Optical fibre is a long cylindrical wire that consists of an inner code and a cladding layer on top of inner core. The inner core is made of a glass of higher refractive index and the cladding core is made of lower refractive index.
An electrical signal is converted into an optical signal by use of (transducer) optical LEDs. They glow at the frequency of the incoming signal. The LED is kept at one end of the fibre.
When a light wave is incident on the circular cross section of the fibre at one end, it is refracted on entering inner core and it is incident on the interface between inner core and the cladding. The angle is arranged to be more than the critical angle and so the light wave is reflected totally. into the inner core. The light wave travels along the fibre cable with innumerable number of total internal reflections.
When the light wave reaches the other side (receiver), the optical-diodes sense the light and convert the signal into electrical pulses. These pulses are given for digital processing.
Optical fibres can transmit with very small loss of energy over kilometers. The fibres are flexible inside the covering material. Many fibres are packed together in to a cable. Each fibre has a very large optical bandwidth and high response time. It is suitable for super fast transmission systems and for computer networking and for on board connections too. One fibre can be used to transmit waves of many frequencies simultaneously.
One advantage of optical fibre is that they are electrically and magnetically not affected by any external interference unlike copper wires. Multiple grades of the fiber is fabricated suitable for different applications.
These cables are some what expensive, but the advantages are many fold. So they are used in telecommunications. They are also used as table top decorative (colorful) lighting articles.