JPH0477690B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a single mode glass optical fiber preform.

(Prior art) Single mode glass optical fiber base material (hereinafter referred to as
(simply referred to as fiber base material), the shape of the refractive index distribution of the core is a step type, and in order to reduce the influence of the spread of light energy propagating through the core to the cladding layer, the thickness of the core layer is Therefore, it is necessary to increase the thickness of the cladding layer.

In today's world where there is a demand for lower loss, it is 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 plurality of burners are arranged in the radial direction in an apparatus as shown in FIG. There is a method of simultaneously depositing fiber preforms to obtain a single mode fiber preform.

Although this method can certainly obtain a cladding/core ratio of 10 times or more and an optical fiber with low loss, it is inevitable that the porous base material will have a large diameter.
As a result, cracks are likely to occur in the porous base material,
There is a problem that a porous base material cannot be manufactured stably with good reproducibility. Therefore, in order to stably produce a porous base material with good reproducibility, the diameter of the core porous body should be made as small as possible, the outer diameter of the porous base material should be made relatively thin, and the clad/core ratio should be increased. It needs to be 10 times or more.

Incidentally, according to the results of the studies conducted by the present inventors, this kind of problem does not occur much with porous base materials of 100 mmφ or less;
It has been found that cracks are more likely to occur if the

However, until now, there has been no method for manufacturing a porous base material that fully satisfies the above requirements, and in order to reduce the diameter of the core porous body, a porous base material with a clad/core ratio of about 4 to 6 times has been created. After manufacturing and making transparent glass, a porous layer is formed around the outer periphery of the transparent base material (external attachment), or after external attachment, a jacket tube is used to make up for the deficiency in the cladding layer.
A base material with a core ratio of more than 10 times was obtained.

However, the refractive index distribution of the core portion of the base material obtained in this way tends to be irregular as shown in Figure 8, and furthermore, when a jacket tube is used, the mechanical strength when made into a fiber is not always satisfactory. There was a problem in that it was not possible to obtain a value that was too high.

(Problems to be Solved by the Invention) As described above, when a fiber base material is produced using a plurality of glass fine particle synthesis burners, in the case of the conventional core synthesis method, it has a uniform refractive index distribution and Because it is difficult to form a porous material with a sufficiently small diameter, it is difficult to produce a base material that can yield a fiber with a large cladding/core diameter ratio and excellent transmission characteristics through a simple process. There was a problem.

The purpose of the present invention is to solve such technical problems by reducing the outer diameter of the porous body obtained with conventional burners to 10 mm or less, and solving the problem of irregular refractive index distribution. The purpose of the present invention is to provide a method for manufacturing a fully synthetic porous base material without using a jacket tube or the like.

(Means for Solving the Problems) The present inventors have completed the present invention as a result of intensive studies to solve the above problems.

That is, in the method of manufacturing a fiber base material, the present invention removes glass fine particles forming a glass fine particle aggregate by using a burner provided with inert gas nozzles adjacent to both sides of a raw material gas outlet nozzle to remove only a portion of the raw material gas flow. The method is characterized in that a flow of fine glass particles is produced by reacting with an acid-hydrogen flame and deposited on the starting member or the end face of the porous body.

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 acid-hydrogen flame, and at the same time the H ₂ O present in the flame is heated. or surplus
It undergoes a hydrolysis reaction or thermal oxidation reaction with O ₂ gas to become SiO ₂ and GeO ₂ . Of 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 and gasifies in the high-temperature part of the flame. It is thought that it reaches the vicinity of the growth end face of the core porous body, is cooled, and is precipitated onto the SiO ₂ fine particles as GeO ₂ again, with some of it becoming a solid solution.

The present invention basically introduces the above reaction process.

In the burner used in the present invention, the inert gas nozzles are provided adjacent to both sides of the raw material gas outflow nozzle. A method in which only a portion of the glass particles is reacted with an acid-hydrogen flame to create a flow of fine glass particles, which are deposited on the starting member or on the end face of the porous body to reduce the diameter of the core porous body formed. It is. This will be explained based on FIG.

FIG. 4 schematically shows the diameter reduction mechanism of this glass particle flow. In the figure, 12 is the outflow nozzle for raw material gas, and 16 and 17 are the outflow nozzles 12.
an inert gas outflow nozzle adjacent to the inert gas outflow nozzle, 18 is _O2 gas surrounding each of these outflow nozzles, inert gas,
The outflow nozzle part of _H2 gas is shown. In addition, 19 shows a schematic cross section of the gas formed in the acid-hydrogen flame by the gas emitted from each nozzle, of which 20 shows the acid-hydrogen flame surrounding the source gas and the adjacent inert gas flow. Hydrogen flame, 21 is a portion of the raw material gas flow that is in direct contact with the acid-hydrogen flame, 22 is a portion of the raw material gas that is prevented from contacting the acid-hydrogen flame by the adjacent inert layer gas flow, 23
indicates an inert gas stream adjacent to the feed gas stream.

As is clear from FIG. 4, by arranging the inert gas nozzles adjacent to both sides of the raw material gas nozzle, a considerable portion of the raw material gas flow exiting from the raw material gas outlet nozzle is enveloped by the inert gas flow 23. The acid-hydrogen flame 20
The reaction with the acid is blocked, and only the non-encapsulated portion 21
Direct contact with the hydrogen flame 20 promotes hydrolysis and thermal oxidation reactions, creating a stream of fine glass particles. If the burner position is set so that this flow is deposited on the starting member or the end face of the porous body,
A core porous body with a small diameter that cannot be obtained by conventional methods can be obtained.

In this way, in the burner used in the present invention, rather than simply narrowing the raw material gas outflow nozzle to narrow the flow of glass particles, the burner attempts to obtain thin glass particles by reacting only a part of the raw material gas flow. It is something to do. Even if an attempt is made to obtain a thin glass particle flow by making the raw material gas outflow nozzle smaller, the flow of glass particles will become faster and the glass particles will collide with the starting member or the growth end of the core porous body. Diffusion becomes large, and the diameter of the resulting core porous body cannot be reduced.

By the way, the nozzle for outflowing O ₂ gas or H ₂ gas to obtain the acid-hydrogen flame 20 described above is configured to provide O ₂ gas around the outer periphery of the raw material gas outflow nozzle 12 and the inert gas outflow nozzles 16 and 17 adjacent thereto. As described above, an outflow nozzle for a gas or a mixed gas of _O2 gas and an inert gas is provided in this order, followed by an outflow nozzle for an inert gas, and an outflow nozzle for a _H2 gas or a mixed gas of _H2 gas and an inert gas. This is preferable for forming a uniform GeO ₂ dopant concentration distribution inside the small-diameter core porous body.

The reason for this will be explained based on FIG.

Figure 5 shows the inside of this narrow core porous body.
This figure schematically shows the mechanism by which the concentration distribution of GeO ₂ is formed, and 24 is the burner outlet, 21 is a narrow flow of glass particles before and after, and 25 is the growth end portion of the core porous body. In FIG. 5, the portion of the raw material gas flow exiting from the burner outlet 24 that comes into contact with the acid-hydrogen flame becomes fine particles of SiO ₂ and GeO ₂ at a temperature of 600° C. or higher mainly through the hydrolysis reaction described 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 The raw material gas stream containing
Cooled to ~800℃. Among the above fine particles generated in the relatively low temperature part of the flame, SiO ₂ is stable even in the high temperature part, 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.
_GeO2 fine particles are unstable at high temperatures, so
The above decomposes into GeO gas as follows.

GeO ₂ (S)→GeO(G)+1/20 ₂ (S) This GeO gas is cooled at the same time as it reaches the growth end face of the core porous body, and due to the above reverse reaction,
Precipitates on the surface of SiO ₂ particles as GeO ₂ particles,
A part of it dissolves in solid solution.

GeO(G)+1/20 ₂ (G)→GeO ₂ (G) Depending on where on the surface of the core porous body the GeO ₂ precipitation reaction due to the residual GeO gas occurs, the GeO ₂ inside the soot The concentration distribution of

In the burner used in the present invention, an O ₂ gas outflow nozzle is first provided outside the raw material gas outflow nozzle 12 and the inert gas outflow nozzles 16 and 17 adjacent thereto, and H2 gas is supplied to the outermost layer through the inert gas outflow nozzle. ₂ It is preferable to provide a gas outflow nozzle because it is necessary to oxidize the GeO gas mentioned above.
By placing O ₂ gas adjacent to the flow of glass 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 then with H ₂ gas flowing to the outer periphery of the flame, This is because its reducing power suppresses the oxidation and export reactions of GeO ₂ gas that has arrived at parts other than the growth end face of the core porous body.

In this way, an O ₂ gas outflow nozzle is provided at a position where O ₂ gas contacts the raw material gas flow, and an H ₂
By providing the H ₂ gas outflow nozzle at the farthest position from the raw material gas outflow nozzle so that the gas is in contact with the outer periphery of the flame, a uniform GeO ₂ concentration distribution is achieved inside the small-diameter core porous body. can be formed.

In addition, in the present invention, the above O ₂ gas and H ₂
The gas composition flowing out from the gas outflow nozzle is
A mixed gas of O ₂ gas and inert gas or a mixed gas of H ₂ gas and inert gas may be used. In this way, it is possible to prevent a rise in temperature on the side surface of the growth end of the core porous body that is hit by the flame, and to suppress the oxidation raw material of GeO gas in this part more effectively.

Further, in the present invention, normally one inert gas nozzle is provided on each side of the raw material gas outflow nozzle, but two or more inert gas nozzles may be provided. Furthermore, although a square glass particle synthesis burner is usually used, the same effect can be obtained by using a circular or elliptical synthesis burner as shown in FIGS. 1B and 1C.

(Examples) Hereinafter, the present invention will be further explained based on the present invention, but the present invention is not limited only to these Examples.

FIG. 1 shows a glass particle synthesis burner 26 according to an embodiment of the present invention, in which 12 is a frit gas outflow nozzle, 16 and 17 are inert gas outflow nozzles, and 15 is an inert gas outflow nozzle.
O ₂ outflow nozzle, 13 is an inert gas outflow nozzle. Further, Table 1 shows the flow rate of gas discharged from each gas discharge nozzle of this burner 26. In the apparatus shown in FIG. 6, the core synthesis burner 1 is replaced with the synthesis burner 26 of the present invention, SiCl ₄ and GeCl ₄ are supplied from the center nozzle 12, Ar gas is supplied from the nozzles 16 and 17 adjacent to the center nozzle 12, Ar gas is From nozzle 14
A porous base material for a single mode optical fiber was prepared by letting H ₂ gas flow out.

Table 2 shows the materials supplied as glass raw materials to the three cladding layer synthesis burners 2, 3, and 4 shown in Figure 6.
This shows the flow rate of SiCl ₄ gas. Figure 2 shows
This figure shows the shape of the porous base material produced in this way, and the core porous body diameter was 7 mm and the outer diameter of the porous body was 100 mm. The obtained porous base material was heated to 1500℃
The material was heated to make it transparent and vitrified to form a fiber base material.
The refractive index distribution of this base material was as shown in FIG.

In Figure 3, 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% under the gas conditions shown in Table 1 a, and 0.3% under the gas conditions shown in Table 1 b. was 1.0%, and in both cases, the refractive index distribution of 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 embodiment of the present invention, a burner whose structure is shown in FIG. 7 described in JP-A No. 56-54240 was used for core synthesis in the apparatus shown in FIG. A porous base material with an outer diameter of 10 mm was similarly prepared using it as a burner. In this case, the raw material gas, inert gas, and
The flow rates of each of O ₂ gas and H ₂ gas and the flow rate of SiCl ₄ gas supplied to the three cladding layer synthesis burners were the same as in the above example. The outer diameter of the porous base material thus produced was 17 mmφ. 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 Figure 8, the clad diameter is 48mm and the core diameter is
The diameter was 7.2 mm, and the ratio of the cladding diameter to the core diameter was 6.7 times. The relative refractive index distribution difference △ at the center of the core is
0.3%, but the refractive index distribution in the core part is highly irregular, and in order to draw this base material and make it into a fiber, it is necessary to use an external method or to apply a cladding layer using a jacket tube after external application. It is necessary to increase

Table 3 shows the fiber obtained by directly drawing the fully synthetic base material according to the present invention (Figure 3), the fully synthetic base material externally attached to the base material according to the comparative example (Figure 8), and the base material according to the comparative example. The results of measuring the optical loss value at a wavelength of 1.55 μm of the fiber obtained by drawing the quartz tube and the jacket-processed base material, respectively, are shown in FIG. 8. The fiber obtained from the fully synthetic base material according to the present invention clearly has a lower loss value than the fiber obtained from the base material produced by the conventional method. In particular, the loss value at a wavelength of 1.55 μm is 0.2 dB/ Km or less, the loss was clearly reduced.

(Effects of the Invention) By using the glass fine particle synthesis burner according to the present invention, it is possible to produce a core porous body having a diameter of 7 to 8 mm and having a uniform GeO ₂ concentration distribution. Therefore, around the outer periphery of this core porous body
By depositing and forming a cladding layer consisting only of SiO ₂ fine particles using a conventional burner, creating a porous base material with an outer diameter of approximately 100 mm, and then converting the entire material into transparent glass, it is easy to create a fully synthetic single-mode optical fiber. It becomes possible to produce a base material for use.

By drawing the base material for a fully synthetic optical fiber obtained by the present invention to make a fiber, it has superior mechanical strength compared to a fiber using a quartz jacket tube, and also has good refraction properties in terms of transmission characteristics. It has the advantage that transmission loss due to Leley scattering is reduced compared to fibers with irregular rate distribution. Moreover, since such a high-quality base material 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 the drawing]

Fig. 1 is a glass particle synthesis burner according to an embodiment of the present invention, A is a structural diagram of a square burner, B is a structural diagram of a circular burner, C is a structural diagram of an elliptical burner, and Fig. 2 is a structural diagram of an elliptical burner. FIG. 3 is a diagram of the shape of the obtained porous base material, FIG. 3 is a refractive index distribution diagram of the glass base material of the present invention, and FIG. 4 is a diagram showing the diameter reduction of the glass particle flow forming the core portion by the burner according to the present invention. A schematic diagram of the mechanism; FIG. 5 is a schematic diagram of the mechanism for uniformly doping the cross section of the small-core porous body formed by the burner according to the present invention with GeO ₂ concentration; FIG. 6 is the production of a glass particle aggregate The apparatus, Fig. 7 shows the burner for producing a small-diameter porous body proposed in JP-A No. 56-54240, and Fig. 8 shows the burner for producing a small porous body.
It is a refractive index distribution map of the glass base material obtained with the burner of a figure. Explanation of symbols 1... Burner for core synthesis, 2,
3, 4... Burner for forming a cladding layer, 5... Glass fine particle porous body, 6... Core porous body, 7...
Clad layer porous body, 8... Starting member, 9... Rotating/pulling device, 10... Protective container, 11... Exhaust regulator, 12... Glass raw material outflow nozzle, 13...
...Inert gas outflow nozzle, 14... _H2 gas outflow nozzle, 15... _O2 gas outflow nozzle, 16,1
7...Inert gas outflow nozzle adjacent to frit outflow nozzle 12, 18...Nozzle 12 Nozzle 1
A collective part of the outer layer nozzle surrounding 6 and 17, 19
...Schematic cross-sectional view of the gas formed in the acid-hydrogen flame, 20...Acid-hydrogen flame section surrounding the source gas and inert gas, 21...Glass in the source gas that is in direct contact with the acid-hydrogen flame Particulate flow, 22...
...part of the raw material gas that is prevented from contacting the acid-hydrogen flame, 23...inert gas flow, 24...
... Burner outlet, 25 ... Growth end portion of core porous body, 26 ... Glass fine particle synthesis burner according to the present invention.

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Claims (1)

[Claims] 1. In a method for producing a glass base material for an optical fiber, in which high-temperature glass particles are rotated and grown while being deposited on the tip of a pulled-up starting member to obtain a glass particle aggregate, the glass particles forming the glass particle aggregate are grown as a raw material. A burner with inert gas nozzles adjacent to both sides of the gas outlet nozzle
A method for producing a porous glass preform for an optical fiber, characterized in that only a portion of the raw material gas flow is reacted with an acid-hydrogen flame to form a flow of fine glass particles, which are deposited on the starting member or the end face of the porous body. .