Optical fibres have graded index, or are clad with a material with slightly lower refractive index, so as to reduce pulse broadening. This page illustrates how cladding reduces the pulse broadinging. It supports the multimedia tutorial Geometrical Optics.
An animated cartoon showing ray diagrams for an unclad optical fibre, and the arrival times of different rays
In the simplified cartoon above, the fibre is surrounded by air or some material with a much lower refractive index. Consequently, the critical angle (see Total internal reflection) is substantially less than 90°. This means that rays that are substantially off-axis are reflected by total internal reflection. Because they travel much further than rays nearly parallel to the axis, they arrive later and broaden the pulse of light rays. This limits the number of pulses that can be sent per second, and thus the information-carrying capacity of the fibre.
Optical fibre with cladding to reduce pulse broadening
Cartoon of a fibre with a cladding whose refractive index is slightly less than that of the core.
When the cladding has a refractive index only slightly beow that of the core, the critical angle for total internal reflection is close to 90°. Consequently, only rays travelling nearly parallel to the axis are totally internally reflected. The others are partially reflected (not shown here) but, after many partial reflections, their intensity is much reduced.