Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-02-23
2003-08-19
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S046012, C372S050121
Reexamination Certificate
active
06608850
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device.
2. Description of the Related Art
A conventional semiconductor laser device will be described with reference to
FIGS. 6A through 6C
.
FIGS. 6A and 6B
are diagrams illustrating a front and face and a rear end face of a conventional semiconductor laser device
600
, respectively.
FIG. 6C
is a diagram illustrating a cross-sectional view of the semiconductor laser device
600
, as taken along the
6
C—
6
C line indicated in
FIGS. 6A and 6B
.
As shown in
FIGS. 6A and 6B
, the semiconductor laser device
600
includes: a semiconductor substrate
1
made of an n-type InP material and having a mesa structure; a light confinement layer
2
, provided on a mesa region of the substrate
1
, made of an n-type InGaAsP (composition wavelength: about 1.05 &mgr;m) and having a thickness of about 600 nm; an active layer
3
having a multiple quantum-well structure; a light confinement layer
4
made of a p-type InGaAsP material and having a thickness of about 600 nm; and a cladding layer
5
made of a p-type InP material and having a thickness of about 400 nm. The layers
2
through
5
are provided in this order on the semiconductor substrate
1
. The active layer
3
includes seven InGaAsP well layers (not shown) each having a thickness of about 6 nm and a compressive distortion of 1.0% or less, and seven InGaAsP (composition wavelength: about 1.05 &mgr;m) barrier layers (not shown) each having a thickness of about 10 nm and no compressive distortion, which are alternately layered on one another.
The semiconductor laser device
600
also includes: a first buried layer
6
made of a p-type InP whose carrier density is 7.0×10
17
cm
−3
, a second buried layer
7
made of an n-type InP whose carrier density is 2.0×10
18
cm
−1
, a third buried layer
8
made of an p-type InP whose carrier density is 7.0×10
17
cm
3
, and a contact layer
9
made of a p-type InGaAsP (composition wavelength: about 1.3 &mgr;m). The layers
6
through
9
are provided in this order in the vicinity of the active layer
3
so as to surround the mesa region of the substrate
1
.
In order to reduce the parasitic capacity in the buried layers
6
through
8
so as to improve the frequency response characteristic of the semiconductor laser device
600
, grooves
14
are provided by using an etching technique. The grooves
14
extend into the first buried layer
6
via the contact layer
9
, the third buried layer
8
, and the second buried layer
7
.
On the contact layer
9
, a SiO
2
film
10
is formed having a thickness of about 0.3 &mgr;m with an aperture therein. A metal multilayer film
11
including three layers (i.e., an Au layer, a Zn layer, and an Au layer) is formed in the aperture, and a p-type electrode
12
is formed on the metal multilayer film
11
.
An n-type electrode
13
is provided on the back side of the semiconductor substrate
1
.
Referring to
FIG. 6C
, a cross-sectional view of the active layer
3
of the semiconductor laser device
600
is shown. The active layer
3
has a width of about 0.6 &mgr;m within about 25 &mgr;m from the front end face, while it has a width of about 1.6 &mgr;m within about 25 &mgr;m from the rear end face. The distance between the front end face and the rear end face is about 400 &mgr;m, and the cross section of the active layer
3
has a stripe structure. The width of the active layer
3
having this stripe structure continuously decreases from the rear end face toward the front end face. Thus, the stripe width of the active layer
3
at the front end face is narrower than that at the rear end face. This is a structure of a semiconductor laser device for implementing a narrow output angle characteristics and a low operation current characteristics at a high temperature (Y. Inaba et al., IEEE JSTQE, vol. 3, 1399-1404, 1997). With this structure, the effect of confining light within the active layer
3
continuously decreases from the rear end face toward the front end face. Therefore, a large amount of light leaks out of the active layer
3
into the first and third buried layers
6
and
8
) in the area adjacent to the front end face.
FIG. 7
is a graph illustrating the relationship between an operation environment temperature and an output angle of the semiconductor laser device
600
. As seen from
FIG. 7
, when the operation environment temperature changes from about −40° C. to about 85° C., the output angle changes from about 14.0° to about 10.2° (i.e., about 3.8°). Therefore, in a case where light output from the semiconductor laser device
600
is coupled to an optical fiber
16
as shown in
FIG. 8
, for example, such temperature conditions change the coupling efficiency between the light from the semiconductor laser device
600
and the optical fiber
16
, thereby changing the intensity of the light propagated through the optical fiber
16
. This will adversely affect the transmission characteristics of the optical fiber
16
. The amount of change in the intensity of the light propagated through the optical fiber
16
should satisfy the practical standard in optical communications (i.e., 1 dB or less for a temperature change for about −40° C. to about 85° C.). Otherwise, the transmission characteristics of the optical fiber would be very poor. Thus, the semiconductor laser device
600
may not be usable without optical components for focusing light (e.g., a lens) between the semiconductor laser device
600
and the optical fiber
16
.
The conventional semiconductor laser device
600
, however, does not satisfy the above-mentioned standard, and the amount of change in the intensity of the light propagated through the optical fiber is 2 dB.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a semiconductor laser device, including: a semiconductor substrate of a first conductivity type; an active layer having a stripe structure formed on the semiconductor substrate; a first buried layer of a second conductivity type formed on the semiconductor substrate and in a vicinity of the active layer; a second buried layer of the first conductivity type formed on the first buried layer and in the vicinity of the active layer; and a third buried layer of the second conductivity type formed on the second buried layer and in the vicinity of the active layer.
In this semiconductor laser device, a difference between a refractive index of the first buried layer and a refractive index of the second buried layer is about 0.02 or less, and a difference between the refractive index of the second buried layer and a refractive index of the third buried layer is about 0.02 or less.
According to the present invention, the difference between the refractive index of the first buried layer and that of the second buried layer, and the difference between the refractive index of the second buried layer and that of the third buried layer are set to be small, thereby reducing the amount of light propagating through the active layer which leaks into the first buried layer or the third buried layer.
In one embodiment of the present invention, the semiconductor laser device further includes a pair of light confinement layers formed so as to sandwich the active layer.
In another embodiment of the present invention, a cross section of the active layer at a front end face is smaller than that of the active layer at a rear end face.
In still another embodiment of the present invention, the semiconductor laser device further includes an optical fiber provided adjacent to the front end face of the active layer into which light output from the front end face of the active layer is input.
In still another embodiment of the present invention, a carrier density of the first buried layer and that of the third buried layer are substantially equal to each other.
In still another embodiment of the present invention, the first conductivity type is n-type and the following inequality is satisfied:
log
10
f
n
(
Flores Ruiz Delma R.
Ip Paul
Matsushita Electric - Industrial Co., Ltd.
RatnerPrestia
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