JPS6350291B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing an optical fiber, and more specifically, the present invention relates to a method of manufacturing an optical fiber, and more specifically, the cross section is composed of a plurality of layers, and the outer peripheral shape of some of the plurality of layers is different from that of the other. The present invention relates to a method of manufacturing an optical fiber having a different peripheral shape of the layers.

[Prior art]

With the development of optical integrated circuits, the concept of light polarization has become important as well as the concept of light intensity.

This is necessary because, for example, an optical switch operates only in a certain direction of the polarization plane. The optical fiber coupled to such an optical integrated circuit must be capable of transmitting only the component in the polarization direction in which the optical integrated circuit operates. Such fibers are called polarization preserving fibers, and they are becoming important not only for optical communication transmission lines but also for measurement purposes.

As a method of manufacturing such a polarization-maintaining fiber, a method is known in which the core or cladding of the optical fiber is made into an ellipse and strain is applied by utilizing the difference in thermal expansion between the core and cladding materials. (Literature
Electronics Letters October 1979 issue, page 677 “Strain birefringlance in single polarization
germanosilicate optical fibers”IPKaminow)
This method involves creating a perfectly circular preform rod, then polishing some of the sides and drawing it at high temperature to create an optical fiber with an elliptical core.

[Problem that the invention seeks to solve]

This conventionally known manufacturing method requires the work of polishing the glass tube or glass rod during the manufacturing process, which wastes time and money in mass production and is a major obstacle to practical application. .

[Means for solving problems]

Therefore, an object of the present invention is to realize an optical fiber in which at least a portion of the core or cladding has a desired ellipticity through a simple process. That is, it is an object of the present invention to realize a method of manufacturing an optical fiber, which does not require a process such as polishing, and in which the outer circumference of the optical fiber is circular, and either the core or the cladding is a desired ellipse.

In order to achieve the above-mentioned object, the present invention forms a core, or a core and a glass thin film serving as a clad material, on the inner wall of a base material tube such as a quartz tube, and then heats and melts this to form a solid glass rod. In the method of making a preform and then heating and drawing it to make an optical fiber, in the step of making the preform, a part of the glass tube on which the glass thin film is formed is heated and smoothed to make it solid; The preform is made by gradually moving the heating point of the solid part while reducing the pressure at a constant degree from the other opening end.

[Effect]

According to the method of manufacturing an optical fiber of the present invention, in the process of melting a glass tube with another glass thin film formed on the inside and making it into a solid rod, the inside of the glass tube is depressurized in a heated and softened state. There is a force acting on the tube that tries to make it flat, and a force that tries to make it circular due to centrifugal force due to rotation, surface tension, etc.
Furthermore, based on the differences in the softening point temperatures of the materials used for glass tubes and glass thin films, we have found that although the outermost periphery is circular, there is an elliptical core inside, or a core with a circular core and an intermediate layer. The outer layer is elliptical and the outermost layer is circular, which previously required a complicated process.
An optical fiber structure is realized.

〔Example〕

The present invention will be explained in detail below with reference to Examples.
FIG. 1 is an explanatory diagram of the process of making a preform in the optical fiber manufacturing method according to the present invention. Before starting the process of making this preform, a quartz tube (thickness
1.5mm (inner diameter 17mm) to a thickness of approximately 3.2mm and a radius of 12
Make it into a mm quartz tube.

The purpose of reducing the diameter and increasing the thickness of the quartz tube is to prevent the axis of the ellipse from fluctuating if the preform is made thin and with a large diameter.

A core is formed on the inner wall of the quartz tube by the conventionally known chemical deposition method on the quartz tube whose diameter has been reduced.
Form a glass thin film of 10 μm consisting of 30 mol% GeO ₂ and 70 mol% SiO ₂ .

Both ends of the glass tube thus formed are attached to a glass lathe. Although the mounting base is not shown in FIG. 1, a part of the mounted tube 1 (the end of the glass tube) is heated and crushed with an oxyhydrogen burner 2.
An exhaust tank 4 is provided at the opening at one end of the glass tube, and the air inside is removed from the exhaust pipe 3 while adjusting the exhaust adjustment valve 6 to keep the internal pressure constant. The amount of pressure reduction is measured by the difference in the liquid level of the liquid 8 using a U-shaped tube with one end inserted into the quartz tube interior 5. Note that the pressure may be reduced when initially crushing a portion of the glass tube.

The quartz tube was moved at a constant speed of 50 per minute as described above.
While rotating, gradually turn the oxyhydrogen burner 2 to 0.17
A solid preform is formed when moving at a speed of mm/sec.

Figures 2a, b, c and d are traced photographs of cross sections of preforms made according to the present invention, and the manufacturing conditions for each are as follows.

The initial quartz tubes have the same outer diameter of 20 mm and thickness of 1.5 mm.

A is made of silica glass doped with germa as a core, the amount of pressure reduction is 9 mm at the height of water, and an ellipticity of about 50% is obtained. Figure b shows the amount of pressure reduction in the pipe.
The thickness of the germanium layer doped into the core of the hollow preform is approximately 15 μm. c and d have silica glass in the core,
This relates to silica glass with a boron-doped cladding, and is shown in figure c, in which the core is circular and the cladding is oval, and in contrast, figure d, in which the core is elliptical and the cladding is circular. These manufacturing methods involve shrinking the starting quartz tube to some extent to prevent rotation of the ellipse axis, as described above, and can be accomplished by optimally selecting the amount of shrinkage and the thicknesses of the core layer and cladding layer.

When these preforms are heated and drawn to form optical fibers, the central part maintains almost a similar shape, but only the outer periphery changes slightly, approaching a circular shape, and the ellipticity of the outer periphery is within 1 to 2% at most, making it virtually invisible. It becomes circular.

FIG. 3 shows the measurement results of the ellipticity when the thickness of the quartz tube and the degree of vacuum were changed, and the operating conditions for each curve are as follows. In curves 9, 10, and 11, the core is made of silica glass doped with germa and has a two-layer structure as shown in FIGS. 2a and 2b. The concentration of germa is approximately 15 mol%. For each curve, the outer diameter of the starting quartz tube is 20 mm and the inner diameter is 17 mm, and the thickness of the germanium layer on the hollow preform is approximately
It is 10μm. This shows the ellipticity of the core of the solid preform obtained by shrinking this solid preform to an outer diameter of 13.5 mm, 12.8 mm, and 9.7 mm before collapsing by changing the amount of decompression. curves 9, 10, and 11 are 13.5 mm, 12.8 mm, and 9.7 mm, respectively.
This is an experimental example of mm. Curve 12 is a fiber made of silica glass doped with boron and silica as the core, and has a three-layer structure as shown in FIGS. 2c and d. Boron was doped at approximately 12 mol % and the hollow preform had a thickness of 18 μm. The core silica layer has a thickness of approximately 8 μm, and the starting quartz tube has the same outer diameter of 20 μm as described above.
mm, with an inner diameter of 17 mm. After forming a cladding layer and a core layer on this tube, it was shrunk to about 13.1 mm. After that, the collapse was performed by changing the amount of pressure reduction, and curve 12 shows the ellipticity of the resulting cradle as a function of the amount of pressure reduction. At this time, the shape of the core was slightly elliptical, with an average ellipticity of 4.2%.

〔Effect of the invention〕

As is clear from the above description of the embodiments, the present invention adds depressurization to the conventional optical fiber manufacturing method, thereby easily producing optical fibers of any desired ellipticity without requiring any other special steps. Optical fibers with cores or cruts can be realized.

The factors that determine the ellipticity (when the length of the major axis is a and the length of the minor axis is b, the ellipticity is γ = ^ab _{a + b} ) are the degree of reduced pressure in the pipe, surface tension, Although it is difficult to theoretically analyze the centrifugal force due to the rotation of the tube, the force due to thermal expansion, and the material radius of the glass tube and glass thin film, etc., it is difficult to analyze the material, degree of vacuum, and radial dimension. Once specified, an optical fiber having a core or cladding with the same ellipticity can be reliably reproduced.

What is particularly noteworthy is that, as shown in the example shown in Fig. 2c, a first layer of a material containing B ₂ O ₃ , which is lower than the material of the quartz tube, is first formed on the inner wall of the quartz tube. Furthermore, if a second thin film layer made of a material with a higher softening point than the material of the first layer is made by the CVD method on top of the first layer, and the inner pressure is lower than the external pressure as in the present invention, the center It is possible to create an optical fiber with a special cross-sectional structure in which the layer is circular, the middle layer is elliptical, and the outermost layer is circular.

In addition, the above example shows the case where the degree of depressurization is 20 (water mm) or more, but if the degree of depressurization is extremely large, it will not form an ellipse that is symmetrical with respect to the two orthogonal axes. , is often impractical in practice. However, in all cases below 20 (water mm), an ellipse with a shape symmetrical about two axes is formed.

Conventionally, in the process of directly drawing optical fiber from a glass tube, rather than the process of making a preform,
There are some documents that describe reducing the pressure, but this is simply to quickly realize a solid optical fiber.
There is no known method for making the core or cladding elliptical; rather, means have been proposed for applying pressure to the core or cladding to make it circular when making the preform.

The present invention is based on experiments that show that in the process of making a preform with an elliptical core or cladding, by reducing the pressure, the outer periphery of the fiber can be circular, and only the core or cladding can be made with any desired ellipticity and with good reproducibility. This method was discovered in 1995 and provides an effective means for realizing an optical fiber that can preserve the plane of polarization.

[Brief explanation of the drawing]

Fig. 1 is a diagram explaining a part of the manufacturing process of the present invention, Fig. 2 is a cross-sectional view of a preform formed during the process of the present invention, and Fig. 3 is a diagram showing the relationship between the amount of pressure reduction and the ellipticity of the preform. It is a graph showing measurement results.

Claims (1)

[Claims] 1. A first step of forming a glass layer of a material different from the material of the glass tube on the inner wall of a circular glass tube;
The internal pressure of the circular glass tube on which the glass layer is formed is reduced to be lower than the external pressure, and the circular glass tube on which the glass layer is formed is heated while rotating. a second step of moving the glass rod relative to the glass tube to form a solid glass rod; and a third step of heating and drawing the glass rod to form an optical fiber. A method for manufacturing an optical fiber in which at least a portion of the middle of the outermost layer is elliptical. 2. In the method for manufacturing an optical fiber according to item 1, the second step includes a step of heating and crushing a part of the circular glass tube obtained in the first step, and a glass tube with the part crushed. A method for manufacturing an optical fiber, comprising the step of reducing pressure from an opening of the fiber. 3. The method for manufacturing an optical fiber according to item 1 or 2, wherein the reduced pressure is 4 to 20 mm lower in water column height than the external pressure. 4. In the method for manufacturing an optical fiber according to item 1 or 2, the glass layer formed in the first step includes a first thin film layer formed on the inner wall of the circular glass tube, and a first thin film layer formed on the inner wall of the circular glass tube. a second thin film layer formed on the thin film layer, the softening temperature of the first thin film layer is lower than that of the glass tube, and the softening temperature of the second thin film layer is lower than that of the first thin film layer. A method of manufacturing optical fiber that is higher than that of layers.