JPH05105467A - Production of base material for optical fiber - Google Patents

Abstract

PURPOSE:To produce a base material for an optical fiber used for optical fiber laser, an optical amplifier, etc. CONSTITUTION:When a base material for an optical fiber with a core doped with a dopant compd. is produced, an SiO2-based coating film contg. the dopant compd. is formed by a sol-gel process on the inner wall of a core-clad type glass tube with a hole at the center of the core and this film is converted into transparent glass by heating while feeding a gaseous chlorine-O2 mixture into the glass tube. This glass tube is then collapsed. The dopant compd. such as a compd. of a rare earth element is added to the center of the core at high concn. in a uniformly dispersed state and a base material for an optical fiber having satisfactory transmitting characteristics can be produced.

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 an optical fiber preform used for, for example, an optical fiber laser, an optical amplifier fiber and the like.

[0002]

2. Description of the Related Art Conventionally, a vapor phase addition method and a solution impregnation method are known as a method for adding a rare earth element to a silica fiber. As an example of the vapor phase addition method, when synthesizing the core portion inside the quartz pipe by the MCVD method, the glass raw material S
The rare earth compound is heated at around 1000 ° C. together with the vapor of iCl ₄ and GeCl _4, the vapor is transferred to a quartz pipe, heated by a burner to vitrify, and the pipe thus obtained is collapsed. There is a method of forming a preform and drawing it into a fiber.

As an example of the solution impregnation method, VAD
There is one that uses a soot for a core produced by the method. In this method, the rare earth compound is first dissolved in alcohol, and the soot base material for core is immersed in this. Next, the alcohol in the soot is evaporated at room temperature to deposit only the rare earth compound in the core soot, and the transparent glass compound is sintered in a normal He gas atmosphere to obtain an optical fiber preform.

[0004]

For application to optical fiber lasers and fibers for optical amplifiers, it is desirable that the concentration of the rare earth element added be high. However, in the conventional method, since a high temperature of 1500 ° C. or higher is required for vitrification, even if an attempt is made to add it at a high concentration, the rare earth element is crystallized in the glass and a uniformly added glass cannot be obtained. There was a problem. In view of such a current situation, an object of the present invention is to provide a method for producing an optical fiber preform capable of adding a rare earth element at a high concentration.

The present invention relates to a method of manufacturing an optical fiber preform in which a dopant compound is doped into a core. SiO ₂ containing the dopant compound is formed on the inner wall of a glass tube having a clad and a core, the center of which is open. A coating film having a main component is formed by a sol-gel method, and then a method characterized by transparent vitrification and collapse by heating while flowing a mixed gas of chlorine and oxygen in the glass tube, to solve the above problems To do. As a particularly preferred embodiment of the present invention, water and alcohol are added to one or more metal alkoxides selected from silicon alkoxide, titanium alkoxide, and germanium alkoxide, and a sol-gel solution obtained by further dissolving a dopant compound is used as an inner wall of a glass tube. A coating method can be mentioned.

The present inventors have found that a glass synthesis process called Zokugel method is suitable for the synthesis of glass doped with a rare earth element at a high concentration, and completed the present invention. That is, a glass film in which a rare earth element is highly doped with a rare earth element is formed on the inner wall of a glass tube by a sol-gel method using a metal alkoxide such as silicon alkoxide as a glass raw material, which is made transparent glass, and the tube is collapsed to form a core rod. To do. At this time, it is preferable that the glass tube is composed of a core and a clad, and the center of the core is open. Further, when the coating film is vitrified and collapsed, it is important to flow a mixed gas of chlorine and oxygen in the tube.

[0007]

The vapor phase method and the solution impregnation method are known as methods for doping glass with a rare earth element composition. In these methods, a vapor of a rare earth element or a solution of a rare earth element is diffused in a porous body of silica glass synthesized by a vapor phase method to turn the porous body into a transparent glass. In this way, when passing through the silica glass porous body, a high temperature of 1500 ° C. or higher is required for transparent vitrification. The rare earth element added to the glass is uniformly dispersed in the case of a low concentration, but is easily crystallized in an attempt to add it in a high concentration. This tendency is more remarkable at higher temperatures, making it difficult to manufacture high-concentration added glass.

On the other hand, the sol-gel method as a method for synthesizing glass is characterized in that glass is obtained by a relatively low temperature process of about 1000 ° C. using a silicon alkoxide as a glass raw material. According to this method, it is possible to synthesize a glass doped with a rare earth element at a much higher concentration than the conventional method, and a method for producing an optical fiber having a rare earth element-doped glass produced by the sol-gel method in its core The present invention was developed.

According to the present invention, as an optical fiber preform composed of a core and a clad, a hole is formed in the center of the core and a glass coating film doped with a rare earth element is formed on the inner wall of the core by the sol-gel method. At the time of transparent vitrification of this film and collapsing of a glass tube, a mixed gas of chlorine and oxygen is flown into the tube. The coating film formed by the sol-gel method may cause the deterioration of the transmission loss due to the penetration of the OH substrate into the core during solidification. By flowing chlorine gas, the water content of the coating film can be removed, and glass with low transmission loss can be obtained. Further, the reason for flowing oxygen is to prevent the rare earth element in the glass from vaporizing and to dope the glass in a stable manner at a high concentration. As described above, the optical fiber preform manufactured according to the present invention has a core and a clad, and has a portion in which a rare earth element is highly doped in a part of the center of the core.
It goes without saying that the core, the clad, and the added region of the rare earth element can be arbitrarily adjusted. By this sol-gel method, a glass film to which a rare earth element is added is formed on the inner wall of the glass tube forming the core, and the glass tube is heated and sintered to be solid to obtain a glass rod for core. According to this method, it is possible to obtain a glass material in which a large amount of rare earth elements is present in the central portion.

As the metal of the metal alkoxide used in the hydrolyzed solution of the sol-gel method according to the present invention, various metal alkoxides can be used, and particularly preferable examples include silicon, germanium, aluminum and the like. Preferred alkoxides include methoxide, ethoxide, propoxide, butoxide and the like. The alcohol used in the sol-gel method of the present invention is, for example, methanol or ethanol. As a mixed solution of metal alkoxide, water, and alcohol, for example, a molar ratio of 5:53:42 is used, but these ranges may be within the range usually performed in the technical field. An acid or alkali can be added as a catalyst. For example, hydrochloric acid, nitric acid or the like can be used as the acid, and ammonia water or the like can be used as the alkali. Examples of the dopant compound of the present invention include lanthanoid rare earth elements, and particularly preferably rare earth elements such as Er, Nd, and pr. These rare earth elements may be added to the sol-gel solution (hydrolysis solution) in the form of chloride, nitrate or the like. As a method of attaching the sol-gel solution to the inner wall of the tube,
There is a dipping method. After the attachment, it is dried and gelled by a technique known in this kind of field. For example, there is a method of drying at a temperature of 150 ° C. and then heating at about 1000 ° C. As described above, a coating film containing SiO ₂ containing a dopant compound as a main component can be formed in the glass tube. Subsequently, for example, a glass rod is obtained by sintering and solidifying at a temperature of 1500 ° C. to 1800 ° C., and when this film is transparent vitrified and a glass tube is collapsed, a mixed gas of chlorine and oxygen is flown into the tube. The oxygen gas concentration in the mixed gas is preferably 10 to 60%. As a result, a core rod in which the dopant compound is added to the center of the core is obtained.

As a subsequent process, a conventionally known optical fiber preform manufacturing process may be applied. That is, if a glass material having a core / clad and having a hole provided in the core portion is used as a starting material, a dopant compound is added to the center of the core portion to obtain a glass base material having a clad around the core. Further, if a glass rod serving as a core with a hole formed at the center is used as a starting material, a core rod having a dopant compound added at the center can be obtained. It is sufficient to form a low clad.

An Er-doped fiber according to the present invention was prepared according to the operation of Example 1 described later. That is, E
A core rod doped with r at the center was obtained, and this was drawn as a fiber preform by the rod-in-tube method. Several kinds of fibers were obtained by changing the added concentration. Separately, Er-doped fiber was prepared by a conventional solution impregnation method. The fluorescence lifetime of these Er-doped fibers was measured. A 2 × liquid of Nd.YAG laser was used as an excitation light source, and 1.55 μm fluorescence was observed by a sampling oscillograph. The dependency of the fluorescence lifetime on the Er concentration is investigated, and the result of comparison between the conventional method and the present invention is shown in the graph of FIG. In Figure 1
The mark means the present invention, and the mark x means the conventional method. Figure 1
As is clear from the above, according to the present invention, the fluorescence lifetime is long even at high concentrations. The longer the fluorescence lifetime, the more suitable it is for applications such as lasers and optical amplifiers. As described above, the optical fiber obtained by drawing the optical fiber preform according to the present invention has a high additive concentration, excellent functionality, and good transmission loss characteristics.

[0013]

【Example】

Example A coating solution was prepared by the sol-gel method. First, 10 ml of silicon ethoxide and 10 ml of water were mixed with 20 ml of ethanol in a beaker to dissolve 0.93 g of erbium chloride as a rare earth compound in the mixed solution. V
A glass rod was produced by AD method in which the cocoa was made of pure silica and the clad was made of silica doped with fluorine so that the refractive index of the clad was 0.34% lower than that of the core. This glass rod had an outer diameter of 16.5 mmφ and a core diameter of 2 mmφ, and a 1.5 mmφ hole was formed in the center of the core. The inner surface of this opening was coated with a sol-gel method solution, and the film thickness was 2 μm. Next, while flowing chlorine gas at 50 cc / min and oxygen gas at 50 cc / min in the glass tube, the coating film was transparentized by heating with an oxyhydrogen burner from the outside of the tube, and the glass tube was collapsed and solidified. The optical fiber base material was used. When the obtained base material was drawn, a fiber having a clad diameter of 125 μm and a core diameter of 10 μm was obtained, and when analyzed by EPMA, Er (erbium) was doped in the center of the core within a diameter of 1 μm.
Its concentration was 5% by weight. According to the above-mentioned conventional method, it can be seen that the method of the present invention is superior to the fact that only a concentration of about 1% by weight can be achieved at most.

[0014]

As described above, according to the present invention,
It is very effective for producing an optical fiber used for an optical fiber laser and an optical amplifier, which is doped with a large amount of rare earth elements and uniformly dispersed therein and has a good transmission loss characteristic.

[Brief description of drawings]

FIG. 1 is a graph showing the results of measuring the Er concentration dependence of the fluorescence lifetime of the Er-doped fiber according to the present invention and the Er-doped fiber according to the conventional method.

Claims (3)

[Claims] 1. A method of manufacturing an optical fiber preform in which a dopant compound is doped into a core, wherein SiO ₂ containing the dopant compound is mainly formed on an inner wall of a glass tube which is composed of a clad and a core and whose center is open. A method for producing an optical fiber preform, which comprises forming a coating film as a component by a sol-gel method and then heating the mixture while flowing a mixed gas of chlorine and oxygen into the glass tube for vitrification and collapsing. 2. A glass tube inner wall is coated with a sol-gel solution prepared by adding water and alcohol to one or more metal alkoxides selected from silicon alkoxide, titanium alkoxide and germanium alkoxide, and further dissolving a dopant compound. The method for producing an optical fiber preform according to claim 1, which is characterized in that. 3. The method for producing an optical fiber preform according to claim 1, wherein the dopant compound is a rare earth element.