Semiconductor laser apparatus

Details Semiconductor laser apparatus Semiconductor laser apparatus

2000-08-23

2003-04-08

Lee, Eddie (Department: 2815)

Coherent light generators

Particular active media

Semiconductor

C372S046012, C372S050121, C257S094000

Reexamination Certificate

active

06546032

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser apparatus for use as a light source for pumping a solid-state laser, a fiber laser, a fiber amplifier or the like, or for use in various laser instrumentation.
2. Description of the Related Art
In realizing a higher power semiconductor laser, it is important to inhibit catastrophic optical mirror damage at the light-emitting end face of the laser. For the purpose of a higher power semiconductor laser, the present applicant has proposed a semiconductor laser provided with a carrier blocking layer having a bandgap larger than an active layer and a small thickness on each of the opposite sides of the active layer, thereby offering widened design freedom as to the thickness of a waveguide layer and the bandgap of a cladding layer which are formed outside each carrier blocking layer (refer to International Publication No. WO93/16513).
In such a structure, carriers injected into the active layer can efficiently be confined by the carrier blocking layers, while laser light generated at the active layer passes through each thin carrier blocking layer and is propagated along an optical waveguide defined mainly by the waveguide layers and cladding layers. By designing the optical waveguide so as to confine light existing in the waveguide layers to a greater extent, the intensity of light in the active layer lowers and, hence, an optical output power at which the catastrophic optical mirror damage (COMD) occurs on the light-emitting facet can be raised, thus leading to a higher power operation.
Japanese Unexamined Patent Publication JP-A 10-303500 (1998) describes a structure such as to enhance confinement of light in the waveguide layers in order to lower the intensity of light existing in the active layer formed in the separate-confinement structure thereby raising an optical output power at which the COMD occurs on the light-emitting facet.
In the case of such enhanced confinement of light in the waveguide layers as in JP-A 10-303500, guided modes which can be propagated along the optical waveguide include not only a fundamental mode but also higher-order modes.
From the standpoint of the lasing conditions, the intensity of light in the active layer creating a gain reaches the highest value in the fundamental mode, and in addition the free carrier absorption and the substrate radiation loss become smaller in the fundamental mode. This means that lasing is likely to occur in the fundamental mode.
However, the present inventors have discovered the fact that the optical waveguide can propagate a higher-order mode as well as the fundamental mode in which lasing occurs and, hence a undesirable wavelength separation occurs in the lasing spectrum of laser light. Such a wavelength separation causes a full width at half maximum to increase and inhibits the oscillation wavelength to continuously vary even when the temperature or the injection current is continuously varied. For this reason, use of such a semiconductor laser as a light source for pumping a solid-state laser, fiber laser, fiber amplifier or the like results in a decrease in pumping efficiency and an unstable output. Thus, problems will be raised in applications.
SUMMARY
It is an object of the invention to provide a semiconductor laser apparatus which is capable of a stabilized high power operation by inhibiting wavelength separation caused by a higher-order mode in an epitaxial direction.
The invention provides a semiconductor laser apparatus comprising an active layers, n-type and p-type optical waveguide layers provided on both sides of the active layer, the n-type and p-type optical waveguide layers each having a bandgap larger than that of the active layer, and n-type and p-type cladding layers disposed so that the active layer and the optical waveguide layers are sandwiched therebetween, the n-type and p-type cladding layers each having a bandgap larger than that of each of the waveguide layers,
wherein a condition represented by the following formula (A) is satisfied:
nc
1≦
Re
(
ne
1)   (A)
wherein nc1 represents a smaller one of refractive indices of the n-type and p-type cladding layers, Re(ne1) represents a real part of an effective refractive index of a first-order guided mode of an optical waveguide including the optical waveguide layers and the cladding layers, and
wherein compositions of the n-type and p-type cladding layers are made asymmetrical, whereby no higher-order waveguide mode exists in an epitaxial direction.
According to the invention, by setting the real part Re(ne1) of the effective refractive index of the first-order guided mode to be equal to or larger than that of the smaller one of the refractive indices of the n-type and p-type cladding layers the intensity of light existing in the active layer can be lowered, thereby raising an optical power at which the catastrophic optical mirror damage occurs on the light-emitting facet. Thus, a higher power operation can be realized.
Furthermore, according to the invention, by making the compositions of the n-type and p-type cladding layers asymmetrical, a higher-order waveguide mode in the thickness direction is avoided. Accordingly only a fundamental mode can be propagated with low loss and wavelength separation in oscillation spectrum can be prevented.
In the invention it is preferable that an optical waveguide including the optical waveguide layers and the cladding layers satisfies a condition represented by the following formula (1):
t
λ
⁢
nw
2
-
nc1
2
≧
1
2
(
1
)
wherein &lgr; represents a laser wavelength in a vacuum, t represents a thickness between the n-type and p-type cladding layers, nw represents a refractive index of the n-type and p-type optical waveguide layers, nc1 represents the smaller one of the refractive indices of the n-type and p-type cladding layers, and nc2 represents a larger one of the refractive indices of the n-type and p-type cladding layer.
According to the invention, the condition represented by the formula (1) is satisfied, so that the intensity of light existing in the active layer can be lowered, thereby raising an optical output at which the COMD occurs on the light-emitting facet. Thus, a higher power operation can be realized.
In the invention it is preferable that a condition represented by the following formula (2) is satisfied:
Re
(
ne
1)<
nc
2
≦ne
0   (2)
wherein ne0 represents an effective index of a fundamental guided mode in the optical waveguide, and Re(ne1) represents the real part of the effective index of a first-order guided mode.
According to the invention, the condition represented by the formula (2) is satisfied, so that the fundamental guided mode can be propagated in middle layers and the first-order guided mode leaks into the cladding layer having the refractive index nc2. As a result only the fundamental guided mode can be propagated with low loss and wavelength separation in oscillation spectrum can be prevented.
The principle of the invention is as follows.
FIG. 4
is a sectional view illustrating a typical structure of a semiconductor laser apparatus. On a substrate
111
of n-type GaAs are sequentially formed using metalorganic vapor phase epitaxy (MOVPE) or a like process an n-type cladding layer
112
(AlGaAs, Al context x=0.20, thickness t=1.2 &mgr;m), an n-type optical waveguide layer
113
(GaAs, t=0.49 &mgr;m), an n-type carrier blocking layer
114
(AlGaAs, x=0.40, t=0.03 &mgr;m), an active layer
115
(composed of an In
0.28
Ga
0.82
As quantum well layer and GaAs barrier layers), a p-type carrier blocking layer
116
(AlGaAs, x=0.40, t=0.03 &mgr;m), a p-type optical waveguide layer
117
(GaAs, t=0.49 &mgr;m), and a p-type cladding layer
118
(AlGaAs, x=0.20, t=1.2 &mgr;m). On the p-type cladding layer
118
is further formed a p-type cap layer
120
(GaAs), in which a pair of n-type current blocking layers
119
(GaAs) are buried to form a current injecti

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