Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-05-21
2003-04-22
Paladini, Albert W. (Department: 2827)
Coherent light generators
Particular active media
Semiconductor
C372S044010, C372S045013, C372S050121
Reexamination Certificate
active
06553046
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device which is used in the fields of image processing, printing, medicine, and the like.
2. Description of the Related Art
Recently, high-power semiconductor laser devices are used in the fields of image processing, printing, medicine, and the like. The semiconductor laser devices used in those fields are required to operate with an output power of 1 W or higher and high reliability, and there are demands for increasing the output power of the semiconductor laser devices.
Generally, when a semiconductor laser device operates with an output power exceeding a certain level, non-radiative recombination occurs at the end facet, and the energy generated by the non-radiative recombination is absorbed by the crystal lattice, i.e., heat is generated. Since the temperature rise at the end facet decreases the energy gap, the light absorption is enhanced, and the temperature at the end facet is further raised. When the above cycle of operations is repeated, the so-called catastrophic optical mirror damage (COMD) occurs. Thus, the reliability of the semiconductor laser device in the high output power operation is impaired.
In particular, recombination centers are likely to be generated in AlGaAs-based semiconductor laser devices due to the inclusion of aluminum. Therefore, the AlGaAs-based semiconductor laser devices are prone to COMD, and not reliable in the high output power operation.
On the other hand, since recombination centers are not likely to be generated in InGaP-InGaAsP-based semiconductor laser devices, it is possible to increase the critical output power of the InGaP-InGaAsP-based semiconductor laser devices. However, electrical resistances at GaAs/InGaP hetero interfaces in the InGaP-InGaAsP-based semiconductor laser devices are great. Therefore, the characteristics of the InGaP-InGaAsP-based semiconductor laser devices are poor, and the reliability of the InGaP-InGaAsP-based semiconductor laser devices is low.
In order to solve the above problems, Japanese Unexamined Patent Publication (JPP) No. 6(1994)-302910 discloses a semiconductor laser device in which electrical resistance is reduced by using a graded-index type light-carrier-separate-confinement structure and unsymmetrically formed optical waveguide layers.
However, due to the miscibility gap, it is impossible to produce good-quality crystals in the manufacturing process of the semiconductor laser device disclosed by JPP No. 6(1994)-302910. Therefore, the electrical resistance of the entire semiconductor laser device cannot be effectively reduced. In addition, the electrical resistance can be reduced in only the vicinity of the active layer, and the electrical resistances at the interface between a GaAs substrate and a cladding layer and the interface between a contact layer and another cladding layer remain great. As a result, the electrical resistance of the entire semiconductor laser device cannot be sufficiently decreased.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a semiconductor laser device which has low electrical resistance, and is reliable even when the semiconductor laser device operates with high output power.
According to the present invention, there is provided a semiconductor laser device comprising: a substrate made of a GaAs material having a first energy gap; a lower cladding layer formed on the substrate, and made of a material having a second energy gap; a lower optical waveguide layer formed on the lower cladding layer; an active layer formed on the lower optical waveguide layer; an upper optical waveguide layer formed on the active layer; an upper cladding layer formed on the upper optical waveguide layer, and made of a material having a third energy gap; and a contact layer formed on the upper cladding layer, and made of a material having a fourth energy gap. In addition, the semiconductor laser device comprises at least one of first and second resistance reduction layers, where the first resistance reduction layer is arranged between the substrate and the lower cladding layer, and made of an InGaAsP material having a fifth energy gap which is greater than the first energy gap, and smaller than the second energy gap, and the second resistance reduction layer is arranged between the upper cladding layer and the contact layer, and made of an InGaAsP material having a sixth energy gap which is greater than the fourth energy gap, and smaller than the third energy gap.
That is, according to the present invention, a resistance reduction layer is arranged between the substrate and the lower cladding layer and/or between the upper cladding layer and the contact layer, where the resistance reduction layer has an energy gap which is intermediate between the energy gaps of the layers located immediately above and below the resistance reduction layer. Thus, the difference in the energy gap between at least one pair of adjacent layers of the semiconductor laser device is reduced.
Specifically, each resistance reduction layer should have at least one energy gap which is intermediate between the energy gaps of the layers located immediately above and below the resistance reduction layer. In addition, each resistance reduction layer may have an energy gap which varies stepwise or gradually between the energy gaps of the layers located immediately above and below the resistance reduction layer. Further, each resistance reduction layer may be constituted by one or more sublayers.
According to the present invention, due to the provision of the resistance reduction layer, the energy gap varies stepwise or gradually between the layers located immediately above and below the resistance reduction layer. Therefore, the height of the potential barrier caused by a band offset between adjacent layers is reduced by the provision of the resistance reduction layer. Thus, the electrical resistance of the entire semiconductor laser device can be reduced. Due to the reduction of the electrical resistance of the semiconductor laser device, temperature rise in a high output power operation can be suppressed. Since the temperature rise can be suppressed, the facet degradation can also be suppressed. As a result, the critical output power and the lifetime of the semiconductor laser device increase.
In addition, generally, when the temperature of the semiconductor laser device rises, the oscillation wavelength is shifted to a longer wavelength side. Since the temperature rise can be suppressed according to the present invention, the wavelength shift due to the temperature rise can be reduced. Thus, the semiconductor laser device according to the present invention is reliable even in a high output power operation.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) to (v).
(i) Each of the lower cladding layer and the upper cladding layer is made of an aluminum-free material. In this case, generation of surface recombination centers at the end facet can be suppressed. Therefore, the reliability in the high output power operation can be further increased.
(ii) The lower optical waveguide has a seventh energy gap which is greater than the fifth energy gap. In this case, the electrical resistance of the entire semiconductor laser device can be further decreased.
(iii) The upper optical waveguide has an eighth energy gap which is greater than the sixth energy gap. In this case, the electrical resistance of the entire semiconductor laser device can be further decreased.
(iv) The lower cladding layer has a first carrier density, and the first resistance reduction layer has a second carrier density which is greater than the first carrier density. In this case, the electrical resistance of the entire semiconductor laser device can be further decreased.
(v) The upper cladding layer has a third carrier density, and the second resistance reduction layer has a fourth carrier density which is gr
Akinaga Fujio
Fukunaga Toshiaki
Wada Mitsugu
Fuji Photo Film Co. , Ltd.
Paladini Albert W.
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