JPH02236507A - Wide band optical fiber coupler - Google Patents

Abstract

PURPOSE:To improve the yield in a single mode optical fiber coupler, and also, to realize the conversion to a wide band in a polarization maintaining optical fiber coupler by obtaining the coupler by welding and drawing optical fibers in which the outside diameters of the clads are different from each other. CONSTITUTION:The part corresponding to an optical coupling part 4 of optical fibers 1a, 1b in which the outside diameters of clads are different from each other is welded (weak welding) by an oxyhydrogen burner to each other, and subsequently, by weakening heating power, the vicinity of a welded part is brought to drawing, while waving the burner in the lengthwise direction of the optical fiber, and an optical coupling part 4 is manufactured. In this case, since the outside diameters of the clads are different, the shape of the optical coupling part is specified with high accuracy as a matter of course. In such a way, a wide band coupler of an asymmetrical structure can be obtained uniformly and with high accuracy without executing pre-drawing in a single mode optical fiber.

Description

[Detailed Description of the Invention] Industrial Applications The present invention relates to an optical fiber coupler used for branching and coupling optical signals in optical communication systems, etc., and particularly relates to an optical fiber coupler used for branching and coupling optical signals in optical communication systems. The present invention relates to a low-bandwidth optical fiber coupler.

2. Background Art and its Problems In recent years, with the advancement of optical signal transmission technology, various types of optical communication systems have been proposed and put into practical use. Against this background, it is possible to split one or more types of optical signals propagating through one optical transmission line into multiple transmission lines, or to combine and superimpose one or more types of optical signals propagated through multiple transmission lines. Demand is increasing rapidly.

One of the components that performs this function is an optical fiber coupler. This optical fiber coupler has low loss and excellent connectivity with optical fibers that are light transmission lines. In addition, it is necessary to satisfy the demand for low cost, and several methods of manufacturing optical fiber couplers have been considered for this purpose, but
The stretching method has the highest productivity, and the price is expected to continue to drop rapidly in the future.

Conventionally, optical fiber couplers have been manufactured by combining and stretching optical fibers with the same outer diameter and a diameter, and the optical branching ratio of this symmetrical optical fiber coupler with the same outer diameter and diameter is shown in Figure 9. (As shown in al, there is a characteristic that it changes periodically between 0% and 100% depending on the wavelength of the optical signal.

On the other hand, it is also desired that a so-called broadband optical fiber coupler, in which the optical branching ratio does not easily change depending on the wavelength and the wavelength dependence of the optical branching ratio is small, is also desired. This has the advantage of being excellent in compatibility and compatibility with branching multiple wavelengths.

Such a broadband optical fiber coupler is disclosed in the international application PCT/GB86/00445. That is, one of the two single mode optical fibers is mainly drawn in advance (referred to as pre-stretching) and then fused to obtain a broadband optical fiber. By changing the propagation constant of single mode optical fiber by pre-stretching, the maximum value of optical coupling degree is 10.
A specific value of less than 0% is set, and the wavelength flatness of the branching ratio in the vicinity thereof is utilized. Figure 9(b) is an example of the characteristics of a broadband optical fiber coupler with a wavelength of 1.3 μm and a branching ratio of 50%. Change flattens out.

However, when producing a broadband optical fiber coupler by this pre-stretching, there is a drawback that uniform pre-stretching is difficult.

In other words, if pre-stretching fails, strict shape control cannot be achieved, in other words, it is not possible to control the outer diameter distribution in the length direction, and as a result of the inability to obtain precision, the optical fiber may bend during fusion. There are problems in that it is difficult to control the set value of the branching ratio, the degree of flattening, or the excessive loss of the output light relative to the input light. On the other hand, there is a problem in that the production yield of asymmetric optical fiber couplers due to this bristle drawing is extremely low.

Next, when focusing on a polarization-maintaining optical fiber instead of the single-mode optical fiber as described above, the optical branching ratio of this polarization-maintaining optical fiber coupler will differ due to the difference between the X and Y polarizations. , as shown in FIG. 10, has the characteristic of periodically changing between 0% and 100% depending on the wavelength of the optical signal.

In the most common 3 dB coupler, the optical branching ratio becomes most rapid depending on the wavelength, so the optical branching ratio changes significantly due to variations in the light source wavelength or temperature changes in the wavelength. Therefore, it is impossible for one polarization-maintaining optical fiber coupler to support multiple types of light sources.

As a result, a broadband optical fiber coupler is desired for polarization-maintaining optical fiber couplers as well, but to date, there has been no polarization-maintaining optical fiber coupler that has achieved a wide band.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present invention aims to provide a broadband optical fiber coupler that improves the yield of a single mode optical fiber coupler and is the first to realize a broadband polarization maintaining optical fiber coupler. purpose.

Means for Solving the Problems> The basic idea of the present invention for achieving the above object is to obtain a coupler by fusing and stretching optical fibers whose claddings have different outer diameters.

Since the outer diameters of the claddings are different, the shape of the optical coupling part must be specified with high precision, making it possible to use broadband couplers with an asymmetric structure uniformly without pre-stretching for single-mode optical fibers. and can be obtained with high precision.

Embodiments Here, embodiments of the present invention will be explained with reference to FIGS. 1 to 8. Note that FIGS. 1 to 5 show examples of single-mode optical fiber couplers, and FIGS. 6 to 8 show examples of polarization-maintaining optical fiber couplers.

(Example 1) Relative refractive index difference Δ=0.3%, cutoff wavelength 1.20 μm
An optical fiber coupler was fabricated using two single mode optical fibers with cladding outer diameters of 125 μm and 110 μm.

In other words, the parts of these two single mode optical fibers that correspond to the optical coupling part are fused together (weakly fused) using an oxyhydrogen burner, and then the burner is heated in the longitudinal direction of the optical fibers by weakening the heating power. Wavelengths of 1.3 μm and 1.5 μm were applied near the fused portion while shaking the area.
An optical coupling part was produced by stretching until the peak of the optical coupling degree at 55 μm became the same.

Figure 1 shows a cross section of the optical coupling section that was fabricated, with la, lb
are single mode optical fibers, 2m and 2b are their cores,
3a and 3b are cladding.

The optical coupling part was then fixed to a quartz glass substrate and housed in a metal case to form a broadband optical fiber coupler.

The entire optical fiber coupler is shown in Figure 2, with la, l
5 is a quartz glass substrate, and 6 is a metal case.

The broadband optical fiber coupler thus formed had a low excess loss of 0.2 dB. This means that by using optical fibers with different cladding outer diameters, such as 125 μm and 110 μm, the controllability of the outer diameter is much better than in conventional bristle drawing, and non-uniformity in shape can be suppressed. This is thought to be because the optical fiber is less deformed due to fusion splicing, and the increase in loss caused by this is suppressed.

FIG. 3 shows the wavelength dependence of the optical branching ratio of the broadband optical fiber coupler of this example. Optical branching ratio is wavelength 1.2-1
．． It is within 20±3% at 6 μm, and it can be seen that the wavelength dependence of the branching ratio is flattened. The controllability of the branching ratio in this case was significantly improved to 80% or more in this example, compared to the poor yield of several tens of percent in the case of conventional bristle drawing.

(Example 2) Specific refractive index difference Δ=0.3%, cutoff wavelength 1420μ
An optical fiber coupler was fabricated using a single mode optical fiber with an outer diameter of 125 μm.

Here, a part of one optical fiber (approximately 3 centimeters long) is immersed in hydrochloric acid for about 2 minutes, and the outer diameter of this optical fiber is 120 μm.
After etching, a broadband optical fiber coupler was manufactured using the same steps as in Example 1.

The cross section of the optical coupling part of this broadband optical fiber coupler is
As shown in the figure, 1c is a single mode optical fiber with an outer diameter of 125 μm, ld is a single mode optical fiber with an outer diameter of 120 μm, 2c and 2d are their cores, and 3c and 3d are claddings.

The broadband optical fiber coupler thus obtained had a low excess loss of 0.25 dB.

This means that by etching one optical fiber, the uniformity of the outer diameter is improved compared to conventional pre-stretching, and the length of the etched part can be made much longer than the pre-stretched part. This is thought to be because there is no change in the outer diameter of the fused portion, so there is little deformation of the optical fiber due to fusion, and the increase in loss due to this <<ζ> is suppressed.

FIG. 5 shows the wavelength dependence of the optical branching ratio of the broadband optical fiber coupler of this example. It can be seen that the wavelength dependence of the optical branching ratio is flat within 50±4% within the wavelength range of 1.2 to 1.5 μm. In this case, the control of the branching ratio was improved compared to the case of conventional pre-stretching, and the yield was 70% or more.

As described above, in Examples 1 and 2, the branching characteristics were broadened not by pre-stretching, but by using optical fibers with different cladding outer diameters, or by etching one of the optical fibers in advance. We were able to significantly improve the controllability of the outer diameter and improve the productivity of optical fiber couplers like never before.

Although the present example shows the case where the optical branching ratio is set to 20% or 50%, a desired branching ratio can be obtained by changing the fusion/stretching conditions when creating the joint.

(Example 3) FIG. 6 shows a cross section of a polarization-maintaining optical fiber constituting a coupler. In this figure, lla, 1lb are cores, 1
2 a and 1 2 b are clads, and 13 a and 13 b are stress applying parts. Both relative refractive index difference Δ-0.24%
, the core diameter is 6.5 μm, the stress applying part diameter is 41 μm, and the boron concentration in the stress applying part is 15 moj%, but only the outer diameter of the cladding is different, 12a is 125 μm, 12b is 120 μm.
It is μm.

In manufacturing, these two optical fibers are aligned parallel to each other, and the stress applying portions 13a,
13b are parallel to each other, and 2
The two cores 11a and llb are aligned perpendicular to the fiber surface and are fused using an oxyhydrogen burner (weak fusion), then the burner is moved in the length direction of the optical fiber by weakening the heating power. While shaking the area near the fused part, use X-polarized wave with a wavelength of 1.
An optical coupling part was produced by stretching until the peaks of the degree of optical coupling at 3 μm and 1.55 μm were the same.

FIG. 7 shows a cross section of the optical coupling portion of this broadband wave-maintaining optical fiber coupler. In the figure, 14a and 14b are Oi wave holding light beams 77, lla and llb are their cores, and 12a
, 12'b are claddings, and 13a, '13b are stress applying parts.

Then, this optical coupling part was fixed to a quartz glass substrate and housed in a metal case to form a broadband polarization-maintaining optical fiber coupler, with a structure similar to that shown in FIG. 2.

FIG. 8 shows the wavelength dependence of the optical branching ratio of this broadband polarization-maintaining optical fiber coupler. Optical branching ratio is wavelength 1.2-1
．． Within the range of 6 μm, it is within 50±3% for X-polarized waves and 60±3% for Y-polarized waves, indicating that the wavelength dependence of the branching ratio is flattened. In addition, this polarization-maintaining optical fiber coupler 1; LC for both EX and Yl waves is 1.2-71.55 μm.
In the wavelength range of , the excess loss was less than 0.8 dB under crosstalk of 20 dB, II, t.

(Example 4) Relative refractive index difference Δ=0.24%, core diameter 6.5 μm 1 stress applying part diameter 41 μm, poron concentration in stress applying part 15 moI%
A broadband return-maintaining optical fiber coupler was fabricated using a polarization-maintaining optical fiber with an outer diameter of 125 μm.

Here {yo, part of one optical fiber (length about 3c+T
+) was immersed in hydrochloric acid for about 2 minutes, and the outer diameter of this optical fiber was etched to 120 μm, and then a broadband optical fiber coupler was produced in the same process as in Example 3.

This broadband polarization-maintaining optical fiber coupler had a crosstalk of 18' dB or less and an excess loss of 1.0 dB or less in the wavelength range of 1.2 to 1.55 μm for both X and Y damaged waves. The wavelength dependence of the optical branching ratio showed the same change as in Example 3, and was flattened.

Effects of the Invention As mentioned above, in the present invention, optical fibers are not stretched, but instead optical fibers with different cladding shapes are used, or the outer shape of the optical fibers is changed by etching. The uniformity of the outer diameters of the two optical fibers is significantly improved, so there is less deformation of the optical fibers due to fusion, the controllability of the optical branching ratio is improved, and reliability is increased, and the cause of increased loss is suppressed. , the reproducibility of branching characteristics is improved and the yield is high.

Furthermore, in the present invention, polarization-maintaining optical fibers with different cladding outer diameters are used, or the outer shape of one polarization-maintaining optical fiber is changed by etching, so that optical branching can be achieved without increasing crosstalk or excessive loss. It becomes possible to flatten the wavelength property.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of the optical coupling part of the broadband optical fiber coupler of Example 1, Figure 2 is a diagram showing the broadband optical fiber coupler, and Figure 3 is the optical branching ratio of the broadband optical fiber coupler of Example 1. 4 is a cross-sectional view of the optical coupling part of the broadband optical fiber coupler of Example 2. FIG. 5 is a characteristic diagram showing the wavelength dependence of the optical branching ratio of the broadband optical fiber coupler of Example 2. FIG. 6 is a cross-sectional view of the polarization-maintaining optical fiber used in Example 3, and FIG. 7 is a characteristic diagram showing Example 3.
FIG. 8 is a characteristic diagram showing the wavelength dependence of the optical branching ratio of the broadband sidewave-maintaining optical fiber coupler of Example 3, and FIG. A characteristic diagram showing the wavelength dependence of the optical branching ratio of the symmetrical optical fiber coupler (al and broadband optical fiber coupler (b)). Figure 10 shows the wavelength dependence of the optical branching ratio of the conventional symmetrical polarization-maintaining optical fiber coupler. In the drawing, la, lb, lc, and ld are single mode optical fibers, 2a, 2b, 2c, 2d, lla, and llb are cores, and 3a, 3b, 3c, 3d, 12a, and 12b are cladding, 4 is an optical coupling part, 5 is a quartz glass substrate, 6 is a metal case, 13a and 13b are stress applying parts, 14a and 14bl; t are wave-maintaining optical fibers.

Claims (3)

[Claims] (1) An optical fiber coupler formed by fusing and stretching two optical fibers, characterized in that the optical fiber coupler is formed of optical fibers whose claddings have different outer diameters. (2) The broadband optical fiber coupler according to claim (1), wherein a part of one optical fiber is thinned by etching to make the optical fibers have different outer diameters of cladding. (3) The broadband optical fiber coupler according to claim (1) or (2), wherein the two optical fibers are polarization maintaining optical fibers.