JP2007091579A - Method of manufacturing optical fiber preform - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing an optical fiber preform by which uniform characteristics are attained through the longitudinal direction of an optical fiber. <P>SOLUTION: The method of manufacturing the optical fiber preform comprises a first step for preparing a glass tube consisting essentially of quartz to become clad glass, a second step for producing glass fine particles and depositing them inside the glass tube by introducing a raw material gas and gaseous oxygen in the glass tube while heating the glass tube, a third step for introducing the glass tube on which the glass fine particles are deposited into a heating oven and increasing the bulk density of the glass fine particle layer deposited on the glass tube, a fourth step for adding an additive into the glass fine particle layer inside the glass tube after heat-treated, and a fifth step for vitrifying the glass fine particle layer by heating the glass tube after the adding step and collapsing a hallow part to solidify. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

 The present invention relates to a method for manufacturing an optical fiber preform, and more particularly to a method for manufacturing an optical fiber preform used for manufacturing an optical fiber in which a rare earth element or Al is added to the core of the optical fiber.

 As an optical fiber preform manufacturing method used for manufacturing an optical fiber in which a rare earth element or the like is added to the core, there is known an MCVD method (also called an internal method) in which a rare earth element or the like can be easily added to a portion to be a core. (For example, refer to Patent Document 1 and Non-Patent Document 1).

In a conventional method of manufacturing an optical fiber preform, first, a glass tube made of quartz glass is prepared, and then SiCl ₄ gas, GeCl ₄ gas, and O ₂ are rotated from the end surface of the glass tube to the hollow portion while rotating the glass tube. Introduce gas. The glass tube is heated while traversing the burner along the glass tube in parallel with the introduction of these source gases. With this heating, the temperature of the hollow portion rises, and SiCl ₄ and GeCl ₄ are oxidized in the hollow portion to generate fine particulate soot, which is deposited on the inner surface of the glass tube and baked to form a glass fine particle layer. The Subsequently, Er is added to the glass fine particle layer by the immersion method and dried, and then the glass tube is heated to make the porous glass layer transparent, thereby forming a transparent glass layer. Subsequently, the glass tube is stretched and solidified to obtain an optical fiber preform.

Additives added to the core by the immersion method include, for example, rare earth elements such as Er and Yb, and elements such as Co and Al.
Japanese Patent No. 3517848 Ed. Sudo, “Erbium-doped fiber amplifier”, Optronics, p. 75

 The production of the optical fiber preform by the conventional MCVD method is performed as described above, but it is difficult to uniformly add the additive added to the core by the immersion method over the longitudinal direction of the optical fiber preform. Met. For this reason, the optical fiber obtained by drawing the optical fiber preform has variations in the amount of the additive in the longitudinal direction, and the optical characteristics of the optical fiber also differ depending on each part in the longitudinal direction. For example, when an additive added to the core by the immersion method is an element having an optical amplification effect such as Er or Yb, the optical amplification characteristic of an optical fiber manufactured from the optical fiber preform is affected. In addition, when the additive added to the core by the immersion method is an element such as Al that has a function of changing the refractive index in the glass, the cutoff wavelength and mode field diameter of the optical fiber made from this optical fiber preform are used. It affects the optical fiber optical characteristics.

In addition, Er-added optical fibers in which Er and Al are added to the core can broaden and smooth the Er ³⁺ fluorescence cross-sectional area by adding Al, and clustering that occurs when Er ³⁺ is added at a high concentration. It is known to have an action to prevent. Therefore, it affects the broadband property / smoothness of the Er ³⁺ fluorescence cross section of the optical fiber produced from the optical fiber preform produced by adding Al and Er to the core by the immersion method.

 As described above, in the conventional method for manufacturing an optical fiber preform, it is difficult to uniformly add an additive element along the longitudinal direction of the core, and an optical fiber manufactured from an optical fiber preform manufactured by a conventional method is difficult. Since there are variations in the amount of additive in each part in the longitudinal direction, the optical amplification characteristics, optical fiber optical characteristics, broadness and smoothness of the fluorescence cross section as described above are also found in each part in the longitudinal direction of the optical fiber. It will be different. Therefore, in the prior art, it is difficult to obtain a large amount of optical fibers having desired optical fiber characteristics from one optical fiber preform.

 The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a manufacturing method for an optical fiber preform that can obtain uniform characteristics in the longitudinal direction of the optical fiber.

 In order to achieve the above object, the present invention provides a first step of preparing a glass tube mainly composed of quartz, which is a clad glass, and a glass raw material gas and an oxygen gas are added to the glass tube while heating the glass tube. A second step of introducing and generating glass fine particles in the glass tube and depositing the glass tube on which the glass fine particles are deposited in a heating furnace to perform heat treatment, and the glass deposited in the glass tube A third step of increasing the bulk density of the fine particle layer, a fourth step of adding an additive into the glass fine particle layer in the glass tube after the heat treatment, and heating the glass tube after the addition step And a fifth step of making the glass fine particle layer transparent and crushing the hollow portion to make it solid.

 In the optical fiber preform manufacturing method of the present invention, the heating means in the second step is an oxyhydrogen burner, the heating furnace in the third step is an electric furnace or an induction heating furnace, and the fifth step The heating means is preferably an oxyhydrogen burner.

In the method for manufacturing an optical fiber preform of the present invention, it is preferable bulk density of the glass fine particle layer after the third step is in the range of ^{0.1g / cm 3 ~0.4g / cm 3} .

 In the method of manufacturing an optical fiber preform according to the present invention, the heating furnace used in the third step includes a plurality of heating means so that at least a portion of the glass tube on which the glass fine particles are deposited can be simultaneously heated. Or a heating furnace having heating means having a length sufficient to simultaneously heat at least all of the glass tube where glass fine particles are deposited.

 In the method for producing an optical fiber preform of the present invention, the additive added in the fourth step is preferably one or more selected from the group consisting of rare earth elements and Al.

 According to the present invention, it is possible to make the bulk density of the glass fine particles uniform over the entire length in the longitudinal direction of the glass tube, and in the subsequent step of adding the additive into the glass fine particle layer, Er, Al, etc. The additive can be added uniformly over the entire length in the longitudinal direction. As a result, the optical fiber preform obtained by the manufacturing method of the present invention can produce a rare earth-doped optical fiber having a uniform rare earth concentration in the longitudinal direction.

 The method for producing an optical fiber preform of the present invention includes a first step of preparing a glass tube mainly composed of quartz, which is a clad glass, and a glass raw material gas and an oxygen gas while heating the glass tube. The second step of generating and depositing glass fine particles in the glass tube and the glass tube on which the glass fine particles are deposited are put into a heating furnace and subjected to heat treatment, and deposited in the glass tube. A third step of increasing the bulk density of the glass fine particle layer, a fourth step of adding an additive into the glass fine particle layer in the glass tube after the heat treatment, and the glass tube after the addition step. And a fifth step of making the glass fine particle layer transparent by heating and crushing the hollow portion to make it solid.

 In the conventional optical fiber preform manufacturing method, glass particles are generated and deposited in a glass tube, and then the glass tube is heated using an oxyhydrogen burner, and the glass particles are baked and hardened to increase the bulk density of the glass particle layer. increase. At this time, baking is performed while the oxyhydrogen burner is moved along the longitudinal direction of the glass tube. However, in such a conventional method, the oxyhydrogen burner itself is locally heated, and the glass tube is heated by the movement of the oxyhydrogen burner, so the entire glass tube is not uniform. The bulk density is not uniform. Therefore, in the subsequent step of adding the additive into the glass fine particle layer, there is a problem that the amount of the additive added becomes non-uniform in the longitudinal direction.

FIG. 1 shows the change in bulk density with respect to the heating temperature when the glass fine particles are baked and solidified to increase the bulk density of the glass fine particle layer. As an example, FIG. 2 shows how the Al concentration of the core portion of the optical fiber preform manufactured by adding Al in the fourth step changes with respect to the bulk density.
FIG. 2 shows that the Al concentration decreases as the bulk density increases. Further, FIG. 1 shows that the bulk density increases as the heating temperature increases. Therefore, the amount of the additive added to the glass fine particle layer in the fourth step is affected by the heating temperature when increasing the bulk density of the glass fine particle layer.

A process for increasing the bulk density of the glass fine particle layer by baking the glass fine particles in the conventional method for producing an optical fiber preform will be described with reference to FIG.
While controlling the flow rate of oxygen and hydrogen so that the temperature of the portion heated by the oxyhydrogen burner 12 reaches the target temperature, the glass tube 10 is heated while reciprocating the oxyhydrogen burner 12 to the left and right to thereby glass. The bulk density of the fine particle layer 11 is increased. Since it is such a heating method, the whole glass tube 10 cannot be heated simultaneously and uniformly. Therefore, the bulk density of the glass fine particle layer 11 in the longitudinal direction of the glass tube 10 is not uniform.

 On the other hand, in the method for producing an optical fiber preform according to the present invention, the glass tube on which the glass fine particles are deposited is put into a heating furnace, and heat treatment is performed to increase the bulk density of the glass fine particle layer deposited in the glass tube. Since the process is incorporated, it is possible to heat the entire glass tube simultaneously and uniformly. Therefore, in the subsequent step of adding the additive into the glass fine particle layer, the amount of additive added can be made uniform in the longitudinal direction.

 FIG. 4 shows the bulk density of the glass fine particle layer 11 in the longitudinal direction of the glass tube 10. When the glass fine particles are baked and hardened by a conventional method to increase the bulk density of the glass fine particle layer 11, the bulk density is not uniform throughout the glass tube 10. On the other hand, when an optical fiber preform is produced by the method for producing an optical fiber preform of the present invention, the bulk density is substantially uniform throughout the glass tube 10. As shown in FIG. 2, since the amount of the additive added to the glass fine particle layer 11 in the fourth step affects the bulk density of the glass fine particle layer 11, the method for producing an optical fiber preform of the present invention is used. The produced optical fiber preform can make the additive amount in the longitudinal direction uniform.

Hereinafter, an example of the manufacturing method of the optical fiber preform of the present invention will be described with reference to FIGS.
In the first step of the present invention, a glass tube 10 containing quartz as a main component and serving as a clad glass is prepared. The glass tube 10 is made of quartz glass having a refractive index lower than that of the glass constituting the core portion 17 of the optical fiber preform 19 to be manufactured, such as pure quartz glass or quartz glass doped with fluorine. Can be mentioned. The glass tube 10 preferably has a thickness that constitutes a part of the cladding region 18 of the optical fiber preform to be manufactured, so that the process of crushing and solidifying the hollow portion is facilitated. It is good also as the thickness which comprises all the clad area | regions 18. FIG.

Next, the glass tube 10 is set in an MCVD apparatus (the detailed apparatus configuration is not shown), and an oxyhydrogen burner 12 is disposed outside the glass tube 10 as shown in FIG. A glass raw material gas (SiCl ₄ , GeCl ₄ ) and oxygen gas (O ₂ ) are introduced into the glass tube 10 while heating the glass tube 10 to generate and deposit glass fine particles in the glass tube 10. Perform the process. In FIG. 5A, GeO ₂ is used as the refractive index adjusting dopant of the core portion 17, but the present invention is not limited to this example. For example, fluorine is added to the cladding region 18. In the case where the low refractive index quartz glass is used and quartz glass is used for the core portion, GeCl ₄ may be omitted. In the second step, the glass fine particle layer 11 having a small bulk density is formed on the inner surface of the glass tube 10.

Next, as shown in FIG. 5B, a third step of increasing the bulk density of the glass fine particle layer 11 is performed by putting the glass tube 10 on which the glass fine particle layer 11 is formed into a heating furnace 13 and performing a heat treatment. I do. As shown in FIG. 5B, the heating furnace 13 used in the third step has a sufficient length to simultaneously heat all of the glass tube 10 where at least the glass fine particle layer 11 is formed. An electric furnace or induction furnace is desirable. The bulk density of the glass fine particle layer 11 after the third step ^is preferably in the range of ^{0.1g / cm 3 ~0.4g / cm 3} . When the bulk density is less than 0.1 g / cm ³ , the glass fine particle layer 11 is easily peeled off in the fourth step of adding the additive by the immersion method, which is not preferable. On the other hand, if the bulk density exceeds 0.4 g / cm ³ , the amount of the additive added by the immersion method decreases, which is not preferable.

FIG. 7 shows the base material defect rate with respect to the bulk density of the glass fine particle layer.
In the manufacturing method of the optical fiber preform, the base material defect is the first step “step of preparing the glass tube 10” to the fifth step “step of crushing the hollow portion and performing fidelity” During this period, it is difficult to continue the production of the base material for some reason, and it is defined as a state in which the base material cannot be made into an optical fiber. The base material defect rate is the number of base material defects / the number of base material manufactured.
A broken line portion in FIG. 7 indicates a defect rate of 5%. 7, in the range bulk density of 0.1g ^/ cm ³ ~0.4g ^/ cm 3 of the glass fine particle layer 11, it can be seen that the parent material produced defective rate of 5% or less. On the other hand, when the bulk density is smaller than 0.1 g / cm ³ , the defect rate increases greatly as the bulk density decreases. The low bulk density means that the glass fine particles are weakly bound, and when the aqueous solution is immersed in the glass fine particle layer 11 in the fourth step, the glass fine particle layer 11 is easily dissolved in the aqueous solution. Peels from the glass tube 10 and becomes a base material defect.
Also, when the bulk density is larger than 0.4 g / cm ³ , the defect rate increases with the increase in bulk density. In this case, the glass fine particle layer 11 is not peeled off from the glass tube 10 even if the glass fine particle layer 11 is immersed in the fourth step, but the glass in the drying step shown in FIGS. 6 (D) and (E). The base material defect that the fine particle layer 11 is cracked easily occurs. Therefore, when the bulk density is larger than 0.4 g / cm ³ , it is preferable from both viewpoints that the amount of the additive to be added by the immersion method is reduced and the optical fiber preform production failure rate is also increased. Absent. As in the present invention, it is possible to bulk density of the glass fine particle layer 11 if the range of ^{0.1g / cm 3 ~0.4g / cm 3} , is sufficiently low preform defect rate.

 In this third step, as shown in FIG. 5B, when a heating furnace 13 such as an electric furnace or an induction heating furnace is used when heating the glass tube 10, the glass tube 10 is heated with an oxyhydrogen burner 12. Compared with the case where it does, it becomes possible to heat the glass fine particle layer 11 whole in the glass tube 10 very uniformly, and it is possible to make not only the longitudinal direction of the glass tube 10 but also the temperature inside and outside the glass tube 10 uniform. In addition, the bulk density of the glass fine particle layer 11 deposited in the glass tube 10 can be increased more uniformly.

Next, the 4th process of adding an additive by the immersion method in the glass fine particle layer 11 in the glass tube 10 which finished heat processing is performed.
For example, as shown in FIG. 5 (C), the additive addition process by this immersion method is performed by attaching a stopper 15 on one side of the glass tube 10 with the opening facing upward, and adding the additive solution 14 from the opening. It can be performed by pouring into a tube and leaving it for a predetermined time.

The additive added in the fourth step is preferably one or more selected from the group consisting of rare earth elements and Al. As the additive solution 14, it is desirable to use a solution in which a compound (solute) containing these rare earth elements and Al is dissolved in a solvent. As this solute, for example, rare earth elements and Al chlorides, nitrates, complexes and the like are used. For example, when Er and Al are added, ErCl ₃ , AlCl _{3 and the} like are preferable. Moreover, water, alcohol, etc. can be used as a solvent, and water is especially preferable.

 After the addition of the additive, the glass tube 10 removes moisture by flowing oxygen gas or the like into the tube as shown in FIG. 6 (D), and further, as shown in FIG. 6 (E), chlorine gas, While flowing helium gas and oxygen gas, the outer surface of the glass tube 10 is heated by the oxyhydrogen burner 12 to be sufficiently dried.

Thereafter, the glass tube 10 is heated with the oxyhydrogen burner 12 to make the glass fine particle layer transparent, and a fifth step of crushing and solidifying the hollow portion of the glass tube 10 is performed.
In the fifth step, it is desirable to separately perform the formation of the transparent glass layer 16 shown in FIG. 6 (F) and the solidification shown in FIG. 6 (G). In the step of forming the transparent glass layer 16 shown in FIG. 6F, the glass fine particle layer 11 is heated at a heating temperature suitable for transparent vitrification, and a sufficient time is taken so that no bubbles remain in the transparent glass layer 16. It is desirable to heat. At this time, in order to promote transparent vitrification, it is preferable to flow helium gas or oxygen gas into the tube. After the formation of the transparent glass layer 16, the inside of the tube is evacuated as necessary, and the heating temperature of the oxyhydrogen burner 12 is increased and stretched to crush the hollow portion of the glass tube as shown in FIG. 6 (G). Make it solid.

The optical fiber preform 19 (preform) obtained in this way is subjected to a jacket process in which a quartz tube serving as a cladding is covered and integrated by heating to obtain an optical fiber preform for manufacturing an optical fiber. In addition, about the synthesis | combination of a clad glass layer, the external method can also be used besides the method by the said jacket process.
The manufactured optical fiber preform 19 includes a core portion 17 made of quartz glass containing one or more additive elements selected from the group consisting of rare earth elements and Al, and a clad portion made of quartz glass surrounding the core portion 17. It consists of 18. The manufactured optical fiber preform can be set in a conventionally known optical fiber drawing device (spinning device), drawn in the same manner as in the production of a conventional optical fiber, and used for producing an optical fiber.

 According to this manufacturing method, it is possible to make the bulk density of the glass fine particles uniform over the entire length in the longitudinal direction of the glass tube 10, and in the subsequent step of adding the additive into the glass fine particle layer 11, Er , Al and other additives can be added uniformly over the entire length in the longitudinal direction. As a result, the optical fiber preform 19 obtained by this manufacturing method can produce a rare earth-doped optical fiber having a uniform rare earth concentration in the longitudinal direction.

An Er-doped optical fiber preform was manufactured by the manufacturing method of the present invention shown in FIGS.
First, SiCl ₄ , GeCl ₄ , O ₂ gas is supplied into a glass tube 10 having an outer diameter of 32 mm and a wall thickness of 2.5 mm, and the glass tube 10 is heated with an oxyhydrogen burner 12 to cause an oxidation reaction in the tube, Glass particulates on a soot composed of SiO ₂ and GeO ₂ were generated and deposited on the inner surface of the glass tube to form a glass particulate layer 11 (FIG. 5A).
When soot-like glass fine particles were generated and deposited on the inner surface of the glass tube 10, heating was performed while moving the oxyhydrogen burner 12 along the glass tube longitudinal direction so that it could be deposited in the longitudinal direction of the glass tube. At this time, it is necessary to carefully control the heating temperature of the oxyhydrogen burner 12 so that the deposited soot-like glass fine particles are not baked and solidified to become a transparent glass. Deposition was performed at a temperature as low as about 200-400 ° C.
By reciprocating the oxyhydrogen burner 12 once or a plurality of times, a glass fine particle layer 11 made of SiO ₂ and GeO ₂ was formed.

Next, the supply of the raw material gas and the oxyhydrogen burner 12 are stopped, the glass tube 10 on which the glass fine particle layer 11 is formed is set in the heating furnace 13 of FIG. 5B, heated, and the bulk density of the glass fine particle layer 11 is set. Increased. In this case, the bulk density of the glass fine particle layer 11 was adjusted to a temperature of the heating furnace 13 to be in the range of ^{0.1g / cm 3 ~0.4g / cm 3} . The heating furnace 13 used in the present embodiment has a heating region in which all of the glass fine particle layer 11 on the inner surface of the glass tube 10 can be heated simultaneously.

After the heat treatment by the heating furnace 13 is finished, the glass tube is taken out from the heating furnace 13, the stopper 15 is put on one side of the glass tube 10, and an aqueous solution containing a rare earth element is poured from the other end face, and the glass particle layer 11 This aqueous solution was immersed in the solution, and a rare earth element in the aqueous solution was added (FIG. 5C).
In this example, the solute of the aqueous solution containing the rare earth element is ErCl ₃ and AlCl ₃ , and the solvent is H ₂ O. The Er concentration in the solution is, for example, 0.01 to 1% by mass.
The solution concentration to obtain the desired Er concentration in the optical fiber is determined empirically.
After the glass fine particle layer 11 was immersed in this aqueous solution containing Er for about 3 hours, the solution was extracted, dried O ₂ gas was fed into the glass tube to evaporate the moisture, and dried (FIG. 6D). .
This drying process was performed for about 6 hours.
Further, in order to remove the remaining moisture, Cl ₂ , O ₂ , and He were fed into the glass tube 10 and heated with the oxyhydrogen burner 12 to sufficiently remove the moisture (FIG. 6E).
Also at this time, the operation was performed at a sufficiently low heating temperature so that the glass fine particle layer 11 was not transparent.

Thereafter, He and O ₂ were fed into the glass tube 10 and the thermal power of the oxyhydrogen burner 12 was raised to make the glass particulate layer 11 transparent (FIG. 6 (F)).
Subsequently, the heating power of the oxyhydrogen burner 12 was further increased to solidify the glass tube 10 to produce a preform rod (FIG. 6G).

At the center of the obtained preform rod, there is a glass layer doped with Al, Ge, Er, which corresponds to the core portion 17 of the optical fiber obtained from this preform.
An optical fiber preform 19 was manufactured by covering the outer surface of the preform rod obtained in this manner with a quartz tube serving as a clad glass layer and performing a heating and integrating jacket process.
Further, the obtained optical fiber preform was set in an optical fiber drawing device and drawn to produce an Er-doped optical fiber.

 The Er-doped optical fiber (hereinafter referred to as the present invention) obtained as described above has a core relative refractive index difference Δ of about 1.2%, a mode field diameter at a wavelength of 980 nm of about 3.5 μm, and a cut. The off wavelength was about 0.9 μm. The absorption amount at a wavelength of 1530 nm was 10 dB / m. This absorption amount is a value proportional to the Er content.

 On the other hand, for comparison, an Er-doped optical fiber was also produced by a conventional manufacturing method. In this conventional optical fiber manufacturing method, the step of increasing the bulk density of the glass fine particle layer after the deposition step is performed using the same oxyhydrogen burner as the deposition step. An Er-doped optical fiber manufactured by a conventional method (hereinafter referred to as a conventional product) has a core relative refractive index difference Δ of about 1.2% and a mode field diameter at a wavelength of 980 nm of about 3. The cutoff wavelength was 5 μm and about 0.9 μm. The absorption amount at a wavelength of 1530 nm was 10 dB / m.

FIG. 8 shows the results of measuring the amount of absorption at a wavelength of 1530 nm at each site in the longitudinal direction for the optical fibers of the present invention and the conventional product.
FIG. 8 shows that the product of the present invention has a stable absorption amount at a wavelength of 1530 nm in the longitudinal direction, that is, Er is doped at a substantially uniform concentration over the entire length of the optical fiber. The variation in the absorption amount at a wavelength of 1530 nm in the longitudinal direction is about 0.3 dB / m with a standard deviation.
On the other hand, in the conventional product, the absorption amount at a wavelength of 1530 nm is not stable in the longitudinal direction of the optical fiber, and the absorption amount at a wavelength of 1530 nm is particularly small on the drawing start side (side close to 0 km). The variation in the absorption amount at a wavelength of 1530 nm in the longitudinal direction is about 1.5 dB / m with a standard deviation.

4 is a graph showing changes in bulk density with respect to heating temperature when glass fine particles are baked and solidified to increase the bulk density of the glass fine particle layer in the method for producing an optical fiber preform of the present invention. In the manufacturing method of the optical fiber preform of this invention, it is a graph which shows the change with respect to the bulk density of Al density | concentration of the core part of the optical fiber preform produced by adding Al at the 4th process. It is sectional drawing which shows an example of the manufacturing method of the conventional optical fiber preform, and shows each process from formation of a glass fine particle layer to liquid immersion. It is a graph which shows the bulk density of the glass fine particle layer in the longitudinal direction of a glass tube in the manufacturing method of the optical fiber preform of the present invention, and the manufacturing method of the conventional optical fiber preform. It is sectional drawing which shows an example of the manufacturing method of the optical fiber preform | base_material of this invention, and shows each process from formation of a glass fine particle layer to liquid immersion. It is sectional drawing which shows each process after the process of FIG. It is a graph which shows the base material defect rate with respect to the bulk density of a glass fine particle layer. It is a graph which shows the result of an Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Glass tube, 11 ... Glass fine particle layer, 12 ... Oxyhydrogen burner, 13 ... Heating furnace, 14 ... Additive solution, 15 ... Plug, 16 ... Transparent glass layer, 17 ... Core part, 18 ... Cladding part, 19 ... Optical fiber preform.

Claims (5)

 A first step of preparing a glass tube mainly composed of quartz, which becomes a clad glass, and while heating the glass tube, a glass raw material gas and an oxygen gas are introduced into the glass tube, and glass fine particles are introduced into the glass tube. A second step of generating and depositing, a third step of increasing the bulk density of the glass particulate layer deposited in the glass tube by performing heat treatment by placing the glass tube on which the glass particulate is deposited in a heating furnace. A step, a fourth step of adding an additive into the glass fine particle layer in the glass tube after the heat treatment, and heating the glass tube after the addition step to make the glass fine particle layer transparent. And a fifth step of crushing the hollow portion and solidifying the hollow portion.  The heating means in the second step is an oxyhydrogen burner, the heating furnace in the third step is an electric furnace or an induction heating furnace, and the heating means in the fifth step is an oxyhydrogen burner. The method for producing an optical fiber preform according to claim 1. Manufacturing the optical fiber preform according to claim 1 or 2 bulk density of the glass fine particle layer after the third step is characterized by a range of 0.1g ^/ cm ³ ~0.4g ^/ cm 3 Method.  The heating furnace used in the third step is provided with a plurality of heating means so that at least a portion of the glass tube on which the glass fine particles are deposited can be simultaneously heated, or at least the glass fine particles of the glass tube The optical fiber preform according to any one of claims 1 to 3, which is a heating furnace having heating means having a length sufficient to simultaneously heat all the deposited portions. Method. The optical fiber preform manufacturing method according to claim 1, wherein the additive added in the fourth step is one or more selected from the group consisting of rare earth elements and Al. .

Images