Method for producing glass preform for optical fiber

Abstract: A glass preform for an optical fiber containing a dopant in a core portion at a high concentration is produced by providing a glass tube, forming a coating film of a sol-gel solution containing a dopant compound on an inner wall of the glass tube, and vitrifying and collapsing the glass tube having the coating of the sol-gel solution while flowing a mixture of chlorine and oxygen through the glass tube to obtain a glass preform.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for producing a glass preform for use in the fabrication of an optical fiber, in particular, an optical fiber which is used as an optical fiber laser or an light amplifying fiber. Description of the Related Art An optical fiber comprising a core to which a rare earth element such as Er or Nd is added is mainly used as a light amplifying fiber. As a method for adding a rare earth element to a quartz optical fiber, there are known a vapor phase addition method and a solution-impregnation method. In one example of the gas phase addition method, when a core part is formed on an inner wall of a quartz pipe by the MCVD method, a compound of a rare earth element is heated around 1000°C and a vapor of the compound is supplied in the quartz pipe together with a vapor of a glass-forming material such as SiCl 4 and GeCl 4 , then the pipe is heated to a high temperature to vitrify the compound and also collapse the pipe to obtain a preform, which is drawn to fabricate an optical fiber. One example of the solution-impregnation method utilizes a soot preform for a core part which is produced by the VAD method. In this method, the soot preform for the core part is dipped in a solution of a compound of a rare earth element dissolved in an alcohol. After removing the soot preform from the solution, the alcohol is evaporated off from the soot preform at room temperature to deposit the compound of the rare earth element on the glass soot. Then, the glass soot carrying the compound of the rare earth element is sintered and vitrified to produced a transparent preform. In order to use the quartz optical fiber containing the rare earth element as the optical fiber laser or the optical amplifying fiber, a higher concentration of a dopant compound of a rare earth element is preferred. However, in the conventional methods, since the vitrification of the glass soot requires a high temperature of 1500°C or higher, the rare earth element tends to be associated and crystallized in the glass so that the rare earth element cannot be uniformly added to the glass. EP-A-0 443 781, published on 28 September 1991, and EP-A-0 469 795, published on 5 February 1992 and falling within the terms of Article 54(3)EPC, both disclose methods for producing glass preforms for use in the fabrication of optical fibers. On an inner surface of a glass tube, comprising MCVD cladding and optionally MCVD doped core material, is formed a coating film derived from a sol-gel solution. This solution comprises a metal alkoxide dissolved in an alcoholic or aqueous solvent. The resulting coated glass tube is dehydrated whilst flowing a mixture of chlorine and oxygen through it. EP-A-0 469 795 also discloses the option of MCVD of further doped core material after dehydration of the coating film. OFC Conference 1991, paper WA2 from 20/02/91, Technical Digest page 62 discloses sol-gel dip coating to apply thin doped SiO 2 layers inside MCVD preforms for the fabrication of rare-earth-doped fiber amplifiers. The MCVD substrate tube, comprising MCVD cladding and optionally MCVD core, is filled with a doped alkoxide sol and then drained at a constant rate. The resulting glass tube may be dehydrated in a chlorine and oxygen atmosphere at 900°C without significant loss of rare-earth ions. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a glass preform for use in the fabrication of an optical fiber by which the rare-earth element can be added to the preform at a high concentration. Another object of the present invention is to provide a method for producing a glass preform for use in the fabrication of an optical fiber by which the rare-earth element can be added to the preform uniformly. According to the present invention, there is provided a method for producing a glass preform for use in the fabrication of an optical fiber which comprises the steps of: applying a sol-gel solution to form a coating film comprising SiO 2 and a dopant compound on the inner wall of a glass tube whose inner wall forms a core part and whose outer wall forms a cladding part; flowing a vapor of a metal compound through the glass tube after the formation of said coating film, and vitrifying and collapsing said glass tube having the coating of the sol-gel solution while flowing a mixture of chlorine and oxygen through the glass tube to form the glass preform. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a refractive index profile of the optical fiber fabricated in Example 1. DETAILED DESCRIPTION OF THE INVENTION As explained above, as the vitrification temperature increases, the rare-earth elements associate with each other. To obtain a glass in which a large amount of the rare-earth is dispersed in an ionic state, a low vitrification temperature is desired. The sol-gel method uses a metal alkoxide as a raw material, and the metal alkoxide is hydrolyzed with water in an alcoholic solution and a hydroxide is changed to a silica glass at a comparatively low temperature of about 1000°C. By the sol-gel method, a glass containing the uniformly added rare-earth element in a high concentration is produced by dissolving the rare-earth element in the solution. While the maximum concentration of the rare-earth element is 1% by weight in a method using a porous soot preform, it is about 5% by weight when using the sol-gel method. According to the present invention, the glass tube consists of the core portion and the cladding portion, and the glass coating containing the dopant is formed by the sol-gel method. The glass tube consisting of the core portion and the cladding portion may be produced by the VAD method or the MCVD method. In the case of the VAD method, the soot preform is vitrified and then bored along its center axis. In the case of the MCVD method, the cladding portion and the core portion are formed on an inner wall of a silica glass tube. As the metal alkoxide to be used in the sol-gel method, any of the conventionally used ones may be used. Preferred examples of the alkoxide are methoxide, ethoxide, propoxide and butoxide of silicon, germanium and aluminium. As an alcohol to be used in the sol-gel method, methanol, ethanol, propanol, isopropanol and butanol are preferred. A molar ratio of the metal alkoxide, water and the alcohol is not critical and may be the same as that used in this field. For example, a molar ratio of the metal alkoxide, water and the alcohol is 5:53:42. As a catalyst, an acid or an alkali may be added to the solution. Preferred examples of the acid are hydrochloric acid, nitric acid, sulfuric acid and phosphoric acid, and preferred example of the alkali is aqueous ammonia. A composition of an aqueous solution used in the sol-gel method may be in a conventional range in this art field. For example, a molar ratio of the alcohol to water is from 0.5:1 to 5:1, a molar ratio of water to be added during hydrolysis to the metal alkoxide is from 2:1 to 5:1. The acid as a catalyst is used in an amount such that pH of the solution is from 1 to 5. As the rare earth element as the dopant, lanthanoids, in particular, Er, Nd and Pr are preferred. The rare earth element is used in the form of a salt such as chloride or nitrate. To apply the sol-gel solution on the inner surface of the glass tube, a dip coating method is preferred. For example, the glass tube is dipped in the sol-gel solution and pulled up at a constant speed, or the sol-gel solution is filled in the glass tube and drained at a constant rate. After the application of the sol-gel solution, the glass tube is dried and the solution is gelled by conventional methods. For example, the glass tube is dried at a temperature of about 150°C and heated at a temperature of about 1000°C. After the formation of the glass coating, a vapor of a metal compound is flowed through the glass tube and then the glass coating is vitrified and the glass tube is collapsed while flowing the mixture of chlorine and oxygen therethrough. The metal compound is used to adjust the refractive index of the glass coating formed by the sol-gel method. Examples of the metal are silicon, germanium, titanium, boron, aluminium, phosphorus and the like. The metal is used in the form of a chloride, alkoxide and the like. The compound of the metal is flowed in the glass tube using an inert gas (e.g. argon, helium, nitrogen, etc.) as a carrier gas. Since the glass coating formed by the sol-gel method is porous, it absorbs the metal compound. By adjusting the concentration of the metal compound, the refractive index of the glass coating is controlled easily. The glass tube may be collapsed and solidified at a temperature of 1500 to 1800°C by a conventional method for the production of the glass rod to obtain a glass rod. Then, the glass rod may be drawn to a specific size or its outer surface is cleaned by a conventional method. The glass tube having the coating of the sol-gel solution is vitrified and collapsed while flowing a mixture of chlorine and oxygen through the glass tube to form the glass preform. The concentration of oxygen in the mixture is preferably from 10 to 60% by volume. The use of chlorine removes moisture from the glass coating and decreases the transmission loss of the finally fabricated optical fiber. The use of oxygen oxidizes the rare-earth element in the glass coating and the metal used for adjusting the refractive index and stabilizes them. Also, oxygen prevents evaporation of the metal compound during heating the glass tube to vitrify the glass coating. PREFERRED EMBODIMENTS OF THE INVENTION The present invention will be illustrated by the following examples. Example 1 To a mixture of silicon ethoxide (10 ml), water (10 ml) and ethanol (20 ml), erbium chloride (0.93 g) was added and mixed to obtain a solution. A glass rod having an outer diameter of 16.5 mm and a core diameter of 2 mm and a refractive index of a core of 0.9 % higher than that of a cladding was produced by the VAD method and bored at a bore diameter of 1.5 mm along its axis. In the bore, the above solution was filled and drained to coat the inner surface of the tube with the solution at a thickness of 2 µm. Then, the coated solution was heated with an oxyhydrogen flame while flowing SiCl 4 and GeCl 4 at flow rates of 30 cc/min. and 60 cc/min. respectively in the bore and then vitrified while flowing chlorine and oxygen at flow rates of 50 cc/min. and 50 cc/min. respectively, followed by collapsing the tube to obtain a preform. The glass preform was drawn to fabricate an optical fiber having a cladding diameter of 125 µm and a core diameter of 10 µm and a refractive index profile of Fig. 1, which was analyzed by EPMA to find that the core was doped with erbium at a concentration of 5 % by weight in a diameter of 1 µm around the center axis. Example 2 To a mixture of silicon methoxide (10 ml), aluminum ethoxide (1 g), water (10 ml) and propanol (20 ml), neodymium chloride (1 g) was added and mixed to obtain a homogeneous solution. In a silica tube, a core layer containing germanium was formed by the MCVD method. The core layer had a refractive index 1.5 % higher than that of cladding silica. This glass tube was dipped in the above solution and pulled up to coat the inner surface of the tube with the solution. Then, the coated solution was heated with an oxyhydrogen flame while flowing SiCl 4 and GeCl 4 at flow rates of 30 cc/min. and 90 cc/min. respectively in the bore and then vitrified at a higher temperature while flowing chlorine and oxygen at flow rates of 50 cc/min. and 50 cc/min. respectively, followed by collapsing the tube to obtain a preform. The glass preform was drawn to fabricate an optical fiber having a cladding diameter of 125 µm and a core diameter of 10 µm, which was analyzed by EPMA to find that the core was doped with aluminum and neodymium at concentrations of 3 % by weight and 5 % by weight, respectively in a diameter of 3 µm from its center axis.

Claims: A method for producing a glass preform for use in the fabrication of an optical fiber which comprises the steps of: applying a sol-gel solution to form a coating film comprising SiO 2 and a dopant compound on the inner wall of a glass tube whose inner wall forms a core part and whose outer wall forms a cladding part; flowing a vapor of a metal compound through the glass tube after the formation of said coating film, and vitrifying and collapsing said glass tube having the coating of the sol-gel solution while flowing a mixture of chlorine and oxygen through the glass tube to form the glass preform. A method according to Claim 1, wherein said metal compound is at least one of silicon chloride, germanium chloride, titanium chloride, boron chloride, phosphorous chloride and aluminium chloride. A method according to Claim 1 or Claim 2, wherein silicon alkoxide is used as the source of SiO 2 . A method according to any preceding claim, wherein said sol-gel solution comprises a metal alkoxide, water, an alcohol and said dopant compound. A method according to Claim 4, wherein the metal alkoxide is at least one of silicon alkoxide, titanium alkoxide, germanium alkoxide and aluminium alkoxide A method according to any preceding claim, wherein said dopant is a rare earth element.

Inventors: HOSHINO SUMIO, ISHIKAWA SHINJI, ITO MASUMI, HOSHINO, SUMIO, ISHIKAWA, SHINJI, ITO, MASUMI, HOSHINO, SUMIO, C/O YOKOHAMA WORKS OF SUMITOMO E., ISHIKAWA, SHINJI, C/O YOKOHAMA WORKS OF SUMITOMO, ITO, MASUMI, C/O YOKOHAMA WORKS OF SUMITOMO ELEC.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1992
Main classification: C03B37
IPC codes: C03C13, C03B37