JP2024077986A - Base material for optical fiber, method for manufacturing base material for optical fiber, and method for manufacturing optical fiber - Google Patents

Abstract

To provide a base material for an optical fiber capable of manufacturing an optical fiber with distortion from a circle on an outer peripheral surface of a clad restrained.SOLUTION: An adjusting through hole 20A having a center of gravity in a specific area AR and having a cross sectional area 0.5 or more and 2 or less times larger than a cross sectional area of a gap between an inner periphery of a clad rod 20P forming an inserting through hole 20H and an outer periphery of a core rod 10P inserted in the inserting through hole 20H, is formed on the clad rod 20P of a base material 1P of a multi-core fiber. The specific area AR is surrounded by a common external tangent ECT contacting a pair of inserting through holes 20H, a pair of center passage tangents CPT passing through a center 20PC of the clad rod 20P and contacting the pair inserting through holes 20H, and an outer peripheral line part POC sandwiched by the pair of center passage tangents CPT on an outer periphery of the clad rod 20P.SELECTED DRAWING: Figure 2

Description

The present invention relates to a base material for optical fiber, a method for manufacturing a base material for optical fiber, and a method for manufacturing an optical fiber.

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, multicore fibers, in which the outer circumference of multiple cores is surrounded by a single cladding, are being used. Multicore fibers can transmit multiple signals using light propagating through each of the multiple cores, which increases the transmission capacity per optical fiber.

The following Patent Document 1 describes a method for manufacturing a multicore fiber. In Patent Document 1, core rods are inserted into a plurality of through holes formed in a cladding rod that becomes the cladding of the multicore fiber, the cladding rod and the core rod are integrated, and the fiber is drawn. Patent Document 1 also describes a method for manufacturing a polarization-maintaining fiber. In this method for manufacturing a polarization-maintaining fiber, stress-applying rods that become stress-applying parts are inserted into a pair of through holes formed in a cladding rod that becomes the cladding of the polarization-maintaining fiber, the cladding rod and the stress-applying rod are integrated, and the fiber is drawn.

JP 2017-14078 A

When inserting glass rods into multiple through holes in a clad rod, as in the manufacturing method of optical fibers such as multicore fibers and polarization-maintaining fibers in Patent Document 1, a gap is generated between the inner wall of the through hole in the clad rod and the outer peripheral surface of the glass rod. This gap is crushed before the optical fiber preform becomes an optical fiber. Therefore, the portion of the outer peripheral surface of the clad rod facing the through hole into which the glass rod is inserted is more likely to move inward than other positions due to the crushing of the gap, and the cross-sectional shape of the outer peripheral surface of the clad is more likely to be distorted from a circular shape. For this reason, it is difficult to stably measure the diameter of the optical fiber. When manufacturing an optical fiber, generally, the optical fiber is drawn while measuring its diameter, and the drawing speed is adjusted based on the measured diameter. Therefore, if the cross-sectional shape of the clad is distorted from a circular shape, the outer diameter fluctuation during the manufacturing of the optical fiber will be large. For this reason, it is preferable that the cross-sectional shape of the outer peripheral surface of the clad is close to a circular shape.

The present invention aims to provide an optical fiber preform capable of producing an optical fiber in which distortion of the outer peripheral surface of the cladding from the circular shape is suppressed, a method for manufacturing an optical fiber preform, and a method for manufacturing an optical fiber.

Aspect 1 of the present invention for solving the above problem is an optical fiber preform comprising a clad rod having a plurality of insertion through holes and a plurality of glass rods inserted into the insertion through holes, the clad rod having a center of gravity located in a specific region and an adjustment through hole whose cross-sectional area is 0.5 to 2 times the cross-sectional area of the gap between the inner peripheral surface of the clad rod forming the insertion through hole and the outer peripheral surface of the glass rod inserted into the insertion through hole, the specific region being a region surrounded by a common circumstantial line that is in contact with a pair of adjacent insertion through holes on the outer peripheral side of the clad rod, a pair of center passing tangents that pass through the center of the clad rod and are in contact with the pair of insertion through holes on the opposing sides between the center and the common circumstantial line, and an outer peripheral line portion that is sandwiched between the pair of center passing tangents and faces the common circumstantial line on the outer peripheral surface of the clad rod.

According to such an optical fiber preform, in the process of manufacturing an optical fiber from the optical fiber preform, the gap between the inner wall of the insertion through hole in the clad rod and the outer peripheral surface of the glass rod is crushed. The adjustment through hole is also crushed. The adjustment through hole is a through hole whose center of gravity is located in a specific region and whose cross-sectional area is 0.5 to 2 times the cross-sectional area of the gap. Therefore, the part of the outer peripheral surface of the clad rod facing the insertion through hole moves inward, and at least a part of the outer peripheral surface of the clad rod in the specific region also moves inward. Therefore, according to the optical fiber preform of the present invention, an optical fiber can be manufactured in which the distortion from the circular shape of the outer peripheral surface of the clad is suppressed compared to when the clad rod does not have an adjustment through hole.

Aspect 2 of the present invention is the optical fiber preform of aspect 1, characterized in that the entire adjustment through hole is located in the specific region.

This optical fiber preform can be used to produce optical fibers in which distortion caused by deformation of the outer surface of the cladding is more effectively suppressed.

Aspect 3 of the present invention is an optical fiber preform according to aspect 1 or 2, characterized in that the center of gravity of the adjustment through hole is located inside a virtual circle that is inscribed in the midpoint of the outer periphery line portion, circumscribed in a virtual circle having the same area as the cross-sectional area of the adjustment through hole, and centered at the center of the clad rod.

With this optical fiber base material, even if the outer peripheral surface of the cladding is deformed due to the adjustment through-hole being crushed, the center of gravity of the adjustment through-hole is located toward the center of the cladding rod, so the deformation can be made gentle.

Aspect 4 of the present invention is an optical fiber preform according to any one of aspects 1 to 3, characterized in that the center of gravity is located on a straight line passing through the center of the clad rod and the midpoint of the outer periphery line portion.

With this optical fiber preform, even if the outer peripheral surface of the cladding is deformed due to the adjustment through-holes being crushed, the deformation can be made approximately symmetrical with respect to the pair of insertion through-holes.

Aspect 5 of the present invention is an optical fiber preform according to any one of aspects 1 to 4, characterized in that the adjustment through-hole is made up of a plurality of through-holes.

With this optical fiber base material, even if the outer peripheral surface of the cladding is deformed due to the adjustment through-hole being crushed, the deformation can be dispersed and the deformation can be made gentle.

Aspect 6 of the present invention is the optical fiber preform of aspect 5, characterized in that at least two of the through holes are located on a virtual circle centered on the center of the clad rod.

With this optical fiber base material, even if the outer peripheral surface of the cladding is deformed due to the crushing of the through holes arranged on the imaginary circle, the degree of deformation due to the crushing of the through holes can be made roughly constant, and the overall deformation can be made gentle.

Aspect 7 of the present invention is a method for manufacturing an optical fiber preform, comprising an insertion step of inserting a glass rod into a clad rod having a plurality of insertion through holes, the clad rod having a center of gravity located in a specific region and an adjustment through hole whose cross-sectional area is 0.5 to 2 times the cross-sectional area of the gap between the inner peripheral surface of the clad rod forming the insertion through hole and the outer peripheral surface of the glass rod inserted into the insertion through hole, the specific region being a region surrounded by a common circumstantial line that is in contact with a pair of adjacent insertion through holes on the outer peripheral side of the clad rod, a pair of center passing tangents that pass through the center of the clad rod and are in contact with the pair of insertion through holes on the opposing sides between the center and the common circumstantial line, and an outer peripheral line portion that is sandwiched between the pair of center passing tangents and faces the common circumstantial line on the outer peripheral surface of the clad rod.

According to this method for manufacturing an optical fiber preform, similar to embodiment 1, it is possible to manufacture an optical fiber preform capable of manufacturing an optical fiber in which distortion due to deformation of the outer peripheral surface of the cladding is suppressed compared to when the cladding rod does not have an adjustment through hole.

Aspect 8 of the present invention is a method for manufacturing an optical fiber, characterized by including a drawing step of drawing the optical fiber preform manufactured by the method for manufacturing an optical fiber preform of aspect 7.

Aspect 9 of the present invention is a method for manufacturing an optical fiber, characterized by comprising a collapse step of collapsing the optical fiber preform manufactured by the method for manufacturing an optical fiber preform of aspect 7 into an optical fiber intermediate, and a drawing step of drawing the optical fiber intermediate.

According to these optical fiber manufacturing methods, similar to aspect 1, it is possible to manufacture an optical fiber in which distortion due to deformation of the outer peripheral surface of the cladding is suppressed compared to when the cladding rod does not have an adjustment through hole.

As described above, the present invention provides an optical fiber preform capable of producing an optical fiber in which distortion from the circular shape of the outer peripheral surface of the cladding is suppressed, a method for manufacturing an optical fiber preform, and a method for manufacturing an optical fiber.

1 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber according to a first embodiment of the present invention. FIG. 2 is a diagram showing an optical fiber preform for manufacturing the optical fiber of FIG. 1; FIG. 3 is a flow chart for manufacturing the optical fiber preform of FIG. 2 and the optical fiber of FIG. 1. FIG. 13 is a diagram showing the state after the preparation process. FIG. FIG. 4 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber according to a second embodiment of the present invention. 7 is a diagram showing an optical fiber preform for manufacturing the optical fiber of FIG. 6.

Below, preferred embodiments of the optical fiber preform, the method for manufacturing the optical fiber preform, and the method for manufacturing the optical fiber according to the present invention will be described in detail with reference to the drawings. The embodiments exemplified below are intended to facilitate 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 without departing from the spirit of the invention. Note that for ease of understanding, the scale of each figure may differ from the scale described in the following description.

First Embodiment
Fig. 1 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber according to this embodiment. In this embodiment, a multi-core fiber will be described as an example of the optical fiber. The multi-core fiber 1 of this embodiment includes a plurality of cores 10, claddings 20 surrounding the outer circumferential surfaces of the cores 10 without gaps, an inner coating layer 31 coating the outer circumferential surface of the cladding 20, and an outer coating layer 32 coating the outer circumferential surface of the inner coating layer 31. In the example of Fig. 1, an example having four cores 10 is shown.

The outer diameter of the cladding 20 in a cross section perpendicular to the longitudinal direction is approximately circular. In this embodiment, the distance between each core 10 is equal to each other, and each core 10 is arranged at a position that is approximately four-fold rotationally symmetric about the center 20C of the cladding 20. Therefore, the distance from the center 20C of each core 10 is approximately the same. The diameter of the core 10 is, for example, 4 μm or more and 10 μm or less.

The refractive index of each core 10 is higher than that of the cladding 20, and the relative refractive index difference of each core 10 with respect to the cladding 20 is, for example, 0.2% to 2.0%. Such a core 10 is made of silica glass doped with a dopant such as germanium that increases the refractive index, and the cladding 20 is made of silica glass with no dopant added. Alternatively, the core 10 may be made of silica glass with no dopant added, and the cladding 20 may be made of silica glass doped with a dopant such as fluorine that decreases the refractive index.

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.

Next, we will explain the multicore fiber preform used to manufacture the multicore fiber 1 shown in Figure 1.

Figure 2 is a diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber preform for manufacturing the multicore fiber 1 of Figure 1. The multicore fiber preform 1P includes a clad rod 20P and a plurality of core rods 10P. Note that, in order to make the figure easier to see, hatching indicating the clad rods 20P and the core rods 10P has been omitted in Figure 2.

The clad rod 20P is a glass rod that forms the clad 20 of the multicore fiber 1 and has a generally circular outer periphery. The clad rod 20P has multiple insertion through holes 20H. The cross-sectional shape of each insertion through hole 20H is circular, and the cross-sectional areas of each insertion through hole 20H are generally equal to each other. In this embodiment, the distance between each insertion through hole 20H is generally equal to each other, and each insertion through hole 20H is formed at a position that is generally four-fold rotationally symmetric about the center 20PC of the clad rod 20P. Therefore, the distance from the center 20PC of each insertion through hole 20H is generally the same.

A core rod 10P is inserted into each insertion through hole 20H. Each core rod 10P is a glass rod with an approximately circular outer shape that becomes the core 10 of the multicore fiber 1. The diameter of the core rod 10P is smaller than that of the insertion through hole 20H. Therefore, when the core rod 10P is inserted into the insertion through hole 20H, a gap is formed between the inner peripheral surface of the clad rod 20P that forms the insertion through hole 20H and the outer peripheral surface of the core rod 10P. The cross-sectional areas of the core rods 10P are approximately equal to each other. Therefore, the cross-sectional areas of the gaps in each insertion through hole 20H are approximately equal to each other. The core rod 10P may be configured such that the outer peripheral surface of the glass body that becomes the core 10 is covered with a glass layer that becomes part of the clad 20.

Here, in the clad rod 20P, a specific area AR is defined that is surrounded by a common external tangent ECT, a pair of center passing tangents CPT, and a peripheral line portion POC. In FIG. 2, the specific area AR is indicated by diagonal lines, and only the common external tangent ECT, a pair of center passing tangents CPT, and a peripheral line portion POC for defining one specific area AR are shown. The common external tangent ECT is a tangent that is tangent to a pair of adjacent insertion through holes 20H on the outer periphery side of the clad rod 20P. The center passing tangent CPT is a tangent that passes through the center 20PC of the clad rod 20P and is tangent to the insertion through hole 20H. The pair of center passing tangents CPT that define the specific area AR are tangent to the insertion through hole 20H on the opposing sides of the pair of insertion through holes 20H. The point of contact between the center passing tangent CPT and the insertion through hole 20H is between the center 20PC and the common circumscribing tangent ECT. The outer peripheral line portion POC is a portion of the outer peripheral surface of the clad rod 20P that is sandwiched between a pair of center passing tangents CPT and faces the common circumscribing ECT. In FIG. 2, a dashed line indicating the outer peripheral line portion POC is drawn slightly inside the outer peripheral surface of the clad rod 20P so that the outer peripheral line portion POC can be seen.

The clad rod 20P has an adjustment through hole 20A in the specific area AR defined as above. In this embodiment, the adjustment through hole 20A is composed of two through holes 20A1 having the same cross-sectional area. The two through holes 20A1 are located entirely within the specific area AR. That is, in this embodiment, the adjustment through hole 20A is located entirely within the specific area AR. Therefore, the center of gravity of the adjustment through hole 20A is also located within the specific area AR. In addition, the two through holes 20A1 in this embodiment are formed on a virtual circle 20CR centered on the center 20PC of the clad rod 20P. In addition, the two through holes 20A1 in this embodiment are formed at symmetrical positions with respect to the straight line BST passing through the center 20PC of the clad rod 20P and the midpoint of the outer periphery line portion POC. Therefore, the center of gravity of the adjustment through hole is located on the straight line BST. Here, a virtual circle 20CS is assumed to be inscribed in the midpoint of the outer periphery line portion POC and to have the same area as the cross-sectional area of the adjustment through hole 20A. In FIG. 2, 20CS is shown by a dashed line. Furthermore, a virtual circle 20CL is assumed to be circumscribed in the virtual circle 20CS and centered on the center 20PC of the clad rod 20P. The two through holes 20A1 of this embodiment are located inside this virtual circle 20CL. That is, the entire adjustment through hole 20A, including its center of gravity, is located inside the virtual circle 20CL. However, this is not limited to a configuration in which the entire adjustment through hole 20A, including its center of gravity, is located inside the virtual circle 20CL, and the center of gravity of the adjustment through hole 20A may be located inside the virtual circle 20CL, and a part of the adjustment through hole 20A may be located outside the virtual circle 20CL.

Unlike the insertion through hole 20H, no glass rod is inserted into the adjustment through hole 20A.

The cross-sectional area of the adjustment through hole 20A, which is the sum of the cross-sectional areas of the two through holes 20A1, is 0.5 to 2 times the cross-sectional area of the gap between the inner surface of the clad rod 20P that forms the insertion through hole 20H and the outer surface of the core rod 10P, which is a glass rod inserted into the insertion through hole 20H.

Next, we will explain the manufacturing method for manufacturing the multicore fiber preform 1P and multicore fiber 1 shown in Figure 1.

Figure 3 is a diagram showing a flowchart for manufacturing the multicore fiber preform 1P of Figure 2 and the multicore fiber 1 of Figure 1. As shown in Figure 3, the manufacturing method of the multicore fiber preform 1P of this embodiment includes a preparation process P1 and an insertion process P2, and the manufacturing method of the multicore fiber 1 further includes a drawing process P3.

(Preparation process P1)
This step is a step of preparing a cladding rod 20P having a plurality of insertion through holes 20H and adjustment through holes 20A and serving as the cladding 20 of the multi-core fiber 1, and a plurality of core rods 10P that can be individually inserted into the insertion through holes 20H and serve as the cores 10 of the multi-core fiber 1. Fig. 4 is a diagram showing a state after the preparation step. As shown in Fig. 4, the number of core rods 10P is the same as the number of insertion through holes 20H. In this embodiment, the length of the cladding rod 20P and the length of the core rod 10P are equal to each other. In this step, the core rod 10P and the cladding rod 20P may be prepared by manufacturing or by purchasing.

(Insertion process P2)
This step is a step of inserting the core rod 10P into the insertion through hole 20H. Through this step, a multicore fiber preform 1P shown in Fig. 2 is obtained. In this state, a gap is generated between the inner peripheral surface of the cladding rod 20P that forms the insertion through hole 20H and the outer peripheral surface of the core rod 10P inserted into the insertion through hole 20H.

(Wire drawing process P3)
This step is a step of manufacturing a multi-core fiber 1 by drawing a multi-core fiber preform 1P. FIG. 5 is a diagram showing this step. In this step, first, dummy glass is fused to one end of the multi-core fiber preform 1P, and a glass tube is fused to the other end. Then, the multi-core fiber preform 1P is placed in a spinning furnace 110, and the inside of each through hole of the clad rod 20P is degassed via the glass tube. Next, the multi-core fiber preform 1P is heated by the heating unit 111 of the spinning furnace 110. By this heating, the lower end of the multi-core fiber preform 1P becomes a molten state, and glass is drawn from the multi-core fiber preform 1P. At this time, the gap of the insertion through hole 20H is crushed, and the adjustment through hole 20A is crushed. When the drawn molten glass comes out of the spinning furnace 110, it immediately solidifies, and each core rod 10P becomes each core 10, and the clad rod 20P becomes a clad 20. In this manner, a bare wire of a multi-core fiber composed of a plurality of cores 10 and a cladding 20 is obtained. Then, this bare wire of the multi-core fiber passes through a cooling device 120 and is cooled to an appropriate temperature. The cooled bare wire of the multi-core fiber passes through a coating device 130, where an inner coating layer 31 and an outer coating layer 32 are formed, to obtain the multi-core fiber 1 shown in Fig. 1. Then, the direction of the multi-core fiber 1 is changed by a turn pulley 141, and the multi-core fiber 1 is taken up by a reel 142. In this manner, the multi-core fiber 1 is manufactured.

As described above, in the multicore fiber preform 1P, which is the optical fiber preform of this embodiment, the clad rod 20P has a center of gravity located in the specific region AR and an adjustment through hole 20A whose cross-sectional area is 0.5 to 2 times the cross-sectional area of the gap between the inner surface of the clad rod 20P that forms the insertion through hole 20H and the outer surface of the core rod 10P, which is a glass rod inserted into the insertion through hole 20H.

According to this multi-core fiber preform 1P, in the process of manufacturing the multi-core fiber 1 from the multi-core fiber preform 1P, the gap between the inner wall of the insertion through hole 20H in the clad rod 20P and the outer peripheral surface of the core rod 10P is crushed. Therefore, if the adjustment through hole 20A is not provided, the cross-sectional shape of the outer peripheral surface of the clad 20 of the manufactured multi-core fiber 1 tends to be close to a rectangle and has a large distortion from a circular shape. However, in the multi-core fiber preform 1P of this embodiment, since the clad rod 20P has the adjustment through hole 20A, the adjustment through hole 20A is also crushed. This adjustment through hole 20A is a through hole whose center of gravity is located in a specific region and whose cross-sectional area is 0.5 to 2 times the gap. Therefore, the outer peripheral surface of the clad rod 20P facing the insertion through hole 20H moves inward, and at least a part of the outer peripheral surface of the clad rod 20P in the specific region AR moves inward. Therefore, according to the multicore fiber preform 1P, the manufacturing method for the multicore fiber preform 1P, and the manufacturing method for the multicore fiber 1 of this embodiment, it is possible to manufacture a multicore fiber 1 in which distortion from the circular shape of the outer peripheral surface of the cladding 20 is suppressed compared to when the cladding rod 20P does not have the adjustment through hole 20A.

In addition, in the multicore fiber preform 1P of this embodiment, the entire adjustment through hole 20A is located in the specific area AR. Therefore, compared to a case where a portion of the adjustment through hole 20A is located outside the specific area AR, an optical fiber can be manufactured in which distortion due to deformation of the outer peripheral surface of the cladding is more suppressed.

In addition, in the multicore fiber preform 1P of this embodiment, the entire adjustment through hole 20A is located inside a virtual circle 20CL that is inscribed in the midpoint of the outer periphery line portion POC, circumscribed in a virtual circle 20CS having the same area as the cross-sectional area of the adjustment through hole 20A, and centered on the center 20PC of the clad rod 20P. Therefore, compared to when the adjustment through hole 20A is formed outside the virtual circle 20CL, the center of gravity of the adjustment through hole 20A is located on the center 20PC side of the clad rod 20P, so even if the outer peripheral surface of the clad 20 is deformed due to the adjustment through hole 20A being crushed, the deformation can be made gentle.

In addition, in the multicore fiber preform 1P of this embodiment, the center of gravity of the adjustment through hole 20A is located on a straight line BST that passes through the center 20PC of the clad rod 20P and the midpoint of the outer periphery line portion POC. Therefore, even if the outer periphery of the clad is deformed by crushing the adjustment through hole 20A, the deformation can be made approximately symmetrical with respect to the pair of insertion through holes 20H.

In addition, in the multicore fiber preform 1P of this embodiment, the adjustment through hole 20A is composed of multiple through holes 20A1. Therefore, even if the outer peripheral surface of the cladding 20 is deformed due to the adjustment through hole 20A being crushed, the deformation can be dispersed and the deformation can be made gentle. Note that when the adjustment through hole 20A is composed of multiple through holes 20A1, the cross-sectional area of the adjustment through hole 20A is the sum of the cross-sectional areas of the respective through holes.

In addition, in the multicore fiber preform 1P of this embodiment, the two through holes 20A1 are located on a virtual circle 20CR centered on the center 20PC of the clad rod 20P. Therefore, even if the outer peripheral surface of the clad 20 is deformed due to the crushing of the through holes 20A1 located on the virtual circle 20CR, the degree of deformation due to the crushing of the through holes 20A1 can be made roughly constant, and the overall deformation can be made gentle.

Second Embodiment
Next, a second embodiment of the present invention will be described in detail with reference to Figures 6 and 7. Note that components that are the same as or equivalent to those in the first embodiment are given the same reference numerals and will not be described again unless otherwise specified.

Figure 6 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber according to this embodiment. In this embodiment, a polarization-maintaining fiber is used as an example of the optical fiber. As shown in Figure 6, the polarization-maintaining fiber 2 has one core 10, which is located at 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.

The stress applying parts 12 are made of, for example, glass having a thermal expansion coefficient different from that of the cladding 20, and stress is applied to the core 10 from a pair of stress applying parts 12 arranged to sandwich the core 10. The core 10 to which tensile stress or compressive stress is applied from the pair of stress applying parts 12 has birefringence induced by the photoelastic effect, and has different propagation constants for the polarization modes in these two mutually perpendicular directions.

An example of a material that can be used to form the stress applying portion 12 is quartz glass doped with a dopant such as boron.

Figure 7 is a diagram showing a cross section perpendicular to the longitudinal direction of a polarization-maintaining fiber preform 2P, which is an optical fiber preform for manufacturing the polarization-maintaining fiber 2 in Figure 6. The polarization-maintaining fiber preform 2P includes a clad rod 20P, a core rod 10P, and a pair of stress-applying rods 12P. In this example, the core rod 10P is surrounded by the clad rod 20P without any gaps. Note that for the same reason as in Figure 2, the hatching indicating the clad rod 20P, the core rod 10P, and the stress-applying rods 12P is omitted in Figure 7.

The clad rod 20P has a pair of insertion through holes 20H. In this embodiment, the cross-sectional shape of the pair of insertion through holes 20H is also circular, and the cross-sectional areas of the respective insertion through holes 20H are approximately equal to each other. In this embodiment, the pair of insertion through holes 20H are formed at positions sandwiching the core. Specifically, the insertion through holes 20H are formed at positions symmetrical with respect to the center 20PC of the clad rod 20P. Therefore, the distance from the center 20PC of each insertion through hole 20H is approximately the same.

A stress-applying rod 12P is inserted into each insertion through hole 20H. Each stress-applying rod 12P is a glass rod with an outer circumferential shape that becomes the stress-applying portion 12 of the polarization-maintaining fiber 2 and has a substantially circular outer circumferential shape. The diameter of the stress-applying rod 12P is smaller than that of the insertion through hole 20H. Therefore, when the stress-applying rod 12P is inserted into the insertion through hole 20H, a gap is formed between the inner circumferential surface of the clad rod 20P that forms the insertion through hole 20H and the outer circumferential surface of the stress-applying rod 12P. The cross-sectional areas of the stress-applying rods 12P are substantially equal to each other. Therefore, the cross-sectional areas of the gaps of each insertion through hole 20H are substantially equal to each other. The stress-applying rod 12P may be configured such that the outer circumferential surface of the glass body that becomes the stress-applying portion 12 is covered with a glass layer that becomes part of the clad 20.

In this embodiment, a specific area AR is defined that is surrounded by a common external tangent ECT, a pair of center passing tangents CPT, and a peripheral line portion POC, as in the first embodiment. In FIG. 7, only the common external tangent ECT, a pair of center passing tangents CPT, and a peripheral line portion POC for defining one specific area AR are shown. In this embodiment, the common external tangent ECT contacts a pair of insertion through holes 20H on the outer periphery side of the clad rod 20P. In FIG. 7, for the same reason as in FIG. 2, the dashed line indicating the peripheral line portion POC is drawn slightly inside the outer periphery of the clad rod 20P.

The clad rod 20P has an adjustment through hole 20A in the specific region AR. In this embodiment, as in the first embodiment, the adjustment through hole 20A is composed of two through holes 20A1. The position of the through holes 20A1 is explained as in the first embodiment. No glass rod is inserted into the adjustment through hole 20A.

Also, in this embodiment, the cross-sectional area of the adjustment through hole 20A, which is the sum of the cross-sectional areas of the two through holes 20A1, is 0.5 to 2 times the cross-sectional area of the gap between the inner peripheral surface of the clad rod 20P that forms the insertion through hole 20H and the outer peripheral surface of the stress-applying rod 12P, which is a glass rod inserted into the insertion through hole 20H.

The manufacturing method of the polarization-maintaining fiber preform 2P and the manufacturing method of the polarization-maintaining fiber 2 of this embodiment are generally similar to the manufacturing method of the multicore fiber preform 1P and the manufacturing method of the multicore fiber 1 of the first embodiment. Therefore, in the explanation of the manufacturing method of the multicore fiber preform 1P and the manufacturing method of the multicore fiber 1 of the first embodiment, the multicore fiber 1 is replaced with the polarization-maintaining fiber 2, and the multicore fiber preform 1P is replaced with the polarization-maintaining fiber preform 2P.

(Preparation process P1)
This embodiment differs from the preparation step P1 of the first embodiment in that it prepares a cladding rod 20P having a pair of insertion through holes 20H and an adjustment through hole 20A and serving as the cladding 20 of the polarization-maintaining fiber 2, and a pair of stress-applying rods 12P which can be individually inserted into the insertion through holes 20H and serve as the stress-applying portions 12 of the polarization-maintaining fiber 2. In this step, the stress-applying rods 12P and the cladding rods 20P may be prepared by manufacturing or by purchasing.

(Insertion process P2)
This step differs from the insertion step P2 of the first embodiment in that the stress-applying rod 12P is inserted into the insertion through-hole 20H.

(Wire drawing process P3)
This step differs from the drawing step P3 in the first embodiment in that a polarization-maintaining fiber preform 2P is drawn to manufacture a polarization-maintaining fiber 2.

The polarization-maintaining fiber preform 2P, which is the optical fiber preform of this embodiment, is similar to the multicore fiber preform 1P of the first embodiment in that the clad rod 20P has a center of gravity located in the specific region AR and an adjustment through hole 20A whose cross-sectional area is 0.5 to 2 times the cross-sectional area of the gap between the inner peripheral surface of the clad rod 20P that forms the insertion through hole 20H and the outer peripheral surface of the stress-applying rod 12P, which is a glass rod inserted into the insertion through hole 20H. Therefore, according to the polarization-maintaining fiber preform 2P of this embodiment, the manufacturing method of the polarization-maintaining fiber preform 2P, and the manufacturing method of the polarization-maintaining fiber 2, it is possible to manufacture a polarization-maintaining fiber 2 in which distortion from the circular shape of the outer peripheral surface of the clad 20 is suppressed compared to when the adjustment through hole 20A is not formed.

The present invention has been described above using examples of embodiments, but the present invention is not limited to the above embodiments.

For example, in the first embodiment, the number of cores 10 is four, but the number is not particularly limited as long as there are multiple cores 10 arranged other than at the center 20C of the cladding 20. Also, a core 10 may be arranged at the center of the cladding 20.

In the above embodiment, the multicore fiber 1 and the polarization-maintaining fiber 2 are described as examples of optical fibers. However, the glass rods inserted into the multiple insertion through holes 20H formed at locations other than the center 20PC of the cladding rod 20P are not limited to the core rod 10P or the stress-applying rod 12P, as long as they are glass rods that serve as predetermined locations within the cladding 20 of the optical fiber.

In the above embodiment, an example was described in which the entire adjustment through hole 20A is located in the specific area AR. However, in the present invention, it is sufficient that the center of gravity of the adjustment through hole 20A is located in the specific area AR, and a portion of the adjustment through hole 20A may be located outside the specific area AR.

In the above embodiment, an example was described in which the entire adjustment through hole 20A is located inside the imaginary circle 20CL in the specific region AR. Also, it was described that the center of gravity of the adjustment through hole 20A may be located inside the clad rod 20P of the imaginary circle 20CL in the specific region AR, and a part of the adjustment through hole 20A may be located outside the clad rod 20P of the imaginary circle 20CL. However, in the present invention, unlike these descriptions, the center of gravity of the adjustment through hole 20A may be located outside the clad rod 20P of the imaginary circle 20CL, and the entire adjustment through hole 20A may be located outside the clad rod 20P of the imaginary circle 20CL.

In the above embodiment, the center of gravity of the adjustment through hole 20A is located on the straight line BST passing through the center 20PC of the clad rod 20P and the midpoint of the outer periphery line portion POC. However, the center of gravity of the adjustment through hole 20A does not have to be located on the straight line BST.

In addition, in the above embodiment, an example was described in which the adjustment through hole 20A is composed of two through holes 20A1. However, the adjustment through hole 20A may be composed of three or more through holes 20A1, or the adjustment through hole 20A may be composed of one through hole. When the adjustment through hole 20A is composed of one through hole, for example, the two through holes 20A1 described in the above embodiment may be connected.

In the above embodiment, the two through holes 20A1 are located on a virtual circle 20CR centered on the center 20PC of the clad rod 20P. However, when the clad rod 20P has multiple through holes 20A1, the multiple through holes 20A1 do not have to be located on a common virtual circle 20CR. In other words, when the clad rod 20P has multiple through holes 20A1, the distance from the center 20PC of the clad rod 20P to the through holes 20A1 may be different for at least two through holes 20A1. In addition, when the adjustment through hole 20A is composed of three or more through holes 20A1, it is preferable that two or more through holes 20A1 are located on a common virtual circle 20CR.

In the above embodiment, after the insertion step P2, the multicore fiber preform 1P or the polarization-maintaining fiber preform 2P was drawn to manufacture the multicore fiber 1 or the polarization-maintaining fiber 2. That is, the optical fiber preform was drawn in a state in which a glass rod was inserted into the insertion through hole 20H and there was a gap in the insertion through hole 20H to manufacture the optical fiber. However, the present invention is not limited to this. For example, in the first embodiment, the multicore fiber preform 1P may be collapsed to perform a collapse step of collapsing the gap in the insertion through hole 20H of the multicore fiber preform 1P and the adjustment through hole 20A, to manufacture an optical fiber intermediate that is an intermediate of the multicore fiber 1, and the drawing step P3 of drawing the optical fiber intermediate may be performed. Similarly, in the second embodiment, a collapse process may be performed in which the polarization-maintaining fiber preform 2P is collapsed to eliminate the gaps in the insertion through-holes 20H and the adjustment through-holes 20A of the polarization-maintaining fiber preform 2P, an optical fiber intermediate that is an intermediate of the polarization-maintaining fiber 2 is manufactured, and a drawing process P3 may be performed in which the optical fiber intermediate is drawn. If a collapse process is included, degassing in the drawing process P3 is not necessary.

In the above embodiment, the cross-sectional areas of the insertion through holes 20H are equal to each other, and the cross-sectional areas of the core rods 10P and stress-applying rods 12P inserted into the insertion through holes 20H are equal to each other. Therefore, the cross-sectional areas of the gaps in the insertion through holes 20H are also equal to each other. However, the cross-sectional areas of adjacent insertion through holes 20H may be different from each other, or the cross-sectional areas of the core rods 10P and stress-applying rods 12P inserted into these insertion through holes 20H may be different from each other, and the cross-sectional areas of the gaps in adjacent insertion through holes 20H may be different from each other.

As described above, the present invention provides an optical fiber preform capable of producing an optical fiber in which distortion from the circular shape of the outer peripheral surface of the cladding is suppressed, a method for manufacturing an optical fiber preform, and a method for manufacturing an optical fiber, which can be used in the field of optical communications and other devices that use optical fibers.

1. Multicore fiber (optical fiber)
2... Polarization-maintaining fiber (optical fiber)
10: Core 12: Stress applying portion 20: Cladding 1P: Preform for multi-core fiber (preform for optical fiber)
2P: Polarization-maintaining fiber base material (optical fiber base material)
10P...Core rod (glass rod)
12P: Stress applying rod (glass rod)
20P: Clad rod 20H: Insertion through hole 20A: Adjustment through hole P1: Preparation step P2: Insertion step P3: Wire drawing step

Claims (9)

a clad rod having a plurality of insertion through holes;
A plurality of glass rods to be inserted into the insertion through holes;
Equipped with
the clad rod has a center of gravity located in a specific region and an adjustment through hole whose cross-sectional area is 0.5 to 2 times the cross-sectional area of a gap between an inner peripheral surface of the clad rod forming the insertion through hole and an outer peripheral surface of the glass rod to be inserted into the insertion through hole,
The specific region is an optical fiber preform characterized in that it is an area surrounded by a common circumferential tangent in the clad rod that touches a pair of adjacent insertion through holes on the outer periphery of the clad rod, a pair of center passing tangents that pass through the center of the clad rod and touch the pair of insertion through holes on opposing sides between the center and the common circumferential tangent, and an outer periphery line portion on the outer periphery of the clad rod that is sandwiched between the pair of center passing tangents and faces the common circumferential tangent. 2. The optical fiber preform according to claim 1, wherein the entire adjustment through hole is located in the specific region. 2. The optical fiber preform according to claim 1, characterized in that the center of gravity of the adjustment through hole is located inside a virtual circle that is inscribed in the midpoint of the outer circumferential line portion, circumscribed in a virtual circle having the same area as the cross-sectional area of the adjustment through hole, and centered on the center of the clad rod. 4. The optical fiber preform according to claim 1, wherein the center of gravity is located on a straight line passing through the center of the clad rod and a midpoint of the outer circumferential line portion. 4. The optical fiber preform according to claim 1, wherein the adjustment through-hole comprises a plurality of through-holes. 6. The optical fiber preform according to claim 5, wherein the at least two through holes are located on an imaginary circle centered on the center of the clad rod. An insertion step of inserting a glass rod into a plurality of insertion through holes of a clad rod,
the clad rod has a center of gravity located in a specific region and an adjustment through hole whose cross-sectional area is 0.5 to 2 times the cross-sectional area of a gap between an inner peripheral surface of the clad rod forming the insertion through hole and an outer peripheral surface of the glass rod to be inserted into the insertion through hole,
A method for manufacturing an optical fiber preform, characterized in that the specific region is a region surrounded by a common circumferential tangent in the clad rod that touches a pair of adjacent insertion through holes on the outer periphery of the clad rod, a pair of center passing tangents that pass through the center of the clad rod and touch the pair of insertion through holes on opposing sides between the center and the common circumferential tangent, and an outer periphery line portion on the outer periphery of the clad rod that is sandwiched between the pair of center passing tangents and faces the common circumferential tangent. 8. A method for producing an optical fiber, comprising the step of drawing the optical fiber preform produced by the method for producing an optical fiber preform according to claim 7. a collapsing step of collapsing the optical fiber preform manufactured by the method for manufacturing an optical fiber preform according to claim 7 to obtain an optical fiber intermediate;
A method for producing an optical fiber, comprising the step of drawing the optical fiber intermediate.

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