High-power surface-emitting semiconductor injection laser with etched internal 45 degree and 90 degree micromirrors

A surface-emitting semiconductor injection laser for use in fabricating high-power two-dimensional monolithic laser arrays. The surface-emitting semiconductor laser includes a substrate and an active layer and a pair of cladding layers formed on the substrate. A folded resonator cavity is formed by highly-reflective 45.degree. and 90.degree. micromirrors that are etched at either end of the active layer and by a partially-reflective reflector that is positioned between the 45.degree. micromirror and the substrate for outcoupling the laser light from the resonator cavity. The semiconductor laser is mounted junction down on a heat sink to position the active layer close to the heat sink for good heat dissipation at high power levels. In one preferred embodiment of the present invention, the substrate is optically opaque and an opening is etched in the substrate for outcoupling the laser light. In another preferred embodiment of the invention, the substrate is optically transparent and a microlens is formed on the substrate to collimate the laser light.

BACKGROUND OF THE INVENTION 
This invention relates generally to semiconductor injection lasers and, 
more particularly, to high-power surface-emitting semiconductor injection 
lasers. 
High-power laser light sources are required for a variety of optical 
systems, such as optoelectronic logic circuits, fiber optic communication 
systems, and for optically pumping solid state lasers. Semiconductor 
injection lasers are particularly well suited as laser light sources for 
these optical systems because of their small size, reliability, and 
operation at wavelengths having low transmission and dispersion losses in 
glass fiber optics. 
A semiconductor injection laser is a diode device in which a forward bias 
voltage is applied across an active layer formed between n-doped and 
p-doped cladding layers. Excess electrons from the n-doped layer and 
excess holes from the p-doped layer are injected into the active layer by 
the bias voltage, where the excess electrons and holes recombine. At low 
current levels, the electrons recombine with the holes to produce 
spontaneous emission of photons in all directions. At current levels above 
the laser threshold value, the excess carrier density becomes high enough 
to produce an inverted population, yielding a positive gain. Stimulated 
emission occurs and a monochromatic, highly-directional beam of light is 
emitted from the active layer. A resonant cavity can be formed in the 
active layer by cleaved facets at either end of the device, one being a 
highly-reflective surface and the other being a partially-reflective 
surface through which the beam emerges. The resonant cavity can also be 
bounded by etched side surfaces, to prevent emission in the lateral 
direction, and by the cladding layers, which have indexes of refraction 
that are lower than the active layer to confine the light to the plane of 
the active layer. 
The power output of a single semiconductor injection laser is rather small 
and is inadequate for most types of optical systems. Therefore, 
semiconductor injection lasers are typically combined in large 
two-dimensional arrays to provide increased power levels. Surface-emitting 
semiconductor injection lasers are particularly well suited for 
fabricating large two-dimensional monolithic laser arrays because of their 
emission of laser light from a top or bottom surface of the device. 
However, many surface-emitting semiconductor lasers exhibit poor thermal 
characteristics because of poor thermal conduction between the active 
layer and the heat sink. Accordingly, there has been a need for a 
surface-emitting semiconductor injection laser having good thermal 
conduction properties for operation at high power levels. The present 
invention is directed to this end. 
SUMMARY OF THE INVENTION 
The present invention resides in a surface-emitting semiconductor injection 
laser for use in fabricating high-power two-dimensional monolithic laser 
arrays. The surface-emitting semiconductor laser includes a substrate and 
an active layer and a pair of cladding layers formed on the substrate. A 
folded resonator cavity is formed by highly-reflective 45.degree. and 
90.degree. micromirrors that are etched at either end of the active layer 
and by a partially-reflective reflector that is positioned between the 
45.degree. micromirror and the substrate for outcoupling the laser light 
from the resonator cavity. The semiconductor laser is mounted junction 
down on a heat sink to position the active layer close to the heat sink 
for good heat dissipation at high power levels. In one preferred 
embodiment of the present invention, the substrate is optically opaque and 
an opening is etched in the substrate for outcoupling the laser light. In 
another preferred embodiment of the invention, the substrate is optically 
transparent and a microlens is formed on the substrate to collimate the 
laser light. 
It will be appreciated from the foregoing that the present invention 
represents a significant advance in the field of surface-emitting 
semiconductor injection lasers. Other features and advantages of the 
present invention will become apparent from the following more detailed 
description, taken in conjunction with the accompanying drawings, which 
illustrate, by way of example, the principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in the drawings for purposes of illustration, the present 
invention is embodied in a surface-emitting semiconductor injection laser 
for use in fabricating high-power two-dimensional monolithic laser arrays. 
In accordance with the present invention, the surface-emitting 
semiconductor laser includes a substrate and an active layer and a pair of 
cladding layers formed on the substrate. A folded resonator cavity is 
formed by highly-reflective 45.degree. and 90.degree. micromirrors that 
are etched at either end of the active layer and by a partially-reflective 
reflector that is positioned between the 45.degree. micromirror and the 
substrate for outcoupling the laser light from the resonator cavity. The 
semiconductor laser is mounted junction down on a heat sink to position 
the active layer close to the heat sink for good heat dissipation at high 
power levels. In one preferred embodiment of the present invention, the 
substrate is optically opaque and an opening is etched in the substrate 
for outcoupling the laser light. In another preferred embodiment of the 
invention, the substrate is optically transparent and a microlens is 
formed on the substrate to collimate the laser light. 
As illustrated in FIG. 1, one preferred embodiment of the surface-emitting 
semiconductor injection laser of the present invention includes an 
optically opaque substrate 10, an n-doped buffer/stack reflector layer 12, 
an n-doped cladding layer 14, an undoped active layer 16, a p-doped 
cladding layer 18, and a p-doped cap layer 20. These layers 12, 14, 16, 
18, 20 are grown on the substrate 10 by conventional epitaxial processes, 
such as molecular beam epitaxy (MBE) or metalorganic chemical vapor 
deposition (MOCVD). A folded resonator cavity is then formed by etching a 
highly-reflective 90.degree. micromirror 22 at one end of the active layer 
16 and a highly-reflective 45.degree. micromirror 24 at the other end of 
the active layer 16. The 90.degree. micromirror 22 can be etched using any 
conventional reactive ion etching process and the 45.degree. micromirror 
24 can be etched using any conventional ion beam etching process. An 
opening 26 is then etched through the optically opaque substrate 10 
adjacent the 45.degree. micromirror 24 for outcoupling the laser light 
from the resonator cavity. One end of the resonator cavity is formed by 
the 90.degree. micromirror 22 and the other end is formed by a 
partially-reflective reflector, which includes the stack reflector in the 
buffer/stack reflector layer 12 and/or the interface between the opening 
26 and the buffer layer 12. Electrical contacts 28, 30 are then deposited 
on the substrate 10 and cap layer 20, respectively, and the semiconductor 
laser is soldered, by solder layer 32, to a heat sink 34. 
In this preferred embodiment of the present invention, the substrate 10 is 
a GaAs substrate having a thickness of approximately 100 microns. The 
buffer/stack reflector layer 12 includes alternating n-doped GaAs and 
n-doped GaAlAs layers, with the number and thickness of the layers being 
selected to provide a desired stack reflectivity and resonant frequency. 
The cladding layer 14 is an n-doped Ga.sub.0.6 Al.sub.0.4 As layer having 
a thickness of approximately 1.5 microns and the active layer 16 is a thin 
undoped GaAlAs layer. The cladding layer 18 is a p-doped Ga.sub.0.6 
Al.sub.0.4 As layer and the cap layer 20 is a p-doped GaAs layer, with a 
combined thickness of approximately 1.5 microns. A half-wavelength thick 
film 36 of a dielectric material, such as Si.sub.3 N.sub.4, is deposited 
on the 45.degree. and 90.degree. micromirrors 22, 24 for passivation and a 
highly-reflective coating is then deposited on the dielectric film 36. The 
electrical contacts 28, 30 are Ni-AuGe-Ni-Au and Ti-Pt-Au ohmic contacts, 
respectively. The solder layer 32 is an indium layer having a thickness of 
approximately 10 microns and the heat sink 34 is copper. The 45.degree. 
and 90.degree. micromirrors 22, 24 are approximately 450 microns apart and 
an antireflective coating is deposited in the opening 26 on the exposed 
buffer/stack reflector layer 12. 
As illustrated in FIG. 2, another preferred embodiment of the 
surface-emitting semiconductor injection laser of the present invention 
includes an active layer 16' that emits laser light at a longer wavelength 
than the absorption band edge of the GaAs substrate. This provides an 
optically transparent substrate 10', which eliminates the need for the 
opening 26 and allows a microlens 38 to be formed on the substrate. The 
microlens 38 can be used to collimate the emitted beam for high intensity 
incoherent applications or to fill the aperture in coherent external 
cavity configurations. 
In this preferred embodiment of the present invention, the active layer 16' 
is a strained InGaAs/GaAs quantum well structure having a radiation 
wavelength greater than 0.9 micron, a wavelength at which the GaAs 
substrate 10' is optically transparent. The substrate 10' is a GaAs 
substrate having a thickness of approximately 100 microns. The 
buffer/stack reflector layer 12 includes alternating n-doped GaAs and 
n-doped GaAlAs layers which are doped with silicon to about 
3.times.10.sup.18 cm.sup.-3, with the number and thickness of the layers 
being selected to provide a desired stack reflectivity and resonant 
frequency. The cladding layer 14 is an n-doped Ga.sub.0.5 Al.sub.0.5 As 
layer having a thickness of approximately 1.0 micron, which is doped with 
silicon to about 5.times.10.sup.17 cm.sup.-3. The active layer 16' 
includes a linearly-graded (20%-50% Al) undoped GaAlAs active layer having 
a thickness of 0.15 micron, a GaAs spacer layer having a thickness of 0.01 
micron, an undoped InGaAs quantum well layer having a thickness of 0.005 
micron, a GaAs spacer layer having a thickness of 0.01 micron, and a 
linearly-graded (20%-50% Al) undoped GaAlAs layer having a thickness of 
0.15 micron. The cladding layer 18 is a p-doped Ga.sub.0.5 Al.sub.0.5 As 
layer having a thickness of 1.0 micron, which is doped with zinc to about 
5.times.10.sup.17 cm.sup.-3. The cap layer 20 is a p-doped GaAs layer 
having a thickness of 0.25 micron, which is doped with zinc to about 
1.times.10.sup.19 cm.sup.-3. A half-wavelength thick film 36 of a 
dielectric material, such as Si.sub.3 N.sub.4, is deposited on the 
45.degree. and 90.degree. micromirrors 22, 24 for passivation and a 
highly-reflective coating is then deposited on the dielectric films 36. 
The electrical contacts 28, 30 are Ni-AuGe-Ni-Au and Ti-Pt-Au ohmic 
contacts, respectively. The solder layer 32 is an indium layer having a 
thickness of approximately 10 microns and the heat sink 34 is copper. The 
45.degree. and 90.degree. micromirrors 22, 24 are approximately 450 
microns apart and an antireflective coating is deposited on the substrate 
10'. 
From the foregoing, it will be appreciated that the present invention 
represents a significant advance in the field of surface-emitting 
semiconductor injection lasers. Although several preferred embodiments of 
the invention have been shown and described, it will be apparent that 
other adaptations and modifications can be made without departing from the 
spirit and scope of the invention. Accordingly, the invention is not to be 
limited, except as by the following claims.