JPS59207845A - Manufacture of optical fiber preform - Google Patents

Abstract

PURPOSE:To manufacture an optical fiber preform capable of giving a continuous optical fiber having ideal refractive index distribution, by forming porous preforms corresponding to the inside and outside of core and clad with specific and respective silicon oxides, and vitrifying to transparent glass. CONSTITUTION:The vapor of glass raw materials is subjected to the vapor-phase chemical reaction in the flame of the burner 1 to form fine powder of glassy SiO2, Al2O3, ZrO2, etc., and deposited to the lower end of the rod-like substrate 2 forming a porous glass preform 3 to form the core. When the preform 3 is grown to a certain degree, the burner 4 is ignited to form the porous preform 5 corresponding to the inner layer of the clad, and then, the combustion of the burner 6 is started to form the porous preform 7 corresponding to the outer layer of the clad. The resultant porous preform having triple layer structure is if necessary dehydrated with chlorine gas, and vitrified and clarified with heat to obtain the objective optical fiber preform.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing an optical fiber preform particularly suitable for obtaining a single mode optical fiber having a depressed land structure.

In order to further improve the transmission characteristics of a single mode optical fiber, a single mode optical fiber having a depressed cladding structure and having a refractive index distribution as shown in FIG. 1 has been proposed. As is clear from Figure 1, in this single-mode optical fiber with a tipped cladding structure, the refractive index of the cladding changes in a stepwise manner in the radial direction, with the refractive index of the portion close to the core being low and the refractive index of the portion far from the core being low. The refractive index of the part is high. A single-mode optical fiber with a depressed clad structure having such a refractive index distribution has the features of low transmission loss and excellent band characteristics.

Conventionally, single mode optical fibers with such depressed cladding structures have been manufactured by the dcVD method.

According to the CVD method, the refractive index distribution can be precisely adjusted, so it is possible to obtain a material with a sharp refractive index distribution as shown in Figure 1, but the size of the optical fiber base material is limited, and long It is impossible to obtain optical fiber.

On the other hand, the %VAD method is not limited by the thick blindness of the optical fiber base material and can obtain long optical fibers.
When a single-mode optical fiber with a depressed cladding structure is manufactured using the D method, the refractive index distribution is depressed and a completely step-like refractive index distribution can be obtained, for example, as shown in Figure 2. It's impossible. This is because when forming the porous preform that will become the cladding, a part of the dopant such as germanium oxide in the porous preform that will become the core that was formed first evaporates and reacts with the glass particles that will become the cladding. This is because it reacts, evaporates, or dries due to heating during dehydration and transparent vitrification.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing an optical fiber preform that can produce a long single-mode optical fiber having a depressed cladding structure and having an ideal refractive index distribution. The purpose is to
This method is characterized by specifying dopants for porous preforms that will become the core and cladding, and by using a plasma jet to form the porous preforms that will become the inner part of the cladding.

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

FIG. 3 shows a manufacturing apparatus suitably used in the manufacturing method of the present invention, and reference numeral 1 in the figure is a first burner for forming a porous preform that becomes a core. This first burner 1 is an oxyhydrogen flame burner with a multi-tube structure, and is S
H2 gas, 0□ gas, Ar gas, etc. are separately supplied to glass raw material gases such as iC Swamp 4 and MCA3hZrc-134 gas. The glass raw material gas undergoes gas phase chemical reactions such as thermal hydrolysis reaction and thermal oxidation reaction in a flame, and then undergoes S10□5
The powder becomes fine glass powder such as M203 and zrO□, which adheres to and accumulates on the tip of the rod-shaped base material 2 that moves upward while rotating, forming a porous glass preform 3 that becomes the core.

Here, 5to2. The key point is to use oxides with a high melting point such as M2O3 and ZR02. M2O3, ZrO□, etc. have high melting points, for example, over 1600°C, and may react with other glass components or volatilize during dehydration using chlorine (J2) gas in the subsequent process or heating during transparent vitrification. Also, during the formation of the porous preform that will become the next cladding, it will not react with the SIO□ of the glass particles.Therefore, A-82
By using a high melting point oxide such as 03, zrO2 as a dopant, the outside of the core preform IIt! l
There is no droop in the refractive index due to the Fa component, and an ideal step-type refractive index distribution can be achieved.

On the other hand, when GeO2, which is commonly used, is used as a dopant, the melting point of GeO2 is as low as 1100°C, so GeO□ reacts or evaporates in the above-mentioned post-process, resulting in only a product with a dull refractive index distribution. I can't get it.

When the porous glass preform 3 that becomes the core grows to a certain extent, the second burner 4 provided above the first burner 1 starts combustion.

This second burner 4 forms a porous preform on the inner side of the cladding, and is a cylindrical frequency plasma torch. That is, a high frequency coil 4b is wound around the outer periphery of the main body 4a of the burner 4, and is connected to a high frequency power source (not shown) to supply high frequency current. In addition, the burner body 4
Ar gas, N2 gas, horse gas, etc. as plasma generating gas are pumped into the main body 4a from the base of the burner 4a, and a plasma jet is thereby emitted from the nozzle of the burner 4. At the same time, 5ick is applied to the burner body 4a.
Frit gas such as 4, 0□ gas, Ar gas, and fluoride gas are supplied. This fluoride gas is B
F, OF, 5tF4. PF4゜4 AgF2. It is a fluoride of non-metallic elements such as SF6, which is in a gas state at around room temperature, especially C.
F4 and SF6 are preferred. The above gas supplied to the burner 4 undergoes a gas phase chemical reaction in the plasma, resulting in 5t-F
The glass particles become bonded and adhere to and accumulate on the porous preform 3 that becomes the core, forming the porous preform 5 that becomes the inner part of one crat. At this time, the burner 4 emits an ultra-high temperature plasma jet of 10,000 to 20,000', so the burner 4 is installed sufficiently away from the previously formed porous preform 3 that will serve as the core. 12
Care must be taken to ensure that glass fine particles cooled to about 00 to 1500° C. adhere and deposit.

Here, a plasma torch was used for the second burner 4 because, as mentioned above, the plasma has a very high temperature of 10,000 to 20,000 χ, so Sl and F react, forming a 51-F bond. This is because glass fine particles are generated. In a general oxyhydrogen flame burner, the flame temperature is approximately 2480°C, so %5i
-F bond does not occur and F decomposed from fluoride gas is 5i
When a porous preform on which such glass particles are deposited is dehydrated or made into transparent vitrification, F evaporates in the high temperature atmosphere, making it difficult to sufficiently lower the refractive index. At the same time, the refractive index changes between the outer portion and the inner portion, and the refractive index no longer becomes a flat step shape. On the other hand, porous preforms formed by depositing glass fine particles having Si-'FF bonds have strong bonds between Si and F, so dehydration and
F does not volatilize during transparent vitrification, and a glass base material whose refractive index does not change even after these post-processes can be obtained.

Once the porous preform 5, which will become the inner part of the cladding, is formed by the second burner 4 in this way, the third burner 6 provided above the second burner 4
is combusted, forming a porous preform 7 which becomes the outer part of the cladding.

The third burner 6 has a multi-tube structure with an oxyhydrogen flame/(-
s t ci 4 Gas, horse Gas, 0□ Gas, A
r gas etc. are supplied, and glass fine particles consisting only of SIO□ are produced in the flame. These glass particles adhere and accumulate on the previously formed porous preform 5 which will become the inner part of the cladding, and become the porous preform 7 which will become the outer part of the cladding.

The porous preform having a three-layer structure thus obtained is subjected to a dehydration treatment using chlorine gas as necessary, and then heated to be transparently vitrified and used as an optical fiber base material. During this transparent vitrification, since the porous preform 3 serving as the core contains a high melting point oxide, the heating temperature is about 1600 to 1700°C.

Next, the transparent vitrified optical fiber base material.

After adding a jacket to the Sekihei glass tube as necessary,
The fiber is melt-spun using a conventional method to obtain the desired single-mode optical fiber with a depressed-clamp structure. Since high melting point oxides are included in this melt spinning,
It is necessary to raise the melting temperature to 1,600 to 1,700° C. or higher, and to perform high-speed spinning in order to rapidly cool the optical fiber and prevent recrystallization of the high-melting-point oxide.

According to the manufacturing method of such an optical fiber base material, M2O3, zrO is added to the dopant of the porous preform 3 serving as the core.
Since a high melting point oxide such as No. 2 is used, the dopant does not react or volatilize during heating in the post-process, and has a uniform refractive index. In addition, since glass fine particles having 5l-F bonds were deposited on the porous preform 5, which is the inner part of the gland, using a plasma torch, there is also a risk of F reacting during heating in the post-process. There is no volatilization, the refractive index is effectively lowered (depressed), and the surface becomes flat. Therefore,
The optical fiber obtained from this optical fiber base material becomes a single mode optical fiber with a depressed clad structure having an ideal refractive index distribution as shown in FIG.

Hereinafter, a specific explanation will be given by showing examples.

Example An optical fiber base material was manufactured using the manufacturing apparatus shown in FIG.

St (?i412 occ/min b Z
rC42rro=, Mouse/min, H24, OJ3/min, 0□
10.0437 minutes and Ar 1.5-137 minutes were supplied to grow a porous preform 3 having a core diameter of 12 nD at the tip of the rod-shaped base material 2 at a growth rate of 90 mm/hour.

As the second burner 4, a high frequency coil 4b having a diameter of 30 mm is wound around the outer periphery of a quartz tube, and this coil 4blC
A high frequency power with a frequency of 13.5 RaHz and an ffi power of 30 KW (7) was supplied to generate a plasma jet. 5i (J. 1000cc/min, Ar4.0
．． 8/min, O□5.0437min, CF42ocr:4
/min to each at the same time to generate glass fine particles and form a porous preform 5 having an outer diameter of 601 Lm and having a size of 10 &f1 minutes. The distance between the second burner 4 and the preform 5 (';' l 80 vs.

Next, in the third burner 6, SiC41800QC-7
min, H2S, 5.137 min, 0□ 87/min, Ar2
．． 0-g/min, respectively, to form a porous preform 7 which becomes the outer part of the crund consisting only of SiO2. The finished outer diameter was 907 uL.

The obtained porous preform with a three-layer structure was heated at 1680° C. and heated with transparent glass, then stretched and sealed in a quartz pipe to obtain an optical fiber base material. The refractive index distribution of the 2t optical fiber obtained by melt-spinning this optical fiber base material has an almost perfect depressed clad structure as shown in Figure 4, and this optical fiber is different from a normal single mode optical fiber. In comparison, the bending properties were excellent and the transmission loss was stable.

2. The second burner 4 is not limited to a high-frequency plasma torch, but any type of non-transfer type direct current plasma torch may be used, but it may cause a reaction with the electrode and cause serious contamination of the porous preform. Please be careful as there may be a risk of

As explained above, the method for manufacturing the optical fiber base material of the present invention is to use the VAD method (1) to form the porous Prefor J as the core with silicon oxide doped with a high melting point oxide such as aluminum oxide or zirconium oxide. Then, a porous preform that will become the inner part of the gland is formed of silicon oxide doped with fluorine using Plasma Jets Y, a porous preform that will become the outer part of the gland is formed only of silicon oxide, and then a transparent It is something that turns into a crow. Therefore, even if this porous preform was exposed to 1K in a high temperature atmosphere during dehydration treatment or quick vitrification, the respective dopants in the core and cladding preforms reacted with other glass components. It does not evaporate or evaporate, and does not cause fluctuations in the refractive index. Therefore, the optical fiber obtained from the optical fiber base material of ζ is
For example, a single mode optical fiber having an ideal depressed cladding structure as shown in Figure 1 is obtained. Also.

It is possible to obtain not only a single mode optical fiber but also a general multimode step index type optical fiber, and one having a particularly sharp refractive index distribution can be obtained.

[Brief explanation of the drawing]

Figure 1 is a graph showing the refractive index distribution of an ideal single-mode optical fiber with a depressed cladding structure, and Figure 2 is a graph showing the refractive index distribution of a single-mode optical fiber with a depressed cladding structure obtained by the conventional VAD method. 3 is a schematic perspective view showing an example of a manufacturing apparatus suitably used in the manufacturing method of the present invention, and FIG. 4 is a graph showing the refractive index distribution of an optical fiber obtained in an example of the present invention. be. 1... First burner% 2... Rod-shaped base material, 3... Porous preform serving as the core, 4.
...Second burner, 5...Porous preform that becomes the inner part of the cladding, 6...Third
burner, 7... Porous preform that becomes the outer part of crat 0. Figure 4 → Um 1-48% ← □125ノΔ Taneyoi Yashu 2

Claims (1)

[Claims] By the VAD method, a porous preform as a core is formed of silicon oxide doped with a high melting point oxide such as aluminum oxide or zirconium oxide, and a porous preform as an inner part of the cladding is formed using plasma. A method for producing an optical fiber base material, which is characterized in that it is formed from silicon oxide doped with fluorine using a jet, a porous preform that becomes the outer part of the cladding is formed from silicon oxide, and then it is made into transparent glass.