666
23 Electricity-to-Light Conversion
(a)
1.0
0.5
0 -1.0
-0.5
0
5
.
0
0
.
1
AlGaAs/
GaAs
0.1
0.075
0.2
(b)
Fig. 23.38 a Optical mode (relative intensity) for various values of the thickness d of the active layer as labeled of a
GaAs/Al 0.3 Ga 0.7 As DHS laser, b Optical confinement factor as a function of the thickness of the active layer and the
Al concentration x of the barrier as labeled in a GaAs/Al x Ga 1−x As DHS laser. Adapted from [1976]
Fig. 23.39 a Various
geometries of the active
layer of a DHS laser with
quantum wells as active
medium, (i) single QW
(separate confinement
heterostructure, SCH), (ii)
multiple QW SCH, and c
GRINSCH (graded-index
SCH) structure. b Layer
sequence for a separate
confinement
heterostructure laser
(a)
E C
E V
E C
E V
E C
E V
(i)
(ii)
(iii)
SCH
MQW-SCH
GRINSCH
(b)
p -GaAs
+
p-Al Ga As
0.3
0.7
p-GaAs
+
p-Al Ga As
0.1
0.9
n-Al Ga As
0.1
0.9
n-Al Ga As
0.3
0.7
E g
n
2
(i)
(ii)
(iii)
(iv)
g mod = g mat .
(23.14)
In order to allow simultaneous optimization of the light mode and the carrier confinement, the separate
confinement heterostructure (SCH) has been designed. Here, a single or multiple quantum well of a
third material with even smaller band gap is the active medium (Fig. 23.39a,b,d). A single quantum well
has an optical confinement factor of a few per cent only. It offers, however, efficient carrier capture and
efficient radiative recombination. An increase in the carrier capture efficiency can be achieved using a
graded index in the barrier (GRINSCH, Fig. 23.39c).
The thin wave-guiding layer leads to large divergence of the laser beam along the vertical direction,
typically about 90
◦ . The strong confinement of light also limits the maximum achievable output power
due to catastrophic optical damage (COD). Several ideas have been realized to overcome this problem
and achieve much smaller divergence of about 18
◦ . The waveguide can be designed to be very thick
(large optical cavity, LOC) that leads to an increase of threshold. Other schemes are insertion of a
23 Electricity-to-Light Conversion
(a)
1.0
0.5
0 -1.0
-0.5
0
5
.
0
0
.
1
AlGaAs/
GaAs
0.1
0.075
0.2
(b)
Fig. 23.38 a Optical mode (relative intensity) for various values of the thickness d of the active layer as labeled of a
GaAs/Al 0.3 Ga 0.7 As DHS laser, b Optical confinement factor as a function of the thickness of the active layer and the
Al concentration x of the barrier as labeled in a GaAs/Al x Ga 1−x As DHS laser. Adapted from [1976]
Fig. 23.39 a Various
geometries of the active
layer of a DHS laser with
quantum wells as active
medium, (i) single QW
(separate confinement
heterostructure, SCH), (ii)
multiple QW SCH, and c
GRINSCH (graded-index
SCH) structure. b Layer
sequence for a separate
confinement
heterostructure laser
(a)
E C
E V
E C
E V
E C
E V
(i)
(ii)
(iii)
SCH
MQW-SCH
GRINSCH
(b)
p -GaAs
+
p-Al Ga As
0.3
0.7
p-GaAs
+
p-Al Ga As
0.1
0.9
n-Al Ga As
0.1
0.9
n-Al Ga As
0.3
0.7
E g
n
2
(i)
(ii)
(iii)
(iv)
g mod = g mat .
(23.14)
In order to allow simultaneous optimization of the light mode and the carrier confinement, the separate
confinement heterostructure (SCH) has been designed. Here, a single or multiple quantum well of a
third material with even smaller band gap is the active medium (Fig. 23.39a,b,d). A single quantum well
has an optical confinement factor of a few per cent only. It offers, however, efficient carrier capture and
efficient radiative recombination. An increase in the carrier capture efficiency can be achieved using a
graded index in the barrier (GRINSCH, Fig. 23.39c).
The thin wave-guiding layer leads to large divergence of the laser beam along the vertical direction,
typically about 90
◦ . The strong confinement of light also limits the maximum achievable output power
due to catastrophic optical damage (COD). Several ideas have been realized to overcome this problem
and achieve much smaller divergence of about 18
◦ . The waveguide can be designed to be very thick
(large optical cavity, LOC) that leads to an increase of threshold. Other schemes are insertion of a