Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
1999-06-10
2001-03-13
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S124000, C385S126000, C065S417000, C065S431000, 62
Reexamination Certificate
active
06201917
ABSTRACT:
The present invention relates to an optical fiber preform based on silica and comprising a core of radius R, and a sheath, said sheath comprising an optical inner portion in direct contact with the core, and referred to as the “cladding”, and an outer portion referred to as the “outer sheath”, said outer sheath being made of silica doped with alumina. The present invention also relates to a method of manufacturing said preform and to optical fiber manufactured by performing fiber-drawing on said preform.
BACKGROUND OF THE INVENTION
Optical conductors are commonly used in telecommunications. Optical fibers based on silica generally conveyed data at a wavelength approximately in the range 1300 nanometers to 1550 nanometers. Such an optical fiber is formed of an optically-active portion constituted by a core that conveys most of the light wave, and by cladding, the core and the cladding having different refractive indices. Such a fiber usually also has an outer peripheral portion that is not optically active and that is referred to as the “outer sheath” which, together with the cladding, forms the sheath of the optical fiber. Since an optical fiber is drawn down from a preform which is geometrically similar in section, the same terms “core”, “cladding”, and “outer sheath” are used for the preform from which the optical fiber is manufactured. Each fiber is protected by covering made of a polymer material, and such protective covering is often covered with other covering made of a colored polymer. A set of optical fibers may be assembled together to form a ribbon. In which case, the material holding together the fibers of the ribbon, which material is referred to as the “matrix”, is also a polymer. In a telecommunications cable, the individual optical fibers or the ribbons of optical fibers are generally disposed in a tube made of metal or of a plastics material.
It is known that optical fibers must not be exposed to hydrogen because it degrades their transmission properties. The higher the hydrogen partial pressure to which the fiber is subjected, the worse the degradation. Hydrogen comes, in particular, from decomposition of the polymers of which the fiber covering or ribbon matrix is made. Hydrogen can also come from decomposition of the filler substance generally disposed in the cable in order to hold the fibers in the tube containing the optical fibers, and to prevent moisture from advancing in the event that the tube is broken or damaged. Such types of decomposition occur naturally due to aging.
Patent GB-B-2,145,240 mentions manufacturing optical fiber by performing fiber-drawing on an optical preform, and it describes said optical fiber comprising an optically-active portion made up of a core and of cladding, with the core and the cladding having different refractive indices, and of an outer sheath which, in addition to silica, contains a dopant belonging to a list of oxides. Among the list of oxides, alumina is mentioned and boron oxide is preferred. Neither the method of manufacturing the preform nor the structure of such a preform are specified. The structure of the final optical fiber is given by way of example and shown in a figure. That optical fiber comprises a core of radius R, made of doped silica, and of diameter 50 &mgr;m, corresponding to 2 R, and, around the core, a sheath that takes the diameter of the optical fiber to 125 &mgr;m, i.e 5 R, and that comprises cladding taking the diameter to 90 &mgr;m, i.e. 3.6 R, and an outer sheath, of thickness equal to 17.5 &mgr;m, i.e. 0.7 R. The concentration of oxide in said outer sheath preferably lies in the range 1% to 20% by weight of oxide relative to the composition of the outer sheath.
The presence of a large quantity of alumina in the outer sheath formed in that way makes it problematic to manufacture optical fiber from the preform under suitable conditions, in particular in terms of diameter stability during fiber-drawing, because the viscosity of the doped outer sheath is too low. With only 1% by weight of Al
2
O
3
in silica, it is difficult to remain in the generally accepted tolerance bracket which is 125±0.5 &mgr;m, and the tolerance becomes equal to 125±1.5 &mgr;m. That makes it problematic to connect the optical fibers together.
Furthermore, it is important for the outer sheath obtained to offer good performance in terms of mechanical properties, and an outer sheath that is too thick, such as the sheath described in Patent GB-B-2,145,240 can cause a reduction in performance, in particular when subjected to traction testing.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to make it possible for an optical fiber that is almost impermeable to hydrogen and that has good strength to be manufactured industrially.
To this end, the invention provides an optical fiber preform based on silica and comprising a core of radius R, and a sheath, said sheath comprising an optical inner portion in direct contact with the core, and referred to as the “cladding”, and an outer portion referred to as the “outer sheath”, said outer sheath being made of silica doped with alumina, wherein the outer sheath itself comprises an inner portion in direct contact with the cladding and referred to as the “inner zone”, and an outer portion in direct contact with the inner zone and referred to as the “peripheral zone” mainly comprising silica doped with alumina, wherein the thickness of said peripheral zone lies in the range 0.08 R to 2.2 R, and wherein the proportion by weight of alumina in the peripheral zone is such that the equivalent concentration of alumina in the outer sheath as a whole lies in the range 100 ppm to 1000 ppm by weight of aluminum relative to silica.
Preferably, the proportion by weight of alumina in the peripheral zone is such that the equivalent concentration of alumina in the outer sheath as a whole lies in the range 100 ppm to 500 ppm by weight of aluminum relative to silica.
Preferably, the thickness of the peripheral zone lies in the range 0.08 R to 1.5 R.
The equivalent concentration may be defined as the quantity of alumina contained in the peripheral zone in % weight of the volume of the outer sheath as a whole (and not only the volume of the peripheral zone). Since the preform is generally drawn down into an optical fiber of section that is geometrically similar, the equivalent concentration is the same in the preform and in the optical fiber. By way of example, in Patent GB-B-2,145,240, the minimum concentration of 1% by weight of alumina relative to the overall composition of the outer sheath gives a concentration of 1.01% by weight of alumina relative to silica in the outer sheath. That then gives an equivalent concentration of 10,100 ppm, in % weight of alumina relative to the silica contained in the outer sheath. Thus, since the ratio of the molar mass of alumina divided by the molar mass of aluminum is equal to 1.888, computation gives an equivalent concentration of alumina of about 5,350 ppm, by weight of aluminum relative to the silica contained in the outer sheath.
The alumina concentration of the peripheral zone is generally constant over the entire thickness of said peripheral zone, but it may also vary radially in continuous manner or in discontinuous manner.
The optical fiber preform of the invention makes use of a range of alumina concentrations and alumina thicknesses in the outer sheath that mitigates the drawbacks of the prior art, i.e. the outer sheath obtained offers good performance in terms of mechanical properties, and viscosity such that optical fiber can be manufactured in suitable manner from said preform, and in particular with an optical fiber diameter that is stable within the generally accepted tolerance bracket.
The preform of the invention advantageously has a peripheral zone of silica doped with alumina, which zone, once drawn down in geometrically similar manner, imparts high strength to the optical fiber manufactured from said preform. It should be noted that such a peripheral zone is generally of low roughness because it is melted
Campion Jean-Florent
Dubois Sophie
Orcel Gerard
Alcatel
Sanghavi Hemang
Sughrue Mion Zinn Macpeak & Seas, PLLC
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