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Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance traveled through a transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the very high quality of transparency of modern optical transmission media. The medium is usually a fiber of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a signal across large distances. In optical fibers the main attenuation source is scattering from molecular level irregularities (Rayleigh scattering) due to structural disorder and compositional fluctuations of the glass structure. This same phenomenon is seen as one of the limiting factors in the transparency of infrared missile domes. Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding. Light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation.
• Multi-phonon absorption
The design of any optically transparent device requires the selection of materials based upon knowledge of their properties and limitations. The lattice absorption characteristics observed at the lower frequency regions (mid-infrared to far-infrared wavelength range) define the long-wavelength transparency limit of the material. They are the result of the interactive coupling between the motions of thermally induced vibrations of the constituent atoms and molecules of the solid lattice and the incident light wave radiation. Hence, all materials are bounded by limiting regions of absorption caused by atomic and molecular vibrations (bond-stretching) in the far-infrared spectral region (>10 µm).
The concepts of temperature and thermal equilibrium associated with ionic solids are based on individual atoms and molecules in the system possessing vibrational motion. The frequencies of the normal modes of a system are known as its natural frequencies or resonant frequencies. These thermal vibrational modes are associated with atomic and molecular displacements, producing both longitudinal and transverse waves of atomic and molecular displacement.