JP3498590B2 - Manufacturing method of preform for optical fiber - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an optical fiber preform by applying a soot around the glass rod for core and making it into transparent glass.

[0002]

2. Description of the Related Art Conventionally, as a method of manufacturing a base material for an optical fiber, a glass rod for a core is manufactured, and soot is attached around the glass rod for a core by an external attachment method to form an external soot body. There is a method of heating and transparentizing to form a preform for optical fibers. Here, as a method of manufacturing the external soot body, while rotating the glass rod for the core, the glass fine particles synthesized by the burner for synthesizing the glass fine particles are reciprocally moved in the axial direction relative to the burner for synthesizing the glass fine particles. OVD that sprays on the outer circumference of the rod and deposits in layers
Method (layered external attachment method), the burner for synthesizing glass fine particles is moved in the axial direction while rotating the glass rod for core arranged vertically, and glass fine particles are gradually deposited from the upper part of the glass rod for core to the lower part. The VAD method is being carried out. The external soot body produced by these methods is a method of gradually passing the external soot body through a heating zone from one end using a zone heating furnace and gradually making it transparent from one end, using a soaking furnace. Then, the external soot body is heated and made transparent by a method of uniformly heating and making the whole soot body transparent to obtain a base material for an optical fiber.

[0003]

SUMMARY OF THE INVENTION In the above method,
There is a problem that bubbles tend to remain on the boundary surface between the glass rod for core and the soot attached externally when the external soot body is heated and made transparent. This is because in the external technology, a tensile force is generated between the external soot that contracts significantly during the transparentization and the core glass rod that does not contract, and it is easy to create voids in the glass structure. Therefore, there is a tendency that the surface becomes transparent, and if the surface of the entire base material becomes transparent first, the residual gas existing in the voids will not escape and will remain as bubbles. In addition, since the distance from the heat source is long in the ineffective portions at both ends, the situation may be slightly different depending on the balance between the heat from the surface of the inclined portion and the heat conducted from the inside of the base material. Especially VA
In the case of an external soot body formed by the D method, the soot and glass are brought into close contact with each other at the upper sooting start portion, so that the bulk density of soot is often higher than that of the cylindrical effective portion. There is a case where the part becomes transparent first, and the escape path of gas that becomes bubbles may be closed, and it is easy for many bubbles to remain near the non-effective part on the heating end side when transparentizing by the zone heating furnace. . Such bubbles remain unfavorable in light transmission characteristics when used as an optical fiber and must be removed as much as possible.

Japanese Laid-Open Patent Publication No. 49-9523 discloses a blank (b) used for high-quality optical waveguides, lenses, etc.
A method of producing optically high quality fused silica containing no water in the form of a land) is sufficient to deposit a silicon-containing soot on a core material to form a preform and sinter the soot. A layer of soot at a rate that allows the entrapped gas to escape through the portion of the soot that is not yet in the thick zone of the furnace as the soot is sintered and contracts around the core. Is gradually inserted into the furnace. With this method, the gas escape path can be maintained in the cylindrical effective portion by using the zone heating method, but there is the problem that air bubbles are easily trapped in the ineffective portion where heating is terminated. In addition, JP-A-53
JP-A-67447 discloses that a glass-forming gas phase raw material is oxidized or hydrolyzed to synthesize glass soot, which is laminated on a rotating and moving rod-shaped starting member, and the soot laminate is sintered to obtain a transparent material. In a method of obtaining a glass body and using this as a base material for an optical fiber, a glass film is formed while irradiating a focused infrared ray (CO ₂ laser) at a position after laminating the soot on a rod-shaped starting member. A method of manufacturing a preform for an optical fiber is disclosed. This method has a slightly different technical content from the method of the present invention, but in this method, the transparency by the laser determines the speed of the process, and it is difficult to improve the productivity (deposition rate).

In view of the state of the art as described above, the present invention intends to provide a method of manufacturing a base material for an optical fiber in which air bubbles from the external soot body are less likely to remain.

[0006]

The present invention includes the following aspects (1) to (6) as means for solving the above problems. (1) In a method for producing an optical fiber preform in which soot is attached around the glass rod for a core by an external attachment method, and the obtained externally attached soot body is heat-treated to be transparent vitrified. A method for producing an optical fiber preform, characterized in that at least a part of one or both of the ineffective portions at both ends of the optical fiber preform is left in an opaque glass state. (2) The soot attachment by the external attachment method is performed by the VAD method of arranging the glass rod for the core in the vertical direction and growing the soot in the vertical direction. Manufacturing method of optical fiber preform. (3) The method for producing an optical fiber preform according to (2) above, wherein the sooting start portion located above is selected as an ineffective portion that leaves the opaque glass state portion. (4) The method for producing an optical fiber preform according to the above (1), wherein the soot attachment by the external attachment method is performed by the OVD method in which the soot is layered around the glass rod for core. (5) The opaque glass state portion is formed by performing transparent vitrification of the external soot body using a zone heating furnace and adjusting the heating end position of the external soot body. (4) A method for manufacturing an optical fiber preform according to any one of (4). (6) By performing transparent vitrification of the external soot body using a soaking furnace, at least part of either or both of the ineffective portions at both ends is prevented from entering the soaking range, thereby making the ineffective portion. An opaque glass state portion is formed in the portion, and the method for producing an optical fiber preform according to any one of (1) to (4) above.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, first, a glass rod for core is prepared and synthesized on the surface thereof (around the glass rod) by a burner for synthesizing glass particles by an external method such as VAD method or OVD method. Soot is applied to deposit fine glass particles. In the obtained external soot body (glass particulate deposit body), a glass particulate deposit layer (soot layer) 3 is formed around a core glass rod 2 as schematically shown in FIG. The external soot body 1 has a non-effective portion (a portion which cannot be used as a product due to unstable characteristics when used as an optical fiber) at both ends of an effective portion (a portion which can be used as a product when used as an optical fiber) in the central portion. Is formed.

Next, the external soot body 1 is heated in a heating furnace such as a zone heating furnace or a soaking furnace to be transparent vitrified into an optical fiber preform. At this time, as shown in FIG. Bubbles 5 tend to remain at the boundary between the glass rod 2 for core and the transparent glass layer 4. Note that FIG. 2 shows an example in which bubbles remain over the entire length of the optical fiber preform, but when a zone heating furnace is used, it tends to concentrate near the ineffective portion at the end of heating. The method of the present invention is characterized in that, in order to suppress such bubbles from remaining, at least a part of one or both of the ineffective portions of both ends of the external soot body is left in an opaque glass state. As a result, an escape route for residual gas can be secured until the final stage of transparency, and bubbles can be prevented from remaining. Here, the opaque glass state means a state in which the soot is still attached, to a state in which some sintering is in progress but the remaining gas is porous enough to pass through. Further, the proportion of the portion left in the opaque glass state does not have to be strictly defined and may be an amount capable of discharging the residual gas, but as a guide, it is 0.5 to 50%, preferably 5 to 5% of the ineffective portion volume. 30%
It is a range of degrees.

An example of a pattern in which an opaque glass state portion is left in an ineffective portion is schematically shown in FIG. In FIG. 3, 2 is a glass rod for core, 4 is a transparent glass layer, and 6 is an opaque glass state portion. The shape of the opaque glass state portion depends on the heating method, the radial direction density distribution of the soot body, and the like, and a typical example is shown here. FIG. 3 (a) shows that the vicinity of the surface of one ineffective portion is not made into transparent glass but is left in an opaque glass state. This pattern uses a zone heating furnace and gradually inserts into the heating zone from the lower end of the external soot body and allows it to pass through the heating zone without sufficiently vitrifying the ineffective part at the upper end. It is formed when leaving. Specifically, as in the prior art, it is possible to adopt a method such as ending the heating before the entire external soot body including the ineffective portion passes through the heating zone, and lowering the temperature of the furnace near the end of heating. . FIG. 3 (b) shows that most of the non-effective portion is left as an opaque glass state without being made transparent glass. This pattern is formed, for example, when a soaking furnace is used and one of the ineffective portions is heated outside the soaking range to form transparent glass. In FIG. 3 (c), a zone heating furnace is used, and the external soot body is positioned below the heating zone to gradually raise the transparency so that the ineffective portion at the lower end is in an opaque glass state. It is the one that remains. In the case of an external soot body produced by the VAD method, gas tends to remain at the soot starting portion of the upper part, so this method should be used to make the upper ineffective part transparent first to prevent gas residue. Can be considered.

Although FIG. 3 shows an example in which the opaque glass state portion is left on only one of the non-effective portions, the opaque glass state portion may be left on both of the non-effective portions if necessary.
If the external soot body is made by the VAD method, the soot and glass are brought into close contact with each other at the upper part of the soot attachment, so that the bulk density of the soot is made higher than that of the cylindrical effective portion. In many cases, this part becomes transparent first in the usual clearing method, and it may close the escape path of gas that becomes bubbles, so while preventing peeling at the time of clearing, generation of bubbles is prevented. In order to prevent this, it is preferable to select the ineffective portion at the end where the sooting is started as the ineffective portion that leaves the opaque glass state portion. Further, the method of leaving the opaque glass state portion in the ineffective portion is not limited to the above example, and it is opaque to at least a part of one or both of the ineffective portions formed at both ends of the external soot body. Any method can be adopted as long as it can leave the glass state portion.

It is desirable to carry out the transparent vitrification in an atmosphere of a gas having a molecular diameter as small as possible to remove the air, moisture and the like remaining at the time of forming the soot body. Even in the case of
An e atmosphere is preferable. Moreover, since He is expensive, it is also effective to lower the atmospheric gas pressure in the furnace body to perform transparent vitrification.

[0012]

EXAMPLES The method of the present invention will be described in more detail below with reference to examples. (Example 1, Comparative Example 1) As an effective part of the glass rod for core, an optical core part having a stepwise refractive index distribution manufactured by the VAD method (specific refractive index 0.35%, a small amount of germanium dioxide in pure silica glass) (Added) and a quartz glass rod of a two-layer structure having an optical cladding (pure silica glass) around the effective portion length: 500 mm, diameter 27
mm, and pure silica glass rods for ineffective portions were connected to both ends thereof. Soot was applied around the rod by the VAD method. To generate soot glass particles, S
A hydrolysis method was used in which iCl ₄ was vaporized and introduced into an oxygen-hydrogen flame. As a result, an external soot body having an effective portion diameter of 165 mm was obtained. The effective length was 500 mm, which was the same as the effective length of the rod, and the ineffective length was about 150 mm for both the upper and lower parts. The external soot body was vitrified into a transparent glass by using a zone heating furnace. FIG. 4 is a schematic diagram showing the heating method and the state of the obtained optical fiber preform.
As shown in FIG. 4A, an external soot body 1 in which a soot layer 3 of fine glass particles is formed around a glass rod 2 for a core.
Is set in the zone heating furnace with the upper end portion where the sooting is started up, and 163 at a descending speed of 5 mm / min from the lower end portion.
It was passed through a heating zone adjusted to 0 ° C. During this period, the heating zone was kept in He atmosphere.

As shown in FIG. 4 (b), a comparative example in which the soot layer 3 is entirely made into transparent glass by passing through a heating zone to a position 30 mm from the end of the upper non-effective portion (prior art).
Then, it was confirmed that the bubbles 5 remained at the portion near the upper end near the boundary with the transparent glass layer 4 around the glass rod for core 2. On the other hand, as shown in FIG. 4 (c), the method of the present invention terminated the heat treatment 40 mm above the comparative example of FIG. In the example in which the quality state portion (opaque glass state portion 6) was left (the vicinity of the glass rod interface for the core was made into transparent glass), no bubbles were observed in the effective portion of the optical fiber preform.

(Example 2, Comparative Example 2) As the glass rod for core, a quartz glass rod having the same structure as that used in Example 1 was used, and the effective portion length was 550.
mm and diameter 25 mm, and pure silica glass rods for ineffective portions were connected to both ends thereof. O around this rod
Sooting was performed by the VD method. To produce soot glass particles, a hydrolysis method in which SiCl ₄ is vaporized and introduced into an oxygen-hydrogen flame is used, and 10 reciprocating depositions are performed in the longitudinal direction to form an external soot body having an effective portion diameter of 120 mm. Obtained. The effective length is 550m, which is the same as the effective length of the rod.
m, and the length of the ineffective part is about 170 m for both upper and lower parts
It was m. This soot body attached to the outside was made into transparent glass by using a soaking furnace. 5 and 6 are schematic diagrams showing the heating method and the state of the obtained optical fiber preform in Examples and Comparative Examples. First, as a comparative example, as shown in FIG. 5A, an external soot body 1 having a soot layer 3 of glass fine particles formed around a glass rod 2 for a core is uniformly heated over the non-effective portion and the effective portion. The glass was set in a soaking furnace in a He atmosphere maintained at 1500 ° C. so as to be within the range, and kept for 90 minutes to be transparent vitrified. In the optical fiber preform obtained as a result, as shown in FIG. 5 (b), bubbles 5 were observed to remain near the boundary with the transparent glass layer 4 around the core glass rod 2. On the other hand, as shown in FIG. 6A, by the method of the present invention, the external soot body 1 is set 110 mm above the comparative example of FIG. 5, and most of the upper non-effective portion is outside the soaking range. Heat treatment as shown in FIG.
In the example in which most of the upper non-effective portion was left as the opaque glass state portion 6 as shown in (4), no bubbles remained in the effective portion of the optical fiber preform. In this embodiment, the production of the external soot body by the OVD method was performed using SiCl ₄ +.
An example of using an oxyhydrogen flame was shown, but this was performed using SiCl ₄ +
Similar results were obtained when using methane gas (CH ₄ ).

When the optical fiber preforms obtained in Examples 1 and 2 were made into a fiber, the glass diameter of the optical fiber was stable within the range of 125 μm ± 1 μm, and the optical fiber of 1550 nm band was used. When analyzed by OTDR, the transmission loss step was extremely small, and an optical fiber of good quality was obtained. On the other hand, when the optical fiber preform obtained in Comparative Examples 1 and 2 in which the bubbles remain was made into a fiber, the glass diameter of the optical fiber suddenly changed locally at the portion corresponding to the bubbles, and the glass diameter was 125 μm ± 1 μm. This exceeds the range, and it is necessary to remove this in order to obtain a good product, resulting in a decrease in yield. Further, when the fiber including these bubbles was analyzed by OTDR in the 1550 nm band, a transmission loss step of about 0.1 dB was observed in the portion corresponding to the bubbles.

[0016]

According to the method of the present invention, a method for producing an optical fiber preform by heat-transparentizing an external soot body, which is sooted around the glass rod for core by the OVD method or the VAD method, It is possible to suppress the remaining of bubbles that are likely to occur around the core glass rod.

[Brief description of drawings]

FIG. 1 is an explanatory view schematically showing the structure of an external soot body.

FIG. 2 is an explanatory view schematically showing bubbles that remain when an external soot body is heated and made transparent by a conventional method to form an optical fiber preform.

FIG. 3 is an explanatory view schematically showing an example of a pattern which leaves an opaque glass state portion in a non-effective portion in the method of the present invention.

FIG. 4 is a schematic diagram showing a heating method in Example 1 and Comparative Example 1 and a state of the obtained optical fiber preform.

FIG. 5 is a schematic diagram showing a heating method in Comparative Example 2 and a state of the obtained optical fiber preform.

FIG. 6 is a schematic diagram showing a heating method in Example 2 and a state of the obtained optical fiber preform.

[Explanation of symbols]

1 External soot body (glass particulate deposit body) 2 Glass rod for core 3 Soot layer (glass particulate deposit layer) 4 Transparent glass layer 5 Bubbles 6 Opaque glass state part

Front page continuation (72) Inventor Takehiko Kitou 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries, Ltd. Yokohama Works (56) Reference JP-A-5-58648 (JP, A) JP-A-61-86431 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) C03B 37/00-37/16 C03B 20/00

Claims (6)

(57) [Claims] 1. A method for producing an optical fiber preform in which soot is attached to the periphery of a glass rod for a core by an external attachment method, and the obtained externally attached soot body is heat-treated to be transparent vitrified. At this time, at least a part of one or both of the ineffective portions at both ends of the optical fiber preform is left in an opaque glass state, and a method for producing an optical fiber preform. 2. The optical fiber mother board according to claim 1, wherein the soot attachment by the external attachment method is performed by a VAD method in which a glass rod for core is arranged in the vertical direction and the soot is grown in the vertical direction. Method of manufacturing wood. 3. The method for producing an optical fiber preform according to claim 2, wherein the sooting start portion located above is selected as the ineffective portion that leaves the opaque glass state portion. 4. The method for producing an optical fiber preform according to claim 1, wherein the soot attachment by the external attachment method is performed by the OVD method in which the soot is layered around the glass rod for core. . 5. An opaque glass state portion is formed by performing transparent vitrification of an external soot body using a zone heating furnace and adjusting a heating end position of the external soot body. The method for manufacturing a preform for an optical fiber according to any one of 1 to 4. 6. A soaking furnace is used for transparent vitrification of an external soot body so that at least a part of one or both of the ineffective portions at both ends does not fall within the soaking range. The method for producing an optical fiber preform according to any one of claims 1 to 4, wherein an opaque glass state portion is formed in the non-effective portion.