An optical fibre is essentially a very long length of extremely high purity Quartz glass rod (hence the name “fibre”), enclosed within a protective wrapper. The Quartz glass propagates any light through it at high speeds with very low loss of light. The levels of purity required to achieve long lengths of optical fibres are only possibly using synthetic Quartz, as any impurity within the fibre is a possible source of light loss, as well as (for bigger defects) potential for breakages under tension.
Within the fabrication process for optical fibres, natural (ie. non-synthetic) Quartz parts are also used to help create the pre-formed mass which will subsequently be drawn into a long fibre. The requirements for Quartz within the fibre optic fabrication process are purity (as explained above), straightness (as any misalignments will get accentuated once the geometries are shrunk), and the ability to withstand very high temperatures (similar to semiconductor processes).