JPH03214108A - Production of fiber fusion type optical coupler - Google Patents

Abstract

PURPOSE:To facilitate the production of the coupler by fusing the side faces of two pieces of single mode fibers varying in structural parameters, then stretching the fused part until a prescribed outside diameter is attained. CONSTITUTION:The side faces of two pieces of the single mode fibers 16, 18 varying in the structural parameter are fused and the fused part is stretched until the outside diameter of the fused part is reduced to about 1/4 to 1/2 the outside diameter before the stretching. The max. coupling ratio in a required wavelength range is no longer increased to 100% and the wavelength at which this max. coupling ratio is attained can be set according to the outside diameter or length of the final fused and stretched part. Namely, the branching by approximately the same branching ratio with light of respective wavelengths is executed. The production of the fiber fusion type optical coupler of a broad wavelength region is facilitated in this way.

Description

[Detailed Description of the Invention] Overview Regarding the manufacturing method of a fiber fused optical coupler, the present invention aims to facilitate the manufacturing method of a fiber fused optical coupler with a wide wavelength range. The sides are fused and the fused portion is stretched until it reaches a predetermined outer diameter.

Industrial applications The present invention relates to a method for manufacturing a fiber fusion type optical coupler.

In the field of optical communication or optical transmission that uses optical fibers as transmission paths, it is necessary to distribute multiple optical signals from multiple channels to a single optical fiber in order to distribute the transmitted optical signals to multiple devices or for multiplex transmission. For introduction, an optical coupler is used. This type of optical coupler includes a micro-optical type that uses optical elements such as lenses and half mirrors, a waveguide type, and a coupler that fuses and stretches multiple (for example, two) optical fibers. A fiber fusion type optical fiber coupler (optical fiber coupler) is well known. In particular, when the optical transmission line is a single-mode optical fiber, if it is a micro-optical type or a waveguide type, there will be a large loss when converting the optical beam or when connecting to the optical transmission line, so it is not suitable for such applications. It is said that a fiber fusion type optical coupler that can be directly connected to an optical transmission line is advantageous. By the way, fiber fusion type optical couplers exhibit wavelength dependence of the branching ratio due to their operating principle, so some kind of ingenuity is required in order to provide fiber fusion type optical couplers with a wide wavelength range. Optimization is being sought.

Conventional technology FIG. 7(a) is a diagram for explaining the configuration and function of a boat fishing fiber fusion type optical coupler, and FIG.
)-(b) is a sectional view taken along the line. This fiber fusion type optical coupler includes input parts 2 and 4, a fusion/stretching part 6,
Output section 8. It is composed of IO.

In the fusion/stretching section 6, the diameters of the portions 6a and 6b corresponding to the core of each optical fiber are smaller than the original core diameter, and these portions 5a and 5b are sufficiently close to each other.
Mode coupling occurs depending on the length of the fused/stretched portion 6, etc., and a predetermined branching ratio can be obtained. That is, when a signal light having an optical power of 20 is input from the input section 2 or 4, optical signals having optical powers PI and P2 are outputted from the output sections 8 and 10 at a predetermined branching ratio, respectively.

With this configuration, it is possible to connect directly to a single-mode optical fiber, so the loss of optical power is small, and since the only component is the optical fiber, it is highly reliable against usage environmental conditions such as temperature and humidity. be able to. The branching ratio (coupling rate) of the coupler is 100・P + / (P I + P2) or 10
It is expressed as 0.P2/(PI+P2).

Conventionally, this type of fiber-fused optical coupler has been manufactured by side-welding two single-mode fibers, stretching the fused portion, and appropriately reinforcing the fused/stretched portion. FIG. 8(a) shows the relationship between the coupling ratio and the coupling length (corresponding to the length of the fused/stretched portion). For light of a given wavelength, as the compling length becomes longer,
The coupling rate changes continuously like 100%, 0%, 1100%, and so on. Based on this relationship, in order to realize a 3 dB optical coupler with low wavelength dependence, for example, the stretching should be stopped when the coupling rate first reaches 50% (see Figure 8 (a). )
point A). In addition, when attempting to realize a fiber fusion type optical multiplexer/demultiplexer by actively utilizing the wavelength dependence of the coupling rate, the coupling rate becomes 100% once or more times during stretching. The stretching is stopped after a certain point (for example, point B in FIG. 8(a)).

Problems to be Solved by the Invention When the stretching is stopped at point A in FIG. 8(a), the wavelength dependence of the coupling rate is smaller than when the stretching is stopped at point B. In fact, the wavelength dependence described above cannot be ignored, as shown by C in FIGS. That is, for a wavelength λ that gives a coupling rate of 50%, it is 3 dB.
Although it functions as an optical coupler, it does not function as a 3 dB optical coupler for the wavelength λ2 that provides a coupling rate of 100%. In other words, it functions as a 3[IB optical coupler only for light with a wavelength near wavelength λ1, and this optical coupler does not have a wide wavelength range.

A conventional manufacturing method for a fiber fusion type optical coupler with a wide wavelength range will be explained with reference to FIG. This conventional method uses two single mode fibers 12. with the same core diameter, relative refractive index difference, etc.
14, one of these fibers 14 is heated and drawn in advance, and the single mode fiber 14 with a reduced outer diameter and the single mode fiber 12 that is not reduced in outer diameter are side fused and then drawn. This is how I did it. According to this method, the diameters at the fused and stretched parts are different, so the maximum coupling rate is 100%.
It will no longer be. As a result, as shown by the broken line in FIG. 8(b), the wavelength λ1. The coupling ratio for λ2 is approximately the same, for example, about 50%, and characteristics over a wide wavelength range can be obtained.

However, in this conventional method, it is necessary to draw one single mode fiber in advance.
There is a problem that the manufacturing process becomes complicated. Further, if the diameter of one of the single mode fibers drawn in advance varies, there is a problem that the obtained characteristics also vary.

Therefore, an object of the present invention is to facilitate the manufacturing method of a fiber fusion type optical coupler having a wide wavelength range.

Means for Solving the Problems FIG. 1 is an explanatory diagram of the method of the present invention. First, the same figure (a)
As shown in Figure 2, two single mode fibers 16 and 18 with different structural parameters are side fused. Next, as shown in the same figure, the fused portion is stretched until it reaches a predetermined outer diameter.

Here, the predetermined outer diameter is, for example, an outer diameter such that the branching ratios for light of specific wavelengths that can be used (for example, 1.3 μm and 1.55 μm) are approximately equal.

Function: Since the method of the present invention uses single mode fibers with different structural parameters, it is not necessary to draw one of the fibers in advance as in the conventional method, and the manufacturing method is simplified. The reason why a single mode fiber is used in the method of the present invention is because multimode fibers would cause unstable coupling at the fusion/stretching section. By using two single mode fibers with different structural parameters, a wide wavelength range can be achieved in accordance with the same principle as when two identical single mode fibers are used and one of them is pre-stretched.

Example The present invention will be explained in detail below.

An apparatus for manufacturing a fiber fusion type optical coupler that can be used to carry out the method of the present invention will be explained with reference to FIG. The single mode fiber 16.18 mentioned above is connected to the fine movement table 20.2.
2, and are in close contact with each other on the sides. The fine movement table 20 is composed of a fixed part 20a and a movable part 20b that holds the single mode fiber 16.18 and is movable in the horizontal direction in the figure with respect to the fixed part 20a. It is composed of a fixed part 22a and a moving part 22b. A light source 24 such as a semiconductor laser module is connected to one end of the single mode fibers 16 and 18, for example, the single mode fiber 18.

Furthermore, optical power meters 26 and 28 are connected to the other ends of the single mode fibers 16 and 18, respectively.

30 is a burner such as a 02 H2 burner, which partially heats the single mode fiber 16.18 between the portions held by the fine movement table 20.22. burner 30
When the movable parts 20b and 22b of the fine movement table are slightly moved while partially heating the single mode fiber 16.18, the sides of the single mode fiber 16.18 are fused together, and the fused portion and its vicinity are is stretched,
Since the light intensity measured by the optical power meter 26, 28 changes depending on the degree of fusion/stretching, heating and stretching are performed until a desired coupling ratio is achieved. The outer diameter of the fused/stretched part (for example, the short axis of the long axis and short axis) is
It can be measured using a non-contact type laser outer diameter measuring device 32.

In the first embodiment, as the two single mode fibers 16 and 18 having different structural parameters, two single mode fibers having different cutoff wavelengths are used. The cutoff wavelength is a wavelength such that only the fundamental mode propagates at a wavelength larger than the wavelength, and other modes also propagate at a wavelength smaller than the wavelength, and this cutoff wavelength λC is given by the following equation.

λc=2πa(n, -n2')"' /2.405
...(1) Here, a is the core diameter, nl is the core refractive index, and n2 is the cladding refractive index.

FIG. 3 shows an example of two single mode fibers with different cutoff wavelengths λ. Now, let us assume that one single mode fiber 16 has a core diameter of a1, a core refractive index of nl, and a cladding refractive index of n12, and the other single mode fiber 18 has a core diameter of a2, a core refractive index of 121,
Assume that the cladding refractive index is 122. At this time, the necessary and sufficient condition for the cutoff wavelengths of both fibers 16.18 to be different is at(n,, '-i1,22)I/2
≠a2(n2+2-n2a'ν/2 ,,,
(2). A specific example of the cutoff wavelength is:
1.15 μm and 1.20 μm can be selected.

In this way, two single mode fibers 16.18 with different structural parameters are side fused and stretched until the outer diameter of the fused part becomes about 1/4 to 1/2 of the outer diameter before stretching. Then, the maximum coupling rate will not be 100% in the required wavelength range, and the wavelength that provides this maximum coupling rate can be set depending on the outer diameter or length of the final fused/stretched part. It becomes like this. If stretching is performed until the outer diameter of the fused/stretched portion is reduced to 1/4 or less of the original diameter, effects due to differences in structural parameters will be less likely to occur, and this is not suitable for implementing the method of the present invention. In addition, if the stretching is performed only until the outer diameter of the fused/stretched part becomes 1/2 or more of the original outer diameter, good coupling will not occur, which is also inappropriate for implementing the method of the present invention. .

FIG. 4 shows the relationship between the coupling rate (%) and wavelength (μm) of the optical coupler obtained in the first embodiment. A, B,
The characteristic curve indicated by C is a characteristic curve for the case where the outer diameter after fusing and stretching increases in this order. In this example, stretching is performed until the outer diameter becomes such that the branching ratio (coupling ratio) for light with wavelengths of 13 μm and 1.55 μm becomes approximately equal. By doing this,
When performing wavelength division multiplexing transmission at the above wavelengths, it becomes possible to perform branching using approximately the same branching ratio for light of each wavelength.

By the way, when two single mode fibers with different structural parameters are used, at least one of the structural parameters will be different from the structural parameters of the single mode fiber used as an optical transmission line. When connecting songle mode fibers with different structural parameters, the connection loss may increase, so it is desirable to address this problem. Therefore, a second embodiment will be described as a configuration that can prevent such an increase in loss.

In this embodiment, one single mode fiber 16 has a first core 16a as shown in FIG. 5(a).
and a second core 16b having a lower refractive index than the first core 16a.
and a γ-doped 16C having a lower refractive index than the second core 16b.

The other single mode fiber 18 also includes a first core 18a, a second core 8b, and a cladding 18c having a similar refractive index relationship.

Then, the second core 1 of one single mode fiber
6b has a diameter of 2r3, and the second core 18b of the other single mode fiber has a diameter of 2r2, then rl
≠r2.

When two single mode fibers with different second core diameters are fused and stretched in this way, the mode field will extend beyond the first core as the first core diameter decreases. However, the mode field that protrudes from this first core becomes limited by the second core. Since the diameter of this second core is different for both single mode fibers, it is possible to realize a state in which the maximum coupling rate is not 100% within the required wavelength range, as in the previous embodiment, and a wide wavelength range. is achieved. In this case, since the diameter and refractive index of the first cores 16a and 18a can be made equal in the portions that are not fused and stretched, there is no fear that the splice loss will increase in such a case.

An example of the single mode fiber used in the second embodiment may have a common mode field diameter of 10 μm, which is equivalent to a normal single mode fiber, and a second core diameter of 25 μm and 35 μm. As a result of stretching until the outer diameter of the fused/stretched part was approximately 15 μm, as shown in FIG.
An optical coupler with characteristics such that the coupling rate was 50% at 5 μm was obtained. By varying the outer diameter of the fused/stretched portion, various coupling ratios in the range of about 30% to 80% can be obtained, but the mode-trapped state in the first core is maintained. In order to prevent the effect of the second core from occurring, it is desirable that the outer diameter of the fused/stretched portion be ⅓ or less of that before stretching.

As described in detail, the present invention has the effect of simplifying the manufacturing method of a fiber fusion type optical coupler having a wide wavelength range.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the method of the present invention, Fig. 2 is an explanatory diagram of a manufacturing apparatus for a fiber fusion type optical coupler that can be used to carry out the method of the present invention, and Fig. 3 is an explanatory diagram of the method of the present invention.
FIG. 4 is a graph showing the characteristics in the first example. FIG. 5 is an explanatory diagram of the structural parameters in the second example. FIG. 6 is a graph showing the characteristics in the second example. Figure 7 is an explanatory diagram of a fiber fusion type optical coupler, Figure 8 is an explanatory diagram of a manufacturing method of a fiber fusion type optical coupler, and Figure 9 is a diagram of a conventional fiber fusion type optical coupler with a wide wavelength range. It is an explanatory view of a manufacturing method. 16.18...Single mode fiber. 8 (0) 6 8 (b) 16.18' Shin 2゛1 Beat Fiber Hon Shigiro f
J Yuo's SE 9th area, Figure 1, Tsubaki (, flJM explanatory diagram, Figure 2, Obake Ita 11 Tewa [Suru Kyoushin), Lameno's Obake, Tsuki area, Figure 3, Number 1 In the loquat ita J, it shows v eye 1 and shows J Cluff figure 4 figure 2 husband Jya ii 13 E8 also the illusion of the old (original) smell is also the sun moon country figure 5 second tip 1 jl += Wakab Teyu/Eboji 7 Graph No. 6
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Claims (1)

[Claims] 1. Two single mode fibers (16, 18) having different structural parameters are side fused and the fused portion is stretched until it reaches a predetermined outer diameter. A method for manufacturing a fiber fusion type optical coupler. 2. The above structural parameters are λ_C=2πa(n_1^2-n_2^2)^1^/^, where the core diameter is a, the core refractive index is n_1, and the cladding refractive index is n_2.
2. The method for manufacturing a fiber fusion type optical coupler according to claim 1, wherein the cutoff wavelength is λ_C expressed as 2/2.405. 3. The single mode fibers (16, 18) are the first
cores (16a, 18a) and the first core (16a, 1
8a) A second core (16b, 18b) having a lower refractive index and a cladding (16c, 18c) having a lower refractive index than the second core (16b, 18b), and the structural parameter is the same as that of the second core. Core (16b, 18b
2. The method for manufacturing a fiber fusion type optical coupler according to claim 1, wherein the fiber fusion type optical coupler has a diameter of . 4. The above predetermined outer diameter is the wavelength of 1.3 μm and 1.55 μm.
4. The method for manufacturing a fiber fusion type optical coupler according to claim 1, wherein the outer diameter is such that the branching ratios for the light are approximately equal.