Optical fibre is typically constructed of a transparent core surrounded by a transparent cladding material with a lower index of refraction. The core is usually made from synthetic silica, while the transparent cladding material can be made from glass of natural high purity quartz. Only the purest available natural high purity quartz or synthetic quartz can be used for this application. As shown in the image below from Wikipedia, the thickness of the cladding and core will determine the output pulse of the fibre.
Fibres are also used for illumination, and are wrapped in bundles to carry images, thus allowing viewing in tight spaces. Specially designed fibres are used for a variety of other applications, including light transmission, sensors, and fibre lasers.
More detailed information can be found on our blog.