JP3439258B2 - Method for producing glass preform for optical fiber - Google Patents

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 glass preform which is an intermediate product for producing an optical fiber having a low loss, and is particularly suitable for producing an optical fiber having an effect of suppressing stimulated Brillouin scattering. The present invention relates to a method for producing a glass base material that can be used.

[0002]

2. Description of the Related Art In recent years, optical fiber communication has enabled long-distance communication between continents by reducing transmission loss of the optical fiber or putting an optical fiber amplifier into practical use. However, the power of light used for communication is limited due to stimulated Brillouin scattering in the optical fiber, and there is a problem that a high-power light source cannot be used. As a method for solving this problem, conventionally, a method of suppressing stimulated Brillouin scattering by providing a strain distribution in the longitudinal direction of the optical fiber has been proposed. For example, Electronics Lett
ers Vol.27, No.12, p1100 to 1101 (19), a method of suppressing stimulated Brillouin scattering by applying stress to the fiber at the time of forming a cable is proposed. Also, 19
Proceedings of the 1991 IEICE Fall Conference, Separate Volume 4, 4
Pp. 6, Lecture No. B-546, a method is proposed in which the stress in the axial direction of the fiber is changed by changing the tension during drawing to suppress the stimulated Brillouin scattering. However, when stress is applied to the fiber, it is not necessarily a preferable method because it affects the transmission characteristics or strength characteristics.

On the other hand, Conference on Optical Fiber
r Communication '93 Techinical Digest, 115-118 proposes a method of suppressing stimulated Brillouin scattering by changing the composition in the longitudinal direction of the fiber without applying stress to the fiber. In this method, the composition is changed by changing the fluorine concentration in the cladding in the longitudinal direction of the fiber, so that the strength characteristics and transmission characteristics of the fiber are not affected.

[0004]

In order to put into practical use the above method of suppressing stimulated Brillouin scattering by changing the composition in the longitudinal direction of the fiber, the development of a method of changing the fluorine concentration in the longitudinal direction of the fiber with good controllability. is necessary. As a method of adding fluorine to glass, a method of uniformly adding it in the longitudinal direction of the fiber has been studied and reported, as in the conventional production of pure silica core single mode fiber. For example, in JP-A-60-90842,
A method is disclosed in which glass particle aggregates are synthesized by a vapor phase synthesis method and heat treated in a fluorine atmosphere at the time of making transparent. When this method is applied, it is considered that the fluorine concentration in the glass base material can be changed by changing the fluorine concentration or the heating temperature of the atmosphere during the heat treatment in the longitudinal direction of the glass base material. However, when changing the fluorine concentration during heat treatment or changing the heat treatment temperature, a method of controlling the concentration distribution or temperature distribution in the glass particulate deposit in a desired form is required. Since these distributions change depending on the diameter and the bulk density of the base material, it is difficult to control, and it is extremely difficult to provide a large fluorine concentration distribution especially in a short distance. The present invention has been made as a subject of a technique capable of producing a glass base material in which the fluorine concentration is changed in the longitudinal direction of the fiber with good controllability in order to suppress such stimulated Brillouin scattering.

[0005]

Means for Solving the Problems The constitution of the present invention for solving the above-mentioned problems is to carry out an oxidation reaction or a flame hydrolysis reaction in a flame formed from a gaseous glass raw material by a burner,
A glass particle deposit is synthesized by depositing the generated glass particles on the tip or outer circumference of the starting rod,
In a method for producing a glass base material for optical fibers having a different fluorine concentration in the longitudinal direction of the base material by heat-treating the glass particle deposit body in a fluorine atmosphere, a firing having a heater shorter than the length of the glass particle deposit body is performed. In the furnace, a first heat treatment for transparentizing at least one part of the glass fine particle deposit in advance, then a second heat treatment for adding fluorine by heating in a fluorine-containing atmosphere, and finally sintering the whole. Is characterized by performing a third heat treatment for making the transparent. Further, in the case of a glass particle deposit having a large outer diameter, a bulk density distribution is often already formed in the radial direction when the base material is synthesized, and the glass particle deposit is transparent before the first heat treatment. It is effective to make the bulk density uniform by temporarily contracting at a relatively low temperature at which it does not change. In the method of the present invention, the core or the refractive index portion near the core through which the signal light of the optical fiber passes needs to be dehydrated to improve the transmission characteristics, and the first heat treatment or the temporary shrinkage for adjusting the bulk density distribution. Before the above, it is effective to heat the glass particle deposit body in advance in the presence of halogen or a halogen compound for dehydration. In the present invention, in consideration of long-distance communication, the core base material has a relatively high refractive index first core doped with GeO ₂ at the center of the glass particle deposit body, and a central part doped with GeO ₂ at the outer periphery thereof. A so-called base material for dispersion-shifted fibers, which is composed of a second core having a lower refractive index and a part of a clad of pure silica around the second core, is preferably used. However, the present invention is not limited to the above fiber structure, but may be a single core fiber made of GeO ₂ cladding, or B
It can be suitably used for a material in which ₂ O ₂ , TiO ₂ , P ₂ O ₅ or the like is doped to adjust the refractive index or the glass viscosity, or a pure silica (pure SiO ₂ ) base material.

[0006]

The structure of the present invention will be described based on an embodiment. The manufacturing process is shown in FIG. First, as shown in FIG. 1 (a), a glass particle deposit 2 is synthesized by a glass particle synthesizing burner 1 by a vapor phase synthesis method. Although FIG. 1 (a) shows one burner, a similar effect can of course be expected with a plurality of burners. Glass raw materials include SiCl ₄ , SiHCl ₃
Etc. are used, and G is used as a dopant for controlling the refractive index.
eCl ₄ , POCl ₃ , BCl ₃ , TiCl ₄ or the like is used. A suitable dopant can be appropriately used for controlling the refractive index, and may not be supplied in the case of synthesizing a glass particle deposit of pure silica.

The glass particulate deposit thus synthesized is optionally heated at a temperature at which the glass particulate deposit does not shrink, for example, in a temperature range of 1000 to 1100 ° C. in an atmosphere of an inert gas containing halogen or a halogen compound. It is heat-treated and dehydrated. The dehydration treatment is particularly necessary for the glass base material near the core through which the signal light of the optical fiber passes. Generally, chlorine and SOCl are used as dehydration gas.
A halogen gas such as ₃ , CCl ₄ or a halogen compound is used.

Then, a first heat treatment for making a part of the glass fine particle deposit transparent as shown in FIG. 1 (b) in a sintering furnace (zone furnace) having a heater 3 shorter than the length of the glass fine particle deposit. To do. Although (b) is a view showing a state in which a part of the tip 5 is made transparent, the central portion or the upper portion may be made partially transparent, or a plurality of portions may be similarly made transparent. By this first heat treatment, the transparent glass portion and the glass particle deposit body portion are formed adjacent to each other in the axial direction of the glass particle deposit body, and the adjacent portion from the transparent portion to the glass particle deposit body portion is bulky. Boundaries with continuously varying densities are formed as shown in FIG.

A second heat treatment for doping the fluorine by heating the partially transparent base material in a fluorine-containing atmosphere and diffusing and reacting the fluorine raw material inside the glass particle deposit body. As shown in FIG. 1 (c).
At this time, the doping amount of fluorine is determined in proportion to the surface area of the glass particles. The smaller the bulk density, the larger the surface area.
Therefore, at the boundary where the bulk density changes in the axial direction as shown in FIG. 2, a fluorine concentration distribution is formed in proportion to the bulk density. Fluorine is not doped in the portion that has been made transparent in advance, and the fluorine concentration is highest in the portion of the glass particle deposit body, so an advantage that the difference in fluorine concentration can be made large can be considered. The change in the axial fluorine concentration gradient can be controlled by changing the size of the boundary where the bulk density of the transparentized portion and the glass fine particle deposit portion changes. For example, the length of the heater can be changed, or the speed at which the glass particle deposit moves in the heater can be changed. Thus, according to the configuration of the present invention,
It is possible to easily form a fluorine concentration distribution in the axial direction of the glass base material. Fluorine raw materials include SF ₆ , CF ₄ ,
SiF ₄ or the like is used. The heat treatment temperature for fluorine addition is preferably in the range of 1200 to 1450, and particularly 13
It is preferably from 0 to 1450 ° C. If the temperature is lower than 1200 ° C, the progress of the fluorine doping reaction is slow, and if it exceeds 1450 ° C, the vitrification of the base material rapidly proceeds.
This is because the reaction is inhibited and it becomes difficult to dope fluorine. The gas concentration at the time of fluorine addition is arbitrarily determined by the fluorine addition amount set value. Generally, since there is a relation of [amount of fluorine added ∝ (concentration of fluorine raw material) ^1/4 ], the concentration of fluorine raw material is determined according to this. Actually, the amount of fluorine added is −0.1% to −0.7% in relative refractive index difference, but the concentration of the fluorine raw material is about 3% to 100% by volume.

Finally, a third heat treatment for making the whole base material to which fluorine is added transparent is performed as shown in FIG. 1 (d).
Clarification temperature 1450 to 1600 ° C in an inert gas atmosphere containing a fluorine raw material so that the doped fluorine does not volatilize.
The heat treatment is preferably performed in the temperature range of. 1450
If the temperature is lower than 0 ° C, vitrification hardly progresses, while if the temperature is higher than 1600 ° C, the fluorine-doped glass is softened and stretched by its own weight, which is not preferable as a transparent condition. Examples of the inert gas include helium, argon, nitrogen, or a mixed gas thereof such as a mixed gas of helium and argon. Particularly, helium is preferable because it has a high diffusion coefficient in glass and is less likely to generate bubbles. .

When the outer diameter of the glass particulate deposit is large, a density distribution is formed in the radial direction during the first heat treatment, and a fluorine concentration distribution is formed not only in the axial direction but also in the radial direction. It may be done. The bulk density distribution in the radial direction may be formed during the synthesis of the glass particulate deposit, and in order to solve such a problem, a temperature at which the glass is not vitrified once before the first heat treatment, for example, 1250.
℃ ~ 1450 ℃ temperature, more preferably 1300 ℃
It is effective to temporarily shrink at -1400 ° C. to make the bulk density uniform. The preferable bulk density range at the time of temporary shrinkage is 0.25 to 1.0 g / cm ³ , and more preferably 0.3 to
It is 0.6 g / cm ³ . If it is higher than this range, the fluorine raw material will not diffuse and fluorine cannot be added. On the other hand, if it is too low, the bulk density distribution in the base material changes greatly, resulting in a difference in the amount of fluorine added. At this time, it is desirable that the atmosphere is an inert gas, such as helium, argon, or nitrogen, whose purity can be controlled. In particular, helium diffuses faster in the glass than other gases, and is therefore preferable for preventing the formation of bubbles in the subsequent process.

In order to maintain the heating atmosphere of the base material, it is desirable to perform heat treatment inside the furnace core tube of quartz or high purity carbon. As the core tube of high-purity carbon, it is also conceivable to use one coated with SiC to maintain airtightness.

[0013]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited thereto. Also,
In the following examples, a zone furnace is used and the base material is lowered and heat-treated while maintaining a constant temperature, so the heating conditions are defined by the temperature and the traverse speed. Example 1 Fluorine was added in the configuration shown in FIG.
First, a glass fine particle deposit was synthesized using three burners as shown in FIG. SiCl ₄ was used as the glass raw material, GeCl ₄ was introduced as a dopant into the central burner 31 and the second burner 32, and the base material 4 having the refractive index distribution shown in FIG. 4 was synthesized. Combined length of about 500 mm
Of the base material at 1050 ° C and length of 200 at a speed of 10 mm / min.
It was inserted into a zone furnace having a mm heater and a quartz furnace core tube, and dehydrated. As the atmosphere gas, a mixed gas of chlorine (Cl ₂ ) and helium (He) was used. No shrinkage of the base metal was observed at this point. Next, after raising the temperature of the furnace to 1400 ° C., the dehydrated base material was lowered at a speed of 5 mm / min to make a part of the tip transparent as shown in FIG. 1 (b) (first heat treatment). ). This base material was lowered at 1350 ° C. in a mixed gas of SiF ₄ and He (flow rate ratio SiF ₄ : He = 1: 1) at a speed of 6 mm / min to perform a second heat treatment with fluorine addition. Then, a third heat treatment for making transparent was performed. The third heat treatment was performed by raising the furnace temperature to 1600 ° C. and lowering the base material at 5 mm / min in the same atmosphere gas as in the second heat treatment. The time required for the above processing was 6.6 hours including the standby time for raising the furnace temperature. When the fluorine concentration in the axial direction of the obtained glass base material was measured, it was confirmed that the relative refractive index difference from pure quartz had an axial concentration distribution as shown in FIG. The fluorine concentration in the radial direction was slightly distributed as shown in FIG.

Example 2 With the same structure as in Example 1, a glass preform was manufactured by inserting a temporary shrinkage process before the first heat treatment. In the temporary shrinking process, the base material was lowered at 1400 ° C. at 10 mm / min to shrink the whole. The outer diameter of the base material shrank to about 80 pipes of glass particulate deposits. He was used as the atmosphere gas. The time required for the above processing was 7.3 hours including the standby time for raising the furnace temperature. When the vitrified base material was evaluated, the distribution of fluorine in the axial direction was similar to that in the example, but the fluorine concentration in the radial direction was as shown in FIG.
It was confirmed that as shown in FIG.

Example 3 As shown in FIG. 8, a glass fine particle deposit body 3 was synthesized on the outer circumference of a core glass 6 using a glass fine particle synthesizing burner 1. Si as a glass raw material
Cl ₄ was used. This base material is 7 mm / at a furnace temperature of 1400 ° C.
At the speed of a minute, a part of the tip was made transparent as shown in FIG. 1 (b) and the first heat treatment was performed. This base material is 1300 ℃
Then, the second heat treatment was performed by adding fluorine to the mixed gas of SiF ₄ and He at a speed of 5 mm / min. Then, a third heat treatment for making transparent was performed. The third heat treatment was performed by raising the furnace temperature to 1600 ° C. and lowering the base material at 6 mm / min in the same atmosphere gas as in the second heat treatment. The time required for the above processing was 4.9 hours in total including the standby time for raising the furnace temperature. When the fluorine concentration in the axial direction of the obtained glass base material was measured, it was confirmed that the relative refractive index difference from pure quartz had a concentration distribution formed in the axial direction as shown in FIG.

[Embodiment 4] With the configuration shown in FIG.
We synthesized glass particles of pure silica. SiCl ₄ was used as the glass raw material. The furnace temperature was set at 1050 with a quartz furnace tube in which this base material was kept in a He atmosphere containing Cl _2.
While maintaining the temperature at 0 ° C., it was dehydrated by descending at a speed of 6 mm / min. Then, the furnace temperature was raised to 1450 ° C. in a He atmosphere, the base material was lowered halfway at a speed of 8 mm / min, and a first heat treatment for making the tip end transparent was performed. This base material is the furnace temperature 13
4mm / speed in mixed gas of SiF ₄ and He at 50 ℃
Then, the second heat treatment for fluorine doping was performed. Next, in a mixed gas (1: 1, flow rate ratio) atmosphere of SiF ₄ and He, the furnace temperature was raised to 1500 ° C. and lowered at a rate of 4 mm / min to perform a third heat treatment for making the whole transparent.
The time required for the above processing was 4.9 hours in total including the standby time for raising the furnace temperature. When the refractive index in the longitudinal direction of the base material obtained as described above was evaluated, it was found that a gradient of the refractive index in the longitudinal direction could be formed as in the case of Example 3.

[0017]

As described above, according to the present invention, it is possible to easily form a bulk density distribution in the glass particulate deposit body in the axial direction, and by utilizing this bulk density distribution, the axial direction It is possible to easily manufacture a glass base material having a different fluorine concentration. The base material manufactured according to the present invention is very effective as an intermediate product that is preferably used for manufacturing an optical fiber capable of suppressing stimulated Brillouin scattering.

[Brief description of drawings]

FIG. 1 is a diagram schematically illustrating the present invention in the order of steps.

FIG. 2 is a diagram showing the bulk density distribution in the axial direction formed by the first heat treatment in the present invention.

FIG. 3 is a diagram showing the synthesis of a glass particulate deposit in Example 1 of the present invention.

FIG. 4 is a diagram showing the refractive index distribution of the glass particulate deposit material synthesized in Example 1 of the present invention.

FIG. 5 is a diagram showing a refractive index distribution in the axial direction of the glass preform manufactured in Example 1 of the present invention.

FIG. 6 is a diagram showing the refractive index distribution in the radial direction of the glass preform manufactured in Example 1 of the present invention.

FIG. 7 is a diagram showing the refractive index distribution in the radial direction of the glass preform manufactured in Example 2 of the present invention.

FIG. 8 is a schematic diagram illustrating the synthesis of a glass particulate deposit in Example 3 of the present invention.

FIG. 9 is a diagram showing the refractive index distribution in the axial direction of the glass preform manufactured in Example 3 of the present invention.

[Explanation of symbols]

1 Burner for synthesizing fine glass particles 2 Glass particulate deposit 3 heater 4 core tube 5 Part of the transparent tip Glass for 6 cores 31 center burner 32 Second Burner 33 Third Burner 34 Base material

Front page continued (72) Sumitomo Hoshino, Sumio Hoshino 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor Hikaru Tachida ▲ Hiro ▼ 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Shoji Ohashi 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Inside Nippon Telegraph and Telephone Corporation (56) Reference JP-A-5-249329 (JP, A) JP-A-4 -367539 (JP, A) JP-A-2-201403 (JP, A) JP-A 63-222042 (JP, A) JP-A 62-182129 (JP, A) JP-A 61-17432 (JP, A) ) Tetsuro Nozawa (4 others), optical fiber that suppresses stimulated Brillouin scattering, Technical Report of IEICE, Japan, The Institute of Electronics, Information and Communication Engineers, June 25, 1991, Vol. 91, No. 108, pp. 21-26 Kazuyuki Shiraki (2 others), Characteristics of SBS suppression fiber, 1994 IEICE Fall Conference-Society Leading Conference-Lecture Collection Electronics 1, Japan, Electronic Information The Institute of Communications, September 5, 1994, pp. 137 (C-137) Kazuyuki Shiraki (2 others), Distortion-free stimulated Brillouin scattering suppression optical fiber, Proceedings of the 1993 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers, Japan, The Institute of Electronics, Information and Communication Engineers, August 15, 1993, pp. 4-342 (C-262) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02B 6/00-6/02 G02B 6/10 G02B 6/16-6/22 G02B 6/44 C03B 37 / 014-37/16 C03C 1/00-14/00 JISST file (JOIS) IEEE / IEEE Electronic Library

Claims (4)

(57) [Claims] 1. A glass fine particle deposit is synthesized by subjecting a gaseous glass raw material to an oxidation reaction or a flame hydrolysis reaction in a flame formed by a burner, and depositing the generated glass fine particles on the tip or outer periphery of a starting rod. In a method for producing a glass base material for optical fibers having a different fluorine concentration in the longitudinal direction of the base material by heat-treating the glass particle deposit body in a fluorine atmosphere, a firing having a heater shorter than the length of the glass particle deposit body is performed. In the furnace, a first heat treatment for transparentizing at least one part of the glass fine particle deposit in advance, then a second heat treatment for adding fluorine by heating in a fluorine-containing atmosphere, and finally sintering the whole. A method of manufacturing a glass preform for an optical fiber, which comprises performing a third heat treatment for making the glass transparent. 2. The glass base material for an optical fiber according to claim 1, wherein before the first heat treatment, the glass particulate deposit is temporarily contracted at a relatively low temperature at which it is not made transparent. Manufacturing method. 3. The heat treatment for dehydrating the glass particle deposit body in advance in the presence of halogen or a halogen compound before the first heat treatment or the temporary shrinkage. 2. The method for producing a glass base material for an optical fiber according to 2. 4. The glass particulate deposit has a central portion of G
eO ₂ doped relatively high refractive index of the first core, having a refractive index lower than the center portion, which is doped with GeO ₂ in its outer periphery a second core, and the second core pure silica of class of the circumference of
The glass base material for optical fiber according to any one of claims 1 to 3, wherein the glass base material comprises a part of a pad.