JPS60260432A - Manufacture of preform for optical fiber - Google Patents

Abstract

PURPOSE:To prevent the cracking of a preform for an optical fiber by using a starting member having a specified coefft. of thermal expansion when a base material for an optical fiber is formed on a starting member by a vapor axial deposition method. CONSTITUTION:SiCl4, GeCl4 for a dopant, He as a carrier gas, H2 and O2 for generating an oxyhydrogen flame are blown on a starting rod 4 rotating in a reactor 6 from a burner 1 to form a porous base material 2 for an optical fiber on the rod 4 by hydrolysis in a flame, and the base material 2 is made transparent to manufacture a preform for an optical fiber. At this time, the starting rod 4 is made of quartz or SiC contg. a dopant such as GeO2 so that the coefft. of thermal expansion is made higher than or equal to that of the base material formed by deposition, and the tip of the rod 4 is made conic. Cracking is prevented when the porous base material 2 is made transparent by heating to manufacture a preform.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an improved method for manufacturing optical fiber preforms, and is suitable for manufacturing all optical fibers such as single mode and multimode optical fibers. It can be used as a method.

(Prior Art) FIG. 3 shows an outline of a conventional method for manufacturing an optical fiber preform using the six gas phase shafting methods. This involves introducing frit gas, )k, Ox, inert gas, etc. into a flame hydrolysis burner 1 to generate a flame 5 containing glass fine 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 rond-shaped porous glass preform 2. This method is then heated and sintered to produce a transparent optical fiber preform. This method has the feature that a long, large-diameter, broadband, and low-loss optical fiber can be easily obtained.

(Problems to be Solved by the Invention) However, in the above method, after the porous base material is sintered, cracks occur at the contact surface between the starting material and the optical fiber preform. Cracks may also propagate during renovation. This results in the optical fiber preform bursting.

The inventors of the present invention have conducted various studies and considered the causes of the difficulty of cracks occurring at the contact surface between the starting material and the optical fiber preform in the conventional method, and have found that the cause is as follows. .

FIG. 2 (al indicates the connection between the starting material and the optical fiber preform in the conventional method. 12 indicates the quartz starting material, 1
3 is the part of the optical fiber preform that contains a lot of Goot, and 14 is the Gem of the optical fiber preform.
, and the coefficient of thermal expansion of each part is 12≦14 (13).Here, B-Q (5f
In other words, if we pay attention to the seeding part, there is a quartz starting material 12 with a small coefficient of thermal expansion in the center, and around it there are parts 13 and 14 with a large coefficient of thermal expansion of the optical fiber preform. . Therefore, when cooling the porous base material after making it transparent, the parts 13 and 14 of the optical fiber preform having a large coefficient of thermal expansion are hindered by the starting material 12 and cannot be sufficiently shrunk. Cracks occur.

In order to prevent the occurrence of cracks in the optical fiber base material, conventionally, a quartz fiber glass rod with a refractive index distribution similar to that of the optical fiber in the radial cross section is used as a starting material to grow a porous base material. JP-A-58-135147 proposes a method using a starting material in which the tip of the starting material is elongated so as to have a tapered conical shape.

However, as a result of the inventors' study of the method described in the above-mentioned publication, it has been found that there are the following problems. Taking as an example a step-index fiber doped with Ge to increase the refractive index, Figure 2 (b)
I will explain based on K.

In FIG. 2 (1)), 1 is a portion of the starting rod made of pure silica, and 2 is a portion in which Gems are mixed and has a high coefficient of thermal expansion. Since the tip is formed by stretching, 1.2
The diameters of the two are similarly narrow. Further, 3 is a portion of the porous base material made of pure silica, and 4 is a portion where Ge- is mixed, and the refractive index distribution of the porous base material is a step index type. At this time, the relationship between the coefficient of thermal expansion of each part is 1
．． 3 (2=, 4. At this time, considering the A cross section, the part 2 with a higher coefficient of thermal expansion than the inside, the lower part 1, and the higher part A1
They are arranged in order of lowest part 3. As described above, since the portion 1 with a low coefficient of thermal expansion is located inside the portion with a high coefficient of thermal expansion, Ic, 4 cannot be sufficiently shrunk during cooling after completion of sintering. Therefore, this method is insufficient for the purpose of preventing rupture.

Furthermore, since the tip of the starting rod is made by elongating into a tapered shape, it is difficult to control the angle of the taper, and the reproducibility of the optical fiber preform made by depositing it on this soil is poor.

As described above, with the conventional technology, it is difficult to reduce the falling of the porous base material due to the occurrence of cracks and the rupture of the optical fiber preform, and it is also difficult to reproduce high-quality optical fiber preforms. There was a problem.

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

(Means for Solving the Problems) The present invention uses a material whose thermal expansion coefficient is equal to or larger than that of an optical fiber preform as a starting material, and the seeding portion has a radial thermal expansion coefficient from the center. This method solves the problems of the conventional method by decreasing the size toward the outer periphery.

That is, in the present invention, a burner is placed in the lower part of a reaction vessel equipped with an exhaust device, and a rotating starting material is placed in the upper part, and glass fine particles generated from the burner are sprayed onto the starting material to cause the porous base material to move in the axial direction. In a method for producing an optical fiber preform by growing a base material and later heating this base material,
A method for producing an optical fiber preform, characterized in that the starting material used is a material whose thermal expansion coefficient in the portion that contacts the optical fiber preform is greater than or equal to that of the dazzling fiber preform. It is.

(Function) It will be explained below that the method of the present invention produces the effect of preventing the generation of cracks in optical fiber preforms.

1) FIG. 1(a) shows a connection between a starting material and an optical fiber preform according to the method of the present invention. In the figure, 9 is a starting material whose thermal expansion coefficient is larger than or equal to that of the optical fiber preform, and 10 is a starting material for the optical fiber preform (
A portion 11 that does not contain DGeO is a portion that contains Ge01 of the optical fiber preform and has a larger coefficient of thermal expansion than 10. In this case, the relationship between the coefficients of thermal expansion of each part is 1o<11≦9. That is,
The thermal expansion coefficients of the starting material 9 and the optical fiber preform are:
In all cross sections, the center part is sharper than the outer periphery.

Therefore, when the optical fiber preform is cooled after sintering, the outer circumferential portion of the preform can contract without being hindered by the central portion, and stress that may cause cracks is not generated.

2) When using quartz whose thermal expansion coefficient has been adjusted by dopant (K) as a starting material, the thermal expansion coefficient of the starting material can be set in accordance with the thermal expansion coefficient of the optical fiber preform. Furthermore, it is possible to provide a radial distribution of the coefficient of thermal expansion of the preform for optical fiber, and to provide a distribution of the coefficient of thermal expansion to the deposited material.

As a dopant for adjusting the coefficient of thermal expansion, Ge, Island,
A~, self etc. are used for A.

3) In item 2) above, when the tip of the starting material is ground into a conical shape, the surface area is large and the heat capacity of the tip of the starting material to be heated is reduced, so it is difficult to heat the tip of the starting material to a high temperature. The starting material and the porous glass base material are firmly attached.

Also, as you move toward the center, GeO! For optical fiber preforms in which the ratio of Can be used as a starting material. In FIG. 1(b), the relationship between the coefficients of thermal expansion of each part is 5=7<6=8. Therefore, defective optical fiber preforms can be used as starting materials, and the manufacturing cost of optical fiber preforms can be reduced.

In the conventionally known method described in JP-A-58-13.5147, in which a starting material with a single refractive index distribution is simply stretched, one portion with a small coefficient of thermal expansion is used. Therefore, it is not possible to gradually reduce the coefficient of thermal expansion from the center to the outer periphery on all sides as shown in Figure 2 (b), and it is not possible to prevent the occurrence of cracks in the optical fiber preform. .

4) 81c can also be used as a starting material in the present invention. Since the linear expansion of SiO is 4.6 x 10 times 1/, when SiO is used as a starting material, if the mole fraction of Ge in the optical fiber preform is up to about 0.4 (5
It is effective in preventing optical fiber preforms from falling (because it is smaller than the coefficient of linear expansion of tCO).

Furthermore, when using SiO as a starting material, it is difficult to process the shape of the tip, but by shaping and sintering the shape, a starting material with a certain shape can be made. Also, SiO is EF
Since it is insoluble, by removing the remaining glass portion after removing the optical fiber preform from the starting material by washing with hydrofluoric acid, the starting material can be reused without changing its shape. Thereby, it is possible to apply a constant pressure to the shape of the optical fiber preform formed by depositing it on the starting material, and it is possible to stably and economically produce high-quality optical fiber preforms.

(Effects of the Invention) As described above in detail and as is clear from the following examples, the method of the present invention can prevent the occurrence of cracks in the optical fiber preform and prevent the optical fiber preform from bursting. In addition to being able to stably produce high-quality optical fiber preforms, depending on the type of preform, defective products can be ground and used as starting materials, and when SIC is used as a starting material, This is an excellent method that is advantageous in terms of cost as it allows reuse.

(Example) The present invention will be specifically explained below with reference to Examples, and its effects will be shown.

Example 1 Using the optical fiber preform manufacturing apparatus shown in FIG. I made this prototype. The conditions for creating the porous base material are: center pipe 1o IK 51cz of concentric quadruple pipe burner 1 with outer diameter 18-
, (α51/m1n), Gem! /, (α28//m
1n) with He as a carrier gas, % (3//min) into the outer tube 102, and the next tube 1.
Ar (1,2//min) in 03, outermost tube 104
Ox (6//m1n) was flowed into each chamber. The pressure inside the reaction vessel 6 was adjusted to -71' g.

Under these conditions, a porous base material having an outer diameter of about 70W and a length of 450cm was prepared. This porous base material was sintered at 1620°C.
The obtained optical fiber preform having a diameter of 32, III + length 23α was drawn on a glass lathe using the S10 type rod as a support rod without removing it.

As a result, the porous base material never fell, and no cracks occurred at the contact surface of the optical fiber preform with the starting material.

In addition, the glass part of the S10 rod used was cleaned with hydrofluoric acid and the same rod was used repeatedly.
The variation in the shape of the deposition surface of the porous matrix was negligible.

As a result of drawing this optical fiber preform, the wavelength is 0.
85μ poison, loss less than 3 dB/km, band 600 M
It was possible to obtain an optical fiber having high quality characteristics of H2 or higher.

Example 2 A stretched quartz glass iron doped with GeO2 having a relative refractive index difference of 2.2 was used as a starting material, and its tip was ground into a conical shape at 60°C.

The conditions for creating the porous base material and the sintering conditions are the same as in Example 1. Although 15 optical fiber preforms were made under the same conditions, none of the optical fiber preforms cracked at the point of contact with the starting material No. 11 during cooling after sintering. When the refractive index distribution of this optical fiber preform was measured, the relative refractive index difference was 1.8 at the peak.
±0.2, and the average value was within the range of 1.2±α15. When an optical fiber preform with such a refractive index distribution was fabricated by depositing it on a quartz starting material, cracks often occurred in the inventor's gauze experiments; High (=large relative refractive index difference) C1eO
By using quartz glass doped with , we were able to create an optical fiber preform without cracking.

The present invention is not limited to the above-described embodiments, but can be applied to any starting material by selecting the material of the starting material so that the coefficient of thermal expansion of the seeding portion gradually decreases from the center toward the outer periphery. Starting materials may also be used. The method for manufacturing an optical fiber preform is not limited to the method shown in FIG. 1, but can be applied to all conventional vapor phase mounting methods using starting materials.

[Brief explanation of the drawing]

FIGS. 1 and 2 are diagrams for explaining the relationship between the contact surfaces of the starting material and the optical fiber preform and the coefficients of thermal expansion of each part, and FIGS. 1 (a) and (b) are The case according to the invention method and FIG. 2 (al and Y) show the case according to the conventional method. 1 is a diagram schematically illustrating an example of a preform manufacturing apparatus. Agents 1) Akira Agent Ryo Hagiwara - Figure 1 (α) (b) -2:

Claims (4)

[Claims] (1) A burner is placed at the bottom of a reaction vessel equipped with an exhaust device, 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 move the porous base material in the axial direction. In the method of manufacturing an optical fiber preform by growing the base material and later heating this base material, the starting material has a coefficient of thermal expansion of a portion that contacts the optical fiber preform compared to that of the optical fiber preform. Is it big?
or a method for manufacturing an optical fiber preform, characterized in that an equivalent material is used. (2) The method for manufacturing an optical fiber preform according to claim 1, wherein quartz whose thermal expansion coefficient is adjusted with a dopant is used as the starting material. (3) Manufacturing the optical fiber preform according to claim (1), in which quartz whose thermal expansion coefficient is adjusted with a dopant is used as the starting material, and the tip thereof is ground into a conical shape. Method. (4) The method for manufacturing an optical fiber preform according to claim 1, wherein BIC is used as the starting material.