An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length by confining as much light as possible in a propagating form. Fiber optics are useful to send and actually make light fast without spending much energy. On big fibers the light is propageted by total internal reflaction, in small is based on estabilishing a waveguide. The diagram showing how light is guided through an multimode fiber (core diameter bigger than 10 micrometers). Incident rays which fall within the acceptance cone of the fiber are transmitted, whereas those which fall outside of the acceptance cone are lost in the cladding. The rays that enter the core with an angle grater than the critical angle, that depends on what the material and the diameter of the fiber is are completely reflected and the ones that enter with a smaller angle are refracted and keep going on into the core. The angle of acceptance is called also numerical aperture and with a big numerical aperture we can have a good propagation of light close and far from the axis but we’ll have a problem of dispersion of light into the fiber, so big fiber= a lot of light , But a lot of energy. In smaller fibers (usually from 8 to 10 micrometers of diameter) the light does propagates differently than in the multimode, in fact the light travels just around the axis of the core and doesn’t reflect into the cladding, and higher order signal is absent from this single mode. In this process very little energy is lost and light can propagate for very long with a little amount of input energy. ( into the core there is very little dispersion). Most of fiber optics are made of plastic or glass.
Most of the particular kind of glasses used have a refractive index of about 1.5, and typically the difference between core and cladding is less than one percent. Plastic optical fiber (POF) is commonly used for multimode fiber, with core diameter of 1 mm or larger. In plastic fibers usually the amplitude of the signal decreases faster. .