JPH0416418B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a method for manufacturing an optical fiber preform, which efficiently and stably manufactures a large optical fiber preform at a high speed.

[Conventional method] Conventionally, the MCVD method (Modified Chemical Vapor
Deposition), OVD method (Outside Vapor
Deposition), VAD method (Vapor-phase Axial
Deposition) etc. At present, the industry is concerned with how to economically produce optical fibers with excellent properties obtained by these methods in large quantities in a short period of time, thereby reducing the cost of the optical fibers. In particular, manufacturing large-sized base materials at high speed is expected to be highly effective in making optical fibers more economical.

Conventionally, as a high-speed synthesis method in the VAD method, consideration has been given to using multiple stages of burners or improving the use of a single burner. In multi-stage burners,
Since a plurality of burners are installed around the growing end of the porous base material, the synthesis speed increases as the number of burners increases. However, due to the interference of each burner flame, the characteristics,
It is inferior in terms of stability and reproducibility compared to the single burner case. In the case of a single burner, it is necessary to supply a larger amount of glass raw material in order to synthesize the porous base material at high speed, and as the amount of glass raw material supplied increases, the amount of unreacted glass raw material increases and the flame flow becomes turbulent. There was a problem that this resulted in a decrease in the synthesis yield.

Furthermore, attempts were made to optimize the flow rate in order to manufacture thick porous base materials at high speeds (H. Suda et al.'s “Fine Glass Particle-
Deposition Mechanism in the VAD
Process”, Fiber and Integrated Optics,
Vol. 4, No. 4, pp. 427-437), in that case, it could only be achieved with a poor yield and was not suitable as a method for producing a porous base material.

In the VAD method, in order to synthesize large base materials at high speed, it was necessary to increase the amount of glass raw materials supplied. Therefore, in order to improve the reaction efficiency of glass raw materials, it has been considered to use multiple flames. That is, a burner that increases the effective flame length and controls the particle size of glass particles by multiplexing flames and regressing the inner flame has been proposed in Japanese Patent Application No. 58-219380 entitled ``Burner for Synthesizing Glass Fine Particles.'' .

The structure of a double flame burner as one of such multiple flame burners is shown in FIG. In Fig. 1, 1 is a frit supply port for the inner flame, 2 is a combustion gas supply port for the inner flame arranged around the glass raw material amount port 1 for the inner flame, and 3 is the combustion gas supply port 2 for the inner flame. Outer flame frit supply ports 4 arranged around the outer flame frit supply ports 4 are outer flame combustion gas supply ports arranged around the outer flame frit supply ports 3 . 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 frit layer reacting within the inner flame 8, 9 indicates an outer flame, 10 indicates generated glass fine particles, and 11 indicates a growing porous base material. a is the flame length of the inner flame,
b indicates the flame length of a double flame consisting of an inner and outer flame. By making the inner flame nozzle 5 retractable from the outer flame nozzle 6, the inner flame 8 is configured to be able to retreat by a distance l from the outer flame 9, and the regression distance l is It can be adjusted according to the amount of supply.

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

This is thought to be because the longer flame accelerates the decomposition of the glass raw materials, which lengthens the residence time of the glass particles in the flame, increasing the particle size of the glass particles produced. .

Next, FIG. 2 is a graph showing the relationship between the residence time in the flame of a double flame burner, the specific surface area of glass particles, and the particle size of 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 decreases, and conversely, the particle size of the glass particles increases. Therefore, it has been found that by increasing the flame length, the residence time of the glass fine particles in the flame can be increased, and as a result, the particle size of the glass fine particles can be increased.

That is, by regressing the inner nozzle using a double flame burner, the particle size of the glass particles is increased, thereby increasing the diameter of the particles and increasing the deposition rate, thereby increasing the synthesis rate of the optical fiber base material. It was believed that this could be achieved.

As described above, although the effect of controlling the particle size of glass fine particles has been confirmed in multiple flame burners, sufficient results have not been obtained in terms of increasing the glass raw material reaction efficiency. Furthermore, when preparing the actual base material, growth instability due to uneven flame temperature (cracks in the porous base material, disordered shape of the growth surface of the porous base material) must be avoided.
However, 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. Therefore, it is important to understand the conditions for manufacturing base materials with good reproducibility and to establish basic manufacturing methods.

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

[Structure of the Invention] In order to achieve the above object, the present invention provides a method for controlling the flow velocity of each of the multiplexed flame flows to a predetermined value when producing a porous base material by supplying glass raw materials to multiplexed flame flows. and the flow rate of the frit stream is less than or equal to the flow rate of the flame stream.

In order to achieve such an object, in the present invention, when producing a porous base material by supplying glass raw materials to multiplexed flames, the flow velocities of each of the multiplexed flame flows are set in a predetermined relationship, and The flow rate of the frit stream is supplied at a rate less than or equal to the flow rate of the flame stream.

That is, the manufacturing method of the present invention decomposes a glass raw material in a flame to form glass fine particles, deposits the glass fine particles on a seed rod to form a porous base material,
In the method for producing an optical fiber preform in which the porous preform is transparently vitrified to form an optical fiber preform, a raw material supply nozzle for supplying the glass raw material and sequentially arranged around the raw material supply nozzle are provided. and a plurality of flame forming nozzles that sequentially form individual flame flows, and the flow velocity of the k-th flame among the plurality of flames is determined.
V _k , when 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 _n , 0.1V _k+1 ≦V _k ≦2.5 V _k+1 V _n ≦V _k+1 V _n ≦ V _k is satisfied, and the k-th flame flow is regressed from the (k+1)-th flame flow, resulting in multiple flames. The method is characterized in that the glass raw material is supplied to produce glass fine particles.

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

First, a deposition experiment was conducted to examine the flow rate of combustion gas and the growth stability of the porous matrix. In this experiment, we used a burner with a double flame structure consisting of multiple concentric flame nozzles as shown in Figure 1.
As shown in FIG. 3, the regression distance l was set to 60 mm, and the relationship between the flow velocity Vo of the outer flame flow, the flow velocity Vi of the inner flame flow, and the flow velocity Vm of the raw material flow was investigated.

The flame flow velocity of the outer flame is set at 2m/2m to obtain a stable flame, taking into consideration the number of flame flow Ray nozzles, etc.
sec. Here, the flame flow velocity is the flow rate of each of hydrogen gas and oxygen gas, which predominantly constitute the flame, divided by the cross-sectional area of the nozzle from which each gas is blown out. The burner position was set to the position used in normal base material production. Since the amount of glass fine particles deposited depends on the pseudo base material because a pseudo base material is used, it was set on an arbitrary scale.

Silicon tetrachloride (SiCl ₄ ) as a glass raw material is supplied to the inner flame center layer at a flow rate of 0.7 m/sec using argon as a carrier at a flow rate of 2200 c.c./min, and the flame flow velocity Vi of the inner flame is changed to form glass particles. The amount of deposition was measured. The flame flow rate was adjusted by changing the oxygen and hydrogen gas flow rates in equal proportions.

The results are shown in FIG. Inside fire flow velocity Vi
As ViVo increases, a double flame effect occurs, the amount of glass particles deposited increases, and ViVo=2
The maximum value was obtained at m/sec, which was the most suitable. When the inner flame flow velocity Vi is further increased, the inner flame starts to disturb the outer flame, a stable flame cannot be obtained, the base material surface temperature becomes uneven, and the glass particle deposition rate decreases. In particular, when the inner flame flow velocity Vi exceeded around 5 m/sec (equivalent to 2.5 Vo), a stable growth surface could no longer be obtained.

On the other hand, as the inner flame flow velocity Vi decreases,
When Vi was around 0.2 m/sec (equivalent to 0.1 Vo) or less, it no longer functioned as a burner, and a stable growth surface could no longer be obtained.

Next, by adjusting the flow rate of the carrier gas, the flow rate Vm of the frit layer supplied to the inner flame is determined.
FIG. 5 shows the results of determining the glass raw material yield with respect to the glass raw material flow rate by varying the amount of glass and measuring the weight of the deposited glass particles.

As can be seen from Figure 5, the glass raw material laminar flow velocity
The yield drops sharply when Vm exceeds around 0.5 Vo = 1 m/sec, and when Vm exceeds around Vo = 2 m/sec, the glass raw material hardly reacts with the flame and stable growth is not observed. It was found that in order to grow the porous base material stably, it is necessary to set Vm≦Vo in the burner.

From the above, in order to obtain stable growth of the base metal at high speed, the flow velocity Vo of the inner flame flow should be 0.1Vo.
It can be seen that it is preferably in the range of ≦Vi≦2.5Vo and that Vm≦Vo, and in particular, it is most preferable that Vo=Vi≧Vm.

In addition, in the above experiment, Vo = 2m/sec,
The relationship between Vi, Vm, and Vo was investigated, and the setting of Vo can be varied within a range that does not cause turbulence near the growth area 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.

Furthermore, a similar study was conducted on a triple flame with another set of flames on the outside. FIG. 6 shows the relationship between the flow velocity Vi of the innermost first layer flame and the flow velocity Vo(2) of the outermost third layer flame. Here, 2
Assuming the flow velocity of heavy flame as Vo(1), Vo(1)=Vo(2),
The case where Vo(1)=0.2Vo(2) is illustrated. A similar relationship was found for triple flames as well. In particular, when flames with different flow velocities are formed, as shown in the case of Vo(1) = 0.2Vo(2), the range of stable growth becomes narrower and the amount of glass particles deposited tends to decrease overall. Ta.

As described above, the present invention is applicable not only to double flames and triple flames but also to multiple flames in general, and for each of these flames, the flow velocity V _k of the kth flame and the flow velocity V k of the (k+1)th flame are determined. When focusing on _the flow velocity _{V k + 1 of} A porous base material can be grown at high speed, stably, and with good reproducibility.

Adjustment of the flow velocity in the inner flame or outer flame section can be achieved by adjusting the burner dimensions and nozzle gap, as well as the flow rates of oxygen gas and/or hydrogen gas.

Next, the determination of the regression distance l of the inner flame when the above-mentioned flame flow reaches a suitable flow velocity will be explained 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 reaction of the glass raw material, and below this temperature Tc, the glass raw material does not react and remains in the same state.

In state (A) with regression distance l1 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 continuously higher than the lower limit temperature Tc. There is. Here, continuous means that the flame temperature is always higher than the lower limit temperature Tc. State where regression distance l is increased to l2 (B)
In this case, the inner flame 8 and the outer flame 9 will be separated. Therefore, the regression distance l is determined so that the inner flame 8 and the outer flame 9 are substantially continuous under the preferable conditions of Vo, Vi, and Vm determined as described above. Moreover, by making the regression distance l as long as possible within the range determined in this way, the flame length becomes longer, so that the deposition efficiency of glass particles can be increased.

The effect of the double flame under such flow velocity conditions is shown in FIG. Figure 8 shows the relationship between the amount of SiCl ₄ supplied as a glass raw material and the volume of glass fine particles with and without double flame formation, and the deposition rate when SiO ₂ was deposited on a pseudo base material with a diameter of 150 mm. It is shown relative to the supply amount of SiCl ₄ , which is a glass raw material. Here, the solid line represents the case where there is a double flame, and the broken line represents the case where there is only an inner flame.

Here, when the feed rate of SiCl ₄ was 5000 c.c./min, a deposition rate of 5 g/min was obtained with the dual flame burner, and the yield was 60 to 70%.

Next, embodiments of the present invention will be described.

FIG. 9 shows an embodiment of a burner for synthesizing glass particles used in the present invention. Here, 21 is a nozzle for supplying glass raw materials, 22 is a nozzle for supplying flammable gas such as H ₂ or hydrocarbon fuel such as propane or butane, and 23 is a nozzle for supplying inert gas such as Ar, He, N ₂ etc. , 24 is a nozzle for supplying combustion-supporting gas such as _O2 , and 25 is a nozzle for supplying inert gas. These nozzles 22 to 25 are arranged concentrically in this order around the nozzle 21 to form an inner multiple nozzle, and an inner flame is formed by the combustible gas and the combustion-supporting gas. Furthermore, 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, 29 is a combustion-supporting gas supply nozzle, and these nozzles 26 29 are concentrically arranged in this order around the nozzle 25 to form an outer multiple nozzle, and an outer flame is formed by the combustible gas and the combustion-supporting gas. Here, the inner multiple nozzle is moved back by a distance l from the outer multiple nozzle. 30 is a hood arranged around the outer multiple nozzle.

Here, the frit supply nozzle 21 and the outer frit supply nozzle 26 are located on the upstream side of the gas flow with respect to each set of combustible gas supply nozzle and combustion-supporting gas supply nozzle located outside thereof. the opening. This is to prevent the glass 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 in the flame and becomes glass particles. That is, if glass particles adhere to the nozzle tip, the synthesis conditions will change over time.

Further, it is desirable that the tip of each nozzle is machined into a single-edged shape as shown in FIG. When using a nozzle that is made of a thick material and has an end cut at a right angle, the gas flow will be disturbed at the end, causing problems such as adhesion of glass particles to the nozzle tip. It is. Note that even if the supply locations of the combustible gas and the combustion-supporting gas are switched in each set of nozzles, there is no significant change in the characteristics of the burner.

Next, we will show an example in which the manufacturing method of the present invention was applied to the double flame burner shown in Figure 9, with a burner outer diameter of 53 mm and a regression distance l = 60 mm, in which a porous base material was manufactured. .

Example 1 Example 1 relates to the production of a porous matrix consisting only of _SiO2 , and as a multiple flame burner,
A double flame burner with a concentric multiple nozzle structure used in the flow velocity model experiment described above was used.

In the double flame burner shown in Figure 9, the outer flame flow velocity, inner flame flow velocity and raw material layer flow velocity are 2.1 m/sec, 2.1 m/sec and 0.7 m/sec, respectively.
sec. Silicon tetrachloride was supplied at 2200 c.c./min to the raw material supply nozzle using argon gas as a carrier. The base material synthesis rate was 3.5 g/min, and the glass raw material yield was 65%. The matrix growth was very stable, and a large porous matrix with a diameter of 120 mm and an effective length of 800 mm was obtained in 10 hours. Even during the long time of preparing the base material for 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 manufacturing a porous base material for a base material having a refractive index distribution doped with GeO ₂ . Here, compared to the case where GeO ₂ is not added to form solid solution germanium dioxide,
It was necessary to lower the flame temperature. Further, in order to increase the synthesis rate and adjust the refractive index distribution, a glass raw material was also supplied to the outer flame as shown below.

As the burner, the same double flame burner shown in Figure 9 as in Example 1 was used, and the outer flame flow velocity, inner flame flow velocity, and raw material flow velocity were set to 2.0 m/min, respectively.
sec, 2.0 m/sec and 0.8 m/sec, and as glass raw materials, in addition to silicon tetrachloride under the same conditions as in Example 1, germanium tetrachloride was supplied at 200 c.c./min with argon gas 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 synthesizing a glass base material containing a dopant for controlling the refractive index.

Note that if the glass raw material is also supplied to the outer flame, the deposition rate will be further improved, but in the outer flame, there is no effect of extending the residence time of the synthesized glass particles in the flame, so the total amount of glass raw material supplied is It was confirmed that the yield of the glass raw material for the glass was slightly decreased. In this example, the raw material yield was 55%. Here, the reason for the decrease in the yield of frit is that the yield of germanium tetrachloride is lower than that of silicon tetrachloride, and the yield of frit supplied to the outer flame is lower than the yield of frit supplied to the inner flame. This is due to the fact that it is lower.

[Effects] As explained above, according to the present invention, flames are multiplexed, the flame velocity of each flame is set to be equal to each other, and the inner flame is caused to regress more than the outer flame. The shape base material can be manufactured at high speed and stably. As a result, the base material manufacturing efficiency is improved and the cost of the optical fiber can be reduced.

[Brief explanation of drawings]

FIG. 1 is a diagram schematically showing a flame multiplexing configuration;
Figure 2 is a characteristic diagram showing the relationship between the particle size of glass fine particles and the residence time in the flame, and Figure 3 is a characteristic diagram showing the relationship between the particle size of glass particles and the residence time in the flame.
An explanatory diagram of the flow velocity and regression distance at each part in a heavy flame burner. Figure 4 is a characteristic diagram showing the dependence of the amount of glass particles deposited on the flame flow velocity of the inner flame in the present invention. Figure 5 is a diagram showing the dependence of the glass particle yield on the glass raw material. FIG. 6 is a characteristic diagram showing the dependence of the laminar flow velocity on the amount of deposited glass particles in the present invention, and FIG. 7 is a characteristic diagram showing the dependence of the flame flow velocity of the inner flame in the burner axial direction and flame temperature. Figure 8 is a characteristic diagram showing the relationship between glass raw material supply amount and glass particle deposition amount in single flame and double flame burner, Figure 9 is an example of a double flame burner. It is a sectional view showing an example. 1... Glass raw material supply port for inner flame, 2... Combustion gas supply port for inner flame, 3... Glass raw material supply port for outer flame, 4... Combustion gas supply port for outer flame, 5... For inner flame Nozzle, 6... Outer flame nozzle, 7... Glass raw material layer, 8... Inner flame,
9... Outer flame, 10... Generated glass particles, 11... Porous base material, 21... Glass raw material supply nozzle, 22... Flammable gas supply nozzle,
23... Nozzle for supplying inert gas, 24... Nozzle for supplying combustion-supporting gas, 25... Nozzle for supplying inert gas, 26... Nozzle for supplying outer glass raw material (or inert gas), 27... Flammable gas supply nozzle, 28... Inert gas supply nozzle, 29
...Nozzle for supplying combustion-supporting gas, 30...Hood.

Claims (1)

[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 base material, and the porous base material is made into transparent glass. A method for manufacturing an optical fiber preform that forms a preform for an optical fiber includes: a raw material supply nozzle that supplies the glass raw material; Using a burner having a plurality of flame forming nozzles, the flow velocity of the k-th flame among the plurality of flames is V _k , and the (k+1)-th flame outside the k-th flame is V k .
When the flow velocity of the second flame is V _k+1 and the flow velocity of the glass raw material is V _n , 0.1V _k+1 ≦V _k ≦2.5V _k+1 V _n ≦V _k+1 V _n ≦V _k glass particles are produced by supplying the glass raw material into multiple flames obtained by regressing the k-th flame flow from the (k+1)-th flame flow so as to satisfy the following conditions. A method for manufacturing an optical fiber base material. 2. A method for manufacturing an optical fiber preform according to claim 1, characterized in that the following condition is satisfied: V _k+1 =V _k ≧V _n .