JPH06347664A - Composite type optical fiber coupler and its production - Google Patents

Abstract

PURPOSE:To provide a composite type optical fiber coupler which has four output terminals capable of acting as a 2X2 optical fiber coupler and demultiplexing/multiplexing optical fiber coupler which have small temp. characteristics, are further intensified in the mechanical strength of the optical coupling parts and are formable into smaller size. CONSTITUTION:This composite type optical fiber coupler 1 is produced by simultaneously fusion stretching total four pieces of optical fibers f1 to f4 consisting of respectively two pieces of single mode optical fibers having two kinds of different propagation constants. In the optical coupling part 2 of the optical fiber coupler, four pieces of the optical fibers are arranged symmetrically to the center line. The optical fibers having the different propagation constants are disposed adjacent to each other. In addition, the respective optical fibers are formed by fusion stretching only with two pieces of the respectively adjacent optical fibers.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to an optical fiber coupler used for optical communication, optical measurement, etc., and particularly to two single mode optical fibers each having two different propagation constants. And a composite type optical fiber coupler having four output ends, which is formed by fusion-spreading a total of four optical fibers each of which is composed of 4 and which can operate as a 2 × 2 optical fiber coupler and a demultiplexing / multiplexing optical fiber coupler. The present invention relates to a manufacturing method thereof.

[0002]

2. Description of the Related Art Optical fiber couplers made of quartz single-mode optical fibers and having various functions, each having two input and two output terminals, have been conventionally known. For example, among optical fiber couplers, the branching ratio of optical output at a specific wavelength is 50: 5 at each optical output end.
“2 × 2 optical fiber coupler” set to 0, or the branching ratio of the optical output at substantially two specific wavelengths is 0:10.
The "demultiplexing / multiplexing optical fiber coupler" set to 0 is produced by fusion splicing two optical fibers having the same optical propagation constant. Usually, in the optical fiber coupler required for optical communication, the above specific wavelength is 1.3.
Laser light of 1 μm and 1.55 μm is used.

As described above, the 2 × 2 optical fiber coupler and the demultiplexing / multiplexing optical fiber coupler have different wavelength characteristics of the branching ratio, and conventionally, the 2 × 2 optical fiber couplers have different wavelength characteristics of the branching ratio.
When a composite of the optical fiber coupler and the demultiplexing / combining optical fiber coupler is required, the 2 × 2 optical fiber coupler and the demultiplexing / combining optical fiber coupler are individually prepared and individually fixed to the support plate, After that, a method of housing these two optical fiber couplers in the same package has been adopted.

[0004]

However, the composite type optical fiber coupler having two optical fiber couplers of the 2 × 2 optical fiber coupler and the demultiplexing / multiplexing optical fiber coupler having such a structure is manufactured. In addition to the problem of poor workability of (1), since the 2 × 2 optical fiber coupler and the demultiplexing / multiplexing optical fiber coupler are fixed to different support plates, the size of the package accommodating them is Grow bigger by myself. (2) Generally, the 2 × 2 optical fiber coupler and the demultiplexing / multiplexing optical fiber coupler have different optical coupling lengths, and the temperature characteristics, that is, the branching ratio due to the change of the optical coupling length with the change of the ambient temperature. Are different from each other, and it is difficult to make the temperature characteristics of both couplers the same when housed in the same package, and the temperature characteristics cannot be reduced. (3) Both the 2 × 2 optical fiber coupler and the demultiplexing / multiplexing optical fiber coupler have an optical coupling part formed by two optical fibers, and the mechanical strength of each optical fiber coupler is
It is not strengthened by compounding. I have a problem.

In other words, the conventional composite type optical fiber has a merit in that it is simply housed in a single package, and its handling is advantageous.

Therefore, an object of the present invention is to provide a 2 × 2 optical fiber coupler and a demultiplexing / multiplexing optical fiber coupler which have small temperature characteristics, further enhance the mechanical strength of the optical coupling portion, and can achieve miniaturization. An object of the present invention is to provide a composite optical fiber coupler having four output ends, which is operable.

[0007]

The above object is achieved by a composite type optical fiber coupler having four outputs according to the present invention. In summary, the present invention relates to an optical fiber coupler having an optical coupling section in which a total of four optical fibers each consisting of two single mode optical fibers having two different propagation constants are fused and stretched. The four optical fibers are symmetrically arranged, and the optical fibers having different propagation constants are adjacent to each other, and each optical fiber is fused and drawn only with two adjacent optical fibers. Is a composite optical fiber coupler.

In this composite optical fiber coupler, one optical fiber set having the same propagation constant operates as a 2 × 2 optical fiber coupler, and the other optical fiber set having the same propagation constant has two wavelengths. It operates as a demultiplexing / combining optical fiber coupler that can combine and demultiplex light.

The above-mentioned composite optical fiber coupler comprises: (a) preparing four single-mode optical fibers having the same core diameter and clad outer diameter; and (b) at least two of the four optical fibers. The optical fiber has a clad outer diameter reduced to the same diameter. (C) Next, the four optical fibers are symmetrically arranged, and optical fibers having different clad outer diameters are adjacent to each other. Each optical fiber is brought into contact with only two adjacent optical fibers, and the four optical fibers are collectively fused along the longitudinal direction of the respective optical fibers, and then fused and stretched. (D) At the time of fusion-stretching of each of the optical fibers, monitor light of a predetermined wavelength is incident on the input side of one of the optical fibers, and an optical output detection means is connected to the output side of each of the optical fibers. The above each light The manufacturing method is preferably characterized in that the output from the fiber is monitored, and (e) the collective fusion drawing is stopped when the output from each of the optical fibers reaches a desired value. To be done.

At this time, preferably, when the cladding outer diameters before fusion drawing of the single mode optical fibers having the two different cladding outer diameters are D ₁ and D ₂ , the cladding outer diameter ratio D ₂ / D ₁ is selected in the range of 0.24 or more and 0.85 or less, particularly preferably 0.75 or more and 0.81 or less.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A composite optical fiber coupler having four output ends and a method for manufacturing the same according to the present invention will be described below in more detail with reference to the drawings.

Referring to FIG. 1, in a composite optical fiber coupler 1 according to the present invention, four single mode optical fibers f _{1 to} f ₄ are collectively fused and extended to form an optical coupling portion 2. According to the present invention, the four single-mode optical fibers f _{1 to} f ₄ are single-mode optical fibers having two different propagation constants, and in this embodiment, the optical fibers f ₁ and f ₃ have the same propagation constant. And the optical fibers f ₂ and f ₄ have the same propagation constant. In the optical coupling section 2 of the composite optical fiber coupler 1 of the present invention, as shown in the cross section of FIG. 1B, the four optical fibers f _{1 to} f ₄ are arranged symmetrically, and It is characterized in that optical fibers having different propagation constants are adjacent to each other, and each optical fiber is fused and drawn only with two adjacent optical fibers.

With such a configuration, the composite optical fiber coupler 1 of the present invention is an optical fiber coupler that combines, for example, a 2 × 2 optical fiber coupler and a demultiplexing / combining optical fiber coupler. That is, in this embodiment, the optical fibers f ₁ and f _{3 form a} 2 × 2 optical fiber coupler, and the optical fiber f ₂
And f _{4 form a} demultiplexing / combining optical fiber coupler.

That is, in the optical coupling section 2, two single mode optical fibers each having two kinds of different propagation constants are symmetrically arranged, so that each two optical fibers having the same propagation constant are input / output terminals. Then, the optical branching ratio can be substantially 0: 100 at a certain wavelength. The other two optical fibers having different propagation constants have an optical branching ratio of substantially 0: 100 at different wavelengths. Therefore, when one of the two optical fiber groups, that is, the optical fibers f ₂ and f ₄ in this embodiment, are used as the optical input / output ends when properly selecting the amount of extension, the branching ratio is substantially equal to at least two wavelengths. At 0:10
It can be 0, and it can be a demultiplexing / combining optical fiber coupler. In addition, the other two optical fiber sets f ₁ , f ₃
When the optical input and output terminals are, the optical branching ratios can be made equal at a specific wavelength, and at that time, the optical output to the other _two optical fiber groups f ₂ and f ₄ becomes sufficiently small. Therefore, it is substantially a 2 × 2 coupler.

The composite optical fiber coupler 1 of the present invention
The characteristic construction of will be better understood with reference to the manufacturing method described below.

According to this embodiment, as shown in FIG. 2, first, the same single mode optical fiber having a core diameter of d and a cladding outer diameter of D is cut into an appropriate length to prepare four pieces. . For example, two optical fibers are the first and the third.
The optical fibers f ₁ and f ₃ are used as they are without reducing the cladding outer diameter. That is, the optical fibers f ₁ and f
Outer diameter of ₃ D _1, D ₃ is D ₁ = D ₃ = D. on the other hand,
The clad outer diameter D of the other two optical fibers, that is, the second and fourth optical fibers f ₂ and f ₄ is a predetermined diameter D ₂ (=
The diameter is reduced to D ₄ ), and a reduced diameter portion f ₂ ′ (f ₄ ′) is formed. When this diameter reducing operation is performed by etching, an etching solution such as hydrofluoric acid or a mixed solution of hydrofluoric acid and an etch pit inhibitor (for example, ammonium fluoride) is used, and the optical fibers f ₂ and f are added to the etching solution. It can be performed by bringing ₄ into contact. The etching conditions can be variously changed according to the desired diameter reduction amount,
Usually, etching temperature is 30 to 60 ° C., processing time is 5 to 30
Considered as minutes.

The optical fibers f ₂ and f ₄ can be reduced in diameter by etching as described above, but the invention is not limited to this. For example, mechanical polishing with abrasive grains can be employed. Furthermore, a method of previously reducing the outer diameter of the cladding by melt-drawing a part of the optical fiber can also be adopted. Further, if the cladding outer diameter D of the optical fiber is originally manufactured with the outer diameter D ₂ (= D ₄ ) after the diameter reduction, this can be used as it is and the above diameter reduction step can be omitted. You can

Further, the optical fibers f ₂ and f ₄ are connected to a predetermined system D
Not only is the diameter reduced to ₂ (= D ₄ ), but the optical fiber f
₁ and f ₃ are also reduced to a predetermined diameter D ₁ ′ (= D ₃ ′),
For example, by setting D ₁ ′> D ₂ , it is possible to have two types of optical fibers having different cladding outer diameters in the fusion-stretched portions, that is, two different propagation constants.

Next, each of the optical fibers f _{1 to} f ₄ is reduced in diameter with respect to the optical fibers f ₂ and f ₄ , f ₂ ′ and f.
_As shown in FIG. 3, ₄ ′ contacts only two adjacent optical fibers, that is, the first optical fiber f
With respect to ₁ , the second and fourth optical fibers f ₂ , f
₄ of reduced diameter portion f ₂ ', f _4' in contact only, the third optical fiber f ₃ without contact, also a third As for the optical fiber f _3, the second and fourth light fiber f _2, reduced diameter portion f ₂ of f ₄ ', f _4' in contact only with the further second and fourth reduced diameter portion f ₂ of the optical fiber f _2, f ₄ ', f _4'
Also, the respective optical fibers are arranged parallel to the longitudinal direction so that they do not contact each other, that is, the optical fiber f.
A line L ₁ connecting the core centers of ₁ and f _{3 and} an optical fiber f ₂
And f ₄ are arranged symmetrically with respect to a line L ₂ connecting the center of the cores and attached to the fusion drawing stand. Then, first, each optical fiber f _{1 to} f ₄ is maintained in the above-described arrangement state, for example, by using a flame torch, 1000 to 2
Heat to 000 ° C. and fuse over a predetermined length. Then, subsequently, the fusion-bonded portion is collectively subjected to fusion-bonding and stretching treatment. Usually, the fusion bonding and the subsequent fusion stretching process are performed in the same step.

The fusion-stretching treatment can be carried out according to a usual method, for example, flame burner, heater laser,
Using a suitable heating device such as a small electric furnace, generally 1300
The fusion-stretching stage can be carried out by pulling the optical fiber axially on both sides in the axial direction at a rate of, for example, 0.05 to 10 mm / minute while heating at a temperature of ˜2000 ° C., for example, via a rack-pinion mechanism.

According to this embodiment, the optical fibers f _{1 to} f _{4 are}
At the time of fusion splicing and drawing, a monitor light of a predetermined wavelength, for example, 1.55 μm is incident from the LD light source on the input side of any one of the optical fibers, in this embodiment the first optical fiber f ₁ , ₁ to the output side of ~f ₄ connected optical output detecting means, for example, a light receiving element such as a photodiode, for monitoring the output from the optical fibers f ₁ ~f _4. And
When the output from each of the optical fibers f _{1 to} f ₄ reaches a desired value, for example, the outputs from the optical fibers f ₁ and f ₃ are stopped at the first stretched amount. Alternatively, the second
A monitor light of a predetermined wavelength, for example 1.31 μm, is incident on the input side of the optical fiber f ₂ from the LD light source, and the output side of each of the optical fibers f _{1 to} f ₄ has a light output detecting means, such as a photodiode. connect Do light receiving elements, to monitor the output from the optical fibers f ₁ ~f _4. Then, when the output from each of the optical fibers f _{1 to} f ₄ reaches a desired value, for example, the output from the optical fiber f ₄ is stopped at the first stretched amount.

According to the present invention, as described above, the composite optical fiber coupler 1 having four input ends and four output ends as shown in FIG. It can be manufactured.

In the composite optical fiber coupler 1 of the present invention as described above, as will be understood from the above, each of the optical fibers f _{1 to} f of the fusion extending portion, that is, the fusion extending.
It is extremely important to perform fusion-bonding stretching while maintaining ₄ in a specific positional relationship. At the same time, the relationship of the optical fibers f ₁ ~f before fusing stretching ₄ cladding diameter D ₁ to D _4, it is also important.

According to the results of the research conducted by the present inventors, the composite optical fiber coupler 1 of the present invention has the first, second, third and fourth optical fibers f _{1 to} f ₄ constituting the composite optical fiber coupler 1. of the fused and extended previous cladding diameter D ₁ to D _4, at least two in an optical fiber f ₁ ~f 4 ₄ present before fused and extended, for example,
It is preferable to reduce the cladding outer diameter of the optical fibers f _{2 to} f _{4 and set} the cladding outer diameter ratio D ₂ (= D ₄ ) / D ₁ (= D ₃ ) to 0.24 or more and 0.85 or less. It can be manufactured. In particular, the cladding outer diameter ratio D ₂ (= D ₄ ) / D ₁ (= D
It was found that when ₃ ) is 0.75 or more and 0.81 or less, it can be manufactured very suitably.

Clad outer diameter ratio D ₂ (= D ₄ ) / D ₁ (=
If D ₃ ) exceeds 0.85, the difference in the propagation constants is not sufficiently large, so that when the optical fiber f ₁ (or the optical fiber f ₃ ) is used as the input end, the output ends of the optical fibers f ₂ and f ₄ are used. The branching ratio of is not sufficiently small, and is substantially 2 ×
It is not preferable because it does not work as a two-fiber coupler. In addition, the cladding outer diameter ratio D ₂ (= D ₄ ) / D ₁ (= D ₃ ) is 0.24.
If it is smaller, the difference in propagation constant is sufficiently large, but it becomes difficult to perform fusion splicing while maintaining the respective optical fibers f _{1 to} f ₄ in the specific positional relationship shown in FIG. 4, that is, when fusion fusion stretching proceeds. 3, the voids surrounded by the optical fibers f _{1 to} f ₄ are often filled with the melted cladding, which is not preferable.

On the other hand, the cladding outer diameter ratio D ₂ (= D ₄ ) / D
₁ (= D ₃ ) is 0.85 or less, more preferably 0.81
By setting the following, the difference in propagation constant can be made sufficiently large,
For example, when the optical fiber f ₁ (or the optical fiber f ₃ ) is used as the input end, the branching ratio of the output ends of the optical fibers f ₂ and f ₄ can be made sufficiently small, and a substantially 2 × 2 optical fiber coupler is used. Will be easier. Also, the clad outer diameter ratio D ₂
When (= D ₄ ) / D ₁ (= D ₃ ) is 0.24 or more, more preferably 0.75 or more, while maintaining the respective optical fibers f _{1 to} f ₄ in the specific positional relationship shown in FIG. The fusion-stretching becomes extremely easy, and even if the fusion-stretching proceeds, the voids surrounded by the optical fibers f _{1 to} f ₄ in FIG. 3 are not filled with the melted cladding.

As described above, the cladding outer diameters D _{1 to} D ₄ of the first, second, third and fourth optical fibers f _{1 to} f ₄ are selected and manufactured so as to have the above-mentioned extremely suitable relationship. Was done,
The wavelength dependence of the branching ratio in the composite optical fiber coupler of the present invention is shown in FIGS. FIG. 4 shows the first or third
FIG. 5 shows the branching ratios of the output ends of the optical fibers f _{1 to} f ₄ when input to the input ends of the optical fibers f ₁ and f ₃ .
The branching ratios at the output ends of the respective optical fibers f _{1 to} f ₄ when input to the input ends of the second or fourth optical fibers f ₂ and f ₄ are shown. From FIG. 4 and FIG. 5, the composite optical fiber coupler 1 of the present invention has the first and third optical fibers f ₁ and f ₃ operating as a 2 × 2 optical fiber coupler at a wavelength of 1.55 μm. Further, the second and fourth optical fibers f ₂ and f ₄ operate as a demultiplexing / combining optical fiber coupler capable of multiplexing and demultiplexing the wavelength of 1.31 μm and the wavelength of 1.55 μm. I understand.

In the composite type optical fiber coupler 1 of the present invention, since the optical coupling portion 2 is constructed by extending four single mode optical fibers at a time, the mechanical strength is increased. Further, the change amount of the optical coupling length is equal to and small with respect to the change of the ambient temperature, and as a result, the temperature characteristic is reduced when fixing to the support plate.

Further, the present invention will be described in detail with reference to Examples.

Example 1 In accordance with the present invention, a composite optical fiber coupler was manufactured by the following procedure.

As the single mode optical fibers f _{1 to} f ₄ , the core diameter (d) is 10 μm and the cladding outer diameter (D) is 1.
A 25 μm single mode optical fiber was used. Each optical fiber was cut to an appropriate length, and then the coating coating on the predetermined fiber length portion of each optical fiber was removed. The clad portion exposed by this was washed and cleaned to remove residues such as UV coating.

Then, the two optical fibers, that is, the first and third optical fibers f ₁ and f ₃ are used as they are, that is, D ₁ = D ₃ = 125 μm, and the other two optical fibers, That is, the second and fourth optical fibers f ₂ and f ₄ are
Etching was performed using hydrofluoric acid or a mixed solution of hydrofluoric acid and an etch pit inhibitor at an etching temperature of 30 ° C. for a processing time of 10 minutes to reduce the cladding diameters (D ₂ , D ₄ ) to 96 μm.

The optical fibers f _{1 to} f ₄ thus obtained
In particular, regarding the optical fibers f ₂ and f ₄ , the reduced diameter portions f ₂ ′ and f ₄ ′ are arranged along the longitudinal direction, and as shown in FIG. 3, each optical fiber has two adjacent optical fibers. Each optical fiber f ₁ is made to contact only with the fiber.
To f ₄ are the first optical fiber f ₁ and the third optical fiber f
_The line connecting the core centers of ₃ and the line connecting the core centers of the second optical fiber f ₂ and the fourth optical fiber f ₄ are symmetrically arranged. While maintaining the state, a flame torch was used to heat to 1200 ° C. for fusion. The fusion length was 15 mm. Then, the fused portion 15 mm is heated to 1500 to 16 using a flame torch.
While being heated to 00 ° C., the film was stretched at a rate of 1.6 mm / min to collectively perform the fusion drawing process.

[0034] when fused and extended from LD light source to a first input side of the optical fiber f _1, incident monitor light 1.55 .mu.m, the light output of the output side of the optical fibers f ₁ ~f ₄ to photodiode Was detected and the optical output of each optical fiber was monitored, and the fiber was collectively stretched. Then, the fusion splicing of the optical fibers was stopped at the point where the optical output values of the first optical fiber f ₁ and the third optical fiber f ₃ first became equal. The optimum stretch amount was 13.0 mm.

The composite optical fiber coupler manufactured in this manner was fixed to one and the same support plate with resin and housed in one package. In the conventional method, a 2 × 2 optical fiber coupler and a demultiplexing / multiplexing optical fiber coupler are individually manufactured and individually fixed to a support plate, and then these two optical fiber couplers are housed in the same package. In comparison with the produced composite optical fiber coupler, for example, in the case of housing in a cylindrical package, the diameter of the package has been reduced from 50 mm of the conventional package to 38 mm. Furthermore,
The time required to manufacture one composite optical fiber coupler could be reduced to 2/3 of the conventional one.

More importantly, according to the present invention, in the composite optical fiber coupler of the present invention, a single mode optical fiber having four optical coupling portions is collectively fused and drawn to form an optical coupling portion. The temperature characteristics of the 2 × 2 optical fiber coupler and the demultiplexing / multiplexing optical fiber coupler are almost the same, and since the optical coupling portion is formed by four optical fibers, the mechanical strength of the optical fiber coupler itself is increased. I was able to.

When the wavelength dependence of the branching ratio of the thus obtained composite optical fiber coupler of the present invention was investigated, the composite optical fiber coupler 1 of the present invention inputs the _first optical fiber f ₁ . When the wavelength dependence of the branching ratio selected at the end is measured, as shown in an example in FIG. 4, when the light is input from the first optical fiber f ₁ or the third optical fiber f ₃ , Of the optical fibers f ₂ and f ₄ of the first optical fiber f ₁ are substantially small.
, And the third optical fiber f ₃ can be regarded as an optical fiber coupler having two terminals at the input and output ends.
Of the first and third optical fibers f ₁ and f ₃ at the wavelength
It operates as a two-optical fiber coupler, and when the second optical fiber f ₂ is selected as the input end and the wavelength dependence of the branching ratio is measured, the second optical fiber f 2 is shown as an example in FIG.
When light is input from the _second or fourth optical fiber f ₄ , the first
And the branching ratios of the output ends of the third optical fibers f ₁ and f ₃ are sufficiently small, and the input and output ends of the second optical fiber f ₂ and the fourth optical fiber f ₄ are 2 each. Considered as the optical fiber coupler of the terminal, the second and fourth optical fibers f ₂ and f ₄ have a wavelength of 1.31 μm and a wavelength of 1.55 μm.
It operated as a demultiplexing / combining optical fiber coupler capable of multiplexing and demultiplexing.

In this embodiment, the insertion loss at the wavelength of 1.55 μm for the 2 × 2 optical fiber coupler is
The maximum is 3.59 dB, and the excess loss for the demultiplexing / multiplexing optical fiber coupler is 0.21 for a wavelength of 1.31 μm.
Within dB and 0.21d for wavelength 1.55 μm
It was within B. Further, regarding the temperature stability, the maximum insertion loss change amount in the temperature range of −20 ° C. to 70 ° C. is 0.20 dB.

[0039]

As described above, the composite optical fiber coupler according to the present invention is an optical coupling in which two single mode optical fibers each having two different propagation constants, a total of four fibers are fused and drawn at one time. In this optical coupling part, each of the four optical fibers is symmetrically arranged, and the optical fibers having different propagation constants are adjacent to each other. Each optical fiber is two adjacent optical fibers. Since it is constructed by fusion splicing and drawing only two optical fibers with the same propagation constant as the optical input and output ends, it operates as a 2 × 2 optical fiber coupler with the same branching ratio at the optical output end. However, when two other optical fibers having the same propagation constant are used as optical input / output ends, they can operate as a demultiplexing / combining optical fiber coupler. Further, the composite optical fiber coupler according to the present invention is significantly larger in size than the conventional composite optical fiber coupler in which the 2 × 2 optical fiber coupler and the demultiplexing / multiplexing optical fiber coupler are housed in the same package. In addition, the structure is such that the optical coupling portion is formed by collectively fusion-stretching four single-mode optical fibers at the same time, which has the advantage that the temperature characteristics are substantially the same. Further, since the optical coupling portion is formed by four optical fibers, there is also an advantage that the mechanical strength of the coupler is large.

[Brief description of drawings]

1 (A) is a diagram showing a schematic configuration of a composite optical fiber coupler according to the present invention, and FIG. 1 (B) is FIG.
It is an expanded sectional view of the optical coupling part taken along the line AA of (A).

FIG. 2 is a perspective view of each optical fiber used when manufacturing the composite optical fiber coupler according to the present invention.

FIG. 3 is an enlarged cross-sectional view of the optical coupling portion before collective fusion drawing.

FIG. 4 is a diagram showing the branching ratio at the output end of each optical fiber when inputting to the first or third optical fiber in the composite optical fiber coupler according to the present invention.

FIG. 5 is a diagram showing a branching ratio at the output end of each optical fiber when inputting to the second or fourth optical fiber in the composite optical fiber coupler according to the present invention.

[Explanation of symbols]

 1 Composite type optical fiber coupler 2 Optical coupling section

Claims (7)

[Claims] 1. An optical fiber coupler having an optical coupling section in which a total of four optical fibers each consisting of two single-mode optical fibers having two different propagation constants are fused and stretched together. The four fibers are symmetrically arranged, and the optical fibers having different propagation constants are adjacent to each other, and each optical fiber is fused and drawn only with two adjacent optical fibers. Type optical fiber coupler. 2. The composite optical fiber coupler according to claim 1, wherein the single mode optical fibers having two kinds of different propagation constants have different cladding outer diameters in the fusion spliced portions. 3. A clad outer diameter ratio D ₂ / D ₁ of 0.75, where D ₁ and D ₂ are the clad outer diameters of the single mode optical fibers having different propagation constants before fusion drawing. 3. The composite optical fiber coupler according to claim 2, wherein the composite optical fiber coupler is selected so as to be 0.81 or less. 4. One optical fiber set having the same propagation constant operates as a 2 × 2 optical fiber coupler, and the other optical fiber set having the same propagation constant combines the two wavelengths of light, 4. The composite optical fiber coupler according to claim 1, which operates as a demultiplexing / combining optical fiber coupler capable of demultiplexing. 5. (a) 4 having the same core diameter and clad outer diameter
A book of single-mode optical fibers, (b)
At least two of the four optical fibers have a cladding outer diameter reduced to the same diameter, (c)
Then, the four optical fibers are arranged symmetrically, and optical fibers having different cladding outer diameters are adjacent to each other, and each optical fiber is in contact with only two adjacent optical fibers, 4 optical fibers are fused together at a time along each of the optical fibers parallel to the longitudinal direction, and then fused and stretched; and (d) when the optical fiber is fused and stretched, one of the optical fibers Monitor light of a predetermined wavelength is incident on the input side of the fiber, and optical output detection means is connected to the output side of each optical fiber to monitor the output from each optical fiber; and (e) each of the above A method for manufacturing a composite type optical fiber coupler, characterized in that when the output from the optical fiber reaches a desired value, the collective fusion drawing is stopped. 6. The cladding outer diameter before fusing stretching a single-mode optical fiber having a cladding diameter of different the two when the D _1, D _2, cladding outer diameter ratio D ₂ / D ₁
Is selected to be 0.75 or more and 0.81 or less. 6. The method for manufacturing a composite optical fiber coupler according to claim 5, wherein 7. The method for manufacturing a composite optical fiber coupler according to claim 5, wherein the outer diameter of the clad of the two optical fibers is reduced by etching.