Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-02-16
2003-10-21
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06636542
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a surface emitting semiconductor laser, a surface emitting semiconductor laser array, and a method for manufacturing a surface emitting semiconductor laser, and more particularly, relates to a surface emitting semiconductor laser, a surface emitting semiconductor laser array, and a method for manufacturing a surface emitting semiconductor laser which are of polarization plane control type used as a light source of optical information processing devices, optical communication devices, and image forming devices using light.
2. Description of the Related Art
While a circular emitting spot is usually required in the laser application technology, a surface emitting semiconductor laser, featured in that the emitting spot is formed easily in a circular shape, has attracted much attention recently because of convenience as a light source which is capable of two-dimensional accumulation. A surface emitting semiconductor laser usually has the isotropic physical configuration in the perpendicular direction to the emitting surface (high axial symmetry), and such configuration leads to no difference in characteristics with respect to two orthogonal axial directions (referred to as two axial directions hereinafter) in the plane horizontal to the emitting surface. As the result, it is known that the polarization plane is directed to the two axial directions in the same probability.
The direction to the two axial directions in the same probability results in impossibility to use polarization-dependent optical elements such as lenses having no special coating and polarization beam splitters as they are, and when many elements are used in the same optical system, the difference of polarization plane between elements is reflected disadvantageously on the light quantity variation, and to avoid such problem various methods have been developed to stabilize the polarization plane in one direction.
Japanese Patent Publication No. 2701596 is an example of such method. In this method, as shown in
FIG. 25
, the first reflection mirror layer
52
doped with n-type impurity comprising a multi-layer film formed by laminating GaAs layers and AlAs layers alternately, an active layer
70
having an InGaAs layer
56
which is served as a quantum well and provided between two AlGaAs layers
54
and
58
, and the second reflection mirror layer
64
doped with p-type impurity comprising a multi-layer film formed by laminating GaAs layers and AlAs layers and a metal electrode layer are formed on a GaAs substrate
50
, and the second reflection mirror layer
64
and the active layer
70
are mesa-etched to form a square post
60
served as a vertical resonator structure, and metal films
8
a
and
8
b
are provided partially on the mesa side of the post
60
to cause the difference in resonator loss, and thus the light is oscillated in the specified polarization direction and the polarization plane is directed in one direction.
Japanese Published Examined Patent Application No. Hei 7-73139 is another example of such method. In this method, as shown in FIG.
26
(
b
) and FIG.
26
(
c
), an n-type AlAs/GaAs distribution Bragg reflecting mirror (referred to as DBR hereinafter)
52
, an n-type Al
0.4
Ga
0.6
As layer
54
, an In
0.2
Ga
0.8
As active layer
56
, a p-type Al
0.4
Ga
0.6
As layer
58
, and a p-type AlAs/GaAs DBR
64
are formed successively on an n-type GaAs substrate
50
, and the n-type Al
0.4
Ga
0.6
As layer
54
, the In
0.2
Ga
0.8
As active layer
56
, the p-type Al
0.4
Ga
0.6
As layer
58
, and the p-type AlAs/GaAs DVR
64
are mesa-etched to form a square post
60
served as a vertical resonator structure, and when SiN film
72
is formed on the mesa-sides of the post
60
, the temperature is kept at 300° C. on one pair of facing sides and at 100° C. on another pair of facing sides.
Because the tensile stress exerted on the post
60
is different between the above-mentioned two axial directions when the temperature is cooled down to the room temperature, the stress exerted in the two axial directions in the active layer plane horizontal to the emitting surface is different, one polarization mode out of two axial directions is diminished to stabilize the polarization plane to one direction.
However, because conventional surface emitting semiconductors have the air post structure, if the post diameter is reduced to reduce the threshold current then the optical-output is reduced undesirably, and on the other hand, if the post diameter is increased to obtain desired optical output then the threshold current increases and the horizontal mode becomes unstable un:desirably, so it is difficult to obtain a surface emitting semiconductor laser with reduced threshold current.
In the methods described hereinabove, the metal films
8
a
and
8
b
or SiN film
72
are formed on the mesa side which is approximately perpendicular to the semiconductor substrate plane, however, since it is difficult to form an even film reproducibly on a side approximately perpendicular to the semiconductor substrate plane and it requires special high level technique, these methods are not preferable from the viewpoint of reliability for accurate manufacturing of elements.
It is the first object of the present invention to provide a surface emitting semiconductor laser and a surface emitting semiconductor laser array which are capable of controlling the polarization plane of a laser beam to one direction and obtaining a low threshold current. It is the second object of the present invention to provide a method for manufacturing a surface emitting semiconductor laser which is capable of controlling the polarization plane of a laser beam to one direction and enhancing element performance.
SUMMARY OF THE INVENTION
To accomplish the above-mentioned first object, A surface emitting semiconductor laser, comprising: a semiconductor substrate; a first reflection mirror layer of the first conduction type formed on the main plane of the semiconductor substrate; an active layer having a quantum well laminated above the first reflection mirror layer; a post having a second reflection mirror layer of the second conduction type different from the first conduction type for constituting an resonator structure together with the first reflection mirror; a plurality of peripheral high resistance layers having a periphery of high resistance and inserted between the first reflection mirror layer and the second reflection mirror layer; and wherein at least two layers of the plurality of peripheral high resistance layers are different in the proportion of non-high resistance therein.
As described above, because plurality of peripheral high resistance layers having the periphery of high resistance inserted between the first reflection mirror layer and the second reflection mirror layer are formed and at least two layers out of the plurality of peripheral high resistance layers are different from each other in proportion of non-high resistance to give the stress different in reflectance distribution and intensity between orthogonal two axial directions, as the result, anisotropy in oscillation threshold gain is caused, only the mode in the direction of smaller threshold gain is obtained selectively, and the polarization plane of the laser beam is fixed in the constant direction.
Because the high resistance area is not conductive, a refractive index waveguide in which a current is narrowed and a beam is confined in the different proportion between the two axial directions orthogonal in the plane parallel to the main plane of the semiconductor substrate is formed, and the element performance of improved low threshold current is obtained.
The proportion of non-high resistance termed in the present invention means the proportion of non-high resistance determined depending on the depth or thickness of the high resistance area and chemical composition of the high resistance area, and includes all the factors that influence the stress exerted on the active layer.
The surface emitting se
Fuji 'Xerox Co., Ltd.
Leung Quyen
Oliff & Berridg,e PLC
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