JPS62108744A - Transparent vitrification method of porous glass base material - Google Patents

Abstract

PURPOSE:To change fluorine content in the radium direction of a porous glass base material and to easily change the distribution of refractive index by controlling the conc. of fluorine-contg. gas contained in the atmosphere in case of subjecting the porous glass base material to transparent vitrification in the fluorine-contg. gas atmosphere. CONSTITUTION:A porous glass base material 1 consisting of an accumulated material of a sooty fine glass grain is inserted to the inside of a reactor core pipe 5 and a fluorine-contg. gas (e.g. gaseous mixture of SF6, He and Cl2) is introduced into the inside of the pipe 5. The base material 1 is heated by an electric furnace 2 in the fluorine-contg. gas atmosphere and fluorine is doped to the inside of the base material 1 while subjecting it to transparent vitrification. On this occasion, fluorine content in a range over the radium direction of the base material 1 is changed by controlling the concn. of the fluorine contg. gas contained in the atmosphere and the distribution of refractive index in the radium direction of the base material 1 is changed. Thereby a glass base material consisting of a pure quartz core and a quarts cladding doped with fluorine can easily be prepared in a VAD process.

Description

DETAILED DESCRIPTION OF THE INVENTION 1. Field of Industrial Application The present invention relates to a method for transparent vitrifying a porous glass preform used in obtaining a glass preform for communication and optical applications.

[Prior art j] When producing glass base materials used in communication and non-communication fields, such as for optical fibers and rod lenses, by VAD method, OVD method, etc., soot generated by flame hydrolysis reaction or thermal oxidation reaction. The microscopic glass particles are deposited in a desired shape, and the deposited porous glass base material is turned into transparent vitrification.

Generally, the above-mentioned transparent vitrification is performed by placing a porous glass base material in an electric furnace into which helium, chlorine gas, etc. are introduced and heat-treating the base material.
The porous glass matrix is dehydrated.

It becomes transparent glass through the sintering process.

In this way, when a porous glass base material is made into transparent glass, sulfur hexafluoride, freon (
A method of doping fluorine into the base material is by mixing a fluorine-containing gas such as fluorinated hydrocarbon (trade name).

Conventionally, when fluorine is doped into the base material for transparent vitrification as described above, the refractive index distribution of the base material is 4th.
It will look like the figure.

FIG. 5 shows the amount of fluorine doped in the base material in the conventional example. That is, in the conventional example, the amount of fluorine doped in the base material is uniform in the radial direction.

At this time, the fluorine-containing gas concentration in the transparent vitrification atmosphere is set to 1z or more.

The reason why the amount of fluorine doped in the base material is uniform is that when the fluorine-containing gas flowing into the electric furnace is, for example, sulfur hexafluoride, the refractive index of the base glass is (SF ) 1 as shown in Figure 6.
This is because it is proportional to /4.

rProblems to be solved by the invention j As mentioned above, in the conventional example, fluorine is doped into the porous glass base material when the base material is made into transparent vitrification, but this is because the fluorine is doped in the radial direction of the base material. This is a fluorine doping means that makes the doping amount uniform, so it is not suitable for fluorine doping in which the refractive index distribution in the radial direction of the base material changes, and it is not possible to obtain, for example, an optical fiber base material with a square refractive index distribution.

In view of the above problems, the present invention is capable of changing the amount of fluorine doped in the radial direction of the porous glass base material when doping fluorine into the base material in synchronization with the transparent vitrification of the porous glass base material. The present invention aims to provide a new transparent vitrification method that can change the refractive index distribution in the radial direction of the base material.

Means for Solving Problem 1 The present invention heat-treats a porous glass base material made of deposits of soot-like glass particles in a fluorine-containing gas atmosphere to make it transparent, while adding fluorine to the base material. The method for transparent vitrification of a porous glass preform to be doped is characterized in that the fluorine content in the radial direction of the preform is changed by controlling the concentration of a fluorine-containing gas in the atmosphere.

"Function" In the method of the present invention, the porous glass base material is made into transparent vitrification in a fluorine-containing gas atmosphere, and fluorine is doped into the base material at the time of transparent vitrification, which is the same as in the conventional method. Since the concentration of the element-containing gas in the fluorine-containing gas atmosphere is controlled, the fluorine content in the radial direction of the base material changes, and the refractive index distribution in the radial direction of the base material also changes.

At this time, the concentration of the fluorine-containing gas in the fluorine-containing gas atmosphere is preferably controlled to less than 1 $, and by controlling the gas concentration in such a minute amount, for example, the fluorine content of the base material can be The number increases as you move toward the outer periphery.

This is a unique phenomenon that is not observed in conventional examples in which the gas concentration is set to tX or more.

“Examples” Specific examples of the method of the present invention will be described below with reference to the drawings.

In FIG. 1, l is a quartz-based porous glass base material, and this porous glass base material 1 is a soot-like glass produced by a flame hydrolysis reaction or a thermal oxidation reaction in a VAD method, an OVD method, etc. It is made by depositing fine particles in a desired shape.

Reference numeral 2 denotes an electric furnace for converting the porous glass base material 1 into transparent vitrification. It consists of a ring-shaped heater B placed around the outer periphery of the tube 5.

When the porous glass base material 1 is made into transparent vitrification using the electric furnace 2 shown in FIG.
E, C12, SF6, etc. are supplied, and the inside of the furnace core tube 5 is heated via the heater 8.

The porous glass base material 1 is inserted into the furnace core tube 5 in a rotating state, where it is dehydrated, sintered, and made into transparent vitrification, and the base material is doped with fluorine.

In a more specific embodiment of the method of the present invention, a porous glass base material 1 made of S r 02 produced by the AD method is rotated and rotated at a temperature of 14 at a speed of 120+s/h.
It was inserted into the furnace core tube 5 of the electric furnace 2 which was set at 00°C.

At this time, 300cc/win of He was added to the core tube 5.
, O,.

and SF6 were supplied at 85 cc/win and 2 cc/win, respectively.

The concentration of SF8 in the fluorine-containing gas is 0.0
07%.

The base material made into transparent glass under these conditions had a refractive index distribution as shown in FIG.

This is because the amount of fluorine doped increases from the center of the base material to the outer periphery of the base material, and the refractive index decreases in areas where the amount of fluorine doped is large.

In the above specific example, by more precisely controlling the fluorine-containing gas concentration in the fluorine-containing gas atmosphere,
It has been found that a transparent glass base material having a refractive index distribution as shown in FIGS. 3(a), (b), and (c) can be obtained.

As the fluorine-containing gas, the already mentioned freon can also be used.

As a comparative example, 300 cc/inch of He was added in the core tube 5.
, S i 02 porous glass base material 1 was made into transparent vitrification in the same manner as in the above specific example except that SF6 was supplied at a rate of 300 ml/min.

In this comparative example, the concentration of SF6 in the fluorine-containing gas is 1 kg.

In the case of this comparative example, as expected, the refractive index distribution of the base material after transparent vitrification was as shown in FIG. 4 above.

"Effects of the Invention J As explained above, 1. When the method of the present invention is used, the concentration of fluorine-containing gas in the atmosphere is controlled when a porous glass base material is made into transparent vitrification. The rate can be varied so that
A glass base material made of a pure quartz core and a fluorine-doped quartz cladding, which was considered difficult in the VAD method, OVD method, etc., has become possible, and the refractive index distribution in the base material can be arbitrarily changed to a rectangular distribution, square distribution, triangular distribution, etc. Can be set.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram schematically showing an embodiment of the method of the present invention, and FIG.
The figure is a refractive index distribution diagram of a base material made transparent by the method of the present invention, and Figure 3 (a), (b), and (c) are refractive index distribution diagrams of various base materials made transparent by the method of the present invention. Figure 4 is a refractive index distribution diagram of the base material made into transparent glass by the conventional method.
Figure 5 is a refractive index distribution diagram showing the amount of fluorine doped in the base material made transparent by the conventional method, and Figure 6 is the relationship between the refractive index difference of the silica-based glass base material and the degree of sulfur hexafluoride moat. FIG. l ... Porous glass base material 2 ... Electric furnace 5 ... Electric furnace core tube 6 ... Electric furnace heater agent Patent attorney Yoshio Saito Figure 4 Figure 1 flash @ Figure b

Claims (2)

[Claims] (1) Transparent vitrification of a porous glass base material consisting of deposits of soot-like glass particles is made transparent by heat-treating it in a fluorine-containing gas atmosphere and doping fluorine into the base material. A method for transparent vitrification of a porous glass preform, characterized in that the fluorine content in the radial direction of the preform is varied by controlling the concentration of a fluorine-containing gas in the atmosphere. (2) The method for transparent vitrification of a porous glass base material according to claim 1, wherein the fluorine-containing gas concentration in the fluorine-containing gas atmosphere is less than 1%.