JP2003277070A - Method for manufacturing porous preform - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide such a method for manufacturing a porous preform which can uniformly dope a core soot 4 with a dopant and can suppress the variation and fluctuation in the refractive index distribution of a glass preform to a lower level. <P>SOLUTION: The method for manufacturing the porous preform comprises manufacturing the porous preform by depositing the glass particulates synthesized by the gaseous raw materials jetted out of a burner 2 for cladding while forming the core soot 4 by depositing the glass particulates synthesized by the gaseous raw materials jetted out of the burner 1 for the core, thereby forming a clad soot 5. A multitubular burner which has flow passages for the gaseous raw materials, gaseous fuel and gaseous oxygen in a laminar form and has the external diameter of the outermost gaseous oxygen layer greater than 0.6 times the external diameter of the core soot 4 is used as the burner 1 for the core. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a VAD method capable of uniformly doping a dopant even when a large-sized porous base material is manufactured. 2. Description of the Related Art A porous preform used for producing a silica-based optical fiber is produced by various methods, and as one of the production methods, the VAD method is well known. I have. In the VAD method, glass particles synthesized by a core burner are deposited on the tip of a vertically supported starting member while rotating the starting member axially, and a core soot that becomes a core of an optical fiber grows in a rod shape. At the same time, glass fine particles synthesized by a cladding burner are deposited on the outer periphery of the core soot to form a cladding soot that becomes a part or all of the cladding, and is used as a porous base material. Then, the obtained porous preform is heated at a high temperature to perform a dehydration treatment and a transparent vitrification treatment to form a glass preform, and an optical fiber can be manufactured by drawing the glass preform. [0003] In order to synthesize glass particles by a core burner and a clad burner, the burner includes silicon tetrachloride (SiCl ₄ ) or germanium tetrachloride (GeC).
l ₄ ), 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 cladding burner is made different, and the core is doped with a dopant such as GeO ₂ at a predetermined concentration so that the refractive index is higher than that of the cladding. Further, in order to impart a desired refractive index distribution (profile) to the optical fiber, the heating conditions of the core burner are controlled to appropriately change the surface temperature of the glass fine particle layer, and a dopant such as GeO ₂ is used. Is adjusted. This is because the doping efficiency taken into the core soot may vary greatly depending on the surface temperature of the core soot depending on the dopant. For example, the doping efficiency of GeO ₂ increases when the surface temperature of the core soot is in the range of about 500 to 800 ° C., but the doping efficiency decreases when the surface temperature of the core soot is outside this range. I have. In addition, B
It is said that doping efficiency of Br ₃ , SnCl ₄ or the like as a raw material is easily affected by the surface temperature of the core soot. Therefore, for example, a radiation thermometer is installed around the core soot to measure the surface temperature distribution of the core soot,
Based on the measured values, the heating conditions such as the relative position between the core burner and the core soot and the flow rate of the fuel gas supplied to the core burner are controlled so that the dopant is uniformly incorporated into the core soot. Is appropriately reduced. [0007] In recent years, the size of the porous preform tends to increase in size in order to reduce the manufacturing cost of optical fibers. However, since the outer diameter of the core soot is increased due to the increase in the size of the porous base material, it has become difficult to control the surface temperature distribution of the core soot. For this reason, the doping amount of the dopant in the core soot becomes non-uniform in the peripheral direction, and the fluctuation and fluctuation of the refractive index distribution of the glass base material manufactured from the porous base material may increase. As a result, there is a problem that the characteristics of the obtained optical fiber deteriorate. The present invention has been made in view of the above circumstances, and is directed to a porous mother material capable of uniformly doping a dopant into a core soot and suppressing fluctuation and fluctuation of a refractive index distribution of a glass base material. It is an object to provide a method for manufacturing a material. The object of the present invention is to form a core soot by depositing glass fine particles synthesized by a raw material gas ejected from a core burner on a tip of a starting member, and forming a core soot from the clad burner. By depositing glass fine particles synthesized by the ejected raw material gas around the core soot to form a clad soot, in a method of manufacturing a porous base material to manufacture a porous base material, as the core burner, This problem can be solved by using a multi-tube burner having a flow path of a source gas, a fuel gas, and an oxygen gas in a layered form, wherein the outer diameter of the outermost oxygen gas layer is at least 0.6 times the outer diameter of the core soot. . Hereinafter, the present invention will be described in detail based on embodiments. FIG. 1 shows an example of a manufacturing apparatus for performing a method for manufacturing a porous base material according to the present invention. In FIG. 1, reference numeral 1 denotes a core burner, and reference numeral 2 denotes a clad burner. Although only one clad burner 2 is shown in FIG. 1, a plurality of burners may be used. A fuel gas such as hydrogen, an oxygen gas, and a source gas such as SiCl ₄ and GeCl ₄ are supplied to the core burner 1 and the clad burner 2 from a gas supply source (not shown). Glass particles are synthesized by the reaction or the flame hydrolysis reaction. The glass particles synthesized by the core burner 1 are deposited on the tip of a vertically suspended starting member 3 to form a core soot 4 and grow in the axial direction. The glass particles synthesized by the cladding burner 2 are deposited on the outer periphery of the core soot 4 to form the cladding soot 5. The core burner 1 is provided below the clad burner 2 and the core soot 4 is provided.
Are formed so as to hang down below the cladding soot 5. The starting member 3 can be rotated and moved up and down by driving means (not shown). By rotating the starting member 3 axially, the core soot 4 and the clad soot 5 can be deposited axially symmetrically. Further, the starting member 3 is pulled up in accordance with the growth of the core soot 4 in the axial direction. In the present embodiment, a multi-tube burner is used as at least the core burner 1, and the outer diameter of the outermost oxygen gas layer of the multi-tube burner is at least 0.6 times the outer diameter of the core soot 4. It is assumed that By doing so, a sufficiently large flame is obtained, the temperature gradient on the surface of the core soot 4 is reduced, and the dopant can be uniformly doped. The upper limit of the outer diameter of the outermost oxygen gas layer of the core burner 1 is not particularly limited, but if it is too large, the ejection area of the glass fine particles synthesized by the core burner 1 becomes too large, and the core soot diameter becomes larger than necessary. In general, it is preferable to set the outer diameter of the core soot 4 to be equal to or less than the outer diameter of the core soot because it is not preferable because an unstable deposition state is required to obtain a required core soot diameter. In the present invention, the multi-tube burner is a burner in which a plurality of pipes are arranged in layers, a gas is supplied to a gap between the pipes, and a gas layer can be divided into a plurality of layers. The structure of the multi-tube burner, the number of tubes and the type of gas flowing through each tube are not particularly limited, but generally have at least four tubes, and a raw material gas layer is provided between the tubes. It is preferable that at least one fuel gas layer, oxygen gas layer, and inert gas layer are provided. An oxygen gas layer is provided outside of any fuel gas layer via an inert gas layer or without an inert gas layer. Here, the outermost oxygen gas layer refers to the outermost one if there is only one oxygen gas layer, and refers to the outermost one of the oxygen gas layers if there are a plurality of oxygen gas layers. The source gas is SiCl ₄ , GeCl ₄ , S
It is a gas that becomes a glass constituent material such as iF ₄ and BBr ₃ . Generally, SiCl ₄ is used as a main component, and as a dopant, one or a mixture of one or more of GeCl ₄ , SiF ₄ , and BBr ₃ is used as appropriate.
In addition, a carrier gas such as argon may be mixed, diluted, and allowed to flow so that the source gas is easily supplied to the burner stably. Examples of the fuel gas include hydrogen (H ₂ ), propane, butane, and natural gas, and a mixture of a plurality of these gases at a predetermined ratio. However, in general, it is preferable to use hydrogen to form an oxyhydrogen flame because a high temperature is obtained and the amount of by-products is small. Examples of the inert gas include argon (Ar), helium (He), and nitrogen. From the viewpoint of stability and cost, it is preferable to use argon. As the multi-tube burner, for example, a burner having four gas layers in a quadruple tube structure as shown in FIG. 2 is exemplified. In this multi-tube burner, the first tube 1
The raw material gas is stored in the first layer 21 inside the first tube 11, the fuel gas is stored in the second layer 22 in the gap between the first tube 11 and the second tube 12, and the second tube 12 and the third tube 13 In the third layer 23 of the gap between the third tube 13 and the fourth tube 14, an inert gas is supplied.
The layer 24 is supplied with oxygen gas. In this multi-tube burner, the outermost oxygen gas layer is the fourth layer 24.
Therefore, the outer diameter of the outermost oxygen gas layer is
Of the tube 14. A multi-tube burner having a quintuple tube structure as shown in FIG. 3 is also known, and such a multi-tube burner can be used in the present embodiment. In this multi-tube burner, the first layer 21 inside the first pipe 11 contains the raw material gas, the second layer 22 in the gap between the first pipe 11 and the second pipe 12 contains the fuel gas, Second tube 12 and third
In the third layer 23 in the gap with the tube 13, an inert gas is contained.
Oxygen gas is supplied to the fourth layer 24 in the gap between the fourth tube 14 and the fourth tube 14, and inert gas is supplied to the fifth layer 25 in the gap between the fourth tube 14 and the fifth tube 15. You. In this multi-tube burner, since the outermost oxygen gas layer is in the fourth layer 24, the outer diameter of the outermost oxygen gas layer corresponds to the inner diameter of the fourth pipe 14. A double flame burner having an eight-tube structure as shown in FIG. 4 can be used as a multi-tube burner. A double flame burner is a double flame provided with two layers of fuel gas and oxygen gas to obtain a large flame called a double flame, raise the temperature of the flame, and increase the particle diameter of the glass fine particles synthesized in the flame. Is increased, the adhesion to the object to be deposited is facilitated, and a larger amount of glass particles can be synthesized than in a conventional flame, whereby the production of the porous base material can be accelerated. In the double flame burner shown in FIG. 4, the raw material gas is contained in the first layer 21 inside the first tube 11 and the second layer in the gap between the first tube 11 and the second tube 12. The fuel gas is stored in the third layer 2 in the gap between the second pipe 12 and the third pipe 13.
3 is an inert gas, oxygen gas is in a fourth layer 24 in the gap between the third tube 13 and the fourth tube 14, and the fourth tube 14 and the fifth
In the fifth layer 25 in the gap with the pipe 15, an inert gas is contained.
The fuel gas is supplied to the sixth layer 26 in the gap between the pipe 15 and the sixth pipe 16, the inert gas is supplied to the seventh layer 27 in the gap between the sixth pipe 16 and the seventh pipe 17, Oxygen gas is supplied to the eighth layer 28 in the gap between the seventh tube 17 and the eighth tube 18. In this double flame burner, since the outermost oxygen gas layer is in the eighth layer 28, the outer diameter of the outermost oxygen gas layer corresponds to the inner diameter of the eighth tube 18. The method for producing a porous preform of the present embodiment is as follows.
Except for using the above-described multi-tube burner as the core burner 1, it can be carried out in the same manner as the conventional method of manufacturing a porous preform by the VAD method. For example, the type and number of the burners 2 for cladding, the types of fuel gas and raw material gas supplied to the burners 1 for core and the burners 2 for cladding, and the mixing ratio thereof are determined according to the characteristics such as the refractive index distribution of the target optical fiber. And appropriate ones can be used. The porous preform manufactured according to the present embodiment can be made into an optical fiber by heating and clearing the porous preform into a glass preform and then drawing the same as in the prior art. Next, the present invention will be described based on specific examples. A porous base material was manufactured using the manufacturing apparatus shown in FIG. As the core burner 1, a multi-tube burner having a quadruple tube structure as shown in FIG. 2 is used. As a source gas, SiCl ₄ and GeCl ₄ are passed through the first layer 21, and hydrogen and
Argon was supplied to the layer 23 and oxygen was supplied to the layer 24. Here, the flow rate of the hydrogen gas is 7 to 16 liters / minute, the flow rate of the oxygen gas is 4 to 18 liters / minute, the flow rate of SiCl ₄ is 1.9 liters / minute, and the flow rate of GeCl ₄ is 0.1 liter. / Minute, and the flow rate of argon was 3.0 liter / minute. Outer diameter φo of outermost oxygen gas layer of multi-tube burner
₂ and the outer diameter φcore of the core soot were changed in five ways as shown in Table 1 to produce a porous base material. [Table 1] After the porous preforms manufactured under the respective conditions were heated and made transparent to obtain glass preforms, the refractive index distributions of these glass preforms were measured, and the refractive index distribution values were used to obtain optical fibers. The mode field diameter (hereinafter referred to as MFD) was determined. In order to quantify the fluctuation of the refractive index distribution of the glass base material, here, the amount of “MFD fluctuation” is represented by M
The difference between the maximum value and the minimum value of the FD estimation value was defined and used as a percentage of the average value. This "MFD fluctuation" can be measured according to the following procedure. First, the refractive index distribution is measured from one of six directions in one longitudinal direction of the same glass base material.
That is, one measurement position is set in the longitudinal direction of each glass preform, and the rotation angle of the glass preform is changed by 60 ° with respect to this measurement position, and the glass preform is used by using a preform analyzer. The refractive index distribution is measured six times over the entire circumference. While changing the measurement position sequentially in the longitudinal direction,
The same procedure is repeated five times at different longitudinal positions. The MFD can be determined by estimating the MFD from each of the refractive index distributions measured a total of 30 times and calculating the percentage of the difference between the maximum value and the minimum value with respect to the average value. In this case, if the fluctuation of the refractive index distribution of the glass base material is small, the fluctuation of the estimated MFD value is extremely small even when the rotation angle of the glass base material is changed and at each position in the longitudinal direction. It is considered that the “MFD fluctuation” becomes smaller. Conversely, when the fluctuation of the refractive index distribution of the glass base material is large, the fluctuation of the MFD estimated value becomes large when the rotation angle of the glass base material is changed or when the measurement is performed at different longitudinal positions, and the “MFD fluctuation” Is considered to be larger. Therefore, by using the “MFD fluctuation” measured as described above, the fluctuation of the refractive index of the glass base material can be quantified. In order to obtain an optical fiber having excellent characteristics, it is preferable that the “MFD fluctuation” be 1.5% or less. Table 1 shows the measurement results of the "MFD fluctuation".
As is clear from the results shown in Table 1, the "MFD fluctuation" was reduced to 1.5% or less by controlling the heating conditions using the core burner 1 so as to satisfy the above manufacturing conditions. From this, it is understood that the use of the manufacturing method of the present embodiment has made the doping amount of the dopant uniform and suppressed the fluctuation of the refractive index distribution of the glass base material to be extremely small. As described above, according to the method for producing a porous preform of the present invention, it is possible to uniformly dope a dopant such as GeO ₂ into a glass fine particle layer serving as a porous preform. Therefore, the fluctuation of the refractive index distribution and the fluctuation of the refractive index of the glass base material manufactured from the porous base material can be suppressed to a very small value, and an optical fiber having excellent characteristics can be manufactured. Further, the present invention can be easily implemented by simply making the core burner appropriate without requiring a special device.

BRIEF DESCRIPTION OF THE 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 of the present invention. FIG. 2 is a cross-sectional view showing a first example of a multi-tube burner used as a core burner in the present embodiment. FIG. 3 is a cross-sectional view showing a second example of a multi-tube burner used as a core burner in the present embodiment. FIG. 4 is a cross-sectional view showing a third example of a multi-tube burner used as a core burner in the present embodiment. [Description of Signs] 1 ... Core burner, 2 ... Clad burner, 3 ... Starting member, 4 ... Core soot, 5 ... Clad soot.

Claims (1)

Claims: 1. A material gas ejected from a cladding burner while depositing glass fine particles synthesized by a material gas ejected from a core burner on a tip of a starting member to form a core soot. In the method for producing a porous base material by forming a clad soot by depositing the glass particles synthesized by the above around the core soot to form a clad soot, a raw material gas and a fuel gas as the core burner And a flow path of oxygen gas in a layered form, wherein the outer diameter of the outermost oxygen gas layer is a multi-tube burner whose outer diameter is 0.6 times or more the outer diameter of the core soot. Production method.