JPH0524873A - Production of glass preform for optical fiber - Google Patents

Abstract

PURPOSE:To reduce residual stress in core of optical fiber consisting of quartz glass and core in production of quartz-based optical fiber. CONSTITUTION:In producing a glass preform by dehydrating a porous preform consisting essentially of quartz and vitrifying transparently, the porous preform is heat-treated in an atmosphere comprising a halogen-containing gas in the dehydrating process and/or the transparently vitrifying to add 0.1-1wt.% halogen to quartz glass and the prepared glass preform is used as core of optical fiber to produce glass preform for optical fiber. Chlorine is preferable as the halogen. The core formed in this way has small residual stress and variability of specific refractive index difference, lambdac, MFD, etc., from set values is small.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of a silica-based optical fiber preform, and in particular, it is applied to the production of an optical fiber having a core of pure silica glass to reduce the residual stress in the core. It's about how you can do it.

[0002]

2. Description of the Related Art VAD (Vapor Phase Axial Deposition) is a general method for mass-producing optical fiber preforms.
The law is known. In the VAD method, glass particles produced in an oxyhydrogen flame are deposited on a rotating starting member such as a glass rod to form a columnar porous base material, and the porous base material is sintered to produce a transparent light. It is a method of manufacturing a glass preform for fibers. In the VAD method, the porous base material is sintered,
To make transparent glass, the base material should be 1 in an inert gas atmosphere.
It needs to be heated to 600 ° C or higher. In addition, since the loss characteristics in the communication wavelength range of the obtained optical fiber are impaired when water is mixed in, the porous base material is usually dehydrated before or at the same time as it is made transparent. As a dehydration treatment, there is known a method in which a porous base material is heated at a high temperature in an inert gas atmosphere to which a chlorine-based gas is added.

[0003]

By the way, in recent years, especially from the viewpoint of reducing the loss of an optical fiber, a single-mode fiber using pure silica glass for the core and fluorine-doped silica glass for the cladding is regarded as promising. The optical fiber having pure silica as a core does not have GeO ₂ added to the core, so that scattering loss can be reduced, which is extremely advantageous for lowering the loss. However, in the case of an optical fiber composed of a pure quartz core and a fluorine-added quartz clad, since the viscosity of the core portion having a small cross-sectional area is larger than that of the clad portion, most of the tension during drawing is applied to the core. As a result, tensile stress remains in the core during glass solidification, causing various problems. That is, the tensile stress lowers the refractive index of the core part (photoelastic effect) and decreases the relative refractive index difference between the core and the clad. As a result, λc (cutoff wavelength) and MFD (mold field) predicted at the preform stage are obtained. Since the diameter) and λ ₀ (zero dispersion wavelength) are deviated, and the residual tensile stress strongly depends on the manufacturing conditions (drawing conditions), it is necessary to pay attention to the stabilization of the conditions. The present invention has been made for the purpose of solving the above-mentioned problems of the fiber having the pure quartz core-fluorine-doped clad structure and providing a method capable of manufacturing an optical fiber preform having a designed structure. .

[0004]

DISCLOSURE OF THE INVENTION In view of the above, the inventors of the present invention reduce the viscosity of pure quartz glass as a core so that the difference from the viscosity of fluorine-doped quartz glass as a clad becomes relatively small. However, we thought that this would alleviate the stress concentration on the core during drawing, and as a result of various studies,
The inventors have found that the problem can be solved by containing a slight amount of halogen in pure quartz glass, and have reached the present invention. That is, the present invention which solves the above-mentioned problem is a method for obtaining a glass base material by subjecting a porous base material containing quartz as a main component to a dehydration treatment and a transparent vitrification, wherein a halogen is used in the dehydration treatment step and / or the transparent vitrification step. It is characterized in that 0.1 to 1% by weight of halogen is contained in quartz glass by heat treatment in an atmosphere containing a system gas, and the glass preform thus obtained is used as a core of an optical fiber. Chlorine is particularly preferable as the halogen contained in the quartz glass in the present invention.

[0005]

In order to investigate the effect of the above-mentioned viscosity of pure quartz glass, Cl is used as a typical halogen, and the amount of Cl in the quartz glass is analyzed by ion chromatography.
In addition to quantifying, a single mode fiber was manufactured by using the quartz base material as a core to obtain a structure (λc, MFD),
The relative refractive index difference between the core and the clad was measured from the residual stress and the refractive index distribution of the fiber. It was revealed that the residual stress in the fiber became smaller as the amount of Cl increased, and as a result, the relative refractive index difference between the core and the clad approaches the value measured in the preformed state (FIGS. 1 and 2). Further, the fiber structure (λc, MFD) also almost matched the predicted value at the preform stage (FIG. 3). From the above, by increasing the amount of Cl in the pure silica glass, the viscosity of the silica glass can be lowered, so that the fiber structure does not deviate from the predicted value at the preform stage and the fiber drawing Since the dependence is relaxed, it works very effectively for manufacturing a pure silica core single mode fiber. The amount of Cl in pure quartz glass is 0.2% by weight
It is preferably ˜1% by weight. If it is less than 0.1% by weight, the effect of containing Cl does not appear and it greatly deviates from the predicted value at the preform stage, which is not preferable. Further, since the content of about 1% by weight substantially agrees with the predicted value of the preform, it is not necessary to exceed the content. The same was true for the case of adding another halogen element having the effect of lowering the viscosity of quartz glass. Since fluorine also has a function of lowering the refractive index, when fluorine is added, it is necessary to add more fluorine to the cladding than the amount added to the core. In addition, iodine (I) and bromine (Br) are also effective, but it is advantageous to use a chlorine-based gas in that it has the largest dehydration effect and does not affect the refractive index. Examples of chlorine-based gas include Cl ₂ , CCl ₄ , SiCl ₄ , SOCl ₂ , and SCl _2.
Etc. can be mentioned. However, for example, it is possible to use only chlorine-based gas for dehydration and use other halogen-based gas for clearing treatment, or use other halogen-based gas for dehydration and chlorine-based gas for clearing. Is.

[0006] Actually, halogen of 0.
In order to make the content within the range of 1% by weight to 1% by weight, an appropriate one is selected from the halides, and the halide and H
It is heated at a temperature in the range of 1000 ° C. to 1700 ° C. in an atmosphere of an inert gas such as e to a pressure of about 3 kg / cm ² from normal pressure to react with the porous base material to form a transparent glass. Specifically, the heat treatment of the porous base material in an atmosphere containing a halide gas includes a) a dehydration processing step of the porous base material, b) a transparent vitrification processing step of the porous base material, and c). It may be carried out in any of a) to c) of the dehydration treatment step and the transparent vitrification treatment step, particularly preferably a) or c). The addition amount can be controlled by adjusting the gas concentration, the processing temperature, etc., but in the experiments conducted by the present inventors, the range of adjustment was at most about 1.5 times. Rather, selection of the type of halide gas is more effective in adjusting the amount of addition, for example, when chlorine gas was used, it was 0.2 to 0.4% by weight, but when SiCl ₄ gas was used, it was 0.5. ~ 0.8% by weight
Became. That is, the content can be greatly changed depending on the gas species used. The porous base material used in the present invention may be one prepared by means known in the technical field of this kind, for example, VAD.
However, the present invention is not limited thereto. The bulk density of the porous base material is not particularly limited, but is generally about 0.2 to 0.4 g / cm ³ . Further, although a porous body made of pure quartz is shown in the above description, a porous body containing quartz as a main component may be used.

[0007]

【Example】

Example 1 First, as shown in FIG. 5, a burner 2 was made to face a lower end of a starting member 3 vertically supported by a VAD method, and 1400 cc / min of SiCl ₄ was supplied into this burner 2, and H
₂ 35 l / min, O ₂ 35 l / min, Ar10
Lit / min was supplied to deposit the SiO ₂ glass fine particles (porous base material) 1 on the lower end of the starting member 3. The obtained porous base material had a diameter of 125 mmφ and a length of 500 mm. The porous base material was inserted into a sintering furnace having a furnace core tube 5 and a heater 4 shown in FIG. The porous base material 1 inserted into the closed system was He 10 liters / minute in a furnace maintained at 1200 ° C., and SiC was used as a halide gas (chlorine gas).
It was dehydrated by supplying 200 cc / min of l ₄ . Subsequently, the furnace temperature was raised to 1600 ° C., He 10 liter / min, S
A transparent glass rod for a core was obtained by supplying 200 cc / min of iCl ₄ . When the amount of Cl in this base material was analyzed by ion chromatography, it was 0.8% by weight. Next, this core rod was stretched to have a diameter of 5 mm, and SiO ₂ glass fine particles were deposited to a thickness of 140 mm around the core rod by an external attachment method. The base material was inserted into a furnace maintained at 1350 ° C., He (10 l / min) and SiF _{4 (} 300 cc / min) were supplied, and fluorine was added.
The temperature was raised to 50 ° C. to make transparent glass. The preform having a core-clad thus obtained was made into a fiber having an outer diameter of 125 μm, and the characteristics were investigated. FIG. 4 shows the refractive index profile of the obtained fiber.

Example 2 In the same manner as in Example 1, pure quartz glass was prepared using Cl ₂ as a chlorine-based gas. The atmosphere in the furnace during the dehydration treatment and the clarification treatment was He 10 liter / min, Cl ₂ 2
It is 00 cc / min. The amount of Cl in this base material is 0.3% by weight
Met. Next, this core rod was stretched to have a diameter of 5 mm, and SiO ₂ glass fine particles were deposited to a thickness of 140 mm around the core rod by an external attachment method. The base material was inserted into a furnace maintained at 1350 ° C., He 10 liter / min, SiF
_After supplying 300 cc / min of _{4 and} adding fluorine, the furnace temperature was raised to 1550 ° C. in the same atmosphere to form a transparent glass. The preform having a core-clad thus obtained was made into a fiber having an outer diameter of 125 μm, and the characteristics were investigated.

Comparative Example 1 In the same manner as in Example 1, CCl ₄ was used as a chlorine-based gas.
Was used to prepare pure quartz glass. The atmosphere in the furnace at the time of dehydration treatment and clarification is He 10 liter / min, CCl ₄
20 cc / min and O ₂ 60 cc / min. C of this base material
The amount was 0.05% by weight. Next, this core rod was stretched to have a diameter of 5 mm, and SiO ₂ glass fine particles were deposited to a thickness of 140 mm around the core rod by an external attachment method. The base material was inserted into a furnace maintained at 1350 ° C., and He 10
L / min and SiF ₄ 300 cc / min were supplied and fluorine was added, and then the furnace temperature was raised to 1550 ° C. in the same atmosphere to obtain transparent vitrification. The preform having a core-clad thus obtained was made into a fiber having an outer diameter of 125 μm, and the characteristics were investigated.

Comparison of Fiber Properties Produced from Examples 1 and 2 and Comparative Example 1 The residual stress, relative refractive index difference and fiber structure (λ
c, MFD) is shown in comparison with FIGS. 1 to 3. As can be seen from FIG. 1, the larger the amount of Cl in pure quartz glass, the smaller the residual stress after drawing. That is, Cl
It is estimated that the larger the amount of base material, the smaller the viscosity of the quartz glass. As a result of less residual stress in the fiber, Fig. 2
As shown in (1), the relative refractive index difference between the core and the clad increases as the amount of Cl increases, and is close to the measured value of the preform. Figure 3 shows the λc, M of each fiber with different Cl content.
It shows the FD measurement result. As is clear from FIG. 3, the higher the Cl content, the closer the relative refractive index difference is to the measured value at the preform stage. Therefore, λc, MFD
Is close to the predicted value and the fluctuation range is small.

[0011]

As described above, according to the present invention, it is possible to reduce the viscosity of the core portion of a pure silica core single mode fiber by increasing the amount of halogen in pure silica glass, especially Cl. As a result, residual stress generated during drawing can be reduced. Therefore, the core-clad relative refractive index difference after drawing is almost the same as the actually measured value at the preform stage, and the λc and MFD characteristics can be almost the same as the predicted value, which is extremely effective.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the amount of Cl during vitrification of pure quartz and the residual stress of the fiber.

FIG. 2 is a diagram showing the relationship between the amount of Cl during the vitrification of pure silica and the relative refractive index difference between the core and the clad of the fiber.

FIG. 3: Cl content in pure silica vitrification and λ of fiber
It is a figure which shows the relationship with c and MFD.

FIG. 4 is a diagram showing a refractive index distribution of a pure quartz core-fluorine-doped quartz clad fiber.

FIG. 5 is a schematic view showing a method for producing glass fine particles.

FIG. 6 is a schematic view showing a step of sintering a porous base material.

[Explanation of symbols]

1 Glass fine particles (porous matrix) 2 burners 3 Starting material 4 heater 5 core tube

Claims (2)

[Claims] 1. A method for obtaining a glass base material by subjecting a porous base material containing quartz as a main component to dehydration treatment and transparent vitrification, in an atmosphere containing a halogen-based gas in the dehydration treatment step and / or the transparent vitrification step. Production of a glass base material for optical fiber, characterized in that the glass base material obtained by containing 0.1 to 1% by weight of halogen in the quartz glass by heat treatment is used as a core of the optical fiber. Method. 2. The method for producing a glass preform for an optical fiber according to claim 1, wherein the halogen is chlorine.