JPH1072227A - Optical fiber preform and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent cracks from being generated at the time of performing a cut-off work of a front end part of a preform by using a specific optical fiber preform which has a core that is composed of a glass contg. added germanium and a clad that is formed on the periphery of the core and is composed of glass having a lower refractive index than that of a glass of a core. SOLUTION: This preform has a core that is composed of glass contg. added germanium and a clad that is formed on the periphery of the core and is composed of glass having a lower refractive index than that of the glass of the core. In the preform, a region having a reduced added germanium content as compared with that of the central part in the axial direction of the preform is placed in a front end part in the axial direction of the preform. At this time, as the preform, an optical fiber preform having such a region which has a reduced added germanium content and the length of which in the axial direction of the preform is preferably 0.1 to 1 time of the outer diameter of the preform, is preferred.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a preform for an optical fiber,
In particular, the present invention relates to an optical fiber preform which does not crack when cutting the tip of the optical fiber preform, and a method of manufacturing the same.

[0002]

2. Description of the Related Art It is known that a core burner and a clad burner are arranged in a method of manufacturing a porous base material by a VAD method, and germanium tetrachloride is supplied to the core burner as an additive for increasing the refractive index ( For example, Japanese Patent Application Laid-Open No. Hei 5-193973). However, glass to which germanium has been added has a larger coefficient of thermal expansion than pure silica, so that distortion tends to remain during cooling and cracks are likely to occur during processing.

In general, when the optical fiber preform is drawn and reduced in diameter, it is necessary to weld and connect a dummy rod to be attached to a drawing machine at both ends, but the optical fiber preform manufactured by the VAD method rotates. Since the core portion obtained by adding germanium to the tip of the starting rod and the surrounding clad portion are simultaneously synthesized in the axial direction, the tip portion has a shape in which the core portion protrudes. Therefore, when the dummy rod is to be welded and connected, it is necessary to cut the tip end or the like so as to facilitate the connection. However, since the tip is doped with germanium, cracks occur when cut,
There is a problem that welding cannot be performed. 3A and 3B show a state in which the dummy rods are connected to both ends of the optical fiber preform in the order of steps. FIG. 3A shows an optical fiber preform manufactured by the VAD method with a tip protruding from the core. (B) shows a processed state, and (c) shows a state where a dummy bar is connected.

[0004]

As described above, cracks are likely to be formed when cutting the tip of the optical fiber preform because germanium is added.
The optical fiber preform is obtained by synthesizing a porous preform and then heating the porous preform in a sintering furnace to form a transparent glass. During the vitrification, the glass is heated to about 1500 ° C., and when the vitrification is completed, it is cooled to room temperature. Since glass to which germanium is added has a larger expansion coefficient than pure silica, distortion tends to remain during cooling. Therefore,
Strain remains in the core portion to which germanium is added, and cracks easily occur when the tip portion is cut. Therefore, an object of the present invention is to prevent the generation of cracks when processing the base metal tip by controlling the addition of germanium in the core portion in the VAD method or the OVD method.

[0005]

The above objects can be achieved by the following optical fiber preform and a method of manufacturing the optical fiber preform. (1) A base material for an optical fiber having a core portion made of glass to which germanium is added, and a clad portion made of glass having a lower refractive index than the core portion at the outer periphery of the core portion, the base material for an optical fiber being provided. A preform for an optical fiber, characterized in that at an axial end of the material, there is a region where the amount of germanium added is smaller than the amount of germanium added at the center in the axial direction of the core. (2) The light according to (1), wherein the axial length of the region where the amount of germanium added at the axial end is reduced is 0.1 to 1 times the outer diameter of the base material. Base material for fiber. (3) In the region where the amount of added germanium decreases at the axial end, the amount of added germanium gradually decreases from the center to the end. ) Or the optical fiber preform according to (2).

(4) In any one of the above (1) to (3), in the region where the amount of germanium added at the end in the axial direction is reduced, the amount of germanium added is 0 at the end. Preform for optical fiber. (5) At least in the flame formed by the burner,
In a method of synthesizing a glass material containing a silicon compound and a germanium compound, synthesizing glass fine particles to which germanium is added, and depositing the glass particles on a rotating starting material to synthesize a base material for an optical fiber, A method for manufacturing a preform for an optical fiber, comprising: synthesizing an axial end portion of a material from a central portion toward an end portion while gradually decreasing a set flow rate of a germanium compound. (6) By supplying at least a glass material made of a silicon compound and a germanium compound into the flame formed by the core synthesizing burner, glass particles to which germanium is added are synthesized, and this is used as a starting material to be rotated. Deposited on the tip and growing the core base material in the axial direction, and at the same time as synthesizing the core base material, supply at least a glass raw material made of a silicon compound into the flame formed by the cladding burner around the core base material By synthesizing glass fine particles by forming a cladding layer to produce a preform for an optical fiber comprising a core clad,
A method of manufacturing an optical fiber preform, wherein the growth end of the preform is synthesized by gradually decreasing the set flow rate of the germanium compound. (7) The optical fiber preform according to the above (5) or (6), wherein the flow rate of the germanium compound is reduced and at the same time, the flow rate of the glass raw material and / or the flow rate of the hydrogen are changed. Production method. An optical fiber preform having a core portion made of glass to which germanium is added, and a cladding portion made of glass having a lower refractive index than the core portion at the outer periphery of the core portion, and an axial growth portion of the core portion An optical fiber preform, characterized in that there is a portion to which germanium is not added at the tip of

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The optical fiber preform (1)
In order to avoid cracks that are likely to enter when cutting the preform tip, a dummy rod is welded and connected to the tip of the preform for optical fiber. It is characterized in that there is a region where the amount of germanium added is reduced with respect to the amount of germanium added in the portion. Here, in order to provide a region where the amount of added germanium is reduced with respect to the amount of added germanium in the central portion in the axial direction of the core portion, as described in the above (5) to (7), In the synthesis of the end portion, the set flow rate of the germanium compound is gradually reduced from the central portion toward the end portion, or the growth end of the base material is synthesized by gradually decreasing the set flow rate of the germanium compound. Alternatively, at the same time as decreasing the flow rate of the germanium compound, one or both of the flow rate of the glass raw material and the hydrogen flow rate are changed.

FIG. 1 is a schematic view showing a method of manufacturing a porous preform 1 for an optical fiber by a VAD method.
Further, a gas such as hydrogen, oxygen, silicon tetrachloride, germanium tetrachloride or the like is supplied to the core burner 3. Before the porous base material reaches a predetermined length, the supply amount of germanium tetrachloride is gradually reduced from the set flow rate, and as shown in the graph of FIG.
The e concentration is reduced to create a portion that does not contain Ge. Here, the predetermined length is generally in a range of 300 to 2000 mm. The present invention can be applied not only to the VAD method but also to the OVD method under almost the same conditions. The optical fiber preform (2) specifies the length of the region where Ge is not added to a preferable range, thereby preventing the occurrence of cracks at the time of cutting the tip. Therefore, the axial length of the region is set to a range of 0.1 to 1 times the outer diameter of the base material. If the ratio is less than 0.1 times, the effect of preventing cracks is not sufficient.
If it exceeds twice, the region where the Ge addition concentration changes may extend to the portion where the cladding outer diameter is steady,
The portion that can be used as an optical fiber becomes short.
That is, the tip portion where the clad outer diameter is tapered is a portion that cannot be used as an optical fiber because the ratio of the core outer diameter to the clad outer diameter has changed, and the length of the Ge concentration change portion is By setting the outer diameter of the clad to be equal to or less than the length of the tapered portion, the portion used as the optical fiber can be made equal.

In the above optical fiber preform (3), Ge
It defines that the amount of germanium added gradually decreases from the center to the end. In the optical fiber preform (4), it is shown that the Ge addition amount is preferably set to 0 at the end. According to the method for manufacturing a preform for an optical fiber of the above (5), the supply amount of the germanium compound is gradually reduced at the time of synthesizing the porous preform by the OVD method, particularly, at the time of synthesizing the end portion in the axial direction. With such a method, the germanium concentration at the tip of the core portion is gradually changed toward the tip portion. In some cases, it is possible to obtain a preform for an optical fiber in which germanium is not added to the tip. According to the method (6) for producing a preform for an optical fiber, when the porous preform is synthesized by the VAD method, particularly when the growth end of the porous preform is synthesized, the length of the porous preform is desired. Before becoming length,
By reducing the supply of the germanium compound, by adopting such a method, the germanium concentration at the tip of the core gradually changes as it approaches the tip, and in some cases, the optical fiber does not contain germanium at the tip. A base material for use can be obtained.

However, when only the germanium raw material is reduced during the production of the porous base material, the growth rate of the core portion changes, and the growth balance with the clad is lost. If the growth balance between the clad and the core breaks down, the outer diameter ratio of the core and the clad will change, and ultimately the core diameter when used as an optical fiber will change, so the characteristics will change only in that part. I will. In order to prevent this, in the method (7) for manufacturing the optical fiber preform, when changing the flow rate of the germanium raw material, the flow rate of the glass raw material is also changed in order to keep the growth rate of the core portion constant. Increasing the glass material flow rate also increases the growth rate, and decreasing the glass material flow rate also decreases the growth rate. Further, if the bulk density suddenly changes in that portion, a problem such as cracking occurs, so the hydrogen density is adjusted to keep the bulk density constant. This adjustment is usually performed by measuring the surface temperature using a non-contact thermometer or the like, and adjusting the hydrogen flow rate so that the surface temperature becomes the same as before changing the germanium raw material flow rate. Increasing the hydrogen flow increases the surface temperature and increases the bulk density, and decreasing the hydrogen flow decreases the surface temperature and reduces the bulk density. However, if the flow rate of hydrogen is changed, the growth rate also changes. Therefore, it is necessary to adjust the flow rate of the glass raw material if necessary. In general, when the flow rate of the germanium raw material is reduced, the growth rate often decreases. On the contrary, the growth rate may increase. Therefore, it is necessary to adjust the glass raw material and the hydrogen flow rate according to the change in the growth rate.

[0011]

The present invention will be described in more detail with reference to the following examples. (Example 1) As shown in FIG. 1, a core burner and a clad burner are installed, and silicon tetrachloride and germanium tetrachloride are supplied as glass raw materials into a flame formed by supplying oxygen gas and hydrogen gas to the core burner. Then, oxygen gas and hydrogen gas are supplied to the clad burner, silicon tetrachloride flows in the formed flame, and glass particles are deposited in the axial direction at the tip of the rotating starting rod to add germanium to the core. And a porous base material comprising a clad portion to which germanium was not added. At this time, the flow rate of germanium tetrachloride to be supplied to the core burner was gradually decreased from the point where the axial length of the porous base material became 400 mm from the tip of the starting rod, and the supply was completely stopped when it reached 440 mm. I let it. At that time, the growth rate was about 70 mm / H before the germanium raw material flow rate was reduced. However, the growth rate gradually decreased by reducing the germanium raw material flow rate.
It was around 0 mm / H. Thereafter, when the porous base material was deposited 500 mm in the axial direction from the tip of the starting rod, the supply of silicon tetrachloride to the core burner and the clad burner was stopped, and the deposition of the glass base material was stopped. The obtained porous base material was dehydrated and sintered, then its tip was cut, and a dummy bar was welded and connected by a glass lathe, and attached to a stretching machine for stretching. At the time of cutting the tip portion, there was no abnormality such as a crack in the cut portion, and the welding connection of the dummy rod was successfully performed.

(Example 2) A porous preform was synthesized with the same burner configuration as in Example 1. At this time, while decreasing the supply flow rate of germanium tetrachloride and increasing the flow rate of silicon tetrachloride supplied to the core burner, the growth rate of the core portion is kept constant at about 70 mm / h, and the porous base material is reduced. Synthesis could be performed. (Comparative Example) At the time of manufacturing a porous preform, predetermined amounts of silicon tetrachloride and germanium tetrachloride were supplied until the length became 400 mm, and a porous preform having a predetermined length was synthesized. At that time, the growth rate of the core portion was about 70 mm / hour. Thereafter, dehydration sintering and tip cutting were performed in the same manner as in Example 1. However, cracks were generated from the core portion at the time of cutting the tip, and the dummy rods could not be welded and connected.

[0013]

According to the present invention, a dummy rod is welded to the tip of the optical fiber preform by providing a portion of the porous preform for optical fiber to which no germanium is added at the tip of the axial growth portion of the core. In addition, cracks can be prevented when cutting the base metal tip.

[Brief description of the drawings]

FIG. 1 is a schematic view of a method for producing a porous preform for an optical fiber by a VAD method.

FIG. 2 is a graph showing how the supply flow rate of GeCl ₄ is reduced after a predetermined time has elapsed.

FIG. 3 is a schematic view showing steps (a) to (c) of welding and connecting a dummy rod to a porous preform for an optical fiber manufactured by a VAD method.

[Explanation of symbols]

 1: Porous preform for optical fiber 2: Burner for clad 3: Burner for core

Claims (7)

[Claims] An optical fiber preform having a core portion made of glass to which germanium is added, and a cladding portion made of glass having a lower refractive index than the core portion at the outer periphery of the core portion, At the axial end of the base material,
A preform for an optical fiber, characterized in that there is a region where the amount of germanium added is reduced with respect to the amount of germanium added at the center in the axial direction of the core. 2. The axial length of the region where the amount of germanium added at the axial end is reduced is from 0.1 to 1 of the outer diameter of the base material.
2. The optical fiber preform according to claim 1, wherein the preform is doubled. 3. In a region where the amount of added germanium is reduced at the axial end, the amount of added germanium is gradually reduced from the center toward the end. Item 3. An optical fiber preform according to item 1 or 2. 4. In a region where the amount of added germanium is reduced at the end in the axial direction, the amount of added germanium is 0 at the end.
The optical fiber preform according to any one of claims 1 to 3, wherein: 5. At least a glass material composed of a silicon compound and a germanium compound are supplied into a flame formed by a burner, thereby synthesizing glass particles to which germanium is added, and depositing the particles on a rotating starting material. In the method of synthesizing the optical fiber preform by performing the above, the synthesis of the axial end portion of the preform is performed by gradually decreasing the set flow rate of the germanium compound from the center to the end. A method for producing a preform for an optical fiber, comprising: 6. At least a glass raw material comprising a silicon compound and a germanium compound are supplied into a flame formed by a core synthesis burner, thereby synthesizing glass particles to which germanium is added, and rotating the glass particles. At the same time as depositing on the tip of the material and growing the core material in the axial direction, at the same time as the synthesis of the core material, the flame formed by the cladding burner on the outer periphery of the core material, at least a glass material consisting of a silicon compound In the method of synthesizing glass microparticles by supplying and forming a cladding layer to produce a base material for an optical fiber comprising a core clad, the synthesis of the growth end end of the base material is performed by gradually increasing the set flow rate of the germanium compound. A method for producing a preform for an optical fiber, wherein 7. The optical fiber preform according to claim 5, wherein at least one of the flow rate of the glass raw material and the hydrogen flow rate is changed at the same time as the flow rate of the germanium compound is reduced. Method.