JPH0327493B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an optical fiber preform, which is a material that is spun into an optical fiber.

The vapor deposition method (VAD) is known as one of the manufacturing methods for optical fiber base material.
SiCl ₄ , GeCl ₄ , POCl ₃ and
By supplying volatile glass-forming raw materials such as BBr ₃ and subjecting these glass-forming raw materials to a hydrolysis reaction in a flame, glass-forming fine particles (hereinafter simply referred to as soot) are produced, and the starting group is This is a method of obtaining a substantially cylindrical optical fiber base material by depositing and growing the soot in the axial direction from the tip of the material. Since this optical fiber preform is usually an unfinished product in the form of a porous preform, it can be made into a finished optical fiber preform by heating and making it transparent.

As a further development of this VAD, a core burner for synthesizing the glass-forming fine particles constituting the core of the optical fiber base material is placed near the growth end of the porous preform, and the glass forming the cladding is placed near the growth end of the porous preform. A clad burner for synthesizing the forming fine particles is placed above the core burner,
Japanese Patent Application Laid-Open No. 1989-1999 discloses a method of integrally synthesizing the core part and the cladding part by growing soot for the cladding on the circumferential surface of a porous preform for the core.
Proposed as No. 134133.

However, in the above-mentioned core-clad integral synthesis method, (1) the soot for the core and the soot for the clad are mixed in space, and the refractive index distribution near the boundary between the core and the clad is achieved as desired. Since the shape is different, the desired transmission performance cannot be obtained.

(2) The cladding is formed immediately after the core is formed, and the core is not shrunk during the formation, so the ratio of the core diameter to the cladding is made small, as in the production of single mode fibers. In this case, the diameter of the porous preform after manufacture becomes too large. For this reason, when the temperature is lowered or heated, the temperature difference becomes large between the axial center and the peripheral surface of the porous preform, increasing the risk that the porous preform will crack due to the difference in shrinkage or expansion. The overall size of the reaction vessel, heater for transparency, etc. also needs to be increased.

(3) If the core burner and cladding burner are separated from each other to prevent the soots from mixing with each other, the porous preform cools between them, increasing the temperature difference between the core surface and the cladding soot. For this reason, the bulk density of the cladding portion becomes low, creating a difference between the bulk density of the core portion and the bulk density of the cladding portion. Therefore, the porous preform is susceptible to cracking due to differences in shrinkage during cooling after the cladding soot is deposited, differences in expansion during heating for transparency, and the like.

There are some problems.

In order to solve these problems, we have created separate exhaust pipes for the core and cladding to prevent soot from mixing, and also moved the burners for the core and cladding closer to each other. A method of reducing the temperature gradient of a porous preform located between both burners has been proposed in JP-A-55-23074.

However, even with this method, it is still insufficient to prevent the core and cladding soot from mixing because the burners for the core and cladding are placed close to each other, and furthermore, the diameter of the porous preform becomes too large, so the cladding part is There still remains the problem that it cannot be adhered very thickly.

On the other hand, an electric furnace or the like is placed between the core burner and the cladding burner, and the cylindrical porous preform for the core on which glass-forming fine particles are attached and grown is heated in the electric furnace or the like, and then they are mutually heated. The diameter of the porous preform for the core is reduced by slightly melting the fused soot to increase the adhesion area between the soots and reduce the gap between the soots, and then the outer periphery is reduced using a burner for the cladding. A method of forming transparent glass by depositing cladding soot on a surface and then heating it was disclosed in JP-A-55-
Proposed as No. 154336.

According to this method, since the cladding portion is attached after the core porous preform is once shrunk, there is no problem that the diameter of the finally obtained porous preform becomes too large.

However, even with this method, the boundaries between the core and cladding parts are still unclear because no measures are taken to prevent the soots for the core and the cladding from mixing, and also because electric furnaces are generally complex and large. In addition, the entire manufacturing equipment is complicated and large, making it impractical.Furthermore, the presence of such large electric furnace materials results in the mist produced by the hydrolysis reaction.
This furnace material is eroded by HCl, and there is a high risk that its products may be mixed into the porous preform as foreign matter, resulting in problems such as a decrease in the transmission performance of the optical fiber.

In view of the above-mentioned problems, the present invention prevents the mixing of the soot for the core and the soot for the cladding in the vicinity of the boundary between the core and the cladding, although it is relatively small and has a simple configuration. Since the refractive index distribution can be made into a desired shape, it is possible to obtain the desired transmission performance, and the temperature gradient of the optical fiber base material during manufacturing can be reduced to prevent cracking, and the resulting optical fiber base material can be Since the ratio of the core diameter to the cladding diameter can be made small without increasing the diameter of the material, it is suitable for manufacturing single mode fibers, etc., and is also suitable for use as a reaction vessel for obtaining an optical fiber base material by depositing and growing glass-forming particles. There is no need to increase the overall size of the heater for transparency, and there is less risk of foreign matter getting into the optical fiber base material, so transmission performance is excellent. It is an object of the present invention to provide a method capable of obtaining an optical fiber base material with excellent transmission performance even when the cladding part is thickened, since the formed fine particles can be volatilized and removed before the cladding part is formed.

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

FIG. 1 shows an example of an optical fiber preform manufacturing apparatus for carrying out the present invention. As shown in FIG. 1, the reaction vessel 1 has a substantially cylindrical shape with a closed upper and lower bottom surface, and is arranged substantially vertically. A substantially cylindrical base material 2 is held approximately at the center of the upper bottom surface of the reaction vessel 1 by a holder (not shown) that can be moved up and down while rotating. Also, near the center of the bottom surface, there are 4 different diameters.
A core burner 3 is provided in which two cylinders are arranged concentrically to form a multi-cylindrical shape. That is, this core burner 3 includes a center nozzle, a second layer nozzle surrounding the center nozzle, a third layer nozzle surrounding the second layer nozzle, and an outermost layer nozzle surrounding the third layer nozzle. have.

Further, near the core burner 3 on the circumferential surface of the reaction vessel 1, two cylindrical high-temperature gas blowing nozzles 4 and 5 and a crud burner 6 having the same structure as the core burner 3 are installed. They are sequentially arranged at predetermined intervals in the axial direction of the container 1.

Further, two cylindrical exhaust hoods 7 and 8 are provided on the circumferential surface of the reaction vessel 1 at positions substantially facing the high temperature gas blowing nozzle 4 and the cladding burner 6, respectively.
is provided.

In order to manufacture a porous preform of an optical fiber base material using the manufacturing equipment described above, first, the base material 2 is
is lowered to near the bottom of the reaction vessel 1. Next, combustible gas and core glass forming raw material gas such as SiCl ₄ and GeCl ₄ are supplied to the core burner 3, respectively.
The glass-forming raw material gas is subjected to a hydrolysis reaction in the flame generated by the combustion of this combustible gas, and the core soot 11 produced thereby is attached and grown on the lower end of the base material 2. A porous preform 12 is obtained.

The core burner 3 has a multi-cylindrical shape as described above, and it supplies glass forming raw material gas from the center nozzle, inert gas such as Ar from the second layer nozzle, and H ₂ from the third layer nozzle. This is to further jet out O ₂ from the outermost layer nozzle. Therefore, the inert gas acts as a barrier between glass forming raw material gas and
It is possible to prevent H ₂ and O ₂ from immediately reacting near the outlet of the core burner 3 and clogging the core burner 3.

Thereafter, the porous preform 12 for the core is pulled up while being rotated using a holder not shown, and is heated and contracted by the jet gas 13 from the high temperature gas blowing nozzles 4 and 5, so that the porous core preform 12 has a reduced diameter. A quality preform 14 is obtained.

At this time, the hot gas blowing nozzles 4 and 5 are
By supplying halogen gas or halogen compound gas such as Cl ₂ or SOCl ₂ , the crystallized GeO ₂ attached to the low temperature part of the porous core preform 12 when the core soot 11 is attached is removed by volatilization as GeCl ₄ . are doing.

That is, the temperature of the adhesion growth surface of the fiber soot 11 is highest at the surface intersecting the central axis of the core burner 3, that is, at the center of the adhesion growth surface, and decreases toward the periphery. And it is one of the raw material gases for glass formation.
GeO ₂ produced by the hydrolysis reaction of GeCl ₄ is
It is known that in high temperature areas it becomes amorphous and adheres as a solid solution with SiO ₂ , and in low temperature areas it adheres alone as crystallized GeO ₂ . In other words, a large amount of this crystallized GeO ₂ is attached to the low temperature area around the attached growth surface.

This crystallized GeO ₂ remains opaque even when the porous preform is heated to make it transparent, and may foam during heating for spinning, significantly reducing the transmission performance of the optical fiber.

Since crystallized GeO ₂ has a higher vapor pressure than amorphous GeO ₂ attached as a solid solution with SiO ₂ , it is usually removed by volatilization during heating for transparency. However, in the so-called core-clad integral growth method in which soot for the cladding is attached and grown on the peripheral surface of the porous preform for the core, if the cladding part is attached too thickly, the crystallized GeO ₂ will be completely volatilized when the porous preform is heated and made transparent. It is difficult to remove. Therefore, as in this embodiment, it is necessary to volatilize and remove the crystallized GeO ₂ before forming the cladding portion.

In this way, the crystallized GeO ₂ is removed by volatilization and the diameter of the core porous preform 14 is reduced.
is further rotated and pulled up using a holder (not shown). Then, a flammable gas and a glass-forming raw material gas for the cladding mainly composed of SiCl ₄ are supplied to the burner 6 for the cladding, and the glass-forming raw material gas is heated in the flame generated by the combustion of the combustible gas. A hydrolysis reaction is carried out, and the soot 15 for the cladding thus produced is attached to the circumferential surface of the porous preform 13 for the core to form a cladding part,
A porous preform 16 is obtained.

The reason why the cladding burner 6 has a multi-cylindrical shape like the core burner 3 is due to the same reason as the core burner 3.

When the base material 2 is gradually pulled up while being rotated in the manner described above, a porous preform 16 having a cladding portion formed around the outer periphery of the core portion is gradually formed from the lower end of the base material 2.

Porous preform 1 thus obtained
6 is heated in a separate heating step to form a transparent optical fiber preform, and this transparent optical fiber preform is further spun to form the final optical fiber.

In addition, in the above-mentioned process for obtaining the porous preform 16, the core soot 1 that did not adhere to the base material 2 or the porous preforms 12, 13
1 and the cladding soot 1 are immediately evacuated by exhaust hoods 7 and 8, respectively.

Comparative examples and examples of the present invention will be shown below, which were carried out using an apparatus substantially similar to the optical fiber preform manufacturing apparatus shown in FIG.

Comparative example 1 [Core burner] Center nozzle SiCl ₄ = 140 c.c./min, GeCl ₄ = 40 c.c./min, POCl ₃ = 2 c.c./min, Ar = 360 c.c./min 2nd Layer nozzle Ar = 400c.c./min Third layer nozzle H ₂ = 3000c.c./min Outermost layer nozzle O ₂ = 6000c.c./min [Clad burner] Center nozzle SiCl ₄ = 150c.c./min min, POCl ₃ = 2c.c./min, Ar = 200c.c./min 2nd layer nozzle Ar = 500c.c./min 3rd layer nozzle H ₂ = 3000c.c./min Outermost layer nozzle O ₂ =5000c.c./min Position l=80mm [High-temperature gas blowing nozzle] Not used [Result] Cracks occurred in the porous preform 16.

Comparative Example 2 [Burner for core] [Burner for cladding] [Nozzle for blowing out high-temperature gas] Same as Comparative Example 1 except l = 40 mm [Results] Porous preform 16 with diameter ≒ 90 mm and bulk density ≒ 0.16 g/cm ³ Obtained. After sintering this at a high temperature of approximately 1550℃, we measured the refractive index distribution in the radial direction, and as shown in Figure 2A, the difference in refractive index was unclear at the boundary between the core and cladding. I was getting used to it.

Example 1 [Core burner] [Clad burner] [High-temperature gas blowing nozzle] Same as Comparative Example 1 One piece used Ar = 8000 c.c./min Temperature immediately after nozzle blowing = 1000°C [Results] Diameter ≒ 80 mm A porous preform 16 having a bulk density of ≈0.18 g/cm ³ was obtained. When the refractive index distribution was measured in the same manner as in Comparative Example 2, it was found that the difference in refractive index became clear at the boundary between the core portion and the cladding portion, as shown in FIG. 2B.

Example 2 [Burner for fiber] [Burner for cladding] [High-temperature gas blowing nozzle] Same as Comparative Example 1 Two pieces used Ar = 4000 c.c./min, N ₂ = 2000 c.c./min, Cl ₂ = 200c.c./min Temperature immediately after nozzle blowout = 1100°C [Results] A porous preform 16 having a diameter of 76 mm and a bulk density of 0.19 g/cm ³ was obtained. When the refractive index distribution was measured in the same manner as in Comparative Example 2, the results shown in FIG. 2C were obtained.

As described above, according to the embodiments 1 and 2 of the present invention, the means for heating and shrinking the core porous preform 12 are the high temperature gas blowing nozzles 4 and 5. It is compact in size and has a simple configuration.

In addition, by arranging the high temperature gas blowing nozzles 4 and 5 between the core burner 3 and the cladding burner 6, both burners 3 and 6 are separated from each other, and the high temperature gas blowing nozzles 4 and 5 are Since the ejected gas 13 acts as a barrier, mixing of the core soot 11 and the cladding soot 15 is prevented. Furthermore, mixing of these can also be prevented by immediately exhausting the core soot 11 and the clad soot 15 that have not adhered to the base material 2 or the core porous preform 12 through the exhaust hoods 7 and 8. be done. Therefore, the refractive index distribution near the boundary between the core part and the cladding part can be made into a desired shape.
It is possible to produce optical fiber preforms with desired transmission performance.

Furthermore, even if the core burner 3 and the cladding burner 6 are separated as described above, the gas 13 emitted from the high-temperature gas blowing nozzles 4 and 5 disposed between them is high temperature, so both burners The temperature gradient between 3 and 6 is small. Therefore, it is possible to prevent the porous preform during manufacture from cracking due to temperature gradients.

Further, the core porous preform 12 is heated and shrunk by the jetting gas 13 from the hot gas blowing nozzles 4 and 5 to form a core porous preform 14 having a small diameter. Therefore, the ratio of the core diameter to the cladding diameter can be made small without increasing the diameter of the resulting optical fiber base material too much.
It is also suitable for manufacturing single mode fibers, etc.
There is no need to increase the overall size of the reaction vessel, the heater for transparency, etc.

In addition, unlike electric furnaces, there is no relatively large furnace material, so the mist of HCl generated by the hydrolysis reaction
Less erosion due to Therefore, there is little risk that products of erosion will be mixed into the porous preform as foreign matter, and therefore an optical fiber base material with excellent transmission performance can be manufactured.

Furthermore, Cl ₂ is added to the high temperature gas blowing nozzles 4 and 5.
Since halogen gas or halide compound gas such as SOCl ₂ is supplied, crystallized GeO ₂ attached to the core porous preform 12 can be removed by volatilization as GeCl ₄ before the formation of the cladding portion. Therefore, even if the cladding portion is thickly deposited, no crystallized GeO ₂ remains, so it is possible to manufacture an optical fiber base material with excellent transmission performance.

The present invention has been described above based on one embodiment, but
The present invention is not limited to this example,
Various changes are possible.

For example, in the above embodiment, two hot gas blowing nozzles 4 and 5 are used, but the number may be three or more, or may be one in some cases.

In addition, in the above-mentioned embodiment, a heating means for transparentizing the optical fiber preform obtained by the optical fiber preform manufactured by this manufacturing apparatus is required separately from the optical fiber preform manufacturing apparatus shown in FIG. However, if an appropriate heating means is provided in the structural apparatus shown in FIG. 1, a transparent optical fiber preform can be obtained using only this manufacturing apparatus.

Since the present invention has the above-described configuration, it is relatively small and simple, but it prevents the core soot and the clad soot from mixing and prevents the soot from being mixed with the soot near the boundary between the core and the cladding. It is possible to obtain the desired transmission performance because the refractive index distribution of the optical fiber can be shaped into the desired shape, and it is also possible to reduce the temperature gradient of the optical fiber base material during manufacturing to prevent its cracking.
In addition, since the ratio of the core diameter to the cladding diameter can be made small without increasing the diameter of the obtained optical fiber base material, it is suitable for manufacturing single mode fibers, etc., and it is also possible to make the optical fiber base material by adhering and growing glass-forming fine particles. To provide an optical fiber base material which has excellent transmission performance since there is no need to increase the overall size of a reaction vessel, a heater for transparency, etc. for obtaining the optical fiber, and there is little risk of foreign matter being mixed into the optical fiber base material. Can be done. In addition, halogen gas or halogen compound gas is supplied to the high-temperature gas blowing nozzle, and the glass-forming fine particles that have crystallized and adhered to the porous preform for the core can be removed by volatilization before the cladding part is formed. It is possible to manufacture an optical fiber base material that has excellent transmission performance even if it is thickened.

[Brief explanation of drawings]

FIG. 1 is a schematic vertical sectional view showing an example of an optical fiber preform manufacturing apparatus for carrying out the present invention, and FIG. 2 is an optical fiber preform obtained in comparative examples and examples for explaining the present invention. The refractive index distribution of the material is shown, and A corresponds to Comparative Example 2, B corresponds to Example 1, and C corresponds to Example 2. Regarding the symbols used in the drawings, 3...
Burner for core, 4, 5... Nozzle for blowing out high temperature gas, 6... Burner for cladding, 12, 14... Porous preform for core, 16... Finally obtained porous preform.

Claims (1)

[Claims] 1. A core burner for synthesizing the glass-forming fine particles constituting the core portion of the optical fiber base material, and a cladding burner for synthesizing the glass-forming fine particles constituting the clad portion of the optical fiber base material. By supplying a raw material gas and a combustible gas, and causing the glass-forming raw material gas to react in a flame in which these combustible gases are burned, the glass-forming fine particles are allowed to adhere and grow, forming a bond with the core portion. In a method for producing an optical fiber preform, the method comprises obtaining an optical fiber preform consisting of a core portion and the cladding portion integrally formed around the core portion,
A high-temperature gas blow-off nozzle is disposed between the core burner and the clad burner, and while halogen gas or halogen compound gas is supplied to the high-temperature gas blow-off nozzle, the core section is A method for producing an optical fiber base material, characterized in that a high-temperature gas is blown onto the fiber.