Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2000-05-19
2004-04-06
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S513000, C065S510000
Reexamination Certificate
active
06715323
ABSTRACT:
The present invention relates to a method and apparatus for cooling an optical fiber during the drawing process of said fiber.
In the production of optical fibers, a glass preform is first prepared according to known technologies, comprising VAD (vapor axial deposition), OVD (outside vapor deposition) and MCVD (modified chemical vapor deposition), by depositing a soot of glass particles; said soot-glass preform is then consolidated before drawing the fiber.
The optical fiber is obtained from the consolidated preform by heating the bottom end of the preform at its softening temperature into a so-called “drawing furnace” and drawing the fiber from said softened preform under controlled conditions, according to known procedures. Upon cooling, the glass solidifies into the optical fiber, which is very fragile. Thus, during the drawing phase, before collecting it, the fiber is normally coated with one or more layers of synthetic coating material—preferably two layers—for instance urethane-acrylate resins) for protecting it.
In general the coating of the fiber is performed by passing the fiber through a “coating-die”, which contains a liquid resin. The fiber, which in general has a temperature of about 2000° C. at its outlet from the drawing furnace, has to be cooled before entering into the coating-die at a temperature compatible with the one of the coating application technique, (in general below 100° C., preferably at about 25° C.-60° C.) for avoiding possible inconveniences during the coating phase due to the high temperature of the fiber, resulting in irregular deposition of the coating layer. By increasing the draw speed, the fiber requires longer distances for cooling down to a temperature suitable for the coating application. For instance, as reported in U.S. Pat. No. 4,437,870, a distance of 120 cm is required for natural cooling a fiber with a 125 &mgr;m diameter from 1780° C. down to 50° C., with a draw speed of 0.75 m/sec; when the draw speed is increased up to 5 m/sec, a 800 cm cooling distance is required. With the increasing rate of the drawing speed the distance between the drawing furnace and the coating die would increase too much when only a natural cooling is applied; it has thus been suggested to employ cooling means for force-cooling the fiber at a suitable temperature for the coating application, allowing to employ shorter cooling distances.
U.S. Pat. No. 4,437,870 discloses an apparatus for cooling the fiber which consists of a vertical tube through which the fiber is drawn, said tube being provided at its bottom end with a cylindrically-shaped porous member. Cooling gas is supplied into a chamber surrounding the porous member, and then, through said porous member, it flows upwardly along the fiber to the top of the cooling tube. A chamber containing liquefied gas (nitrogen) surrounds the cooling tube. According to an alternative embodiment the fiber is drawn through a vertical tube, which may be surrounded by a layer of insulating material, having an annular opening at its bottom through which cooling gas emanates into the tube, flowing upwardly to the top of the cooling tube.
U.S. Pat. No. 4,514,205 discloses an apparatus for cooling the fiber consisting of a cooling tube surrounding the fiber, which tube is centrally disposed in a reservoir containing liquefied gas. The cooling gas flows first through a coil disposed into the reservoir, thus being cooled by the liquefied gas contained in said reservoir, and then into the cooling tube axially along the fiber.
U.S. Pat. No. 4,913,715 discloses a cooling apparatus wherein the fiber is drawn through a forced-cooled double-walled tube. The inner space of the tube, through which the fiber passes, contains a gas with good heat-transporting properties, having a flow which is reduced but sufficient to prevent the penetration of the surrounding atmosphere into the tube and to compensate loss of gas. According to the method disclosed in this patent, the fiber is thus cooled substantially by heat transfer to the cooled wall by means of the heat-transporting gas surrounding the fiber.
U.S. Pat. No. 4,966,615 discloses a cooling tube surrounded by a cooling jacket. A number of ring-shaped partition plates, spaced from each other, are mounted within the tube. The partition plates allows breaking the laminar flow of the gas through the tube, in order to increase the heat transfer between gas and fiber.
U.S. Pat. No. 4,838,918 discloses a method for cooling an optical fiber wherein said fiber is passed between two parallel plates cooled with nitrogen, a laminar flow of inert gas being directed onto said fiber along a plane centrally located between said plates, said laminar flow being generated by a ½ inch tube provided on its surface with a number of holes of {fraction (1/16)} inches diameter, spaced one inch apart from each other.
EP 319,374 discloses a cooling device comprising a pair of parallel plates between which the fiber is passed, said plates being optionally cooled for absorbing the heat radiating from the fiber, and a pair of vertically oriented copper tubes delivering a laminar flow of room temperature nitrogen gas between the parallel plates to surround the downwardly moving fiber.
GB 2,287,244 discloses the cooling device comprising an elongated water-cooled body member provided with a through hole opening out abruptly into a succession of spherical chambers in which a cyclonic flow of gas is induced by the tangential injection of helium, preferably with opposite cyclonic rotation in successive chambers.
DE 4,412,563 discloses a cooling device having a plurality of gas-flow openings in a structure surrounding longitudinal axis of the fiber, said plurality of gas-flow being positioned at respective different heights along said structure.
U.S. Pat. No. 4,664,689 relates to a method and apparatus for rapidly cooling an optical fiber comprising passing the optical fiber through an enclosure having a flat back internal surface, the walls of said enclosure having symmetrically oriented perforations or other symmetrically oriented means of passing cryogenic gas through the walls to contact the optical fiber within the enclosures.
The applicant has noticed that the above cooling devices and methods have some drawbacks in their use, in particular as they can not be easily adapted to the variations of the drawing conditions.
Furthermore, the applicant has also observed that with conventional cooling methods employing an axial flow of cooling gas, the possibility to increase the drawing speed is also limited from a critical value of the flow rate of the cooling gas (depending on its initial temperature and on the length of the tube), above which there is a saturation of the cooling efficiency of the gas, with no substantial increase in the cooling capacity of the gas.
In addition, in the method and apparatus disclosed in U.S. Pat. No. 4,838,918, the applicant has observed that the efficiency of the cooling of the fiber can be reduced by the fact that the cooling gas is provided by means of small holes spaced from each other and that there are no means for effectively removing the inert gas from the apparatus.
It has now been found that according to the present invention, the efficiency of the cooling of the optical fiber can be improved by passing said fiber through a hollow elongated body, said body being provided with at least a first longitudinal opening and at least a second longitudinal opening, both said openings being provided substantially on the whole length of said elongated body and said second opening being positioned at substantially the opposite side with respect to said first opening, wherein the cooling gas is passed through said first opening, directed onto the fiber and removed from the opposite second opening. Accordingly, the cooling method of the invention provides a flow of cooling gas which is substantially transversal with respect the longitudinal axis of the drawn fiber, for the whole path of the cooling gas inside the elongated hollow body. In the present description the wording “flow
Pata Roberto
Roba Giacomo Stefano
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hoffmann John
Pirelli Cavi E Sistemi S.p.A.
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