Download High Power Laser Diodes

Risø National Laboratory
High Power Laser Diodes
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
The diode laser
+
P-type
active
junction
n-type
_
•Laser action is obtained using the natural cleaving surfaces of the crystal (30% reflectivity)
•Typical dimensions: 0.1 x 0.2 x 0.002 mm
•Cheap laser – but divergent laser beam with
significant astigmatism
•The diode laser is used in a large number of applications
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Diode lasers (advantages)
•Small dimensions: 300 µm x 50 µm x 10 µm
•Laser action with a simple power supply (1A, 2 V)
•Large efficiency η=30-40% (electricity to light)
•Long lifetime (up to 40.000 hours)
•Well suited for optics communication (10-100 GHz)
•The semiconductor technology is well established
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Energy levels in semiconductors
Energy
Conduction band
Ef
bandgap
Valence band
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Light emission from semiconductors
energy
electron
Conduction band
conduction
band
Light emission
Band gap
If electrons are excited into
the conduction band ligth is
emitted. The frequency
depends on the band gap
Valence band
Hole
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Doping of semiconductors
Doping in Si:
Doping with group V atoms
electrons (n-type)
Doping with group III atoms
electrons (p-type)
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Semiconductors (n-type og p-type)
n-type
p-type
CB
EF
CB
VB
VB
EF
electron
hole
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
pn-junction in semiconductors
(A)
n-type
(A) Doping of semiconductors:
n-type and p-type semiconductors
p-type
CB
EF
CB
VB
VB
EF
(B) pn-junction without applied voltage
The Fermi energy remains constant
everywhere. The conduction bands and
the valence band are bended. This
potential barrier prevents electrons
from diffusing into the p-type material.
Electron
Hole
(B)
p-type
n-type
EF
( C)
Efp
p-type
n-type
E=eV
Efn
(C) pn-junction with applied electric
field. The potential barrier is reduced
and electrons diffuse into the p-type
material where they recombine and
stimulated emission can take place.
Recombination
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Generation of carriers
Generation of carriers is governed by
the rate Equation:
dN
I
N
2
=
− + D∇ 2 N − g ( N ) E0
dt
qV
τs
where N(x,y,z) is the excited carrier population,
s is the carrier recombination time,I is the injected
current
D the ambipolar diffusivity, E0 is the total optical field
g(N)=a(N-N0) is the gain.
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Lateral antiguiding in semiconductor lasers
The refractive index of the semiconductor material depends on the
concentration of free carriers. The antiguiding parameter is given by:
α c = − 2k0
∆n
∆g
The complex refractive index is given by:
∆n = ∆n + i
∆g
2 k0
Since β is positive an increase in gain will reduce the refractive index. The effect
leads to self-defocusing in the lateral intensity distribution. For broad-area lasers
the lateral antiguiding leads to a far field with two lobes.
Self-defocusing
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Threshold for population in
semiconductor lasers
. The
gain in the active area is:
λ2A
g (ν ) =
N 2 S (ν )
2
8π n
Assuming homogeneously broadened transition
with Lorentzian linewidth δv0 we have:
λ 2 AN 2
g (ν 0 ) = 2 2
8π n δν 0
The threshold population for laser action:
N 2,t
8π 2 n 2δν 0
1
1
(
ln
)
since
g
ln r1r2
=
−
=
−
a
r
r
a
1 2
t
2
λ A
2l
2l
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Pumping threshold
The injection rate is equal to the loss rate:
J
= Re N 2
ed
The rate per volume at which electrons
are injected is equal to the loss rate:
Inserting the threshold value of N2 leads to:
Jt =
8π 2 n 2δν 0
λ2
Re
1
(eD) (a − ln r1r2 )
2l
A
The threshold
current for laser action
Example: GaAs laser, λ=8400 Å , n=3.6, δν0=1013 Hz, A/Re=1,
D=2µm, l=0.5 mm, a=10 cm-1 we find Jt =500 amp/cm2.
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
The heterojunction laser diode
Due to additional layers with AlAsGaAs alloys in the nonactive layer the
band gap is changed just outside the
active layer.
This effect leads to a high index in the
active layer and, consequently, the
losses are significantly reduced.
Double heterostructure diode lasers
have extended the lifetime of laser
diode significantly.
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Gain-guided and index-guided
lasers
Gain-guided lasers:
A stripe electrode creates
carriers that guide the light
Index-guided lasers:
A waveguide groove with high
index guides the light
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
The output beam from a high-power
diode
axis of high
coherence
emitting
junction
axis of low
coherence
1 mm
100 µs
Broad- area lasers and laser arrays have a high coherence
axis and a low coherence axis with different divergence
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
Gainguiding in high-power laser
diodes
The applied voltage creates carriers that guide the light in
the pn-junction in the semiconductor
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
High-power multiple stripe arrays
metallization
GaAs (p-type)
GaAlAs (p-type)
GaAlAs
GaAlAs (n-type)
GaAs (n-type)
gain
proton implantation
High-power multiple stripe arrays have increased the output
of laser diodes significantly
Proton implantation is used to define a periodic voltage
The coherence of these lasers, however, is rather poor.
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school
Risø National Laboratory
High-power diode laser system
Broad area laser
Laser diode array
Laser diode bar
Schematic diagrams of the different types of highpower laser diodes
Paul Michael Petersen, Biophotonics 05, 2nd International Graduate summer school