JPH0791082B2 - Method and apparatus for manufacturing optical fiber preform - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to a method and apparatus for manufacturing an optical fiber preform.

[Prior art]

Conventionally, as one of the methods for manufacturing an optical fiber preform, glass fine particles to be used as a clad are deposited on a transparent glass rod to be a core (or a part of the core and the clad). A method is known in which a composite preform of glass particulate deposits is heated in a heating furnace to produce a glass preform that is entirely transparent. Especially, so-called VAD as a transparent glass rod that should be the core
It is expected that an extremely low-loss optical fiber can be obtained by using a glass rod produced by the method as it is and depositing glass fine particles to form a clad around the glass rod to produce an optical fiber preform. That is, at present, the loss of the optical fiber preform produced by the VAD method is 0.5 dB / km or less, which means that the concentration of impurities contained in the fiber preform produced by this method is very low, and This is because it shows that the surface condition of the just-prepared base material is also very good.

[Problems to be Solved by the Invention]

However, if the surface of the glass rod to be the core is contaminated with dust, transition metal or the like, there is a problem that the loss of the optical fiber is greatly adversely affected. Therefore, in order to prevent this, before depositing the glass fine particles to be the cladding, the surface of the glass rod to be the core is treated under conditions such as vapor phase or liquid phase to clean the contaminated surface mechanically or chemically. However, if the glass rod to be the core is treated with an oxygen / hydrogen flame or a natural gas / oxygen flame for that purpose, the OH groups contained in the flame will be absorbed inside the core glass surface. The resulting optical fiber will suffer a large absorption loss due to the OH group. Further, even if it is treated with a flame in which a gas containing no hydrogen is burned, the presence of some moisture contained in the outside air poses a problem of mixing OH groups into the core glass rod. For example, when the amount of OH group mixed is 1 ppm, it is several dB / dB at the wavelength of 1.3 μm, which is currently mainly used in optical fiber communication.
It will increase km loss. Considering that the loss at the same wavelength when the OH group is not contained at all is 0.5 dB or less, it must be said that this is an extremely large number. Furthermore, the so-called VA is used as the transparent glass rod that should become the core.
In addition to the above problems, when the glass rod made by the D method is used as it is, and glass particles to be used as a clad are deposited to form the optical fiber preform, the glass particle deposition for the core The conditions for vitrification of the body and vitrification after depositing the glass particles for cladding are problems. Specifically, it is the consistency between the diameter of the glass particle deposit and the size of the heating furnace. For example, it is not always easy to heat a glass particulate deposit having an outer diameter of 30 mm into a transparent glass by heating it in a heating furnace having an inner diameter of 150 mm. As one of the reasons, when the difference between the glass particle deposit and the inner diameter of the heating furnace is too large, the gap between them acts as a chimney, and a so-called chimney effect causes a flow of atmospheric gas from below to above, As a result, sufficient heat is not applied to the glass particle deposit body. From this also, it is understood that proper dimensional matching is required between the outer diameter of the glass fine particle deposit to be made transparent and the inner diameter of the heating furnace. However, when the size of the heating furnace was designed according to the size of the deposit in order to make the glass fine particle deposit for the core made by the VAD method into transparent vitrification, the composite of the glass particulates to be the clad was deposited after that. The inner diameter of the heating furnace is too small for the preform, and the composite preform cannot be passed through the heating furnace for vitrification into transparent glass. This invention solves the problem of contamination of the surface of the glass rod for core and mixing of OH group, and makes the glass rod for core transparent glass formation by the VAD method so that each process is performed under optimal conditions. It is an object of the present invention to provide a manufacturing method that solves the problem of conditions and that can also be used in combination as a material that can be used in combination to reduce waste and manufacture an optical fiber preform at low cost, and a manufacturing apparatus used therefor. To do.

[Means for solving problems]

In the optical fiber manufacturing method according to the present invention, glass particles are deposited on the lower end of a seed rod suspended from above in a glass particle deposition chamber to form a glass particle deposit for a core that is grown in the axial direction. And a step of moving the glass particle deposit body downward and continuously provided below the glass particle deposit chamber having a small inner diameter.
Heating the glass particulate deposit in the heating furnace for dehydration and transparent vitrification, and pulling up the glass rod for core obtained by the transparent vitrification to the above chamber for depositing glass particulate. A step of depositing glass particles for clad around the core glass rod to form a glass particle deposit for clad, and a composite preform of the glass rod and glass particle deposit after the glass particle deposition. In a second heating furnace having a large inner diameter, which is moved upward and is continuously provided above the glass particle deposition chamber, the composite preform of the glass rod and the glass particle deposit is heated to dehydrate and clear glass. It is characterized by having a process of converting into. Further, the optical fiber preform manufacturing apparatus according to the present invention includes a glass fine particle deposition chamber that is used both for depositing glass fine particles for core and for depositing glass fine particles for clad,
A first heating furnace having a small inner diameter, which is coaxially arranged under the chamber so as to be continuous with the chamber, in order to perform dehydration and transparent vitrification by heating the glass fine particle deposit for core, and for cladding And a second heating furnace having a large inner diameter, which is coaxially arranged above the chamber and is continuous with the chamber for heating the glass fine particle deposit to dehydrate and transparent vitrify. Has become.

[Action]

Forming glass fine particle deposit for core by VAD method,
The transparent vitrification step, the step of depositing glass fine particles for clad around the glass rod for core which has become a transparent glass, and the step of vitrifying the glass fine particle deposit for clad to transparent glass are sequentially performed. These steps can be performed continuously and consistently in a single closed system without touching the outside air, so that the surface of the glass rod for core is contaminated or OH groups are mixed. Can be prevented. Therefore, an extremely low loss optical fiber can be obtained from the optical fiber preform thus produced. Moreover, since each process is continuously and consistently performed in one closed system, the manufacturing efficiency can be improved. further,
Dehydration and transparent vitrification of the glass fine particle deposit for core are performed in the first heating furnace with a small inner diameter, and dehydration and transparent vitrification of the glass fine particle deposit for clad are performed in the second heating furnace with a large inner diameter. , Each of these dehydration and clear vitrification steps can be optimized. Also, one glass particle deposition chamber is used for both glass particle deposition for the core and glass particle deposition for the clad, so that waste can be eliminated and the process can be streamlined, and the optical fiber preform can be manufactured at low cost. Be able to manufacture.

【Example】

As shown in FIG. 1, a heating furnace 1, a glass particle deposition chamber 3 and a heating furnace 2 are arranged coaxially and continuously from bottom to top. In this embodiment, the glass particle deposition chamber 3 serves both as a chamber for depositing the glass particles for the core and a chamber for depositing the glass particles for the clad, and includes a burner 31 for the core glass and a burner 32 for the clad glass. Exhaust port connected to waste gas treatment system
It has 33 and. That is, in this embodiment, the columnar glass fine particle deposit to be the core is produced by the VAD method. The heating furnace 1 has a core tube 11 having an inner diameter of 50 mm, and the heating furnace 2 has a core tube 21 having an inner diameter of 150 mm. The former is suitable for transparent vitrification of the glass fine particle deposit for core and surface treatment. The latter is sized to make the glass particulate deposit for cladding transparent glass. These two heating furnaces 1 and 2 and the chamber 3 are coaxially continuous vertically, and the rotating seed rod 4 can be traversed so as to penetrate them. The heating furnaces 1 and 2 are provided with cutters 12 and 22, respectively, so that the atmosphere therein is filled only with the gas supplied from the gas supply ports 13 and 23. First, as shown in FIG. 1, the seed rod 4 held and suspended at the upper end is lowered, and the lower end thereof reaches the chamber 3. At this time, silicon tetrachloride is introduced into the oxyhydrogen flame of the burner 31 to generate fine particles of quartz glass, and the fine glass particles are deposited on the lower end of the seed rod 4. Then, the seed rod 4 is gradually traversed upward while rotating, and the glass fine particle deposit body 5 to be a core later is grown in a columnar shape. In this example, the flow rate condition of the gas supplied to the burner 31 was H _{2 4} l / min O ₂ 5 l / min SiCl ₄ 80 cc / min Ar 120 cc / min. As a result, a cylindrical glass fine particle deposit body 5 having an outer diameter of 30 mm and a length of 300 mm was obtained. Next, as shown in FIG. 2, the seed rod 4 having the core glass fine particle deposit 5 formed at the lower end is traversed downward,
The glass particulate deposit 5 is inserted into the furnace core tube 11 of the heating furnace 1. The glass fine particle deposit body 5 is heated in the heating furnace 1 to perform dehydration and transparent vitrification to obtain a transparent glass rod 6 for a core. At this time, the shutter 12 is closed and the inside of the heating furnace 1 is kept airtight. First, in this, He from the gas supply port 13
A gas containing 99% by volume and 1% by volume of SOCl ₂ is supplied, and the temperature in the core tube 11 is maintained at about 1000 ° C. to remove the OH groups remaining in the glass particulate deposit body 5. Then, the inside of the heating furnace 1 was set to a 100% He atmosphere and the temperature was raised to 1600 ° C. to make the glass particle deposit body 5 transparent, thereby obtaining a transparent glass rod 6 for a core. This transparent glass rod 6 is directly transferred into the chamber 3 without being pulled out to deposit glass particles for cladding, which is a method that should be obtained in an actual manufacturing process. Experimentally, the transparent glass rod 6 was pulled up and observed. Then, it was confirmed that the finished transparent glass rod 6 had a cylindrical shape with a diameter of 17 mm and a length of 150 mm, and was sufficiently heated to have its surface very clean. It is considered that this is because the size of the core tube 11 was appropriate for the outer diameter of the glass fine particle deposit body 5 for core. If the heat treatment of the surface of the core transparent glass rod 6 is insufficient, the heat treatment can be performed again in the heating furnace 1. At that time, if necessary, in addition to an inert gas such as He or Ar, SOCl ₂ , Cl ₂ , CF ₄ , C ₂ F ₆ , C ₃ F ₈ , CCl.
It is also possible to introduce a halogen-containing gas such as ₂ F ₂ or CCl _{4 into} the furnace core tube 11 and heat it to etch the surface of the transparent glass rod 6. The transparent glass rod 6 thus obtained is obtained by pulling the seed rod 4 upwards, as shown in FIG.
Is placed in a chamber 3 continuous with the above, and the process proceeds to the next step of depositing glass fine particles for cladding without contact with the outside air. That is, the burner 32 for clad glass is ignited, and the fine particles of quartz glass generated there are deposited on the outer periphery of the transparent glass rod 6 for core. At this time, the burner 32
The flow rate condition of the gas to be supplied to H ₂ was ₂₀ l / min O ₂ 25 l / min SiCl ₄ 3 l / min Ar 2 l / min (carrier). Then, if necessary, a dopant gas such as GeCl ₄ , POCl ₃ , AlCl ₃ or a halogen-based gas is added to adjust the refractive index of the cladding glass. As a result, a cylindrical glass fine particle deposit 7 for clad having an outer diameter of 120 mm and a length of 150 mm could be grown around the transparent glass rod for core 6. Next, the seed rod 4 is pulled up further, and as shown in FIG. 4, the composite preform including the glass fine particle deposits for cladding 7 formed around the glass rod for core 6 is placed in the heating furnace 2. Then, the glass fine particle deposit 7 is dehydrated and transparentized to obtain a transparent glass 8 for cladding. At this time, the shutter 22 is closed and the fluorine-containing gas such as CF ₄ , SF ₆ , SiF ₄ , C ₂ F ₆ , C ₃ F ₈ is supplied from the gas supply port 23 into the heating furnace 2 for dehydration and dehydration. Fluorine is added to the clad glass. When it is not necessary to add fluorine to the clad glass, another halogen-containing gas such as SOCl ₂ , Cl ₂ and CCl ₄ is caused to flow with He to remove the OH group from the clad glass. Thus, the transparent vitrification was completed, and the optical fiber preform having the glass to be the core and the clad was obtained. The shape was a cylinder with a diameter of 45 mm and a length of 130 mm. The ratio of core-clat diameter of this base material was 1: 3, but the clad thickness is not sufficient as it is. Therefore, by heating and stretching the above base material, the diameter is 10 mm and the length is 700 m.
A columnar shape of about m is formed, and the fine glass particles are deposited on the outer periphery of the cylinder 3 in the chamber 3 using the burner 32 for the clad glass in the same manner as in the step of FIG. Dewatering and transparent vitrification of the glass particulate deposits were performed in the heating furnace 2. Also at this time, fluorine is added into the glass as needed. When the refractive index distribution of the resulting optical fiber preform was measured, it was as shown in FIG. Further, when the wavelength loss characteristic of the optical fiber obtained by drawing and fiberizing this base material was measured, it was as shown in FIG. The optical fiber is a single-mode fiber, the core is made of fused silica containing some germanium, and the cladding is made of fused silica. In addition, in this specification, the transparent glass rod 6 as the central member is the portion that will later become the core, but this mainly means the portion that becomes the core, and in addition to the core, the portion of the clad that is part of it. It also implies the inclusion of. That is,
In the above-described embodiment, first, the glass fine particles are generated by the burner 31 for the core glass in the chamber 3 to deposit the glass fine particles to obtain the glass fine particle deposit body for the core, but at the same time, the burner 32 for the clad glass is used. It is also possible to generate the glass fine particles by and deposit the glass fine particles forming a part of the clad around the glass deposit body for the core.

【The invention's effect】

According to the method and apparatus for manufacturing an optical fiber preform of the present invention, the step of forming a glass fine particle deposit for core by the VAD method, its transparent vitrification step, and the cladding around the glass rod for core that has become transparent glass The step of depositing glass fine particles, the step of vitrifying the glass fine particle deposit for cladding, and the step of performing these steps continuously and continuously without touching the outside air in one closed system, It is possible to effectively prevent the surface of the glass rod for core from being contaminated and the OH group being mixed in, so that an extremely low loss optical fiber can be obtained from the produced optical fiber preform. Moreover, since each process is continuously and consistently performed in one closed system, the manufacturing efficiency can be improved. Further, dehydration and transparent vitrification of the glass fine particle deposit for the core are carried out in the first heating furnace having a small inner diameter, and dehydration and transparent vitrification of the glass fine particle deposit for the clad are carried out in the second heating furnace having a large inner diameter. As such, each of these dehydration and clear vitrification steps can be optimized. Also, one glass particle deposition chamber is used for both glass particle deposition for the core and glass particle deposition for the clad, so that waste can be eliminated and the process can be streamlined, and the optical fiber preform can be manufactured at low cost. Be able to manufacture.

[Brief description of drawings]

FIG. 1 is a schematic view in a certain step of one embodiment of the present invention, and FIGS. 2, 3, and 4 are schematic views in respective steps different from FIG. 1 of the same embodiment, FIG. Is a graph showing the refractive index distribution in the diameter direction of the optical fiber base material obtained in the example,
FIG. 6 is a graph showing loss wavelength characteristics of an optical fiber manufactured from the optical fiber preform obtained in the same example. 1, 2 ... heating furnace, 11, 21 ... furnace tube, 12, 22 ... shutter, 13, 23 ... gas supply port, 3 ... glass particulate deposition chamber, 31 ... core glass burner, 32 …… Clad glass burner, 33 …… Exhaust port, 4 …… Seed rod, 5…
… Deposited glass particles for core, 6 …… Transparent glass rod for core, 7 …… Deposited glass particle for clad, 8 …… Transparent glass for clad.

Claims (2)

[Claims] 1. A step of depositing glass fine particles on a lower end of a seed rod suspended from above in a chamber for depositing glass fine particles to form a glass fine particle deposit body for an axially grown core, The glass particle deposit is moved downward to heat the glass particle deposit in a first heating furnace having a small inner diameter continuously provided below the glass particle deposition chamber for dehydration and transparent vitrification. Steps to be carried out, and by pulling up the glass rod for core obtained by being made into vitrified glass in the above chamber for depositing glass particles, depositing glass particles for cladding around the glass rod for core. The step of forming the glass fine particle deposit for clad and the composite preform of the glass rod and the glass fine particle deposit after the glass fine particle deposition are moved upward, A second heating furnace having a large inner diameter continuously provided above the child deposition chamber, and heating the composite preform of the glass rod and the glass fine particle deposit to perform dehydration and transparent vitrification. And a method for manufacturing an optical fiber preform. 2. A glass fine particle deposition chamber which is used both for depositing glass fine particles for a core and glass fine particles for a cladding, and a glass fine particle deposit for a core are heated for dehydration and transparent vitrification. A first heating furnace having a small inner diameter, which is arranged coaxially below the chamber so as to be continuous with the chamber, and a glass fine particle deposit for cladding are heated to perform dehydration and transparent vitrification. An apparatus for manufacturing an optical fiber preform, comprising: a second heating furnace having a large inner diameter, which is coaxially arranged above the chamber and is continuous with the chamber.