Distributed feedback semiconductor laser

Details Distributed feedback semiconductor laser Distributed feedback semiconductor laser

2001-09-28

2003-05-06

Davie, James (Department: 2828)

Coherent light generators

Particular resonant cavity

Distributed feedback

C372S046012

Reexamination Certificate

active

06560266

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a distributed feedback semiconductor laser comprising an active layer sandwiched between the upper side and lower side cladding layer and a diffraction grating formed along said active layer. More specifically, the present invention relates to a distributed feedback semiconductor laser characterized by its structure to enable stable oscillation in the single mode and in addition enable high output operation.
DESCRIPTION OF THE RELATED ART
In recent years, trunk line optical communication systems that enable transmission of a large amount of data for a long distance have been established. Therefore, a light source of such optical communication is requested to satisfy both excellent single wavelength oscillation as well as high output operation.
For a light source to realize the above requested characteristics, a distributed feedback semiconductor laser (hereinafter, referred to as DFB laser), particularly a {fraction (&lgr;/4)} phase-shifted DFB laser has been used. This {fraction (&lgr;/4)} phase-shifted DFB laser is provided with a diffraction grating having a projected and recessed shape along an active layer and having a {fraction (&lgr;/4)} phase shift region near the center of the laser resonator. The active layer is formed within an optical waveguide in the resonator.
However, in the refractive index coupled DFB laser explained above, it is difficult to simultaneously attain stability of single mode oscillation and high output operation, and resistance for the reflected beam returning from external sides is rather small. Therefore, a gain coupled DFB laser has been proposed as a DFB laser to overcome the problems of the refractive index coupled DFB laser explained above, and development of such DFB laser is now continued.
In the gain coupled DFB laser, mode selectivity of an oscillation wavelength is enhanced by adding a periodical perturbation of the gain in the guiding direction of an optical beam. Therefore the stable single mode oscillation is possible even if there is no phase shift region and no anti-reflection film at the end surface of the resonator in the refractive index coupled DFB laser.
Moreover, it is experimentally confirmed that a large resistance is also assured for the reflected beam returning from the external sides.
Moreover, since stable single mode oscillation is possible without any anti-reflection films, the anti-reflection film and the high-reflection film can be provided at both end surfaces of the laser. Therefore, high output operation can be realized.
Here, a first conventional art gain coupled DFB laser that can be assumed to have relationship with the present invention will be explained with reference to FIGS.
1
(
a
)-
1
(
d
).
FIG.
1
(
a
) is a cross-sectional view of the gain coupled DFB laser of the first conventional art along the guiding direction of an optical beam. In this figure, numeral
31
designates an n-type InP cladding layer;
32
, an active layer comprising a multiple quantum well (MQW);
33
, a p-type InP cladding layer; and
34
, n-type InP current blocking portions. A curve given the arrow mark schematically indicates the flow of a current. FIGS.
1
(
b
) to
1
(
d
) respectively illustrate distributions of gain, refractive index and optical field intensity corresponding to the cross-sectional view of FIG.
1
(
a
) along the guiding direction of an optical beam.
In the DFB laser of FIGS.
1
(
a
)-
1
(
d
), the n-type portions
34
are periodically formed within the p-type cladding layer
33
. The n-type portions
34
keep a constant distance from the active layer
32
formed in the uniform thickness. Since this n-type portion
34
is embedded in the p-type layer
33
, it plays a role of the current blocking layer.
Since a current is pinched in the regions between respective current blocking portions with the effect of this n-type current blocking portion
34
, as schematically illustrated in FIG.
1
(
a
), distribution of density of the current flowing into the active layer
32
is lowered in the regions of the active layer just under the current blocking portions
34
and is raised, on the contrary, in the regions where there are no current blocking portions
34
thereon.
This is the reason why the gain of the DFB laser of
FIG. 1
is periodically distributed as illustrated in FIG.
1
(
b
). Therefore gain coupling occurs and the laser starts to operate as the gain coupled DFB laser.
Next, a second conventional art gain coupled DFB laser that is assumed to have relationship with the present invention will be explained with reference to FIGS.
2
(
a
)-
2
(
d
).
FIG.
2
(
a
) is a cross-sectional view of the gain coupled DFB laser of the second conventional art along the guiding direction of an optical beam. The elements like those of
FIG. 2
are designated with the like reference numerals. As in the case of
FIG. 2
, FIGS.
2
(
b
) to
2
(
d
) respectively illustrate distributions of gain, refractive index and optical field intensity corresponding to the cross-sectional view of FIG.
2
(
a
) in the guiding direction of an optical beam.
In the DFB laser of
FIG. 2
, the active layer itself is periodically etched to form periodical projected and recessed shapes as illustrated in FIG.
2
(
a
). Therefore, in this shape, thickness of the active layer is periodically changed along the guiding direction of an optical beam. In the region (projected region) where the active layer is thick, the generated gain is larger than that of the thin region (recessed region).
Accordingly, since the periodical gain distribution is formed as illustrated in FIG.
2
(
b
) in the DFB laser of
FIG. 2
, gain coupling occurs and the laser starts to operate as the gain coupled DFB laser.
A third conventional art gain coupled DFB laser that is assumed to have the relationship with the present invention will be explained with reference to FIG.
3
.
FIG. 3
is a cross-sectional view of the gain coupled DFB laser of the third conventional art along the guiding direction of an optical beam. The elements like those in FIGS.
1
(
a
)-
1
(
d
) and FIGS.
2
(
c
)-
2
(
d
) are designated with like reference numerals. Numeral
35
designates embedded portions.
The DFB laser of
FIG. 3
has a structure almost similar to the DFB laser of the second conventional art of FIGS.
2
(
c
)-
2
(
d
). However, it is different only in the point that the recessed region of the active layer
32
having periodical projected and recessed shapes is embedded with p-type InGaAsP. This p-type InGaAsP is the quaternary compound semiconductor material having a band gap which is smaller than that of the p-type InP cladding layer
33
, namely the refractive index which is larger than that of such material.
Moreover, in the DFB laser of
FIG. 3
, distribution of the refractive index in the guiding direction of an optical beam is kept small by adjusting a composition of the InGaAsP embedded portions
35
so that the refractive index of the embedded portions
35
becomes close to the average refractive index of the active layer
32
.
However, the gain coupled DFB laser structures of the first to third conventional arts explained above respectively have the following problems.
First, the DFB laser of the first conventional art has a problem that it is actually difficult to increase the ratio of gain coupling to refractive index coupling. This will cause a generation of a fluctuation in the single wavelength characteristic of the laser oscillation thereby lowering the stability of the single mode oscillation.
Namely, in the process of forming a groove for current blocking portions
34
by etching the p-type cladding layer
33
, it is required to keep a constant margin for the remaining thickness of the cladding layer
33
in order to prevent the active layer
32
from being etched. Therefore, it is actually difficult to reduce the distance between the current blocking portion
34
and the active layer
32
.
Therefore, the pinching effect of currents by the current blocking portions
34
becomes insufficient and a current dispers

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