An optical cable of the type described above is known, for example, from European Patent No. 0 136 913. According to this publication, optical fibres are placed in secondary sheathing tubes reinforced with glass fibres, and these secondary sheathing tubes are positioned around a central strength and support member. The cable thus requires secondary sheathing tubes reinforced with glass fibres and a separate strength and support member, although the cable may be thinner than conventional cables, since the secondary sheathing tubes reinforced with glass fibres contribute to the tensile strength of the cable. Such elements for receiving tensile and compression stresses are necessary simply because the coefficient of heat expansion of the plastic material in the secondary sheathing tube is high as compared with the almost nonexistent heat expansion of optical fibres.
As the aim has been metal-free optical cables, constructions similar to the one disclosed in European Patent No. 0 136 913 cited above have been provided wherein elements with tensile strength are positioned in a cable or its sheathing, e.g. in the form of longitudinally oriented bundles of glass, carbon or plastic fibres. Such fibre-reinforced cables are known, for instance, from Finnish Patent No. 91 680 and European Patent Application No. 0 461 871. The problem with the fibre-reinforced cables of these publications is that they are complicated and expensive to manufacture. The fibre bundles must at first be twisted around the cable core and then be impregnated with plastic, hot-melt adhesive or the like to provide an integral sheathing. This renders the manufacturing process of the cable complicated and expensive.