JPH01148724A - Production of polarization retaining fiber - Google Patents

Abstract

PURPOSE:To obtain the title fiber having a high extinction ratio irrespective of the absence of a stress imparting part by inserting a quadrangular prismatic core into the quadrangular prismatic hollow part formed at the center of a clad vitrifying the materials, regulating the clad outer diameter, and then drawing the materials. CONSTITUTION:The sooty clad 21 of fiber made by the CVD, VAD, duplex crucible methods, etc., is bored at its center to form the part 22 in the form of a quadrangular prism. The core 11 in the form of a quadrangular prism is inserted into the hollow part. The materials are heated in a high-temp. furnace, and vitrified to obtain a preform. The preform is then pressed from the outside, if necessary, to eliminate the gap between the hollow part 22 and the core 11, ground into a circle, and further heated to shape the surface. The preform is drawn while keeping its circular profile, and the fiber having the rectangular core 11 is obtained. The clad is then coated, as required, with a protective primary coat, a buffer layer, a jacket layer, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] ' The present invention relates to a method of manufacturing polarization-maintaining fiber used in optical fiber communications and optical fiber sensors.

[Conventional technology]

Conventionally, optical fibers have been used in optical communications and optical fiber sensors to not only transmit light intensity signals, but also information including light intensity, light phase, and polarization characteristics.
Polarization-maintaining fibers in which the core portion of the optical fiber has birefringence, as shown in FIGS. 6a), b), and c), have been considered. Here, 1 is a core, 2 is a cladding, and 3 is a stress applying part.

[Problem that the invention seeks to solve]

However, in the conventional polarization maintaining fiber as mentioned above,
There is a restriction that the cross-sectional shape of the core part is close to a circle, and there are also restrictions on the degree of birefringence due to the material and structure, or from the single mode condition, which limits the performance of polarization-maintaining fiber. There was a problem.

In order to solve this problem, it is theoretically possible to create a core with a rectangular cross-sectional shape, but there is a problem in that the manufacturing method has not been established and this has not yet been realized.

The present invention has been made in view of these conventional problems, and it is possible to reduce the polarization-maintaining characteristic, that is, the extinction ratio or crotalk property, to the extremes without losing the single mode property and transmission loss of the polarization-maintaining fiber. good. Another object of the present invention is to obtain a method of manufacturing a polarization-maintaining fiber having a rectangular core shape.

[Means for solving problems]

In order to solve the above problems, in the present invention, the part forming the core is cut into a square prism shape in advance, a square prism-shaped hollow part into which the square prism core can be inserted is bored in the center of the part forming the cladding, and A columnar cladding is inserted into the hollow part and heated to make it transparent vitrified, and then the outer shape of the cladding is cut into a perfect circular columnar or prismatic shape, and then lines are drawn so that the outer shape of the cladding maintains a perfect circular columnar or prismatic shape. I did it like that.

[Function] In the present invention, since the cross-sectional shape of the fiber core is not close to a circle but rather close to a rectangle, the incident light is decomposed into linearly polarized light that is parallel to two adjacent sides of the rectangle. The resulting light is not rotated within the fiber and can be propagated with each polarization state maintained. This is considered to be almost the same effect as when microwaves propagate in a rectangular waveguide.

In the present invention, before the wire drawing process, the outer shape of the cladding is shaved into a perfect cylindrical shape and drawn while maintaining the outer shape of a perfect cylinder, so the rectangular shape of the core and the perfect circular shape of the cladding are maintained, which is almost ideal. fibers can be produced.

[Example] The first example is the first example of the present invention, in which the core (1) of the fiber is rectangular and uniformly arranged in the center of the cladding (2).

The cladding (2) is covered with, for example, a primary coat (41) and a buffer layer or jacket layer (42) around it for protection.

When light is incident on the core (1) of the present invention in such a state that one of its polarization axes is parallel to one side of the core (1), this polarized light does not change its state until it exits at the other end of the fiber.

Needless to say, the width of the rectangular core is set to a value that provides a single mode for the wavelength used for each axis.

For example, when the wavelength is 0.85 μm and the refractive index difference between the core and the cladding is 0.0035, the width of the core is about 5 μm.

Figure 2 shows a second embodiment of the present invention, which is a so-called polarization-maintaining fiber in which stress-applying parts (3) are provided on both sides of the core in the cladding to further enhance its characteristics. be. It is also made of a rectangular core. The shape and size of this stress applying part is not limited to the panda shape as shown in FIG. 2, but may also be an elliptical clad shape or a bow tie shape as shown in FIG.

Next, an embodiment of the fiber manufacturing method of the present invention will be described. FIG. 3 shows an example of the manufacturing process of the fiber of the present invention.

First, a soot-like fiber is manufactured using the CVD method, VAD method, double crucible method, etc. (Fig.
21) Figure 3 (b-IN, core part (1)
1) was prepared separately and cut into a square prism shape (No. 31F (b-
2) After inserting it into the cladding part, the whole is made into transparent glass in a high-temperature furnace, for example, to produce a preform (FIG. 3(C)). It should be noted that during vitrification, the entire structure shrinks and the lower tip becomes thinner. At this time, if there is a gap between the hollow part and the core part, it is crushed from the sides; in this case, pressure is applied from all sides along with the square prism faces of the core. When the preform is deformed into a rectangular prism by the pressure applied from all sides, it is shaved into a circular shape, and then heated to create a preform with a shaped surface. An optical fiber having a rectangular core shape is manufactured by heating the preform and drawing it while maintaining a circular outer shape.

The drawing, primary coat, and jacket coating are performed in the same manner as in ordinary fiber manufacturing.

Figure 4 shows the preform of the polarization maintaining fiber, the state before it, or the completed state, and shows the cladding part (2
), the stress applying part (3) in addition to the core part (1) is a combination according to the method of the invention and the usual method of incorporating the stress applying part.

FIG. 5 shows a polarization-maintaining fiber in which the shape of the cladding portion is close to a rectangle with the same plane as the rectangle of the core. A preform or finished fiber is shown. This shape is used when winding this fiber into a small size and using it as a polarizer,
It is effective when you want to know the optical axis of other parts, such as when connecting or fusing the polarization axes.

〔Effect of the invention〕

As described above, according to the present invention, the core of the fiber can be made square, so when the polarization axis of the light incident on the core is aligned with the sides of the square core, the polarization axis of the light will change until it is emitted. It has the same effect.

Although this effect does not require the stress-applying part that conventional polarization-maintaining fibers have as shown in Figure 6, it has a higher polarization-maintaining ability than those, that is, a higher extinction ratio, and is comparable to absolute polarization fibers. It becomes possible to manufacture polarization-maintaining fibers.

Furthermore, it goes without saying that if a stress applying section is provided as shown in FIG. 2 and the core section is made birefringent, the ability to maintain polarization, that is, the extinction ratio, will be further strengthened.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of one embodiment of the polarization-maintaining fiber according to the present invention, FIG. 2 is a cross-sectional view of another embodiment of the polarization-maintaining fiber, FIG. 3 is an explanatory diagram showing the manufacturing process of the present invention, and FIG. Figure, 5th
The figure shows a preform of another embodiment, and FIG. 6 is a sectional view showing a conventional polarization-maintaining fiber. [Explanation of symbols of main parts] 1... Core of fiber, 2... Clad of fiber, 3... Stress applying part of fiber, 11... Core of fiber Section 21...Fiber cladding section. Applicant Nippon Kogaku Kogyo Co., Ltd.

Claims (1)

[Claims] A step of cutting the core portion into a square columnar shape, a step of boring a square columnar hollow portion in the center of the cladding portion, a step of inserting the square columnar core portion into the hollow portion, and a step of heating the core portion. A method for manufacturing a polarization-maintaining fiber, comprising: a step of converting the fiber into transparent glass, a step of shaping the outer shape of the cladding, and a drawing step.