Manufacturing method for preforms for multicore fibers

Abstract

This method for producing a base material (1P) for a multicore fiber includes: a preparation step (P1) for preparing a plurality of core rods (10P) and a cladding rod (20P) in which an even number, 4 or more, of through holes (20PH) formed on the periphery (20PC) are positioned so as to form a plurality of sets (PA1), (PA2) aligned in the radial direction; a determination step (P3) for determining a combination of a core rod (10P) and a through hole (20PH) so that in a case where the core rod (10P) is inserted into the through hole (20PH), when the sum total of the clearance between the core rod (10P) and the inner wall of the through hole (20PH) is determined for the sets (PA1), (PA2), the residual sum of squares is minimized; and an insertion step (P4) for inserting the core rods (10P) into the through holes (20PH) for the determined combinations of core rods (10P) and through holes (20PH).

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a base material for a multi-core fiber.

Background Art

[0002] In recent years, with the spread of optical fiber communication systems, the amount of information transmitted by optical fibers has increased dramatically. Against this background, multi-core fibers in which the outer peripheries of a plurality of cores are surrounded by a single cladding are being used. Since a multi-core fiber can transmit a plurality of signals by the light propagating through each of the plurality of cores, the transmission capacity per optical fiber is increased.

[0003] The following Patent Document 1 describes a method for manufacturing a multi-core fiber. Specifically, core rods are inserted into a plurality of through-holes formed in a cladding rod that becomes the cladding of the multi-core fiber, and while integrating the cladding rod and the core rods, wire drawing is performed to manufacture the multi-core fiber.

[0004]

Patent Document 1

Summary of the Invention

[0005] When inserting a core rod into a through-hole of a cladding rod as in the method for manufacturing a multi-core fiber of Patent Document 1, a clearance occurs between the inner wall of the through-hole of the cladding rod and the outer peripheral surface of the core rod. This clearance is crushed before the multi-core fiber is manufactured. Therefore, the portion of the outer peripheral surface of the cladding rod facing the through-hole into which the core rod is inserted is more likely to move inward than other positions by the amount by which the clearance is crushed. Further, the greater the clearance, the more likely the above-mentioned portion of the outer peripheral surface of the cladding rod is to move inward. However, there is a demand to suppress deviation from the circular shape of the outer shape of the cladding of the multi-core fiber.

[0006] Therefore, the present invention aims to provide a method for manufacturing a matrix material for multicore fibers that can produce multicore fibers in which deviation from the circular shape of the cladding is suppressed.

[0007] One aspect of the present invention for solving the above problems is a clad rod which will be the clad of a multicore fiber, wherein four or more even-numbered through holes formed on a predetermined circumference centered on the central axis of the clad rod are positioned to form a plurality of sets aligned in the radial direction of the clad rod, and a plurality of core rods which will be the core of the multicore fiber and can be individually inserted into the through holes; a determination step for determining a combination of the through holes and the core rod to be inserted into the through holes; and an insertion step for inserting each of the core rods into each of the through holes in the determined combination of core rod and through hole, wherein in the determination step, the combination of the core rod and the through hole is determined such that when the sum of the clearances between the core rod and the inner wall of the through hole is taken for each set, the sum of the residuals of each sum is minimized.

[0008] When the clearances of the through-holes aligned radially in the clad rod are compressed, the portion of the outer surface of the clad rod facing the through-holes moves inward, as described above, causing the clad rod to shrink in the radial direction in which these through-holes are aligned. However, in the present invention, as described above, the sum of the clearance sizes in each set is taken, and the sum of the squared residuals of each sum is minimized, making it possible to make the sum of the clearance sizes in each set more uniform than in the conventional method. Therefore, even if the clad rod shrinks in the radial direction in which the through-holes in each set are aligned due to the compression of the clearances, the degree of shrinkage in the direction of alignment of each set can be made more uniform than in the conventional method. In other words, it is possible to suppress the extreme reduction in diameter of one part of the clad rod compared to another part. Therefore, by using a multicore fiber base material manufactured by this method for manufacturing multicore fiber base materials, it is possible to manufacture a multicore fiber in which deviation from the circular shape of the clad outer surface is suppressed.

[0009] Aspect 2 of the present invention is a method for manufacturing a matrix material for multicore fibers according to aspect 1, further comprising a measurement step of measuring the outer diameter of each of the core rods and the diameter of the through hole into which the core rods are inserted, wherein the determination step is performed based on the measurement results of the measurement step.

[0010] In this case, even if the outer diameter of the prepared core rod and the diameter of the through-hole in the clad rod are unknown or contain errors, these can be clarified through the measurement process. Therefore, the determination process can be performed more accurately.

[0011] A third aspect of the present invention is a method for manufacturing a matrix material for multicore fibers according to aspect 1 or 2, characterized in that the outer diameters of each of the core rods are different from each other, and the hole diameters of each of the through holes are different from each other.

[0012] In this case, glass rods with different outer diameters due to manufacturing variations can be effectively used, potentially improving the manufacturing yield of optical fiber preforms.

[0013] As described above, the present invention provides a method for manufacturing a matrix material for multicore fibers that can produce multicore fibers in which deviation from the circular shape of the cladding is suppressed. [Brief explanation of the drawing]

[0014] [Figure 1] This figure shows a cross-section perpendicular to the longitudinal direction of a multicore fiber according to an embodiment of the present invention. [Figure 2] This figure shows the base material for multicore fibers used to manufacture the multicore fiber shown in Figure 1. [Figure 3] This figure shows the matrix material for multicore fibers shown in Figure 2, and a flowchart for manufacturing the multicore fibers shown in Figure 1. [Figure 4] This diagram shows the situation after the preparation process. [Figure 5] This diagram shows the size of the clearance. [Figure 6] This is a diagram showing the line drawing process. [Modes for carrying out the invention]

[0015] Hereinafter, preferred embodiments of the method for manufacturing a multicore fiber matrix according to the present invention will be described in detail with reference to the drawings. The embodiments illustrated below are provided to facilitate understanding of the present invention and are not intended to limit its interpretation. The present invention can be modified and improved from these embodiments without departing from its spirit. Note that for ease of understanding, the scale of each figure may differ from the scale described in the following description.

[0016] Figure 1 shows a cross-section perpendicular to the longitudinal direction of a multicore fiber according to this embodiment. The multicore fiber 1 of this embodiment comprises four or more even-numbered cores 10, a cladding 20 that completely surrounds the outer surface of each core 10, an inner covering layer 31 that covers the outer surface of the cladding 20, and an outer covering layer 32 that covers the outer surface of the inner covering layer 31. In this embodiment, an example with four cores 10 is shown.

[0017] The outer diameter in a cross-section perpendicular to the longitudinal direction of the clad 20 is generally circular. In the present embodiment, each core 10 is arranged on a virtual circumference 20C centered on the central axis 20R of the clad 20. Also, each core 10 forms a pair arranged in the radial direction of the clad 20, and a plurality of such pairs are formed. In the present embodiment, the distances between the respective cores 10 are equal to each other, and each core 10 is arranged at a position that is generally rotationally symmetric about the central axis 20R approximately four times. The diameter of the core 10 is, for example, 4 μm or more and 10 μm or less.

[0018] The refractive index of each core 10 is higher than that of the clad 20, and the difference in relative refractive index of each core 10 with respect to the clad 20 is, for example, 0.2% or more and 2.0% or less. Such a core 10 is made of, for example, silica glass to which a dopant such as germanium that increases the refractive index is added, and the clad 20 is made of, for example, silica glass to which no dopant is added. Also, the core 10 may be made of silica glass to which no dopant is added, and the clad 20 may be made of silica glass to which a dopant such as fluorine that decreases the refractive index is added.

[0019] 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 from each other.

[0020] Next, a manufacturing method for manufacturing the multi-core fiber 1 of FIG. 1 will be described.

[0021] FIG. 2 is a diagram showing a cross-sectional view perpendicular to the longitudinal direction of a base material for a multi-core fiber for manufacturing the multi-core fiber 1 of FIG. 1. The base material 1P for a multi-core fiber includes a clad rod 20P and a plurality of core rods 10P.

[0022] The clad rod 20P serves as the clad 20 of the multi-core fiber 1. The outer peripheral shape of the clad rod 20P is circular, and four or more even-numbered through holes 20PH are formed in the clad rod 20P. Each through hole 20PH is formed at equal intervals on a virtual circumference 20PC centered on the central axis 20PR of the clad rod 20P. In the present embodiment, since the number of cores 10 in FIG. 1 is four, the number of through holes 20PH is four. The through holes 20PH are formed at equal intervals on the circumference 20PC, and a plurality of sets PA1, PA2 of through holes 20PH arranged in the radial direction of the clad rod 20P are formed. In each set PA1, PA2, the through holes 20PH are arranged in the radial direction of the clad rod 20P with the central axis 20PR interposed therebetween. In FIG. 2, the through holes 20PH surrounded by the dotted lines indicating the sets PA1, PA2 are the sets.

[0023] Also, a core rod 10P is inserted into each through hole 20PH. In the present embodiment, each core rod 10P is a glass rod that serves as the core 10 of the multi-core fiber 1. Note that the core rod 10P may have a configuration in which the outer peripheral surface of the glass body serving as the core 10 is covered with a glass layer that is part of the clad 20.

[0024] FIG. 3 is a diagram showing a flowchart for manufacturing the multi-core fiber preform 1P of FIG. 2 and the multi-core fiber 1 of FIG. 1. As shown in FIG. 3, the method for manufacturing the multi-core fiber preform 1P of the present embodiment includes a preparation step P1, a measurement step P2, a determination step P3, and an insertion step P4, and the method for manufacturing the multi-core fiber 1 further includes a drawing step P5 in addition to these.

[0025] (Preparation step P1) This process involves preparing cladding rods 20P, which have four or more even-numbered through-holes 20PH formed on them and become the cladding 20 of the multicore fiber 1, and a plurality of core rods 10P, which can be individually inserted into the through-holes 20PH and become the cores 10 of the multicore fiber 1. Figure 4 shows the state after the preparation process. As shown in Figure 4, the number of core rods 10P is the same as the number of through-holes 20PH. Also, in this embodiment, the length of the cladding rods 20P and the length of the core rods 10P are equal to each other. In this process, the core rods 10P and cladding rods 20P may be prepared by manufacturing or by purchase.

[0026] (Measurement process P2) This step involves measuring the outer diameter of each core rod 10P and the diameter of the through hole 20PH into which the core rod 10P is inserted. The outer diameter of the core rod 10P is preferably measured at multiple locations along its longitudinal direction. In this case, the minimum, median, average, or maximum value of the outer diameter may be used as the outer diameter of the core rod 10P. Similarly, the diameter of the through hole 20PH is preferably measured at multiple locations along its longitudinal direction. In this case, the minimum, median, average, or maximum value of the hole diameter may be used as the diameter of the through hole 20PH.

[0027] The outer diameter of the core rod 10P is measured using, for example, a caliper, a laser outer diameter measuring machine, or a 3D measuring machine. Similarly, the diameter of the through hole 20PH is measured using, for example, a caliper, a laser outer diameter measuring machine, or a 3D measuring machine.

[0028] Note that this step does not need to be performed if the outer diameter of the core rod 10P and the diameter of the through hole 20PH are known. Alternatively, if a specific core rod 10P or a specific clad rod 20P is selected and prepared in preparation step P1, the outer diameter of the core rod 10P and the diameter of the through hole 20PH of the clad rod 20P may be measured before selection, and then preparation step P1 may be performed by selecting the core rod 10P or clad rod 20P.

[0029] (Decision process P3) This step determines the combination of core rod 10P and through hole 20PH such that the sum of the squared residuals of the clearances PA1 and PA2 between the core rod 10P and the inner wall of the through hole 20PH is minimized when inserting the core rod 10P into the through hole 20PH. Figure 5 shows the size of the clearance. As shown in Figure 5, the size of the clearance CS is the maximum distance between the core rod 10P and the inner wall 20W when the core rod 10P is pressed against the inner wall 20W of the through hole 20PH. Therefore, it is approximately equal to the difference between the diameter of the through hole 20PH and the outer diameter of the core rod 10P. For this reason, it is preferable to perform this step based on the measurement results of measurement step P2.

[0030] Furthermore, considering cases where the clearance size CS varies along the longitudinal direction, it is preferable to measure the clearance size CS at multiple points along the longitudinal direction. In this case, the minimum, median, average, or maximum value of the size CS may be used as the clearance size CS. Also, the clearance size CS may be measured based on the total length of the clad rod 20P in the longitudinal direction, or it may be measured based on a part of the clad rod 20P in the longitudinal direction (for example, the part that becomes the effective portion of the optical fiber preform). In the latter case, the total length of the manufactured optical fiber preform cannot be the product due to waste of preform volume as yield during preform processing or spinning. For example, if the central region of the manufactured optical fiber preform (for example, a region with a length of 30 to 1000 mm on one end and the other end of the optical fiber preform centered on the longitudinal center position of the optical fiber preform) is the effective region of the optical fiber preform, it may be better to measure the clearance size CS based on the average of that central region. Therefore, it is possible to achieve this effect at a specific position in the longitudinal direction of the clad rod 20P. Furthermore, by setting the clearance size CS to its maximum value, it may be possible to maximize the reduction in the positional deviation of the fabricated optical fiber base material from the design value of a specific core.

[0031] Specifically, if we are manufacturing a multicore fiber base material 1P for producing a multicore fiber 1 with four cores as described above, we determine the difference between the diameter of the through-hole 20PH and the outer diameter of the core rod 10P for each combination of core rod 10P and each through-hole 20PH. Then, we find the combination of core rod 10P and through-hole 20PH that minimizes the sum of squared residuals of the sum of these differences for each set PA1 and PA2. If we define the sum of these differences for each set PA1 and PA2 as "Sum of Clearances SUM1" and "Sum of Clearances SUM2", respectively, then in this embodiment, with j set to 1 or 2, the sum of squared residuals PRS can be expressed as follows. PRS = Σ(Sum of clearances SUMj) 2

[0032] In this process, the combination of core rod 10P and through hole 20PH that minimizes the PRS (Pressure Rating System) is determined. In this way, the through hole 20PH into which the core rod 10P is inserted is determined.

[0033] Furthermore, if the difference at each through-hole 20PH is denoted as "clearance size CS1", "clearance size CS2", "clearance size CS3", and "clearance size CS4", then the sum of squared residuals RS of the clearances for each through-hole 20PH can be expressed as follows, with i set to 1 to 4. RS = Σ(clearance CSi) 2

[0034] Furthermore, as described above, in the combination of core rod 10P and through hole 20PH that minimizes the sum of squared residuals PRS in the determination process P3, it is preferable that the sum of squared residuals RS is minimized. In this case, the size of each clearance CS can be made more uniform than in the conventional method. Therefore, by using this multi-core fiber base material 1P, it is possible to manufacture a multi-core fiber 1 in which the positional displacement of the core 10 within the cladding 20 is made more uniform than in the conventional method.

[0035] (Insertion process P4) This process involves inserting each core rod 10P into its respective through-hole 20PH, according to the determined combination of core rod 10P and through-hole 20PH. This process yields the multi-core fiber base material 1P shown in Figure 2. In the thus manufactured multi-core fiber base material 1P, the combination of through-hole 20PH and core rod 10P is the one that minimizes the sum of squared residuals of the clearances for each set PA1 and PA2.

[0036] Furthermore, considering the possibility of the core rod being bent, it is preferable that the clearance size be 0.1 mm or more.

[0037] (Drawing process P5) This process involves drawing a wire from a multicore fiber base material 1P to manufacture a multicore fiber 1. Figure 6 shows this process. In this process, first, a dummy glass is welded to one end of the multicore fiber base material 1P, and a glass tube is welded to the other end. Then, the multicore fiber base material 1P is placed in a spinning furnace 110, and the clearance is degassed through the glass tube. Next, the multicore 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 fiber base material 1P to melt, and glass is drawn from the multicore fiber base material 1P. As soon as the drawn molten glass leaves the spinning furnace 110, it solidifies, with each core rod 10P becoming a core 10 and the cladding rod 20P becoming cladding 20. In this way, a bare wire of a multicore fiber composed of multiple cores 10 and cladding 20 is obtained. Subsequently, the bare wire of the multicore fiber passes through the cooling device 120 and is cooled to an appropriate temperature. The cooled bare wire of the multicore fiber then passes through the coating device 130, where the inner coating layer 31 and the outer coating layer 32 are formed, resulting in the multicore fiber 1 shown in Figure 1. The multicore fiber 1 is then reoriented by the turn pulley 141 and wound up by the reel 142. In this way, the multicore fiber 1 is manufactured.

[0038] In this embodiment, after the insertion step P4, the multicore fiber 1 was manufactured by drawing the base material 1P for the multicore fiber. That is, the multicore fiber 1 was manufactured by drawing the base material 1P for the multicore fiber, which had a clearance in the through hole 20PH with the core rod 10P inserted into it. However, the present invention is not limited thereto. For example, the multicore fiber base material 1P may be collapsed in a collapse step to crush the clearance in the through hole 20PH of the multicore fiber base material 1P, thereby manufacturing a multicore fiber intermediate, which is an intermediate of the multicore fiber 1, and then a drawing step P5 may be performed to draw the multicore fiber intermediate. If a collapse step is included, degassing in the drawing step P5 is unnecessary.

[0039] As described above, in the manufacturing method of the multicore fiber base material 1P of this embodiment, in the determination step P3, when inserting the core rod 10P into the through hole 20PH, the combination of core rod 10P and through hole 20PH is determined such that the sum of the clearances PA1 and PA2 between the core rod 10P and the inner wall of the through hole 20PH is minimized. In the insertion step P4, each core rod 10P is inserted into the respective through hole 20PH using the determined combination of core rod 10P and through hole 20PH.

[0040] According to the manufacturing method of the multicore fiber base material 1P of this embodiment, the sum of the clearance sizes in sets PA1 and PA2 can be made more uniform than in the conventional method. Therefore, even if the clad rod 20P shrinks in diameter in the radial direction connecting the through holes 20PH in each set PA1 and PA2 due to the collapse of the clearance, the degree of diameter reduction in the direction of each set PA1 and PA2 can be made more uniform than in the conventional method. Therefore, it is possible to suppress the extreme reduction in diameter of a part of the clad rod 20P compared to other parts. For this reason, by using the multicore fiber base material 1P manufactured by this manufacturing method, it is possible to manufacture a multicore fiber 1 in which the deviation of the outer shape of the clad 20 from the circular shape is suppressed.

[0041] Furthermore, this embodiment includes a measurement step P2 for measuring the outer diameter of each core rod 10P and the diameter of the through hole 20PH, and the determination step P3 is performed based on the measurement results of the measurement step P2. Therefore, even if the outer diameter of the prepared core rod 10P and the diameter of the through hole 20PH of the clad rod 20P are unknown or contain errors, these can be clarified by the measurement step P2. Consequently, the determination step P3 can be performed more accurately.

[0042] Although the present invention has been described above with reference to embodiments, the present invention is not limited to the above embodiments.

[0043] For example, in the above embodiment, the number of cores 10 was set to 4, but they were arranged on the circumference 20C. The number of cores 10 is not particularly limited, as long as it is an even number of 4 or more. Also, a core may be placed in the center of the cladding 20. For example, a multicore fiber may have multiple cores 10 in a so-called 1-6 arrangement, where one core 10 is placed in the center of the cladding 20 and six cores are placed around the said core 10. In this case, the through holes 20PH formed in the cladding rod 20P of the multicore fiber base material 1P for manufacturing the multicore fiber will also be in a 1-6 arrangement, and three sets of through holes 20PH aligned radially in the cladding rod 20P will be formed. In this case, in the determination step P3, i will be between 1 and 3 in the formula for calculating the sum of squared residuals PRS.

[0044] Furthermore, in this invention, it is preferable that the outer diameters of each core rod 10P are different from each other, and the hole diameters of each through hole 20PH into which the core rod 10P is inserted are different from each other. This allows for the effective use of glass rods with different outer diameters due to manufacturing variations, and can improve the manufacturing yield of optical fiber base materials.

[0045] As described above, the present invention provides a method for manufacturing a preform for multicore fibers that can produce multicore fibers in which deviation from the circular shape of the cladding is suppressed, and can be used in the field of optical communications and other devices that utilize multicore fibers.

Claims

1. A preparation step of preparing a clad rod which will serve as the clad for a multicore fiber, wherein four or more even-numbered through holes formed on a predetermined circumference centered on the central axis of the clad rod are arranged in multiple sets aligned in the radial direction of the clad rod, and a plurality of core rods which will serve as the core of the multicore fiber and can be individually inserted into the through holes, A determination step of determining the combination of the through hole and the core rod to be inserted into the through hole, An insertion step is to insert each of the core rods into each of the through holes in the determined combination of core rods and through holes, Equipped with, In the determination step, when inserting the core rod into the through hole, the combination of the core rod and the through hole is determined such that the sum of the squared residuals of the respective sums of the clearances between the core rod and the inner wall of the through hole is minimized. A method for manufacturing a matrix material for multicore fibers, characterized by the above.

2. The measurement step further comprises measuring the outer diameter of each of the core rods and the diameter of the through hole into which the core rods are inserted. The aforementioned determination step is performed based on the measurement results of the aforementioned measurement step. A method for producing a matrix material for multicore fibers according to claim 1.

3. Each of the core rods has a different outer diameter, and each of the through holes into which the core rods are inserted has a different diameter. A method for producing a matrix material for multicore fibers according to claim 1 or 2, characterized in that it is the method described in claim 1 or 2.

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