Thursday, March 1, 2018

Signal Distortion in Optical Fibers Part 1: Attenuation

. Attenuation is a measure of decay of signal strength or loss of light power that occurs as light pulses propagate through the length of the fiber.
. In optical fibers the attenuation is mainly caused by two physical factors absorption and scattering losses. Absorption is because of fiber material and scattering due to structure imperfection s within the fiber. Nearly 90 % of optical fiber also contributes to the attenuation of signal.
. The rate at which light is absorbed is dependent on the wave length of the light and the characteristics of particular glass. Glass is a silicon compound, by adding different additional chemicals to the basic silicon dioxide the optical properties of the glass can be changed.
. The Rayleigh scattering is wavelength dependent and reduces rapidly as the wavelength of the incident radiation increases.
. The attenuation of fiber is governed by the materials from which it is fabricated, the manufacturing process and the refractive index profile chosen. Attenuation loss is measured in dB/km.
1. Attenuation Units
. As attenuation leads to a loss of power along the fiber, the output power is significantly less than the coupled power. Let the coupled optical power is P(0) i.e. at origin (z=0).
Then the power at distance z is given by,

Where, αp is fiber attenuation constant (per km),

This parameter is known as fiber loss or fiber attenuation.
. Attenuation is also a function of wavelength. Optical fiber wavelength as a function of wavelength is shown in Fig. 1.

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