JP2848832B2 - Broadband optical fiber coupler - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical signal input to one optical fiber, branched to a port of another optical fiber, or input to two optical fibers. More particularly, the present invention relates to an optical fiber coupler that is used when coupling optical fibers, and particularly to an optical fiber coupler that can easily realize a wide band.

2. Description of the Related Art Conventionally, as one of optical fiber couplers used for coupling or splitting optical signals, a type called a fusion-splicing type is known.

FIG. 3 is a diagram showing an example of a conventional fusion-stretched optical fiber coupler. This optical fiber coupler 1 heats and fuses a part of two optical fibers 2 and further stretches. It is constituted by forming a fusion-stretched portion 3.

In the fusion-spreading section 3, the diameter of each optical fiber 2 is reduced, and at the same time, the core diameter is also reduced. As a result, the light propagating in the core of each optical fiber 2 causes a large amount of optical power to seep into the cladding as the core diameter decreases, and the distance between the cores of each optical fiber also decreases. In addition, the coupling between modes propagating in the core of each optical fiber becomes very large. This allows the light incident on one optical fiber to be branched to the port of the other optical fiber,
Light incident on both optical fibers is coupled.

By the way, in the optical fiber coupler 1, for example, the third
Port A among ports indicated by symbols A to D in the drawing
The rate at which the light incident on the port D is coupled to the port D (referred to as the degree of coupling) fluctuates in a pseudo sine wave shape as shown in FIG. 4 depending on the wavelength of the incident light.

On the other hand, as an optical fiber coupler capable of flattening the variation of the coupling degree due to the wavelength of the incident light, a broadband optical fiber coupler having a weak coupling degree with wavelength has been proposed. FIG. 5 is a view showing an example of a conventional wide-band optical fiber coupler. This optical fiber coupler 4 is composed of an ordinary optical fiber 5 which has been pre-drawn and has a portion with a reduced fiber diameter, and a normal optical fiber coupler. The optical fiber 2 and the optical fiber 2 are additionally provided, and a fusion-stretched portion 6 is formed by fusion-stretching. By this pre-stretching process, a difference in propagation constant (hereinafter referred to as Δβ) can be generated between each of the optical fibers 2 and 5 before the fusion and stretching, whereby each of the optical fibers 2 and 5 is fusion-stretched. It is possible to increase the coupling coefficient (hereinafter, referred to as S) between the modes propagating between the fibers in the fusion-spreading section 6 as it progresses, and obtain a desired degree of coupling. In the fiber coupler 4 configured as described above, as shown in FIG. 6, a flat coupling degree can be obtained in a considerably wide wavelength width. FIG. 6 is a diagram showing an example of the wavelength dependence of the degree of coupling from the port A (incident side) to the port D of the optical fiber coupler 4 configured as shown in FIG.

“Problems to be Solved by the Invention” However, the conventional broadband optical fiber coupler has the following problems.

Since the two optical fibers are originally the same optical fiber, if only one of them is extended, the two fibers can only move on the curve shown in FIG. 7, so each optical fiber is extended. Accordingly, Δβ becomes smaller than the value before stretching. Therefore, there has been a problem that sufficient wideband characteristics cannot be obtained. FIG. 7 is generally called a dispersion curve. The vertical axis represents the propagation constant β of the mode of the optical fiber, and the horizontal axis represents the normalized frequency V.
This normalized frequency V is obtained by the following equation (1).

(Where λ is the wavelength (μm), a is the core radius (μm), n ₁
Is the core refractive index, Δ is the relative refractive index difference between the core and the clad). In FIG. 7, β is the propagation constant, n clad is the cladding refractive index, and K ₀ = 2π / λ. The above Δ (specific refractive index difference) is Δ = (n ₁ −n clad) / n ₁ .

Also, one of the two optical fibers to be fused is
First, if the optical fibers are thinned by preliminary stretching, deformation occurs when the optical fibers are aligned and fused and stretched, and in particular, asymmetry occurs in a plane perpendicular to the fiber axis, thereby causing a problem that fiber bending loss easily occurs.

The present invention has been made in view of the above circumstances, and has as its object to provide a broadband optical fiber coupler that is easy to manufacture and has low insertion loss.

"Means for Solving the Problems" As a means for solving the above problems, the invention according to claim 1 is an optical fiber formed by fusing and stretching a part of two optical fibers to form a fused and stretched portion. In the coupler, the relative refractive index difference between the two optical fibers is relatively different by 15% or more, and the cutoff wavelength of the optical fiber having the larger relative refractive index difference is set to the cutoff wavelength of the other optical fiber. 10% for
This is a broadband optical fiber coupler characterized by being long.

According to a second aspect of the present invention, there is provided the wide-area optical fiber coupler according to the first aspect, wherein the wavelength in the fusion extending portion is 1.3 to 1.5.
A wide-area optical fiber coupler characterized in that the degree of coupling of optical power to a partner fiber in a region of 5 μm is set in a range of 5 to 50%.

 Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing an example of the present invention, and reference numeral 11 denotes a broadband optical fiber coupler (hereinafter, referred to as a coupler). The coupler 11 is formed by fusing and extending a part of two optical fibers 12 and 13 whose relative refractive index differences are relatively different from each other by 15% or more to form a fused and stretched portion 14. This fusing and stretching part
In FIG. 14, the optical fibers 12 and 13 are thinned, but the optical fibers 12 and 13 having the same fiber diameter are uniformly drawn without performing pre-drawing when the respective optical fibers 12 and 13 are fused and drawn. It is in a state where it has been done. Note that the amount of stretching of the fusion stretching portion 14 is appropriately adjusted by S (coupling coefficient).

In the coupler 11, two optical fibers 12, 13 having a relative refractive index difference of 15% or more are fusion-stretched and fused.
, The Δβ between each optical fiber 12 and 13
(Difference in propagation constant). Where Δ
If β and S have an appropriate ratio, the peak value of the coupling degree can be set to an arbitrary value.

Here, two different relative refractive index difference delta ₁ and delta ₂ is relatively more than 15% of the optical fiber 12, 13 based on FIG. 2 Factors coupler wideband type can be obtained by fusing stretching explain. Figure 2 is a when fused and extended optical fibers 12, 13 of the relative refractive index difference delta ₁ and delta ₂ is relatively least 15% different two beta (propagation constant) and V (normalized frequency) FIG. The β value of each of the optical fibers 12 and 13 moves on a different curve as shown by the curves of β ₁ and β _{2 in} the figure due to the difference in relative refractive index Δ ₁ and Δ ₂ . Based on the difference between the β values of the optical fibers 12 and 13, a certain amount of Δβ can be generated between the optical fibers 12 and 13 before drawing.

Then, after fusion-stretching each optical fiber 12, 13, based on the difference in β value of each optical fiber 12, 13, each optical fiber 12, 13,
Even if 13 is stretched in an even state, each optical fiber 1
Propagation constant beta ₁ between 2,13 can Motas some difference ([Delta] [beta]) to the beta _2.

The value of Δβ is determined by the relative difference between the relative refractive index differences (Δ) of the optical fibers 12 and 13. If the relative difference between the relative refractive index differences is 15% or more, each optical fiber 12 Fiber 12,1
A practical coupling degree (coupling degree of about 5 to 50%) can be obtained by fusing and stretching 3 as a broadband coupler, but it is practical if the relative refractive index difference is relatively 15% or less. It is difficult to obtain a high degree of bonding.

Accordingly, the coupler 11 can have an appropriate ratio between Δβ and S, and as a result, as shown in FIG. 6, a wide-band coupler having a flat variation in the degree of coupling depending on the wavelength.

As described above, the coupler 11 is formed by fusion-stretching a part of the two optical fibers 12, 13 whose relative refractive index difference is relatively different by at least 15% to form a fusion-stretched portion. Thereby, even if the diameters of the optical fibers 12 and 13 are the same, a sufficient propagation constant difference can be obtained, and a broadband coupler having a desired degree of coupling can be obtained.

In addition, since the pre-drawing of the optical fibers 12 and 13 can be unnecessary, it is possible to prevent the occurrence of bending loss due to the asymmetry of each of the optical fibers 12 and 13 at the time of fusion-drawing, and to realize a coupler having a low insertion loss. Can be easily manufactured.

In the coupler 11 configured as described above, the wavelength of light
The degree of coupling of the mating fiber in the 1.3 to 1.55 μm
It is desirable to set the range to 50%. The degree of bonding is appropriately adjusted depending on the amount of stretching during fusion stretching. If the degree of coupling is less than 5%, the exchange of optical signals becomes inadequate,
There is a possibility that the branching / coupling of the optical signal having a small power may not be performed. On the other hand, if the degree of coupling exceeds 50%, the wavelength dependence of the degree of coupling will increase, and sufficient broadband characteristics will not be obtained as a broadband coupler. Further, the range of the coupling degree, the light wavelength range of 1.3 to 1.55 μm is a preferable wavelength range of transmission light in the silica-based optical fiber used in general optical communication, as the optical fibers 12 and 13 constituting the coupler 11 When using a silica-based fiber, as described above, the light wavelength is 1.3 to 1.5.
It is necessary to set the range of the coupling degree at 5 μm to 5 to 50%.

By the way, the above-mentioned optical fiber 1
2,13, the relative refractive index difference is relatively different by 15% or more, and the cutoff wavelength (hereinafter referred to as λc) of the optical fiber having the larger relative refractive index difference is larger than λc of the other optical fiber. It is desirable to make it longer than 10%. As a specific method for making the λc of each of the optical fibers 12 and 13 relatively different, there is a method of changing the core diameter of each of the optical fibers 12 and 13. At this time, even if the core diameters of the optical fibers 12 and 13 are different, it is desirable that the outer diameters of the fibers are the same so that the optical fibers 12 and 13 are uniformly drawn during the fusion drawing.

In fused and extended portion 14 of the coupler 11, the core diameter of the optical fibers 12, 13 is reduced, [lambda] c of the LP ₁₁ mode is basically the second order mode is reduced in proportion to the core diameter. At this time, if the optical fibers 12 and 13 have the same λc, the value of Δβ will be low. For this reason, in order to obtain a certain Δβ value, it is necessary to make the core diameter of each optical fiber different so that the relative difference of λc is 10% or more. If this relative difference of λc is less than 10%, sufficient Δβ cannot be obtained, and good broadband characteristics cannot be obtained.

“Example” The first having a relative refractive index difference of 0.4%, a normalized frequency of 3.2 at a wavelength of 1.3 μm, a core diameter of 10.2 μm, and a fiber outer diameter of 125 μm.
Optical fiber, relative refractive index difference is 0.3%, normalized frequency is
2.67, a second optical fiber having a core diameter of 9.8 μm and a fiber outer diameter of 125 μm was arranged in parallel and fused and drawn to form a coupler having a coupling degree of 10%. The fused part of this coupler has a minimum outer diameter of about 25μ.
m, and the fusion-stretched portion length was 15 mm.

As a result of measuring the relationship between the coupling degree and the wavelength of the obtained coupler, it was found that the wavelength dependence of the coupling degree became flat, and that the coupler had sufficient characteristics as a broadband coupler. Also, as a result of measuring the insertion loss of the obtained coupler, 0.15 dB
And was very good.

“Effects of the Invention” The present invention has the following effects by being configured as described above.

The relative refractive index difference is relatively different by 15% or more, and the cutoff wavelength of the optical fiber having the larger relative refractive index difference is 10% or more longer than the cutoff wavelength of the other optical fiber.
A part of the optical fiber is fusion-stretched to form a fusion-stretched portion, so that a sufficient propagation constant difference can be obtained even if the diameter of the optical fiber is the same, and a broadband type having a desired degree of coupling can be obtained. Can be obtained.

Also, since the preliminary drawing of the optical fiber can be made unnecessary, it is possible to prevent the occurrence of bending loss due to the asymmetry of each optical fiber at the time of fusion drawing, and to easily manufacture a coupler having a low insertion loss. Can be.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of the invention described in claim 1,
FIG. 2 is a side view of a broadband optical fiber coupler, FIG. 2 is a dispersion curve of each optical fiber constituting the broadband optical fiber coupler of FIG. 1, FIG. 3 is a side view of a conventional optical fiber coupler, and FIG. FIG. 3 is a graph showing the relationship between the degree of coupling and the wavelength of the optical fiber coupler shown in FIG. 3, FIG. 5 is a side view showing an example of a conventional broadband optical fiber coupler, and FIG. 6 is the coupling of the optical fiber coupler shown in FIG. Graph showing the relationship between the degree and the wavelength, FIG.
The figure shows a dispersion curve of an optical fiber constituting the optical fiber coupler shown in FIG. 11 ... Optical fiber coupler 12,13 ... Optical fiber 14 ... Fused and drawn part.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Noboru Kawakami 1440 Mutsuzaki, Sakura-shi, Chiba Pref. Fujikura Electric Cable Co., Ltd. Sakura Plant (56) References JP-A-60-57303 (JP, A) JP-A-63-63 216008 (JP, A) Special table 63-501427 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G02B 6/28

Claims (2)

(57) [Claims] 1. An optical fiber coupler wherein a part of two optical fibers is fusion-stretched to form a fusion-stretched portion, wherein the relative refractive index difference between the two optical fibers is relatively 15%. A broadband optical fiber coupler characterized in that a cutoff wavelength of an optical fiber having a different relative refractive index difference and a larger relative refractive index difference is 10% or more longer than a cutoff wavelength of the other optical fiber. 2. A wide-area optical fiber coupler according to claim 1, wherein the degree of coupling of the optical power to the counterpart fiber in the wavelength range of 1.3 to 1.55 μm in the fusion drawing section is set in the range of 5 to 50%. A wide-area optical fiber coupler characterized by the following.