Basically we are compelled to use a mathematical description of light, be it in terms of rays, waves or photons. A pulse of light will be a burst of light waves. Now light waves oscillate at incredibly high frequencies, so a very large number of pulses can be fitted into a second. Despite those great demands stated earlier, the bits required for several TV signals can be fitted onto a single light beam. This is why optical communications are so successful.
Light travels along the fibre by bouncing off the core-cladding interface.
This is rather like the way sound waves travel along a tube, by reflecting off the inside walls. Thus a pulse, originally launched into one time slot, may weaken by losing energy or by spreading into neighbouring time slots, in the process contaminating them. The fibre designer makes a mathematical model of pulse propagation. Mathematics gives a precise definition.