 nm (the wavelengths at which the fiber loss is minimum), the only modes that can propagate are the LP01. 4.2.2 Fiber Losses We have seen that the primary reason for the success of optical fiber as communication medium is the extremely low loss. Thus, studying loss mechanisms is very important. When the light is injected into the fiber from an external light source and propagates along the fiber for a long span, the output light power is reduced with respect to the source emitted power by two kinds of losses: coupling losses and propagation losses. Coupling losses are due to the fact that the field shape of the incoming light on the input fiber facet does not coincide with the fundamental mode profile, SO that both radiation modes and below-threshold propagation modes are excited. After a very short fiber length Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices only the fundamental mode remains, and all the power coupled with other modes is radi- ated out of the fiber core. During propagation along the fiber, the fundamental mode power also decreases, due to propagation losses, to produce at the fiber output the attenuated output power. 4.2.2.1 Coupling Losses Generally in a transmission system, light is injected into a single-mode fiber from a single- mode semiconductor laser source [4,5]. We will see dealing with lasers that the section of the waveguide composing the active region of a single-mode semiconductor laser is quite smaller than the section of a fiber (reference dimensions are 1 um X 100 nm for the square section active waveguide of a laser versus a diameter of 10 um for the fiber core). In this condition, a focusing system is needed to achieve good light injection into the fiber. The focus system can be designed to focus the laser beam at the center of the input facet of the fiber. In this condition, light injection is performed as from a point source. Since a point source emits a spherical wave, the coupling between the injected field and the suitably normalized fund