JPH0653589B2 - Method for manufacturing preform for optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to a novel method for easily and stably producing a preform for an optical fiber having a high numerical aperture.

<Prior Art> Conventionally, as an example of a method for manufacturing a preform for an optical fiber, a so-called rod-inch yuve method has been known in which a core rod having a high refractive index is inserted into a cladding pipe having a lower refractive index, and this is melted and integrated by heating. ing. In general, the high refractive index core rod is formed by doping glass containing silicon dioxide as a main component with one or more kinds of dopants such as germanium dioxide, titanium dioxide, aluminum trioxide, phosphorus pentoxide, etc. for increasing the refractive index. Get by. on the other hand,
The low-refractive-index cladding pipe is obtained by using a glass pipe made of extremely high-purity silicon dioxide, or by adding an additive such as fluorine to lower the refractive index to glass containing silicon dioxide as a main component. Use the one.

Among the dopants that increase the refractive index, germanium dioxide can obtain a high refractive index with a relatively small amount,
Germanium tetrachloride as the raw material is most often used because of its high volatility and the fact that germanium dioxide can be easily obtained by a hydrolysis reaction.

By the way, quartz glass composed of silicon dioxide and germanium dioxide exhibits a high refractive index depending on the concentration of germanium dioxide contained therein. Therefore, when trying to obtain an optical fiber preform having a core showing a particularly high relative refractive index difference with respect to the cladding part, that is, an optical fiber refform having a high numerical aperture, the quartz glass used as the core part is extremely high. The method of doping a high concentration of germanium dioxide has been used.

<Problems to be Solved by the Invention> As described above, when a high numerical aperture high fiber preform is manufactured by the rod-inch tube method, quartz glass doped with germanium dioxide at an extremely high concentration is used as a core rod. The core rod has a high refractive index, and the fact that it is doped with germanium dioxide causes the physical properties such as the melting point and softening point temperature, the coefficient of thermal expansion, and the viscosity coefficient to change from those of pure quartz glass. It is full of. This means a decrease in the physical property matching between the pure silica glass most often used as a cladding pipe and the high refractive index core rod, and the core portion and the cladding portion of the optical fiber preform obtained by using both of them. This leads to an increase in strain generated at the boundary.

Further, germanium dioxide has a property of being easily decomposed and volatilized at high temperature, and the phenomenon becomes more remarkable as the concentration of germanium dioxide is higher or the temperature is higher. Therefore, when a high fiber preform having a high numerical aperture is manufactured by a method that requires processing at an extremely high temperature such as the rod-inch uve method, it is highly doped in the silica glass rod that is the core material. A large amount of germanium dioxide was decomposed and volatilized, and remained as bubbles at the boundary between the core and the cladding after integration, and thermal stress due to the difference in the physical properties of the core and the cladding was concentrated in the bubbles, which was obtained. The phenomenon that high fiber preforms burst and damage during cooling frequently occurs.

As a result of intensive studies by the present inventors, generally, the silica glass has high stress resistance, and in a normal state, the probability that the preform for an optical fiber will be ruptured and damaged by only the thermal stress caused by the difference in the physical properties between the core and the cladding. Is extremely low, but if there are stress concentration parts such as bubbles in the preform, the rupture /
We have found that the probability of reaching damage increases rapidly. Therefore, when manufacturing a preform for optical fibers, it is necessary to prevent the generation of bubbles and the like.

As an example of a specific method for preventing the generation of bubbles, Japanese Patent Laid-Open No. 59-59
As disclosed in Japanese Patent Laid-Open No. 137336, there is already known a method of using a core rod which is coated beforehand by directly vitrifying a pure silica glass layer on the outer periphery thereof. This method attempts to prevent bubbles from remaining by matching the physical properties of the cladding pipe with the physical properties of the surface of the core rod to be integrated, and is based on the facts described below.

The phenomenon that air bubbles remain in the optical fiber preform (hereinafter abbreviated as "preform") is often inevitable when a core rod (hereinafter abbreviated as "rod") is inserted into a cladding pipe (hereinafter abbreviated as "pipe"). The resulting contact scratches on the outer surface of the rod and the inner surface of the pipe were left unsatisfactorily smoothed during the rectification due to the mismatch of the physical properties of the two, and also the gas volatilized from the rod was trapped in the scratched part. It occurs because it remains at. Therefore, it is possible to prevent bubbles from remaining by sufficiently smoothing the contact scratches at the time of integration. By the way, it was stated that the contact scratches that were not smoothed sufficiently were due to the inconsistencies in the physical properties of the rod and the pipe, but more specifically, silica glass with a high dopant concentration generally had a softening temperature higher than that of pure silica glass. For example, when the rod has a high dopant concentration, the rod and the pipe are integrated with each other while the viscosity of the pipe is high, and contact scratches on the inner surface of the pipe remain. Therefore, by preliminarily coating the outer circumference of the rod with pure silica glass, the outer circumference of the rod and the pipe can be sufficiently softened at the same time, and the preform can be manufactured while smoothing contact scratches between them.
Here, a pure quartz glass layer (hereinafter simply
It is abbreviated as a SiO ₂ layer. ) Is limited to a linear vitrification method using a plasma flame or an oxyhydrogen flame, but this is a method other than the above two methods, for example, by depositing glass fine powder on the surface of the rod when the concentration of the dopant in the rod is high. This is because when the SiO ₂ layer is applied, air bubbles will already remain in the sintering method and the like.

When the method described above is used, for example, when the germanium dioxide concentration of the rod is 1.5% in terms of the relative refractive index difference of the silica glass with respect to the pure silica glass, the probability that the preform will rupture / damage is about 35%. However, in the case where the SiO ₂ layer was not provided on the outer circumference of the rod, the probability of burst damage of the preform exceeded 90%.

The number expressed as the relative refractive index difference is an average value of the relative refractive index differences in the core cross section. In this way, the average value of the relative refractive index difference of the core is used as a reference because the volatilization phenomenon of germanium oxide from the core depends on the concentration of germanium dioxide contained in the entire core. In the case where germanium dioxide is doped at a high concentration, but the germanium dioxide concentration in the whole core is relatively low, the amount of germanium oxide volatilized is not a problem.
The reason is that the concentration of germanium dioxide in the entire core has a substantially linear relationship with the relative refractive index difference in the core cross section.
Therefore, if the refractive index distribution in the core is a perfect step type, the relative refractive index difference shown here and the maximum value of the actual relative refractive index difference match, but in the case of the distributed refractive index type, Depending on the distribution state, a difference occurs between the relative refractive index difference shown here and the actual maximum relative refractive index difference of the core. In the detailed description below, similarly, the number shown as the relative refractive index difference means the average value of the relative refractive index differences in the core cross section.

However, the effect of applying a SiO ₂ layer to the outer circumference of this rod is also
There was a problem that as the germanium dioxide concentration in the core increased, it decreased. For example, when the germanium dioxide concentration of the rod is 1.8% in terms of the relative refractive index difference of the quartz glass with respect to the pure quartz glass, the preform will be ruptured or damaged even if a SiO ₂ layer is applied to the outer circumference of the rod. The probability reaches 60%. This is inside the rod and SiO ₂
Due to the difference in the softening temperature of the layer or pipe, the vapor pressure inside the core rises during the process of heating to soften the SiO ₂ layer and the pipe sufficiently, and when the SiO ₂ layer softens, germanium oxide blows out all at once. And the SiO ₂ layer around the rod is very thin (this is because the method of direct vitrification is inefficient. Therefore, if productivity is ignored, obtain a certain thickness. Therefore, as the temperature rises, the stress balance inside the rod collapses at a part of the outer peripheral direction even if the pipe is not sufficiently softened, and integration progresses. It is inferred for one reason.

On the other hand, in addition to the method of forming the SiO ₂ layer on the outer circumference of the lot as described above, there is a method of manufacturing a preform having a high numerical aperture while preventing bubbles from remaining. The method is a rod with a high refractive index not provided with a SiO ₂ layer and silica glass whose refractive index has been lowered by adding fluorine as a pipe material, thereby forming a preform by the rod-in-tube method. It is to manufacture. This method makes use of the fact that the softening point of silica glass becomes lower than that of pure silica glass by adding fluorine. For example, the amount of fluorine added to silica glass is higher than that of pure silica glass. When converted to a relative refractive index difference of −0.3%, the softening point temperature of the quartz glass is lowered by about 120 ° C. as compared with that of pure quartz glass. Therefore, the softening temperatures of the high-refractive index rod and the low-refractive index pipe are close to each other, and even when the preform is manufactured by the rod tube method, the contact scratch between the rod and the pipe is sufficiently smoothed by using such rod and pipe. After that, integration proceeds. Furthermore, since the integration is possible at a lower temperature than when using a pure quartz glass pipe, the amount of germanium oxide volatilized from the inside of the rod is relatively small. For this reason, when a rod containing a large amount of germanium oxide and a pipe containing a large amount of fluorine are used, the probability that the preform will rupture or be damaged when the preform is manufactured by the rod-inch uve method is extremely high. It becomes lower, and the determined preform can be obtained.

However, the optical fiber obtained by heating and spinning the preform produced in this way has a shorter wavelength of 0.8 μm or less than the optical fiber in which the core is quartz glass containing a large amount of germanium dioxide and the cladding is pure quartz glass. It has become clear that the transmission loss in the area is several dB / km to several tens of dB / km, which is not suitable for applications requiring long-distance transmission. As a result of examination by the present inventors, the cause of this loss increase is mainly concentrated in the vicinity of the boundary between the core portion and the cladding portion, and the binding of germanium contained in the core and the fluorine added to the cladding and the spinning It is speculated that the absorption may be due to a type of defect caused by the breaking of the bond.

Also, when the preform is produced by the latter method, the stability of the obtained preform gradually decreases as the concentration of germanium dioxide in the core increases. For example, the concentration of germanium dioxide in the core and the concentration of fluorine in the cladding are
When the relative refractive index difference of the quartz glass with respect to the pure quartz glass was 1.8% and -0.3%, respectively, the obtained preform had a probability of burst damage of about 40%. Further, when the fluorine concentration of the cladding is the same as the above and is -0.3% in terms of relative refractive index difference conversion, and the concentration of germanium dioxide in the core is 2.0% in terms of relative refractive index difference conversion, the obtained preform ruptures. The probability of damage is over 50%. In this way, the stability of the preform decreases as the concentration of germanium oxide in the core increases, and the increase in the amount of germanium oxide volatilized at high temperature and the fluorine concentration that can be added to the cladding are technically high. This is because the difference in the physical property values between the core and the cladding widens because it is limited to.

In view of the above problems, the present invention aims to provide a method for easily and stably manufacturing a preform for optical fibers having a high numerical aperture.

<Means for Solving the Problems> The present invention inserts a core rod having a high refractive index into a cladding pipe having a lower refractive index, and heats the core rod and the cladding pipe inserted from the outer peripheral portion of the cladding pipe into which the core rod is inserted. In the method for producing a preform for optical fiber by melting and integrating, the core rod is
1/60 to 1/125 of the outer diameter of the rod is formed on the outer circumference of the rod made of quartz glass containing germanium dioxide in an amount corresponding to a relative refractive index difference of 1.5% or more with respect to pure quartz glass.
The above-mentioned cladding pipe is a pipe made of quartz glass to which fluorine is added in an amount corresponding to a relative refractive index difference of -0.3% or less with respect to pure quartz glass. And a method for producing a preform for optical fiber.

As a result of diligent studies by the present inventors, when a preform is obtained by the rod-inch tube method using the fluorine-added cladding pipe described in the preceding paragraph, a thin layer made of pure quartz glass should be applied to the outer periphery of the core rod in advance. It has been found that it is possible to reduce the increase in loss. The reasons for this are as follows. By providing a pure quartz glass layer between the core and the cladding, it is possible to prevent direct contact between the high germanium dioxide concentration and the high fluorine concentration, and reduce the induction of defects due to the reaction between germanium and fluorine. On the other hand, the influence of the pure silica glass layer between the core and the cladding on the transmission characteristics can be reduced to a negligible level by reducing the thickness of the pure silica glass layer. it can. As a result of the inventors of the present invention also examining this point, if the thickness of the pure silica glass layer is in the range of 1/60 to 1/125 of the core diameter, there is no practical problem with the effect on the transmission characteristics, and It has been found that it is possible to obtain a good preform with almost no residual air bubbles, which was a problem when a high numerical aperture preform for an optical fiber is manufactured by the conventional rod-inch Ube method.

According to the present invention, since the outer peripheral portion of the rod is made of pure quartz glass and the pipe is made of fluorine-containing quartz glass, there is a physical difference between the two. However, the effect of reducing residual bubbles is substantially improved. There are two possible reasons for this. Firstly, the integration temperature is relatively low, and secondly, the SiO ₂ layer on the surface of the rod is so thin that it deforms as the inside of the rod softens and does not hinder the adhesion between the rod and the pipe.
For these reasons, it is thought that the amount of volatilization of germanium oxide from the rod during heating / melting integration is reduced, and the contact scratches on the outer surface of the rod and the inner surface of the pipe, which cause bubbles to remain, are sufficiently smoothed. .

The method for obtaining a rod made of quartz glass containing germanium dioxide in an amount corresponding to a relative refractive index difference of 1.5% or more with respect to pure quartz glass used in the present invention is not particularly limited, but, for example, water in a flame is used. Derivation of a silicon compound (such as silicon tetrachloride) that can be converted to silicon dioxide by a decomposition or oxidation reaction and a germanium compound (such as germanium tetrachloride) that can be converted to germanium dioxide to form quartz glass fine particles containing germanium dioxide,
A so-called vapor phase axis-attachment method (VAD method) in which this is deposited on the tip of the starting material and the porous material is gradually grown in the axial direction while pulling the starting material upward, and other MCVD methods can be used.

Further, there is no particular limitation on the method for obtaining a pipe made of quartz glass to which a relative refractive index difference with respect to pure quartz glass added with an amount of fluorine corresponding to −0.3% or less, used in the present invention, is used. Pure silica glass porous body manufactured by the vapor phase method was used for high temperature fluorine compound gas (SF6, CC
l ₂ F ₂ etc.) Heat treatment in an atmosphere and shrink-transparent vitrification to obtain a glass rod obtained by perforating the glass rod concentrically with the axis, or with a vapor axis Examples of the method include a method of introducing a fluorine-based raw material such as CCl ₂ F _{2 and the} like to add fluorine when producing a glass porous body by the method, and a method of producing a quartz pipe to which fluorine is added by the MCVD method.

In the present invention, as a method of directly applying a pure silica glass thin film having a thickness of 1/60 to 1/125 of the rod outer diameter by a vitrification method, there is a method using a plasma flame or a method using an oxyhydrogen flame.

In the method using a plasma flame, a raw material compound such as silicon tetrachloride and O ₂ gas are introduced into the thermal plasma generated on the torch to generate pure quartz glass fine particles, and the glass rod is placed in the thermal plasma. At the same time as it is deposited on the outer circumference, it vitrifies directly by utilizing the high sound of thermal plasma. An example of specific conditions at this time is SiCl ₄ 100
_It 's like _SCCM , power 20KW.

On the other hand, in the method using the oxyhydrogen flame, the H ₂ gas is excessively supplied to raise the flame temperature to directly vitrify the deposited glass particles. As an example of specific conditions,
SiCl ₄ 100 _SCCM , H ₂ / O ₂ 25SLM / 16SLM. Comparing the two methods, the method using plasma flame is slightly more efficient.

To control the thickness to 1/60 to 1/125 of the rod outer diameter,
By monitoring the weight. In the case of plasma and oxyhydrogen flame, the base material is supported in parallel and traverses left and right to form an extremely thin glass layer. Although the thickness of the glass layer formed by one traverse is on the order of 0.1 mm, depending on the outer diameter of the starting base material, the thickness can be controlled to some extent by the number of traverses.

<Examples> The effects of the present invention will be described below with reference to the drawings.

During oxyhydrogen flame by Example 1 VAD method (H ₂ / O ₂ flow rate of 3.0 / 8.2 / min), introduced SiCl ₄ 200c.c./ min, a GeCl ₄ 410c.c./ min, GeO ₂ -SiO ₂ Glass fine particles are formed and deposited on the tip of the starting material to grow in the axial direction.
A glass porous base material of 00 mm was obtained. The porous base material was heated at a temperature of 1520 ° C. in a 100% He atmosphere (He flow rate 10 / min)
Shrink transparent glass inside, outer diameter 32mm, length 180mm
Of the sintered body. SiCl ₄ 100 _{SCCM on} the outer periphery of the sintered body,
For a core with a diameter of 15 mm and a length of 400 mm, after traversing twice with a plasma flame under the condition of a power of 20 KW to directly vitrify and providing a pure glass layer with a thickness equivalent to 1/125 of the glass rod outer diameter, A silica glass rod containing GeO ₂ was obtained. The relative refractive index difference of the core rod with respect to pure silica glass was 1.8%.

On the other hand, similarly to the above, pure SiO ₂ glass with an outer diameter of 90 mm and a length of 400 mm was prepared by introducing SiCl ₄ 10 / min into an oxyhydrogen flame (H ₂ / O ₂ flow rate 5.2 / 14 / min) by the VAD method. A porous body is formed and treated in a fluorine-helium atmosphere (fluorine gas 150 c.c./minute, He flow rate 5 / minute) at a temperature of 1500 ° C. to shrink and become transparent, and the outer diameter is 45 mm and the length is 45 mm. 240 mm
4. Fluorine was added to obtain a sintered body having a relative refractive index difference of -0.3% with respect to pure quartz glass, which was pierced into a pipe and stretched to have an outer diameter of 28 mm and a thickness of 5. 7mm, length 4
A 50 mm cladding pipe was obtained.

As shown in FIG. 7, the silica glass rod 1 obtained above was inserted into a glass pipe 2, and while being heated by a burner 3, the glass rod 1 and the glass pipe 2 were concentric with each other as shown by the arrow. Rotate in one direction to uniformize the temperature distribution in the outer peripheral direction. In addition, by depressurizing the gap 4 between the glass rod 1 and the glass pipe 2 to make them easy to integrate, the glass rod 1 and the glass pipe 2 are moved in parallel to each other by moving the glass rod 1 and the glass pipe 2 in parallel with each other. While moving the heating / melting portion of the glass pipe 2, the glass rod 1 and the glass pipe 2 are integrated over the entire length to have an outer diameter of 26 mm, a length of 400 mm, a core / clad diameter ratio of 0.5, and a refractive index distribution. An almost step index type preform having an α coefficient of 5 or more was obtained.

In the preform thus obtained, almost no air bubbles remained, and the probability of bursting / damage was less than 5%. Also, the cladding diameter obtained by heating and spinning the preform was 165 μm.
When the transmission loss of an optical fiber having a m and a core diameter of 82.5 μm was measured, it was 12 dB / km at a wavelength of 0.6 μm.

Example 2 In the manufacture of the glass rod 1 for a core of Example 1, the same procedure was performed except that the GeCl ₄ flow rate with soot was 600 c.c./min and the number of traverses of the plasma flame was changed to 5. , The relative refractive index difference with respect to pure quartz glass is 2.
1/60 of the outer diameter of the glass rod is placed on the outer circumference of the silica glass rod containing germanium oxide in an amount equivalent to 3%.
13mm in diameter with a layer of pure quartz glass with a thickness equivalent to
A glass rod having a length of 400 mm was obtained.

Further, in the production of the glass pipe 2 for cladding of Example 1, the same procedure was applied except that the F gas flow rate during sintering was 300 c.c./min and the He gas flow rate was 1 / min. Two
A quartz glass pipe having a thickness of 8 mm, a thickness of 5.7 mm, a length of 450 mm, and a fluorine content of -0.5% in terms of the relative refractive index difference with respect to pure silica glass was obtained.

Then, the glass rod and the glass pipe were integrated by the same operation as in Example 1 to obtain a step index type optical fiber preform having the same size as in Example 1.

Also in this case, as in Example 1, there was almost no air bubbles remaining in the preform, and the probability of burst damage was 10% or less. The transmission loss of an optical fiber having a cladding diameter of 165 μm and a core diameter of 82.5 μm obtained by heating and spinning the preform was 14 dB / km at a wavelength of 0.6 μm.

The graph of FIG. 2 is a graph showing the relationship between the combination of the relative refractive index difference of the cladding pipe of pure silica glass and that of the core rod of the preform manufactured according to the present invention and that of the manufactured preform, together with comparative examples. Is.

The absolute value of the relative refractive index difference horizontal axis of FIG. 2 is for pure silica glass Kuratsudopaipu △ ^- ordinate axis represents the relative refractive index difference △ ⁺ for pure silica glass Koarotsudo, for optical fiber was produced by combining these preform The quality judgment of is indicated by a symbol. "○" indicates that the probability of burst damage to the optical fiber preform is extremely low, and the transmission loss of the optical fiber obtained by spinning the optical fiber preform is at a level that poses no practical problems. "X" indicates that it is for optical fiber. If the preform has a high probability of burst damage, "△" indicates that the probability of the optical fiber preform bursting or damaging is 25% to 50%.
25% in the middle of what is indicated by "x" for%
To 50% are shown. As can be seen from Figure 2, △ ^- and a more than 0.3% Kuratsudopaipu, △ ⁺ is Yotsute to be used by more than 1.5% Koarotsudo, stable and good preform having a high numerical aperture manufacture can do. The numerical value in parentheses attached to each symbol indicates the ratio of the thickness of the SiO ₂ layer to the rod outer diameter.

In the above embodiment, the burner is used for heating and melting, but it is also possible to use an electric furnace or the like as the heat source. Further, the pressure in the gap 4 between the glass rod 1 and the glass pipe 2 is reduced, but this is to facilitate the integration.
This means that the pressure in the gap between the glass rod 1 and the glass pipe 2 is set to be lower than the pressure in the outer circumference of the glass pipe 2, and accordingly, the same effect can be obtained by increasing the outer circumference of the glass pipe 2.

<Effect of the Invention> As described above, by doping germanium dioxide, the relative refractive index difference with respect to pure silica glass is 1.
1/60 to 1/125 of the outer diameter of the core rod made of quartz glass adjusted to 5% or more
And a silica-based glass rod having a structure in which a pure silica glass layer having a thickness equivalent to is previously coated, and by adding fluorine, the relative refractive index difference with respect to pure silica glass is −
By using a cladding pipe made of silica-based glass adjusted to 0.3% or less, rupture / damage occurs when a low loss optical fiber preform having a high numerical aperture is manufactured by the rod inch tube method. It has become possible to reduce the probability of doing.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an embodiment of the present invention, and FIG. 2 is a graph showing the effect of the present invention. The combined SiO ₂ layer thickness of the F-containing cladding pipe and GeO ₂ core rod and the rupture damage of the obtained preform. The probability relationship is shown.

Claims (1)

[Claims] 1. A core fiber having a high refractive index is inserted into a cladding pipe having a lower refractive index, and the core rod and the cladding pipe, which are heated by heating from the outer peripheral portion of the cladding pipe into which the core rod is inserted, are melted and integrated at the same time for an optical fiber. In the method for producing a preform, the core rod is formed on the outer circumference of a rod made of quartz glass containing germanium dioxide in an amount corresponding to a relative refractive index difference of 1.5% or more with respect to pure quartz glass. 1/60 to 1 /
It is a pure quartz glass thin layer with a thickness of 125,
A method for manufacturing a preform for optical fiber, wherein the cladding pipe is a pipe made of quartz glass to which an amount of fluorine corresponding to a relative refractive index difference with respect to pure quartz glass is -0.3% or less is added.