JPH1171125A - Production of preform for optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a preform for an optical fiber capable of providing an optical fiber having a slight transmission loss without causing soot cracking. SOLUTION: This method comprises connecting both ends or one end of a core preform composed of a core or a part of the core and a clad to dummy rods 2, carrying out the flame polishing of the surface of the core preform with a burner 3 for flame polishing, then synthesizing a porous clad glass on the outer periphery of the core preform containing the dummy rods 2, forming a composite preform of the core preform and the porous clad glass, heat-treating the composite preform and thereby producing a transparent preform for an optical fiber. In this case, the difference between the outside diameter of the core preform and the outside diameter of the dummy rods 2 connected to the core preform is <=2 mm.

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 high-purity glass base material used for optical communication, and more particularly to a method of manufacturing a porous clad glass by reducing the difference between the outer diameters of a dummy rod and a core base material. The present invention relates to a method for manufacturing a preform for an optical fiber, which can uniformly deposit a dummy portion and a core base material portion when depositing the same to prevent occurrence of cracks.

[0002]

2. Description of the Related Art Conventionally, as a method of manufacturing an optical fiber, glass fine particles having a high refractive index serving as a core are deposited on the surface of a rod-shaped starting member, a cladding layer is deposited thereon, and the starting member is removed. A soot attachment method (OVD method) and an axis attachment method (VAD method) in which glass fine particles serving as a core and a clad are axially deposited are known.

In these known methods, for example, an outer periphery of a cylindrical starting rod is coated with glass having a lower refractive index than the starting rod, a preform made of a core and a clad is formed, and this is drawn to form an optical fiber. When manufacturing,
Method of flame polishing a starting rod (Japanese Patent Laid-Open No. 48-7352)
No. 2) or a method of depositing a soot body while flame-polishing the outer periphery of a starting rod when depositing a porous body on the outer periphery of an optical fiber intermediate preform consisting of a core and a part of a clad (Japanese Patent Laid-Open No. 57-1979). No. 160927). Its purpose is to improve the interface between the core glass and the clad glass to prevent excessive light absorption loss, prevent bubbles from forming at the interface between the core and the clad, and remove foreign matter from adhering. Is to do.

Further, a dummy rod of a homogeneous material is connected to both ends of a base material having a refractive index distribution in a radial direction, and when the base material is subjected to flame polishing, heat treatment is performed on the dummy rod to prevent cracks due to the heat treatment. It has been proposed to stop at an intermediate point (Japanese Patent Laid-Open No. 2-80338).

[0005]

As described above, the core,
Alternatively, a dummy rod is connected to both ends of a core preform consisting of a core and a part of a clad, and a preform for an optical fiber is formed by a method of forming a clad glass including a part of the dummy rod on the outer periphery of the core preform. When combined, the following problems occurred. When synthesizing the clad glass, especially when synthesizing near the connection point between the core base material and the dummy rod, there is a problem that the clad glass is easily broken, especially when the porous clad glass is deposited. If you had a problem.

[0006] Dummy rods are sometimes broken during processing such as depositing and sintering a porous clad glass around the outer periphery of a core rod. After making the optical fiber, the transmission loss was evaluated.
Transmission loss sometimes became abnormally high.

[0007] The above problem is particularly high when the difference between the outer diameters of the core rod and the dummy rod is larger than a certain value, and the frequency of cracking is high, and soot is deposited when soot synthesis is performed at locations having different outer diameters. It is considered that the soot is distorted and easily broken. In particular, the local heat capacity of the rod differs due to the different outer diameter, and the surface temperature when depositing the porous clad glass changes, so the adhesion to the core base material also changes rapidly. Therefore, it is considered that a crack occurs.

[0008] The problem is that the outer periphery of the core base material is flame-polished or etched with a solution such as hydrofluoric acid to clean the surface. A dummy rod connected to this is generally flame-polished. It is highly probable that minute scratches and dirt are still attached to the surface of the dummy rod because it has not been performed, and this is considered to have an effect on cracks in the dummy rod and transmission loss.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for manufacturing an optical fiber preform which does not cause soot cracking and can obtain an optical fiber with a small transmission loss. And

[0010]

The above objects can be effectively achieved by the following inventions and embodiments. (1) Dummy rods are connected to both ends or one end of a core or a core base material composed of a part of the core and the clad, and after the surface of the core base material is polished by flame, the dummy rods are placed on the outer periphery of the core base material including the dummy rods. A method for producing a transparent optical fiber preform by synthesizing a porous clad glass to form a composite preform of a core preform and the porous clad glass, and heat treating the composite preform, A method for manufacturing an optical fiber preform, wherein a difference between an outer diameter of a base material and an outer diameter of a dummy rod connected to the core base material is set to 2 mm or less.

(2) The method for producing an optical fiber preform according to the above (1), wherein a part of the dummy rod is flame-polished by flame polishing before synthesizing the porous clad glass. (3) The method for producing a preform for an optical fiber as described in (2) above, wherein when the dummy rod is flame-polished, at least the area covered with the porous clad glass in the next step is flame-polished.

(4) When the dummy rod is flame-polished, the region on the connection point side which is 60% or more of the area covered with the porous clad glass in the next step is flame-polished. A method for producing a preform for an optical fiber according to the above.

By reducing the difference between the outer diameter of the dummy rod and the outer diameter of the core base material by the method (1), the dummy portion and the core base material portion can be uniformly synthesized in the adhesion of the porous clad glass. Therefore, no distortion can occur in the clad glass,
The cause of the cracks can be eliminated, and stable production of the optical fiber preform can be realized. Here, the outer diameter difference means a difference between the outer diameter of the glass rod including a part of the clad and the outer diameter of the dummy rod in some cases.

In the method (2), the surface of the dummy rod can be cleaned by flame polishing to the dummy portion,
It is possible to prevent impurities adhering to the surface of the dummy rod from being mixed into the base material, thereby preventing an increase in transmission loss. In the method of (3) above, in particular, by cleaning the portion of the dummy rod covered with the porous clad glass,
Mixing into glass can be efficiently suppressed. Furthermore, as described in (4) above, a limited area near the core rod, that is, a minimum required area of 60%
Effective treatment can be performed by flame polishing the above, preferably 80% or more.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will be described below. A core base material composed of a core or a part of a core and a clad is prepared by a general VAD method as shown in FIG. In FIG. 3, 31 is a core burner, 32
Indicates a burner for cladding, 33 indicates a core base material, and 34 indicates a flame for synthesizing glass fine particles. This core material is generally VA
After synthesizing by the method D, it is synthesized by dehydration and sintering. In addition, it is synthesized by a general method such as the MCVD method or the OVD method. This base material is generally stretched once in a subsequent step to a convenient outer diameter. For example, at this stage, the refractive index difference between the core and the clad is 0.3 to 0.4%, and the clad / core magnification is 2.5 to 5 times.

[0016] Dummy rods are fusion-connected to both ends of the core base material. This process is referred to as damping. At this time, it is preferable that the dummy rod to be in contact with the dummy has an outer diameter whose dimensional difference with respect to the outer diameter of the core base material is within 2 mm. More preferably, the outer diameter is within 1 mm. If the difference in outer diameter is 5 mm or more, cracks occur almost during soot synthesis. If possible, hold down the connection point after contacting with a heat-resistant plate, such as a high-purity carbon plate, while heating it, and process it into a smooth shape.
The effects of the present invention can be obtained efficiently. When the cladding glass is subsequently synthesized in a vertical facility, a dummy bar having one end shaped as shown in FIG. 2 is used so as to fit the tip of the base material supporting member. Sometimes. In FIG. 2, reference numeral 21 denotes a dummy rod, 22 denotes a dummy rod fit, 23 denotes a base material support rod, and 24 denotes a support rod fit.

The thus prepared core base material having dummy rods connected to both ends (in some cases, a dummy rod may be connected to only one side) is flame-polished as shown in FIG. In the flame polishing, the oxyhydrogen flame temperature is usually adjusted so that the glass surface becomes 1200 to 1600 ° C. (preferably 1400 to 1500 ° C.), and the surface of the base material is heat-treated. At this time, a part of the dummy bar close to the connection point with the core base material is also flame-polished. In FIG. 1, 1 is a core rod, 2 is a dummy rod, 3 is a burner for flame polishing, 4 is a flame, and 5 is a chuck. In the flame polishing of the dummy rod, it is preferable that the portion where the clad is coated on the outer periphery in the subsequent step is flame-polished. However, if the area (connection side) of 60% or more, preferably 80% or more of the length of this portion is flame-polished, the influence on the transmission loss can be reduced. If it is less than 60%, the effect gradually decreases.

After flame polishing, the base material is vertical or horizontal as shown in FIG. 4 or FIG.
It is set in a VD apparatus, and a porous clad glass is deposited on the outer periphery of a core base material having dummy rods connected to both ends. At this time, the portions where the outer diameters of both ends are changed are hooked on the dummy rods, so that the outer diameter of the clad of the core base material becomes constant, and the core base material can be used effectively. FIG.
And 41 is a core base material, 42 is a dummy bar, 43 is a support bar,
44 is a burner, 45 is a flame, 46 is a base material, and in FIG. 5, 51 is a core base material, 52 is a dummy bar, 53 is a burner, 54
Represents a flame, 55 represents a chuck, and 56 represents a base material.

The base material having the porous glass deposited on the outer periphery is then set in a sintering furnace and subjected to a heat treatment. Although the heat treatment depends on the thickness of the clad included in the core base material, there are cases where dehydration and transparency are performed and cases where only transparency is performed. When dehydration is performed, light propagating to the synthesized clad portion is transmitted. When the core base material prepared first covers the range in which light is transmitted, dehydration is not necessary. Generally, when dehydration is not required, the ratio of the outer diameter of the core base material to the core is 3.
Dehydration is not necessary if it is 5 to 4 times or more, but dehydration is considered necessary if it is less than 5 times. By performing the dehydration treatment, OH groups in the glass can be reduced, and transmission loss can be reduced. Dehydration is usually 1000-1200 ° C
Heat treatment is performed at a temperature at which the porous body does not shrink by heat. Transparency is achieved by performing heat treatment at a temperature of about 1500 to 1650 ° C. Thus, an optical fiber preform can be synthesized. This preform is stretched to an appropriate outer diameter, heated and melted, and drawn into a 125 μm fiber, whereby a high-performance optical fiber used for communication or the like can be obtained. The drawn base material may be drawn as it is without stretching.

[0020]

The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto. (Example 1) A core base material including a core and a clad was synthesized by a VAD method as shown in FIG. This base material was heat-treated at 1100 ° C. in a sintering furnace, and then heated at 1600 ° C. to produce a transparent glass base material. When the refractive index inside the base material was examined, the refractive index difference between the core and the clad was 0.36%, and the clad / core magnification was 5 times. This base material was stretched to 14 mm, and dummy rods of 14 mm were connected to both ends. The machining accuracy of both the core rod and the dummy rod is within 0.2 mm, and the difference in outer diameter is at most 0.
4 mm. The surface of this rod was heated with an oxyhydrogen flame and subjected to flame polishing. The flame polishing temperature was set at 1470 ° C. Flame polishing of the dummy rod is 50mm from the contact position
Range. One of the dummy rods had a distal end having a fitting portion shown in FIG. This base material was attached to the tip of a base material supporting rod of a VAD apparatus, and a porous clad glass was laminated on the outer periphery in the configuration shown in FIG. The outer diameter of the porous clad glass was substantially constant at the outer periphery of the core base material, and the outer diameter changed to a tapered shape at the position of the dummy rod. At this time, the length of the tapered shape was about 45 mm, which was within the flame polishing range.

The base material thus synthesized is transferred to a sintering furnace,
Heat treatment was performed at a temperature of 1600 ° C. to make the porous clad glass transparent. The base material prepared in the above steps did not cause soot cracking, and a preform having an excellent outer diameter of 46 mm and high transparency was obtained. This was stretched to 36 mm, and a 125 μm optical fiber was manufactured by drawing, and the transmission loss was measured. As a result, it was 0.34 dB / km at 1.3 μm.
Good characteristics of 0.2 dB / km at 1.55 μm were exhibited.

(Example 2) Using the same rod as described above, a cladding glass for porous was synthesized by the OVD method shown in FIG. With the base material prepared by the OVD method, a porous clad glass having a substantially uniform outer diameter could be synthesized on the outer periphery of the core rod, but the taper length at both ends was 65 mm, which was slightly longer than VAD. The length of this length was about 77% by Kenken.
The same characteristics as were obtained. (0.3 μm and 0.1 μm).
342 dB / km, 0.201 dB / k at 1.55 μm
m)

(Comparative Example 1) For comparison with Example 1, a trial production was performed using a rod having a different outer diameter of a dummy rod. Example 1
A core base material including a core and a clad is synthesized by the VAD method in the same manner as described above, and heat-treated at 1100 ° C. in a sintering furnace.
A transparent glass preform was produced by heating at ° C.
In this base material, the refractive index difference between the core and the clad was 0.36%, and the clad / core magnification was 5 times. This base material is 14mm
And a 19 mm dummy rod was connected to both ends. The surface of this rod was heated with an oxyhydrogen flame and subjected to flame polishing. The flame polishing temperature was set at 1470 ° C. The flame polishing of the dummy rod was within a range of 50 mm from the contact position. One of the dummy rods had a fitting portion shown in FIG. This base material was attached to the tip of a base material supporting rod of a VAD apparatus, and a porous clad glass was laminated on the outer periphery in the configuration shown in FIG. At this time, cracks occurred in the porous clad glass when the porous clad glass was laminated on the contact portion, and a good base material could not be obtained. Set the outer diameter of the dummy rod to 17
In some cases, the base material could be synthesized without cracking when it was reduced to mm, but almost all cracked. Also,
Even when the porous clad glass was not cracked during synthesis, cracking occurred during sintering, and a good base material could not be obtained.

Example 3 A base material was synthesized with the same configuration as in Example 1 without flame polishing of the dummy rod. When a fiber was formed in the same configuration and the transmission loss was evaluated, the transmission loss in the range close to the dummy contact portion was 1.55 μm and 0.25 d.
B / km was found to be slightly higher. The transmission loss of 1.3 μm was as high as 0.36 dB / km.

[0025]

According to the present invention, according to the OVD method, dummy rods are connected to both ends of a core or a core base material composed of a core and a part of a clad using the VAD method. In the method for forming the cladding glass including the portion, the difference between the outer diameter of the core base material and the outer diameter of the dummy rod is made as small as possible to prevent soot cracking, and if necessary, the porous clad glass A part of the dummy rod is also flame-polished by flame polishing before synthesis, so that the transmission loss of the obtained optical fiber can be reduced.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of one embodiment for implementing the method of the present invention.

FIG. 2 is a conceptual diagram showing a state where a dummy bar is fitted to a base material support bar in one embodiment of the present invention.

FIG. 3 is a conceptual diagram showing a manufacturing mode of a core base material according to the present invention by a VAD method.

FIG. 4 is a conceptual diagram showing a configuration for synthesizing a clad glass by a VAD method according to the present invention.

FIG. 5 is a conceptual diagram showing a configuration for synthesizing a clad glass by an OVD method according to the present invention.

[Explanation of symbols]

 1: Core rod 2: Dummy rod 3: Burner for flame polishing 4: Flame 5: Chuck

Claims (4)

[Claims] 1. A dummy rod is connected to both ends or one end of a core or a core base material composed of a part of a core and a clad, and the surface of the core base material is flame-polished. Synthesizing a porous clad glass on the outer periphery to form a core matrix and a composite matrix of the porous clad glass,
A method for producing a transparent optical fiber preform by heat treating the composite base material, wherein a difference between an outer diameter of a core base material and an outer diameter of a dummy rod connected to the core base material is set to 2 mm or less. A method for producing a preform for an optical fiber. 2. The method for producing an optical fiber preform according to claim 1, wherein a part of the dummy rod is flame-polished by flame polishing before synthesizing the porous clad glass. 3. The method for producing an optical fiber preform according to claim 2, wherein when the dummy rod is flame-polished, at least the area covered with the porous clad glass in the next step is flame-polished. . 4. The method according to claim 2, wherein, when the dummy rod is flame-polished, the region on the connection point side, which is at least 60% of the area covered by the porous clad glass in the next step, is flame-polished. A method for producing the optical fiber preform according to the above.