JP2003277069A - Method for manufacturing porous preform - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a porous preform which is uniformly doped with GeO<SB>2</SB>, capable of suppressing the variation in a refractive index distribution and the fluctuation in a refractive index to a lower level. <P>SOLUTION: The method for manufacturing the porous perform comprises forming a clad soot 5c around a core soot 5a by using a burner for cladding while forming the core soot 5a by using a burner 3 for a core. The surface temperature distribution near 5b at the front end of the core soot 5a is measured and the heating conditions by the burner 3 for the core are controlled in accordance with such surface temperature distribution so that (1) the distance Rt between the point m of the maximum surface temperature near 5b the front end and the central point c at the front end of the core soot 5a attains half or below the radius r of the core soot 5a or (2) the circumference M around the central point c at the front end of the core soot 5a passing the point m is determined and that the difference between the maximum temperature and minimum temperature on the circumference M attains ≤200°C. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a VA capable of suppressing fluctuations in the refractive index distribution and evenly depositing glass particles even when manufacturing a large-sized porous glass preform.
Regarding the improvement of the D method.

[0002]

2. Description of the Related Art The VAD method is well known as one of methods for manufacturing a silica optical fiber. In the VAD method, a vertically supported starting member is axially rotated, glass fine particles synthesized by a core burner are deposited on the tip of the starting member, and a core soot to be a core of an optical fiber is grown in a rod shape. At the same time, the glass soot synthesized by the burner for clad is deposited on the outer periphery of the core soot to form a clad soot which is a part or the whole of the clad, and is used as a porous base material. The obtained porous base material is heated at a high temperature to perform dehydration treatment and transparent vitrification treatment to obtain a glass base material, and if necessary, a glass for the clad portion is added, and then the glass base material is drawn. Thus, an optical fiber can be manufactured.

In order to synthesize fine glass particles with a burner for core and a burner for clad, the burner contains silicon tetrachloride (SiCl ₄ ) and germanium tetrachloride (GeC).
l ₄ ) etc., a source gas such as hydrogen, a fuel gas such as hydrogen, an oxygen gas for assisting combustion, and an inert gas such as argon. Further, in order to impart a refractive index distribution to the optical fiber, the composition of the raw material gas supplied to the core burner,
The composition of the raw material gas supplied to the clad burner is different, and the core soot has a predetermined concentration of GeO ₂
A dopant such as is doped to form a refractive index distribution.

In order to give a desired refractive index distribution (profile) to the optical fiber, when the porous preform is manufactured by the VAD method, the heating condition by the core burner is controlled to appropriately change the surface temperature of the core soot. Then, the doping amount of GeO ₂ is adjusted. This is because the doping efficiency with which GeO ₂ is incorporated into the core soot strongly depends on the surface temperature of the core soot, and the surface temperature of the core soot is about 600-8.
When the temperature is 00 ° C., the doping efficiency of GeO ₂ is high,
If the surface temperature of the core soot is outside this range, GeO ₂
This is because the doping efficiency of is reduced.

Therefore, a radiation thermometer is installed on the side of the core soot to measure the surface temperature distribution of the core soot, and based on the measured value, GeO ₂ is uniformly doped so that the core burner and the core soot can be uniformly doped. The heating conditions such as the relative position of the fuel gas and the flow rate of the fuel gas supplied to the core burner are controlled.

[0006]

By the way, in recent years, in order to reduce the manufacturing cost of the optical fiber, the size of the manufactured porous preform tends to increase. However, as the outer diameter of the core soot becomes thicker, the change in the surface temperature in the circumferential direction near the tip of the core soot becomes larger, and accordingly, G
The doping amount of eO ₂ becomes non-uniform. Thus, when the doping amount of GeO _{2 in} the circumferential direction becomes non-uniform, it appears as fluctuations in the refractive index distribution, but it is possible to estimate the characteristics of the optical fiber by measuring the refractive index distribution in the state of the optical fiber preform. As the measurement error of the reform analyzer increases,
There is a problem that the characteristics of the optical fiber are changed.

The present invention has been made in view of the above circumstances, and it is possible to uniformly dope GeO ₂ and to suppress fluctuations and fluctuations in the refractive index distribution of the glass base material and the optical fiber to be small. An object of the present invention is to provide a method for manufacturing a quality base material.

[0008]

[Means for Solving the Problems] The problem is to produce a porous base material by forming a clad soot around the core soot by using a clad burner while forming a core soot by using a core burner. In the method for producing a porous base material, the surface temperature distribution near the tip of the core soot is measured, and based on this surface temperature distribution, the point where the surface temperature near the tip is the highest and the center of the tip. The problem can be solved by controlling the heating condition by the burner for core such that the distance to the point is 1/2 times or less of the radius of the core soot. Alternatively, the surface temperature distribution in the vicinity of the tip of the core soot is measured, and based on this surface temperature distribution, a circle passing through a point where the surface temperature is the highest in the vicinity of the tip with the center point of the tip as the center. Further, the heating condition by the burner for core may be controlled so that the difference between the maximum temperature and the minimum temperature on the circumference is 200 ° C. or less.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail based on the embodiments. FIG. 1 shows an example of a manufacturing apparatus for carrying out the method for manufacturing a porous base material according to the present invention. In FIG. 1, reference numeral 1 is a starting member. The starting member 1 is suspended vertically in the chamber 2 and can be axially rotated and vertically moved by a driving means (not shown).

A core burner 3 and a clad burner 4 are provided in the chamber 2. Although only one clad burner 4 is shown in FIG. 1, a plurality of clad burners 4 may be used. The core burner 3 and the cladding burner 4 synthesize glass fine particles from a fuel gas such as oxygen and hydrogen and a material gas such as SiCl ₄ and GeCl ₄ supplied from a gas supply source (not shown). . The glass particles synthesized by the core burner 3 are deposited on the tip of the vertically suspended starting member 1 to form the core soot 5a, which grows in the axial direction. Further, the glass fine particles synthesized by the clad burner 4 are deposited on the outer periphery of the core soot 5a to become the clad soot 5c, which finally form the porous base material 5. At least, the flow rates of the fuel gas and the material gas supplied to the core burner 3 can be controlled by using a flow rate adjusting device (not shown) or the like, and the core burner 3 is moved horizontally or vertically by a moving means (not shown). It can move in any direction.

A first radiation thermometer 6a is installed beside the core soot 5a, and a second radiation thermometer 6b is installed vertically below. First and second radiation thermometers 6a, 6
b is connected to the image processing data recording device 7. The heating condition by the core burner 3 can be controlled based on the surface temperature distribution of the core soot 5a near the tip portion 5b measured by the first and second radiation thermometers 6a and 6b. There is.

Further, a laser outer diameter measuring device 8 is installed beside the core soot 5a. This laser outer diameter measuring instrument 8
Irradiates the core soot 5a with a laser beam and scans the core soot 5a to obtain an outer diameter (radius) r of the core soot 5a.
Is measured.

In the present embodiment, the surface temperature distribution in the vicinity 5b of the tip of the core soot 5a is measured using the manufacturing apparatus shown in FIG. 1, and the core burner 3 uses the measured value of the surface temperature distribution. The heating conditions for the core soot 5a are controlled. FIG. 2 shows an example of the surface temperature distribution in the vicinity 5b of the tip of the core soot 5a measured using the second radiation thermometer 6b. In this example, the center point c of the tip of the core soot 5a shown in FIG. 1 corresponds to the center of the surface temperature distribution. The surface temperature becomes highest at the position m closest to the burner 3 for cores, and the surface temperature decreases as the distance from the burner 3 increases. Therefore, an isotherm centering on the position m is drawn. In FIG. 2, the circle shown by the broken line represents the outer circumference of the core soot 5a, and the shaded area is a region where temperature measurement by the radiation thermometer 6b is substantially difficult.

When controlling the heating conditions of the core burner 3 based on the surface temperature distribution of the core soot 5a,
(1) In the vicinity 5b of the tip of the core soot 5a, the distance Rt between the point m where the surface temperature is highest and the center point c of the tip of the core soot 5a is 1/2 times or less the radius r of the core soot 5a. To be (2) The center point c of the tip of the core soot 5a is used as the center, and the vicinity 5b of the tip
The circumference M passing through the point m at which the surface temperature is highest is obtained, and the difference ΔT between the highest temperature Tmax and the lowest temperature Tmin on the circumference M is set to 200 ° C. or less. A method of controlling the heating condition by the core burner 3 so as to satisfy the condition

By using either of these conditions, GeO ₂ can be more uniformly doped into the core soot 5a. Therefore, it is possible to suppress variations in the refractive index distribution of the glass base material and the optical fiber manufactured from the porous base material 5 to be small. In particular, it is preferable to control the heating conditions so that all of these conditions are satisfied. If these conditions are not satisfied, the doping of GeO ₂ becomes uneven in the longitudinal direction or the radial direction, which is not preferable.

The lower limit value of the difference ΔT between the maximum temperature Tmax and the minimum temperature Tmin on the circumference M is not particularly limited, but according to the heating conditions that can be usually carried out, the Δ
T is usually 30 ° C. or higher.

The reason why the GeO ₂ doping amount becomes extremely uniform by controlling the heating conditions by the core burner 3 as described above is considered as follows. The main cause of the fine fluctuation (fluctuation) in the refractive index distribution of the glass base material and the optical fiber manufactured from the porous base material 5 is that the doping of GeO ₂ into the core soot 5a is non-uniform in the circumferential direction. This is because. It is generally known that the doping amount of GeO ₂ is strongly influenced by the surface temperature distribution of the core soot 5a on which glass particles are deposited.

In general, GeO ₂ produced by thermal oxidation reaction or flame hydrolysis reaction of GeCl ₄ has much higher saturated vapor pressure and volatility than SiO ₂ , and when core soot 5a is exposed to flame and is deposited. , A process in which GeO ₂ synthesized in a flame penetrates into glass fine particles to form a solid solution with SiO _2, and from the glass fine particles to GeO ₂
Is in equilibrium with the process of volatilization. For this reason, when the surface temperature of the core soot 5a is high, GeO ₂ is volatilized from the glass fine particles, so that the doping concentration of GeO ₂ is lowered. On the other hand, if the surface temperature of the core soot 5a is low,
It becomes difficult for GeO ₂ to form a solid solution in SiO ₂ , and crystalline Ge
Since it precipitates as O _{2 and} is volatilized by heating during the subsequent dehydration and transparency treatment, the doping concentration of GeO ₂ decreases.
Therefore, the temperature range in which GeO ₂ is efficiently doped is limited, but it is generally said that the doping efficiency of GeO ₂ is highest when the surface temperature of the core soot 5a is about 600 to 800 ° C.

Therefore, the vicinity 5b of the tip of the core soot 5a
If the surface temperature distribution of No. 2 has a large unevenness, the doping amount of GeO ₂ will be small in the region where the surface temperature is extremely high and the region where the surface temperature is extremely low. Noticeably larger. Therefore, it is considered preferable to reduce the temperature gradient of the surface temperature distribution near the tip portion 5b.

From this point of view, first, the above condition (1) will be considered. In the vicinity 5b of the tip portion of the core soot 5a, the point C in FIG. 2 (the central portion of the core soot 5a) is a location where the temperature change hardly occurs even if the starting member 1 rotates. On the other hand, in FIG. 2, on the line of the circle M passing through the point m where the surface temperature is the highest, the temperature change due to the rotation of the starting member 1 is the largest, and GeO ₂
The fluctuation of the doping amount of is also maximum. Fine fluctuations in the refractive index due to fluctuations in the doping amount of GeO _{2 in} the circumferential direction of the core soot 5a cause a decrease in the measurement accuracy of the preform analyzer and fluctuations in the characteristics of the optical fiber as the distance from the center to the outside of the refractive index distribution increases. It will be easier. The reason is as follows. In the vicinity of the center of the core suit 5a,
Even if the refractive index distribution fluctuates to some extent, the fluctuation region is narrow and has little influence on the intensity distribution of the signal light transmitted through the optical fiber. However, when the fluctuation of the refractive index distribution becomes large at a location away from the center, the area also becomes wider, which causes a remarkable change in the intensity distribution of the signal light transmitted through the optical fiber.

Therefore, as illustrated in FIG. 3, Rt /
When r is 1/2 or less, even if some fluctuation of the refractive index distribution occurs in the radial direction around the circumference M, the measurement accuracy of the preform analyzer is not deteriorated and the characteristics of the optical fiber are hardly changed. . However, as illustrated in FIG.
When Rt / r is larger than 1/2, fluctuations in the refractive index in the radial direction that occur around the circumference M cause a decrease in measurement accuracy of the preform analyzer, and further cause fluctuations in the characteristics of the optical fiber. Also, Rt / r is 1/2
If it becomes larger, the so-called "swelling of soot", which makes the deposition of the glass particles in the radial direction uneven, is likely to occur, and the production of the porous base material 5 itself may become impossible.

Similarly, the above condition (2) will be considered. On the line of the circumference M, when the temperature change becomes large, the change of the doping amount of GeO _{2 in} the circumferential direction of the core soot 5a, that is, the fluctuation of the refractive index distribution becomes large, and the measurement accuracy of the preform analyzer and the characteristics of the optical fiber are increased. It will have a noticeable effect. Furthermore, also in this case, so-called "swelling of soot", which makes uneven deposition of the glass particles in the radial direction, easily occurs, and the production of the porous base material 5 itself may be impossible.

Therefore, by controlling the heating conditions by the core burner 3 so as to satisfy the above condition (1) or (2), the GeO ₂ can be more evenly distributed in the core soot 5a.
It is possible to dope into the glass base material, and it is possible to suppress fluctuations in the refractive index distribution of the glass base material and the optical fiber manufactured from the porous base material 5 to be small.

Core soot 5a by the core burner 3
As the heating conditions for the above, the flow rate of the fuel gas such as oxygen and hydrogen, the relative position between the core burner 3 and the core soot 5a, etc. are exemplified. These conditions can be controlled by using an appropriate control device or the like.

There are the following methods for changing the relative positions of the core burner 3 and the core soot 5a. For example, FIG. 5 is a view of the manufacturing apparatus of FIG. 1 seen from below. As shown in FIG. 5, by moving the core burner 3 in the horizontal direction, the central axis of the core burner 3 and the core soot 3 are moved. The horizontal distance X between the central axis of the core 5a and the horizontal distance Y between the tip of the core burner 3 and the central axis of the core soot 5a can be changed. In addition, the core burner 3 is vertically moved vertically (change in vertical distance Z between the tip of the core burner 3 and the tip of the core soot 5a shown in FIG. 1), or the core burner 3 is moved closer to the direction. Or you may keep away.

In the present embodiment, the core soot 5a
A second radiation thermometer 6b is also installed vertically below.
The reason for doing this is as follows. As the size of the porous base material 5 increases, the outer diameter of the core soot 5a increases and the surface area of the core soot 5a increases. For this reason,
In order to know the situation of the accumulation of glass particles more accurately,
It is necessary to measure the surface temperature distribution of the vicinity 5b of the core soot 5a.

However, it is generally known that the emissivity of the surface of an object depends on the direction of radiation. That is, as shown in FIG. 6, regarding the radiant flux radiated from the surface F of the object, if the radiation angle φ is defined as the angle formed by the radiation direction and the normal to the surface F of the object, the present invention is concerned. In the case of the porous base material 5, when the radiation angle φ is about 55 ° or less, the emissivity has a substantially constant value. However, the radiation angle φ is about 5
If it exceeds 5 °, the emissivity is remarkably reduced and the measurement accuracy of the radiation thermometer 6 is deteriorated.

Therefore, if the surface temperature of the core soot 5a is measured by installing the first radiation thermometer 6a only on the side of the core soot 5a as in the prior art, the vicinity 5b of the tip end of the core soot 5a is the first temperature. Radiation angle φ for radiation thermometer 6a
Therefore, the accuracy of measuring the surface temperature distribution may be reduced. Therefore, the core soot 5a is vertically
The radiation thermometer 6b is installed.

The present inventors have shown in FIG. 1 to confirm how the positions of the first and second radiation thermometers 6a and 6b affect the measurement of the surface temperature of the tip portion of the core soot 5a. By using the first and second radiation thermometers 6a and 6b of the manufacturing apparatus, the vicinity 5b of the tip of the core soot 5a is used.
The surface temperature distribution was measured. Then, the measured value by the first radiation thermometer 6a installed on the side of the core soot 5a and the measured value by the second radiation thermometer 6b installed vertically below the core soot 5a have a difference of about 200 ° C. or more. Appeared. From this, it is considered that by installing the second radiation thermometer 6b vertically below the core soot 5a, the surface temperature distribution in the vicinity of the tip portion 5b can be measured more accurately.

When the second radiation thermometer 6b is installed vertically below the core soot 5a, the flame of the core burner 3 has a great influence. Therefore, it is preferable that the measurement wavelength of the second radiation thermometer 6b is in the 3.0 to 5.3 μm band in order to remove the absorption by the flame and the atmosphere. If the measurement wavelength is out of the above range, the absorption due to the flame of the core burner 3 and the absorption due to the atmosphere become remarkably large, and the surface temperature distribution cannot be accurately measured.

The manufacturing method of the porous base material of the present embodiment is as follows:
As described above, the surface temperature distribution in the vicinity 5b of the tip of the core soot 5a is measured, and the heating condition by the core burner 3 is controlled based on this surface temperature distribution.
It can be carried out in the same manner as the method for producing the porous base material by the method D. For example, regarding the material and shape of the chamber 2,
There are no particular restrictions. In addition, the kind and blending ratio of the fuel gas and the material gas supplied to the core burner 3 and the clad burner 4 should be selected appropriately according to the characteristics such as the refractive index distribution of the target optical fiber. You can Generally, hydrogen and oxygen are used as the fuel gas. As the material gas, in addition to SiCl ₄ , GeCl ₄ , SiF ₄ , POCl ₃ or the like can be added and used. Porous base material 5 manufactured according to the present embodiment
In the same manner as in the prior art, after heating and performing transparent vitrification treatment to obtain a glass preform, an optical fiber can be obtained by drawing the glass preform.

Next, the present invention will be described based on specific examples. The porous base material 5 was manufactured using the manufacturing apparatus shown in FIG. The measurement wavelengths of the first and second radiation thermometers 6a and 6b are 3.0 to 5.3 μm. As the core burner 3, a multi-tube burner having layered hydrogen, oxygen, and argon outlets around the material gas outlet was used. Here, the flow rate of hydrogen gas is 28 liters / minute, the flow rate of oxygen gas is 21 liters / minute, the flow rate of SiCl ₄ is 1.8 liters / minute, the flow rate of GeCl ₄ is 0.12 liters / minute, and argon is used. The flow rate is 8.2 l / min.

The horizontal distance X between the central axis of the burner 3 for core and the central axis of the core soot 5a and the horizontal distance Y between the tip of the burner 3 for core and the central axis of the core soot 5a are shown in Table 1. The porous base material was manufactured by changing in three ways as shown in FIG.

Further, Table 1 shows the parameters of the surface temperature distribution in the vicinity 5b of the tip of the core soot 5a under the respective manufacturing conditions. In Table 1, Rt is the distance between the point m where the surface temperature is highest and the center point c of the tip of the core soot 5a. r is the radius of the core soot 5a. Rt / r is the ratio of Rt and r. Tmax
Is the maximum temperature of the surface temperature distribution on the circumference M. T
min is the minimum temperature of the surface temperature distribution on the circumference M. ΔT is the difference between Tmax and Tmin.

[0035]

[Table 1]

The porous preforms 5 produced under the respective conditions were made transparent and made into glass preforms, and then the refractive index distributions of these glass preforms were measured. An estimated value of the field diameter (hereinafter referred to as MFD) was obtained. In order to confirm the uniformity of the refractive index distribution of the glass base material, for one location in the longitudinal direction of the same base material,
The refractive index distribution was measured from 6 directions. The details of this procedure are as follows.

One measuring position is set in the longitudinal direction of the glass base material, and the glass preform analyzer is used to change the rotation angle of the glass base material by 60 ° with respect to the measuring position. The refractive index distribution was measured 6 times over the entire circumference of the base material. Then, the position in the longitudinal direction was sequentially changed, and the same procedure was repeated 5 times.

In this case, if the fluctuation of the refractive index distribution of the glass base material is small, even if the rotation angle of the glass base material is changed,
Also, at each position in the longitudinal direction, the variation of this estimated value should be extremely small. On the contrary, when the estimated value fluctuates greatly, it can be interpreted that the fluctuation of the refractive index of the glass base material is large and the measurement accuracy of the preform analyzer is deteriorated.

The results of the estimated values of the respective MFDs were produced under the conditions of test number 1 in FIG. 7, in the case of production under the conditions of test number 2 in FIG. 8, and under the conditions of test number 3. The case is shown in FIG. Furthermore, in order to quantify the fluctuation of the refractive index distribution of the glass base material, "MF
The amount of "D variation" was defined as the percentage of the difference between the maximum and minimum MFD estimated values with respect to the average value, and the values in each test are shown in Table 1. In order to obtain an optical fiber preform with excellent characteristics, and further an optical fiber, it is desirable that this "MFD fluctuation" be 1.5% or less. As is clear from this result, by controlling the heating conditions by the core burner 3 so as to satisfy the above manufacturing conditions, the fluctuation in the estimated value of the MFD (MFD fluctuation) became extremely small. From this, it can be seen that the fluctuation of the refractive index distribution of the glass base material becomes extremely small.

[0040]

As described above, according to the method for producing a porous preform of the present invention, GeO ₂ can be uniformly doped in the core soot, which is the core of the optical fiber.
It is possible to suppress fluctuations in the refractive index distribution and fluctuations in the refractive index of the glass base material and the optical fiber manufactured from the porous base material to an extremely small level, and manufacture an optical fiber having excellent characteristics.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an example of a manufacturing apparatus used to carry out a method for manufacturing a porous base material according to the present invention.

FIG. 2 is a diagram showing an example of a surface temperature distribution in the vicinity of the tip of the core soot.

FIG. 3 is a schematic diagram illustrating the state of core soot when the heating conditions of the present embodiment are satisfied.

FIG. 4 is a schematic diagram illustrating the state of the core soot when the heating conditions of the present embodiment are not satisfied.

FIG. 5 is a partial schematic view of an example of a manufacturing apparatus used for carrying out the method for manufacturing a porous base material according to the present invention, as viewed from below.

FIG. 6 is a cross-sectional view illustrating a radiation angle.

FIG. 7 is a graph showing an example of fluctuations in the estimated value of MFD of the optical fiber preform manufactured under the heating condition of test number 1.

FIG. 8 is a graph showing an example of variation in the estimated value of MFD of the optical fiber preform manufactured under the heating condition of test number 2.

FIG. 9 is a graph showing an example of variations in the estimated value of MFD of the optical fiber preform manufactured under the heating condition of test number 3.

[Explanation of symbols]

1 ... Starting member, 3 ... Burner for core, 5 ... Porous base material, 5
a ... core soot, 5b ... near the tip of the core soot, 6a
... 1st radiation thermometer, 6b ... 2nd radiation thermometer, 8 ... Laser outer diameter measuring device.

Claims (2)

[Claims] 1. A method for producing a porous base material, wherein a core soot is formed by using a core burner, and a clad soot is formed around the core soot by using a clad burner. In, measuring the surface temperature distribution near the tip of the core soot,
Based on this surface temperature distribution, the distance between the point where the surface temperature is the highest in the vicinity of the tip and the center point of the tip is 1/2 times or less the radius of the core soot for the core. A method for producing a porous base material, which comprises controlling heating conditions by a burner. 2. A method for producing a porous base material, wherein a core soot is formed by using a core burner, and a clad soot is formed around the core soot by using a clad burner. In, measuring the surface temperature distribution near the tip of the core soot,
Based on this surface temperature distribution, the circumference passing through the point where the surface temperature is highest in the vicinity of the tip with the center point of the tip as the center is obtained, and the difference between the highest temperature and the lowest temperature on this circumference. Is controlled to be 200 ° C. or lower, the heating conditions by the burner for core are controlled, and the method for producing a porous base material.