 distances of 1000 km. Moreover, a large bandwidth exists around the minimum attenuation wavelength where the attenuation remains very low. Thus, several channels can be transmitted at different wavelengths, realizing wavelength division multiplexing (WDM) systems. A commercial WDM system for long-haul applications can convey, just to give an order of magnitude, up to 150 channels at different wavelengths around 1.55 um each of which transmits a signal at 10 Gbit/s up to a distance of 3000 km. The main limitations to the reachable distance are the amplifiers' induced noise, single- mode propagation dispersion, and the nonlinear propagation effects. Optical amplifiers work generally in a quantum noise limit regime; thus, there is no means of avoiding noise generation. Noise power accumulates at each amplification and at the end overcomes the signal. Since the signal is modulated, different frequencies of the signal spectrum experience a different group velocity SO that further signal distortion is generated. Moreover, due to the small core area of fibers in which the main part of the optical power is confined, the transmission cannot be considered linear in several practical cases. Nonlinear effects create signal distortions, coupling among signals at different wave- lengths, and signal-noise coupling constituting another limitation to the reachable dis- tance and transmission speed. In this chapter, we will present a review of fiber optics technology in the two main appli- cations that are relevant for telecommunication networks: transmission fibers and optical fiber amplifiers. Telecommunication Networks We will add to the bulk of the chapter devoted to fiber technology the review of a set of filtering and wavelength management devices that are of wide use in fiber-based telecom- munication equipment. 4.2 Optical Fibers for Transmission An optical fiber is a circular dielectric glass waveguide that has a wide number of applica- tions in telecommunications, in sensor systems, in medicine, and SO on. Each applicat