Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Optical fiber – waveguide – or preform
Reexamination Certificate
2000-06-02
2002-12-31
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Optical fiber, waveguide, or preform
C264S001290, C425S114000
Reexamination Certificate
active
06500365
ABSTRACT:
This invention relates to a process for the manufacture of an optical telecommunications cable.
More particularly, the invention relates to a process for the manufacture of an optical core for a telecommunications cable, wherein the optical core comprises a support consisting of a central, traction-resistant reinforcing member, a first polymeric coating layer applied around the central member, a plurality of optical fibres arranged around the first layer and a second polymeric coating layer extruded around the first layer and around the optical fibres.
Numerous processes are known for the manufacture of telecommunications cables comprising a core with optical fibres where the optical fibres, typically coated with one or more layers of acrylic resin, are completely encapsulated in thermoplastic materials.
A process is known, for example, from GB patent application 2,176,905 according to which optical fibres with acrylic coatings are forced through a die together with a central reinforcing member before being encapsulated in a thermoplastic casing of a material commercially designated “Hytrelo®”.
This process is known as a “one-shot operation”, indicating that the core is formed in a single step in the extrusion die.
A method is described in GB patent no. 2,136,350 for constructing an optical core whereby a first central strength member is heated and a first thermoplastic elastomer layer is extruded on this member. A plurality of optical fibres is arranged in a helical pattern on the first layer with a planetary motion obtained from a special revolving cage whereon drums containing the fibres are arranged. A second layer of thermoplastic material is extruded around the fibres. This method requires particularly complex equipment, consisting of a revolving cage with which rotating drums of fibres are associated and also guides for directing the fibres from the cage to the extrusion head.
GB patent no. 2,113,903 describes a method for making a telecommunications cable according to which a plurality of optical conductors is encapsulated at least partially in the outer periphery of a central matrix of thermoplastic material, polyethylene for example, extruded around a central steel or nylon filament member. More particularly, the conductors are forced against the central member which is softened by heat. The pressure on the conductors is such that a predetermined depositing thereof takes place in the matrix by which they are kept apart from each other before being coated with a further second extrusion layer. US document no. 4,902,097 describes a method according to which a central reinforcing member is heated and provided with a first layer of thermoplastic elastomer. The support thus formed is heated and thereby softened to such an extent as to enable partial encapsulation therein of the optical fibres guided to the first layer through a plate with through-holes that the fibres pass through on their way to the extrusion head. A second layer of thermoplastic material is then extruded on the support and on the fibres. GB 2,303,938 discloses a method for producing an optical cable by disposing a plurality of fibers on the surface of an inner polymeric layer and embedding them into an outer polymeric layer. EP 646 819 discloses a method for reducing the PMD in fiber optic cable by imparting a controlled twist to the fiber being disposed around a coated strength member. During the manufacturing process, the strength member passes through a helically rotating closing die which applies radially inward forces on the cable core.--
The applicant has observed that the production of optical cores according to the known methods may result in attenuation of the fibre transmission properties, due to the stresses that the fibres are subjected to during production of the optical core.
For example, with the so-called “one shot” process, it is difficult to control the relative positions of the fibres during extrusion of the polymeric layer and the fibres are subjected to undesired and uncontrolled stresses on account of the high level of pressure exerted by the polymeric material in the extrusion head.
The Applicant has also noted that partial encapsulation of the optical fibres in a first layer of thermoplastic material extruded around a central reinforcing member and subsequently covered with a second polymeric layer, as described in GB patent 2,113,903 or U.S. Pat. No. 4,902,097, may be one of the causes of signal transmission attenuation in the fibres. In practical terms, it was seen that embedding fibres to a greater or lesser extent in a first thermoplastic layer, as described in the above patents to maintain the fibres in the desired configuration around the extrusion zone of the second layer, requires a certain mechanical compression action to be exerted on the fibres, which thus remain in the cable in a state of mechanical stress which, if high, results in attenuation of the signal. It was also observed that it is difficult to continuously keep this compression at a low level in view of the considerable lengths, running into kilometres, usually required in optical core manufacture.
The Applicant also noticed that, in the absence of a certain control over extrusion parameters, such as temperature of the molten polymer or dimension of the extruder, extrusion of the second polymeric layer onto the optical fibres arranged around the first coat may cause non-uniform distribution of pressure on the fibres, with the risk of moving the fibres from their desired configuration and of increasing attenuation of the signal when the transmission cable is in operation.
The Applicant has now found that a “tight” cable can be made simply and effectively, in which a plurality of optical fibres are encapsulated in a core of polymeric material consisting of at least two concentric and contiguous layers of polymer. This result may be conveniently obtained by arranging the fibres around a central support, so that they are free of the interface formed between the two contiguous polymeric layers and by controlling the extrusion parameters so that the geometric configuration of the fibres is maintained in a predetermined position.
Accordingly, one aspect of the present invention concerns a process for the manufacture of an optical core for a telecommunications cable, comprising at least one central support coated with a first polymeric coating layer, a plurality of optical fibres arranged longitudinally around the support and a second polymeric coating layer extruded around said first layer and around said optical fibres, which comprises the following steps:
a) arranging said optical fibres longitudinally around said first polymeric coating layer so that the optical fibres are substantially tangential to the surface of the coating and circumferentially separated from each other in a predetermined way; and
b) extruding the second layer around said first layer and around said optical fibres, maintaining a condition of substantial tangency and of circumferential separation at least as far as the exit of the extruder.
In the course of this description, the phrase “condition of substantial tangency of the fibres to the support” means a configuration wherein the fibres are placed in a position such that the interface between the two polymeric layers does not cross through the fibres. This condition generally includes both the case where the fibres are placed in substantial contact with the inner polymeric layer arranged around the central support, and the condition where the fibres are totally encapsulated in the second polymeric layer, thereby placing a thin layer of the second polymeric coat between the optical fibres and the first coating layer. This substantial tangency is typically obtained without applying any particular pressure on the fibres, either radially or longitudinally, to compress the fibres against the support or encapsulating them partially in it, as for example the mechanical type compression exerted by suitable equipment or compression obtained by winding the fibre helically under tension around the s
Brandi Giovanni
Cecchi Feliciano
Finnegan Henderson Farabow Garrett & Dunner LLP
Pirelli Cavi e Sistemi S.p.A.
Vargot Mathieu D.
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