tt503a – dispositivos fotônicos aula 5 - fontes Ópticas.gallep/tt708/a5.pdf · • light...

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tt503A – Dispositivos Fotônicos Aula 5 - fontes Ópticas.

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Sources for Optical Transmitters

• Many types of optical sources are available –  Light Emitting Diodes (LEDs) –  Solid state lasers –  Gas lasers –  Semiconductor lasers –  Fiber lasers

• Semiconductor Lasers are preferred –  Powered by electrical energy –  Directly converts electrical signals to optical signals

Copyright VPIsystems. All rights reserved. 2

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Semiconductor Laser Features • High modulation bandwidth (> 10 Gbit/s) • Small size

–  Packaged: ~ 2×1×1 cm –  Unpackaged: ~ grain of salt, 0.5mm × 200µm × 100µm

•  Intense single spatial mode • Energy efficient • Narrow spectral linewidth • Can be single longitudinal mode (monochromatic) • Reliable operation • Can be integrated


What does LASER stand for?

LASER is an acronym for:

LIGHT

AMPLIFICATION by

STIMULATED

EMISSION of

RADIATION

•  How does it work? •  Why amplification? •  What is stimulated emission? Copyright VPIsystems. All rights reserved. 4

How does a Laser work?

A functional view of a laser: 4 main parts

3) Optical Resonator (Cavity)

Mirrors Mirrors 2) Energy Pump (Current Source)

Mirror Loss

Mirror Loss

4) Losses: 1) Active (Gain) Medium

Material and Waveguide


Basic Laser Structure

Fabry-Perot Laser

Light

Cleaved facet (partial mirror)

Active Region

Heat Sink

Laser

-

Wires (connection to current source)

+


A Basic Laser Structure

Fabry-Perot Laser (longitudinal section)

+

-

Energy Pump (Current Source)

Active region (provides gain)


P-layer

N-layer


PN Junction

photons

Holes

Electrons


Optical Emission Processes

Light produced by spontaneous emission:

Excited state

Ground state

E2

E1

–  Random propagation direction –  Random phase –  Random frequency –  is incoherent (broad linewidth)


Optical Emission Processes

• Stimulated Emission

Excited state

Ground state E1

E2

• A photon with energy > (E2 – E1): triggers transition of an excited electron

photon

à identical photon is emitted

Identical photon

• Produced light is coherent (desirable)


Population Inversion

• Stimulated emission – coherent light, desirable

• Need electrons to be mostly in state E2

• Population inversion achieved by electrical pumping

Population N2

Population N1

Excited state

Ground state E1

E2

• N2 >> N1, stimulated emission dominates


Optical Cavity and Feedback

• Light amplification by stimulated emission… is not strong, especially if the active region is short

• Optical cavity provides feedback into active region

• Light is reflected repeatedly and greatly amplified

Excited state

Ground state E1

E2

along the laser

N2

N1


Optical Losses and Laser Output

• One mirror is partially transmitting, to get output

• Photons transmitted out are lost (mirror loss)

• Other losses: scattering in the material, nonradiative processes

Excited state

Ground state E1

E2

along the laser

N2

N1


Laser Spectrum (Gain vs. λ Curve)

• Laser oscillation (lasing) occurs over a range of λ • Due to distribution of E2 and E1 around mean values

BANDGAP

Gain

hf

E1

E2

Electron Population Distribution

E

Probability

Hole Population Distribution E

Probability

A

B

C Lowest Energy Transition (A)

Most Probable Energy Transition (B)

Highest EnergyTransition, more likely to be absorbed (C)


Lasing Conditions For lasing to initiate in a cavity, two conditions need to be satisfied:

–  Gain condition: The electric field of the light, after completing one round trip inside the cavity, should have the same amplitude

–  Phase condition: The electric field of the light, after completing one round trip inside the cavity, should have the same phase

one round trip


Phase Condition

Discrete modes that can be supported by the cavity are called longitudinal modes

P


Laser Spectrum: Gain & Phase Conditions met Pow

er spectral density Pow

er spectral density

Cavity mode

Gain Curve

Dominant mode Gain ≡ Loss

Side modes Gain < Loss

Lasing spectrum

λ

λCopyright VPIsystems. All rights reserved. 18

Another Laser Structure: DFB Laser

frequency

frequency

Lasing spectrum

Cavity modes

Gain Curve

DFB Gain Curve

Power spectral density


Refractive Index Grating


Another Laser Structure: DFB Laser

λ / 4

frequency

frequency



Lasing spectrum

Cavity modes

Gain Curve

1/4-wave shifted DFB Gain Curve

1/4-wave shifted Grating


Rate Equation Rate equation describe the dynamic interaction between excited electrons and photons

Change in carrier density:

Change in photon density:

Contribute from Current

Decay of electrons

Usage of electron by

Stimulated emission

Photon generated by stimulated emission

Decay of photon

Photon generated by Spontaneous emission


Light-Current Characteristics

• The L-I curve: output light power vs. input current • Diode characteristics, threshold current Ith

• For I > Ith, light power increases linearly with I

current

Light output (power)

Ith

Spontaneous emission region

Stimulated emission region


Direct-Current Modulation

The information is encoded on semiconductor lasers by current modulation

current


Ith

Modulated Laser Light

Current modulation


Laser Dynamics: Chirp

• Chirping of the laser output signal (pulses) • Instantaneous frequency of the signal changes • Leads to increased dispersion (broadening)

t

t

Frequency

power


Laser Dynamics: Modulation Bandwidth

A

B

100MHz 10GHz 2GHz 500MHz

+10

0

-10

-20

A B

current


• Determines maximum direct modulation speed • Increases with increasing drive (bias) current • Within practical limits, get multi-GHz modulation


Laser Dynamics: Turn On Delay

Delay between injection of current and generation of light

t

t

t

Current

Carrier density


Turn on delay

B

C

D

E

A

FH

G


Laser Dynamics: Relative Intensity Noise

Copyright VPIsystems. All rights reserved.

Current


1

2

3

Frequency

RIN

1 2 3

Frequency shape of RIN depends on laser driving conditions

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0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Blue

Gre

enO

rang

eY

ello

w

Red

λ

1.7Infrared

Vio

let

GaA

s

GaA

s 0.55

P 0.45

GaAs1-yPy

InP In0.

14G

a 0.86

As

In1-xGaxAs1-yPyAlxGa1-xAs

x = 0.43

GaP

(N)

GaS

b

Indirectbandgap

InG

aNSi

C(A

l)

In0.

7Ga 0.

3As 0.

66P 0.

34

In0.

57G

a 0.43

As 0.

95P 0.

05

Free space wavelength coverage by different LED materials from the visible spectrum to theinfrared including wavelengths used in optical communications. Hatched region and dashedlines are indirect Eg materials.

In0.49AlxGa0.51-xP

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

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p n

BC

BV

Fp Fn

p n E

Barreira de

potencial

Região de

depleção

elétron lacuna

íon fixo aceitador íon fixo doador

BC: banda de condução BV: banda de valência Fp: nível quasi-Fermi do semicondutor tipo p Fn: nível quasi-Fermi do semicondutor tipo n

1

2

3 p n

Barreira de

potencial

V I

Região ativa

emissão de luz em junções pn

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Dispositivos semicondutores ópticos LED vs. laser

l  Diodo emissor de luz (LED): luz incoerente, largura espectral entre 30 e 60 nm, grande espalhamento de luz, potência de saída < 10 mW para correntes > 100 mA e estável com a temperatura, modulação até centenas de MHz

l  Laser : luz coerente, multimodo e monomodo, largura espectral + estreita, modulação máxima de centenas GHz, pequeno espalhamento de luz, potência de saída pode chegar a dezenas de mW para correntes inferiores a 50 mA, porém dependente da temperatura

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metalização

região ativacontatometálico

contatode ouro

contatometálico

fibra óptica

resina

n-GaAs

LED de Emissão superfcial (SLED)

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LED Emissão lateral (ELED)

p-AlGaAs

região ativa (AlGaAs)

n-AlGaAs

p+-AlGaAs

n-GaAs

eletrodo

eletrodo n+-GaAs

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Laser - Características gerais l  Amplamente utilizado em sistemas de comunicação

óptica l  Estrutura capaz de amplificar a luz no interior de uma

cavidade refletiva, levado-a a oscilação através de realimentação positiva

l  A potência óptica suficiente para permitir transmissões, sem repetidores ou amplificadores ópticos, a distâncias, em média, entre 100 e 150 km

l  Modulação da intensidade luz através da modulação de sua corrente em até dezenas de GHz → com a utilização de técnicas coerentes, a modulação da fase e da freqüência do laser é possível

l  A freqüência de operação do laser varia com a corrente e a temperatura → possibilidade de sintonia

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Laser FP Corrente de limiar e a temperatura

Corrente (mA)

Potência óptica (mW

)

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Laser FP - Resposta à modulação direta digital

P (mW)

I (mA)

Po

t (s)

t (s)

Io

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Laser SQW e MQW Estruturas

BC

BV

BC

BV

SQW

SQW com índice gradual

Região gradual

MQW

MQW modificado

BC

BV

BC

BV

Barreira Ativa Casca

Barreira Ativa Casca

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Laser Monomodo Comercial-Exemplo 2

Po

tênc

ia (d

Bm)

Freqüência de modulação (MHz)

Potê

ncia

Rel

ativ

a (d

B)

Comprimento de onda (nm)

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Modulação direta do laser Exemplo de circuito

Sinal de modulação

Opcional para DC

IDC

VPOL

VREF

Fotodetector para monitoração Laser

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Exercícios

Dado:

Assuma:

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Dado:

Assuma:

Dado:

tt503a – dispositivos fotônicos aula 5 - fontes Ópticas.gallep/tt708/a5.pdf · • light...

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