JP2958179B2 - Optical fiber coupler and manufacturing method thereof - Google Patents

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 used for an optical fiber communication system, an optical fiber sensor, optical measurement, and the like, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, there is an optical fiber coupler as a passive device that simply branches an input optical signal to a plurality of output ports or conversely couples an optical signal input from a plurality of ports to one optical fiber. In particular, optical fiber couplers produced by fusing and stretching a plurality of optical fibers have been receiving attention because of their low loss, compatibility with optical fibers as optical transmission lines, and ease of production.

However, this type of optical fiber coupler cannot be used in a wavelength multiplex communication system using a plurality of wavelengths because various optical characteristics (branch ratio, insertion loss, etc.) have wavelength dependence. Was. In recent years, studies have been made on fiber-fused optical fiber couplers in which such wavelength dependence has been reduced (it is also called a broadband optical fiber coupler because it can be used in a wider wavelength band than before). For example, as disclosed in US Pat. No. 4,798,436, a method of reducing the wavelength dependence by providing a difference in the propagation constant of an optical fiber is well known. Generally USP
As disclosed in U.S. Pat. No. 4,798,438, a technique is used in which one optical fiber is drawn in advance and then fused and drawn with the other optical fiber to provide a difference in the propagation constant between the two optical fibers. (Hereinafter, this method is referred to as a pre-stretching method). According to this pre-stretching method, for example, the wavelength 1.
The branching ratio at 3 μm and the wavelength of 1.55 μm can be made equal, whereby the branching ratio in a wide wavelength band around 1.3 to 1.55 μm becomes close.

[0004]

However, in an optical fiber coupler manufactured by the pre-drawing method, since at least one optical fiber is drawn in advance to reduce the diameter, the outer shape of the fusion-bonded drawn portion is reduced. Is largely asymmetric and has a problem in mechanical strength, and lacks long-term reliability.

Further, this optical fiber coupler has a close branching ratio in a wide wavelength band as described above, but in recent years, the branching ratio in this wide wavelength band is more uniform, and the wavelength dependence of the branching ratio is high. A very small optical fiber coupler is desired.

The present invention has been made in view of the above problems, and provides a broadband optical fiber coupler which is mechanically stable and has a very small wavelength dependence of a branching ratio.

[0007]

In order to solve the above-mentioned problems, an optical fiber coupler according to the present invention is an optical fiber coupler formed by fusing and stretching a part of a plurality of optical fibers to form a joint. A plurality of optical fibers are pre-heated and stretched by substantially the same length and then melt-stretched to form a joint, and the refractive index distribution between at least one optical fiber and another optical fiber in the joint is a dopant. This is an optical fiber coupler whose branching ratio is made substantially uniform over a wide wavelength band by being made different by the thermal diffusion process.

Further, in order to manufacture the optical fiber coupler, in a method for manufacturing an optical fiber coupler, a part of a plurality of optical fibers is fused and drawn to form a joint.
A heat diffusion step of a dopant in which a refractive index distribution of a portion to be a coupling portion of at least one optical fiber and a portion to be a coupling portion of another optical fiber are preliminarily heated and changed, and simultaneously with the heat diffusion step. Or, independently, a stretching step of heating a portion to be a joint portion of each optical fiber and pre-stretching by approximately the same length in advance, and by stretching and stretching the stretched portions of the stretched optical fibers in close contact with each other. This is a method for manufacturing an optical fiber coupler comprising a fusion-stretching step of forming a joint.

When the stretching step and the thermal diffusion step are performed simultaneously, at least one optical fiber and another optical fiber are stretched by heating at substantially the same length with different stretching speeds, thereby obtaining a refractive index. This is a method for manufacturing an optical fiber coupler in which the distribution is made different and the stretched portions are closely adhered to each other and fused and stretched to form a joint.

[0010]

In the optical fiber coupler of the present invention, the conditions of the thermal diffusion treatment of the dopant are made different, and the difference in the refractive index distribution causes a difference in the propagation constant. As a result, the mode coupling at the coupling portion becomes incomplete and the branching occurs. The wavelength dependence of the ratio can be reduced. Furthermore, by pre-drawing each optical fiber by approximately the same length before fusion drawing, not only the wavelength dependence of the branching ratio can be further reduced, but also the shape of the coupling portion becomes symmetric, so that the mechanical strength is excellent. Can be.

Further, the above optical fiber coupler can be easily manufactured by the manufacturing method of the present invention.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. This embodiment relates to an embodiment in which the same optical fiber is used and the heat diffusion step and the pre-stretching step are performed simultaneously in the optical fiber coupler according to the present invention.

FIG. 1 shows the optical fibers 100 and 200 previously drawn at different speeds and the A of the narrowest diameter portions 103 and 203 of the optical fibers 100 and 200.
It is the figure which showed the refractive index distribution in -A and BB cross section. FIG. 4A shows an optical fiber 100 drawn at a high temperature at a temperature higher than the softening point of the fiber, and the dopant in the core 102 having a higher refractive index than the cladding 101 is hardly thermally diffused. As shown in ()), the refractive index distribution in the AA cross section remains stepwise, and only the core is reduced in diameter by heating and stretching.

FIG. 2B shows that the core 202 is stretched during stretching.
The core 202 having a higher refractive index than the cladding 201 is reduced in diameter by heating and stretching the optical fiber 200 at a temperature at which the dopant is thermally diffused at a low speed over a period of several minutes over a period of several minutes. Is diffused and the dopant of the core is diffused to form a Gaussian distribution as shown by the solid line in FIG.

Here, since the two optical fibers are extended by almost the same length, their outer shapes are substantially the same.

Next, the optical fibers 100 and 200 drawn in advance under the different heat diffusion conditions are connected to each other as shown in FIG.
The alignment jigs 301 and 301 are aligned as shown in FIG. 2A, and the alignment jigs 301 and 301 are rotated in opposite directions as shown in FIG.
3 and 203 are brought into close contact. Then, as shown in FIG. 3 (c), the contact portions are connected to heat sources 401 and 40 such as an oxyhydrogen burner.
1 and heat to fuse. Further, as shown in FIG. 4D, the joining portion is formed by heating and stretching the fused portion while sliding the heat source 401 while changing the temperature. still,
Although not shown in the drawing, during fabrication, one end of one optical fiber is excited using a stabilized light source or the like, and light emitted from the other end of the two fibers is monitored with a power meter, and When the branching ratio is obtained, the stretching is stopped.

The branch ratio of the optical fiber coupler thus manufactured is shown by the solid line (a) in FIG. The two optical fibers used in the above examples are the same commercially available products, and the propagation constant of the coupling part due to the thermal diffusion of the dopant in the core despite the fact that the shape of the extended coupling part is substantially the same. And incomplete mode coupling is realized. In addition, by pre-stretching, the branching ratio of the broadband optical fiber coupler manufactured by the conventional pre-stretching method shown by the one-dot chain line (b) in FIG. It can be seen that the wavelength dependence has been further reduced. The branching ratio of an optical fiber coupler in which a bonding portion is formed by directly fusing and stretching an optical fiber having a different refractive index distribution by previously performing a thermal diffusion treatment of a dopant without heating and stretching in advance as in the present invention. Experiment 3
Only the same level as that shown by the alternate long and short dash line (b) was obtained.

As described above, in the optical fiber coupler according to the present invention, the refractive index distribution is changed by the thermal diffusion treatment, and each optical fiber is pre-stretched by substantially the same length before the fusion-stretching. Can be made substantially uniform over a wide band.

Further, since the pre-stretching is performed on each optical fiber by substantially the same length, the symmetry of the outer diameter of the coupling portion is not impaired, and when tension is generated in the axial direction of the optical fiber,
No shear stress or the like due to a stress difference is generated at the fusion starting point, and the mechanical strength is high and the long-term reliability is excellent.

Further, if the manufacturing method is such that the thermal diffusion conditions are varied by varying the heating and stretching speed as in the above embodiment, the thermal diffusion step and the stretching step can be performed by the same apparatus.

In the above embodiment, an example in which two optical fibers are used has been described. However, the present invention is not limited to this, and at least one optical fiber may be used even if two or more optical fibers are used. If the refractive index distribution of the portion to be joined is made different by the thermal diffusion treatment of the dopant, and the thermal diffusion portion of each optical fiber is pre-stretched by approximately the same length and fusion-stretched, the wavelength dependence is the same as above. The property can be extremely reduced.

Further, in the above embodiment, an example in which the stretching step and the thermal diffusion step are performed simultaneously is shown. However, the present invention is not limited to this. For example, the thermal diffusion step and the thermal stretching step may be performed independently. In the case of performing this independently, for example, in a heat diffusion step, a heat diffusion condition such as a heating time or a heating temperature is changed so that a portion to be a coupling portion of at least one optical fiber among a plurality of optical fibers and another optical fiber. It is conceivable that the refractive index distributions of the portions to be joined are made different in advance, the portions to be joined are aligned for each optical fiber, and the optical fibers are simultaneously stretched by the same length at the same speed. As described above, regardless of whether the thermal diffusion step and the heating and stretching step are performed simultaneously or independently, various modes can be considered depending on differences in conditions such as heating time, heating temperature, and stretching speed.

[0023]

As described above, the present invention can provide a broadband optical fiber coupler having high mechanical strength, excellent long-term reliability, and a smaller wavelength dependency than conventional ones.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 (a-1) and (b-1) are pre-drawn optical fibers used in producing an optical fiber coupler in one embodiment of the present invention, and (a-2) and (b-2) are It is a figure which shows the refractive index distribution in section AA and BB.

FIG. 2 is an explanatory view showing one embodiment of a method for manufacturing an optical fiber coupler of the present invention.

FIG. 3 is a graph showing the wavelength dependence of the branching ratio of a broadband optical fiber coupler manufactured using the conventional pre-stretching method and the manufacturing method of the present invention.

[Explanation of symbols]

 100, 200 optical fiber 101, 201 clad 102, 202 core

Claims (3)

(57) [Claims] 2. An optical fiber coupler comprising: a plurality of optical fibers, wherein a plurality of optical fibers are fused and drawn to form a coupling portion; The coupling portion is formed, and the refractive index distribution of at least one optical fiber and another optical fiber in the coupling portion is made different by the thermal diffusion treatment of the dopant, so that the branching ratio is wide. An optical fiber coupler characterized in that the optical fiber coupler is made substantially uniform over the entire area. 2. A method of manufacturing an optical fiber coupler, comprising forming a coupling portion by fusing and stretching a part of a plurality of optical fibers, wherein a portion to be a coupling portion of at least one optical fiber and another optical fiber. A thermal diffusion step of a dopant that pre-heats and changes the refractive index distribution of a portion to be a coupling portion of a fiber, and simultaneously or independently heats a portion to be a coupling portion of each optical fiber simultaneously or independently of the thermal diffusion step. An optical fiber coupler comprising: a drawing step of drawing in advance by substantially the same length; and a fusion drawing step of forming a joint by fusing and stretching the drawn portions of the drawn optical fibers. Manufacturing method. 3. The method according to claim 1, wherein, when the stretching step and the thermal diffusion step are performed simultaneously, the refractive index distribution is varied by varying the stretching speed of at least one optical fiber and another optical fiber. A method for manufacturing the optical fiber coupler according to claim 2.