Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2001-01-19

2003-11-04

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S040000, C372S045013

Reexamination Certificate

active

06643307

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device, and more particularly relates to a self-sustained pulsation type semiconductor laser device that emits red laser light.
A red-light-emitting semiconductor laser is widely used as a light source for a DVD apparatus. Also, a self-sustained pulsation type semiconductor laser device does not need an external high frequency modulation circuit for reducing external optical feed back noise and therefore can be a key device in terms of size and cost reduction.
Hereinafter, a typical known structure for a self-sustained pulsation type semiconductor laser device will be described with reference to FIG.
8
.
FIG. 8
illustrates a cross-sectional structure for a known semiconductor laser device
10
. N-type cladding layer
12
made of an n-type AlGaInP layer; active layer
13
with a multiple quantum well structure; first p-type cladding layer
14
made of a p-type AlGaInP layer; saturable absorption layer
15
; and second p-type cladding layer
16
, which is made of a p-type AlGaInP layer and has a ridge portion, are stacked in this order over an n-type GaAs substrate
11
. That is to say, the saturable absorption layer
15
is inserted between the first and second p-type cladding layers
14
and
16
.
A current blocking layer
17
of an n-type GaAs layer is formed on the second p-type cladding layer
16
to cover both sides of the ridge portion. A contact layer
18
is formed on part of the ridge portion of the second p-type cladding layer
16
, which is sandwiched by the current blocking layer
17
. And a cap layer
19
of a p-type GaAs layer is formed on the current blocking and contact layers
17
and
18
.
Further, an n-side electrode
20
is formed on the lower surface of the n-type GaAs substrate
11
, while a p-side electrode
21
is formed on the upper surface of the cap layer
19
.
In order to fabricate the known semiconductor laser device, n-type AlGaInP layer to be the n-type cladding layer
12
; active layer
13
; p-type AlGaInP layer to be the first cladding layer
14
; saturable absorption layer
15
; p-type AlGaInP layer to be the second p-type cladding layer
16
; and contact layer
18
are stacked in this order over the n-type GaAs substrate
11
by a crystal growth process (i.e., first growth process). Thereafter, the second cladding layer
16
and contact layer
18
are etched and patterned to form a ridge portion. Next, an n-type GaAs layer to be the current blocking layer
17
are selectively formed on both sides of the ridge of the second cladding layer
16
by another crystal growth process (i.e., second growth process). Subsequently, a p-type GaAs layer to be the cap layer
19
is formed on the contact and current blocking layers
18
and
17
by another crystal growth process (i.e., third growth process).
The distribution
22
of laser light, emitted from the active layer
13
, is confined in a part of the active layer
13
under the ridge portion. However, self-sustained pulsation is realized because the saturable absorption layer
14
exists within the range in which the light is distributed.
FIG. 9
illustrates the optical output-current characteristic of the known semiconductor laser device. In
FIG. 9
, curves a, b, c, d and e represent the characteristics of the semiconductor laser device at temperatures of 20° C., 30° C., 40° C., 50° C. and 60° C., respectively. As can be seen from
FIG. 9
, non-continuous characteristics resulting form the self-sustained pulsation is observable in the vicinity of the threshold current. It should be noted that the operating current is 86.6 mA when the optical output is 5 mW at room temperature (25° C.).
However, in the known semiconductor laser device, the current blocking layer
17
is made of GaAs and therefore absorbs a great deal of red laser light emitted from the active layer
13
. For that reason, the internal loss at the optical waveguide is as large as about 20 cm
−1
, thus causing a problem that the operating current of the semiconductor laser device increases.
Furthermore, increased heat is generated from the semiconductor laser device due to the large operating current, and the known semiconductor laser device cannot be built in an optical pickup apparatus for a DVD, which is in the highest demand now. As a result, the known semiconductor laser device is not suitable for practical use.
Moreover, the known self-sustained pulsation type semiconductor laser device needs to perform three crystal growth processes as described above. Accordingly, the device has a problem that it is difficult to cut down the cost required.
SUMMARY OF THE INVENTION
In view of the foregoing, it is a first object of the present invention to realize a self-sustained pulsation type semiconductor laser device having a low operating current. It is a second object of the present invention to get the device fabricated by two crystal growth processes.
To achieve these objects, a semiconductor laser device according to the present invention includes: an active layer; a first cladding layer, which is formed on the active layer and is made of (Al
X1
Ga
1−X1
)
Z1
In
1−Z1
P (where 0≦X
1
≦1 and 0<Z
1
<1) of a first conductivity type; a current blocking layer, which is formed on the first cladding layer and is made of (Al
Y
Ga
1−Y
)
Z2
In
1−Z2
P (where 0≦Y≦1 and 0<Z
2
<1) of a second conductivity type and has a striped region; and a second cladding layer, which is formed at least in the striped region and is made of (Al
X2
Ga
1−X2
)
Z3
In
1−Z3
P (where 0≦X
1
≦1 and 0<Z
3
<1) of the first conductivity type. X
1
, X
2
and Y have relationships represented as Y>X
1
and Y>X
2
. A saturable absorption region for absorbing laser light produced from the active layer is formed in part of the active layer. The part is located under the current blocking layer.
In the semiconductor laser device of the present invention, the aluminum mole fraction (Y) of the current blocking layer is greater than the aluminum mole fraction (X
1
) of the first cladding layer or the aluminum mole fraction (X
2
) of the second cladding layer. Therefore, the bandgap energy of each of the first cladding layer, current blocking layer and second cladding layer can be made greater than the energy corresponding to the oscillation wavelength of the laser light produced from the active layer.
Thus, the first cladding layer, current blocking layer and second cladding layer are transparent to the laser light emitted from the active layer, and it is possible to prevent the laser light from being absorbed into the first cladding layer, current blocking layer and second cladding layer, or the current blocking layer among other things. As a result, the semiconductor laser device of the present invention can reduce its operating current.
Also, the current blocking layer is transparent to the laser light, and the distribution of the laser light emitted from the part of the active layer located under the striped region can be easily expanded to other parts of the active layer located under the current blocking layer. Accordingly, the saturable absorption region for absorbing the laser light produced from the active layer can be formed in those parts of the active layer located under the current blocking layer. As a result, the semiconductor laser device of the present invention realizes self-sustained pulsation.
Further, in this structure, the current blocking layer has the striped region and the second cladding layer is formed in the striped region. Accordingly, only two crystal growth processes are needed, and the fabrication cost of the semiconductor laser device can be reduced.
In the semiconductor laser device of the present invention, an effective refractive index difference between the inside and outside of the striped region, which is a difference between first and second effective refractive indices, is preferably equal to or greater than 2×10
−3
and equal to or smaller than 5×10
−3
. The fi

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