JP2015197638A - Optical device, fiber laser, and manufacturing method of optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical device, having a fusion splicing part, which can prevent a decrease in beam quality.SOLUTION: In an optical device 1, fusion-spliced optical fibers F1 and F2 are fixed to a reinforcement member 13. The optical fiber F2 is a multimode optical fiber. A tension producing a higher magnitude strain than 1 microstrain is imparted to the optical fibers F1 and F2.

Description

  The present invention relates to an optical device having optical fibers connected to each other and a method for manufacturing the optical device. The present invention also relates to a fiber laser provided with such an optical device.

  As fiber lasers increase in output in recent years, the occurrence of nonlinear phenomena has become a problem. As a countermeasure for this problem, it is common practice to suppress the nonlinear phenomenon by enlarging the core of the optical fiber to lower the intensity of the laser beam. As the core is expanded, the beam quality of the fiber laser deteriorates from single mode to multimode. In response to this, the laser beam can propagate in a single mode in a multimode optical fiber by devising how to bend the optical fiber, how to excite it, and the addition region in the core of the amplification medium. Thus, high beam quality can be maintained.

  In a fiber laser using such a multimode optical fiber, it is a problem to maintain high beam quality by suppressing high-order mode excitation that may occur at the fusion splicing portion between the optical fibers.

  For such a problem, a method of reinforcing the fusion splicing part is important. Conventionally, even when connecting multi-mode optical fibers excited by a single mode, the optical fiber is kept in a straight line in the axial direction of the optical fiber in the same manner as a method for reinforcing a fusion splicing portion between normal single-mode optical fibers. A method of reinforcing the fusion spliced portion by applying a small tension is adopted. In addition, the structure which reinforces a fusion splicing part so that the fusion splicing part of optical fibers may be kept linear is described in patent document 1, for example.

JP 2013-174583 A (published September 5, 2013)

  However, in a configuration in which multimode optical fibers are connected to each other, if the core is bent due to inclination or chipping of the end face of the fusion splicing part of the optical fiber or pushing of the optical fiber at the time of connection, Even if the light propagating through the optical fiber on the input side with respect to the connection portion is only the fundamental mode, the higher-order mode is excited in the optical fiber on the output side with respect to the fusion splice portion. Such a slight bending of the core cannot be corrected with the above-described conventional configuration in which the fusion spliced portion is reinforced by applying a tension low enough to keep the optical fiber in a straight line. As a result, in the above-described conventional configuration, high-order mode excitation in the fusion spliced portion cannot be avoided, and the beam quality, which is an important characteristic of the fiber laser, is deteriorated.

  In particular, a fiber laser has a plurality of fusion splicing portions between optical fibers. Therefore, the occurrence of higher-order modes as described above will lead to a significant decrease in beam quality when viewed from the whole fiber laser, even if the influence at each fusion splice is minimal, and should be ignored. I can't.

  In addition, the above-mentioned problems are caused by at least propagating light, such as when connecting multimode optical fibers, or when connecting a single mode optical fiber on the input side of propagating light and a multimode optical fiber on the output side of propagating light. This occurs when the output side optical fiber is a multimode optical fiber.

  Therefore, the present invention provides an optical device capable of preventing the deterioration of beam quality by suppressing the excitation of higher-order modes at the fusion splicing portion between optical fibers whose output side of propagating light is a multimode optical fiber. It aims at providing the manufacturing method of a fiber laser and an optical device.

  In order to solve the above problems, an optical device of the present invention includes a first optical fiber on the input side of propagating light and a second optical fiber on the output side of propagating light, which are fusion-connected at a fusion splicing portion. In the optical device in which these optical fibers are fixed to the reinforcing member, at least the second optical fiber is a multimode optical fiber, and the first and second optical fibers are the first and second optical fibers. It is characterized by being fixed to the reinforcing member in a state in which a tension that generates a strain larger than 1 microstrain is applied in the axial direction.

  According to said structure, since the 1st and 2nd optical fiber is being fixed to the reinforcement member in the state to which the tension | tensile_strength which produces a distortion larger than 1 microstrain was given to the axial direction of these optical fibers, It is possible to correct a slight bend generated in the fusion splicing portion of the core due to the inclination or chipping of the end surface of the fusion splicing portion, or by pressing the first and second optical fibers at the time of connection.

  As a result, when the first and second optical fibers are subjected to single mode excitation, it is possible to suppress high-order mode excitation caused by minute bending of the fusion splicing portion and to prevent deterioration of the beam quality.

  The method for manufacturing an optical device of the present invention includes a first optical fiber on the input side of propagating light and a second optical fiber on the output side of propagating light, which are fusion-connected at the fusion splicing portion. In the method of manufacturing an optical device in which an optical fiber is fixed to a reinforcing member, at least the second optical fiber is a multimode optical fiber, and the first and second optical fibers are used with respect to the first and second optical fibers. A step of applying a tension that generates strain larger than 1 microstrain in the axial direction of the fiber, and a step of fixing the first and second optical fibers to the reinforcing member in a state where the tension is applied. It is characterized by having.

  According to said structure, there exists an effect similar to the said optical device.

  In the above optical device, the tension applied to the first and second optical fibers is equal to or greater than a force causing strain of 10 microstrain and less than a force causing the first and second optical fibers to be cut. It is good also as a structure.

  According to said structure, tension | tensile_strength more than the force which produces distortion of the optical fiber 10 microstrain in an axial direction is less than the force which cut | disconnects the said 1st and 2nd optical fiber in the 1st and 2nd optical fiber. Is granted. Therefore, when the first and second optical fibers are single-mode excited, high-order mode excitation caused by minute bending of the fusion splicing portion of the core is surely suppressed, and deterioration of the beam quality is surely prevented. be able to.

  The optical device of the present invention includes a first optical fiber on the input side of propagating light and a second optical fiber on the output side of propagating light, which are fusion-connected at the fusion splicing portion. In the optical device fixed to the reinforcing member, at least the second optical fiber is a multimode optical fiber, and the first and second optical fibers are in a tensioned state in the axial direction of the optical fibers. The tension is fixed to the reinforcing member, and the tension is, in the direction in which the tension increases, sequentially, a first region where the rate of change of the beam quality is small with respect to the applied tension, and the beam quality with respect to the applied tension. Can be divided into a second region having a rate of change of the first region greater than the first region, and a third region having a rate of change of the beam quality smaller than the second region with respect to the applied tension. Before the two optical fiber It may be configured to tension the second region is applied.

  According to said structure, the tension | tensile_strength of the 2nd area | region where the change rate of beam quality is relatively largest with respect to the tension | tensile_strength provided is given to the 1st and 2nd optical fiber. That is, suitable tension is applied to the first and second optical fibers in order to prevent deterioration of the beam quality. Thereby, it is possible to reliably prevent the beam quality from deteriorating.

  Note that a fiber laser in which the optical device according to the present invention is inserted into a subsequent optical fiber that functions as a transmission medium is also included in the scope of the present invention. In such a fiber laser, when the first and second optical fibers are excited in a single mode, excitation in a higher-order mode can be suppressed and deterioration in beam quality can be prevented.

  According to the configuration of the present invention, when the first and second optical fibers are excited in the single mode, the excitation of the higher mode caused by the minute bending of the fusion splicing portion of the core is suppressed, and the beam quality is reduced. Can be prevented.

It is a three-view figure which shows the structure of the optical device which concerns on embodiment of this invention. FIG. 2 is a cross-sectional view taken along line A-A ′ in FIG. 1. It is a schematic diagram which shows the principal part of the optical device shown in FIG. It is a graph which shows the measured value of the distortion in each position of the optical fiber shown in FIG. It is explanatory drawing which shows an example of the manufacturing apparatus used for manufacture of the optical device shown in FIG. It is a graph which shows the relationship between the tension | tensile_strength provided to the optical fiber shown in FIG. 3, and beam quality M2. It is a block diagram which shows the structure of the fiber laser containing the optical device shown in FIG.

  Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, an example in which the present invention is applied to an optical device constituting a fiber laser will be described.

[Optical device]
FIG. 1 is a three-view diagram (upper left: top view, upper right: front side view (right side view), lower left: front view) showing a configuration of an optical device 1 according to an embodiment of the present invention. 2 is a cross-sectional view taken along line AA ′ in FIG.

  As shown in FIGS. 1 and 2, the optical device 1 includes a pedestal 11, a grooved plate 12, a reinforcing member 13, a cover plate 14, an optical fiber F1 (first optical fiber), and an optical fiber F2 (second optical fiber). And a high refractive index resin layer 21. The top view of FIG. 1 omits the cover plate 14 in order to show the internal structure of the optical device 1.

(Pedestal 11)
The pedestal 11 is a rectangular plate-like member, and is formed of, for example, a metal such as aluminum whose surface is black anodized. A grooved plate 12 is provided on the pedestal 11. A screw hole 11a is formed in the pedestal 11 at an outer edge protruding from the grooved plate 12 in the width direction (a direction parallel to the surface of the pedestal 11 and perpendicular to the axial direction of the optical fibers F1 and F2). . The optical device 1 can be fixed to the fiber laser with screws provided in the screw holes 11a.

(Grooved plate 12)
The grooved plate 12 is a rectangular plate-like member, and is formed of, for example, a metal such as aluminum whose surface is black anodized. The grooved plate 12 is disposed on the pedestal 11 so that the upper surface thereof is parallel to the upper surface of the pedestal 11, and the lower surface is fixed (for example, bonded) to the upper surface of the pedestal 11. The length of the grooved plate 12 in the longitudinal direction (axial direction of the optical fibers F1 and F2) is the same as the length of the pedestal 11 in the longitudinal direction (axial direction of the optical fibers F1 and F2). On the upper surface of the grooved plate 12, a groove portion 12 a reaching from the one end in the longitudinal direction of the grooved plate 12 to the other end is formed.

(Reinforcing member 13)
The reinforcing member 13 has a rectangular parallelepiped shape and is formed of a metal such as aluminum. When the reinforcing member 13 is made of aluminum, the inner surface of the reinforcing member 13 may be black anodized.

  The reinforcing member 13 is disposed in the groove portion 12 a so as to be parallel to the groove portion 12 a of the grooved plate 12. The lower surface of the reinforcing member 13 is fixed (for example, bonded) to the bottom surface of the groove 12a. The length of the reinforcing member 13 is shorter than the length of the grooved plate 12, and the width is narrower than the width of the groove 12a. On the upper surface of the reinforcing member 13, a groove portion 13 a reaching from the one end portion in the longitudinal direction of the reinforcing member 13 to the other end portion is formed.

(Optical fibers F1, F2, high refractive index resin layer 21)
In the groove portion 13a of the reinforcing member 13, optical fibers F1 and F2 whose end faces are fusion-connected at the fusion-connecting portion 19 are disposed. In the optical fibers F1 and F2, it is assumed that the propagation light propagates from the optical fiber F1 toward the optical fiber F2.

  The optical fibers F1 and F2 are multimode optical fibers. Alternatively, the optical fiber F1 on the input side of propagating light is a single mode optical fiber, and the optical fiber F2 on the output side of propagating light is a multimode optical fiber. That is, at least the optical fiber F2 on the output side of the propagation light is a multimode optical fiber.

  The optical fibers F1 and F2 disposed in the groove 13a of the reinforcing member 13 are covered with a high refractive index resin layer 21 filled in the groove 13a. The refractive index of the high refractive index resin layer 21 is equal to or higher than the refractive index of the cladding of the optical fibers F1 and F2.

  End resin layers 22 are provided at both ends in the axial direction of the optical fibers F1 and F2 in the groove 13a of the reinforcing member 13, and the optical fibers F1 and F2 are fixed to the reinforcing member 13 by the end resin layers 22, respectively. ing. In the optical fibers F 1 and F 2, the coating 15 is removed around the fusion splicing portion 19 and its periphery to expose the bare fiber 16, and the portion fixed to the end resin layer 22 has the coating 15.

  Further, end resin layers 23 are provided at both ends of the optical fiber F1 and F2 in the axial direction in the groove 12a of the grooved plate 12, respectively, and the optical fibers F1 and F2 are grooved by the end resin layer 23. 12 is fixed.

(Cover plate 14)
The lid plate 14 is a rectangular plate-like member, and is formed of a metal such as aluminum. When the cover plate 14 is formed of aluminum, the inner surface of the cover plate 14 may be black anodized. The cover plate 14 is disposed on the grooved plate 12 so as to cover the groove portion 12a. The lower surface of the cover plate 14 is fixed (for example, bonded) to the upper surface (a portion other than the groove portion 12a) of the grooved plate 12.

(Details of reinforcing state of optical fibers F1 and F2 by reinforcing member 13)
FIG. 3 is a schematic diagram showing a main part of the optical device 1 shown in FIG. As shown in FIG. 3, the optical fibers F1 and F2 are fixed to the reinforcing member 13 by the end resin layer 22 in a state where tension (T) is applied (a state in which the optical fibers F1 and F2 are pulled in directions opposite to each other in the axial direction). ing. The axial length of the optical fibers F1, F2 in the end resin layer 22 is about 5 mm.

  The tension applied to the optical fibers F1 and F2 is a value at which the strain (elongation of the optical fibers F1 and F2) measured by the strain measuring instrument is larger than 1 microstrain (μ strain). In this case, the measurement limit of the strain measuring instrument is, for example, 10 microstrain.

  On the other hand, conventionally, the optical fibers F1 and F2 are given a tension that is small enough to be kept straight. In this case, in the optical fibers F1 and F2, the coating 15 is mainly pulled, and the bare fiber 16 is hardly distorted (elongated). Therefore, the strain of the bare fiber 16 is estimated to be about 0.5 microstrain.

  Specifically, conventionally, the optical fibers F1 and F2 (fiber strands in which glass optical fibers are covered with a resin coating) are fixed in order to keep the optical fibers F1 and F2 in a straight line. It is carried out in a state where it is gripped and pulled with a tension of about 5 g. It is known that when the fiber strand is pulled in this manner, the coating (coating 15) extends 0.5 μm / cm (50 μm / 1 m = 50 μ strain). Here, since the elastic modulus (about 100 GPa) of the glass is about 100 times the elastic modulus (about 1 GPa) of the resin (coating 15), the elongation of the fiber strands (optical fibers F1, F2) at this time is , 0.005 μm / cm (0.5 μm / m = 0.5 μ strain).

  In the present embodiment, the relationship between each position and strain of the optical fibers F1 and F2 shown in FIG. 3 is as shown in FIG. Note that the position of 6 cm shown in FIG. 4 corresponds to the position of the fusion splicing portion 19 of the optical fibers F1 and F2. In the example of FIG. 4, tension is applied to the optical fibers F1 and F2 so that a strain of about 35 microstrain is generated in the bare fiber.

(Manufacturing method of the optical device 1)
In the above configuration, a method for manufacturing the optical device 1 will be described below. FIG. 5 is an explanatory diagram showing an example of a manufacturing apparatus 31 used for manufacturing the optical device 1. The manufacturing apparatus 31 shown in FIG. 5 is used in a process of applying tension to the optical fibers F1 and F2 and fixing the optical fibers F1 and F2 to the reinforcing member 13.

  As shown in FIG. 5, the manufacturing apparatus 31 includes bases 41 to 43 arranged in one direction. The base 41 located at one end has an optical fiber fixing portion 41a at the top. The central base 42 is provided with the reinforcing member 13 of the optical device 1 on the upper surface, and the arranged reinforcing member 13 can be fixed. The base 43 at the other end has a rail 43b and an optical fiber fixing portion 43a at the top. The optical fiber fixing portion 43a is movable in the direction in which the bases 41 to 43 are arranged on the rail 43b. A string 44 is connected to the optical fiber fixing portion 43a, and a weight 45 is attached to the tip of the string 44. The weight 45 is in a suspended state via a pulley 46 attached to the base 43.

  When attaching the optical fibers F1 and F2 to the reinforcing member 13, the reinforcing member 13 is placed on the base 42 and fixed, and the optical fibers F1 and F2 in the state of being connected by the fusion splicing portion 19 The fiber F2 is fixed by the fiber fixing part 41a, and the optical fiber F1 is fixed by the optical fiber fixing part 43a. Further, the weight 45 is suspended from the optical fiber fixing portion 43a via the pulley 46, and the fusion splicing portion 19 of the optical fibers F1 and F2 is disposed at a predetermined position of the reinforcing member 13.

  In the above state, a predetermined tension is applied to the optical fibers F1 and F2 by the weight 45, and a desired strain (elongation) is generated in the fusion splicing portion 19. Next, in this state, a resin that covers the optical fibers F1 and F2 and becomes the end resin layer 22 is disposed at the end position of the reinforcing member 13 and cured. Thereby, the optical fibers F1 and F2 are fixed to the reinforcing member 13 in a state where a desired tension is applied. Thereafter, the reinforcing member 13 is filled with a resin that becomes the high refractive index resin layer 21 and cured.

(Advantages of optical device 1)
As described above, in the optical device 1, a tension is applied to the optical fibers F1 and F2 such that the strain (elongation) of the bare fiber 16 measured by the strain measuring instrument is larger than 1 microstrain. Therefore, it is possible to correct a slight bend generated in the fusion splicing portion 19 of the core due to inclination or chipping of the end surface of the splicing splicing portion 19 or pressing of the optical fibers F1 and F2 at the time of connection.

  As a result, when the optical fibers F1 and F2 are excited in a single mode, the excitation of the higher-order mode due to the slight bending of the fusion splicing portion 19 is suppressed, and the optical device 1 and the fiber laser including the optical device 1 are suppressed. Reduction in beam quality can be suppressed.

  Here, the relationship between the tension applied to the optical fibers F1 and F2 and the beam quality M2 is shown in FIG. In FIG. 6, “M2 before connection” indicates a state in which the fusion splicing portion 19 does not exist in the optical device 1 and the optical device 1 is in single mode excitation. “After connection M2” indicates a state in which the fusion splicing portion 19 of the optical fibers F1 and F2 exists in the optical device 1. Further, the tension range “conventional” is a range of 0 microstrain measured by a strain measuring instrument (a tension is 0 or a minute range in which the tension exceeds the measurement limit of the strain measuring instrument).

  As shown in FIG. 6, when the tension applied to the optical fibers F1 and F2 increases, the value of M2 decreases and the beam quality of the optical device 1 improves. This is because, as the tension increases, the correction of the minute bending occurring in the fusion splicing portion 19 of the optical fibers F1 and F2 progresses, and the optical fibers F1 and F2 are excited in a single mode. This is because the ratio of the higher-order mode in the device 1 (the ratio at which the higher-order mode is excited in the fusion splicing portion 19) decreases and the ratio of the single mode (basic mode) increases.

  In addition, the tension applied to the optical fibers F1 and F2 is sequentially divided into a first area (corresponding to the “conventional” above), a second area, and a third area in the direction in which the tension increases. can do. The first region is a region where the change rate of the beam quality is small with respect to the applied tension. The second region is a region where the change rate of the beam quality is the largest with respect to the applied tension. The third region is a region in which the rate of change in beam quality is smaller than the second region with respect to the applied tension. From FIG. 6, the tension applied to the optical fibers F1 and F2 can be effectively prevented from degrading the beam quality, and the unnecessary load is not applied to the optical fibers F1 and F2. It is preferable to set.

(Example)
In the conventional optical device corresponding to the optical device 1, for example, both the optical fibers F1 and F2 have a core diameter of 30 μm, a cladding diameter of 125 μm, and a core NA of 0.06. The propagating light propagating through the optical fiber F1 (propagating light input side) propagated in a single mode, and the beam quality M2 was 1.05. In this optical device, the beam quality M2 of the optical fiber F2 (propagating light output side) deteriorated to 1.2 due to the excitation of the higher-order mode at the fusion splicing portion 19.

  Therefore, like the optical device 1, the optical fibers F 1 and F 2 were fixed by applying a tension of about 40 g (weight of the weight 46) and reinforced by the reinforcing member 13. As a result, the beam quality M2 of the optical fiber F2 on the output side was improved to about 1.07.

  In the optical device 1, the degree of deterioration of the beam quality M2 at the fusion splicing part 19 and the degree of improvement when tension is applied to the optical fibers F1 and F2 depend on the state of the fusion splicing part 19 (connection conditions and fusion splicing). The angle varies depending on the angle of the end face of the connecting portion 19. However, the beam quality M2 can be improved by increasing the tension applied to the optical fibers F1 and F2 (however, the optical fibers F1 and F2 are not cut).

[Fiber laser]
Next, the fiber laser 51 including the optical device 1 will be described. FIG. 7 is a block diagram showing the configuration of the fiber laser 51.

  As shown in FIG. 7, the fiber laser 51 includes an optical device 1, a light source device 2, an optical fiber F3, a residual light removing unit 3, and optical fibers F1 and F2 (rear stage optical fibers). The fiber laser 51 originally has a plurality of fusion splicing portions between optical fibers, and the optical device 1 is applied to at least one of them.

  The optical fiber F3 functions as an amplification medium. The optical fiber F3 is a double-clad fiber including a core to which an active element is added, an inner cladding that surrounds the core, and an outer cladding that surrounds the inner cladding. The active element added to the core of the optical fiber F3 is output from the light source device 2 and transitions to the inverted distribution state by the excitation light propagating through the inner cladding of the optical fiber F3. The propagating light output from the light source device 2 and propagating through the core of the optical fiber F3 is amplified by the active element that has transitioned to the inverted distribution state. In particular, two fiber Bragg gratings FBG are written in the optical fiber F3, and propagating light propagating through the core of the optical fiber F is recursively amplified between the two fiber Bragg gratings FBG.

  The residual light removing unit 3 removes residual light from the output light of the optical fiber F3. Residual light refers to light other than propagating light propagating through the core, such as pumping light propagating through the inner clad and propagating light leaking into the clad due to axial misalignment between the optical fibers.

  The propagating light output from the residual light removing unit 3 propagates through the optical fibers F1 and F2 functioning as transmission media and is output to the outside from the output end out. The optical device 1 according to this embodiment is inserted into the optical fibers F1 and F2.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims. That is, embodiments obtained by combining technical means appropriately modified within the scope of the claims are also included in the technical scope of the present invention.

  INDUSTRIAL APPLICABILITY The present invention is an optical device provided in a fiber laser and in which optical fibers are connected to each other, and can be suitably used as an optical device in which at least an optical fiber on the output side of propagating light is a multimode optical fiber. .

DESCRIPTION OF SYMBOLS 1 Optical device 12 Grooved plate 12a Groove part 13 Reinforcement member 13a Groove part 15 Coating | cover 16 Bare fiber 19 Connection part 21 High refractive index resin layer 22 End part resin layer 23 End part resin layer 31 Manufacturing apparatus
41-43 units
41a, 43a Fiber fixing part 43b Rail 45 Weight 46 Pulley 51 Fiber laser F1 Optical fiber (first optical fiber, rear optical fiber)
F2 optical fiber (second optical fiber, rear optical fiber)
F3 optical fiber

Claims (5)

An optical device having a first optical fiber on the input side of propagating light and a second optical fiber on the output side of propagating light, which are fusion-connected at the fusion splicing portion, and these optical fibers are fixed to a reinforcing member In
At least the second optical fiber is a multimode optical fiber;
The optical device is characterized in that the first and second optical fibers are fixed to the reinforcing member in a state in which tension is generated in the axial direction of the optical fibers so as to generate strain larger than 1 microstrain. .   The tension applied to the first and second optical fibers is greater than or equal to a force that causes a strain of 10 microstrain and less than a force that cuts the first and second optical fibers. Item 4. The optical device according to Item 1. An optical device having a first optical fiber on the input side of propagating light and a second optical fiber on the output side of propagating light, which are fusion-connected at the fusion splicing portion, and these optical fibers are fixed to a reinforcing member In
At least the second optical fiber is a multimode optical fiber;
The first and second optical fibers are fixed to the reinforcing member in a tensioned state in the axial direction of the optical fibers,
The tension is, in the direction in which the tension increases, sequentially, a first region in which the change rate of the beam quality is small with respect to the applied tension, and the change rate of the beam quality in the first region with respect to the applied tension. A second region that is larger than the second region, and a third region in which the rate of change of the beam quality with respect to the applied tension is smaller than the second region, and the first and second optical fibers include the second region. The optical device according to claim 1, wherein a tension in the region is applied. A fiber laser comprising a front-stage optical fiber that functions as an amplification medium, and a rear-stage optical fiber that functions as a transmission medium that transmits light output from the front-stage optical fiber,
A fiber laser, wherein the optical device according to any one of claims 1 to 3 is inserted into the rear optical fiber. An optical device having a first optical fiber on the input side of propagating light and a second optical fiber on the output side of propagating light, which are fusion-connected at the fusion splicing portion, and these optical fibers are fixed to a reinforcing member In the manufacturing method of
At least the second optical fiber is a multimode optical fiber;
Applying tension to the first and second optical fibers to generate strain larger than 1 microstrain in the axial direction of the first and second optical fibers;
And a step of fixing the first and second optical fibers to the reinforcing member in a state where the tension is applied.

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