Process of drawing optical fiber

Abstract: There is provided a process of melt drawing an optical fiber (119) from a preform (101) therefor in which a heating furnace (1) comprising a muffle tube (2) inside thereof through which the optical fiber (119) is passed is provided immediately below a melt drawing furnace (105) to further draw the optical fiber (119) while heated, and an interior of the muffle tube (2) is kept in an atmosphere selected from the group consisting of an inert atmosphere, an atmosphere comprising oxygen gas and an atmosphere comprising hydrogen gas.

Description: The present invention relates to a process of drawing an optical fiber. In particular, the present invention relates to a process of drawing an optical glass fiber having increased reliability and reduced transmission loss by preventing formation of defects and decreasing thermal fluctuations in a molecular order or an atomic order in the glass fiber. When an optical glass fiber is conventionally drawn with melting a glass preform for the optical fiber, the preform is heated to a temperature above its softening point in a drawing furnace (or a melt drawing furnace) and the optical fiber is drawn from the preform followed by coating the fiber with a resin composition with a die after allowing the fiber to be air-cooled. As shown concretely in Fig. 1 which schematically illustrates a drawing apparatus used in a conventional process of drawing the optical fiber, a muffle tube 102 into which a preform 101 for the optical fiber is inserted is supported by a furnace body 104 together with a carbon heater 103 surrounding the tube. To an upper end of the furnace body 104, is connected a purging gas tube 106 which supplies an inert gas such as nitrogen in the drawing furnace 105 in order to prevent degradation of the muffle tube 102 and the carbon heater 103 due to oxidation. A sealing ring 107 is fixed around an upper end surface of the furnace body 104 to be in sliding contact with an outer surface of the preform 101, and a shutter 108 is provided at a lower end of the furnace body 104, whereby an inert atmosphere can be kept in the drawing furnace 105. The optical fiber 109 drawn in the furnace 105 is passed through a coating die 111 and applied with a first coating of, for example, an ultraviolet curing resin. The ultraviolet curing resin which passes through the coating die 111 together with the optical fiber is cured by irradiation of ultraviolet in an ultraviolet curing device 113 so as to integrate the resin with the optical fiber 109. At a lower end of the device 113, a tube 114 is provided which supplies a purging gas such as nitrogen to exclude oxygen which adversely affects the curing of the resin. In connection with the gas tube, a shutter 115 is provided at an upper end of the device 115 in order to prevent outflow of the purging gas from the device 113. In addition, along the downstream from the curing device 113, another coating die 116 for a second coating and another curing device 117 are provided in turn, so that the optical fiber 118 coated with the first coating is converted to an optical fiber 119 which is further coated with the second coating. The optical fiber 119 is wound up by a winder (not shown) via a direction changing roller 120. In the conventional process, since the optical fiber is drawn directly into an atmosphere at an ambient temperature from the drawing furnace, defects are fixed and remain in the fiber due to quenching of the optical fiber. In addition, since the glass preform is kept in the furnace at a high temperature (higher than 2000 °C), thermal fluctuations in the glass are promoted. Solutions to overcome these problems are highly desired in order to reduce a transmission loss of the optical fiber and to make the reliability of the optical fiber higher by means of preventing the increase of the transmission loss over a long period. JP-A-60 221 337 reveals a process for preparing an optical fiber containing little oxygen deficiency and little OH groups. In this process, the optical fiber is heat-treated immediately after spinning in an oxygen atmosphere. Thereby the optical fiber is drawn from a parent material and introduced into a heating furnace while flowing oxygen continuously therethrough. The fiber is heat-treated in this furnace at about 1200°C. In US-A-4 396 409 a method of forming a low loss optical waveguide fiber is disclosed. This method comprises the steps of drawing a fiber and heating said fiber, while said fiber is being drawn. The step of heating said fiber is preferably effected at a temperature lying between 900 and 1300°C. In the above process, an elongated tubular chamber having an inlet pipe and and outlet pipe can be located within the furnace. Filtered air or nitrogen is flowed into the pipe during the fiber heat treatment. JP-A-63-129 035 includes a process for the manufacturing of an optical fiber, wherein a bare optical fiber is spun and heat-treated immediately in a hydrogen atmosphere at at least 800°C. The residence time of the fiber in the heating furnace is higher than 10 seconds, and preferably higher than 20 seconds. The above process should inactivate the non-crosslinked oxygen fault (≡Si-O·) into Si-OH. It is an object of the present invention to provide a process of drawing an optical fiber having higher reliability and reduced transmission loss which overcomes the problems as described above. According to the present invention, there is provided a process of melt drawing an optical fiber from a preform comprising the steps i) and ii) as defined in claim 1. Fig. 1 schematically shows an apparatus of drawing an optical fiber with which the conventional process is carried out, Fig. 2 schematically shows an apparatus of drawing the optical fiber with which the present process is carried out, Fig. 3 is a graph which compares temperature distributions between the present process and the conventional process, Fig. 4 is a graph showing comparison between resistance properties against γ-ray of the optical fiber made in accordance with the present process and one made by the conventional process, and Fig. 5 schematically shows an optical fiber drawing apparatus used in the present process in which a plurality of heaters are used each heating temperature of which can be controlled independently. In the present process the temperature in the heating furnace is in the range of 500 to 1500 °C, the temperature of the optical fiber immediately before entering the heating furnace is in the range of 500 to 1500 °C, and the residence time of the optical fiber in the heating furnace is in the range of 0.1 to 10 seconds. In addition, at least one heating furnace is provided immediately below the melt drawing furnace in the present process. When a plurality of the heating furnaces are provided, it is particularly preferred for an efficient operation that a temperature of each heating furnace can be independently controlled. The muffle tube in the present invention is preferably made of quartz, high purity carbon, pyrolytic graphite-coated high purity carbon or SiC-coated high purity carbon. Proper scattering in glass can be expressed by the following equation (see S. SAKKA, I. KITANO (Editor), "Glass Handbook," ASAKURA SHOTEN, Tokyo, JAPAN, 1975, p. 954): τ = 8π³/3λ⁴(n²-1)κTβ + 16π³n/3λ⁴( n/ c)²Δc²δV wherein n is a refractive index of the glass, κ is the Boltzmann's constant, β is a compression ratio, T is a hypothetical temperature fixed to around an annealing temperature, Δc² is a mean square value of a concentration fluctuation and δV is a volume of the fluctuating portion. The first term of the above equation (1) is based on a density fluctuation derived from a temperature fluctuation kept in a supercooled condition upon solidification of the glass from a liquid state. The second term is based on the concentration fluctuation derived from a composition variation. According to the present invention, quenching of the optical fiber from a higher temperature is avoided by re-heating the optical fiber immediately below the melt drawing furnace. As a result, the hypothetical temperature of the first term of the equation (1) is lowered, which efficiently serves to decrease the proper scattering τ in the fiber. When an defect density of the optical fiber is measured with ESR (Electron Spin Resonance), the optical fiber drawn by the conventional process had an E' center (≡Si·) density of 1.09 x 10¹⁴ spins/g, while the optical fiber drawn with re-heating in the heating furnace immediately below the drawing furnace according to the present invention had the E' center density of 6.76 x 10¹³ spins/g. That is, with the re-heating treatment, the E' centers were seemed to be re-bonded each other in accordance with the following formula (2): ≡Si· + ·Si≡ → (heat) → ≡Si-Si≡ (2) In view of the above, it is certified that the re-heating of the optical fiber efficiently works so as to reduce the defects therein. It is also assumed that, in addition to the E' center, a non-crosslinked oxygen radical such as ≡Si-O·, ≡Si-O-O· and so on also can be re-bonded to form ≡Si-O-Si≡ and ≡Si-O-O-Si≡, whereby the optical fiber having a stable structure can be produced. In the present process, the atmosphere comprising hydrogen in which the optical fiber is reheated is used by providing the muffle tube in the heating furnace. The re-heating atmosphere comprising hydrogen is efficient for reducing the defects. That is, for example, following three reactions (3) to (5) seem to proceed: ≡Si· + 1/2H₂ → ≡Si-H (3) ≡Si-O· + 1/2H₂ → ≡Si-OH (4) 2≡Si· + 1/2O₂ → ≡Si-O-Si≡ (5) Since a diffusion coefficient of H₂ is larger than that of O₂, H₂ diffuses into a core portion of the fiber more easily than O₂ so that the defects (≡Si·) present in a pure silica core portion seemed to be killed by H₂. The re-heating temperature in the heating furnace in the present process is in the range of from 500 to 1500 °C. When the temperature is lower than 500 °C, sufficient effect due to the re-heating cannot be expected. On the contrary, when the temperature is above 1500 °C, the optical fiber is deformed due to such high temperature, whereby the drawing is impossible. From the above view points, the temperature of the optical fiber which enters the heating furnace is also in the range of from 500 to 1500 °C. As to the re-heating time, i.e. a residence time, of the optical fiber in the heating furnace, when the time is less than 0.1 sec., the sufficient effects by the re-heating treatment cannot be expected. When the optical fiber is gradually cooled over a period longer than 10 sec., it is not practical in view point of designing the drawing apparatus. At least one heating furnace is provided in series below the drawing furnace. Alternatively, at least one heater is provided in the heating furnace. When a plurality of heating furnaces or heaters are provided, it is preferable that the temperature of each furnace (or heater) can be independently controlled whereby stepwise annealing effects can be promoted. The muffle tube used in the present invention is preferably made of, for example, quartz, high purity carbon, pyrolytic graphite-coated high purity carbon, SiC-coated high purity carbon. Though the present invention will be described with reference to an example, it is not not limited to them. Example Fig. 2 schematically shows an apparatus used in the present invention, in which the reference numbers corresponding to those in Fig. 1 represent the corresponding parts of the apparatus in Fig. 1. In this apparatus, a heating furnace 1 of a length of 60 cm was provided just and immediately below the melt drawing furnace 105, and a die 111 was separated from the heating furnace 1 by 60 cm. The heating furnace comprised a muffle tube 2 made of quartz with a gas inlet and a gas outlet so that a desired atmosphere can be kept in the muffle tube. As the heating furnace 1, a SiC (Siliconit®) furnace (commercially available from Siliconit Konetsu Kogyo Co. Ltd., Japan) was used. A glass preform 101 for a single mode optical fiber consisting of a core of pure silica and a cladding of fluorine-doped silica was heated to a temperature higher than its softening point in the drawing furnace and the optical fiber 10 was drawn from the preform. The temperature of the optical fiber just before entering the heating furnace was 1100 °C. Then, the heating furnace was controlled to a temperature of 1200 °C. A quartz tube having an outer diameter of 40 mm was used as the muffle tube 2, and the atmosphere in the heating furnace consisted of 100% hydrogen. The optical fiber was passed through the heating furnace at a controlled linear velocity of 30 m/min., so that the residence time of the optical fiber in the heating furnace was 1.2 sec. A solid line in Fig. 3 shows a temperature distribution along a running direction of the optical fiber through the drawing furnace and the heating furnace to an ultraviolet curing device. A broken line in Fig. 3 represents a temperature distribution in the conventional process where the same conditions as described above were employed except that the heating furnace was not present. Transmission losses of the optical fibers according to the present invention and the conventional process were measured, and proper scatterings were evaluated. In addition, the E' center (Si·) density, which is a measure of the defect density, was determined with ESR and found to be less than the detection limit of ESR. It is assumed that hydrogen molecule diffused into the glass and a reaction occurred as follows: Si· + 1/2H₂ → SiH The optical fiber produced in the Example was tested on γ-ray resistance. The fiber was irradiated to a dose rate of 10⁵ R/hour for one hour. The results are shown in a graph of Fig. 4 in which a solid line represents the results from the fiber of the Example and a broken line represents those from the fiber of the conventional process. It is confirmed that the present process suppress the increase of the transmission loss due to γ -ray irradiation. Though, in above Example, the SiC Siliconite furnace was used, any other heating furnace can be used as long as it can heat the fiber to a desired temperature. For example, an electric resistance furnace with a carbon heater and a Kanthal super-heatering furnace (with a quartz made muffle tube) can be used. As to the muffle tube, a proper material can be selected from the group consisting of carbon, quartz, pyrolytic graphite-coated carbon, SiC-coated carbon and so on. Though, in above Example, the present invention has been described with reference to the pure silica core/fluorine-added silica cladding single mode optical fiber as an example, the same results as described above are obtained in the case where the core and/or the cladding of the optical fiber contain other additives such as Ge, P, F or Al. As described above, according to the present invention, the optical fiber having the stable transmission properties and the stable resistive properties against circumstances can be obtained since the just drawn optical fiber is gradually cooled through the heating furnace without being quenched from the drawing temperature so that the thermal fluctuations and the defects during softening of the glass are not fixed in the glass.

Claims: A process of melt drawing an optical fiber from a preform comprising the steps of: i) heating the preform in a drawing furnace (105) and ii) passing the fiber drawn from the preform in the drawing furnace through at least one heating furnace (1) which is immediately below the drawing furnace (105), the heating furnace being kept at a temperature in the range of from 500°C to 1500°C and containing a muffle tube (2) in which an atmosphere consisting essentially of hydrogen gas is maintained, whereby the fiber is drawn such that said fiber immediately before entering said at least one heating furnace has a temperature in the range of from 500°C to 1500°C and has a residence time within the heating furnace in the range of from 0.1 to 10 seconds. A process as claimed in claim 1, wherein in the case of more than one heating furnace (1) the temperature of each heating furnace (1) is controlled independently.

Inventors: ISHIKAWA SHINJI C O YOKOHAMA W, KANAMORI HIROO C O YOKOHAMA WO, OHGA YUICHI C O YOKOHAMA WORKS, YOKOTA HIROSHI C O YOKOHAMA WO, YOSHIMURA ICHIRO C O YOKOHAMA, ISHIKAWA, SHINJI, C/O YOKOHAMA WORKS, KANAMORI, HIROO, C/O YOKOHAMA WORKS, OHGA, YUICHI C/O YOKOHAMA WORKS, YOKOTA, HIROSHI, C/O YOKOHAMA WORKS, YOSHIMURA, ICHIRO, C/O YOKOHAMA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: C03B37
IPC codes: C03B37, G02B6