Ch 6: Optical SourcesCh 6: Optical Sources Variety of sourcesVariety of sources LS considerations:LS considerations:

WavelengthWavelength Output powerOutput power ModulationModulation CouplingCoupling Cost/performanceCost/performance

LEDLED OperationOperation

Spontaneous emissionSpontaneous emission

LEDLED

Recent advances:Recent advances: Porous siliconPorous silicon

Semiconductor polymersSemiconductor polymers

=hc/=hc/PHPH

LED: Light EmissionLED: Light Emission

LED: CouplingLED: Coupling

LED: CharacteristicsLED: CharacteristicsLow costLow costLow power: 100 uW (recently: 75 mW)Low power: 100 uW (recently: 75 mW)Wide spectrum: 50-100 nmWide spectrum: 50-100 nm Incoherent light: neither directional nor coherentIncoherent light: neither directional nor coherentDigital modulation: up to 300 MbpsDigital modulation: up to 300 MbpsAnalog modulation: simplyAnalog modulation: simply

LDLD LASER: Light LASER: Light

Amplification by the Amplification by the Stimulated Emission of Stimulated Emission of RadiationRadiation

Laser active mediaLaser active media Laser action: Absorption, Laser action: Absorption,

spont. Emission, spont. Emission, stimulated emissionstimulated emission

For stimulated emission to happen:For stimulated emission to happen:

Material capable of st. emission (have a metastable Material capable of st. emission (have a metastable high energy statehigh energy state

Population inversion: pumpingPopulation inversion: pumping

LDLD Lasing requires:Lasing requires:

Active mediumActive medium Supplying energySupplying energy ConfinementConfinement

Fabry-Perot LaserFabry-Perot Laser LED with couple of LED with couple of

mirrorsmirrors Fabry-Perot resonatorFabry-Perot resonator OperationOperation

L=L=x/2nx/2n

FPLFPLModes

Spectral Characteristics

FPLFPLGain threshold

DFB LaserDFB Laser Disadvantages of FPL:Disadvantages of FPL:

Spectral width of 5-8 nmSpectral width of 5-8 nm Mode hoppingMode hopping WDMWDM

DFB: OperationDFB: Operation

DFB LaserDFB Laser DFB: Phase shifted gratingDFB: Phase shifted grating

DFB: Spectral characteristicsDFB: Spectral characteristics

DFB CharacteristicsDFB CharacteristicsVery narrow linewidthsVery narrow linewidthsLow chirpLow chirpLow relative intensity noise (RIN)Low relative intensity noise (RIN)Sensitive to reflectionsSensitive to reflectionsTemperature sensitiveTemperature sensitiveOutput power fluctuationsOutput power fluctuationsHigh costHigh costSpeed?Speed?

DFBDFB Integrated Electro-Integrated Electro-

Absorption ModulatorAbsorption Modulator

Q-SwitchingQ-Switching

Stability?Stability?

Distributed Bragg Reflector (DBR)Distributed Bragg Reflector (DBR) More stableMore stable Less efficientLess efficient

Quantum Well Lasers (QW)Quantum Well Lasers (QW)

When light is confined to an area ~ When light is confined to an area ~ , it behaves like a particle, it behaves like a particle

Fundamental differences of SQW over non QWFundamental differences of SQW over non QW

Reduction in lasing threshold Reduction in lasing threshold Low output powerLow output power No lateral modesNo lateral modes Narrower linewidthNarrower linewidth

MQW/Tunable DBRMQW/Tunable DBR MQW vs SQWMQW vs SQW

High powerHigh power Low thresholdLow threshold Broader linewidthBroader linewidth

Strained layer QWStrained layer QW ConstructionConstruction AdvantagesAdvantages

Breadth of materialsBreadth of materials TunabilityTunability

Tunable DBR LasersTunable DBR Lasers 3 section tunable DBR3 section tunable DBR

Complex electronicsComplex electronics 10 nm range10 nm range

Tunable DBRTunable DBR Sampled grating TDBRSampled grating TDBR

External Cavity TDBRExternal Cavity TDBR

100 nm range100 nm range

Wide rangeWide range SlowSlow ExpensiveExpensive

Frequency stabilized DBRFrequency stabilized DBR External fiber cavity DBRExternal fiber cavity DBR

Coherence collapse operationCoherence collapse operation

Low costLow cost Accurate wavelength controlAccurate wavelength control Narrow linewidthNarrow linewidth

Temepreture control is shifted outTemepreture control is shifted out

Difficult positioningDifficult positioning

Vertical Cavity Surface Emitting LaserVertical Cavity Surface Emitting Laser Low powerLow power Scarce of wavelengthsScarce of wavelengths MM and SM lasersMM and SM lasers Easy couplingEasy coupling

Low threshold currentLow threshold current Simple electronicsSimple electronics High modulation BWsHigh modulation BWs

Very stableVery stable

In Fiber LasersIn Fiber Lasers CharacteristicsCharacteristics

Upconversion (double pumping)Upconversion (double pumping)

High output powerHigh output power Low noiseLow noise TunabilityTunability Very narrow linewidthsVery narrow linewidths Good soliton generationGood soliton generation External modulation is requiredExternal modulation is required Preselected wavelengthPreselected wavelength Mode hoppingMode hopping

ComparisonComparison

Optical Amplifiers: EDFAOptical Amplifiers: EDFA

Wavelengths:1480980

Materials:Praseodymium

Neodymium

L Band:CoDopantsLonger fiber

2nd generation:Gain control

Gain equalization

EDFA: GainEDFA: Gain

EDFAEDFA

Raman AmplifiersRaman Amplifiers

MultistageMultistage