JPH01264940A - Production of glass base material for optical fiber - Google Patents

Abstract

PURPOSE:To obtain optical fiber having a large difference of specific refractive index between a core and a clad by adjusting a bulk density of a core corresponding part at the outermost peripheral part to be higher than the bulk density of internal part, and heat-treating a heaped body in an atmosphere contg. F-contg. gas for a certain period in a stage for making the heaped body transparent by heating. CONSTITUTION:O2 and H2 are ejected by feeding the gas from a feeding port 4 and 5, respectively, using a multicore tube burner 3A, 3B. At the same time, SiCl4 as feed gas, GeCl4 as dopant gas, etc. are fed through a feeding port 7 together with Ar gas as carrier gas. A reaction is thus caused, and simultaneously, shielding Ar gas is fed through a feeding port 6, causing successive deposition of fine glass particles from a tip end of a starting base material 8 toward an axial direction. Thus, a bulk density of a core corresponding part at an outermost peripheral part is adjusted to a larger bulk density than that of the internal part by elevating a surface temp. Then, obtd. soot base material is heated, if necessary in an atmosphere contg. Cl2, then heated for at least a specified period at 1,150+ or -150 deg.C in an atmosphere contg. F-contg. gas. Thus, F is added easily to only a clad corresponding part.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method of manufacturing a glass preform for optical fibers.

In particular, the present invention provides a glass for optical fibers in which fluorine can be easily added only to the portion corresponding to the cladding when a glass particle deposit for optical fibers, that is, a soot base material, is sintered in a high-temperature furnace to become transparent vitrified. The present invention relates to a method for manufacturing a base material.

As shown in FIG. 2, a conventional optical fiber usually consists of a central portion 1 called a core portion through which light passes, and a peripheral portion 2 called a cladding portion. The refractive index n of the core portion 1 is higher than the refractive index n2 of the cladding portion 2, as shown in the refractive index distribution in FIG. 2, for the convenience of transmitting light.

Then, the relative refractive index difference Δn between the core and the cladding: Δn=1
712-n, l/n+ can be increased, the receiving angle of total reflection on the cladding surface can be increased, and there are advantages such as reducing power loss when bending the optical glass fiber. This Δn
As a method for increasing the temperature, GeO2, Al2O3, Tie,
There is a method of increasing the refractive index n of the core portion 1 by adding a metal oxide dopant such as, or a method of lowering the refractive index n2 of the cladding portion by adding a fluorine-based gas to a portion of the soot base material corresponding to the cladding. Conceivable.

However, in the case of the former method of increasing the refractive index n of the core portion 1 by adding a dopant, the following problems occur as the amount of the dopant increases: (1) When the amount of dopant is increased, the dopant addition Light scattering (Rayleigh scattering) occurs, which increases transmission loss, which is unfavorable for optical transmission. Note that the magnitude of this light scattering is proportional to the amount of dopant.

(2) Adding a large amount of dopants tends to cause bubbles and single crystals in the glass base material. For example, GeO2
When using GeOz gas, bubbles may be generated due to GeOz gas. A1□o3 tends to produce clusters of A1□03 crystals.

The presence of such bubbles and crystal phases causes loss in optical transmission (ie, scattering loss), which is undesirable. In addition, it may cause the optical fiber to break.

On the other hand, the latter method of lowering the refractive index n2 by adding fluorine-based gas to the portion corresponding to the cladding is very effective in solving the above-mentioned disadvantages. In this method, G
After adding a dopant such as eO2 to increase the refractive index and forming a predetermined refractive index difference between the cladding and the cladding, fluorine is added to the cladding by heating at a high temperature in an atmosphere containing a fluorine-based gas for at least a period of time. In this method, the refractive index of the cladding portion is lowered, and a transparent glass base material with a high Δn is finally obtained.

However, this method also had the following problems: If the soot material was simply heated at high temperature in a fluorine-based gas atmosphere, fluorine would not be uniformly added to the core of the soot base material. As a result, Δn does not become high.

Moreover, since fluorine is highly reactive, it is extremely difficult to control the temperature in the high-temperature furnace, the fluorine-based gas concentration, and the treatment time in order to add fluorine only to the cladding.

Therefore, in order to add fluorine only to the portion corresponding to the cladding of a soot base material with a conventional dopant distribution and bulk density distribution, it is necessary to strictly control the temperature within a range of about ±30°C. It has been extremely difficult to produce an optical glass fiber having a desired value of Δn by this method.

Problems to be Solved by the Invention The purpose of the present invention is to easily add fluorine only to the cladding portion of the soot base material so that fluorine is not added to the core portion of the soot base material during high-temperature heating. An object of the present invention is to provide a method for manufacturing a glass preform for optical fibers.

Composition of the Invention The method for manufacturing a glass preform for optical fiber provided by the present invention is such that when producing a glass particle deposit for an optical fiber consisting of a core-corresponding part and a clad-corresponding part surrounding it, the most important part of the core-corresponding part is The method is characterized in that the bulk density of the outer peripheral part is adjusted to be higher than that of the inner part, and then, at least part of the process of heating and transparentizing the deposited body, heat treatment is performed in an atmosphere containing a fluorine-based gas.

Effect When manufacturing the soot base material, the bulk density of the outermost periphery of the core-equivalent part of the soot base material is locally increased, and when that soot base material is heat-treated at a high temperature, the core-equivalent part of the soot base material with high bulk density is The sintering of the outermost portion of the cladding progresses faster than that of the surrounding cladding portion. Therefore, even if a fluorine-based gas is introduced into the atmosphere at some point during the subsequent high-temperature heat treatment, the outermost periphery of the core-corresponding part that sintered quickly in the soot base material has the effect of suppressing the penetration of fluorine into the core. Therefore, fluorine is not added to the inside of the core, but only to the portion corresponding to the cladding.

In addition, by locally increasing the bulk density at the outermost periphery of the core-equivalent part in the soot base material so that the outermost periphery of the core-equivalent part is sintered fastest during sintering, fluorine-based The allowable temperature range in which fluorine can be added only to the portion corresponding to the cladding during gas injection can be increased to ±150°C. As a result, temperature control becomes easy and the reaction temperature can be lowered.

Furthermore, the present inventors have confirmed through experiments that fluorine is not added to the soot base material when the relative density of the soot base material (ratio of the bulk density of the soot base material to the density of the transparent glass) is 0.45 or more. .

That is, in the soot base material having the above configuration, when the relative density of the outermost peripheral part of the core equivalent part reaches 0.45, the relative density of the cladding part is 0.45 or less, so from this point on, the fluorine-based By introducing gas, fluorine can be added to the cladding portion without adding fluorine to the core portion. As a result, a refractive index distribution with a high Δn can be obtained.

Hereinafter, the present invention will be explained in more detail with reference to the accompanying drawings.

FIG. 3 is a schematic diagram showing a method of producing the soot base material used in the present invention by a fire hydrolysis reaction.

Using the multicore tube burners 3A and 3B, oxygen and hydrogen are supplied as combustion gases from the supply port 4 and the supply port 5, respectively, and are ejected from the outermost annular port of the multicore tube burner and its inner port. At the same time, 5iC1, as a raw material gas,
Further, GeCl4 or the like as a dopant raw material is supplied from a supply lower with an inert gas such as Ar gas used as a carrier gas, and fed through the center port of the multi-core tube burner to cause a reaction.

Further, so that the raw material gas reacts in a space several mm away from the tip of the burner, Ar gas for shielding is supplied from the supply port 6 and is ejected from the annular port next to the central port.

In this state, glass particles are sequentially deposited in the axial direction from the tip of the rotating starting base material 8 so as to obtain a rod of glass fine particles, that is, a soot base material.

Then, according to the method of the present invention, the bulk density is locally increased.

That is, in the present invention, in order to obtain a locally high bulk density as shown in FIG. Ar is supplied respectively at the ratio, and hydrogen is supplied from the supply port 5 at a rate of 31~i/
In addition to 5iCI4 from the supply lower, add 10 GeC1,
Feed at a rate of ~50 cc/min. The upper burner 3A has the function of locally increasing the bulk density and also has the function of synthesizing a portion corresponding to the cladding.

In order to locally increase the bulk density, the flow of hydrogen may be increased to increase the surface temperature of the portion corresponding to the core.

On the other hand, the lower burner 3B has the function of synthesizing the core part, and simultaneously supplies GeCl4 from the supply lower at a rate of 20 cc/min and 5iC1n at a rate of 200 cc/min. You can burn it with

Note that the above conditions are merely examples and do not limit the present invention.

In the manner described above, a soot base material can be manufactured in which the bulk density of the outermost peripheral portion of the core-corresponding portion is higher than that of the inside thereof.

Next, the soot base material is inserted into a heat-resistant core tube such as a pure quartz core tube or an alumina core tube, and is heated to a high temperature to dehydrate and sinter it.

At this time, first, the main focus is on dehydrating the soot base material, and for example, the fle
It is preferable to perform the treatment in an atmosphere in which the gas is supplied at a flow rate of about 10β/min and the chlorine gas is supplied at a flow rate of about 100 cc/min.

The purpose of chlorine gas is dehydration, and C12,5OC12,C
0C12, CCl4, etc. can be used.

If the dehydration temperature is below 800° C., impurities in the soot base material cannot be removed and dehydration takes time, which is disadvantageous. In addition, when heated above 1100℃ in a chlorine gas atmosphere, the soot base material begins to shrink, and even if fluorine-based gas is introduced in the second stage, the bulk density of the soot base material increases throughout. , it becomes difficult to add fluorine to the soot matrix.

Furthermore, since chlorine gas volatilizes the dopant as chloride, it can also be used as a refractive index distribution adjuster, and the temperature is most preferably in the range of 1000 to 1100°C.

Following the first-stage heat treatment, a second-stage heat treatment is performed that focuses on adding fluorine. The temperature in this case is
Although it depends on the distribution of additives such as GeO7 in the soot matrix obtained by fire hydrolysis reaction and bulk density distribution, it is approximately 11
A range of 50°C±150°C is preferred. That is, by configuring the soot base material as described above, the outermost peripheral part of the core-corresponding part is sintered at around 1000°C, and then during the heat treatment at around 1600°C until the entire soot base material is sintered. Furthermore, by setting the atmosphere to a fluorine-based gas, it is possible to add sulfur S only to the portion corresponding to the crund.

In other words, in order to introduce fluorine-based gas under conditions such that fluorine is not added to the core-corresponding part of the soot base material, the relative density of the outermost part of the core must be It is sufficient to start supplying the fluorine-based gas when the value becomes 0.45 or more.

Therefore, when heat treatment is performed in a furnace tube with a uniform temperature distribution and a heating rate of α (t'/min), the temperature T at which the relative density of the outermost peripheral part of the core-corresponding part becomes 0.45! The fluorine-based gas may be introduced between the temperature T2 and the temperature T2 at which the relative density of the cladding portion becomes 0.45. In this case, temperature difference ΔT =
The larger T2-TI, the larger the tolerance for the temperature at which the fluorine-based gas is introduced, making temperature control easier.

Conversely, the smaller ΔT is, the more strict control of the input temperature is required, which is disadvantageous.

In order to increase ΔT, it is preferable that the local height Δ2 of the bulk density shown in FIG. 1 be as high as possible. However, if Δ2 is set too high, bubbles are likely to be generated during sintering, which is not preferable. Therefore, it is preferable to perform fire hydrolysis so that Δ2=0.1 to 0.3.

It is also observed that the concentration distribution of additives such as GeO7 added during fire hydrolysis exhibits a so-called "sagging phenomenon" in the innermost peripheral portion 11A of the cladding portion, depending on the manufacturing conditions. This can be dealt with by adding chlorine in advance to adjust the refractive index distribution.

FIG. 5 shows an example of the soot base material treatment process according to the present invention.

Examples of the method of the present invention are shown below.

Example A soot base material for a single mode fiber having the relative density distribution shown in FIG. 4(a) is heated to a heating temperature of 800°C, and chlorine gas is supplied at a rate of 100 cc/min and He gas at a rate of 101/min. 105 at a heating rate of 3℃/min in a snowy atmosphere
Heat to 0°C, then stop the chlorine gas, introduce SF6 gas at a flow rate of 150 cc/min and heat to 1300°C, then heat to 1500°C or higher in a pure He snow atmosphere to create transparent vitrification. Ta.

The refractive index distribution of the obtained glass base material is as shown in Fig. 4), and the core-corresponding part has a refractive index distribution corresponding to the Ge○2 dopant concentration, while the cladding-corresponding part has a refractive index distribution corresponding to the Ge○2 dopant concentration. On the other hand, the portion corresponding to the cladding showed a decrease in refractive index corresponding to the amount of fluorine added. In fact, when XMA analysis was performed, it was confirmed that the core-corresponding portion contained no fluorine element at all. The core part has a △n value of +0.3%, and the cladding equivalent part has a - value.
It was 0.15%.

In addition, although the above-mentioned example was a soot base material for a single mode fiber, the present invention can be implemented in the same way with a soot base material for a multimode fiber. or,
The present invention can be similarly carried out using a soot base material produced by the "external sooting method" in which a porous base material is laminated on a cylindrical mandrel.

Effects of the Invention As is clear from the above, according to the method for producing a glass preform for optical fiber of the present invention, fluorine is not added to the core-corresponding portion of the soot preform during high-temperature heating, and the cladding-corresponding portion of the soot preform is not added. Fluorine can be easily added to Therefore, it is possible to effectively manufacture an optical fiber with a large Δn.

Further, according to the method for manufacturing a glass preform for optical fiber of the present invention, when a fluorine-based gas is used at least temporarily to add fluorine to a portion corresponding to the cladding when the soot preform is heated and sintered at high temperature, the manufacturing method Adjustment of conditions is extremely easy.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the bulk density distribution of the soot base material according to the present invention, Fig. 2 is a schematic diagram showing the refractive index distribution structure of the optical fiber, and Fig. 3 is a diagram showing the bulk density distribution of the soot base material according to the present invention. Schematic diagram of the soot base material manufacturing method, FIGS. 4(a) and (b)
) is a graph showing the relative density distribution of the soot base material and the refractive index difference of the glass base material in Examples of the present invention. FIG. 5 is a graph showing the pattern of soot base material treatment in the present invention. (Main reference numbers) 1... Core equivalent part of soot base material, 2... Clad equivalent part of soot base material, 3A, 3B
...Multi-core tube burner, 4...0□ supply port, 5...H2 supply port, 6...
... Shielding Ar gas supply port, 7... Raw material gas supply port, 8... Starting base material 9... Core equivalent portion of soot base material, 10... Cladding of soot base material Corresponding part, 11A...
・Innermost periphery of the clad equivalent part, 11B...outermost periphery of the core equivalent part, Patent applicant: Sumitomo Electric Industries, Ltd., Nippon Telegraph and Telephone Ltd.

Claims (3)

[Claims] (1) When producing a glass particle deposit for optical fiber consisting of a core-equivalent part and a clad-equivalent part surrounding it, the bulk density at the outermost periphery of the core-equivalent part is adjusted to be higher than the inside. and then, in at least one period of the step of heating and transparentizing the deposit, heat treatment is performed in an atmosphere containing a fluorine-based gas. (2) The glass particle deposit for optical fibers is heat-treated in advance in an atmosphere containing a chlorine-based gas before being heat-treated in an atmosphere containing a fluorine-based gas. A method for manufacturing a glass base material for optical fibers. (3) According to claim 1 or 2, in the heat treatment process of the glass particle deposit, the heat treatment is performed in an atmosphere containing the fluorine-based gas at a heating temperature in the range of 1150°C ± 150°C. A method for manufacturing a glass base material for optical fiber.