JPH0532725B2 - - Google Patents

Description

[Detailed Description of the Invention] Outline A first optical coupler and a second optical coupler, each consisting of two fused and drawn optical fibers, are connected in series so that their branching ratio is 0.5 at different wavelengths. settings to configure a fiber fused optical coupler. This cancels out the wavelength dependence of the branching ratios of the first optical coupler and the second optical coupler, and significantly reduces the dependence as a whole. Also, two relatively close
At two wavelengths (e.g. 1.3 μm and 1.55 μm),
The branching ratio can be 0.5 for each.

INDUSTRIAL APPLICATION FIELD The present invention relates to a fiber-fused optical coupler made by fusing and stretching optical fibers.

In an optical communication system using an optical fiber as a transmission path, an optical coupler is used to branch a transmitted optical signal to a plurality of devices. An optical coupler basically consists of an input section and an output section for connecting to an optical fiber, and a functional section that branches an input signal at a predetermined ratio. The requirements for a practical optical coupler are (a) low insertion loss, that is, the total output power does not decrease with respect to the input power, and (b) a simple configuration and high reliability under usage environmental conditions. That, etc.

BACKGROUND ART Conventionally, optical couplers include micro-optical type ones constructed using optical elements such as lenses and half mirrors, waveguide type ones, and multiple optical couplers (for example, two optical couplers).
The fiber fused type, which is made by fusing and stretching optical fibers, is mainly used. In particular, when the transmission line is a single-mode optical fiber, if it is a micro-optical type or a waveguide type, the insertion loss is about 1 dB due to the increased loss during conversion and connection of the optical beam. fiber-fused optical couplers that can be directly connected to single-mode optical fibers (insertion loss is
0.3dB or less) is said to be advantageous.

FIG. 6 is a diagram showing the general configuration of a fiber fusion type optical coupler.
and a fused/stretched part 1e. When an optical signal of P ₀ is input from the input section 1a or 1b,
Optical signals with powers P ₁ and P ₂ are outputted from the output units 1c and 1d at predetermined branching ratios, respectively. With such a configuration, insertion loss is small because it can be directly connected to a single mode optical fiber, and since it is composed only of optical fibers and the configuration is simple, it can be said to have high reliability against temperature, humidity, etc. . In addition, P ₁ /P ₀ , P ₂ /P ₀
Alternatively, the branching ratio expressed as P ₁ :P ₂ is
Diameter and length of extended portion 1e and fused/stretched portion 1e
It is said that it is determined according to the refractive index of the cladding part and its surroundings.
Electronics letters, Vol.21, No.11, 23rd May,
1985, pp461-462, “Modelling Fused Single−
Mode-Fiber Couplers”.

Problems to be Solved by the Invention However, the structure of the conventional fiber fused optical coupler shown in FIG. 6 has the disadvantage that the branching ratio changes depending on the wavelength of the signal light. For example, for signal light with a wavelength of 1.3 μm, the branching ratio is
If it is set to 0.5, the wavelength is 1.5 μm.
For the signal light, the branching ratio is, for example, 0.7. If the branching ratio is wavelength dependent in this way, a problem arises in that different branching ratios are obtained during wavelength division multiplexing communication or the like. Furthermore, since this optical coupler cannot be used for signals with significantly different wavelengths, there is also the problem of lack of versatility.

The present invention was created in view of these circumstances, and its purpose is to provide a fiber-fused optical coupler that has a branching ratio of 0.5 at two different wavelengths. Another object of the present invention is to provide a fiber-fused optical coupler in which the wavelength dependence of the branching ratio is small.

Means for Solving the Problems The problems of the prior art described above are solved by a fiber fusion type optical coupler whose basic configuration is shown in FIG.

This fiber fusion type optical coupler has two
The first one is made by fusing and stretching the optical fibers of a book.
It is composed of an optical coupler 10 and a second optical coupler 20.

The first optical coupler 10 includes input sections 11 and 12, a fusing/stretching section 15, and output sections 13 and 14.

The second optical coupler 20 includes input sections 21 and 22, a fusing/stretching section 25, and output sections 23 and 24.

The first optical coupler 10 and the second optical coupler 20 are connected in series. That is, output sections 13 and 14 of the first optical coupler 10 are connected to input sections 21 and 22 of the second optical coupler 20, respectively.

The branching ratios of both optical couplers 10 and 20 are set to 0.5 at different wavelengths.

Operation It is now assumed that an optical signal with a power of P ₀ is input from one input section of the first optical coupler 10, for example, the input section 12. The connection portion between the output section 13 of the first optical coupler 10 and the input section 21 of the second optical coupler 20, and the connection section between the output section 14 of the first optical coupler 10 and the second optical coupler 20
Assuming that there is no loss at the connection with the input section 22, the optical power passing through each connection is
P _a1 and P _a2 are expressed as P _a1 = P ₀ S ₁ and P _a2 = P ₀ (1-S ₁ ) using the branching ratio S ₁ of the first optical coupler 10. Further, if the powers of the optical signals output from the output sections 23 and 24 of the second optical coupler 20 are respectively P _b1 and P _b2 , then using the branching ratio S ₂ of the second optical coupler 20, P _b1 = P _a2 S ₂ + P _a1 (1-S ₂ ) = P ₀ (S ₁ + S ₂ -2S ₁ S ₂ ) P _b2 = P ₀ - P _b1 . Therefore, the overall branching ratio S is S=S ₁ +S ₂ −2S ₁ S ₂ .

Here, if either S ₁ or S ₂ is 0.5, S = 0.5 from the above equation, so S ₁ and S ₂ are set to 0.5 at different arbitrary wavelengths (for example, 1.3 μm and 1.55 μm). By setting the branching ratio of the fiber fused optical coupler to 0.5 at both wavelengths,
It can be done. Also, each branching ratio
By appropriately setting the wavelength dependence of S ₁ and S ₂ , the wavelength dependence of the branching ratio S as a whole can be reduced.

Embodiments Hereinafter, preferred embodiments of the present invention will be described based on the drawings.

FIG. 2 is for explaining an example of the manufacturing process of the first and second optical couplers. First, in order to fuse and stretch the two optical fibers 26 and 27 in parallel, they are each twisted 0.5 times and supported so that they are in contact with each other, as shown in FIG. 2a. , electrodes 29,2 are connected to this contact portion.
From step 9, arc discharge is performed to fuse. Then, as shown in FIG. 2b, the optical fibers 26, 27
Stretching is performed by stretching in the direction of the arrow in the figure.

The first and second optical couplers configured in this way are
When connected in series as shown in FIG. 3, a fiber fused optical coupler is formed. This fiber fusion type optical coupler includes input parts 31, 32, a first fusion/stretching part 33, branch parts 34, 35, a second fusion/stretching part 36, and output parts 37, 38. Become. The first and second fusions have lengths L ₁ and L ₂ , respectively.
The cross-sectional configuration of the extending portions 33 and 36 is as shown in FIGS. 4a and 4b. Since the optical fiber is stretched after being fused, it has a roughly elliptical shape with a depression in the center. Note that instead of configuring the first and second optical couplers as separate bodies as described above, they are continuously fused and bonded to the same optical fiber.
A fiber fusion type optical coupler may be constructed by providing a stretched portion.

FIG. 5 shows that the length L ₁ of the first fused/stretched portion 33 is
1.27 mm, the minor axis a ₁ = 20 μm in the cross section,
The wavelength dependence of the branching ratio in the _first optical coupler and _the second optical coupler (respectively A , B) and the fiber-fused optical coupler formed by connecting these (represented by C). It is clear from the figure that although the branching ratio of each individual optical coupler is largely dependent on the wavelength, this dependence is reduced by connecting these in series. In this example, 1.25~
In the wavelength range of 1.60 μm, the variation width of the branching ratio was 5% upward and downward.

Furthermore, since the branching ratio of the first optical coupler is 0.5 at a wavelength of 1.3 μm and the branching ratio of the second optical coupler is 0.5 at a wavelength of 1.55 μm, the branching ratio of the entire fiber-fused optical coupler is 0.5, increasing versatility. In this way, the first
In order to set the wavelength dependence of the branching ratio in the second optical coupler to a desired value, the following procedure may be used. Generally, the branching ratio S ₁ of the optical coupler obtained by fusing and drawing two optical fibers, for example, the first optical coupler in the present invention, is according to the literature listed in the prior art section of the specification of the present application. , S ₁ = sin(C ₁ L ₁ ). Here, C ₁ = 3πλ/32n ₂ a ₁ ² (1 + 1/V) ² V = 2πa ₁ (n ₂ ² − n ₃ ² ) 1/2/λ, where λ is the wavelength of the signal light and n ₂ is Fusion/stretching part 3
3, the refractive index of the cladding part 3, n ₃ is the fused/stretched part 33
is the refractive index of the surrounding medium.

In this way, by controlling the length of the fused/stretched portion, the small diameter of its cross section, and the refractive index of each portion, the wavelength dependence of the branching ratio can be freely set. Note that it is also possible to obtain a desired branching ratio at a predetermined wavelength without setting the above parameters, for example, by stretching the fused portion while actually measuring the branching output of the first optical coupler.

Effects of the Invention As detailed above, according to the present invention, the two-input, two-output type first and second optical couplers are connected in series, so that the branching ratio of each is 0.5 at different wavelengths. By setting , the overall branching ratio of the fiber-fused optical coupler becomes 0.5 at both wavelengths, and a highly versatile optical coupler is provided. Furthermore, in the wavelength range between the two wavelengths, there is also the effect that the wavelength dependence of the branching ratio is significantly reduced.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the principle configuration of the present invention, FIG. 2 is an embodiment diagram of the present invention, and is a manufacturing process diagram of the first and second optical couplers, and FIG. 3 is a fiber fusion diagram showing an embodiment of the present invention. FIG. 4 is a cross-sectional view taken along lines aa and b-b in FIG. A graph showing the relationship, FIG. 6, is a block diagram of a general fiber fusion type optical coupler. 10...First optical coupler, 20...Second optical coupler, 26, 27...Optical fiber, 33...First fusion/stretching section, 36...Second fusion/stretching section.

Claims (1)

[Claims] 1. Two optical fibers are fused and stretched so that the branching ratio for light of the first wavelength is 0.5, and the first and second input parts 11, 12 and the first and second input parts 2
A first optical coupler 10 having output parts 13 and 14, and two optical fibers fused and stretched so that the branching ratio for light having a second wavelength different from the first wavelength is 0.5. 1 and second input section 21, 2
2 and a second optical coupler 20 having first and second output sections 23 and 24, the first and second output sections 1 of the first optical coupler 10
3 and 14 are the first optical couplers of the second optical coupler 20, respectively.
and a fiber fusion type optical coupler, which is connected to second input sections 21 and 22.