Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
1998-08-12
2001-11-20
Font, Frank G. (Department: 2877)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S096000, C372S045013, C372S046012, C372S050121, C372S092000
Reexamination Certificate
active
06320893
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a surface emitting semiconductor laser (referred to as surface emitting laser hereinafter) used as a light source for optical information processing and optical communication, a light source of image processing devices using light, and a light source of electrophotographic copy machines, and more particularly relates to a surface emitting semiconductor laser whose transverse mode is stable, threshold current is small, and output is high.
2. Description of the Related Art
Recently, surface emitting semiconductor lasers, which are used easily for structuring a two-dimensional array, have attracted attentions in optical communication or optical interconnection technical field. Among surface emitting semiconductor lasers, the vertical cavity surface emitting laser (referred to as VCSEL hereinafter), the basic structure of which is manufactured through an integrated semiconductor process, is now being developed for practical application because it is suitable for fabrication of large scale wafers. In the VCSEL, the threshold current is reduced relatively easily by narrowing the emission area (light emission area), therefore the VCSEL is advantageous in that power consumption is reduced in the field of optical switching devices which require simultaneous parallel driving of many elements.
However, in the surface emitting semiconductor laser, it is difficult to increase the output because of small volume of the active area. Currently, multi-mode optical fiber, which is supplied at low cost, is used for optical communication, however, single mode optical fiber will be used mainly in the future because the number of lines is increased greatly. The single mode optical fiber needs the single mode laser to be used together, and the single mode laser is required to be developed.
However, the surface emitting laser is involved in a trade-off problem that if the transverse mode stability is increased, the threshold current increases to result in poor response characteristics or low output, and the surface emitting semiconductor laser which satisfies all the requirements has not been realized.
For example, the proton injection surface emitting laser having the gain waveguide structure, from which stable transverse mode oscillation is easily obtained, causes the slight effective refractive index difference between the current passage area and the peripheral area of the current passage area due to the thermal lens effect to result in weak optical confinement condition, therefore stable transverse mode is obtained event though the diameter of a non-proton injection area (current passage) is enlarged to approximately 10 &mgr;m. However, because of weak optical confinement, limited emission efficiency improvement, and significant heat generation, the threshold current is relatively high and response characteristics are poor under the condition without bias.
To cope with this problem, recently the selective oxidation type surface emitting laser having the refractive index waveguide structure has been developed. The selective oxidation type surface emitting laser has a refractive index waveguide passage formed by selective oxidation of a portion of a semiconductor multilayer reflection film adjacent to an active layer, this structure brings about current narrowing effect and strong optical confinement effect, and the threshold current of sub-milliampere order is easily obtained and prompt response characteristics are obtained. However, because the optical confinement effect is significant, the diameter of an emission area should be reduced to 5 &mgr;m or smaller in order to stabilize the transverse mode, therefore it is difficult to increase output.
It is reported that the effective refractive index difference of approximately 0.015 between a light emission area and the peripheral area of the light emission area is effective to realize the surface emitting laser which has a large light emission area, emits high output, and has stable transverse mode.
In the above-mentioned selective oxidation type surface emitting laser, the refractive index difference between a selectively oxidized layer (for example, a layer of oxide of AlAs) and a spacer layer (for example, Al
0.5
Ga
0.5
As layer) is large. For example, in the case of the above-mentioned materials, the refractive index of the former AlAs oxide layer is approximately 1.6 and the refractive index of the latter Al
0.5
Ga
0.5
As layer is approximately 3.35, therefore the refractive index difference between these materials is 1.8. As the result, great optical confinement effect is brought about between the selectively oxidized layer and the spacer layer.
To achieve the above-mentioned condition, an optical confinement layer formed of a material having a refractive index somewhat larger than the one of the selectively oxidized layer (the refractive index larger than the one of the selectively oxidized layer and smaller than the one of the spacer layer) may be provided on the peripheral area of the light emitting area instead of the selectively oxidized layer.
The surface emitting semiconductor laser disclosed in Japanese Published Unexamined Patent Application No. Hei 6-69858 is an example of the surface emitting laser having a structure similar to the structure of the surface emitting laser based on this concept. The disclosed surface emitting laser is described with reference to
FIG. 19. A
bottom DBR mirror (Distributed Bragg reflector mirror )
104
composed of laminates of an n-AlAs layer and an n-GaAs layer is formed on an n-GaAs substrate
101
, and thereafter a bottom spacer layer
105
composed of an n-Al
0.2
Ga
0.8
As, a bottom barrier layer
106
composed of an undoped GaAs layer, a top barrier layer
108
composed of an undoped GaAs layer, and a top spacer layer
109
composed of a p-Al
0.2
Ga
0.8
As layer are orderly deposited, and additionally an n-InGaP layer with a thickness of 200 nm is formed thereon. The n-InGaP layer is removed from a light emission area using wet-etching to form a current narrowing layer
110
having an aperture
112
, which is served as a current passage, then a p-Al
0.2
Ga
0.8
As layer
113
is formed to a sufficient thickness for obtaining the flat surface on the top spacer layer
109
exposed to the aperture
112
and the current narrowing layer
110
. The p-Al
0.2
Ga
0.8
As layer
113
functions to adjust the distance between the top and bottom resonators (distance from the top surface of the bottom DBR mirror
104
to the bottom surface of the top DBR mirror
116
described hereinafter) to a triple half-wave length of oscillation wavelength &lgr;. The optical distance from the center of the single quantum well active layer
107
(center plane between the top surface and bottom surface) to the top surface of the bottom DBR mirror
104
is a half of the oscillation wavelength &lgr;, the thickness is designed so that the amplitude of the standing wave is a maximum at the center of the single quantum well active layer
107
. Next, a top DBR mirror
116
composed of laminates of a p-AlAs layer and a p-GaAs layer is formed, and electrodes (not shown in the drawing) is formed on the top surface
115
and the bottom surface to complete the laser.
As mentioned partially hereinabove, the design concept of this surface emitting laser is presumed as described hereunder.
(1) A current is concentrated to the light emission area and the threshold current is reduced because of the current narrowing layer
110
.
(2) The material of the current narrowing layer
110
is n-InGaP, the forbidden band width of the layer is larger than the forbidden bandwidth of p-Al
0.2
Ga
0.8
As which is the composition of the above-mentioned spacer layer
109
and-the conduction type is reversed, the larger forbidden band width functions to narrow the current and also the reversed conduction type functions similarly in the aspect of energy barrier.
(3) The thickness of the current narrowing layer
110
is made as thick as 280 nm in order to prevent tunneling and to
Flores Ruiz Delma R.
Font Frank G.
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
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