Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-06-08
2004-06-22
Dunn, Drew (Department: 2872)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S127000, C385S128000
Reexamination Certificate
active
06754423
ABSTRACT:
The present invention relates to a method for the manufacture of a single mode optical fibre comprising a light-conductive core portion, an internal cladding portion surrounding said core portion and a jacketing portion surrounding said internal cladding portion, in which the refractive index of the core portion is larger than those of the cladding and jacketing portion areas, and in which the refractive indices of the cladding and jacketing portion areas are practically equal, by which method a silica substrate tube used as jacketing portion, is internally flushed with one or more reactive gases to form the internal cladding portion and core portion, respectively, after which the substrate tube thus provided with layers is collapsed and drawn into a single mode optical fibre. Furthermore, the present invention relates to a single mode optical fibre comprising a light-conductive core portion, a cladding portion surrounding said core portion and a jacketing portion surrounding said internal cladding portion.
Optical fibres of this type are well known and are mainly applied in the field of telecommunications technology. See, for example, European Patent Application 0 127 227, U.S. Pat. No. 5,242,476 and U.S. Pat. No. 5,838,866. The term ‘single mode’ used in the present description is generally known to experts in this field and needs therefore no further explanation here. Because of their characteristic low attenuation and dispersion such optical fibres are particularly suitable for the formation of long-distance data links, often spanning many thousands of kilometers. Over such considerable distances it is of vital importance that the cumulative signal losses in the optical fibre be kept to a minimum, if transmission of optical signals is to occur with a small number of intermediate amplification stations. At the commonly employed transmission wavelength of 1550 nm the telecommunications industry conventionally requires that the total attenuation in such optical fibres does rot exceed 0.25 dB/km, and preferably does not exceed 0.2 dB/km.
Although the presently manufactured fibres may meet all such requirements with regard to permissible attenuation, it is nevertheless often observed that, after elapse of time, the same optical fibres demonstrate considerable attenuation increases, Extensive investigation has shown that this phenomenon is attributable to the gradual seepage of hydrogen gas into the fibre from its surroundings, with the consequent formation of groups like SiH and SiOH within the fibre. These compounds demonstrate strong infra-red absorption, with attenuation peaks at wavelengths of about 1530 and 1385 nm.
A solution to overcome the problem of such hydrogen-induced attenuation is known from European Patent Application 0 477 435. In the method therein disclosed, a molten optical fibre is extensively exposed to a hydrogen-containing gas during its manufacture, so as to ensure that all structural defect sites in the fibre have already been presented with a hydrogen atom before the actual implementation of the fibre. A disadvantage of this known method is, however, that it only addresses the symptoms of hydrogen-induced attenuation and not the causes thereof. Moreover, this known measure considerably complicates the manufacturing process, and introduces an additional risk of contamination of the product fibre by the hydrogen-containing gas employed.
From U.S. Pat. No. 5,090,979 a method for the manufacture of an optical fibre is known, subsequently comprising of a pure silicon dioxide core portion, an outer layer of fluorine-doped silicon dioxide, a substrate layer of fluorine-doped silicon dioxide, and a carrier layer of pure silicon dioxide, in which the refractive index of the core portion is practically equal to that of the carrier layer.
From U.S. Pat. No. 5,033,815 an optical fibre of the multi-mode type is known, which fibre substantially differs from the present single mode optical fibre. Furthermore, the multi, mode optical fibre known from said publication subsequently contains a GeO
2
- or Sb
2
O
2
-doped core portion, an F-doped cladding portion and finally a possibly TiO
2
-doped jacketing portion, resulting in the refractive index of the core portion being higher than those of the cladding- and jacketing portion areas, and the refractive index of the jacketing portion being substantially lower than that of the cladding portion, which refractive index profile substantially differs from the present profile. No data with regard to compressive axial stress are known from said publication.
From European Patent Application 0 762 159 a dispersion-compensating fibre is known, subsequently comprising a core portion with at least 10 mol % of GeO
2
and a cladding portion, which cladding portion comprises a first fluorine-doped cladding portion, a second chlorine-doped cladding portion, and a third chlorine- or fluorine-doped cladding portion. The doping of the third cladding portion is chosen such that the glass viscosity at the moment of drawing is lower than that of pure silicon dioxide glass, which allows a relatively low temperature during drawing. No data with regard to compressive axial stress are known from this application.
It is therefore an objective of the present invention to provide a method for the manufacture of a single mode optical fibre, in which the hydrogen-induced attenuation at a wavelength of 1550 nm is sufficiently low to ensure the total attenuation at that wavelength to be at most 0.25 dB/km, and preferably to be at most 0.2 dB/km.
As mentioned in the preamble, in accordance with the present invention this objective is achieved because the present method for the manufacture of a single mode optical fibre is characterised in that the internal cladding portion is built up from SiO
2
comprising a fluorine doping within a range of 0.1-8.5 wt. %, thus resulting in the core portion to be subjected to a compressive axial stress over its full cross section.
The present inventors suppose that the presence of axial compression in the fibre core prevents the occurrence of the defects mentioned before, thus resulting in a significantly lowered hydrogen-induced attenuation. Since, according to the present inventors, the presence of axial tension in a fibre core facilitates the formation of structural defects in the silicon dioxide core, the presence of axial compression in a fibre core will essentially inhibit the occurrence of such defects, thus leading to a substantially lowered hydrogen-induced attenuation.
The present inventors have carried out a number of experiments in which a preform was manufactured by subsequently providing the internal surface of a substrate tube with an internal cladding portion of silicon oxide, which cladding portion is built up of SiO
2
, comprising fluorine-doping, and a second doped layer of silicon oxide, which second layer has a higher refractive index than that of the internal cladding portion and forms the final core of the fibre. The substrate tube thus provided with a core portion and internal cladding portion was subsequently thermally subjected to a collapse procedure to form a rod, which rod eventually was drawn into the required fibres at one of its molten extremities.
In the present invention the internal cladding portion is preferably doped with fluorine within a range of 0.1-8.5 wt. %, and preferably of 0.2-2.0 wt. %. Fluorine-doping of more than 8.5 wt. % is undesirable since then problems will arise in the deposition of such layers. A fluorine amount of less than 0.1 wt. % does not give a noticeable result with regard to the required compressive axial stress in the core portion. A maximum doping of 2.0 wt. % is particularly preferred if very low attenuation losses are required, which attenuation losses are negatively influenced by the increase of Rayleigh scattering. The fact is that experiments have shown that part of the internal cladding portion also functions as a light path for the light that is transported inside the fibre's core.
The application of fluorine doping in the internal claddin
Breuls Antonius Henricus Elisabeth
Simons Dennis Robert
Draka Fibre Technology B.V.
Dunn Drew
Pritchett Joshua L
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