JPH0357058B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing optical fiber preforms, which can produce all types of optical fibers, including single mode and multimode optical fibers.

A conventional method for manufacturing an optical fiber base material is shown in FIG. This involves a flame hydrolysis burner 1 generating a flame 5 containing fine glass particles. This is sprayed onto the starting material 4 moving in the direction of arrow 7 while rotating in the direction of arrow 8 to grow a rod-shaped porous glass base material 2. This method is followed by sintering with a heat source 3 to form a transparent optical fiber base material 14. This method uses long
It has the characteristics of being able to easily obtain a large diameter, broadband, and low loss optical fiber, but the inventor of the present invention conducted experiments using this device configuration with various starting material shapes, and found the following results. It turns out that there are some problems.

(1) Figures 2 a to d were examined as the shapes of the starting materials. A shows a simple case using a solid glass rod, but in this case, as shown in Fig. 3, the flow of flame 5 from burner 1 is reversed as shown by arrow 21 near the starting material. will occur. When such a backflow occurs, the growth rate of glass fine particles in the radial direction increases as shown in Figure 3b, and as shown in Figure 3c, the shoulder part of the deposited porous base material has a shape like that shown in A. A constriction occurs. Since the bulk density of such a constricted part is low, distortion occurs due to mechanical shock or sharp temperature changes, and the porous base material 2 tends to fall as shown in Figure 3d. was unstable. In addition, since the bottom surface shape of the starting material is different from the bottom surface shape of the porous base material created, it takes time to maintain the porous base material in a steady state.
There was a problem with its reproducibility.

(2) In Figure 2 b and c, the bottom shape of the starting material is made larger in advance in order to solve the problem in (1) above, and the problem in (1) is almost solved. However, after the porous base material is sintered, cracks occur at the contact surface between the starting material and the optical fiber base material, and the cracks propagate into the optical fiber base material. This crack can be caused by using silica-based glass as the starting material and using SiO ₂ -GeO ₂ -P ₂ O ₅ -based glass (glass with a refractive index difference of 1% or more higher than that of SiO ₂ ) as the optical fiber base material. This is due to the difference in thermal expansion coefficients between the two glasses because they are deposited, and the contact area between the two glasses is too large.

(3) Figure 2d shows a quartz glass rod 4'' (the surface of which has been roughened with sandpaper or the like to prevent the occurrence of cracks as described in (2) above) at the tip of the quartz glass tube 4'. ) with the knock pin 22
It is fixed at In this method, the same starting material 4' can be used each time, and by replacing only 4'', it is possible to obtain an optical fiber base material with fewer cracks, but it is necessary to align the center axes of 4' and 4''. The reproducibility of the optical fiber base material is poor because it is difficult to do so.

As described above, with conventional techniques, it is difficult to produce high-quality optical fiber preforms with good reproducibility, and cracks may occur, causing the porous preform to fall and optical fiber preforms to be lost, resulting in reduced productivity. bad.

An object of the present invention is to provide a method for solving the above-mentioned conventional problems. That is, the object of the present invention is to provide a method for manufacturing an optical fiber preform that can prevent the porous preform from falling, prevent the occurrence of cracks in the optical fiber preform, and improve the manufacturing yield of high-quality optical fiber preforms.

The present invention is directed to a quartz glass rod or a quartz glass tube, which is processed so that the shape of the tip bottom surface (the side on which the porous base material is grown) of the starting material is ^l∝r〓s , or even a quartz glass tube that has a refractive index distribution. This method solves the problems of the conventional method by using a quartz-based glass rod or glass tube. However, l: distance from the bottom of the starting material in the pulling direction, r: position in the radial direction of the starting material, α _s : bottom surface shape distribution coefficient. By stretching the tip of the starting material into a tapered conical shape in this way, the following effects can be expected.

(1) By stretching the tip of the starting material onto the central axis, the central axis of the starting material becomes clear, making it easy to set initial conditions such as setting the central axes of the burner and the starting material.

(2) When set as in (1), the flame from the burner is 5
The flow uniformly flows along the outer periphery of the starting material, and a backflow as shown in FIG. 3 does not occur near the bottom of the starting material. As a result, uneven stress is not applied to the porous base material, and cracks do not occur in the porous base material.

(3) If α _s of the starting material is set in advance near the value of the bottom shape distribution coefficient (this is defined as α _s ′) of the porous base material to be created, the process of creating the porous base material can be A porous base material having a desired α _s ' can be easily produced without requiring any extra control (for example, controlling the distance between the burner and the bottom surface of the starting material, the pulling speed of the starting material, etc.).

(3) When the tip of the starting material is stretched into a tapered conical shape, the glass particles deposited near this point shrink (compress) when they shrink due to sintering.
As shown in Figure 4a, the stress is almost uniform in the radial and axial directions, whereas when the bottom surface area of the conventional starting material is large, the stress is as shown in Figure 4b. As shown in the figure, the stress distribution changes greatly in the radial and axial directions, resulting in a non-uniform distribution, which causes strain at the contact area between the starting material and the optical fiber base material (due to the above strain and the difference in the thermal expansion coefficients of the two glasses). cracks in the optical fiber base material. In extreme cases, cracks occur in more than half of the optical fiber base material, making it unusable as an optical fiber base material. In the case of the present invention, almost no cracks occur.

(4) By giving the starting material a refractive index distribution similar to that of an optical fiber, the laser beam can be incident on the other end of the starting material and emitted from the sharp end of the bottom surface. The relative position between the central axis of the material and the central axis of the burner can be easily set, and it can also be used to monitor the transparency process during sintering of a porous base material. Furthermore, the center of the starting material is SiO ₂ or
Dopants (Ge, P, etc.) that increase the refractive index
By using SiO ₂ or SiO ₂ containing a dopant (B, F, etc.) that lowers the refractive index in the _outer periphery, the coefficient of thermal expansion can be made close to that of the optical fiber base material. It is possible to prevent distortion from occurring due to this difference in thermal expansion coefficients.

Figures 5a and 5b show a cross-sectional view and a front view of the starting material of the present invention, and c is a graph showing the relationship between the axial length l of the starting material and the radial position r at that l. It is. The starting material is a commercially available quartz glass rod (outer diameter of several mm to several tens of mm, preferably 10 mm).
mm, length 1 m), and its tip was heated with a burner and stretched. α _s of the lower surface is from 0.3
A range of 3.5 is preferred. This is preferable from the standpoint of preventing backflow as shown in FIG. This α _s was determined by observing the bottom surface of the starting material with a television camera and comparing it with the desired α _s screen. Note that the tip and position 0 are preferably aligned with the central axis of the starting material, and are preferably concentric with position 0 in the r direction.

In FIG. 6, a glass tube is used as the starting material, and the tip thereof is stretched. In the examples, quartz glass (outer diameter 12 mm, inner diameter approximately 10 mm, length 1 m) was used, but since it has better straightness than a quartz glass rod, it was more convenient as a starting material. The leading edge 0 point did not have to be completely sealed; for example, a hole of about 1 mm could be present. It was also easy to set the relative positional relationship between the central axis of the tube and the central axis of the burner by passing a laser beam into the glass tube from one side. The diameter of the glass tube can range from several mm to several tens of mm. The same value as above can be applied to α _s of the bottom surface portion.

Figure 7 shows the refractive index distribution in a radial cross-sectional view when a glass rod is used as the starting material. , d can also be used. It is clear from the above description that the refractive index difference Δn can be selected from within the range of several 0.0% to several percent, and that it is more effective if it is selected according to the refractive index distribution of the optical fiber base material. As a method for producing a starting material with a refractive index distribution of b to d, SiO ₂ glass containing B ₂ O ₃ was attached to the outer periphery of a quartz glass rod using an external CVD method (a method of spraying glass particles with a flame hydrolysis burner). It can be obtained by depositing and sintering, or by immersing a quartz glass rod in an aqueous solution of boric acid and then heat-treating it, or by applying an organic silane-based alcohol solution (for example, the product name is Silica Film) to a quartz glass rod.
Generally well-known methods such as obtaining by sintering can be applied.

FIG. 8 shows the refractive index distribution in a radial cross-sectional view when a glass tube is used as the starting material. a is the case of a quartz glass tube, and a to d are the cases where a dopant that increases the refractive index is deposited on the inner surface of the glass tube. A quartz glass tube in which a dopant (B, F, etc.) that lowers the refractive index is deposited on the outside of the quartz glass tube may be used. The method of depositing a glass rod film on the inner surface of a glass tube is a well-known internal attachment method.
CVD method may be used. In addition, starting materials with various refractive index distributions can be made by the rod incubation method using the glass rod shown in FIG. 7 and the glass tube shown in FIG. 8, and in this case, one of the starting materials of the present invention constituted as one.

Next, the characteristics of the optical fiber preform when the starting material of the present invention is used in the optical fiber preform manufacturing apparatus shown in FIG. 1 will be described. Using a quartz glass rod (outer diameter 12 mm, length 1 m) as shown in FIG. 5 as a starting material, 28 optical fiber base materials were prepared by setting α _s of the tip portion to 2.0 to 2.5. However, the same starting material is used, and after sintering the optical fiber base material, the contact area between the starting material and the optical fiber base material is burnt off with a burner, and the tip is processed again to the desired α _s . The method used was used. First, we will show the conditions for creating the porous base material.
Center tube 1 of concentric quadruple tube burner 1 with an outer diameter of 17.8 mm
01 SiCl ₄ (0.5/min), GeCl ₄ (0.18/min),
POCl ₃ (0.1/min) is fed with Ar as a carrier gas, and Ar (0.9/min) is fed into the outer pipe 102.
min) into the next tube ₁₀₃ .
min), and O ₂ (6/min) was flowed into the outermost tube 104, respectively. A protective tube consisting of concentric triple tubes was used around the burner (not shown in the figure), and N ₂ gas was flowed at 10/min into each protective tube.
The output of the differential pressure gauge 15 was fed back to the pumping speed regulator 17 through the control circuit 16 so that the pressure inside the reaction vessel 6 was -0.1 mmAq. Under these conditions, a porous base material (outer diameter approximately 70 mm, length 300 mm) was created by aligning the central axes of the starting material and the burner.
This porous base material was sintered in an atmosphere of He and Cl ₂ at 1650°C (the value measured by the surface temperature of the heater with an optical pyrometer). As a result, the porous base material never fell due to cracks. Furthermore, no cracking occurred at the contact surface between the optical fiber base material and the starting material. The variation in α _s ′ of the porous base material was also less than ±10%. In addition, this optical fiber base material was put into a quartz glass tube and made into a rod inch tube.
%), the loss is 3dB/Km at a wavelength of 0.85μm.
Below, we were able to obtain an optical fiber with high quality characteristics with a bandwidth of 600MHz/Km or more.

The invention is not limited to the above embodiments. The length of the starting material may be from several tens of centimeters to several meters. The material of the starting material may be one containing SiO ₂ as the main component.
Vycor glass (trade name, manufactured by Corning Glass) may also be used. The method for manufacturing the optical fiber base material is not limited to the method shown in FIG. 1, but can be applied to all gas phase shaft dimensions using conventionally used starting materials.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of a conventional optical fiber base material manufacturing method, Fig. 2 is a schematic diagram of the shape (cross-sectional view) of the starting material that the present inventor has conventionally used, and Fig. 3 is a schematic diagram of the conventional method for producing an optical fiber base material. Figure 4 is a qualitative illustration of the shrinkage stress generated during sintering of the porous base material, where a is the case of the present invention and b is the conventional case. , FIGS. 5 and 6 are schematic diagrams of the starting material of the present invention, and FIGS. 7 and 8 show the refractive index distribution of the starting material. 6... Reaction container, 9, 10... Gas introduction pipe, 1
DESCRIPTION OF SYMBOLS 1, 12... Gas flow direction, 13... Pulling device, 18... Starting material mounting chuck, 19... Burner mounting support part, 20... Burner fine movement device.

Claims (1)

[Claims] 1 A burner is placed at the bottom of a reaction vessel equipped with an exhaust system, and a rotating and moving starting material is placed at the top, and glass fine particles generated from the burner are sprayed onto the starting material to grow the porous base material in the axial direction. , a method for manufacturing an optical fiber base material by inserting this base material into a heating furnace placed coaxially with the starting material,
As the starting material, one of a quartz glass rod having a refractive index distribution similar to that of an optical fiber in the radial cross section and a quartz glass tube having a refractive index distribution in the radial cross section is used. 1. A method for producing an optical fiber preform, characterized in that a starting material on which the fiber preform is grown is elongated so that the tip of the starting material is elongated into a tapered conical shape.