Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
1999-11-12
2001-11-06
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C385S123000, C385S142000, C385S144000
Reexamination Certificate
active
06314227
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the design of liquid lightguides, e.g. for use in medical endoscopy and in the curing of epoxy in dentistry and in industry, and more specifically to the selection of materials for tube, interior coating layer and for the transparent liquid.
2. Description of the Prior Art
Liquid lightguides with internal coating layers are already known since 1988.
In EP 0246 552 Fitz describes the possibility of coating internally flexible tubes made from thermoplastic or elastomeric material with an internally reflecting layer, made out of an amorphous fluoropolymer, called HOSTAFLON® TFB (Hoechst) or THV (3M Company), for use as a cladding of liquid core lightguides. The “highly amorphous” THV material from 3M can be dissolved in certain liquids, which allows for a simple coating process from solution by the way of dip coating.
In DE 423 3087 the present applicant describes a liquid core lightguide consisting of a tube made form a carbon fluoropolymer material, coated internally with a thin layer of TEFLON® AF, which is a new amorphous fluoropolymer, developed by the DuPont Company, comprising a copolymer of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole (PDD) with tetrafluorethylene (TFE).
Like THV from 3M, TEFLON® AF is also soluble in certain fluorinated liquids, thus allowing for a simple coating procedure via dip coating.
The advantages of internally coating plastic tubes, used for liquid lightguides with amorphous fluoropolymers are: Improved transmission due to the lack of crystallinity inside the reflective layer, and, especially when using TEFLON® AF, a higher numerical aperture of the lightguide, due to the extreme low value of the refractive index of TEFLON® AF, and as a consequence of the increased aperture, less dependence of the transmission on the bending radius of the tube.
The internal coating of tubes with TEFLON® AF, however, has also some disadvantages: DuPont, being the sole producer of TEFLON ® AF, charges 10 US$ per gram of the material. This means already considerable cost per meter of liquid lightguide, even with the small thickness of the applied layers being in the range of a few microns.
Furthermore, the adhesion of thin layers of TEFLON® AF applied from solution to the surface of plastic materials other than fluorocarbon polymers is still insufficient.
Especially the baking procedure at temperatures up to 250° C., required by DuPont to achieve good adhesion to the plastic surface, does not leave much of a choice for others than fluorocarbon polymers as a substrate material, being one of the few plastic materials, that can be heated up to such high temperatures.
The adhesion of TEFLON® AF to substrate materials of glass is also insufficient.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a more economical method to make an internal coating layer for a liquid lightguide tube without sacrificing the excellent optical properties of proven materials like TEFLON® AF. Another objective is to improve the adhesion of the coating layer resulting in a prolonged lifetime of the liquid lightguide, especially where frequent flexing is required, e.g. in medical endoscopy.
Briefly, the preferred embodiment of the present invention includes a illumination system comprising a high power lightsource with a reflecting mirror, a condensor lens and a filter which generates a cone of light of a selected wavelength, from the ultraviolet to the infrared, for acceptance by the aperture of a liquid lightguide. This lightguide comprises a tube, e.g. made out of a plastic like a fluoropolymer, which carries on its inside a thin amorphous coating layer, preferably also made out of at least one fluoropolymer. The tube is closed at both ends with transparent plugs to confine an optically transparent liquid within.
If the coating layer is at least a few wavelengths thick, say 3-20&mgr;, and if its index of refraction is lower than that of the transparent liquid within the tube, the liquid lightguide acts as a low loss optical waveguide. Besides for medical endoscopic applications, the liquid lightguide may be used for example for the transmission of high power ultraviolet light (up to several watts) for curing epoxy at a distance of several meters from the lightsource or for curing fillings in dentistry.
The invention is the result of a large number of experiments with different materials for tube, coating layer and transparent liquids. One specific aspect of the invention is to propose a mixture of solid (TEFLON® AF) amorphous fluoropolymers and liquid amorphous fluoropolymers (e.g. perfluorinated liquids like perfluoropolyether, PFPE) and to create thus a coating layer with the equivalent performance of one amorphous solid fluoropolymer (TEFLON® AF) for a much smaller cost. This mixture advantageously provides a better adhesion of the coating layer to the tube while yielding the same optical properties as pure TEFLON® AF. The nonpolar hydrophobic nature of the perfluorinated liquids together with their low surface energy of 15-20 dyn/cm contrasts favorably with the high surface energy of the aqueous, inorganic solutions, used as the transparent liquid which are frequently employed for their high transmissivity in the ultraviolet region, e.g. for curing epoxy. This hydrophobic nature of the perfluorinated liquids cause them to remain within the mixture of the coating layer inspite of frequent bending of the tube e.g. in endoscopic applications. These properties features extend the lightguide lifetime especially for larger diameter tubing, a highly desirable feature for industrial application.
An advantage of the present invention is the substitution of expensive TEFLON® AF with a mixture of TEFLON® AF and much less expensive perfluorinated liquids like PFPE as the coating layer while achieving the same index of refraction, thus achieving substantially lower material costs.
Another advantage of the present invention is the enhanced elasticity and enhanced adhesion of the mixture to the tube, resulting in a longer lifetime for the coating layer and thus the lightguide inspite of frequent bending.
Another advantage of the present invention is the hydrophobic nature of the perfluorinated liquids, like PFPE, within the coating layer, which prevents them from being diluted into the aqueous transparent liquid within the tube inspite of frequent bending, thus maintaining the reflective properties of the coating layer over a longer time.
Another advantage of the present invention is the possibility to use, besides TEFLON® AF, also another solid amorphous fluoropolymer, like TEFLON® SF Soluble
Fluoropolymer (TEFLON® TE-5169A) as additive component for the production of the reflective layer.
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patent: 0 246 552 (1987-11-01), None
patent: 95/12138 (1995-05-01), None
Bovernick Rodney
Kinberg Robert
Pak Sung
Venable
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