JPS61236626A - Production of base material for optical fiber - Google Patents

Abstract

PURPOSE:To obtain a base material for an optical fiber wherein fluorine is doped in a synthesized glass layer uniformly and efficiently by decreasing gradually the amount of fluorine added in a raw material in case of accumulating a porous glass layer on an outside periphery of a quartz series glass rod. CONSTITUTION:The fine glass grains injected from a reaction burner 2 are successively accumulated on an outside periphery of a quartz series glass rod 7 and a porous glass layer 8a is formed. In this case, the feed amount of a doped raw material for the reaction burner 2 is gradually decreased and the following porous glass layer 8a is formed wherein fluorine (or a fluorine compd.) is contained in large quantities as the position is near to the outside peripheral surface of the glass rod 7 and the fluorine content is small as the position is further from the outside periphery. The quartz series glass rod 7 stuck with the porous glass layer 8a is inserted to the inside of a furnace core pipe 5 of a transparent vitrification furnace 4 to vitrify transparently the porous glass layer 8a. In a transparent glass layer 8b, the distribution of fluorine is uniform in the diameter direction and therefore the required base material for an optical fiber is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for manufacturing optical fiber preforms suitable for communication applications.

1. Prior Art As is known, in the external VAD method (OVD method), a porous glass layer is deposited on the outer periphery of a high-purity quartz rod, a synthetic quartz glass rod, etc., and then the porous glass layer is Generally, transparent glass is formed by high-temperature treatment, and when doping fluorine into the deposited glass layer in such OVD method,
Conventionally, methods such as hoggi have been adopted.

One method is to add fluorine or a fluorine compound to the glass raw material used to deposit and form the porous glass layer, and deposit a fluorine-doped silica-based porous glass layer around the outer periphery of a silica-based glass rod. It is.

The other method is to convert a porous glass layer deposited around the periphery of a quartz-based glass rod into transparent vitrification in an inert gas atmosphere containing fluorine or a fluorine compound, and at the same time as converting the glass layer to transparent vitrification. This method involves doping fluorine inside.

"Problems to be Solved by the Invention" In the former method described above, the deposition rate and deposition efficiency of glass particles in the OVD method decrease due to the etching effect of fluorine, and there are other problems in the glass layer during transparent vitrification. Since fluorine volatilizes from the glass layer, the fluorine distribution in the radial direction of the glass layer is not uniform.

In the case of the latter method, the amount of etching is small, but on the other hand, the porous glass layer near the outer periphery of the quartz-based glass rod is damaged early (before receiving a predetermined amount of fluorine doping) by the heat from the glass rod. Therefore, this method also does not provide a uniform fluorine distribution in the radial direction of the glass layer.

In this way, each of the conventional methods cannot produce a glass layer that has a refractive index lower than that of quartz and exhibits a flat refractive index distribution. When making a mode optical fiber,
Since the refractive index distribution of the cladding is not flat, light leaks from that part.

Therefore, unless the composite rad/core ratio is set to a theoretical value or higher, a low-loss optical fiber cannot be obtained, and there is also the problem that bending loss of the optical fiber is likely to occur.

In view of the above-mentioned problems, the present invention seeks to provide a method for manufacturing an optical fiber preform, which allows fluorine to be doped uniformly and efficiently into a synthetic glass layer manufactured by an OVD method.

``Means for Solving the Problems J'' The present invention forms a porous glass layer by depositing glass fine particles produced by mixing and burning glass raw materials and combustion gas on the outer periphery of a rotating quartz-based glass rod. After that, in the method for producing an optical fiber base material, the porous glass layer is treated at high temperature in an atmosphere containing fluorine or a fluorine compound to become transparent vitrified. When doing so, fluorine or a fluorine compound is added to the glass raw material, and as the deposition of the porous glass layer progresses, the amount of fluorine or fluorine compound added to the glass raw material is gradually reduced. There is.

"Operation" In the method of the present invention, fluorine or a fluorine compound is added to the glass raw material when depositing and forming a porous glass layer around the outer periphery of a silica-based glass rod, and as the deposition of the porous glass layer progresses, Gradually reduce the amount of fluorine or fluorine compounds added to the glass raw material.

In the case of the porous glass layer formed in this way, the portion close to the outer circumferential surface of the silica-based glass rod has a large amount of fluorine doped, and the portion far from the outer circumference of the silica-based glass rod has a large amount of fluorine doped.
The amount of dope is reduced.

Next, when the porous glass layer is made into transparent glass by high-temperature treatment in an atmosphere containing fluorine or a fluorine compound, the amount of fluorine doped is The amount of fluorine doped increases in the portions farther away from the outer periphery of the quartz-based glass rod.

- As mentioned above, when forming a transparent glass layer around the outer periphery of a quartz-based glass rod through two predetermined steps, in the first step the amount of fluorine doped increases from the outer periphery toward the center, and in the next step Since the amount of fluorine doped increases from the center toward the outer periphery, these two types of fluorine doping make the fluorine distribution in the radial direction of the glass layer uniform.

Moreover, the amount of fluorine doped when forming the porous glass layer is sufficient to compensate for the uneven distribution of fluorine during transparent glass formation, so the glass etching effect due to fluorine during the formation of the porous glass layer is small; Since the glass etching effect is small even during etching, a desired glass layer can be formed efficiently.

1 Example 1 Examples of the method of the present invention will be described below with reference to the drawings.

In FIG. 1, 1a and 1b are rotatable chucks, and these chucks 1a and 1b are installed in a glass lathe (not shown).

Reference numeral 2 denotes a reaction burner having a multi-tube structure, and this burner 2 is designed to reciprocate along an axial line connecting both chucks 1a and lb.

In FIG. 2, 4 is a transparent vitrification furnace, and this furnace 4
is equipped with a furnace core tube 5 and a heater 8 on the outer periphery of the furnace core tube.

In the figure, 7 is a quartz-based glass rod, and 8a is the glass rod 7.
The porous glass layer 8b deposited on the outer periphery of the porous glass layer 8b is a glass layer obtained by converting the porous glass layer 8a into transparent glass.

When the porous glass layer 8d is deposited on the outer periphery of the quartz-based glass rod 7 through the OVD method shown in FIG.
For example, the reaction burner 2, which has a multi-tube structure and rotates clockwise and reciprocates along the longitudinal direction of the glass rod 7, contains a gaseous glass raw material (silicon tetrachloride) and a gaseous dope raw material (silicon tetrachloride).
Gases such as sulfur hexafluoride (for example, sulfur hexafluoride), hydrogen, and oxygen are supplied, and glass particles generated by burning these gases are injected from the tip of the reaction chamber 2 toward the outer periphery of the glass rod 7.

In this way, the glass particles sprayed from the reaction burner 2 are sequentially deposited on the outer periphery of the glass rod 7, and a porous glass R8a having a predetermined layer thickness is formed. Since the amount of dope raw material supplied to the quartz glass rod 7 is gradually reduced, the porous glass layer 8a contains a large amount of fluorine in the portion close to the outer circumference of the silica glass rod 7, and the portion far from the outer circumference of the silica glass rod 7 contains less fluorine. The content will be reduced.

Next, the quartz glass rod 7 with the porous glass layer 8a is
It is inserted into the furnace tube 5 of the transparent vitrification furnace 4 shown in FIG. 2, and helium, sulfur hexafluoride, chlorine gas, etc. are supplied into the furnace tube 5 to turn the porous glass layer 8a into transparent vitrification. .

At this time of transparent vitrification, the inner layer portion of the porous glass layer 8a receives heat from the quartz-based glass rod 7, thereby becoming transparent vitrified earlier than the outer layer portion.

Therefore, in the porous glass layer 8a of the furnace tube 5, the amount of fluorine doped is small in the inner layer, which is quickly transformed into transparent glass, and increases in the amount of fluorine doped toward the outer layer.

As described above, since the fluorine doping tendency in the porous glass deposition process and the transparent vitrification process of the porous glass layer are different, the transparent glass layer 8b after the porous glass layer 8a has been made into transparent vitrification is The distribution of fluorine becomes uniform throughout the area.

Next, specific examples of the method of the present invention and comparative examples thereof will be explained.

Specific Example When producing a base material for a single mode optical fiber by the above-mentioned method, the quartz-based glass rod 7 serving as the core has an OH group content of 0. A high purity quartz rod with a diameter of 1 h+m and less than 1 ppm was used.

When externally attaching a porous glass layer to the outer periphery of this quartz rod, each reaction burner with a multi-tube structure is filled with 10 H2.
1/win, 02 to 1041/+sin, 5il1
4 was supplied at 500 cc/sin, and SF8 was employed as the fluorine compound, which was supplied together with the S + Cl 4 described above.

The amount of SF6 supplied is reduced each time a porous glass layer is deposited, and the amount of SF6 supplied for the nth layer is reduced to (300/n).
cc/win, and after the 50th layer (7) the SF6 supply amount was set to zero.

” 1. The rotation speed of the quartz rod during glass deposition is 60 r,
The moving speed of the reaction burner is 80 mm/win, and the average thickness of the glass deposited layer is 0.25 m layer/layer.

After the porous glass layer is deposited on the outer periphery of the quartz rod in this way, the quartz rod with the porous glass layer is placed in the highest temperature section 14.
The porous glass layer was placed in a core tube of a transparent vitrification furnace heated to 50° C. to vitrify the porous glass layer.

At this time, in the core tube of the transparent vitrification furnace, 20 sentences/wi
n He, 500cc/win SF8.200cc/
Supply each C12 of win, descending speed 3mm1I
As a sin, the quartz rod with a porous glass layer was lowered to the highest temperature part of the furnace tube.

FIG. 3 shows the refractive index distribution of the optical fiber preform thus obtained.

As is clear from Figure 3, the Δ- of the transparent glass layer is 0.3
The transparent glass layer is uniformly doped with fluorine and exhibits a flat refractive index distribution.

The glass deposition rate was 0.23 g/in as measured from the glass weight after transparent vitrification.

Comparative Example 1 When forming a porous glass layer on the outer periphery of a quartz rod in the same manner as in the specific example, 300 cc of SF6 was added to the reaction/hener.
It was supplied at a flow rate of /min from beginning to end.

The glass deposited layer at this time was 0.1 mm/layer, and the glass yield was significantly reduced.

This is due to etching with fluorine.

In the same manner as in the specific example, but without supplying SF6, the above-mentioned quartz rod with the porous glass layer was placed in a transparent vitrification furnace, and the porous glass layer was transformed into transparent vitrification.

FIG. 4 shows the refractive index distribution of the optical fiber preform thus obtained.

As is clear from FIG. 4, the refractive index distribution of the transparent glass layer is not flat, and the refractive index of the outer peripheral portion increases due to the volatilization of F.

The glass deposition rate in this comparative example is 0.1 g/sin.

Comparative Example 2 A porous glass layer was deposited on the outer periphery of a quartz rod in the same manner as in the specific example except that SF6 was not supplied to the reaction burner.

The average thickness of the glass deposited layer in this comparative example is 0.26 mm.
/ single layer, and was almost the same as the specific example.

Furthermore, the porous glass layer was made into transparent glass in the same manner as in the specific example.

FIG. 5 shows the refractive index distribution of the optical fiber preform thus obtained.

As is clear from FIG. 5, the refractive index distribution of the transparent glass layer is not flat in this comparative example as well.

This is because the glass layer near the glass rod was not doped with F.

The glass deposition rate in this comparative example was 0.24 g/sin.

Optical fiber preforms with an outer diameter ratio of transparent glass layer for cladding/quartz rod for core of 8.5 are prepared from each side as described above, and these optical fiber preforms are spun to form a depressed clad single unit. A one-mode optical fiber was manufactured.

The transmission characteristics of the optical fiber thus obtained are shown below.

[In the case of specific examples]

Loss at wavelength 1.30mm: 0.5dB/km wavelength 1
．． Loss at 55pLi: 0.3dB/km wavelength 1.3
Loss at 11 gm: 2.3 dB/km [for comparative example 1] Loss at wavelength 1.301 Lm: 0.5 dB/+.

Loss at wavelength 1.55 pLm: 4-2 dB/km Loss at wavelength 1.39 gm: 2. '5dB/+m [in case of comparative example 2] Loss at wavelength 1.30JL11: 0.8dB/km Loss at wavelength 1.55pm: 3. OdB/km wavelength 1.
391L+m Te loss: ? , OdB/km As is clear from the above characteristic values, the cladding of the optical fiber in the specific example is almost completely flat, so it is resistant to microbending and has low loss at a wavelength of 1.55 gm.

Moreover, in the case of the specific example, since fluorine is contained inside the porous glass layer, dehydration at the core:clad interface is good, and as a result, the loss at the wavelength of 1.39 p■ is less than that of the comparative example 1. The wavelength is the same as that of 1.39. In terms of power, it shows better transmission characteristics than Comparative Example 2.

"Effect of the Invention 1 As explained above, when using the method of the present invention, in the step of forming a porous glass layer on the outer periphery of a quartz-based glass rod and the step of making the porous glass layer transparent vitrification, it is possible to Since the fluorine layer is doped with the fluorine layer, when manufacturing a desired optical fiber preform, fluorine can be contained in the glass layer in a uniform distribution without reducing the glass deposition efficiency.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram schematically showing the process of forming a porous glass layer according to the present invention, FIG. 2 is an explanatory diagram schematically showing the process of making the porous glass layer transparent vitrification, and FIG. FIGS. 4 and 5 are refractive index distribution diagrams of optical fiber preforms manufactured as comparative examples of the present invention. 2... Reaction burner 4 for glass particle generation...
Transparent vitrification furnace 7 of porous glass layer...Quartz-based glass rod 8a...Porous glass layer 8b @ fist/transparent glass layer

Claims (1)

[Claims] Glass particles produced by mixing and burning glass raw materials and combustion gas are deposited on the outer periphery of a rotating quartz-based glass rod to form a porous glass layer, and then the porous glass layer is treated with fluorine or a fluorine compound. In a method for manufacturing an optical fiber base material, which is made into transparent glass by high temperature treatment in an atmosphere containing
When depositing and forming a porous glass layer around the outer periphery of the silica-based glass rod, fluorine or a fluorine compound is added to the glass raw material, and as the porous glass layer is deposited, fluorine or a fluorine compound is added to the glass raw material. A method for producing an optical fiber preform, characterized by gradually reducing the amount of fluorine or fluorine compound added.