optical drivers

8/13/2019 Optical Drivers

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OPTICAL DRIVERSOPTICAL DRIVERS

ANDAND

DETECTORSDETECTORS


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Light sourcesLi

ght sources

Characteristics required Effective coupling to fibres down to 8.5

microns diameter

Easily modulated with linear characteristics igh output power

igh reliability

!mall si"e and weight #ow cost


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#ight emitting diodes $#EDs%

&ade from p' and n'type semiconductor

substrates &ost common materials

( )allium aluminium arsenide $)a*l*s% for 8++

to ,++ nm wavelengths

( )allium arsenide $)a*s% for ,-+ nm

( )allium arsenide phosphorous $)a*s% for //+

nm $red for plastic fibres%

( Indium gallium arsenide phosphide $In)a*s%

for 0-++ and 055+ nm


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' type substrate

( &ore protons than electrons ' holes

(*pply 1ve voltage 2 ' type substrate

( &ore electrons than protons

(*pply 've voltage

Electrons and holes flow toward the 3unction and

combine4 emitting photons

orbidden bandgap determines the energy of the

emitted photon


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Basic LED operationBasic LED

operation

1

1

'

1'

1'

'

6ias7oltage

1

'n ' region

p ' region

#ight Emission

ecombination

9unction


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Bandgap determines photonBan

dgap determines photon

energy emission

energy emission

Conduction 6and$ree Electrons%

7alence 6and$6ound Electrons ' ree oles%

6andgap $w% hf

*mount of Energyreleased when electrondrops to valence bandand recombination occurs11 '

'

Energy$e7

%


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#ED geometry

!tandard #EDs emit light in all directions

ibre #EDs must emit light in a narrower

confined beam

:ypes

( 6urrus * hole is etched into the substrate for fibre

( Edge emitting diodes ;ses a very thin active region to confine the light


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Edge emitting LEDEdge emitting LED

&etal

' egion

*ctive #ayer

n ' egion$substrate%

!i


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DIODE AND LASERCHARACTERISTICS


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&odulation

( &ostly digital ' $#ED turned on and off%

(*nalogue possible $requires forward DC bias%

:emperature effects

(


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Practica LED pac!agesPractica LED pac!ages

#E2! coupling$for large fibres A0+++ microns%

&icrolensed #ED$5+ micron fibres%

a%

b%

ibre

ibre

&etalCap

&etalCap

:


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Diode Packaging


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Integrated Connectors


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ANALOG AND DIGITAL DIODE CONTROLS


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Laser diodesLaser diodes #aser light amplification by stimulated

emission of radiation


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Laser diode operationLaser diode operation


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&odulation

( &ostly digital !witched onBoff close to threshold

(*nalogue possible *bove threshold current

:emperature effects

( :hreshold current increases with temperature '0.5 H per degree C

(


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Laser diode po"er # current #Laser diode po"er # current #

temperature cur$estemperature cur$es


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VCSEL

7ertical Cavity !urface Emitting #aser

Cheap laser diode

#ess energy required for laser operation

Emits narrow4 more circular beam


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VCSEL Chip


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ANALOG AND DIGITAL LASER CONTROL CIRCUITS


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Laser diode transmitter modueLaser diode transmitter modue

)rooved6locF

ibre

.hotodetector

Diode :erminal#aser Diode


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Optica detectorsOptica detectors

Converts photon light energy to

electrical energy

&ust be very low inherent noise device

rovide amplification of low level

signals


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in photodiodes

;se reverse process of the #ED

Common response times of +.5 to 0+ ns $?

)" to 0++ &"%.

!pecial devices down to 0+ ps

$0++ )"%


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Pin photodiodePin photodiode

'

1

1 n17


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*valanche photodiodes

igh reverse bias voltages cause an increasing

avalanche of charges in the semiconductor areas.*dvantages

( Increased sensitivity to incoming light

( Internal amplification

( :wice the response time for standard devices

Disadvantages

( &ore comple>

( Increase costs

( #ess reliable

;sed for low signal to noise requirements and long

distance telecommunications.


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A$aanche photodiodesA$aanche photodiodes


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APD Recei#er


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*mplifiers equired because the very low levels of

light produce tiny currents in the detectors(* light signal of 0 = will produce /++ n* in a

pin diode ( need to amplify..

Can be E:

( )reater sensitivity at lower data rates lowest

noise


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Optical amplifiers*mplification without conversion to electrical

energy ;se stimulated emission principle for single

photon pass

Doped fibres

( Erbium for 055+ nm

( raseodymium for 0-++ nm

!emiconductor lasers

( roblems matching wide diameter fibre cores tosmall laser areas

( *ctive layer narrow ' , m to 0 m


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Doped 'i(re Ampi%ierDoped 'i(re Ampi%ier

Incident #ight05@+nm

!timulates Emissionat 05@+nm

*mplified


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ED$A


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Semiconductor Laser Ampi%ierSemiconductor Laser Ampi%ier

Core Core

!emiconductor#aser *mplifier

Incident

light beam*ctive#ayer

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