Imagine we launch two different rays of light into a fiber. Now, since both rays are traveling in material of the same refractive index they must be moving at the same speed. If we follow two different rays of light which have entered the core at the same time, we can see that one of them, Ray A in Figure below, will travel a longer distance than the other, Ray B. The effect of this is to cause the pulse of light to spread out as it moves along the fiber – as the ray taking the shorter route overtakes the other. This spreading effect is called dispersion (see below)

The data can be corrupted by dispersion. If we send a sequence of ON-OFF-ON pulses, it would start its life as an electronic signal with nice sharp edges as in Figure below:

These pulses are used to switch a light source, usually an LED or a laser and the resultant pulses of light are launched into the fiber.

Dispersion causes the pulses to spread out and eventually they will blend together and the information will be lost (Figure below).

We could make this degree of dispersion acceptable by simply decreasing the transmission frequency and thus allowing larger gaps between the pulses.

This type of dispersion is called intermodal dispersion. At this point, we will take a brief detour to look at the idea of modes.