TTQ 2-3 Signal Degradation
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Transcript of TTQ 2-3 Signal Degradation
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Optical communications
Chapter 2.3Chapter 2.3
Pham Quang Thai
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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
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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
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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)
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Mie scattering
Elastic
Irregularities interface
Rayleightscattering
Elastic
Density fluctuation
Raman scattering
Inelastic
Molecule vibration
Both
Brillouinscattering
Inelastic
Propagating acoustic wave
Forward BackwardBoth
directionsBackward
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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
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SPM: cause pulse broadening
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Cross phase modulation (XPM): power fluctuation in
one channel is converted to phase fluctuation in other
channels
XPM causes pulse broadening and crosstalk
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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
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FWM: causes crosstalk in WDM system
With equal channel spacing and fiber dispersion are small
enough
Mitigate with:
Unequal spacing
Dispersion management
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