Multicore Fiber

Abstract

The purpose of the present invention is to provide a multicore fiber in which increases in transmission loss are suppressed. This multicore fiber comprises a plurality of first glass regions including a core and a first cladding having a lower refractive index than the maximum refractive index of the core, and a cladding region formed circumferentially around the plurality of first glass regions, wherein the plurality of first glass regions are subjected to compressive stress. The cladding region is subjected to tensile stress. The multicore fiber further comprises a second glass region which is surrounded by the plurality of first glass regions and which includes a second cladding in contact with the first cladding of each of the plurality of first glass regions, wherein, in a distribution of the stress produced between the cores of the plurality of first glass regions facing each other with the second glass region in between, the average value of the compressive stress in the first glass regions is less than the average value of the compressive stress in the second glass region.

Description

[Technical Field]

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

[0002] Conventionally, multi-core fibers, which are optical fibers having a plurality of core portions, have been known. Generally, when manufacturing a multi-core fiber, a multi-core fiber preform is used that includes a cylindrical cladding rod that becomes a cladding portion, and a plurality of core rods that have core portions and cladding portions formed on the outer peripheries of the core portions. The multi-core fiber is manufactured by integrating and drawing the multi-core fiber preforms (or by drawing them while integrating them).

[0003] As a method for manufacturing such a multicore fiber preform, for example, a drilling method is known in which a plurality of holes are formed (bored) in a cylindrical cladding rod using a drill, and a plurality of core rods are inserted into these holes, respectively (see Patent Document 1). In this drilling method, the cladding rod in which a plurality of holes has been drilled and the plurality of core rods inserted into these holes, respectively, are integrated by heat treatment. As a result, the multicore fiber preform becomes a preform in which a cladding portion and a plurality of core portions are integrated. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-209702 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in a multi-core fiber preform manufactured by the hole-punching method, when the cladding rod after the hole-punching process and the core rods in the plurality of holes are integrated by heat treatment, tensile stress concentrates on each of the plurality of core portions toward the cladding portion due to the influence of thermal contraction of the cladding portion, etc. As a result, in a multi-core fiber drawn from the multi-core fiber preform, tensile stress is applied from the cladding portion to each of the plurality of core portions, which causes a problem of an increase in transmission loss of the multi-core fiber.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a multi-core fiber in which an increase in transmission loss is suppressed. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems and achieve the object, the multicore fiber according to the present invention is a multicore fiber including a plurality of first glass regions each having a core region and a first cladding region having a refractive index lower than the maximum refractive index of the core region, and a cladding region formed on the outer periphery of the plurality of first glass regions, wherein compressive stress is applied to the plurality of first glass regions.

[0008] Moreover, in the multi-core fiber according to the present invention, a tensile stress is applied to the cladding region.

[0009] Furthermore, the multicore fiber according to the present invention comprises a plurality of first glass regions each having a first core portion and a first cladding portion having a refractive index lower than the maximum refractive index of the first core portion; a second glass region having a second cladding portion in contact with the first cladding portion of each of the plurality of first glass regions and surrounded by the plurality of first glass regions; and a cladding region formed on the outer periphery of the plurality of first glass regions and the second glass region, wherein in a stress distribution generated between the first core portions of the plurality of first glass regions facing each other across the second glass region, an average value of compressive stress in the first glass regions is smaller than an average value of compressive stress in the second glass regions.

[0010] Moreover, in the multi-core fiber according to the present invention, in the above invention, the difference between the maximum value and the minimum value of the stress in the stress distribution is 30 MPa or less.

[0011] Moreover, in the multicore fiber according to the present invention, in the above invention, the second glass region has a second core portion having a maximum refractive index higher than the refractive index of the second cladding portion.

[0012] Moreover, in the multi-core fiber according to the present invention, in the above invention, the second cladding portion contains a low-softening point glass having a softening point lower than that of the first cladding portion.

[0013] Moreover, in the multi-core fiber according to the present invention, the stress applied to the core portion from the first cladding portion includes a tensile stress component.

[0014] Moreover, in the multi-core fiber according to the present invention, in the above invention, the stress distribution does not have an inversion between tensile stress and compressive stress.

[0015] Moreover, in the multi-core fiber according to the present invention, the maximum value of the stress in the stress distribution is 80 MPa or less. [Effects of the Invention]

[0016] The present invention has an effect of realizing a multi-core fiber in which an increase in transmission loss is suppressed. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a cross-sectional view showing an example of the configuration of a multi-core fiber preform used for manufacturing a multi-core fiber according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a flowchart showing an example of a method for manufacturing a multi-core fiber according to the first embodiment of the present invention. [Figure 3] FIG. 3 is a cross-sectional view showing an example of a cladding in which holes are formed by the hole forming step of the first embodiment. [Figure 4] FIG. 4 is a cross-sectional view illustrating a state in which a glass rod is inserted into a hole in the cladding by the insertion step of the first embodiment. [Figure 5] FIG. 5 is a schematic diagram for explaining the drawing step in the first embodiment of the present invention. [Figure 6] FIG. 6 is a cross-sectional view showing an example of the configuration of a multi-core fiber according to the first embodiment of the present invention. [Figure 7] FIG. 7 is a diagram illustrating the difference between the maximum value and the minimum value in the stress distribution of the multi-core fiber according to the first embodiment of the present invention. [Figure 8] FIG. 8 is a diagram illustrating the difference between the maximum value and the minimum value in the stress distribution of a conventional multi-core fiber manufactured from a multi-core fiber preform by the punching method. [Figure 9] FIG. 9 is a cross-sectional view showing an example of the configuration of a multi-core fiber according to the second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to these embodiments. In the drawings, identical or corresponding elements are appropriately designated by the same reference numerals. It should be noted that the drawings are schematic, and the dimensional relationships and ratios of elements may differ from the actual figures. The dimensional relationships and ratios may differ between drawings. In this specification, the term "cutoff wavelength" refers to the cable cutoff wavelength defined in ITU-T (International Telecommunication Union) G.650.1. Other terms not specifically defined in this specification will be defined and measured in accordance with the definitions and measurement methods in ITU-T G.650.1 and G.650.2.

[0019] (Embodiment 1) [Configuration of multi-core fiber preform] First, the configuration of a multicore fiber preform used in manufacturing a multicore fiber according to embodiment 1 of the present invention will be described. Fig. 1 is a cross-sectional view showing an example of the configuration of a multicore fiber preform used in manufacturing a multicore fiber according to embodiment 1 of the present invention. As shown in Fig. 1, the multicore fiber preform 1 according to embodiment 1 includes a plurality of (for example, four) first glass rods 2, a second glass rod 3, and a cladding 4. Note that in the multicore fiber preform 1 shown in Fig. 1, the plurality of first glass rods 2, second glass rods 3, and cladding 4 are integrated by an integration step described later.

[0020] As shown in FIG. 1, each of the multiple first glass rods 2 has a first core portion 2a and a first cladding portion 2b as core portions. The first core portion 2a is made of, for example, silica-based glass doped with a dopant (such as germanium) to increase the refractive index. The first cladding portion 2b is made of glass having a refractive index lower than the maximum refractive index of the first core portion 2a. For example, the glass constituting the first cladding portion 2b may be pure silica glass not doped with a dopant for adjusting the refractive index. As shown in FIG. 1, the first cladding portion 2b is formed on the outer periphery of the first core portion 2a. A contact surface 2c, which is a flat surface that comes into contact with the second glass rod 3, is formed on a part of the outer periphery of the first cladding portion 2b.

[0021] As shown in Fig. 1, the second glass rod 3 has a second cladding portion 3b and is surrounded by a plurality of first glass rods 2. More specifically, as shown in Fig. 1, the second cladding portion 3b is in contact with the first cladding portion 2b of each of the plurality of first glass rods 2. The second cladding portion 3b has a contact surface 3c, which is a flat surface that contacts the first cladding portion 2b, at each portion of the outer periphery facing each of the plurality of first glass rods 2. That is, the contact surface 2c of the first glass rod 2 and the contact surface 3c of the second glass rod 3 are flat surfaces that abut against each other.

[0022] The second cladding region 3b may be made of silica-based glass similar to that of the first cladding region 2b. However, it is preferable that the second cladding region 3b contain a low-softening-point glass having a lower softening point than the first cladding region 2b. For example, the low-softening-point glass may be made by adding a dopant for lowering the softening point, such as potassium (K), phosphorus (P), chlorine (Cl), fluorine (F), or germanium (Ge), to glass having a refractive index lower than the maximum refractive index of the first core region 2a, such as pure silica glass without any dopant for adjusting the refractive index. When F and Ge are co-doped as the dopants, the amounts of F and Ge added may be adjusted so that the refractive index of the doped glass is the same as or close to the refractive index of pure silica glass (i.e., approximately 1.444 at a wavelength of 1550 nm).

[0023] 1, the cladding 4 is formed on the outer periphery of a plurality of first glass rods 2 and second glass rods 3. For example, the cladding 4 is made of silica-based glass similar to the first cladding portion 2b described above. Note that the refractive indices of the first cladding portion 2b, the second cladding portion 3b, and the cladding 4 may be the same as or different from one another.

[0024] In the first embodiment, as shown in Fig. 1, the first core portions 2a are arranged to face each other across the central axis A of the second cladding portion 3b in the second glass rod 3. In this case, the distances between the central axes of the first core portions 2a and the central axis A of the second cladding portion 3b may be the same or different. The central axis A of the second cladding portion 3b may or may not coincide with the central axis of the cladding 4. In the present invention, the "central axis" means the central axis in the longitudinal direction of the element in question.

[0025] [Method for manufacturing multi-core fiber] Next, a method for manufacturing a multicore fiber according to embodiment 1 of the present invention will be described. Fig. 2 is a flowchart showing an example of the method for manufacturing a multicore fiber according to embodiment 1 of the present invention. In embodiment 1, by performing each of steps S101 to S106 shown in Fig. 2, first, a multicore fiber preform 1 is manufactured, and then a target multicore fiber is manufactured from this multicore fiber preform 1.

[0026] 2, a preparation step is first performed to prepare members for manufacturing the multi-core fiber preform 1 (step S101). In this step S101, a plurality of cylindrical first glass rods 2 (four in the first embodiment) each having the above-described first core portion 2a and first cladding portion 2b are prepared. Also, a cylindrical second glass rod 3 having the above-described second cladding portion 3b and a cylindrical cladding 4 are prepared. The first glass rod 2, the second glass rod 3, and the cladding 4 can be manufactured using well-known methods such as a VAD (Vapor Phase Axial Deposition) method, an OVD (Outside Vapor Deposition) method, or an MCVD (Modified Chemical Vapor Deposition) method.

[0027] After the preparation step of step S101 is performed, a hole forming step (step S102) is performed in which holes for inserting the plurality of first glass rods 2 and the plurality of second glass rods 3 are formed in the cylindrical clad 4. In step S102, a plurality of holes for inserting the plurality of first glass rods 2 and a hole for inserting the second glass rod are formed in the cylindrical clad 4.

[0028] Fig. 3 is a cross-sectional view showing an example of a cladding in which holes are formed by the hole forming step of the first embodiment. As shown in Fig. 3, a plurality of first holes 4a (four in the first embodiment) for inserting a plurality of first glass rods 2 and a second hole 4b for inserting a second glass rod 3 are formed inside the cylindrical cladding 4 so as to extend along the longitudinal direction of the cladding 4. In this case, the first holes 4a are formed at positions in the cladding 4 where the first glass rods 2 are to be disposed. The second holes 4b are formed at positions in the cladding 4 where the second glass rods 3 are to be disposed. Furthermore, as shown in Fig. 3, the first holes 4a and the second holes 4b are formed so as to communicate with each other via openings 4c.

[0029] The above-described plurality of first pores 4a and second pores 4b can be simultaneously formed, for example, by using a non-cutting method and applying heat to the cylindrical cladding 4. Alternatively, a cladding 4 in which the plurality of first pores 4a and second pores 4b are formed in advance may be prepared by a powder molding method or the like.

[0030] After the hole forming step S102, an abutment surface forming step is performed (step S103), in which abutment surfaces are formed on the outer peripheries of the plurality of first glass rods 2 and the plurality of second glass rods 3. In step S103, a portion of the outer periphery of each of the plurality of cylindrical first glass rods 2 (the portion facing the second glass rod 3) is ground along the longitudinal direction of the first glass rod 2. This forms an abutment surface 2c (see FIG. 1) on each of the plurality of first glass rods 2. Furthermore, a portion of the outer periphery of each of the cylindrical second glass rods 3 (the portion facing the plurality of first glass rods 2) is ground along the longitudinal direction of the second glass rod 3. This forms an equal number of abutment surfaces 3c (see FIG. 1) on the second glass rod 3 as there are first glass rods 2.

[0031] In the present invention, the first glass rod 2 and the second glass rod 3 may be prepared with the contact surfaces 2c and 3c formed in advance by a powder molding method, etc. The contact surface forming step S103 may be performed before the hole forming step S102.

[0032] Next, an insertion step is performed in which a plurality of first glass rods 2 and a plurality of second glass rods 3 are inserted into the holes formed in the cladding 4 (step S104). In this step S104, a plurality of first glass rods 2 are inserted into the plurality of first holes 4a formed in the cladding 4, and a second glass rod 3 is inserted into the plurality of second holes 4b formed in the cladding 4.

[0033] Fig. 4 is a cross-sectional view illustrating the state in which glass rods are inserted into the holes in the cladding by the insertion step of Embodiment 1. As shown in Fig. 4, a plurality of (e.g., four) first glass rods 2 are inserted into a plurality of (e.g., four) first holes 4a so that the abutment surfaces 3c of the second glass rods 3 inserted into the second holes 4b in the cladding 4 abut against the abutment surfaces 2c of the first glass rods 2. Note that the order in which these first glass rods 2 and second glass rods 3 are inserted is not particularly important in the present invention.

[0034] After the insertion step of step S104, an integration step (step S105) is carried out to integrate the above-mentioned plurality of first glass rods 2, second glass rods 3, and clad 4. In step S105, the intermediate structure (see FIG. 4) obtained by inserting the first glass rods 2 and second glass rods 3 into the holes in the clad 4 in the above-mentioned insertion step is heated, for example, in a heating furnace. This heating process closes (collapses) the gaps between the plurality of first glass rods 2, second glass rods 3, and clad 4 in the intermediate structure, and the first glass rods 2, second glass rods 3, and clad 4 are integrated as shown in FIG. 1.

[0035] By performing each of the above-described steps S101 to S105, the desired multicore fiber preform 1 is manufactured. Note that, in the present invention, the integration step of the above-described step S105 may be omitted, and integration of the intermediate structures and drawing of the multicore fiber preform 1 may be performed simultaneously in the drawing step described next.

[0036] Next, a drawing step is performed in which the obtained multi-core fiber preform 1 is drawn to manufacture a desired multi-core fiber (step S106). Fig. 5 is a schematic view for explaining the drawing step in embodiment 1 of the present invention. Fig. 5 illustrates an example of an optical fiber manufacturing apparatus 10 for drawing the multi-core fiber preform 1 to manufacture a desired multi-core fiber 15.

[0037] 5 , in step S106, the multicore fiber preform 1 is set in the drawing furnace 11 of the optical fiber manufacturing apparatus 10, one end of the set multicore fiber preform 1 is heated and melted by a heater 11a in the drawing furnace 11, and a glass optical fiber 12 is drawn vertically downward from one end of the multicore fiber preform 1. Thereafter, an ultraviolet-curable resin is applied to the outer circumferential surface of the glass optical fiber 12 by a coating device 13, and the applied ultraviolet-curable resin is hardened by irradiation with ultraviolet rays from an ultraviolet irradiation device 14. As a result, the glass optical fiber 12 becomes a multicore fiber 15 whose outer circumferential surface is coated with resin. Then, a guide roller 16 guides the multicore fiber 15 to a winding machine 17, and the winding machine 17 winds the multicore fiber 15 around a bobbin. In this manner, the multicore fiber 15 is manufactured.

[0038] Note that, before setting the multicore fiber preform 1 in the optical fiber manufacturing apparatus 10, a tapered member having an outer diameter of the welded portion substantially equal to that of the multicore fiber preform 1 may be welded to the drawing start end of the multicore fiber preform 1. This reduces the manufacturing loss at the start of drawing the multicore fiber preform 1, and also allows most of the assembled multicore fiber preform 1 to be used as a product part.

[0039] [Multicore fiber configuration] Next, the configuration of the multicore fiber according to the first embodiment of the present invention will be described. Fig. 6 is a transverse sectional view showing an example of the configuration of the multicore fiber according to the first embodiment of the present invention. Fig. 6 illustrates an example of the configuration of the transverse cross section of a multicore fiber 15 manufactured using the above-mentioned multicore fiber preform 1 (see Fig. 1 ). Fig. 6 also illustrates an example of a stress distribution L1 showing the relationship between the radial position in the transverse cross section of the multicore fiber 15 and the generated stress. In the XY coordinate system showing this stress distribution L1, the X axis is a coordinate axis indicating the radial position of the multicore fiber 15. The Y axis is a coordinate axis indicating the magnitude of each of the tensile stress and compressive stress in the transverse cross section of the multicore fiber 15. Specifically, a positive value on the Y axis is a value of tensile stress, and a negative value on the Y axis is a value of compressive stress. The value of this tensile stress increases in the positive direction of the Y axis, and the value of this compressive stress increases in the negative direction of the Y axis. Furthermore, a stress that changes in the positive direction of the Y axis is a component of tensile stress, and a stress that changes in the negative direction of the Y axis is a component of compressive stress.

[0040] 6 also illustrates a stress distribution L2 of a conventional multi-core fiber manufactured from a multi-core fiber preform by the punching method as a stress distribution for comparison with the stress distribution L1 of the multi-core fiber 15. This conventional multi-core fiber is the same as the multi-core fiber 15 according to the first embodiment, except that its preform is manufactured by the punching method.

[0041] As shown in Fig. 6, the multi-core fiber 15 according to the first embodiment includes a plurality of (for example, four) first glass regions 22, a second glass region 23, and a cladding region 24. The plurality of first glass regions 22 are glass regions corresponding to the plurality of first glass rods 2 in the multi-core fiber preform 1 described above. Compressive stress is applied to the plurality of first glass regions 22. The second glass region 23 is a glass region corresponding to the second glass rod 3 in the multi-core fiber preform 1 described above. The cladding region 24 is a glass region corresponding to the cladding 4 in the multi-core fiber preform 1 described above. Tensile stress is applied to the cladding region 24. Although not particularly shown, a coating is applied to the outer periphery of the cladding region 24. A coating that is normally used for optical fibers is used as this coating.

[0042] Specifically, as shown in Fig. 6, each of the plurality of first glass regions 22 has a first core portion 22a and a first cladding portion 22b having a refractive index lower than the maximum refractive index of the first core portion 22a. The first core portion 22a is a core portion corresponding to the first core portion 2a of the first glass rod 2 in the multicore fiber preform 1 (see Fig. 1) described above, and although it has a different size from the first core portion 2a, it is made of the same silica-based glass as the first core portion 2a. The first cladding portion 22b is a cladding portion corresponding to the first cladding portion 2b of the first glass rod 2 in the multicore fiber preform 1 described above, and as shown in Fig. 6, the first cladding portion 22b is formed on the outer periphery of the first core portion 22a. Although it has a different size from the first cladding portion 2b of the multicore fiber preform 1, it is made of the same silica-based glass as the first cladding portion 2b. In addition, a contact surface 22c, which is a flat surface that comes into contact with the second cladding portion 23b of the second glass region 23, is formed on part of the outer periphery of the first cladding portion 22b.

[0043] As shown in Fig. 6, the second glass region 23 has a second cladding portion 23b corresponding to the second cladding portion 3b of the second glass rod 3 in the multicore fiber preform 1 described above, and is surrounded by a plurality of (four in the first embodiment) first glass regions 22. In detail, as shown in Fig. 6, the second cladding portion 23b is in contact with the first cladding portion 22b of each of the plurality of first glass regions 22. A contact surface 23c, which is a flat surface in contact with the first cladding portion 22b, is formed on each portion of the outer periphery of this second cladding portion 23b that faces each of the plurality of first glass regions 22. That is, the contact surface 22c of the first cladding portion 22b and the contact surface 23c of the second cladding portion 23b are flat surfaces that abut against each other.

[0044] Furthermore, the second cladding portion 23b has dimensions different from those of the second cladding portion 3b of the multicore fiber preform 1, but is made of the same silica-based glass as the second cladding portion 3b. For example, the second cladding portion 23b may be made of the same silica-based glass as the first cladding portion 22b of the first glass region 22, but preferably contains a low-softening-point glass having a softening point lower than that of the first cladding portion 22b. Furthermore, the second cladding portion 23b is more preferably made of a low-softening-point glass. This low-softening-point glass is the same as that in the second cladding portion 3b of the multicore fiber preform 1 described above.

[0045] As shown in Fig. 6, the cladding region 24 is formed on the outer periphery of the plurality of first glass regions 22 and second glass regions 23. The cladding region 24 corresponds to the cladding 4 of the multicore fiber preform 1 described above, and although it has different dimensions from the cladding 4, it is made of the same silica-based glass as the cladding 4. For example, the cladding region 24 is made of the same silica-based glass as the first cladding portion 22b described above. Note that the refractive indices of the first cladding portion 22b, the second cladding portion 23b, and the cladding region 24 described above may be the same as or different from one another.

[0046] 6, for example, the plurality of first core portions 22a are arranged to face each other across the central axis A of the second cladding portion 23b in the second glass region 23. In this case, the distances between the central axes of the plurality of first core portions 22a and the central axis A of the second cladding portion 23b may be the same as each other or may be different from each other. The central axis A of the second cladding portion 23b may or may not coincide with the central axis of the cladding region 24. The central axis A of the second cladding portion 23b is the same as the central axis of the second glass rod 3 in the multicore fiber preform 1 described above.

[0047] In the multi-core fiber 15 having the above-described configuration, a stress distribution L1 occurs along the radial direction (X-axis direction), as shown in Fig. 6, for example. The stress distribution L1 of the multi-core fiber 15 includes tensile stress and compressive stress occurring in the first core portion 22a, the first cladding portion 22b, the second cladding portion 23b, and the cladding region 24. In this stress distribution L1, stress at a position where the tensile stress component is larger than the compressive stress component becomes tensile stress, and stress at a position where the tensile stress component is smaller than the compressive stress component becomes compressive stress. Specifically, as shown by the stress distribution L1 in Fig. 6, tensile stress occurs in the cladding region 24, and compressive stress occurs in the plurality of first glass regions 22 and second glass regions 23.

[0048] Furthermore, stress is applied from the first cladding portion 22b to the first core portion 22a in each of the multiple first glass regions 22. The stress applied from the first cladding portion 22b to the first core portion 22a includes a tensile stress component from the first core portion 22a side to the first cladding portion 22b side.

[0049] Here, when the multi-core fiber 15 is manufactured from a multi-core fiber preform by a punching method, the stress in the first core portion 22a increases toward the tensile stress side (the positive direction of the Y-axis) as exemplified by the stress distribution L2 in Fig. 6. That is, an excessively large tensile stress component is applied to this first core portion 22a from the surrounding cladding portion.

[0050] In contrast, in the multi-core fiber 15 according to the first embodiment, the multiple first glass regions 22 and the second glass regions 23 are configured so that each of the multiple first cladding regions 22b, to which a large tensile stress is applied, is in contact with the second cladding region 23b. This relieves stress concentration on the first core region 22a, and reduces the tensile stress component applied to the first core region 22a from the first cladding region 22b. As a result, a compressive stress is applied to each of the multiple first glass regions 22, and therefore the magnitude relationship between the tensile stress component and the compressive stress component is not reversed. Furthermore, in the multi-core fiber 15 according to the first embodiment, from the viewpoint of reducing the tensile stress component applied to the first core region 22a, it is preferable that the second cladding region 23b of the second glass region 23 contains a low-softening point glass.

[0051] Furthermore, when the multi-core fiber 15 is manufactured from a multi-core fiber preform by a punching method, as exemplified by the stress distribution L2 in Fig. 6, in a region R1 between the first core portions 22 a-1, 22 a-2 facing each other in the radial direction of the multi-core fiber 15, tensile stress and compressive stress are reversed (see region R2 in Fig. 6). In contrast, in the multi-core fiber 15 according to the first embodiment, as exemplified by the stress distribution L1 in Fig. 6, there is no reversal of tensile stress and compressive stress in the stress distribution generated between the first core portions 22 a-1, 22 a-2 of the plurality of first glass regions 22 facing each other across the second glass region 23. This stress distribution is generated in the region R1 between the first core portions 22 a-1, 22 a-2 out of the entire stress distribution L1 of the multi-core fiber 15.

[0052] Fig. 7 is a diagram illustrating the difference between the maximum value and the minimum value in the stress distribution of the multi-core fiber according to the first embodiment of the present invention. As shown in Fig. 7, in the multi-core fiber 15 according to the first embodiment, in the stress distribution occurring in the above-mentioned region R1 (see Fig. 6) of the entire stress distribution L1, the average value of the compressive stress in the first glass region 22 is smaller than the average value of the compressive stress in the second glass region 23. Furthermore, the difference ΔS1 between the maximum value and the minimum value of the stress in the stress distribution is preferably 30 MPa or less. The difference ΔS1 is more preferably 20 MPa or less. Furthermore, the maximum value of the stress is preferably 80 MPa or less.

[0053] In the first embodiment, the stress distribution is a stress distribution occurring between the first core portions 22a-1 and 22a-2 of the multiple first glass regions 22 that face each other across the second glass region 23. The maximum value in the stress distribution is the maximum compressive stress value Sa, and the minimum value in the stress distribution is the minimum compressive stress value Sb (see FIG. 7).

[0054] Fig. 8 is a diagram illustrating the difference between the maximum value and the minimum value in the stress distribution of a conventional multi-core fiber manufactured from a multi-core fiber preform by a piercing method. As shown in Fig. 8, in this conventional multi-core fiber 115, the difference ΔS2 between the maximum value and the minimum value of stress in the stress distribution generated in the same region R1 as in the multi-core fiber 15 according to the first embodiment within the entire stress distribution L2 is greater than 30 MPa. That is, in the conventional multi-core fiber 115, it is more difficult to reduce the stress concentrated in the core portions than in the multi-core fiber 15 according to the first embodiment described above. [Example]

[0055] Next, an example of the present invention will be described. In this example, a sample of a multi-core fiber 15 (hereinafter referred to as an example sample) was fabricated by, for example, drawing the multi-core fiber preform 1 according to the above-described first embodiment using the optical fiber manufacturing apparatus 10 shown in Fig. 5. The structure and optical properties of the fabricated example sample are as follows.

[0056] Specifically, in the example sample, the core shape of the first core portion 22a was unimodal, and the relative refractive index difference Δ of the first core portion 22a with respect to the first cladding portion 22b was 0.35%. The core diameter of the first core portion 22a was 8.1 μm, and the diameter of the cladding region 24 (cladding diameter) was 125.0 μm. The number of first core portions 22a (number of cores) was four, and the interval between the first core portions 22a (core pitch) was 40.1 μm. The cutoff wavelength was 1267 nm, and the mode field diameter (MFD) at a wavelength of 1550 nm was 10.6 μm.

[0057] Furthermore, as a comparative example for this example, a multicore fiber sample (hereinafter referred to as a comparative example sample) was fabricated from a multicore fiber preform fabricated by the hole-punching method. This comparative example sample is similar to the above-described example sample except that a multicore fiber preform fabricated by the hole-punching method was used.

[0058] The example samples and comparative example samples prepared as described above were evaluated for optical propagation loss. The evaluation results of the structures, optical properties, and propagation loss of these example samples and comparative example samples are summarized in Table 1.

[0059] [Table 1]

[0060] As shown in Table 1, the propagation loss of the example sample was 0.190 dB / km. This propagation loss value is comparable to that of a single-core optical fiber having one core portion similar to that of the example sample. That is, in the example sample, the propagation loss of light could be suppressed to a level comparable to that of a single-core optical fiber. On the other hand, the propagation loss of the comparative example sample was 0.205 dB / km. From this evaluation result, it was confirmed that the propagation loss of light in the comparative example sample was increased compared to that of the example sample, and that it was difficult to suppress the increase in propagation loss.

[0061] As described above, the first embodiment of the present invention provides a multicore fiber including: a plurality of first glass regions each having a first core portion and a first cladding portion whose refractive index is lower than the maximum refractive index of the first core portion; a second glass region having a second cladding portion in contact with the first cladding portion of each of the first glass regions and surrounded by the plurality of first glass regions; and a cladding region formed on the outer periphery of the plurality of first glass regions and the second glass region. In this multicore fiber, in a stress distribution generated between the first core portions of the plurality of first glass regions facing each other across the second glass region, the average value of compressive stress in the first glass regions is smaller than the average value of compressive stress in the second glass regions. In this case, the difference between the maximum and minimum values ​​of stress in the stress distribution is, for example, 30 MPa or less.

[0062] Therefore, it is possible to alleviate the concentration of stress on each of the multiple first core portions and reduce the tensile stress component applied to the first core portions from the first cladding portion side, thereby suppressing an increase in the transmission loss of the multi-core fiber, and as a result, it is possible to realize a multi-core fiber that can reduce the propagation loss of light to the same level as that of a single-core optical fiber.

[0063] Furthermore, in the first embodiment of the present invention, the second cladding portion is configured to contain a low-softening point glass having a softening point lower than that of the first cladding portion. Therefore, it is possible to more easily relieve the concentration of stress on each of the plurality of first core portions, and further reduce the tensile stress component applied to the first core portions from the first cladding portion side. This makes it possible to further suppress an increase in the transmission loss of the multicore fiber, and therefore it is possible to easily realize a multicore fiber that can reduce the propagation loss of light to the same level as that of a single-core optical fiber.

[0064] (Embodiment 2) Next, a second embodiment of the present invention will be described. Fig. 9 is a cross-sectional view showing one configuration example of a multi-core fiber according to the second embodiment of the present invention. As shown in Fig. 9, a multi-core fiber 15A according to the second embodiment includes a second glass region 23A instead of the second glass region 23 of the multi-core fiber 15 (see Fig. 6) according to the first embodiment described above. The other configurations are the same as those of the first embodiment, and the same components are denoted by the same reference numerals.

[0065] 9, the second glass region 23A has a second cladding portion 23b similar to that of the above-described embodiment 1, and a second core portion 23a having a maximum refractive index higher than that of the second cladding portion 23b. Furthermore, the second glass region 23A is surrounded by a plurality of (for example, four) first glass regions 22, similar to the above-described embodiment 1.

[0066] The second core portion 23a is made of, for example, silica-based glass doped with a dopant (such as germanium) to increase the refractive index. As shown in Fig. 9, the second core portion 23a is disposed at the same position as the central axis A of the second cladding portion 23b in the second glass region 23A. The refractive index of the second core portion 23a may be the same as or different from the refractive index of the first core portions 22a in the plurality of first glass regions 22. The central axis of the second core portion 23a may or may not coincide with the central axis A of the second cladding portion 23b.

[0067] Although not particularly shown, the multi-core fiber preform according to the present embodiment 2 includes a core portion that becomes the second core portion 23a in the second glass rod 3 of the multi-core fiber preform 1 (see FIG. 1) according to the above-described embodiment 1, and other than this configuration, it is the same as embodiment 1. Furthermore, the multi-core fiber 15A is manufactured from the multi-core fiber preform according to the present embodiment 2 using the optical fiber manufacturing apparatus 10 (FIG. 5) in the same way as in the above-described embodiment 1.

[0068] As described above, in the second embodiment of the present invention, the second glass region in the multicore fiber has a second core portion whose maximum refractive index is higher than the refractive index of the second cladding portion, and the other parts are configured in the same manner as in the first embodiment. Therefore, in a multicore fiber having a second core portion in an inner region surrounded by a plurality of first core portions, it is possible to enjoy the same effects as in the first embodiment described above.

[0069] In the above-described first and second embodiments, a multicore fiber having four first glass regions each having a first core portion and a first cladding portion is exemplified, but the present invention is not limited to this. In the multicore fiber according to the present invention, the number of the first glass regions (the number of first core portions and first cladding portions) may be two or more.

[0070] Furthermore, in the above-described second embodiment, a multi-core fiber in which one core portion (second core portion 23a) is arranged in the second glass region surrounded by a plurality of first glass regions has been exemplified, but the present invention is not limited to this. In the multi-core fiber according to the present invention, when core portions are arranged in the second glass region, the number of core portions arranged may be one or more.

[0071] In the above-described first and second embodiments, the cores (first and second cores) are made of silica-based glass doped with a dopant such as germanium, and the claddings (first and second claddings, cladding regions) are made of pure silica glass, but the present invention is not limited to this. For example, the cores may be made of pure silica glass, and the claddings may be made of silica-based glass doped with a dopant (e.g., fluorine) that reduces the refractive index.

[0072] In addition, in the above-described first and second embodiments, the holes for inserting the glass rods are formed in the cylindrical cladding constituting the multicore fiber preform by a cut-and-remove method, but the present invention is not limited to this. For example, the multicore fiber preform may be manufactured by a stack method in which a plurality of glass rods are arranged inside a glass tube having circular holes. Alternatively, the multicore fiber preform may be manufactured by preparing a cladding in which holes are formed by a powder molding method or the like, and inserting a plurality of glass rods into the holes in the cladding.

[0073] In the first and second embodiments described above, the first cladding sections and the second cladding section surrounded by the first cladding sections are in surface contact with each other, but the present invention is not limited to this. For example, the first cladding sections and the second cladding section described above may be in line contact with each other.

[0074] Furthermore, the present invention is not limited to the above-described first and second embodiments, and also includes configurations in which the above-described components are appropriately combined. In addition, other embodiments, examples, operational techniques, etc. made by those skilled in the art based on the above-described first and second embodiments are all included in the scope of the present invention. [Industrial Applicability]

[0075] As described above, the multi-core fiber according to the present invention is useful as an optical fiber having multiple cores. [Explanation of symbols]

[0076] 1. Multicore fiber preform 2. First glass rod 2a First Core 2b First clad section 2c Contact surface 3. Second glass rod 3b Second cladding section 3c Contact surface 4. Clad 4a 1st hole 4b Second hole 4c opening 10 Optical fiber manufacturing equipment 11 Wire drawing furnace 11a Heater 12 Glass optical fiber 13 Coating equipment 14 Ultraviolet irradiation device 15, 15A multi-core fiber 16 Guide roller 17 Winder 22 First Glass Area 22a, 22a-1, 22a-2 First core part 22b First cladding section 22c Contact surface 23, 23A Second glass area 23a Second Core 23b Second cladding section 23c Contact surface 24 Cladding region 115 Conventional multicore fiber A center axis L1, L2 stress distribution R1, R2 area

Claims

1. a plurality of first glass regions each having a first core portion and a first clad portion having a refractive index lower than the maximum refractive index of the first core portion; a second glass region having a second cladding portion in contact with the first cladding portion of each of the plurality of first glass regions and surrounded by the plurality of first glass regions; a cladding region formed on the outer periphery of the plurality of first glass regions and the second glass region; Equipped with In a stress distribution occurring between the first core portions of the plurality of first glass regions facing each other across the second glass region, an average value of compressive stress in the first glass region is smaller than an average value of compressive stress in the second glass region, Compressive stress is applied to the entire stress distribution. A multicore fiber characterized by:

2. a plurality of first glass regions each having a first core portion and a first clad portion having a refractive index lower than the maximum refractive index of the first core portion; a second glass region having a second cladding portion in contact with the first cladding portion of each of the plurality of first glass regions and surrounded by the plurality of first glass regions; a cladding region formed on the outer periphery of the plurality of first glass regions and the second glass region; Equipped with In a stress distribution occurring between the first core portions of the plurality of first glass regions facing each other across the second glass region, an average value of compressive stress in the first glass region is smaller than an average value of compressive stress in the second glass region, The stress distribution does not have a reversal between tensile stress and compressive stress. A multicore fiber characterized by:

3. The difference between the maximum and minimum stress values ​​in the stress distribution is 30 MPa or less. The multicore fiber according to claim 1 or 2.

4. the second glass region has a second core portion having a maximum refractive index higher than the refractive index of the second cladding portion; The multicore fiber according to any one of claims 1 to 3.

5. the second cladding portion contains a low-softening point glass having a softening point lower than that of the first cladding portion; The multicore fiber according to any one of claims 1 to 4.

6. the stress applied from the first cladding portion to the first core portion includes a tensile stress component; The multicore fiber according to any one of claims 1 to 5.

7. The stress distribution does not have a reversal between tensile stress and compressive stress. The multi-core fiber according to any one of claims 1, 3, 4, 5 and 6.

8. The maximum stress value in the stress distribution is 80 MPa or less. The multicore fiber according to any one of claims 1 to 7.

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