The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. Consequently, this type of fiber is best suited for transmission over short distances, in an endoscope, for instance. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding. The shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the same time as the slow but straight rays in the core axis. These fibers often become the physical medium for local-area networks.
Light thus travels parallel to the axis, creating little pulse dispersion.
Telephone and cable television networks install millions of kilometers of this fiber every year.