JPH08240729A - Optical fiber and manufacturing method thereof - Google Patents

Abstract

PURPOSE: To obtain such an optical fiber that ultrasonic waves or light can be easily inputted through the side face of the fiber, fibers can be densely packed, problems of twisting or bending are not caused, and the fiber can be accurately aligned and fixed to the substrate of an optical waveguide, by forming the optical fiber having the cross section of a regular polygon. CONSTITUTION: An optical fiber preform is formed to have the part corresponding to the core in the center of the optical fiber preform and the part corresponding to the clad in point symmetry around the core part. Therefore, the cross section of the clad part is a regular polygon, especially, a square, regular hexagon or octagon. Therefore, when the optical fiber preform is heated and drawn into an optical fiber, the preform is drawn in the axial direction while uniform force is applied in the circumference direction of the core so as to prevent production of double refraction in the optical fiber. When the optical fiber thus produced is used as a bundle fiber such in a matrix state or the like, it is preferable to make the cross section of the core a regular polygon rather than a circle.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber having a regular polygonal cross section suitable for optical communication and optical measurement.

[0002]

2. Description of the Related Art In the fields of optical communication, optical measurement, etc., an optical fiber terminal is used by being connected to a planar optical waveguide. However, it is technically difficult to connect the optical fiber and the optical waveguide, and thus the cost of the device for optical communication and optical measurement is increased. The cause is that the connection between the optical fiber and the optical waveguide is performed on the substrate on which the optical waveguide is formed, but at this time, it is difficult to accurately arrange the optical fiber at a predetermined position. Further, this phenomenon is caused by the fact that the ordinary optical fiber has a circular cross section, and thus is easy to roll and bend. Also, in the planar optical waveguide,
Since the cross section of the core is not a circle but a quadrangle, it is desirable that both the optical fiber and the core have a quadrangle for efficient optical coupling with this.

Recently, an external modulator has been developed, which applies an external signal to the light from a DC light source propagating in an optical fiber to indirectly modulate the light. The external modulator has a sectional structure as shown in FIG. 6, for example. In the figure, reference numeral 1
Is an optical fiber, reference numeral 2 is a quartz substrate, reference numeral 3 is a piezoelectric element made of ZnO, reference numeral 4 is an Au electrode, and reference numeral 5 is a coating portion which also serves as an adhesive made of silica gel. Also in this external modulator, the optical fiber 1 needs to be accurately arranged on the quartz substrate 2, and an ordinary optical fiber having a circular cross section is easy to roll and bend, so that the optical fiber 1 can be accurately arranged. Difficult to place. Further, this external modulator modulates an optical signal propagating in the optical fiber by the ultrasonic wave from the piezoelectric element ZnO, but the substrate of the piezoelectric element and the optical fiber are merely in contact with each other at a circular contact point, The contact points are few. Therefore, the input of ultrasonic waves from the piezoelectric element to the optical fiber is small. To improve this,
It is desired that the cross section of the optical fiber is flat and contacts the substrate. Further, recently, an optical fiber grating has been developed. This is manufactured by irradiating ultraviolet rays from the side surface of an optical fiber, but it is difficult to irradiate ultraviolet rays to a conventional circular optical fiber. For this reason, it is desired that the optical fiber has a flat cross section.

Further, recently, as shown in FIG. 7, an optical component in which a large number of optical fibers 1 are arranged in a matrix in a resin 7 is also required. Even with such components, it is difficult to arrange them in a matrix with the conventional circular optical fibers for the reasons described above. In the figure, reference numeral 6 is a coating portion made of resin. Further, when the optical fibers are arranged in a matrix in this manner, the conventional circular optical fibers have a small packing density because voids are formed between the circles. Also in this case, an optical fiber having a cross section such as a square or a regular hexagon is desired. Furthermore, if the cross section of the core of the optical fiber is not a conventional circular shape but a square shape, there is an advantage that a larger amount of light can be transmitted.

Here, it is easily conceivable that the above-mentioned drawback can be compensated by using an optical fiber having a rectangular cross section instead of the conventional optical fiber having a circular cross section. As a conventional non-circular optical fiber having a cross section, for example, a fiber having a flat clad cross section as shown in FIG. 8 has been announced (see Documents 1 and 2). The optical fiber 11 is a polarization maintaining optical fiber called a PANDA type optical fiber having a flat clad 14. The optical fiber 11 is manufactured by providing a stress imparting portion 13 and applying different stresses to the core 12 during drawing to increase the birefringence and easily identify the polarization axis of the optical fiber. It is possible. In order to manufacture the optical fiber 11 having the flat clad, a preform having a flat clad equivalent portion is drawn. When a preform having a flat clad equivalent portion is drawn, even if the stress imparting portion is not provided, the stretching amounts of the major axis and the minor axis of the optical fiber differ, and a birefringence occurs. Reference 1: IEICE Technical Report OQE88-2
2, p71 Reference 2: IEICE Technical Report OQE85-1
3, p7

[0006]

Since the conventional circular optical fiber easily rolls and bends, it is difficult to accurately dispose the optical fiber on the substrate on which the optical waveguide is formed, and the matrix-shaped optical fiber is difficult. There was a problem that it was difficult to place it in. Further, in an optical fiber having a flat clad, there is a problem that polarization dependence occurs due to birefringence. In addition, there are problems that it is difficult to input ultrasonic waves and light from the side surface of the optical fiber and that it is difficult to densely fill them. It is an object of the present invention to provide an optical fiber that can be accurately and easily placed on a substrate and is free of birefringence. This optical fiber also has the effect of easily inputting ultrasonic waves or light waves from the side surface of the optical fiber, and the present invention provides an optical fiber having many of these advantages.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides an optical fiber and a method for manufacturing the same which solve the above-mentioned problems. An optical fiber having a regular polygonal cross section is provided.
Invented. Further, the method for producing an optical fiber according to the first invention, which comprises heating and stretching a rod-shaped optical fiber preform having a regular polygonal cross section, is defined as a second invention. In the second invention, the optical fiber preform is A third invention is to manufacture by a powder molding method.

In the optical fiber preform of the present invention, it is desirable that the core-corresponding part is located at the center of the optical fiber preform and that the clad-corresponding part is point-symmetric in the circumferential direction around the core-corresponding part. Therefore, it is preferable that, among the regular polygons, the cross section of the portion corresponding to the clad is square, regular hexagonal, regular octagonal or the like. The reason for this is that when the optical fiber preform is heated and drawn from the optical fiber preform, the optical fiber preform is stretched in the axial direction so that a uniform force is applied in the circumferential direction of the core, and birefringence does not occur in the optical fiber. This is because it is easy to do so. Therefore, as shown in FIG.
To the anisotropy of the clad, for example, by inserting the stress applying portions 13 on the left and right of the core 12 or by providing an elliptical inner clad 15c between the circular inner clad 15b and the outer clad 15a as shown in FIG. It is not preferable to add it.
Even when inserting a foreign glass such as a stress applying part into the clad part, insert it not only on the left and right of the core but also on the top and bottom,
Alternatively, even when the inner clad is provided, it is necessary that the clad portion is not anisotropic in any case, such as a circular clad instead of an ellipse.

Further, in the present invention, it is important to manufacture a similar regular polygonal optical fiber from a regular polygonal optical fiber preform, for example, to manufacture a square optical fiber from a square optical fiber preform. Is. However, when a rectangular optical fiber (which may be a square optical fiber depending on the drawing conditions) is manufactured from the rectangular optical fiber preform, the ratio of the long side to the short side of the rectangle changes due to the drawing.
This means that different strains occur in the long-side direction and the short-side direction of the cross section of the optical fiber, resulting in birefringence. Of course, there is a concern that the core may be deformed,
It is desirable to draw a regular polygonal optical fiber preform to produce a similar regular polygonal optical fiber. In the present invention,
The core of the optical fiber may be circular or regular polygonal. To manufacture these optical fibers, an optical fiber preform having a circular or regular polygonal core may be used. In order to use the optical fiber as a matrix-like bundle fiber, it is preferable that the core has a regular polygonal cross section rather than a circular cross section. This is because the core occupancy rate becomes large and a large amount of light can be transmitted.

The optical fiber preform used for manufacturing the optical fiber of the present invention can be manufactured by various methods. Specifically, an optical fiber preform having a circular cross section is used for VAD method, MC
An optical fiber preform having a circular cross section is manufactured by a well-known vapor phase method such as the VD method or OVD method, and the side surface of the transparent glass preform obtained by transparentizing and vitrifying the optical fiber having the circular cross section is machined. The optical fiber preform used in the production of the optical fiber of the present invention can be manufactured by mechanically cutting. As a more economical manufacturing method, there is a method using a powder molding method. This method is, for example, a pressure molding method (1992 Japanese Patent Application No. 304418, 1991 Japanese Patent Application No. 12672).
3, JP-A-61-256937), extrusion molding method (see JP-A-4-12042),
Slip cast molding method (Japanese Patent Laid-Open No. 1-5633)
No. 1, 1992 patent application No. 189851), MSP method (publication patent publication No. 4-502256, published patent publication Sho 6)
No. 1-266325), the silica-based glass fine particles are mechanically formed into a porous preform, and further made into transparent vitreous to produce an optical fiber preform. When using these methods, it is most preferable to directly form a regular rod-shaped porous rod-shaped body, and it is also possible to manufacture a cylindrical porous body and cut the side surface thereof to form a regular polygonal shape. This is the preferred method. Of course, the side surface of the transparent glass base material can be mechanically cut. The reason why the powder molding method is economically advantageous is that the regular polygon can be directly produced, or the regular polygon can be easily cut out from the cylindrical porous body.
In the present invention, the cross section of the optical fiber is a regular polygon, and the cross section of the core is preferably a regular polygon. However, the method for producing the optical fiber preform using the powder molding method is the same. Especially suitable for. That is, in order to manufacture a core having a regular polygonal cross section, a cylindrical core that is usually manufactured must be processed by cutting the side surface thereof into a rod having a regular polygonal cross section. Next, in order to give a clad portion having a regular polygonal cross section to this, a method of forming a glass for clad from the silica powder on the outer periphery of the glass rod for a core by using a powder molding method is almost technically possible. This is the only way.

The method of drawing the optical fiber preform when manufacturing the optical fiber of the present invention can be carried out by using a normal optical fiber drawing apparatus. As a drawing condition, it is better to draw at a lower temperature than the conventional method. This drawing temperature is
It can be easily determined in preliminary experiments. The specific drawing temperature is lower than the drawing temperature of an optical fiber having a normal circular cross section, but the lower limit must be about 1830 ° C. When drawing at a temperature lower than this, the drawing speed becomes slow, which is economically disadvantageous, the loss of the optical fiber is high, and the mechanical strength is deteriorated. In addition, the drawing stress may remain in the optical fiber. Further, it is not preferable to raise the drawing temperature indiscriminately. This is because when the drawing temperature is increased, the side surface of the fiber collapses from the flat surface and becomes rounded, and the roundness of the corners also increases. Therefore, it is necessary to select a drawing temperature of 2000 ° C. or less.

Further, the regular polygonal rod-shaped optical fiber preform has sharp edges and may be damaged during manufacture. Therefore, more preferable optical fiber preforms include regular polygonal preforms having rounded corners or chamfered. Also in this case, it is preferable that chamfering is performed in a point-symmetrical manner with the portion corresponding to the core as center as possible. By using the optical fiber preform as described above, it is possible to manufacture an optical fiber that is neither damaged nor birefringent. Further, in manufacturing an optical fiber having a regular polygonal cross section, it is particularly important to chamfer the core preform. That is, normally, a columnar core manufactured by the vapor phase method must be processed into a rod shape with a regular polygonal cross section by cutting the side surface, but this rod-shaped core is chamfered at the corners and used. , Because the damage is drastically reduced. It should be noted that from the chamfered optical fiber preform, it is possible to manufacture an optical fiber having a shape similar to that of the chamfered cross section. It is possible to obtain an optical fiber having the same. For example, when the optical fiber preform is drawn at a high temperature, an optical fiber having a chamfered cross section is obtained. This method is effective when the side surface of the optical fiber is not required to be a perfect flat surface. An optical fiber having such a chamfered cross section is preferable because its strength is increased.

Since the optical fiber of the present invention has a regular polygonal cross section, it can easily input ultrasonic waves and light from the side surface of the optical fiber, can be filled with high density without voids, and has no fear of rolling or bending. It can be accurately placed and fixed on the substrate. When a rod-shaped optical fiber preform having a regular polygonal cross section is heated and drawn to form a regular polygonal optical fiber having a similar cross-section, the optical fiber has no birefringence and there is no risk of polarization dependence.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below based on the embodiments of the invention. The manufacturing process of the optical fiber of the present embodiment is as follows. That is, 1) core equivalent part diameter: clad equivalent part diameter = 1: 4.88
Then, VA the core base material 24 having an outer diameter of 6.98 mm and a length of 250 mm, which is composed of a core-corresponding part and a part of the clad-corresponding part.
It was made by the D method. In the core base material 24, the core is made of silica glass doped with germanium oxide, and the clad is made of pure silica glass. As shown in FIG. 1, quartz glass rods are fusion-bonded to both ends of the core preform 24 as support rods 25, and then the core preform 24 is placed at the center of a cylindrical molding rubber die 23. Reference numeral 21 is an inner lid, reference numeral 22 is an upper lid, and reference numeral 26 is a lower lid. Further, the molded rubber mold 23 is
The whole is a urethane rubber tubular object with an outer diameter of 60 mm,
The inner diameter is 50 mm and the length is 300 mm. 2) Next, the void between the molding rubber mold 23 and the core base material 24 was filled with granulated powder obtained by granulating silica-based glass powder. This granulated powder was manufactured as follows. First, silica powder having an average particle size of 8 μm produced by a gas phase synthesis method is used as a raw material, and silica powder 100 is mixed with pure water 66, polyvinyl alcohol 1.6, and glycerin 1.0 in a weight ratio,
The mixture was stirred to form a slurry, and a spray dryer was used to produce granulated powder. The average particle size of the granulated powder was about 100 μm. 3) After filling the molded rubber mold 23 with the granulated powder, the molded rubber mold 23 was put into a hydrostatic pressure device, and the molded rubber mold 23 was pressed at 98 MPa to mold the granulated powder. After the pressurization, the porous molded body in the molded rubber die 23 was taken out. The outer diameter of the molded body was 38 mm. 4) The molded body was mechanically cut on all four sides so that the core-corresponding part was the center, and as shown in FIG. 2, one side was 25.5 m.
A rod-shaped porous body 27 having a square cross section of m was formed. 5) The porous body 27 is dried at 500 ° C. in an atmosphere of dry air.
The temperature was raised to 3, and heat treatment was performed for 3 hours. Subsequently, chlorine 10% by volume
Was heat-treated for purification at 1000 ° C. in an atmosphere containing Then, it was heated to 1690 ° C. in a helium atmosphere to form a transparent glass body as a whole. In this way, an optical fiber preform having a square cross section with one side of 21.4 mm and having a core equivalent part in the center was obtained. 6) The optical fiber preform is supplied at a constant rate to an optical fiber drawing apparatus equipped with a carbon resistance heating furnace, and 1930
An optical fiber 31 having a square cross section with a side of 124.4 μm and a core diameter of 7.8 μm was manufactured by drawing the optical fiber at a drawing speed of 20 m / min. FIG. 3 shows the optical fiber 31 manufactured in this way.
FIG. In FIG. 3, reference numeral 32 is a core and reference numeral 33 is a clad. The cross-sectional shape of the optical fiber of the present invention is not limited to the above embodiment, and as shown in FIGS. 4 and 5, may be a regular hexagon or a regular polygon having an even number of angles such as a regular octagon.

On the other hand, as a comparative example, an optical fiber having a rectangular cross section was manufactured. In this comparative example, a silica glass base material having long sides and short sides of 30 mm and 15 mm, respectively (long side: short side =
2: 1) at a drawing temperature of 1950 ° C or 1900 ° C,
Drawing at a drawing speed of 20 m / min, the short side is 12
It is an optical fiber of 5 μm. The resulting optical fiber had a short side of 125 μm, but a long side: short side ratio of 1.90: 1 or 1.94: 1, respectively. As described above, the optical fiber obtained from the non-regular polygonal base material contained strain and caused anisotropy in the core.

[0016]

As described above, according to the present invention, since the cross section of the optical fiber is a regular polygon, it is possible to easily input ultrasonic waves or light from the side surface of the optical fiber, and to fill it with high density without voids. The optical fiber can be accurately placed and fixed on the substrate of the optical waveguide without fear of rolling or bending, and the optical fiber base material of a regular polygonal rod having a similar cross section can be heated and drawn. When manufactured by the above method, the obtained optical fiber has an excellent effect that it has no birefringence and does not cause polarization dependence.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a molded rubber mold used for manufacturing an embodiment of an optical fiber according to the present invention.

FIG. 2 is a cross-sectional view of a porous body formed during the manufacturing process of the above embodiment.

FIG. 3 is a cross-sectional view of the optical fiber according to the above embodiment.

FIG. 4 is a sectional view of an optical fiber according to another embodiment.

FIG. 5 is a sectional view of an optical fiber according to still another embodiment.

FIG. 6 is a cross-sectional view of an external modulator.

FIG. 7 is a sectional view of an optical component.

FIG. 8 is a cross-sectional view of a conventional optical fiber.

FIG. 9 is a cross-sectional view of another conventional optical fiber.

[Explanation of symbols]

 21 Middle Lid 22 Upper Lid 23 Molded Rubber Mold 24 Base Material 25 Support Rod 26 Lower Lid 27 Porous Body 31 Optical Fiber 32 Core 33 Clad

Front page continuation (72) Inventor Ken Yagi 2-6-1, Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd.

Claims (3)

[Claims] 1. An optical fiber having a regular polygonal cross section. 2. The method for producing an optical fiber according to claim 1, wherein a rod-shaped optical fiber preform having a regular polygonal cross section is heated and drawn. 3. The method of manufacturing an optical fiber according to claim 2, wherein the optical fiber preform is manufactured by a powder molding method.