Optical waveguides – Optical transmission cable – Tightly confined
Reexamination Certificate
2002-01-24
2003-06-10
Nguyen, Khiem (Department: 2839)
Optical waveguides
Optical transmission cable
Tightly confined
Reexamination Certificate
active
06577795
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fiber optic cables and apparatus used for broadband communications, and more particularly, to providing a fiber optic transmission systems and cables which have improved lifetime attenuation performance.
BACKGROUND OF THE INVENTION
Recently fiber optic transmission has become a preferred method of providing broadband communications because of its extremely low attenuation and large available bandwidth for providing advanced digital communications. In long distance transmission applications individual optical fibers are typically bundled together to form fiber optic cables; these cables usually interface with repeaters at periodic intervals so that the optical signals carried by the fibers can be restored to their desired levels after having suffered attenuation over long propagation distances. Further, the optical transmission path may include optical filters to shape and flatten composite signal response. Often the individual fibers carry multiple independent channels through the use of, for example, dense wavelength division multiplexing (DWDM) technology, with each of the channels occupying a sub-band contained within the main transmission band. The gain vs. wavelength responses of the amplifiers contained within the repeaters are typically quite sensitive to small (age-induced) changes in input signal levels. These changes in input signal levels are most often due to age-related attenuation changes in the cabled fibers. It is for this reason that the current invention is aimed at minimizing long-term, hydrogen-induced increases in attenuation that might otherwise occur.
The fiber optic cables referred to above may be used to transmit information for telephone, television and computer data in indoor and outdoor environments. These fiber optic cables are made in various configurations and designs. Some example of a few fiber optic cable designs are illustrated in U.S. Pat. Nos. 4,156,104, 5,930,431, 4,439,632, 4,477,298, 4,557,560, 4,569,704, 4,729,629 and the article by Raymond D. Tuminaro,
Materials Aspects of the SL Lightguide Undersea Cable Design
, MRS Bulletin, July 1988. Outside fiber optic cables may be used to span various types of geography and environments which may include terrestrial, subterranean, and submarine (e.g., undersea) applications.
Due to the cost of manufacturing and installing the fiber optic cables, it is desired that the useful life of the fiber optic cables be as long as possible, often times greater than 25 years. One of the characteristics which affects the useful life of fiber optic cables is age-dependent increases in attenuation of signals at wavelengths used for signal transmission (e.g., 1550 nm). Further, some of the characteristics which affect the usefulness of the fiber optic transmission cable systems in, for example, DWDM systems is the overall spectral response resulting from the combination of attenuation vs. wavelength response of the fibers and the gain vs. wavelength response of the amplifiers deployed along the cabled fiber transmission path. As noted above, changes in fiber attenuation give rise to changes in input levels to the amplifiers; optical amplifiers, such as those based on erbium-doped fibers, experience changes in their gain vs. wavelength responses when their input signals carried by the cabled fibers are changed. As greater bandwidth and more and more channels are carried by individual optical fibers, stability of the attenuation behavior of the fiber is an ever increasing concern. Hydrogen-induced increase in fiber attenuation over the lifetime of the system is one of the major causes for concern with respect to the composite amplifier plus fiber spectral response.
Extensive experience in the area of optical fiber cable design has shown that, while it is often possible to reduce the amount of hydrogen inside of fiber optic cables to very low levels (for example, much less than 0.01 atmosphere partial pressure), this small amount of residual hydrogen is capable of chemically reacting with the constituents of the optical fibers to a sufficient degree so as to cause a small, but measurable, increase in fiber attenuation, with an attendant degradation in the composite spectral response of the cable plus repeater iterative structure. Further, there is evidence that, as a result of some of the fiber optic cable designs and some of the environments in which fiber optic cables are installed, the amount of hydrogen in the core of optical fibers within the fiber optic cables may increase over time. Therefore, there is a need to provide an efficient and cost effective manner for reducing the fiber optic cable susceptibility to signal degradation which may be caused by hydrogen over time.
Analysis has shown that molecules of ordinary hydrogen (having individual atoms comprised of a single electron and single proton), can readily combine with the chemical constituents of optical fibers and give rise to resonant attenuation peaks, whose tails extend into the portion of the spectral band, or bands, used for the transmission of signals, thereby degrading signal transmission. Analysis has also shown that by introducing a common isotope of hydrogen, deuterium (with individual atoms comprised of an electron, a proton, and a neutron), into the process of forming individual optical fibers, the deuterium will combine with the constituents of optical fibers and give rise to resonant attenuation peaks; however, in the case of deuterium, these resonant peaks and their tails lie well outside the commonly used portion of the spectrum used for transmission, and are therefore non-degrading, or minimally degrading, to signal transmission.
SUMMARY OF THE INVENTION
The present invention is directed to a method and apparatus for improved long term signal attenuation performance of fiber optic cable and cable and/or fiber interface components. The improved long term signal attenuation performance of the fiber optic cable and apparatus is achieved by introducing an additive after the optical fibers and apparatus have been formed as part of, or precursor to, the cable and apparatus making process. For example, an additive such as deuterium, may be introduced before, during, or after the fiber optic cable or apparatus assembly process. The additive will react with and occupy available chemically active defect sites in the optical fibers and apparatus that might otherwise adversely react with ordinary hydrogen. In some preferred embodiments, deuterium may be introduced into the cabling process and/or components anywhere from the initial fiber or component storage and preparation process to the later processing stages in which optical fiber bundles are encased within the structural elements of the cable or components are deployed within an amplifier or repeater housing. The partial or complete fiber optic cable structure or apparatus housing may then act as a reaction chamber so that the additive which has been introduced during the fiber optic cable or apparatus assembly process will react with the optical fibers or components so as to occupy defect site locations in the optical fibers or fiber-interfacing components, and thereby make them unavailable for chemical combination with ordinary hydrogen. Alternatively, the deuterium may be introduced into the optical fibers just prior to forming a fiber optic cable and components or subsequent to forming the fiber optic cable and components, as long as the deuterium is made to react with available defect sites. In any case, the occupation of defect sites by deuterium reaction products improves the long term signal attenuation characteristics of the fiber optic cables and/or components because the number of defect sites that are available for chemical combination with ordinary hydrogen will have been reduced as a result of prior combinations with deuterium.
According to one exemplary embodiment of the invention deuterium is introduced into a fill material used in the fiber optic cable and/or apparatus. According to one variation of the invention, th
Nguyen Khiem
Tyco Telecommunications (US) Inc.
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