JP2003306342A - Method for producing glass preform for optical fiber, and glass preform for optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a glass preform for an optical fiber that can control bubble formation in the glass preform for an optical fiber having a layer containing fluorine and to produce glass preform for an optical fiber having less bubbles, and the glass preform for an optical fiber. <P>SOLUTION: In the method for producing the glass preform for an optical fiber having a step where a porous body comprising quartz glass powder is vitrified and added with fluorine by heat treatment at a vitrifying point in an atmosphere containing a fluorine compound, the above porous body is vitrified at a vitrifying point of 1,400°C or lower and then heated to a fixed temperature of 1,500°C or more at a rate of 10°C/min or less after reaching the above vitrifying point at the latest in an atmosphere free of the fluorine compound and kept at the temperature for 2 hours or more. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a glass preform for optical fibers and a glass preform for optical fibers manufactured by the manufacturing method.

[0002]

2. Description of the Related Art An optical fiber is composed of a core for propagating light and a clad for preventing the exudation of the light, and it is required to have low loss in order to propagate the light through a long-distance optical transmission line. In order to achieve low loss, it is important that the material used for the optical fiber has the property of not absorbing light. At present, the material of the optical fiber that has been put to practical use for optical transmission lines is quartz glass. .

Further, as a material added to the silica glass for changing the refractive index distribution of the optical fiber, (Si
Germanium (Ge) for increasing the refractive index and fluorine (F) for decreasing the refractive index are generally used (as compared with quartz glass made of O ₂ ).

The simplest refractive index distribution of an optical fiber is a so-called unimodal type in which the core has a uniform refractive index, is larger than the cladding, and has a step shape. In this case, it is general that Ge is added to the central core and the cladding is made of pure quartz. However, in order to achieve low loss and increase resistance to radiation and ultraviolet rays by eliminating the additives of the core and increasing the purity, the core is made of pure quartz and F is added to the clad to refract the refraction of the clad. Sometimes the rate is relatively low.

The speed of light propagating in an optical fiber depends on the wavelength and the composition of the glass. This phenomenon is called dispersion. Due to this dispersion, if the signal light source has a wavelength distribution, the speed of light is not constant depending on the wavelength, and thus the signal may be disturbed. However, if the refractive index distribution of the optical fiber is set appropriately, the wavelength dependence of the dispersion can be suppressed, so that a complicated refractive index distribution may be selected in the optical fiber for high-speed communication.

Recent WDM (Wavelength Division Mult)
In iplexing) transmission, signal light is propagated over a long distance by an optical amplification method, so that dispersion design is particularly important to prevent signal waveform distortion. The optical fiber used for WDM transmission has a refractive index distribution shape that is not a simple structure such as a single peak type, but a layered W-shaped refractive index (see FIG. 1) or W segment type (see FIG. 2). (See). In order to lower the refractive index and obtain a concave layer, it is necessary to add F, and in order to increase the refractive index, Ge is generally added. However, when the additive is added in this way, the optical loss characteristic and the non-linear characteristic may deteriorate. Therefore, the amount of Ge added to the portion where the refractive index is increased may be suppressed or set to zero, and a large amount of F may be added to the surrounding layers to give a relative refractive index difference.

The optical fiber having the above structure can be manufactured by several manufacturing methods, but the VAD method is suitable from the viewpoint of productivity. Hereinafter, an example of the steps of this manufacturing method will be described. That is, 1) First, a burner having a multiple structure is prepared, and oxygen gas, hydrogen gas, and silicon tetrachloride as a glass raw material are vaporized and made to flow, ignited, and hydrolyzed in a flame (called VAD flame). The reaction produces fine glass particles. The fine glass particles are sprayed onto a rotating quartz seed rod to grow a porous body, and the quartz seed rod is gradually pulled up to obtain a porous body of a desired size. 2) Next, the porous body is placed under He atmosphere, for example, at 160
It is made transparent by heating at 0 ° C. to make a transparent glass body. 3) Next, the transparent glass body is stretched into an elongated rod shape, and a VAD flame is sprayed over the entire length of the transparent glass body while moving the transparent glass body and the burner relatively to form a porous layer. To generate. This step is repeated as necessary to thicken the porous layer. 4) After that, heating is performed at, for example, 1350 ° C. in an atmosphere in which He and a fluorine compound are coexistent, F is added to the porous layer, and the porous layer is made into a transparent glass base material.

Since the addition of F is influenced by the partial pressure of F, etc., if it is carried out at the same time as the hydrolysis reaction in a flame, it will be restricted by the reaction conditions and F will escape during the subsequent clearing treatment. It is difficult to add F to the. Therefore, as described above, it is easier to increase the amount of F to be added and control it by adding F during the transparent vitrification.

A quartz layer may be further formed on the outer periphery of the transparent glass base material. In that case, a porous layer is attached by an external method to make it transparent by heating, or it is put in a quartz tube and heat-fused to be integrated. The glass base material for an optical fiber thus obtained is heated to, for example, 2000 ° C. by a drawing machine and drawn. At the time of drawing, an ultraviolet curable resin or a thermosetting silicone resin is applied to obtain an optical fiber.

[0010]

By the way, in the case of making a porous body in which fine glass particles are densely deposited on the outer periphery of the transparent glass base material transparent, a gas is generated between the transparent glass base material and the porous body. If it exists, there is a problem that it is trapped and becomes a bubble. For the purpose of preventing this, it is common to make transparent in a He gas atmosphere that is easy to melt into glass. However, when the concentration of the fluorine compound coexisting in the He gas atmosphere for adding F is increased, the fluorine compound is less likely to diffuse into the solid glass than He, and this gas component remains. As a result, there is a problem that bubbles are generated when heat is further applied by drawing or the like after vitrification.

As a means for preventing the above-mentioned generation of bubbles, it is conceivable to reduce the concentration of the fluorine compound coexisting in the He gas atmosphere during the transparent vitrification, but it is the minimum necessary fluorine compound to obtain the desired refractive index difference. There is a concentration, and it may be difficult to reduce the concentration of the fluorine compound to the point where the generation of bubbles is suppressed.

Further, it is considered that the maximum temperature to be exposed in the manufacturing process of the glass preform for optical fiber is generated in the transparent vitrification process so that the temperature is not higher than this temperature in other processes. To be However, since F-added glass has a low softening point, there is a risk that the viscosity will decrease and the transparent glass base material will deform under a high transparent vitrification temperature. It cannot be raised. Specifically, the F-containing vitrification temperature in the transparent vitrification step is
In order to prevent the deformation of the glass base material, 1400 ° C. or lower is desired. However, when pure quartz is further added after the transparent vitrification step, the transparent vitrification temperature at that time is higher than the softening temperature, and therefore, a treatment at 1500 ° C. or higher is required. Further, a temperature of 1800 ° C. or higher is required for softening for drawing. Therefore, the treatment temperature in the vitrification step of adding F cannot be made higher than the treatment temperature in the subsequent step.

[0013]

The present invention has been achieved as a result of earnest and experimental studies in order to solve the above problems. That is, the invention according to claim 1 is a glass for an optical fiber, which has a step of heating a porous body composed of silica glass fine particles at a transparent vitrification temperature in an atmosphere containing a fluorine compound to form a transparent vitrification and adding fluorine. In the method for producing a base material, the porous body is vitrified at a transparent vitrification temperature of 1400 ° C. or lower, and then 10 ° C./at least after exceeding the transparent vitrification temperature in an atmosphere containing no fluorine compound gas. It is characterized in that the temperature is raised to a predetermined temperature of 1500 ° C. or higher at a speed of not more than a minute, and the temperature is maintained at the predetermined temperature for 2 hours or longer.

According to a second aspect of the invention, in the invention of the first aspect, the porous body has a density of 0.4 g /
cm ³ or less, and is characterized in that it is pre-heat treated in an atmosphere containing the 1100 ° C. or less chlorine.

The invention of claim 3 is the same as the invention of claim 1 or 2, wherein the fluorine compound is SiF.
It is characterized by being ₄ .

The invention according to claim 4 is characterized in that, in the invention according to any one of claims 1 to 3, the atmosphere not containing the fluorine compound gas is Ar gas. .

The invention according to claim 5 is the invention according to any one of claims 1 to 4, wherein the atmospheric pressure of the atmosphere containing no fluorine compound gas is reduced by 10 kPa or more with respect to the atmospheric pressure. It is characterized by being present.

Further, the invention according to claim 6 is the same as claim 1.
A glass base material for an optical fiber manufactured by the manufacturing method according to any one of 1 to 5, wherein the center portion is doped with Ge and has the highest refractive index, and the layer around it has at least fluorine. Is added, and the relative refractive index difference is 0.4% or more lower than that of pure quartz.

According to the first aspect of the invention, the transparent vitrification temperature is set to 1400 ° C. or lower, and after the vitrification of the porous body to the transparent vitrification, at least the transparent vitrification temperature is exceeded in an atmosphere containing no fluorine compound. Is heated to a predetermined temperature of 1500 ° C. or higher at a rate of 10 ° C./min or lower and kept at the predetermined temperature for 2 hours or longer, by heating the gas remaining in high concentration in the porous body. It is possible to diffuse and suppress the generation of bubbles in the subsequent process. Here, the predetermined temperature means a temperature higher than or equal to 1500 ° C. and higher than the maximum temperature exposed in the subsequent steps before the drawing step after the transparent vitrification.

In the present invention, the transparent vitrification temperature is set to 14
The reason why the temperature is set to 00 ° C or lower is that the transparent vitrification temperature is 1400.
This is because if the temperature exceeds C, the F-added glass has a low softening point and may be deformed. Further, in order to diffuse the residual gas in the porous body and suppress the generation of bubbles in the subsequent process, the higher the treatment temperature and the longer the treatment time, the more effective. Therefore, as described above, after the transparent vitrification, in a fluorine compound-free atmosphere, at least after the transparent vitrification temperature is exceeded, 1500 ° C./min at a rate of 10 ° C./min or less.
The temperature is raised to a predetermined temperature of ℃ or more, and at the predetermined temperature, 2
When it is held for a time or more, generation of bubbles in this step and the subsequent steps can be suppressed. Here, the temperature rising rate is 10
The reason for setting the temperature to not more than ° C / min is that at a temperature rising rate higher than this, gas may expand and bubbles may be generated before diffusion.

When the density of the porous body is 0.4 g / cm ³ or less as in the second aspect of the invention, F can be added efficiently. The reason is that the addition of F in the vitrification step depends on the reaction between the solid SiO ₂ and the fluorine compound.
This is because it is desired that the fine particles forming the porous body be fine and have a gap so that the gas can sufficiently flow inside. This means that the density of the porous body is low. As a result of the examination, as described above, the density is 0.4 g / c.
When it is m ³ or less, F can be efficiently added. Further, when the porous body contains a large amount of hydroxyl groups, an absorption peak appears at around 1385 nm when used as an optical fiber. For the purpose of reducing this, the porous body may be heat-treated in a chlorine (including chlorine compound) atmosphere, but if the heat-treatment temperature is high, sintering proceeds in that step and the density increases. Therefore, it is desirable that the heat treatment temperature is 1100 ° C. or less, as in the second aspect of the invention.

According to the third aspect of the invention, the fluorine compound contained in the atmosphere during the vitrification is made into SiF.
_{When it is 4} , F can be added more efficiently than SF ₆ , C ₂ F _{4, and the} like.

When the atmosphere containing no fluorine compound gas is Ar as in the fourth aspect of the invention, it is less likely to be dissolved in the glass as compared with air, which has an effect of suppressing bubble generation. When the atmospheric pressure of the atmosphere containing no fluorine compound gas is reduced by 10 kPa or more with respect to the atmospheric pressure, the amount of gas dissolved in the glass is further reduced, and the generation of bubbles is further suppressed. it can.

Furthermore, since the optical fiber manufactured from the optical fiber preform having the structure as described in claim 6 can have a negative dispersion slope, the dispersion compensation of the optical fiber for a line can be performed efficiently. be able to.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below. The glass preform for optical fibers having the W-type refractive index distribution shown in FIG. 1 was manufactured by the following steps. That is, 1) A core having a two-layer structure is divided and synthesized twice. First VA
A D-device is used to synthesize a porous body to which Ge corresponding to the center-core 1 is added. Then, this porous base material is made into transparent glass and stretched to a desired outer diameter. 2) Then, pure quartz fine particles are generated and deposited on the outer circumference of the transparent glass body by a flame hydrolysis reaction to newly synthesize a porous body. This porous base material is made into transparent glass by a two-step heat treatment of a first step (dehydration treatment) and a second step (transparent vitrification) in an atmosphere containing a fluorine compound, and F is added. Table 1 shows the conditions of the first and second stage heat treatments.

Table 1

Regarding the refractive index distribution of the transparent glass body thus manufactured, the relative refractive index difference of the center core 1 with respect to pure quartz is 2%, and the relative refractive index difference of the side core 2 around the center core 1 with respect to pure quartz. The rate difference was -0.6%.

3) Next, the following heat treatment is performed for the purpose of diffusing the gas remaining in the transparent glass body.
That is, the transparent glass body is divided into five after being drawn, and four of them are heat-treated in an Ar atmosphere. The heat treatment conditions are such that the temperature is raised from 1350 ° C. and the temperature is maintained at 1500 ° C. for 2 hours. However, in order to investigate the effect of the heating rate, the heating rate was 5 ° C / min, 8 ° C / min, 11 ° C.
/ Min, 15 ° C / min. The rate of temperature decrease from 1500 ° C. is 10 ° C./min in all cases. No heat treatment was applied to the remaining one transparent glass body.

4) Next, a pure quartz porous clad layer was formed under the same conditions on the outer periphery of the five transparent glass bodies. This porous clad layer was made into transparent glass to form clad 3, which was used as a glass preform for optical fibers.

Regarding the above glass base material for optical fiber, F
The number of bubbles generated in the addition layer was visually counted. The results are shown in Table 2. Bubbles having a diameter of 0.2 mm or more were counted. As can be seen from Table 2, when the heating rate is slowed down to 10 ° C./min or less, the generation of bubbles can be suppressed to a level at which there is no problem in the product (approximately 0.003 cells / cm ³ or less).

Table 2

Next, four F-containing transparent glass base materials prepared in the same manner as above were subjected to a heat treatment for the purpose of diffusing the remaining gas. The conditions for this heat treatment are 12
The temperature was raised from 00 ° C. at a rate of 10 ° C./min, held at a holding temperature of 1500 ° C., and then lowered at a temperature dropping rate of 10 ° C./min. Here, in order to investigate the influence of the holding time at 1500 ° C. on the generation of bubbles, the holding time of the four transparent glass base materials was changed to 30 minutes, 60 minutes, 90 minutes, and 150 minutes, and the generation of bubbles was changed. The states were compared. A porous clad layer of pure quartz was formed on these four transparent glass bodies under the same conditions. After making this clad layer transparent glass, F
The number of bubbles generated in the addition layer was visually counted. The results are shown in Table 3. As can be seen from Table 3, when the holding time is 2 hours or more, the generation of bubbles can be suppressed to a problem-free level (approximately 0.003 cells / cm ³ or less).

Table 3

Optimal values were examined for the density of the porous material corresponding to the F-added layer and the temperature conditions in the first step (dehydration step) of Table 1. The density of the porous body is 0.2
Samples were prepared by changing the amount in the range of 0.5 g / cm ³ and transparent vitrified under the conditions of Table 1, and the relative refractive index difference of the F-added layer with respect to pure quartz was measured. The result is shown in FIG. As can be seen from FIG. 3, the density of the porous body is 0.
When the density becomes higher than 4 g / cm ³ , the decrease in the relative refractive index becomes small, so the density of the porous body is 0.4 g / cm ^3.
It is desirable that it is not more than cm ³ .

The density of the porous body is 0.4 g / c.
Four samples of m ³ were prepared, and the temperature condition of the first step in Table 1 was changed from 900 ° C. to 1200 ° C. in 100 ° C. increments to form a transparent vitrified F-added layer. Other conditions were the same as in Table 1. The relative refractive index difference of the F-doped layer of these samples with respect to pure quartz was measured. Further, a cladding layer was added and drawn, and the absorption loss at a wavelength of 1385 nm was measured. The results of these measurements are shown in FIG.

As can be seen from FIG. 4, when the processing temperature in the first step exceeds 1100 ° C., the decrease in relative refractive index difference is small, and when the processing temperature in the first step is lower than 1000 ° C., 1385 nm. The loss will increase. Therefore, in order to increase the decrease amount of the relative refractive index difference and reduce the loss of 1385 nm, the treatment temperature in the first step is 1000 ° C to 11 ° C.
It is preferably in the range of 00 ° C.

In the second stage heat treatment of the porous body, when the initial temperature exceeds 1400 ° C., bubbles that are considered to be caused by rapid heating are generated, and the bubbles are generated even if the heating time or the heating rate is changed. It was not possible to suppress the occurrence of.
In addition, the atmosphere of the heat treatment for diffusing the residual gas of the transparent glass body (not containing the fluorine compound gas) is air,
When N ₂ or He was used, the generation of bubbles could not be suppressed, and it was found that Ar is desirable as the atmosphere gas.
When the atmospheric pressure of the heat treatment for diffusing the residual gas of the transparent glass body was reduced to 10 kPa or more with respect to the atmospheric pressure, no bubbles were observed and a good glass preform for optical fibers was obtained. Further, the present invention is not limited to the glass preform for optical fibers having the W-type refractive index distribution shown in FIG. 1, but as shown in FIG. 2, a W segment including a center core 11, side cores 12, 13 and a clad 14. It goes without saying that it is also applied to a glass preform for optical fibers having a type refractive index distribution.

[0038]

As described above, according to the present invention, it is possible to suppress the generation of bubbles in the glass base material for an optical fiber having a layer containing fluorine, and the glass for an optical fiber according to the present invention. By drawing the base material, there is an excellent effect that an optical fiber in which bubbles contained therein are reduced can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a W-shaped refractive index profile of an embodiment of an optical fiber glass preform manufactured by a method for manufacturing an optical fiber glass preform according to the present invention.

FIG. 2 is a diagram showing another W segment type refractive index profile shape to which the present invention is applicable.

FIG. 3 is a diagram showing a relationship between a density of a porous body before adding fluorine, a relative refractive index difference of a transparent glass body after adding fluorine, and a loss of 1385 nm in the embodiment.

FIG. 4 is a first stage (dehydration step) in the above embodiment.
FIG. 3 is a diagram showing the relationship between the furnace temperature, the relative refractive index difference of the transparent glass body after the addition of fluorine, and the loss of 1385 nm of the optical fiber obtained by drawing.

[Explanation of symbols]

1, 11 Center core 2, 12, 13 Side core 3,14 Clad

Claims (6)

[Claims] 1. A method for producing a glass preform for an optical fiber, which comprises a step of heating a porous body composed of silica glass fine particles to a transparent vitrification by heating at a transparent vitrification temperature in an atmosphere containing a fluorine compound, and adding fluorine. In the above, the porous body is made into transparent vitrification at a transparent vitrification temperature of 1400 ° C. or lower, and then in an atmosphere containing no fluorine compound gas,
10 ° C at least after exceeding the transparent vitrification temperature
A method for producing a glass preform for an optical fiber, which comprises raising the temperature to a predetermined temperature of 1500 ° C. or higher at a rate of / min or less and holding the glass at the predetermined temperature for 2 hours or longer. 2. The porous body has a density of 0.4 g / cm.
^The method for producing a glass preform for an optical fiber according to claim 1, wherein the glass base material for an optical fiber is preheated in an atmosphere containing chlorine at ³ ° C or lower and 1100 ° C or lower. 3. The method for producing a glass base material for an optical fiber according to claim 1, wherein the fluorine compound is SiF ₄ . 4. The method for producing a glass base material for an optical fiber according to claim 1, wherein the atmosphere containing no fluorine compound gas is Ar gas. 5. The glass fiber for optical fiber according to claim 1, wherein the atmospheric pressure of the atmosphere containing no fluorine compound gas is reduced by 10 kPa or more with respect to the atmospheric pressure. Method of manufacturing wood. 6. A glass base material for an optical fiber manufactured by the manufacturing method according to claim 1, wherein Ge is added to the center portion and has the highest refractive index. A glass preform for an optical fiber, characterized in that at least fluorine is added to the surrounding layer, and the relative refractive index difference is 0.4% or more lower than that of pure quartz.