Hi Umer,
Dispersion is the spreading of light pulse as its travels down the length of an optical fiber. Dispersion limits the bandwidth or information carrying capacity of a fiber. The bit-rates must be low enough to ensure that pulses are farther apart and therefore the greater dispersion can be tolerated.
There are three main types of dispersion in a fiber:
Modal Dispersion.
Material dispersion.
Waveguide dispersion.
Modal dispersion occurs only in Multimode fibers. It arises because rays follow different paths through the fiber and consequently arrive at the other end of the fiber at different times.
Different wavelengths also travel at different velocities through a fiber, in the same mode, as
n = c/v.
where n is index of refraction, c is the speed of light in vacuum and v is the speed of the same wavelength in the material.
Waveguide dispersion, most significant in a single- mode fiber, occurs because optical energy travels in both the core and cladding, which have slightly different refractive indices.
A signal is said to be undistorted if it is delayed in time and scaled version of the original signal.
So, in general the signals get distorted on an optical fiber because different frequency components undergo different attenuation and different delays.
Thanks