Post on 08-Jan-2016
description
Optical communications
Chapter 2.3Chapter 2.3
Pham Quang Thai
Pqthai.hcmut@gmail.com
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Content
Geometrical optics
Optical propagation in fiber
Fiber fabrication
Signal degradation in fiber Signal degradation in fiber
Types of fiber
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Think-pair-share
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Signal degradation in fiber
Dispersion
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DispersionDispersion
IntermodalIntermodal Intramodal (chromatic)Intramodal (chromatic)Polarization
modePolarization
modeIntermodalIntermodal Intramodal (chromatic)Intramodal (chromatic)
MaterialMaterial WaveguideWaveguide
modemode
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Intermodal dispersion: different travel
times of modes
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Material dispersion: different travel
speed of wavelengths
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Waveguide dispersion: different travel
speed inside core and cladding
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Polarization mode dispersion: different
travel speed of polarization modes
Polarization mode dispersion (video)
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Polarization mode dispersion
Total pulse broadening due to fiber dispersion
DIM (ps/km)
Dm (ps/nm.km)
Dwg (ps/nm.km)wg
DPDM (ps/km)
L: length (km)
RMS: spectra width (nm)
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Dispersion
Total dispersion in a fiber
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Bragg reflection
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Sir William Lawrence BraggSir William Henry Bragg
Dispersion compensation
Fiber Bragg grating
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Dispersion compensation
Linearly chirped fiber grating
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Dispersion compensation
Linearly chirped fiber grating
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Dispersion compensation
Fabricate fiber Bragg grating (video)
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Wavelengths are delayed and reflected
at the same time
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Think-pair-share
Dim light
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Signal degradation in fiber
Attenuation
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AttenuationAttenuation
AbsorptionAbsorption ScatteringScattering BendingBendingAbsorptionAbsorption
ExtrinsicExtrinsic IntrinsicIntrinsic
ScatteringScattering
Mie scattering
Mie scattering
Rayleigh scatteringRayleigh
scatteringRaman
scatteringRaman
scatteringBrillouin
scatteringBrillouin
scattering
BendingBending
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Absorption Extrinsic
Impurity ions: Fe, Cu, Co, Ni, Cr, Mn
OH- ions
Intrinsic
Infrared absorption by silicon-oxygen bonds
Ultraviolet absorption by silica
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Mie scattering
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Rayleigh scattering
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Raman scattering: interaction between light waves and
the vibrational modes of silica molecules
Spontaneous process
Arbitrary direction
Brillouin scattering: light scatter from acoustic
waves
Thermal motions of atoms in a material
create acoustic vibrations, which lead to
density variations.
The incident light is reflected from a three-
dimensional diffraction grating (Bragg
reflection)
Mie scattering
Elastic
Irregularities interface
Rayleightscattering
Elastic
Density fluctuation
Raman scattering
Inelastic
Molecule vibration
Both
Brillouinscattering
Inelastic
Propagating acoustic wave
Forward BackwardBoth
directionsBackward
Total attenuation in fiber
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Nowadays, most optical devices utilize the C and L bands
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Bending loss
Video: signal degradation in fiber
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Content
Geometrical optics
Optical propagation in fiber
Fiber fabrication
Signal degradation in fiber others nonlinear Signal degradation in fiber others nonlinear
effects
Types of fiber
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Self phase modulation (SPM): signal phase changes
with signal intensity (Kerr effect)
Kerr effect: refractive index changes with optical power
intensity
SPM: cause pulse broadening
Cross phase modulation (XPM): power fluctuation in
one channel is converted to phase fluctuation in other
channels
XPM causes pulse broadening and crosstalk
Four wave mixing (FWM): three optical frequencies mix
to produce a fourth intermodulation frequency
Channel spacing and fiber dispersion are small enough
( ) products mixing 121
channels N
2
+=
NN
ffff kjiijk
FWM: causes crosstalk in WDM system
With equal channel spacing and fiber dispersion are small
enough
Mitigate with:
Unequal spacing
Dispersion management