Optical waveguides – Optical transmission cable – Tightly confined
Patent
1998-06-12
2000-03-07
Ngo, Hung N.
Optical waveguides
Optical transmission cable
Tightly confined
385147, 385104, G02B 644
Patent
active
060350865
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an optical fibre cable and to its method of manufacture and more particularly to the reduction in polarisation mode dispersion in such cables.
Deformities of optical fibre cross section modify the internal birefrigence of the fibre and result in the normally single polarisation mode for the fibre becoming two orthogonal polarised modes. If the dispersion between these two modes, called polarisation mode dispersion, becomes large enough then bit errors can occur in digital systems.
One method of manufacturing optical fibre cables is described in EP-A-0250173 which method is to extrude a thermoplastic polymer coating over a kingwire to provide a surface for receiving the fibres. The surface of the layer is melted and the fibres are embedded into the surface prior to extrusion of a second layer of thermoplastic polymer to cover the fibres. We have experimented with such a method where we have used HYTREL 40D as the thermoplastic material (HYTREL is a Trade Mark of Dupont). We have found that such a construction results in a significant level of polarisation mode dispersion in the fibres and it has been discovered that stresses induced as a consequence of extrusion of the elastomer onto the fibre causes the deformation of the otherwise circular cross section of the fibre and contributes significantly to the polarisation mode dispersion.
The present invention seeks to provide an improved construction and method of manufacture of optical fibre cable in which polarisation mode dispersion is reduced.
According to the invention there is provided a method of manufacturing an optical fibre cable comprising the steps of, extruding a thermoplastic elastomer layer around a kingwire, setting the elastomer layer, laying a plurality of optical fibres along the set elastomer and extruding a further layer of thermoplastic elastomer over the fibres.
The method may be carried out in one processing line, wherein after extruding the thermoplastic elastomer around the kingwire the coated kingwire is cooled to a temperature below the melt temperatures of the elastomer and then the optical fibres are layed therealong. Alternatively, after extruding the thermoplastic elastomer around the kingwire, the coated kingwire is cooled to solidify the coating and is subsequently heated to soften the layer at a temperature below the melt point of the elastomer prior to laying the optical fibres therealong. The latter method may be carried out in two separate production processes wherein the coated kingwire is produced and stored and is subsequently employed in the process of laying the plurality of optical fibres including the step of preheating the elastomer coating to a temperature below its melt point to soften the layer prior to laying the optical fibres therealong.
The temperature of the first elastomer layer may be arranged to be up to 26.degree. below the melt point when laying the optical fibres and is preferably within 6.degree. C. of the melt point of the elastomer which melt point may be approximately 156.degree. C.
Preferably, the thermoplastic elastomer employed has a Youngs Modulus value less than 100 MPa at 20.degree. C.
The invention also includes an optical fibre cable, comprising a kingwire provided with inner and outer extruded layers of a thermoplastic elastomer and a plurality of optical fibres disposed on the outer surface of the inner layer and embedded in the outer layer.
The thermoplastic material may be HYTREL 35D. A suitable extrusion temperature for the further layer of thermoplastic elastomer is between 156.degree. C. and 200.degree. C.
The fibres may be twisted about their own axis, and/or twisted helically about the first layer prior to extrusion of the further layer. The or each twist may be a continuous twist or a to and fro twist through a predetermined angle. In one advantageous construction each twist is one turn between 1 and 10 meters of fibre length.
In order that the invention and its various other preferred features may be understood more easily, embodiment thereof and their m
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patent: 4765711 (1988-08-01), Obst
patent: 4902097 (1990-02-01), Worthington et al.
patent: 5291573 (1994-03-01), Yoshida et al.
patent: 5604833 (1997-02-01), Kambe et al.
Barnes Stuart R.
Curtis David
Dixon Timothy J.
Henderson Keith
Norman Philip A.
Alcatel
Ngo Hung N.
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