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