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 solgel solution containing a dopant compound on an inner wall of the glass tube, vitrifying and collapsing the glass tube having the coating of the sol-gel solution to obtain a glass rod.

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 (designating DE, FR and GB), EP-A-0 433 643 and EP-A-0 469 795, each published after the priority date of the present application and falling within the terms of Art. 54(3) EPC, disclose methods for producing glass preforms including a dopant of a rare earth element using a sol-gel technique. US-A-4 141 710 discloses the production of optical fibers by the deposition on the interior of a glass tube of a sol comprising silicon oxide, alcohol, water and a lead compound. OFC Conference 1991, Paper WA2, Technical Digest page 62 discloses the use of a silicon oxide sol, doped with a rare earth compound, to form a coating on the interior of a tube and collapsing the tube. 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 comprising a silicon alkoxide and a dopant compound on an inner wall of a glass tube, drying and vitrifying the glass tube to cause the sol-gel solution to form a coating film comprising the dopant compound and SiO 2 derived from the silicon alkoxide on the inner wall, collapsing said dried glass tube to form a glass rod; and providing a cladding part around the periphery of said glass rod to form the glass preform whose core part is formed from the glass rod whereby the central axis of the preform is formed from material derived from the coating film. In an embodiment of this method, the glass rod is inserted in a cladding glass tube having a refractive index lower than that of the core glass rod and they are fused together to form a glass preform. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the dependency of fluorescence life on a concentration of erbium in the core 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 each other. To obtain a glass in which a large amount of the rare earth element 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 the method using the porous soot preform, it is about 5 % by weight by the sol-gel method. According to the present invention, the glass film containing the added rare earth element is formed on an inner surface of the glass tube which forms the core part by the sol-gel method. Then, the glass tube is heated to solidify the tube to form a rod which is used as a core. By this method, the core glass rod contains the rare earth element at a higher concentration at or near an axis of the rod than a peripheral part of the rod. As the metal alkoxide to be used in the sol-gel method, any of conventionally used ones may be used. Preferred examples of the alkoxide are methoxide, ethoxide, propoxide and butoxide of silicon, germanium and aluminum. 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 above steps, the glass tube is 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. During the formation of the gel and the collapsing of the glass tube, preferably a mixture of chlorine and oxygen is flowed in the glass tube. A concentration of the oxygen gas in the mixture is preferably from 10 to 60 % by volume. Thereafter, a cladding portion is formed around the periphery of the glass rod. For example, the glass rod is inserted in a glass tube for a cladding having a lower refractive index than that of the core glass rod and fused together by a so-called rod-in-tube method to obtain a glass preform for an optical fiber. Alternatively, the cladding portion may be formed by the VAD or CVD method. The present invention will be illustrated by the following examples. Example 1 Silicon ethoxide (10 ml) as a glass-forming raw material was mixed with water (10 ml) and ethanol (20 ml). To the mixture, erbium chloride (0.93 g) was added and mixed to obtain a homogeneous solution. A glass tube having an outer diameter of 10 mm and a inner diameter of 6 mm was produced by synthesizing silica glass rod by the VAD method and boring it along its center axis. In the bore of the glass tube, the above solution was flowed to apply the solution on the inner surface of the glass tube. The coated solution was dried at 100°C for 24 hours to form a coating film. The coating film was further heated at 1000°C to vitrify it. The glass tube was then heated with an oxyhydrogen burner to collapse the bore while flowing a gaseous mixture of helium, chlorine and oxygen in a volume ratio of 4:3:3 to obtain a glass rod. The glass rod was drawn to an outer diameter of 2 mm to obtain a core rod. The core rod was inserted in a fluorine-doped glass tube which was produced by the VAD method and had an outer diameter of 25 mm and an inner diameter of 2 mm. The fluorine-doped glass tube contained fluorine in an amount such that its refractive index was 0.34 % lower than that of the core rod. The fluorine-doped glass tube carrying the glass rod therein was heated and collapsed with the oxyhydrogen flame to obtain a glass preform. The glass preform was drawn to fabricate an optical fiber, which was analyzed by EPMA to find the following results: The optical fiber had a cladding diameter of 125 µm and a core diameter of 10 µm, and contained erbium at the center of the core. The erbium-containing part had a diameter of 0.3 µm, and a concentration of erbium was 5 % by weight. Erbium was dispersed in the ionic state. The method of the present invention provides an optical fiber in which ionic erbium is uniformly present at the center of the core. By changing the concentration of erbium, several optical fibers were fabricated, and their fluorescence life was measured. As an excitation light source, a frequency doubling of a Nd-YAG laser was used and fluorescence having a wavelength of 1.55 µm was measured by a sampling osciloscope. Dependency of the fluorescence life on the erbium concentration was studied. By the conventional solution-impregnation method, some optical fibers containing erbium were fabricated, and the same experiment as above was carried out. The result are shown in Fig. 1, in which O stands for the results for the optical fibers according to the present invention, and X stands for the results for the conventional optical fiber. According to the present invention, the fluorescence life is long up to the high concentration of erbium. As the fluorescence life is longer, the optical fiber is more suitable for an optical fiber laser or an light amplifying fiber. Example 2 In the same manner as in Example 1 except that a sol-gel solution was prepared using silicon methoxide (10 ml), water (10 ml), ethanol (20 ml) and neodymium chloride (0.852 g), a core rod was produced and then a preform was produced. The preform was drawn to fabricate an optical fiber having an cladding diameter of 125 µm and a core diameter of 10 µm. The neodymium-containing part had a diameter of 0.3 µm, and a concentration of neodymium was 5 % by weight. Neodymium was dispersed in the ionic state.

Claims: Claims for the following Contracting States : DE, FR, GB 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 comprising a silicon alkoxide and a dopant compound on an inner wall of a glass tube, drying and vitrifying the glass tube to cause the sol-gel solution to form a coating film comprising the dopant compound and SiO 2 derived from the silicon alkoxide on the inner wall, collapsing said dried glass tube to form a glass rod; and providing a cladding part around the periphery of said glass rod to form the glass preform whose core part is formed from the glass rod whereby the central axis of the preform is formed from material derived from the coating film. A method according to Claim 1, wherein said cladding part is provided around the periphery of said glass rod by inserting said glass rod in a cladding glass tube having a refractive index lower than that of said core glass rod and fusing them together. A method according to Claim 1 or Claim 2, wherein the glass tube is collapsed while flowing a mixture of chlorine and oxygen throught it. A method according to any preceding claim, wherein said sol-gel solution further comprises a metal alkoxide, water and an alcohol. A method according to Claim 4, wherein the metal alkoxide is germanium alkoxide and/or aluminium alkoxide. A method according to any preceding claim, wherein said dopant compound includes a rare earth element. Claims for the following Contracting States : IT, NL, SE 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 comprising a silicon alkoxide and a dopant compound on an inner wall of a glass tube, drying and vitrifying the glass tube to cause the sol-gel solution to form a coating film comprising the dopant compound and SiO 2 derived from the silicon alkoxide on the inner wall, collapsing said dried glass tube to form a glass rod; and providing a cladding part around the periphery of said glass rod to form the glass preform whose core part is formed from the glass rod whereby the central axis of the preform is formed from material derived from the coating film. A method according to Claim 1, wherein said cladding part is provided around the periphery of said glass rod by inserting said glass rod in a cladding glass tube having a refractive index lower than that of said core glass rod and fusing them together. A method according to Claim 1 or Claim 2, wherein the glass tube is collapsed while flowing a mixture of chlorine and oxygen throught it. A method according to any preceding claim, wherein said sol-gel solution further comprises a metal alkoxide, water and an alcohol. A method according to Claim 4, wherein the metal alkoxide is germanium alkoxide and/or aluminium alkoxide. A method according to any preceding claim, wherein said dopant compound includes a rare earth element. A glass preform for use in the fabrication of an optical fiber, the preform comprising cladding and core parts, the central axis of the core part being formed from a material derived from a silicon alkoxide and a dopant compound, the glass preform being obtainable by a process according to any preceding claim.

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