Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-11-12
2004-09-21
Wong, Don B (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012, C372S043010
Reexamination Certificate
active
06795469
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser element having an optically-nonabsorbent window structure at the end facets.
2. Description of the Related Art
Maximum optical output power of semiconductor laser elements is known to be limited by catastrophic optical mirror damage (COMD), wherein end facets are damaged by a cycle in which currents generated by optical absorption at the end facets raise the temperature at the end facets, the raised temperature reduces the semiconductor bandgaps at the end facets, and therefore the optical absorption is further enhanced. The optical output power level, at which the COMD occurs (COMD level), decreases with degradation of the end facet caused by aging. In addition, there are cases in which the semiconductor laser suddenly shuts down. Accordingly, high output power and high reliability are known to be obtained by forming an optically-nonabsorbent window structure at the end facet of semiconductor laser element.
Kazushige Kawasaki et al. (“0.98 &mgr;m band ridge-type window structure semiconductor laser (1),” Abstracts of the Spring Meeting of the Japan Society of Applied Physics, 1997, 29a-PA-19) disclose a semiconductor laser element which emits laser light in the 980 nm band and has a window structure at its end facets. The window structure is formed by injecting Si ions into near-edge regions of a ridge structure and disordering an In
0.2
Ga
0.8
As quantum well by thermal diffusion.
H. Horie et al. (in “Reliability improvement of 980-nm laser diodes with a new facet passivation process”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5 (1999), No. 3, pp. 832-838) disclose a semiconductor laser element having an internal current confinement structure. The semiconductor laser element comprises an InGaAs active layer, GaAs optical waveguide layers, AlGaAs cladding layers, and an AlGaAs current confinement layer. In addition, cleaved end facets are irradiated with Ar ions having energy not higher than 35 eV, and coated with silicon by vapor deposition. Then, AR/HR coatings are administered on the end facets by an ion assist vapor deposition method, where the average acceleration voltage for Ar ions is 110 eV. Thus, this semiconductor laser element can achieve high output power and reliability. Further, Horie et al. report that since the upper and lower optical waveguide of the semiconductor laser element are formed by GaAs, the crystal quality is maintained even if the temperature is raised and lowered during GaAs growth, and since InGaAs of the active layer can be grown in low temperatures, the crystal quality can be improved.
However, in the former report, there is a drawback that a process for injecting Si ion in the vicinities of the active layer is required and the fabrication process becomes longer. In addition, since a diffusion process is required, obtaining an accurate window structure is difficult. In the latter report, the low-energy ion acceleration requires expensive equipment, and the cost increases.
SUMMARY OF THE INVENTION
The present invention has been developed in view of the above circumstances. The object of the present invention is to provide a semiconductor laser element which is reliable in operation from low to high output power, having an optically-nonabsorbent window structure at its end facets.
According to the present invention, there is provided a semiconductor laser element having optical waveguide layers whose bandgap energies are greater than a bandgap energy of an active layer, comprising: a substrate of GaAs of a first conductive type; a lower cladding layer of the first conductive type, formed above the GaAs substrate; a lower optical waveguide layer of the first conductive type or an undoped type, formed above the lower cladding layer; a lower GaAs layer formed above the lower optical waveguide layer; an active layer of In
x1
Ga
1-x1
As
1-y1
P
y1
(0.49y1<x1≦0.4, 0≦y1≦0.1), formed above the lower GaAs layer; an upper GaAs layer formed above the active layer; an upper optical waveguide layer of a second conductive type or an undoped type, formed above the upper GaAs layer; an upper cladding layer of the second conductive type formed above the upper GaAs layer; and a contact layer of the second conductive type being formed above the upper cladding layer; wherein:
at least the upper GaAs and the active layer among the lower GaAs Layer, the active layer, and the upper GaAs layer, are formed in regions except at least a vicinity of one emission end facet of a resonator, and the upper optical waveguide layer is formed so that the end portions thereof bury the vicinity of the emission end facet.
Note that at the two end facets of the resonator, the vicinity of the end facets may be removed.
The contact layer of the second conductive type can be formed in a region except the vicinity of the end facets, and in this case, it is desirable that an insulative film having an opening for current injection is formed from the upper surface of the upper cladding layer of the second conductive type to the upper surface of the contact layer of the second conductive type, and an electrode is formed at least on the opening mentioned above. That is, the electrode may be formed on apart of the insulative film corresponding to the contact layer of the second conductive type, or on the entirety of the insulative film, so as to cover the opening.
A GaAs layer with a thickness of approximately 20 nm may be formed under the upper optical waveguide layer.
A semiconductor laser element according to the present invention may have a refractive index waveguide mechanism formed by a ridge structure. In addition, a semiconductor laser element according to the present invention may have a refractive index waveguide mechanism formed by an internal current confinement structure.
A semiconductor laser element according to the present invention may have a first etching block layer of the second conductive type of In
x9
Ga
1-x9
P (0≦x9≦1), a second GaAs etching block layer, a current confinement layer of the first conductive type of In
0.5
(Ga
1-z4
Al
z4
)
0.5
P (0≦z4≦1), and a cap layer of an InGaP family crystal, lattice matched to GaAs, are formed in this order on the upper optical waveguide layer. There may be provided an opening with a width from approximately 1 &mgr;m to approximately 4 &mgr;m between the two facets of the resonator in the cap layer, the current confinement layer and the second etching block layer, and the upper cladding layer of the second conductive type and the contact layer of the second conductive type may be provided so as to bury the opening. Note that there may be another upper cladding layer made of the second conductive type Al
z1
Ga
1-z1
As (0.25≦z1≦0.8) between the upper optical waveguide layer and the first etching block layer.
In addition, in a semiconductor laser element according to the present invention, the upper cladding layer of the second conductive type may be made of a first upper cladding layer of the second conductive type and a second upper cladding layer of the second conductive type. The first upper cladding layer of the second conductive type of Al
z1
Ga
1-z1
As (0.25≦z1≦0.8), first etching block layer of the second conductive type of GaAs, a second etching block layer being made of In
x8
Ga
1-x8
P (0≦x8≦1), a current confinement layer of the first conductive type being made of Al
z3
Ga
1-z3
As (z1≦z3≦0.8) and a GaAs cap layer may be formed above the upper optical waveguide layer in this order. An opening of from about 1 &mgr;m to about 4 &mgr;m width may be provided between the two facets of the resonator in the cap layer, the current confinement layer and the second etching block layer, and said opening may be filled in by the second upper cladding layer of the second conductive type with the contact layer of the second conductive type formed above the cladding layer.
In addition, in a semiconductor laser element according to the present invention, the upper claddi
Nguyen Dung T
Wong Don B
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