JPH06219765A - Production of preform for optical fiber - Google Patents

Abstract

PURPOSE:To produce low-loss optical fiber of a pure quartz core and a fluorine- doped quartz clad. CONSTITUTION:Pure quartz glass having a quartz glass part doped with fluorine on the surface is used as a rod for a core. A fine particle layer of pure quartz glass is deposited on the surface of the rod by outer deposition method. The fine particle layer of the glass is dehydrated and transparently vitrified in a fluorine-containing gas atmosphere to form a preform for optical fiber. The rod for a core is exposed to high temperature in piling the fine particle layer of the glass on the surface of the rod by outer deposition method. Since the quartz glass part doped with fluorine exists on the surface of the rod, water is made into HF and is vaporized even if water is attached to the outer peripheral face of the rod. Consequently, existence of water in the preform is suppressed and prepared optical fiber has low absorption loss by OH group.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a pure quartz core-fluorine-doped quartz clad type optical fiber preform, and provides a method for obtaining an optical fiber having a small absorption loss due to OH groups. is there.

[0002]

2. Description of the Related Art Pure silica glass is sometimes used for the core when attempting to achieve low loss of an optical fiber.
The reason is that pure quartz glass has a smaller loss due to Rayleigh scattering as compared with Ge-doped quartz glass.
Generally, a method of producing a pure quartz core fiber is to prepare a pure quartz glass rod, deposit glass fine particles made of pure quartz by a so-called external attachment method around the rod, dehydrate the glass fine particles, and then add a fluorine-containing gas. For example, it is a method of forming a core-clad type optical fiber preform by forming a vitrified quartz clad in a SiF ₄ atmosphere to form a fluorine-doped quartz clad.

[0003]

However, since this method is a method of directly attaching the clad material to the core material,
Impurities are likely to be mixed in, and in particular, water attached to the core material remains and becomes an OH group, and its absorption appears at a wavelength of 1.39 μm, resulting in a large loss. Therefore, as a result of various investigations by the present inventors, an optical fiber preform was formed by forming a fluorine-doped quartz clad to be a clad by an external attachment method in a similar manner to a core material containing fluorine. It was found that the fiber obtained by melting and drawing the fiber had a small absorption loss due to the OH group. This is because the fluorine-doped core material scatters fluorine from the surface when exposed to high temperature, and reacts even in the presence of water to generate H.
It is thought that it is because it becomes F and is skipped.

[0004]

The present invention has been made from the above viewpoints, and the invention according to claim 1 is characterized in that the surface thereof is a pure silica glass containing a silica glass portion doped with fluorine. Prepare a core rod consisting of
It is a method of manufacturing an optical fiber having a quartz glass clad layer around which fluorine is doped by an external method. Here, the external attachment method means that a glass layer is formed on the outer periphery of the core rod, and the formed silica glass layer is formed by once forming a silica glass fine particle layer by a fire hydrolysis method,
It may be transparent vitrified, or may be vitrified directly by the plasma method. Further, the fluorine doping means, when obtaining transparent glass from quartz glass fine particles, for example, it is possible to make the vitreous silica fine particles transparent vitrification in a fluorine-containing gas atmosphere. The oxygen gas and the fluorine-containing gas may be supplied and reacted. Note that the fluorine-doped portion formed on the surface of the pure quartz core can be made a part of the clad by adjusting the doping amount of fluorine so as to finally match it with that of the clad. Further, pure silica core rod having a portion fluorine on its surface is doped, using a central burner and side burners by a so-called VAD method, the central burner supply only SiCl ₄ for instance, the side burner with SiCl ₄ It can be produced by supplying a fluorine-containing gas.

[0005]

[Function] Since the core rod made of pure silica glass has a silica glass portion containing fluorine on the surface thereof, the rod is formed in the process of depositing a pure silica glass fine particle layer on the rod by an external method. Even when exposed to high temperatures, water on the outer peripheral surface of the rod becomes HF and scatters.
The obtained fiber has a small absorption loss due to the OH group.

[0006]

Example 1 The VAD method was adopted for the production of core fine particles. Specifically, pure silica glass fine particles were deposited in a rod shape on the tip of a starting member vertically supported by using a central burner, while fluorine-doped quartz glass fine particles were deposited around the fine particles by a side burner. S for the central burner
0.1 mol / min of iCl ₄ , 4 liters / min of H ₂ , O
₂ is supplied at 2 liters / minute, and the side burner is SiCl
₄ , 0.1 mol / min, SiF ₄ 0.05 mol / min, H
₂ was supplied at 4 liters / minute, and O ₂ was supplied at 2 liters / minute.
After that, the fine particles for the core were mixed with 0.5% Cl ₂ and He 9
It was dehydrated in an atmosphere of 9.5% and then vitrified into a transparent glass rod having a length of 200 mm and an outer diameter of 30 mmφ. The pure quartz portion of the obtained core rod is 25 mm
The thickness of φ, the quartz portion outside which was doped with fluorine was 2.5 mm, and the relative refractive index difference between them was 0.3%. This rod is stretched to 10 mmφ, and pure silica glass fine particles are deposited around the rod by an external attachment method so that the outer diameter is 7
It was set to 0 mmφ. At that time, SiCl ₄ was added to the burner in an amount of 0.
1 mol / minute, H ₂ at 4 liters / minute, and O ₂ at 2 liters / minute were supplied. Then, the glass fine particle layer is mixed with Cl
_{2 It} was dehydrated in an atmosphere of 0.5% and He 99.5%, and subsequently it was transparent vitrified in an atmosphere of SiF ₄ 0.3% and He 99.6% to obtain a 35 mmφ transparent glass rod. This rod was further stretched to 10 mmφ, and a fluorine-doped quartz layer was again formed thereon by the external attachment method under the same conditions as above, and dehydration and transparent vitrification were carried out in the same manner to finally obtain 30
A base material for an optical fiber having a diameter of mmφ and a length of 500 mm was used. The base material was melt-drawn from one end to a fiber having an outer diameter of 125 μm, which was coated with UV resin. The loss of the obtained optical fiber is 1 dB / km at the wavelength of 1.39 μm where the absorption loss due to the OH group appears, and the wavelength is 1.55.
In μm, it was a low loss of 0.2 dB / km. By the way,
If the starting core material is not doped with fluorine, the wavelength is 1.3.
60 dB / km at 9 μm, wavelength 1.55 μm
The loss at m was 0.8 dB / km.

[0007]

The method for producing an optical fiber according to the method of the present invention comprises a pure silica glass having a silica glass portion doped with fluorine on the surface thereof as a core rod, and therefore it is formed by an external attachment method. Even when the rod is exposed to a high temperature when depositing the glass fine particle layer to be the cladding, the water on the surface of the rod scatters as HF and does not remain in the fiber. A small fiber is obtained.

Claims (4)

[Claims] 1. A core rod made of pure silica glass including a silica glass portion doped with fluorine on its surface is prepared, and a silica glass clad layer doped with fluorine is formed around the core rod by an external attachment method. And a method for manufacturing an optical fiber preform. 2. A core rod made of pure silica glass containing a silica glass portion doped with fluorine on its surface is prepared, and pure silica glass fine particles are deposited around the core rod by an external attachment method. A method for producing an optical fiber preform, wherein the glass fine particle layer is dehydrated, and then this pure silica glass fine particle layer is made into a transparent vitrification in a fluorine-containing gas atmosphere to form a fluorine-doped Rad layer. 3. The method for producing an optical fiber preform according to claim 1, wherein the silica glass portion doped with fluorine on the surface of the core rod forms a part of the clad. 4. A VAD method is used to deposit pure silica glass particles as a core on the tip of a vertically supported starting member in a rod shape, while depositing fluorine-doped silica glass particles in a layer form around the rod. The optical fiber glass fine particles are formed into a core rod made of pure silica glass containing a silica glass portion doped with fluorine on the surface by dehydration and transparent vitrification. The method for producing an optical fiber preform according to claim 2.