lasers lecturer: mauro mosca ( university of palermo –deim a.a. 2014-15
TRANSCRIPT
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LasersLasers
Lecturer: Mauro MoscaLecturer: Mauro Mosca
(www.dieet.unipa.it/tfl)(www.dieet.unipa.it/tfl)
University of Palermo –DEIM
A.A. 2014-15A.A. 2014-15
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Emission and absorption in radiationEmission and absorption in radiation
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Einstein relationsEinstein relations
( )
coefficienti di Einstein
=
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Einstein relationsEinstein relations
=
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Einstein relationsEinstein relations
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EsercizioEsercizioQuale temperatura si dovrebbe Quale temperatura si dovrebbe raggiungere per ottenere emissione raggiungere per ottenere emissione stimolata per delle frequenze ottiche stimolata per delle frequenze ottiche all’equilibrio termico (es. all’equilibrio termico (es. = 632,8 nm)? = 632,8 nm)?
Dalla 2kT
h
e
K 804322ln1062,6
1074,41062,6
2ln 23
1434
k
hT
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Two-levels laserTwo-levels laser
? ? ? ?
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Two-levels laserTwo-levels laser
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Three-levels laserThree-levels laser
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GainGain
mezzoomogeneo
!
=
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-levels laser-levels laserThreeThreeFourFour
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mezzo attivo(amplificatore)
L A S E Rlight amplification by stimulated emission of radiationOSCILLATOR
FEEDBACK POSITIVO
parzialmente riflettente
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Optical resonatorsOptical resonators
risonatore di Fabry-Perot
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Laser oscillationsLaser oscillations
=
gain in excess of loss
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Laser propertiesLaser properties
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Modi longitudinaliModi longitudinali
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Doppler shiftDoppler shift
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EsercizioEsercizioQuanti modi sono presenti a 300 K all’interno Quanti modi sono presenti a 300 K all’interno dello spettro d’uscita di un laser He-Ne, la cui dello spettro d’uscita di un laser He-Ne, la cui cavità è lunga 50 cm?cavità è lunga 50 cm?Il Il Doppler shiftDoppler shift dell’emissione dell’He-Ne a 300 K dell’emissione dell’He-Ne a 300 K risulti pari a 1,5 GHzrisulti pari a 1,5 GHz
distanza tra i modi
numero di modi dentro lo spettro =
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Modi trasversaliModi trasversali
off-axis rays also may interfere…
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Pulsed laser: Q-switchingPulsed laser: Q-switching
Laser medium with high loss:
What does it happen if
?
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Pulsed laser: Q-switchingPulsed laser: Q-switching
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Argon laser Argon laser
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Nd:YAG laserNd:YAG laser
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Semiconductor Laser: Semiconductor Laser: giunzione giunzione p-np-n all’equilibrio all’equilibrio
EC
EC
EV
EV
EF EF
p n
= 0,7 – 0,8 V
p n
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EC
EV
EF EF
p n
EA
ED
Semiconductor Laser: Semiconductor Laser: drogaggiodrogaggio
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Semiconductor Laser: Semiconductor Laser: polarizzazionepolarizzazione
p n
EC
EV
EF EF
p n
V0
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Semiconductor Laser: Semiconductor Laser: polarizzazionepolarizzazione
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Semiconductor laserSemiconductor laser
E
EC
EV
x
transition that occursspontaneously
transition stimulatedby incoming photons(same Eg)
positive feedback to thestimulated transition
LED
opticalamplifier
LASER but…
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Semiconductor laser:Semiconductor laser:lasing conditionslasing conditions
1)
Solution)
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Semiconductor laser:Semiconductor laser:lasing conditionslasing conditions
2)
Solution)edge-emitting laser
g
V
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LossesLosses
Why so high losses?
g
nGaAs = 3,35 R = = 0,292
dipendenza lineare da R
EC
EV
EF EF
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Losses:Losses:free carrier absorptionfree carrier absorption
free carrier absorption
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Heterojunction laserHeterojunction laser
homojunction heterojunction
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Light-Current characteristicLight-Current characteristic