A distinction must be made between the optical fibre – a single strand of glass fibre – and the optical-fibre cable consisting of one or more strands of fibre and various protective coverings.
Bare optical fibre is fragile and vulnerable, and the cabling must provide the properties given below.
Tensile strength: The cable should prevent the fibre being strained when the cable is under tension. When the cable is being laid, for example, pulling it through a duct results in considerable tension.
Abrasion resistance: Abrasion on the surface of the fibre will weaken it significantly because cracks, starting at surface scratches, will readily propagate right across the fibre. The cable around the fibre must protect it from such abrasion.
Resistance to radial compression: There must be sufficient radial strength in the cable to shield the fibre from the expected radial force.
Bend resistance: If the fibre is bent such that its radius of curvature is less than a critical minimum, its attenuation starts to increase as a result of light loss. The cable must ensure that the fibre is never bent that severely.
Environmental protection (chemical resistance): Certain chemicals can react with the glass of the fibre to change its optical properties. For example, hydrogen can increase the attenuation. The cable must prevent such chemicals coming into contact with the fibre.
These are the principal characteristics required of the cable, but there are other hazards against which the fibre must be protected in particular circumstances. For example, frequent vibration or flexing of the fibre could weaken it so that it eventually breaks.
The first stage of protection of the fibre is the addition of one or more polymer layers as soon as the fibre is manufactured, before it is wound onto a drum. This is necessary to protect against abrasion, which can very easily damage the bare fibre. The polymer-coated fibre is then cabled to provide various degrees of protection depending upon the application. At one extreme the cabling could be just further plastic covering to produce a cable for use within a rack of equipment. At the other extreme many fibres could be put together in an armoured cable for use in a submarine optical-fibre link.