Optical semiconductor device

An optical semiconductor device includes a semiconductor laser diode having a first electrode for receiving a current for driving the laser diode, and a grounding electrode; a modulator for modulating light emitted from the semiconductor laser diode, the modulator having a second electrode for receiving a current for driving the modulator, and a grounding electrode connected to the grounding electrode of the semiconductor laser diode; a first resistor having a terminal connected to the first electrode of the laser diode; a second resistor connected between the second electrode of the modulator and the grounding electrode of the modulator; and a third resistor having a first terminal connected to the grounding electrode of the modulator and a grounded second terminal. Therefore, the laser diode driving current can be made larger when the modulator absorbs the laser light than when the modulator does not absorb the laser light, so that the oscillating wavelength is shortened, resulting in an inexpensive optical semiconductor device that hardly degrades optical pulse transmission performance.

FIELD OF THE INVENTION 
The present invention relates to an optical semiconductor device employed 
for optical fiber communication. 
BACKGROUND OF THE INVENITON 
In optical fiber communication, as a light source for broad-band 
transmission exceeding 2.5 Gb/s, an integrated electroabsorption modulator 
and semiconductor laser diode is employed. 
FIG. 10 is a perspective view, partly broken away, illustrating a 
conventional integrated modulator and laser diode (hereinafter referred to 
as LD). In FIG. 10, an integrated modulator and LD 14 comprises a 
modulator 100 and a distributed feedback (DFB) LD 200. Reference numeral 1 
designates an n type InP substrate. An n type InP cladding layer 2 is 
disposed on the substrate 1. An InGaAsP active layer 3 is disposed on the 
cladding layer 2. A first p type InP cladding layer 4 is disposed on the 
active layer 3. The n type InP cladding layer 2, the active layer 3, and 
the first p type InP cladding layer 4 form a ridge in the center of the 
structure. Intrinsic (hereinafter referred to as i type) InP current 
blocking layers 5 are disposed on the substrate 1, contacting both sides 
of the ridge. A second p type InP cladding layer 6 is disposed on the 
ridge and on the current blocking layers 5. A p type InGaAsP contact layer 
7 is disposed on the second p type InP cladding layer 6. An insulating 
film 10, such as SiO.sub.2, is disposed on the contact layer 7. A surface 
electrode 8 for the modulator 100 (hereinafter referred to as a modulator 
surface electrode, and a surface electrode 9 for the LD 200 (hereinafter 
referred to as an LD surface electrode) are disposed on the insulating 
film 10. A grounding rear electrode 11 common to the modulator 100 and the 
LD 200 is disposed on the rear surface of the substrate 1. Reference 
numeral 40 designates a diffraction grating. The modulator 100 and the LD 
200 are separated from each other by a separation groove 41 that reaches 
the second p type InP cladding layer 6. The modulator surface electrode 8 
and the LD surface electrode 9 are electrically insulated from each other 
by the separation groove 41. The insulation resistance r is several 
k.OMEGA.. 
FIG. 8 is a perspective view illustrating a conventional optical 
semiconductor device 303 in which an integrated modulator and LD 14 as 
shown in FIG. 10 is mounted on a submount comprising a dielectric, such as 
Al.sub.2 O.sub.3. FIG. 8, the same reference numerals as those shown in 
FIG. 10 designate the same or corresponding parts. The optical 
semiconductor device 303 includes an Al.sub.2 O.sub.3 substrate 16 having 
opposed front and rear surfaces. The rear surface of the substrate 16 is 
metallized for grounding (not shown). A strip line 20 for grounding is 
disposed on the front surface of the substrate 16 and connected to the 
metallized rear surface of the substrate 16 via a through-hole 15. An LD 
feeding strip line 17 and a modulator feeding strip line 18 are disposed 
on the front surface of the substrate 16. A resistor 19 is disposed on a 
portion of the grounding strip line 20. An electrode 21 for mounting is 
disposed on the front surface of the substrate 16, and an integrated 
modulator and LD 14 is disposed on the electrode 21. A surface electrode 8 
of the modulator is connected to the feeding strip line 18 and to the 
resistor 19 with Au wires 22. A surface electrode 9 of the LD is connected 
to the feeding strip line 17 with an Au wire 22. A common grounding 
electrode 11 of the integrated modulator and LD 14 contacts the mounting 
electrode 21, whereby it is grounded via the grounding strip line 20 and 
the through-hole 15. 
FIG. 9 is an equivalent circuit diagram of the optical semiconductor device 
shown in FIG. 8. In FIG. 9, the same reference numerals as those shown in 
FIG. 8 designate the same or corresponding parts. Reference character 
I.sub.0 denotes a DC current source, and reference character V denotes a 
modulation signal applied to the modulator. Further, reference character 
R.sub.3 denotes a resistance of the resistor 19. The resistor 19 is a 
terminal resistor of a supply source of the modulation signal V. 
A description is given of the operation of the optical semiconductor 
device. When a forward current flows from the DC current source I.sub.0 
toward the LD surface electrode 9, laser light is produced in the active 
layer 3 of the LD 200. When a modulation signal V that reversely biases 
the pn junction within the modulator 100 (V&lt;0) is applied to the modulator 
surface electrode 8, the active layer 3 in the modulator 100 absorbs the 
laser light produced in the active layer 3 in the LD 200. The state in 
which the modulator 100 absorbs the laser light is hereinafter called a 
"0" pattern. On the other hand, when the modulation signal V is 0 V, the 
laser light emitted from the LD 200 is not absorbed in but travels through 
the modulator 100, and this state is hereinafter called a state where the 
optical semiconductor device 303 is in a "1" pattern. 
In the conventional optical semiconductor device including the integrated 
modulator and LD 14, although the modulator 100 and the LD 200 are 
separated from each other by the separation groove 41 reaching the second 
p type InP cladding layer 6, since the modulator 100 and the LD 200 are 
connected by the cladding layer 6 and the semiconductor layers under the 
cladding layer 6, the insulation resistance r between the modulator 100 
and the LD 200 is not sufficient. Therefore, when a modulation signal 
having a voltage V&lt;0 is applied to the modulator 100 to operate the 
optical semiconductor device in the "0" pattern, the current flowing in 
the LD 200 is reduced from I.sub.0 to I.sub.0 +V/r (V&lt;0). When the LD 
driving current is reduced, the carrier concentration in the active layer 
3 is reduced, whereby the refractive index of the active layer 3 is 
increased. As a result, the oscillating wavelength shifts toward the 
longer wavelength. Therefore, when the light intensity is changed between 
"1" and "0" by changing the modulation signal V, the wavelength of light 
emitted from the LD 200 through the modulator 100 is changed. 
Generally, the velocity of light traveling through an optical fiber depends 
on the wavelength of the light. When a 1.3 .mu.m zero dispersion optical 
fiber is used in a wavelength band of 1.55 .mu.m, the velocity of light 
traveling through the fiber decreases with an increase in the wavelength 
of the light. So, the longer the wavelength of the light traveling through 
the fiber is, the wider the width of the optical pulse becomes as it 
travels a long distance, whereby the waveform is unfavorably distorted, 
resulting in a degradation of the transmission performance. Therefore, in 
optical fiber communication using the conventional optical semiconductor 
device 303 as a light source, when a modulation signal having a voltage 
V&lt;0 is applied to the modulator 100 to operate the optical semiconductor 
device in the "0" pattern, the oscillating wavelength unfavorably shifts 
toward a longer wavelength, the width of the optical pulse increases, and 
the waveform is distorted, resulting in a degradation of the optical pulse 
transmission performance. 
In order to solve the problems mentioned above, the injection current to 
the LD 200 is increased when a modulation signal is applied to make the 
modulator 100 absorb the laser light, and the injection current is 
decreased when a modulation signal is applied to make the modulator 100 
not absorb the laser light. Thereby, the wavelength of the modulated light 
becomes longer when the optical semiconductor device 303 is operated at 
the "1" level than when the optical semiconductor device 303 is operated 
at the "0" level. When light having such a wavelength variation is 
transmitted through the optical fiber mentioned above, the width of the 
optical pulse is reduced, so that the waveform is hardly distorted. 
In order to realize the operation mentioned above, conventionally, the 
power supply for driving the LD 200 is provided with a special circuit for 
changing the LD driving current synchronously with the circuit outputting 
the modulation signal that drives the modulator 100. 
Although the circuit for changing the LD driving current can prevent 
degradation of transmission performance, the circuit is very expensive 
because it includes complicated electronic circuits, such as a 
synchronizing circuit and a current modulating circuit. As a result, the 
device cost is significantly increased. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an optical 
semiconductor device that hardly degrades optical pulse transmission 
performance, at a low cost. 
Other objects and advantages of the invention will become apparent from the 
detailed description that follows. The detailed description and specific 
embodiments described are provided only for illustration since various 
additions and modifications within the scope of the invention will be 
apparent to those of skill in the art from the detailed description. 
According to a first aspect of the present invention, an optical 
semiconductor device comprises a semiconductor laser diode having a first 
electrode receiving a current for driving the laser diode, and a grounding 
electrode; a modulator for modulating light emitted from the semiconductor 
laser diode, the modulator having a second electrode receiving a current 
for driving the modulator, and a grounding electrode connected to the 
grounding electrode of the semiconductor laser diode; a first resistor 
having a terminal connected to the first electrode of the laser diode; a 
second resistor connected between the second electrode of the modulator 
and the grounding electrode of the modulator; and a third resistor having 
a first terminal connected to the grounding electrode of the modulator and 
a grounded second terminal. Therefore, the laser diode driving current can 
be made larger when the modulator absorbs the laser light than when the 
modulator does not absorb the laser light, whereby the oscillating 
wavelength is shortened, resulting in an inexpensive optical semiconductor 
device that hardly degrades the optical pulse transmission performance. 
According to a second aspect of the present invention, the above-mentioned 
optical semiconductor device further includes an inductor connected, in 
series, between the first electrode of the laser diode and the first 
resistor. Therefore, the phase of the current flowing through the laser 
diode can be delayed from the phase of the modulation signal that drives 
the modulator. 
According to a third aspect of the present invention, the above-mentioned 
optical semiconductor device further includes an inductor connected 
between the grounding electrode of the modulator and a node of the second 
resistor and the third resistor. Therefore, the phase of the current 
flowing through the laser diode can be delayed from the phase of the 
modulation signal that drives the modulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a perspective view illustrating an optical semiconductor device 
according to a first embodiment of the present invention. In FIG. 1, an 
optical semiconductor device 300 includes a dielectric substrate 16, such 
as Al.sub.2 O.sub.3, having opposed front and rear surfaces. The rear 
surface of the substrate 16 is metallized for grounding. An integrated 
modulator and LD 14 is disposed on the front surface of the dielectric 
substrate 16 via a mounting electrode 21. The structure of the integrated 
modulator and LD 14 is shown in FIG. 3. The integrated modulator and LD 14 
according to the first embodiment of the invention is identical to the 
conventional one already described with respect to FIG. 10 and, therefore, 
does not require repeated description. A grounding strip line 20 
comprising Au or the like is connected to the metallized rear surface of 
the substrate 16 via a through-hole 15. An LD feeding strip line 17 and a 
modulator feeding strip line 18 are disposed on the front surface of the 
substrate 16 and connected to the LD surface electrode 9 and the modulator 
surface electrode 8, respectively, with wires 22 comprising Au or the 
like. Reference numerals 31, 32, and 33 designate first, second, and third 
resistors comprising a material having a resistance sufficiently higher 
than the resistance of the material of the strip lines, for example, 
tantalum nitride (Ta.sub.2 N). 
FIG. 2 is an equivalent circuit diagram illustrating the optical 
semiconductor device shown in FIG. 1. In FIG. 2, the same reference 
numerals as those shown in FIG. 1 designate the same or corresponding 
parts. Reference character V.sub.0 denotes a DC voltage source, and 
character V denotes a modulation signal applied to the modulator 100. 
Further, reference characters R.sub.0, R.sub.1, and R.sub.2 denote 
resistances of the first, second, and third resistors 31, 32, and 33, 
respectively. 
In the optical semiconductor device 300 according to the first embodiment 
of the invention, the integrated modulator and LD 14 is mounted on the 
dielectric substrate 16 comprising Al.sub.2 O.sub.3 or the like. The 
grounding rear electrode 11 is common to the modulator 100 and the LD 200. 
Further, the LD surface electrode 9 is connected to the first resistor 31 
with the wire 22, and the first resistor 31 is connected through the LD 
feeding strip line 17 to the DC voltage source V.sub.0. The modulator 
surface electrode 8 is connected to the modulator feeding strip line 18 to 
which the modulation signal V is applied and, further, it is connected 
through the second resistor 32 and the mounting electrode 21 to the common 
grounding electrode 11 of the integrated modulator and LD 14. Furthermore, 
the common grounding electrode 11 is grounded through the mounting 
electrode 21, the third resistor 33, the grounding strip line 20, and the 
through-hole 15. 
A description is given of the operation of the optical semiconductor device 
300. When a forward current flows from the DC source V.sub.0 through the 
first resistor 31 toward the LD surface electrode 9, laser light is 
produced in the active layer 3 of the LD 200. This laser light travels 
through the active layer 3 of the modulator 100 and is emitted from a 
facet of the integrated modulator and LD 14. When a modulation signal V 
that reversely biases the pn junction within the modulator 100 (V&lt;0) is 
applied through the modulator feeding strip line 18 to the modulator 
surface electrode 8, the active layer 3 of the modulator 100 absorbs the 
laser light produced in the active layer 3 of the LD 200. When the laser 
light is absorbed in the modulator 100, the optical semiconductor device 
300 is in the "0" pattern, as described for the conventional device. When 
the modulation signal V is 0 V, laser light produced in the LD 200 is not 
absorbed in but travels through the modulator 100. In this state, the 
optical semiconductor device 300 is in the "1" pattern. 
Although there is an insulation resistance r between the modulator surface 
electrode 8 and the LD surface electrode 9 in the optical semiconductor 
device 300 as described for the conventional device, this insulation 
resistance r can be ignored because R.sub.1 &lt;&lt;r. So, a current I flowing 
in the LD 200 is given by 
##EQU1## 
Since R.sub.1 + R.sub.2 becomes a terminal resistor for a modulator power 
supply (not shown) that outputs the modulation signal V, when an internal 
impedance of the modulator power supply is given by Z, R.sub.1 +R.sub.2 is 
equal to Z. Further, since R.sub.0, R.sub.1, R.sub.2 &gt;0, V.sub.0 &gt;0, and 
V.ltoreq.0, the current flowing through the LD 200 when the integrated 
modulator and LD 14 is at the "0" level, i.e., when the voltage V applied 
to the modulator surface electrode 8 is smaller than 0 and the modulator 
100 absorbs laser light, is larger than the current flowing through the LD 
200 when the integrated modulator and LD 14 is at the "1" level, i.e., 
when the voltage V is equal to 0 and the modulator 100 does not absorb 
laser light. This difference in the current values is given by 
##EQU2## 
Therefore, the current flowing through the LD surface electrode 9 when the 
integrated modulator and LD 14 is operated at the "0" level can be always 
larger than the current flowing through the electrode 9 when the 
integrated modulator and LD 14 is operated at the "1" level. 
For example, when Z=50.OMEGA., V=-1V, R.sub.0 =45.OMEGA., R.sub.1 
=20.OMEGA., and R.sub.2 =30.OMEGA. in formula (2), .DELTA.I=10mA is 
attained as a change of the current I. Usually, the rate of change in the 
LD oscillating wavelength to the injected current is about -0.1 .ANG./mA, 
so that a wavelength change of about -1 A is realized when 
.vertline..DELTA.I.vertline.=10 mA. That is, when the integrated modulator 
and LD 14 is operated at the "0" level, the oscillating wavelength is 
shortened by about 1 .ANG. as compared to the case where it is operated at 
the "1" level. 
As described above, according to the first embodiment of the invention, the 
optical semiconductor device 300 includes the first resistor 31 connected 
to the LD surface electrode 9 of the integrated modulator and LD 14, the 
second resistor 32 connected between the modulator surface electrode 8 and 
the common grounding electrode 11, and the third resistor 33 connected 
between the common grounding electrode 11 and the ground. Therefore, the 
current flowing through the LD surface electrode 9 is larger when the 
integrated modulator and LD 14 is operated at the "0" level than when the 
integrated modulator and LD 14 is operated at the "1" level, whereby the 
wavelength is shortened. In this case, unwanted reduction in the LD 
driving current when the integrated modulator and LD 14 is at the "0" 
level is avoided without using a special circuit for changing the current 
in response to the modulation signal, resulting in an inexpensive optical 
semiconductor device that hardly degrades the optical pulse transmission 
performance. 
Embodiment 2! 
FIG. 4 is a perspective view illustrating an optical semiconductor device 
in accordance with a second embodiment of the present invention. In FIG. 
4, the same reference numerals as those shown in FIG. 1 designate the same 
or corresponding parts. Reference numeral 23 designates an inductor, for 
example, a wire inductor comprising Au and having a diameter of 25 .mu.m. 
FIG. 5 is an equivalent circuit diagram of the optical semiconductor device 
shown in FIG. 4. In the figure, the same reference numerals and characters 
as those shown in FIG. 2 designate the same or corresponding parts. 
Reference character L.sub.1 denotes an inductance of the inductor 23. 
The optical semiconductor device according to this second embodiment is 
identical to the optical semiconductor device according to the first 
embodiment except that the inductor 23 is connected in series between the 
first resistor 31 and the LD surface electrode 9. 
A description is given of the operation of the optical semiconductor 
device. When the frequency of the modulation signal V driving the 
modulator 100 is denoted by f, the driving current I(f) for the LD 200, 
which changes with a change of the modulation signal V, is given by 
##EQU3## 
wherein j is the imaginary unit. At this time, the phase delay of the 
driving current I(f) from the phase of the modulation voltage V is given 
by 
##EQU4## 
Therefore, although the difference in phases between the modulation signal 
V and the LD driving current that changes in response to the modulation 
signal V is zero in the first embodiment of the invention, since the 
inductor 23 having an inductance L.sub.1 is included in the optical 
semiconductor device according to this first embodiment, the phase of the 
LD driving current can be delayed from the phase of the modulation signal 
V. 
For example, when a wire inductor comprising Au and having a diameter of 25 
.mu.m and a length of 1 mm is employed as the inductor 23, the inductance 
L.sub.1 of the wire inductor 23 is 0.87 nH. In this case, when R.sub.0 
=45.OMEGA., V=-1 V, Z=50.OMEGA., R.sub.1 =20.OMEGA., R.sub.2 =30.OMEGA., 
and f=2.5 GHz in formula (4), .theta.=13.5.degree. stands. Further, from 
formula (2), the absolute value of the difference in current values 
flowing through the LD 200 between when the integrated modulator and LD 14 
is at the "1" level and when it is at the "0" level, i.e., 
.vertline..DELTA.I.vertline., is 10 mA. 
As described above, according to the second embodiment of the invention, 
since the optical semiconductor device includes the inductor 23 between 
the LD surface electrode 9 and the first resistor 31, the phase of the LD 
driving current can be delayed from the phase of the modulation signal V. 
Embodiment 3! 
FIG. 6 is a perspective view illustrating an optical semiconductor device 
in accordance with a third embodiment of the present invention. In FIG. 6, 
the same reference numerals as those shown in FIG. 4 designate the same or 
corresponding parts. Reference numeral 24 designates an inductor, for 
example, a wire inductor comprising Au and having a diameter of 25 .mu.m. 
FIG. 7 is an equivalent circuit diagram of the optical semiconductor device 
shown in FIG. 6. In the figure, the same reference numerals and characters 
as those shown in FIG. 5 designate the same or corresponding parts. 
Reference character L.sub.2 denotes an inductance of the inductor 24. 
The optical semiconductor device according to this third embodiment is 
identical to the optical semiconductor device according to the second 
embodiment except that the common grounding electrode 11 is connected 
through the inductor 24 to the resistor 32 and the resistor 33 whereas the 
inductor 23 is connected between the LD surface electrode 9 and the first 
resistor 31. 
Also in this optical semiconductor device, the driving current I(f) for the 
LD 200 is given by 
##EQU5## 
and the delay of the phase of the driving current I(f) from the phase of 
the modulation voltage V is given by 
##EQU6## 
Therefore, as in the second embodiment of the invention, the phase of the 
change of the LD driving current can be delayed from the phase of the 
modulation signal V. 
In the foregoing description, emphasis has been placed upon an optical 
semiconductor device including a DFB-LD and a modulator integrated on a 
semiconductor substrate. However, the present invention may be applied to 
any optical semiconductor device as long as it includes an LD and a 
modulator for modulating light output from the LD, with the same effects 
as described for the first to third embodiments.