vcsel vertical cavity surface-emitting laser
DESCRIPTION
VCSEL Vertical Cavity Surface-Emitting Laser. Docente: Mauro Mosca (www.dieet.unipa.it/tfl). A.A. 2013-14. Ricevimento: alla fine della lezione o per appuntamento. Università di Palermo – Facoltà di Ingegneria (DEIM). Edge-emitting e surface-emitting laser. - PowerPoint PPT PresentationTRANSCRIPT
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VCSELVCSELVertical Cavity Surface-Emitting LaserVertical Cavity Surface-Emitting Laser
Docente: Mauro MoscaDocente: Mauro Mosca
(www.dieet.unipa.it/tfl)(www.dieet.unipa.it/tfl)
Ricevimento: alla fine della lezione o per appuntamento
Università di Palermo – Scuola Politecnica - DEIM
A.A. 2015-16A.A. 2015-16
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Edge-emittingEdge-emitting e e surface-emittingsurface-emitting laser laser
- Applications: local area networks, color displays, bio-sensing, printing applications (using VCSEL arrays would increase printing throughput) and optical data storage
- Advantage over edge emitting LDs: micrometric size, that allows larger devices density on substrates
- No additional technology steps for the realization of good reflective facets (for edge emitting LDs: chemical assisted ion beam etching, cleaving focused ion beam polishing or wet chemical polishing)
- Circularly shaped, low numerical aperture beam (ideal sources for fiber coupling and free space optics), single longitudinal mode operation (due to a cavity length of the order of one λ), low power dissipation and significantly lower operating currents
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Struttura di una cavità VCSEL planareStruttura di una cavità VCSEL planare
/2n
Only a few single longitudinal modes are supported by a VCSEL cavity
The emission wavelength is not determined anymore by the maximum gain of the active material but rather by the geometry of the cavity. Thus, VCSELs can lase only if the QW emission wavelength approximately coincides with the cavity mode
Lasing in a VCSEL critically depends on the reflectivity of both the top and bottom mirrors.
99%
epitaxially grownor dielectric
tipically /n
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Condizione di soglia (threshold)Condizione di soglia (threshold)
- The lasing condition in a laser is reached when the amplitude of the optical field is maintained after a round trip in the cavity
- This condition is reached when the optical gain in the cavity is sufficient to compensate all the losses the field experiences in the cavity during a round trip
INTERNAL LOSSES EXTERNAL LOSSES
absorption scattering diffraction mirror reflectivity
threshold modal gain threshold material gain
The confinement factors (Γxy and Γz) accounts for the volume actually occupied by photons in the cavity, that is usually larger than the active region volume
standing wave enhancement factor
= 1 in edge-emitting laser
z is subject to an enhancement dependent on the position of the active region into the cavity
’
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Distribuzione di campo Distribuzione di campo longitudinale dentro la cavitàlongitudinale dentro la cavità
enh = 2EZ
z
= 0 actactQWsactQWsact
ll
dzl
dzE
E
l
222
12
0
2
0
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Guadagno e corrente di soglia in Guadagno e corrente di soglia in funzione della riflettivitàfunzione della riflettività
The semiconductor material becomes “transparent” (material transparency) when the rate of absorption just equals the rate of stimulated emission.
One incident photon produces exactly one photon in the output
The transparency current density is defined as the minimal current densityfor which the material becomes transparent for any photon energy largerthan or equal to Eg,qw
gain parameter per quantum-wellsnumber of quantum-wells
transparency current
’
The gain upon transparency is g = 0 (Jth = Jtr , for NQW = 1)
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Guadagno e corrente di soglia in Guadagno e corrente di soglia in funzione della riflettività (InGaAs QW)funzione della riflettività (InGaAs QW)
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Guadagno e corrente di soglia in Guadagno e corrente di soglia in funzione della riflettività (InGaAs QW)funzione della riflettività (InGaAs QW)
R
g th1
ln
a large number of QWs decreasesgth, since lact increases
very high R with a single QW we havethe lowest Jth becauseit requires the smallest currentdensity to be pumped to thetransparency
lower R gth is higher and it is still higherfor only one QWJth will be lower with 3 QWsthan one
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Guadagno e corrente di soglia in Guadagno e corrente di soglia in funzione della riflettività (GaN QW)funzione della riflettività (GaN QW)
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Guadagno e corrente di soglia in Guadagno e corrente di soglia in funzione della riflettività (GaN QW)funzione della riflettività (GaN QW)
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DBR (Distributed Bragg Reflectors)DBR (Distributed Bragg Reflectors)normal incidence
++
the two contributions sum in phase at the design wavelength λ0
a mirror with a wide stop-band ensures a higher tolerance with respect to emission wavelength variations
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DBR: lunghezza efficace della cavitàDBR: lunghezza efficace della cavità
The penetration of the optical mode into the DBR stack has to be taken into account, as it defines the effective cavity length and, subsequently, the wavelength of the lasing mode
The penetration depth leff of the DBR is defined as the depth into the
mirror, at which the optical field intensity is equal to 1/e of its value at the input of the mirror
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DBR: effetti delle perdite per assorbimentoDBR: effetti delle perdite per assorbimento
reflectivity ofan m-pair DBR
absorptioncoefficientof the DBR
high n helps to reduce the absorption losses of the mirrors
low n and high DBR
and 0 for longwavelength VCSEL
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Effetto interfacce gradualiEffetto interfacce graduali
- Interfaces not abrupt!
- The material composition islinearly graded over a distanceof some tens of nm in orded toreduce the electrical resistanceacross the interface
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Resistenza degli specchiResistenza degli specchi
heating
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Resistenza degli specchiResistenza degli specchi
doping
fabricationtechnology
simpler
barrier for holes(high R)p-doped
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Specchi gradualiSpecchi graduali
valenceband
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Efficienza differenzialeEfficienza differenziale
fattore di normalizzazione1.
2.
mirror losses
total losses
3.
MECHANISMS THAT REDUCE d :
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Efficienza differenzialeEfficienza differenziale
depends on cavity design
depends on mirror design
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Wall-plug efficiencyWall-plug efficiency
top mirror: 19 periods
thermal roll-over
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Wall-plug efficiencyWall-plug efficiency
W
Inoltre… se la riflettività diminuisce troppo,aumenta la corrente di soglia!!!
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Confinamento lateraleConfinamento laterale
Injections schemes for GaAs-based VCSELs: - conductive DBRs with and without current confinement layers;- annular intra-cavity contacts in combination with one or two current confinement layers
sacrificial layer
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Ossidazione laterale dell’AlAsOssidazione laterale dell’AlAs
.
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Ossidazione laterale dell’AlAsOssidazione laterale dell’AlAs
W
?
TEnergia libera di Gibbs a 698 K
< 0, vuol dire che la reazione è spontanea nella direzione indicata
With GaAs (instead of AlAs): G > 0!!
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Ossidazione laterale dell’AlAsOssidazione laterale dell’AlAs
Controlled Evaporated and Mixing systemprocesso lineare… ma diventa dipendente dalla radicequadrata
i gas reagenti penetrano difficilmenteattraverso l’ossido(per alti spessori dello stesso ossido)
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Ossidazione laterale dell’AlGaAsOssidazione laterale dell’AlGaAs
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Fabbricazione di un VCSEL con Fabbricazione di un VCSEL con ossidazione lateraleossidazione laterale
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Ossidazione laterale in VCSEL a nitruriOssidazione laterale in VCSEL a nitruri
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Ossidazione laterale in VCSEL a nitruriOssidazione laterale in VCSEL a nitruri
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Ossidazione laterale in VCSEL a nitruriOssidazione laterale in VCSEL a nitruri