JPH0419174B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method of manufacturing a glass base material for optical fibers, and in particular to a method for manufacturing a glass base material for optical fibers, in particular a single mode fiber with excellent characteristics and low optical transmission loss, using pure silica as the core and fluorine-doped silica as the cladding. Suitable for fiber manufacturing methods. (Prior Art) Examples of the refractive index distribution of a pure silica core type single mode fiber are shown in FIGS. 1A and 1B. In principle, the pure silica core type single mode fiber shown in Figure 1 is expected to have extremely low loss characteristics because the core is pure silica and there is no Rayleigh scattering loss due to dopants, but it is difficult to manufacture. That's funny. For example, the present inventors have already applied for patent application No. 58-194104.
In the specification, pure silica rod is prepared as the core glass, silica soot is deposited on the outer periphery of the rod by flame hydrolysis reaction, and the deposit is dehydrated and sintered in an atmosphere containing fluorine compound gas. proposed a method of doping fluorine into silica glass to form a pure silica core and a fluorine-doped cladding layer. (Problems to be Solved by the Invention) The disadvantages of the above manufacturing method are that the process is long, and moisture tends to remain on the outer periphery of the core glass, and this moisture cannot be sufficiently removed during the dehydration and sintering process. There will be cases where it is not possible to do so. For this reason, the fiber obtained by the above manufacturing method remains affected by light absorption (wavelength: 1.39 μm) due to OH groups. The present invention solves this problem and provides a method for producing a single mode fiber, particularly a pure silica core type single mode fiber, which has an extremely simple process and is stably affected by less absorption by OH groups. (Means for Solving the Problems) The present invention further sinters the inner wall of the cylinder by inserting a heater into the hollow part of the cylinder made of a semi-sintered body of particulate SiO ₂ glass and heating it. The present invention relates to a method for manufacturing a glass preform for optical fibers, which comprises heating the cylindrical body in an atmosphere containing at least a fluorine compound gas to melt and convert it into transparent glass. The process of manufacturing an optical fiber according to the method of the present invention is generally carried out as follows. A cylindrical body (soot pipe) made of a semi-sintered body of particulate glass is created. A heater is inserted into the soot pipe to heat the inner wall of the soot pipe, thereby progressing the sintering of the inner wall of the pipe while completely dewatering the soot pipe. Fluorine doped sintering. Collapse (solidification of pipe). Jacketing. Draw a line to make fiber. In order to manufacture the fiber having the structure shown in FIG. 1A, the above-mentioned steps 1 to 3 may be used, and to manufacture the fiber having the structure shown in FIG. As a method for producing the soot pipe, the so-called flame hydrolysis external attachment method (for example, the method described in JP-A-48-73522) is suitable. That is,
SiCl ₄ is used as a gas phase raw material for forming glass, and SiO ₂ glass particles (soot) are generated by H ₂ −O ₂ flame.
This soot is deposited in a semi-sintered state on the outer periphery of a rotating and moving starting rod made of a refractory material such as a quartz rod, an alumina rod, or a metal rod, and after a predetermined amount has been deposited, the starting rod is pulled out. , get a soot pipe. A heater is inserted into the soot pipe produced as described above, and chlorine-based gas is introduced from the outside while heating the soot pipe, thereby dehydrating the soot pipe and increasing the degree of sintering of the inner wall of the pipe. FIG. 2 illustrates an example of a method of heating the inner wall of a soot pipe. In the figure, 4 is a soot pipe, 5 is a spacer made of quartz glass or high-purity alumina, etc., and 6 is a protection tube made of quartz glass or high-purity alumina, etc. provided to prevent impurities from entering the heater. spacer 5 in the gap between
is inserted. 7 is a heating element made of a material such as carbon, SiC, zirconia, etc., and generates heat by high frequency induction. 8 is a matsufuru made of quartz glass, and 9 is a work coil for applying high frequency. The heating conditions for the soot pipe 4 are as follows: First, while introducing chlorine gas and helium gas into the Matsufuru 8, the temperature of the heating element 7 is increased, and after the heating element 7 is raised to a predetermined temperature _T1 , , a fluoride gas such as CF ₄ , CCl ₂ F ₂ or SF ₆ is introduced into the Matsufuru 8. Thereafter, the temperature of the heating element 7 is further increased to turn the soot pipe into transparent glass. The above process basically involves first dehydrating the soot pipe with chlorine-based gas to increase the degree of sintering of the inner wall of the soot pipe, and then doping the soot pipe with fluorine by introducing fluoride gas. Become. By taking the above process, a glass pipe can be obtained in which the amount of fluorine added is small in the inner wall portion of the soot pipe and large in the outer layer portion. An example of the heating conditions in the above step is shown schematically in FIG. In the figure, the horizontal axis shows the elapsed time (t) and the gas atmosphere at that time, and the vertical axis shows the heating element temperature (T).
, where τ is the temperature increase rate and Δt is the holding time when the temperature of the heating element reaches _T1 . The temperature of the heating element is increased from T ₀ to T ₁ at a heating rate τ in a He atmosphere containing chlorine gas, and the heating element temperature is increased from T 1 to _{t 2} at a temperature of T ₁ .
The glass pipe is completely dehydrated by holding it for a period of △t, and the degree of sintering near the inner wall of the pipe is increased. Then, the supply of chlorine gas is stopped and the temperature is raised to T ₂ in a He atmosphere containing fluoride gas. Warm from time t ₃
Fluorine addition and transparent vitrification are performed by maintaining the temperature at T ₂ for t ₄ . Moreover, the refractive index distribution in the radial direction of the obtained glass pipe is shown in FIG. In FIG. 4, the refractive index near the inner wall of the glass pipe is equal to that of pure silica. In the above explanation, we have described a method in which the process up to transparent vitrification is carried out in one furnace, but after the sintering of the inner wall of the soot pipe has progressed, another furnace is used to dope the fluorine dope and convert it into transparent vitrification. You may do so. The fluorine concentration distribution in the radial direction of the glass pipe can be controlled by the above-mentioned temperature increase rate τ, T ₁ and Δt. The glass pipe obtained through the above steps is made into a rod shape by crushing the holes in the pipe according to the final step of the well-known MCVD method, and is used as a glass base material for optical fiber. (Effects of the Invention) As described in detail above, the method for producing a glass base material for optical fibers of the present invention can form a core and a glade while dehydrating a soot body (soot pipe), so the process is simple and no residue remains. A core with extremely low moisture content can be formed. Furthermore, a fiber with a pure silica core that does not contain a dopant in the core can be easily produced. (Example) Using an aluminum metal rod with an outer diameter of 15 mm as a starting rod, SiO ₂ fine particles (soot) were deposited on the starting rod by flame hydrolysis of SiCl ₄ to synthesize a soot body with an outer diameter of 150 mm. After that, the starting aluminum rod was pulled out to make a soot pipe, and a heating element with an alumina protection tube was set inside the soot pipe.
The soot pipe was heat-treated under the conditions shown in the table. Note that the meanings of τ, Δt, T ₁ , and T ₂ in the table and the method of flowing the atmospheric gas are the same as shown in FIG. 3.

[Table] The obtained molten glass tube (glass pipe) was crushed (solidification) by heating it in a resistance furnace at approximately 2000°C while introducing chlorine gas into the tube. When the refractive index distribution of the obtained rod cross section was measured, it was found that a pure silica core with a refractive index difference of 0.3% was formed in the center. A single mode fiber was produced by covering the outer layer of the rod with a silica tube, melting it and drawing it. The transmission loss of the obtained fiber is that the OH peak at a wavelength of 1.39 μm is
Less than 10dB/Km, residual OH groups are low, and the wavelength is
The transmission loss at 1.3 μm was small, less than 1 dB/Km, making it a single mode fiber with excellent characteristics.

[Brief explanation of drawings]

Figures 1A and 2B are diagrams showing the refractive index distribution of a single mode fiber, Figure 2 is a diagram explaining one embodiment of the soot pipe heating method of the present invention, and Figure 3 is a diagram showing the heating conditions of the soot pipe in the present invention. FIG. 4 is a diagram showing the refractive index distribution of the glass pipe obtained by the present invention.

Claims (1)

[Claims] 1. A heater is inserted into the hollow part of a cylindrical body made of a semi-sintered body of particulate SiO ₂ glass, and the area near the wall of the cylindrical body is further sintered by heating, and then the cylindrical body is heated to contain at least a fluorine compound gas. 1. A method for producing a glass base material for optical fibers, which comprises heating in an atmosphere to melt and transform into transparent glass.