In telecommunications, a signal can be transported in different ways: by air with radio links (electromagnetic waves), by an electrical signal (copper wire), but also by light (optical fiber). An optical fiber is made to transport light. Rays of light obey the laws of geometric optics. A ray propagates in a straight line in the fiber as long as the refractive index does not change. The laws of refraction apply as soon as the ray changes medium.
There are two fiber optic technologies. Step-index fibers and graded-index fibers.
Depending on the angle of the incident radius and its wavelength, the light follows a more or less long path. If the transmitted signal is an overlay of several rays (multimode), the difference in path for each of the rays causes dispersion. The reconstruction of the signal a few kilometers away is marred by errors (signal distortion). Each fiber technology has its advantages and disadvantages.