JPH02232608A - Wide band polarization maintaining optical fiber coupler - Google Patents

Abstract

PURPOSE:To make the wavelength dependency of branching ratio small and to obtain nearly constant branching ratio in a wide waveguide range by forming an optical coupling part between polarization maintaining single mode optical fibers. CONSTITUTION:The coupler is provided with two polarization maintaining single mode optical fibers 1a and 1b having at least one different value among the values of a mode field diameter, a cutoff wavelength and a mode double refractive index and the optical coupling part 5 is formed between the polarization maintaining single mode optical fibers. In this case, the maximum branching ratio can be designed to be an optional value from 0% to 100% corresponding to the degree of the difference of fiber parameter between two optical fibers. Since the wavelength dependency of the branching ratio is small near the maximum branching ratio, the optical fiber coupler having wide band characteristic is obtained.

Description

[Detailed Description of the Invention] Industrial Application Fields The present invention relates to a broadband polarization-maintaining optical fiber coupler, which is useful for branching or merging optical signals in various optical communication systems. .

<Prior Art> A polarization-maintaining optical fiber coupler is required for coherent optical communication or the distribution of optical fiber sensors. A polarization-maintaining optical fiber coupler is manufactured using a polarization-maintaining optical fiber by a fusion drawing method or a polishing method.

The fusion-stretching method is a method in which the side surfaces of two optical fibers are fused together, and then the fused portion is stretched into a long and thin shape to form an optical coupling portion at the fused portion of the two optical fibers. In the optical fiber coupler formed by the fusion-drawing method described in Part B, the cores of the two optical fibers are close to each other at the optical coupling part, so when an optical signal is input from one end of one optical fiber, this optical signal is A portion of the optical signal leaks into the other optical fiber when passing through the coupling section. Therefore, this optical fiber coupler can be used as an optical signal branching type or merging device.

Furthermore, when using a birefringent single mode fiber, that is, a return wave maintaining type optical fiber, which can maintain normal S polarization over a long distance, when fusion-stretching it, A polarization-maintaining optical fiber coupler can be fabricated by aligning the principal axes of birefringence of the optical fibers.

FIG. 4 is a structural diagram showing a polarization-maintaining optical fiber coupler manufactured by the fusion drawing method.

In the figure, la and lb are return-maintaining optical fibers,
2m, 2b is core, 3g, 3b is clad, 4a, 4b
5 is a stress applying part, and 5 is an optical coupling part. In the polarization-maintaining optical fiber coupler shown in the figure, the input optical signal I0 is branched into optical signals 1, , I, and output.

Furthermore, the polishing method is a method of joining two optical fibers whose cladding surfaces have been partially polished, and provides the same characteristics as the fusion-stretching method.

FIG. 5 (Jl) is a structural diagram showing a polarization-maintaining optical fiber coupler manufactured by a polishing method, and FIG. 5 (Jl) is a cross-sectional view of the optical coupling portion thereof. In both figures, lc and ld are polarization-maintaining optical fibers, 2c and 2d are cores, 3c and 3d are claddings, 4c and 4d are stress applying parts, 5 is an optical coupling part, and 6a and a
d is a quartz block.

The polarization-maintaining optical fiber coupler as described above has conventionally been formed by two optical fibers la and lb or la and ld having the same fiber parameters.

FIG. 6 is a graph showing an example of the branching ratio one-wavelength characteristic of a polarization-maintaining optical fiber coupler manufactured by the fusion drawing method according to the prior art, that is, the fiber parameters of two optical fibers la and lb are equal. . The optical fiber coupler having the characteristics shown in the figure was manufactured as a 3 dB splitter for the wavelength band of 1.3 μm.

As is clear from FIG. 6, the polarization maintaining optical fiber coupler according to the above-mentioned prior art has a wavelength dependence in the branching ratio, so that the polarization maintaining optical fiber coupler according to the above-mentioned prior art has a wavelength dependence of 3 dB only in a narrow wavelength range. There was a problem that it did not function as a branched moromi.

SUMMARY OF THE INVENTION In view of the above-mentioned prior art, it is an object of the present invention to provide a polarization-maintaining optical fiber coupler that has a branching ratio that has a small wavelength dependence and has a substantially constant branching ratio over a wide wavelength range.

Means for Solving the Problems> The configuration of the present invention that achieves the above object includes two polarization-maintaining units in which at least one of the mode field diameter, cutoff wavelength, and mode birefringence has a different value. It is characterized in that it has one-mode optical fibers and that an optical coupling portion is formed between these polarization-maintaining single-mode optical fibers.

Function> A conventional polarization-maintaining optical fiber coupler consists of two optical fibers with the same fiber parameters. Therefore, as shown in Figure 6, the branching ratio wavelength characteristic changes with the wavelength. It exhibits fully bonded characteristics in which the branching ratio varies from 0% to 100%. On the other hand, the polarization-maintaining optical fiber coupler of the present invention having the above configuration is designed to have a maximum branching ratio of any value between 0% and 100% depending on the degree of difference in fiber parameters between the two optical fibers. be able to. Furthermore, since the wavelength dependence of the branching ratio is small near the maximum branching ratio, the optical fiber coupler has broadband characteristics.

Example of implementation Embodiments of the present invention will now be described in detail based on the drawings.

Since the optical fiber coupler according to the embodiment of the present invention is formed by the same fusion drawing method as that shown in FIG. 4, its external shape is the same as that of the optical fiber coupler shown in FIG. Table 1 also shows polarization-maintaining optical fibers la and l having various fiber parameters used in the same example.
This shows the specifications of b.

In the same table, A written in the fiber section is an optical fiber of 1 m, and Bl ~ B3, Cl ~ C3 and D1 ~ D3 are A
This is the optical fiber 1b to be combined with the optical fiber 1b. Well, Bl~B3 have different mode field diameters than A, 01~C
3 have different cutoff wavelengths, and D1 to D3 have different mode birefringence.

First Embodiment> FIG. 1 is a graph showing the branching ratio single wavelength characteristic of the first embodiment of the present invention. This figure shows the characteristics of a polarization-maintaining optical fiber obtained by drawing the In the figure, the solid line, broken line, and dashed-dotted line indicate mode field diameters of 7.5 pm and 8.0 pm.
pm and 8.5 μm optical fiber 1b are shown.

Referring to FIG. 1, it can be seen that near the maximum branching ratio, the wavelength dependence of the branching ratio is small, and as the difference in mode field diameter increases, the maximum branching ratio decreases from 100%. Therefore, the polarization-maintaining type 2
By selecting the values of the mode field diameters of the optical fibers la and lb, a broadband polarization-maintaining optical fiber coupler with a desired branching ratio can be obtained.

<Second Example> 2s is a graph showing the branching ratio single wavelength property of the second example of the present invention, in which optical fibers la and lb have different cutoff wavelengths for fiber A and fibers C1 to C3. This figure shows the characteristics of a 4ll wave-maintaining optical fiber obtained by fusion-drawing. In the figure, the solid line, broken line, and dashed-dotted line indicate cutoff wavelengths of 1.05 μm and 1.10 pm.
, 115 pm light 77 pm] b).

Referring to FIG. 2, it can be seen that near the maximum branching ratio, the wavelength dependence of the branching ratio is small, and the larger the difference in cutoff wavelength is, the lower the maximum branching ratio is. therefore,
A broadband return-maintaining optical fiber coupler can be obtained by changing the cutoff wavelength.

Third Embodiment FIG. 3 is a graph showing the branching ratio single-wavelength characteristics of the third embodiment of the present invention, in which fiber A and fibers D1 to D3 are used to fuse optical fibers with different mode birefringences. This figure shows the characteristics of a polarization-maintaining optical fiber obtained by stretching. In the figure, solid lines, broken lines, and dotted chain lines indicate moat birefringence of 0.5×10, 1. OX10, 2x10
- Indicate the case of the optical fiber 1b.

Referring to Figure 3, near the maximum branching ratio, the wavelength dependence of the branching ratio is small, and the decrease in the maximum branching ratio is not so remarkable. A fiber coupler can be obtained.

Incidentally, the first to third embodiments of the present invention were all produced by the fusion stretching method. ! The fact that it can also be realized by lm is a natural consequence of the fact that the feature of the present invention is that it is composed of two optical fibers la and lb with different fiber parameters.

In addition, the different fiber parameters need not be limited to just one of the mode field diameter, cutoff wavelength, and mode birefringence. However, the reduction in maximum branching ratio may be greater.

Effects of the Invention> As specifically explained above in conjunction with the embodiments, in the present invention, two wavelength-maintaining optical fibers with different fiber parameters are used, so that polarized light with small wavelength dependence of the branching ratio can be obtained. A retaining optical fiber coupler can be obtained. As a result, when the coupler is applied to co-current optical communications or optical fiber sensors, there is an advantage that it can be used in a wide range of wavelength ranges without changing the specifications.

[Brief explanation of the drawing]

Figures 1 to 3 are graphs showing the branching ratio single wavelength characteristics of the first to third embodiments of the present invention, respectively, and Figure 1 is a graph of two polarization maintaining types with different mode field diameters. When using optical fibers, Figure 2 shows the case where two polarization-maintaining optical fibers with different cutoff wavelengths are used, and Figure 3 shows the case where two polarization-maintaining optical fibers with different mode birefringences are used. This is the case using an optical fiber. Figures 4 and 5 (a) both show structures 4m, 4b, 4c, and polarization-maintaining optical fiber couplers.
4d is a stress applying part, 5 is an optical coupling part, 6c and ad are quartz blocks〇

Claims (1)

[Claims] It has two polarization-maintaining single-mode optical fibers in which at least one of the mode field diameter, cutoff wavelength, and mode birefringence has a different value, and an optical fiber is formed between these polarization-maintaining single-mode optical fibers. 1. A broadband polarization-maintaining optical fiber coupler, characterized in that a polarization-maintaining optical fiber coupler is formed.