Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

1998-12-18

2001-12-25

Davie, James W. (Department: 2881)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06333945

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims benefit of priority of Japanese Patent Applications No. Hei-9-358192 filed on Dec. 25, 1997, and No. Hei-10-256645 filed on Sep. 10, 1998, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device having a double heterostructure, in which plural semiconductor layers including an active layer are formed on a substrate, and more particularly to such a semiconductor laser device emitting a high power laser beam.
2. Description of Related Art
A semiconductor laser device is used for measuring a distance, for example, in a vision system of a robot and in a radar system. A laser device emits a laser beam toward an object and receives a reflected beam therefrom. A distance between the object and the laser device is measured based on a delay time of the reflected beam which depends on the distance. Since a measurable distance by a laser device depends on its power, a high power laser is necessary to measure a long distance. For example, to measure a distance of 100 m between two cars, a pulse driven laser device having an output of several tens watts is required. To obtain the light output of several tens watts, the laser device has to be driven with pulse current of several tens amperes.
A semiconductor laser device shown in
FIGS. 12A and 12B
is known as a high power device. This laser device includes an active layer having a multi-quantum-well structure and optical guide layers and clad layers disposed on both sides of the active layer. This structure is proposed to effectively confine light and current. In
FIG. 12A
, a depth from a top surface is shown on the abscissa and an aluminum-mixing ratio in the layers is shown on the ordinate. On an n-GaAs (n-type gallium arsenide) substrate
102
, a first clad layer
103
, a first optical guide layer
104
, an active layer
105
, a second optical guide layer
106
, a second clad layer
107
and a p-GaAs (p-type gallium arsenide) layer
108
are laminated in this order. The active layer
105
has a multi-quantum-well structure in which layers made of an AlGaAs-based (aluminum-gallium-arsenide) material and layers made of a GaAs-based material are alternately laminated. Each of such layers in the active layer
105
is made sufficiently thin to a level of an wave-length of de Broglie of an electron and a hole, or less. A total thickness of the active layer
105
is made around 0.1 &mgr;m to effectively confine electric current therein. The clad layers
103
,
107
and optical guide layers
104
,
106
are made of an AlGaAs-based material in which an Al-mixing ratio (a ratio of Al in AlGa) is properly selected so that each layer performs a desired function.
In
FIG. 12B
, the depth from the top surface is shown on the abscissa and a refractive index is shown on the ordinate. A band gap of each layer depends on the Al-mixing ratio, and a refractive index thereof depends on the band gap. Therefore, each layer has its refractive index as shown in FIG.
12
B. Thus, a SCH structure (separate confinement heterostructure) having a desired refractive index distribution is obtained. Among layers
103
,
104
,
105
,
106
and
107
, the active layer
105
has the highest refractive index, the optical guide layers
104
,
106
formed on both sides of the active layer
105
have an intermediate refractive index, and the clad layers
103
,
107
have the lowest refractive index. Light generated in the active layer
105
is amplified in a region of the active layer
105
and optical guide layers
104
,
106
and is distributed as shown by a dotted line in FIG.
12
B. Since the light density is distributed, energy concentration to the active layer
105
is alleviated.
A semiconductor laser device having a so-called GRIN-SCH structure (graded index separate confinement heterostructure) is shown in U.S. Pat. No. 4,905,246. In this device, the refractive index of the optical guide layers formed on both sides of the active layer is varied continuously between the active layer and the clad layer. Energy concentration in the active layer is alleviated by distributing light density to both optical guide layers.
Though the light density is distributed in those conventional devices, a peak of the light density is still in the active layer which is made very thin and overlaps with a peak of carriers (current) injected into the active layer. Therefore, the energy concentration in the active layer is not sufficiently reduced. The energy concentration causes dislocation in a crystal of the active layer, which in turn deteriorates the active layer during a long time operation. This results in output power decrease and shortening a life of the device. It is important to avoid such a energy concentration especially in a high power laser device, such as a pulse driven device having an output of several tens watts.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide a semiconductor laser device in which a light density distribution peak is shifted from the active layer to avoid overlapping of the light density peak with a current distribution peak in the active layer. In other words, an object of the present invention is to avoid the deterioration of the device caused by energy concentration to the active layer, thereby to enhance its reliability and to prolong its life time.
Plural semiconductor layers including an active layer in which light is generated are laminated on a semiconductor substrate. The active layer has a multi-quantum-well structure in which two layers each having a different energy band gap are alternately laminated. The active layer is sandwiched between upper layers and lower layers. Preferably, the lower layers include a first clad layer and a first optical guide layer, the first optical guide layer being formed in contact with the active layer, and the upper layers include a second clad layer and a second optical guide layer, the second optical guide layer being formed in contact with the active layer. Preferably, the plural layers are made of an AlGaAs-based material. An aluminum-mixing ratio in AlGa of the first optical guide layer is set at a level which is different from that of the second optical guide layer, so that refractive indices thereof are different from each other. Alternatively, the aluminum-mixing ratio of the first clad layer is set at a level which is different from that of the second clad layer for the same purpose.
Since laser light is generated in the active layer by injecting driving current therein, and the generated light is distributed in the layers with a distribution peak in the active layer, energy of the driving current and the generated light concentrates in the active layer if the layers located at both sides of the active layer have symmetrical refractive indices with respect to the active layer. According to the present invention, the refractive indices of layers located at both sides of the active layer are made asymmetrical. The generated light is confined or distributed more in a layer having a higher refractive index than in a layer having a lower refractive index. Accordingly, a peak of the generated light is shifted from the active layer into a neighboring layer having a higher refractive index. Thus, energy concentration to the active layer is avoided, and deterioration of the active layer caused by the energy concentration is greatly alleviated, and thereby a life time of the semiconductor laser device is prolonged.
The energy peak shift is further enhanced by making the layer having a higher refractive index thicker than the layer having a lower refractive index. Avoidance of the energy concentration to the active layer is especially important when the laser device is a high power device which is driven by pulse current having several tens amperes to output several tens watts.
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