JPH0577617B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing a single mode glass optical fiber preform. [Prior art and problems to be solved by the invention] Single-mode glass optical fiber base material (hereinafter referred to as
(simply referred to as fiber base material), the refractive index distribution of the core is step-shaped, and the thickness of the core layer is Therefore, it is necessary to increase the thickness of the cladding layer. Nowadays, there is a demand for lower loss, and it is considered desirable that the ratio of the thickness of the cladding layer to the thickness of the core layer (hereinafter referred to as cladding/core ratio) be at least 10 times or more. As a method for producing such a fiber base material, for example, a burner for core synthesis and a burner for cladding synthesis are arranged in the radial direction in an apparatus as shown in FIG. There is a method to obtain a single mode fiber matrix by simultaneously depositing the porous matrix of the cladding section and the cladding section. In this method, in order to increase the cladding/core ratio to 10 times or more, it is conceivable to keep the core portion constant and increase the cladding portion, or to keep the cladding portion constant and make the core portion smaller. However, if the cladding portion is made excessively large, the diameter of the entire porous base material becomes large and cracks are likely to occur in the porous base material. This tendency becomes particularly noticeable when the diameter of the porous base material exceeds 100 mmφ. Therefore, in order to stably produce a porous base material with good reproducibility while maintaining a clad/core ratio of 10 times or more, the diameter of the core porous body should be made as small as possible (preferably 10 mmφ or less), and the porous It is necessary to prevent the outer diameter of the base material from becoming too large. For this reason, various burners for core synthesis that form small-diameter core porous bodies have been developed (for example,
No. 56-54240), but no matter which burner was used, it was not possible to obtain a core porous body having a sufficiently small diameter. Another method of increasing the clad/core ratio to 10 times or more is to first manufacture a porous base material with a clad/core ratio of about 4 to 6 times, and then vitrify it and attach it externally to the outer periphery. A method of using a jacket tube after external attachment to compensate for the lack of cladding layer was considered, but there were problems such as complicating the process and reducing the mechanical strength when made into a fiber. It was not suitable for practical use. For this reason, the present inventors first installed a raw material gas outflow nozzle, two inert gas outflow nozzles provided at opposing positions with this raw material gas outflow nozzle in between, and a method that surrounded these three nozzles. We developed a burner for core porous material synthesis consisting of a gas outflow nozzle for forming an oxyhydrogen flame, and one of the two streams of glass particles obtained by this burner is used to form the core porous material. Invented a method for manufacturing a porous base material for optical fibers (Japanese Patent Application No. 1983-80372,
-180617, patent application No. 180618). However, as a result of intensive research in order to stably obtain a core porous material with a smaller diameter, the present inventors discovered that it is possible to do so by adjusting the flow rate of the gas flowing out from the nozzle. , we have completed the present invention. An object of the present invention is to be able to stably obtain a core porous body with a smaller diameter by using the above burner for synthesizing a core porous body. The present invention provides a method for manufacturing a porous base material for optical fiber. [Means for Solving the Problems] The present invention provides a method for pulling up a starting member while rotating it,
In this method for producing a porous base material for an optical fiber in which an aggregate of glass particles is formed at the tip of a starting member, a core porous material synthesis burner is used, and this burner has a raw material gas outlet nozzle and a raw material gas outlet nozzle. The above-mentioned oxyhydrogen flame forming gas outflow nozzle is composed of two inert gas outflow nozzles provided at opposite positions with the nozzle in between, and an oxyhydrogen flame forming gas outflow nozzle surrounding these three nozzles. injects three layers of gas flow inside and outside, the middle layer is an inert gas flow,
One of the remaining two is an oxygen gas flow and the other is a hydrogen gas flow, and the flow rate of the raw material gas flowing out from the raw material gas outflow nozzle and the inert gas flowing out from the intermediate layer of the oxyhydrogen flame forming gas outflow nozzle. The ratio of the flow rate to the flow rate is set to 1:0.3 to 3.0, and one of the two streams of glass fine particles obtained by such a burner is used for forming the core porous body. In the production method of the present invention, by setting the ratio of the flow rate of the raw material gas to the flow rate of the inert gas flowing out from the intermediate layer of the oxyhydrogen flame forming nozzle to 1:0.3 to 3.0, a good oxyhydrogen flame can be generated. condition is formed and only a portion of the feed gas stream is reacted with this well formed oxyhydrogen flame, thereby obtaining 2
One of the fine glass particle streams is applied to the starting member or the end face of the porous body, and the glass particles are deposited there, thereby making it possible to stably obtain a core porous body with a narrow diameter. The mechanism for the above will be explained in detail below. Generally, the glass raw material gas (mixed gas of SiCl ₄ and GeCl ₄ ) discharged from the raw material gas outflow nozzle of the burner for core synthesis is heated by an oxyhydrogen flame, and at the same time, the H ₂ O present in the oxyhydrogen flame is heated. At the same time, it undergoes a hydrolysis reaction with excess oxygen gas and a thermal oxidation reaction, resulting in fine particles of SiO ₂ and GeO ₂ . Among these, SiO ₂ directly reaches the starting member or the growth end face of the core porous body in the form of glass particles, and adheres and deposits thereon, whereas GeO ₂ decomposes in the high temperature part of the oxyhydrogen flame and becomes a gas. After reaching the growth end face of the core porous body, it is cooled and GeO ₂
It is thought that SiO ₂ precipitates on fine particles, and some of it becomes a solid solution. That is, only the portion of the raw material gas flow that undergoes a hydrolysis reaction or thermal oxidation reaction upon contact with the oxyhydrogen flame becomes fine particles forming the core porous body.
Therefore, even if the raw material gas flow coming out of the nozzle has a large diameter, if the part that comes into contact with the oxyhydrogen flame and substantially reacts is small, the diameter of the fine particle flow forming the core porous body can be reduced. This makes it possible to form a core porous body with a small diameter. In the core synthesis burner used in the present invention,
Two inert gas outflow nozzles are provided adjacent to both sides of the raw material gas outflow nozzle, and oxyhydrogen flame forming nozzles are provided to surround these three nozzles, so that the raw material discharged from the raw material gas outflow nozzle is The gas flow is divided into two streams by adjacent inert gas streams,
Only a portion of the stream comes into contact with the oxyhydrogen flame and reacts, creating two streams of fine glass particles. Furthermore, in the core porous material synthesis burner used in the present invention, the gas outflow nozzle for forming an oxyhydrogen flame injects gas flows in three layers, the inner and outer layers, the middle layer being an inert gas flow, and the remaining two layers being an inert gas flow. They are an oxygen gas flow and a hydrogen gas flow, respectively, and the ratio of the flow rate of the raw material gas flowing out from the raw material gas outflow nozzle to the flow rate of the inert gas flowing out from the intermediate layer of the oxyhydrogen flame forming gas outflow nozzle is 1: By adjusting from 0.3 to 3.0,
Oxyhydrogen flame can be formed in good condition.
Therefore, one of the two glass particle streams mentioned above
By applying a book to the starting member or the end face of the porous body and depositing glass fine particles thereon, a small-diameter core porous body can be stably formed. This will be further explained based on FIG. In the figure, 1 is a burner for core synthesis used in the present invention, 3 is a raw material gas outlet nozzle, 5 and 7 are inert gas outlet nozzles sandwiching the raw material gas outlet nozzle 3,
Reference numeral 9 denotes a gas outflow nozzle for forming an oxyhydrogen flame, which consists of an annular nozzle that surrounds each of these outflow nozzles and is partitioned into three layers: an innermost layer 11, an intermediate layer 13, and an outermost layer 15. Further, 17 is a schematic cross section showing the state of the gas emitted from each of the above-mentioned nozzles in an oxyhydrogen flame. Of these, 19 are oxyhydrogen flames surrounding the feed gas and adjacent inert gas flow; 2
1 and 21 are parts of the raw material gas flow that are in direct contact with the oxyhydrogen flame, 23 and 23 are parts of the raw material gas that are prevented from contacting the oxyhydrogen flame by the adjacent inert gas flow, and 25 and 25 are the raw material gases. An inert gas stream is shown adjacent to the gas stream. As is clear from FIG. 2, the gas flow flowing out from each nozzle advances while expanding slightly. Adjacent gas streams overlap and mix with each other around the contact area. Therefore, by arranging the two inert gas outlet nozzles 5 and 7 adjacent to both sides of the raw material gas outlet nozzle 3, a considerable portion of the raw material gas flow discharged from the raw material gas outlet nozzle 3 can be made unused. It is surrounded by active gas streams 25, 25 and is cut off from contact with the oxyhydrogen flame 19. Then, only the two parts 21, 21 that are not surrounded by the inert gas flows 25, 25 are exposed to the oxyhydrogen flame 1.
9, a hydrolysis reaction and a thermal oxidation reaction occur, producing two streams of fine glass particles. By setting the position and direction of the burner 1 so that one of the particles hits the starting member or the end face of the porous body (for example, Fig. 3), it is possible to create a core porous material with a small diameter that cannot be obtained with conventional methods. You can get a body. In order to stably obtain a thin stream of glass particles, it is necessary to form an oxyhydrogen flame in good condition.
For this purpose, the ratio of the flow rate of the raw material gas to the flow rate of the inert gas flowing out from the intermediate layer of the gas outflow nozzle for forming an oxyhydrogen flame must be 1:0.3 to 3.0. The flow rate of the inert gas is 1 for the raw material gas flow rate.
If it is less than 0.3, the glass particle flow becomes thicker,
The diameter of the core porous body formed also increases. Conversely, if the flow rate of the inert gas is greater than 3.0 with respect to the flow rate of 1 of the raw material gas, the reaction between oxygen and hydrogen will be inhibited, and the supply of H ₂ O necessary for the production of glass particles will be reduced. Therefore, sufficient glass particles are not produced, the absolute amount of glass particles in the glass particle flow decreases, and the growth rate of the core porous body decreases. Furthermore, the glass particles that have once adhered to the growth end of the core porous body are blown away by the inert gas with a high flow rate, and the growth of the core porous body is finally stopped. Note that the ratio of the flow rate of the raw material gas to the flow rate of the inert gas is preferably 1:0.5 to 2.0, more preferably 1:0.8 to 1.5. [Examples] The present invention will be further described below based on Examples, but the present invention is not limited to these Examples. FIG. 1 shows an example of a core porous body synthesis burner used in the present invention, in which 3 is a frit gas outflow nozzle, and 5 and 7 are provided at opposing positions with the raw material gas outflow nozzle 3 in between. The inert gas outflow nozzle 9 is a gas outflow nozzle for forming an oxyhydrogen flame that surrounds these three nozzles. The gas outflow nozzle for forming an oxyhydrogen flame in this embodiment is partitioned into a total of three layers: an innermost layer 11 for oxygen gas outflow, an intermediate layer 13 for inert gas outflow, and an outermost layer 15 for hydrogen gas outflow. There is. By configuring the oxyhydrogen flame forming gas outflow nozzle 9 in this way, inside the small-diameter core porous body,
This is preferable for forming a uniform GeO ₂ dopant concentration distribution. Note that a mixed gas of oxygen gas and inert gas may be flowed out from the oxygen gas outflow nozzle 11.
Further, a mixed gas of hydrogen gas and inert gas may be flowed out from the hydrogen gas outflow nozzle 15. When forming the core porous body in the present invention, the burner is heated so that one of the two glass fine particle streams 21, 21 obtained by the core synthesis burner 1 hits the starting member or the end face of the porous body. What is necessary is to set the position and direction of 1. An example of this is shown in FIG. Next, the results of manufacturing a porous preform for optical fiber by the method of the present invention will be shown. In the apparatus shown in FIG. 4, the position and orientation of the core porous body synthesis burner 1 shown in FIG. 1 are set as shown in FIG. The gases of the type and flow rate shown are respectively discharged, and are supplied as glass raw materials to the three cladding layer synthesis burners 31, 33, and 35.
SiCl ₄ gas was supplied at a flow rate shown in Table 3 to produce a porous base material 39 for a single mode optical fiber. Table 2 shows the diameter of the core portion 41 of the finished porous preform for optical fiber, the outer diameter of the preform, and the ratio of the cladding diameter to the core diameter after vitrifying the preform. In addition, in FIG. 4, 43 is a starting member, and 45 is a starting member.
47 indicates a rotation/lifting device, 47 indicates a protective container, and 49 indicates an exhaust regulator.

【table】

【table】

[Table] As shown in Table 2, in the porous base material for optical fiber obtained by the method of the present invention, the diameter of the core portion is 10 mm or less, and this base material is vitrified. The later clad/core ratio was more than 10 times. On the other hand, in Comparative Example 1, since the flow rate ratio of the raw material gas and the inert gas is 1:0.1, the diameter of the core portion is 10 mm or more, and the cladding/
The core ratio was also below 10. In Comparative Example 2,
The flow rate ratio of the raw material gas and the inert gas was 1:5.0, and since the inert gas flow rate was high, the generation of glass particles was inhibited, and the porous base material was not stably formed. [Effects of the Invention] The method of the present invention makes it possible to stably produce a core porous body having a diameter of 10 mm or less and having a uniform GeO ₂ concentration distribution. And at the same time,
A cladding layer consisting only of SiO ₂ fine particles is deposited around the outer periphery of this core porous body using a conventional burner, and after the outer diameter of the porous base material is approximately 100 mm, the entire body is made into transparent glass. , it becomes possible to easily produce a base material for a high-quality fully synthetic single mode optical fiber. Moreover, since such a base material for a fully synthetic optical fiber can be produced in a simple process using relatively small-scale equipment, the economical contribution is extremely large.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an example of a glass particle synthesis burner used in the present invention, FIG. 2 is a schematic diagram showing a mechanism in which a glass particle flow is formed by the burner of FIG. 1, and FIG. FIG. 1 is a schematic view showing how a core porous body is formed by the flow of glass particles formed by the burner, and FIG. 4 is a front view showing an example of an apparatus for producing a glass particle aggregate. Explanation of symbols, 1... Burner for core porous body synthesis, 3... Raw material gas outflow nozzle, 5, 7... Inert gas outflow nozzle, 9... Gas outflow nozzle for forming oxyhydrogen flame, 13... Acid Intermediate layer of gas outflow nozzle for hydrogen flame formation, 21...Glass particle flow, 39...
Porous base material for optical fiber, 41... core porous body, 43... starting member.

Claims (1)

[Claims] 1. In a method for producing a porous preform for an optical fiber in which a starting member is pulled up while rotating and an aggregate of glass particles is formed at the tip of the starting member, a core porous body synthesis burner is used, and this burner It consists of a gas outflow nozzle, two inert gas outflow nozzles provided at opposite positions sandwiching this raw material gas outflow nozzle, and an oxyhydrogen flame forming gas outflow nozzle surrounding these three nozzles, The gas outflow nozzle for forming an oxyhydrogen flame injects three layers of gas flow inside and outside, with the middle layer being an inert gas flow and the remaining two layers being an inert gas flow.
One is an oxygen gas flow and the other is a hydrogen gas flow, and the flow rate of the raw material gas flowing out from the raw material gas outflow nozzle and the flow rate of the inert gas flowing out from the intermediate layer of the oxyhydrogen flame forming gas outflow nozzle are the same. A porous base material for optical fiber, characterized in that the ratio is 1:0.3 to 3.0, and one of the two streams of glass particles obtained by such a burner is used to form a core porous body. Production method.