JPH01188439A - Production of optical fiber preform - Google Patents

Abstract

PURPOSE:To improve the quality of the subject preform and to prolong the service life of a furnace core tube by using a quartz furnace core tube having a low content of carbon, heating the preform in the atmosphere of a halogen or its compd., dehydrating and sintering the preform, and adjusting the refractive index. CONSTITUTION:The quartz furnace core tube 3 contg. about 0.1-5wt.% carbon is prepared and set in the main furnace body 5, and a heating element 4 is provided. The tube 3 is heated to >=1500 deg.C by the heating element 4, and a quartz-based soot preform 1 is dehydrated and sintered in the atmosphere of SiF4, etc., diluted with an inert gas. The refractive index is adjusted with a raw gas such as CF4 and SiF4. Since the tube 3 having such a low content of carbon is used, the damage of the tube 3 is reduced, and the tube 3 is not etched. As a result, the purity of the preform is enhanced, hence the quality is improved, and the service life of the tube 3 is prolonged because of the reduced damage of the tube 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for stably producing a high-quality base material for a base fiber that can prevent contamination of impurity elements into a glass base material.

[Conventional technology]

As a general method for mass producing base materials for optical fibers'
/AD method is known. This l/AD method involves depositing glass fine particles produced in an oxyhydrogen flame on a rotating starting member, such as a glass plate or a glass plate, and then forming a cylindrical porous base material. This is a method of manufacturing a transparent optical fiber base material by sintering. In this method, in order to sinter the porous base material and make it transparent, it is necessary to heat the base material to +500 C' or higher in an Ha or Ar gas atmosphere. A carbon furnace is usually used as this heating furnace. During sintering in such a heating furnace, special attention must be paid to the prevention of contamination of transition elements such as Cu + Fa and contamination of moisture. This is because if more than + ppb of transition elements are mixed, the loss wavelength characteristics of the optical fiber will be significantly impaired over all wavelengths, and if more than 0.1 ppm of water is mixed, the customization in the long wavelength range will be impaired. Therefore, the porous base material is usually dehydrated, and a known method is to heat the porous base material at a high temperature in an inert gas atmosphere containing chlorine-based gas or fluorine-based gas. . Among these, the method using fluorine gas not only dehydrates the porous base material but also has the effect of adding fluorine. Adding fluorine to the porous base material has the advantage that the refractive index distribution, which is an essential element of optical fibers, can be adjusted. Regarding this point, the Special Publick Act of 1973-
It is explained in detail in Japanese Patent Application Laid-open No. 15682 and Japanese Patent Application Laid-open No. 55-67555.

The above-mentioned treatment with fluorine gas is usually carried out in a carbon furnace simultaneously with sintering or as a preliminary step. In order to prevent the carbon heating element from being consumed by moisture and oxygen generated during the heat treatment of the base material, the carbon furnace is equipped with a furnace tube that isolates the carbon heating element from the sintering atmosphere. was used. However, when an alumina core tube is used, alkaline components contained in the alumina are scattered around at high temperatures and adhere to the surface of the porous base material, resulting in contamination or loss of the fiber base material. There was a problem that transparent crystals formed (forming a cristobalite layer). Therefore, quartz glass core tubes have been put into practical use.

The use of quartz glass furnace tubes has the following advantages over the use of alumina furnace tubes.

■ High precision machining ensures good airtightness and almost complete dehydration of the soot base material.

■It has higher purity than alumina and contains almost no impurities such as Fa and fluoride.

■ The glass base material obtained using this does not cause surface devitrification due to alkali.

■ Thermal damage (no failure due to thermal shock).

■ When using fluorine compound gas, All? There is no generation of impurity gases such as 3. However, although 81F4 gas is generated, it does not have an adverse effect as an impurity on the glass base material.

Regarding the use of quartz furnace tube,
No. 299, No. 5B-42156, JP-A-60-860
It is explained in detail in each publication No. 49.

[Problem to be solved by the invention]

However, while having the above-mentioned advantages, quartz tubes also have the disadvantage of being easily deformed by high heat. By the way, the temperature goes up and down between +500C and room temperature every day9.
If you return it, the deformation will become larger after about a week. Furthermore, when SF6 or 0F40 fluorine gas is used, the quartz may be etched, and in severe cases, pinholes may even be formed.However, this may also cause outside air to enter or atmospheric gas to leak outside. This will result in a negative impact on the manufacturing process.

The purpose of the present invention is to solve the problems in the dehydration, sintering, and fluorine addition treatment of optical fiber base materials by conventional methods as described above, and to develop a method for manufacturing base materials for fibers that can be used for long lifespans of furnace tubes. It is on offer.

[Means to solve the problem]

As a result of intensive research into means for solving the above-mentioned problems, the inventors of the present invention have concluded that by using a quartz furnace core tube to which a small amount of carbon is added, it is possible to use the furnace core tube at high temperatures. Finally, we have arrived at the present invention. Furthermore, it has been found that when using a fluorine compound gas, selecting SiF4 gas is effective in extending the life of the reactor core tube.

That is, the present invention heats a quartz-based soot base material in a quartz furnace tube containing a small amount of carbon in an atmosphere containing halogen or halogen compound gas to dehydrate and dehydrate it.
This method of manufacturing an optical fiber preform is characterized in that at least one of sintering and refractive index adjustment treatment is performed.

Particularly preferred embodiments of the present invention include the above-mentioned method in which the compound Gade 12 or S 24 is used.

In this specification, the quartz-based soot base material refers to a glass soot base material containing quartz as a main component, which is produced by, for example, using a flame hydrolysis reaction to collect glass raw material gas and additive gas using an inert gas or the like as a carrier. It can be introduced into a flame and the generated glass particles can be deposited, or it can be carried out by the so-called sol-gel method, that is, hydrolysis of Alcolade.
method), and these techniques are known.

In the present invention, the quartz-based soot base material is dehydrated, sintered,
At least part of the refractive index adjustment is performed under heating in an atmosphere to which a halogen or halogen compound gas is added.

In the case of dehydration, it is carried out in an atmosphere in which chlorine or chlorine compound gas such as C12,5OC12, CCl4 is diluted with an inert gas such as He gas. Also, instead of chlorine or chlorine compound gas, CF4, CC1!2F2, a21F
It is also preferable to use a fluorine compound gas such as 6, SF6, SiF4, etc. In this case, it is of course possible to perform sintering at the same time as dehydration. When the purpose is dehydration, the amount of the above halogen or halogen compound gas used is 1% by volume in the atmospheric gas.
This is generally sufficient.

On the other hand, when adjusting the refractive index, for example when adding fluorine, OF4, SF6.02F are used as the additive source gas.
6. SiF4 etc. may be used, and the concentration of these gases,
By determining the processing temperature, the refractive index can be adjusted. For example, the addition of fluorine to lower the refractive index
By using a fluorine compound gas with a concentration of 2 to 20 volumes as the atmospheric gas, Δn (relative refractive index difference) -1-0
, 5 to -065%. Further, to add boron, Bob, BBr3, etc. may be used, and to add both fluorine and boron, BP may be used.

Refractive index adjustment may also be performed by dehydration treatment or sintering, or refractive index adjustment, dehydration, and sintering may be performed simultaneously. Further, the atmosphere may be a mixture of one or more of the following gases:

In any of the above-mentioned dehydration, refractive index adjustment, and sintering treatments, the temperature is preferably 1100C or higher and lower than the temperature at which the quartz-based soot base material becomes transparent.

By using a quartz tube to which a small amount of carbon (approximately 0.1 to 5 percent) is added as the furnace core tube for heat treatment of the quartz-based soot base material under the above-mentioned conditions, a high-quality quartz-based glass base material can be produced. This means that the material can be manufactured with less damage to the furnace core tube.

The experiments and concepts that formed the basis of the present invention will be explained below. It should be noted here that the concepts described below could only be explained after obtaining experimental knowledge useful for the present invention, and cannot be easily deduced in advance.

Experiment 1 We made trial quartz glass by changing the amount of carbon added as shown in Table 1, and processed them into glass rods with a diameter of 5 cranes and a diameter of 200 mm. When the amount of carbon added was 5x, processing by heating was difficult, and a portion of the material devitrified during this processing. Each glass rod obtained was hung at +50°C (suspended in a furnace at Ic) and held for 24 hours, and the amount of elongation of the glass rod was measured and the results were summarized in a table at °C.
show.

Table 1 Experiment 2 A carbon-added quartz piece of 1 weight X with a thickness of 5 fl was heated to 19F.
It was placed in 6 gas, heated at +500C, and heated for about 6 hours, and the thickness became 30 or less.

Experiment 3 The same quartz piece as in Experiment 2 was heated to 1500°C in 13) F4 gas.
When heated at C, there was almost no change in the thickness even after 6 hours of heating.

From the above experiments 1 to 5, the following was revealed.

1) A material in which 0.1 to 3 weight X of carbon is added to quartz can withstand extremely high temperatures compared to pure quartz.

I) When using a fluorine compound gas, if 5zF4 gas is selected, the core tube shown in I) above will not be etched.

Based on this result, the inventors decided to use a quartz core tube with a small amount of carbon added, and to use a fluorine-based gas when heat-treating the soot base material in the furnace core tube at temperatures higher than tsooc. It depends on the conclusion that it is preferable to use fi5IF4.

The above fact can be explained as follows.

The improvement in the heat resistance of quartz tubes due to the addition of carbon is thought to be due to the bonding of carbon in the glass, which increases its viscosity.

Manen 81 F a gives good results with quartz (Sin).
2) The relationship between the reactor core tube and porous base material and 8F6 is as follows (11 formula % formula % where S: solid, g: gas) and reacts like a reactor core tube, whereas SxF
(As shown in Equation 21, no product gas is generated between SiO2, 5io2(s) + sty4(g)-M-No product gas... (2) In other words, the core tube is not etched. It is.

〔Example〕

The present invention will be specifically explained below with reference to examples.

FIG. 1 is a schematic structural diagram showing an apparatus for manufacturing a glass preform for an original fiber, which is one embodiment of the present invention. In Fig. 1, 1 is the soot base material, 2j/i support rod, 3 is the furnace tube, 4 is the heating element, 5 is the furnace body, 6 is the inert gas inlet, 7 is the atmospheric gas (for example, 5iCI!4, 5xF4.He, etc.).

The following Examples and Comparative Examples were carried out using the apparatus shown in FIG.

Example 1 The heating element 4 turns carbon into I! ft% doped quartz furnace tube 3 is heated to +600C, and 5% of S engineering F4 is added into the tube 3.
0cc/min and H・51! The soot base material 1 was inserted into the flow at a rate of 2/min. When the obtained transparent glass preform was subsequently spun into a fiber, the residual moisture in the fiber was 0.1 ppm.
Absorption derived from p Fe was not observed at all. Approximately 1% by weight of fluorine was added to the glass base material. There was almost no stretching even after continuous use for one month under these conditions.

Example 2 The same porous base material used in Example 1 was 0/2'i
When dehydrated at 1200C and then treated under the same conditions as in Example 1, 0. A fiber with an extremely low moisture content of less than 5 ppm O was obtained. The results other than the water content were the same as in Example 1.

Comparative Example A fiber was manufactured under the same conditions as in Example 1, except that a pure quartz tube was used as the furnace core tube, and the residual moisture in the obtained fiber was 0.1 ppm. Elongation occurred and the core tube broke after 20 uses.

The above explanation exemplifies the case using the vAD method, but is of course not limited to this, and the method of the present invention can be applied to all other porous base materials obtained by the external method etc. There is.

Further, the furnace structure is just one example, and excellent results similar to those of this example can be obtained even with a homogeneous heating furnace that does not require moving the porous base material.

〔Effect of the invention〕

The present invention can prevent contamination of impurity elements into the glass base material, and can extend the life of the furnace tube compared to the conventional method. In particular, when adding fluorine, using SiF4 can prevent etching of the furnace tube, improving economic efficiency. It also has the effect of producing high-purity glass articles, and is advantageous when applied to the production of base materials for original fibers.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating an embodiment of the method for manufacturing a base material for L:D original fibers according to the present invention.

Claims (3)

[Claims] (1) A quartz-based soot base material is heated in a quartz furnace tube containing a small amount of carbon in an atmosphere to which halogen or halogen compound gas is added, and at least one of dehydration, sintering, and refractive index adjustment treatment is performed. 1. A method for manufacturing an optical fiber base material, the method comprising: (2) The method for manufacturing an optical fiber preform according to claim (1), wherein the halogen compound gas is SiF_4. (3) The method for manufacturing an optical fiber preform according to claim (1), wherein the halogen gas is Cl_2.