1. Field of the Invention
The present invention relates to a method of manufacturing an optical fiber; and, more specifically, to a method of manufacturing an optical fiber in which an optical fiber preform is drawn while being softened upon heating.
2. Related Background Art
Optical fibers have been employed in communication lines since they have advantages over copper wire cables because of their smaller size, lighter weight, lower transmission loss, higher band transmission, and the like.
A drawing furnace such as the one shown in FIG. 4 is usually used in a conventional optical fiber manufacturing step. Namely, an optical fiber preform 1 is inserted into a core tube 4 of a drawing furnace 3 and, while a leading end of the optical fiber preform 1 is softened upon heating with a heater 6 disposed about the outer periphery of the core tube 4 within a furnace body 5, the optical fiber preform 1 is drawn with a predetermined tension being applied thereto by a capstan (not depicted) or the like, whereby an optical fiber 2 having a desirable diameter is obtained.
In order to improve characteristics of thus obtained optical fiber, various manufacturing methods and apparatus have been proposed. For example, Japanese Patent Application Laid-Open No. HEI 4-198036 and Japanese Utility Model Application Laid-Open No. SHO 61-147233 disclose a heating furnace in which an annealing heater is provided in the upper part thereof, and a heating furnace equipped with a coil-shaped heater, respectively. Japanese Patent Publication No. HEI 6-2603 discloses an optical fiber manufacturing apparatus comprising a heat-treating furnace disposed between a drawing furnace and a coating unit, and a manufacturing method using the same; while stating that defects in the optical fiber can be reduced when the heat-treating furnace has such a temperature distribution that temperature becomes higher toward the optical fiber preform. Japanese Patent Publication No. HEI 8-9490 discloses a method in which an optical fiber preform is drawn such that its length and line speed of drawing an optical fiber satisfy a predetermined condition therebetween.
Meanwhile, demands have recently been increasing for optical fibers whose center core has a relative refractive index difference (Δ+) of at least 1% with respect to a cladding, such as dispersion-compensating fibers. Means for enhancing the relative refractive index difference of an optical fiber is exemplified by a method in which a core part is doped with a germanium compound such as germanium oxide (GeO2), whereby the core part of thus obtained optical fiber is formed with a skeleton in which a silicon atom (Si) and a germanium atom are combined to each other by way of an oxygen atom.
However, when an optical fiber whose core part contains a germanium compound at a high concentration is made by the above-mentioned conventional method, defects such as non-bridging oxygen hole center may occur due to thermal dissociation of Si—O—Ge bonds in the drawing step, whereby the characteristic that the optical fiber does not increase transmission loss for light having a wavelength of 1.38 μm in a hydrogen atmosphere (hereinafter referred to as “hydrogen characteristic”) becomes insufficient. The occurrence of non-bridging oxygen hole center is remarkably seen when the optical fiber is drawn from larger preform or with a higher line speed, whereby it has been very difficult to mass-produce efficiently and reliably optical fibers having a center core with a large relative refractive index difference.
For example, even when the heating furnaces disclosed in Japanese Patent Application Laid-Open No. HEI 4-198036, Japanese Utility Model Application Laid-Open No. SHO 61-147233, Japanese Patent Publication No. HEI 8-9490, and the like are used, Si—O—Ge bonds in thus drawn optical fiber cannot be prevented from thermally dissociating, whereby the resulting optical fiber may not be suitable for practical use in terms of the hydrogen characteristic. On the other hand, though the method disclosed in Japanese Patent Publication No. HEI 6-2603 reduces defects remaining in the optical fiber, the time during which the optical fiber stays within the heat-treating furnace and the length of the heat-treating furnace in the drawing direction are required to be very long, whereby it may not be sufficient yet in terms of production efficiency.