JP2001240424A - Manufacturing method of base material of optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing an optical fiber base material that contains plural regions with different refractive indexes in a core, by which a process adding a dopant to a clad region is simplified. SOLUTION: Double-cored base material 1a is formed from a first core region 10 to which a first dopant Ge raising the refractive index is added, and a second core region 20 consisting of pure SiO2. A fine particle glass layer for the clad is formed over the material 1a by using a raw material gas which contains a second dopant F decreasing the refractive index, then is sintered, thus causing the result that optical fiber base material 1 having a clad region 30 in which the dopant F is added to the outside of the double core is formed. A cost reduction by simplifying the process is realized because there is no impregnation process for adding the second dopant F before the sintering process.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an optical fiber preform having a plurality of regions having different refractive indexes in a core.

[0002]

2. Description of the Related Art As an optical fiber used for WDM transmission or the like, a refractive index such as a double core type optical fiber formed by including a first core region and a second core region having different refractive indexes in a core. An optical fiber having a refractive index profile having a plurality of different regions in a core is used. At this time,
In many cases, the first core region and the second core region of the double-core type optical fiber are both formed to be higher than the cladding region outside the core and to have different refractive indexes from each other.

The refractive index in each region in an optical fiber is:
It is determined by the type and concentration of the dopant added to those regions. The method of adding the dopant for realizing the above-described refractive index profile to the first core region, the second core region, and the cladding region of the double-core optical fiber includes the following. A method of adding a first dopant for increasing the refractive index to each of the two core regions at a different concentration, and (2) adding the first dopant for increasing the refractive index to the first core region and increasing the refractive index to the cladding region. For example, a method of adding a second dopant to lower the concentration can be considered.

[0004]

Among the above two addition methods, in the case of (1), in order to obtain a necessary refractive index difference between the cladding region and Ge (Germanium) in the core region. It is necessary to increase the additive concentration of the first dopant. In this case, there is a problem that the transmission loss of the optical fiber increases due to the dopant. On the other hand, in the case of (2), by adding a second dopant such as F (fluorine) to the cladding region, the concentration of the dopant added to the core region can be reduced, and the transmission loss of the optical fiber is reduced. It is possible.

As a method of adding the second dopant to the cladding region, a method of impregnating and adding F to the base material in a sintering furnace when a porous glass base material is heated and sintered after synthesis is used. (See JP-A-5-124831). When such a method is used, a step of impregnating the second dopant F is added to the sintering step, so that the number of manufacturing steps and the cost of the manufacturing apparatus need to be increased, and the step in the sintering furnace is required. There is a problem that the time becomes longer and the manufacturing cost increases.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and a process of adding a dopant to a cladding region of an optical fiber preform having a plurality of regions having different refractive indexes in a core is simplified. It is an object of the present invention to provide a method for manufacturing an optical fiber preform.

[0007]

In order to achieve the above object, a method for manufacturing an optical fiber preform according to the present invention comprises:
A core forming step of forming a core base material having at least a first core region to which a first dopant for increasing the refractive index is added and a second core region having a lower refractive index than the first core region; Forming a cladding glass fine particle layer on the outer periphery of the material using a source gas containing a second dopant for lowering the refractive index to form a cladding region outside the first core region and the second core region; and It is characterized by having.

In the above manufacturing method, at least 2
After preparing a core base material (transparent glass base material) including a core having two core regions, a glass fine particle layer for cladding is deposited and synthesized on the outside thereof, and a porous glass composite base material having the glass fine particle layer is synthesized. Sintering to obtain an optical fiber preform. At this time, the method of adding the second dopant such as F to the cladding region is not impregnated and added in the sintering step of making the cladding glass fine particle layer transparent and forming the cladding region. It is added during synthesis.

As described above, by adding the second dopant to lower the refractive index of the cladding region, it is possible to reduce the doping concentration of the dopant in the core region and suppress the transmission loss when the optical fiber is formed. it can. Further, at the time of synthesizing the glass fine particle layer, by using a source gas containing the second dopant as the synthesis burner gas and performing the addition, the impregnation step is added to the sintering step. Two dopants can be added. Therefore, the manufacturing process is simplified, for example, the process time in the sintering furnace is shortened, and the manufacturing cost of the optical fiber preform is reduced.

In the cladding forming step, it is preferable that the glass fine particle layer for cladding is synthesized by a VAD method or an OVD method.

According to these synthesizing methods, the second
The addition of the dopant can be suitably performed. Also, V
The AD method and the OVD method are low-cost production methods, and the production cost can be particularly suppressed by applying the above-described production method to these synthesis methods.

In addition, before the cladding forming step, a core processing step of performing processing including stretching on the core base material is provided, and the heat treatment on the core base material in the core processing step is performed by using a source gas containing hydrogen. The heat treatment is performed by a method other than heating with a flame using the method.

In an optical fiber, it is important to reduce the amount of OH groups contained in order to suppress the transmission loss. Here, in the above-described manufacturing method, since the cladding region (glass fine particle layer for cladding) is directly synthesized on the outer periphery of the core base material, OH groups on the outer peripheral surface of the core base material pose a problem. On the other hand, the heat treatment performed in the core processing step for elongating the core base material or the like is performed using a heating means other than a flame using a raw material gas containing hydrogen, so that the core base material has an outer peripheral surface. Can reduce the generation amount of OH groups, thereby suppressing the transmission loss of the optical fiber. Heating means other than a flame using a hydrogen-containing source gas include an electric heater (electric resistance furnace), a heating light source such as a laser, and a flame using a hydrogen-free source gas.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the method for manufacturing an optical fiber preform according to the present invention will be described below in detail with reference to the drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description. Also, the dimensional ratios in the drawings do not always match those described.

FIG. 1 is a diagram showing the refractive index profiles of (a) a core preform and (b) an optical fiber preform produced in the first embodiment of the optical fiber preform manufacturing method according to the present invention. . In FIG. 1, reference symbol C indicates a central axis of the core preform and the optical fiber preform, a horizontal axis indicates a radial position perpendicular to the central axis C, and a vertical axis indicates a refractive index at each part. ing. In the present embodiment, the relative refractive index difference Δn in each region is determined by the refractive index n in that region and pure Si.
From the refractive index n0 at O ₂ , Δn = (n−n0) / n0
(%).

In the method of manufacturing an optical fiber preform according to the present embodiment, first, a core preform 1a having a refractive index profile shown in FIG. 1A is prepared by using the VAD method (core forming step). The core base material 1a has a first core region 10 including the central axis C and doped with a first dopant for increasing the refractive index, and a lower refractive index than the first core region 10 provided on the outer periphery of the first core region 10. And has a second core region 20 in the core. Specifically, the core glass fine particles are prepared using a glass synthesis burner in which the raw material gas and the like are controlled so as to obtain the above configuration. Then, the obtained core glass fine particles (porous glass base material) are dehydrated in a predetermined atmosphere, for example, an atmosphere containing chlorine, and then are transparentized by heat sintering to obtain a core base material 1a.

The core base material 1a is formed mainly of SiO ₂ (quartz glass), and the refractive index in each region is controlled by the type and concentration of the added dopant. Here, as a first embodiment, the first core region 10 on the center side of the core is formed by adding GeO ₂ (germanium) as a first dopant for increasing the refractive index to SiO ₂ at a predetermined concentration. The outer peripheral side second core region 20 is formed as pure SiO ₂ without addition of a dopant. In addition, Ge into the first core region 10
Addition concentration of O _2, the second core region made of pure SiO ₂ 2
The relative refractive index difference of the first core region 10 with respect to 0 is Δn1 =
It is set to be 0.45%.

Next, the core base material 1a is subjected to necessary processing such as stretching by heat treatment using a burner (core processing step). And when the processing is finished,
A glass fine particle layer for cladding is synthesized on the outer periphery by using the VAD method to form a cladding region 30 which is a region outside the core including the first core region 10 and the second core region 20 (cladding forming step).

The glass fine particle layer for cladding is synthesized by the VAD method by adding H ₂ to the burner for glass synthesis at 40 l / min.
70 l / min O ₂ , 0.9 l / min CF ₄ and SiCl
₄ is performed by supplying a raw material gas having a gas composition and a flow rate of 1.5 l / min. This source gas contains F (fluorine), which is a second dopant that lowers the refractive index.
The glass particle layer for cladding is formed by adding F at a predetermined concentration to SiO ₂ . Then, the obtained porous glass composite base material is dehydrated and transparentized by heating and sintering, thereby producing the optical fiber preform 1. At this time, the clad glass fine particle layer becomes the clad region 30 of the optical fiber preform 1.

FIG. 1B shows a refractive index profile of the optical fiber preform 1. Note that the diameter of the core base material 1a and its respective regions 10 and 20 changes due to stretching or the like.
In (a) and (b), in order to show the configuration of the refractive index profile in the core preform 1a and the optical fiber preform 1 correspondingly, the diameters of the portions corresponding to the core preform 1a are equalized. It is.

The optical fiber preform 1 has a core composed of a first core region 10 and a second core region 20 corresponding to the core preform 1a, and is externally attached to the outer periphery of the second core region 20 of the core preform 1a by the VAD method. And a clad region 30 formed. The second core region 20 (pure S) of the first core region 10 to which the first dopant GeO ₂ for raising the refractive index is added.
The relative refractive index difference with respect to iO ₂ ) is Δn1 = 0.45% as described above for the core base material 1a. The relative refractive index difference between the cladding region 30 to which the second dopant F for lowering the refractive index is added and the second core region 20 is Δn2 = −0.09%.

An optical fiber was prepared by drawing this optical fiber preform 1 and its characteristics were examined.
effective area A _eff 1550 nm = 65.7 μm in nm
m ² , dispersion value ＠ 1550 nm = 7.89 ps / nm / k
m, dispersion slope @ 1550 nm = 0.0595 ps /
nm ² / km, cable cutoff wavelength λc = 1340
transmission loss α _1.55 = 0.19 nm at a wavelength of 1550 nm
A characteristic value of 0 dB / km was obtained.

In the above-described method of manufacturing the optical fiber preform, the refractive index of the cladding region 30 formed outside the first core region 10 and the second core region 20 constituting the double core is reduced. 2 F is added as a dopant. At this time, the first core region 10 and the second core region 2
The relative refractive index difference of 0 with respect to SiO ₂ is reduced, and the concentration of the first dopant GeO ₂ added to the first core region 10 and the second core region 20 can be reduced. Here, as the concentration of GeO ₂ added to SiO ₂ increases, the transmission loss of the optical fiber increases due to Rayleigh scattering or the like. Therefore, by adopting the above configuration in which the additive concentration is reduced, it is possible to suppress transmission loss when the optical fiber is used.

The cladding region 3 of the optical fiber preform 1
F is added to the cladding glass fine particle layer, which becomes 0, using a source gas containing the second dopant F during synthesis. At this time, it is not necessary to add an impregnating step of impregnating the clad with F to the sintering step, so that the manufacturing process and the manufacturing apparatus are simplified, and the process time in the sintering furnace can be shortened, and the optical time can be reduced. The manufacturing cost of the fiber preform is reduced.

In this embodiment, the second core region 20 is formed of pure SiO ₂ . At this time, the reproducibility of the refractive index and the refractive index difference can be improved as compared with the case where the dopant is added to both the first core region 10 and the second core region 20 to set the refractive index. . However, a configuration in which a dopant is added to the second core region 20 is also possible.

As a comparative example with respect to the first embodiment, a core preform and an optical fiber preform were prepared according to the refractive index profiles shown in FIGS. 2 (a) and 2 (b). The core base material 2a in this comparative example is formed by using the VAD method, and as shown in FIG. 2A, is formed into a double core type having a first core region 11 and a second core region 21. The first cladding region 31 is formed around the outer periphery of the second core region 21.
a is provided in the core base material 2a. Further, while the first cladding region 31a is made of pure SiO ₂ ,
Each of the core region 11 and the second core region 21 is doped with a first dopant GeO _{2 at a} predetermined concentration.

After stretching this core base material 2a, a glass fine particle layer for cladding is formed on the outer periphery thereof by VAD method, and dehydration and heat sintering are performed to obtain a refractive index profile shown in FIG. 2 (b). Is obtained. Here, the cladding glass fine particle layer, the gas composition and flow rate supplied to the glass synthesizing burner, the H ₂ 39l / min, O ₂ 70
1 / min, and 1.5 l / min of SiCl ₄ , and formed as pure SiO ₂ without adding the second dopant F. Then, the cladding region 3 of pure SiO ₂ is formed from the second cladding region 31b in which the glass particle layer for cladding is sintered and the first cladding region 31a of the core base material 2a.
1 is configured. The relative refractive index difference between the first core region 11 and the cladding region 31 with respect to the second core region 21 is as follows:
Δn1 = 0.45, each equal to the embodiment shown in FIG.
%, Δn2 = −0.09%.

An optical fiber was prepared by drawing the optical fiber preform 2 and its characteristics were examined. The effective cross-sectional area and the like were almost the same as those in the above embodiment, but the wavelength
The transmission loss α _1.55 at 50 nm is 0.19 in the above embodiment.
0 dB / km, whereas in this comparative example, it was 0.2 dB.
The value was as large as 03 dB / km.

That is, in this comparative example, the first dopant added to the first core region 11 and the second core region 21 to obtain a predetermined refractive index difference from the cladding region 31 formed of pure SiO ₂ . The added concentration of GeO ₂ is higher than that of the embodiment shown in FIG. Therefore, the effect of Rayleigh scattering by the dopant on the light transmitted through the core increases, and the transmission loss deteriorates.

On the other hand, in the above-described first embodiment, the cladding region 30 is doped with the second dopant F to lower the refractive index, and thereby the first core region 10 and the second core F The concentration of the first dopant GeO ₂ added to the region 20 is reduced, and the transmission loss in the optical fiber is suppressed.

Here, in the first embodiment, a burner is used as a heat source for the heat treatment in the core processing step of extending the core base material 1a. On the other hand, by not performing the heat treatment by the flame using the raw material gas containing hydrogen in the core processing step, the core base material 1a
Can be suppressed from occurring on the outer periphery of. O
The H group has a light absorption peak at a wavelength of 1380 nm and causes an increase in transmission loss in an optical fiber.

As a second embodiment of the method of manufacturing an optical fiber preform, the core preform 1a is stretched by an electric heater utilizing electric resistance heating instead of a burner, and the other steps are performed according to the first method. An optical fiber preform was prepared in the same manner as in the example (see FIG. 1).

An optical fiber was prepared by drawing this optical fiber preform, and its characteristics were examined. The effective sectional area and the like were almost the same as those of the first embodiment. On the other hand, OH
The transmission loss peak Δα _1.38 at the wavelength of 1380 nm corresponding to the light absorption peak by the base (the height of the transmission loss peak at the wavelength of 1380 nm with respect to the baseline of the curve of the transmission loss α with respect to the wavelength λ) is 0 in the first embodiment. 0.075dB
/ Km, whereas in the second embodiment, it is 0.
004 dB / km, and it was confirmed that the influence of the OH group was reduced.

Regarding the reduction of the OH group, various heating sources other than the above-described electric heater can be used as long as the heat treatment is performed by means other than heating with a flame using a raw material gas containing hydrogen. . Examples of such a heating source include a heating light source such as a laser and a flame using a raw material gas containing no hydrogen.

Further, even when heating is performed using a flame using a gas containing hydrogen as a raw material gas, the surface of the core base material is etched with an HF solution before forming the glass fine particle layer for cladding, thereby forming a layer containing OH groups. By removing OH
It is possible to reduce the effect of the group.

As a method for adding fluorine during the synthesis of the glass fine particles, there is a production method disclosed in Japanese Patent Application Laid-Open No. 6-219765. However, the method disclosed in the above document forms a layer to which F is added at the time of synthesis on the surface side of a core base material (a rod for a core) having a single core region, and forms an OH on the surface of the core base material. It reduces the group. As for the clad layer, a pure silica glass fine particle layer is deposited on the outer periphery of the core base material by an external method, and then sintered in an F-containing gas atmosphere to form the clad layer. On the other hand, the above-described method for manufacturing an optical fiber preform does not perform F addition using an F-containing gas atmosphere when sintering the glass fine particle layer to be the cladding layer, and synthesizes the glass fine particle layer for cladding during the synthesis. By adding F, simplification of the manufacturing process and reduction of the manufacturing cost are realized.

In addition, F at the time of synthesis by the above-mentioned production method is used.
Comparing the addition with the addition of F during sintering by the conventional manufacturing method, the addition of F during synthesis has the advantage that, in addition to the simplification of the process, the obtained refractive index profile is more stable. I have. Such an effect of stabilizing the refractive index profile is particularly remarkable when the relative refractive index difference between the second core region and the cladding region is small and F added to the cladding region has a low concentration.

FIG. 3 shows an example of the refractive index profile of an optical fiber preform obtained when F is added by impregnation during sintering. The porous glass base material at the time of impregnation does not necessarily have a uniform bulk density distribution in the base material. Here, the impregnation of F into the porous glass base material is affected by this bulk density distribution. Therefore, when the F adding method by impregnation is used, F is not uniformly added to the cladding region.

At this time, as shown in FIGS. 3A and 3B, the change in the refractive index at the interface between the core and the cladding (the interface between the second core region 20 and the cladding region 30) becomes gentle and sufficient. No difference in refractive index was obtained, or FIG.
As shown in (1), there is a problem of disorder in the refractive index profile, such as occurrence of swell of the refractive index near the interface. If such a disorder in the refractive index profile occurs in the optical fiber preform, the characteristics of the optical fiber may be varied individually, or the measured waveform of the cutoff wavelength of the optical fiber may be abnormal, such that it cannot be used as a product. There are cases.

On the other hand, in the above-mentioned manufacturing method, since the distribution of the F concentration is not affected by the bulk density distribution or the like, the occurrence of abnormalities and variations in the refractive index profile in the cladding region is suppressed. And a stable refractive index profile can be obtained.

The method for manufacturing an optical fiber preform according to the present invention is not limited to the above-described embodiments and examples, and various modifications are possible. For example, regarding the dopant added to each region, in the above-described embodiment, the first dopant is used.
G as a first dopant for increasing the refractive index of the core region 10
e, F is used as the second dopant for lowering the refractive index of the cladding region 30, but other dopants may be used. Further, if the second core region 20 is set to have a lower refractive index than the first core region 10, a dopant such as Ge or F may be added instead of pure SiO ₂ .

Further, the configuration of the core base material 1a is not limited to the double core type in the above-described embodiment, but includes a first core region to which a first dopant for increasing the refractive index is added and a first core region. Various configurations are possible as long as the configuration includes at least a second core region having a low refractive index.

FIG. 4 is a diagram showing a refractive index profile of an optical fiber preform in a second embodiment of the method for producing an optical fiber preform according to the present invention. In the present embodiment, the core preform 1a includes the first core region 10, the second core region 20, the third core region 25, and the first clad region 30 from the central axis C side.
a).

Of these, the first core region 10 and the third core region 25 are doped with a dopant such as Ge for increasing the refractive index, and the second core region 20 and the first cladding region 30a are A dopant such as F that lowers the refractive index is added. That is, in the present embodiment, a central core composed of the first core region 10 and a ring core composed of the third core region 25 formed with the low refractive index second core region 20 interposed between the central core. A core.

On the outer periphery of the core base material 1a, a second cladding region 30b to which a second dopant such as F for lowering the refractive index is added at the time of synthesis by the VAD method is formed in the same manner as in the first embodiment. You. The entire clad region 30 is constituted by the first clad region 30a and the second clad region 30b of the core base material 1a. In the present embodiment, a dopant for lowering the refractive index from pure SiO ₂ is added to the second core region 20 so as to have a refractive index substantially equal to that of the cladding region 30. The second core region 20 may have a higher refractive index than that of the cladding region 30 by further increasing the dopant concentration, or may be formed of pure SiO ₂ without adding a dopant.

FIG. 5 is a diagram showing a refractive index profile of an optical fiber preform in a third embodiment of the method for producing an optical fiber preform according to the present invention. In the core base material 1a according to the present embodiment, the first core region 10 to which the first dopant for increasing the refractive index is added is formed as a ring core, and the portion including the central axis C inside the first core region 1a is formed as a ring core.
The second core region 20 having a refractive index lower than 0 is formed. The entire cladding region 30 is constituted by the first cladding region 30a of the core base material 1a and the second cladding region 30b formed by adding a second dopant to the outer periphery thereof at the time of synthesis. Note that the refractive index of the second cladding region 20 can be variously changed as in the above-described second embodiment.

FIG. 6 is a view showing a refractive index profile of an optical fiber preform in a fourth embodiment of the method for producing an optical fiber preform according to the present invention. In the core base material 1a in the present embodiment, the first core region 10 to which the first dopant for increasing the refractive index is added is formed including the central axis C. A dopant for lowering the refractive index is added to the second core region 20 outside the cladding region 3 formed on the outer periphery of the core base material 1a.
It is set to be lower than 0.

FIG. 7 is a diagram showing a refractive index profile of an optical fiber preform in a fifth embodiment of the method for producing an optical fiber preform according to the present invention. The core base material 1a in the present embodiment is different from the refractive index profile in the second embodiment shown in FIG.
0 is a ring core, and a low refractive index region 26 is added inside the ring core.

Also in these second to fifth embodiments, the cladding region 3 formed on the outer periphery of the core base material 1a is formed.
By adding a second dopant such as F to the second cladding region 30b that becomes 0 or a part thereof at the time of synthesis, the manufacturing process can be simplified and the manufacturing cost can be reduced. Further, transmission loss in an optical fiber obtained from the optical fiber preform can be suppressed.

In addition to the above, various modifications are possible for the configuration of the core preform or the optical fiber preform and for each step. In addition, for synthesizing the core glass base material and the clad glass fine particle layer formed on the outer periphery of the core base material, an OVD method may be used instead of the VAD method. The VAD method and the OVD method are low-cost production methods, and the production cost can be particularly reduced by applying the above-described production method to these synthesis methods.

[0051]

The method for manufacturing an optical fiber preform according to the present invention has the following effects as described in detail above. That is, after preparing a core base material having at least a first core region to which a first dopant for increasing the refractive index is added and a second core region having a lower refractive index than the first core region, the core base material is formed on the outer periphery thereof. Then, a cladding glass fine particle layer is synthesized with a raw material gas containing a second dopant for lowering the refractive index to form a cladding region of the optical fiber preform.

At this time, by adding the second dopant to the cladding region, the concentration of the dopant added to the core region can be reduced, and the transmission loss in the optical fiber can be suppressed. Also, by adding the second dopant at the time of synthesizing the glass particle layer for cladding,
Since it is not necessary to add a second dopant impregnating step to the sintering step, the manufacturing steps can be simplified and the manufacturing cost of the optical fiber preform can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing refractive index profiles of (a) a core preform and (b) an optical fiber preform in a first embodiment of a method for manufacturing an optical fiber preform.

FIG. 2 is a diagram showing the refractive index profiles of (a) a core preform and (b) an optical fiber preform in a comparative example of a method for manufacturing an optical fiber preform.

FIG. 3 is a diagram showing an example of a refractive index profile of an optical fiber preform obtained when F addition is performed by impregnation during sintering.

FIG. 4 is a diagram illustrating an example of a refractive index profile of an optical fiber preform in a second embodiment of the method of manufacturing an optical fiber preform.

FIG. 5 is a diagram illustrating an example of a refractive index profile of an optical fiber preform in a third embodiment of the method for manufacturing an optical fiber preform.

FIG. 6 is a diagram illustrating an example of a refractive index profile of an optical fiber preform in a fourth embodiment of the method for manufacturing an optical fiber preform.

FIG. 7 is a diagram illustrating an example of a refractive index profile of an optical fiber preform in a fifth embodiment of the method of manufacturing an optical fiber preform.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical fiber preform, 1a ... Core preform, 10 ... 1st core area, 20 ... 2nd core area, 30 ... Cladding area.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masashi Onishi 1 Tayacho, Sakae-ku, Yokohama, Kanagawa Prefecture Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor Toshihiro Oishi 1 Tayacho, Sakae-ku, Yokohama, Kanagawa Prefecture Sumitomo Electric Ki Kogyo Co., Ltd. Yokohama Works F-term (reference) 2H050 AA01 AB04Y AB05X AB10Y AC28 4G021 BA00 EA01 EA03 EB06

Claims (3)

[Claims] 1. A core forming step of forming a core base material having a first core region to which a first dopant for increasing a refractive index is added and a second core region having a lower refractive index than the first core region. And synthesizing a glass fine particle layer for cladding on the outer periphery of the core base material using a source gas containing a second dopant for lowering the refractive index, and forming a cladding layer outside the first core region and the second core region. And a cladding forming step of forming a region. 2. The method for producing an optical fiber preform according to claim 1, wherein in the cladding forming step, the glass fine particle layer for cladding is synthesized by a VAD method or an OVD method. 3. A core processing step for performing processing including stretching on the core base material before the clad forming step, and a heat treatment on the core base material in the core processing step is performed using hydrogen. The heat treatment is performed by means other than heating with a flame using a raw material gas containing
A method for producing the optical fiber preform according to the above.