Physically, an optical fibre is a very thin and flexible medium, having a cylindrical shape, consisting of three sections: (Fig.1.)
The Core: The core is the innermost section having diameter approximately 5 mm - 100 mm. The fibres are silica (SiO2) based.
The Cladding: The core is surrounded by cladding of slightly lower refractive index. Diameter usually 125 mm.
The Jacket: The outer section is called the jacket consisting of primary coating followed by another layer i.e., secondary coating. These are made of plastic or polymer and is provided for protection against moisture, abrasion, crushing and other environmental dangers.
Plastic optical fibres are used as well as glass fibres. Plastic fibre, though it is inexpensive, has some limitations considering life expectancy, Humidity resistance, heat, bending sensitivity, compatibility with environment. Even though an optical fibre is usually made of glass, it is surprisingly tough, in fact, it can be bent and twisted just like wire.