Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-03-29
2003-05-20
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012, C372S050121
Reexamination Certificate
active
06567444
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device having an end-facet window structure, i.e., a structure which makes an end facet nonabsorbent of oscillation light.
2. Description of the Related Art
J. K. Wade et al. (“6.1 W continuous wave front-facet power from Al-free active-region (&lgr;=805 nm) diode lasers,” Applied Physics Letters, vol. 72, No. 1 (1998) pp.4-6) disclose a semiconductor laser device which emits light in the 805 nm band. The semiconductor laser device comprises an Al-free InGaAsP active layer, an InGaP optical waveguide layer, and InAlGaP cladding layers. In addition, in order to improve the characteristics in the high output power range, the semiconductor laser device includes a so-called large optical cavity (LOC) structure in which the thickness of the optical waveguide layer is increased so as to reduce the light density, and increase the maximum light output power. However, when the optical power is maximized, currents generated by optical absorption in the vicinity of end facets generate heat, i.e., raise the temperature at the end facets. In addition, the raised temperature reduces the bandgap at the end facets, and therefore the optical absorption is further enhanced to damage the end facet. That is, a vicious cycle is formed. This damage is the so-called catastrophic optical mirror damage (COMD). When the optical power reaches the COMD level, the optical output deteriorates with time. Further, the semiconductor laser device is likely to suddenly break down due to the COMD. Therefore, the above semiconductor laser device is not reliable when the semiconductor laser device operates with high output power.
In addition, T. Fukunaga et al. (“Highly Reliable Operation of High-Power InGaAsP/In
0.48
Ga
0.52
P/AlGaAs 0.8 &mgr;m Separate Confinement Heterostructure Lasers,” Japanese Journal of Applied Physics, vol. 34 (1995) L1175-L1177) disclose a semiconductor laser device which comprises an Al-free active layer, and emits light in the 0.8 &mgr;m band. In the semiconductor laser device, an n-type AlGaAs cladding layer, an intrinsic (i-type) InGaP optical waveguide layer, an InGaAsP quantum well active layer, an i-type InGaP optical waveguide layer, a p-type AlGaAs cladding layer, and a p-type GaAs cap layer are formed on an n-type GaAs substrate. S. O'Brien, H. Zhao, and R. J. Lang report, in Electronics Letters, vol. 34, No. 2 (1998) p.184, that the maximum output power of the above semiconductor laser device disclosed by Fukunaga et al, is 1.8 W. They also report the maximum breakdown light output power of multiple-transverse-mode semiconductor laser devices having a stripe width of 50 micrometers or greater. For example, at the wavelength of 0.87 micrometers, the maximum breakdown light output power of a multiple-transverse-mode semiconductor laser device having a stripe width of 100 micrometers is reported to be 1.3 W, and the maximum breakdown light output power of a multiple-transverse-mode semiconductor laser device having a stripe width of 200 micrometers is reported to be 16.5 W.
Further, the present inventor, T. Hayakawa, and others report, in Applied Physics Letters, Vol. 75, No. 13 (1999) p. 1839, that the practical light output power of 1.5 W is achieved in continuous oscillation of a semiconductor laser device having a stripe width of 50 micrometers when the semiconductor laser device is designed to increase the beam width in the direction perpendicular to the active layer, lower the peak optical strength, and minimize the temperature raise at a light-exit end facet. However, it is difficult to increase the reliability and the practical light output power of the semiconductor laser device by a large amount.
In order to solve the above problems, the Japanese Patent Application No. 11(1999)-348527 and the copending U.S. patent application Ser. No. 09/731,702, “HIGH-POWER SEMICONDUCTOR LASER DEVICE IN WHICH NEAR-EDGE PORTIONS OF ACTIVE LAYER ARE REMOVED”, corresponding to the Japanese patent application and being filed on Dec. 8, 2000 by Toshiaki Fukunaga and assigned to the same assignee as the present patent application, disclose a semiconductor laser device in which transparent regions are formed in vicinities of end facets with Al-free material. However, the transparent regions are required to be formed by crystal regrowth, and the crystal regrowth is initiated from a surface of an optical waveguide layer located near to the quantum well active layer. That is, portions of the regrowth boundary are near the quantum well. In addition, there is no energy barrier between the quantum well active layer and the other portions of the regrowth boundary. In the semiconductor laser device formed as above, carriers leaked from the active layer and diffused to the regrowth boundary cause non-radiative recombination at the regrowth boundary. Therefore, the efficiency of the semiconductor laser device decreases, and degradation is promoted. Further, before the regrowth, the regions in the vicinities of the end facets must be etched to the depth of a crystal layer which is located immediately below the active layer. Therefore, it is not easy to control the depth of the etching.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a semiconductor laser device in which regions in vicinities of end facets are made of a material being non-absorbent of oscillation light so that non-radiative recombination at a regrowth boundary is prevented, and the reliability and performance of the semiconductor laser device are improved.
According to the present invention, there is provided a semiconductor laser device comprising a substrate, an active region formed above the substrate, and a non-absorbing layer formed over the active region, and made of a semiconductor material having a bandgap greater than the photon energy of laser light which oscillates in the semiconductor laser device. The active region includes a first lower optical waveguide layer formed above the substrate, an etching stop layer formed on the first lower optical waveguide layer except for near-edge areas of the first lower optical waveguide layer, a second lower optical waveguide layer formed on the etching stop layer, a quantum well active layer formed on the second lower optical waveguide layer, a first upper optical waveguide layer formed on the quantum well active layer, an electron barrier layer formed on the first upper optical waveguide layer and made of a semiconductor material having a bandgap greater than a bandgap of the first upper optical waveguide layer, and a second upper optical waveguide layer formed on the electron barrier layer, where the near-edge areas are located adjacent to opposite end facets which are perpendicular to the direction of the laser light. The etching stop layer has such a chemical property that the etching stop layer can be maintained when the second lower optical waveguide layer, the quantum well active layer, the first upper optical waveguide layer, and the electron barrier layer are etched, and the first lower optical waveguide layer can be maintained when the etching stop layer is etched.
When the semiconductor laser device according to the present invention is produced, first, a first lower optical waveguide layer, an etching stop layer, a second lower optical waveguide layer, a quantum well active layer, a first upper optical waveguide layer, an electron barrier layer, and a second upper optical waveguide layer are formed above the substrate in this order. Next, near-edge portions (i.e., portions in vicinities of end facets which are perpendicular to the light axis) of the second lower optical waveguide layer, the quantum well active layer, the first upper optical waveguide layer, the electron barrier layer, and the second upper optical waveguide layer are removed by selective etching. Then, near-edge portions of the etching stop layer are also removed by selective etching. The etching stop layer has such a chemical property that the etching stop layer ca
Fuji Photo Film Co. , Ltd.
Ip Paul
Nguyen Tuan
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