Multipath dispersion deals with the path (mode) of each light ray. Most transmitters emit
many different modes. Some of these light rays will travel straight through the center of
the fibre (axial mode) while others will repeatedly bounce off the cladding/core boundary
to zigzag their way along the waveguide.
The modes that enter at sharp angles are called high-order modes. These modes take much
longer to travel through the fiber than the low-order modes and therefore contribute to
modal dispersion. One way to reduce multipath dispersion is to use graded-index fibre.
Unlike the two distinct materials in a step-index fibre, the graded-index fibre’s cladding is
doped so that the refractive index gradually decreases over many layers. With a graded-index fibre, the light follows a more curved path. The high-order modes spend most of the
time traveling in the lower-index cladding layers near the outside of the fibre. These
lower-index core layers allow the light to travel faster than in the higher-index centre
layers. Therefore, their higher velocity compensates for the longer paths of these high order modes. A good waveguide design appreciably reduces multipath dispersion