JPH0672025B2 - Method for manufacturing optical fiber preform - 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 of manufacturing a large-sized optical fiber preform efficiently, stably and at high speed.

[0002]

2. Description of the Related Art Conventionally, as a method of manufacturing an optical fiber preform, an MCVD method (Modified Chemical) is used.
al Vapor Deposition, OVD method (Outside Vapor Deposition)
n), VAD method (Vapor-phase Axial)
Deposition) and the like. At the present time, the concern in the industry is how to economically manufacture the optical fibers having excellent characteristics obtained by these methods, that is, in large quantities in a short time to reduce the cost of the optical fibers. In particular, it is expected to manufacture a large-sized base material at a high speed because it has a great effect on the economicalization of the optical fiber.

As a high-speed synthesis method in the conventional VAD method,
It has been considered to have multiple burners or to improve with a single burner. In the multi-stage burner, since a plurality of burners are installed around the growth edge of the porous base material, the synthesis speed increases as the number of burners increases. However, due to the interference of each burner flame, it is inferior to the single burner in terms of characteristics, stability and reproducibility. In the case of a single burner, more glass raw material must be supplied in order to synthesize the porous base material at high speed, and as the glass raw material supply increases, the unreacted glass raw material increases and the flame flow is disturbed. However, there is a problem in that the synthetic yield decreases.

Further, in order to manufacture a thick porous base material at a high speed, an attempt was made to optimize it by paying attention to the flow rate (see "Fine Glass P" by H. Suda et al.
article-Deposition Mechan
ism in the VAD Process ", F
iber and IntegratedOptic
s, Vol. 4, No. 4, pp. 427-437),
In that case, it could be realized only in a poor yield, and it was not suitable as a method for producing a porous base material.

In the VAD method, in order to synthesize a large-sized base material at high speed, it was necessary to increase the amount of glass raw material supplied. Therefore, in order to improve the reaction efficiency of the glass raw material, it is considered to multiplex the flames. That is, a burner for controlling the glass particle size by increasing the effective flame length by making the flame multiple and retreating the inner flame was proposed in "Glass Particle Synthesis Burner" of Japanese Patent Application No. 58-219380. There is.

A structure of a double flame burner as one of such multiple flame burners is shown in FIG. In FIG.
Reference numeral 1 denotes an inner flame glass raw material supply port, 2 denotes an inner flame combustion gas supply port disposed around the inner flame glass raw material supply port 1, and 3 denotes an outer flame disposed around the inner flame combustion gas supply port 2. Glass raw material supply ports 4 are outer flame combustion gas supply ports arranged around the outer flame glass raw material supply port 3. Reference numeral 5 is an inner flame nozzle, 6 is an outer flame nozzle, and the inner flame nozzle 5 and the outer flame nozzle 6 are independent. Reference numeral 7 indicates a glass raw material layer which is reacting inside the inner flame 8, 9 indicates an outer flame, 10 indicates generated glass fine particles, and 11 indicates a growing porous base material. a
Indicates the flame length of the inner flame, and b indicates the flame length of the double flame composed of the inner flame and the outer flame. Here, the inner flame nozzle 5
Can be retracted from the outer flame nozzle 6 so that the inner flame 8 can retract from the outer flame 9 by a distance l, and the retreat distance l can be adjusted according to the amount of glass raw material supplied. Is done.

When the flame is duplicated, the amount of glass particles deposited increases due to the increase in flame length due to the outer flame. In other words, due to the double flame, the deposition rate of glass particles increases. In particular, the greater the amount of glass raw material supplied, the greater the effect of the double flame.

It is thought that this is because the flame becomes longer and the decomposition of the glass raw material is promoted, which causes the residence time of the glass particles in the flame to become longer and the particle size of the glass particles produced to become larger. Has been.

Next, FIG. 2 is a graph showing the relationship between the residence time in the flame by the double flame burner, the specific surface area of the glass particles and the particle size of the glass particles. As is clear from FIG. 2, as the residence time of the glass particles in the flame becomes longer, the specific surface area of the glass particles becomes smaller, and conversely, the particle size of the glass particles becomes larger. Therefore, it has been found that by making the flame length longer, the residence time in the glass fine particle flame can be made longer, and as a result, the particle diameter of the glass fine particles can be made larger.

That is, by retreating the inner nozzle using a double flame burner, the particle size of glass particles is increased, thereby increasing the particle size of the particles and improving the deposition rate, thereby improving the synthesis rate of the optical fiber preform. It was thought that speeding up could be achieved.

[0011]

As described above, the effect of controlling the particle size of fine glass particles has been confirmed in the multiple flame burner, but sufficient results have not been obtained with respect to the increase in the reaction efficiency of the glass raw material. Furthermore, in the actual production of the base material, there are drawbacks such as growth instability due to nonuniform flame temperature (cracking of the porous base material, disorder of the shape of the growth surface of the porous base material) and non-controllability of the refractive index distribution. Depending on the supply conditions of various gases when producing the porous base material, the growth of the porous base material may become extremely slow or unstable, and the conditions for manufacturing the base material with good reproducibility should be understood. The establishment of a basic manufacturing method has become an important issue.

In view of the above points, an object of the present invention is to provide a method for producing an optical fiber preform capable of producing a large-sized porous preform at high speed, stably and with good reproducibility. .

[0013]

In order to achieve the above object, in the present invention, when a glass raw material is supplied to a multiplexed flame to produce a porous base material, each of the multiplexed flame flows is produced. The flow velocity is set to a predetermined relationship, and the flow velocity of the glass raw material flow is supplied at a flow velocity of the flame flow or less.

That is, in the production method of the present invention, the glass raw material is decomposed in a flame to form glass fine particles, and the glass fine particles are deposited on a seed rod to form a porous base material. In a method for manufacturing an optical fiber preform by vitrification to form an optical fiber preform, a raw material supply nozzle for supplying the glass raw material, and a raw material supply nozzle are sequentially arranged around the raw material supply nozzle, and individual flames are provided. A burner having a plurality of flame forming nozzles for sequentially forming a flow is used, and the flow velocity of the k-th flame among the plurality of flames is V
_k , where the flow velocity of the (k + 1) th flame outside the kth flame is V _{k + 1} and the flow velocity of the glass raw material is V _m , The above-mentioned k-th flame flow is regressed from the (k + 1) -th flame flow, and the downstream end of the k-th flame flow and the (k + 1) -th flame flow are satisfied. The retreat distance is determined so that the upstream end of the flame flow is substantially continuous, and the glass raw material is supplied into the resulting multiple flames to generate glass fine particles.

[0015]

According to the present invention, the flames are multiplexed, the flame velocities of the respective flames are made equal, and the inner flame is made to retreat more than the outer flame. And it can be manufactured stably.

[0016]

The present invention will be described in detail below with reference to the drawings.

First, a deposition experiment was conducted on the combustion gas flow rate and the growth stability of the porous base material. In this experiment,
A burner having a double flame structure composed of concentric multiple flame nozzles as shown in Fig. 3 is used, the retreat distance 1 is set to 60 mm as shown in Fig. 3, the flow velocity Vo of the outer flame flow and the flow velocity of the inner flame flow The relationship between Vi and the flow rate Vm of the raw material stream was investigated.

The flame velocity of the outer flame is 2 m / se in which a stable flame can be obtained in consideration of the flame flow Reynolds number and the like.
c. Here, the flame flow velocity is obtained by dividing the respective flow rates of hydrogen gas and oxygen gas that predominantly form the flame by the cross-sectional area of the nozzle that blows out each gas. The burner position was set to the position in the usual base material production. The deposition amount of glass particles depends on the pseudo base material because the pseudo base material is used.

Silicon tetrachloride (SiC as a glass raw material)
l ₄ ) with argon gas as a carrier and a flow rate of 0.7 m /
It was supplied to the inner flame central layer at 2200 cc / min for sec, and the flame flow velocity Vi of the inner flame was changed to measure the glass particulate deposition amount. The flame flow rate was adjusted by changing the flow rates of oxygen gas and hydrogen gas at the same rate.

The results are shown in FIG. Inner flame flow velocity V
As i is increased, the effect as a double flame is generated, and the glass particulate deposition amount is increased.

[0021]

[Equation 2]

In the case of, the maximum value was taken, which was the most preferable. When the inner flame flow velocity Vi is further increased, the inner flame begins to disturb the outer flame, a stable flame cannot be obtained, the base material surface temperature becomes non-uniform, and the glass particulate deposition rate decreases. Especially the inner flame flow velocity Vi is 5 m / sec
A stable growth surface cannot be obtained when the voltage exceeds (corresponding to 2.5 Vo).

On the other hand, when the inner flame flow velocity Vi is decreased, when Vi is in the vicinity of 0.2 m / sec (corresponding to 0.1 Vo) or less, it does not function as a burner and a stable growth surface cannot be obtained. .

Next, the flow rate Vm of the glass raw material layer supplied to the inner flame is changed by adjusting the flow rate of the carrier gas, and the weight of the deposited glass fine particles is measured to determine the glass raw material with respect to the flow rate of the glass raw material. The result of obtaining the yield is shown in FIG.

As can be seen from FIG. 5, the yield decreases sharply when the flow rate Vm of the glass raw material layer exceeds about 0.5 Vo = 1 m / sec, and when Vm exceeds about Vo = 2 m / sec, the glass raw material Does not react with the flame, stable growth is not seen, and in order to grow the porous base material stably, Vm ≦ Vo for Vm and Vo in the burner.
I found it necessary to.

From the above, in order to obtain high-speed and stable growth of the base metal, the inner flame flow velocity Vo is 0.1 V
It can be seen that the range of o ≦ Vi ≦ 2.5 Vo and the range of Vm ≦ Vo are preferable, and the range of Vo = Vi ≧ Vm is particularly preferable.

In the above experiment, Vo = 2 m / sec
As a result, the relation between Vi and Vm and Vo was investigated.
The setting of o can be changed within the range where turbulent flow does not occur near the growth part of the porous base material. Vo varies depending on the diameter and shape of the porous base material to be produced, and is usually set in the range of 0.5 to 5 m / sec. In this range, the same tendency as described above was obtained.

Further, the same examination was conducted for a triple flame having another set of flames on the outside. Innermost one
Flow velocity Vi of heavy flame and flow velocity Vo of outermost triple flame
The relationship with (2) is shown in FIG. Here, Vo (1) = Vo (2), Vo, where the flow velocity of the double flame is Vo (1).
The case where (1) = 0.2 Vo (2) is illustrated. It was found that the same relationship was found in the triple flame. Especially V
When flames with different flow velocities are formed as shown in the case of o (1) = 0.2Vo (2), the range of stable growth becomes narrower and the amount of deposited glass particles tends to decrease overall. It was

[0029] Thus, the present invention is also applicable to general multi-flame not only double flame or triple flame, the flow rate of each, and the flow velocity V _k of the k-th flame the (k +
When focusing on the 1) -th flame velocity V _{k + 1} , By satisfying the above condition, the porous base material can be grown at high speed, stably and with good reproducibility.

The flow velocity in the inner flame or the outer flame can be adjusted by adjusting the burner size and the nozzle gap, and by adjusting the flow rates of oxygen gas and / or hydrogen gas.

Next, the determination of the retreat distance l of the inner flame in the state of the above-mentioned suitable flow velocity of the flame flow will be described with reference to FIG. FIG. 7 shows the flame temperature distribution in the axial direction of the burner in contrast to the inner and outer flames. here,
Tc is a temperature indicating the lower limit of the reaction of the glass raw material, and below this temperature Tc, the glass raw material does not react and remains as it is.

In the state (A) of the retreat distance 11 shown in FIG. 7, the downstream end of the inner flame 8 and the upstream end of the outer flame 9 are substantially continuous, and the flame temperature is higher than the lower limit temperature Tc. is doing. Here, continuous means that the flame temperature is always higher than the lower limit temperature Tc. When the retreat distance 1 is increased to 12 (B), the inner flame 8
And the outer flame 9 are separated. Therefore, the retreat distance l
Is the preferred Vo, Vi and Vm defined above.
Under these conditions, the inner flame 8 and the outer flame 9 are set to be substantially continuous. In addition, the retreat distance 1 is made as long as possible within the range defined in this way, so that the flame length becomes long, so that the deposition efficiency of the glass particles can be enhanced.

FIG. 8 shows the effect of the double flame under such a flow velocity condition. FIG. 8 shows the relationship between the supply amount of SiCl ₄ as a glass raw material and the volume amount of glass fine particles in the case of double flame formation and in the case of non-double flame formation in a pseudo base metal having a diameter of 150 mm.
The deposition rate when iO ₂ is deposited is shown with respect to the supply amount of SiCl ₄ which is a glass raw material. Here, the solid line represents the case of double flame formation, and the broken line represents the case of the inner flame alone.

Here, the supply amount of SiCl ₄ is set to 5000 c.
c / min, 5g / min for double flame burner
And a yield of 60-70%.

Next, examples of the present invention will be described.

FIG. 9 shows an embodiment of the burner for synthesizing glass particles used in the present invention. Here, 21 is a glass material supply nozzle, 22 is a combustible gas supply nozzle such as H ₂ or hydrocarbon fuel such as propane or butane, and 23 is A
Nozzle for supplying an inert gas such as r, He, N ₂ ; 24 is a nozzle for supplying a combustion-supporting gas such as O ₂ ; and 25 is a nozzle for supplying an inert gas. These nozzles 22 to 25 are arranged concentrically around the nozzle 21 in this order to form inner multiple nozzles, and an inner flame is formed by the combustible gas and the combustion supporting gas. Further, 26 is an outer glass raw material (or inert gas) supply nozzle, 27 is a combustible gas supply nozzle, 28 is an inert gas supply nozzle, and 29 is a combustible gas supply nozzle. .About.29 are arranged concentrically around the nozzle 25 in this order to form an outer multiple nozzle, and the outer flame is formed by the combustible gas and the combustion supporting gas. Here, the inner multiple nozzle is retracted from the outer multiple nozzle by a distance l. 30 is a hood arranged around the outer multiple nozzles.

Here, the glass material supply nozzle 21 and
Each of the outer glass raw material supply nozzles 26 is arranged so as to have an opening on the upstream side of the gas flow with respect to each set of the combustible gas supply nozzle and the combustible gas supply nozzle located outside thereof. This is to prevent the glass fine particles from adhering to the tip of the raw material supply nozzle when the glass raw material supplied in the form of chloride or the like is decomposed into a glass fine particle in a flame. That is, when the glass particles adhere to the tip of the nozzle, the synthesis conditions change with time.

Further, it is desirable that the tip of each nozzle is machined into a single edge as shown in FIG. If a nozzle made of thick material and having an end cut at a right angle is used, the gas flow is disturbed at that end, which causes problems such as adhesion of glass particles to the nozzle tip. Is. The characteristics of the burner do not change significantly even if the supply locations of the combustible gas and the combustion-supporting gas in the nozzles of each set are exchanged.

Next, in the double flame burner shown in FIG. 9, the burner had an outer diameter of 53 mm and a retreat distance of 1 = 60 mm, and the manufacturing method of the present invention was applied to this burner to prepare a porous base material. Here is an example:

Example 1 Example 1 relates to the production of a porous base material composed only of SiO ₂ , and the multiple flame burner is the double flame burner having the concentric circular multiple nozzle structure used in the above velocity model experiment. used.

In the double flame burner shown in FIG. 9, the outer flame flow velocity, the inner flame flow velocity and the raw material layer flow velocity are 2.1 m / sec, 2.1 m / sec and 0.
It was set to 7 m / sec. 2200 cc / min of silicon tetrachloride with argon gas as a carrier for the raw material supply nozzle
Was supplied. The base material synthesis rate was 3.5 g / min, and the glass raw material yield was 65%. The growth of the base material was very stable, and a large-sized porous base material having a diameter of 120 mm and an effective length of 800 mm was obtained in 10 hours. Even when the base material was manufactured for a long time of 10 hours, the shape of the growth surface did not change, and stable growth was achieved.

Example 2 Example 2 is an example of a method for producing a porous base material for a base material having GeO ₂ added and having a refractive index distribution. Here, it was necessary to lower the flame temperature as compared with the case where GeO ₂ was not added in order to form the solid-dissolved germanium dioxide. Further, in order to increase the synthesis rate and adjust the refractive index distribution, the glass raw material was also supplied to the outer flame as shown below.

As the burner, the double flame burner shown in FIG. 9 which is the same as that of the first embodiment is used, and the outer flame flow velocity, the inner flame flow velocity and the raw material flow velocity are 2.0 m / sec, respectively.
The glass raw materials were 2.0 m / sec and 0.8 m / sec. In addition to silicon tetrachloride under the same conditions as in Example 1, germanium tetrachloride was supplied at 200 cc / min, and argon gas was also supplied as a carrier. The base material synthesis rate was 4.5 g / min, and a relative refractive index difference of 1.1% was obtained.

According to this example, it was revealed that the manufacturing method of the present invention is also effective for the synthesis of the glass base material containing the dopant for controlling the refractive index.

If the glass raw material is supplied to the outer flame, the deposition rate is further improved. However, in the outer flame, there is no effect of extending the residence time of the synthesized glass fine particles in the flame, so that the entire glass is not affected. It was confirmed that the glass raw material yield with respect to the raw material supply amount was slightly reduced. In this example, the raw material yield was 55%. Here, the glass raw material yield decreased because the yield of germanium tetrachloride was lower than that of silicon tetrachloride, and the glass raw material yield supplied to the outer flame was the glass raw material yield supplied to the inner flame. It is due to the lower.

[0046]

As described above, according to the present invention,
It is possible to manufacture a large base material at high speed and stably by multiplexing flames and making the flame velocities of the respective flames equal to each other, and by making the inner flame recede from the outer flame. As a result, the base material manufacturing efficiency is improved, and the optical fiber price can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a flame multiplexing arrangement.

FIG. 2 is a characteristic diagram showing the relationship between the particle size of glass particles and the residence time in a flame.

FIG. 3 is an explanatory diagram of flow velocity and retreat distance of each part in the double flame burner according to the present invention.

FIG. 4 is a characteristic diagram showing the flame flow velocity dependence of the inner flame of the glass particulate deposition amount in the present invention.

FIG. 5 is a characteristic diagram showing the glass raw material layer flow rate dependence of the glass fine particle yield.

FIG. 6 is a characteristic diagram showing the flame flow velocity dependence of the inner flame of the glass particulate deposition amount in the present invention.

FIG. 7 is a flame temperature distribution diagram showing the relationship between the retreat distance of the inner flame in the axial direction of the burner and the flame temperature.

FIG. 8 is a characteristic diagram showing a relationship between a glass raw material supply amount and a glass particulate deposition amount in a single flame and a double flame.

FIG. 9 is a sectional view showing an embodiment of the double flame burner used in the present invention.

[Explanation of symbols]

1 Inner Flame Glass Raw Material Supply Port 2 Inner Flame Combustion Gas Supply Port 3 Outer Flame Glass Material Supply Port 4 Outer Flame Combustion Gas Supply Port 5 Inner Flame Nozzle 6 Outer Flame Nozzle 7 Glass Raw Material Layer 8 Inner Flame 9 Outside flame 10 Generated glass particles 11 Porous base material 21 Glass raw material supply nozzle 22 Combustible gas supply nozzle 23 Inert gas supply nozzle 24 Combustible gas supply nozzle 25 Inert gas supply nozzle 26 Outer glass Raw material (or inert gas) supply nozzle 27 Combustible gas supply nozzle 28 Inert gas supply nozzle 29 Combustible gas supply nozzle 30 Hood

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Motohiro Nakahara 162 Shirahone, Shirahoji, Tokai-mura, Naka-gun, Ibaraki Prefecture Nippon Telegraph and Telephone Corporation, Ibaraki Telecommunications Research Institute (56) Reference JP-A-60-112636 ( JP, A)

Claims (2)

[Claims] 1. A glass raw material is decomposed in a flame to form glass fine particles, the glass fine particles are deposited on a seed rod to form a porous preform, and the porous preform is made into transparent glass to produce an optical fiber. In a method of manufacturing an optical fiber preform for forming a preform, a raw material supply nozzle for supplying the glass raw material,
A burner having a plurality of flame forming nozzles sequentially arranged around the raw material supply nozzle and sequentially forming individual flame flows is used, and the flow velocity of the kth flame among the plurality of flames is used. Is V _k , the flow velocity of the (k + 1) th flame outside the kth flame is V _{k + 1,} and the flow velocity of the glass raw material is V _m , Is satisfied, the k-th flame flow is made to recede from the (k + 1) -th flame flow, and the downstream end of the k-th flame flow and the (k + 1) -th flame flow are Manufacture of an optical fiber preform characterized in that the retreat distance is determined so that the upstream end of the flame flow is substantially continuous, and the glass raw material is supplied into the obtained multiple flames to generate glass fine particles. Method. 2. The method for manufacturing an optical fiber preform according to claim 1, wherein: A method for manufacturing an optical fiber preform characterized by satisfying the following condition.