Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Optical fiber – waveguide – or preform
Reexamination Certificate
2000-06-30
2003-06-10
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Optical fiber, waveguide, or preform
C425S072200, C425S174400, C264S001360
Reexamination Certificate
active
06576166
ABSTRACT:
The present invention relates to a method of manufacturing a graded-index plastics optical fiber.
BACKGROUND OF THE INVENTION
Graded-index plastics optical fibers, suitable for use in a spectrum range covering the visible and the near infrared, are advantageous in that they can be applied to broadband access networks. A graded-index plastics optical fiber can comprise at least one polymer and at least one other compound, with the polymer content being substantially the same throughout the fiber, while the concentration of the other compound varies from the core to the periphery of the fiber in such a manner as to form the desired index gradient. Said compound, which can be referred to as a “dopant” or as a “diluant”, comprises at least one substance. Manufacturing such plastics optical fibers is difficult in that it is necessary to obtain a distribution of said compound that varies from the core to the periphery of a plastics optical fiber. The fiber needs to have a refractive index profile of the graded-index type that is as smooth as possible, with total variation in refractive index between the center and the periphery of the fiber generally lying in the range 0.01 to 0.03.
There are two main techniques for making said index gradient.
In a first technique, the index gradient is made at least in part before the optical fiber is spun through a die by diffusing compound(s) in at least one given polymer. Said compound can possibly itself comprise another polymer, at least in part.
In a first implementation of that first technique, such diffusion can take place m said given polymer while it is in the molten state. Thus, patent U.S. Pat. No. 5,593,621 describes manufacturing a plastics optical fiber with an index gradient by forming a cylinder of a first molten polymer and then injecting a transparent material into the central portion of said cylinder, said transparent material being diffusible and non-polymerizahle, and possibly mixed with a second molten polymer that is transparent and of refractive index that is different from that of the first polymer. After said nonpolymerizable material has diffused in the first polymer, the cylinder is spun into an optical fibers. In such all implementation, it is possible to install a plurality of nozzles nested in one another so as to improve the diffusion of said material.
In a second implementation of that first technique such diffusion can take place in said diluted polymer. Thus, patent application JP A-9 243 835 describes the manufacture of a graded-index plastics optical fiber comprising a matrix-forming transparent polymer, a nonpolmerizable second compound having a refractive index higher than that of the transparent polymer, and a nonpolymerizable third compound of refractive index smaller than that of the transparent polymer. The second compound is more concentrated in the core of the fiber. The third compound is more concentrated in the periphery of the fiber. Manufacture consists in preparing layers of various compositions of the second and third compounds, together with a monomer constituting a source for the transparent polymer, said compositions being delivered via a multi-orifice nozzle. After diffusion between the layers and spinning, where diffusion can take place before and during spinning, fiber manufacture is finalized by polymerizing said layers so as to transform the monomer into a polymer.
In the second technique for making a graded-index fiber, the index gradient is made almost entirely after the optical fiber has been spun through a die by applying at least one coating layer. The coating layer can comprise at least one polymer in the molten state. Thus, patent application JP-A-9 133 819 describes manufacturing a graded-index plastics optical fiber continuously by applying molten plastics materials of at least two different types and of decreasing refractive indices around the plastics fiber constituting the higher-index core of the fiber.
Alternatively, the coating layer can also include at least one partially polymerizable compound. Thus, patent application JP-A-9 138 313 discloses a method of manufacturing a graded-index plastics optical fiber by applying a solution obtained by diluting a polymer with a low refractive index compound in a monomer around a plastics core of higher index. The polymerized monomer constitutes the first layer of the polymer of refractive index lower than that of the core. Further layers are deposited in the same manner with refractive indices that decrease progressively from the center towards the periphery. Each layer is polymerized by ultraviolet (UV) treatment after it has been applied. In the example, the viscosity of the solutions applied to the core is adjusted to lie in the range 5000 poises to 10,000 poises, i.e. in the range 500 Pa.s to 1000 Pa.s. To implement the invention, the low refractive index compound is not polymerizable.
Patent application JP-A-6 003 533 describes manufacturing a multilayer graded-index plastics optical fiber continuously by preparing substances having different refractive indices and then applying separate layers, allowing diffusion to take place,between the layers, and then curing said layers.
All of the above manufacturing methods rely on compounds diffusing between adjacent layers, which means that said methods are slow and not very reproducible. This gives rise to major problems in manufacturing graded-index plastics optical fibers on an industrial scale.
OBJECTS AND SUMMARY OF THE INVENTION
The method of manufacture of the invention seeks to manufacture graded-index plastics optical fibers in a manner that is as effective as possible, mainly in terms of speed and reproducibility.
The present invention provides a method of manufacturing a graded-index plastics optical fiber whose refractive index varies continuously between its center and its periphery, said fiber being made from at least one polymer P and at least one compound M
1
enabling refractive index to be varied, said method comprising:
preparing two compositions of different refractive indices, the refractive index difference between the two compositions being not less than 5×10
−3
, each composition comprising at least the polymer P, and a “first” one of said compositions also having at least the compound M
1
, a cross-linking photo-initiator being present in at least one of said compositions;
the two compositions are diffused into each other; and
said mixture is spun so as to obtain a graded-index plastics optical fiber;
wherein:
the polymer P and the compound M
1
are selected in such a manner that:
said polymer P is of molecular mass lying in the range 1000 to 20,000 and said compound M
1
is of molecular mass lying in the range 100 to 1000;
said compound M
1
has at least one reactive functional group selected from the group formed by vinyls and acrylates;
the polymer P has at least one reactive functional group selected from the group formed by vinyls and acrylates; and
at least one of the two compounds P and M
1
is at least mono-functional, the other of the two compounds P and M
1
being at least bi-functional;
wherein
prior to spinning, the two compositions are subjected to active mixing using at least one mixing means so as to obtain continuous variation in the refractive index of the optical fiber; and
wherein
spinning of the graded-index plastics optical fiber is followed by curing giving rise to a cross-linked three-dimensional lattice.
In the invention it is preferred and found more appropriate to use the term “spinning” instead of the term “extrusion”, even though these two terms are often capable of being confused, particularly when documents are translated. Extrusion applies in general to materials that are solid, whereas spinning applies in general to materials that are not solid, and often liquid. It is not possible to extrude a liquid.
Such active mixing is mixing that takes place with assistance, i.e. does not rely on diffusion only, and the assistance can be given either statically by forcing two compositions to mix using static diffusion means, gene
Andrieu Xavier
Morichere Dominique
Pastouret Alain
Perrin Bernard
Nexans
Sughrue & Mion, PLLC
Vargot Mathieu D.
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