JPH05221678A - Production of preform of optical fiber and equipment for producing preform of optical fiber - Google Patents

Abstract

PURPOSE:To produce the subject preform of an optical fiber, having the clad layer enabling fluorine addition without infiltrating the fluorine into the core layer and low in transmission loss. CONSTITUTION:An SiO2-GeO2 skin layer is uniformly deposited on a pure SiO2 core soot in an amount of <=3% on molar ratio base and a clad is further deposited on the skin layer. Both the skin layer and the core layer of the resultant porous preform prepared by the above-mentioned method are subjected to dehydration treatment with Cl2. The treated material is then heat treated at a sintering temperature (about 1400 deg.C) of SiO2-GeO2 lower than that of SiO2 so as to sinter only the skin layer. Further heat treatment is carried out at a clarification temperature (about 1450 deg.C) of SiO2 while charging an SF6- containing atomospheric gas so as to sinter the core layer and the clad layer.

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 an optical fiber preform which produces glass fine particles by a flame hydrolysis reaction and deposits them on a starting material to obtain a porous glass body. The present invention relates to an optical fiber preform manufacturing method and an optical fiber preform manufacturing apparatus capable of obtaining an optical fiber preform having excellent transmission loss and transmission band.

[0002]

2. Description of the Related Art Generally, in an optical fiber, light propagates in the core while being totally reflected at the boundary surface between the core and the clad of the optical fiber. The way of propagation varies depending on the refractive index distribution of the core, and when the core diameter is several tens of μm or more in both the step index type and the graded index type, the light path varies depending on the incident angle of light, and many optical paths occur. Further, when the outer diameter of the core is set to about 5 to 15 μm, the light does not reflect on the boundary surface between the core and the clad and goes straight through the core, and the optical path becomes one. Such an optical fiber is a single mode optical fiber.

In such an optical fiber, it is ideal that the intensity of the incident light is emitted without being attenuated, but while the light propagates through the core of the optical fiber, various kinds of light are emitted. Transmission loss occurs due to the cause. The degree to which the intensity of light becomes weaker while propagating in an optical fiber is the transmission loss of the optical fiber. Transmission loss of an optical fiber is mainly due to absorption by electron transition in the ultraviolet region, absorption by molecular vibration in the infrared region, Rayleigh scattering inversely proportional to the fourth power of the wavelength, and impurities, especially water (hydroxy ion / OH group) absorption. The total of these has wavelength dependency.

Further, the wider the transmission band of the optical fiber, the larger the information transmission capacity. That is, the wider the transmission band, the more signals can be sent at one time. In this transmission band, a signal with a constant amplitude having a band from DC to high frequency is input to one end of an optical fiber with a length of 1 km, and the amplitude of the received signal at the output end is shown by the value of the frequency reduced by 6 dB. is there.

Since the transmission loss and the transmission band affect the performance of the optical fiber, a method for improving the transmission loss and the transmission band has been conventionally devised in the optical fiber manufacturing method. It is known to add Ge to the core in order to increase the refractive index of the core. But this Ge
Is an impurity in the core of the optical fiber, and Rayleigh scattering of the light propagating in the core occurs to cause a transmission loss. Therefore, it is attempted to prevent the expansion of transmission loss by adding fluorine to the clad layer to reduce the scattering and attenuation of light from the core.

As a method for manufacturing an optical fiber preform by adding fluorine to this cladding layer, conventionally, an external attachment method (O
There is a method as disclosed in Japanese Patent Laid-Open No. 1-230445 by VD method Outside Vapor Deposition). In this conventional example, as shown in FIG. 4, a porous base material 120 having SiO ₂ glass particles deposited on a pure quartz or Ge-containing quartz rod 100 is transparent vitrified in an atmosphere containing SF ₆ by a torch 110. Fluorine is added to the cladding layer.

Similarly, as a method for manufacturing an optical fiber preform by adding fluorine to the cladding layer, a conventional axial attachment method (V
There is also a method as disclosed in JP-A-1-270533 by AD method Vapor Phase Axial Deposition). In this manufacturing method, as shown in FIG. 5, the core torch 200 hydrolyzes the vapor of SiCl ₄ using combustible gas (H ₂ ) and oxygen to deposit SiO ₂ soot containing a large amount of air in multiple layers to form a porous structure. The core part 210 is obtained. On the porous core portion 210, a cladding torch 220 is used to form SiCl
The vapor of ₄ is hydrolyzed using combustible gas (H ₂ ) and oxygen to deposit SiO ₂ containing a large amount of air to obtain the porous clad portion 230. At this time, by changing the manufacturing conditions when manufacturing the core and the manufacturing conditions when manufacturing the clad, the density distribution of the entire porous base material becomes high in the core part and low in the clad part as shown in FIG. Control is performed to make it transparent in the atmosphere of SF ₆ . By doing so, SF ₆ in the atmosphere is prevented from entering the core.

[0008]

However, according to the former method of the prior art, SiO ₂ forming the core portion is formed.
In order to deposit glass particles, pure quartz or Ge is used in advance.
Since it is necessary to prepare the containing quartz rod, there is a problem that the manufacturing process of the optical fiber preform becomes complicated. In addition, pure quartz or G which is a starting material for depositing the core portion
The e-containing quartz rod has a problem in that, when SiO ₂ glass fine particles are deposited, the surface thereof is exposed to the flame 130 and OH groups are mixed in the core portion, which causes an increase in transmission loss of the optical fiber. have.

According to the latter method of the prior art, it is possible to prevent SF ₆ in the atmosphere from entering the core by forming a step in the density distribution of the entire porous base material, but Gases such as Cl ₂ for the purpose of removing the OH group contained in the core cannot penetrate into the core,
The H group cannot be removed. That is, vapor of SiCl ₄ is converted into combustible gas (H ₂ ) by the core torch 200.
However, impurities such as OH groups contained in the soot of SiO ₂ remain in the porous core portion 210 itself when it is hydrolyzed and deposited using oxygen and oxygen, which increases the transmission loss. ing.

The present invention provides a method for producing an optical fiber preform and an apparatus for producing the optical fiber preform, in which fluorine does not penetrate into the core portion even if fluorine is added to the cladding layer and the transmission loss is small. Has a purpose.

[0011]

In order to achieve the above object, in the method of manufacturing an optical fiber preform, pure SiO _{2 is used.}
A porous base material formed by uniformly depositing a SiO ₂ —GeO ₂ skin layer on the core soot and depositing a predetermined amount of clad is subjected to flame hydrolysis with Cl ₂ and dehydration treatment, and then firing of SiO ₂ Only the skin layer is sintered by heating at a temperature lower than the binding temperature, and then the cladding layer is sintered by heating at the transparentizing temperature of SiO ₂ while introducing an atmosphere gas containing SF _6. It was done like this.

In order to achieve the above object, in a manufacturing apparatus for an optical fiber preform, a core torch for depositing pure SiO ₂ core soot, and SiO on the pure SiO ₂ core soot formed by the core torch. _2- Ge
Porous base material manufacturing apparatus provided with a torch for a cladding layer for depositing an O ₂ cladding layer, and dehydration / clearing treatment apparatus for dehydrating / clearing the porous base material manufactured by the porous base material manufacturing apparatus In an apparatus for producing an optical fiber preform constituted by: and S on a pure SiO ₂ core soot formed by the core torch in the porous preform production apparatus.
The skin layer torch for depositing the iO ₂ -GeO ₂ skin layer is provided.

[0013]

[Function] 3% as a mole fraction on pure SiO ₂ core soot
It is uniformly deposited following SiO ₂ -GeO ₂ skin layer,
Further, a clad is deposited on this skin layer. The porous base material thus prepared, including the skin layer and the core, is dehydrated with Cl ₂ . Then, below the sintering temperature of the SiO ₂ SiO ₂ -GeO ₂ sintering temperature (1
Only the skin layer is sintered by heat treatment at about 400 ° C. Further, while introducing an atmosphere gas containing SF ₆ , S
The core and clad layers are sintered by heat treatment at the transparentizing temperature of iO ₂ (about 1450 ° C.).

[0014]

EXAMPLES Examples of the present invention will be described below. FIG. 1 is a process flowchart showing an embodiment of the method for manufacturing an optical fiber preform according to the present invention. A method of manufacturing an optical fiber preform will be described according to the procedure shown in the figure.

First, in step 1, the vapor of SiCl ₄ is hydrolyzed using combustible gas (H ₂ ) and oxygen to deposit pure SiO ₂ core soot. The pure SiO ₂ core obtained at this time is a porous core containing a large amount of air. When the porous core is formed in step 1, SiCl ₄ and GeCl ₄ are formed in step 2.
Is hydrolyzed with combustible gas (H ₂ ) and oxygen to uniformly deposit a SiO ₂ —GeO ₂ skin layer having a molar fraction of 3% or less on the porous core. The skin layer does not need to be thick. And in step 3,
The vapor of SiCl ₄ is hydrolyzed using combustible gas (H ₂ ) and oxygen to deposit SiO ₂ to form a clad layer,
A porous matrix is formed. This clad layer is a porous clad containing a large amount of air.

Next, in step 4, the formed porous base material is dehydrated by mixing He gas and oxygen and heating with Cl ₂ gas. At this time, the clad layer, the skin layer, and the core portion are also dehydrated. When the dehydration process is performed in step 4, in step 5, below the sintering temperature of the SiO _2, sintering only the skin layer is heat treated at a sintering temperature of SiO ₂ -GeO _2. This is SiO
_This utilizes the property that the sintering temperature of _2- GeO ₂ is lower than the sintering temperature of SiO ₂ . The temperature of this heat treatment is about 1400 ° C. The SiO ₂ -GeO ₂
By the heat treatment at the sintering temperature of, the core and the clad layer remain in a porous state, and since the dehydration treatment has already been performed in step 4, the OH group has already been removed from the core and the clad layer. It is in the state of being.

Then, when only the skin layer is sintered in step 5, in step 6, the sintered porous base material of the skin layer is continuously supplied with He, Cl ₂ and SF ₆ gases. It is placed in a gas atmosphere and heat-treated for a predetermined time at the transparentizing temperature of SiO ₂ to sinter the core and clad layers. The clearing temperature of this SiO ₂ is about 1450 ° C. At the time of the transparentizing heat treatment in step 6, fluorine is added to the cladding layer, and at the same time fluorine is diffused into the skin layer. However, since the skin layer has already been semi-sintered in step 5, the fluorine added to the cladding layer is protected by the skin layer to prevent it from entering the core. Further, in the skin layer itself, since fluorine is mixed with GeO ₂ remaining in the skin layer, the refractive index is canceled by the two components of Ge and fluorine, and the optical fiber matrix having a good RI distribution as a whole. Get the wood. In step 6, the transparentizing heat treatment is performed, and in step 7, when the core, the skin layer, and the clad layer are transparentized, the production of the optical fiber preform having a favorable RI distribution is completed.

2 to 3 show an embodiment of an apparatus for manufacturing an optical fiber preform according to the present invention.

In the figure, 10 is a porous base material manufacturing apparatus, in which a SiO ₂ --GeO ₂ skin layer is deposited on a pure SiO ₂ core soot, and a SiO ₂ clad layer is deposited on this skin layer to form a porous layer. It is for producing a quality base material.
Numeral 11 is a core torch, which is pure Si from bottom to top.
It supplies O ₂ core soot. That is, the core torch 11 uses vapor of SiCl ₄ as a combustible gas (H ₂ ).
Oxygen to generate pure SiO ₂ core soot was hydrolyzed using the is intended for depositing pure SiO ₂ core soot on the tip of a quartz rod to be species. When this quartz rod is pulled up while rotating, pure SiO ₂ core soot containing a large amount of air supplied from the core torch 11 is continuously deposited and grows. The pure SiO ₂ core obtained at this time is
The porous core 14 contains a large amount of air.

Reference numeral 12 is a skin layer torch, which is provided above the core torch 11 and is made of SiO ₂ --GeO _2.
Is continuously supplied. That is, the skin layer torch 12 hydrolyzes vapors of SiCl ₄ and GeCl ₄ using combustible gas (H ₂ ) and oxygen to form a SiO ₂ —GeO ₂ skin layer 15 having a molar fraction of 3% or less. It is for uniformly depositing on the porous core 14.
SiO ₂ -G supplied from this skin layer torch 12
By pulling up the quartz rod while rotating, eO ₂ becomes a thin film and is continuously and uniformly deposited on the porous core 14 to grow. The skin layer 15 does not need to be thick.

A cladding layer torch 13 is provided above the skin layer torch 12 and continuously supplies SiO ₂ . That is, the cladding layer torch 13 converts the vapor of SiCl ₄ into a combustible gas (H ₂ ).
And for hydrolyzing with oxygen to deposit SiO ₂ on the skin layer 15. S obtained at this time
The iO ₂ clad is a porous clad layer 16 containing a large amount of air.

The porous core 14, the skin layer 15, and the porous clad layer 16 constitute a porous base material 20.

Reference numeral 30 denotes a dehydration / transparency treatment apparatus for dehydrating / transparentizing the porous base material 20 produced by the porous preform producing apparatus 10 to produce a preform (optical fiber preform). Reference numeral 31 is a quartz tube, which is a container for dehydrating and making the porous base material 20 transparent. Reference numeral 32 is a heating element for heating the porous base material 20 inserted in the quartz tube 31 to perform dehydration treatment / heat sintering treatment of the clad layer 16, the skin layer 15, and the core 14.

Reference numeral 33 denotes an atmosphere gas inlet, which is the cladding layer 1
6, the atmospheric gas sent into the quartz tube 31 when the skin layer 15 and the core 14 are dehydrated, and the quartz gas 31 inside the quartz tube 31 when the clad layer 16, the skin layer 15 and the core 14 are heat-sintered. This is for introducing the atmospheric gas to be sent. Reference numeral 34 denotes an atmosphere gas discharge port for discharging the used atmosphere gas used for dehydration in the quartz tube 31 and the used atmosphere gas used for heating and sintering treatment to the outside of the quartz tube 31. .. Reference numeral 35 is an elevating device, which is the porous base material 20.
In order to dehydrate / transparent, the optical fiber preform 40 produced by spin-drying the porous preform 20 and pulling it up is rotated as shown by an arrow A.

Next, the operation of the dehydration / clearing processing device 30 will be described. First, the porous base material 20 manufactured in the porous base material manufacturing apparatus 10 is housed in a quartz tube 31,
He and Cl ₂ are introduced into the quartz tube 31 through the atmospheric gas inlet 33.
The mixed gas of is continuously fed together with oxygen. The porous base material 20 is dehydrated by heating with the heating element 32 while feeding this gas. By this dehydration treatment, the porous base material 20
Is subjected to dehydration treatment including the clad layer 16, the skin layer 15, and the core 14. When this dehydration treatment is performed, SiO ₂ −
Utilizing the property that the sintering temperature of GeO ₂ (about 1400 ° C.) is lower than the sintering temperature of SiO ₂ (about 1450 ° C.),
The heating element 32 is operated to heat the inside of the quartz tube 31 at a sintering temperature of SiO ₂ —GeO ₂ (about 1400 ° C.) lower than the sintering temperature of SiO ₂ (about 1450 ° C.) to burn only the skin layer 15. Conclude. At the stage where only the skin layer 15 is sintered, the core 14 and the clad layer 16 remain in a porous state.

When only the skin layer is sintered, the quartz tube 31
A mixed gas of He, Cl ₂ , and SF ₆ is continuously supplied from the atmosphere gas inlet 33 to activate the heating element 32 and the quartz tube 31 is heated at a transparentizing temperature of SiO ₂ (about 1450 ° C.) for a predetermined time. The core 14 and the clad layer 16 are sintered by heating.
The core 14 and the clad layer 1 at the time of performing the transparentizing heat treatment
In No. 6, the OH group has already been removed in the dehydration treatment in the previous step. When this heat treatment for clearing is performed, fluorine is added to the cladding layer 16, and at the same time, fluorine is diffused into the skin layer (SiO ₂ —GeO ₂ ) 15 and GeO ₂ is volatilized from the skin layer 15. But,
The fluorine added to the clad layer 16 is in a state where the skin layer 15 is already semi-sintered, is protected by the skin layer 15, and does not enter the core 14. Also, the skin layer 1
In 5 itself, since fluorine is mixed with GeO ₂ remaining in the skin layer 15, the refractive index is offset by the two components of Ge and fluorine, and a good RI distribution is obtained as a whole. In this way, the optical fiber preform 40 is manufactured, and the rod-shaped optical fiber preform 40 is obtained by pulling the optical fiber preform 40 upward by the elevating device 35.

[0027]

Since the present invention is constructed as described above, it has the following effects.

[0028] SiO ₂ -Ge on the pure SiO ₂ core soot
A porous base material formed by uniformly depositing an O ₂ skin layer and depositing a predetermined amount of clad is subjected to flame hydrolysis with Cl ₂ and dehydration treatment, and then a temperature lower than the sintering temperature of SiO _2. And heat only to sinter the skin layer, and then while introducing an atmosphere gas containing SF ₆ , Si
Since the cladding layer is manufactured by heat treatment at a transparentizing temperature of O ₂ , it is possible to add fluorine to the cladding layer without invading the core portion, and an optical fiber with less transmission loss. The base material can be obtained.

[0029] The porous having a core torch to deposit a pure SiO ₂ core soot, the cladding layer torch for depositing a cladding layer of SiO ₂ -GeO ₂ on pure SiO ₂ core soot formed by the core torch Of the optical fiber preform, which is composed of a high-quality preform manufacturing device and a dehydration / clearing treatment device for dehydrating / clearing the porous preform manufactured by the porous preform manufacturing device. Since the material manufacturing apparatus is provided with the skin layer torch for depositing the SiO ₂ —GeO ₂ skin layer on the pure SiO ₂ core soot formed by the core torch, the skin layer can be formed on the core portion. Since this skin layer can prevent fluorine from entering the core when fluorine is added to the clad layer, fluorine is added only to the clad layer. It is possible to obtain an optical fiber preform with less transmission loss.

[Brief description of drawings]

FIG. 1 is a process flowchart showing an embodiment of a method for manufacturing an optical fiber preform according to the present invention.

FIG. 2 is a schematic view showing an embodiment of a porous preform manufacturing apparatus of an optical fiber preform manufacturing apparatus according to the present invention.

FIG. 3 is a schematic view showing an embodiment of a dehydration / clearing treatment device of an optical fiber preform manufacturing device according to the present invention.

FIG. 4 is a diagram showing a method for producing an optical fiber preform by adding fluorine to a cladding layer by a conventional external attachment method.

FIG. 5 is a diagram showing a method of manufacturing an optical fiber preform by adding fluorine to a cladding layer by a conventional VAD method.

FIG. 6 is a diagram showing a density distribution of the entire porous base material shown in FIG.

[Explanation of symbols]

10 ………………………………………………………… Porous base material manufacturing equipment 11 …………………………………………………… Torch for core 12 ……………………………………………………… Skin layer torch 13 …………………………………………………… Cladding layer torch 14 …… ………………………………………………… Porous core 15 …………………………………………………… Skin layer 16 ………………… …………………………………… Porous clad layer 20 …………………………………………………… Porous matrix 30 ………………………… ………………………… Dewatering / clearing treatment equipment 31 …………………………………………………… Quartz tube 32 ………………………………… …………………… Heating element 33 …………………………………………………… Atmosphere gas introduction 34 ………………………………………………………… Atmosphere gas outlet 35 ……………………………………………… Lifting device 40 ……… ……………………………………………… Optical fiber base material

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[Procedure amendment]

[Submission date] July 10, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Correction content]

Then, when only the skin layer is sintered in step 5, in step 6, the sintered porous base material of the skin layer is continuously supplied with He, Cl ₂ and SF ₆ gases. It is placed in a gas atmosphere and heat-treated for a predetermined time at the transparentizing temperature of SiO ₂ to sinter the core and clad layers. The clearing temperature of this SiO ₂ is about 1450 ° C. At the time of the transparentizing heat treatment in step 6, fluorine is added to the cladding layer, and at the same time fluorine is diffused into the skin layer. However, since the skin layer has already been semi-sintered in step 5, the fluorine added to the cladding layer is protected by the skin layer to prevent it from entering the core. Further, in the skin layer itself, since fluorine is mixed with GeO ₂ remaining in the skin layer, the refractive index is canceled by the two components of Ge and fluorine, and the optical fiber matrix having a good RI distribution as a whole. Get the wood. No clearing heat treatment in step 6
Is, core, skin layers, the clad layer has finished the production of optical fiber preform having a good RI distribution when transparent.

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

[Figure 1]

Claims (2)

[Claims] 1. SiO ₂ -G on pure SiO ₂ core soot.
The eO ₂ skin layer is uniformly deposited, and the porous base material prepared by depositing a predetermined amount of cladding is hydrolyzed by flame hydrolysis with Cl ₂ and then subjected to a temperature lower than the sintering temperature of SiO _2. Heat treatment to sinter only the skin layer,
Then, while introducing an atmosphere gas containing SF ₆ , S
A method for producing an optical fiber preform, characterized in that the cladding layer is sintered by heat treatment at a transparentizing temperature of iO ₂ . 2. A core torch for depositing pure SiO ₂ core soot, and pure S formed by the core torch.
A porous base material manufacturing apparatus equipped with a torch for a cladding layer for depositing a cladding layer of SiO ₂ —GeO ₂ on an iO ₂ core soot, and dehydration / transparency of the porous base material manufactured by the porous base material manufacturing apparatus. In the manufacturing apparatus of the optical fiber preform, which is composed of the dehydrating / clearing processing apparatus
SiO ₂ —GeO ₂ is formed on the pure SiO ₂ core soot formed by the core torch in the porous base material manufacturing apparatus.
An apparatus for producing an optical fiber preform, comprising a skin layer torch for depositing a skin layer.