684
23 Electricity-to-Light Conversion
Fig. 23.64 Linewidth f
of a cw GaAs/AlGaAs
laser diode at various
temperatures as a function
of the inverse output power
P
−1
out . At room temperature
α ≈ 5. Adapted
from [2018]
200
150
100
50
0
1/P (mW )
out
1
0
0.5
1.0
1.5
2.0
2.5
T=
273K
195K
77K
23.4.14 Surface-Emitting Lasers
Surface-emitting lasers emit their beam normal to the surface. They can be fabricated from horizontal
(edge-) emitters by reflecting the beam with a suitable mirror into the surface direction. This technology
requires tilted facets or micro-optical components but allows for high power per area. In Fig. 23.65, a
schematic cross section of a horizontal-cavity surface-emitting laser (HCSEL) and the light emission
from an array of 220 such lasers are shown. The laser contains a 45
◦ mirror that steers the light through
the substrate and a Bragg mirror to provide the cavity mirror. The facet can also be fabricated such
that the emission is to the top surface (Fig. 23.66). Another possibility to couple the beam out of a
horizontal cavity is a surface grating.
Now, surface-emitting lasers with vertical-cavity (VCSEL), as shown in Fig. 23.29b, will be discussed. A detailed treatment can be found in [2021]. VCSELs are of increasing importance after many
issues regarding their technology and fabrication have been solved. VCSEL fabrication is essentially
a planar technology and VCSELs can be fabricated as arrays (Fig. 23.68). An on-wafer test of their
properties is possible. They offer a symmetrical (or possibly a controlled asymmetrical) beam profile
(Fig. 23.67) with possible polarization control or fixation.
The cavity is formed by two highly reflecting Bragg mirrors with a distance of λ/2 or 3λ/2 forming
a microcavity (see Sect. 19.1.7). A high index contrast can be obtained from GaAs/AlAs Bragg mirrors
in which the AlAs layers have been selectively oxidized in a hot moist atmosphere. Pure semiconductor
Bragg mirrors suffer typically from small index contrast and require many pairs. This poses a problem,
(a)
SiO 2
p-AlGaAs
n-AlGaAs
DBR
active
medium
n-metal
p-metal
AR
SiO 2
(b)
Fig. 23.65 a Principle of a surface-emitting laser. Light generated in the active region is internally reflected by the 45 ◦
angled mirror and directed through the substrate; ‘AR’: antireflection coating, ‘DBR’: epitaxial Bragg mirror. b Light
emission from a 10 × 22 surface-emitting diode array. The light emission appears as stripes due to the broad beam
divergence in the vertical direction. Part b reprinted with permission from [2019]
23 Electricity-to-Light Conversion
Fig. 23.64 Linewidth f
of a cw GaAs/AlGaAs
laser diode at various
temperatures as a function
of the inverse output power
P
−1
out . At room temperature
α ≈ 5. Adapted
from [2018]
200
150
100
50
0
1/P (mW )
out
1
0
0.5
1.0
1.5
2.0
2.5
T=
273K
195K
77K
23.4.14 Surface-Emitting Lasers
Surface-emitting lasers emit their beam normal to the surface. They can be fabricated from horizontal
(edge-) emitters by reflecting the beam with a suitable mirror into the surface direction. This technology
requires tilted facets or micro-optical components but allows for high power per area. In Fig. 23.65, a
schematic cross section of a horizontal-cavity surface-emitting laser (HCSEL) and the light emission
from an array of 220 such lasers are shown. The laser contains a 45
◦ mirror that steers the light through
the substrate and a Bragg mirror to provide the cavity mirror. The facet can also be fabricated such
that the emission is to the top surface (Fig. 23.66). Another possibility to couple the beam out of a
horizontal cavity is a surface grating.
Now, surface-emitting lasers with vertical-cavity (VCSEL), as shown in Fig. 23.29b, will be discussed. A detailed treatment can be found in [2021]. VCSELs are of increasing importance after many
issues regarding their technology and fabrication have been solved. VCSEL fabrication is essentially
a planar technology and VCSELs can be fabricated as arrays (Fig. 23.68). An on-wafer test of their
properties is possible. They offer a symmetrical (or possibly a controlled asymmetrical) beam profile
(Fig. 23.67) with possible polarization control or fixation.
The cavity is formed by two highly reflecting Bragg mirrors with a distance of λ/2 or 3λ/2 forming
a microcavity (see Sect. 19.1.7). A high index contrast can be obtained from GaAs/AlAs Bragg mirrors
in which the AlAs layers have been selectively oxidized in a hot moist atmosphere. Pure semiconductor
Bragg mirrors suffer typically from small index contrast and require many pairs. This poses a problem,
(a)
SiO 2
p-AlGaAs
n-AlGaAs
DBR
active
medium
n-metal
p-metal
AR
SiO 2
(b)
Fig. 23.65 a Principle of a surface-emitting laser. Light generated in the active region is internally reflected by the 45 ◦
angled mirror and directed through the substrate; ‘AR’: antireflection coating, ‘DBR’: epitaxial Bragg mirror. b Light
emission from a 10 × 22 surface-emitting diode array. The light emission appears as stripes due to the broad beam
divergence in the vertical direction. Part b reprinted with permission from [2019]