Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Optical fiber – waveguide – or preform
Reexamination Certificate
2000-08-29
2004-08-17
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Optical fiber, waveguide, or preform
C264S001290, C264S001700, C264S002100
Reexamination Certificate
active
06776932
ABSTRACT:
BACKGROUND OF THE INVENTION
Optical resin materials which are characterized by a distributed refractive index have proved useful in the construction of optical conductors such as, optical fibers, optical waveguides, optical integrated circuits, and the corresponding preforms from which these optical conductors are fabricated. In general, plastic or polymeric optical fibers (POF) are considered an attractive alternative to copper cable and glass optical fibers. Typically, the plastic optical fiber (or thin, flexible optical rod) has an elongated core within which the majority of the light travels in a generally axial direction and a sheathing layer which coaxially surrounds the core and confines the light to the core due to its having an index of refraction less than that of the core.
The refractive index distribution of plastic optical fibers can be classified as either a gradient (or graded) index or step index. However, gradient index plastic optical fibers (GI POF) are preferred over step index fibers for many data communication applications due to their superior bandwidth capacity. The index of refraction in a gradient index plastic optical fiber has a distribution that continuously changes within the core of the fiber, generally decreasing radially from a maximum value at the core central axis outwardly until it approaches the lower index of refraction of the sheathing at or near the core-sheathing interface. Due to this continuously varying refractive index within the core, the optical fiber acts like a lens tending to refocus light rays, reducing their propagation in non-axial directions, so that light rays entering the core at a small angle, with respect to the axis, follow undulating paths with relatively small deviations from the axial direction when compared to light propagation in a step index type fiber. In addition, the speed of the light rays following undulating paths is higher in the regions of lower refractive index so that the total travel time for light rays following undulating paths is nearly equal to those following a straight axial path. This results in, for example, a fiber with a wider bandwidth of transmission with minimal modal dispersion and a more rapid information flow than that obtained with step index plastic optical fibers.
In general, typical methods of fabricating gradient index plastic optical fibers involve preparation of a polymeric sheathing and a polymeric core disposed within the sheathing in a coaxial configuration. The refractive index of the core and sheathing are different and, for most optical conducting applications, the refractive index of the core is greater than that of the sheathing. Frequently, the core is made of the same polymer as that which comprises the sheathing but, in addition, further includes a non-polymeric substance (commonly referred to as a dopant) which increases the refractive index of the core so that it is greater than that of the sheathing. (See for example, U.S. Pat. No. 5,541,247 to Koike.)
However, currently available methods of fabrication have significant shortcomings. For example, the type and amount of dopant substances which can be incorporated into the core and still provide a gradient index plastic optical article which maintains both sufficient optical transparency and an acceptable difference in the refractive index between the sheathing and the core, are limited. Therefore, a need exists for methods and materials useful for fabricating improved gradient index plastic optical articles.
SUMMARY OF THE INVENTION
One aspect of the present invention is based upon the discovery that a gradient index plastic optical article having excellent optical characteristics can be produced using a method of fabrication that incorporates a low refractive index dopant (i.e. having a refractive index lower than that of the polymer comprising the sheathing but without the dopant) in the sheathing of the article.
The present invention in another aspect relates to a gradient index plastic optical article, and methods of processing the article. The methods of the invention provide for the use of a significantly broader selection of dopant and polymeric materials which can be used to produce a functional gradient index plastic optical article with excellent optical characteristics. For example, the methods of the invention allow for control of the gradient refractive index of the material and for a wider range of differences in refractive indicies between the core and sheathing for a given concentration of core dopant thereby producing a gradient index plastic optical article with a low loss due to light attenuation and broad transmission bandwidth, having a high level of transparency, a substantial absence of bubbles and good environmental stability, for example, enhanced thermal stability and resistance to humidity.
One method for forming a gradient index plastic optical article according to the invention comprises the steps of: (a) forming a transparent tube of sheathing material including at least one sheathing polymer and at least one sheathing dopant; and (b) forming a transparent core within the sheathing tube produced in step (a) by: (i) filling the interior space of the sheathing tube with a core solution including at least one polymerizable core monomer which upon polymerization has a refractive index greater than that of the sheathing tube; and ii) allowing the polymerizable core monomer to polymerize thereby forming a polymeric core having a refractive index greater than that of the sheathing tube such that the article is suitable to conduct light at at least one wavelength with an attenuation less than 500 dB/km. The core solution can include an optional core dopant. When present, the core dopant will have a refractive index greater than that of the polymer obtained upon polymerization of a core monomer solution polymerized under the same conditions but not including the core dopant. The product thus obtained, is a gradient index plastic optical article having an outer sheathing and an inner core both at least partially transparent to light at at least one wavelength. The refractive index of the central axis of the core will be greater than that of the sheathing such that the article is suitable to conduct light at at least one wavelength with an attenuation less than about 500 dB/km, with the refractive index of the core preferably gradually decreasing in a radial direction from the central axis of the core to the periphery of the core at the core-sheathing interface. In general, the article is fabricated in the shape of a preform rod. Preferably, the preform rod has a cylindrical shape which can be drawn into fibers.
In one embodiment, the sheathing tube is made by extrusion methods. Alternatively, the sheathing tube can be produced by: (a) placing into a polymerization container a sheathing solution including at least one sheathing polymerizable monomer and at least one sheathing dopant, the sheathing dopant having a refractive index lower than that of the polymer obtained by the polymerization of a sheathing monomer solution under the same conditions but not including the sheathing dopant; and (b) causing the sheathing monomer of the sheathing solution to polymerize within the polymerization container into a cylindrical sheathing tube at least partially transparent to light at at least one wavelength. The invention further provides a method for forming a gradient index plastic optical fiber. In the method, the gradient index plastic optical article is prepared, for example as described above, in the shape of a preform rod which is then be subjected to hot-drawing at a predetermined temperature and speed suitable to produce a fiber useful as an optical conductor. In one embodiment, the monomer of the sheathing solution and the monomer of the core solution are the same. Suitable monomers include those which form polymers that are substantially amorphous and capable of conducting light at the desired wavelength(s). For embodiments where the core polymer and the sheathing polymer are the same, when a cor
Getronics Wang Co. LLC
Vargot Mathieu D.
Wolf Greenfield & Sacks P.C.
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