Distributed feedback semiconductor laser device

A distributed feedback semiconductor laser device comprising an active layer positioned between a first cladding layer and a second cladding layer, and an absorption layer positioned between the active layer and one of the cladding layers, the absorption layer having grooves to form a diffraction grating with a given periodicity in the direction of the propagation of laser light.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a distributed feedback semiconductor laser 
device, the absorption loss of which has been adjusted to the same 
periodicity as the wavelength of laser light in the direction of the 
propagation of the laser light, thereby allowing oscillation in a single 
longitudinal mode. 
2. Description of the Prior Art 
It has been found in recent years that distributed feedback semiconductor 
laser devices, the refractive index of which has been changed to match the 
periodicity of the wavelength of laser light in the direction of the 
propagation of the laser light, are an effective means for laser light 
oscillation in a single longitudinal mode. In a semiconductor laser with a 
distributed feedback the refractive index of which has been changed, there 
are essentially two longitudinal modes with equivalent gain. However, when 
uniform electrical current is injected into the device, that is, when 
device operation is under a steady state, oscillation is attained in only 
one longitudinal mode because of some asymmetrical characteristics of the 
resonator. 
In most semiconductor laser devices with distributed feedback, the facets 
of both ends of the devices do not result in those of a Fabry-Perot 
resonator, but rather, one of the facets is etched diagonally, so that the 
asymmetry of the resonator is increased greatly, and stable single-mode 
oscillation can be achieved. 
However, when the above-mentioned conventional semiconductor lasers with 
distributed feedback operate under an unsteady state where the excitation 
current is rapidly modulated as when they are being used as a light source 
for optical communication, the above-mentioned asymmetry of the resonator 
is probably changing constantly. Thus, oscillation is not necessarily in a 
single mode, and in practice it involves broadening of the oscillation 
spectrum and noncontinuous change in the oscillation mode. 
SUMMARY OF THE INVENTION 
The semiconductor laser device with a distributed feedback of this 
invention which overcomes the above-discussed and numerous other 
disadvantages and deficiencies of the prior art, comprises an active layer 
positioned between a first cladding layer and a second cladding layer, and 
an absorption layer positioned between the active layer and one of the 
cladding layers, the absorption layer having grooves to form a diffraction 
grating with a given periodicity in the direction of the propagation of 
laser light. A buffer layer having the same polarity as one of the 
cladding layers is, in a preferred embodiment, positioned between said 
active layer and said absorption layer. 
In another preferred embodiment, the absorption layer has an energy gap 
which is equal to or is somewhat narrower than that of the active layer 
and has the same polarity as one of the cladding layers. 
Alternatively, in a preferred embodiment, the absorption layer has a 
different polarity from that of one of the cladding layers and a reverse 
bias is applied to the junction at the interface between the buffer layer 
and the absorption layer, resulting in a current blocking structure having 
a diffraction grating with the same periodicity as the diffraction grating 
of the absorption layer. 
Thus the invention described herein makes possible the objects (1) of 
providing a distributed feedback semiconductor laser device in which an 
absorption layer is disposed between the active layer and one of the two 
cladding layers to achieve a change of the absorption coefficient of laser 
light at the same periodicity as the wavelength of the laser light in the 
direction of the propagation of the laser light thereby allowing for the 
existence of only one longitudinal mode with maximum gain, so that the 
laser device can oscillate in a single longitudinal mode in operation not 
only under a steady state but also under an unsteady state , and neither 
broadening of the oscillation spectrum nor noncontinuous change in the 
oscillation mode arise; (2) a distributed feedback semiconductor laser 
device in which, since a buffer layer with a limited thickness is disposed 
between the active layer and the absorption layer, laser light is 
sensitive enough to the periodic structure of the absorption layer to be 
oscillated in a single longitudinal mode; (3) a distributed feedback 
semiconductor laser device in which reverse bias voltage is applied to the 
heterojunction at the interface between the buffer layer and the 
absorption layer to achieve a change of the absorption coefficient of the 
laser light at the same periodicity as the wavelength of the laser light 
in the direction of the propagation of the laser light thereby attaining 
oscillation in a single longitudinal mode even in operation under an 
unsteady state; and (4) a distributed feedback semiconductor laser device 
which attains stabilized oscillation of high output power laser light 
which can be used as a light source for optical communication.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The semiconductor laser device of this invention is a distributed feedback 
semiconductor laser device of a double heterostructure type designed in 
such a manner that an absorption layer, having a limited thickness with a 
regular periodicity along the direction of the propagation of laser light, 
is disposed between the active layer and one of the cladding layers, 
thereby achieving a change of gain or the absorption coefficient to match 
the periodicity of the wavelength of the laser light. In this distributed 
feedback semiconductor laser device, there is only one longitudinal mode 
with maximum gain, and oscillation is attained in a single longitudinal 
mode even in operation under an unsteady state where the excitation 
current is rapidly modulated as when it is being used as a light source 
for optical communication. Distributed feedback semiconductor laser 
devices in which the gain or the absorption coefficient changes 
periodically have not been proposed. 
EXAMPLE 1 
FIG. 1 shows a distributed feedback semiconductor laser device of this 
invention, which is produced as follows: On a p-GaAs substrate 2, an 
n-GaAs current blocking layer 3 is grown by liquid phase epitaxy, followed 
by etching with an etchant (H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 :H.sub.2 
O=1:2:50) in such a manner that a V-shaped channel 11 reaches the p-GaAs 
substrate 2 by the use of a photo-resist mask (not shown). On the current 
blocking layer 3 including the channel 11, a p-Ga.sub.0.75 Al.sub.0.25 As 
cladding layer (the first cladding layer) 4, a Ga.sub.0.99 Al.sub.0.01 As 
active layer 5, an n-Ga.sub.0.75 Al.sub.0.25 As buffer layer 6 and an 
n-GaAs absorption layer 7 are successively grown by liquid phase epitaxy. 
The energy gap of the GaAs absorption layer 7 is smaller than that of any 
Ga.sub.x Al.sub.1-x As layer (namely, the energy gap Eg of the GaAs 
absorption layer 7 is smaller than that of the Ga.sub.0.99 Al.sub.0.01 As 
active layer 5), and accordingly the resulting laser light resonator has 
an effective absorption coefficient. The buffer layer 6 is not necessarily 
required, but when it is formed with a limited thickness, a desired 
oscillation in a single longitudinal mode can be attained even in 
operation under an unsteady state as described in detail below. 
Then, a photo-resist is coated on the n-GaAs absorption layer 7 to form a 
photo-resist mask in the form of a diffraction grating with the same 
periodicity as the wavelength of laser light by means of a holographic 
exposing system (not shown). The GaAs absorption layer 7 is then etched 
with an etchant (ethylene glycol (C.sub.2 H.sub.6 O.sub.2):H.sub.3 
PO.sub.4 :H.sub.2 O.sub.2 =8:1:1) to form diffraction grating shaped 
grooves with the same periodicity (i.e., center-to-center spacing) as the 
wavelength of laser light in the direction of the propagation of the laser 
light. None of the grooves should pass through the absorption layer 7, 
because the buffer layer 6 is composed of a GaAlAs layer and once it is 
exposed to air it is oxidized to thereby make it difficult to be subjected 
to the subsequent liquid phase epitaxial treatment. Then, on the etched 
absorption layer 7, an n-Ga.sub.0.75 Al.sub.0.25 As cladding layer (the 
second cladding layer) 8 and an n-GaAs cap layer 9 are successively grown 
by liquid phase epitaxy. Electrodes 1 and 10 are formed on the back face 
of the substrate 2 and on the upper face of the cap layer 9, respectively, 
by vacuum evaporation. 
The influence of the laser light on the periodic structure of the 
above-mentioned absorption layer 7 depends upon both the amplitude at the 
point of the periodical change of the thickness of the absorption layer 7 
and the thickness of the buffer layer 6. When the thickness of the buffer 
layer 6 is extremely thin, the laser light becomes sensitive to the 
periodic structure of the absorption layer 7, but the laser light is 
absorbed to a large extent in the optical waveguide so that the threshold 
current level for oscillation will become high. Such an undesirable 
phenomenon can be prevented if the thickness of the absorption layer 7 is 
sufficiently thin. However, it is desirable that the amplitude at the 
point of the periodical change of the thickness of the absorption layer 7 
is great. For example, when the center-to-center spacing of the 
diffraction grating of the absorption layer 7 is 2400 .ANG., the amplitude 
must be 800 .ANG. at maximum. However, the formation of the buffer layer 6 
with a limited thickness makes it possible to make the laser light 
sensitive to the periodic structure of the absorption layer 7, so that 
oscillation in a single longitudinal mode can be attained even in 
operation under an unsteady state. This is because the absorption 
coefficient of the device changes to the same center-to-center spacing as 
the wavelength of the laser light in the direction of the propagation of 
the laser light and thus there is only one longitudinal mode with maximum 
gain. 
EXAMPLE 2 
FIG. 2 shows another semiconductor laser device of this invention, which is 
the same structure as disclosed in Example 1 except that the p-GaAlAs 
buffer layer 13 and the p-GaAs absorption layer 12 are positioned under 
the active layer 5. The polarity of the GaAs absorption layer 12 must be 
the same p-type as that of the p-Ga.sub.0.75 Al.sub.0.25 As cladding layer 
(the first cladding layer) 4. 
Although each of the above-mentioned Examples 1 and 2 discloses only an 
inner striped GaAlAs/GaAs double-heterostructure semiconductor laser 
device, it is not limited thereto. Semiconductor laser devices using other 
materials such as InGaAsP/InP, etc., can be applied. Also, semiconductor 
laser devices having the same sectional view as a buried type laser device 
can be applied. 
EXAMPLE 3 
FIG. 3 shows another distributed feedback semiconductor laser device of 
this invention, which comprises a first n-InP cladding layer 4, an 
In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y (0.47&lt;x&lt;1 and 0&lt;y&lt;1) active layer 
5, a p-InP buffer layer 6, an n-InP absorption layer 7 formed in a 
diffraction grating and a second p-InP cladding layer 8, in sequence. A 
reverse bias is applied to the junction at the interface between the p-InP 
buffer layer 6 and the n-InP absorption layer 7. 
All of the p-InP buffer layer 6, the n-InP absorption layer 7 and the p-InP 
cladding layer 8 are not necessarily of the same materials, but the p-InP 
buffer layer 6 must be the same polarity as the p-InP cladding layer 8 and 
the energy gap of each of the buffer layer 6 and the cladding layer 8 must 
be greater than that of the active layer 5. The polarity of the absorption 
layer 7 must be different from that of each of the InP buffer layer 6 and 
the InP cladding layer 8. 
This semiconductor laser device is produced as follows: On an n-InP 
substrate 2, the first n-InP cladding layer 4, the active layer 5 having 
the composition ratio required to create an oscillation wavelength ranging 
from approximately 1.1 .mu.m to approximately 1.6 .mu.m, the p-InP buffer 
layer 6 having a thickness of approximately 0.08 .mu.m and the n-InP 
absorption layer 7 having a thickness of approximately 0.1 .mu.m are 
successively grown by liquid phase epitaxy. Then, a photo-resist is coated 
on the n-InP absorption layer 7 to form a photo-resist mask (not shown) in 
the form of a diffraction grating with the same periodicity as the 
wavelength of laser light by the exposure of an interference fringe 
pattern with a center-to-center spacing in a range of 1700 .ANG. to 2500 
.ANG. by means of a holographic exposing system or the like. The n-InP 
absorption layer 7 is then etched into a grating with an etchant 
(saturated bromine water:H.sub.3 PO.sub.4 :H.sub.2 O=2:1:15). On the 
etched n-InP absorption layer 7, the second p-InP cladding layer 8 and an 
n-InP cap layer 9 are successively grown, followed by a plasma chemical 
vapor deposition treatment to form a SiNx film 15 having a thickness of 
approximately 2000 .ANG.. The SiNx film 15 is then etched with an etchant 
(HF:NH.sub.4 F=1:40) using a photo-resist mask (not shown) so that the 
stripe in the direction of the propagation of laser light is removed. An 
impurity element such as Zn is then diffused into the cap layer 9 and the 
cladding layer 8 to form a p-channel 11 functioning as an electric current 
path while the SiNx film 15 is used as a diffusion mask. Then, electrodes 
1 and 10 are formed on the upper face of the SiNx film 15 including the 
channel 11 and the back face of the substrate 2, respectively, resulting 
in a distributed feedback semiconductor laser device. 
When a reverse bias is applied to the junction at the interface between the 
p-InP buffer layer 6 and the n-InP absorption layer 7 while the electrode 
10 is maintained at positive potential against the electrode 1, an 
electric current blocking structure is formed into a grating at the 
junction area so that gain of the active layer 5 varies in magnitude 
corresponding to the diffraction grating of the n-InP absorption layer 7. 
That is, the portions of the active layer 5 corresponding to the convex 
portions of the absorption layer 7 exhibit lower gain than the other 
portions of the active layer 5 corresponding to the grooved portions of 
the absorption layer 7. Due to the reduction of gain in the active layer, 
the semiconductor laser device of this invention achieves oscillation in a 
stabilized single longitudinal mode in operation not only under a steady 
state but also under an unsteady state. 
Although the above-mentioned Example 3 discloses only an InP/InGaAsP/InP 
double-heterostructure semiconductor laser device, it is not limited 
thereto. Other semiconductor laser devices such as GaAlAs/GaAs/GaAlAs 
double-heterostructure semiconductor laser devices can be applied. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty which reside in the present 
invention, including all features which would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.