3. Active components and non-linear effects

In this section, we present a number of components that are active or use non-linear effects involving changes in optical frequency. They use either phenomena induced by changes in fiber composition (rare-earth doping, microstructured fibers) or inelastic scattering due to inhomogeneities within the material. In the case of rare-earth-doped fibers, the material itself becomes a gain medium, offering the possibility of amplifying the signal to be transmitted and obtaining a lasing effect by looping the cavity. If the fiber is undoped, light scattering becomes stimulated when the fluctuations at the origin of the scattering are induced by the presence of electromagnetic waves, which then modify the optical characteristics of the medium.