Optical signal transmitter

An optical transmitter may obtain suitable optical output waveform within a wide temperature range from a semiconductor laser diode with an APC free system. The optical transmitter may be composed of a semiconductor laser diode, a driver circuit operatively connected to the semiconductor laser diode for supplying a bias current and a pulse current corresponding to the level of an input signal to the semiconductor laser diode, a bias current controller operatively connected to the driver circuit for controlling the level of the bias current, a pulse current controller operatively connected to the driver circuit for controlling the level of the pulse current, and a duty variable circuit operatively connected to the driver circuit for setting the duty of the input signal to make the duty of an optical output from the semiconductor laser diode more than 100% at an initial adjustment.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical signal transmitter. More 
particularly, it relates to an optical signal transmitter having a driver 
circuit composed of a semiconductor laser diode without including a feed 
back loop for optical output control. 
2. Description of the Related Art 
FIG. 12 shows a general optical power and current characteristic of a 
semiconductor laser diode, which varies with temperature. To generate a 
constant optical output P.sub.0 without depending on the temperature, it 
is required to control, at least, one of a bias current and a pulse 
current, according to the temperature fluctuation. 
More particularly, in the case where the optical output P.sub.0 can be 
obtained by the bias current I.sub.B and the pulse current I.sub.P on 
normal temperature. It is required for generating the optical output 
P.sub.0 as the same as that on high temperature, for example, to make the 
currents I.sub.B and I.sub.P to a bias current I.sub.B, and a pulse 
current I.sub.P', respectively. 
In a conventional driver circuit for semiconductor laser diode, is provided 
on each of a bias current supply section and a pulse current supply 
section not shown in the diagram, a semiconductor element such as a diode 
having a negative temperature characteristic, which is the characteristic 
of decreasing a voltage between terminals of the semiconductor element due 
to the rise of temperature. The driver circuit is constructed to increase 
a bias current and a pulse current in accordance with decreasing the 
voltage and to obtain constant output power. 
Meanwhile, to obtain a suitable transmission characteristic, optical output 
waveform having a large optical quenching ratio, i.e., a ratio of optical 
output of "1" and "0", and an eye opening characteristic (eye pattern) is 
desired. Therefore, the conventional semiconductor laser driver circuit 
controls a bias current to be Ib=Ith (Ith means a threshold value for 
outputting light, when exceeding the value). 
In recent years, an optical transmitter and receiver has been demanded for 
use in a data transmitting system, which transmits optical parallel 
signals between computers. In an optical transmitter having a 
semiconductor laser diode (LD), it is general to employ an Automatic Phase 
Control (APC) system using a feed back loop, which controls an optical 
output from the semiconductor laser diode to be a constant, based on 
information obtained by monitoring a part of the optical output. 
However, in an optical transmitter and receiver for transmitting the 
above-described parallel signals, there has been inconvenience to employ 
the Automatic Phase Control (APC) system, which employs the feed back 
loop. 
A plurality of semiconductor laser diodes should be used to transmit 
parallel signals. In the APC system, it is required to monitor optical 
output power of all semiconductor laser diodes. Therefore, a plurality of 
photo diodes are required to be installed to monitor each of the outputs 
of the plurality of the semiconductor laser diodes, and then cause 
problems for mounting spaces. 
In an optical output control system, which does not employ the APC system, 
i.e., an APC free system; 
an turn-on delay time t.sub.d of LD, can be calculated with the expression 
of 
EQU Td=.tau.s ln ((If-Ib)/(If-Ith) (1) 
where .tau.s is a carrier life time, If is a LD current, and Ib is a bias 
current. The threshold value (Ith) of a semiconductor laser diode is 
increased by the expressed turn-on delay time (T.sub.d) of LD, according 
to the rise of temperature. As shown in FIG. 13, the turn-on delay time 
T.sub.d of the optical output is generated thereby. Then, the turn-on 
delay time Td becomes large as the difference between Ith and I.sub.B 
becomes large. Therefore, optical output waveform in a phase direction is 
crushed, so that an optical output cannot be stably obtained within a wide 
temperature range. 
Further, even if a duty is controlled based on temperature in the 
semiconductor laser driver circuit, the duty of the output is changed 
according to the input waveform. Therefore, it is impossible to obtain 
stable optical output waveform. 
More particularly, it is difficult to install photo diodes system for 
monitoring, when employing the APC system in an optical parallel 
transmission. Even if the APC free system is employed, problems occur, 
such that suitable optical output waveform cannot be obtained within a 
wide temperature range. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an optical 
signal transmitter employing a semiconductor laser modulating system to 
obtain suitable optical output waveform within a wide temperature range. 
Further, it is another object of the present invention to provide an 
optical signal transmitter employing a semiconductor laser modulating 
system, in which suitable optical waveform can be obtained within a wide 
temperature range for not only a continuous signal but a burst signal with 
no dependence to an input signal. 
It is other object of the present invention to provide an optical signal 
transmitter employing a semiconductor laser modulating system to obtain a 
suitable optical output waveform within a wide temperature range from a 
semiconductor laser in an APC free system. 
It is further object of the present invention to provide an optical signal 
transmitter employing a semiconductor laser modulating system to obtain a 
suitable optical output waveform within a wide temperature range with no 
dependence to an input signal in an APC free system. 
A basic structure of the present invention to realize the above-described 
objects of the present invention may have a semiconductor laser diode, a 
driver circuit, which supplies a bias current and a pulse current 
corresponding to a size of an input signal to the semiconductor laser, a 
pulse current controller, which controls a size of the pulse current, a 
bias current controller, which controls the size of the bias current, and 
a duty variable circuit, which changes the duty of the input signal. The 
duty variable circuit sets the duty of the input signal so as that the 
duty of optical output waveform from the semiconductor laser diode becomes 
100% or more at the time of initial adjustment. 
Therefore, it becomes possible to reduce distortion of output optical 
waveform, even if light output delay is generated according to temperature 
fluctuation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Examples of the present invention will be explained according to the 
drawings. Throughout the following descriptions, the same reference 
numerals are used to denote and identify corresponding or identical 
components. 
To better understand the present invention, a principle of the present 
invention will be explained. FIGS. 1A and 1B show a principle of the 
present invention. More particularly, FIG. 1A show optical waveform in an 
APC free system in the case where the optical output waveform is 
T=T.sub.0, and the duty is set to 100%. 
It is well-known for an ordinary skilled person in the art that the optical 
output waveform develops a tendency to reduce the duty according to the 
turn-on delay time, due to temperature fluctuation, as shown in FIG. 13. 
The turn-on delay time T.sub.d (T.sub.1) is marked as a deterioration of 
an optical output in a phase direction on the highest temperature 
(T.sub.1). 
It is apparent from FIG. 1A that the turn-on delay T.sub.d is generated and 
waveform distortion is generated on a temperature T=T.sub.1, higher than 
the temperature T=T.sub.0. More particularly, a failure of bits is 
generated on high temperature side according to the turn-on delay T.sub.d. 
That causes hindrance in communication. 
FIG. 1B shows the case where the duty of an output of the semiconductor 
laser driver circuit is changed to initially set optical output waveform 
to more than 100%. As shown in FIG. 1B, for example, an initial phase is 
set fast by an adjustment Td (T.sub.1)/2 on the side of low temperature 
T(=T.sub.0) to make the duty of the optical output, 100+.alpha. %. 
Therefore, when the temperature rises (T=T.sub.1), it becomes possible that 
deterioration of an optical output in a phase direction be less than Td 
(T.sub.1)/2. 
FIG. 2 is a block diagram for explaining a first example of the present 
invention. A semiconductor laser 1 is given a bias current and a pulse 
current corresponding to an input data by the driver circuit 4. Further, a 
controller 6, which controls the bias current and a controller 7, which 
controls the pulse current are provided as the driver circuit. 
Further, in FIG. 2, a duty variable circuit 3 varies the duty of waveform 
of an electrical signal inputted corresponding to the set of resistance 
value in the duty controller 5. Further, reference numeral 23 is a flip 
flop circuit. 
With the structure, in FIG. 2, the F/F circuit 23 forms the waveform of an 
electrical signal of the input data to be the duty of 100%, at first. 
Further, the output waveform of the F/F circuit 23 is set to have the duty 
of more than 100% by the duty variable circuit 3. 
In this way, the semiconductor laser diode 1 is driven by the driver 
circuit 4, according to the data, of which duty is changed and set. 
In the structure shown in FIG. 2, it is possible to make the optical output 
duty not to depend on an input waveform by once forming or shaping the 
waveform in the F/F circuit. Further, the duty controller 5 includes a 
temperature-sensitive element 8, such as a thermistor or the like, and 
supplies a current change due to a temperature rise to the duty variable 
circuit 3. The duty controller 5 controls the duty variable circuit 3 to 
compensate the decrease of the optical output waveform duty according to 
an turn-on delay time due to the temperature rise by increasing the output 
waveform duty. Therefore, the fluctuation of the optical waveform 
according to temperature can be reduced. 
FIGS. 3A and 3B show the above-described mode. In FIGS. 3A and 3B, an 
electrical waveform T=T.sub.0 and T=T.sub.1, and optical output waveform 
at the temperature T=T.sub.0 and T=T.sub.1 are shown, respectively. 
It becomes apparent from FIG. 3A that the duty of the electrical output 
waveform of the duty variable circuit 3 is controlled to more than 100% 
according to temperature fluctuation. Therefore, as shown in FIG. 3B, 
illustrating the diagram of optical output waveform, when the temperature 
T=T.sub.1 (&gt;T.sub.0), as compared with the case where there is no 
compensation of duty temperature as the result of providing the 
temperature compensating element 8, such as a thermistor, in the duty 
controller 5 shown with the dotted line, waveform distortion due to the 
waveform delay can be reduced. 
FIGS. 4A and 4B further show a structural example of the duty variable 
circuit 3 and the duty controller 5 in the structural block diagram shown 
in FIG. 2. Further, FIG. 5 is a structural example of the driver circuit 4 
shown in FIG. 2. As referring to these figures, the operation of the first 
mode will be further explained. 
In FIG. 4, the duty variable circuit 3 comprises a differential pair 
composed of a pair of transistors TR.sub.1 and TR.sub.2, of which emitters 
are commonly connected to a constant current source 30 and collectors are 
connected to resistances R.sub.1 and R.sub.2. Data is inputted to a base 
of the transistor TR.sub.1, which is one of the differential pair. A duty 
controller 5 is connected to a base terminal of the other transistor 
TR.sub.2. 
As one example, the duty controller 5 is formed of a serial circuit 
composed of a temperature-sensitive element 8, such as a thermistor or the 
like, and a variable resistor R.sub.v. A potential of the connecting point 
between the temperature-sensitive element 8 and the variable resistor 
R.sub.v is supplied to a base terminal of the transistor TR.sub.2. 
Further, when the temperature T=T.sub.0, the variable resistance value is 
adjusted so that the output of the duty variable circuit 3 has more than 
100% duty as an initial value. 
Accordingly, the potential of the connecting point between the 
temperature-sensitive element 8 and the variable resistance R.sub.v 
becomes a reference potential of the data inputted to the base of the 
transistor TR.sub.1, which is one of the differential pair. The collector 
of the transistor TR.sub.2, which is the other of the differential pair, 
is further inputted to the base of transistor TR.sub.3 having a grounded 
collector on the next stage. The emitter of the transistor TR.sub.3 is 
connected to a constant current source 32, and outputs the duty-controlled 
data. 
In FIG. 4A, when alternating data signal, of which waveform is shaped in 
the flip flop FF 23 (see FIG. 2), is inputted to the transistor TR.sub.1, 
and the base potential of the transistor TR.sub.1 exceeds a reference 
voltage supplied from the duty controller 5, the transistor TR.sub.1 is 
switched to ON, and the transistor TR.sub.2 is switched to OFF. On the 
other hand, when the base potential of the transistor TR.sub.1 is smaller 
than the reference voltage supplied from the duty controller 5, the 
transistor TR.sub.1 is switched to OFF, and the transistor TR.sub.2 is 
switched to ON. 
Then, a data signal, which is an electrical signal, has a leading time 
refer to a waveform shown in FIG. 4B!. Consequently, the size of the 
reference voltage supplied from the duty controller 5 causes the change of 
timing for switching the transistors TR.sub.1 and TR.sub.2. That mean, for 
example, the reference voltage from the duty controller 5 is V.sub.ref1, 
the transistor TR.sub.1 is switched to ON with a timing of .tau.1. In 
addition, when the reference voltage is V.sub.ref2, the transistor 
TR.sub.1 is switched to ON with a timing of .tau.2. 
By adjusting the variable resistance R.sub.v in the duty controller 5, as 
shown in FIG. 3, when the temperature is high (T =T.sub.1), it is possible 
to set the duty to more than 100% on the step of the electrical signal, 
according to a principle of the present invention shown in FIG. 1, without 
receiving distortion due to light output delay. 
In the circuit shown in FIG. 4A, the duty controller 5, which includes a 
temperature-sensitive element 8, such as a thermistor, develops a negative 
resistance value against the temperature fluctuation. Accordingly, the 
potential of a connecting point of the temperature-sensitive element 8 and 
the variable resistance R.sub.v is fluctuated according to the 
temperature. However, the fluctuation gives a direction for removing 
influence of light output delay according to the temperature. Thereby, as 
explained later, even if the APC free system is employed, it is possible 
to give duty, which can solve the above-described problem of light optical 
delay due to the temperature fluctuation. 
In this way, the data signal, of which duty is controlled, is inputted to 
the driver circuit 4. A structure of the driver circuit 4 shown in FIG. 5 
includes an input side differential circuit 40, an emitter-follower 
circuit 41, and the bias current supplier 43. Further, the pulse current 
controller 6 and the bias current controller 7 are connected to the driver 
circuit 4. 
The input side differential circuit 40 is composed of a pair of transistors 
Q.sub.1 and Q.sub.2, of which level is alternated according to the data of 
"1" or "0", and to which an input data signal DT and the reference voltage 
V.sub.ref are inputted. The duty of the input data signal and the 
reference voltage are controlled in the above-described duty variable 
circuit 3. 
The emitter follower circuit 4 includes transistors Q.sub.3 and Q.sub.4, 
which adjust the level of a differential output of the input side 
differential circuit 40. Further, an output side differential circuit 42 
is composed of a pair of transistors Q.sub.5 and Q.sub.6. 
The above-described data signal DT is inputted to a base terminal of a 
first transistor Q.sub.1 of the input side differential circuit 40, and a 
constant reference voltage, i.e. fixed reference voltage V.sub.ref, is 
inputted to a base terminal of a second transistor Q.sub.2. Further, each 
of the emitter terminals of the transistors Q.sub.1 and Q.sub.2 are 
connected to the negative source voltage V.sub.SS through a constant 
current source I.sub.O, and each of the collector terminals is connected 
to a current voltage V.sub.DD, i.e., a ground, through resistors R.sub.1, 
R.sub.2, and R.sub.3. 
When the input data signal DT is larger than the reference voltage 
V.sub.ref, the transistor Q.sub.1 is switched to ON, and the transistor 
Q.sub.2 is switched to OFF. Further, the input data signal is smaller than 
the reference voltage V.sub.ref, the transistor Q.sub.1 is switched to 
OFF, and the transistor Q.sub.2 is switched to ON. 
Several of diodes D1 to D4 are inserted to emitter terminals of each of the 
transistors Q.sub.3 and Q.sub.4, which form an emitter follower circuit 
41, in order to adjust the level. Each collector of the transistors 
Q.sub.1 and Q.sub.2 in the input side differential circuit 40 is connected 
to each of the base terminals of the transistors Q.sub.3 and Q.sub.4. 
Additionally, each of the collector terminals of the transistors Q.sub.3 
and Q.sub.4 is connected to the source voltage V.sub.DD (ground), and each 
of the emitter terminals are connected to the negative source voltage 
V.sub.SS through diodes for controlling the level and constant sources 
I.sub.I and I.sub.2. 
The collector terminal of the first transistor Q.sub.5 of the output side 
differential circuit 42 is connected to the source voltage V.sub.DD 
(ground), and the collector terminal of the second transistor Q.sub.6 is 
connected to a semiconductor laser diode 1. Each of the base terminals of 
the transistors Q.sub.5 and Q.sub.6 is connected to each of the emitter 
terminals of the transistors Q.sub.3 and Q.sub.4 in the emitter follower 
circuit 41. Further, the emitter terminals of the transistors Q.sub.5 and 
Q.sub.6 are connected to the source voltage V.sub.SS through the third 
transistor Q.sub.7 and the diode D5. 
Further, the base terminal of the third transistor Q.sub.7 is connected to 
the pulse current controller 6 to control the bias current I.sub.B 
according to a voltage V.sub.gs between the base and the emitter. The 
pulse current controller 6 including a variable resistance VR1 connected 
between the ground and the source voltage V.sub.SS controls the pulse 
current I.sub.P by adjusting a voltage between the base and the emitter of 
the transistor Q.sub.7. 
The driver circuit 4 further includes a bias current supplier 43. The bias 
current supplier 43 provides a transistor Q.sub.8 and a diode D6. A 
collector terminal of the transistor Q.sub.8 is connected to the 
semiconductor laser diode 1. The base terminal of the transistor Q.sub.8 
is connected to the bias current controller 7 to control the bias current 
I.sub.B with the voltage V.sub.gs between the base and the emitter. 
The pulse current controller 6 provides a variable resistor VR1 connected 
between a ground and a source voltage V.sub.SS to control the pulse 
current I.sub.P by adjusting a voltage between the base and the emitter of 
the transistor Q.sub.7 in the pulse current supplier 42. The bias current 
controller 7 provides a ground and a variable resistor VR2 connected 
between the ground and the source voltage V.sub.SS to control the bias 
current I.sub.B by adjusting a voltage between the base and the emitter of 
the transistor Q.sub.8 in a bias current supplier 43. 
When the level of the data signal DT, which is an output of the duty 
variable circuit 3, is larger than the reference voltage V.sub.ref 
(data="1"), the transistor Q.sub.1 in the differential circuit 40 is 
switched to ON, and the transistor Q.sub.2 is switched to OFF. As the 
result, the transistor Q.sub.5 in the output side differential circuit 42 
is switched to ON, and the transistor Q.sub.6 is switched to OFF, thus the 
pulse current I.sub.P could not flow to the semiconductor laser diode 1, 
and the light cannot be outputted. 
Meanwhile, when the level of the data signal DT is smaller than the 
reference voltage V.sub.ref (data="0"), the transistor Q.sub.1 in the 
differential circuit 40 is switched to OFF, and the transistor Q.sub.2 is 
switched to ON. In this respect, the transistor Q.sub.5 in the output side 
differential circuit 42 is switched to OFF, and the transistor Q.sub.6 is 
switched to ON, thus the pulse current I.sub.P is flowed to the 
semiconductor laser diode 1 and, then the light is outputted. In this 
case, the desired optical output P.sub.0 can be obtained by adjusting 
variable resistor VR1 in the pulse current controller 6 and a variable 
resistor VR2 in the bias current controller 7. 
FIG. 6 is a block diagram for explaining the second example of the present 
invention. In the second example, as compared with the first example shown 
in FIG. 2, the duty variable circuit 3 and the duty controller 5 are 
omitted. Further, it is different from the structure of the driver circuit 
4 shown in FIG. 2 to allocate FETs Q.sub.1 to Q.sub.8, instead of the 
transistors Q.sub.1 to Q.sub.8. 
Temperature-sensitive elements 60 and 70, such as a thermistor, are 
respectively inserted to each of the pulse current controller 6 and the 
bias current controller 7, serially with the variable resistors VR1 and 
VR2. It is also possible to have only one of the temperature-sensitive 
elements 60 and 70, according to the mode of the later-explained 
embodiment. 
As similar with FIG. 2, an input signal is made to the duty of 100%, and 
the pulse current and the bias current are set to current values for each 
temperature according to the principle explained with respect to FIGS. 1 
and 3, respectively, by the pulse current control circuit 6, which is 
composed of thermistor 60 and variable resistor VR1 and bias current 
control circuit 7, which is composed of thermistor 70 and variable 
resistor VR2. With this structure, it is possible to individually control 
each of the pulse current and the bias current according to the 
temperature fluctuation. 
FIG. 7 shows a first control example based on a feature of making it 
possible to respectively control to each of the pulse current and bias 
current. In this control example, it is controlled so as to fix the pulse 
current and change only the size of the bias current according to the 
temperature. In FIG. 6, a thermistor 70 in the bias current controller 7 
is omitted. Accordingly, when the temperature varies from T.sub.0 to 
T.sub.1, the bias current I.sub.b is increased. On the other hand, the 
pulse current I.sub.P is set to a predetermined value according to the 
existence of the temperature-sensitive element 70. 
In this way, the bias current I.sub.b is controlled to vary according to 
I-L characteristic of the semiconductor laser 1 and the characteristic of 
a threshold current Ith of the semiconductor laser diode 1, according to 
the temperature. As stabilization of optical output may be performed not 
by optical output data but by the temperature, the stable optical output 
can be obtained without dependence to the input signal and photo diode for 
monitoring. 
Accordingly, the optical output of the bias current I.sub.b becomes 
constant on each temperature according to the I-L characteristic of the 
semiconductor laser diode 1, and it is set as I.sub.b .ltoreq.Ith. 
Consequently, even if the I-L characteristic of the semiconductor laser 
diode 1 is changed due to the rise of the time of rising the temperature, 
it becomes apparent from the above-described equation (1) that the optical 
output is approximately constant, and the turn-on delay time T.sub.d 
becomes smaller than the fixed bias current I.sub.b, as the bias current 
I.sub.b becomes larger according to the rise of temperature. 
FIG. 8 shows a second control example based on a feature of making it 
possible to respectively control to each of the pulse current and the bias 
current. In this control example, the bias current is fixed, i.e., it is 
controlled so that only the size of the pulse current is changed according 
to the temperature. 
In FIG. 6, a thermistor 70 in the pulse current controller 7 is omitted. 
Accordingly, compensation is not performed with respect to the increase of 
the pulse current due to the rise of the temperature. Therefore, when the 
temperature varies from T.sub.0 to T.sub.1, the pulse current I.sub.P is 
increased. On the other hand, the bias current I.sub.b is set to a 
predetermined value according to the existence of the 
temperature-sensitive element 60. 
In this way, the pulse current I.sub.P is changed based on the I-L 
characteristic of the semiconductor laser diode 1 and the characteristic 
of a threshold current Ith of the semiconductor laser 1, according to the 
temperature. As stabilization of an optical output is performed not only 
by optical output data but by the temperature, the stable optical output 
can be obtained, without dependence to the input signal, even to a burst 
signal. 
Accordingly, the pulse current I.sub.P is set so that an optical output 
becomes constant and the relation is set as I.sub.P .ltoreq.Ith, according 
to the I-L characteristic of the semiconductor laser diode 1. 
Consequently, the oscillation delay time T.sub.d becomes smaller than the 
fixed bias current I.sub.P, because the optical output becomes 
approximately constant and I.sub.P becomes large due to the rise of 
temperature, according to the already explained equation (1), even where 
the I-L characteristic of the semiconductor laser diode 1 is changed due 
to the rise of the temperature. 
FIG. 9 shows a block diagram showing a third example of the present 
invention, which shows other structural example of the pulse current 
controller 6 and the bias current controller 7 shown in FIG. 6. In the 
structural example, each of the pulse current controller 6 and the bias 
current controller 7 is composed of a current absolute value controller, 
which decides an absolute value of each control voltage, and a temperature 
characteristic controller, which decides temperature characteristic, 
according to the characteristic of the thermistor, to make it easy to 
adjust the current value. 
That is, the current absolute value controller, which decides the absolute 
value of the control voltage, is composed of a serial circuit of a 
variable resistor VR and a resistor R.sub.1 inserted between reference 
voltages, and a temperature characteristic controller for deciding 
temperature characteristic is composed of a serial circuit of a thermistor 
8 and a resistance R.sub.2. Then, the current absolute value controller 
and the temperature characteristic controller are separated by the 
arithmetic amplifier 80. 
FIG. 10 shows a fourth embodiment of the present invention. The pulse 
current and the bias current are controlled for the semiconductor laser 1 
according to the outputs of the pulse current controller 6 and the bias 
current controller 7. Then, when the pulse current and the bias current 
are large, as thermal-runaway is generated on the current of the 
semiconductor laser diode 1, the semiconductor laser diode 1 may be 
destroyed. 
That is, FIG. 10 shows an embodiment for solving such the problem. The 
structure shown in FIG. 10 includes a comparator 90 and an analog switch 
91. A limit voltage V.sub.Lim and the output of the pulse current 
controller 6 or the bias current controller 7 are inputted to the 
comparator 90. Simultaneously, these inputs are inputted to the analog 
switch 91. 
When the comparator 90 detects that the output of the pulse current 
controller 6 or the bias current controller 7 exceeds to the limit voltage 
V.sub.Lim, an alarm signal is outputted. The alarm signal notifies a 
possibility of thermal runaway of the semiconductor laser diode 1 to 
operators. Simultaneously, the analog switch 91 is switched, and the 
output of the pulse current controller 6 or the bias current controller 7 
is stopped, and the limit voltage V.sub.Lim is outputted. 
The limit voltage V.sub.Lim is sent to the driver circuit 4, and the pulse 
current and the bias current corresponding to the limit voltage V.sub.Lim 
are limited, so that the thermal runaway of the semiconductor laser diode 
1 can be prevented. 
FIG. 11 shows a fifth embodiment of the present invention, which shows a 
structure of adjusting equipment for an optical signal transmitter 
according to the present invention, employing an APC free modulating 
system. In FIG. 11, reference numeral 50 is an optical signal transmitter 
according to the present invention. An optical power meter 51 monitors an 
optical output power outputted from the optical signal transmitter 50 
through an optical fiber 52. A signal generator 53 generates an input 
signal, and a variable voltage source 54 sets initial values of a current 
value setting voltage, a pulse current value setting voltage and the duty. 
Further, a computer 55 controls the output signal of the signal generator 
53 and controls a variable current source 54 with monitoring the signal 
outputted from the power meter 51. The bias current, the pulse current, 
and the duty in the optical signal transmitter 50 are automatically set 
initially. 
In this embodiment, the voltage value of the variable voltage source 54 is 
adjusted so that the value of the optical power meter 51, when changing a 
mark ratio of the output of the above-described signal generator 53 to 0, 
1/2, 1, becomes a desired value. 
Further, it is possible to obtain a compact device by making circuitry 
parts of the optical signal transmitter according to the present invention 
into a monolithic IC. Then, it is also possible to make it more compact by 
employing Band Gap Reference (BGR) as a reference voltage source when 
making the circuitry parts to a monolithic IC. 
Alternatively, a timer provided in a CPU is employed on the device. A used 
time of the optical signal transmitter according to the present invention 
can be detected by the timer. Accordingly, it becomes possible to control 
the bias current, the pulse current, and the duty according to the used 
time. 
As explained according to the above-described embodiments of the present 
invention, a suitable optical output characteristic within a wide 
temperature range in an optical signal transmitter may be obtained, and 
more particularly, an APC free system, which is a semiconductor laser 
modulating system effective to optical parallel transmission may be 
obtained. Accordingly, it becomes possible to construct a system, which 
can use an optical signal transmitter using the semiconductor laser diode 
effectively, in order to send and receive data transmission of the 
computer as an optical signal. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential characteristic thereof. The present 
embodiment is therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.