JP4690956B2 - Optical fiber preform manufacturing method - Google Patents

Description

  The present invention relates to an optical fiber preform, and more particularly to a method for producing an optical fiber preform used for producing a single mode optical fiber having a zero dispersion wavelength near 1310 nm.

With the rapid spread of the Internet in recent years, the expansion of optical fiber line networks is progressing rapidly. In this optical fiber line network, the most frequently used optical fiber is a single mode optical fiber (SMF) having a zero dispersion wavelength near 1310 nm. This single-mode optical fiber is an optical fiber having excellent characteristics that can realize high capacity at low cost and low loss.
Further, the single mode optical fiber (SMF) can be applied as an optical fiber for low density wavelength division multiplexing (CWDM) transmission by reducing the OH. Therefore, further improvement in characteristics is demanded.

  By the way, the refractive index profile of the single mode optical fiber (SMF) is a so-called simple step type refractive index profile, and the ideal is that the refractive index of the core is increased in a rectangular shape at the interface between the core and the cladding. It is considered good. With such a refractive index distribution, it is possible to obtain an optical fiber with improved dispersion characteristics and small signal waveform deterioration while maintaining a desired mode field diameter (MFD) and cutoff wavelength λc. Are known.

However, a porous base material (hereinafter referred to as a core soot body) including a part of a core and a clad is formed by using a vapor phase synthesis method such as the VAD method, and this is dehydrated and sintered (transparent vitrification). When manufacturing a glass rod for a core, germanium contained in the core diffuses into the cladding around the core during dehydration and sintering, and a skirt of the refractive index distribution is formed at the interface between the core and the cladding.
When the bottom of this refractive index distribution (hereinafter simply referred to as the bottom) increases, the dispersion characteristics of the optical fiber deteriorate. That is, if the mode field diameter (MFD) and the cutoff wavelength λc are maintained at desired values, there is a problem that the zero dispersion wavelength cannot be kept within a desired range.

As a method for solving this problem, as described in Patent Document 1, the relative refractive index difference at the boundary between the core region and the cladding region of the optical fiber preform after transparent vitrification is 0.8 × Δn ( Δn is the relative refractive index normalized by the core radius r of 0.5 × Δn, where d is the radial thickness of the optical fiber, which varies from the relative refractive index difference of the core region) to 0.3 × Δn. A method has been proposed in which only the optical fiber preform whose rate of change (0.5 × Δn) / (d / r) falls within the range of 0.4% to 4.0% is selected to form an optical fiber. Yes.
According to Patent Document 1, when this method is adopted, it is possible to make a judgment at a stage before drawing, rather than judging whether the optical fiber is good or not after drawing the optical fiber from the optical fiber preform. It is said that an optical fiber having a small number of desired characteristics, specifically, a narrow bottom of the refractive index distribution at the boundary between the core and the clad can be obtained with certainty.

Japanese Patent Publication No. 2000-026709

However, this method is only a method for selecting an optical fiber preform that has already been made into a transparent glass, and is not a fundamental solution.
Of course, Patent Document 1 also discloses that the evaluation result of the optical fiber preform after the transparent vitrification is reflected in the optical fiber preform synthesis condition, but simply fine-tunes the synthesis condition in the general-purpose synthesis method. To the extent, it cannot be a fundamental solution to reduce the skirt at the boundary between the core and the clad.

  In view of the above problems, an object of the present invention is to easily and surely obtain an optical fiber that maintains a desired mode field diameter (MFD) and a cutoff wavelength λc, has excellent dispersion characteristics, and has little signal waveform deterioration. To provide an optical fiber preform manufacturing method, that is, an optical fiber preform manufacturing method capable of reliably obtaining an optical fiber having a small skirt of the refractive index distribution at the boundary between the core and the clad. is there.

In order to achieve the above object, an optical fiber manufacturing method according to claim 1 of the present invention includes a step of synthesizing a core soot body having a core portion and a portion of a cladding portion outside the core portion by a gas phase synthesis method, A step of dehydrating the core soot body in a dehydrating atmosphere and then forming a transparent glass in an inert gas atmosphere, and a method for producing an optical fiber preform, wherein the refractive index distribution at the boundary between the core and cladding of the optical fiber is obtained In order to reduce the bottom of the gas, a clad burner for synthesizing a part of the clad part is supplied with a fluorine-containing gas, a core soot body containing fluorine is formed at the boundary with the core part, and the fluorine-containing gas is formed. The concentration distribution of fluorine contained in the skirt is a skirt-shaped gradient distribution having a higher concentration as it is closer to the core portion .

Further, the supply amount of the fluorine-containing gas to the cladding burner per unit time is supplied in an inclined distribution so that the fluorine concentration of the soot to be synthesized becomes higher toward the side closer to the core burner for synthesizing the core portion. It is also characterized.
Furthermore, the soot is composed of a plurality of clad burners, and the amount of fluorine-containing gas supplied to each clad burner per unit time is so synthesized as to be closer to the core burner that synthesizes the core part. Another feature is that the gradient distribution is supplied so that the fluorine concentration of the carbon dioxide increases.

  According to the present invention as described above, an optical fiber having excellent dispersion characteristics and small signal waveform deterioration can be easily and reliably maintained while maintaining a desired mode field diameter (MFD) and a cutoff wavelength λc. An optical fiber preform that can be obtained, that is, a method for producing an optical fiber preform that can reliably obtain an optical fiber having a small skirt of the refractive index distribution at the boundary between the core and the clad is provided.

Hereinafter, the manufacturing method of the optical fiber preform of the present invention will be described in detail with reference to the drawings.
The feature of the optical fiber preform manufacturing method of the present invention is that, for example, using the apparatus shown in FIG. 1, a core soot body 1 including a core and a part of a cladding is manufactured by a vapor phase synthesis method, specifically, a VAD method. The point is to improve the method.
More specifically, a so-called multiple pipe having a cylindrical multiple introduction hole at the front end portion of the starting base material 2 that rises at a constant speed while rotating so that the opening portion faces the front end of the starting base material 2. A type core burner 3 is arranged.
A first cladding burner 4 is disposed adjacent to the core burner 3, and a second cladding burner 5 is adjacent to the first cladding burner 4 and a second cladding burner 5 is adjacent to the cladding burner 5. Three-cladding burners 6 are respectively arranged. The cladding burners 4, 5 and 6 are basically multi-tube burners as well as the core burner 3.

Each of these cladding burners 4, 5 and 6 contains an inert gas such as Ar, a combustion gas comprising oxygen gas, hydrogen gas, etc., a raw material gas comprising a glass raw material gas such as silicon tetrachloride (SiCl ₄ ), and A fluorine-containing gas, which is a feature of the method of the present invention, is supplied to a predetermined layer of each multiple tube. All of these gases are supplied to each burner in a gas state, glass fine particles are formed by a flame hydrolysis reaction, and this is blown toward the starting base material 2 to produce the core soot body 1.

Here, for example, a multi-tube burner as shown in FIG. 2 is used as the core burner 3.
Specifically, silicon tetrachloride gas is used for the central layer 31 of the core burner 3 shown in FIG. 2, germanium tetrachloride gas and hydrogen gas are used for the second layer 32, argon gas is used for the third layer 33, and 4 A predetermined amount of oxygen gas was supplied to each layer 34.
Further, as the cladding burners 4, 5 and 6, multi-tube burners as shown in FIG. 3 were used.
Specifically, silicon tetrachloride gas and SiF ₄ gas are used for the central layer 41 of the first cladding burner 4, hydrogen gas is used for the second layer 42, and argon gas is used for the third layer 43. Is supplied with oxygen gas, 5th layer 45 with argon gas, 6th layer 46 with hydrogen gas, 7th layer 47 with argon gas, and 8th layer 48 with oxygen gas. did.

The second cladding burner 5 and the third cladding burner 6 were supplied with the same type of gas as the first cladding burner 4 for each layer in the same manner as the first cladding burner 4 described above.
However, the most important thing here is that the supply amount of the fluorine-containing gas to each cladding burner per unit time is the fluorine concentration of the soot synthesized in the cladding burner closer to the core burner 3 for synthesizing the core part. The gradient distribution is supplied so as to increase.

In order to increase the fluorine concentration of the soot to be synthesized as the cladding burner closer to the core burner 3, for example, the SiF ₄ gas supplied to the central layer of the second cladding burner 5 per unit time The supply amount is made smaller than that supplied to the center layer 41 of the first cladding burner 4, and the amount of SiF ₄ gas supplied to the center layer of the third cladding burner 6 is set to the center of the second cladding burner 5. The amount is smaller than the amount of SiF ₄ gas supplied to the layer. Alternatively, the ratio of the supply amount of SiF ₄ gas and the supply amount of SiCl ₄ gas per unit time supplied to the center layer of the second cladding burner 5 is supplied to the center layer 41 of the first cladding burner 4. The ratio of the supply amount of SiF ₄ gas and the supply amount of SiCl ₄ gas per unit time supplied to the center layer of the third cladding burner 6 is smaller than that supplied to the center layer of the second cladding burner 5. Just make it smaller.
That, SiF ₄ concentration of soot synthesized by SiF ₄ concentration> third cladding burner 6 of soot synthesized by SiF ₄ concentration> second cladding burner 5 of soot synthesized by the first cladding burner 4, To be.
Thus, the feature of the present invention is that so-called gradient distribution is supplied in which the soot synthesized by the cladding burner close to the core burner 3 increases the concentration of SiF ₄ .

When the core soot body 1 was formed as described above, it was dehydrated in a known technique, that is, in a dehydrating atmosphere, and then transparent vitrified in an atmosphere containing an inert gas. Thereafter, if necessary, this is stretched to reduce the outer diameter, and then the remaining cladding is synthesized by, for example, the OVD method. When a predetermined amount of the clad portion was synthesized, it was dehydrated and made into a transparent glass by a known technique, and finally drawn to obtain an optical fiber.
In the obtained optical fiber, the skirt formed at the boundary between the core and the clad was surely smaller than that obtained by the conventional method. This is because the refractive index of the boundary between the core and the clad is lowered by fluorine by the fluorine-containing gas supplied to the above-described clad burners 4, 5 and 6, and as a result, the skirt portion becomes small. In other words, fluorine is added to the skirt portion in addition to germanium oxide, and both are co-doped. As a result, the refractive index of this portion is lower than that of the conventional one.

  Thus, by reducing the bottom of the refractive index distribution at the boundary between the core and the clad, the obtained optical fiber has a desired mode field diameter (MFD) and a cutoff wavelength λc while maintaining dispersion characteristics. The signal waveform was less degraded.

Incidentally, the fluorine-containing gas is not limited to the above-described SiF ₄ , and for example, a gas such as SF ₆ or CF ₄ can be used.
Examples of the present invention and comparative examples are shown below.

The core soot body 1 was manufactured by arranging the core burner 3 and the cladding burners 4, 5 and 6 as shown in FIG.
Incidentally, silicon tetrachloride was supplied to the central layer 31 in the core burner 3 at 360 cc / min, and hydrogen gas was added to the second layer 32 together with 86 cc / min of germanium tetrachloride. Argon gas was supplied to the third layer 33 and oxygen gas was supplied to the fourth layer 34 in a predetermined amount.
Further, 0.4 l / min of silicon tetrachloride is supplied to each central layer of the first cladding burner 4 and the second cladding burner 5, and SiF ₄ is added to the center layer of the first cladding burner 4 in addition to this. 300 cc / min, SiF ₄ was supplied at 75 cc / min to the center layer of the second cladding burner 5. Further, only 0.5 l / min of silicon tetrachloride was supplied to the center layer of the third cladding burner 6 and SiF ₄ was not supplied.
A predetermined amount of each gas described above, that is, oxygen gas, hydrogen gas, and argon gas, which is an inert gas, was supplied from the supply ports other than the central layer of each of the cladding burners 4, 5 and 6.

As described above, the remaining clad portion was externally attached to the core soot body 1 obtained under such conditions, followed by dehydration and transparent vitrification to obtain an optical fiber preform, which was drawn to obtain an optical fiber. The refractive index profile of the obtained optical fiber is shown in FIG.
In FIG. 4, the portion indicated by the dotted line at the boundary between the core 10 and the clad 20 indicates the bottom 11 of the optical fiber obtained by the conventional method. The solid line shows the refractive index distribution of the optical fiber obtained by using the core soot body 1 obtained by the method shown in Example 1 described above.
As can be seen from FIG. 4, in the optical fiber obtained by the method of the present invention, the skirt 11 is clearly reduced.
The reason is that at the boundary between the core 10 and the clad 20, the first clad is formed at the bottom 11 of the refractive index profile formed in the vicinity of the interface between the core 10 and the clad 20, that is, where GeO ₂ is added. Since fluorine that lowers the refractive index is added by the burner 4 and the second cladding burner 5, the bottom 11 is smaller than the conventional optical fiber (SMF) indicated by the dotted line due to the influence of the fluorine.

When the characteristics of this optical fiber were measured, the cutoff wavelength λc was 1250 nm, the mode field diameter (MFD) was 9.2 μm, and the zero dispersion wavelength was 1312 nm. The maximum refractive index difference Δ1 of the core 10 with respect to the clad 20 was 0.4%, and the core diameter was 7.9 μm.
Incidentally, in the optical fiber obtained by the conventional method, the cutoff wavelength λc is 1250 nm, the mode field diameter (MFD) is 9.3 μm, and the zero dispersion wavelength is 1317 nm.
Thus, in the optical fiber obtained in Example 1, the zero-dispersion wavelength is greatly improved without deteriorating the cutoff wavelength λc and the mode field diameter (MFD), that is, almost unchanged from the conventional one. We were able to. As described above, it was possible to obtain an optical fiber that maintains the mode field diameter (MFD) and the cut-off wavelength λc within the desired ranges and also has excellent dispersion characteristics and small signal waveform deterioration.

In FIG. 1, only one first cladding burner 4 having a multi-tube structure as shown in FIG. 5 is arranged adjacent to the core burner 3. The burner 4 is characterized in that the center layer 41 is vertically divided into four stages as shown in FIG.
Incidentally, the conditions for the core burner 3 were the same as those in Example 1. For the first cladding burner 4, only 0.64 l / min of silicon tetrachloride is supplied to the center layer 41a, and SiF ₄ is supplied to the center layer 41b by adding 0.15 l / min of silicon tetrachloride to 23cc / min. did. More central layer 41c, four of SiF ₄ in the silicon chloride 0.25 l / min was supplied by the addition of 76cc / min, silicon tetrachloride in the bottom of the central layer 41d 0.3 l / min of SiF ₄ in 230 cc / min To be supplied. Thus, the gas was supplied so that the SiF ₄ concentration of the soot to be synthesized became higher, that is, an inclined distribution, from the upper stage to the lower stage, that is, closer to the core burner 3.
5 is positioned so that the side described as “upper” is farther from the core burner 3 and the side written as “lower” is closer to the core burner 3 in FIG. .

After subjecting the optical fiber preform obtained under such conditions, that is, the core soot body 1, to a known treatment such as dehydration and transparent vitrification, the remaining cladding portion is externally attached as described above, and this is further dehydrated, An optical fiber was obtained by drawing an optical fiber preform obtained by forming transparent glass. The refractive index profile of the obtained optical fiber is shown in FIG.
In FIG. 6, the portion indicated by the dotted line at the boundary between the core 10 and the clad 20 shows the skirt 11 of the optical fiber obtained by the conventional method as in FIG. The solid line shows the refractive index distribution of the optical fiber obtained using the core soot body 1 obtained under the conditions shown in Example 2.
As shown in FIG. 6, it can be seen that the bottom 11 is obviously small even in the optical fiber obtained under the conditions of Example 2.

Further, the characteristics of this optical fiber were measured, and almost the same as in Example 1 was obtained.
Specifically, the cutoff wavelength λc was 1250 nm, the mode field diameter (MFD) was 9.1 μm, and the zero dispersion wavelength was 1311 nm. The maximum refractive index difference Δ1 of the core 10 with respect to the clad 20 was 0.4%, and the core diameter was 7.8 μm.
Incidentally, in the optical fiber obtained by the conventional method, as described above, the cutoff wavelength λc is 1250 nm, the mode field diameter (MFD) is 9.3 μm, and the zero dispersion wavelength is 1317 nm.
Thus, even in the optical fiber obtained in Example 2, only the zero dispersion wavelength is increased without degrading the values of the cutoff wavelength λc and the mode field diameter (MFD), that is, almost unchanged from the conventional one. It was possible to improve.
Thus, the mode field diameter (MFD) and the cut-off wavelength λc were completely satisfactory, and an optical fiber having excellent dispersion characteristics and small signal waveform deterioration could be obtained.

(Comparative Example 1)
1, only one first cladding burner 4 as shown in FIG. 3 is arranged adjacent to the core burner 3 shown in FIG.
The conditions of the gas supplied to each layer of the core burner 3 were the same as in the first and second embodiments. However, the center layer 41 of the first cladding burner 4 was supplied with 1.35 l / min of silicon tetrachloride and 350 cc / min of SiF ₄ . Gases similar to those in Example 1 were supplied from the other layers. However, the supply amount was adjusted to some extent according to the supply amount of the raw material gas, and then similar gases, that is, oxygen gas, hydrogen gas and argon gas were supplied.

  After subjecting the optical fiber preform obtained under such conditions, that is, the core soot body 1, to a known treatment such as dehydration and transparent vitrification, the remaining cladding portion is externally attached as described above, and this is further dehydrated, An optical fiber was obtained by drawing an optical fiber preform obtained by forming transparent glass. The refractive index profile of the obtained optical fiber is shown in FIG.

In FIG. 7, the portion indicated by the dotted line at the boundary between the core 10 and the clad 20 shows the skirt 11 of the optical fiber obtained by the conventional method. The solid line shows the refractive index distribution of the optical fiber obtained using the core soot body 1 obtained under the conditions shown in Comparative Example 1.
As shown in FIG. 7, in the optical fiber obtained under the conditions of Comparative Example 1, not only the boundary between the core 10 and the clad 20, but also the refractive index of the clad 20 itself is lowered, and the clad 20 is used as a reference. The hem 11 is not small.
When the characteristics were measured, the cutoff wavelength λc was 1250 nm, the mode field diameter (MFD) was 9.1 μm, and the zero dispersion wavelength was 1315 nm. The maximum refractive index difference Δ1 of the core 10 with respect to the clad 20 was 0.4%, and the core diameter was 7.9 μm.
Incidentally, in the optical fiber obtained by the conventional method, as described above, the cutoff wavelength λc is 1250 nm, the mode field diameter (MFD) is 9.3 μm, the zero dispersion wavelength is 1317 nm, and the improvement width of the zero dispersion wavelength is as follows. It was small.

The major difference between the conditions of Example 1 and Example 2 and Comparative Example 1 is that in the former, the fluorine concentration of the soot synthesized is higher as the supply amount of the fluorine-containing gas supplied from the cladding burner is closer to the core burner. Although the so-called gradient distribution was supplied, in Comparative Example 1, since the supply layer of the fluorine-containing gas is the central layer 41 and one place, the gradient distribution could not be supplied.
On the other hand, in Example 1, by providing a plurality of cladding burners, the fluorine concentration of the soot to be synthesized can be increased as the supply amount of the fluorine-containing gas is closer to the core burner 3. In the second embodiment, the center layer 41 of the first cladding burner 4 is partitioned vertically so that the fluorine concentration of the soot synthesized in the lower stage closer to the core burner 3 can be increased.

  In this way, the refractive index distribution at the boundary between the core and the clad is supplied by supplying a gradient distribution that supplies the fluorine-containing gas so that the fluorine concentration of the soot to be synthesized becomes higher as the supply amount of the fluorine-containing gas is closer to the core burner 3. And the influence on the refractive index distribution on the clad side can be reduced.

In Example 1, the third cladding burner 6 does not supply SiF ₄ at all, but the amount of SiF ₄ supplied to each central layer of the first cladding burner 4 and the second cladding burner 5 is as follows. SiF ₄ may also be supplied from the central layer of the third cladding burner 6 with a value different from that of the embodiment, for example, less. These conditions may be determined in consideration of the size of the skirt to be formed, and the supply amount and supply position of fluorine may be determined so that the skirt becomes smaller.
That is, the amount of supplied SiF ₄ is synthesized by SiF ₄ concentration> third cladding burner 6 of soot synthesized by SiF ₄ concentration> second cladding burner 5 of soot synthesized by the first cladding burner 4 This is the condition of the soot SiF ₄ concentration.

  As described above, according to the present invention, it is possible to easily and surely provide an optical fiber that maintains a desired mode field diameter (MFD) and a cutoff wavelength λc, has excellent dispersion characteristics, and has little signal waveform deterioration. To provide an optical fiber preform manufacturing method that can reliably obtain an optical fiber having a small bottom of the refractive index distribution at the boundary between the core and the clad. Can do.

It is a schematic diagram which shows each arrangement | positioning of the burner for cores which is one Example of the manufacturing method of the optical fiber preform | base_material of this invention, and the burner for clads. It is a top view which shows an example of the burner for cores used for the manufacturing method of the optical fiber preform | base_material of this invention. It is a top view which shows an example of the burner for clads used for the manufacturing method of the optical fiber preform | base_material of this invention. It is a graph which shows the refractive index distribution of the optical fiber obtained using the optical fiber preform | base_material obtained in Example 1 of this invention. It is a top view which shows another example of the burner for a clad used for the manufacturing method of the optical fiber preform | base_material of this invention. It is a graph which shows the refractive index distribution of the optical fiber obtained using the optical fiber preform | base_material obtained in Example 2 of this invention. 6 is a graph showing a refractive index distribution of an optical fiber obtained using the optical fiber preform obtained in Comparative Example 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Core soot body 2 Starting base material 3 Core burner 4 First cladding burner 5 Second cladding burner 6 Third cladding burner 10 Core 11 Bottom 20 Cladding

Claims (3)

A step of synthesizing a core soot body having a core part and a part of a clad part outside the core part by a vapor phase synthesis method, and dehydrating the core soot body in a dehydrating atmosphere, and then forming a transparent glass in an inert gas atmosphere In a method for producing an optical fiber preform having a step,
In order to reduce the bottom of the refractive index distribution at the boundary between the core and the cladding of the obtained optical fiber, a fluorine-containing gas is supplied to the cladding burner for synthesizing a part of the cladding, and the boundary with the core Forming a core soot body containing fluorine in the part ,
A method for manufacturing an optical fiber preform , wherein the concentration distribution of fluorine contained in the fluorine-containing gas is a skirt-shaped inclined distribution having a higher concentration as it is closer to the core portion .   The supply amount of the fluorine-containing gas per unit time to the cladding burner is supplied in an inclined distribution so that the fluorine concentration of the soot to be synthesized becomes higher as the side closer to the core burner for synthesizing the core portion. The method of manufacturing an optical fiber preform according to claim 1.   The clad burner includes a plurality of clad burners, and the amount of fluorine-containing gas supplied to each clad burner per unit time is such that the closer the burner for the clad to the core burner for synthesizing the core part, the more the soot fluorine synthesized. 3. The method of manufacturing an optical fiber preform according to claim 1, wherein the gradient distribution is supplied so as to increase the concentration.

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