JPH02207203A - Optical fiber coupler - Google Patents

Abstract

PURPOSE:To eliminate a difference in coupling degree due to a polarized wave having single wavelength by arranging two polarization maintaining optical fibers, which each have a core, stress inducing parts on both its sides, and a clad surrounding them, in parallel, forming a fusion drawn part as part of them, and sectioning the cores elliptically. CONSTITUTION:The two polarization maintaining fibers 13 each have the elliptically sectioned core 12, the stress inducing parts 5 arranged on both short-diameter sides of the core 12, and the clad 6 surrounding them and are arranged so that the (y)- polarization axes of the both are aligned and the (x)-polarization axes are parallel to each other. The fusion drawn part 14 is formed by reducing the diameters of both the polarization maintaining fibers 13 by fusion drawing so that the cores 12 are put close to each other. The size difference between the long-diameter side and short- diameter side of the sectional shape of each core 12 is reduced by the drawing and the core section changes from the elliptic shape to a nearly circular shape. Thus, the optical fiber coupler 11 has both the cores 12 sectioned almost circularly at the fusion drawn part 14, an increase in the propagation constant difference of each polarized wave can be suppressed, and the difference in coupling degree between an (x) and a (y) polarized wave having single wavelength is eliminated.

Description

[Detailed Description of the Invention] "Field of Industrial Application" This invention relates to an optical fiber coupler used for branching and combining optical signals in optical fiber communication systems, optical fiber sensors, etc. In particular, the present invention relates to an optical fiber using a stress-applied polarization-maintaining optical fiber.

"Prior art and its problems" FIG. 8 is a diagram showing an example of a conventional polarization-maintaining optical fiber coupler, in which symbol I is an optical fiber coupler, 2 is a polarization-maintaining optical fiber, and 3 is a fusion spliced optical fiber. This is a stretching part. As shown in FIG. 9, this polarization-maintaining optical fiber 2 is composed of a core 4 having a circular cross section, stress applying sections 5, 5 provided on both sides of the core 4, and a crat 6 surrounding them.

Normally, in a stress-applied polarization-maintaining optical fiber, birefringence is imparted by the non-axisymmetric stress distribution inside the core 4, and as shown in FIG. The propagation constants are different.

This optical fiber coupler I, as shown in FIG.
It is produced by heat-sealing a part of it using a high-temperature heating source such as an oxyhydrogen burner, and then stretching it to form the fused and stretched portion 3 .

In the fusion-stretching section 3 of the optical fiber coupler 1 produced in this way, as shown in FIG. (in example ① shown in this figure, it is extended in the chi polarization axis direction), and the cross section becomes an ellipse. Along with this shape change, the difference in the propagation constant in each polarization axis direction described above increases, and as a result, as shown in Figure 12, there is a difference in the degree of coupling between χ polarization and y polarization at a single wavelength. There was a problem that occurred.

When this difference in coupling degree is used in an actual device, there is either no difference in coupling degree depending on the polarization at a single wavelength, or, on the contrary, each polarization is used as a polarization beam splitter. Preferably separated.

In addition, the difference in the degree of coupling due to polarization at a single wavelength can be adjusted to some extent by adjusting the fusion length and the stretching length, but generally the difference in the degree of coupling is about 10 to 50%. There are many things to put away.

The present invention was made in view of the above circumstances, and aims to provide an optical fiber coupler that has no difference in coupling degree due to polarization at a single wavelength, or an optical fiber coupler that can accurately separate each polarization. It is said that

"Means for Solving the Problems" As a means for achieving the above object, the present invention provides two parallel polarization maintaining optical fibers each having a core, stress applying sections provided on both sides of the core, and a cladding surrounding them. The optical fiber coupler is formed by forming a fused and stretched portion in a part thereof, and the optical fiber coupler is constructed by making the core of the polarization maintaining optical fiber have an elliptical cross-sectional shape.

``Action'' By making the core elliptical, ■A non-axisymmetric stress distribution can be created inside the core by applying stress.

■The core has an elliptical shape and the refractive index distribution is non-axisymmetric.

An optical fiber coupler is constructed using a polarization-maintaining optical fiber that has been given birefringence due to the two effects of Wave-maintaining optical fibers are arranged in parallel so that their y-polarization axes match and their χ-polarization axes are parallel, and a fused and stretched portion is formed in a part of the fibers to form an optical core 4 to an optical coupler? Therefore, it is possible to suppress an increase in the difference in propagation constant of each polarized wave at the fusion-stretching part, and it is possible to obtain a structure in which there is no difference in the degree of coupling between the χ polarized wave and the y polarized wave at a single wavelength.

In addition, two polarization-maintaining optical fibers with the long diameter side of the core arranged along the χ polarization axis direction are arranged in parallel so that their y polarization axes coincide and the χ polarization axes are parallel. By forming a fused and stretched portion on the optical fiber coupler,
It is possible to separate χ polarization and y polarization at a single wavelength.

Embodiment FIGS. 1 to 3 are diagrams showing a first embodiment of the present invention, and reference numeral 11 in the figures is an optical fiber coupler.

As shown in FIG. 2, this optical fiber coupler 11 includes a core 12 having an elliptical cross section, stress applying parts 5 arranged on both sides of the short diameter side of this core 12, and a cladding 6 surrounding them. It is constructed by arranging two polarization-maintaining optical fibers 13 in parallel and forming a fused and stretched portion 14 in a part thereof.

The two polarization-maintaining optical fibers 13 are arranged in parallel so that their y-polarization axes match and their chi-polarization axes are parallel.

In the fusion-stretching section 14, as shown in FIG. 3, both polarization-maintaining optical fibers 13 are fused and stretched to reduce their diameters, and the respective cores 12 are brought close to each other. In addition, the cross-sectional shape of these cores 12 has a smaller dimensional difference between the longer diameter side and the shorter diameter side due to stretching, and the core cross section changes from an elliptical shape to a shape close to a circular shape.

In the optical fiber coupler If configured in this way, the cross-sectional shape of both cores I2 in the fusion-stretching part 14 becomes close to circular, and it is possible to suppress an increase in the difference in propagation constant of each polarized wave, and it is possible to suppress the increase in the propagation constant difference of each polarized wave. It is possible to eliminate the difference in the degree of coupling between the χ polarization and the y polarization.

FIGS. 4 and 5 are diagrams showing other embodiments of the present invention. In this example, as shown in FIG. 4, two polarization-maintaining optical fibers 15 each having a stress applying section 5 on both sides of the major axis of an elliptical core 12 are connected so that their y-polarization axes coincide. , χ polarization axes are parallel to each other, and a fused and stretched portion 16 is formed in a part of the optical fiber coupler to form a single optical fiber coupler.

As shown in FIG. 5, in this fused and stretched portion 16, both cores 12 are further stretched into an elongated shape along the χ polarization axis direction, with their short diameter sides facing each other. By making the cross-sectional shape of both cores I2 elongated in this way,
In this fused and stretched portion I6, there is a large difference in the propagation constants of the x-polarized wave and the y-polarized wave.

The optical fiber coupler configured in this manner has both cores 12 having a substantially elliptical cross-sectional shape that is more elongated than the core I2 of the polarization-maintaining optical fiber 15, so that the χ polarization and the By increasing the difference in the propagation constants of the y-polarized waves, it is possible to separate the χ-polarized waves and the y-polarized waves at a single wavelength.

(Manufacturing Example 1) An optical fiber coupler was manufactured using a polarization-maintaining optical fiber having the same configuration as that shown in FIG. 2.

The polarization-maintaining optical fiber has an outer diameter of 125 μm, a relative refractive index difference of 0.38%, a Ge-doped core, the core has a major axis in the y polarization axis direction, and the ratio of major axis to minor axis is approximately 2:1 (8 ,7μm
:4.571m) was used.

Two polarization-maintaining optical fibers were aligned so that their y-polarization axes matched and their χ-polarization axes were parallel, and they were fused together by approximately 3 mm using an oxyhydrogen burner. , 5mm, as shown in Figure 6, the wavelength was approximately 5mm at a wavelength of 13μm.
A polarization-maintaining optical fiber with a coupling degree of 0% and almost no difference in coupling degree with respect to wavelength between chi-polarized waves and y-polarized waves was obtained.

Note that even in the fused and drawn cross-section at this time, the core shape had a slightly longer diameter (about I I 2) in the y polarization axis direction.

(Manufacturing Example 2) An optical fiber coupler was manufactured using a polarization maintaining optical fiber having a configuration similar to that shown in FIG.

The polarization maintaining optical fiber has an outer diameter of 125 μm and a relative refractive index difference of 0.
33%, Ge-doped core, the core has a major axis in the χ polarization axis direction, and the ratio of major axis: minor axis is approximately 2: I (9,1 μ
m・4.471 m) was used.

These two polarization-maintaining optical fibers are placed in parallel and a part of them is heated with an oxyhydrogen burner to fuse approximately 2 mm, and then approximately 3 mm is fused.
When stretched to 5.6 mm, as shown in Figure 7, a polarization-maintaining optical fiber coupler (polarization beam splitter) was obtained with a characteristic that χ polarization and y polarization were almost separated at a wavelength of 13 μm. .

In addition, in the fusion-stretched cross section at this time, the core shape had a long axis to short axis ratio of about 3.1, and the long axis was in the χ polarization axis direction.

"Effects of the Invention" As explained in "2" below, the present invention achieves the following effects by having the configuration described in "1".

As a polarization-maintaining optical fiber with an elliptical cross-section core,
Two polarization-maintaining optical fibers with the short diameter side of the core arranged along the χ polarization axis direction are arranged in parallel so that their y polarization axes coincide and the χ polarization axes are parallel. By forming an optical fiber coupler by forming a fusion-stretched part in the fusion-stretched part, it is possible to suppress the increase in the propagation constant difference of each polarized wave at the fusion-stretched part. It is possible to obtain an optical fiber coupler with no difference in coupling degree between the two.

In addition, two polarization-maintaining optical fibers with the long diameter side of the core arranged along the χ polarization axis direction are arranged in parallel so that their y polarization axes coincide and the χ polarization axes are parallel. By forming a fused and stretched portion on the optical fiber coupler,
It is possible to obtain an optical fiber coupler (polarization beam splitter) that can separate chi-polarized light and y-polarized light at a single wavelength.

[Brief explanation of the drawing]

1 to 3 are diagrams showing a first embodiment of the present invention, in which FIG. 1 is a side view of an optical fiber coupler, FIG. 2 is a sectional view taken along line A-A in FIG. 1, and FIG. is a sectional view taken along line B-B in Figure 1,
4 and 5 are diagrams showing other embodiments of the present invention, in which FIG. 4 is a cross-sectional view of a polarization-maintaining optical fiber, FIG. 5 is a cross-sectional view of a fused and stretched portion, and FIG. 7 is a graph that does not explain the present invention in detail, FIG. 8 is a side view showing an example of a conventional optical fiber coupler, and FIGS. 9 to 11 explain a method of manufacturing a conventional optical fiber coupler. FIG. 12 is a graph for explaining the difference in the degree of coupling of each polarized wave i:l in a conventional optical fiber coupler. ...Optical fiber coupler...Core, 15...Polarization maintaining optical fiber 16...Fusion and stretching part.

Claims (1)

[Claims] An optical fiber coupler in which two polarization-maintaining optical fibers each having a core, stress-applying parts provided on both sides of the core, and a cladding surrounding them are arranged in parallel, and a fused and stretched part is formed in a part of the polarization-maintaining optical fibers. , an optical fiber coupler characterized in that the core of the polarization-maintaining optical fiber has an elliptical cross-sectional shape.