23.4 Lasers
667
Fig. 23.40 Scheme of
gain-guided lasers with
stripe contact: a oxide
stripe, b proton implanted
with shadow mask from
tungsten wire (∼ 10 µm).
Adapted from [1983]
metal
metal
oxide
p -GaAs
p-AlGaAs
n-AlGaAs
+
S
d
L
light
(a)
(b)
metal
substrate
protons
protons
p -GaAs
p-AlGaAs
n-AlGaAs
+
Fig. 23.41 Schematic
cross section of
index-guided lasers: a
shallow ridge, b deep etch
and regrowth. Black areas
are metal contacts
(a)
(b)
p -InGaAsP
+
n-InP substrate
n-InP substrate
n-InP
n-InP
InGaAsP
n - In P
p - In P
p-InP
p-InP
InGaAsP
dielectric
p -InGaAsP
+
InGaAsP
low-index layer into the confinement layer, insertion of a high-index layer into the cladding layer or
the use of high-index quarter-wavelength reflecting layers [1984].
Lateral Mode Guiding
Lateral waveguiding can be achieved with gain guiding and index guiding (or a mixture of the two). In
the gain-guiding scheme (Fig. 23.40), the current path that is defined by the stripe contact and the current
spreading underneath it, defines the gain region and therefore the volume of amplification that guides
the optical wave. Since a high carrier density reduces the index of refraction, a competing antiguiding
effect can occur. For index guiding, the lateral light confinement is caused by a lateral increase of
the index of refraction. This index modulation can be achieved by using a mesa-like contact stripe
(Fig. 23.41a). A shallow mesa reaches down into the upper cladding, a deep mesa reaches down into or
through the active layer. Possible problems with surface recombination can be avoided by regrowth of
the structure (Fig. 23.41b) with a wide band gap material (compared to the active layer). Optimization
of regrowth is targeted to achieve a well-defined surface for subsequent contact processing. A lateral
pn-diode can be incorporated that avoids current spreading in the upper part of the structure.
Depending on the width of the lateral mode, it can be monomode or multimode (Fig. 23.42a,b). For
laterally monomode lasers, the stripe width may only be a few µm. In particular for such lasers, the
current spreading must be controlled. Problems can arise for wide stripe widths due to current filamentation and inhomogeneous laser emission from the facet. Since the optical mode is typically more
confined in the growth direction than in the lateral direction, the far field is asymmetric (Fig. 23.43a,b).
The vertical axis has the higher divergence and is called the fast axis. The lateral axis is called the slow
axis. The asymmetric beam shape is detrimental when the laser needs to be coupled into an optical
fiber or a symmetric beam profile is needed for subsequent optics. The beam can be made symmetric
using special optic components such as anamorphic prisms (Fig. 23.43c) and graded-index lenses. The
beam from a laterally monomode laser is diffraction limited and can therefore generally be refocused
efficiently (beam quality M
2
1).
667
Fig. 23.40 Scheme of
gain-guided lasers with
stripe contact: a oxide
stripe, b proton implanted
with shadow mask from
tungsten wire (∼ 10 µm).
Adapted from [1983]
metal
metal
oxide
p -GaAs
p-AlGaAs
n-AlGaAs
+
S
d
L
light
(a)
(b)
metal
substrate
protons
protons
p -GaAs
p-AlGaAs
n-AlGaAs
+
Fig. 23.41 Schematic
cross section of
index-guided lasers: a
shallow ridge, b deep etch
and regrowth. Black areas
are metal contacts
(a)
(b)
p -InGaAsP
+
n-InP substrate
n-InP substrate
n-InP
n-InP
InGaAsP
n - In P
p - In P
p-InP
p-InP
InGaAsP
dielectric
p -InGaAsP
+
InGaAsP
low-index layer into the confinement layer, insertion of a high-index layer into the cladding layer or
the use of high-index quarter-wavelength reflecting layers [1984].
Lateral Mode Guiding
Lateral waveguiding can be achieved with gain guiding and index guiding (or a mixture of the two). In
the gain-guiding scheme (Fig. 23.40), the current path that is defined by the stripe contact and the current
spreading underneath it, defines the gain region and therefore the volume of amplification that guides
the optical wave. Since a high carrier density reduces the index of refraction, a competing antiguiding
effect can occur. For index guiding, the lateral light confinement is caused by a lateral increase of
the index of refraction. This index modulation can be achieved by using a mesa-like contact stripe
(Fig. 23.41a). A shallow mesa reaches down into the upper cladding, a deep mesa reaches down into or
through the active layer. Possible problems with surface recombination can be avoided by regrowth of
the structure (Fig. 23.41b) with a wide band gap material (compared to the active layer). Optimization
of regrowth is targeted to achieve a well-defined surface for subsequent contact processing. A lateral
pn-diode can be incorporated that avoids current spreading in the upper part of the structure.
Depending on the width of the lateral mode, it can be monomode or multimode (Fig. 23.42a,b). For
laterally monomode lasers, the stripe width may only be a few µm. In particular for such lasers, the
current spreading must be controlled. Problems can arise for wide stripe widths due to current filamentation and inhomogeneous laser emission from the facet. Since the optical mode is typically more
confined in the growth direction than in the lateral direction, the far field is asymmetric (Fig. 23.43a,b).
The vertical axis has the higher divergence and is called the fast axis. The lateral axis is called the slow
axis. The asymmetric beam shape is detrimental when the laser needs to be coupled into an optical
fiber or a symmetric beam profile is needed for subsequent optics. The beam can be made symmetric
using special optic components such as anamorphic prisms (Fig. 23.43c) and graded-index lenses. The
beam from a laterally monomode laser is diffraction limited and can therefore generally be refocused
efficiently (beam quality M
2
1).