Multi-core optical fiber

Abstract

Provided is a multi-core optical fiber in which an increase in transmission loss is suppressed. The multi-core optical fiber includes a plurality of first glass regions each having a core portion and a first cladding portion having a refractive index lower than a maximum refractive index of the core portion, and a cladding region formed around the plurality of first glass regions, wherein a compressive stress is applied to the plurality of first glass regions. In addition, a tensile stress is applied to the cladding region. Further, the multi-core optical fiber includes 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 an average value of the compressive stress in the first glass regions is smaller than an average value of the compressive stress in the second glass region in a stress distribution between the core portions of the plurality of first glass regions across the second glass region.

Description

Technical Field

[0001] This invention relates to a multi-core optical fiber. Background Technology

[0002] Previously, optical fibers with multiple cores, known as multi-core optical fibers, were known. Typically, in manufacturing multi-core optical fibers, a multi-core fiber mother material is used, comprising a cylindrical clad rod serving as the cladding portion and multiple core rods, each core rod having a core and a cladding portion formed around the core. Multi-core optical fibers are manufactured by integrating the multi-core fiber mother material and then drawing it (or integrating and drawing it simultaneously).

[0003] As a method for manufacturing such multi-core optical fiber preforms, a perforation method is known, for example, which involves forming (perforating) multiple holes in a cylindrical cladding rod using a drill bit and inserting multiple core rods into the holes respectively (see Patent Document 1). In this perforation method, the cladding rod with the perforated holes and the multiple core rods inserted into the holes are integrated by heat treatment. As a result, the multi-core optical fiber preform becomes a preform where the cladding and multiple cores are integrated.

[0004] Prior art literature

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2011-209702 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] However, in multi-core optical fiber preforms manufactured using the perforation method, when the cladding rod and the core rods within the multiple holes are integrated through heat treatment, the tensile stress towards the cladding portion concentrates in each of the multiple cores due to factors such as thermal shrinkage of the cladding. As a result, in the multi-core optical fiber drawn from the preform, tensile stress is applied from the cladding portion to each of the multiple cores, thus increasing the transmission loss of the multi-core optical fiber.

[0009] The present invention was made in view of the above circumstances, and its object is to provide a multi-core optical fiber that suppresses the increase of transmission loss.

[0010] Solution for solving the problem

[0011] To address the aforementioned issues and achieve the objective, the multi-core optical fiber of the present invention comprises: a plurality of first glass regions having a core and a first cladding portion having a refractive index lower than the maximum refractive index of the core; and a cladding region formed on the outer periphery of the plurality of first glass regions, wherein the multi-core optical fiber is characterized in that compressive stress is applied to the plurality of first glass regions.

[0012] Furthermore, based on the above invention, the multi-core optical fiber of the present invention is characterized in that tensile stress is applied to the cladding region.

[0013] Furthermore, the multi-core optical fiber of the present invention is characterized by comprising: a plurality of first glass regions having a first core and a first cladding portion having a refractive index lower than the maximum refractive index of the first core; 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 being 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 the stress distribution generated between the first cores of the plurality of first glass regions opposite each other across the second glass region, the average value of the compressive stress in the first glass region is less than the average value of the compressive stress in the second glass region.

[0014] Furthermore, based on the above invention, the multi-core optical fiber of the present invention is characterized in that the difference between the maximum and minimum stress values ​​in the stress distribution is less than 30 MPa.

[0015] Furthermore, based on the above invention, the multi-core optical fiber of the present invention is characterized in that the second glass region has a second core with a maximum refractive index higher than that of the second cladding portion.

[0016] Furthermore, based on the above invention, the multi-core optical fiber of the present invention is characterized in that the second cladding portion includes low softening point glass with a softening point lower than that of the first cladding portion.

[0017] Furthermore, based on the above invention, the multi-core optical fiber of the present invention is characterized in that the stress applied from the first cladding portion to the first core portion includes a component of tensile stress.

[0018] Furthermore, based on the above invention, the multi-core optical fiber of the present invention is characterized in that there is no reversal of tensile stress and compressive stress in the stress distribution.

[0019] Furthermore, based on the above invention, the multi-core optical fiber of the present invention is characterized in that the maximum stress value in the stress distribution is below 80 MPa.

[0020] Invention Effects

[0021] According to the present invention, it achieves the effect of enabling multi-core optical fibers that suppress the increase of transmission loss. Attached Figure Description

[0022] Figure 1 This is a cross-sectional view showing a structural example of a multi-core optical fiber preform used in the manufacture of a multi-core optical fiber according to Embodiment 1 of the present invention.

[0023] Figure 2 This is a flowchart illustrating an example of a method for manufacturing a multi-core optical fiber according to Embodiment 1 of the present invention.

[0024] Figure 3 This is a cross-sectional view showing an example of a cladding with pores formed by the pore forming process of Embodiment 1.

[0025] Figure 4 This is a cross-sectional view illustrating the state in which the glass rod is inserted into the cavity of the cladding through the insertion process of Embodiment 1.

[0026] Figure 5 This is a schematic diagram illustrating the wire drawing process in Embodiment 1 of the present invention.

[0027] Figure 6 This is a cross-sectional view showing a structural example of a multi-core optical fiber according to Embodiment 1 of the present invention.

[0028] Figure 7 This is a diagram illustrating the difference between the maximum and minimum stress distribution in the multi-core optical fiber of Embodiment 1 of the present invention.

[0029] Figure 8 This is a diagram illustrating the difference between the maximum and minimum stress distributions in existing multi-core optical fibers manufactured from perforated multi-core optical fiber preforms.

[0030] Figure 9 This is a cross-sectional view showing a structural example of a multi-core optical fiber according to Embodiment 2 of the present invention. Detailed Implementation

[0031] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that the present invention is not limited to these embodiments. Furthermore, in the accompanying drawings, the same or corresponding elements are appropriately labeled with the same reference numerals. The drawings are schematic, and it should be noted that the dimensional relationships and ratios of the elements may sometimes differ from reality. The drawings may also include portions with different dimensional relationships and ratios. Additionally, in this specification, the cutoff wavelength refers to the cable cutoff wavelength defined by ITU-T (International Telecommunication Union) G.650.1. Furthermore, for other terms not specifically defined in this specification, the definitions and measurement methods in ITU-T G.650.1 and G.650.2 shall apply.

[0032] (Implementation Method 1)

[0033] [Structure of multi-core optical fiber parent material]

[0034] First, the structure of the multi-core optical fiber parent material used in the manufacture of the multi-core optical fiber of Embodiment 1 of the present invention will be described. Figure 1 This is a cross-sectional view illustrating a structural example of a multi-core optical fiber preform used in the manufacture of a multi-core optical fiber according to Embodiment 1 of the present invention. For example... Figure 1 As shown, the multi-core optical fiber mother material 1 of this embodiment 1 includes multiple (e.g., 4) first glass rods 2, second glass rods 3, and cladding 4. It should be noted that... Figure 1 In the multi-core optical fiber mother material 1 shown, multiple first glass rods 2, second glass rods 3 and cladding 4 are integrated through an integration process described later.

[0035] like Figure 1 As shown, each of the plurality of first glass rods 2 has a first core portion 2a and a first cladding portion 2b as a core portion. The first core portion 2a is, for example, made of quartz glass to which a dopant (such as germanium) for increasing the refractive index has been added. The first cladding portion 2b is made of glass with a refractive index lower than the maximum refractive index of the first core portion 2a. For example, pure quartz glass without a dopant for adjusting the refractive index can be cited as a glass constituting the first cladding portion 2b. Furthermore, as... Figure 1 As shown, the first cladding portion 2b is formed on the outer periphery of the first core portion 2a. A plane, namely the contact surface 2c, that contacts the second glass rod 3 is formed on a portion of the outer periphery of such the first cladding portion 2b.

[0036] like Figure 1 As shown, the second glass rod 3 has a second cladding portion 3b, which is surrounded by a plurality of first glass rods 2. More specifically, as... Figure 1 As shown, the second cladding portion 3b is in contact with the first cladding portions 2b of each of the plurality of first glass rods 2. On the outer periphery of the second cladding portion 3b, each portion facing the plurality of first glass rods 2 has a plane, i.e., an abutment surface 3c, that contacts the first cladding portion 2b. That is, the abutment surfaces 2c of the first glass rods 2 and 3c of the second glass rods 3 are mutually abutting planes.

[0037] Alternatively, the second cladding portion 3b can also be made of the same quartz-based glass as the first cladding portion 2b described above, but preferably includes a low softening point glass with a lower softening point than the first cladding portion 2b. For example, the low softening point glass is formed by adding dopants such as potassium (K), phosphorus (P), chlorine (Cl), fluorine (F), and germanium (Ge) to a glass with a refractive index lower than the maximum refractive index of the first core portion 2a, such as pure quartz glass without added dopants for adjusting the refractive index. It should be noted that when both F and Ge are added as the above-mentioned dopants, the amount of each can be adjusted so that the refractive index of the added glass is the same as or close to the refractive index of pure quartz glass (i.e., approximately 1.444 at a wavelength of 1550 nm).

[0038] like Figure 1 As shown, cladding 4 is formed on the outer periphery of a plurality of first glass rods 2 and second glass rods 3. For example, cladding 4 is made of the same quartz glass as the first cladding portion 2b described above. It should be noted that the refractive indices of the first cladding portion 2b, the second cladding portion 3b, and cladding 4 may be the same or different from each other.

[0039] Furthermore, in this embodiment 1, for example, Figure 1 As shown, multiple first cores 2a are arranged opposite each other with their central axes A sandwiching the second cladding portion 3b in the second glass rod 3. In this case, the distances between the central axes of the multiple first cores 2a and the central axis A of the second cladding portion 3b can be the same or different. Furthermore, the central axis A of the second cladding portion 3b can be aligned with or not aligned with the central axis of the cladding 4. In this invention, "central axis" refers to the central axis along the length direction of the element being considered.

[0040] [Manufacturing method of multi-core optical fiber]

[0041] Next, the manufacturing method of the multi-core optical fiber according to Embodiment 1 of the present invention will be described. Figure 2 This is a flowchart illustrating an example of a method for manufacturing a multi-core optical fiber according to Embodiment 1 of the present invention. In this Embodiment 1, by performing... Figure 2 Each step in the steps S101 to S106 shown first involves manufacturing a multi-core optical fiber mother material 1, and then manufacturing the multi-core optical fiber as the target from the multi-core optical fiber mother material 1.

[0042] In detail, such as Figure 2 As shown, firstly, a preparation process (step S101) is performed to prepare components for manufacturing the multi-core optical fiber mother material 1. In this step S101, multiple (four in this embodiment 1) cylindrical first glass rods 2, each having the aforementioned first core portion 2a and first cladding portion 2b, are prepared. Additionally, cylindrical second glass rods 3 and cylindrical cladding 4, each having the aforementioned second cladding portion 3b, are prepared. The first glass rods 2, second glass rods 3, and cladding 4 can be manufactured using known methods such as VAD (Vaporphase Axial Deposition), OVD (Outside Vapor Deposition), and MCVD (Modified Chemical Vapor Deposition).

[0043] After performing the preparation step S101, a hole-forming step (step S102) is performed to form holes for inserting a plurality of first glass rods 2 and second glass rods 3 in the cylindrical cladding 4. In this step S102, a plurality of holes for inserting a plurality of first glass rods 2 and holes for inserting second glass rods are formed in the cylindrical cladding 4.

[0044] Figure 3 This is a cross-sectional view showing an example of a cladding layer formed by the void forming process of Embodiment 1. For example... Figure 3 As shown, multiple first holes 4a (four in this embodiment 1) for inserting multiple first glass rods 2 and second holes 4b for inserting second glass rods 3 are formed inside the cladding 4 in such a way that they extend along the length direction of the cylindrical cladding 4. At this time, the multiple first holes 4a are formed at various positions in the cladding 4 where the multiple first glass rods 2 are arranged. The second holes 4b are formed at positions in the cladding 4 where the second glass rods 3 are arranged. Furthermore, as... Figure 3 As shown, these multiple first holes 4a and second holes 4b are formed in a manner that they are interconnected via openings 4c.

[0045] The aforementioned multiple first pores 4a and second pores 4b can be formed simultaneously, for example, by heating the cylindrical cladding 4 using a non-drilling method. Alternatively, a cladding 4 with multiple first pores 4a and second pores 4b pre-formed using a powder forming method or the like can be prepared.

[0046] After performing the hole-forming process in step S102, an abutment surface forming process (step S103) is performed to form abutment surfaces on the outer peripheries of the plurality of first glass rods 2 and second glass rods 3. In this step S103, for each of the plurality of first glass rods 2, a portion of the outer periphery of the cylindrical first glass rod 2 (the portion opposite to the second glass rod 3) is ground along the length direction of the first glass rod 2. As a result, an abutment surface 2c (refer to) is formed on each of the plurality of first glass rods 2. Figure 1 Additionally, a portion of the outer periphery of the cylindrical second glass rod 3 (each portion opposite to the plurality of first glass rods 2) is ground along the length direction of the second glass rod 3. As a result, the second glass rod 3 forms the same number of abutment surfaces 3c as the first glass rods 2 (see reference). Figure 1 ).

[0047] It should be noted that, in this invention, a first glass rod 2 and a second glass rod 3, having abutment surfaces 2c and 3c pre-formed using powder forming methods, may also be prepared. Furthermore, the abutment surface forming process in step S103 can be performed before the void forming process in step S102 described above.

[0048] Next, an insertion process (step S104) is performed in which multiple first glass rods 2 and second glass rods 3 are inserted into the cavities formed in the cladding 4. In this step S104, multiple first glass rods 2 are inserted into the multiple first cavities 4a formed in the cladding 4, and second glass rods 3 are inserted into the second cavities 4b formed in the cladding 4.

[0049] Figure 4 This is a cross-sectional view illustrating the state in which the glass rod is inserted into the cavity of the cladding through the insertion process of Embodiment 1. Figure 4 As shown, multiple (e.g., four) first glass rods 2 are inserted into multiple (e.g., four) first holes 4a respectively, such that the abutting surface 3c of the second glass rod 3 inserted into the second hole 4b of the cladding layer 3 abuts against the abutting surface 2c of the first glass rod 2. It should be noted that the insertion order of the first glass rods 2 and the second glass rods 3 is not particularly limited in this invention.

[0050] After the insertion process in step S104, an integration process (step S105) is performed to integrate the aforementioned plurality of first glass rods 2, second glass rods 3, and cladding 4. In this step S105, an intermediate structure (refer to) is formed by inserting the first glass rods 2 and second glass rods 3 into the cavities of the cladding 4 through the aforementioned insertion process. Figure 4 For example, heating in a furnace. This heating process blocks (collapses) the gaps between the multiple first glass rods 2, second glass rods 3, and cladding 4 in the intermediate structure, causing the first glass rods 2, second glass rods 3, and cladding 4 to... Figure 1 As shown, it is integrated.

[0051] By performing each of the above-described steps S101 to S105, the multi-core optical fiber mother material 1 is manufactured. It should be noted that, in this invention, the integration process in step S105 can also be omitted, and in the drawing process described below, the integration of the above-described intermediate structure and the drawing of the multi-core optical fiber mother material 1 are performed simultaneously.

[0052] Next, a drawing process is performed on the obtained multi-core optical fiber mother material 1 to manufacture the multi-core optical fiber as the target (step S106). Figure 5 This is a schematic diagram illustrating the wire drawing process in Embodiment 1 of the present invention. Figure 5 The illustration shows an example of an optical fiber manufacturing apparatus 10 used to draw a multi-core optical fiber mother material 1 to manufacture a multi-core optical fiber 15.

[0053] In step S106, as Figure 5As shown, a multi-core optical fiber mother material 1 is placed in the drawing furnace 11 of the optical fiber manufacturing apparatus 10. A heater 11a within the drawing furnace 11 heats and melts one end of the multi-core optical fiber mother material 1, drawing a glass optical fiber 12 vertically downwards from this end. Then, an ultraviolet-curable resin is coated onto the outer surface of the glass optical fiber 12 using a coating device 13. This ultraviolet-curable resin is cured by ultraviolet light irradiation from an ultraviolet irradiation device 14. As a result, the glass optical fiber 12 becomes a multi-core optical fiber 15 with its outer surface coated with resin. A guide roller 16 guides the multi-core optical fiber 15 to a winding machine 17, which winds the multi-core optical fiber 15 onto a spool. Thus, a multi-core optical fiber 15 is manufactured.

[0054] It should be noted that, before placing the multi-core optical fiber mother material 1 into the optical fiber manufacturing apparatus 10, a tapered member with an outer diameter approximately equal to that of the multi-core optical fiber mother material 1 can be fused at the starting end of the drawing process of the multi-core optical fiber mother material 1. This reduces manufacturing loss at the start of the drawing process of the multi-core optical fiber mother material 1, allowing a large portion of the assembled multi-core optical fiber mother material 1 to be used as a product component.

[0055] [Structure of multi-core optical fiber]

[0056] Next, the structure of the multi-core optical fiber in Embodiment 1 of the present invention will be described. Figure 6 This is a cross-sectional view illustrating a structural example of a multi-core optical fiber according to Embodiment 1 of the present invention. Figure 6 The example shown is the use of the multi-core optical fiber mother material 1 described above (refer to...). Figure 1 The cross-sectional structure of the multi-core optical fiber 15 manufactured. Additionally, in Figure 6 The diagram illustrates a stress distribution L1 that shows the relationship between the radial position in the cross-section of the multi-core optical fiber 15 and the generated stress. In the XY coordinate system illustrating this stress distribution L1, the X-axis represents the radial position of the multi-core optical fiber 15. The Y-axis represents the magnitudes of the tensile and compressive stresses in the cross-section of the multi-core optical fiber 15. Specifically, positive values ​​on the Y-axis represent the tensile stress, and negative values ​​represent the compressive stress. The tensile stress increases towards the positive direction of the Y-axis, and the compressive stress increases towards the negative direction of the Y-axis. Furthermore, the stress varying towards the positive direction of the Y-axis is the component of the tensile stress, and the stress varying towards the negative direction of the Y-axis is the component of the compressive stress.

[0057] In addition, Figure 6 In this example, the stress distribution L2 of a conventional multi-core optical fiber manufactured from a multi-core optical fiber preform based on a perforation method is shown as a stress distribution used for comparison with the stress distribution L1 of the multi-core optical fiber 15. This conventional multi-core optical fiber is identical to the multi-core optical fiber 15 of Embodiment 1, except that its preform is manufactured using a perforation method.

[0058] like Figure 6 As shown, the multi-core optical fiber 15 of this embodiment 1 includes a plurality of (e.g., four) first glass regions 22, second glass regions 23, and cladding regions 24. The plurality of first glass regions 22 correspond to the plurality of first glass rods 2 in the multi-core optical fiber parent material 1. Compressive stress is applied to the plurality of first glass regions 22. The second glass regions 23 correspond to the second glass rods 3 in the multi-core optical fiber parent material 1. The cladding regions 24 correspond to the cladding 4 in the multi-core optical fiber parent material 1. Tensile stress is applied to the cladding regions 24. It should be noted that, although not specifically illustrated, the outer periphery of the cladding regions 24 is covered. This covering uses a material commonly used in optical fibers.

[0059] In detail, such as Figure 6 As shown, each of the plurality of first glass regions 22 has a first core 22a and a first cladding portion 22b with a refractive index lower than the maximum refractive index of the first core 22a. The first core 22a is associated with the aforementioned multi-core optical fiber mother material 1 (see reference). Figure 1 The core portion 22b corresponds to the first core portion 2a of the first glass rod 2 in the multi-core optical fiber mother material 1. Its dimensions differ from the first core portion 2a, but it is made of the same quartz-based glass as the first core portion 2a. The first cladding portion 22b is the cladding portion corresponding to the first cladding portion 2b of the first glass rod 2 in the multi-core optical fiber mother material 1, such as... Figure 6 As shown, the first cladding portion 22b is formed on the outer periphery of the first core portion 22a. The first cladding portion 22b has a different size from the first cladding portion 2b of the multi-core optical fiber mother material 1, but is made of the same quartz-based glass as the first cladding portion 2b. In addition, a plane, namely an abutment surface 22c, is formed on a portion of the outer periphery of the first cladding portion 22b, which contacts the second cladding portion 23b of the second glass region 23.

[0060] like Figure 6 As shown, 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 multi-core optical fiber parent material 1, and is surrounded by a plurality of (four in this embodiment 1) first glass regions 22. More specifically, as... Figure 6 As shown, the second cladding portion 22b is in contact. On the outer periphery of the second cladding portion 23b, in each portion opposite to each of the plurality of first glass regions 22, a plane, i.e., an abutment surface 23c, is formed that contacts the first cladding portion 22b. That is, the abutment surface 22c of the first cladding portion 22b and the abutment surface 23c of the second cladding portion 23b are mutually abutting planes.

[0061] Furthermore, the second cladding portion 23b differs in size from the second cladding portion 3b of the multi-core optical fiber mother material 1, but is made of the same quartz-based glass as the second cladding portion 3b. For example, the second cladding portion 23b may be made of the same quartz-based glass as the first cladding portion 22b of the first glass region 22, but preferably includes a low softening point glass with a lower softening point than the first cladding portion 22b. Moreover, the second cladding portion 23b is more preferably made of low softening point glass. This low softening point glass is the same as the low softening point glass in the second cladding portion 3b of the multi-core optical fiber mother material 1 described above.

[0062] like Figure 6 As shown, cladding regions 24 are formed on the outer periphery of multiple first glass regions 22 and second glass regions 23. Cladding regions 24 correspond to the cladding 4 of the multi-core optical fiber mother material 1 described above, but have different dimensions than cladding 4; however, they are made of the same quartz-based glass as cladding 4. For example, cladding regions 24 are made of the same quartz-based glass as the first cladding portion 22b described above. It should be noted that the refractive indices of the first cladding portion 22b, the second cladding portion 23b, and cladding regions 24 can be the same or different from each other.

[0063] Furthermore, in this embodiment 1, for example, Figure 6 As shown, multiple first cores 22a are arranged opposite each other with their central axes A sandwiching the second cladding portion 23b in the second glass region 23. In this case, the distances between the central axes of the multiple first cores 22a and the central axis A of the second cladding portion 23b can be the same or different. Furthermore, the central axis A of the second cladding portion 23b can be aligned with or not aligned with the central axis of the cladding region 24. It should be noted that 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 multi-core optical fiber parent material 1 described above.

[0064] In a multi-core optical fiber 15 having the structure described above, for example... Figure 6 As shown, a stress distribution L1 is generated along the radial direction (X-axis). The stress distribution L1 of the multi-core optical fiber 15 includes tensile stress and compressive stress generated in the first core 22a, the first cladding 22b, the second cladding 23b, and the cladding region 24. In this stress distribution L1, the stress at locations where the tensile stress component is larger than the compressive stress component is called tensile stress, and the stress at locations where the tensile stress component is smaller than the compressive stress component is called compressive stress. Specifically, as... Figure 6 As shown in stress distribution L1, tensile stress is generated in cladding region 24, and compressive stress is generated in multiple first glass regions 22 and second glass regions 23.

[0065] Furthermore, in each of the plurality of first glass regions 22, stress is applied to the first core 22a by the first cladding portion 22b. The stress applied to the first core 22a by the first cladding portion 22b includes a component of tensile stress from the first core 22a side to the first cladding portion 22b side.

[0066] Here, in the case where the multi-core optical fiber 15 is manufactured from a multi-core optical fiber mother material based on the perforation method, such as Figure 6 As illustrated in stress distribution L2, the stress at the first core 22a increases towards the tensile stress side (positive Y-axis side). That is, the cladding portion surrounding the first core 22a applies an excessive component of tensile stress to the first core 22a.

[0067] In contrast, in the multi-core optical fiber 15 of Embodiment 1, the plurality of first glass regions 22 and second glass regions 23 are configured such that the plurality of first cladding portions 22b subjected to large tensile stress are in contact with the second cladding portions 23b respectively. This mitigates the concentration of stress on the first core 22a, thereby reducing the component of the tensile stress applied to the first core 22a by the first cladding portions 22b. As a result, compressive stress is applied to each of the plurality of first glass regions 22, so the magnitude relationship between the components of tensile stress and compressive stress does not reverse. Furthermore, in the multi-core optical fiber 15 of Embodiment 1, from the viewpoint of reducing the component of the tensile stress applied to the first core 22a, it is preferable that the second cladding portion 23b of the second glass region 23 comprises a low softening point glass.

[0068] Furthermore, when the multi-core optical fiber 15 is manufactured from a multi-core optical fiber mother material based on the perforation method, such as Figure 6 As illustrated in stress distribution L2, in region R1 between the radially opposed first cores 22a-1 and 22a-2 of the multi-core optical fiber 15, tensile stress and compressive stress are reversed (see Figure 1). Figure 6 Region R2 in this embodiment). In contrast, in the multi-core optical fiber 15 of this embodiment 1, such as Figure 6 As illustrated in stress distribution L1, in the stress distribution generated between the first cores 22a-1, 22a-2 of the plurality of first glass regions 22 opposite each other with the second glass region 23, there is no reversal of tensile stress and compressive stress. This stress distribution is the stress distribution generated in the region R1 between the aforementioned first cores 22a-1, 22a-2 in the overall stress distribution L1 of the multi-core optical fiber 15.

[0069] Figure 7 This is a diagram illustrating the difference between the maximum and minimum stress values ​​in the multi-core optical fiber of Embodiment 1 of the present invention. (See diagram for example.) Figure 7As shown, in the multi-core optical fiber 15 of this embodiment 1, the aforementioned region R1 (refer to) in the overall stress distribution L1 Figure 6 In the resulting stress distribution, the average compressive stress in the first glass region 22 is less than the average compressive stress in the second glass region 23. Furthermore, the difference ΔS1 between the maximum and minimum stress values ​​in this stress distribution is preferably 30 MPa or less. More preferably, this difference ΔS1 is 20 MPa or less. Additionally, the maximum stress value is preferably 80 MPa or less.

[0070] In this embodiment 1, the stress distribution described above is the stress distribution generated between the first core portions 22a-1 and 22a-2 of a plurality of first glass regions 22 opposite to each other with the second glass region 23. Furthermore, the maximum value in the stress distribution is the maximum compressive stress Sa, and the minimum value in the stress distribution is the minimum compressive stress Sb (refer to...). Figure 7 ).

[0071] Figure 8 This is a graph illustrating the difference between the maximum and minimum stress values ​​in the stress distribution of existing multi-core optical fibers manufactured from perforated multi-core optical fiber preforms. (See figure.) Figure 8 As shown, in this existing multi-core optical fiber 115, the difference ΔS2 between the maximum and minimum stress values ​​in the stress distribution generated in the same region R1 as the multi-core optical fiber 15 of Embodiment 1 is greater than 30 MPa. That is, in the existing multi-core optical fiber 115, compared with the multi-core optical fiber 15 of Embodiment 1, it is difficult to reduce the stress concentrated in the core.

[0072] Example 1

[0073] Next, embodiments of the present invention will be described. In this embodiment, using Figure 5 The optical fiber manufacturing apparatus 10 shown above produces a sample of multi-core optical fiber 15 (hereinafter referred to as the embodiment sample) by drawing the multi-core optical fiber mother material 1 of Embodiment 1 described above. The structure and optical characteristics of the manufactured embodiment sample are shown below.

[0074] Specifically, in the embodiment sample, the core shape of the first core 22a is set to a single peak, and the relative refractive index difference Δ between the first core 22a and the first cladding portion 22b is set to 0.35%. The core diameter of the first core 22a is set to 8.1 μm, and the diameter of the cladding region 24 (cladding diameter) is set to 125.0 μm. The number of first cores 22a is set to 4, and the spacing between the first cores 22a is set to 40.1 μm. In addition, the cutoff wavelength is set to 1267 nm, and the mode field diameter (MFD) at a wavelength of 1550 nm is set to 10.6 μm.

[0075] In addition, as a comparative example relative to this embodiment, a sample of multi-core optical fiber was fabricated from a multi-core optical fiber preform based on the perforation method (hereinafter referred to as the comparative example sample). This comparative example sample is the same as the embodiment sample described above, except that it uses a multi-core optical fiber preform based on the perforation method.

[0076] The light propagation loss of the example samples and comparative samples prepared as described above was evaluated. The evaluation results of the structure, optical properties, and propagation loss of these example samples and comparative samples are summarized in Table 1.

[0077] Table 1

[0078] Example Sample Comparative sample Core shape Single-peak type Single-peak type Relative refractive index difference Δ 0.35％ 0.35％ Core diameter 8.1μm 8.1μm Cladding diameter 125.0μm 124.9μm Number of chips 4 4 Core pitch 40.1μm 39.8μm Cutoff wavelength 1258nm 1267nm MFD 10.6μm 10.6μm Propagation loss 0.190dB / km 0.205dB / km

[0079] 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 the same core as the example sample. That is, in the example sample, the propagation loss of light can be suppressed to the same level as 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 can be confirmed that the propagation loss of light in the comparative example sample is increased compared to the example sample, and it is difficult to suppress this increase in propagation loss.

[0080] As explained above, in Embodiment 1 of the present invention, a multi-core optical fiber is constructed, comprising: a plurality of first glass regions having a first core and a first cladding portion having a refractive index lower than the maximum refractive index of the first core; 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 being 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. Furthermore, in this multi-core optical fiber, in the stress distribution generated between the first cores of the plurality of first glass regions facing each other across the second glass region, the average compressive stress in the first glass regions is less than the average compressive stress in the second glass regions. In this case, the difference between the maximum and minimum stress values ​​in this stress distribution is, for example, set to 30 MPa or less.

[0081] Therefore, it is possible to mitigate the stress concentration in each of the multiple first cores and reduce the component of tensile stress applied from the first cladding side to the first core. Consequently, it is possible to suppress the increase in transmission loss of the multi-core optical fiber, resulting in a multi-core optical fiber where the propagation loss is reduced to the same level as that of a single-core optical fiber.

[0082] Furthermore, in Embodiment 1 of the present invention, the second cladding portion is configured to include a low softening point glass with a softening point lower than that of the first cladding portion. Therefore, it is easier to mitigate stress concentration in each of the plurality of first cores, and the component of tensile stress applied from the first cladding portion side to the first core can be further reduced. Consequently, the increase in transmission loss of the multi-core optical fiber can be further suppressed, and thus it is easy to realize a multi-core optical fiber with propagation loss reduced to the same level as that of a single-core optical fiber.

[0083] (Implementation Method 2)

[0084] Next, Embodiment 2 of the present invention will be described. Figure 9 This is a cross-sectional view illustrating a structural example of a multi-core optical fiber according to Embodiment 2 of the present invention. Figure 9 As shown, the multi-core optical fiber 15A of this embodiment 2 has a second glass region 23A instead of the multi-core optical fiber 15 of embodiment 1 described above (see Figure 1). Figure 6 The second glass region 23. The other structures are the same as in Embodiment 1, and the same reference numerals are used to label the same structural parts.

[0085] like Figure 9 As shown, the second glass region 23A has a second cladding portion 23b, the same as in Embodiment 1 described above, and a second core portion 23a, whose maximum refractive index is higher than that of the second cladding portion 23b. Furthermore, like in Embodiment 1 described above, the second glass region 23A is surrounded by a plurality of (e.g., four) first glass regions 22.

[0086] The second core 23a is, for example, made of quartz glass to which dopants (such as germanium) have been added to increase the refractive index. Figure 9 As shown, the second core 23a is positioned at the same location as the central axis A of the second cladding portion 23b in the second glass region 23A. It should be noted that the refractive index of the second core 23a may be the same as or different from the refractive index of the first core 22a in the plurality of first glass regions 22. Furthermore, the central axis of the second core 23a may or may not coincide with the central axis A of the second cladding portion 23b.

[0087] Although not specifically illustrated, the multi-core optical fiber mother material of this embodiment 2 is based on the multi-core optical fiber mother material 1 of embodiment 1 described above (refer to...). Figure 1 The second glass rod 3 contains a core that forms the second core 23a, and is otherwise identical to Embodiment 1. Furthermore, the multi-core optical fiber 15A is also similar to that in Embodiment 1, using an optical fiber manufacturing apparatus 10 ( Figure 5 It is manufactured from the multi-core optical fiber mother material in Embodiment 2.

[0088] As explained above, in Embodiment 2 of the present invention, the second glass region in the multi-core optical fiber is configured as a second core having a refractive index higher than that of the second cladding portion, and otherwise configured the same as in Embodiment 1. Therefore, in a multi-core optical fiber having a second core in the region inside surrounded by multiple first cores, the same effect as in Embodiment 1 can be obtained.

[0089] It should be noted that in the above embodiments 1 and 2, a multi-core optical fiber with four first glass regions is illustrated, each first glass region having a first core and a first cladding portion; however, the present invention is not limited to this. In the multi-core optical fiber of the present invention, the number of the aforementioned first glass regions (the number of first cores and first cladding portions) may also be two or more.

[0090] Furthermore, in Embodiment 2 described above, a multi-core optical fiber with a core (second core 23a) disposed in a second glass region surrounded by multiple first glass regions was illustrated, but the present invention is not limited to this. In the multi-core optical fiber of the present invention, when the core is disposed in the second glass region, the number of such cores can be one or multiple.

[0091] Furthermore, in embodiments 1 and 2 described above, the core (first core, second core) is composed of quartz-based glass with added dopants such as germanium, and the cladding (first cladding, second cladding, cladding region) is composed of pure quartz glass; however, the present invention is not limited to this. For example, the core may be composed of pure quartz glass, and the cladding may be composed of quartz-based glass with added dopants (such as fluorine) that lower the refractive index.

[0092] Furthermore, in embodiments 1 and 2 described above, a non-drilling method is used to form a cavity for inserting glass rods in the cylindrical cladding constituting the multi-core optical fiber mother material, but the present invention is not limited to this. For example, the multi-core optical fiber mother material can also be manufactured by stacking multiple glass rods arranged inside a glass tube with circular cavities. Alternatively, a cladding with cavities can be formed by a powder forming method or the like, and multiple glass rods can be inserted into the cavities of the cladding to manufacture the multi-core optical fiber mother material.

[0093] Furthermore, in embodiments 1 and 2 described above, the plurality of first cladding portions and the second cladding portion surrounded by the plurality of first cladding portions are in surface contact with each other, but the present invention is not limited to this. For example, the plurality of first cladding portions and the second cladding portions described above may also be in line contact with each other.

[0094] Furthermore, this invention is not limited to the embodiments 1 and 2 described above; any configuration by appropriately combining the aforementioned constituent elements is also included in this invention. Additionally, all other embodiments, examples, and applications made by those skilled in the art based on the embodiments 1 and 2 described above are included within the scope of this invention.

[0095] Industrial availability

[0096] As described above, the multi-core optical fiber of the present invention is useful for optical fibers having multiple cores.

[0097] Explanation of reference numerals in the attached figures:

[0098] 1. Multi-core optical fiber mother material

[0099] 2 First glass rod

[0100] 2a First Core

[0101] 2b First cladding section

[0102] 2c contact surface

[0103] 3. Second glass rod

[0104] 3b Second cladding section

[0105] 3C contact surface

[0106] 4. Cladding

[0107] 4a First Hole

[0108] 4b Second Hole

[0109] 4c Opening

[0110] 10. Optical fiber manufacturing equipment

[0111] 11 Wire Drawing Furnace

[0112] 11a heater

[0113] 12 Glass optical fibers

[0114] 13. Coating device

[0115] 14. Ultraviolet irradiation device

[0116] 15, 15A multi-core optical fiber

[0117] 16 guide rollers

[0118] 17 Winding Machine

[0119] 22 First Glass Area

[0120] 22a, 22a-1, 22a-2 First Core

[0121] 22b First cladding section

[0122] 22c contact surface

[0123] 23, 23A Second Glass Area

[0124] 23a Second Core

[0125] 23b Second cladding section

[0126] 23c contact surface

[0127] 24. Cladding region

[0128] 115 Existing multi-core optical fibers

[0129] A central axis

[0130] Stress distribution at L1 and L2

[0131] Regions R1 and R2.

Claims

1. A multi-core optical fiber, characterized in that, The multi-core optical fiber has the following characteristics: Multiple first glass regions, each having a first core and a first cladding region with a refractive index lower than the maximum refractive index of the first core; A second glass region having a second cladding portion that contacts the first cladding portion of each of the plurality of first glass regions, and being surrounded by the plurality of first glass regions; and The cladding region is formed on the outer periphery of the plurality of first glass regions and second glass regions. In the stress distribution generated between the first cores of a plurality of first glass regions opposite each other across the second glass region, the average compressive stress in the first glass region is less than the average compressive stress in the second glass region.

2. The multi-core optical fiber according to claim 1, characterized in that, The difference between the maximum and minimum stress values ​​in the stress distribution is less than 30 MPa.

3. The multi-core optical fiber according to claim 1 or 2, characterized in that, The second glass region has a second core with a maximum refractive index that is higher than that of the second cladding portion.

4. The multi-core optical fiber according to claim 1 or 2, characterized in that, The second cladding portion comprises low softening point glass, which has a lower softening point than the first cladding portion.

5. The multi-core optical fiber according to claim 1 or 2, characterized in that, The stress applied from the first cladding portion to the first core portion includes a component of tensile stress.

6. The multi-core optical fiber according to claim 1 or 2, characterized in that, There is no reversal of tensile stress and compressive stress in the stress distribution.

7. The multi-core optical fiber according to claim 1 or 2, characterized in that, The maximum stress value in the stress distribution is below 80 MPa.

Images