WO2020066190A1

WO2020066190A1 - Method for fusion splicing optical fiber, optical fiber, and fusing device

Info

Publication number: WO2020066190A1
Application number: PCT/JP2019/025703
Authority: WO
Inventors: 大介小西; 政直村上; 茂樹時田; 日和上原; 賢治合谷
Original assignee: 三星ダイヤモンド工業株式会社; 国立大学法人大阪大学
Priority date: 2018-09-28
Filing date: 2019-06-27
Publication date: 2020-04-02
Also published as: JPWO2020066190A1; CN112823302A

Abstract

The present invention makes it possible to increase the strength of an optical fiber created by fusion splicing two optical fibers that have different melting points. A method for fusion splicing an optical fiber comprises: a step of placing a first optical fiber 2 that has a first diameter d1 and a first melting point and a second optical fiber 3 that has a second diameter d2 less than the first diameter d1 and a second melting point higher than the first melting point; a step of heating at least the tip end of the first optical fiber 2 to a temperature that is higher than or equal to a first temperature that allows the first optical fiber 2 to soften and lower than a second temperature that allows the first optical fiber 2 to crystallize; a step of inserting the tip end of the second optical fiber 3 into the first optical fiber 2 by moving the second optical fiber 3 in a first direction in which the second optical fiber 3 approaches the first optical fiber with the tip end of the first optical fiber 2 heated; and a step of moving the second optical fiber 3 in a second direction opposite the first direction after the tip end of the second optical fiber 3 is inserted into the first optical fiber 2.

Description

Optical fiber fusion method, optical fiber, and fusion device

The present invention provides a fusion method for fusing two optical fibers made of materials having different melting points at the tip thereof, and a method for fusing two optical fibers made of different materials using the method. Optical fiber.

する When an optical system such as an optical circuit is configured by optical fibers, it is necessary to connect optical fibers made of different materials at the tip. The connection of the optical fiber is generally performed by fusion.

Optical fibers have different physical properties depending on the material. For example, the melting point of quartz used in general optical fibers is higher than the melting point of a material other than quartz (for example, fluoride glass).

As a method of connecting such optical fibers of different materials having different melting points, the diameter of the optical fiber having a high melting point is made smaller than the diameter of the optical fiber having a low melting point, and the optical fiber having a high melting point is heated to thereby form an optical fiber having a low melting point. A method is known in which these two optical fibers are fused by pressing them together (see, for example, Patent Document 1).
In this fusion method, the low-melting-point optical fiber is softened by the heat of heating the high-melting-point optical fiber, and the high-melting-point optical fiber is fused while being inserted into the low-melting-point optical fiber tip. The strength of the part has been improved.

JP 2003-156652 A

However, the fusion strength of the optical fiber fused by the above-mentioned fusion method is about 80 kPa, which is insufficient for forming an optical system using this optical fiber. This is because with such a fusion strength, there is a high possibility that the fusion of the optical fiber is released when handling the optical fiber.

Further, it is considered that the optical fiber fused by the above fusion method is easily damaged by bending or the like.
Specifically, in the optical fiber fused by the above-mentioned fusion method, a step portion whose diameter changes rapidly is formed at the connection portion. For example, when an optical fiber having such a step portion is bent, stress is concentrated on the step portion, and the optical fiber at or near the step portion may be damaged.

An object of the present invention is to improve the strength of an optical fiber formed by fusion splicing two optical fibers having different melting points.

Hereinafter, a plurality of modes will be described as means for solving the problems. These embodiments can be arbitrarily combined as needed.
An optical fiber fusion method according to one aspect of the present invention is a method of fusing a first optical fiber having a first melting point and a second optical fiber having a second melting point higher than the first melting point. The fusing method includes the following steps.
Disposing a first optical fiber having a first diameter and a second optical fiber having a second diameter smaller than the first diameter;
Heating at least the tip of the first optical fiber to a temperature equal to or higher than a first temperature at which the first optical fiber softens and lower than a second temperature at which the first optical fiber crystallizes;
◎ a step of moving the second optical fiber in a first direction to approach the first optical fiber with at least the distal end of the first optical fiber being heated, and inserting the distal end of the second optical fiber into the first optical fiber;
Moving the second optical fiber in a second direction opposite to the first direction after inserting the tip of the second optical fiber into the first optical fiber;

In the above optical fiber fusion method, after the first optical fiber is softened and the tip of the second optical fiber is inserted into the first optical fiber, the second optical fiber is further inserted into the first optical fiber. It is moved in a second direction opposite to the one direction. That is, the second optical fiber is moved in a direction away from the first optical fiber.
Thus, when the second optical fiber is moved in a direction away from the first optical fiber, the tip of the first optical fiber on the side connected to the second optical fiber is elongated. Due to this stretching, the diameter of the distal end portion of the first optical fiber decreases, and the connecting portion between the first optical fiber and the second optical fiber becomes tapered.

As described above, by forming the tapered shape at the connecting portion between the first optical fiber and the second optical fiber, the strength of the optical fiber manufactured by connecting these two optical fibers can be improved. For example, when a manufactured optical fiber is bent, stress can be less likely to be concentrated on a connection portion between two optical fibers, so that strength can be improved.

In addition, by heating at least the tip of the first optical fiber having a low melting point, it is easy to form a tapered shape at the connection portion when the second optical fiber is moved in a direction away from the first optical fiber.

The above fusion method may further include a step of gradually cooling the first optical fiber and the second optical fiber after moving the second optical fiber in the second direction.
Thus, when the first optical fiber and the second optical fiber are cooled, stress is generated at a connection portion between the two optical fibers due to a difference in expansion coefficient between the two optical fibers, so that breakage can be suppressed.

In the step of inserting the distal end of the second optical fiber into the first optical fiber, an insertion distance of the distal end of the second optical fiber into the first optical fiber may be a distance in a range of 20% to 40% of the second diameter.
With this, the insertion depth of the tip of the second optical fiber into the first optical fiber is set to an appropriate depth, and these two optical fibers can be connected with a sufficiently high fusion strength.

In the moving the second optical fiber in the second direction, the moving distance of the second optical fiber may be a distance in a range of 50% to 70% of the second diameter.
Thereby, the taper angle of the tip of the first optical fiber can be set to an appropriate angle at which stress is less likely to be concentrated on the connection portion between the first optical fiber and the second optical fiber.

In the step of disposing the first optical fiber and the second optical fiber, the distal end of the first optical fiber and the distal end of the second optical fiber may be disposed close to each other.
Thereby, not only the tip of the first optical fiber but also the tip of the second optical fiber can be heated. As a result, when the second optical fiber comes into contact with the first optical fiber, the temperature of the first optical fiber can be prevented from lowering.

The first diameter may be in the range of 1.5 to 3 times the second diameter. Thereby, the first optical fiber and the second optical fiber can be connected with sufficient strength.

The first temperature may be a softening point of the first optical fiber. Thereby, the first optical fiber is reliably softened, and the insertion of the tip of the second optical fiber and the formation of the tapered shape can be appropriately performed.

The second temperature may be a crystallization temperature of the first optical fiber. Accordingly, it is possible to reliably prevent the first optical fiber from being crystallized and the transparency of the first optical fiber from being reduced. As a result, the fusion loss at the fusion portion between the first optical fiber and the second optical fiber can be reduced.

The first optical fiber may be a ZBLAN fiber. Thus, the optical fiber connecting the first optical fiber and the second optical fiber can be applied to a fiber laser, a fiber amplifier, and the like.

The second optical fiber may be an optical fiber made of quartz. Thus, the optical fiber connecting the first optical fiber and the second optical fiber can be applied to a fiber laser, a fiber amplifier, a fiber sensor, a transmission fiber, and the like.

An optical fiber according to another aspect of the present invention includes a first optical fiber and a second optical fiber.
The first optical fiber has a tapered portion. The tapered portion is a portion whose diameter decreases from the first diameter toward the tip.
The second optical fiber has a second diameter smaller than the first diameter. The second optical fiber is connected to the first optical fiber with the tip inserted into the tip of the tapered portion.

光 In an optical fiber configured by connecting the first optical fiber and the second optical fiber, the strength of the optical fiber can be improved by forming a tapered shape at the connection portion between the first optical fiber and the second optical fiber. For example, when this optical fiber is bent, stress can be less likely to be concentrated on the connection portion between the two optical fibers.

The insertion depth of the tip of the second optical fiber into the tapered portion of the first optical fiber may be in the range of 4% to 16% of the second diameter.
Thereby, the first optical fiber and the second optical fiber can be connected with a sufficiently high fusion strength.

The taper angle between the side surface of the tapered portion and the length direction of the first optical fiber may be in a range of 20 degrees to 50 degrees.
Thereby, the tapered portion can be formed into an optimal shape in which stress is less likely to be concentrated on the connection portion between the first optical fiber and the second optical fiber.

The first diameter may be twice as large as the second diameter, and a taper angle between a side surface of the tapered portion and a length direction of the first optical fiber may be in a range of 22 degrees to 30 degrees.
The first diameter may be three times the second diameter, and a taper angle between a side surface of the tapered portion and a length direction of the first optical fiber may be in a range of 35 degrees to 50 degrees.
Thus, by setting the taper angle in an appropriate range in accordance with the ratio between the first diameter and the second diameter, the tapered portion is formed into an optimal shape in which stress is less likely to be concentrated on the connection portion between the first optical fiber and the second optical fiber. it can.

A fusion device according to still another aspect of the present invention is a device for fusing a first optical fiber having a first melting point and a second optical fiber having a second melting point higher than the first melting point. The fusion device includes a heating light source and a fiber moving device.
The heating light source heats the distal end of the first optical fiber having the first diameter to a temperature equal to or higher than a first temperature at which the first optical fiber softens and lower than a second temperature at which the first optical fiber crystallizes.
The fiber moving device moves a second optical fiber having a second diameter smaller than the first diameter in a first direction closer to the first optical fiber in a state where at least the distal end of the first optical fiber is heated, and moves the distal end of the second optical fiber. Is inserted into the first optical fiber, the tip of the second optical fiber is inserted into the first optical fiber, and then the second optical fiber is moved in a second direction opposite to the first direction.

In the above fusion device, after the first optical fiber is softened and the tip of the second optical fiber is inserted into the first optical fiber, the second optical fiber is further inserted into the first optical fiber in the first direction in which the tip of the second optical fiber is inserted. Are moved in the second direction opposite to the above. That is, the second optical fiber is moved in a direction away from the first optical fiber.
Thus, when the second optical fiber is moved in a direction away from the first optical fiber, the tip of the first optical fiber on the side connected to the second optical fiber is elongated. Due to this stretching, the diameter of the distal end portion of the first optical fiber decreases, and the connecting portion between the first optical fiber and the second optical fiber becomes tapered.

As described above, by forming the tapered shape at the connecting portion between the first optical fiber and the second optical fiber, the strength of the optical fiber manufactured by connecting these two optical fibers can be improved. For example, when a manufactured optical fiber is bent, stress can be less likely to be concentrated on a connection portion between two optical fibers, so that strength can be improved.
In addition, by heating at least the distal end of the first optical fiber having a low melting point, it is easy to form a tapered shape at the connection portion when the second optical fiber is moved in a direction away from the first optical fiber.

により By forming a tapered shape at the connecting portion of the two optical fibers, the strength of the optical fiber manufactured by connecting the two optical fibers can be improved.

The figure which shows the structure of the optical fiber manufactured by fusion. The figure which shows the whole structure of a fusion device. The figure which shows the structure of a fiber moving apparatus. The figure which shows typically the fusion | fusion method of an optical fiber. FIG. 2 is a diagram illustrating an optical microscope image of the optical fiber of the first embodiment. FIG. 8 is a diagram showing an optical microscope image of the optical fiber of Example 2. FIG. 3 is a diagram illustrating a relationship between a taper angle and a fusion loss of the optical fiber of the first embodiment. FIG. 9 is a diagram illustrating a relationship between a taper angle and a fusion loss of the optical fiber of the second embodiment. FIG. 5 is a view showing a transmission optical microscope image of the optical fiber manufactured in Comparative Example 1. FIG. 9 is a view showing an optical microscope image of the optical fiber of Comparative Example 2.

1. First Embodiment (1) Optical Fiber A method for fusing an optical fiber according to the first embodiment will be described below.
First, the structure of the optical fiber 1 manufactured by the fusion method according to the present embodiment will be described with reference to FIG. FIG. 1 is a diagram showing a structure of an optical fiber manufactured by fusion.
As shown in FIG. 1, the optical fiber 1 has a first optical fiber 2 and a second optical fiber 3. The first optical fiber 2 and the second optical fiber 3 are fusion-spliced at their distal ends by the fusion method of the present embodiment.

The first optical fiber 2 is an optical fiber made of a material having a first melting point. In the present embodiment, the first optical fiber 2 is a ZBLAN fiber. The first melting point is about 300 degrees, which is the softening point of the ZBLAN fiber. When the first optical fiber 2 is a ZBLAN fiber, the melting point is about 440 degrees, the crystallization point is about 350 degrees, and the glass transition point is about 260 degrees. This ZBLAN fiber is often used as a laser medium of a fiber laser.
The core of the first optical fiber 2 which is a ZBLAN fiber may be doped with an element such as erbium (Er).

In addition, glass fibers such as aluminum fluoride, chalcogenide glass (for example, arsenic trisulfide (As ₂ S ₃ ), Ge ₃₃ As ₁₂ Se _55, etc.), tellurite (TeO) glass, and the like are used as the first optical fiber. Can be 2. These glasses also have a melting point lower than the melting point of quartz, which is a material of the second optical fiber 3 described later.

The first optical fiber 2 has a circular cross section perpendicular to the length direction having a first diameter d1. The first optical fiber 2 has a tapered portion 21 at a distal end which is a connection portion with the second optical fiber 3.
The diameter of the tapered portion 21 decreases from the first diameter d1 toward the distal end. The diameter of the tip of the tapered portion 21 that is the connection portion with the second optical fiber 3 is substantially the same as the diameter of the second optical fiber 3 (second diameter d2).

The optimum value of the taper angle θ of the tapered portion 21 varies depending on the ratio between the first diameter d1 and the second diameter d2, as described later, but is in the range of 20 to 50 degrees. In the present embodiment, the taper angle θ is defined as an angle between the side surface of the tapered portion 21 and the length direction of the first optical fiber 2.

The second optical fiber 3 is an optical fiber made of a material having a second melting point. In the present embodiment, the second optical fiber 3 is an optical fiber made of quartz. The second melting point is about 1700 degrees, which is the melting point of quartz.

The cross section of the second optical fiber 3 perpendicular to the length direction is a circle having a second diameter d2. In the present embodiment, the second diameter d2 is smaller than the first diameter d1. Specifically, the first diameter d1 can range from 1.5 times to 3 times the second diameter d2.
Thereby, the first optical fiber 2 and the second optical fiber 3 can be connected with sufficient strength by a fusion method described later.

{Circle around (2)} The second optical fiber 3 is fusion-spliced to the first optical fiber 2 with its tip inserted into the tip of the tapered portion 21 of the first optical fiber 2. The insertion depth of the tip of the second optical fiber 3 into the tapered portion 21 can be in the range of 4% to 16% of the second diameter d2. For example, when the second diameter d2 of the second optical fiber 3 is 125 μm, the insertion depth can be in the range of 5 μm to 20 μm.

With the above configuration, the optical fiber 1 manufactured by fusion splicing the first optical fiber 2 and the second optical fiber 3 makes it difficult for the second optical fiber 3 to be separated from the first optical fiber 2 during handling, and is bent. Hard to be destroyed when
Specifically, the first optical fiber 2 and the second optical fiber 3 are sufficiently strong by inserting the distal end of the second optical fiber 3 into the distal end of the tapered portion 21 of the first optical fiber 2 by a depth within the above range. Connection can be made with fusion strength.
Further, by providing the tapered portion 21 having the above-described taper angle θ at the connection portion between the first optical fiber 2 and the second optical fiber 3, for example, when the optical fiber 1 is bent, stress is applied to the connection portion between the two optical fibers. It can be difficult to concentrate.

The optical fiber 1 in which two optical fibers made of different materials are fusion-spliced can be applied as, for example, a fiber laser, a fiber amplifier, a fiber sensor, a transmission fiber, or the like, depending on the material of the first optical fiber 2.

(2) Fusing Apparatus Next, the fusing apparatus 100 will be described with reference to FIGS. FIG. 2 is a diagram showing the overall configuration of the fusion device. FIG. 3 is a diagram illustrating a configuration of the fiber moving device.
The fusion device 100 according to the present embodiment heats and softens the first optical fiber 2 having a low melting point by irradiating the heating laser beam HL, and presses the second optical fiber 3 against the softened first optical fiber 2 to thereby fuse the first optical fiber 2. To wear.
The fusion device 100 mainly includes a fiber moving device 4, a heating light source 6, and a shutter 8.

(2-1) Fiber moving device The fiber moving device 4 moves the first optical fiber 2 and / or the second optical fiber 3 in a first direction (FIG. 3) and a second direction (FIG. 3) opposite thereto. It is. Specifically, the fiber moving device 4 includes a first moving unit 41 and a second moving unit 43.
The first direction is defined as a direction parallel to the length direction of the first optical fiber 2 and approaching the first optical fiber 2. On the other hand, the second direction is defined as a direction parallel to the length direction of the first optical fiber 2 and away from the first optical fiber 2.

The first moving unit 41 moves the first optical fiber 2 in the first direction or the second direction. The first moving section 41 has a first moving stage 41a and a first holding section 41b.
The first moving stage 41a is a stage that can move in the first direction or the second direction. The first moving stage 41a is, for example, an XY stage. By allowing the first moving stage 41a to move two-dimensionally, the tip of the first optical fiber 2 can be accurately positioned. The movement of the first movement stage 41a can be controlled with high accuracy in units of μm by, for example, a piezo element.

The first holding unit 41b holds the first optical fiber 2. The first holding part 41b is, for example, a clamp for an optical fiber that presses and holds the first optical fiber 2 against the bottom of the V-shaped groove with a screw or the like.
The first holding unit 41b is fixed on the upper part of the first moving stage 41a, and moves in the first direction or the second direction along with the movement of the first moving stage 41a.

The second moving unit 43 moves the second optical fiber 3 in the first direction or the second direction. The second moving section 43 has a second moving stage 43a and a second holding section 43b.
The configurations and functions of the second moving stage 43a and the second holding unit 43b are the same as those of the first moving stage 41a and the first holding unit 41b, respectively, and thus description thereof will be omitted.

(2-2) Heating Light Source The heating light source 6 is a light source that outputs the heating laser light HL. As the heating light source 6, for example, a CO ₂ laser light source that outputs infrared light can be used. The heating light source 6 can precisely control the intensity of the heating laser beam HL.
The path of the heating laser beam HL output from the heating light source 6 is changed by the first mirror 61 and is incident on the light splitting member 62 (for example, a half mirror). The heating laser beam HL incident on the light branching member 62 is branched in two directions.

One of the heating laser beams HL branched by the light branching member 62 has its path changed by the second mirror 63 and reaches the position where the tips of the first optical fiber 2 and the second optical fiber 3 are arranged from one predetermined direction. I do.
The one heating laser beam HL is focused by the first lens 64 at a position (or a vicinity thereof) where the tips of the first optical fiber 2 and the second optical fiber 3 are arranged.

The other one of the heating laser beams HL branched by the light branching member 62 is changed in path by the third mirror 65 and is positioned at the position where the tips of the first optical fiber 2 and the second optical fiber 3 are arranged in the above-mentioned predetermined one direction. Arrives from the opposite direction.
The other heating laser beam HL is focused by the second lens 66 at a position (or a vicinity thereof) where the tips of the first optical fiber 2 and the second optical fiber 3 are arranged.

By irradiating the heating laser beam HL to the tip of the first optical fiber 2 and the tip of the second optical fiber 3 in two directions opposite to each other, the tip of the first optical fiber 2 and the tip of the second optical fiber 3 can be uniformly heated. .

It is preferable that the irradiation diameter (diameter) of the heating laser beam HL applied to the first optical fiber 2 and the second optical fiber 3 is sufficiently larger than the diameter of the first optical fiber 2. For example, the irradiation diameter of the heating laser beam HL can be set to about several hundred μm.

(2-3) Shutter The shutter 8 is arranged on or near the optical path of the heating laser beam HL from the heating light source 6 to the light branching member 62. In the present embodiment, as shown in FIG. 2, the shutter 8 is arranged between the heating light source 6 and the first mirror 61.

When the shutter 8 is disposed on the optical path of the heating laser beam HL, the heating laser beam HL by the shutter 8 is shut off, and the heating of the distal ends of the first optical fiber 2 and the second optical fiber 3 is stopped.
On the other hand, when the shutter 8 deviates from the optical path of the heating laser light HL, the heating laser light HL reaches the tips of the first optical fiber 2 and the second optical fiber 3, and the tips are heated.

That is, the shutter 8 controls the execution and stop of the heating of the tips of the first optical fiber 2 and the second optical fiber 3. By controlling the execution and stop of the heating by the shutter 8, it is possible to heat the fiber tip for a short time.

The fusion device 100 further includes a control unit (not shown). The control unit is a computer system having a CPU, a storage device (RAM, ROM, etc.) and various interfaces, and controls each unit of the fusion device 100. Specifically, the control unit controls the fiber moving device 4, the heating light source 6, and the shutter 8.
The control unit may execute control of each unit of the fusion device 100 by a program stored in a storage device of a computer system constituting the control unit.

(2-4) Configuration for Monitoring the Fusion State and the Like In addition, the fusion device 100 has a configuration for monitoring the fusion state and the like of the first optical fiber 2 and the second optical fiber 3.
Specifically, the fusion device 100 includes a pair of cameras 10. The camera 10 acquires the arrangement state of the first optical fiber 2 and the second optical fiber 3, the fusion step of these optical fibers, and the fusion state of the optical fibers based on visual information (for example, a moving image, a still image).
Each of the pair of cameras 10 acquires the state of the first optical fiber 2 and the second optical fiber 3 from each of the two directions in which the heating laser light HL is irradiated.

Further, the fusion splicer 100 has a configuration for measuring the light transmittance of the optical fiber 1 manufactured by fusion splicing the first optical fiber 2 and the second optical fiber 3. Specifically, the fusion device 100 includes an inspection light source 12 and a light receiving device 14.
The inspection light source 12 outputs inspection light IL that enters from the second optical fiber 3 side of the optical fiber 1. The inspection light source 12 is, for example, a laser diode.
The light receiving device 14 measures the intensity of the inspection light IL transmitted through the first optical fiber 2 and the second optical fiber 3. The light receiving device 14 is, for example, a power meter that measures the intensity of the inspection light IL.

In the above configuration, the light transmittance of the optical fiber 1 can be calculated based on the ratio between the intensity of the inspection light IL output from the inspection light source 12 and the intensity of the inspection light IL measured by the light receiving device 14. .
If the light transmittance of the optical fiber 1 is low, it means that the loss due to fusion splicing of the first optical fiber 2 and the second optical fiber 3 is large, and it is determined that the connection of these two optical fibers is inappropriate. it can.
The case where the connection between the two optical fibers is inappropriate is, for example, a case where the tip of the first optical fiber 2 is crystallized and the transparency is reduced, so that the light transmittance of the optical fiber 1 is reduced. In this case, since the temperature of the first optical fiber 2 has been equal to or higher than the crystallization temperature for a long time, it can be assumed that the connection has become inappropriate.
In addition, when the fusion of these optical fibers is released by stress when the first optical fiber 2 and the second optical fiber 3 are heated and gradually cooled, optical coupling loss occurs due to a defective shape of the fusion spliced portion of these optical fibers. In this case, the transmittance of the optical fiber 1 may be reduced even when a foreign substance is mixed into the fusion surface. Even in such a case, it can be assumed that the fusion splicing has become inappropriate.

(3) Method for Fusing Optical Fiber A method for fusing an optical fiber according to the first embodiment will be described below with reference to FIG. FIG. 4 is a diagram schematically showing a method for fusing optical fibers. Hereinafter, a method for fusing two optical fibers having different melting points will be described using a process of fusing the first optical fiber 2 and the second optical fiber 3 to manufacture the optical fiber 1 as shown in FIG.
First, the first optical fiber 2 and the second optical fiber 3 are arranged in the fusion device 100.
Specifically, first, the first optical fiber 2 is held by the first holding unit 41b, and the second optical fiber 3 is held by the second holding unit 43b.
Thereafter, the first optical fiber 2 and / or the second optical fiber 3 is moved by the first moving stage 41a and / or the second moving stage 43a, and the first optical fiber 2 and the second optical fiber 3 are aligned. For example, as shown in FIG. 4A, the center line of the first optical fiber 2 (the dotted line in FIG. 4A) coincides with the center line of the second optical fiber 3. Align.

Further, the first optical fiber 2 and / or the second optical fiber 3 is moved by the first moving stage 41a and / or the second moving stage 43a so that the tip of the first optical fiber 2 and the tip of the second optical fiber 3 are close to each other. Let it.

After the first optical fiber 2 and the second optical fiber 3 are arranged in the fusion splicer 100, the heating laser light HL is output from the heating light source 6 and the shutter 8 is removed from the optical path of the heating laser light HL. As shown in (), the tip of the first optical fiber 2 is irradiated with the heating laser beam HL. In FIG. 3, the irradiation area of the heating laser beam HL is indicated by a dotted circle.

As described above, since the distal end of the first optical fiber 2 and the distal end of the second optical fiber 3 are arranged close to each other, the heating laser light HL is applied to the distal end of the first optical fiber 2 and the distal end of the second optical fiber 3. Both are irradiated. As a result, not only the tip of the first optical fiber 2 but also the tip of the second optical fiber 3 is heated.
By heating both the distal end portion of the first optical fiber 2 and the distal end portion of the second optical fiber 3, in the step of moving the second optical fiber 3 in the first direction, which will be described later, the heated first optical fiber 2 is connected to the second optical fiber 2. When the contacts 3 come in contact with each other, the second optical fiber 3 takes away the heat of the first optical fiber 2 and the temperature of the first optical fiber 2 can be prevented from lowering.

With the intensity of the heating laser beam HL output from the heating light source 6 adjusted to a predetermined intensity, the heating laser beam HL is applied to the tip of the first optical fiber 2 and the tip of the second optical fiber 3 for several seconds.
Thereby, the tip portion of the first optical fiber 2 is heated to a temperature equal to or higher than the first temperature at which the first optical fiber 2 softens and lower than the second temperature at which the first optical fiber 2 crystallizes.
The first temperature is the softening point of the first optical fiber 2. The second temperature is a crystallization temperature of the first optical fiber 2. For example, when the first optical fiber 2 is ZBLAN glass, the first temperature is about 300 ° C. and the second temperature is about 370 ° C.

By setting the distal end portion of the first optical fiber 2 in the above-mentioned temperature range, it is possible to reliably avoid the crystallization of the first optical fiber 2 and the decrease in the transparency while reliably softening the first optical fiber. As a result, it is possible to reduce the fusion loss of the fusion portion while ensuring the fusion connection between the first optical fiber 2 and the second optical fiber 3.
It should be noted that the second optical fiber 3 does not soften at a temperature in the range from the first temperature to the second temperature.

By irradiating the heating laser beam HL, the distal end portions of the first optical fiber 2 and the second optical fiber 3 are heated, and the distal end portion of the first optical fiber 2 is maintained in the above-mentioned temperature range, and as shown in FIG. As described above, the second optical fiber 3 is moved in the first direction with respect to the first optical fiber 2, and the tip of the second optical fiber 3 is inserted into the first optical fiber 2.
That is, the second optical fiber 3 is moved in a direction approaching the first optical fiber 2, and the tip of the second optical fiber 3 is inserted into the first optical fiber 2.

At this time, while the first optical fiber 2 is moved in the second direction by the first moving unit 41, the second optical fiber 3 is moved in the first direction by the second moving unit 43, thereby connecting the second optical fiber 3 to the first optical fiber. 2 relative to the first direction.

In the above process, the insertion distance of the tip of the second optical fiber 3 into the first optical fiber 2 is preferably in the range of 20% to 40% of the second diameter d2, and is preferably 25% to 35% of the second diameter d2. A range distance is more preferable, and a distance of 32% of the second diameter d2 is most preferable.
For example, when the second diameter d2 is 125 μm, the insertion distance of the tip of the second optical fiber 3 into the first optical fiber 2 is preferably 25 μm to 50 μm, more preferably 30 μm to 45 μm, and more preferably 40 μm. Is most preferable.
Thus, the insertion depth of the tip of the second optical fiber 3 into the first optical fiber 2 is set to an appropriate depth, and these two optical fibers can be connected with a sufficiently high fusion strength.

The speed at which the second optical fiber 3 is moved in the first direction is, for example, about several tens of μm per second.

When the tip of the second optical fiber 3 is inserted into the first optical fiber 2, the second optical fiber 3 is pushed into the first optical fiber 2. Therefore, the most distal end of the first optical fiber 2 is shown in FIG. As a result, a slightly expanded state is obtained.
Further, as shown in FIG. 4C, in a state where the tip of the second optical fiber 3 is inserted into the first optical fiber 2, the boundary portion (connection portion) between the first optical fiber 2 and the second optical fiber 3 is: It has a step shape.

When the optical fiber is cooled in a state where the connecting portion between the first optical fiber 2 and the second optical fiber 3 has a stepped shape, the two optical fibers are connected while the connecting portion between the first optical fiber 2 and the second optical fiber 3 remains in the stepped shape. Will be done. When the optical fiber is bent, stress is concentrated on the connection portion having such a step shape, and the connection portion is easily damaged.

Therefore, in the present embodiment, the connecting portion between the first optical fiber 2 and the second optical fiber 3 is formed into a tapered shape in which the strength is stronger than the stepped shape. Specifically, the connection portion is formed in a tapered shape as described below.
First, as shown in FIG. 4C, irradiation with the heating laser light HL is performed for several seconds with the tip of the second optical fiber 3 inserted into the first optical fiber 2.
After that, the second optical fiber 3 is moved in the second direction while the irradiation of the heating laser beam HL is continued. That is, the second optical fiber 3 is moved in a direction away from the first optical fiber 2.

At this time, while the first optical fiber 2 is moved in the first direction by the first moving unit 41, the second optical fiber 3 is moved in the second direction by the second moving unit 43, whereby the second optical fiber 3 is moved to the first optical fiber. 2 relative to the second direction.

In the above step, the tip of the first optical fiber 2 on the side connected to the second optical fiber 3 is moved in the second direction while the second optical fiber 3 is attached to the tip of the second optical fiber 3 by surface tension. Move with. As a result, the tip portion of the first optical fiber 2 is elongated by the movement of the second optical fiber 3 in the second direction.
Due to this stretching, the diameter of the tip of the first optical fiber 2 on the side connected to the second optical fiber 3 becomes smaller than the first diameter d1 and becomes substantially the same as the second diameter d2. As a result, as shown in FIG. 4D, the connecting portion between the first optical fiber 2 and the second optical fiber 3 has a tapered shape.

As described above, in the present embodiment, the distal end portion of the first optical fiber 2 having a low melting point is heated. As a result, the viscosity of the first optical fiber 2 on the side to be stretched becomes low, so that when the second optical fiber 3 is moved away from the first optical fiber 2, the connection between the first optical fiber 2 and the second optical fiber 3 is made. It becomes easy to form a tapered shape in the portion.

In the above step, the moving distance of the second optical fiber 3 in the second direction is preferably in the range of 50% to 70% of the second diameter d2, and is preferably in the range of 55% to 65% of the second diameter d2. Is more preferable, and a distance of 64% of the second diameter d2 is most preferable.
For example, when the second diameter d2 is 125 μm, the moving distance of the second optical fiber 3 in the second direction is preferably 62 μm to 88 μm, more preferably 68 μm to 82 μm, and more preferably 80 μm. Is most preferred.
Accordingly, the taper angle θ at the tip of the first optical fiber 2 can be set to an appropriate angle at which stress is less likely to be concentrated on the connection portion between the first optical fiber 2 and the second optical fiber 3.

移動 Further, the moving speed of the second optical fiber 3 when forming the tapered shape is, for example, about several tens of μm per second.

By moving the second optical fiber 3 in the second direction, the distal end portion of the first optical fiber 2 is elongated to form a tapered shape at the connection portion, and then the heating laser beam HL is applied as shown in FIG. Is stopped, the first optical fiber 2 and the second optical fiber 3 are cooled, and the first optical fiber 2 and the second optical fiber 3 are connected.
In this embodiment, the first optical fiber 2 and the second optical fiber 3 are gradually cooled by gradually lowering the intensity of the heating laser beam HL over several tens of seconds to several minutes to finally reduce it to zero.

Generally, there is a large difference in the coefficient of thermal expansion between the first optical fiber 2 having a low melting point and the second optical fiber 3 having a high melting point. For example, the expansion coefficient of ZBLAN glass used for the first optical fiber 2 is 20 × 10 ⁻⁶ / K, whereas the expansion coefficient of quartz used for the second optical fiber 3 is 0.55 × 10 ⁻⁶ / K. There is a difference of nearly 40 times.
Therefore, when the first optical fiber 2 and the second optical fiber 3 are rapidly cooled, a stress is generated at a connection portion between the first optical fiber 2 and the second optical fiber 3 due to the difference in the coefficient of thermal expansion, and the connection is made during cooling. Parts may be damaged.

Therefore, as described above, by gradually cooling the first optical fiber 2 and the second optical fiber 3 for about several tens of seconds to several minutes, the two optical fibers are different due to the difference in expansion coefficient between the first optical fiber and the second optical fiber. Can be prevented from being damaged due to the occurrence of stress at the connection portion.

(4) Example In order to confirm whether the fusion splicing method of the optical fiber described above can achieve high-strength fusion splicing, two optical fibers were actually fusion spliced by the fusion method described above. 1 was produced. Hereinafter, an example of manufacturing the optical fiber 1 will be described.

In the following examples, an optical fiber made of ZBLAN glass was used as the first optical fiber 2 having a low melting point. On the other hand, an optical fiber made of quartz was used as the second optical fiber 3 having a high melting point.
The second diameter d2 of the second optical fiber 3 was 125 μm (core diameter: 100 μm, clad diameter: 125 μm).
On the other hand, the first diameter d1 of the first optical fiber 2 was set to two types: 240 μm (core diameter: 170 μm, clad diameter: 240 μm) and 330 μm (core diameter: 280 μm, clad diameter: 330 μm). That is, the first diameter d1 was 1.9 times and 2.6 times the second diameter d2.
Hereinafter, an example in which the first diameter d1 is 240 μm is referred to as “Example 1”, and an example in which the first diameter d1 is 330 μm is referred to as “Example 2”.

Further, in the present embodiment, in order to examine the strength of the fusion splicing, a tensile load is applied to the optical fiber 1 in a state where both ends of the optical fiber 1 after fusion are gripped, and the tensile load when the joint is broken is fused. It was measured as the strength of the connection. This tensile test can be performed using, for example, a fiber cleaver.

5 and 6 show optical microscope images of a connection portion of the optical fiber 1 manufactured in the present example. FIG. 5 is a diagram illustrating an optical microscope image of the optical fiber of the first embodiment. FIG. 6 is a diagram illustrating an optical microscope image of the optical fiber of the second embodiment.
FIGS. 5 and 6 show an optical microscope image ((A) of each drawing) of the outer periphery of the optical fiber 1 and a transmission optical microscope image ((B) of each drawing) of the optical fiber 1. 5 and 6, the first optical fiber 2 is labeled as "2" and the second optical fiber 3 is labeled as "3".

As shown in FIGS. 5 and 6, in each of the optical fibers 1 of the embodiments, the distal end of the first optical fiber 2 on the side connected to the second optical fiber 3 has a reduced diameter and is tapered. . No step at which the diameter changes suddenly at the connection between the two optical fibers is observed.
From the transmission optical microscope image, it can be seen that the tip of the second optical fiber 3 is inserted into the core of the first optical fiber 2 and the tip of the second optical fiber 3 is covered by the core of the first optical fiber 2 from three directions. I understand.

As a result of a tensile test of the optical fibers 1 of Examples 1 and 2, the connection portion of the optical fiber 1 of the present example can withstand a load of 200 gf to 250 gf (in terms of pressure, 1.6 MPa to 2 MPa). I understood that.
This indicates that the connection portion of the optical fiber 1 of the present embodiment has about 200 times the strength of the conventional one, as compared with the result of the proof test of the optical fiber in Patent Document 1 (the strength of the connection portion: 80 kPa). .
In addition, even when compared with the optical fiber of a comparative example described later, the strength of the connection portion was improved by a factor of two or more by forming a tapered shape in the connection portion, that is, by having the tapered portion 21 in the first optical fiber 2. .

Further, in order to investigate the fusion loss at the connection portion of the optical fiber 1 manufactured in the present embodiment and the optimum taper angle θ, the optical fiber 1 having the tapered portions 21 having various taper angles θ was manufactured. The fusion loss of the optical fiber 1 was measured.
The fusion loss is obtained by measuring the intensity of the inspection light IL on the side of the first optical fiber 2 with the light receiving device 14 by inputting the inspection light IL output from the inspection light source 12 from the second optical fiber 3 side, and calculating from the following equation. did.
In the following equation, “the transmitted light intensity of the second optical fiber 3 alone” is an actual measurement obtained by measuring the intensity of the inspection light IL incident from one of the second optical fibers 3 and output from the other by the light receiving device 14. It may be a value or a theoretical value calculated from the transmittance of the second optical fiber 3.
Fusion loss = 1− (intensity of inspection light IL measured by light receiving device 14 / intensity of transmitted light by second optical fiber 3 alone)

Hereinafter, the relationship between the taper angle θ of the tapered portion 21 and the fusion loss will be described with reference to FIGS. FIG. 7 is a diagram illustrating the relationship between the taper angle and the fusion loss of the optical fiber according to the first embodiment. FIG. 8 is a diagram illustrating a relationship between the taper angle and the fusion loss of the optical fiber according to the second embodiment.
7 and 8, “*” indicates that the fusion loss is extremely large, and the optical fiber 1 is broken at the connection portion (for example, at the time of fusion splicing of the optical fiber (at the time of heating / cooling). At the time of handling the optical fiber 1 after the fusion splicing was released.

(7) As shown in FIGS. 7 and 8, it can be seen that the fusion loss of the optical fiber 1 in which the connection portion was not broken is extremely low at 0.2 dB or less. That is, it can be seen that the fusion method according to the first embodiment described above is an excellent method that generates only a fusion loss that does not interfere with actual use.

Further, as shown in FIGS. 7 and 8, when the taper angle θ was out of the specific range, the connection portions of all the optical fibers 1 were broken. From this, it can be seen that if the taper angle θ of the tapered portion 21 is set to an appropriate angle, the strength of the connection portion can be made sufficient. Specifically, if the taper angle θ is set to 20 to 50 degrees, the strength of the connection portion can be improved.

Further, it can be seen that the optimum value of the taper angle θ for increasing the strength of the connection portion differs between the case of the first embodiment and the case of the second embodiment.
Specifically, in the case of Example 1 shown in FIG. 7, that is, when the first diameter d1 (240 μm) is about twice as large as the second diameter d2 (125 μm), the optimum taper angle θ is 22 degrees or less. The range was 30 degrees.
On the other hand, in the case of the second embodiment shown in FIG. 8, that is, when the first diameter d1 (330 μm) is about three times the second diameter d2 (125 μm), the optimum taper angle θ is 35 degrees to 50 degrees. Was in the range.

As described above, by setting the taper angle θ in an appropriate range according to the ratio between the first diameter d1 of the first optical fiber 2 and the second diameter d2 of the second optical fiber 3, the tapered portion 21 can be formed into the first optical fiber 2 It is possible to obtain an optimum shape in which stress is not easily concentrated on the connection portion between the first optical fiber 3 and the second optical fiber 3.

(5) Comparative Example Hereinafter, a comparative experiment was performed to confirm the effectiveness of the optical fiber 1 according to the first embodiment and the method of fusing the optical fiber.
First, as Comparative Example 1, fusion splicing of these two optical fibers was attempted with the diameter of the first optical fiber 2 and the diameter of the second optical fiber 3 being approximately the same (first diameter d1 = second diameter d2). The result is shown in FIG. FIG. 9 is a view showing a transmission optical microscope image of the optical fiber manufactured in Comparative Example 1.

As shown in FIG. 9, when the first diameter d1 and the second diameter d2 were substantially the same, the second optical fiber 3 was not inserted into the first optical fiber 2. In addition, the connection portion cannot be formed in a tapered shape.
In the optical fiber of Comparative Example 1, the connection portion was broken by a load smaller than 100 gf (pressure conversion: 0.8 MPa).

As described above, in order to insert the second optical fiber 3 into the first optical fiber 2 and improve the strength of the connection portion, the first diameter d1 is sufficiently larger than the second diameter d2, for example, the second diameter d2. It can be seen that it is preferably in the range from 1.5 times to 3 times of.

Next, as Comparative Example 2, in the above fusion method, the second optical fiber 3 was inserted in the second direction while the second optical fiber 3 was inserted into the first optical fiber 2 as shown in FIG. The first optical fiber 2 and the second optical fiber 3 were fusion-spliced without being moved.
The first optical fiber 2 and the second optical fiber 3 used were the same as those used in Example 1 above.

FIG. 10 shows an optical microscope image of an optical fiber manufactured without moving the second optical fiber 3 in the second direction from the state where the second optical fiber 3 is inserted into the first optical fiber 2. FIG. 10 is a diagram illustrating an optical microscope image of the optical fiber of Comparative Example 2.
As shown in FIG. 10, in the optical fiber of Comparative Example 2, the first optical fiber 2 and the second optical fiber 3 are fusion-spliced. Are shown).
The optical fiber having a step at the connection portion breaks at or near the step at the connection portion when it is taken out of the fusion device 100 after fusion, and its handling is difficult.

分かる Thus, in order to improve the strength of the connection portion between the first optical fiber 2 and the second optical fiber 3, it is understood that the connection portion between these two optical fibers is preferably tapered.

(6) Common Items of the Embodiment The first embodiment has the following configurations and functions in common.
The fusion method of the optical fiber includes a first optical fiber 2 having a first melting point (an example of a first optical fiber) and a second optical fiber 3 having a second melting point higher than the first melting point (an example of a second optical fiber). This is a fusion method. The fusing method includes the following steps.
A step of arranging a first optical fiber 2 having a first diameter d1 (an example of a first diameter) and a second optical fiber 3 having a second diameter d2 (an example of a second diameter) smaller than the first diameter d1. .
Heating at least the tip of the first optical fiber 2 to a temperature equal to or higher than a first temperature at which the first optical fiber 2 softens and lower than a second temperature at which the first optical fiber 2 crystallizes;
◎ a step of moving the second optical fiber 3 in the first direction to approach the first optical fiber 2 with the distal end of the first optical fiber being heated, and inserting the distal end of the second optical fiber 3 into the first optical fiber 2;
◎ After inserting the tip of the second optical fiber 3 into the first optical fiber 2, moving the second optical fiber 3 in the second direction opposite to the first direction.

In the optical fiber fusion method according to the first embodiment, after the first optical fiber 2 is softened and the tip of the second optical fiber 3 is inserted into the first optical fiber 2, the second optical fiber 3 is further connected to the first optical fiber 2. The tip of the second optical fiber 3 is moved in a second direction opposite to the first direction in which it is inserted. That is, the second optical fiber 3 is moved in a direction away from the first optical fiber 2.
As a result, when the second optical fiber 3 is moved in a direction away from the first optical fiber 2, the end of the first optical fiber 2 on the side connected to the second optical fiber 3 is elongated. Due to this stretching, the diameter of the distal end portion of the first optical fiber 2 decreases, and the connecting portion between the first optical fiber and the second optical fiber becomes tapered.

As described above, by forming a tapered shape at the connection portion between the first optical fiber 2 and the second optical fiber 3, the strength of the optical fiber 1 (an example of an optical fiber) manufactured by connecting these two optical fibers can be improved. For example, when the manufactured optical fiber 1 is bent, stress can hardly be concentrated on a connecting portion between the two optical fibers, so that the strength can be improved.

(4) By heating the tip of the first optical fiber 2 having a low melting point, when the second optical fiber 3 is moved in a direction away from the first optical fiber 2, it is easy to form a tapered shape at the connection portion.

2. Other Embodiments One embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment, and various modifications can be made without departing from the gist of the invention. In particular, a plurality of embodiments and modifications described in this specification can be arbitrarily combined as needed.
(A) The tip of the first optical fiber 2 and the tip of the second optical fiber 3 can be heated not only by the irradiation of the heating laser beam HL but also by another heating method. For example, discharge heating and heating by a heater are possible.

(B) The fusion splicing of the first optical fiber 2 and the second optical fiber 3 can be performed in a controlled atmosphere. For example, when a deliquescent glass such as ZBLAN glass is used as the first optical fiber 2, in order to prevent the crystallization of the ZBLAN glass from being promoted by the humidity (moisture) in the atmosphere, the first optical fiber 2 should be placed in a dry atmosphere. To perform fusion splicing.
In addition, fusion splicing can be performed in a controlled atmosphere such as an atmosphere with a small amount of oxygen (eg, a nitrogen atmosphere or an inert gas atmosphere) or a dry atmosphere, depending on the properties of the material used as the optical fiber.
Furthermore, in order to heat the two optical fibers efficiently and accurately, fusion splicing can be performed in a windless atmosphere.

INDUSTRIAL APPLICABILITY The present invention can be widely applied to a fusion method in which two optical fibers made of materials having different melting points are fused at a tip thereof, and an optical fiber in which two optical fibers made of materials having different melting points are fusion-spliced. .

Reference Signs List 1 optical fiber 2 first optical fiber 21 tapered portion θ taper angle d1 first diameter 3 second optical fiber d2 second diameter 100 fusion device 4 fiber moving device 41 first moving portion 41a first moving stage 41b first holding portion 43 second Moving unit 43a Second moving stage 43b Second holding unit 6 Heating light source HL Heating laser beam 61 First mirror 62 Light splitting member 63 Second mirror 64 First lens 65 Third mirror 66 Second lens 8 Shutter 10 Camera 12 Inspection light source 14 Light receiving device IL inspection light

Claims

A fusion method for fusing a first optical fiber having a first melting point and a second optical fiber having a second melting point higher than the first melting point,
Disposing the first optical fiber having a first diameter and the second optical fiber having a second diameter smaller than the first diameter;
Heating at least the tip of the first optical fiber to a temperature equal to or higher than a first temperature at which the first optical fiber softens and lower than a second temperature at which the first optical fiber crystallizes;
Moving the second optical fiber in a first direction closer to the first optical fiber while at least heating the distal end of the first optical fiber, and inserting the distal end of the second optical fiber into the first optical fiber;
Moving the second optical fiber in a second direction opposite to the first direction after inserting the tip of the second optical fiber into the first optical fiber;
A fusion method comprising:
The method according to claim 1, further comprising: gradually moving the first optical fiber and the second optical fiber after moving the second optical fiber in the second direction.
In the step of inserting the tip of the second optical fiber into the first optical fiber, an insertion distance of the tip of the second optical fiber into the first optical fiber is a distance in a range of 20% to 40% of the second diameter. The fusion method according to claim 1.
The method according to any one of claims 1 to 3, wherein in the step of moving the second optical fiber in the second direction, a moving distance of the second optical fiber is a distance in a range of 50% to 70% of the second diameter. The fusion method described.
The fusion method according to any one of claims 1 to 4, wherein in the step of disposing the first optical fiber and the second optical fiber, the distal end of the first optical fiber and the distal end of the second optical fiber are disposed close to each other. .
The method according to any one of claims 1 to 5, wherein the first diameter is in a range from 1.5 times to 3 times the second diameter.
The method according to any one of claims 1 to 6, wherein the first temperature is a softening point of the first optical fiber.
The fusion method according to any one of claims 1 to 7, wherein the second temperature is a crystallization temperature of the first optical fiber.
The method according to any one of claims 1 to 8, wherein the first optical fiber is a ZBLAN fiber.
The method according to any one of claims 1 to 9, wherein the second optical fiber is a quartz optical fiber.
A first optical fiber having a tapered portion whose diameter decreases from the first diameter toward the tip;
A second optical fiber having a second diameter smaller than the first diameter and connected to the first optical fiber with a tip inserted into the tip of the tapered portion;
An optical fiber.
The optical fiber according to claim 11, wherein an insertion depth of the tip of the second optical fiber into the tapered portion is in a range of 4% to 16% of the second diameter.
The optical fiber according to claim 11, wherein a taper angle formed between a side surface of the tapered portion and a length direction of the first optical fiber is in a range of 20 to 50 degrees.
The optical fiber according to any one of claims 11 to 13, wherein the first diameter is in a range from 1.5 times to 3 times the second diameter.
15. The method according to claim 14, wherein the first diameter is twice as large as the second diameter, and a taper angle between a side surface of the tapered portion and a length direction of the first optical fiber is in a range of 22 degrees to 30 degrees. An optical fiber according to claim 1.
15. The method according to claim 14, wherein the first diameter is three times the second diameter, and a taper angle formed by a side surface of the tapered portion and a length direction of the first optical fiber is in a range of 35 degrees to 50 degrees. An optical fiber according to claim 1.
The optical fiber according to any one of claims 11 to 16, wherein the first optical fiber is a ZBLAN fiber.
18. The optical fiber according to claim 11, wherein the second optical fiber is an optical fiber made of quartz.
A fusion device for fusing a first optical fiber having a first melting point and a second optical fiber having a second melting point higher than the first melting point,
A heating light source that heats a tip of the first optical fiber having a first diameter to a temperature equal to or higher than a first temperature at which the first optical fiber softens, and lower than a second temperature at which the first optical fiber crystallizes;
In a state where at least the tip of the first optical fiber is heated, the second optical fiber having a second diameter smaller than the first diameter is moved in a first direction closer to the first optical fiber, and the tip of the second optical fiber is moved. Is inserted into the first optical fiber, and after inserting the tip of the second optical fiber into the first optical fiber, a fiber moving device that moves the second optical fiber in a second direction opposite to the first direction,
A fusion device comprising: