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RAJAGIRI SCHOOL OF ENGINEERING AND TECHNOLOGY SUDHEESH P G Light Wave Communication Module 3

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RAJAGIRI SCHOOL OF ENGINEERING AND TECHNOLOGYSUDHEESH P G

Light Wave Communication

Module 3

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

LEDLASER

Optical sources

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

LED

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Simpler fabricationCostReliabilityLess temp dependenceSimpler drive circuitryLinearity

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Surface emitterEdge emitterSuperluminescentResonant cavityPlanardome

LED Structure

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

SimpleLiquid or

Vapour phase Epitaxial process• P type diffusion into n type substrate• Forward current-spontaneous emission

Planar LED

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Surface emitter LED

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

High radiance

Edge emitter LED

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

P-n junction,ridge waveguideOne end lossy-prevent reflections(lasing)

Superluminescent LED

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

High output powerDirectional output beamNarrow spectral line widthinjected current is increased-amplificationNo lasing action-one end is lossy

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Greater energy spread in carrier distribution at higher temp

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Stimulated emission by the recombination of the injected carriers

Semiconductor Injection LASER

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

High radianceNarrow linewidth-less material disp.Temporal coherenceSpatial coherence-for efficient coupling to fiber

adv

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

GaAs homojunction,Fabry-Perot cavity-less carrier containment-large threshold currentHeterojunction-large carrier containment –less thershold current

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Lasing action-mirrors- oscillationMany resonant frequency@ resonant freq- gain> overcome lossOther sides are roughened to avoid emission

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

DBR-distributed Bragg grating-grating in ends

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Gain guidedIndex-guidedQuantum-wellQuantum-dot

LASER structures

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Variation in refractive index

Index guided

Gain guided

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Threshold current temp dependence

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Critical-high speedi/p-current step > switch on delay >damped oscillations (relaxation oscillations RO)

Dynamic response

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Dynamic shift of peak wavelengthLine width broadeningReason-direct current modulation

Frequency chirp

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Phase or frequency noiseInstabilities in operation & self pulsationReflection of light back into deviceMode partition noise

Noise

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Mode hopping to longer wavelength as current is increasedReason-increase in temperature of device junction

Mode hopping

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Catastrophic-mechanical damage of mirror facetsGradual-defect formation in active region, degradation of current confining junctions

Reliability>>> degradation

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RAJAGIRI SCHOOL OF ENGINEERING AND TECHNOLOGYSUDHEESH P G

Photo detection

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

principle

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Absorption coefficientLess than unity as all the photons absorbed are not converted to electron hole pair

Quantum efficiency

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Absorption coeff- dependent on wavelength

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Reverse bias

PIN diode

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Impact ionization-ionize bound e in VB,carrier multiplicationAvalanche effect-gain energy through impact ionization

Avalanche photo diode

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RAJAGIRI SCHOOL OF ENGINEERING AND TECHNOLOGYSUDHEESH P G

Source to fiber power launching

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Source output pattern

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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

Power coupling

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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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RAJAGIRI SCHOOL OF ENGINEERING & TECHNOLOGYSUDHEESH P G

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

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MZI