optical fiber construction and working :
Fibre Optic Construction
Fibre optic cabling has the following components (starting in the centre and working out): core, cladding, coating, strength member, and jacket. The design and function of each of these will be defined.
The core is in the very centre of the cable and is the medium of propagation for the signal. The core is made of silica glass or plastic with a high refractive index. The actual core is very small. Typical core sizes range from 8 microns (millionth of a meter) for single mode silica glass cores and up to 1000 microns for multimode plastic optical fibre (POF).
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The cladding is a material of lower index of refraction which surrounds the core. This difference in index forms a mirror at the boundary of the core and cladding. Because of the lower index, it reflects the light back into the centre of the core, forming an optical wave guide. This is the same effect as looking out over a calm lake and noting the reflection, while looking straight down you see through the water. It is this interaction of core and cladding that is at the heart of how optical fibre works.
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Construction of a Fibre Optic Cable
The coating (also referred to as buffer or buffer coating) is a protective layer around the outside of the cladding. It is typically made of a thermoplastic material for tight buffer construction and a gel material for loose buffer construction. As the name implies, in tight buffer construction, the buffer is extruded directly onto the fibre, tightly surrounding it. Loose buffer construction uses a gel filled tube which is larger than the fibre itself. Loose buffer construction offers a high degree of isolation from external mechanical forces such as vibration. Tight buffer construction on the other hand provides for a smaller bend radius, smaller overall diameter, and crush resistance.
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To further protect the fibre from stretching during installation, and to protect it from expansion and contraction due to temperature changes, strength members are added to the cable construction. These members are made from various materials from steel (used in some multi-strand cables) to Kevlar. In single and double fibre cables, the strength members are wrapped around the coating. In some multi-strand cables, the strength member is in the centre of the bundle.
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The jacket is the last item in the construction, and provides the final protection from the environment in which the cable is installed. Of concern here is the intended placement of the cable. Different jackets provide different solutions for indoor, outdoor, aerial, and buried installations.
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