fiber optic cables are built for outdoor applications. Loose tube cables typically are used for outside-plant installation in aerial, duct, and direct-buried applications. Telecommunication Networks 4.3 Optical Fiber Amplifiers In this section, we will describe optical fiber-based amplifiers for telecommunication applications. A specific attention to optical amplifiers is justified by the fact that design of single ampli- fiers and of an amplifiers cascade is quite different from what happens in the electronics domain. As a matter of fact, optical amplifiers behavior is dominated by the quantum mechanical properties of the light field and a quantum mechanical approach is required at least to set the main amplifier properties. 4.3.1 Basic Theory of Optical Amplifiers Due to the specific properties of optical amplifiers, before dealing with particular devices and technologies it is important to fix a few characteristics that are common to all amplifi- ers working in a regime where thermal noise is negligible and the amplifier signal to noise ratio is determined by quantum noise. These characteristics can be derived directly from the base principles of quantum mechanics applied to a system that has the characteristic to multiply the input signal by a constant greater than one, called gain. Thus, in this section we will review the basic theory of quantum amplifiers [44], thus placing the bases for the analysis of specific amplifiers design. 4.3.1.1 Quantum Noise An optical amplifier can be seen as a system designed to multiply the input optical power by a gain greater than one. In order to transfer this definition in a quantum mechanical formulation, let us start from the expression of the quantized basic mode of the fiber. The modal fiber analysis is not altered by the second quantization of the optical field [45] and we will assume in this SO basic an analysis that propagation will be single polarization and that the fiber mode can be approximated as a plane wave. In the Heisenberg representation [45], a single fre