Fiber Loss

The deterioration of the light signal in transmission through a fiber optic cable.
The various losses in the fiber are due to absorption, scattering, dispersion and bending.
Fibre bending produces at least two loss mechanisms. In multimode fibers, the number of propagating modes is reduced as a function of the fibre bend radius. An additional problem when fibres are bent is electromagnetic radiation loss due to differences in propagation (wave front) velocity. All these losses contribute to increased attenuation. For multimode fibres, this effect is relatively small when compared to the reduction of propagating modes. This type of attenuation can seriously affect single mode fibres when they are bent.
Absorption losses are largely due to impurities in glass material from residual often unwanted atoms and hydrogen/oxygen molecules. Unwanted impurities include metal ions such as Cr3+, Fe2+ and Cu2+, absorbing between 500 nm and 1000 nm.
Dispersion Loss: as an optical signal travels along the fiber, it becomes increasingly distorted. This distortion is a sequence of intermodal and intramodal dispersion. Intermodal dispersion is a distortion mechanism occurring in multimode fibers in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes. Intramodal dispersion is the pulse spreading that occurs within a single mode. Material dispersion or spectral dispersion or chromatic dispersion results because of variation due to the refractive index of the core as a function of wavelength, because of which pulse spreading occurs even when different wavelengths follow the same path. Waveguide Dispersion is whenever any optical signal is passed through the optical fiber, practically 80% of the optical power is confined to the core and the rest 20% optical power into the cladding.
Scattering losses occurs due to microscopic variations in the material density, compositional fluctuations, structural inhomogeneities and manufacturing defects. One significant scattering mechanism at low wavelengths is Rayleigh scattering. Spatially there are high density gradients which alter refraction index and thereby cause scattering. The effect evidences itself in, among other things, strong reverse scattering. Another scattering mechanism is Mie scattering, which mainly results in forward scattering. This mechanism comes from material inhomogeneities at longer wavelengths. Waveguide Scattering Losses is a result of variation in the core diameter, imperfections of the core cladding interface, change in RI of either core or cladding. Finally, Stimulated Raman Scattering and Stimulated Brillouin Scattering are non-linear radiation induced effects, which take place when particular intensity thresholds are exceeded. In practice, such non-linear effects only take place when high intensity laser light is transmitted.


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