Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2000-02-17
2003-07-01
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
Reexamination Certificate
active
06584808
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the fabrication of optical fiber and optical fiber preforms.
While potentially useful in a wide variety of applications, the present invention evolved and was further developed in the field of optical fiber manufacture. Optical fibers are thin strands of glass capable of transmitting a light wave signal containing a large amount of information over long distances with very low loss. An optical fiber typically consists of an inner cylinder made of glass, often referred to as the core, surrounded by a cylindrical shell of glass or plastic of lower refractive index, often referred to as the cladding.
Optical fibers have traditionally been manufactured by first constructing a preform of appropriate composition and then drawing fiber from that preform. A typical preform generally assumes the form of a solid, concentric glass rod having a length of about one meter and a typical diameter of 10-100 mm. The core of this preform is a high purity, low loss glass such as germanium silicate glass having a diameter of about 1-40 mm. The cladding is a layer of glass which surrounds the core and which has a lower index of refraction than the core.
There are a number of fabrication processes in use today to manufacture such a preform. In one process, commonly known as the lateral soot deposition technique and described in U.S. Pat. Nos. 3,711,262 and 3,876,560, glass particulate matter and doped halides are formed in a hydrolysis burner and deposited on a starting member such as a glass rod. Additional layers of glass, including a cladding layer, are deposited on the rod and the combination is consolidated onto a transparent rod by heating in an inert environment. This process, requires many passes (up to 200) of the hot soot stream and is therefore costly and time consuming. In addition, after the soot is deposited, the preform must be sintered in a controlled inert atmosphere such as helium, which is also very costly. Moreover, these additional requirements require extensive process controls that can even further delay production and increase costs.
Another fabrication process is commonly referred to as the modified chemical vapor deposition (MCVD) technique. In this technique, glass precursor vapors are directed through a hollow glass cylinder which is heated sufficiently to start a homogeneous reaction within the glass cylinder. During this reaction, glass particulate matter is formed, deposited on the inside of the glass cylinder, and subsequently fused into the cylinder by traversing the heat source. This technique also has problems related to inefficient deposition rates and starting tube needs which, in turn, negatively impact manufacturing economics and production schedules.
Still another technique for the fabrication of fiber preforms is the vapor.axial deposition process, or more commonly VAD. This process, described in U.S. Pat. No. 4,062,665, involves simultaneous flame deposition of both core and cladding soots onto the end of a rotating fused silica-bait rod. As the porous soot preform grows, it is slowly drawn through a graphite resistance furnace (carbon heater) where it is consolidated into a transparent glass preform by zone sintering. This process has all the disadvantages and problems associated with a flame hydrolysis burner containing doped halides, as found in the lateral soot deposition technique described above, except in this case there are two hydrolysis burners to control. The process control of the finished preform and the control of both burners must be precise.
In still another method of manufacturing an optical fiber preform, the core is manufactured from an inner solid doped silica glass rod and one or more sleeving tubes. In this method, as described in U.S. Pat. Nos. 4,154,591 and 4,596,589, a core rod is placed within a sleeving tube. The tube is then collapsed onto the rod by slowly traversing a heat source over the entire length of the tube. British patent Application GB 22284206 suggests welding a supporting rod (with a sealing-up part for sealing the sleeving, or over-cladding, tube) to a core rod and a supporting tube to the sleeving tube, said supporting tube having a purity different than that of the sleeving tube and including a ring for centering the core rod. The tube is then collapsed onto the rod by slowly traversing a heat source over the entire length of the tube, while rotating the assembly on a lathe. The methods disclosed in the above patents result however in slow and expensive processes in that the tube and rod are completely collapsed into a solid multilayered cylindrical mass prior to the actual drawing of the fiber.
An alternative method for collapsing a sleeving tube onto a glass rod is disclosed in Japanese patent application with publication no. 63-170235. In said patent it is suggested to collapse a first end of the tube onto the rod, then applying a negative pressure to the inside of the tube and eventually collapsing the opposite end of the tube onto the rod. Japanese Patent Application JP 60-155542 discloses a method wherein a core rod and a sleeving tube are disposed into a heating furnace for drawing and the respective bottom ends are softened by heat, fused, joined, and drawn downward to mold a fiber.
According to what observed by the Applicant, when the tube is collapsed onto the rod, particular attention should be paid in not introducing asymmetries into the preform geometry during both the fabrication and/or collapse of the preform into a solid mass, which asymmetries may be reflected in the cross-section of the resulting fiber, with consequent negative impact on the transmission properties of the fiber. In particular, when manufacturing a simple two-layered preform by collapsing a tube onto a rod, attention should be paid in correctly aligning the tube and the rod at the beginning of the process and maintaining the tube centered onto the rod during the whole collapsing process, for avoiding such asymmetries. At this regard, the Applicant has noticed that, according to the prior art methods, the alignment of the tube with the inner rod is achieved directly on the lathe for carrying out the collapsing of the tube onto the rod. This operation is however particularly troublesome, as the lathe is generally in a vertical position and the correct alignment of the tube with the rod generally requires specific glass working skills to be realized. In addition, the applicant has observed that also the temperature of the heat source used for collapsing the tube should be accurately controlled along the entire collapsing process. In particular, when only the opposite ends of the tube are collapsed onto the respective ends or the rod, particular attention should be paid to the heating of the uncollapsed zone of the tube, in order to minimize thermal stress areas in the assembled preform, while avoiding undesired collapsing in this zone.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method of and an apparatus for manufacturing optical fibers by utilizing an improved direct sleeving technology that substantially obviates one or more of the problems observed by the Applicant associated with the prior art methods.
Additional features, objectives and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the method and apparatus particularly pointed out in the written description and claims hereof as well as the appended drawings.
To attain these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the invention provides a fiber optic preform and method for making the same by centrally positioning a core rod within a sleeving tube, thereby providing an annular gap between the outer surface of the rod and the inner surface of the tube, thermally collapsing the extreme ends of the tube onto
Garnham Cheryl Jane
Roba Giacomo Stefano
Zilnyk Anna
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hoffmann John
Pirelli Cavi e Sistemi S.p.A.
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