having a cleanliness higher than the grade 1000.

[0091] In the MCF preform 100 of Embodiment 1, the total of the areas of the gaps G in the cross section of the glass tube 10 orthogonal to the axial direction AD2 before drawing is (S1+S2) / S3x5.66x10 -4 [mm 2 ] or less, or (S1+S2) / S3x1.42x10 -3 [mm 2 ] or less, or (S1+S2) / S3x2.83x10 -3 [mm 2 ] or less, the cleanliness of the gaps G is higher than the grade 1000.

[0092] (Embodiment 2)

[0093] In the case where the management of the grade 1000 is difficult, the environment of the grade 10000 can also be used. Therefore, as Embodiment 2, the MCF preform 100 is considered to be assembled in the environment of the grade 10000 (the number of particles having a particle size of 0.5 μm or more contained per cubic foot is 10000 or less). Based on the same discussion as described above, by making the total of the areas of the gaps G in the cross section of the glass tube 10 orthogonal to the axial direction AD2 before drawing be

[0094] (S1+S2) / S3x0.3048 3 x10 -7 xF[m 2 ]

[0095] Hereinafter, the frequency of the quality abnormality targeted can be achieved.

[0096] The allowable frequency F of the quality abnormality is, for example, 0.02 pieces / km (1 piece per 50 km on average). In this case, by making the total of the areas of the gaps G be

[0097] (S1+S2) / S3x0.3048 3 x10 -7 x0.02[m 2 ]

[0098] =(S1+S2) / S3x5.66x10 -5 [mm 2 ]

[0099] This is sufficient.

[0100] The allowable frequency F of the quality abnormality can also be 0.05 pieces / km (1 piece per 20 km on average). In this case, by making the total of the areas of the gaps G be

[0101] (S1+S2) / S3x0.3048 3 x10 -7× 0.05 [m 2 ]

[0102] = (S1 + S2) / S3 x 1.42 x 10 -4 [mm 2 ]

[0103] The following is also acceptable.

[0104] The allowable frequency F of quality abnormality can also be 0.10 per km (1 per 10 km on average). In this case, as long as the sum of the areas of the gaps G is

[0105] (S1 + S2) / S3 x 0.3048 3 x 10 -7 x 0.10 [m 2 ]

[0106] = (S1 + S2) / S3 x 2.83 x 10 -4 [mm 2 ]

[0107] The following is also acceptable.

[0108] That is, the sum of the areas of the gaps G in the cross section of the glass tube 10 orthogonal to the axial direction AD2 before drawing is set to (S1 + S2) / S3 x 5.66 x 10 -5 [mm 2 ] or less, or (S1 + S2) / S3 x 1.42 x 10 -4 [mm 2 ] or less, or (S1 + S2) / S3 x 2.83 x 10 -4 [mm 2 ] or less, and each of the values of S1, S2, and S3 is set in the manner, and the insertion process S30 of the specific example 2 is performed. The insertion process S30 of the specific example 2 is performed in an environment having a cleanliness higher than the grade 10000.

[0109] In the MCF preform 100 of the specific example 2, the sum of the areas of the gaps G in the cross section of the glass tube 10 orthogonal to the axial direction AD2 before drawing is (S1 + S2) / S3 x 5.66 x 10 -5 [mm 2 ] or less, or (S1 + S2) / S3 x 1.42 x 10 -4 [mm 2 ] or less, or (S1 + S2) / S3 x 2.83 x 10 -4 [mm 2 ] or less, and the cleanliness of the gaps G is higher than the grade 10000.

[0110] The above describes the embodiments, but the present disclosure is not necessarily limited to the above-described embodiments and modified examples, and various changes can be made within the scope of the gist thereof. For example, the MCF 1 can also have a low refractive index portion disposed between the adjacent cores 2 and surrounded by the cladding 3. In this case, the plurality of holes 11 include holes provided at positions corresponding to the low refractive index portion, and the plurality of glass rods 30 include glass rods that become the low refractive index portion. The above-described embodiments and modified examples can also be appropriately combined.

[0111] Explanation of Reference Signs 1 MCF

[0112] 2 Core

[0113] 3 Cladding

[0114] 4 Mark

[0115] 10 Glass Tube

[0116] 10a First End

[0117] 10b Second End

[0118] 11, 11A, 11B Hole

[0119] 20 Dummy Tube

[0120] 21 Hole 30, 30A, 30B Glass Rod 40 Sealing Portion

[0121] 50 Coupler

[0122] 51 Vacuum Pump

[0123] 52 Gas Supply Portion

[0124] 100, 101 MCF Preform

[0125] 102 Optical Fiber Preform

[0126] G Gap

[0127] AD1, AD2 Axial Direction

Claims

1. A method of manufacturing a multi-core optical fiber having a plurality of cores and a cladding that surrounds the plurality of cores, the method of manufacturing a multi-core optical fiber comprising the steps of: inserting a plurality of glass rods into a plurality of holes provided in a glass tube; sealing a first end of the glass tube; and drawing the glass tube while integrating the glass tube with the plurality of glass rods by reducing the pressure inside the plurality of holes from a second end of the glass tube, the step of inserting being performed in an environment having a cleanliness higher than Class 1000.

2. The method of manufacturing a multi-core optical fiber according to claim 1, wherein 3. The method of manufacturing a multi-core optical fiber according to claim 1, wherein 4. A method of manufacturing a multi-core optical fiber having a plurality of cores and a cladding that surrounds the plurality of cores, the method of manufacturing a multi-core optical fiber comprising the steps of: inserting a plurality of glass rods into a plurality of holes provided in a glass tube; sealing a first end of the glass tube; and drawing the glass tube while integrating the glass tube with the plurality of glass rods by reducing the pressure inside the plurality of holes from a second end of the glass tube, the step of inserting being performed in an environment having a cleanliness higher than Class 10000.

5. The method of manufacturing a multi-core optical fiber according to claim 4, wherein when the cross-sectional area of the glass tube before the drawing process is set as S1 [m 2 ], the sum of the cross-sectional areas of the plurality of glass rods before the drawing process is set as S2 [m 2 ], and the cross-sectional area of the multi-core optical fiber after the drawing process is set as S3 [m 2 ], the inserting process is performed in such a manner that the sum of the areas of the gaps between the glass tube before the drawing process and the plurality of glass rods in the cross section of the glass tube orthogonal to the axial direction is (S1+S2) / S3 x 5.66 x 10 -4 [mm 2 ] or less, 6. The method of manufacturing a multi-core optical fiber according to claim 4, wherein 7. A multi-core optical fiber preform that is a preform for a multi-core optical fiber having a plurality of cores and a cladding that surrounds the plurality of cores, the multi-core optical fiber preform comprising: a glass tube provided with a plurality of holes; and a plurality of glass rods inserted into the plurality of holes, the cleanliness of the gap being higher than Class 1000. so that the sum of the areas of the gaps is (S1+S2) / S3 x 1.42 x 10 -3 [mm 2 The insertion process is performed in the following manner.

8. A multi-core optical fiber preform that is a preform for a multi-core optical fiber having a plurality of cores and a cladding that surrounds the plurality of cores, the multi-core optical fiber preform comprising: a glass tube provided with a plurality of holes; and a plurality of glass rods inserted into the plurality of holes, the cleanliness of the gap being higher than Class 10000. so that the sum of the areas of the gaps is (S1+S2) / S3 x 2.83 x 10 -3 [mm 2 The insertion process is performed in the following manner. ​ ​ ​ ​ ​ when the cross-sectional area of the glass tube before the drawing process is set as S1 [m 2 ], the sum of the cross-sectional areas of the plurality of glass rods before the drawing process is set as S2 [m 2 ], and the cross-sectional area of the multi-core optical fiber after the drawing process is set as S3 [m 2 ], the inserting process is performed in such a manner that the sum of the areas of the gaps between the glass tube before the drawing process and the plurality of glass rods in the cross section of the glass tube orthogonal to the axial direction is (S1+S2) / S3 x 5.66 x 10 -5 [mm 2 ] or less, ​ ​ so that the sum of the areas of the gaps is (S1+S2) / S3 x 1.42 x 10 -4 [mm 2 The insertion process is performed in the following manner. ​ so that the sum of the areas of the gaps is (S1+S2) / S3 x 2.83 x 10 -4 [mm 2 The insertion process is performed in the following manner. ​ ​ ​ When the cross-sectional area of the glass tube before drawing is set as S1 [m 2 ], the sum of the cross-sectional areas of the plurality of glass rods before drawing is set as S2 [m 2 ], and the cross-sectional area of the multi-core optical fiber after drawing is set as S3 [m 2 ], the sum of the areas of the gaps between the glass tube before drawing and the plurality of glass rods in the cross section of the glass tube orthogonal to the axis direction is (S1+S2) / S3 x 5.66 x 10 -4 [mm 2 ] or less. ​ ​ ​ ​ when the cross-sectional area of the glass tube before drawing is set as S1 [m 2 ], the sum of the cross-sectional areas of the plurality of glass rods before drawing is set as S2 [m 2 ], and the cross-sectional area of the multi-core optical fiber after drawing is set as S3 [m 2 ], the sum of the areas of the gaps between the glass tube before drawing and the plurality of glass rods in the cross section of the glass tube orthogonal to the axis is (S1+S2) / S3 x 5.66 x 10 -5 [mm 2 ] or less. ​

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