JPH0333661B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing an optical fiber soot. (Prior art) In silica-based optical fibers, in order to make the refractive index of the cladding lower than that of the core, a glass layer for the cladding, which is to become the cladding, is coated at the soot manufacturing stage.
It is generally doped with a dopant such as boron or fluorine. Figure 1 shows the dopant concentration d (mol%),
It shows the relationship with the refractive index r, and as is clear from the figure, fluorine has a particularly large effect of lowering the refractive index compared to boron. This large effect means that it is possible to increase the relative refractive index difference with the core, and by increasing the relative refractive index difference, it becomes possible to obtain an optical fiber with a high numerical aperture. Methods for doping with fluorine include flame hydrolysis, internal CVD, plasma CVD, and the like. In the flame hydrolysis method, HF is produced and this reacts with silica to produce SiF ₄ (gas) as follows; in other words, F becomes SiF ₄ and evaporates.
The deposition efficiency of soot made of SiO ₂ will be significantly reduced, and fluorine will not be doped so much (difference in relative refractive index of about 0.2 to 0.3%). SiO ₄ +4HF→SiF ₄ ↑+H ₂ O In the other two methods, doping is carried out by an oxidation reaction in oxygen gas, but in both cases the doping is carried out at a high temperature of 1400°C or higher, so the amount of fluorine doped is extremely small. . In other words, silica generally has a stable tetrahedral network structure, and it is difficult to introduce anions such as fluorine into this structure, and the amount of anion introduced is affected by temperature and pressure. However, since production is carried out under normal pressure, the amount of fluorine introduced ultimately depends on the temperature. The amount introduced increases as the temperature decreases. However, either method requires a process to turn the generated SiO ₂ soot into transparent glass, and this process requires a high temperature of 1400° C. or higher, which makes it difficult to dope with fluorine. Figure 2 shows the reduction rate m (%) of the fluorine concentration when the fluorine concentration contained in the soot at room temperature is taken as 100%, that is, [fluorine concentration at heating temperature t]/[fluorine concentration at room temperature] Fluorine concentration] m (%)
This is the result of heating fluorine-doped silica and examining F on the surface using an X-ray microanalyzer, and F is
It can be seen that the decrease starts at a low temperature of 550°C.
In other words, the reason why it decomposes at such a low temperature is thought to be because the bonding energy of fluorine in silica is extremely low. In the production of optical fibers, the fibers undergo a thermal history of 2000° C. or more during processes such as collapse, processing, and drawing, so that the disappearance of F is rapidly accelerated. As described above, in the above conventional example, the amount of F doped is small, and therefore an optical fiber with a high numerical aperture cannot be obtained. Furthermore, when heated to high temperatures, F
Not only does the relative refractive index decrease because it evaporates as SiF ₄ , but also because F diffuses into the core layer due to heating, the refractive index distribution in the boundary region with the cladding layer is disturbed. A complete step in refractive index will not be obtained at the interface between the layers and the cladding layer. (Object of the invention) The object of the invention is to improve the doping amount of F and form a thermally stable clad glass layer,
An object of the present invention is to provide a method for producing a good optical fiber soot having an F-doped silica cladding. (Structure of the Invention) The present invention provides a method for manufacturing an optical fiber base material having a cladding having a refractive index slightly lower than that of the core on the outer periphery of the core. By supplying silica glass-based raw materials, fluorine as a dopant, and cations that stably fix an arbitrary amount of fluorine in the synthetic silica, and depositing a glass layer for the cladding, the refractive index can be adjusted. It is now possible to stably fix fluorine in synthetic silica, which is highly effective in lowering It does not decompose easily, and therefore, even when heated, the reduction in fluorine is suppressed as much as possible. (Embodiments) The present invention will be described below with reference to embodiments shown in the drawings. FIG. 3 shows a cross section of an optical fiber, in which a cladding 2 is provided around the outer periphery of a core 1. The refractive index of the cladding 2 is set to be slightly lower than that of the core 1. Setting of such a refractive index is performed at the soot manufacturing stage. That is, fluorine (F) for lowering the refractive index is sprayed onto the outer periphery of the core glass layer, which is to become the core, together with the glass raw material in the vapor phase. This point is similar to the conventional manufacturing method, but in the present invention, cations are sprayed together with the glass raw material and dopant. This cation acts as a network modifying ion in the network structure of silica, and combines with F to form glass as a thermally stable compound. Furthermore, if it is difficult for silica and network modification ions to form glass, the −Si
In order to break down the -O-Si- bond, that is, the oxygen bridge, and facilitate the introduction of network-modifying ions, a third component,
For example, alkali or alkaline earth metal ions may be added. Here, a compound that is thermally stable when combined with F means that it is difficult to decompose or evaporate at the maximum temperature, which is the normal quartz processing temperature, that is, 2100°C. Next, two more specific examples will be described. (Specific Example 1) A quadruple pipe burner as shown in Fig. 4 is used as a torch for soot synthesis, and from the first hole 3 located in the center, SiCl ₄ and O ₂ are supplied as a carrier gas.
A mixture of fine particles with CaCl ₂ is flowed, SF ₆ gas is flowed at a rate of 0.2/min from the second hole 4 on the outer periphery of the first hole 3, and H 2 gas is flowed from the third hole 5 on the outer periphery of the second hole _4. of
from the outermost fourth hole 6 at a rate of 15/min.
Soot was synthesized by flowing O ₂ at a rate of 5/min. In this soot, CaF ₂ is produced by the following reaction. CaCl ₂ + F ₂ → CaF ₂ + Cl ₂ This CaF ₂ has a refractive index of 1.4339, a melting point of 1360°C, and a boiling point of 2500°C, and is thermally stable, and F is fixed in the synthesis soot by combining with Ca. become. The soot thus obtained was heated in an electric furnace or the like in a manner commonly used in the VAD method to turn it into transparent vitrification. The relative refractive index difference of this transparent glass rod was 1%, and when the same rod was made into a fiber, the loss was 5 dB/Km (wavelength λ = 0.85 μm). (Specific Example 2) A cladding layer to be a cladding was synthesized on the outer periphery of a glass rod to be a core using a burner similar to that in Specific Example 1 by an external deposition method. However, SiCl ₄ flows from the first hole 3 of the burner, and from the second hole
O ₂ or Ar gas containing SiF ₆ is used as a carrier gas,
A mixed aqueous solution of NaCl and Ca(No ₃ ) ₂ atomized using an ultrasonic atomizer was flowed into the third hole 5 and fourth hole 6 to synthesize soot in the same manner as in Example 1. In this case, the purpose of Na is to break through the oxygen bridge mentioned above. When the soot thus obtained was vitrified using the usual method, the relative refractive index of the transparent glass rod was
At 2.5%, the loss when fiberized is 4~
6 dB/Km (wavelength λ = 0.8 μm), which was sufficient for practical use. In the above, as an element that fixes fluorine,
Although Ca is shown as an example, other examples include Sr, Mg, Li, and Pb. These combine with F to become SrF ₂ , MgF ₂ , LiF, and PbF ₂ , respectively. In addition to these, there are no limitations on the type of fluoride as long as it is a thermally stable fluoride that forms glass with SiO ₂ . Furthermore, the method of doping these elements is not limited to the above. The properties of the above-mentioned compounds with fluorine are listed in the following table.

[Table] (Effects) As is clear from the above, in the present invention, fluorine is thermally stably fixed in the synthesis soot, so it is possible to increase the amount of fluorine doped, and therefore, a high aperture can be achieved. A large number of optical fibers can be easily obtained. Further, even if the soot and subsequent intermediates are heated to high temperatures, fluorine does not volatilize, and the relative refractive index is therefore stabilized. Furthermore, since the fluorine is completely trapped in the cladding layer, that is, there is little diffusion of fluorine into the core layer, there is no disturbance of the base of the core portion.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the dopant and the refractive index of the cladding, Figure 2 is a graph showing the thermal behavior of fluorine, Figure 3 is a cross-sectional view of the optical fiber, and Figure 4 is the plane of the burner. FIG. 1... Core, 2... Clad.

Claims (1)

[Claims] 1. In a method for producing an optical fiber soot having a cladding having a refractive index slightly lower than that of the core on the outer periphery of the core, a vapor phase quartz glass-based raw material and a dopant are added to the outer periphery of the core glass layer to be the core. A method for producing an optical fiber soot, which comprises depositing a glass layer for a cladding to be a cladding by supplying fluorine and an arbitrary amount of cations that stably fix fluorine in a synthesis soot. .