Download - 3. Light sources - VILNIUS TECH
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
ELEKTRONIKOS ĮTAISAI 2009
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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
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.
ELEKTRONIKOS ĮTAISAI 2009
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Injection luminescence. Internal quantum efficiency
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.
ELEKTRONIKOS ĮTAISAI 2009
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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
ELEKTRONIKOS ĮTAISAI 2009
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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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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.
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.
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.
% 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.
ELEKTRONIKOS ĮTAISAI 2009
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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
ELEKTRONIKOS ĮTAISAI 2009
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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 ≥
ELEKTRONIKOS ĮTAISAI 2009
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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 =
ELEKTRONIKOS ĮTAISAI 2009
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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
ELEKTRONIKOS ĮTAISAI 2009
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UI
I
UI
P
q
h1 tho
Dν
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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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