Variable wavelength semiconductor laser

A semiconductor laser device has an active layer, a cladding layer, and a contact layer successively disposed on a semiconductor substrate. A pair of electrodes for passing a current parallel to the cladding and contact layers and perpendicular to the resonator direction of the active layer for heating the active layer are disposed opposite the stripe-shaped active layer. The pair of electrodes are disposed on the contact layer and the contact layer between the pair of electrodes is missing. One of the pair of electrodes may be disposed directly on the cladding layer. An improved wavelength change response property as a function of the active layer temperature controlling current flowing between the two electrodes is obtained. Further, the production of the laser is easy.

FIELD OF THE INVENTION 
The present invention relates to a semiconductor laser device that can 
oscillate at variable wavelengths and, more particularly, to a 
semiconductor laser that can be produced easily and that has a good 
wavelength change response characteristic as a function of wavelength 
controlling current. 
BACKGROUND OF THE INVENTION 
FIG. 5(a) is a perspective view showing a structure of the prior art 
resistor film heating-type variable wavelength semiconductor laser 
disclosed in Applied Physics, Spring Meeting 1991, Prescript Number 3, 
page 969, 29p-D-8. In FIG. 5(a), reference numeral 101 designates an 
n-type semiconductor substrate. A stripe-shaped active layer 102 is 
disposed on the substrate 101 at the central portion of the laser element. 
A semi-insulating current blocking layer 103 is disposed on the substrate 
101 at both sides of the active layer 102. A p-type semiconductor layer 
104 is disposed on the current blocking layer 103 and the active layer 
102. A p side electrode 105 and an n side electrode 106 for injecting a 
laser driving current are disposed on the front surface of the p-type 
semiconductor layer 104 and on the rear surface of the substrate 101, 
respectively. An insulating film 107 is disposed directly opposite the 
active layer 102 on the p side electrode 105 and a platinum resistor film 
108 is disposed on the insulating film 107 at a position directly opposite 
the active layer 102. 
When a bias voltage in a forward direction is applied to the p-n junction 
of the semiconductor laser through the p side electrode 105 and the n side 
electrode 106, charge carriers are injected into the active layer 102 and 
recombine in the active layer 102 to generate light. Here, the current 
blocking layer 103 is provided so that current is concentrated in the 
active layer 102. The light generated in the active layer 102 is guided 
along the active layer with repeated reflection and amplification, thereby 
producing laser oscillation. 
The oscillation wavelength of a semiconductor laser varies with the 
temperature of the active layer and when the temperature rises, the 
wavelength is generally lengthened because the band gap energy of the 
semiconductor of the active layer is narrowed, thereby shifting the 
wavelength to the longer wave-length side. The variable wavelength 
semiconductor laser of FIG. 5(a) utilizes this effect. In that laser 
device, the resistor film 108 generates heat when a current flows between 
the ends 108a and 108b of the resistor film 108, thereby heating the 
active layer 102. Therefore, when the current flowing through the resistor 
film 108 is increased, the temperature of the active layer 102 rises and 
the oscillation wavelength is shifted to the longer wavelength side, as 
shown in FIG. 5(b). 
FIG. 6 is a perspective view of a prior art variable wavelength 
semiconductor laser disclosed in Japanese Published Patent Application 
1-173686. In FIG. 6, reference numeral 111 designates an n-type GaAs 
substrate. An n-type GaAs buffer layer 112 is disposed on the substrate 
111, an n-type AlGaAs cladding layer 113 is disposed on the buffer layer 
112, a GaAs active layer 115 is disposed on the cladding layer 113, a 
p-type AlGaAs cladding layer 117 is disposed on the active layer 115, and 
a p-type GaAs cap layer is disposed on the cladding layer 117. A 
stripe-shaped laser driving electrode of width w.sub.d 121 is disposed on 
the cap layer 118 at a central portion of the laser element and 
temperature control electrodes of width w.sub.m (w.sub.m is significantly 
larger than w.sub.d) is disposed on the cap layer 118 on both sides of and 
separated from the laser driving electrode 121. In the semiconductor 
laminated structures at regions between the laser driving electrode and 
the temperature control electrodes 122, respectively, ions, such as He 
ions, are implanted from the surface of the cap layer 118, reaching the 
semiconductor substrate 111 and producing high resistance regions 119. A 
common electrode 123 is disposed at the rear surface of the substrate 111. 
Reference numeral 125 designates a laser active region. 
In this prior art device, laser oscillation occurs at the region 125 of the 
active layer 115 directly opposite the driving electrode 121 from an 
excitation current injected from the driving electrode 121. Further, since 
a region 125 of the active layer 115 directly opposite the temperature 
control electrode 122 has a large width, laser oscillation is not likely 
to occur there even if a current flows, as shown by arrows A. By making 
relatively large currents flow from the electrodes 122, the regions 
directly opposite the electrodes 122 can be used as heating materials. 
Accordingly, by controlling the temperature of the laser active region 125 
directly opposite the driving electrode 121 for the active layer 115 with 
the heat generated at regions directly opposite the electrode 122, the 
laser oscillation wavelength can be controlled. 
FIG. 7 is a cross-sectional view of another prior art variable wavelength 
semiconductor laser disclosed in Japanese Published Patent Application 
1-173686. In FIG. 7, reference numeral 131 designates an n-type GaAs 
substrate. An n-type GaAs buffer layer 132 is disposed on the substrate 
131, an n-type AlGaAs cladding layer 133 is disposed on the buffer layer 
132, an n-type AlGaAs-GaAs graded layer 134 is disposed on the cladding 
layer 133, a GaAs active layer 135 is disposed on the graded layer 134, a 
p-type GaAs-AlGaAs graded layer 136 is disposed on the active layer 125, a 
p-type AlGaAs cladding layer 137 is disposed on the graded layer 136, and 
a p-type GaAs cap layer 138 is disposed on the cladding layer 137. A 
stripe-shaped laser driving electrode 141 of width w.sub.d is disposed on 
the cap layer 138 at a central portion of the laser element, and 
temperature control electrodes 142 of width w.sub.m (w.sub.m is 
significantly larger than w.sub.d) are disposed on the cap layer 138 at 
both sides of the laser driving electrode 141. Grooves extending from the 
surface of the cap layer 138 to the cladding layer 137 are produced by 
etching where the laser driving electrode 141 is disposed and where the 
temperature control electrodes 142 are disposed, respectively, and the 
grooves are filled with insulating layers 139 comprising polyimide or the 
like. A common electrode 143 is disposed at the rear surface of the 
substrate 131. Reference numeral 145 designates a laser active region. 
Also in this prior art device, similar to the semiconductor laser of FIG. 
6, laser oscillation occurs at a region 145 directly opposite the driving 
electrode 121 of the active layer 135 from an excitation current injected 
from the driving electrode 141. Further, since the regions directly 
opposite the temperature control electrodes 142 of the active layer 135 
have a large width, even when a current flows, as shown by the arrows B, 
from the electrode 142, laser oscillation is not likely to occur. A 
relatively large current flow from the electrode 142 through the regions 
directly below the electrode 142 can be used as a heat source. By 
controlling the temperature of the laser active region 145 of the active 
layer 135 directly opposite the driving electrode 141 with the heat 
generated at regions directly opposite the electrode 142, the oscillation 
wave-length of the laser can be controlled. 
FIG. 8 is a partly broken away perspective view of a prior art variable 
wavelength semiconductor laser disclosed in Japanese Published Patent 
Application 1-173686. In FIG. 8, reference numeral 151 designates a p-type 
GaAs substrate. A p-type GaAs buffer layer 152 is disposed on the 
substrate 151, a p-type AlGaAs cladding layer 153 is disposed on the 
buffer layer 152, a GaAs active layer 155 is disposed on the cladding 
layer 153, an n-type AlGaAs cladding layer 157 is disposed on the active 
layer 155, and a p-type GaAs cap layer 158 is disposed on the cladding 
layer 157. The laminated structure from the cap layer 158 to the buffer 
layer 152 is formed into a stripe-shaped ridge configuration, and 
insulating layers 159 are disposed on the substrate 151 at both sides of 
the ridge. A stripe-shaped resistance layer 164 is disposed parallel to 
the ridge in the insulating layer 159. An insulating film 165 is disposed 
covering the insulating layer 159 and the resistance layer 164. This 
insulating film 165 has apertures at both ends of the resistance layer 164 
and temperature control electrodes 162 are disposed on the resistance 
layer 164 exposed in the apertures. An n side electrode 161 for driving 
the laser is disposed in contact with the stripe-shaped cap layer 158. 
Further, a p side electrode 163 is disposed on the rear surface of the 
substrate 151. 
In this prior art laser device, laser oscillation occurs at the active 
layer 155 from an excitation current injected from the driving electrode 
161. Further, by making a current flow through the resistance layer 164 
through the electrode 162, as shown by arrows C in the figure, the 
resistance layer 164 generates heat, the temperature of the active layer 
165 is changed, and the laser oscillation wavelength is controlled. The 
striped-shaped resistance layer 164 is produced in the insulating layer 
159 by ion implantation and the resistance value of the insulating layer 
159 can be arbitrarily determined by the ion implantation. 
The prior art resistance film heating-type variable wavelength laser shown 
in FIG. 5(a) requires producing an insulating film 107 on the p side 
electrode 105 and producing a platinum resistance film 108 thereon, 
resulting in high production costs. In addition, the heat conductivity is 
inferior because there exists an insulating film 107 between the 
resistance film 108 and the active layer 102 and, even when the platinum 
resistance film is heated by a current flow, it takes time for the heat to 
be transmitted to the active layer, resulting in a delayed change of 
wavelength in response to the wavelength controlling current. 
The variable wavelength lasers of FIGS. 6 and 7 heat a part of a grown 
layer and require no complicated structure for producing a particular 
region to be heated. However, the regions to be heated are separated by 
ten to several tens of microns from the active region, thereby taking time 
for the transmission of heat to the active region, also delaying a change 
in wavelength in response to the wavelength controlling current. 
Further, the variable wavelength laser of FIG. 8 requires an ion 
implantation process for producing the resistance layer, and that process 
is complicated. As in the other prior art devices, the region to be heated 
is separated by ten to several tens of microns from the active region, 
thereby requiring time for the heat of the region to be transmitted to the 
active region, resulting in a delayed change of wavelength in response to 
wavelength controlling current. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a semiconductor laser 
device that can reduce the number of process steps in manufacturing and 
that has an improved response wavelength change as a function of a 
wavelength controlling current. 
Other objects and advantages of the present invention will become apparent 
from the detailed description given hereinafter. It should be understood, 
however, that the detailed description and specific embodiments are given 
by way of illustration only since various changes and modifications within 
the spirit and scope of the invention will become apparent to those 
skilled in the art from this detailed description. 
In accordance with one aspect of the present invention, in a semiconductor 
laser that has an active layer, a cladding layer, and a contact layer, 
laser oscillation is achieved by injecting a current into a stripe-shaped 
region of an active layer. A pair of electrodes for making a current flow 
in a plane parallel to the layers and in a direction perpendicular to the 
stripe direction are provided at locations directly opposite the 
stripe-shaped active region. In this semiconductor laser, production is 
simplified and an improved wavelength change response characteristic as a 
function of temperature controlling current is obtained. 
In accordance with another aspect of the present invention, in a 
semiconductor laser having a laser light emitting region including an 
active region and a reflector region including a diffraction grating layer 
arranged in series with the laser light emitting region in the resonator 
length direction and in which light having a wavelength matching the 
period of the diffraction grating is selectively amplified to produce 
laser oscillation, a pair of electrodes are provided for establishing a 
current flow in the diffraction grating layer parallel to the layer and in 
the resonator length direction. In this construction, the oscillation 
wavelength of a DBR laser can be changed by a change in the temperature of 
the semiconductor laser including the diffraction grating layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a perspective view of a semiconductor laser device in accordance 
with an embodiment of the present invention. Reference numeral 1 
designates an n-type semiconductor substrate. A stride-shaped active layer 
2 comprising InGaAsP is disposed on a central portion of the substrate 1 
and semi-insulating current blocking layer 3 comprising Fe doped InP is 
disposed at both sides of the active layer 2. A cladding layer 4 
comprising p-type InP containing Zn as a p-type dopant impurity in a 
concentration of an order of 10.sup.18 cm.sup.-3 is disposed on the active 
layer 2 and the current blocking layer 3. A three-part contact layer 5 
comprising p-type InP containing Zn in a concentration of an order of 
10.sup.19 cm.sup.-3 has two parts that are disposed on the cladding layer 
4 and a third part disposed on the cladding layer 4 directly opposite the 
active layer 2. The third part is separated from the first and second 
parts by grooves. A p side electrode 6 comprising Ti(50 nm)/Pt(100 
nm)/Au(250 nm) for injection a laser driving current is disposed on the 
third part of the contact layer 5 directly opposite the active layer 2 and 
electrodes 8a and 8b comprising Ti(50 nm)/Pt(100 nm)/Au(250 nm) for 
injecting a current into the cladding layer 4 serving as a resistance 
layer are disposed on the first and second Darts of the contact layer 5. 
In addition, an n side electrode 7 comprising AuGe(80 nm)/Ni(100 
nm)/Au(250 nm) for injecting a laser driving current is disposed on the 
rear surface of the substrate 1. When a forward direction bias voltage for 
the p-n junction of the laser is applied between the p side electrode 6 
and the n side electrode 7, carriers are injected into and recombine in 
the active layer 2 to generate light. Here, the current blocking layers 3 
confine the current to the active layer 2. The light generated in the 
active layer 2 is guided along the active layer stripe with repeated 
reflection and amplification, thereby leading to laser oscillation. 
The contact layer 5 is a semiconductor layer introduced to reduce the 
contact resistance between the semiconductor and the electrodes 8a and 8b 
and it is a layer of high dopant concentration and low resistance. On the 
other hand, the p-type InP layer 4 functions as a cladding layer confining 
carriers and light in the active layer 2 in the usual laser oscillation 
operation and has a low dopant concentration and high resistance relative 
to the contact layer. In this embodiment, portions of the contact layer 5 
between the electrode 8a and the cladding layer 4 are removed. When a 
current flows between the electrodes 8a and 8b, heat is generated at the 
p-type InP cladding layer 4 where the contact layer 5 is absent. This 
generated heat heats the active layer to change the wavelength of the 
semiconductor laser. In this way, the semiconductor layer in contact with 
the active layer generates heat to heat the active layer whereby quite a 
good wavelength response characteristic as a function of the temperature 
controlling current which flows between the electrodes 8a and 8b is 
obtained 
In producing the semiconductor laser of this embodiment, the same process 
as that for a usual semiconductor laser can be employed through the 
production of the contact layer 5. Thereafter, only partial removal of the 
contact layer 5 is required, making its production quite easy. 
The resistance R1 between the electrode 8a and the electrode 8b of the 
semiconductor laser of FIG. 1 can be obtained from the following formula: 
EQU R1=(d/L.multidot.t).multidot.w. 
Here, d designates a sum (d1+d2) of the lengths of portions of the cladding 
layer 4 where the contact layer 5 is removed in a direction transverse to 
the electrodes 8a and 8b, L is laser length, t is the layer thickness of 
the cladding layer 4, and w is the resistivity of the cladding layer 4. In 
this embodiment, d=20 microns (10 microns+10 microns), w=10.sup.-1 .OMEGA. 
cm.sup.2, and R1=33.OMEGA.. Therefore, when a current of 100 mA flows 
between the electrodes 8a and 8b, the temperature of the cladding layer 
becomes about (100mA).sup.2 .times.33.OMEGA..times. (thermal resistance 
(100.degree. CC/W))=33.degree. C. Because the cladding layer 4 is disposed 
in contact with the active layer 2, the temperature of the active layer 
also rises about 33.degree. C. The oscillation wavelength is shifted 
toward a longer wavelength by about 1 Angstrom for each 1.degree. C. 
temperature rise of the active layer so that the oscillation wavelength is 
lengthened about 33 Angstroms by a temperature rise of 33.degree. C. 
FIG. 9 shows an equivalent circuit of the semiconductor laser of FIG. 1. 
The same reference numerals as those shown in FIG. 1 designate the same or 
corresponding parts. Resistors R.sub.A and R.sub.B between the temperature 
controlling electrode 8a and the p side electrode 6 and between the p side 
electrode 6 and the temperature controlling electrode 8b, respectively, 
are portions of cladding layer 4 where the contact layer 5 of FIG. 1 is 
removed and have a resistance value of 16.5.OMEGA., respectively. The 
RL.sub.D as a serial resistance component of the laser is a value 
significantly smaller than the resistances R.sub.A and R.sub.B, that is, 
16.5.OMEGA.. For example, when the voltage V.sub.LD for laser light 
emission is about 1.5 volts and a temperature controlling current of 100 
mA flows through the cladding layer, as voltages to be applied to 
respective electrodes, supposing that the n side electrode 7 is grounded 
to 0 volts, a V.sub.LD of about 1.5 volts should be applied to the p side 
electrode 6 and V.sub.LD +1.65 volts, that is, about 3.15 volts, and 
V.sub.LV -1.65 volts, that is, about -0.15 volts, should be applied to the 
temperature controlling electrodes 8a and 8b, respectively. 
FIG. 2 is a perspective view of a semiconductor laser according to a second 
embodiment of the present invention. In FIG. 2, the same reference 
numerals designate the same or corresponding parts, reference numeral 8 
designates a resistor electrode, and reference numeral 9 desiqnates an 
electrode serving commonly as a laser electrode and a resistor electrode. 
In this second embodiment, a current is made to flow between the common 
electrode 9 and the n side electrode 7 to produce laser oscillation and a 
current is made to flow between the resistor electrode 8 and the common 
electrode 9 to vary the oscillation wavelength. 
The laser oscillation operation obtained when a forward bias is applied to 
the p-n junction of the laser between the common electrode 9 and the n 
side electrode 7 is the same as in the first embodiment. In the second 
embodiment, the contact layer 5 between the electrode 8 and the electrode 
9 is removed and when a current flows between the electrode 8 and the 
electrode 9, heat is generated at the portion of the p-type InP cladding 
layer 4 where the contact layer 5 was removed. The active layer is thereby 
heated to vary the wavelength of the semiconductor laser. As in the first 
embodiment, the semiconductor layer in contact with the active layer 
generates heat to heat the active layer, thereby resulting in quite a good 
wavelength change characteristic as a function of the temperature 
controlling current flowing between the electrodes 8 and 9. 
The production process for a laser according to the second embodiment, up 
to the production of the contact layer 5, can be carried out by 
conventional processing and only the step of partially removing the 
contact layer 5 to produce the structure of FIG. 2 is added, resulting in 
quite easy production. 
FIG. 10 shows an equivalent circuit of the semiconductor laser of FIG. 2. 
In FIG. 10, the same reference numerals as those shown in FIG. 2 designate 
the same or corresponding parts. A resistor R.sub.c between the resistor 
electrode 8 and the common electrode 9 is produced by a portion of the 
cladding layer 4 where the contact layer 5 is removed in FIG. 2. In the 
semiconductor laser of FIG. 2, when the cladding layer 4 comprises the 
same material as in the first embodiment and, for example, d3=20 microns, 
L=300 microns, and t=2 microns, R.sub.c amounts to 33.OMEGA.. The 
R.sub.LD, which is a series resistance component of the laser, has a value 
significantly smaller than the resistance R.sub.c which is 33.OMEGA.. 
As in the first embodiment, when the voltage V.sub.LD for producing the 
laser light emission is about 1.5 volts and a temperature controlling 
current of 100 mA is made to flow through the cladding layer, as voltages 
to be applied to respective electrodes, supposing the n side electrode 7 
is grounded to 0 volts, a V.sub.LD of about 1.5 volts should be applied to 
the p side electrode 6 and V.sub.LD +3.3 volts, that is, about 4.8 volts, 
should be applied to the resistor electrode 8. 
Since the region to be heated is disposed at only one side of the active 
layer in this second embodiment, as compared with the first embodiment in 
which the region to be heated is disposed at both sides of the active 
layer, the efficiency of heating the active layer is a little inferior to 
the first embodiment. However, the number of electrodes is reduced, 
resulting in easy control of the voltage applied. 
FIG. 3 is a perspective view showing a structure of a semiconductor laser 
in accordance with a third embodiment of the present invention. In FIG. 3, 
the same reference numerals as those in FIG. 2 designate the same or 
corresponding parts. 
The structure of the third embodiment is almost the same as that of the 
second embodiment except that the resistor electrode 8 is disposed 
directly on the cladding layer 4 without placing therebetween a contact 
layer. To operate the device, as in the second embodiment, a current flows 
between the common electrode 9 and the n side electrode 7 to produce laser 
oscillation and, to vary the wavelength, a current flows between the 
resistor electrode 8 and the common electrode 9. 
The laser oscillation operation obtained when a forward bias is applied 
between the common electrode 9 and the n side electrode 7 is the same as 
that of the first and second embodiments. In this embodiment, the contact 
layer 5 at both sides of the stripe-shaped active layer is removed and the 
resistor electrode 8' is directly disposed on the cladding layer 4 at a 
predetermined interval d4 from the common electrode 9. Since the contact 
resistance between the electrode and the semiconductor layer is high when 
the electrodes are disposed directly on the semiconductor layer having a 
low dopant impurity concentration, this contact resistance can also be 
utilized as heating resistance, resulting in an improvement in the thermal 
efficiency of the active layer. In more detail, when a current is made to 
flow between the electrode 8' and the common electrode 9, heat is 
generated at the boundary between the electrode 8' and the resistance 
layer 4 and at the resistance layer between the electrode 8' and the 
electrode 9. The active layer is heated by this heat and the wavelength of 
the semiconductor laser is changed. As in the first and second 
embodiments, the semiconductor layer in contact with the active layer 
heats the active layer, thereby resulting in quite a good response 
characteristic of wavelength change as a function of the temperature 
controlling current flowing between the electrodes 8 and 9. 
In a semiconductor laser according to this embodiment, as in the first and 
second embodiments, the production process up to the production of the 
contact layer 5 can be carried out by the same process steps as in the 
conventional process. Thereafter, only the process steps of partially 
removing the contact layer 5 and forming the electrode 8' on the exposed 
cladding layer are carried out to produce the structure of FIG. 3, 
resulting in quite an easy production. 
FIG. 11 shows an equivalent circuit of the semiconductor laser in FIG. 3. 
In FIG. 11, the same reference numerals as those shown in FIG. 3 designate 
the same or corresponding parts. The resistance R.sub.D between the 
resistor electrode 8 and the common electrode 9 comprises the cladding 
layer 4 between the resistor electrode 8 and the common electrode 9 in 
FIG. 3. In the semiconductor laser of FIG. 3, when the cladding layer 4 
comprises the same material as in the first embodiment and, for example, 
d4=200 microns, L=300 microns, and t=2 microns, R.sub.D =33.OMEGA.. In 
addition, the resistance R.sub.E is the contact resistance between the 
electrode 8 and the cladding layer 4 and, for example, is 10.OMEGA. in 
this embodiment. The R.sub.LD as a serial resistance component of the 
laser is significantly smaller than the resistance R.sub.C which is 
33.OMEGA.. 
As in the first and second embodiments, when the voltage V.sub.LD for laser 
light emission is about 1.5 volts and a temperature controlling current of 
100 mA flows through the cladding layer, as voltages to be applied to 
respective electrodes, supposing the n side electrode 7 is grounded to 0 
volts, V.sub.LD is about 1.5 volts and is applied to the p side electrode 
6 and V.sub.LD +4.3 volts, that is, about 5.8 volts, is applied to the 
resistor electrode 8. 
As is evident from the foregoing description, the temperature controlling 
current flows to a part in contact with the active region of the 
semiconductor laser adjacent to the active layer to heat the active region 
and change the oscillation wavelength, whereby a semiconductor laser 
having a good wavelength response characteristic as a function of the 
temperature controlling current is realized. 
While in the first to third embodiments InP is used as a substrate and 
InGaAsP is used as an active layer, another compound semiconductor, such 
as GaAs, AlGaAs, or the like, may be used, resulting in the same effect as 
in the above-described embodiment. 
A semiconductor laser capable of changing oscillation wavelength is 
disclosed in Japanese Published Patent Application 62-247582. The 
refractive index of a material constituting a diffraction grating is 
changed by injecting a current into a diffraction grating of a distributed 
Bragg reflector (DBR) laser whereby oscillation wavelength is changed. 
This laser utilizes the change in the refractive index according to the 
plasma effect produced when carriers are injected into a semiconductor 
layer forming a diffraction grating. It is well known that the refractive 
index of a semiconductor layer is also changed by a temperature change. 
Accordingly, the oscillation wavelength of a semiconductor laser can be 
changed by heating a semiconductor layer of the diffraction grating of a 
DBR laser. 
FIG. 4(a) shows a semiconductor laser in which a semiconductor layer 
forming a diffraction grating of a DBR laser is heated in accordance with 
a fourth embodiment of the present invention. FIG. 4(a) is a perspective 
view of the semiconductor laser and FIG. 4(b) is a cross-sectional view of 
FIG. 4(a) taken along line IVb--IVb. In these figures, the same reference 
numerals as those shown in FIG. 1 designate the same or corresponding 
parts. A laser light emitting region 20 and a reflector region 21 
including a diffraction grating are arranged serially in the resonator 
length direction of the DBR laser. A stripe-shaped layer 11 comprising, 
for example, InGaAsP, is disposed on an extension of the stripe-shaped 
active layer 2 on the current blocking layer 3 of the reflector region 21 
and a periodic uneven structure, that is, a diffraction grating 12, is 
formed in the resonator length direction on the surface of this InGaAsP 
layer 11. A p-type semiconductor layer 10 comprising, for example, InP, 
covers the active layer 2 and the InGaAsP layer 11 on which the 
diffraction grating is formed. Here, the p-type semiconductor layer 10 in 
the laser light emitting region functions as a cladding layer that 
confines light and charge carriers in the active layer 2. The material of 
the stripe-shaped layer 11 has a different energy band gap from that of 
the p-type semiconductor layer 10. As described above, since the periodic 
uneven structure is formed in the resonator length direction on the 
surface of the stripe-shaped layer 11, a periodic refractive index 
distribution is produced in the resonator length direction in the 
reflector region 21. 
In the DBR laser, only light having a wave-length matching the period of 
the diffraction grating of light emitted from the active layer 2 is 
selectively amplified and produces laser oscillation. In this embodiment, 
as shown in FIG. 4(a), resistor electrodes 8a and 8b are disposed opposite 
the diffraction grating 12 in the reflector region 21. A current flows in 
the direction transverse to the stripe in the p-type semiconductor 
cladding layer 10 disposed on the InGaAsP layer 11 opposite which the 
diffraction grating is formed between the electrodes 8a and 8b, thereby 
making the p-type semiconductor layer 10 generate heat. The refractive 
index of the p-type semiconductor layer 10 is changed by the generated 
heat and, further, the period of the diffraction grating 12 is changed due 
to the thermal expansion of the layers 10 and 11 whereby the oscillation 
wavelength is changed. 
In this fourth embodiment, electrodes for passing a current through the 
semiconductor layer 10 including a diffraction grating of a DBR laser are 
disposed on the semiconductor layer 10 and, thus, the oscillation 
wavelength of the laser can be changed by controlling the current injected 
from the electrodes. 
In the fourth embodiment, a reflector region is provided at one end in the 
laser light emitting region. Reflector regions can be provided at both 
ends of the laser light emitting region with the same effects as in the 
above-described embodiment. 
In the fourth embodiment, InP is used as a substrate and InGaAsP is used as 
an active layer, as in the first and third embodiments. Other compound 
semiconductors, such as GaAs, AlGaAs, or the like, can be used with the 
same effects as in the above-described embodiment. 
As is evident from the foregoing description, according to the present 
invention, in a semiconductor laser which has an active layer, a cladding 
layer, and a contact layer which are successively disposed on a substrate 
and in which laser oscillation is produced by injecting a current into a 
stripe-shaped region of the active layer, a pair of electrodes for passing 
a current in a direction parallel to the layers and in a direction 
perpendicular to the stripe direction are provided in contact with a 
portion of the cladding layer directly opposite the stripe-shaped active 
region. Therefore, a semiconductor laser, the production of which is easy 
and which provides a preferable wavelength change response characteristic 
as a function of a temperature controlling current is obtained. 
According to another aspect of the present invention, in a semiconductor 
laser that has a laser light emitting region including an active layer and 
a reflector region including a diffraction grating arranged in series with 
the laser light emitting region in the resonator length direction and in 
which light having a wavelength matching the period of the diffraction 
grating is selectively amplified to produce laser oscillation, a pair of 
electrodes for passing current in a direction parallel to the layers and 
in a direction perpendicular to the resonator length direction in a 
semiconductor layer including a diffraction grating are provided in the 
reflector region. Therefore, in this semiconductor laser, the oscillation 
wavelength of a DBR laser can be changed by changing the temperature of 
the semiconductor layer including the diffraction grating.