JPS5924741B2 - Manufacturing method of optical fiber base material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a preform for an optical communication fiber.

Typical manufacturing methods for glass fibers for optical communications include the internal soot attaching method, the shaft attachment method, and the head throat incubation method.

In both cases, silica-based glass containing a dopant is used as the core material, and pure silica glass or silica-based glass containing a dopant to lower the refractive index than the core material is used as the cladding or jacket, and is heated to a high temperature of 1000°C or higher and then cooled. A base material (preform) is formed through this process, and this is heated and melted again to obtain a fiber.

Conventionally, in these manufacturing methods, cracks often occur in the base material during the molding, cooling and heating processes of the base material. The reason is. 1 Glass containing dopants has a larger thermal expansion coefficient than pure silica glass, and thermal residual strain or thermal residual stress is generated between the outer periphery and the interior during the cooling and heating process.

2. When forming the base material, a combination of materials with different amounts of dopants is cooled or heated, which causes thermal residual strain or thermal residual stress to occur due to the difference in thermal expansion coefficient between the different materials.

In particular, when manufacturing fibers with a large refractive index difference (numerical aperture) between the core and cladding, the difference in thermal expansion coefficients between the base materials is generally proportionally large, resulting in excessive thermal distortion during cooling and heating of the base material. The frequency of damage is extremely high. Conventionally, measures have been taken to avoid rapid heating and cooling, and to reduce residual strain by slowly heating and cooling over a long period of time.

However, prolonged slow heating and slow cooling is not preferable in terms of production speed and energy cost as an industrial manufacturing process.

The present invention solves these problems and presents a method for molding a base material that does not break even when heated and cooled in a short time.

The feature is that a cylinder or a cylinder made of glass or glass fine particles having a desired refractive index distribution in the radial direction, or a composite thereof is heated and melted to form a transparent glass cylinder, and both ends of the cylinder are heated and melted. is heated and stretched to make the ends needle-like. As an example, the production of the base material by the rod inch tube method will be described below.

There are two methods for the rod incubation method.

One is to insert the core rod into the cladding or jacket tube, heat it as it is, integrate it, and draw it at the same time.In this case, the above-mentioned cracks generally do not occur, but the core and cladding are separated. It is difficult to visually inspect and control irregularities that occur at the boundary (such as air and dust intrusion), and because of the difference in viscosity during heating between the rod and tube, it is difficult to control the flow during heating, and the core Diameter/
The disadvantage is that it is difficult to control the fiber outer diameter. In order to eliminate these drawbacks, it is preferable to collapse the tube to form a base material (preform) that is integrated with the rod before wire drawing. An example of this process is shown in FIG. A silica glass rod 1 containing a dopant is inserted into a pure quartz tube 2, and fed into a heat source 3 at a constant speed to collapse and integrate the rods 2. FIG. 2 shows the base material obtained in this manner, and thermal strain occurs between the core material rod 5 and the clad (jacket) tube during cooling and reheating.

Generally, cracks are generated starting at both ends 0 or 5 where the residual strain (stress) distribution is discontinuous, propagating in the longitudinal direction of the preform and causing damage. The present invention will be explained with reference to FIGS. 3 and 4.

When starting collapse, after collapsing the starting edge 5, immediately stretch only the starting edge, then continue collapsing,
Immediately after the collapse, the end portions are similarly stretched to create the shape shown in FIG. 4 with both ends having a narrow diameter. In the case of Figure 2, there are discontinuous points of residual strain at both ends,
A crack is generated starting from this point, but in the case of FIG. 4, the discontinuity point can be substantially eliminated, so there is no starting point for a crack.

Therefore, this base material will not be damaged even if it undergoes relatively rapid heating and cooling processes. A more specific example will be described below.

GeO2-Si with a refractive index difference of 2.5% produced by VAD method
O2 glass was heated and stretched to form a glass rod with a diameter of 14 mm.

This has an outer diameter of 24mm and an inner diameter of 20m7! It was inserted into a L quartz pipe, one end of the quartz pipe was sealed, and while the inside was depressurized, it was heated from the outside with an oxyhydrogen flame and welded to form a preform. At this time, both ends after welding were not specially processed and were left at room temperature for about 1 hour, and cracks appeared at both ends where the GeO2-SiO2 glass was welded to the quartz pipe.
Furthermore, when this was left for 24 hours, cracks developed throughout the preform and it became unusable.

Next, a base material made by the AD method with the same refractive index difference was collapsed into a quartz pipe of the same size of 24 mmφ/20 m1Lφ in the same manner as above, and immediately after the collapse, both ends were stretched to form a needle shape. . The shape is as shown in FIG. No cracks were observed in this preform even after it was left for 24 hours. By the method of the present invention, it is possible to combine materials with widely different thermal expansion coefficients as the core and cladding materials, and it is possible to manufacture a fiber with a large numerical aperture and a large difference in refractive index between the core and cladding. .

In addition to the rod inch tube method, the present invention can also be applied to base material manufacturing using the internal soot attaching method, the shaft attaching method, and the like.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a conventional manufacturing method, FIG. 2 is a base material manufactured by FIG. 1, and FIG. 3 is a schematic diagram of a manufacturing method according to the present invention.
Figure 4 shows the base materials 1, 11, 2 manufactured according to Figure 3.
1, 41 are core; 2, 12, 22, 42 are clad; 3
, 23 is a heat source; 14 and 15 are the ends of the base material.

Claims (1)

[Claims] 1. When forming a transparent glass cylinder by heating and melting a glass cylinder or cylindrical body having a desired refractive index distribution in the radial direction, or a composite thereof, immediately after forming the cylinder, both ends of the cylinder are heated and stretched. A method for manufacturing an optical fiber preform, characterized in that the end portion is needle-shaped.