semiconductor lasers operating principle : outlinepbrosson.free.fr/eso/01_laser.pdfsemiconductor...

1Philippe Brosson/ESO/2005/Semiconductor Lasers and integrated devices/ 1- S.L. Operating principle/

Semiconductor Lasers operating principle : outline

-> gain and recombination processes, threshold, QW lasers waveguiding in semiconductor lasers Distributed FeedBack lasers (DFB) Semiconductor Laser structures

1


Double-heterojunction laser

refr

activ

ein

dex

profi

le

field

profi

leI

PN

active layer0.2 µm (bulk)9 nm (quant. well)

current flow(a few mAs)

300 µm

100 µm

lightcleaved facet {110}R=0.3-0.4

2


Optical transitions

ENER

GYEN

ERGY h ν

ENER

GY

h ν

h νh ν

Absorption

Spontaneous emission

Stimulated emission

3


Optical gain

φdφ = g dz

φ0

⇒0 z⇒

φ φ0 e g z=

gain region

Φ ( photons s-1cm-2 ) photon fluxg ( cm-1 ) optical gain

4


Carrier confinement in a homo-junction laser, @ 0 K

no bias, V=0

electrons

Fermi level

valence band

conductionband

EgEg

valence band

conductionband

P type N type

}

populationinversion

EFc

VEFvh ν

Carriers can diffuse and are poorly confined

forward bias V

5


Carrier and optical confinement in a double-heterojunction laser

forward biasDouble-heterojunctionlaser

Condition for Net stimulated emissionEg < hν < E g + EF c +EF v

[M.G. A. Bernard, G. Duraffourg, Phys. Status Solidi, 1, 699, 1961]

refra

ctive

inde

x

injected electrons

holes

conduction band

valence band hν ≈ Eg

P NInP InP GaInAsP

d ≈ 0.1-0.3 µm

ener

gy

[I. Hayashi, M. B. Panish, F. K. Reinhart, « GaAs-Alx Ga 1-x As double-heterostructure injection lasers », J. App. Phys., vol. 42, pp 1929-1941, Apr. 1971]

6


Threshold condition for a Fabry-Perot laser

Φ0Φ0 Exp(g-α)L

R2Φ0 Exp(g-α)LR2Φ0 Exp(g-α)2 L

R1R2Φ0 Exp(g-α)2 L

mirror loss

Φ0 = R1R2Φ0 Exp(g-α)2 Lg = α + (1/L) Ln(1/Sqrt(R1R2) )

internal loss

LR1 R2

gain

7


Light-current characteristics

0 20 40 60 80 1000

1

2

3

4

5

I ( mA )

P ( m

W )

Jth(kA/cm2) = 100 Ith(mA)L(µm) w(µm)

Linear gain approximationgth = a ( Nth -Nt)

Case of a Double-Hetero structure laserΓgth = Γαint + (1-Γ)αcladd. + 1/L Ln(1/R)

-> threshold carrier density Nth(cm-3)

Optical confinement

Recombination rateJ/ed = Rrad (N) + Rnon rad (N)

-> threshold current density Jth(kA/cm-2)

gth = 100 cm-1

Nth = 2x10 18 cm-3

Jth = 1 kA/ cm2

Γ= 0.2

8


Spontaneous and external differential efficiencies

Spontaneous radiative efficiency

External differential efficiency(per facet)

9


Optical spectrum for a Fabry-Perot laser

Longitudinal mode spacing Δλqby differentiation of (1)

L = 400 µmλ= 1494 nmn = 3.53dn/dλ = -0.12 µm-1ne = 3.70Δλq = 0.75 nm

Effective group index

q integerλ free space wavelengthL cavity lengthn effective index

10


Threshold current density and active layer thickness

The threshold current density is minimum around 0.15 micrometer

Thre

shol

d cu

rrent

den

sity

(kA/

cm2 )

11


Threshold current density in a Quantum Well laser

The threshold current density is minimum for a 5-well laser

12


Quantum well semiconductor lasers

Lz

E 1c

E 2c

E 1hhE 2hh

E 1lhE 2lhE 3lh

E 3c

E 3hh

Egh ν ≈ Eg + E1c + E1hh

∆Ec

∆Ev

conduction band

valence band

En = h2

2 m* nπLz

2for an infinitely deep well

InP GaInAsP

GaInAs

D2 D2transition with highest gain

multiple quantum well laser

13


Control of the emission wavelength in a quantum qwell laser

the energy hν of the peak gain is determined by the width Lz of the well the emission wavelength is controlled by Lz

Lz = 5 nmλ = 1.4 µm

Lz = 9 nmλ = 1.55 µm

14


Maximum material gain in a quantum well laser

well: GaInAsbarrier: GaInAsP, λg = 1.2 µm

0

500

1000

1500

2000

2500M

axim

um G

ain

(cm

-1)

0 1 2 3 4 5N (10^18 cm-3)

G(N) Data

quantum well

bulk GaInAs

[Zielinski]

in quantum well lasers:• threshold carrier density is lower• higher modulation bandwith (due to higher dg/dN)

15


Temperature dependence of the threshold current density

Split-Off

ConductionBand

HeavyHole

Light Hole

Δ

Eg

E k

1

2

3

hν

hν

acceptor

IVBA

Split-Off

ConductionBand

HeavyHole

Light Hole

Δ

Eg

E k

1

2

3

hν

Auger

origin of the low T0 in GaInAsP• carrier leakage over barriers• Inter Valence Band Absorption• Auger recombination

low T0 <-> high temperature sensitivity

Jth ( T ) ≈ J0 eTT 0

400300200100100

1000

10000

TEMPERATURE(K)

Thre

shol

d cu

rren

tde

nsity

(A/c

m2

)

T0 = 110 K

T0 = 60 K

GaAs T0 = 120 K

GaInAsP

16



gain and recombination processes, thresold, QW lasers->waveguiding in semiconductor lasers Distributed FeedBack lasers (DFB) Semiconductor Laser structures

17


Waveguide structure

Y

x

z

exponential

exponential

cosine {fie

ldpr

ofile

refr

activ

ein

dex

profi

leEy

Electric field

18


Beam ray description of guided modes

θc = sin-1 ( ncladncore )

θ0 = sin-1 ( neff0ncore )

θ1 = sin-1 ( neff1ncore )

cladding nclad

cladding nclad

core nlcore

θc

TE0 TE1

d

critical angle θc for total internal reflection

guided modes:θ > θc

ncore = 3.53nclad = 3.17d = 0.75 µm2 modes TE0, TE1

neffTE0TE1

critical

796764

3.463.253.17

θ (°)

guided

modes

19


Optical field profile (perpend. to junction plane)

dact = 0.4 µmneff = 3.38Γ = 0.77 d I/e2 = 0.65 µm

ncore = 3.53nclad. = 3.17

dact = 0.1 µmneff = 3.20Γ = 0.17 d I/e2 = 1.1 µm

20


6 mW

0.6 mW

-50 +50 -50 +50Angl e(°)

//⊥

10 mW

Beam divergence

Y

x

z

half intensity contour

θ//

θ⊥

θ// = 23 °θ⊥ = 24 °A. Accard, F. Brillouet, E. Duda, B. Fernier, G. Gelly, L. Goldstein, D. Leclerc, D. Lesterlin,

J. Phys. III, France, Vol 2, p1727-1738, 1992

E⊥ (θ ) = E (x) e - j [k sin θ ] x dx

21



gain and recombination processes, QW lasers waveguiding in semiconductor lasers-> Distributed FeedBack lasers (DFB)

22


Principle of DFB lasers (Distributed FeedBack)

Optical feeback is achieved by a periodic perturbation along the propagation direction z (modulation of the effective index)with a grating

case of index coupling:the interaction between waves R and S is described by a real coupling coefficientΚ(cm-1)

optical feedback due to constructive interferences betweenthe two waves R and S travelling in opposite directions

Λ = grating period

confinement layergrating layeractive layerconfinement layer

Λ

R S

z

23


Selective feedback for λ ≅ λ Bragg

Λ cladding layer

cladding layer

grating layeractive layer

Λ

θ

[ Λ + Λ sin θ ] neff = q λ Bragg

θ = π/2

first order grating (q=1)λBragg ≅ 1.5 µm Λ ≅ 0.2 µm

second order grating (q=2)λBragg ≅ 1.5 µm Λ ≅ 0.4 µm

Λ = λBragg

2 neff

24


Modes and respective threshold

Wavelength

L igh

t int

ens it

y(a

rb. u

nit )

thre

shol

d

Wavelength L i

ght i

nten

s ity

(arb

. un i

t )th

resh

old

DFB laser Fabry-Perot laser

25


References (DFB lasers)[1] Kogelnik H., Shank C. V., " Coupled-wave theory of distributed-feedback lasers ", J. Appl. Phys.,1972, 43, pp2327-2335

[2 ] STREIFER, W., SCIFRES, D. R., BURNHAM, R. D., "Coupling coefficients for distributed feedbacksingle-and double-heterostructure diode lasers ", IEEE J. Quantum Electron., 1975, QE-11, pp. 867-873

[3] Kihara K., Soda H., Ishikawa H., Imai H. , "Evaluation of the coupling coefficient of a distributed feedbacklaser with residual facet reflectivity ", J. Appl. Phys.,1987, 62, pp 1526-1527

[4] STREIFER, W., BURNHAM, R. D., SCIFRES, D. R. :"Effect of external reflectors on longitudinal modes ofdistributed feedback lasers", IEEE J. Quantum Electron., 1975, QE-11, pp. 154-161

[5] Wang S., " Design considerations of the DBR injection laser and the waveguiding structure for integratedoptics " ,IEEE J. Quantum Electron., 1977, QE-13, pp. 176-186

[6] P. Brosson, C. Artigue, B. Fernier, D. Leclerc, J. Jacquet, J. Benoit, “A simple determination of the couplingcoefficient in DFB waveguide structures“, Electron. Letters, Vol. 24, pp 990, 1988.

26



gain and recombination processes, QW lasers waveguiding in semiconductor lasers Distributed FeedBack lasers (DFB)-> Semiconductor Laser structures

27


Vertical laser structure(bulk active layer)

GaInAsP contact layer p+

InP p

InP substrate n

InP n 0.2 µm

GaInAsP λg = 1.3 µm

λg = 1.55 µm GaInAsP

0.1 µm

0.1 µm

28


Vertical laser structure(Quantum Well)

GaInAs contact layer p+ InP p

InP substrate n

g grating layer

wells (9 nm) andbarriers (9 nm)

D2 = 9 - 150 nm

D2 = 9 - 150 nm

GaInAsP

GaInAsP λg = 1.2 µm

GaInAsP λg = 1.2 µm InP n D3 = 100 - 400 nm

λg = 1.2 µm

B. FERNIER, L. GOLDSTEIN, A. OLIVIER, A. PERALES, C. STARCK andJ. BENOIT :" Multiquantum welllasers at 1.5µm by GSMBE ",15th ECOC, SWEDEN, WeB 14 - 6, pp. 264 - 267, (1989)

29


Broad area lasers

n InP substrate

n InPGaInAsP active

p InPp GaInAsP

100 µmcontactSimple laser structure to

test the lasers• no lateral current spreading• good determination of

- Jthreshold (kA/cm2)- external efficiency

requires pulsed operationto avoid heating(high Ithreshold = 0.5-3 A)

30


Stripe lasers

n InP substrate

p InP

10 µm Sio2

100 µm

n InP substrate

p InP

p+ GaInAsP

GaInAsP activen GaInAsP passive

≈ ≈

SiO2 InP p, Zn

InP n

InP p, BeW &B

InP n substrate

Index guided∆n =

0.0

01- 0

.01

∆n =

0.1

- 0.3

Gain guided(light is absorbed in

the unpumped region)

31


Buried double-hetero-structure laser processing(1)

n InP substraten InP

p InPp+ GaInAsP

1 epitaxy

2 SiO2 deposition

3 SiO2 engraving

4 mesa engraving

5 epitaxial regrowth

6 AuZn contact &engraving

32


Buried double-hetero-structure laser processing(2)

7 SiO2 deposition 8 TiPtAu p contact 9• lapping & chemicalpolishing• AuGeNi n contact

n InP substrate

GaInAsP activep InP

p+ GaInAsPZn diffusionSiO2

n InP

n InPp InP

x x x x

≈≈

x x x x

33

semiconductor lasers operating principle : outlinepbrosson.free.fr/eso/01_laser.pdfsemiconductor...

Documents