JP2001154058A - Single/multi-modle optical fiber coupler and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a single/multmode optical fiber coupler that has a prescribed branching ratio at wavelengths in both a single-and multi mode areas, and also to provide its manufacturing method. SOLUTION: In two single-mode fibers having the coating removed, their cladding diameters are preliminarily formed arrow at 40μm or less and then, this narrow diameter part is heated, drawn and welded, with a part of this welded part coupled optically by a high-intensity beam. As a result, since heat capacity is small in the part to be heated, the heating and the drawing are performed smoothly. enabling the single/multi mode optical fiber coupler to be easily obtained which has a prescribed branching ratio at both single-and multmode regions.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber coupler formed by fusing and stretching a single mode fiber and a method of manufacturing the same, and more particularly, to both sides of a cutoff wavelength of a single mode fiber used. That is, a single-multimode optical fiber coupler having a predetermined branching ratio in both a single-mode region for transmitting an optical signal having a wavelength longer than the cutoff wavelength and a multimode region for transmitting an optical signal having a wavelength shorter than the cutoff wavelength And a method of manufacturing the same.

[0002]

2. Description of the Related Art One of the transmission characteristics of an optical fiber is a so-called cut-off wavelength as a numerical value representing a single mode condition, that is, a condition in which a higher-order mode is cut off. If this cutoff wavelength is λ _C ,

[0003]

(Equation 1)

Where, v _C : cut-off v value Δ: relative refractive index difference between core and clad ≒ (n ₁ −n ₂ ) / n
₁ n ₁ : Refractive index of core n ₂ : Refractive index of clad d: Core diameter In case of single mode fiber,

[0005]

(Equation 2)

[0006] According to equation (2), the relative refractive index difference Δ
Is plotted, the relationship between the cutoff wavelength λ _C and the core diameter d is 0.4%. Therefore, on the straight line L, a region having a longer wavelength is a single mode region, and a shorter region is a multimode region.

[0007] Since a single mode fiber is usually used for transmitting an optical signal in the single mode region, a fiber coupler used in the transmission line has an optical signal having a wavelength longer than the cutoff wavelength λ _C. There was nothing but branching and combining. FIG. 5 is an explanatory diagram of a main part of a conventional single mode fiber coupler. 5 (a1) and 5 (a2) are explanatory views showing a state in which two single mode fibers 40a and 50a whose coatings have been removed are arranged side by side. FIG. 5 (a1) is a side view and FIG. a2)
Is a YY sectional view thereof. The single-mode fibers 40a and 50a from which the coating has been removed are respectively cores 41a and
It has claddings 42a and 52a on 51a. The single-mode fiber coupler 30 is obtained by heating, stretching and fusing. FIG.
1) is a side view thereof, and FIG. 2 (b2) is a YY sectional view thereof.

FIG. 3 is an explanatory view of a general method of forming a fused portion of a fiber type coupler. The coating of the central portions of the single mode fibers 40 and 50 having a predetermined length is removed, and the central portions 40a and 50 of the single mode fibers from which the coating has been removed.
The both sides of “a” are set on the stretching tools 51, 51. A semiconductor laser (L) is connected to the input end of one single mode fiber 40.
D) and the like, the output terminals of the single mode fibers 40 and 50 are connected to power meters 56 and 57 via light receiving units 54 and 55 such as photodiodes (PD), and the output terminals thereof are operation units. The operation unit 58 is connected to the display unit 59. In this state, while being heated by the gas burner B, the stretching tools 51,
By, fused portion M ₃₀ shown in FIG. 5 (b1) is formed. Although the single mode fibers 40a and 50a become thinner as a whole, the contact portions between them are deformed m, and
b.

From the light source 52 to the single mode fiber 40
The incident optical power P _0, the measured values P _1, P ₂ from the computing unit 58 of the power meter 56 and 57, the branching ratio β is calculated and displayed on the display unit 59. Here, optical coupling C = (P ₂ / P ₀ ) × 100% (3) Branching ratio β = [P ₂ / (P ₁ + P ₂ )] × 100% (4) Also, if excess loss is P _L , P _L = P ₀ − (P ₁ + P ₂ ) (5)

In the single mode region, it is known that the optical coupling C and the branching ratio β become smooth square cosine waves with respect to the wavelength, and the same applies to the stretching amount by the stretching E. Accordingly, the stretching E is performed while monitoring the branching ratio β displayed on the display unit 59, and when the predetermined branching ratio β is obtained, the heating and the stretching are stopped to obtain the desired single mode fiber coupler 30.

[0011]

However, the single-mode fiber coupler 30 has two or more propagating modes in the multi-mode region, and the optical coupling C differs depending on the wavelength of each mode. There was a problem. Assuming that the mode number is i, mode numbers 1 to i
The coupling amount C of the optical signal having i modes of

[0012]

(Equation 3)

Here, C _i : i-mode optical coupling P _i : i-mode shared light quantity ΣP _i = 1 when the total light quantity is 1. Here, the shared light amount _Pi is represented by the angle of incidence of light on the fiber and the state of the fiber (for example, the presence or absence and degree of bending)
And the optical coupling C _i of the _i- mode becomes larger as the mode number is higher. Therefore, the sum C of the optical couplings increases as the shared light amount in the higher-order mode increases, so that stable transmission cannot be performed in a state where the shared light amount _Pi is easily changed.

FIG. 6 is a diagram showing the subordinates shown in FIGS.
The conventional single mode optical fiber coupler 30 (wavelength 0.6
2 with a core diameter of 4 μm and a cladding diameter of 125 μm for 33 μm
This optical fiber was heated and stretched to form a fused part.
FIG. 6A is an explanatory diagram of the wavelength-branch ratio characteristic of FIG.
Explanatory drawing of the use state of the single mode optical fiber coupler,
FIG. 6B is a diagram of wavelength-branch ratio characteristics. Fig. 6 (a) Odor
The single mode fiber coupler 30 has an input port
I₁, I_TwoAnd output port O₁, O_TwoIs provided,
Light P incident from a light source (not shown)₀Is single mode
Optically coupled within the fiber coupler 30 to form a single mode fiber
Output port O₁, O
_TwoFrom each light P₁, P_TwoIs emitted. In FIG. 6 (b)
Curve S₁, S_TwoIs the wavelength of light and the optical power meter (not shown).
Outgoing light P measured by₁, P_TwoCalculated from
Branch ratio β₁, Β_TwoWavelength-branch ratio characteristic curve
With a line, here, a solid line S₁Is the branching ratio β₁= [P₁/ (P₁
+ P _Two)] × 100% wavelength characteristic, dotted line S_TwoIs the branch ratio
β_Two= [P_Two/ (P₁+ P_Two)] X 100% vs. wavelength
Sex.

The curves S ₁ and S ₂ are smooth raised cosine curves at wavelengths longer than the cut-off wavelength λ _C (single mode region), but at wavelengths less than the cut-off wavelength λ _C (multi-mode region). , And changes sharply with a change in wavelength. In the single mode region, it is known that the curve is smooth and that the phase of the raised cosine wave changes when the extension amount is changed. Therefore, a desired branching ratio corresponding to the extension amount can be easily obtained. However, in the multi-mode region, even if the slope of the curve is large and the wavelength and the amount of stretching are slightly different, the branching ratio β fluctuates, and if the angle of incidence of the light on the fiber and the state of the fiber are different, the fluctuation also fluctuates. Also 50 ± 30%
There was a problem that extends to the range.

[0016] And also, since the cladding layer interposed between the core is thick, excess loss P _L is disadvantageously large.

The present invention solves the above problems,
A single / multi-mode optical fiber coupler having a predetermined branching ratio at both a wavelength longer than the cut-off wavelength of a single mode fiber to be used (single-mode region) and a wavelength shorter than the cut-off wavelength (multi-mode region), and a method of manufacturing the same It is intended to provide a single-mode device with a small change in the branching ratio and a small excess loss in the multi-mode region depending on the incident angle of light and the condition of the fiber.
It is an object to provide a multimode optical fiber coupler and a method for manufacturing the same.

[0018]

In order to solve the above problems,
The optical fiber coupler according to claim 1, wherein the single-mode fiber has a cladding diameter of two single-mode fibers from which a coating is removed and heated and drawn to form a fused portion. Is formed by heating / stretching and fusing the small-diameter portion formed to be 40 μm or less, and at least a part of the fused portion is intensely optically coupled, and has a predetermined branching ratio in both the multi-mode region and the single-mode region. It is characterized by having. This allows
Optical signals in both the single mode region and the multimode region can be transmitted with a stable branching ratio, and the excess loss is small.

According to a second aspect of the present invention, there is provided a method of manufacturing an optical fiber coupler, comprising heating and stretching two single mode fibers from which a coating has been removed to form a fused portion. The two single-mode fibers from which the coating has been removed have at least a cladding diameter of at least a portion to be heated and stretched formed into a small diameter of 40 μm or less, and the thin portion is heated and stretched and fused.
It is characterized in that at least a part of the fused portion is strongly light-coupled. As a result, since the clad diameter is reduced to 40 μm or less, and then heated and stretched to form a fused portion, the heat capacity of the portion to be heated is small, and at least a part of the portion can be easily subjected to strong optical coupling. Can be. Accordingly, a single / multimode optical fiber coupler that can transmit optical signals of both wavelengths in the single mode region and the multimode region at a stable branching ratio and has a small excess loss can be easily obtained. When the diameter of the heated part exceeds 40 μm,
Since the heat capacity is large, the heating temperature must be increased or the heating time must be lengthened, so that the stretching is not performed smoothly.

According to a third aspect of the present invention, there is provided a method for manufacturing a single-mode fiber coupler, comprising the steps of:
The cladding diameter of the stretched portion is characterized in that the cladding diameter of the single mode fiber from which the coating has been removed is reduced in advance by etching. Thereby, the cladding diameter can be easily formed to a small diameter of 40 μm or less.

According to a fourth aspect of the present invention, there is provided a method of manufacturing an optical fiber coupler according to the second or third aspect, wherein one of the two single mode fibers is a single mode fiber. The light of the wavelength in the single mode region is made incident on the input end of the single-mode fiber, and the light of the wavelength in the multimode region is made incident on the input end of the other single-mode fiber. At the other output end, only light having a wavelength in the multi-mode region is separated and extracted by a filter, and each optical coupling is calculated and displayed from each of the input light and each output light, and the optical coupling is monitored. While heating and stretching, the optical coupling in the multi-mode region was stabilized at about 50%, and the optical coupling in the single-mode region became the desired value. Wherein the stop heating and stretching. Thus, a fused portion of a single / multimode optical fiber coupler having a stable branching ratio in a multimode region and a desired branching ratio in a single mode region due to strong optical coupling is easily formed.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory view of an embodiment of a single / multi-mode optical fiber coupler and a method of manufacturing the same according to the present invention, and FIGS. 2 and 3 are explanatory views of a method of forming a fused portion. FIG. 1 (a1) is a side view of a portion 10a of a single-mode fiber 10 constituting a single / multi-mode optical fiber coupler 1 from which a coating is removed, and FIG. 1 (a2) is a sectional view taken along the line YY. The single mode fiber 10a has a clad 1 on a core 11a.
2a, usually having a core diameter of 4 to 10 μm,
The cladding diameter is 125 μm, and the longer the wavelength, the larger the core diameter of the fiber suitable for the propagation.

FIG. 1 (b1) is a side view of a single mode fiber 10b whose central portion is etched by hydrofluoric acid (hydrofluoric acid) and reduced in diameter, and FIG. 1 (b2) is a sectional view taken along the line YY. FIG. The diameter of the clad 11b is the core 12
The diameter is reduced to 10 times or less of the diameter of b. Similarly, a single mode fiber 20 is prepared, and the central portion of the portion where the coating is removed is etched to reduce the clad diameter to 40 μm or less as in the case of the single mode fiber 10b.

FIG. 1 (c1) is a side view of a state in which single mode fibers having reduced claddings are juxtaposed and the reduced diameter portions are brought into contact with each other, and FIG.
It is -Y sectional drawing. 10c and 20c are two single mode fibers in that state, and 11c and 21c are
Each core, 12c, 22c indicates a respective cladding.

FIG. 1 (d1) is a small-diameter portion in a state in which the juxtaposed, heated and stretched in the manner described below, fused allowed by single multimode optical fiber coupler to form a fused portion M ₁ 1 (D2) is a sectional view taken along line YY of FIG. Heating and stretching the fused portion M _1, single-mode optical fiber 10d, 20d of the core 11d, 21d
State in which no cladding is present between, i.e., performed to a state in which the strong light coupling of, the fused portion M _1, the core 11d, 2
1d integrally forms a core 2 and a clad 12d,
22d also forms the clad 3 integrally.

Next, with reference to FIG. 2, will be described in detail a method of forming the fused portion M _1. The same parts as those of the general method for forming the fused portion of the single mode optical fiber coupler 30 described with reference to FIG. The coating of the central portions of the single-mode fibers 10 and 20 having a predetermined length is removed, and the single-mode fibers 10c and 20c whose claddings at the central portions are reduced in diameter by etching are set on the stretching tools 51 and 51. Both single mode fibers 10, 20
Light sources 52 and 53 such as semiconductor lasers are connected to the input terminals of the single mode fibers 10 and 20, and the output terminals of the single mode fibers 10 and 20 are connected to light receiving units 54 and 55 such as photodiodes via a low pass filter 60 and a high pass filter 61. The output terminals of the light receiving units 54 and 55 are connected to power meters 56 and 57, and their outputs are input to the calculation unit 62. The operation unit 62, the output optical power P _A0, P _B0 electric signal of been sources 52 and 53 are input, the output optical power P _A0, P
_B0 and received optical power P _B, the optical coupling C _B from P _A, C _A is calculated and displayed on the display unit 63 and 64.

For example, the light source 52 has a wavelength in the multi-mode region, the light source 53 has a wavelength in the single-mode region, the low-pass filter 60 emits light having a wavelength less than the cut-off wavelength, and the high-pass filter 61 emits light having a wavelength longer than the cut-off wavelength. Of light. In this state, by stretching E by stretching tool 51 while heated by the gas burner B, and form a fused portion M ₁ shown in FIG. 1 (d1). At this time, the single mode fiber 10
The optical power P _A0 in the single mode region is incident on the
From the light source 53, the optical power P _B0 having a wavelength in the multi-mode region enters the single mode fiber 20. Since the measured value P _B of the power meter 56 has passed through the low-pass filter 60, the single-mode fiber 20
Out of the optical power P _B0 incident on the single mode fiber 10. Therefore, the optical coupling C _B = (P _B / P
_B0 ) × 100% is calculated. In addition, the power meter 57
The measurements P _A, an optical power that is optically coupled to a single mode fiber 20 of the light power P _A0 which enters the single mode fiber 10. Therefore, from this numerical value, the optical coupling C _A = (P _A / P _A0 ) × 10 in the arithmetic unit 62.
0% is calculated.

As is apparent from the equations (3) to (5), when the excess loss is very small, P ₀ ≒ P ₁ + P ₂ , so that C ≒ β. However, in the single multi-mode optical fiber coupler which is strong optical coupling of, since the excess loss is very small, the optical coupling C _A, C _B are each branching ratio beta _A,
It is almost equal to β _B. Accordingly, while monitoring the branching ratio beta _B shown in the display unit 63, the branching ratio beta _B becomes around 50% stable numerical, and branching ratio beta _A displayed on the display unit 64 reaches a desired numerical value when, by stopping the heating and stretching, the desired fused portion M ₁ is formed.

The method of forming the fused portion is not limited to the method described above.
In the general method of forming a fused portion shown in FIG. 3, a method of heating and stretching until the branching ratio β in the single mode region reaches a desired value may be used. In this embodiment, as a method of forming the cladding diameter of the single mode fiber to be 40 μm or less, a method of etching the cladding of the portion where the coating of the normal single mode fiber is removed by hydrofluoric acid or the like to reduce the diameter is used. However, the method of diameter reduction is not limited to this. Also, the cladding diameter is 40μ from the beginning.
m or less may be used.

The single / multi-mode optical fiber coupler formed as described above can be used, for example, in both the multi-mode region and the single-mode region. Therefore, it is used as an internal mirror with visible light in the multimode region,
It can be used for medical instruments and the like, such as using it as a laser knife with infrared light in a single mode region.

[0031]

EXAMPLE Using a single-mode fiber with a core diameter of 4 μm and a cladding diameter of 125 μm for a wavelength of 0.633 μm, two samples of a predetermined length of about 300 cm were each stripped of about 3 cm of the coating at the center, and then removed at the center. Was etched with hydrofluoric acid to have an outer diameter of 35 μm. While heating the center of the two single mode fibers, about 10
The single-mode optical fiber coupler was manufactured by elongating mm. The wavelength-optical coupling characteristics of the obtained single / multimode optical fiber coupler were as shown in FIG. 4A is an explanatory diagram of the wavelength-branch ratio characteristics of the single / multimode optical fiber coupler 1, and FIG. 4B is an explanatory diagram of a use state of the single / multimode optical fiber coupler 1. FIG. 4B shows the wavelength-optical coupling characteristics. In FIG. 4 (a), the single multi-mode optical fiber coupler 1 input port I _1, I ₂ and an output port O _1, O ₂ is provided, the light P _A0 incident from a light source (not shown) Single Optically coupled in the multimode optical fiber coupler 1 to form a single mode fiber 10d;
Is branched into 20d, it is emitted as light P _A1, P _A2, respectively from the output port O _1, O _2.

The curves SM ₁ and SM _{2 in} FIG. 4B represent the relationship between the wavelength of light and the branching ratios β _A1 and β _A2 calculated from the outgoing lights P _A1 and P _A2 measured by an optical power meter (not shown). wavelength illustrating the - at the branching ratio characteristic curve, where the solid line SM ₁ is branching ratio beta _A1 = _{[P A1 / (P A1 + P} a 2) ] × 100% of versus wavelength characteristic, the dotted line SM ₂ is branching ratio beta _A2 = [P _A2 / (P _A1 +
P _A2 )] × 100%. As is apparent from FIG. 4, the branching ratio in the multimode region is almost 5
It came within the range of 0 ± 20%. This is a great improvement compared to the branching ratio of about 50 ± 30% in the multimode region of the conventional single mode optical fiber coupler, and the use in the multimode region has become possible.

[0033]

As described above, according to the first aspect of the present invention, at least a part of the fused portion is intensely coupled by heating / stretching the narrow portion having a clad diameter of 40 μm or less. And has a predetermined branching ratio in both the multi-mode region and the single-mode region, so that optical signals of both wavelengths in the multi-mode region such as visible light and the single-mode region such as infrared light can be split at a stable branching ratio. An effect that transmission is possible and that excess loss is small is achieved.

According to the second aspect of the present invention, since the clad diameter is formed to be a small diameter of 40 μm or less, and then heated and stretched to form a fused portion, the heat capacity of the portion to be heated is reduced. A part can be easily made into strong light coupling. Therefore, there is an effect that an optical signal having both wavelengths in both the single mode region and the multimode region can be transmitted with a stable branching ratio, and a single / multimode optical fiber coupler with small excess loss can be easily obtained.

According to the third aspect of the present invention, in addition to the effect of the second aspect of the invention, there is an effect that the clad diameter can be easily formed to a small diameter of 40 μm or less.

According to the fourth aspect of the present invention, in addition to the effect of the second or third aspect, a strong branching ratio is obtained in a multimode region by strong light coupling and a desired branching ratio is obtained in a single mode region. This has the effect that the fused portion of the single / multimode optical fiber coupler having a branching ratio can be easily formed.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a single multimode optical fiber coupler according to the present invention.

FIG. 2 is an explanatory diagram of a method for forming a fused portion of a single / multi-mode optical fiber coupler of the present invention.

FIG. 3 is an explanatory view of another method for forming a fused portion of the single / multi-mode optical fiber coupler of the present invention.

FIG. 4 is a diagram showing wavelength-branch ratio characteristics of one example of the present invention.

FIG. 5 is an explanatory view of a conventional single mode optical fiber coupler and a method of manufacturing the same.

FIG. 6 is a diagram showing wavelength-branch ratio characteristics of a conventional single mode optical fiber coupler.

FIG. 7 is a diagram illustrating a relationship between a cutoff wavelength and a core diameter.

[Explanation of symbols]

1 Single-multimode optical fiber coupler of the present invention 2 Strongly optically coupled core of single-multimode optical fiber coupler of the present invention 3 Cladding of single-multimode optical fiber coupler of the present invention 10, 20, 40, 50 Single mode fiber 10a, 20a, 40a, 50a Single mode fiber coating removing section 30 Single mode optical fiber coupler 51 Drawing tool 52, 53 Light source 54, 55 Light receiving section 56, 57 Power meter 62 Operation section 63, 64 Display section

Claims (4)

[Claims] 1. An optical fiber coupler comprising two coated single mode fibers which are heated and drawn to form a fused portion, wherein the cladding diameter of the single mode fiber is 40 μm or less. The portion is heated and stretched to be fused, and at least a part of the fused portion is strongly light-coupled, and has a predetermined branching ratio in both the multi-mode region and the single-mode region. Multimode optical fiber coupler. 2. A method for manufacturing an optical fiber coupler in which two single-mode fibers from which a coating has been removed are heated and drawn to form a fused portion, wherein the two single-mode fibers from which the coating has been removed are:
The cladding diameter of at least the portion to be heated and stretched should be 40
μm or less, and the small diameter portion is heated, stretched and fused, and at least a part of the fused portion is strongly optically coupled. Production method. 3. The method for manufacturing a single mode fiber coupler according to claim 2, wherein the cladding diameter of the portion to be heated and drawn is reduced by etching the cladding diameter of the single mode fiber from which the coating has been removed in advance. A method for manufacturing a single / multi-mode optical fiber coupler. 4. The method of manufacturing an optical fiber coupler according to claim 2, wherein light having a wavelength in a single mode region is incident on an input end of one of the two single mode fibers, The light of the wavelength in the multi-mode region is incident on the input end of the other single-mode fiber, and only the light of the wavelength in the single-mode region is output at one output terminal, and only the light of the wavelength in the multi-mode region is output at the other output terminal. Each is separated and taken out by a filter, and each optical coupling is calculated and displayed from each of the input light and each of the output light, and displayed.
While monitoring the optical coupling, it is heated and stretched, and when the optical coupling in the multi-mode region is stabilized at about 50% and the optical coupling in the single mode region reaches a desired value, the heating and stretching are stopped. A method for manufacturing a single / multi-mode optical fiber coupler.