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, 

Where, 

n = Refractive index 

k = Boltzmann's constant 

ᵦ_T = Isothermal compressibility of material

T_f = 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, 

       Where,

            δ²_n = Mean square refractive index fluctuation 

            δ_v = 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.

Signal Distortion in Optical Fibers Part 2: Absorption

. Absorption loss is related to the material composition and fabrication process of fiber. Absorption loss results in dissipation of some optical power as heat in the fiber cable. Although glass fibers are extremely pure, some impurities still remain as residue after purification. The amount of absorption by these impurities depend on their concentration and light wavelength.

. Absorption is caused by three different mechanisms.

1) Absorption by atomic defects in glass composition.

2) Extrinsic absorption by impurity atoms in glass matts.

3) Intrinsic `absorption by basic constituent atom of fiber.

1. Absorption by Atomic Defects

. Atomic defects are imperfection in the atomic structure of the fiber materials such as missing molecules, high density clusters of atom groups. These absorption losses are negligible compared with intrinsic and extrinsic losses.

. The absorption effect is most significant when fiber is exposed to ionizing radiation in nuclear reactor, medical therapies, space missions etc. the radiation damages the internal structure of fiber. The damages are proportional to the intensity of ionizing particles. This results in increasing attenuation due to atomic defects and absorbing optical energy. The total does a material receives is expressed in rad (Si), this is the unit for measuring radiation absorbed in bulk silicon.

1 rad (Si) = 0.01 J/kg

       The higher the radiation intensity more the attenuation as shown in Fig. 1.

2. Extrinsic Absorption

. Extrinsic absorption occurs due to electronic transitions between the energy levels and because of charge transitions from one ion to another. A major source of attenuation is from transition of metal impurity ions such as iron, chromium, cobalt and copper. These losses can be upto 1 to 10 dB/km. the effect of metallic impurities can be reduced by glass refining techniques.

. Another major extrinsic loss is caused by absorption due to OH (Hydroxil0 ions impurities dissolved in glass. Vibrations occur at wavelengths between 2.7 and 4.2 µm. the absorption peaks occurs at 1400, 950 and 750 nm. These are first, second and third overtones respectively.

. Fig. 2 shows absorption spectrum for OH group in silica. Between these absorption peaks there are regions of low attenuation.

3. Intrinsic Absorption

. Intrinsic absorption occurs when material is in absolutely pure state, no density variation and inhomogenities. Thus intrinsic absorption sets  the fundamental lower limit on absorption for any particular material.

. Intrinsic absorption results from electronic absorption bands in UV region and from atomic vibration bands in the near infrared region.

. The electronic absorption bands are associated with the band gaps of amorphous glass materials. Absorption occurs when a photon interacts with an electron in the valence band and excites it to a higher energy level. UV absorption decays exponentially with increasing wavelength (λ).

. In the IR (infrared) region above 1.2 µm the optical waveguide loss is determined by presence of the OH ions and inherent IR absorption of the constituent materials. The inherent IR absorption is due to interaction between the vibrating band and the electromagnetic field of optical signal this results in transfer of energy from field to the band, thereby giving rise to absorption, this absorption is strong because of many bonds present in the fiber.

. Attenuation spectra for the intrinsic loss mechanism in pure Ge is shown in Fig. 3.

.The ultraviolet loss at any wavelength is expressed as,

       Where, x is mole fraction of GeO₂.

       λ is operating wavelength

       α_uv is in dB/km

. The loss in infrared (IR) region (above 1.2 µm) is given by expression :

       The expression is derived for GeO₂ – SiO₂ glass fiber.

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, α_p 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.