JP2022075176A - Method for manufacturing optical fiber - Google Patents

Abstract

To provide a method for manufacturing an optical fiber that enables a connection part to be reduced in size with achieving highly-accurate alignment of cores.SOLUTION: A method for manufacturing an optical fiber includes: a first step of inserting and fixing an optical fiber 2 being a multi-core fiber into a ferrule 1; a second step of cutting the optical fiber with dividing the ferrule so that alignment can be performed in a circumferential direction of the optical fiber; and a third step of polishing cut surfaces of the optical fibers.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for manufacturing an optical fiber.

In recent years, multi-core fibers having a plurality of cores have been developed. Further, in recent years, a multi-core fiber in which at least one core among a plurality of cores is formed in a spiral shape has been developed. This multi-core fiber is also called a spun multi-core fiber. Multi-core fibers are used, for example, in communication applications, medical applications, contact sensors, shape sensors, and the like. The following Patent Documents 1 and 2 and the following Non-Patent Document 1 disclose conventional span multi-core fibers.

Japanese Unexamined Patent Publication No. 2018-132421 US Pat. No. 8,773,650

P. S. Westbrook et al., “Integrated optical fiber shape senor modules based on twisted multicore fiber grating arrays”, Proc. SPIE 8938, Optical Fibers and Sensors for Medical Diagnostics and Treatment Applications XIV, 89380H (February 20 2014)

By the way, when creating a connection portion for connecting multi-core fibers to each other, high-precision alignment is required for all cores. Further, in recent years, multi-core fibers used for, for example, medical applications and various sensing applications may be used in narrow portions. The connection portion of such a multi-core fiber is required to be small.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing an optical fiber capable of miniaturizing a connection portion while realizing highly accurate alignment of cores.

In order to solve the above problems, the method for manufacturing an optical fiber according to one aspect of the present invention includes a first step of inserting and fixing a multi-core fiber (2, 3) to a ferrule (1, 11, 12). It has a second step of cutting the multi-core fiber while dividing the ferrule so that the alignment in the circumferential direction of the multi-core fiber is possible, and a third step of polishing the cut surface of the multi-core fiber.

In the method for manufacturing an optical fiber according to one aspect of the present invention, in a state where the multi-core fiber is fixed to the ferrule, the multi-core fiber is cut while dividing the ferrule so that the multi-core fiber can be aligned in the circumferential direction. The cut surface of the fiber is polished. Since the ferrule and the multi-core fiber are divided so that the alignment in the circumferential direction of the multi-core fiber is possible, high-precision alignment of the core can be realized. Further, since the connecting portion is formed by a ferrule and does not require a conventional housing, the connecting portion can be miniaturized.

Further, in the method for manufacturing an optical fiber according to one aspect of the present invention, an alignment portion (11a, 12a) is formed in the ferrule, and the alignment portion is divided into two in the second step. It may be a step of cutting the multi-core fiber while dividing the ferrule.

Further, in the method for manufacturing an optical fiber according to one aspect of the present invention, the ferrule is divided into a first ferrule (11) and a second ferrule (12) in advance, and the first step is the alignment unit. The step may be a step of inserting and fixing the multi-core fiber to the first ferrule and the second ferrule whose relationship is adjusted.

Further, in the method for manufacturing an optical fiber according to one aspect of the present invention, the multi-core fiber includes at least one core (32) formed in a spiral shape, and the third step is the axial direction of the multi-core fiber. In the step of polishing by the length nT (n is a natural number) with respect to the period T per turn of the core in the above.

Further, in the method for manufacturing an optical fiber according to one aspect of the present invention, the multi-core fiber includes at least one core (32) formed in a spiral shape, and the ferrules are the first ferrule (11) and the first ferrule. It may have two ferrules (12), and the first step may be a step of displacing the first ferrule and the second ferrule by a predetermined amount in the circumferential direction and fixing them to the multi-core fiber.

Further, in the method for manufacturing an optical fiber according to one aspect of the present invention, the multi-core fiber and the ferrule so that the third step is inclined with respect to a plane orthogonal to the axial direction of the multi-core fiber to form an end face. It may be a step of polishing.

Further, in the method for manufacturing an optical fiber according to one aspect of the present invention, the ferrule may have an exposed portion (14) that exposes the multi-core fiber at a position where the multi-core fiber is scheduled to be cut.

Alternatively, the method for manufacturing an optical fiber according to one aspect of the present invention may be a step in which the second step is a step of cutting the ferrule and the multi-core fiber in an oblique direction with respect to the axial direction of the multi-core fiber.

Here, in the method for manufacturing an optical fiber according to one aspect of the present invention, the third step is a step of diagonally polishing the ferrule and the multi-core fiber with reference to the cut surface of the ferrule and the multi-core fiber. Is also good.

Further, in the method for manufacturing an optical fiber according to one aspect of the present invention, the multi-core fiber includes at least one core (32) formed in a spiral shape, and the third step is the axial direction of the multi-core fiber. The step may be a step of polishing by a length nT (n is a natural number) with respect to the period T per turn of the core.

According to the present invention, there is an effect that the connection portion can be miniaturized while realizing highly accurate alignment of the core.

It is a figure which shows the manufacturing method of the optical fiber by 1st Embodiment of this invention. It is a perspective perspective view which shows the optical fiber used in the 2nd Embodiment of this invention. It is a figure which shows the manufacturing method of the optical fiber by 2nd Embodiment of this invention. It is a figure which shows the manufacturing method of the optical fiber by the 3rd Embodiment of this invention. It is a figure which shows the manufacturing method of the optical fiber by 4th Embodiment of this invention. It is a figure which shows the manufacturing method of the optical fiber by 5th Embodiment of this invention. It is a figure which shows the modification of the manufacturing method of the optical fiber by 5th Embodiment of this invention. It is a figure which shows the manufacturing method of the optical fiber by 6th Embodiment of this invention.

Hereinafter, a method for manufacturing an optical fiber according to an embodiment of the present invention will be described in detail with reference to the drawings. In the drawings referred to below, in order to facilitate understanding, the scale of the dimensions of each member may be appropriately changed and shown.

[First Embodiment]
FIG. 1 is a diagram showing a method for manufacturing an optical fiber according to the first embodiment of the present invention. In the present embodiment, as shown in FIG. 1A, the ferrule 1 is a pair of members including the first ferrule 11 and the second ferrule 12, and the optical fiber 2 is inserted into the ferrule 1. The first ferrule 11 is a substantially cylindrical member formed of zirconia or the like. The first ferrule 11 has a key portion 11a (alignment portion) formed on the peripheral surface of the end surface in contact with the second ferrule 12.

The second ferrule 12 is a substantially cylindrical member formed of zirconia or the like like the first ferrule 11. The outer diameter of the second ferrule 12 is the same as (or substantially the same) as the outer diameter of the first ferrule 11. The second ferrule 12 is formed with a key portion 12a (alignment portion) which is a concave portion formed on the peripheral surface. By aligning the positions of the key portion 11a and the key portion 12a, the first ferrule 11 and the second ferrule 12 can be aligned in the circumferential direction.

In the present embodiment, the optical fiber 2 is a single-mode multi-core fiber. The optical fiber 2 includes, for example, a central core formed parallel to the axis, a plurality of (for example, three) outer peripheral cores formed parallel to the axis, and a cladding common to the central core and the outer peripheral core. Things can be used. The number of cores provided in the optical fiber 2 is arbitrary.

When manufacturing such an optical fiber, first, the first ferrule 11 and the second ferrule 12 are fixed by a jig or the like with the key portion 11a and the key portion 12a aligned. As shown in FIG. 1 (b), the optical fiber 2 is inserted into and fixed to such a ferrule 1 (first step).

Next, as shown in FIG. 1 (c), the optical fiber 2 is cut at a portion between the first ferrule 11 and the second ferrule 12 in a state where the optical fiber 2 is inserted and fixed (second step). After that, the end face of the cut optical fiber 2 is polished together with the ends of the first ferrule 11 and the second ferrule 12, so that the connection portion of the optical fiber 2 is formed (third step). The connection portion of the optical fiber 2 is fixed by a jig or the like at the time of use.

As described above, in the present embodiment, after the key portion 11a of the first ferrule 11 and the key portion 12a of the second ferrule 12 and the key portion 12a are aligned, the optical fiber 2 is inserted into the first ferrule 11 and the second ferrule 12. After fixing the optical fiber 2, the optical fiber 2 is cut. As a result, after the optical fiber 2 is cut, the ferrules 1 can be aligned with each other by the key portions 11a and 12a. Therefore, it is not necessary to provide a resin cover (housing) on the outside of the first ferrule 11 and the second ferrule 12, and the structure is simple, and the ferrule 1 can be miniaturized.

In this embodiment, an example in which the ferrule 1 is divided into the first ferrule 11 and the second ferrule 12 in advance has been described. However, the ferrule 1 does not have to be divided into the first ferrule 11 and the second ferrule 12 in advance. When the ferrule 1 is not divided, in the step of cutting the optical fiber 2, the optical fiber 2 may be cut while dividing the ferrule 1 so that the key portion is divided into two.

[Second Embodiment]
FIG. 2 is a perspective perspective view showing an optical fiber used in the second embodiment of the present invention. Further, FIG. 3 is a diagram showing a method for manufacturing an optical fiber according to the second embodiment of the present invention. In FIGS. 2 and 3, the same components as those shown in FIG. 1 are designated by the same reference numerals.

In the present embodiment, as shown in FIG. 2, the optical fiber 3 is a single-mode multi-core fiber including a central core 31, an outer peripheral core 32 (outer peripheral cores 32a to 32c), and a clad 33. The outer peripheral surface of the clad 33 may be covered with a coating (not shown).

The central core 31 is a core formed in the center of the optical fiber 3 in parallel with the axis of the optical fiber 3. The central core 31 forms an optical path linear with respect to the longitudinal direction of the optical fiber 3 at the center of the optical fiber 3. The central core 31 is made of, for example, quartz glass.

The outer peripheral core 32 is a core formed so as to spirally surround the circumference of the central core 31. Specifically, the outer peripheral cores 32 are separated from the central core 31 by a predetermined distance, and the three outer peripheral cores 32a are arranged at intervals of predetermined angles (for example, 120 °) from each other in a cross section orthogonal to the longitudinal direction. It consists of ~ 32c. These outer peripheral cores 32a to 32c extend in the longitudinal direction of the optical fiber 3 so as to spirally surround the circumference of the central core 31 while maintaining an angular distance from each other. These outer peripheral cores 32a to 32c form three spiral optical paths surrounding the central core 31 in the optical fiber 3.

The outer peripheral cores 32a to 32c are formed of, for example, quartz glass, similarly to the central core 31. The outer peripheral cores 32a to 32c have the same diameter (or substantially the same diameter) as the central core 31.

The clad 33 is a common clad that covers the periphery of the central core 31 and the outer peripheral cores 32a to 32c and has a cylindrical outer diameter. Since the central core 31 and the outer peripheral cores 32a to 32c are covered with the common clad 33, it can be said that the central core 31 and the outer peripheral cores 32a to 32c are formed inside the clad 33. The clad 33 is formed of, for example, quartz glass.

When manufacturing such an optical fiber, first, as shown in FIG. 3A, the first ferrule 11 and the second ferrule 12 are cured with the key portion 11a and the key portion 12a aligned. Fix it with a tool. As shown in FIG. 3B, the optical fiber 3 is inserted into and fixed to such a ferrule 1 (first step).

Then, as shown in FIG. 3C, the optical fiber 3 is cut at a portion between the first ferrule 11 and the second ferrule 12 in a state where the optical fiber 3 is inserted and fixed (second step). Then, as shown in FIG. 3D, the end face of the cut optical fiber 3 and the ends of the first ferrule 11 and the second ferrule 12 are polished as shown by the broken line (third step).

Specifically, the outer peripheral cores 32a to 32c in the axial direction of the optical fiber 3 are polished by the period T per turn (for example, 20 [mm]). As a result, the positions of the outer peripheral cores 32a to 32c on the end face of the optical fiber 3 on the first ferrule 11 side and the end face of the optical fiber 3 on the second ferrule 12 side can be aligned.

In this embodiment, as in the first embodiment, an example in which the ferrule 1 is divided into the first ferrule 11 and the second ferrule 12 in advance has been described. However, the ferrule 1 does not have to be divided into the first ferrule 11 and the second ferrule 12 in advance. When the ferrule 1 is not divided, in the step of cutting the optical fiber 3, the optical fiber 3 may be cut while dividing the ferrule 1 so that the key portion is divided into two.

[Third Embodiment]
FIG. 4 is a diagram showing a method for manufacturing an optical fiber according to the third embodiment of the present invention. In FIG. 4, the same components as those shown in FIGS. 1 to 3 are designated by the same reference numerals.

In the present embodiment, as shown in FIG. 4A, the ferrule 1 is formed by molding or the like together with the first ferrule 11, the second ferrule 12, and the gap holding portion 13. The gap holding portion 13 is a cylindrical portion formed between the first ferrule 11 and the second ferrule 12, and has a smaller outer diameter than the first ferrule 11 and the second ferrule 12. Further, the optical fiber 3 is inserted into the gap holding portion 13. Further, the first ferrule 11 and the second ferrule 12 are in a state where the position of the key portion 11a is displaced by a predetermined angle (for example, 18 °) in the circumferential direction from the position of the key portion 12a by the gap holding portion 13. The length of such a gap holding portion 13 in the axial direction is set to a predetermined length (for example, 19 [mm]).

The optical fiber 3 in this embodiment is the same as that in the second embodiment. That is, it is a single-mode multi-core fiber including a central core 31, an outer peripheral core 32 (outer peripheral cores 32a to 32c), and a clad 33. The outer peripheral surface of the clad 33 may be covered with a coating (not shown).

When manufacturing such an optical fiber, first, as shown in FIG. 4B, the optical fiber 3 is inserted into and fixed to the ferrule 1 (first step). Then, with the optical fiber 3 inserted and fixed, the optical fiber 3 is cut at the position of the gap holding portion 13 (second step). Then, as shown in FIG. 4C, the end face of the cut optical fiber 3 is polished together with the gap holding portion 13 to the ends of the first ferrule 11 and the second ferrule 12 (third step). As a result, the positions of the outer peripheral cores 32a to 32c on the end face of the optical fiber 3 on the first ferrule 11 side and the end face of the optical fiber 3 on the second ferrule 12 side can be aligned.

In the present embodiment, in the state before cutting the optical fiber 3, the first ferrule 11 and the second ferrule 12 are provided so as to be displaced by a predetermined angle in the circumferential direction. As a result, the amount of polishing to align the outer peripheral cores 32a to 32c on the end face of the optical fiber 3 on the first ferrule 11 side and the end face of the optical fiber 3 on the second ferrule 12 side is larger than that in the second embodiment. Can be reduced.

[Fourth Embodiment]
FIG. 5 is a diagram showing a method for manufacturing an optical fiber according to a fourth embodiment of the present invention. In FIG. 5, the same configurations as those shown in FIGS. 1 to 4 are designated by the same reference numerals.

As shown in FIG. 5A, the ferrule 1 in the present embodiment is in a state in which the first ferrule 11 and the second ferrule 12 are integrally formed in the state before the optical fiber 3 is inserted.

The optical fiber 3 in this embodiment is the same as that in the second embodiment. That is, it is a single-mode multi-core fiber including a central core 31, an outer peripheral core 32 (outer peripheral cores 32a to 32c), and a clad 33. The outer peripheral surface of the clad 33 may be covered with a coating (not shown).

When manufacturing such an optical fiber, the optical fiber 3 is inserted into and fixed to the ferrule 1 as shown in FIG. 5 (a) (first step). Then, as shown in FIG. 5B, with the optical fiber 3 inserted and fixed, the optical fiber 3 is cut together with the ferrule 1 in the key portion 11a (12a) (second step).

After that, as shown in FIG. 5 (c), the end face of the cut optical fiber 3 and the end portions of the first ferrule 11 and the second ferrule 12 are attached to one of the outer peripheral cores 32a to 32c in the axial direction of the optical fiber 3. APC (Angled Physical Contact) polishing is performed by the amount of the period T per turn (for example, 20 [mm]) (third step). As a result, the positions of the outer peripheral cores 32a to 32c on the end face of the optical fiber 3 on the first ferrule 11 side and the end face of the optical fiber 3 on the second ferrule 12 side can be aligned. Since the first ferrule 11 and the second ferrule 12 can be aligned in the circumferential direction by APC polishing, the end face polished by APC functions as a key (alignment portion).

[Fifth Embodiment]
FIG. 6 is a diagram showing a method for manufacturing an optical fiber according to the fifth embodiment of the present invention. In FIG. 6, the same components as those shown in FIGS. 1 to 5 are designated by the same reference numerals.

In the present embodiment, as shown in FIG. 6A, the ferrule 1 is in a state in which the first ferrule 11 and the second ferrule 12 are integrally formed. The ferrule 1 is in a state in which the key portion 11a and the key portion 12a are formed. Further, in the ferrule 1, a fiber exposed portion 14 is formed between the first ferrule 11 and the second ferrule 12.

The fiber exposed portion 14 is a rod-shaped member that holds a gap between the first ferrule 11 and the second ferrule 12. Further, by providing the fiber exposed portion 14, when the optical fiber 2 is inserted into the ferrule 1, the optical fiber 2 is exposed at the position of the fiber exposed portion 14. Therefore, the optical fiber 2 can be directly cut, and the cutting becomes easy.

The optical fiber 2 is the same as the optical fiber 2 of the first embodiment. The optical fiber 3 is the same as the optical fiber 3 of the second to fourth embodiments. The optical fiber 2 is provided with a coating 21. Similarly, the optical fiber 3 is provided with a coating 34.

When manufacturing such an optical fiber, first, the coating 21 and the coating 34 at the ends of the optical fiber 2 and the optical fiber 3 are removed. Then, as shown in FIG. 6B, the end portion of the optical fiber 2 and the end portion of the optical fiber 3 are fused, inserted into the ferrule 1, and fixed (first step). Then, as shown in FIG. 6 (c), the optical fiber 2 is cut together with the fiber exposed portion 14 at the portion of the fiber exposed portion 14 of the ferrule 1 (second step). Then, the end face is polished so as to remove the fiber exposed portion 14 (third step).

Further, as shown in FIG. 7, it is also possible to fill a gap with the resin component 4 after attaching the fixing component between the coatings 21 and 34 and the ferrule 1. FIG. 7 is a diagram showing a modified example of the method for manufacturing an optical fiber according to the fifth embodiment of the present invention. In a sensor using an optical fiber 3, the optical fiber 3 may be covered with a metal bellows or the like. In such a case, by mounting a fixing component between the coatings 21 and 34 and the ferrule 1, and then embedding a gap with the resin component 4, it is possible to easily connect the mounting site of the ferrule 1 to the bellows. Become.

[Sixth Embodiment]
FIG. 8 is a diagram showing a method for manufacturing an optical fiber according to the sixth embodiment of the present invention. In FIG. 8, the same reference numerals are given to the configurations corresponding to the configurations shown in FIG.

As shown in FIG. 8A, the ferrule 1 in the present embodiment is in a state in which the first ferrule 11 and the second ferrule 12 are integrally formed in the state before the optical fiber 3 is inserted. However, the ferrule 1 in the present embodiment does not have a key portion (corresponding to the key portions 11a and 12a shown in FIG. 5).

The optical fiber 3 in this embodiment is the same as that in the second embodiment. That is, it is a single-mode multi-core fiber including a central core 31, an outer peripheral core 32 (outer peripheral cores 32a to 32c), and a clad 33. The outer peripheral surface of the clad 33 may be covered with a coating (not shown).

When manufacturing such an optical fiber, the optical fiber 3 is inserted into and fixed to the ferrule 1 as shown in FIG. 8 (b) (first step). Then, as shown in FIG. 8B, with the optical fiber 3 inserted and fixed, the ferrule 1 and the optical fiber 3 are cut in an oblique direction with respect to the axial direction of the optical fiber 3 (second step). The reason why the ferrule 1 and the optical fiber 3 are cut in this way is that the cut surface functions as a key (alignment portion).

Then, as shown in FIG. 8C, the ferrule 1 and the optical fiber 3 are diagonally polished with reference to the cut surface of the ferrule 1 and the optical fiber 3 (third step). For example, the end face of the cut optical fiber 3 and the ends of the first ferrule 11 and the second ferrule 12 are separated by the period T per turn of the outer peripheral cores 32a to 32c in the axial direction of the optical fiber 3 ( For example, 20 [mm]) APC (Angled Physical Contact) polishing is performed.

Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and can be freely changed within the scope of the present invention. For example, the above-mentioned optical fiber 3 includes a linear central core 31 and three spiral outer peripheral cores 32a to 32c, whereas the optical fiber 3 has at least one core among a plurality of cores. Is not limited as long as it is formed in a spiral shape. Further, the multi-core fiber may have the central core 31 omitted.

Further, an FBG (Fiber Bragg Grating) may be formed on the core of the above-mentioned optical fiber 2. Similarly, an FBG may be formed on the core (central core 31 and outer peripheral core 32) of the above-mentioned optical fiber 3. The FBG may be formed over the entire length of the optical fibers 2 and 3 in the longitudinal direction, or may be formed only in a part of the region in the longitudinal direction. Further, the FBG formed on the cores of the optical fibers 2 and 3 may have a fixed period or a continuously changing period (chirp grating).

Further, in the second embodiment described above, in the third step, the optical fiber 3 is provided by the period T per turn of the outer peripheral cores 32a to 32c in the axial direction of the optical fiber 3 (for example, 20 [mm]). An example of polishing has been described. However, the amount of polishing of the optical fiber 3 in the third step is not limited to the amount of one cycle T, and the same effect can be obtained by setting the amount to n cycles (n is a natural number).

Further, in the third embodiment described above, the first ferrule 11 and the second ferrule 12 have a gap holding portion 13 so that the position of the key portion 11a is at a predetermined angle (for example, 18 °) in the circumferential direction from the position of the key portion 12a. ) An example of a displaced state was explained. However, the displacement angle between the key portion 11a and the key portion 12a in the circumferential direction can be appropriately changed. The larger the displacement angle, the smaller the amount of polishing can be.

Further, in the fifth embodiment described above, the fusion point of the optical fibers 2 and 3 may be arranged in the second ferrule 12. Further, a ribbon fiber and a cable including a plurality of fibers can also be manufactured by attaching the ferrule 1 by the same method.

1 ... ferrule, 2, 3 ... optical fiber, 11 ... first ferrule, 11a ... key part, 12 ... second ferrule, 12a ... key part, 14 ... fiber exposed part, 32 ... outer peripheral core

Claims (10)

The first step of inserting and fixing the multi-core fiber to the ferrule,
A second step of cutting the multi-core fiber while dividing the ferrule so that the multi-core fiber can be aligned in the circumferential direction.
The third step of polishing the cut surface of the multi-core fiber and
A method for manufacturing an optical fiber having. An alignment portion is formed in the ferrule.
The second step is a step of cutting the multi-core fiber while dividing the ferrule so that the alignment portion is divided into two.
The method for manufacturing an optical fiber according to claim 1. The ferrule is divided into a first ferrule and a second ferrule in advance.
The method for manufacturing an optical fiber according to claim 2, wherein the first step is a step of inserting and fixing the multi-core fiber to the first ferrule and the second ferrule in which the relationship of the alignment portion is adjusted. The multi-core fiber includes at least one spirally formed core.
The third step is a step of polishing by a length nT (n is a natural number) with respect to a period T per turn of the core in the axial direction of the multi-core fiber.
The method for manufacturing an optical fiber according to any one of claims 1 to 3. The multi-core fiber includes at least one spirally formed core.
The ferrule has a first ferrule and a second ferrule.
The first step is a step of displacing the first ferrule and the second ferrule by a predetermined amount in the circumferential direction and fixing them to the multi-core fiber.
The method for manufacturing an optical fiber according to claim 1. The third step is a step of polishing the multi-core fiber and the ferrule so that an end face is formed so as to be inclined with respect to a surface orthogonal to the axial direction of the multi-core fiber, according to claims 1 to 5. The method for manufacturing an optical fiber according to any one of the above. The method for manufacturing an optical fiber according to any one of claims 1 to 6, wherein the ferrule has an exposed portion that exposes the multi-core fiber at a planned cutting position of the multi-core fiber. The method for manufacturing an optical fiber according to claim 1, wherein the second step is a step of cutting the ferrule and the multi-core fiber in an oblique direction with respect to the axial direction of the multi-core fiber. The method for manufacturing an optical fiber according to claim 8, wherein the third step is a step of diagonally polishing the ferrule and the multi-core fiber with reference to a cut surface of the ferrule and the multi-core fiber. The multi-core fiber includes at least one spirally formed core.
The third step is a step of polishing by a length nT (n is a natural number) with respect to a period T per turn of the core in the axial direction of the multi-core fiber.
The method for manufacturing an optical fiber according to claim 9.

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