JPS5930659B2 - Optical fiber manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a method for manufacturing an optical fiber with excellent long-term reliability.

(Prior Art) Conventionally, since optical fibers are generally very brittle materials, they tend to have surface scratches, which become a stress concentration source and easily break.

In order to solve this problem, a method has been proposed in which a plastic coating is applied immediately after spinning the optical fiber to protect the surface of the fiber, thereby manufacturing an optical fiber with high initial strength. Typically, this plastic coating consists of a primary coating,
It consists of a three-layer structure: a buffer layer and a secondary coating.The primary coating protects the surface of the optical fiber and removes cladding modes.The secondary coating protects the workability when forming cables.The buffer layer protects the secondary coating and removes cladding modes. The purpose of each is to prevent an increase in transmission loss. The tensile strength when primary coating is applied to an optical fiber with extremely few stress concentration sources is 500 kg.
However, since the clad glass for optical fibers is generally a high-purity quartz glass tube (commercially available) made from natural quartz, it has many microbubbles and surface scratches. Therefore, it is nearly impossible to obtain an optical fiber free of stress concentration sources at present.

Therefore, the initial strength of the primary coated optical fiber is 10
It varies from 0 kg/1 to 500 kg/1.

Furthermore, it is known that silica-based optical fibers coated with certain types of plastic among the primary coating materials deteriorate in strength simply by being left in water or in a high-temperature atmosphere. When using an optical fiber that causes variations in initial strength or a decrease in strength as described above as an optical transmission medium, there is a risk that it will not be able to withstand long-term practical use. Furthermore, when connecting optical fibers, the plastic coating is removed before connection, but there is a drawback that it is not easy to remove the primary coating. (Object of the Invention) In order to solve these drawbacks, the present invention is characterized by having a glass layer on the surface of the optical fiber, which has a higher refractive index than the clad glass and a lower coefficient of thermal expansion than the clad glass. The aim is to improve minimum strength, improve long-term reliability, and simplify workability in cable formation and connection.

(Structure and operation of the invention) In view of these points, the present inventors first investigated the strength of a quartz fiber, a quartz fiber having a core and a cladding, and found that the strength is related to the stress in the fiber. found that increasing the strength depends on the compressive stress at the fiber surface and that increasing the compressive stress can increase the strength.

However, in ordinary silica-based optical fibers, the refractive index difference (difference in thermal expansion coefficient) between the core and cladding and the thickness of the cladding layer are determined, so the compressive stress on the fiber surface is only approximately constant. do not have. In the present invention, the surface of the cladding layer has a thermal expansion coefficient smaller than that of cladding glass.
And SiO2-Ti has a higher refractive index than clad glass.
By forming an O2-based glass layer, a larger compressive stress is applied to the surface of the optical fiber compared to a conventional silica-based optical fiber.

FIG. 1 shows a cross section of the structure of a silica-based optical fiber in which the present invention is implemented.

In Figure 1, 1 is the core and this glass is SiO2-
GeO2,2 is the cladding part and this glass is SiO2,3
The glass layer according to the present invention is SiO2-TiO2 glass.

The thickness of the third glass layer according to the present invention can be arbitrarily changed depending on manufacturing conditions, but for a standard optical fiber with a core diameter of 50 μmφ and a cladding diameter of 125 μmφ, a thickness of 3 to 6 μm is optimal. It is. SiO2-TiO2
If the glass layer is made thicker than this, compressive stress will be transmitted to the core portion, resulting in deterioration of the optical fiber loss characteristics. Furthermore, if the thickness is less than 3 μm, sufficient compressive stress will not be applied to increase the strength of the optical fiber. FIG. 2 shows the relationship between TiO2 doping amount and expansion coefficient.

The expansion coefficient of quartz glass is 5.5X10-7/℃,
As TiO2 is doped, the expansion coefficient decreases,
It can be seen that the expansion coefficient becomes negative at a doping amount of about 5m01%. TiO2 doping amount is approximately 25m01 (fl)
In the above case, the expansion coefficient suddenly changes to the positive side and becomes larger than the expansion coefficient of quartz glass. Therefore, the doping amount of TiO2 in the present invention is 5 to 20 m01 (:F
A range of t) is preferred. Furthermore, the relationship between the doping amount of TiO2 and the refractive index is linear, and the refractive index increases as the doping amount increases, so in the present invention, T
What is necessary is to determine the amount of iO2 doping. Next, a method for manufacturing an optical fiber base material having a third glass layer according to the present invention will be described.

FIG. 3 is an explanatory diagram schematically showing a method for manufacturing an optical fiber base material having a core, a cladding, and a third layer according to the present invention. This method is called the vapor phase axial method (VAD method). In FIG. 3, 21, 22 and 23 are quartz glass burners, 24, 25 and 26 are oxyhydrogen flames containing synthesized glass particles, and 27, 28 and 29 are formed porous glass bodies. The gas phase axial method uses oxyhydrogen gas and SiCl4, GeCl4,
A steam glass raw material such as the following is introduced into the core burner 21 and the cladding burner 22, and glass fine particles of SiO2 and GeO2 are synthesized by a flame hydrolysis reaction, and they are attached and deposited on the starting material. SlO2 and GeO2
A porous glass body 27 for the core made of and a porous glass body 28 for the cladding made of SiO2 are formed, and then,
This method heats a porous glass body to obtain a transparent base material for optical fiber. The burner 23 is installed using this manufacturing method,
By forming a porous glass body of SlO2-TlO2, which has a higher refractive index than the cladding and a smaller coefficient of thermal expansion, on the surface of the formed porous glass body 28 for the cladding, an optical fiber having a third layer can be formed. A base material can be manufactured.

Examples are shown below. Example 1 A porous glass body was manufactured under the glass raw material supply conditions shown in Table-1.

SiCl4, GeCl4 as the glass raw material for the core, SiCl4 as the glass raw material for the cladding, 1S1 for the third layer.
C14 and TiC14 are used, each with an oxyhydrogen flame, and burner 21. It was introduced into burners 22 and 23.

First, fine glass particles of SiO2 and GeO2 synthesized by the burner 21 were attached and deposited on the end face of the rotating and rising starting material to form a porous glass body for a core having a diameter of about 30 mm. Next, in a subsequent step, SiO2 glass particles synthesized by the burner 22 were attached and deposited on the surface of the porous glass body for the core.

After that, SiO2 further synthesized by the burner 23
-TiO2 glass particles were attached and deposited on the surface of the porous glass body for cladding. In this way, the diameter is 75m.
A triple-structured porous glass body having a diameter of mφ and a length of 30 degrees was obtained. Next, this porous glass body was degassed in an electric furnace heated to 1500° C. to obtain a transparent optical fiber base material.

This was spun using a spinning device with the outer diameter set to 135 μm.
In a subsequent step, a silicone resin was coated as a buffer layer. In the prior art, a primary coating is applied prior to coating the buffer layer for the purpose of protecting the surface of the optical fiber, that is, suppressing the generation of stress concentration sources and eliminating cladding modes. The structure of the optical fiber thus obtained includes a core at the center, a cladding layer as a second layer, a compressive stress layer as a third layer according to the present invention, and a buffer layer as a fourth layer, each having a diameter of 50 μmφ. , 125μmφ, 135μm
The diameter was 335 μm.

Further, the amount of TiO2 doped in the compressive stress layer was 8m01% as calculated from the refractive index value measured with an interference microscope. FIG. 4 shows the results of measuring the breaking strength of 25 fibers with a length of 10 m. In FIG. 4, characteristic A shows the breaking strength characteristic of the fiber in which the present invention is implemented, and characteristic B shows the characteristic of the fiber manufactured by the prior art.

Further, Table 2 summarizes the average, maximum, and minimum values of the breaking strength obtained from FIG. 4. Figure 4 and table-
As is clear from 2, there is SiO2-T on the surface of the cladding.
Regarding the breaking strength of the fiber on which the iO2 glass layer was formed, the minimum strength was improved, and results were obtained with high strength and very little 1" variation.

Furthermore, as a result of connecting this fiber and examining workability, it was found that the silicone rubber buffer layer can be easily removed, and the strength of the bare fiber has increased, making the work extremely easy and reducing work time. Shortened.

Next, a glass body in which the core and cladding are integrated is formed by a vapor phase axial method, and then examples using the present invention will be described.

Example 2 After forming a porous glass body for the core made of SiO2 and GeO2 and a porous glass body for the cladding made of SiO2 by the vapor phase axising method, they were heated at about 1500°C to reduce the outer diameter. A transparent optical fiber base material having a diameter of 21 mm and a length of 15 mm was obtained.

On the surface of this base material, Si obtained by flame hydrolysis reaction
Glass particles of O2-TiO2 are attached and deposited to a thickness of 0.8m7! A L(7) SiO2-TiO2 glass layer was formed. This was spun using the method described in Example 1, and the strength and other properties of the obtained optical fiber were examined. As a result, the results were very good, similar to those in Example 1. (Effects of the Invention) As explained above, according to the method for manufacturing an optical fiber of the present invention, a quartz glass layer doped with titanium is formed on the surface of an optical fiber base material, and this base material is spun. ,
The minimum strength of the optical fiber can be improved, and as a result, the long-term reliability of the optical fiber can be improved.

Furthermore, since the glass layer according to the invention plays the same role as the primary plastic coating traditionally practiced,
In the process of spinning an optical fiber according to the present invention, a primary coating is unnecessary, and the spinning apparatus and spinning operation can be simplified.

Furthermore, there is an advantage that the working time when connecting the optical fibers is shortened and the handling of the optical fibers is extremely easy.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of the structure of a silica-based optical fiber in which the present invention is implemented, FIG. 2 is a diagram showing the expansion coefficient of SiO2-TiO2 glass, and FIG. 3 is a diagram showing the core, cladding, and third layer according to the present invention. FIG. 4 is a diagram showing the breaking strength of the fiber. 1...SiO2-GeO2 core glass, 2...
...SlO2 clad glass, 3...Si
O2-TiO2 glass, 21, 22, 23...
Burner, 24,2526...Oxyhydrogen flame containing glass particles, 27...Porous glass body for core,
28... Porous glass body for cladding, 29...
...Porous glass body according to the present invention.

Claims (1)

[Claims] 1 SiO_2, Ge obtained by fire hydrolysis reaction
O_2 glass particles are sequentially deposited on the starting material to form Si.
An optical fiber manufacturing method including a step of forming a porous glass body having a core layer made of O_2 and GeO_2 and a cladding layer made of SiO_2, and a step of converting the porous glass body into transparent glass. As a step following the forming step or a step after the transparent vitrification step, SiO_2-TiO_2 is added to the surface of the cladding layer.
1. A method for manufacturing an optical fiber, comprising forming a glass layer.