Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-02-04
2002-11-26
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06487225
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface-emitting laser device suitable for communications and measurements, and in particular suitable for multi-channel optical communication systems.
2. Description of the Related Art
As a technology to transmit and process quickly huge information like image information, parallel information processing systems using two dimensional integrated optical devices have been widely studied. For these systems, a surface-emitting laser device which can be arranged in two dimensional arrays is particularly important.
FIG. 3
is a cross-sectional view showing an example of a conventional surface-emitting laser device. This surface-emitting laser device is described in a paper, IEEE PHOTONICS TECHNOLOGY LETTERS, vol. 7 No. 12, DECEMBER 1995 p1391, and on a GaAs substrate
11
, a cladding layer
10
, an optical waveguide layer
9
, an active layer
8
formed of an InGaAs quantum well layer, an optical waveguide layer
7
, a cladding layer
6
, an etching stop layer
5
and a contact layer
13
are formed in sequence. The active layer
8
, the optical waveguide layers
7
,
9
and the cladding layers
6
,
10
constitute a separate confinement heterostructure of a graded index type, dubbed SCH structure. On the top of the contact layer
13
and on the bottom of the substrate
11
are formed electrodes
4
,
12
for carrier injection, respectively.
The etching stop layer
5
formed between the cladding layer
6
and the contact layer
13
has the function of preventing the cladding layer
6
and layers below the cladding layer
6
from being etched in the process of etching the contact layer
13
to form a horizontal resonator facet
2
a
. Another resonator facet
2
b
is formed by cleavage or the like so as to be perpendicular to each layer.
Furthermore, an inclined facet
3
is formed on a resonator optical axis between the horizontal resonator facet
2
a
and the vertical resonator facet
2
b
. The inclined facet
3
is inclined from the horizontal optical axis by a certain angle, e.g. 45 degrees, and functions as a reflective mirror for bending the optical axis.
With the above constitution, a laser device in which a laser beam having resonated between the resonator facets
2
a
and
2
b
is emitted to the external from the resonator facet
2
a
, functions as a surface-emitting laser device which can output an emitting beam
1
to the external in the normal direction of the substrate
11
. In regard to laser characteristics, in the case of an operation at an operation current of 20 mA, an edge output power of 8 mW and a surface output power of 5 mW are obtained at the facets
2
b
and
2
a
, respectively, wherein a threshold current is 6 mA.
In light sources for communication, not only high speed operation but also high power operation is of great importance. Thermal saturation and facet degradation problems are considered as factors that limit the laser output power. The thermal saturation is such a phenomenon that an elevated device temperature due to loss of Joule heat generated by laser oscillation and other losses leads to reduction of the active layer gain and disturbs continuation of oscillation. On the other hand, the facet degradation is an irreversible phenomenon that generation of surface level on the laser emitting facet, caused by impurities and crystal defects, induces light absorption with the result that facet destruction is generated due to local heating.
As shown in
FIG. 3
, in the surface-emitting laser device with an inclined facet
3
of triangle cross section, heat conduction from the inclined facet
3
to the substrate
11
having large heat capacity is more difficult than that from other facets
2
b
, and hence the thermal saturation and facet degradation are likely to occur around the inclined facet
3
. Furthermore, the inclined facet
3
is likely to be degraded because it is damaged more easily in processing as compared with the other facets
2
a
and
2
b.
The damages in processing will be described in detail in the following. In the case of the surface-emitting laser using an inclined facet, the inclined facet has to be mirror-finished in a flatness of micron order. The practical inclined facet is composed of several laser-constituting layers such as an active layer, waveguide layer and cladding layer exist, and it is necessary to process these layers uniformly to form a mirror surface. As a method of processing the inclined facet is generally employed a dry etching method such as ion beam etching. When the inclined facet is damaged in processing, reabsorption of the laser beam occurs on the inclined facet during laser oscillation, which causes a facet degradation such as so-called catastrophic optical damage.
It is possible to employ a wet etching process as a countermeasure against such problem, in consideration of mirror-finishing while preventing the damage in processing. However, etching characteristics of layers constituting the inclined facet are generally different, and accordingly a step is formed between layers of high etching rate and layers of low etching rate, making it difficult to form a high quality mirror surface. Particularly in the conventional surface-emitting laser device, the Al content of the cladding layer cannot be set much lower so as to keep carrier confinement. In the wet etching process a high-Al-content layer is likely to be oxidized and degraded, causing big barriers to high power laser operation. Furthermore, in the wet etching process a high quality mirror surface has been difficult to form, because differences in etching characteristics between the high-Al-content layers and the low-Al-content layers are likely to cause irregularities on the surface and nonuniformity in orientation of the mirror surface.
Additionally, in order to avoid the thermal saturation and facet degradation in the inclined facet during the laser operation, it is important to reduce the electric resistance and thermal resistance of the device and the optical density thereon.
FIG. 4A
is a cross-sectional view showing a typical example of SCH structure type device, and
FIG. 4B
is a graph showing light intensity distribution thereof. The axis of abscissas indicates positions of the laser device in a layer thickness direction thereof. The axes of ordinates of FIG.
4
A and
FIG. 4B
indicate Al and In contents, and light intensities, respectively. The SCH laser has a layer constitution in which a waveguide layer G is sandwiched between a pair of cladding layers C. The waveguide layer G corresponds to the active layer
8
and optical waveguide layers
7
,
9
in
FIG. 3
, and the cladding layer C corresponds to the cladding layers
6
,
10
in FIG.
3
.
As shown in
FIG. 4A
, for example, the composition of the cladding layer C is Al
0.60
Ga
0.40
As, and Al content of the waveguide layer G decreases continuously toward the center of the active layer in which a quantum well layer formed of InGaAs exists. Regarding AlGaAs type materials, since a band gap tends to increase with increases in Al content thereof, the band gap distribution almost coincides with the graph of FIG.
4
A.
The cladding layer C is mainly involved in the light confinement, but practically has also the function of confining carriers which overflow from the quantum well layer depending on the height of the band gap. Accordingly, to maintain satisfactory temperature characteristics, it is necessary for carriers existing in the active layer to sense that the potential barrier of the cladding layer C is high enough.
FIGS. 5A and 5B
are graphs showing electric resistance and thermal resistance of AlGaAs type materials, respectively. The axis of abscissas indicates Al contents. As seen from
FIG. 5A
, although the electric resistance increases with increase in Al content and particularly the p-type material has higher resistance than the n-type material as a whole, the electric resistance of the n-type material abruptly increases when the Al content thereof is higher than about 0.3. The calorific volu
Birch & Stewart Kolasch & Birch, LLP
Ip Paul
Mitsui Chemicals Inc.
Zahn Jeffrey N
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