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RAJAGIRI SCHOOL OF ENGINEERING AND TECHNOLOGYSUDHEESH P G
Light Wave Communication
Module 3
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LEDLASER
Optical sources
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LED
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Lower current densities than LASERPhotons-random phase-incoherent optical sourceHigh spectral line width-supports many modesLow power, less directionalLower modulation bandwidthHarmonic distortion
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Simpler fabricationCostReliabilityLess temp dependenceSimpler drive circuitryLinearity
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Surface emitterEdge emitterSuperluminescentResonant cavityPlanardome
LED Structure
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SimpleLiquid or
Vapour phase Epitaxial process• P type diffusion into n type substrate• Forward current-spontaneous emission
Planar LED
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Hemisphere of GaAs Diffused p typeDiameter of domeMaximize amount of internal emission reaching surface within critical angle of GaAs interfaceDome>> active recombination areaGreater emission area
Dome LED
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Surface emitter LED
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High radiance-restrict emission to small active region within the deviceHigh current densityEtched well GaAs-prevent heavy absorptionAccommodate fiberActive layer is below the emitting surfacePower coupled into step index multi mode fiber
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High radiance
Edge emitter LED
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Light produced in active layer spreads into transparent guiding layer, reducing self absorption in active layerWave guiding -30 degrees plain perpendicular to active layer
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P-n junction,ridge waveguideOne end lossy-prevent reflections(lasing)
Superluminescent LED
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High output powerDirectional output beamNarrow spectral line widthinjected current is increased-amplificationNo lasing action-one end is lossy
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Based on planar technology containing Fabry- Perot active resonant cavity between distributed Bragg reflector(DBR) mirrors.Quantum well embedded in active cavityCavity-micrometer size
Resonant cavity
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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G
1. Optical output power linearizing
LED characteristics
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Output power-tempInternal quantum efficiency
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Different temperature
Light output against current(SLD)
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Usually 25-40 nm-0.8 to 0.9 um50-160 nm—1.1 to 1.7 um wavelength region
2 . Output spectrum
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Greater energy spread in carrier distribution at higher temp
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Frequency at which output electrical/optical power is reduced by 3 dB(half its const value)
3.Modulation Bandwidth
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Degradation-rapid,slowRapid- growth of dislocations ,precipitative type defect in active regionDark line defects (DLD),Dark spot defect (DSD)Slow- recombination enhanced point defect generation or migrations of impurity into active region
4.Reliability
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Stimulated emission by the recombination of the injected carriers
Semiconductor Injection LASER
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High radianceNarrow linewidth-less material disp.Temporal coherenceSpatial coherence-for efficient coupling to fiber
adv
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GaAs homojunction,Fabry-Perot cavity-less carrier containment-large threshold currentHeterojunction-large carrier containment –less thershold current
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Lasing action-mirrors- oscillationMany resonant frequency@ resonant freq- gain> overcome lossOther sides are roughened to avoid emission
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Similar to fabry-perot except… bragg gratings(reflectors) or periodic variation of refractive index
Distributed Feedback LASER
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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G
DBR-distributed Bragg grating-grating in ends
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Gain guidedIndex-guidedQuantum-wellQuantum-dot
LASER structures
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Small no. of lateral modesStripe geometry….constricts current flow to the stripe
Gain guided LASER
>Proton isolated
>Oxide isolation
>P-n junction isolation
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Variation in refractive index
Index guided
Gain guided
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The carrier motion normal to the active layer in these devices is restricted, resulting in a quantization of the kinetic energy into discrete energy levels for the carriers moving in that direction.Single Quantum well-single active regionMulti quantum well-multiple active region
Quantum well LASER
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Quantum well –with single discrete atomic structure or quantum dotQD- tiny droplet of free electron forming quantum well
Quantum dot
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Threshold current temperature dependenceDynamic responseFrequency chirpNoiseMode hoppingreliability
LASER chara
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Threshold current temp dependence
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Critical-high speedi/p-current step > switch on delay >damped oscillations (relaxation oscillations RO)
Dynamic response
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Dynamic shift of peak wavelengthLine width broadeningReason-direct current modulation
Frequency chirp
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Phase or frequency noiseInstabilities in operation & self pulsationReflection of light back into deviceMode partition noise
Noise
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Mode hopping to longer wavelength as current is increasedReason-increase in temperature of device junction
Mode hopping
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Catastrophic-mechanical damage of mirror facetsGradual-defect formation in active region, degradation of current confining junctions
Reliability>>> degradation
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Photo detection
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High sensitivity at operating wavelengthHigh fidelityLarge electrical response to received optical signalShort response time to obtain a suitable BWMinimum noise by detectorStability of performance charaSmall sizeHigh reliabilityLow cost
Requirements
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principle
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Absorption coefficientLess than unity as all the photons absorbed are not converted to electron hole pair
Quantum efficiency
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Absorption coeff- dependent on wavelength
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Not related to photo energy
Po-incident optical power from fiberIp-output photocurrentGives transfer chara of photo detectorphotocurrent per unit optical incident power
Responsivity
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Responsivity eq with quantum efficiencyInput power / energy of “a” photon
Responsivity-prop to quantum efficiency,fixed λ
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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G
RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G
RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G
RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G
Reverse bias
PIN diode
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Impact ionization-ionize bound e in VB,carrier multiplicationAvalanche effect-gain energy through impact ionization
Avalanche photo diode
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Source to fiber power launching
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Source output pattern
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Power coupling
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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G
Single mode graded
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Power launched into fiber depends only on radiance, not on λ of sourceSource radius,fiber radius, NA , radiance
Wavelength dependency
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Improve coupling efficiencyMiniature lens between lens and fiberMagnifies the emitting area of the source ~ fiberMagnification factor of lens
Lensing scheme
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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G
Mach-Zehnder Interferometer filter Reciprocal device Phase lag +
interference Used for broadband
filtering Crosstalk, non-flat
spectrum, large skirts…
Tunability: by varying temperature (~ few ms)
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MZI
RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G
MZI