3. Light sources - VILNIUS TECH

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ELEKTRONIKOS ĮTAISAI 2009 VGTU EF ESK [email protected] 1 Optical electronics. Light sources Introduction • Injection luminescence • Spectrum of recombination radiation • The double heterojunction • External quantum efficiency of a LED • Light amplification • Principles of laser diode operation • Modern laser diodes

Transcript of 3. Light sources - VILNIUS TECH

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ELEKTRONIKOS ĮTAISAI 2009

VGTU EF ESK [email protected]

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Optical electronics. Light sources

• Introduction

• Injection luminescence

• Spectrum of recombination radiation

• The double heterojunction

• External quantum efficiency of a LED

• Light amplification

• Principles of laser diode operation

• Modern laser diodes

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Light sources and detectors

Light sources:• Light emitting diodes• Laser diodes

Photodetectors:• Heterojunction pin diodes • Avalanche photodiodes

Semiconductors with the wider forbidden band is necessary for light sources

First generation optical systems: wavelength – 0.85 µm, GaAs-GaAlAs light sources, Siphotodetectors.

At λ = 1.3; 1.55 µm: InGaAsP-InP light sources, Ge or InGaAs-InP detectors.

Light emitting diodes (LEDs):

• non-coherent light sources

• wide spectrum oscillations

• wide radiation angle

Laser diodes (LDs):

• coherent light sources

• small radiation angle

• small width of radiation spectrum

• great power from small area

• higher modulation frequency

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Light is emitted as a result of direct radiative recombination of excess electrons and holes.

A parameter that needs to be made as large as possible in an optical source is the internal quantum efficiency . It is defined as the ratio of the number of photons generated to the number of carriers crossing the junction.

Semiconductors such as silicon, germanium and gallium phosphide have indirect band-gap. This means that an electron in an energy state near the bottom of the conduction band has a momentum in the crystal that is quite different from that of an electron in an energy state near to the top of the valence band. Excess momentum must change (must be absorbed) during the recombination in an indirect-band gap semiconductor. But probability of two events simultaneously (producing a phonon as well as a photon) is small. As a result non-irradiative processes tend to dominate in indirect band-gap semiconductors and the internal quantum efficiency is very small.Galium arsenide and other semiconductor materials have direct band-gap. An electron can jump from the conduction band to the valence band and a photon can be emitted. Then the internal quantum efficiency in LEDs can be to 0.5.

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Injection luminescence. Internal quantum efficiency

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Light is emitted as a result of direct radiative recombination of excess electrons and holes.

A parameter that needs to be made as large as possible in an optical source is the internal quantum efficiency . It is defined as the ratio of the number of photons generated to the number of carriers crossing the junction.

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Injection luminescence. Internal quantum efficiency

Some mechanisms of recombination are possible.

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Injection luminescence. Internal quantum efficiency

Semiconductors such as silicon, germanium and gallium phosphide have indirect band-gap. This means that an electron in an energy state near the bottom of the conduction band has a momentum in the crystal that is quite different from that of an electron in an energy state near to the top of the valence band. Excess momentum must change (must be absorbed) during the recombination in an indirect-band gap semiconductor. But probability of two events simultaneously (producing a phonon as well as a photon) is small. As a result non-irradiative processes tend to dominate in indirect band-gap semiconductors and the internal quantum efficiency is very small.Galium arsenide and other semiconductor materials have direct band-gap. An electron can jump from the conduction band to the valence band and a photon can be emitted. Then the internal quantum efficiency in LEDs can be to 0.5.

Internal quantum efficiency:Si: 10-5 GaAs: ...0.5

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Injection luminescence. Internal quantum efficiency

Internal quantum efficiency:Si: 10-5 GaAs: ...0.5

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Injection luminescence. Internal quantum efficiency

ph

phph

ph

k4,2

;]eV[

24.1]µm[;

hc

;

W

T

WW

kTWW

≅=

≅=

+=

λλ∆

γ

λλ

λλλλ/µµµµm γγγγ ∆λ∆λ∆λ∆λ/nm

0.85 0.043 36

1.30 0.065 85

1.55 0.078 120

The distribution of the photon energy and wavelength can be found taking into account the distribution of electrons in the conduction band and the distribution of holes in the valence band.

Researchers intensively work on the problem of silicon laser diodes...

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Silicon laser diodes

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Crystals of pure silicon and rare earth ions in silicon dioxide

Light

Rare-earth ion

Silicon nanocrystal

Electron

Silicon laser diodes

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Silicio šviesos šaltinių veikimas pagrįstas šiais principais:

1. Silicio nanokristalai silicio diokside veikia kaip kvantiniai narvai. Kuo mažesnis nanokristalas, tuo platesnė draudžiamoji juosta. Be to kvantiniai narvai leidžia spręsti momentų problemą ir padidinti spindulinės rekombinacijos tikimybę.

2. Į silicį įterpti retųjų žemės elementų (lantanidų nuo 58 (cerio) iki 71 (lutecio)) jonai spinduliuoja šviesą.

Silicon laser diodes

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Įtaisas spinduliuoja šviesą normalioje temperatūroje. Jo kvantinis efektyvumas gali būti iki 10 % nuo kvantinio efektyvumo, gaunamo panaudojant III-V grupių medžiagas ir šiuolaikiškas technologijas.

Šviesos spalva priklauso tik nuo panaudoto lantanido. Samaris skleidžia raudonos, terbis – žalios, ceris – mėlynos spalvos šviesą, erbis – infraraudonuosius spindulius, taikomus telekomunikacijose.

Kol kas šviesos galia – maža...

Silicon laser diodes

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Application of light for information transmission would increase efectively the transmission rate and revolucionize computing...

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Optical fiberMirrors

Modulators

Electronics

Photodetector

Laser

Optoelectronic IC

Optical waveguides can be used instead of metal conductors for information transmission in optoelectronic ICs... Using silicon technology...

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… pakeisti varinį laidininką optine skaidula, vietoje elektronų naudoti fotonus.

Slicio fotonikos perspektyva – visur taikyti optinių ryšių principus. Gamintojai galės sudaryti optoelektroninius elementus taikydami silicio integrinių grandynų gamybos technologiją. Fotonikos elementų savikaina labai sumažėtų.

… integruotas silicio luste imtuvas-siųstuvas galėtų priimti ir siųsti duomenis 10 ir net 100 gigabitų per sekunde sparta.

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Light emitting diode (LED)

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Light emitting diode (LED)

n p

W

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rrintm

nrrrrrint π2

1 ;

π2

1 ;

111 ;

τη∆

ττττττ

η ==++== Ffd

s

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The double heterojunction

Advantages: 1. High injection efficiency. 2. Confinement of charge carriers in

the active layer.3. Transparency of the wide-band-

gap material.4. Optical guidance.

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2sas

sa2s

2a

intext)(

4;

2;012.0...)1(

nnn

nnt

n

nsst

+===−≅ αηη

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External quantum efficiency

Light is emitted in all directions. Only light emitted in the direction of the semiconductor-air surface is useful.

There is absorption between the point of generation and the emitting surface.Only light reaching the emitting surface at an angle of incidence less than the

critical angle is transmitted through it. Part of this light is reflected at the semiconductor-air surface.

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% 05.0...2

)(2s

22

21int

0intf =

−≅≅

n

nntt

ηΦΦ

ηη

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The cross section area of a fiber core is very small. For this reason, the LED-to-fiber efficiency is also very small – about 0.0005.

LEDs

Combined spectral curves for blue, yellow-green, and high brightness red solid-state semiconductor LEDs. FWHMspectral bandwidth is approximately 24-27 nanometres for all three colors.

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Blue, green and red LEDs. An ultraviolet GaN LED.

LEDs

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LEDs

Special structures of LEDs with small emitting areas are developed and used in telecommunication systems.

Cathode contact layer

p GaAs layer

n GaAs substrate

Anode and radiator

SiO2 layer

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Interaction of photons and electrons. Light amplification

Absorption, spontaneous emission and stimulated emission of light

( ) ( ) ( )νξννα 21hc

nnB

−=

( ) ( ) ( )zPzP νανν −= e,0,

At n1> n2, media absorbs light.

At inverted population, light amplification becomes possible:

( ) ( ) ( ) ( )c

h 12 νξνναν

nnBg

−=−=

( ) ( ) ( )zgPzP ννν e,0, = ( ) ( ) ( )sp

212

2

π8

c

τν

νξν

nng

−=

Normalized power spectral density function

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3 ( W 3 ) 2 ( W 2 )

1 ( W 1 )

Spontaninis spinduliavimas

Stimuliuotasis spinduliavimas

Žadinimas Kaupinimas

W

h ν

a b

Spontaneous emission

Stimulated emission

Excitation Pumping

Light amplification

Inverted population in the three level system and in pn junction

At inverted population, light amplification becomes possible.

At stimulated emission the emitted photon has the same wavelength phase polarization and direction of propagation as the incident one.

Amplification is dependent on spectral line width:

( ) ( ) ( )sp

212

2

π8

c

τννξ

νnn

g−

=

∫∞

=0

1d)( ννξ

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1. Amplifier + positive feedback → generator = laser.

2. Semitransparent mirrors forming optical resonator are used for the feedback.

3. Light spectrum is dependent on natural frequencies of the resonator.

4. Amplitude and phase balance conditions must be satisfied for generation.

( ) ( ) ( )[ ]lgRRPP 2exp00 s212 α−=

( )[ ] 12exp s21 >− lgRR α

+=>

21sΣ

1ln

2

1

RRlg αα ( )

( )22

1

1

+

−=

n

nR

Amplitude balance condition:

LASER – Light Amplification by Stimulated Emission of Radiation

( ) ( ) zag sPzP)(e0 −=

( ) ( )002 PP ≥

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Laser diodes

nlk k =2

λnl

kf

kk 2

cc==

λ nlf

2

c=∆

nlf

f kk 2

2λ∆λλ∆ ==

Due to the resonator … the emission spectrum width of a laser diode is

much less than that of a LED.

The thickness of the active layer is small. Only photons that move along the active region stimulate recombination. For this reason amplified light is emitted in the direction of the active region (in the direction perpendicular to a mirror). .

The phase balance condition is satisfied at the same phase angles of incident wave and wave that appears after the second reflection.

The length of the active region (the distance between mirrors) must equal the integer number of half wave.

fdc

d2λ

λ =

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1954 m.: maser: C.H.Townes, N.Basov, A.Prochorov1960 m.: Solid state (ruby) laser 1962 m.: semiconductor injection lasers, J = … 300–500 A/mm2

1968 – 70 m.: GaAs, GaAlAs technology, heterojunctions, J→ 5 A/mm2

hhFpFn WWW −

<<∆

ν

At , the media is transparent.

At , the media absorbs light.

W∆ν <h

FpFnh WW −>ν

I � ∆λ�

Laser diodes

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UI

I

UI

P

q

h1 tho

ηη

−≅=

Efficiency – to 65 %.

ηην →≅>> Dth ,, hqUII

Luxampere characteristic of a laser diode

q

II sl−≅ηΦ

νην hh tho

q

IIΦP

−≅=

Laser diodes

+=>

21sΣ

1ln

2

1

RRlg αα

At small current the diode operates as LED...

Above the threshold the output powerincreases and spectrum narrows.

The photon flux density:

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A packaged laser diode with penny for scale.

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The structure of the GaAlAs-GaAs laser diode with the

buried heterojunction

Laser diode for telecommunications

n GaAs substrate

Metal o sluoksnis

n GaAs pagrin das

N AlGaAs sluoksnis

Aktyvusis p GaAs sluoksnis

Kontaktinis p + GaAs sluoksnis

SiO 2 sluoksnis

Metalo sluoksnis (diodo anodas)

P AlGaAs sluoksnis

N AlGaAs

Metal layer (anode)

P AlGaAs layer

Active p GaAslayer

N AlGaAs layer

Metal layer

Metal layer (anode)

SiO2 layer

Contact p+ GaAslayer

N AlGaAsn GaAs substrate

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MetalSiO2

Active layer

Laser diode

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Two varieties of telecommunication laser diodes: (a) dual-in-line 14 pin, and (b) butterfly package.

Laser diodes

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Zarlink launches a new line of long-wavelength laser diodes with the industry’s highest level of customization. The ZL60401 laser diodes can be tailored for a broad range of industrial and commercial equipment, as well as telecom and datacom applications.

Laser diode

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Modulation

Change of the drive current causes the frequency to vary, as well as the output power. This is normally referred to as chirp.

Amplitude, phase and frequency modulation is used in coherent systems.

The emitted power is proportional to the diode current. This property is used for amplitude modulation of light. The modulation bandwidth can be to 10 GHz. Usually the diode is biased beyond the threshold.

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Modulation

ENCODING PHOTONS WITH DATA: An optical modulator encodes 1s and 0s by first splitting a laser beam in two and then applying an electric field to the beams, so that one beam is delayed by half a wavelength relative to the other. When the beams recombine, both beams will be out of phase, and they will cancel out.When no voltage is applied, on the other hand, the beams remain in phase when recombined. Encoding the beam with 1s and 0s, then, means making the beams interfere (0) or keeping them in phase (1).

Opticalcoupler(splitter, combiner).

Pockelseffect.

Mach-Zehnderinterfero-meter.

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The key to Intel's continuous silicon laser—the world's first—is a PIN (p-type–intrinsic–n-type) diode placed on either side of the light beam. The diode sweeps free electrons from the path of the light. Without it, the electrons build up and absorb some of the light, killing the amplification.

http://www.spectrum.ieee.org/print/1915

Modulation

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Užduotys

1. Šviesos diodui panaudotas galio arsenidas, kurio draudžiamosios juostos plotis ∆W = 1,42 eV. Raskime šviesos diodo intensyviausiai spinduliuojamos šviesos bangos ilgį ir šviesos spektrinės linijos santykinįplotį 0 0C ir 100 0C temperatūrose. Palyginkime šviesos bangos ilgio santykinį pokytį ir šviesos spektrinės linijos santykinį plotį. Įvertinkime maksimalaus intensyvumo virpesių dažnio absoliutinį pokytį.

2. Sudarykime dvigubos GaAlAs-GaAs-GaAlAs NpP heterosandūrosenergijos lygmenų diagramą, kai neveikia išorinė įtampa. Kokio poliarumo įtampa šiai dvigubai heterosandūrai būtų tiesioginė? Kaip pasikeistųheterosandūros energinė diagrama veikiant tiesioginei įtampai?

3. Dvigubos heterosandūros aktyviojo sluoksnio storis – 0,5 µm, spindulinės rekombinacijos laiko pastovioji – 10 ns, nespindulinės rekombinacijos laiko pastovioji – 30 ns, rekombinacijos greitis heterosandūroje – 10 m/s. Raskime heterosandūros kvantinį našumą ir moduliacijos dažnių juostos viršutinį dažnį.

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4. Šviesos diodo sandūra atvaizduota 2.10 paveiksle, a. Virš aktyviojo sluoksnio yra puslaidininkis, kurio lūžio rodiklis – 3,7. Diodo vidinis kvantinis našumas – 0,7. Raskime diodo išorinį kvantinįnašumą.

5. Šviesos diodo ir skaidulos sąsaja atvaizduota 2.10 paveiksle, b. Skaidulos NA = 0,1. Raskime šaltinio-skaidulos kvantinį našumą.

6. Šviesos diodui panaudota dviguba NpP heterosandūra, kurios storis 0,5 µm. Medžiagų parametrai duoti lentelėje. Kai diodo tiesioginėįtampa 2 V ir per jį teka 100 mA srovė, diodas spinduliuoja 2 mWoptinę galią. Koks diodo, kaip energijos keitiklio, naudingumo koeficientas?

Užduotys

0,42,52,13

0,23,11,42

0,42,52,11

WFi-WviWci-WFiΧi∆Wii