LIGHT LIGHT COMMUNICATIONCOMMUNICATION
Fiber vs Metallic Fiber vs Metallic CablesCables
AdvantagesAdvantagesbull Larger bandwidth Larger bandwidth bull Immune to cross-Immune to cross-
talktalkbull Immune to static Immune to static
interferenceinterferencebull Do not radiate RFDo not radiate RFbull spark freespark freebull No corrosion No corrosion
more environment more environment resistiveresistive
DisadvantagesDisadvantagesbull Initial cost of Initial cost of
installation highinstallation highbull BrittleBrittlebull Maintenance and Maintenance and
repair more repair more difficult and more difficult and more expensiveexpensive
Typical Fiber Optical Typical Fiber Optical Communication SystemCommunication System
Elements of a Fiber Data LinkElements of a Fiber Data Link
Transmitter emits light pulses (LED Transmitter emits light pulses (LED or Laser)or Laser)
Connectors and Cables passively Connectors and Cables passively carry the pulsescarry the pulses
Receiver detects the light pulsesReceiver detects the light pulses
Transmitter ReceiverCable
RepeatersRepeaters For long links repeaters are needed For long links repeaters are needed
to compensate for signal lossto compensate for signal loss
FiberRepeaterRepeater Repeater
Fiber FiberFiber
Optical FiberOptical Fiber CoreCore
bull Glass or plastic with a higher Glass or plastic with a higher index of refraction than the index of refraction than the claddingcladding
bull Carries the signalCarries the signal
CladdingCladdingbull Glass or plastic with a lower Glass or plastic with a lower
index of refraction than the coreindex of refraction than the core
BufferBufferbull Protects the fiber from damage Protects the fiber from damage
and moistureand moisture
JacketJacketbull Holds one or more fibers in a Holds one or more fibers in a
cablecable
Singlemode FiberSinglemode Fiber Singlemode fiber has a core diameter Singlemode fiber has a core diameter
of 8 to 9 microns which only allows of 8 to 9 microns which only allows one light path or modeone light path or modebull Images from arcelectcom (Link Ch 2a)Images from arcelectcom (Link Ch 2a)
Index of refraction
Multimode Step-Index FiberMultimode Step-Index Fiber Multimode fiber has a core diameter Multimode fiber has a core diameter
of 50 or 625 microns (sometimes of 50 or 625 microns (sometimes even larger)even larger)bull Allows several light paths or modesAllows several light paths or modesbull This causes This causes modal dispersionmodal dispersion ndash some modes ndash some modes
take longer to pass through the fiber than others take longer to pass through the fiber than others because they travel a longer distancebecause they travel a longer distance
bull See animation at link Ch 2fSee animation at link Ch 2fIndex of refraction
Multimode Graded-Index FiberMultimode Graded-Index Fiber The index of refraction gradually The index of refraction gradually
changes across the corechanges across the corebull Modes that travel further also move fasterModes that travel further also move fasterbull This reduces This reduces modal dispersionmodal dispersion so the so the
bandwidth is greatly increasedbandwidth is greatly increased
Index of refraction
AttenuationAttenuation AbsorptionAbsorption
bull interaction of light with electrons amp molecule vibrationinteraction of light with electrons amp molecule vibration Rayleigh ScatteringRayleigh Scattering
bull caused by compositional fluctuations in glass material caused by compositional fluctuations in glass material Energy escapes not convertedEnergy escapes not converted
Material FabricationMaterial Fabricationbull caused impurities (transition metal ions)caused impurities (transition metal ions)
Fiber FabricationFiber Fabricationbull caused by fiber imperfections (defectsstresses) Leads caused by fiber imperfections (defectsstresses) Leads
to Mie scattering which is to Mie scattering which is independent independent DeploymentEnvironmentalDeploymentEnvironmental
bull caused by bends and microbends caused by bends and microbends Leads to mode conversions Leads to mode conversions
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Fiber vs Metallic Fiber vs Metallic CablesCables
AdvantagesAdvantagesbull Larger bandwidth Larger bandwidth bull Immune to cross-Immune to cross-
talktalkbull Immune to static Immune to static
interferenceinterferencebull Do not radiate RFDo not radiate RFbull spark freespark freebull No corrosion No corrosion
more environment more environment resistiveresistive
DisadvantagesDisadvantagesbull Initial cost of Initial cost of
installation highinstallation highbull BrittleBrittlebull Maintenance and Maintenance and
repair more repair more difficult and more difficult and more expensiveexpensive
Typical Fiber Optical Typical Fiber Optical Communication SystemCommunication System
Elements of a Fiber Data LinkElements of a Fiber Data Link
Transmitter emits light pulses (LED Transmitter emits light pulses (LED or Laser)or Laser)
Connectors and Cables passively Connectors and Cables passively carry the pulsescarry the pulses
Receiver detects the light pulsesReceiver detects the light pulses
Transmitter ReceiverCable
RepeatersRepeaters For long links repeaters are needed For long links repeaters are needed
to compensate for signal lossto compensate for signal loss
FiberRepeaterRepeater Repeater
Fiber FiberFiber
Optical FiberOptical Fiber CoreCore
bull Glass or plastic with a higher Glass or plastic with a higher index of refraction than the index of refraction than the claddingcladding
bull Carries the signalCarries the signal
CladdingCladdingbull Glass or plastic with a lower Glass or plastic with a lower
index of refraction than the coreindex of refraction than the core
BufferBufferbull Protects the fiber from damage Protects the fiber from damage
and moistureand moisture
JacketJacketbull Holds one or more fibers in a Holds one or more fibers in a
cablecable
Singlemode FiberSinglemode Fiber Singlemode fiber has a core diameter Singlemode fiber has a core diameter
of 8 to 9 microns which only allows of 8 to 9 microns which only allows one light path or modeone light path or modebull Images from arcelectcom (Link Ch 2a)Images from arcelectcom (Link Ch 2a)
Index of refraction
Multimode Step-Index FiberMultimode Step-Index Fiber Multimode fiber has a core diameter Multimode fiber has a core diameter
of 50 or 625 microns (sometimes of 50 or 625 microns (sometimes even larger)even larger)bull Allows several light paths or modesAllows several light paths or modesbull This causes This causes modal dispersionmodal dispersion ndash some modes ndash some modes
take longer to pass through the fiber than others take longer to pass through the fiber than others because they travel a longer distancebecause they travel a longer distance
bull See animation at link Ch 2fSee animation at link Ch 2fIndex of refraction
Multimode Graded-Index FiberMultimode Graded-Index Fiber The index of refraction gradually The index of refraction gradually
changes across the corechanges across the corebull Modes that travel further also move fasterModes that travel further also move fasterbull This reduces This reduces modal dispersionmodal dispersion so the so the
bandwidth is greatly increasedbandwidth is greatly increased
Index of refraction
AttenuationAttenuation AbsorptionAbsorption
bull interaction of light with electrons amp molecule vibrationinteraction of light with electrons amp molecule vibration Rayleigh ScatteringRayleigh Scattering
bull caused by compositional fluctuations in glass material caused by compositional fluctuations in glass material Energy escapes not convertedEnergy escapes not converted
Material FabricationMaterial Fabricationbull caused impurities (transition metal ions)caused impurities (transition metal ions)
Fiber FabricationFiber Fabricationbull caused by fiber imperfections (defectsstresses) Leads caused by fiber imperfections (defectsstresses) Leads
to Mie scattering which is to Mie scattering which is independent independent DeploymentEnvironmentalDeploymentEnvironmental
bull caused by bends and microbends caused by bends and microbends Leads to mode conversions Leads to mode conversions
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Typical Fiber Optical Typical Fiber Optical Communication SystemCommunication System
Elements of a Fiber Data LinkElements of a Fiber Data Link
Transmitter emits light pulses (LED Transmitter emits light pulses (LED or Laser)or Laser)
Connectors and Cables passively Connectors and Cables passively carry the pulsescarry the pulses
Receiver detects the light pulsesReceiver detects the light pulses
Transmitter ReceiverCable
RepeatersRepeaters For long links repeaters are needed For long links repeaters are needed
to compensate for signal lossto compensate for signal loss
FiberRepeaterRepeater Repeater
Fiber FiberFiber
Optical FiberOptical Fiber CoreCore
bull Glass or plastic with a higher Glass or plastic with a higher index of refraction than the index of refraction than the claddingcladding
bull Carries the signalCarries the signal
CladdingCladdingbull Glass or plastic with a lower Glass or plastic with a lower
index of refraction than the coreindex of refraction than the core
BufferBufferbull Protects the fiber from damage Protects the fiber from damage
and moistureand moisture
JacketJacketbull Holds one or more fibers in a Holds one or more fibers in a
cablecable
Singlemode FiberSinglemode Fiber Singlemode fiber has a core diameter Singlemode fiber has a core diameter
of 8 to 9 microns which only allows of 8 to 9 microns which only allows one light path or modeone light path or modebull Images from arcelectcom (Link Ch 2a)Images from arcelectcom (Link Ch 2a)
Index of refraction
Multimode Step-Index FiberMultimode Step-Index Fiber Multimode fiber has a core diameter Multimode fiber has a core diameter
of 50 or 625 microns (sometimes of 50 or 625 microns (sometimes even larger)even larger)bull Allows several light paths or modesAllows several light paths or modesbull This causes This causes modal dispersionmodal dispersion ndash some modes ndash some modes
take longer to pass through the fiber than others take longer to pass through the fiber than others because they travel a longer distancebecause they travel a longer distance
bull See animation at link Ch 2fSee animation at link Ch 2fIndex of refraction
Multimode Graded-Index FiberMultimode Graded-Index Fiber The index of refraction gradually The index of refraction gradually
changes across the corechanges across the corebull Modes that travel further also move fasterModes that travel further also move fasterbull This reduces This reduces modal dispersionmodal dispersion so the so the
bandwidth is greatly increasedbandwidth is greatly increased
Index of refraction
AttenuationAttenuation AbsorptionAbsorption
bull interaction of light with electrons amp molecule vibrationinteraction of light with electrons amp molecule vibration Rayleigh ScatteringRayleigh Scattering
bull caused by compositional fluctuations in glass material caused by compositional fluctuations in glass material Energy escapes not convertedEnergy escapes not converted
Material FabricationMaterial Fabricationbull caused impurities (transition metal ions)caused impurities (transition metal ions)
Fiber FabricationFiber Fabricationbull caused by fiber imperfections (defectsstresses) Leads caused by fiber imperfections (defectsstresses) Leads
to Mie scattering which is to Mie scattering which is independent independent DeploymentEnvironmentalDeploymentEnvironmental
bull caused by bends and microbends caused by bends and microbends Leads to mode conversions Leads to mode conversions
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Elements of a Fiber Data LinkElements of a Fiber Data Link
Transmitter emits light pulses (LED Transmitter emits light pulses (LED or Laser)or Laser)
Connectors and Cables passively Connectors and Cables passively carry the pulsescarry the pulses
Receiver detects the light pulsesReceiver detects the light pulses
Transmitter ReceiverCable
RepeatersRepeaters For long links repeaters are needed For long links repeaters are needed
to compensate for signal lossto compensate for signal loss
FiberRepeaterRepeater Repeater
Fiber FiberFiber
Optical FiberOptical Fiber CoreCore
bull Glass or plastic with a higher Glass or plastic with a higher index of refraction than the index of refraction than the claddingcladding
bull Carries the signalCarries the signal
CladdingCladdingbull Glass or plastic with a lower Glass or plastic with a lower
index of refraction than the coreindex of refraction than the core
BufferBufferbull Protects the fiber from damage Protects the fiber from damage
and moistureand moisture
JacketJacketbull Holds one or more fibers in a Holds one or more fibers in a
cablecable
Singlemode FiberSinglemode Fiber Singlemode fiber has a core diameter Singlemode fiber has a core diameter
of 8 to 9 microns which only allows of 8 to 9 microns which only allows one light path or modeone light path or modebull Images from arcelectcom (Link Ch 2a)Images from arcelectcom (Link Ch 2a)
Index of refraction
Multimode Step-Index FiberMultimode Step-Index Fiber Multimode fiber has a core diameter Multimode fiber has a core diameter
of 50 or 625 microns (sometimes of 50 or 625 microns (sometimes even larger)even larger)bull Allows several light paths or modesAllows several light paths or modesbull This causes This causes modal dispersionmodal dispersion ndash some modes ndash some modes
take longer to pass through the fiber than others take longer to pass through the fiber than others because they travel a longer distancebecause they travel a longer distance
bull See animation at link Ch 2fSee animation at link Ch 2fIndex of refraction
Multimode Graded-Index FiberMultimode Graded-Index Fiber The index of refraction gradually The index of refraction gradually
changes across the corechanges across the corebull Modes that travel further also move fasterModes that travel further also move fasterbull This reduces This reduces modal dispersionmodal dispersion so the so the
bandwidth is greatly increasedbandwidth is greatly increased
Index of refraction
AttenuationAttenuation AbsorptionAbsorption
bull interaction of light with electrons amp molecule vibrationinteraction of light with electrons amp molecule vibration Rayleigh ScatteringRayleigh Scattering
bull caused by compositional fluctuations in glass material caused by compositional fluctuations in glass material Energy escapes not convertedEnergy escapes not converted
Material FabricationMaterial Fabricationbull caused impurities (transition metal ions)caused impurities (transition metal ions)
Fiber FabricationFiber Fabricationbull caused by fiber imperfections (defectsstresses) Leads caused by fiber imperfections (defectsstresses) Leads
to Mie scattering which is to Mie scattering which is independent independent DeploymentEnvironmentalDeploymentEnvironmental
bull caused by bends and microbends caused by bends and microbends Leads to mode conversions Leads to mode conversions
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
RepeatersRepeaters For long links repeaters are needed For long links repeaters are needed
to compensate for signal lossto compensate for signal loss
FiberRepeaterRepeater Repeater
Fiber FiberFiber
Optical FiberOptical Fiber CoreCore
bull Glass or plastic with a higher Glass or plastic with a higher index of refraction than the index of refraction than the claddingcladding
bull Carries the signalCarries the signal
CladdingCladdingbull Glass or plastic with a lower Glass or plastic with a lower
index of refraction than the coreindex of refraction than the core
BufferBufferbull Protects the fiber from damage Protects the fiber from damage
and moistureand moisture
JacketJacketbull Holds one or more fibers in a Holds one or more fibers in a
cablecable
Singlemode FiberSinglemode Fiber Singlemode fiber has a core diameter Singlemode fiber has a core diameter
of 8 to 9 microns which only allows of 8 to 9 microns which only allows one light path or modeone light path or modebull Images from arcelectcom (Link Ch 2a)Images from arcelectcom (Link Ch 2a)
Index of refraction
Multimode Step-Index FiberMultimode Step-Index Fiber Multimode fiber has a core diameter Multimode fiber has a core diameter
of 50 or 625 microns (sometimes of 50 or 625 microns (sometimes even larger)even larger)bull Allows several light paths or modesAllows several light paths or modesbull This causes This causes modal dispersionmodal dispersion ndash some modes ndash some modes
take longer to pass through the fiber than others take longer to pass through the fiber than others because they travel a longer distancebecause they travel a longer distance
bull See animation at link Ch 2fSee animation at link Ch 2fIndex of refraction
Multimode Graded-Index FiberMultimode Graded-Index Fiber The index of refraction gradually The index of refraction gradually
changes across the corechanges across the corebull Modes that travel further also move fasterModes that travel further also move fasterbull This reduces This reduces modal dispersionmodal dispersion so the so the
bandwidth is greatly increasedbandwidth is greatly increased
Index of refraction
AttenuationAttenuation AbsorptionAbsorption
bull interaction of light with electrons amp molecule vibrationinteraction of light with electrons amp molecule vibration Rayleigh ScatteringRayleigh Scattering
bull caused by compositional fluctuations in glass material caused by compositional fluctuations in glass material Energy escapes not convertedEnergy escapes not converted
Material FabricationMaterial Fabricationbull caused impurities (transition metal ions)caused impurities (transition metal ions)
Fiber FabricationFiber Fabricationbull caused by fiber imperfections (defectsstresses) Leads caused by fiber imperfections (defectsstresses) Leads
to Mie scattering which is to Mie scattering which is independent independent DeploymentEnvironmentalDeploymentEnvironmental
bull caused by bends and microbends caused by bends and microbends Leads to mode conversions Leads to mode conversions
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Optical FiberOptical Fiber CoreCore
bull Glass or plastic with a higher Glass or plastic with a higher index of refraction than the index of refraction than the claddingcladding
bull Carries the signalCarries the signal
CladdingCladdingbull Glass or plastic with a lower Glass or plastic with a lower
index of refraction than the coreindex of refraction than the core
BufferBufferbull Protects the fiber from damage Protects the fiber from damage
and moistureand moisture
JacketJacketbull Holds one or more fibers in a Holds one or more fibers in a
cablecable
Singlemode FiberSinglemode Fiber Singlemode fiber has a core diameter Singlemode fiber has a core diameter
of 8 to 9 microns which only allows of 8 to 9 microns which only allows one light path or modeone light path or modebull Images from arcelectcom (Link Ch 2a)Images from arcelectcom (Link Ch 2a)
Index of refraction
Multimode Step-Index FiberMultimode Step-Index Fiber Multimode fiber has a core diameter Multimode fiber has a core diameter
of 50 or 625 microns (sometimes of 50 or 625 microns (sometimes even larger)even larger)bull Allows several light paths or modesAllows several light paths or modesbull This causes This causes modal dispersionmodal dispersion ndash some modes ndash some modes
take longer to pass through the fiber than others take longer to pass through the fiber than others because they travel a longer distancebecause they travel a longer distance
bull See animation at link Ch 2fSee animation at link Ch 2fIndex of refraction
Multimode Graded-Index FiberMultimode Graded-Index Fiber The index of refraction gradually The index of refraction gradually
changes across the corechanges across the corebull Modes that travel further also move fasterModes that travel further also move fasterbull This reduces This reduces modal dispersionmodal dispersion so the so the
bandwidth is greatly increasedbandwidth is greatly increased
Index of refraction
AttenuationAttenuation AbsorptionAbsorption
bull interaction of light with electrons amp molecule vibrationinteraction of light with electrons amp molecule vibration Rayleigh ScatteringRayleigh Scattering
bull caused by compositional fluctuations in glass material caused by compositional fluctuations in glass material Energy escapes not convertedEnergy escapes not converted
Material FabricationMaterial Fabricationbull caused impurities (transition metal ions)caused impurities (transition metal ions)
Fiber FabricationFiber Fabricationbull caused by fiber imperfections (defectsstresses) Leads caused by fiber imperfections (defectsstresses) Leads
to Mie scattering which is to Mie scattering which is independent independent DeploymentEnvironmentalDeploymentEnvironmental
bull caused by bends and microbends caused by bends and microbends Leads to mode conversions Leads to mode conversions
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Singlemode FiberSinglemode Fiber Singlemode fiber has a core diameter Singlemode fiber has a core diameter
of 8 to 9 microns which only allows of 8 to 9 microns which only allows one light path or modeone light path or modebull Images from arcelectcom (Link Ch 2a)Images from arcelectcom (Link Ch 2a)
Index of refraction
Multimode Step-Index FiberMultimode Step-Index Fiber Multimode fiber has a core diameter Multimode fiber has a core diameter
of 50 or 625 microns (sometimes of 50 or 625 microns (sometimes even larger)even larger)bull Allows several light paths or modesAllows several light paths or modesbull This causes This causes modal dispersionmodal dispersion ndash some modes ndash some modes
take longer to pass through the fiber than others take longer to pass through the fiber than others because they travel a longer distancebecause they travel a longer distance
bull See animation at link Ch 2fSee animation at link Ch 2fIndex of refraction
Multimode Graded-Index FiberMultimode Graded-Index Fiber The index of refraction gradually The index of refraction gradually
changes across the corechanges across the corebull Modes that travel further also move fasterModes that travel further also move fasterbull This reduces This reduces modal dispersionmodal dispersion so the so the
bandwidth is greatly increasedbandwidth is greatly increased
Index of refraction
AttenuationAttenuation AbsorptionAbsorption
bull interaction of light with electrons amp molecule vibrationinteraction of light with electrons amp molecule vibration Rayleigh ScatteringRayleigh Scattering
bull caused by compositional fluctuations in glass material caused by compositional fluctuations in glass material Energy escapes not convertedEnergy escapes not converted
Material FabricationMaterial Fabricationbull caused impurities (transition metal ions)caused impurities (transition metal ions)
Fiber FabricationFiber Fabricationbull caused by fiber imperfections (defectsstresses) Leads caused by fiber imperfections (defectsstresses) Leads
to Mie scattering which is to Mie scattering which is independent independent DeploymentEnvironmentalDeploymentEnvironmental
bull caused by bends and microbends caused by bends and microbends Leads to mode conversions Leads to mode conversions
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Multimode Step-Index FiberMultimode Step-Index Fiber Multimode fiber has a core diameter Multimode fiber has a core diameter
of 50 or 625 microns (sometimes of 50 or 625 microns (sometimes even larger)even larger)bull Allows several light paths or modesAllows several light paths or modesbull This causes This causes modal dispersionmodal dispersion ndash some modes ndash some modes
take longer to pass through the fiber than others take longer to pass through the fiber than others because they travel a longer distancebecause they travel a longer distance
bull See animation at link Ch 2fSee animation at link Ch 2fIndex of refraction
Multimode Graded-Index FiberMultimode Graded-Index Fiber The index of refraction gradually The index of refraction gradually
changes across the corechanges across the corebull Modes that travel further also move fasterModes that travel further also move fasterbull This reduces This reduces modal dispersionmodal dispersion so the so the
bandwidth is greatly increasedbandwidth is greatly increased
Index of refraction
AttenuationAttenuation AbsorptionAbsorption
bull interaction of light with electrons amp molecule vibrationinteraction of light with electrons amp molecule vibration Rayleigh ScatteringRayleigh Scattering
bull caused by compositional fluctuations in glass material caused by compositional fluctuations in glass material Energy escapes not convertedEnergy escapes not converted
Material FabricationMaterial Fabricationbull caused impurities (transition metal ions)caused impurities (transition metal ions)
Fiber FabricationFiber Fabricationbull caused by fiber imperfections (defectsstresses) Leads caused by fiber imperfections (defectsstresses) Leads
to Mie scattering which is to Mie scattering which is independent independent DeploymentEnvironmentalDeploymentEnvironmental
bull caused by bends and microbends caused by bends and microbends Leads to mode conversions Leads to mode conversions
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Multimode Graded-Index FiberMultimode Graded-Index Fiber The index of refraction gradually The index of refraction gradually
changes across the corechanges across the corebull Modes that travel further also move fasterModes that travel further also move fasterbull This reduces This reduces modal dispersionmodal dispersion so the so the
bandwidth is greatly increasedbandwidth is greatly increased
Index of refraction
AttenuationAttenuation AbsorptionAbsorption
bull interaction of light with electrons amp molecule vibrationinteraction of light with electrons amp molecule vibration Rayleigh ScatteringRayleigh Scattering
bull caused by compositional fluctuations in glass material caused by compositional fluctuations in glass material Energy escapes not convertedEnergy escapes not converted
Material FabricationMaterial Fabricationbull caused impurities (transition metal ions)caused impurities (transition metal ions)
Fiber FabricationFiber Fabricationbull caused by fiber imperfections (defectsstresses) Leads caused by fiber imperfections (defectsstresses) Leads
to Mie scattering which is to Mie scattering which is independent independent DeploymentEnvironmentalDeploymentEnvironmental
bull caused by bends and microbends caused by bends and microbends Leads to mode conversions Leads to mode conversions
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
AttenuationAttenuation AbsorptionAbsorption
bull interaction of light with electrons amp molecule vibrationinteraction of light with electrons amp molecule vibration Rayleigh ScatteringRayleigh Scattering
bull caused by compositional fluctuations in glass material caused by compositional fluctuations in glass material Energy escapes not convertedEnergy escapes not converted
Material FabricationMaterial Fabricationbull caused impurities (transition metal ions)caused impurities (transition metal ions)
Fiber FabricationFiber Fabricationbull caused by fiber imperfections (defectsstresses) Leads caused by fiber imperfections (defectsstresses) Leads
to Mie scattering which is to Mie scattering which is independent independent DeploymentEnvironmentalDeploymentEnvironmental
bull caused by bends and microbends caused by bends and microbends Leads to mode conversions Leads to mode conversions
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Three Types of DispersionThree Types of Dispersion
Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber
Three typesThree typesbull Modal DispersionModal Dispersionbull Chromatic DispersionChromatic Dispersionbull Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Modal DispersionModal Dispersion
Modal DispersionModal Dispersionbull Spreading of a pulse because different Spreading of a pulse because different
modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times
bull Only happens in multimode fiberOnly happens in multimode fiberbull Reduced but not eliminated with Reduced but not eliminated with
graded-index fibergraded-index fiber
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Chromatic DispersionChromatic Dispersion
Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber
This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion
Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiberbull Larger effect with LEDs than with lasersLarger effect with LEDs than with lasersbull A far smaller effect than modal dispersionA far smaller effect than modal dispersion
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Polarization Mode DispersionPolarization Mode Dispersion
Light with different polarization can Light with different polarization can travel at different speeds if the fiber travel at different speeds if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level
This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable and there is no easy way to cable and there is no easy way to correct itcorrect it
It can affect both singlemode and It can affect both singlemode and multimode fibermultimode fiber
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Light Sources
1048698 Light Emitting Diode (LED)
bull1048698 simple construction and drive circuitrybull1048698 best for short distances modest bit rates and low channel capacity
1048698 Semiconductor Laser Diode
bull1048698 high drive currents and complex circuitrybull1048698 produce high power for higher bit rates and long distances
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Light Sources LED
bull1048698 Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or
bull Gallium-arsenide-phosphide (GaAsP)
bull Spontaneous emission through recombination of electrons and holes
bull Works in forward bias energy released as a photon bull A photon = a quantum of EM wave energy
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Light Sources Laser DiodeLight Sources Laser DiodebullLight Amplification by Stimulated Emission of Radiation
1048698 A laser diode Is an LED with two important differences
1048698 (1) The operating current is much higher in order to produce OPTICAL GAIN
1048698 (2) Two of the ends of the LD are cleaved parallel to each other These ends act as perfectly aligned mirrors which reflect the light back and forth through the gain medium in order to get as much amplification as possible
1048698 The typical response time of a laser diode Is 05 ns The line width is around 2 nm with a typical laser power of 10s of milliwatts The wavelength of a laser diode can be 850 nm 1300 nm or 1500 nm
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Photo-Detectors
1048698 Must detect down to the order of 10-14 W
1048698 Need high conversion efficiency between light and electrical energy
1048698 Must respond fast for high bandwidth
1048698 Must have low-noise power and good light-collecting properties
1048698 Ideally they must operate at low voltage be easy to use be robust and immune to changes in ambient conditions have a long life be reliable and inexpensive
1048698 Two devices stand out1048698 Positive-intrinsic-negative (PIN) diodes
1048698 Avalanche photodiodes (APD)
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Detection ProcedureDetection Procedure1048698 1048698 Photons collide with the electrons in thePhotons collide with the electrons in the
valence bandvalence band
1048698 1048698 The electrons absorb photon energy hv and The electrons absorb photon energy hv and cross the band gap into the conduction band cross the band gap into the conduction band with charge qwith charge q
1048698 1048698 Incident optical power P transfers to theIncident optical power P transfers to the
device with efficiency ηdevice with efficiency η
1048698 1048698 The generated photocurrent isThe generated photocurrent is
1048698 1048698 We resort to mean values because the wholeWe resort to mean values because the whole
photo-detection process is stochasticphoto-detection process is stochastic
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Detectors PIN Diode
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Detectors The APD Device
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength
Detectors Characteristics
1048698 Responsivity1048698 Measure of conversion efficiency a ratio of the output current to the input optical power (AW)
1048698 Dark current1048698 Leakage current flowing with no light input
1048698 Transit time1048698 Time it takes a photo-induced carrier to cross the depletionRegion
1048698 Spectral response1048698 A relative spectral response vs wavelength or frequencycurve displays the range or system length possible for agiven wavelength