Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2003-01-14
2004-11-02
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06813299
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to semiconductor laser devices, and in particular, to a semiconductor laser device capable of achieving especially high power, high reliability, and long operating life and an optical disk reproducing and recording apparatus that employs the device.
Semiconductor laser devices are used for optical communication apparatuses, optical recording apparatuses and so on. In recent years, there have been increasing needs for making the devices have high operating speed and large capacities, and there is promoted research and development for improving various characteristics of the semiconductor laser devices to respond to the needs.
Among them, a 780-nm band semiconductor laser device to be used for an optical disk reproducing (recording) apparatus of CD, CD-R/RW and the like has conventionally been made of an AlGaAs-based material. There have also been growing demands for high-speed write on CD-R/RW, and the semiconductor laser device has been required to yield a high output in order to cope with the demands.
FIG. 11
shows a schematic view of a conventional AlGaAs-based semiconductor laser device. In this semiconductor laser device, an n-GaAs buffer layer
502
, an n-Al
0.458
Ga
0.542
As first lower clad layer
503
, an n-Al
0.5
Ga
0.5
As second lower clad layer
504
, an Al
0.328
Ga
0.672
As lower guide layer
505
, a multiple quantum well active layer
506
where Al
0.115
Ga
0.885
As well layers (layer thickness: 74 Å, two layers) and an Al
0.346
Ga
0.654
As barrier layer (layer thickness: 54 Å, one layer), which are not shown, are alternately arranged, an Al
0.328
Ga
0.672
As upper guide layer
507
, a p-Al
0.476
Ga
0.524
As first upper clad layer
508
and a p-GaAs etching stop layer
509
are successively laminated on an n-GaAs substrate
501
. Further, a mesa-stripe-shaped p-Al
0.476
Ga
0.524
As second upper clad layer
510
is provided on the etching stop layer
509
, and a visor-shaped p-GaAs cap layer
511
is formed on the second clad layer
510
. Moreover, an n-Al
0.685
Ga
0.315
As first current block layer
512
and an n-GaAs second current block layer
513
are laminated on the etching stop layer
509
located on both sides of the second upper clad layer
510
, making the region other than the mesa stripe serve as a current constricting portion. Moreover, a p-GaAs flattening layer
514
is provided on the second current block layer
513
, and a p-GaAs contact layer
515
is laminated on the entire surface.
FIG. 5
shows a graph of the optical output-current characteristic of this semiconductor laser device. As is apparent from
FIG. 5
, this semiconductor laser device has a threshold current of about 35 mA and a COD (Catastrophic Optical Damage) level, i.e., an end surface destruction level of about 160 mW.
However, in the aforementioned conventional semiconductor laser device, the COD level has been able to be improved up to only 160 mW although the device has a high power. In the aforementioned conventional AlGaAs-based semiconductor laser device, end surface destruction due to COD is easily caused by the influence of active Al in a high-output driving time. Therefore, it is a basic design guideline for the obtainment of a high-output semiconductor laser device to weaken optical confinement by reducing a value of &Ggr;/d standardized by dividing an optical confinement coefficient (assumed to be &Ggr;) by a well layer thickness (assumed to be d) or reducing the well layer thickness in order to reduce the optical power density at the end surface.
However, a large gain cannot be obtained if the optical confinement is weakened to make the COD level equal to or larger than 160 mW, and therefore, a threshold current increases to disadvantageously increase the drive current. If the well layer thickness is reduced, then the overflow of carriers easily occurs to disadvantageously degrade the temperature characteristic. Accordingly, there has been a problem that only a semiconductor laser device of a large drive current has consequently been able to be provided if it has been attempted to operate the AlGaAs-based semiconductor laser device with an output of not smaller than 160 mW. Therefore, the aforementioned conventional semiconductor laser device has practically been able to stably operate with a peak optical output of about 120 mW.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to provide a high-output semiconductor laser device that employs a GaAs substrate or, in particular, a 780-nm band high-output semiconductor laser device for CD-R/RW, which has high reliability and long operating life in a high-output drive state and has a small drive current and an optical disk reproducing and recording apparatus that employs the semiconductor laser device.
In order to achieve the aforementioned object, the present invention provides a semiconductor laser device, which is constructed by laminating at least a first conductive type lower clad layer, a quantum well active layer comprised of well layers and barrier layers and a second conductive type upper clad layer on a GaAs substrate and has an oscillation wavelength that is greater than 760 nm and smaller than 800 nm, wherein
the well layers and the barrier layers are made of InGaAsP, InGaP or GaAsP, and
assuming that the well layer has a layer thickness d and an optical confinement coefficient in each one layer of the well layers is &Ggr;, then &Ggr;/d is not smaller than 2.2×10
−4
Å
−1
.
According to the above-mentioned construction, the optical confinement becomes great to allow a large gain to be obtained, because &Ggr;/d is not smaller than 2.2×10
−4
Å
31 1
. Therefore, a high-output semiconductor laser device with a high COD level can be provided with satisfactory threshold characteristic and temperature characteristic maintained.
Moreover, there can be provided a high-output semiconductor laser device, which has high reliability and long operating life and in which oxidation hardly occurs, because the quantum well active layer is constructed of InGaAsP, InGaP or GaAsP and Al is not contained therein.
In the present specification, the first conductive type means the n-type or the p-type. When the first conductive type is the n-type, the second conductive type is the p-type. When the first conductive type is the p-type, the second conductive type is the n-type.
In one embodiment, the layer thickness of the well layer is not smaller than 80 Å and not greater than 200 Å.
According to the above-mentioned embodiment, the well layer has the great layer thickness of not smaller than 80 Å and not greater than 200 Å. Therefore, a high-output semiconductor laser device with a high COD level can be provided with satisfactory threshold characteristic and temperature characteristic maintained by reducing the overflow of carriers.
Moreover, in one embodiment, the well layer has a compressive strain.
According to the above-mentioned embodiment, the well layer has a compressive strain, and the quantum well active layer becomes a compressive strain quantum well active layer. Therefore, a semiconductor laser device, which has high reliability, long operating life and high output in the 780-nm band can be provided.
Moreover, in one embodiment, an amount of the compressive strain owned by the well layer is within 3.5%.
According to the above-mentioned embodiment, the amount of compressive strain is within 3.5%. Therefore, a high-output semiconductor laser device with high reliability and long operating life can be provided.
Moreover, in one embodiment, the barrier layer has a tensile strain.
According to the above-mentioned embodiment, the barrier layer is a tensile strain barrier layer, and the tensile strain of the barrier layer compensates for the amount of compressive strain of the well layer that has the compressive strain. Therefore, a strained quantum well active layer having a more stable crystal can be produced, and a semiconductor laser device with high reliability can
Fujishiro Yoshie
Hirukawa Shuichi
Kawanishi Hidenori
Leung Quyen
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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