Optical fibers

Advantages of optical fiber communicationWide bandwidth 2*1012 Hz to 37*1012 HzLow losses 1dB/kmImmune to cross talk non conductors,no magnetic fieldIntereference immuneno conductive or radiative interferenceLight weightMaterial used is glass and plastic,lesser than copper and aluminiumSmall sizeDiameter of OFC is smaller than copperMore strengthStronger and ruggedSecurityImpossible to tapLong distance transmissionLess attenuation ,long distance transmissionEnviornment immuneNot affected by corrosive liquids or gasesSafe and easy installationNo shock hazardLong term

The Nature of Light Ray Theory: Light travels along astraight line and obeys laws ofgeometrical optics. Ray theory is validwhen the objects are much larger thanthe wavelength.Quantum Theory: Light consists ofsmall particles (photons)Wave Theory: Light travels as atransverse electromagnetic wave

Introduction to Optical fibersAn optical fiber is essentially a waveguide for light.

It consists of a core and cladding that surrounds the core.

The index of refraction of the cladding is less than that of the core, causing rays of light leaving the core to be refracted back into the core.

A light-emitting diode (LED) or laser diode (LD) can be used for the source.

Optical fiber structure

Types of FiberModes :Single mode propagationMultimode propagation

Configuration:Step indexGraded index

Graded-index fiber has less dispersion than a multimode step-index fiber

Comparison of fiber structures

Ray optic representationSkew raysMeridional ray representation

Signal Degradation in Optical Fibers

AttenuationAttenuation means loss of light energy as the light pulse travels from one end of the cable to the other.

It is also called as signal loss or fiber loss.

It also decides the the number of repeaters required between transmitter and receiver.

Attenuation is directly proportional to the length of the cable.

Attenuation is defined as the ratio of optical output power to the input power in the fiber of length L.

Attenuation (fiber loss)

Power loss along a fiber:

The parameter is called fiber attenuation coefficient in a units of for example [1/km] or [nepers/km]. A more common unit is [dB/km] that is defined by:Z=0P(0) mWZ= l

mw

Attenuation:

Variation of specific attenuation with wavelength

Attenuation & WavelengthThe specific attenuation ( power loss in dB per unit length ) actually depends on the wavelength of the radiation travelling along the optic fibreThe graph shows minima at 1310nm and 1550nm, which implies that these are desirable wavelengths for optimal transmissionThese are infra red wavelengths

Intrinsic Absorption:Caused by the interaction with one or more components of the glassOccurs when photon interacts with an electron in the valence band & excites it to a higher energy level near the UV region.Extrinsic Absorption:Also called impurity absorption.Results from the presence of transition metal ions like iron, chromium, cobalt, copper & from OH ions i.e. from water.

Scattering LossesIt occurs due to microscopic variations in the material density, compositional fluctuations, structural in homogeneities and manufacturing defects.Linear ScatteringRayleigh Scattering lossesMie Scattering LossesWaveguide Scattering Losses

Non-linear ScatteringStimulated Brillouin Scattering(SBS)Stimulated Raman Scattering(SRS)

i) Linear ScatteringRayleigh Scattering Losses:These losses are due to microscopic variation in the material of the fiber.

Unequal distribution of molecular densities or atomic densities leads to Rayleigh Scattering losses

Glass is made up of several acids like SiO2, P2O5,etc. compositions, fluctuations can occur because of these several oxides which rise to Rayleigh scattering losses

b) Mie Scattering Losses:These losses results from the compositional fluctuations & structural inhomogenerics & defects created during fiber fabrications, causes the light to scatter outside the fiber.c) Waveguide Scattering Losses:It is a result of variation in the core diameter, imperfections of the core cladding interface, change in RI of either core or cladding.

ii) Non-linear ScatteringSBS Scattering:Stimulated Brillouin Scattering(SBS) may be regarded as the modulation of light through thermal molecular vibrations within the fiber.

Pb =4.4x10-3d22 dB v wattswhere, = operating wavelength md= fiber core diameter m v = source bandwidth in GHz

b) SRS Scattering:Stimulated Raman Scattering is similar to SBS except that high frequency optical phonon rather than acoustic phonon is generated in scattering processes.Pb =5.9x10-2d2 dB watts

Phonon:Collective excitation in a periodic arrangement of atoms or molecules in solid.

Bending Loss (Macrobending & Microbending)Macrobending Loss: The curvature of the bend is much larger than fiber diameter. Lightwave suffers sever loss due to radiation of the evanescent field in the cladding region. As the radius of the curvature decreases, the loss increases exponentially until it reaches at a certain critical radius. For any radius a bit smaller than this point, the losses suddenly becomes extremely large. Higher order modes radiate away faster than lower order modes.

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

Microbending Loss

Microbending Loss: microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables. The power is dissipated through the microbended fiber, because of the repetitive coupling of energy between guided modes & the leaky or radiation modes in the fiber.

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

DispersionDispersion in fiber optics results from the fact that in multimode propagation, the signal travels faster in some modes than it would in others

Single-mode fibers are relatively free from dispersion except for intramodal dispersion

Graded-index fibers reduce dispersion by taking advantage of higher-order modes

One form of intramodal dispersion is called material dispersion because it depends upon the material of the coreAnother form of dispersion is called waveguide dispersionDispersion increases with the bandwidth of the light source

Examples of Dispersion

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