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.
- The step-index fiber is a fiber whose refractive index is uniform in the core (index n1) and in the sheath (index n2) with n1 > n2. As long as the incident radius remains in the acceptance cone, the light propagates along the axis of the fiber in a zigzag pattern by undergoing total reflections at each core/sheath interface.
- The graded-index fiber is a fiber whose refractive index in the core decreases continuously from the center to the edges. Light propagates along the fiber axis by undergoing multiple refractions inside the core. The path of light looks more like a sinusoid than triangles.
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.