Fibre Core

The inner portion of the fibre that transmits light. The index of refraction is higher than that of the cladding that surrounds it.
The core diameter can range from very small, on the order of the wavelength of light, to several millimetres or more. Flexible glass and silica (quartz) fibres have core diameters which range from ∼50 μm to about 1000 μm. Light launched into a fibre will, after a given length, reach the core material boundary and pass to another medium (glass, air, etc). Depending on the incident angle, some of the energy will be refracted outward (leaky modes) and some will reflect back into the core material (guided modes).