ength, we will define the attenuation through the attenuation parameter a defined in such a way that, if L is the fiber length, P the power coupled at the input with the fundamental mode, and P1 the output power, it is P1 = Poe-aL (4.6) Frequently, the fiber loss is also characterized by the attenuation for a kilometer of fiber, measured in dB/km. The two parameters are directly related as can be easily derived. A typical measure of the attenuation parameter of a step index fiber is reported in Figure 4.2, where the different contributions are also evidenced. From the figure, it results that in a standard fiber, the low attenuation zone is divided into two parts by the hydroxyl absorption peak: a zone around 1.3 um and a zone around Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices OH peaks Experiment Infrared absorption Rayleigh scatt. Ultraviolet absorption Fiber defects Wavelength (nm) FIGURE 4.2 Loss contributions and total loss versus wavelength in an SSMF for signal transmission. 1.5 um. For historical reasons, the two zones are called second and third transmission windows. For a few applications, the hydroxyl (OH) absorption peaks are particularly problematic. In this case, the so-called zero water fibers can be used, where the hydroxyl ion content is SO rigorously controlled that the correspondent absorption peaks are eliminated. 4.2.3 Linear Propagation in an Optical Fiber In a real fiber, attenuation is not the only phenomenon causing the difference between the field coupled at the fiber input and the field emerging from the output. In this section, we will assume that the power of the coupled field is sufficiently low that all propagation nonlinear effects are negligible. In this hypothesis, propagation along the fiber is purely linear and the fiber itself can be modeled as a linear distributed system. In this condition, if the field coupled to the fiber at the input facet is a combination of the two main fiber modes, then (4.7) The field at the output will be obtained 