Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
1999-03-30
2001-07-24
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S398000
Reexamination Certificate
active
06263706
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of making preforms for optical waveguides, more particularly to a method for controlling the amount of fluorine doping at various locations across the profile of such preforms.
2. Technical Background
Optical waveguides such as optical fiber are typically made using materials of different refractive indices for the core and clad regions. The core region, wherein the majority of light is transmitted, consists of a region of higher refractive index than the clad region. In silica based optical fibers, this refractive index difference is obtained by adding various dopants. Fluorine is a commonly employed dopant for lowering the refractive index, and germania is a commonly employed dopant for raising the refractive index.
To fluorine dope a silica soot preform, such as can be made from an OVD, MCVD, VAD or other vapor deposition process, a fluorine containing gas such as SiF
4
or CF
4
can be flowed over the surface of the soot preform. The fluorine containing gas then diffuses into the soot preform and the fluorine reacts with the soot, thereby doping the silica soot with fluorine. In the past, fluorine incorporated into the soot has been highest near the preform surface which is in contact with the flow of fluorine containing gas.
Methods have been proposed to achieve a more uniform level of fluorine doping or a higher fluorine doping amount on a surface of the soot preform which is opposite the flow of fluorine dopant gas. For example, U.S. Pat. No. 5,210,816 discloses achieving this result by changing the soot density prior to the consolidation step. Basically, this method works on the principle that the lower the soot density, the higher the fluorine dopant level will be. It would be desirable to develop alternative methods which do not require deposition of various layers having different densities.
SUMMARY OF THE INVENTION
One aspect of the present invention relates to a method for incorporating fluorine internally into a soot preform by exposing the soot preform to a fluorine containing gas for a time and at a temperature which is sufficient to result in a lowering of the fluorine content along the surface of the soot preform which is in contact with the fluorine containing gas. Most preferably, the fluorine containing gas is an organic fluorine containing gas. The methods of the present invention can be used to dope such soot preforms in this manner without having to change the soot density and without having to turn off the fluorine gas during sintering to allow fluorine to escape. The doping methods of the present invention can be employed to form various fluorine doping profiles which have heretofore been difficult or impossible to attain using previous soot preform doping techniques.
The method is particularly useful for doping preforms which are used to form optical fiber and other waveguides. Such preforms are typically formed by chemically reacting at least some of the constituents of a moving stream of gas mixture comprised of at least one glass forming precursor compound and an oxidizing medium, the reaction resulting in the formation of a glassy silica based reaction product. This reaction product is typically employed to form a soot body (i.e., the preform) by depositing at least a portion of the reaction products onto a substrate. The resulting soot bodies are then doped in accordance with the invention.
In one preferred embodiment, the soot body is deposited onto a mandrel and the mandrel is removed to form a cylindrical soot body having a center line hole therethrough. Center line gases, such as helium, can then be flowed down the hole to facilitate the removal of fluorine dopant from the soot preform in the region adjacent to the hole, and thus increase the relative fluorine concentration within the soot body. By flowing a gas such as helium along the centerline hole of the soot preform, and a fluorine containing gas along the outside of the soot preform, the location of the maximum fluorine dopant concentration can be made to be between the outside surface and the centerline surface of the preform.
Alternatively, a glass rod can be inserted into the soot preform prior to the doping step, and the doping step then carried out. The glass rod may or may not be doped with various core layers of index increasing dopants.
Alternatively, the mandrel can be in the form of a glass rod which may or may not be doped with various dopants such as GeO
2
, and the soot body deposited onto the glass rod. In this way, refractive index profiles could be achieved which have a germania updoped central portion surrounded by a fluorine doped annular ring section.
As mentioned above, in the present invention, a dopant gas is employed which, after a sufficient time of exposure to the dopant gas, results in a lowering of the fluorine content at the glass surface which is nearest the flow of dopant gas. Preferred gases for use in the present invention include fluorocarbon gases such as CF
4
, C
2
F
6
, and C
3
F
8
. However, the invention is not necessarily limited to such gases, and it is believed that other gases, such as, for example, SF
6
,would work as well. A particularly preferred gas which can be used in the invention is CF
4
.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.
REFERENCES:
patent: 3981707 (1976-09-01), Araujo et al.
patent: 4693738 (1987-09-01), Hoshikawa et al.
patent: 5210816 (1993-05-01), Iino et al.
patent: 5799123 (1998-08-01), Oyobe et al.
patent: 0 201 937 B1 (1986-11-01), None
patent: 215603 (1987-03-01), None
patent: 311080 (1989-04-01), None
DeLiso Evelyn M.
Lacy Claud E.
Marlatt Deborah L.
Carlson Robert L.
Vincent Sean
LandOfFree
Method of controlling fluorine doping in soot preforms does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of controlling fluorine doping in soot preforms, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of controlling fluorine doping in soot preforms will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2552740