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The light-guiding properties and transmission distance range capability of optical fibers are affected by the waveguide and materials properties (both intrinsic and extrinsic) from which the fibers are made. Silica glass fibers have an intrinsic optical absorption loss that is the result of the interplay of several mechanisms such as UV and IR absorption, Rayleigh scattering, as well as absorption by impurities and metal ions. Typically, the absorptions introduced by these mechanisms intersect each other producing a sort of V-shaped optical transmission spectrum. Figure 6.1 illustrates the optical transmission spectrum for a silica glass fiber, where it can be appreciated that the short-wavelength absorption tail is the result of the UV absorption, while the bottom attenuation is limited by the Rayleigh scattering, and the long wavelength tail is the result of the IR absorption. A minimum absorption wavelength region exists for which maximum transmission distance in a fiber can be achieved. This low-attenuation point occurs around 1.55 μm and has a value of ~0.15 dB/km. Historically, light sources emitting in the 850 nm and 1310 nm windows were first used in fiber optic communication systems and, to date, some fiber sensing systems still operate at those wavelengths. However, most current fiber optic communication and sensing systems operate in the C-band window - for communications band - spanning from 1528 nm to 1561 nm.
Among the multiple factors contributing to the optical loss in a fiber are: absorption from water (OH- ions) and transition metal impurities; scattering produced by inhomogeneities and density irregularities in the fiber's core glass; core-cladding boundary irregularities, micro- and macrobending losses; as well as joint and coupling losses. These loss mechanisms are illustrated in Fig. 6.2, and at each point in the fiber they reduce the amount of energy traveling forward by light lost by radiation or back reflected in the opposite direction-more on this point will be discussed in the next section.
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