Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2000-06-06
2003-03-25
Hoffman, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S421000, C065S427000
Reexamination Certificate
active
06536240
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to optical fibers and, more particularly, to a method and apparatus for making an optical fiber preform using plasma outside vapor deposition process.
Various methods and techniques are known in the relevant art for making silica glass optical fiber preforms. One known method for fabricating preforms starter tubes is to heat silica and extrude it through an aperture. Another method known in the art for forming optical fiber preforms employs steps of depositing silica, either doped or undoped in accordance with desired optical properties of the finished fiber, onto a target. Several techniques for such deposition are known, including modified chemical vapor deposition (MCVD), vapor axial deposition (VAP), and outside vapor deposition (OVD). Each of these deposition methods begins with a rotating target, which can be glass, ceramic or other materials. The target can be a solid rod or a tube, with or without a reinforcing element inserted within it. Depending on the method, the target may become part of the perform and hence of the completed optical fiber, or may be removed by a later step of the fabrication process. A heat source, which can be a chemical reaction type gas burner or a plasma source, is positioned proximal to the rotating target. The position can be beneath, above, or spaced in a horizontal direction relative to the rotating axis of the target. As known in the art, the function of the heat source if to raise the temperature in the deposition zone sufficiently high for the glass-forming reactions to occur, thereby forming the desired glass particles. Depending upon which of the processes is used, the deposited glass particles are dried and sintered by another heat source, which is done by the VAD and OVD methods, or are fused into a vitreous quarts by the same heat source as was used for the deposition, as is done in the MCVD method.
For each of these deposition methods, when the target is mounted horizontally the heat source travels along with the deposition point, along the length of the target. This is done to ensure uniform deposition. If the target is a tube, the glass forming particles and materials may be deposited either on the inside surface of the tube, or on the outside surface. If the deposition is on the inside then the outside diameter remains constant, which deposition on the outside causes the outside diameter to increase.
If, on the other hand, the target is mounted vertically, the heat source is located either vertically above or laterally across. The deposition results in a substantially cylindrical product who's diameter and length increase as deposition continues.
Examples of these and other known deposition methods appearing in the various United States Letters Patent are:
U.S. Pat. No. 3,737,292 to Keck et al. discloses a method of forming optical fibers. Multiple layers with predetermined index of refraction are formed by flame hydrolysis and deposited on the outside wall of a starting rod or member. After these layers of glass are coated on the rod the resulting hollow cylinder is heated and collapsed to form fibers.
U.S. Pat. No. 4,224,046 to Izawa et al. teaches a method for manufacturing an optical fiber preform. Two gaseous raw glass forming materials, oxygen, hydrogen and argon are jetted upwards in a burner towards a vertically mounted, rotating cylindrical start member. Soot-like glass particles are formed by flame hydrolysis and deposited on the lower end of the start member. The start member is gradually withdrawn upwards to maintain a constant spacing between the its growing end and the burner. Upon completion of the deposition, the resulting soot-like glass preform is then dried and sintered to form a transparent glass preform.
U.S. Pat. No. 4,217,027 to MacChesney et al. teaches the fabrication of preforms by what is usually referred to as the Modified Chemical Vapor Deposition (MCVD) process. In this process, a vapor stream consisting of chlorides or hydrides of silicon and germanium with oxygen is directed to the inside of a glass tube. The chemical reactions among these chemicals, which are preferentially induced by a traversing hot zone, will under proper conditions result in the formation of glass on the inner wall of the tube. The particular matter deposited on the tube is fused with each passage of the hot zone.
U.S. Pat. No. 4,412,853 to Partus discloses an MCVD process to form an optical fiber preform starter tube. The process starts with a horizontally mounted, rotating tubular target formed from glass and having a preselected composition and optical characteristics. A vapor stream is fed through the tubular target as a heat source positioned beneath the tubular target, traverses along the latter's length. This causes reaction products of the vapor stream to be deposited on, and fuse to, the interior surface of the tubular target. The deposited material has the same index of refraction as the tubular target, but a different composition. This reference also suggests that one may achieve the same effect by an outside vapor-phase oxidation process or an outside vapor-phase axial deposition process, but does not explicitly teach how this can be done.
U.S. Pat. No. 4,741,747 to Geittner et al. is directed to the Plasma Chemical Vapor Deposition (PCVD) method of fabricating optical fibers. In this PCVD method glass layers are deposited on the inner wall of a glass tube by heating the tube to a temperature between 1100° and 1300° C., before passing the reactive gas mixture at a pressure between 1 and 30 hPa, and moving a plasma back and forth inside the glass tube. After the glass layers are deposited, this glass tube is collapsed to produce a solid preform. Optical fibers can be drawn from this preform.
U.S. Pat. No. 5,522,007 to Drouart et al. teaches the use of plasma deposition to build up an optical fiber preform having high hydroxyl ion concentration. In this reference, hydroxyl ions are deliberately entrained in a plasma generating gas by passing the gas through a water tank before it is introduced into one end of a plasma torch having an induction coil. The plasma torch projects molten silica particles mixed with hydroxyl ions onto a rotating substrate preform. This results in a preform having an average hydroxyl ion concentration lying in the range to 50-100 ppm deposited onto the target preform. According to Drouart et al., this technique results in optical fibers having an attenuation of 0.32 dB/km and 0.195 db/km at 1310 nm and 1550 nm, respectively.
In addition to requiring multiple processing steps to fabricate preforms, other disadvantages of the above processes include are:
1. the MCVD and PCVD processes are slower processes because of their low deposition rate;
2. the preform size is limited by the size of the deposition tube for MCVD and PCVD process; and
3. the outside vapor deposition process and the vertical axial deposition process OVD and VAD processes are based on flame hydrolysis which generates excessive amounts water and requires additional drying and sintering steps to produce high quality optical fiber preforms and require:
a. a deposition of soot particles on a target,
b. a generation of excessive amount of water as by-product, and
c. additional drying and sintering steps to produce high quality optical fiber preforms.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an apparatus and method for producing an optical fiber preform having low hydroxyl content, with an increased rate of deposition, using a lower number of steps, while providing for increased preform diameter and, at the same time, increasing the quality and uniformity of the preforms.
In one embodiment of the present invention, a plasma source or torch is supported in proximity to a target rod formed from a primary material such as, for example, pure silica. The target rod is secured at each of its horizontally opposed ends and is rotated about its longitudinal axis. The plasma source deposits silica doped with a known first doping concentration.
Aslami Mohammad Afzal
Danilov Evgueni Borisovich
Gouskov Mikhail Ivanovich
Mattison John Edward
Wu Dau
Hoffman John
Patton & Boggs LLP
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