JPH04228441A - Production of optical fiber preform - Google Patents

Abstract

PURPOSE:To stably produce a large-sized preform at a high speed by using a multiple flame burner to form multiple flames in a state to retreat the inner flame from the outer flame. CONSTITUTION:The multiple flame burner to be used in the subject process is composed of nozzles 21, 26 for supplying glass raw material (e.g. SiCl4), nozzles 22, 27 for supplying combustible gas (e.g. H2), nozzles 23, 25, 28 for supplying inert gas (e.g. Ar) and nozzles 24, 29 for supplying combustion- sustaining gas (e.g. O2). The nozzles 22-25 are concentrically arranged around the nozzle 21 to form inner flames and the nozzles 27-29 are concentrically arranged around the nozzle 26 to form outer flames. The inner multiple nozzles are retreated from the outer multiple nozzles by a distance (l). A hood 30 is placed around the outer multiple nozzles. For example, a double-flame burner having the above structure was used at an SiCl4 feeding rate of 5,000cc/min to get a deposition rate of 5g/min and a yield of 60-70%.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical fiber preform in an efficient, stable manner and at a high speed.

0002

[Prior Art] Conventionally, as a method for manufacturing optical fiber preforms, the MCVD method (Modified Chemistry
cal Vapor Deposition), O
VD method (Outside Vapor Depos
tion), VAD method (Vapor-phase
Axial Deposition), etc. Currently, the industry is concerned with how to economically produce optical fibers with excellent properties obtained by these methods, that is, how to manufacture them in large quantities in a short period of time to reduce the cost of the optical fibers. In particular, manufacturing large-sized base materials at high speed is expected to have a large effect on economicalization of optical fibers.

[0003] As a high-speed synthesis method in the conventional VAD method,
Consideration has been given to using multiple stages of burners or improving the use of a single burner. In a multi-stage burner, a plurality of burners are installed around the growing end of the porous base material, so as the number of burners increases, the synthesis speed increases. However, due to the interference of each burner flame, the characteristics, stability and reproducibility are inferior compared to the case of a single burner. 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 in that this resulted in a decrease in the synthesis yield.

[0004] Further, attempts have been made to optimize the flow rate by focusing on the flow rate in order to produce a thick porous base material at high speed ("Fine Glass" by H. Suda et al.
Particle-Deposition Mec
hanism in the VAD Pro
cess”, Fiber and Integra
tedOptics, Vol. 4, No. 4, pp. 4
27-437), 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-sized base materials at high speed, it is 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. Specifically, a burner that controls the particle size of glass particles by increasing the effective flame length by multiplexing flames and regressing the inner flame was proposed in Japanese Patent Application No. 58-219380 entitled "Burner for Synthesizing Glass Fine Particles." There is.

FIG. 1 shows the structure of a double flame burner, which is one of such multiple flame burners. In Figure 1,
1 is an inner flame frit supply port, 2 is an inner flame combustion gas supply port arranged around the inner flame frit supply port 1, and 3 is an outer flame arranged around the inner flame combustion gas supply port 2. The frit supply port 4 is an outer flame combustion gas supply port arranged around the outer flame frit supply port 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 the outer flame, 10 indicates the generated glass fine particles, and 11 indicates the growing porous base material. a
is the flame length of the inner flame, and b is the flame length of the double flame consisting of the inner and outer flames. Here, the inner flame nozzle 5
By making it 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 can be adjusted according to the supply amount of the glass raw material. It's like this.

[0007] When the flames are doubled, 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.

[0008] This is thought to be because the longer flame accelerates the decomposition of the glass raw material, which lengthens the residence time of the glass particles in the flame and increases the particle size of the produced glass particles. It is being

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 high speed could be achieved.

[0011]

As described above, although the effect of controlling the particle size of glass fine particles in multiple flame burners has been confirmed, sufficient results have not been obtained in terms of increasing glass raw material reaction efficiency. Furthermore, in actual preparation of the base material, there are drawbacks such as growth instability due to non-uniform flame temperature (cracking of the porous base material, disturbance of the shape of the growth surface of the porous base material) and uncontrollability of the refractive index distribution. Depending on the supply conditions of various gases when manufacturing the porous base material, the growth of the porous base material may become extremely slow or unstable, so it is important to understand the conditions for manufacturing the base material with good reproducibility. Establishing a basic manufacturing method is an important issue.

SUMMARY OF THE INVENTION In view of the above points, an object of the present invention is to provide a method for manufacturing an optical fiber preform in which a large porous preform can be manufactured at high speed, stably, and with good reproducibility. .

[0013]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a method for producing a porous base material by supplying a glass raw material to multiplexed flame flows in each of the multiplexed flame flows. The flow rates are set in a predetermined relationship, and the flow rate of the frit stream is supplied at a rate lower than the flame stream rate.

That is, in the manufacturing method of the present invention, 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 In a method for manufacturing an optical fiber preform in which a preform for optical fiber is formed by transparent vitrification, a raw material supply nozzle for supplying the glass raw material, and separate flames arranged sequentially around the raw material supply nozzle are provided. Using a burner having a plurality of flame forming nozzles that sequentially form flame streams, the k-th flame stream is transferred to the (k-th
+1)th flame flow, and at that time, the distance of the regression is set such that the downstream end of the kth flame flow and the upstream end of the (k+1)th flame flow are substantially continuous. The method is characterized in that glass particles are produced by supplying the glass raw material into the resulting multiple flames.

[0015]

According to the present invention, by multiplexing the flames and causing the inner flame to regress more than the outer flame, a large base material can be manufactured at high speed and stably.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in detail below with reference to the drawings.

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

The flame flow velocity of the outer flame is set to 2 m/sec, which allows a stable flame to be obtained, taking into consideration the flame flow Reynolds number, etc.
c. Here, the flame flow velocity is obtained by dividing the respective flow rates of hydrogen gas and oxygen gas, which predominantly constitute the flame, 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 is dependent on the pseudo base material because a pseudo base material is used, it was set on an arbitrary scale.

Silicon tetrachloride (SiC) as a glass raw material
l4) at a flow rate of 0.7 m using argon gas as a carrier.
/sec to the inner flame center layer at 2200 cc/min, and the amount of glass particles deposited was measured by changing the flame flow velocity Vi of the inner flame. 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 flame flow velocity V
As i increases, a double flame effect occurs, and the amount of glass particles deposited increases.

[0021]

[Math 1]

The maximum value was obtained when ##EQU1## was the most suitable. When the inner flame flow velocity Vi is further increased, the inner flame begins to disturb the outer flame, a stable flame is no longer obtained, the base material surface temperature becomes non-uniform, and the glass particle deposition rate decreases. In particular, the inner flame flow velocity Vi is 5 m/sec (
2.5Vo equivalent), a stable growth surface could no longer be obtained.

On the other hand, as the inner flame flow velocity Vi was decreased, 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 velocity Vm of the frit layer supplied to the inner flame is changed, and the weight of the deposited glass fine particles is measured. The results of determining the yield are shown in FIG.

As can be seen from FIG. 5, the yield sharply decreases when the glass raw material bed flow velocity 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.In order to grow the porous base material stably, Vm and Vo in the burner must be Vm≦Vo.
It was found that it was necessary to

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 set to 0.1V.
It can be seen that it is preferable that o≦Vi≦2.5Vo and Vm≦Vo, and most preferably Vo=Vi≧Vm.

[0027] In the above experiment, Vo=2m/sec
The relationship between Vi and Vm and Vo was investigated as follows, but V
The setting of o can be changed 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.

[0028] Furthermore, a similar study was conducted regarding a triple flame with another set of flames on the outside. innermost one
Flow velocity Vi of the heavy flame and flow velocity Vo of the outermost third flame
The relationship with (2) is shown in FIG. Here, assuming that the flow velocity of the second flame is Vo(1), Vo(1)=Vo(2), Vo
The case where (1)=0.2Vo(2) is illustrated. A similar relationship was found for triple flames. Especially V
As shown in the case of o(1) = 0.2Vo(2), when flames with different flow velocities are formed, 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 the flow velocities, the flow velocity Vk of the kth flame and the (k+
1) By satisfying the condition that the flow velocity of the second flame is between the two flow velocity Vk+1, it is possible to grow the porous base material 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 both or one of the oxygen gas and the hydrogen gas.

Next, the determination of the regression distance l of the inner flame under the condition of the above-described preferred flame 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 as it is.

In state (A) of 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 continuous at a temperature higher than the lower limit temperature Tc. are doing. Here, continuous means that the flame temperature is always higher than the lower limit temperature Tc. In the state (B) where the regression distance l is increased to l2, the inner flame 8
and the outer flame 9 will separate. Therefore, regression distance l
are the preferred Vo, Vi and V defined as described above.
Under the condition m, the inner flame 8 and the outer flame 9 are set to be substantially continuous. 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.

FIG. 8 shows the effect of double flame under such flow velocity conditions. Figure 8 shows the relationship between the supply amount of SiCl4 as a glass raw material and the volume of glass fine particles with and without double flame formation, and the deposition rate when depositing SiO2 on a pseudo base material with a diameter of 150 mm using a glass raw material. The figure is shown for a certain amount of SiCl4 supplied. 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.

[0034] Here, the supply amount of SiCl4 is 5000
cc/min, 5g/min for double flame burner
Deposition rates of n were obtained with yields of 60-70%.

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 combustible gas such as H2, hydrocarbon fuel such as propane, butane, etc., and 23 is a nozzle for supplying a glass raw material.
Nozzle for supplying inert gas such as r, He, N2, etc., 24
25 is a nozzle for supplying a combustion-supporting gas such as O2, and 25 is a nozzle for supplying an 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 the outer glass raw material (
or inert gas) supply nozzle, 27 is a flammable gas supply nozzle, 28 is an inert gas supply nozzle, 29 is a combustion-supporting gas supply nozzle, and these nozzles 26 to 29
are arranged concentrically 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 glass raw material supply nozzle 21 and
Each of the outer frit 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 combustible gas supply nozzles and combustion-supporting gas supply nozzles located outside thereof. 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.

Furthermore, 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, in a double flame burner shown in FIG. 9, the burner outer diameter was 53 mm and the regression distance l was 60 mm, and the manufacturing method of the present invention was applied to this burner to produce a porous base material. Give an example.

Example 1 Example 1 relates to the production of a porous base material consisting only of SiO2, and the double flame burner with the concentric multiple nozzle structure used in the above-mentioned flow velocity model experiment was used as the multiple flame burner. did.

In the double flame burner shown in FIG. 9, the outer flame flow velocity, inner flame flow velocity, and raw material layer flow velocity are set to 2.1 m/sec, 2.1 m/sec, and 0.1 m/sec, respectively.
The speed was set at 7 m/sec. Silicon tetrachloride 2200cc/min is supplied to the raw material supply nozzle using argon gas as a carrier.
was supplied. 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 preparation of the base material 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 having a refractive index distribution doped with GeO2. Here, in order to form germanium dioxide in solid solution, GeO2
It was necessary to lower the flame temperature compared to the case without adding. 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.

A double flame burner shown in FIG. 9, which is the same as in Example 1, was used as the burner, and the outer flame flow velocity, inner flame flow velocity, and raw material flow velocity were set to 2.0 m/sec, respectively.
2.0 m/sec and 0.8 m/sec, and as a glass raw material, in addition to silicon tetrachloride under the same conditions as in Example 1, germanium tetrachloride was supplied at 200 cc/min with argon gas as a carrier. Base material synthesis rate is 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.

[0045] Although the deposition rate can be further improved by supplying the glass raw material to the outer flame, there is no effect of extending the residence time of the synthesized glass fine particles in the flame in the outer flame. It was confirmed that the glass raw material yield with respect to the raw material supply amount decreased somewhat. 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.

[0046]

[Effects of the Invention] As explained above, according to the present invention,
By multiplexing the flames and causing the inner flame to regress more than the outer flame, large-sized base materials can be manufactured at high speed and stably. As a result, the base material manufacturing efficiency is improved and the cost of optical fibers can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing a flame multiplexing configuration.

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 velocities and regression distances at various parts in the dual flame burner according to the present invention.

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

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

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

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

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

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

[Explanation of symbols]

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 Nozzle for inner flame 6 Nozzle for outer flame 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 Combustible gas supply nozzle 23 Inert gas supply nozzle 24 Combustion-supporting gas supply nozzle 25 Inert gas supply nozzle 26 Outer glass Raw material (or inert gas) supply nozzle 27 Flammable gas supply nozzle 28 Inert gas supply nozzle 29 Combustion-supporting gas supply nozzle 30 Hood

Claims (1)

[Claims] Claim 1: A glass raw material is decomposed in a flame to form glass particles, the glass particles are deposited on a seed rod to form a porous base material, and the porous base material is made into transparent glass to form an optical fiber. In the method for manufacturing an optical fiber preform forming a base material for use, a raw material supply nozzle for supplying the glass raw material, and a raw material supply nozzle arranged sequentially around the raw material supply nozzle to sequentially form individual flame streams respectively. Using a burner having a plurality of flame forming nozzles, the k-th flame stream is regressed from the (k+1)-th flame stream outside the k-th flame, and at this time, the k-th flame The regression distance is determined so that the downstream end of the flow and the upstream end of the (k+1)th flame flow are substantially continuous, and the glass raw material is supplied into the resulting multiple flames to generate glass particles. A method for manufacturing an optical fiber preform, characterized in that: