JPS63144138A - Production of optical fiber preform - Google Patents

Abstract

PURPOSE:To attain a refractive index distribution in high controllability and obtain the titled preform having low transmission loss and high quality, by heating a hollow tubular quartz glass containing a specific dopant, thereby evaporating said dopant. CONSTITUTION:A hollow tubular quartz glass 8 containing one or more dopants effective in decreasing refractive index such as fluorine and/or B2O3, etc., and optionally one or more dopants effective in increasing refractive index and selected from Zr, Y, Nb, Ta, Al, Ti and Ga is heated to evaporate the dopants and diffuse the inner dopant toward the inner and outer surface layers having decreased dopant concentration. A desired refractive index distribution can be formed by this process to obtain the titled preform free from the problem of interfacial mismatching.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application of the Invention] The present invention relates to a method of manufacturing a preform for a low-loss optical fiber.

[Prior art to the invention]

In recent years, a sol-gel method has been studied as a method for manufacturing a base material for silica-based optical fibers. In the sol-gel method, the raw material alkoxide is diluted with a solvent and water is added to prepare a sol solution.
This is a method in which the sol solution is gelled by holding it at a constant temperature, and the gel is vitrified after drying. ■ It has a high yield, ■ It is suitable for mass production, and ■ It is possible to add many types of dopants. It has characteristics.

As a method for manufacturing an optical fiber base material by the sol-gel method, there is a sol-gel mold method (Japanese Patent Application No. 11809/1983).
No. 8). As shown in Figure 1, the sol-gel mold method
This is carried out through the following steps. That is, the hollow part forming member 2
The sol solution 1 for cladding is injected into the container 3 equipped with the cladding sol solution 1 (Fig. 1 (al)), and after it becomes a gel (Fig. 1 (b)).
, remove the hollow part forming member 2 (FIG. 1(C)). Next, a core sol solution is injected into the formed hollow part 4, and after the core sol solution is gelled (FIG. 1(e)), it is dried (FIG. 1(f)). This is heated in an electric furnace and vitrified (FIG. 1 (gl)), which is a method for manufacturing an optical fiber base material 7 (FIG. 1 (h)) having a waveguide structure.

If the already known liquid phase fluoride addition is applied to this pull gel mold method (Japanese Patent Application No. 118663/1986), the strength of the gel will be increased, forming a cylindrical kraft gel with no change in diameter in the longitudinal direction. be able to. By drying and vitrifying this clad gel, a cylindrical glass body containing fluorine, which is a dopant that lowers the refractive index, can be easily obtained.

[Problems to be solved by the invention]

However, when forming the core, drying and vitrification are performed while bonding the clad gel and the core gel, which have different compositions, that is, different shrinkage rates, and stress is generated at the core-clad interface due to the difference in shrinkage rates. However, there were disadvantages in that the gel was prone to cracking and the yield was low. Furthermore, due to the same reason, peeling, bubbles, etc. occur at the core-clad interface of the base material, resulting in a large scattering loss of the optical fiber, making it difficult to reduce the loss.

The present invention has been made in view of the above points, and an object of the present invention is to provide a method for economically manufacturing a high-quality optical fiber with no irregularities at the core-cladding interface.

[Means for solving problems]

The present invention is characterized in that it includes a step of heating a quartz-based cylindrical glass body containing at least one kind of dopant that lowers the refractive index, evaporating the dopant that lowers the refractive index, and forming a refractive index distribution. shall be.

By performing this step, the dopant that lowers the refractive index evaporates from the inner surface of the cylindrical glass body, and the refractive index of the inner surface layer increases. Therefore, at the same time as this process or after the completion of this process, this cylindrical glass body is heated and solidified using acid, hydrogen flame, electric furnace, etc., and the core (corresponding to the inner surface layer of the cylindrical glass body before solidification) is ) can be manufactured.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

In the present invention, the sol-gel mold method shown in FIG.
, (The steps corresponding to a are called the first step, second step, third step, fourth step, fifth step, sixth step, and seventh step, respectively.

After removing the hollow part forming member in the third step, without performing coasol injection and gelation in the fourth and fifth steps, drying and vitrification processing in the sixth and seventh steps are performed, A cylindrical glass body having a hollow portion containing a dopant that lowers the refractive index is obtained, and this cylindrical glass body is heated to form a refractive index distribution, and at the same time or after that, it is solidified and used as a base material for optical fiber. do.

FIG. 2 shows a conceptual diagram of the process of forming a refractive index distribution according to the present invention. In the figure, reference numeral 8 denotes a quartz-based cylindrical glass body containing a dopant that lowers the refractive index. When the vapor pressure of the dopant that lowers the refractive index is higher than the vapor pressure of quartz glass, when the cylindrical glass body is heated to a high temperature, part of the dopant in the surface layer of the glass body evaporates. This evaporation lowers the concentration of the dopant in the surface layer and causes the dopant to diffuse along the dopant concentration gradient, ie toward the surface layer with lower concentration.

The concentration distribution of the dopant inside the cylindrical glass body is determined by the vapor pressure of the dopant, the diffusion constant of the dopant, and the heat treatment time. The concentration of the dopant on the surface of the glass body determined by the vapor pressure is Css, the concentration before heat treatment is constant inside the glass body, the CO5 diffusion constant is D, and the heat treatment time is t, the concentration at a point at a distance of χ from the surface after heat treatment is Cx. is represented by . However, φ(''/l is a Gaussian error function.

Figure 3 shows the heat treatment time t and the cylindrical glass body (outer diameter 2).
FIG. 4 is a diagram showing how the radial distribution of the dopant concentration in the inner diameter 2b) changes. FIG. 3 shows the case where the heat treatment time t is 0% Ll (0x11<1.).

If the dopant is a dopant that lowers the refractive index,
The refractive index n of the silica glass inside the cylindrical glass body. relative refractive index difference Δ-(n-nQ)/n.

As shown in FIG. 4, the value is high on the inner and outer walls of the cylindrical glass body. The refractive index of the inner wall of the cylindrical glass body is ns
, the lowest refractive index inside the glass body is n.

Then, the relative refractive index difference Δ between the core and cladding. = (n
r −nt /nt) will be formed.

Next, we will consider dopants.

So far, diboron trioxide (8gO3) and fluorine (P2) are known as dopants that lower the refractive index of silica glass. The boiling point of quartz glass is 2230
℃, whereas the boiling point of B2O3 is as low as about 1500℃, and 8g03 evaporates more easily at a lower temperature than quartz glass. Furthermore, although the evaporation mechanism of fluorine in fluorine-doped quartz glass is not well known, it is thought that fluorine evaporates as a fluorine-based gas such as HF% SIF 4 molecules. In other words, in fluorine-doped quartz glass, fluorine atoms are strongly bonded to silicon atoms, but when heated to high temperatures, the fluorine-silicon bond is broken and fluorine moves freely, producing fluorine gas molecules. Conceivable.

However, since fluorine cannot move at low temperatures due to the fluorine-silicon bond, fluorine-doped quartz glass exists stably at room temperature.

When a cylindrical boron-doped quartz glass or a cylindrical fluorine-doped quartz glass is held at a high temperature, boron or fluorine evaporates and diffuses, forming a dopant distribution and a refractive index distribution as shown in FIGS. 3, 4 and 1. becomes possible.

Regarding the heat treatment temperature, a higher temperature is advantageous because the vapor pressure of the dopant is higher, the refractive index difference between the core, the core, and the node can be increased, and the treatment time can be shortened. This temperature is not preferable because the cylindrical glass body becomes solid before the refractive index distribution is formed. Appropriate heat treatment temperature is 1400℃ or higher and 2200℃
below ℃.

The heat treatment time determines the concentration distribution of the dopant that lowers the refractive index distribution, that is, the refractive index distribution.

From the formula (2), if D % C0% Cs is constant, C is a function of χ/, /''ri''t. Therefore, in order to double the dopant diffusion distance, the heat treatment time must be quadrupled. When heat treatment is performed for a short time, the diffusion distance is short, so the outer diameter/core diameter ratio of the base material increases, resulting in a refractive index distribution suitable for a single mode fiber. Further, when heat treatment is performed for a long time, the outer diameter/core diameter ratio becomes smaller, and a refractive index distribution suitable for a multimode fiber can be formed.

Based on similar considerations, the heat treatment time depends on the diameter of the cylindrical glass body. If a cylindrical glass body with twice the outer diameter and twice the inner diameter is used in order to obtain the same outer diameter/core diameter ratio, the heat treatment time will be four times longer.

During the heat treatment, it is desirable to flow an inert gas or the like into the hollow part of the cylindrical glass body to remove the dopant vapor that lowers the refractive index from the system. If the partial pressure of the dopant vapor in the atmosphere is reduced, the amount of dopant evaporated increases, the dopant concentration Cs on the inner wall surface of the cylindrical glass body can be reduced, and the relative refractive index difference Δ between the core and the cladding. This is because = (ns-n also)/no can be increased.

As explained above, a refractive index distribution can be formed by heating a cylindrical glass body containing a dopant that lowers the refractive index and evaporating the dopant that lowers the refractive index. At that time, by adjusting the heat treatment temperature, heat treatment time, and atmosphere, it is possible to control the refractive index difference between the core and cladding and the outer diameter/core diameter ratio.

If the present invention is applied to the sol-gel method, a refractive index distribution can be formed after manufacturing a cylindrical glass body, and cracking of the gel due to stress at the core-craft interface will not occur. Since the boundary between the core and the craft is formed by dopant diffusion after vitrification, no peeling at the interface, no bubbles, etc. occur.

The method of the present invention has the advantages described above.

Examples will be described below.

Example 1 Cylindrical glass body sample containing fluorine as a dopant that lowers the refractive index (outer diameter 20IIIlφ, inner diameter 8
ms+φ, relative refractive index difference with respect to quartz glass -0.4%)
Three pieces were produced using the sol-gel method, and these glass bodies were mounted on a glass lathe and heat-treated with an acid/hydrogen flame. The heat treatment temperature was 1900°C, and the heat treatment time was 30 minutes for each sample (sample A''), 60 minutes (sample B), and 90 minutes (sample C).
), the atmosphere in the hollow part of the glass body is adjusted at a rate of 100 m/min.
This was carried out by the method of flowing helium gas. After heat treatment,
By raising the temperature further, solidification was achieved in a short time. After cooling the sample, the radial refractive index distribution was measured using the spatial filtering method. The results are shown in Figure 5. The vertical axis represents the relative refractive index difference Δ−(n −no
)/n.

, and the horizontal axis represents the radial coordinate X of each sample in terms of the diameter after solidification. Portion corresponding to the core of each sample (χ−〇)
and the quantitative refractive index difference Δ of the portion corresponding to the craft (χ-r/2). are respectively A 70.24%, B: 0.25%,
C: 0.24%.

Example 2 A cylindrical glass body sample similar to that used in Example 1 was prepared, and this glass body was mounted on a glass lathe, and while the hollow part of the glass body was depressurized with a vacuum pump, it was simultaneously heated at 1900°C. We made it into a solid state. The time required for this was 30 minutes. The refractive index distribution of the obtained rod-shaped glass body is shown in FIG. Relative refractive index difference Δ between core and cladding. is 0.2
7%, and the outer diameter/core diameter ratio was 20. This rod-shaped glass body was spun to obtain an optical fiber with a minimum loss of 1 dB/km or less.

Example 3 The same cylindrical glass body sample as that used in Example 1 was prepared, and the formation of the refraction distribution and solidification of this cylindrical glass body were performed simultaneously with spinning. The spinning was carried out at 2000° C. using a carbon furnace with a soaking section of 3 ctm.

The feed speed of the base material is 1.5 mm per minute + stapling, and the temperature is 200 mm.
The time it took to pass through the 0°C isothermal section was 20 minutes. Spinning was performed while reducing the pressure in the hollow part of the cylindrical glass body to make it solid. The minimum loss of the produced optical fiber was 1 dB/Km or less.

Example 4 The same cylindrical glass sample as used in Example 1 was prepared, and a quartz glass tube (outer diameter 30+n+mφ, inner diameter 20mm
φ), this quartz glass tube was mounted on a glass lathe, and heated to 1700° C. to cover the fluorine-containing cylindrical glass body with the quartz glass tube. Thereafter, while the hollow part of the glass body was depressurized using a vacuum pump, solidification was performed at the same time as heat treatment at 1900°C. The time required for this was 30 minutes. The refractive index distribution of the obtained glass body is shown in FIG.

Since the cylindrical glass body containing fluorine is covered with a quartz glass tube, it is possible to suppress the evaporation of fluorine from the outer surface of the cylindrical glass body and form a cladding layer with a flat refractive index distribution. .

Example 5 Cylindrical glass body (outer diameter 20 wm) containing diboron trioxide and fluorine as dopants to lower the refractive index distribution
φ, inner diameter 8III+1φ, relative refractive index relative to silica glass -0.45%), and the same experiment as in Example 4 was conducted. The relative refractive index difference Δ between the core and cladding of the obtained glass body. was 0.3%.

Example 6 Cylindrical glass containing zirconium oxide and fluorine, which are dopants that increase the refractive index and have a low vapor pressure at high temperatures (boiling point 4399°C), in order to achieve a larger relative refractive index difference between the core and cladding. Body (outer diameter 20...
mφ, inner diameter 3 mmφ, relative refractive index difference Δ-0 with respect to quartz
, 11%) and conducted the same experiment as in Example 4.

FIG. 8 shows the refractive index distribution of the obtained glass body. Because fluorine and silica glass are less susceptible to evaporation, the refractive index of the core (X=O) is larger than the refractive index of silica glass (Δ-0.3%), and the refractive index between the core and cladding is The relative refractive index difference Δ was 0.41%.

In addition, in all the above examples, the cylindrical glass body containing a dopant that lowers the refractive index is also applicable to a glass body produced by the sol-gel method, and it is also applicable to a glass body synthesized by other methods such as the MCVD method or the OvD method. It can also be applied to glass bodies,
There is no limitation in this respect.

Furthermore, as heating methods here, we used a method of holding the glass body in a glass lathe and heating it with an acid/hydrogen flame and a method of heating it in a carbon furnace, but any method that heats it to 1400°C or higher can be used. It can also be used for heating methods, such as silicone electric furnaces and other methods, and is not limited in this respect.

In Example 6, zirconium dioxide was used as a dopant that increases the refractive index with a small vapor pressure at high temperatures, but the vapor pressure of the dopant that increases the refractive index and whose vapor pressure at the heat treatment temperature decreases the refractive index, Any oxide or halide may be used as long as it has a pressure lower than the vapor pressure, and it goes without saying that indium, niobium, tantalum, aluminum, titanium, gallium, etc. can also be used.

C. Effects of the invention] As explained above, the effects of the invention! ! According to the manufacturing method, by heating a cylindrical glass body containing a dopant that lowers the refractive index and evaporating the dopant that lowers the refractive index, it is possible to form a refractive index distribution with good controllability, and to eliminate irregularities at the interface. A problem-free, high-quality base material for optical fiber can be easily manufactured.

[Brief explanation of the drawing]

Fig. 1 is a process diagram of the sol-gel molding method, Fig. 2 is a perspective view of a cylindrical glass body containing a dopant that lowers the refractive index used in the method of the present invention, and Fig. 3 is a diagram of the interior of the cylindrical glass body after heat treatment. FIG. 4 is a diagram showing the dopant concentration distribution in the cylindrical glass body, where Ce shows the dopant concentration before heat treatment, Cs shows the dopant concentration on the surface, and FIG. 4 shows the refractive index distribution in the cylindrical glass body after heat treatment. 5 is a diagram showing the refractive index distribution of three glass body samples A, B, and C prepared in Example 1, and FIG. 6 is a diagram showing the refractive index distribution of three glass body samples prepared in Example 2. 7 is a diagram showing the refractive index distribution of the glass base material covering the quartz glass tube produced in Example 4, and FIG. 8 is a diagram showing the refractive index distribution of the glass base material coated with the quartz glass tube produced in Example 6. FIG. 2 is a diagram showing the refractive index distribution of a base material made from a glass body containing zirconium oxide and fluorine. DESCRIPTION OF SYMBOLS 1... Container, 4... Hollow part, 5... Sol solution for core, 6... Dry gel, 7... Preform material for optical fiber, 8... Cylindrical glass body.

Claims (4)

[Claims] (1) It is characterized by including the step of heating a quartz-based hollow cylindrical glass body containing at least one kind of dopant that lowers the refractive index, evaporating the dopant that lowers the refractive index, and forming a refractive index distribution. A method for manufacturing an optical fiber base material. (2) The dopant that lowers the refractive index is fluorine and/or
or diboron trioxide, the method for producing an optical fiber preform according to claim 1. (3) Manufacturing the optical fiber preform according to claim 1 or 2, wherein the silica-based hollow cylindrical glass body contains a dopant that lowers the refractive index and a dopant that increases the refractive index at the same time. Method. (4) The dopant that increases the refractive index is zirconium,
4. The method for manufacturing an optical fiber preform according to claim 3, wherein the preform is one or more of yttrium, niobium, tantalum, aluminum, titanium, and gallium.