JPH0224605A - Manufacture of optical fiber coupling element - Google Patents

Abstract

PURPOSE:To remove the deformation of the geometric shape in the section of optical fibers accompanying fusion drawing by removing a part of the clad of a polarization holding optical fiber and decreasing the interval between the cores of two optical fibers. CONSTITUTION:A part of a circular clad 4 or 5 is removed from an optical fiber which has strain inducing parts 8 and 9 or 10 and 11 arranged at symmetrical positions about a core 6 or 7 to form the polarization holding optical fibers 2 or 3. The central parts of the optical fibers 2 and 3 are welded to each other to form a fusion drawn part 12, thus obtaining a coupling element 1. Then linear polarized light 17 which enters the optical fiber 2 is propagated along a main axis 13 of double refraction and split at the drawn part 12 even to an optical fiber 3, so that linear polarized light beams 18 and 19 are emitted. Even when the linear polarized light is entered into this optical fiber 3, the same result is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of manufacturing an optical fiber coupling element used in the fields of optical communications and optical fiber sensors.

In particular, it relates to methods for manufacturing optical fiber coupling elements, which are used as multiplexers for locally transmitted optical signals in receiving sections of coherent optical communication systems that require stable polarization maintenance, and as components of optical fiber sensors. .

[Conventional technology]

Optical coherent communication systems for long-distance, high-capacity communications and optical fiber sensor systems for precisely measuring various physical quantities are being developed and put into practical use. In addition to using polarization-maintaining optical fibers, these systems require coupling elements that maintain the polarization state in the optical circuits.

An optical coupling device that maintains the polarization state by fusing two polarization-maintaining optical fibers together and bringing their cores close to each other to cause interaction and achieve optical branching and coupling. There is. According to this optical fiber coupling element,
Linearly polarized light incident on one polarization-maintaining optical fiber propagates along the birefringence principal axis of the optical fiber, and is split into the other polarization-maintaining optical fiber at the fusion-stretching section, maintaining both polarizations as linearly polarized light. Emitted from optical fiber.

The method for manufacturing such an optical fiber coupling device involves arranging two polarization-maintaining optical fibers so that their principal axes of birefringence are aligned parallel to each other in the fusion-stretching section, and then heating and stretching a part of the fibers. Conventionally, a method of forming a fused and stretched portion is known.

[Problem to be solved by the invention]

However, in order to obtain a good coupling ratio, when using a polarization-maintaining optical fiber with a diameter of 125 μm, in the conventional manufacturing method, the diameter is usually 30 μm in the process of forming the fused and stretched portion.
It was necessary to make the two cores closer together by making the cores thinner to a certain degree.

Therefore, the mechanical strength of the fused and stretched portion of the optical fiber coupling element manufactured by the conventional manufacturing method was extremely weak. In addition, when forming the fusion-stretched part, fusion-stretching is continued for a long time, so
There is a problem in that the geometrical shape within the cross section of the polarization maintaining optical fiber progresses and the polarization maintaining characteristics deteriorate.

An object of the present invention is to solve these problems.

[Means to solve the problem]

In order to solve the above problems, the manufacturing method of the present invention includes a first step of removing a part of the cladding from a polarization-maintaining optical fiber with a circular cladding in a chord parallel to one of the principal axes of birefringence in its cross section. Then, prepare two polarization-maintaining optical fibers from which a portion of the cladding has been removed, and arrange these two polarization-maintaining optical fibers in parallel so that the string portions of each cross section are in close contact with each other. The method includes a second step and a third step of forming a fused and stretched portion by heating and stretching two polarization-maintaining optical fibers arranged in parallel.

[Effect]

Since a part of the cladding of the polarization-maintaining optical fiber is removed in the first step, when the two-tree polarization-maintaining optical fibers are arranged in parallel in the second step, the cores of the two polarization-maintaining optical fibers are are closely spaced. therefore,
In the formation of the fused stretch portion in the third step, the required bonding ratio is achieved even with a small amount of stretch. Therefore, there is almost no deformation of the geometrical shape in the cross section of the polarization-maintaining optical fiber due to the fusion-stretching, and the fusion-stretching portion is sufficiently thick.

〔Example〕

FIGS. 1A and 1B are process diagrams showing an embodiment of the manufacturing method of the present invention. FIG. 1(A) is a perspective view showing the polarization maintaining optical fiber 2 or 3 before the fusion process. This polarization-maintaining optical fiber 2 or 3 has two stress-applying parts 8.9 or 1 arranged at symmetrical positions around the core 6 or 7.
0.11 by removing part of the circular cladding 4 or 5 from a stressed polarization maintaining optical fiber. The part to be removed is an arcuate section separated by a chord 24 parallel to the line joining the centers of the two stress-applying parts 8.9 or 10.11 so as to leave two stress-applying parts 8.9 or 10.11. This is part 25. At this time, the cladding is removed by mechanical polishing or chemical etching.

Next, a process of fusion-stretching the polarization-maintaining optical fibers 2.3 produced in this manner to each other to form an optical fiber coupling element will be described. FIG. 1(B) is a perspective view showing the main parts of the optical fiber coupling element manufactured by the manufacturing method of this example. This optical fiber coupling element 1 is shown in FIG.
It has a structure in which the stress-applied polarization-maintaining optical fibers 2.3 shown in FIG. Note that the polarization-maintaining optical fibers 2 and 3 are separated from each other at both ends (not shown).

Polarization-maintaining optical fibers 2 and 3 have principal birefringence axes 13.14 and 15.16 that are perpendicular to each other and determined by the arrangement of stress-applying parts 8.9 and 10.11, respectively. By aligning the two polarization-maintaining optical fibers 2.3 with each other at their strings, the principal axes of birefringence 13 and 15 and the principal axes of birefringence 14 and 16 are aligned parallel to each other. Therefore, according to this embodiment, there is no need for the conventional step of adjusting the parallelism of the birefringence principal axes using a microscope or the like, and the alignment of the birefringence principal axes becomes very easy. Moreover, there is an advantage that the parallelism of the principal axes of birefringence hardly deviates during the fusion-stretching process. FIG. 2 shows the cross-sectional shape of the fusion-stretched portion 12, in which the cores 6 and 7 and the stress-applying portions 8.9 and 10.11 are brought closer to each other by the fusion-stretching.

- The linearly polarized light 17 incident on the optical fiber 2 to be held propagates along the principal birefringent axis 13, and is split into the other polarization maintaining optical fiber 3 at the fusion-stretching section 12, forming one linearly polarized light 18. Each is emitted.

This effect is the same when linearly polarized light is incident on the polarization-maintaining optical fiber 3.

In this example, the polarization-maintaining optical fiber before fusion has two stress-applying parts, one of which has a part of the cladding in a chord parallel to a line connecting the centers of the stress-applying parts, which is the principal axis of birefringence. was used, but the present invention is not limited to this.

For example, as shown in FIG. 3, a cladding portion 2 that includes one stress applying portion 22 of the two stress applying portions is a chord parallel to another principal axis of birefringence that intersects perpendicularly to the principal axis of birefringence.
It is also possible to use a polarization-maintaining optical fiber having a structure in which 6 is removed. In this case, as shown in FIG. 4, in the fused and stretched part, a total of two stress-applying parts 20 and 21 face each other, clearly forming a stress birefringence principal axis, so that a combination with a good extinction ratio is achieved. Ru.

FIG. 5 is a cross-sectional view of a polarization-maintaining optical fiber used in yet another embodiment. This polarization maintaining optical fiber 30 is
This is a stress-applied polarization-maintaining optical fiber with a circular cladding in which a core 31 is surrounded by an elliptical stress-applying part 32, with a part of the cladding 33 removed. A part of the cladding 33 is removed by the string. FIG. 6 shows a cross-sectional shape of a fused and stretched portion of an optical fiber coupling element using this polarization-maintaining optical fiber 30.

A more specific example of the manufacturing method using the polarization maintaining optical fiber shown in FIG. 3 will now be described.

The structural dimensions of the polarization-maintaining optical fiber before removing the arcuate portion 26 are: core diameter 7.5 μm, clad type 125 μm, core/clad ratio refractive index difference 0.36%, stress applying portion diameter 29
The distance between the center of the stress applying part in μm1 and the center of the core is #35 μm, and the stress applying parts 2o and 21 are made of quartz doped with 8203.

The birefringence of this optical fiber to be held is 4X as a result of measurement.
It was 10-'. Then, the arcuate portion 26 was removed from this polarization maintaining optical fiber by polishing.

The amount of polishing was set to a value at which the stress applying portion 22 was just removed.

Specifically, it was polished to about 42 μm from the outer edge toward the center.

Next, two polarization-maintaining optical fibers obtained in this manner were prepared, arranged so that the string portions were in close contact with each other, and stretched while partially heating to form a fused and stretched portion.

Here, the monitor light is input from one end of the polarization-maintaining optical fiber 2, and the emitted light intensity is monitored and stretched at the opposite end of the polarization-maintaining optical fiber 2.3. By stopping the stretching when the strengths became equal, a 50% branched coupling element was produced.

The results of comparing the characteristics of the optical fiber coupling device produced in this manner with those of a coupling device produced by a conventional method are shown. The conventional element used was one in which two polarization-maintaining optical fibers were lined up under the same conditions except that they were not polished, and a fused and stretched portion was formed in the same manner as in this example. In each case, five pieces were produced and the average values were compared.

The diameter of the fused and stretched portion in this example was 68 μm on average, and the tensile strength at break was 350 g. On the other hand, in the conventional element, the amount of stretching is larger than that of this specific example, and the diameter of the fused and stretched portion is 35 μm on average, and the tensile strength at break is 160 μm.
It was g. This improvement in mechanical strength means that when an optical fiber coupling element is actually used, the risk of breakage at the fusion-stretched part is reduced even if linear expansion strain due to vibration external force or temperature change is applied, which increases reliability. It is extremely effective in terms of improvement.

Furthermore, while the conventional element has an average excess loss of 1.2 dB and an extinction ratio of 21 dB, the optical fiber coupling element of this example has an average excess loss of 0.7 dB and an average extinction ratio of 29 dB.
Excellent results of dB were obtained. This result is considered to be due to the fact that the geometric deformation of the cross section of the fused and stretched portion is sufficiently suppressed because the amount of stretching is smaller in this specific example. By the way, when we observed the cross section of the fused and stretched part of the conventional element, we found that the stress applying part 41 to
A deformation occurs in which the cross section of 44 becomes elongated, and
Axial misalignment was also observed.

〔Effect of the invention〕

As explained above, according to the method for manufacturing an optical fiber coupling element of the present invention, a part of the cladding of the polarization-maintaining optical fiber is removed before fusion-stretching, so that the amount of stretching during fusion-stretching can be reduced. It can be suppressed. Therefore, there is almost no deformation of the geometrical shape in the cross section of the polarization maintaining optical fiber due to fusion drawing. Therefore, the optical fiber coupling device manufactured according to the present invention sufficiently maintains polarization maintaining characteristics and has excellent optical propagation characteristics. In addition, since the fused and stretched portion can be made sufficiently thicker than conventional elements, the optical fiber coupling element manufactured according to the present invention has excellent mechanical strength and is resistant to linear expansion due to vibrational external force or temperature change. Breakage due to strain is extremely reduced. Furthermore, by simply aligning the notches of the polarization-maintaining optical fibers, the parallelism of the principal axes of birefringence of the two polarization-maintaining optical fibers can be achieved, thereby simplifying the manufacturing process.

FIG. 3 is a cross-sectional view showing the extending portion.

DESCRIPTION OF SYMBOLS 1... Optical fiber coupling element, 2.3... Polarization maintaining optical fiber, 4.5... Clasode, 6.7... Core, 8-11... Stress applying part, 12・・・Fusion stretching part,
13-16... Birefringence principal axis.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a manufacturing method according to an embodiment of the present invention;
FIG. 2 is a cross-sectional view showing the fused and stretched portion, and FIG.
FIG. 4 is a cross-sectional view of a polarization-maintaining optical fiber used in another example, and FIG. Further, FIG. 6 is a cross-sectional view of a polarization-maintaining optical fiber used in another embodiment, and FIG. , Patent applicant for fusion in conventional devices Yokoyuki Hase, Patent attorney representing Sumitomo Electric Industries, Ltd.
Salt 1) Tatsuya Example (first half) Figure 1 (1) Fusion and stretching part of the example Figure 2 Optical fins used in other examples Figure 3 Fusion and stretching part of other examples Figure 4

Claims (1)

[Claims] 1. A first step of removing a part of the cladding from a polarization-maintaining optical fiber with a circular cladding in a chord parallel to one of the principal axes of birefringence in its cross section; and a part of the cladding. A second step is to prepare two polarization-maintaining optical fibers from which the A method for manufacturing an optical fiber coupling element, comprising: a third step of forming a fused and stretched portion by heating and stretching two arranged polarization-maintaining optical fibers. 2. The method for manufacturing an optical fiber coupling element according to claim 1, wherein a portion of the cladding in the first step is removed by polishing. 3. The method for manufacturing an optical fiber coupling device according to claim 1, wherein the removal of a portion of the cladding in the first step is performed by chemical etching. 4. The polarization-maintaining optical fiber with a circular cladding is a stress-applying type polarization-maintaining optical fiber having two stress-applying parts arranged symmetrically about the core, and the cladding is removed in the first step. 2. The method of manufacturing an optical fiber coupling element according to claim 1, wherein the shape is a bow shape including one stress applying part. 5. The polarization-maintaining optical fiber with a circular cladding is a stress-applying type polarization-maintaining optical fiber having two stress-applying parts arranged symmetrically about the core, and the part of the cladding in the first step 2. The method of manufacturing an optical fiber coupling element according to claim 1, wherein the removal is performed in a chord parallel to a line connecting the centers of the two stress applying parts so as to leave two stress applying parts. 6. The polarization-maintaining optical fiber with a circular cladding is a stress-applying type polarization-maintaining optical fiber in which the core is surrounded by an elliptical stress-applying part, and the part of the cladding in the first step is removed in the elliptical shape. Claim 1: A string parallel to the long axis of the stress-applying part.
A method of manufacturing the optical fiber coupling element described above.