JPH02233529A - Base material for optical fiber - Google Patents

Abstract

PURPOSE:To prevent generation of crack in drawing, etc., by using a rod having a difference of specific refractive index difference with base material smaller than specific value as a glass rod for dummy fusing on base material for a large diameter optical fiber having specific refractive index difference larger than specific value. CONSTITUTION:Base material 1 for a large diameter optical fiber having >=2% specific refractive index difference expressed by (n1-n2)/n1, putting refractive index of core as n1 and refractive index of clad as n2, is prepared. Next, glass rods for dummy 2 having >=2% difference of specific refractive index difference with the base material 1 are fused on at least one end in length direction of the base material 1 and further normal quartz glass rods for dummy 3 are fused on ends of the rods for dummy 2. By said method, inner stress included in the base material 1 is made easy to be released comparing with the case of directly fusing the quartz rod for dummy 3 on the base material 1 for large diameter optical fiber, thus generation of crack in drawing, etc., is able to be steeply decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a preform for an optical fiber that reduces defects such as cracks during drawing of the preform for a large-diameter optical fiber.

``Prior art'' Conventionally, large-diameter optical fibers that require a high numerical aperture (High NA), used in gastrocameras, image scopes, bundle fibers, etc., have been required from a functional standpoint to have a relative refractive index difference. (The relative refractive index difference is defined as (
The value is expressed as n+~nt)/n+. ) is required. For example, when comparing optical fibers used for communications, the relative refractive index difference of optical fibers for communications is typically 1.0 ± 0.2% or 0.3 ± 0.3%. , in large-diameter optical fibers, usually 1.
The relative refractive index difference is about 5%, and some have a relative refractive index difference of 3.0% or more.

In order to increase the relative refractive index difference, it is necessary to increase the difference between the refractive index of the core and the refractive index of the cladding, and as a dopant to increase this difference in refractive index,
Gems etc. are usually used. Therefore, in a large-diameter optical fiber preform for producing a large-diameter optical fiber, it is necessary to add a relatively large amount of G eo , etc. to SiO* as Doban 1. This Gem. etc. have a characteristic of having a larger coefficient of thermal expansion than Sins, which is the material of the fiber end. For this reason, in the preform for large-diameter optical fibers, a large difference in coefficient of thermal expansion occurs between the core portion and the cladding portion, resulting in a large residual internal stress.

On the other hand, when manufacturing an optical fiber from an optical fiber preform, there are several steps of stretching the optical fiber preform.

In these stretching steps, a dummy glass rod is usually welded to one or both longitudinal ends of the optical fiber base material and then stretched. Quartz glass (Show) is generally used for this dummy glass rod (
Hereinafter, this conventional glass rod for a dummy will be abbreviated as a quartz dummy rod. ). Normally, in optical fiber base materials for manufacturing optical fibers used for communication,
In the welding work and stretching work of the quartz dummy rod performed in this stretching process, problems such as cracks in the optical fiber base material do not occur.

``Problem to be solved by the invention'' However, in the case of large-diameter optical fiber base materials for manufacturing large-diameter optical fibers used in medical image scopes, etc., the welding process of the rod for the quartz dummy in the above-mentioned drawing process is difficult. There is a problem in that defects such as cracks frequently occur during the drawing process and subsequent stretching work. This is because, as mentioned above, in the base material for large-diameter optical fibers, a considerable amount of internal stress remains due to the difference in the coefficient of thermal expansion between the core and the cladding. When welding a quartz dummy iron consisting only of Sift with greatly different ratios, it is expected that this internal stress will not be fully released, leading to cracking.

"Means for Solving the Problems" In the present invention, at least one longitudinal end of a preform for a large diameter optical fiber has a relative refractive index difference of 2.0% from the relative refractive index difference of the preform for a large diameter optical fiber. The above problem was solved by using a fiber base material to which an optical fiber base material-type dummy glass rod (hereinafter referred to as base material-type dummy rod) having the following relative refractive index difference was welded. .

1 Effect: The difference in the relative refractive index between the large-diameter optical fiber base material and the base material-type dummy rod to be welded is 2.0% or less, so that the quartz dummy rod can be directly welded. The difference in coefficient of thermal expansion between the large-diameter optical fiber base material and the base material-type dummy rod is smaller than that of the base material for large-diameter optical fibers. Therefore, it becomes easy to release the internal stress of the large-diameter fiber base material, and problems such as cracks are reduced.

Hereinafter, the optical fiber preform of the present invention will be explained in detail.

A feature of the optical fiber preform of the present invention is that at least one end of the large-diameter optical fiber preform in the longitudinal direction has a relative refractive index difference of 2.0. % or less is where the base material type dummy rod is welded.

As this base material type dummy rod, one having the same shape as a normal optical fiber base material having a core part and a cladding part is suitably used, and its material is the same as that of a normal optical fiber base material. Sins. The relative refractive index difference of this base material type dummy rod is G [10 t, P ffio
5. It is regulated by dopant such as fluorine-based substances,
The boron-based substance is Sin. This is not preferable because the coefficient of thermal expansion is significantly different from that of the two. As this base material type dummy rod, an optical fiber base material that has a relative refractive index difference with that of a large-diameter optical fiber base material is suitably used, but is not limited thereto.

This base material type dummy rod is not generally commercially available, so it is necessary to make your own. This manufacturing method is
A method of manufacturing a base material for optical fibers, for example, VAD
method, MCVD method, etc. are preferably used.

The base material type dummy rod manufactured in this way is welded to at least one end in the longitudinal direction of the large diameter optical fiber base material to be stretched, using an oxyhydrogen flame or the like, and the optical fiber of the present invention is Becomes the base material for use.

The optical fiber base material of the present invention may have base material-type dummy rods welded to both ends of the large-diameter optical fiber base material, and a quartz dummy rod may be welded to both ends of the base material in the longitudinal direction. A welded rod may also be used.

When a single large-diameter optical fiber preform is drawn using the optical fiber preform of the present invention, the required length of the preform optical fiber is approximately 10 mm. Therefore, the optical fiber preform of the present invention does not impose a large burden on the manufacturing cost of optical fibers.

"Example" (Example L) FIG. 1 shows an example of the optical fiber preform of the present invention. Reference numeral 1 in the figure indicates a base for large-diameter optical fibers. Base material-type dummy rods 2, 2 are welded to both ends of the large-diameter optical fiber base material 1 in the direction of force. moreover,
Ordinary quartz dummy rods 3, 3 are welded to both ends of the base material type dummy rods 2, 2.

The relative refractive index difference of the large-diameter optical fiber base material l is 3,2
%, and the relative refractive index difference of the base material type dummy rod is 2.
It was 0%. This base material type dummy rod is made of optical fiber GI type base material (G e02 - SiO
t) was used.

When this optical fiber base material was stretched, the frequency of defects such as cracking was about 100% compared to conventional ones! /5 {decreased.

(Example 2) An optical fiber for drawing was produced in the same manner as in Example 1, except that an optical fiber Gl type base material with a relative refractive index difference of 1.5% was used for the base material type dummy rod.

When this optical fiber base material was stretched, the frequency of cracking was reduced to about 1/2 compared to conventional materials.

"Effects of the Invention" The present invention provides a preform for a large-diameter optical fiber having a relative refractive index difference of 2.0% or more. The difference in rate difference values is 2,0
This optical fiber base material is characterized by having a dummy glass rod welded to it, which is less than , the internal stress of the large-diameter optical fiber base material is easily released. Therefore, the optical fiber preform of the present invention has the effect of significantly reducing defects such as cracks during stretching.

[Brief explanation of the drawing]

FIG. 1 is a schematic perspective view showing an embodiment of the optical fiber preform of the present invention. [... Base material for large diameter optical fiber, 2... Rod for base material type dummy. Fujikura Electric Wire Co., Ltd.

Claims (1)

[Claims] A preform for a large-diameter optical fiber having a relative refractive index difference of 2.0% or more, wherein at least one longitudinal end of the preform has a relative refractive index difference of 2.0%. A base material for an optical fiber, characterized in that a dummy glass rod of less than 10% is welded thereto.