JP2000109333A - Production of optical fiber preform - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an optical fiber preform in which bubbles produced on the external deposition of a soot body hardly remain. SOLUTION: In the method for producing an optical fiber preform to deposit soot by the external deposition method around a core glass rod 2 and to heat- treat the externally deposited soot to obtain transparent glass, at least part of one of the ineffective parts on both ends or both of these is left in an opaque glass state in the process of converting the body into transparent glass.

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 a preform for an optical fiber by forming an external soot around a glass rod for a core and turning the glass rod into a transparent glass.

[0002]

2. Description of the Related Art Conventionally, a glass rod for a core is manufactured as a method of manufacturing a preform for an optical fiber, and a soot is formed around the core glass rod by an external method to form an external soot body. Is made transparent by heating to obtain a base material for an optical fiber. Here, as a method for manufacturing the external soot body, the glass fine particles synthesized by the glass fine particle synthesis burner are reciprocated in the axial direction while rotating the glass rod for the core, and the glass fine particles synthesized by the burner for glass fine particle synthesis are turned into glass. OVD sprayed around the rod and deposited in layers
Method (layered external method), the burner for synthesizing glass fine particles is moved in the axial direction while rotating the glass rod for the core arranged in the vertical direction, and the glass fine particles are gradually deposited downward from the upper part of the glass rod for the core. The VAD method is performed. A method in which the external soot body produced by these methods is gradually passed through the heating zone from one end of the external soot body using a zone heating furnace and gradually becomes transparent from one end, using a soaking furnace. Then, the whole external soot body is heated and made transparent by a method of uniformly heating and making the whole transparent soot body to be a preform for an optical fiber.

[0003]

In the above method,
When the external soot body is heated and made transparent, there is a problem that air bubbles are likely to remain on the boundary surface between the core glass rod and the external soot. This is because, in the external technology, a tensile force is generated between the external soot that greatly shrinks during transparency and the glass rod for the core that does not shrink, and it is in a state where it is easy to create a void in the glass structure, heating from the outside For this reason, the surface tends to be transparent, and if the surface of the entire base material is transparent, the escape path of the residual gas existing in the voids disappears and remains as bubbles. In addition, since the distance from the heat source is longer in the ineffective portions at both ends, the appearance may slightly differ depending on the balance between the heat from the surface of the inclined portion and the heat conducted from inside the base material. Especially VA
In the case of an external soot body by the D method, the soot and the glass are brought into close contact with each other at the upper sooting start portion, so that the bulk density of the soot is often higher than that of the columnar effective portion. In some cases, the part becomes transparent first, closing the escape path of gas that becomes bubbles, and when transparentizing with a zone heating furnace, many bubbles are likely to remain near the ineffective part on the heating end side . Such residual bubbles are not preferable in terms of light transmission characteristics when used as an optical fiber, and must be removed as much as possible.

Japanese Patent Application Laid-Open No. 49-9523 discloses a blank (b) used for a high-quality optical waveguide, a lens or the like.
As a method of producing water-free, optically high quality fused silica in the form of rank (lank), silicon-containing soot is deposited on a core material to form a preform and sufficient to sinter the soot. A layer of soot at a rate and at such a rate that when the soot sinters and shrinks around the wick, the entrained gas can escape through the portion of the soot that has not yet entered the thick section of the furnace. Is gradually introduced into a furnace. In this method, a gas escape route can be maintained in a columnar effective portion by using the zone heating method, but there is a problem that air bubbles are easily trapped in an ineffective portion where heating is completed. Also, Japanese Unexamined Patent Publication No.
JP-67-4747 discloses that a glass soot is synthesized by oxidizing or hydrolyzing a gas-phase raw material for forming a glass, and the glass soot is laminated on a rotating and moving rod-shaped starting member. In a method of obtaining a glass body and using the glass body as a base material for an optical fiber, a glass film is formed while irradiating a focused infrared ray (CO ₂ laser) to a portion after the soot is laminated on a rod-shaped starting member. A method of manufacturing a preform for an optical fiber, which is characterized in that it is performed, is disclosed. This method has a slightly different technical content from the method of the present invention. However, in this method, transparency by a laser determines the process speed, and it is difficult to improve productivity (deposition speed).

The present invention has been made in view of the above circumstances of the prior art, and has as its object to provide a method of manufacturing a preform for an optical fiber in which bubbles are less likely to remain from an external soot body.

[0006]

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

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, first, a glass rod for a core is prepared, and the surface thereof (around the glass rod) is synthesized by a burner for synthesizing glass fine particles by an external method such as a VAD method or an OVD method. A soot for depositing glass particles is provided. In the obtained external soot body (glass fine particle deposit), a deposition layer (soot layer) 3 of glass fine particles is formed around the core glass rod 2 as schematically shown in FIG. This external soot body 1 has an ineffective portion (a portion where characteristics are unstable when used as an optical fiber and cannot be used as a product) at both ends of a central effective portion (a portion that can be used as a product when formed into an optical fiber). Is formed.

Next, the external soot body 1 is heated by using a heating furnace such as a zone heating furnace or a soaking furnace, and is vitrified to form an optical fiber preform, as shown in FIG. Bubbles 5 are likely to remain at the boundary between the core glass rod 2 and the transparent glass layer 4. FIG. 2 shows an example in which bubbles remain over the entire length of the optical fiber preform. However, when a zone heating furnace is used, the bubbles tend to concentrate near the ineffective portion at the end of heating. The method of the present invention is characterized in that at least a part of one or both of the ineffective portions at both ends of the external soot body is left in an opaque glass state in order to suppress such bubbles from remaining. As a result, an escape path for the residual gas can be ensured until the final stage of the transparency, and the remaining of bubbles can be suppressed. Here, the opaque glass state means a state from a state in which soot is attached to a state in which sintering is partially progressed but remains porous enough to allow a residual gas to pass. The ratio of the portion left in the opaque glass state does not need to be strictly defined and may be set to an amount that allows the residual gas to be discharged. 30%
Range.

FIG. 3 schematically shows an example of a pattern in which an opaque glass state portion is left in an ineffective portion. In FIG. 3, 2 is a core glass rod, 4 is a transparent glass layer, and 6 is an opaque glass portion. Note that the shape of the opaque glass state portion varies depending on the heating method, the radial density distribution of the soot body, and the like, and a typical example is shown here. FIG. 3 (a) shows a state in which the vicinity of the surface of one of the non-effective portions is left in an opaque glass state without being formed into a transparent glass. This pattern uses a zone heating furnace, and when gradually inserting from the lower end of the external soot body into the heating zone and passing through the heating zone, the ineffective part at the upper end is not sufficiently transparently vitrified It is formed when leaving. Specifically, it is possible to adopt a method of terminating the heating before the entire external soot body including the ineffective portion passes through the heating zone as in the related art, or reducing the temperature of the furnace near the end of the heating. . FIG. 3B shows a case where most of the one ineffective portion is left in an opaque glass state without being formed into a transparent glass. This pattern is formed in a case where a soaking furnace is used, one of the ineffective portions is heated outside the soaking range, and the glass is turned into a transparent glass. FIG. 3 (c) shows the use of a zone heating furnace, in which the external soot body is positioned below the heating zone to gradually increase the transparency while proceeding with transparency, and at the end of the transparency, the ineffective portion at the lower end is in an opaque glass state. It is something that remains. In the case of an external soot body manufactured by the VAD method, gas tends to remain in the upper sooting start portion. Therefore, by using this method, the upper ineffective portion is first made transparent to prevent the gas from remaining. Can be considered.

FIG. 3 shows an example in which an opaque glass state portion is left in only one of the non-effective portions. However, an opaque glass state portion may be left in both of the non-effective portions if necessary.
In the case where the external soot body is manufactured by the VAD method, the soot and the glass are brought into close contact with each other at the upper soot forming start portion, so that the bulk density of the soot is higher than that of the columnar effective portion. In most cases, this part becomes transparent first, and the escape route for gas that becomes bubbles may be closed. In order to prevent this, it is preferable to select the ineffective portion at the end where sooting is started as the ineffective portion leaving the opaque glass state portion. In addition, the method of leaving the opaque glass state portion in the ineffective portion is not limited to the above example, and the opaque portion is formed in 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 a glass state portion.

It is desirable to carry out the vitrification in a gas atmosphere having a molecular diameter as small as possible to remove the air, moisture, etc. remaining during the formation of the soot body. H
Preferably, the atmosphere is e. In addition, since He is expensive, it is effective to reduce the atmospheric gas pressure in the furnace to perform vitrification.

[0012]

EXAMPLES The method of the present invention will be described more specifically with reference to the following examples. (Example 1, Comparative Example 1) As an effective portion of a glass rod for a core, an optical core portion having a step-like refractive index distribution manufactured by a VAD method (relative refractive index: 0.35%, pure silica glass, trace amount of germanium dioxide) And a two-layered quartz glass rod having an optical cladding part (pure silica glass) around the effective part length: 500 mm, diameter 27
mm, and a pure silica glass rod for an ineffective portion was connected to both ends. A soot was formed around the rod by the VAD method. For the generation of glass particles with soot, S
A hydrolysis method in which iCl ₄ was vaporized and introduced into an oxygen-hydrogen flame was used. As a result, an external soot body having an effective portion diameter of 165 mm was obtained. The effective portion length was 500 mm, the same as the effective portion length of the rod, and the ineffective portion length was about 150 mm for both the upper and lower portions. This external soot body was transparently vitrified 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 core glass rod 2.
Was set in a zone heating furnace with the upper end at which sooting was started up, and 163 at a rate of 5 mm / min from the lower end.
It was passed through a heating zone adjusted to 0 ° C. During this time, the heating zone was kept in a He atmosphere.

As shown in FIG. 4B, a comparative example in which the entire soot layer 3 is made vitreous by passing through a heating zone to a position 30 mm from the end of the upper ineffective portion (prior art)
In the sample, bubbles 5 were found to remain near the upper end near the boundary with the transparent glass layer 4 around the core glass rod 2. On the other hand, as shown in FIG. 4 (c), by the method of the present invention, the heat treatment was terminated 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 core glass rod interface was transparently vitrified), no bubbles remained in the effective portion of the optical fiber preform.

Example 2 and Comparative Example 2 As a glass rod for a 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 a diameter of 25 mm, and a pure silica glass rod for an ineffective portion was connected to both ends thereof. O around this rod
Sooting was performed by the VD method. For the generation of soot-coated glass fine particles, a hydrolysis method of evaporating SiCl ₄ and introducing it into an oxygen-hydrogen flame is performed, and deposition is performed 10 times in the longitudinal direction to form an external soot body having an effective portion diameter of 120 mm. Obtained. The effective length is 550m, the same as the effective length of the rod
m and the length of the non-effective part is about 170 m for both the upper and lower parts
m. This external soot body was vitrified transparently using a soaking furnace. FIGS. 5 and 6 are schematic diagrams showing the heating method and the state of the obtained optical fiber preform in this example and the comparative example. First, as a comparative example, as shown in FIG. 5A, an external soot body 1 in which a soot layer 3 of glass fine particles is formed around a core glass rod 2 is heated so that the non-effective portion and the entire effective portion are uniformly heated. It was set in a soaking furnace in a He atmosphere maintained at 1500 ° C. so as to fall within the range, and kept for 90 minutes to form a transparent glass. As shown in FIG. 5B, in the optical fiber preform obtained as a result, bubbles 5 were observed near the boundary between the core glass rod 2 and the transparent glass layer 4. 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. Heat treatment is performed as shown in FIG.
As shown in (1), in the example in which most of the upper non-effective portion was left as the opaque glass state portion 6, no air 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 is performed by using SiCl ₄ +
An example in which an oxyhydrogen flame was used was shown, but this was replaced with SiCl ₄ +
Similar results were obtained when using methane gas (CH ₄ ).

When the optical fiber preforms obtained in Examples 1 and 2 were converted into fibers, the glass diameter of the optical fiber was stable within a range of 125 μm ± 1 μm. Analysis by OTDR showed that the transmission loss step was very small and an optical fiber of good quality was obtained. On the other hand, when the optical fiber preform in which the bubbles obtained in Comparative Examples 1 and 2 were formed into a fiber, the glass diameter of the optical fiber locally fluctuated locally at the portion corresponding to the bubbles, and was 125 μm ± 1 μm. The range was exceeded, and it was necessary to remove this in order to obtain a good product, resulting in a decrease in yield. Further, when the fiber containing these bubbles was analyzed by OTDR in the 1550 nm band, a transmission loss step of about 0.1 dB was found in a portion corresponding to the bubbles.

[0016]

According to the method of the present invention, there is provided a method for producing an optical fiber preform by overheating and transparentizing an external soot body which has been sooted around a core glass rod by an OVD method or a VAD method. It is possible to suppress the remaining of air bubbles that easily occur around the core glass rod.

[Brief description of the 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 remaining 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 that leaves an opaque glass state portion in an ineffective portion in the method of the present invention.

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

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

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

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 External soot body (glass particulate accumulation body) 2 Glass rod for cores 3 Soot layer (glass particulate accumulation layer) 4 Transparent glass layer 5 Bubbles 6 Opaque glass state part

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Haruhiko Aikawa 1 Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Prefecture Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor Takehiko Kito 1-Tagamachi, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Ki Kogyo Co., Ltd. Yokohama Works F-term (reference) 4G021 CA12 EA01 EA03

Claims (6)

[Claims] 1. A method for producing a preform for optical fiber, wherein a soot is formed around a core glass rod by an external method, and the obtained external soot body is heat-treated to form a transparent glass. At this time, a method of manufacturing an optical fiber preform, wherein 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 optical fiber mother according to claim 1, wherein the sooting by the external method is performed by a VAD method in which a glass rod for a core is arranged in a vertical direction and soot is grown in a vertical direction. The method of manufacturing the material. 3. The method for producing a preform for an optical fiber according to claim 2, wherein a sooting start portion located above is selected as an ineffective portion that leaves an opaque glass state portion. 4. The method for producing a preform for an optical fiber according to claim 1, wherein the sooting by the external method is performed by an OVD method in which a soot is layered around the core glass rod. . 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 producing a preform for an optical fiber according to any one of claims 1 to 4. 6. A transparent vitrification of the external soot body is performed by using a soaking furnace, and at least a part of one or both of ineffective portions at both ends is prevented from entering a 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 ineffective portion.