JPS6219367B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a base material for fibers for optical communications, and in particular, to a method for manufacturing a base material for fibers for optical communications, and in particular, to continuously manufacture a base material while minimizing the amount of residual moisture in the base material and controlling the refractive index distribution to an optimum. The present invention relates to a method of manufacturing a base material.

Tokkosho is one of the methods of making glass base material for optical transmission.
There is a vapor phase axial attachment method (VAD method) disclosed in Publication No. 49-145338 and the like. To give an overview of the conventional VAD method, as shown in Fig. 1a, raw material gas for glass particles, fuel gas, and dopant gas are supplied to a burner 2 installed at the bottom of a container 1. , a flame is blown from the burner toward the starting material 3, so that the soot of glass fine particles is attached and deposited on the lower surface of the starting material 3, and while the starting material 3 is rotated and pulled up, this glass fine particle soot body is grown to form an aggregate. 4 is made and used as the base material.

With this VAD method, it is possible to obtain an inexpensive optical transmission fiber with low loss, arbitrary refractive index distribution in the radial direction, and uniform composition in the circumferential and longitudinal directions. A high-purity product can be obtained, and the biggest impurity, water, can be removed.
Because the VAD porous base material is a uniform collection of fine particles, it has features such as easy dehydration during sintering and a small number of steps, which has great practical advantages.

At this time, a large amount of dopant such as GeO ₂ is deposited in the center and less toward the periphery, thereby forming the dopant concentration into a desired distribution, resulting in a refractive index distribution as shown in Figure 1b. of glass base material is obtained. In the figure, n ₀ is the refractive index level of silica glass, △n is the refractive index level increased by GeO ₂ , and α is the refractive index distribution expressed as △n=n[1-(r/r ₀ )〓] is the index of the curve. This α is a factor that greatly affects the important band characteristics of optical fibers, and α = around 2.0 is considered optimal, and its value changes slightly depending on the wavelength range used. For example, at 0.85μm α=2.0, at 1.30μm α=1.80
It is said that Furthermore, in recent years, the demand for low-loss optical fibers with broadband characteristics has increased rapidly, mainly for non-repeater transmission systems over 20 km. Especially in the 1.30μm band
It is said that a fiber having a band characteristic of 0.8 dB/Km or less and a band characteristic of 800 MHzKm or more is desirable. In order to ensure such characteristics and low loss, it is necessary to control the amount of residual water in the fiber to 0.1 ppm or less, and to ensure band characteristics, it is necessary to control α within α ₀ ±0.05.

However, in the conventional known method, when changing the coefficient α of the refractive index distribution, it is necessary to slightly change the ratio of the raw material gas, dopant gas, and combustion gas.
It is very difficult to change the α value to 2.00±0.05 for the 0.85 μm band.

The present inventors previously developed a method for producing a glass base material for optical transmission that can arbitrarily obtain an optimal refractive index distribution, as shown in FIG. in an atmosphere of Cl ₂ /He
proposed a method in which dehydration and profile control were performed simultaneously with sintering using two furnaces 21 (Japanese Patent Application No. 57-4901). In the figure, 21 is a sintering furnace, 22
gas burner, 23 starting materials, 24 aggregates, 25
They are an exhaust port, 26 heaters, and 27 Cl ₂ /He supply ports.

The present inventors have made the following findings as a result of intensive research into the active regions of dehydration reactions and volatilization reactions. Bulk density of soot body is 0.15~0.40g/
In the cm ³ range, the dehydration reaction is active at 1000-1500°C, the lower limit of which comes from the activation of the dehydration reaction, and the upper limit from the fact that the shrinkage rate of the soot body exceeds the dehydration reaction. On the other hand, the volatilization reaction is 1200-1400
It is active at ℃, and the lower limit is due to the activation of the GeO ₂ +2Cl ₂ →GeCl ₄ +O ₂ reaction, and the upper limit is because the soot shrinkage rate is very fast and inhibits the GeO ₂ volatilization reaction. From the above reasons, it can be seen that the temperature range in which both the dehydration reaction and the GeO ₂ volatilization reaction are active is 1200 to 1400°C. Upper temperature limit is 100℃ compared to dehydration reaction
The reason for this decrease is that GeCl ₄ gas diffusion within the soot body is slower than that of HCl gas generated in the dehydration reaction.

On the other hand, a temperature of 1500°C or higher is required to create transparent vitrification. For this reason, it is extremely difficult to produce transparent vitrification using a single furnace and simultaneously performing dehydration and profile control using a volatilization reaction, as in the prior invention mentioned above. degree of freedom is significantly hindered.

The present invention makes it easier to control the profile in the above-mentioned method of the prior application, and also makes it easier to control the profile of the glass base material.
It was developed with the aim of suppressing the generation of bubbles due to Cl ₂ , and is characterized by using two furnaces, first performing dehydration and profile control, and then performing additional dehydration and sintering in the second furnace. That is.

That is, the present invention provides a method for growing a porous preform for optical fiber in the axial direction and continuously converting it into transparent vitrification using a coaxially placed transparent vitrification furnace. A preheating furnace adjusted to a temperature range of 1200 to 1300℃ that does not completely shrink the porous base material is installed between the growth point and the transparent vitrification furnace, and an inert gas (such as He) and dopant are heated in this furnace. Controlling the refractive index distribution by volatilizing the dopant while dehydrating the base material in an atmosphere containing Cl ₂ gas as a dehydrating agent having the effect of reducing and volatilizing the Cl ₂ gas at a concentration of 0.5 to 5% by volume. The present invention relates to a method characterized in that the base material is transparently vitrified in an atmosphere containing an inert gas and a dehydrating agent in a transparent vitrification furnace.

The dehydrating agent used in the present invention includes Cl ₂ gas. He, Ar, _N2 , etc. are used as the inert gas.

In the present invention, _Cl2 gas is used as a dehydrating agent,
The furnace temperature of the preheating furnace is 1200~1300℃, and the atmosphere is an inert gas (such as He) with a _Cl2 gas concentration of 0.5~5% by volume, and the furnace temperature of the transparent vitrification furnace is 1500~1700℃.
By manufacturing an optical fiber base material by the VAD method in a He gas atmosphere with a Cl ₂ gas concentration of 0.1 to 0.2% by volume at 100° C., fibers with good properties can be efficiently manufactured.

On the other hand, there is also a method in which two heating sections or furnaces are used in the production of optical fiber base material, and dehydration is carried out in the first heating section and transparency is carried out in the second heating section, as disclosed in Japanese Patent Application Laid-Open No. 54-134722.
No. 54-134128, No. 54-94050, No. 55-
Although it is described in Publication No. 10412, the first heating section does not perform profile control, and the second
This invention differs from the present invention in that only sintering is performed without a dehydrating agent in the heating section of the optical fiber.In the present invention, this different configuration removes residual moisture in the optical fiber and further improves the band characteristics. The α value, which is important _for
0.05 or less, thereby making it possible to continuously manufacture optical fibers having arbitrary refractive index distributions and to provide optical fibers at low cost.

The difference between the method of the prior application (Japanese Patent Application No. 57-4901), which performs dehydration and sintering using one furnace, and the present invention will be explained using figures.

Figure 3a shows the furnace temperature distribution of the heater and the shrinkage of the soot body when the temperature is set at 1500℃ (the method of the prior application), and the distribution of the furnace temperature of the heater and the contraction of the soot body when the temperature is set at 1100 to 1300℃ (the method of the present invention). is as shown in FIG. 3b. In the figure, the hatched area l is the reaction region, where dopant volatilization and _{subsequent dehydration} take place. It turns out that it is very long.

On the other hand, there are three parameters for profile control (dopant volatilization): temperature T, reaction time t, and chlorine concentration C, which determine profile f.

fi (TtC) → f The larger _{the 1,} the more the dopant volatilizes (1) In the case of zone sintering, t is the rate at which the reaction region l descends.
The value divided by R is expressed as t=l/R (2), and from (2), (1) becomes fi(Tl/R, C). (3) Also, T indicates the activity of the reaction, is displayed.

In the case of the prior application (Fig. 3a), l is small, and in order to increase fi, it is necessary to decrease R or increase C.

On the other hand, T is 1500℃ to make it transparent vitrified.
It needs to be more than that. In such a temperature range, bubbles are likely to occur, and it is necessary to keep C (Cl ₂ concentration) low.In fact, the limit is 0.6 to 1 mol% (varies depending on the soot material), and it must be kept below this level. Therefore, it is difficult to control fi by changing the amount of C.

Also, R needs to be 4 mm/min or less than the heating time required for transparent vitrification, and this is the upper limit. Due to efficiency issues, the rate of descent is limited to 1 mm/min (360 soot takes 6 hours).

As mentioned above, the degree of freedom in profile control is very narrow in the method of the prior application using a single furnace, and profile control is difficult.

On the other hand, in the method of the present invention (FIG. 3b), the reaction region l is long, no bubbles are generated even if C is 5 mol %, and T does not need to be made into transparent glass, so the variable region is large.

The area where fi is efficient is 1000<T<(1300℃)○* 0.5<C<5 mol% R＜10 ○* is determined by other factors.

When T<1300℃ is 130℃ or higher, the soot body contracts, and the contraction speed changes with a slight change in the soot body (P amount, etc.), and even if fi is the same, the phosphorus amount (for example, introduced in the form of POCl ₃ gas) ).

From the above points, it is a method with a wide degree of freedom in that even if any one of the TCRs is fixed, fi can be kept at a constant value (the selection range of T and C is wide).

In the present invention, even if R is selected, fi can be adjusted by T and C, which is very advantageous for soot tandems.
It is possible to remove most of the moisture with this method, and the lack of dehydration can be made up by adding some Cl ₂ in the above-mentioned clarifying furnace.
It is supplemented by adding .

As mentioned above, since the pulling speed (corresponding to descending) is determined by the sooting rate, Figure 3a
The method of the earlier application has a limit when speeding up, but
The method of the present invention shown in FIG. 3b can be applied to cases of high speed, and can be said to be a very advantageous method.

The present invention will be explained in detail below using FIG. 4 using the VAD method as an example. FIG. 4 shows an example of an apparatus configuration for implementing the present invention. Here, reaction vessel 3
A burner 32 is provided at the bottom of the container 31, and a starting material 33 is rotatably suspended inside the container facing the burner 32.
6, 38 are provided. Further, gas supply ports 37 and 39 are provided at the top and side of the container 31 to feed a predetermined gas into the container.
At the bottom of 1, there is an exhaust port 35 for discharging the gas inside the container.
is provided.

The soot of glass particles generated by burning the raw material gas from the burner 32 using the above device is used as the starting material 3.
3 to form a glass fine particle aggregate 34. In this case, a dehydrating agent diluted with an inert gas is supplied through the gas supply port 37, and a Cl ₂ gas diluted with an inert gas, which has a dehydrating action and a dopant volatilization action, is supplied through the gas supply port 39. The inside of the furnace 36 is set to an inert gas atmosphere containing a dehydrating agent, and the inside of the furnace 38 is set to an atmosphere capable of dehydration and dopant volatilization, and the above-mentioned glass fine particle aggregate 34 is formed in these atmospheres, and the glass fine particle body 40 is heated upward. While dehydrating with the heater 38 and controlling the profile (vaporizing the dopant), the heater 36 performs dehydration and transparent vitrification. The heater 38 is installed for the purpose of dehydrating the base material and controlling the profile. In this case, there is one heater, but two heaters may be used. Further, the heater 36 is connected to the particulate matter 3.
4 was installed for the purpose of making transparent vitrification. By changing the heating temperature of the heaters 36 and 38 and the atmospheric gas, it is possible to easily change the coefficient α, which is important for the band characteristics.

Note that the addition of a dehydrating agent such as Cl ₂ gas from No. 37 is to convert the moisture that has entered the outside into hydrochloric acid, and is not to actively dehydrate the base material. 0.1% by volume of Cl ₂ was sufficient, and when it was 0.2% by volume or more, bubbles were sometimes observed in the transparent base material. Also
If Cl ₂ is less than 0.2% by volume, the profile becomes step-like, and if it is more than 5% by volume, the peripheral part will be sloping, both of which will have an adverse effect on the band characteristics.

An example of manufacturing a fiber base material using the apparatus shown in FIG. 4 is shown below.

Example 1 The porous base material generation conditions were kept constant, the heating temperature in the furnace 38 was 1200°C, He gas containing 2% by volume of Cl ₂ was flowed through the gas supply port 39 at 5/min, and the heating temperature in the furnace 36 was 1600℃, Cl ₂ 0.1 from gas supply port 37
When He gas containing % by volume was flowed at a rate of 5/min, a fiber with a residual moisture content of 0.01 ppm and a refractive index distribution α=1.95 was obtained.

Example 2 When He gas 5 containing 1% by volume of Cl ₂ was flowed from the gas supply port 39 and the other conditions were the same as in Example 1, a fiber with residual moisture of 0.05 ppm and α=2.00 was obtained.

Example 3 When the furnace 38 was set at 1100° C. and the other conditions were the same as in Example 1, a fiber with residual moisture of 0.03 ppm and α=1.85 was obtained.

Example 4 When the temperature of the furnace 38 is set to 1100°C, He gas containing Cl ₂ volume % is flowed 5/min from the gas supply port 39, and the other conditions are the same as in Example 1, the residual moisture is 0.04 ppm, α=
A fiber of 1.80 was obtained.

Example 5 In Example 1, when the addition of Cl ₂ through the gas supply port 37 was stopped, the residual moisture content was 0.03 ppm and the refractive index distribution was α=1.95.

[Brief explanation of the drawing]

Figure 1a is a diagram showing the outline of the VAD method, Figure 1b is a diagram showing the refractive index distribution in the VAD method, and Figure 2 is a diagram showing the method of the prior invention using one furnace in the VAD method. FIG. 3a is a diagram showing the heater furnace temperature distribution and soot body contraction in the method of the prior invention shown in FIG. 2, and FIG. 3b is a diagram showing the heater furnace temperature distribution and soot body contraction in the method of the present invention. FIG. 4 is a diagram showing the contraction of the body, and FIG. 4 is a diagram schematically showing the method of the present invention.

Claims (1)

[Claims] 1. In a method of growing a porous preform for optical fiber in the axial direction and continuously converting it into transparent vitrification using a transparent vitrification furnace placed on the same axis, 1200~1300 between the growth point of the material and the transparent vitrification furnace without completely shrinking the porous base material.
A preheating furnace adjusted to a temperature _{range of} Controlling the refractive index distribution by volatilizing the dopant while dehydrating the base material in an atmosphere of 0.5 to 5% by volume,
A method for producing a glass base material for optical fibers, which further comprises vitrifying the base material transparently in an atmosphere containing an inert gas and a dehydrating agent in a transparent vitrification furnace. 2 The furnace temperature of the transparent vitrification furnace is 1500 to 1700℃,
The method for producing a glass base material for optical fiber according to claim 1, wherein the atmosphere is a He gas atmosphere having a Cl ₂ gas concentration of 0.1 to 0.2% by volume.