JPH0535692B2 - - 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 its problems] 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. 4, and glass raw materials having different compositions are ejected. There is a method of simultaneously depositing core and cladding porous preforms to obtain a single mode fiber preform. 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 unnecessarily enlarged, the diameter of the entire porous base material becomes large, and cracks are likely to occur in the porous base material. In particular, the diameter of the porous base material is 100mmφ
This tendency becomes more noticeable when the value exceeds . 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. In addition, once a porous base material with a clad/core ratio of about 4 to 6 is manufactured, it is vitrified and attached externally, or a jacket tube is used after external attachment to compensate for the lack of a clad layer. However, these methods were not suitable for practical use due to the problems of complicating the process and lowering the mechanical strength when made into a fiber. The purpose of the present invention is to solve the above problems,
It is an object of the present invention to provide a method for manufacturing a porous preform for an optical fiber, which makes it possible to obtain a core porous body having a uniform refractive index distribution and a sufficiently small diameter. [Means for solving the problems] The present invention provides a method for pulling up a starting member while rotating it,
In this method for manufacturing 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 raw material gas outlet nozzle and two 2 consisting of an inert gas outflow nozzle and an oxyhydrogen flame forming gas outflow nozzle surrounding these three nozzles.
This method is characterized in that one of the glass particle streams is used to form the core porous body. According to the manufacturing method of the present invention, one of the two thin glass particle streams obtained by reacting only a portion of the raw material gas stream with an oxyhydrogen flame is applied to the starting member or the end face of the porous body. By depositing glass particles thereon, a core porous body with a narrow diameter can be obtained. This mechanism will be explained in detail below. Generally, the raw material gas (mixed gas of SiCl ₄ and GeCl ₄ ) coming out of 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 or Performs a hydrolysis reaction or thermal oxidation reaction with excess O ₂ gas,
It becomes fine particles of SiO ₂ and GeO ₂ . Of these, SiO ₂
GeO 2 directly reaches the starting member or the growth end face of the core porous body in the form of glass fine particles, and adheres and deposits thereon, but GeO ₂ decomposes and gasifies in the high temperature part of the oxyhydrogen flame. It is thought that it reaches the vicinity of the growth end face of the core porous body, is cooled and precipitates again as SeO ₂ on the SiO ₂ fine particles, and a part 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. By applying one of the glass particle streams to the starting member or the end face of the porous body and depositing the glass particles thereon, a small-diameter core porous body can be 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,
9 is a gas outflow nozzle for forming an oxyhydrogen flame that surrounds each of these outflow nozzles. 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 the oxyhydrogen flame surrounding the flow of the raw material gas and the adjacent inert gas, 21, 21 are the parts of the raw material gas flow that are in direct contact with the oxyhydrogen flame, and 23, 23 are the adjacent inert gases of the raw material gas The portions 25, 25 where contact with the oxyhydrogen flame is prevented by the gas flow indicate the inert gas flow adjacent to the source gas flow. 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 enveloped by the active gas flows 25, 25, and contact with the oxyhydrogen flame 19 is cut off. and inert gas flow 25, 25
Two non-enclosed parts 2 that are not included by
Only 1 and 21 come into direct contact with the oxyhydrogen flame 19 to cause a hydrolysis reaction and a thermal oxidation reaction, creating two streams of fine glass particles. By setting the position and direction of the burner 1 so that one of the particulate streams hits the starting member or the porous end face (for example, Fig. 3), a core porous body with a small diameter that cannot be obtained by conventional methods can be created. can be obtained. In addition,
In the portion of the raw material gas flow other than the non-enclosed portions 21 and 21, the raw material gas flow and the inert gas flow overlap and the gases are mixed, and the concentration of the raw material gas is diluted, so that hydrolysis reaction and heat The oxidation reaction does not proceed and the glass particle flow does not spread any further. [Examples] The present invention will be described below based on Examples, but the present invention is not limited to these Examples. FIG. 1 shows an example of a burner for core porous synthesis used in the present invention, where 3 is a frit gas outflow nozzle, and 5 and 7 are provided at opposite 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 surrounding these three nozzles. In this embodiment, the gas outflow nozzle for forming an oxyhydrogen flame is divided into three layers: an O ₂ gas outflow nozzle 11 in the innermost layer, an inert gas outflow nozzle 13 in the middle layer, and an H ₂ gas outflow nozzle in the outermost layer. An outflow nozzle 15 is provided. 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. The reason for this will be explained based on FIG. Figure 3 schematically shows the mechanism by which the concentration distribution of GeO ₂ is formed inside the small-diameter porous body, where 27 is the burner outlet, 21, 21 is a narrow flow of glass particles, and 29 is a The growth end portions of the core porous body are shown respectively. In Fig. 3, the part of the raw material gas flow coming out of the burner outlet 27 that comes into contact with the oxyhydrogen flame becomes fine particles of SiO ₂ and GeO ₂ at a temperature of 600°C or higher mainly through the hydrolysis reaction shown below. . SiCl ₄ (G) + 2H ₂ O(G) →SiO ₂ (S) + 4HCl(G) GeCl ₄ (G) + 2H ₂ O(G) → GeO ₂ (S) + 4HCl(G) SiO ₂ and GeO ₂ thus generated After further passing through a high-temperature flame of 1000 to 1400°C, the raw material gas flow containing 1000 to 1400°C reaches the growth end face of the core porous body, where it is cooled to 600 to 800°C. Among the above fine particles, SiO ₂ is stable even at high temperatures, so it arrives as it is in the form of solid fine particles on the growth end face of the core porous body and is deposited, but GeO ₂ fine particles are unstable at high temperatures, so they are deposited at temperatures above 800°C. decomposes into GeO gas as shown below. GeO ₂ (S)→GeO(G)+1/20 ₂ (G) Then, this GeO gas is cooled at the same time as it arrives at the growth end face of the core porous body, and as shown in the following equation, SiO ₂ is converted into SeO ₂ fine particles. It precipitates on the surface of fine particles, and some of it becomes a solid solution. GeO(G)+1/20 ₂ (G)→GeO ₂ (S) Therefore, depending on where on the surface of the core porous body the precipitation reaction of GeO ₂ due to the oxidation of GeO gas occurs, The concentration distribution of GeO ₂ will be affected. In this embodiment, the raw material gas outflow nozzle 3 and the inert gas outflow nozzles 5 and 7 adjacent thereto are
First, an O ₂ gas outflow nozzle 11 is installed on the outside of the
An inert gas outflow nozzle 13 is provided on the outside, and an H ₂ gas outflow nozzle 15 is provided in the outermost layer, so that the flow of O ₂ gas necessary for oxidizing GeO gas is
By being adjacent to the flows 21, 21 of glass fine particles, the precipitation of GeO ₂ on the surface of the core porous body is concentrated near the growth end face of the core porous body, and the H ₂ flowing to the outer periphery of the oxyhydrogen flame is The gas suppresses the oxidation and precipitation reactions of the GeO gas that has arrived at areas other than the growth end face of the core porous body. In this way, the O ₂ gas outflow nozzle 11 is provided so that the O ₂ gas flow comes into contact with the raw material gas flow, and the H ₂ gas outflow nozzle 11 is provided so that the H ₂ gas flow comes into contact with the outer periphery of the oxyhydrogen flame. 15 at a position farthest from the raw material gas outflow nozzle 3, it is possible to form a uniform concentration distribution of GeO ₂ inside the small-diameter core porous body. Note that the gas compositions flowing out from the O ₂ gas outflow nozzle 11 and the H ₂ gas outflow nozzle 15 are defined as a mixed gas of O ₂ gas and inert gas, and a mixed gas of H ₂ gas and inert gas, respectively. Also good. In this way, it is possible to prevent a rise in temperature of the side surface of the growth end of the core porous body that is hit by the oxyhydrogen flame, and to further effectively suppress the oxidation reaction of the GeO gas in this portion. 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, and examples thereof include those shown in FIGS. 3 and 5. (A in FIG. 5 is a side view of the core synthesis burner 1 seen from the left side of the figure) In this embodiment, the raw material gas outlet nozzle 3 and the inert gas outlet nozzle 5, 7 of the core synthesis burner 1 are The burner 1 is oriented so that the glass particles are lined up in the horizontal direction, and two upper and lower glass particle streams 21, 21 are formed. And the upper particulate flow 2
The position and orientation of the burner 1 are set so that the burner 1 hits the end face of the porous body. In this case, the angle at which the glass particle flow 21 hits the end face of the porous body is not particularly limited, but the angle (θ) is 45 ± 5° with respect to the central axis 37 of the starting member or the porous body at the center of the end face of the porous body. It is preferable to hit with . 6 and 7 show the core synthesis burner 1 set in another direction, and the burner 1
The burner 1 is oriented such that the raw material gas outflow nozzle 3 and the inert gas outflow nozzles 5 and 7 are aligned in the vertical direction. (A in FIG. 6 is a side view of the core synthesis burner 1 viewed from the left side of the figure) In this case, as is clear from FIG. 7, which shows the state of FIG. 6 viewed in the line direction, Two glass particle streams 21, 21 are formed on the left and right sides. The position and orientation of the burner 1 are set so that the right part of the particle flow 21 hits the end face of the porous body. In this case as well, it is preferable that the glass particle stream 21 hits the center of the end face of the porous body at an angle (θ) of 45±5° with respect to the central axis 37 of the starting member or the porous body. Note that 2 formed by the core synthesis burner 1
One of the glass particle streams 21, 21 of the book is
It is only necessary to hit the end face of the porous body, and it is also possible to set the banner 1 in a position and direction other than the above. 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 core porous material synthesis burner 1 shown in FIG. 1 is set in the position and direction shown in FIGS. At the same time, the three cladding layer synthesis burners 31, 33, and 35 were supplied with SiCl ₄ as a glass raw material.
Gas was supplied at the flow rates shown in Table 2 to produce a porous base material for a single mode optical fiber. In addition, in FIG. 4, 43 is a starting member, 45
47 indicates a rotation/lifting device, 47 indicates a protective container, and 49 indicates an exhaust regulator.

【table】

【table】

[Table] Fig. 8 shows the shape of the porous base material 39 produced in this way.The diameter of the core portion 41 of the porous base material 39 is 7 mm, and the outer diameter of the porous base material 39 is
It was 100mm. The obtained porous base material 39 was heated to 1500
It was heated at ℃ to make it transparent and vitrified to obtain a fiber base material. The refractive index distribution of this base material was as shown in FIG. In Figure 9, the clad diameter is 48mm and the core diameter is
3.2 mm, the ratio of the cladding diameter to the core diameter is sufficiently large at 15.0 times, and the relative refractive index difference △ is 0.3% for the gas conditions in Table 1 (a), and for the gas conditions in Table 1 (b). In both cases, the refractive index distribution in the core portion was almost step-like, and could be used as a fully synthetic single-mode optical fiber that did not require a jacket tube. . Next, in order to compare with the example of the present invention, the fourth
In the apparatus shown in the figure, the burner disclosed in JP-A-56-54240 (FIG. 10) was used as a burner for core synthesis, and a porous base material having an outer diameter of 100 mm was prepared. In this case, the respective flow rates of raw material gas, inert gas, O ₂ gas, and H ₂ gas supplied to the core synthesis burner and the three clad layer synthesis burners are
The flow rate of SiCl ₄ gas was the same as in the above example.
The outer diameter of the core porous body prepared in this way was 17mmφ
It was hot. The obtained porous base material was transparently vitrified in the same manner as in the example to obtain a fiber base material. The refractive index distribution of this base material was as shown in FIG. In FIG. 11, the cladding diameter was 48 mm, the core diameter was 7.2 mm, and the ratio of the cladding diameter to the core diameter was 6.7 times. The relative refractive index difference △ at the center of the core is
0.3%, but the refractive index distribution in the core part was highly irregular, and in order to draw this base material and make it into a fiber, it was necessary to use an external attachment method, or use a jacket tube after external attachment. We need to increase the clad layer. Table 3 shows the fibers obtained by drawing the fully synthetic matrix (Fig. 8) synthesized by the method of the present invention.
Fibers obtained by drawing a fully synthetic base material externally attached to a base material according to a comparative example (Fig. 11), and externally attached to a base material according to a comparative example (Fig. 11), and further jacketed with a quartz tube. The results of measuring the optical loss value at a wavelength of 1.5 μm of the fiber obtained by drawing the base material are shown.

[Table] The fiber obtained from the fully synthetic base material produced by the present invention clearly has a lower loss value than the fiber obtained from the base material produced by the conventional method, especially the value at a wavelength of 1.5 μm. , both 0.2dB/
Km or less, clearly reducing the loss. In the above embodiment, a rectangular burner for core synthesis was used, but a circular or elliptical burner as shown in FIGS. 12 and 13 may also be used. [Effects of the Invention] The method of the present invention makes it possible to produce a core porous body having a diameter of 7 to 8 mm and having a uniform GeO ₂ concentration distribution. At the same time, a cladding layer consisting only of SiO ₂ fine particles was attached and formed around the outer periphery of this core porous body using a conventional burner, and the outer diameter of the porous base material was set to about 100 mm.
By making the entire fiber transparent, it becomes possible to easily produce a base material for a fully synthetic single mode optical fiber. Furthermore, the fiber obtained by drawing the base material for a fully synthetic optical fiber synthesized according to the present invention has superior mechanical strength compared to a fiber produced using a quartz jacket tube. Furthermore, in terms of transmission characteristics, it has the advantage that transmission loss due to Leley scattering is lower than that of fibers with irregular refractive index distribution. Moreover, since such a high-quality preform for a fully synthetic optical fiber can be produced in a simple process using relatively small-scale equipment, the degree of economic contribution is extremely large.

[Brief explanation of drawings]

FIG. 1 is a structural 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 diagram showing a mechanism for uniformly doping the cross section of a small-diameter core porous body formed by a burner with GeO ₂ concentration, and Fig. 4 is a front view showing an example of an apparatus for producing a glass particle aggregate. , FIG. 5 is a side view showing an example of the position and orientation of the burner when forming a core porous body using the burner shown in FIG. 7 is a side view showing the state seen in the line direction of FIG. 6, FIG. 8 is a shape diagram of the porous base material obtained by the present invention, and FIG. 9 is a side view showing the state seen in the line direction of FIG.
FIG. 10 is a refractive index distribution diagram of the glass base material obtained by the present invention, and FIG. FIG. 12 is a structural diagram showing another example of the synthetic burner used in the present invention, and FIG. 1 is a structural diagram showing still another example. It is. 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,
21... Glass fine 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 base material 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 raw material gas outflow nozzle and a relative member with the raw material gas outflow nozzle sandwiched therebetween. Two glasses obtained by a core porous body synthesis burner consisting of two inert gas outflow nozzles placed facing each other and a gas outflow nozzle for forming an oxyhydrogen flame surrounding these three nozzles. A method for producing a porous base material for an optical fiber, characterized in that one of the fine particle streams is used to form a core porous body.