Method for manufacturing optical fiber preform and method for manufacturing optical fiber

Abstract

The present invention provides a method for manufacturing an optical fiber matrix that can produce optical fibers with suppressed strength reduction, and a method for manufacturing optical fibers. [Solution] The method for manufacturing an optical fiber preform comprises: an observation step P2 for observing the inside of a clad rod 20P; a drilling step P3 for forming a through hole 20H along the longitudinal direction of the clad rod 20P in a drilling region 20Ha that includes at least a part of the area DP where an abnormal condition was discovered in the observation step P2; and an insertion step P4 for inserting an insertion glass rod 40P, which includes a predetermined glass rod made of glass with different properties from the glass constituting the clad rod 20P, into the through hole 20H.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing an optical fiber preform and a method for manufacturing an optical fiber. [Background technology]

[0002] In some cases, optical fiber preforms are manufactured by inserting glass rods, which are made of glass with different properties from the glass that makes up the cladding rod, into through-holes formed in the cladding rod that forms the cladding of the optical fiber.

[0003] Patent Document 1, described below, describes a method for manufacturing a multicore optical fiber. In Patent Document 1, core rods are inserted into multiple through holes formed in a cladding rod that forms the cladding of the multicore optical fiber, and the cladding rod and core rods are integrated and drawn to manufacture the multicore optical fiber. The core rods are made of glass with a different refractive index from the glass that makes up the cladding rod. Furthermore, Patent Document 1 describes a method for manufacturing a polarization-maintaining optical fiber. In this method for manufacturing a polarization-maintaining optical fiber, stress-applying rods, which are stress-applying parts, are inserted into a pair of through holes formed in a cladding rod that forms the cladding of the polarization-maintaining optical fiber, and the cladding rods and stress-applying rods are integrated and drawn to manufacture the polarization-maintaining optical fiber. The stress-applying rods are made of glass with a different coefficient of thermal expansion from the glass that makes up the cladding rod. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-14078 [Overview of the project] [Problems that the invention aims to solve]

[0005] From the perspectives of enabling long-distance communication using a single optical fiber and reducing the cost of optical fibers, there is a trend towards increasing the length of optical fibers drawn from a single optical fiber matrix. Therefore, the diameter of optical fiber matrix materials is increasing. However, when manufacturing clad rods from optical fiber matrix materials, increasing the diameter can be difficult using the VAD (Vapor-phase Axial Deposition) method. Therefore, a method is being employed to manufacture large-diameter clad rods by externally attaching a glass layer to the outer surface of glass rods manufactured using the VAD method, etc., using the OVD (Outside Vapor Deposition) method, etc.

[0006] However, when a glass layer is attached to the outer surface of a glass rod, there is a concern about the inclusion of bubbles or impurities at the boundary between the glass rod and the glass layer. Such abnormal areas containing bubbles or impurities can lead to a decrease in the strength of the optical fiber, potentially hindering its long lifespan. Furthermore, such abnormal areas can occur not only at the boundary mentioned above, but also in clad rods that do not have such a boundary.

[0007] Therefore, the present invention aims to provide a method for manufacturing an optical fiber matrix and a method for manufacturing an optical fiber, which can produce an optical fiber in which the reduction in strength is suppressed. [Means for solving the problem]

[0008] One aspect of the present invention for solving the above problems is a method for manufacturing an optical fiber preform, comprising: an observation step of observing the inside of a clad rod; a drilling step of forming a through hole along the longitudinal direction of the clad rod in a drilling region that includes at least a part of the area where an abnormal condition was discovered in the observation step; and an insertion step of inserting an insertion glass rod, which includes a predetermined glass rod made of glass having different properties from the glass constituting the clad rod, into the through hole.

[0009] According to this method for manufacturing optical fiber preforms, at least a portion of abnormal areas containing bubbles, impurities, etc., can be removed by inserting predetermined glass rods, which are made of glass with different properties from the glass constituting the cladding rods that form the core and stress-applying parts, through holes. In other words, at least a portion of abnormal areas can be removed by forming the through holes necessary for manufacturing the optical fiber preform. Therefore, by using the optical fiber preform manufactured by this embodiment, it is possible to manufacture optical fibers with suppressed strength reduction.

[0010] Aspect 2 of the present invention is a method for manufacturing an optical fiber preform according to aspect 1, characterized in that, in the observation step, the interior is observed from the end face of the clad rod along the longitudinal direction of the clad rod.

[0011] As described above, in the present invention, through holes are formed along the longitudinal direction of the clad rod. Therefore, no matter where the abnormal part is located along the longitudinal direction of the clad rod, at least a portion of that part can be removed by the through holes. When observing a clad rod from the side, it is difficult to pinpoint the location of the abnormal part unless the effect of light refraction on the side is considered and the clad rod is rotated during observation. According to this embodiment, it is possible to identify where the above part is located on the end face of the clad rod, making the observation process easier than observing the clad rod from the side.

[0012] A third aspect of the present invention is a method for manufacturing an optical fiber preform according to aspect 1 or 2, characterized in that, in the observation step, at least a predetermined annular region surrounding the central axis of the clad rod is observed, and at least a portion of the perforated region is located in the annular region.

[0013] In some cases, a clad rod is produced by depositing glass while rotating a glass rod about its axis, such as by the OVD method. In this case, abnormal regions may occur at different positions at a certain distance from the center of the clad rod. Therefore, by observing the annular region, when an abnormal region occurs in the annular region, it is easier to detect the abnormal region. By removing at least a part of the abnormal region by forming a through hole, an optical fiber with suppressed strength reduction can be manufactured.

[0014] Aspect 4 of the present invention is a method for manufacturing a preform of an optical fiber according to Aspect 3, characterized in that the clad rod includes a central clad rod and an outer clad layer that contacts the outer peripheral surface of the central clad rod and surrounds the central clad rod, the annular region includes the boundary between the central clad rod and the outer clad layer, the hole opening region overlaps the boundary, and the predetermined glass rod is made of glass having characteristics different from those of the glasses constituting the central clad rod and the outer clad layer.

[0015] According to such a method for manufacturing a preform of an optical fiber, by using a central clad rod and an outer clad layer, the diameter of the clad rod can be increased, and a long optical fiber can be manufactured. In addition, crystal misalignment tends to occur at the boundary between the central clad rod and the outer clad layer, impurities are likely to be mixed in, and bubbles are also likely to occur. As in this aspect, by forming through holes so as to overlap the boundary, the boundary can be made smaller compared to the case where the boundary continuously exists for one circumference. Therefore, abnormal regions where impurities are mixed in or bubbles are generated can be removed, and further, a reduction in the strength of the optical fiber due to crystal misalignment at the boundary can be suppressed.

[0016] Aspect 5 of the present invention is a method for manufacturing a preform of an optical fiber according to Aspect 3 or 4, characterized in that in the hole opening step, through holes are formed along the longitudinal direction of the clad rod in a plurality of regions including the hole opening region in the annular region, and in the insertion step, the insertion glass rod is inserted into the plurality of through holes.

[0017] Aspect 6 of the present invention is a method for manufacturing an optical fiber preform according to aspect 5, characterized in that the predetermined glass rod is a core rod.

[0018] In this case, it is possible to manufacture multicore optical fibers with suppressed strength degradation.

[0019] Aspect 7 of the present invention is a method for manufacturing an optical fiber preform according to aspect 5, characterized in that the predetermined glass rod is a stress-applying rod.

[0020] In this case, polarization-maintaining optical fibers with suppressed intensity degradation can be manufactured.

[0021] Furthermore, aspect 8 of the present invention is a method for manufacturing optical fibers, characterized by comprising a drawing step of drawing a fiber optic fiber preform manufactured by any of the optical fiber preform manufacturing methods of aspects 1 to 7.

[0022] This method of manufacturing optical fibers makes it possible to produce optical fibers with suppressed strength degradation. [Effects of the Invention]

[0023] As described above, the present invention provides a method for manufacturing an optical fiber matrix and a method for manufacturing an optical fiber, which can produce an optical fiber in which the reduction in strength is suppressed. [Brief explanation of the drawing]

[0024] [Figure 1] Figure 1 shows a cross-section perpendicular to the longitudinal direction of an optical fiber according to a first embodiment of the present invention. [Figure 2] Figure 2 shows the optical fiber preform for manufacturing the optical fiber shown in Figure 1. [Figure 3] Figure 3 is a flowchart illustrating the process for manufacturing the optical fiber preform shown in Figure 2 and the optical fiber shown in Figure 1. [Figure 4] Figure 4 shows the situation after the preparation process. [Figure 5] Figure 5 shows the observation process. [Figure 6] Figure 6 is a diagram illustrating the perforated area. [Figure 7] Figure 7 is a diagram illustrating the position of the through-hole. [Figure 8] Figure 8 shows the process of drawing lines. [Figure 9] Figure 9 shows a cross-section perpendicular to the longitudinal direction of an optical fiber according to a second embodiment of the present invention. [Figure 10] Figure 10 shows the optical fiber preform for manufacturing the optical fiber shown in Figure 9. [Figure 11] Figure 11 is a diagram illustrating the perforation region in the second embodiment. [Figure 12] Figure 12 shows a modified version of the observation process. [Modes for carrying out the invention]

[0025] Hereinafter, preferred embodiments of the method for manufacturing an optical fiber preform and an optical fiber according to the present invention will be described in detail with reference to the drawings. The embodiments illustrated below are for the purpose of facilitating understanding of the present invention and are not intended to limit the interpretation of the present invention. The present invention can be modified and improved from the embodiments within the scope of the claims without departing from the spirit thereof. Note that for ease of understanding, the scale of each figure may differ from the scale described in the following description.

[0026] (First Embodiment) Figure 1 shows a cross-section of the optical fiber perpendicular to the longitudinal direction according to this embodiment. In this embodiment, a multicore optical fiber is used as an example. The multicore optical fiber 1 of this embodiment comprises a plurality of cores 10, a cladding 20 that surrounds the outer surface of each core 10 without gaps, an inner coating layer 31 that covers the outer surface of the cladding 20, and an outer coating layer 32 that covers the outer surface of the inner coating layer 31. In the example in Figure 1, an example with four cores 10 is shown.

[0027] The outer shape of the cladding 20 in a cross-section perpendicular to the longitudinal direction is approximately circular, and the radius D of the cladding 20 is f For example, the thickness is between 90 μm and 250 μm. In this embodiment, the distance between each core 10 is approximately equal to that of the others, and each core 10 is positioned to be approximately four rotationally symmetric with respect to the center 20C of the cladding 20. Therefore, the distance L from the center 10C of each core 10 to the center 20C of the cladding 20 is... f They are roughly the same. Distance L f For example, the radius d of core 10 is between 23 μm and 32 μm. f For example, the particle size is between 2 μm and 5 μm.

[0028] The refractive index of each core 10 is higher than that of the cladding 20. Therefore, the cores 10 are made of glass with different properties than the glass that makes up the cladding 20. The difference in the relative refractive index of each core 10 relative to the cladding 20 is, for example, 0.2% to 2.0%. Such a core 10 is made of silica glass to which a dopant that increases the refractive index, such as germanium, is added, and the cladding 20 is made of silica glass to which a dopant that decreases the refractive index, such as fluorine, is added. Alternatively, the core 10 may be made of silica glass to which no dopant is added, and the cladding 20 may be made of silica glass to which a dopant that lowers the refractive index, such as fluorine, is added. In other words, the cores 10 and the cladding 20 are made of glass with different compositions from each other.

[0029] The inner coating layer 31 and the outer coating layer 32 are each made of a resin such as an ultraviolet-curable resin, and the inner coating layer 31 and the outer coating layer 32 are made of different resins.

[0030] Next, we will describe the multicore optical fiber preform for manufacturing the multicore optical fiber 1 shown in Figure 1. Since the multicore optical fiber 1 is an optical fiber, the multicore optical fiber preform is an optical fiber preform.

[0031] Figure 2 shows a cross-sectional view perpendicular to the longitudinal direction of the multicore optical fiber base material used to manufacture the multicore optical fiber 1 shown in Figure 1. The multicore optical fiber base material 1P comprises a clad rod 20P and a plurality of insertion glass rods 40P.

[0032] The clad rod 20P is the clad 20 of the multicore optical fiber 1 and is a glass rod with a generally circular outer shape. The clad rod 20P includes a central clad rod 21P and an outer clad layer 22P that surrounds the central clad rod 21P and is in contact with its outer surface. In this embodiment, the refractive index of the central clad rod 21P and the refractive index of the outer clad layer 22P are generally equal to each other, and the central clad rod 21P and the outer clad layer 22P are made of glass with the same properties. The central clad rod 21P and the outer clad layer 22P have the same composition as the clad 20, and in this example, the central clad rod 21P and the outer clad layer 22P are made of glass with the same composition.

[0033] The clad rod 20P has multiple through holes 20H. Each through hole 20H is located between the central clad rod 21P and the outer clad layer 22P. The cross-sectional shape of each through hole 20H is circular, and the cross-sectional areas of each through hole 20H are approximately equal to each other. In this embodiment, the distance between each through hole 20H is also approximately equal to each other, and each through hole 20H is formed in a position that is approximately 4 rotationally symmetrical with respect to the central axis 20PC of the clad rod 20P. Therefore, the distance of each through hole 20H from the central axis PC is approximately the same.

[0034] An insertion glass rod 40P is inserted into each through hole 20H. In this embodiment, the insertion glass rod 40P includes a core rod 10P and an outer glass layer 23P that completely surrounds the outer surface of the core rod 10P. The core rod 10P is a predetermined glass rod that becomes the core 10 and is made of the same glass as the glass that constitutes the core 10. Therefore, the core rod 10P is made of glass with different properties from the glass that constitutes the central cladding rod 21P and the outer cladding layer 22P. The outer glass layer 23P is a glass layer that becomes part of the cladding 20. Therefore, the outer glass layer 23P is made of glass with the same properties as the central cladding rod 21P and the outer cladding layer 22P. In other words, the outer glass layer 23P is made of glass with the same composition as the central cladding rod 21P and the outer cladding layer 22P. The diameter of the insertion glass rod 40P is smaller than the through hole 20H. Therefore, with the insertion glass rod 40P inserted into the through hole 20H, a gap is formed between the inner circumferential surface of the clad rod 20P that forms the through hole 20H and the outer circumferential surface of the insertion glass rod 40P.

[0035] Next, we will describe the manufacturing method for the multicore optical fiber base material 1P shown in Figure 2 and the multicore optical fiber 1 shown in Figure 1.

[0036] Figure 3 is a flowchart showing the manufacturing of the multicore optical fiber preform 1P shown in Figure 2 and the multicore optical fiber 1 shown in Figure 1. As shown in Figure 3, the manufacturing method of the multicore optical fiber preform 1P in this embodiment includes a preparation step P1, an observation step P2, a drilling step P3, and an insertion step P4, and the manufacturing method of the multicore optical fiber 1 includes a wire drawing step P5 in addition to the manufacturing method of the multicore optical fiber preform 1P.

[0037] (Preparation process P1) This process involves preparing multiple insertion glass rods 40P, including cladding rods 20P which will form the cladding 20, and core rods 10P which will form the core 10 of the multicore optical fiber 1. Figure 4 shows the state after this process. As shown in Figure 4, the cladding rods 20P do not have through holes 20H formed in them. The cladding rods 20P prepared in this process have a cylindrical shape with both end faces being generally flat, and the outer cladding layer 22P surrounds the outer surface of the central cladding rod 21P without any gaps. The number of insertion glass rods 40P is the number that can be inserted into each through hole 20H shown in Figure 2. In this embodiment, the length of the cladding rods 20P and the length of the insertion glass rods 40P are generally equal to each other. In this process, the insertion glass rods 40P and cladding rods 20P may be prepared by manufacturing or by purchase.

[0038] When manufacturing a clad rod 20P, a central clad rod 21P is fabricated, and an outer clad layer 22P is provided on the outer surface of the central clad rod 21P. The central clad rod 21P is fabricated, for example, by the VAD method. The outer clad layer 22P is fabricated, for example, by depositing glass soot on the outer surface of the central clad rod 21P using the OVD method and vitrifying the soot. Alternatively, the clad rod 20P may be fabricated by preparing a glass tube that will become the outer clad layer 22P, inserting the central clad rod 21P into the through-hole of the glass tube, and collapsing the gap in the through-hole.

[0039] When manufacturing an insertion glass rod 40P, a core rod 10P is manufactured, and an outer glass layer 23P is provided on the outer surface of the core rod 10P. The core rod 10P is manufactured, for example, by the VAD method. The outer glass layer 23P is manufactured, for example, by depositing glass soot on the outer surface of the core rod 10P using the OVD method and vitrifying the soot. Alternatively, the insertion glass rod 40P may be manufactured by preparing a glass tube that will become the outer glass layer 23P, inserting the core rod 10P into the through-hole of the glass tube, and collapsing the gap in the through-hole. Or, when manufacturing an insertion glass rod 40P, a glass tube that will become the outer glass layer 23P is prepared, glass soot that will become the core rod 10P is deposited in the through-hole of the glass tube, the deposited soot is vitrified, and the remaining gap in the through-hole is collapsed to manufacture the insertion glass rod 40P.

[0040] (Observation process) This process involves observing the inside of the clad rod 20P. Figure 5 shows the process. In the example in Figure 5, a camera 201 is used to observe the clad rod 20P. Specifically, the camera 201 photographs the inside of the clad rod 20P from its side, and the inside of the clad rod 20P is observed using the data output from the camera 201. At this time, light is shone from a light source 202 positioned opposite the camera 201 across the clad rod 20P. The camera 201 photographs the light shone from the light source 202 that passes through the clad rod 20P. At this time, it is preferable to rotate the clad rod 20P while taking the photograph, as shown in the figure, and it is preferable to move the camera 201 along the longitudinal direction of the clad rod 20P while taking the photograph.

[0041] If bubbles or impurities are present within the clad rod 20P, these bubbles or impurities will refract or block the transmitted light, creating a shadow on the light incident on the camera 201. By determining the location of this shadow from the data output from the camera 201, the location of the abnormal condition in the clad rod 20P, such as bubbles or impurities, can be detected.

[0042] In this embodiment, the clad rod 20P consists of a central clad rod 21P and an outer clad layer 22P, and abnormal conditions such as the generation of air bubbles and the incorporation of impurities are likely to occur at the boundary 20BP between the central clad rod 21P and the outer clad layer 22P. Therefore, in this process, at least the annular region 20AR, which includes the boundary 20BP between the central clad rod 21P and the outer clad layer 22P, is observed. In the figure, the annular region 20AR is the region surrounding the central axis 20PC of the clad rod and enclosed by a pair of dashed lines. Alternatively, the entire clad rod 20P, including the annular region 20AR, may be observed.

[0043] In this process, devices other than the camera 201 may be used. For example, the light source 202 may emit laser light, and the camera 201 may consist of a light-receiving device capable of identifying the light-receiving position. In this case, by observing the laser light-receiving position and whether or not light is received while changing the irradiation position of the laser light, the location of abnormal conditions such as bubbles or impurities in the clad rod 20P can be discovered.

[0044] (Drilling process P3) This process involves forming a through-hole along the longitudinal direction of the clad rod 20P in a perforated region that includes at least a portion of the area where an abnormal condition was discovered in the observation process. Figure 6 shows the clad rod 20P before this process. The perforated region 20Ha is the region that will become the through-hole 20H, and the perforated region 20Ha and the through-hole 20H coincide in position and size. The through-hole 20H is formed, for example, by drilling with a drill tool. The drill tool is, for example, a tool with a cylindrical cutting part at the tip of a pipe, and in this case, drilling is performed while water is flowing through the pipe.

[0045] In this process, the region including at least a portion of the area DP where an abnormal condition was discovered in observation process P2 is defined as the perforated region 20Ha. Figure 6 shows an example in which one area DP with an abnormal condition where an impurity is located was discovered in observation process P2. Figure 6 also shows an example in which an impurity adheres to the outer surface of the central clad rod 21P, forming area DP. Therefore, in this process, one of the perforated regions 20Ha is positioned to overlap with at least a portion of area DP. In this example, one of the perforated regions 20Ha overlaps with the entire area DP. Furthermore, in this embodiment, since multiple through holes 20H are formed, multiple regions including the perforated region 20Ha that overlaps with area DP are defined as the perforated region 20Ha to form the through holes 20H.

[0046] In this embodiment, at least the annular region 20AR including the boundary 20BP is observed in observation step P2. Therefore, in this embodiment, at least a portion of the perforated region 20Ha is located on the annular region 20AR. Also, in this embodiment, the perforated region 20Ha overlaps with the boundary 20BP. This positioning of the perforated region 20Ha allows for efficient removal of portion DP on the outer circumferential surface of the central clad rod 21P.

[0047] Next, additional conditions regarding the position and size of the through-hole 20H when the perforated region 20Ha overlaps with the boundary 20BP will be described. In the clad rod 20P of this embodiment, the radius of the central clad rod 21P is indicated by D1, the radius of the outer circumference of the outer clad layer 22P, i.e., the radius of the clad rod 20P, is indicated by D2, the radius of the perforated region 20Ha is indicated by h, and the distance between the center of the central clad rod 21P, i.e., the central axis 20PC of the clad rod 20P and the center 20HC of the perforated region 20Ha is L p The length of the boundary 20BP removed by one through-hole 20H is denoted by s. The radius h and center 20HC of the drilled region 20Ha are also the radius and center of the through-hole 20H. Therefore, in the following description, the radius of the through-hole 20H will be denoted as h, and the center of the through-hole 20H will be denoted as center 20HC.

[0048] When the sizes of each part are defined as described above, the distance L between the center 20HC of the hole opening region 20Ha and the boundary 20BP a , the total length L of the boundary 20BP removed by the through hole 20H s , the length L of the boundary 20BP c is represented by the following equations respectively. Note that the distance La is also the distance between the center 20HC of the through hole 20H and the boundary 20BP to be removed. L a =|L p -D1| L s =Σs L c =2πD1

[0049] The design of the optical fiber preform is based on the design of the optical fiber. As shown in FIG. 1, the radius of the cladding 20 of the multi-core optical fiber 1 is D f , and when the distance from the center 20C of the cladding 20 to the center 10C of the core 10 is L ... f , the multi-core optical fiber preform 1P of the present embodiment is designed to satisfy the following conditions, so that the through hole 20H can be formed overlapping the boundary 20BP. D2·L f / D f -h≦D1≦D2·L f / D f +h

[0050] In this case, the length s of the boundary 20BP removed by one through hole 20H is represented by the following equation. TIFF2026126854000002.tif29170

[0051] When the center 20HC of the through hole 20H is located on the boundary 20BP, L a is zero. Therefore, based on the state where the center 20HC is located on the boundary 20BP, taking the distance L a when the center 20HC is located in the outer cladding layer 22P as positive, and the distance L a when the center 20HC is located in the center cladding rod 21P as negative. The L in this case aWhen taking / h as the horizontal axis and s / 2πh as the vertical axis, the relationship shown in FIG. 7 can be obtained. L a / h is the ratio of the distance L between the center of the through hole 20H and the boundary 20BP a to the radius h of the through hole 20H, and thus indicates the position of the center 20HC of the through hole 20H. Also, since s / 2πh is the ratio of the length of the periphery of one through hole 20H to the length s of the boundary 20BP removed by one through hole 20H, it indicates the efficiency of removing the boundary 20BP by the through hole 20H. In FIG. 7, h / D1 is changed from 0.05 to 1.25. It can be read from FIG. 7 that the first condition or the second condition is good.

[0052] (The first condition) 0 < h / D1 < 1, and L a / h > 0.

[0053] 0 < h / D1 < 1 means that the diameter of the through hole 20H is smaller than the diameter of the central clad rod 21P. Also, L a / h > 0 means that the center 20HC of the through hole 20H is located in the outer clad layer 22P. As described above, since the design of the multi-core optical fiber preform 1P is based on the design of the multi-core optical fiber 1, the position of the through hole 20H is generally determined from the configuration of the multi-core optical fiber 1 to be manufactured. Therefore, the fact that the center 20HC of the through hole 20H is located in the outer clad layer 22P means that the diameter of the central clad rod 21P is small. The manufacture of the central clad rod 21P with both a large diameter and a large length is generally difficult due to problems such as equipment. Also, when the diameter of the central clad rod 21P is large, generally, the thickness of the outer clad layer 22P cannot be increased due to problems such as equipment. However, when the diameter of the central clad rod 21P is small, after manufacturing the thick central clad rod 21P, the central clad rod 21P can be drawn to make it a long and thin rod. In this case, the thickness of the outer clad layer 22P can be increased, and the volume of the multi-core optical fiber preform 1P can be increased. Therefore, by the small diameter of the central clad rod 21P, the multi-core optical fiber 1 that can be manufactured from one multi-core optical fiber preform 1P can be made longer.

[0054] L a In the region where / h>0, L is different from the case where h / D1≧1. a If / h is constant, the value of s / 2πh can be increased. When manufacturing the multicore optical fiber 1, a boundary is created between the insertion glass rod 40P and the cladding rod 20P. Since it is preferable for such a boundary to be small, it is preferable to make the boundary between the insertion glass rod 40P and the cladding rod 20P as small as possible, and to remove as much of the boundary 20BP between the central cladding rod 21P and the outer cladding layer 22P as possible. Therefore, it is preferable to increase the value of s / 2πh. a / h>0 is preferable.

[0055] (Second condition) 0 <h / D1<1、かつ、L a / h ≤ 0.

[0056] L a / h≦0 means that the center 20HC of the through hole 20H is on the boundary 20BP or the central cladding rod 21P. In this case, L a If the absolute value of is the same, the value of s / 2πh can be increased compared to the case where the center 20HC of the through hole 20H is located in the outer cladding layer 22P. Furthermore, it is preferable that the straight line connecting the two intersection points of the boundary 20BP and the outer circumference of the through hole 20H passes through the center 20HC of the through hole 20H, as this allows for the largest possible value of s / 2πh.

[0057] When the through-hole 20H overlaps with the boundary 20BP, it is preferable to define a perforation region 20Ha in the clad rod 20P where the through-hole 20H is not formed, so that the through-hole 20H can be formed in a way that satisfies the first or second condition described above, and then form the through-hole 20H.

[0058] Note L s / L cIt is preferable that the value is ≥0.2. In this case, the boundary between the central cladding rod 21P and the outer cladding rod 22P is sufficiently removed. Therefore, the interface between the central cladding rod 21P and the outer cladding layer 22P can be made smaller, and the reduction in strength of the manufactured optical fiber can be further suppressed.

[0059] (Insertion process P4) This step involves inserting the insertion glass rod 40P into the through hole 20H. This step yields the multicore optical fiber base material 1P shown in Figure 2. In this state, a gap is created between the inner surface of the cladding rod 20P that forms the through hole 20H and the outer surface of the core rod 10P that is inserted into the through hole 20H.

[0060] (Drawing process P5) This process involves drawing a multicore optical fiber 1 from a multicore optical fiber base material 1P. Figure 8 shows this process. In this process, first, a dummy glass is welded to one end of the multicore optical fiber base material 1P, and a glass tube is welded to the other end. Then, the multicore optical fiber base material 1P is placed in a spinning furnace 110, and the air is removed from each through-hole of the clad rod 20P via the glass tube. Next, the multicore optical fiber base material 1P is heated by the heating section 111 of the spinning furnace 110. This heating causes the lower end of the multicore optical fiber base material 1P to melt, and glass is drawn from the multicore optical fiber base material 1P. At this time, the gaps in the through-holes 20H are closed. The drawn molten glass solidifies immediately upon exiting the spinning furnace 110, with the core rods 10P of each insertion glass rod 40P becoming the respective cores 10, and the cladding rods 20P and the outer glass layers 23P of each insertion glass rod 40P becoming the cladding 20. In this way, a bare multicore optical fiber wire composed of multiple cores 10 and cladding 20 is obtained. Subsequently, this bare multicore optical fiber wire passes through the cooling device 120 to cool to an appropriate temperature. The cooled bare multicore optical fiber wire passes through the coating device 130 to form an inner coating layer 31 and an outer coating layer 32, resulting in the multicore optical fiber 1 shown in Figure 1. The multicore optical fiber 1 is then reoriented by the turn pulley 141 and wound up by the reel 142. In this way, the multicore optical fiber 1 is manufactured.

[0061] As described above, the manufacturing method of the multicore optical fiber base material 1P of this embodiment comprises an observation step P2 for observing the inside of the clad rod 20P, a drilling step P3 for forming a through hole 20H along the longitudinal direction of the clad rod 20P in a drilling region 20Ha that includes at least a part of the area DP where an abnormal condition was discovered in the observation step P2, and an insertion step P4 for inserting an insertion glass rod 40P, which includes a core rod 10P, a predetermined glass rod made of glass with different properties from the glass constituting the clad rod 20P, into the through hole 20H.

[0062] According to this method for manufacturing a multicore optical fiber preform 1P, the through-hole 20H through which the core rod 10P, which is made of glass with different properties from the glass constituting the cladding rod 20P, is inserted, can remove at least a portion of the abnormally conditioned portion DP, such as bubbles or impurities. In other words, by forming the through-hole 20H necessary for manufacturing the multicore optical fiber preform 1P, at least a portion of the abnormally conditioned portion DP can be removed. Therefore, by using the multicore optical fiber preform 1P manufactured by the manufacturing method of the multicore optical fiber preform 1P of this embodiment, it is possible to manufacture a multicore optical fiber 1 with suppressed strength reduction.

[0063] Furthermore, in the observation step P2 of this embodiment, at least a predetermined annular region 20AR surrounding the central axis 20PC of the clad rod 20P is observed, and at least a portion of the perforated region 20Ha is located in the annular region 20AR. In some cases, the clad rod 20P is manufactured by depositing glass while rotating a glass rod around its axis, such as in the OVD method. In this case, abnormal areas DP may occur at different positions at a certain distance from the central axis 20PC of the clad rod 20P. For example, abnormal areas DP are likely to occur at the boundary 20PB between the central clad rod 21P and the outer clad layer 22P. Therefore, by observing the annular region 20AR, it is easier to detect abnormal areas DP in the annular region 20AR. By removing at least a portion of these abnormal areas DP by forming through holes 20H, a multicore optical fiber 1 with suppressed strength reduction can be manufactured.

[0064] Furthermore, in this embodiment, the clad rod 20P includes a central clad rod 21P and an outer clad layer 22P that surrounds the central clad rod 21P and is in contact with its outer circumferential surface. The annular region 20AR includes the boundary 20BP between the central clad rod 21P and the outer clad layer 22P. The perforated region 20Ha overlaps the boundary 20BP. The core rod 10P is made of glass with different properties from the glass constituting the central clad rod 21P and the outer clad layer 22P. According to this method for manufacturing a multicore optical fiber base material 1P, the use of the central clad rod 21P and the outer clad layer 22P makes it possible to increase the diameter of the clad rod 20P, and thus enable the production of a long multicore optical fiber 1. However, crystal mismatch tends to occur at the boundary 20BP between the central clad rod 21P and the outer clad layer 22P, making it easy for impurities to be mixed in and for bubbles to be generated. As in this embodiment, by forming the through-hole 20H so as to overlap with the boundary 20BP, the boundary 20BP can be made smaller compared to when the boundary 20BP is continuous around the entire circumference. When the through-hole 20H collapses during the manufacturing of the multicore optical fiber 1, an interface is formed between the insertion glass rod 40P and the cladding rod 20P. However, compared to the case where the through-hole 20H is provided within the central cladding rod 21P or the outer cladding layer 22P, the total amount of glass-to-glass interface can be reduced because the boundary 20BP between the central cladding rod 21P and the outer cladding layer 22P is smaller. Therefore, abnormal areas DP where impurities are mixed in or bubbles are generated can be removed, and further, the reduction in strength of the multicore optical fiber 1 caused by crystal mismatch at the boundary 20BP can be suppressed.

[0065] (Second Embodiment) Next, a second embodiment of the present invention will be described in detail. Note that components identical or equivalent to those in the first embodiment are denoted by the same reference numerals unless otherwise specified, and redundant descriptions will be omitted.

[0066] Figure 9 shows a cross-section of the optical fiber perpendicular to the longitudinal direction according to this embodiment. In this embodiment, a polarization-maintaining optical fiber is described as an example of an optical fiber. As shown in Figure 9, the polarization-maintaining optical fiber 2 has one core 10, the core 10 is located on the center 20C of the cladding 20, and includes a pair of stress-applying portions 12 arranged within the cladding 20 so as to sandwich the core 10.

[0067] The stress-applying section 12 is made of glass having a different coefficient of thermal expansion than the cladding 20, and stress is applied to the core 10 from a pair of stress-applying sections 12 that are arranged to sandwich the core 10. In other words, the stress-applying section 12 is made of glass with different properties from the glass that makes up the cladding 20. When tensile and compressive stress is applied to the core 10 from the pair of stress-applying sections 12, a birefringence is induced due to the photoelastic effect, and these two polarization modes perpendicular to each other have different propagation constants.

[0068] Examples of materials that constitute such a stress-applying section 12 include quartz glass to which dopants such as boron have been added, and the cladding 20 and the stress-applying section 12 are made of glass with different compositions from each other.

[0069] Figure 10 shows a cross-sectional view perpendicular to the longitudinal direction of the polarization-maintaining optical fiber base material 2P, which is the optical fiber base material for manufacturing the polarization-maintaining optical fiber 2 shown in Figure 9. The polarization-maintaining optical fiber base material 2P comprises a cladding rod 20P, a core rod 10P, and a pair of insertion glass rods 40P. In this example, the outer surface of the core rod 10P is surrounded by the cladding rod 20P without any gaps.

[0070] The clad rod 20P of this embodiment has a central clad rod 21P and an outer clad layer 22P, similar to the clad rod 20P of the first embodiment. The central clad rod 21P and the outer clad layer 22P are made of glass with the same properties and composition as in the first embodiment. In this embodiment, a pair of through holes 20H are formed in the clad rod 20P so that a pair of insertion glass rods 40P are inserted into it. The pair of through holes 20H are located between the central clad rod 21P and the outer clad layer 22P.

[0071] In this embodiment, the insertion glass rods 40P inserted into each through-hole 20H include a stress-applying rod 12P and an outer glass layer 23P that completely surrounds the outer surface of the stress-applying rod 12P. The stress-applying rod 12P is a predetermined glass rod that becomes the stress-applying section 12, and is made of the same type of glass as that which constitutes the stress-applying section 12. Therefore, the stress-applying rod 12P is a predetermined glass rod made of glass with different properties than the glass that constitutes the central cladding rod 21P and the outer cladding layer 22P. The outer glass layer 23P is a glass layer that becomes part of the cladding 20, similar to the outer glass layer 23P in the first embodiment. As described above, the central cladding rod 21P and the outer cladding layer 22P are made of glass with the same properties. Therefore, the outer glass layer 23P is also made of glass with the same properties as the central cladding rod 21P and the outer cladding layer 22P. In other words, the outer glass layer 23P is made of glass with the same composition as the central cladding rod 21P and the outer cladding layer 22P.

[0072] The method for manufacturing the polarization-maintaining optical fiber base material 2P and the polarization-maintaining optical fiber 2 in this embodiment is generally the same as the method for manufacturing the multicore optical fiber base material 1P and the multicore optical fiber 1 in the first embodiment. Therefore, the method for manufacturing the polarization-maintaining optical fiber base material 2P and the polarization-maintaining optical fiber 2 can be generally explained by replacing the multicore optical fiber 1 with the polarization-maintaining optical fiber 2 and the multicore optical fiber base material 1P with the polarization-maintaining optical fiber base material 2P in the description of the method for manufacturing the multicore optical fiber base material 1P and the multicore optical fiber 1 in the first embodiment.

[0073] (Preparation process P1) In this step of the embodiment, a pair of insertion glass rods 40P are prepared, including a clad rod 20P that does not have a through-hole 20H formed in the same way as in the first embodiment, and a stress-applying rod 12P that can be individually inserted into the through-hole 20H and will become the stress-applying part 12 of the polarization-maintaining optical fiber 2. In this step, the insertion glass rods 40P and the clad rods 20P may be prepared by manufacturing or by purchase.

[0074] The method for manufacturing the insertion glass rod 40 in this embodiment will be explained by replacing the core rod 10P with the stress-applying rod 12P in the description of the manufacturing of the insertion glass rod 40 in the preparation step P1 of the first embodiment.

[0075] (Observation process P2) The main step of this embodiment is carried out in the same manner as the observation step P2 of the first embodiment. However, in this embodiment, since it is necessary to form a through hole 20H into which a pair of insertion glass rods 40P including the stress-applying rod 12P can be inserted, it is preferable to set the annular region 20AR to overlap with the perforation region 20Ha that coincides with the through hole 20H.

[0076] (Drilling process P3) This step of the embodiment differs from the drilling step P3 of the first embodiment in that it forms through holes 20H into which a pair of insertion glass rods 40P can be inserted, at positions that sandwich the core rod 10P. Figure 11 shows the clad rod 20P before this step. As shown in Figure 11, in this embodiment as well, impurities are attached to the outer surface of the central clad rod 21P, forming an abnormal area DP. Therefore, in this embodiment as well, the drilling region 20Ha is set to overlap with at least a part of the area DP, and through holes 20H are formed in the drilling region 20Ha. Also in this embodiment as well, similar to the first embodiment, the drilling region 20Ha is set to overlap with the boundary 20BP between the central clad rod 21P and the outer clad layer 22P on the clad rod 20P, and through holes 20H are formed in the drilling region 20Ha along the longitudinal direction of the clad rod 20P. Also, in this embodiment as well, it is preferable to satisfy the first or second condition described in the first embodiment.

[0077] (Insertion process P4) This step differs from the insertion step P4 of the first embodiment in that an insertion glass rod 40P, including the stress-applying rod 12P, is inserted into a pair of through holes 20H. In this way, the polarization-maintaining optical fiber base material 2P shown in Figure 10 is obtained.

[0078] (Drawing process P5) This process differs from the drawing process P5 of the first embodiment in that it manufactures the polarization-maintaining optical fiber 2 by drawing a polarization-maintaining optical fiber base material 2P. At this time, it differs from the drawing process P5 of the first embodiment in that the stress-applying rod 12P becomes the stress-applying part 12. In this process of the embodiment, stress is applied to the core 10 in the direction connecting the pair of stress-applying parts 12 due to the difference in the coefficient of thermal expansion between the glass constituting the stress-applying rod 12P and the glass constituting the cladding rod 20P.

[0079] As described above, by using the polarization-maintaining optical fiber matrix 2P manufactured by the method for manufacturing the polarization-maintaining optical fiber matrix 2P of this embodiment, a polarization-maintaining optical fiber 2 with suppressed strength reduction can be manufactured for the same reasons as in the first embodiment.

[0080] Although the present invention has been described above with reference to embodiments, the present invention is not limited to the above embodiments.

[0081] For example, in the first embodiment, the number of cores 10 was set to 4, but the number is not particularly limited as long as there are multiple cores 10 located in places other than the center 20C of the cladding 20 of the multicore optical fiber 1. Furthermore, a core 10 may be located in the center of the cladding 20.

[0082] In the above embodiment, a multicore optical fiber 1 and a polarization-maintaining optical fiber 2 were described as examples of optical fibers. However, the optical fibers manufactured by the optical fiber matrix manufacturing method of the present invention are not limited to the multicore optical fiber 1 and the polarization-maintaining optical fiber 2.

[0083] Furthermore, in the above embodiment, the clad rod 20P consisted of a central clad rod 21P and an outer clad layer 22P, and the perforated region 20Ha was set to overlap with the boundary 20BP, and a part of the boundary 20BP was removed by the formation of the through-hole 20H. However, even when the clad rod 20P consists of a central clad rod 21P and an outer clad layer 22P, as long as the through-hole 20H is formed along the longitudinal direction of the clad rod 20P in the perforated region 20Ha which includes at least a part of the abnormal state portion DP, the perforated region 20Ha does not have to be set to overlap with the boundary 20BP, and a part of the boundary 20BP does not have to be removed by the formation of the through-hole 20H. For example, when a marker is provided on the multicore optical fiber 1, the refractive index of the marker is different from the refractive index of the clad 20. Therefore, in the multicore optical fiber base material 1P, a predetermined glass rod may be used as the marker rod, and a through-hole into which an insertion glass rod including the marker rod is inserted may be formed in a perforated region that includes at least a part of the abnormal region DP. Accordingly, in the multicore optical fiber base material 1P, the predetermined glass rod made of glass with different properties from the glass constituting the cladding rod 20P is not limited to the core rod 10P. Furthermore, there may be just one through-hole formed in the perforated region that includes at least a part of the abnormal region DP.

[0084] Furthermore, unlike the above embodiment, the clad rod 20P may not consist of a central clad rod 21P and an outer clad layer 22P, but may consist of a single glass rod or three or more glass layers.

[0085] Furthermore, in the above embodiment, the insertion glass rod 40P is configured such that the outer surface of a predetermined glass rod, such as the core rod 10P or the stress-applying rod 12P, is covered with an outer glass layer 23P that becomes part of the cladding 20. However, the outer glass layer 23P is not an essential component, and the insertion glass rod may consist only of predetermined glass rods.

[0086] Furthermore, in the above embodiment, the inside of the clad rod 20P was observed from the side during observation step P2. However, observation step P2 may differ from that of the above embodiment. Figure 12 shows a modified example of observation step P2. In this modified example, during observation step P2, the inside of the clad rod 20P is observed from the end face of the clad rod 20P along the longitudinal direction of the clad rod 20P. The through hole 20H is formed along the longitudinal direction of the clad rod 20P. Therefore, even if it is not possible to determine the exact location of the abnormal part DP along the longitudinal direction of the clad rod 20P, if it is known that the part DP is located at any position on the end face of the clad rod 20P, at least a portion of the part DP can be removed by the through hole 20H. When observing the clad rod 20P from the side as in the above embodiment, it is difficult to pinpoint the location of the abnormal part DP unless the effect of light refraction on the side is considered and the clad rod 20P is rotated during observation. However, according to this modified method, it is possible to identify which position the above-mentioned part DP is located at on the end face of the clad rod 20P, and the observation step P2 can be made easier than observing the clad rod 20P from the side.

[0087] Furthermore, in the above embodiment, the perforated region 20Ha includes the entire abnormal part DP, and the entire part DP is removed by the through hole 20H. However, in the present invention, the perforated region 20Ha may include only a part of the abnormal part DP and not the other part of part DP, and only a part of part DP may be removed by the through hole 20H.

[0088] Furthermore, in the above embodiment, after the insertion step P4, the multicore optical fiber 1 and polarization-maintaining optical fiber 2 were manufactured by drawing the multicore optical fiber 1 and polarization-maintaining optical fiber 2. In other words, the insertion glass rod 40P was inserted into the through-hole 20H, and the optical fiber was manufactured by drawing the optical fiber base material while there was a gap inside the through-hole 20H. However, the optical fiber base material may be collapsed before the drawing step P5, and the collapsed state may be used as the optical fiber base material, or the collapsed optical fiber base material may be drawn in the drawing step P5. [Industrial applicability]

[0089] As described above, the present invention provides a method for manufacturing an optical fiber matrix and an optical fiber, which can produce optical fibers with suppressed strength reduction, and can be used in the field of optical communication and other devices that utilize optical fibers. [Explanation of Symbols]

[0090] 1. Multicore optical fiber (optical fiber) 2. Polarization-maintaining optical fiber (optical fiber) 10 cores 12. Stress application section 20.. Clad 1P... Multicore optical fiber preform (optical fiber preform) 2P... Polarization-maintaining optical fiber preform (optical fiber preform) 10P...Core Rod 12P... Stress-applying rod 20P...Clad Rod 20H...Through hole P1...preparation process P2... Observation process P3...hole drilling process P4...Insertion process P5...Drawing process

Claims

1. An observation process to observe the inside of the clad rod, A drilling step in which through holes are formed along the longitudinal direction of the clad rod in a drilling region that includes at least a portion of the area where an abnormal condition was discovered in the observation step, An insertion step of inserting an insertion glass rod, which includes a predetermined glass rod made of glass having different properties from the glass constituting the clad rod, into the through hole; Equipped with A method for manufacturing an optical fiber preform characterized by the following:

2. In the observation step, the interior of the clad rod is observed from the end face of the clad rod along the longitudinal direction of the clad rod. A method for manufacturing an optical fiber preform according to claim 1.

3. In the observation step described above, at least a predetermined annular region surrounding the central axis of the clad rod is observed, At least a portion of the perforated region is located in the annular region. A method for manufacturing an optical fiber preform according to claim 1.

4. The clad rod includes a central clad rod and an outer clad layer that surrounds the central clad rod and is in contact with its outer circumferential surface. The annular region includes the boundary between the central cladding rod and the outer cladding layer, The aforementioned perforated region overlaps with the boundary, The predetermined glass rod is made of glass with different properties from the glass constituting the central cladding rod and the outer cladding layer. The method for manufacturing an optical fiber preform according to claim 3.

5. In the drilling step, through holes are formed in the annular region along the longitudinal direction of the clad rod in a plurality of regions including the drilling region. In the insertion step, the insertion glass rod is inserted into the multiple through holes. A method for manufacturing an optical fiber preform according to claim 3 or 4.

6. The aforementioned predetermined glass rod is a core rod. The method for manufacturing an optical fiber preform according to claim 5.

7. The aforementioned glass rod is a stress-applying rod. The method for manufacturing an optical fiber preform according to claim 5.

8. The invention comprises a drawing step of drawing a fiber optic preform manufactured by the method for manufacturing a fiber optic preform according to any one of claims 1 to 4. A method for manufacturing optical fibers, characterized by the following:

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