Thursday, March 1, 2018

Signal Distortion in Optical Fibers Part 3: Rayleigh Scattering Losses

. Scattering losses exists in optical fibers because of microscopic variations in the material density and composition. As glass is composed by randomly connected network of molecules and several oxides (e.g. SiO2, GeO2 and P2O5, these are the major cause of compositional structure fluctuation. These two effects results to variation in refractive index and Rayleigh type scattering of light. 
. Rayleigh scattering of light is due to small localized changes in the refractive index of the core and cladding material. There are two causes during the manufacturing of fiber. 
. The first is due to slight fluctuation in mixing of ingredients. The random changes because of this are impossible to eliminate completely. 
. The other cause is slight change in density as the silica cools and solidifies. When light ray strikes such zones it gets scattered in all directions. The amount of scatter depends on the size of the discontinuity compared with the wavelength of the light so the shortest wave length (highest frequency) suffers most scattering. Fig. 1 shows graphically the relationship between wavelength and Rayleigh scattering loss. 

. Scattering loss for single component glass is given by, 

n = Refractive index 
k = Boltzmann's constant 
T = Isothermal compressibility of material
Tf = Temperature at which density fluctuations are frozen into the glass as it solidifies (fictive temperature) 
       Another form of equation is 

       Where,  p = Photoelastic coefficient 
. Scattering loss for multicomponent glasses is given by, 

            δ2n = Mean square refractive index fluctuation 
            δv = Volume of fiber 
. Multimode fibers have higher dopant concentrations and greater compositional fluctuations. The overall losses in this fibers are more as compared to single mode fibers. 
Mie Scattering :
 . Linear scattering also occurs at inhomogenities and these arise from imperfections in the fiber's geometry, irregularities in the refractive index and the presence of bubbles etc. caused during manufacture. Careful control of manufacturing process can reduce mie scattering to insignificant levels.

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